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Control of Aldosterone Secretion: A Model for Convergence in Cellular Signaling Pathways

Department of Physiology and Laboratory of Cellular and Molecular Physiology, Semmelweis University and Hungarian Academy of Sciences, Budapest, Hungary

ABSTRACT

I. INTRODUCTION: MULTIPLE CONTROL OF ALDOSTERONE SECRETION—MULTIPLICITY OF SIGNAL TRANSDUCTION PATHWAYS IN THE GLOMERULOSA CELL

II. CALCIUM SIGNAL GENERATION IN GLOMERULOSA CELLS

A. Receptor-Mediated Ca2+ Release

1. ANG II receptors

2. G proteins

3. Phosphoinositide metabolism and formation of inositol phosphates

4. Ca2+ release by IP3

5. Actions of higher inositol phosphates and polyphosphoinositides

B. Ca2+ Transport Through the Plasma Membrane

1. Membrane potential and Ca2+ transport

2. Capacitative influx

3. Voltage-activated Ca2+ currents and Ca2+ channels

4. Action of ANG II on voltage-activated Ca2+ channels

5. DHP-sensitive Ca2+ channels and IP3-induced Ca2+ release

6. Ca2+ influx induced by K+

7. Na+/Ca2+ exchange

8. Ca2+-eliminating mechanisms

C. Diacylglycerol-PKC and Lipoxygenase Pathways

D. Vasopressin: A Transiently Acting Paracrine Agonist

III. EFFECT OF CYTOPLASMIC CALCIUM ON MITOCHONDRIAL FUNCTION

IV. CROSS-TALK BETWEEN CALCIUM AND cAMP-ACTIVATED PATHWAYS

A. Ca2+ Influx Evoked by ACTH

B. Ca2+-Induced Formation of cAMP

V. REGULATION AT THE LEVEL OF PLASMA MEMBRANE RECEPTORS

A. Mechanisms for Regulation of GPCRs

B. Regulation of Adrenal Angiotensin Receptors In Vivo

C. Desensitization of Glomerulosa Cells

D. Intracellular Trafficking of Angiotensin Receptors

E. Receptor Downregulation

VI. LONG-TERM EFFECTS OF ANGIOTENSIN II

A. Activation of Growth Responses by ANG II in the Glomerulosa Cell

B. Role of Receptor and Nonreceptor Tyrosine Kinases

C. ANG II-Induced Activation of MAPKs

VII. THE FINAL ACTION: ROLE OF CALCIUM IN THE CONTROL OF STEROID PRODUCTION

A. Cholesterol Transport

B. Increased Reduction of Pyridine Nucleotides in Mitochondria

C. Induction of Aldosterone Synthase

	ABSTRACT

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	I. INTRODUCTION: MULTIPLE CONTROL OF ALDOSTERONE SECRETION—MULTIPLICITY OF SIGNAL TRANSDUCTION PATHWAYS IN THE GLOMERULOSA CELL

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The steroid hormone aldosterone, secreted by the glomerulosa cells of the adrenal cortex, controls sodium and potassium balance and also influences acid-base homeostasis of the vertebrate organism. Its major physiological targets are the epithelial cells, of which the most important are located in the distal nephron. It augments Na⁺ reabsorption as well as K⁺ and H⁺ excretion. Through changes in sodium balance, it influences the extracellular space and blood pressure. In addition to its epithelial actions, aldosterone influences the function of the cardiovascular system by acting on the heart, vessels, and central nervous system. Aldosterone secretion is increased during acute or chronic sodium depletion or fluid loss, erect postural position, dietary potassium loading, and tissue damage leading to hyperkalemia. In view of its essential role in maintaining extracellular fluid and thereby circulation, it is not surprising that its secretion is controlled by several factors. The list of hormonal and paracrine factors reported to exert a stimulatory effect on aldosterone production in vitro is quite long (Table 1), and the number of proposed inhibitory factors is also remarkable (Table 2). However, under physiological conditions the control of secretion is probably confined to the stimulatory factors corticotrophin (ACTH), angiotensin II (ANG II), and K⁺ and the inhibitory factor atrial natriuretic hormone (ANP). In fact, most or all increases in aldosterone secretion may be attributable to increased activity of the renin-angiotensin system and/or increased plasma level of K⁺. When sodium or fluid loss is severe, ACTH is also secreted and synergizes with ANG II or K⁺ in stimulating glomerulosa cells. ANP secretion is increased in response to sodium and/or water loading, and it in turn inhibits aldosterone secretion. The roles of other factors (shown in Tables 1 and 2) in the physiological control of aldosterone secretion may not be essential.

Signal transduction in the adrenal cortex has been studied for almost half a century. The role of cAMP in the stimulation of steroid production by pituitary trophic hormones was one of the earliest discoveries in this field, leading to the concept of second messengers (232). The formation and mode of action of cAMP have been described in several reviews. Our review focuses on the mechanism and role of Ca²⁺ signaling, paying special attention to the interaction of Ca²⁺ signaling with other signal-transducing mechanisms. The Ca²⁺ dependence of the secretory process was described by Douglas and Rubin four decades ago (142), and it is now well established that Ca²⁺ acts by inducing the exocytosis of secretory vesicles. This principle is not applicable for steroid-producing cells, which lack secretory vesicles and are devoid of an exocytotic mechanism. The steroid precursor cholesterol is stored in lipid droplets, and the rate of hormone secretion depends on the rate of hormone synthesis. However, the activation of hormone synthesis is Ca²⁺ dependent, and the regulatory mechanism involves both Ca²⁺-mediated and Ca²⁺-permitted processes.

Following the pioneering observation of Hokin and Hokin (241) of acetylcholine-induced increase in phospholipid turnover of pancreatic and brain cortical slices in 1955, systematic studies on the role of Ca²⁺ in agonist-induced biological response were initiated in 1975 by Michell's hallmark paper (355). After surveying data on the mode of action of several hormones and neurotransmitters, he hypothesized that Ca²⁺-dependent agonists activate phospholipase C and induce the hydrolysis of phosphatidylinositol and that this breakdown triggers, in turn, the influx of Ca²⁺ from the extracellular fluid. It was only later discovered that also the phosphatidylinositol derivative phosphatidylinositol 4,5-bisphosphate is cleaved (356). Subsequently, it was firmly established that the primary consequence of this breakdown is the formation of inositol 1,4,5-trisphosphate (IP₃), a water-soluble second messenger which primarily induces Ca²⁺ release from intracellular stores, rather than Ca²⁺ influx from the extracellular space (53, 437, 522).

Adrenal glomerulosa cells are a cell type in which Ca²⁺ and cAMP are equally significant in stimulation-secretion coupling. The effect of ACTH is mediated by cAMP, that of ANG II by Ca²⁺ and diacylglycerol (DAG), and that of K⁺ by Ca²⁺. ANP acts by antagonizing Ca²⁺ signaling. The basic framework of the IP₃-Ca²⁺-DAG system in adrenal glomerulosa cells was characterized in the 1980s and reviewed subsequently (32, 189, 503, 509). Therefore, classical data on the phosphoinositide-Ca²⁺-DAG system will only be reviewed concisely here. We will primarily deal with the control of Ca²⁺-releasing and influx mechanisms, the interaction of various signaling pathways, the long-term effects of Ca²⁺-mobilizing agonists, and the mechanism terminating Ca²⁺ signaling. Finally, we summarize current views on the basis of Ca²⁺-induced steroid secretion.

	II. CALCIUM SIGNAL GENERATION IN GLOMERULOSA CELLS

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The two most important physiological stimuli of aldosterone secretion, ANG II and extracellular K⁺ (reviewed in Ref. 570), exert their effects through generating cytoplasmic Ca²⁺ signal. The octapeptide ANG II is formed from the plasma protein angiotensinogen by the sequential action of two proteases, renin and angiotensin-converting enzyme. Under resting conditions its plasma concentration in humans and rats is between 10 and 60 pM (309, 492). ANG II level rises above 100 pM in response to administration of furosemide or dietary sodium depletion, whereas it may attain several hundred picomoles per liter following hemorrhage or water deprivation (337, 492). Plasma levels between 1 and 2 nM were measured under severe pathological conditions (e.g., malignant hypertension) (337).

Renin expression and ANG II production also occur in adrenal glomerulosa cells (371, 372). Within the adrenal cortex, immunolabeling for renin and prorenin is not limited to the steroid-producing cells of the zona glomerulosa but is also present in chromaffin cells, which occur singly or in groups throughout the whole cortex (45, 594). In this latter cell type the expression of angiotensinogen mRNA and the presence of ANG II in chromaffin granules were also shown (594), suggesting a paracrine action of ANG II released from vicinal chromaffin cells. The adrenal expression of renin mRNA is activated by the recognized physiological stimuli of aldosterone secretion, namely, ANG II (593), ACTH (576), and K⁺ (102, 281, 576). Transcription of the (pro)renin gene is also enhanced in dietary sodium depletion (566). In transgenic rats termed TGR(mREN2)27, the murine ren-2 renin gene, originally detected in the submaxillary gland, is expressed predominantly in the adrenal cortex. Although plasma renin and ANG II concentrations are low in these animals, aldosterone secretion is high, indicating that intra-adrenal production of renin can effectively stimulate the glomerulosa cells by stimulating local ANG II production (419). Sodium restriction also increases adrenal renin activity and mRNA in such transgenic rats (480). The observation that the AT₁-type ANG II receptor antagonist DuP 753 attenuates K⁺-stimulated and ACTH-stimulated aldosterone production (209) suggests that the intra-adrenal renin-angiotensin system functions as a local amplifier of systemic stimuli. Renin, in addition to acting as a soluble enzyme, has been found to bind to a 350-amino acid membrane protein. This binding results not only in increased catalytic activity, but also in the activation of mitogen-activated protein kinases (MAPKs) within the respective cell (385). No data are yet available as to whether this binding protein is expressed in the adrenal cortex. Additional studies are required to assess the tissue concentration of ANG II within the glomerulosa zone. Such data would be essential for the evaluation of the physiological relevance of experimental results obtained with exogenous ANG II.

ANG II-stimulated aldosterone production by incubated rat glomerulosa cells exhibits a biphasic dose-response curve. Steroidogenesis is stimulated in the physiological concentration range of 10^–11 M, maximal effect is attained at 10^–9 M, above which level hormone output is reduced (97, 230, 323). In superfusion system ANG II increases aldosterone production of isolated cells by two orders of magnitude (24, 447).

Aldosterone secretion in vivo (76) and production in vitro (74, 180, 238, 563) are stimulated by increases in K⁺ concentration as small as a few tenths of millimolar. Maximal hormone production is attained by [K⁺] around 8–10 mM both in capsular (glomerulosa) tissue (225, 368) and cell suspension experiments (323, 432), whereas decreased hormone production can be observed at [K⁺] between 10 and 20 mM (33, 442). The falling phase of the K⁺-aldosterone dose-response curve is due to the rapid decay of aldosterone output, which follows in time the steep onset of the response (28).

ANG II is a classical Ca²⁺-mobilizing ligand: it induces Ca²⁺ release from IP₃-sensitive intracellular stores, and this release is followed by Ca²⁺ influx from the extracellular space. The mode of action of K⁺ is quite different, because its primary site of action is a voltage-operated Ca²⁺ channel. Nevertheless, how the exceptional sensitivity of glomerulosa cell to K⁺ is achieved is a question still only partially elucidated. In the forthcoming sections we first describe the primary actions of the two agonists and later deal with additional actions that ensure the fine-tuning of regulation.

A. Receptor-Mediated Ca²⁺ Release

Glomerulosa cells do not constitute a homogeneous population. Expression of aldosterone synthase is confined to the two or three outer cell layers of the rat zona glomerulosa (221, 396, 418). The function of glomerulosa cells not expressing aldosterone synthase is not clear; moreover, presently no method is available for correlating aldosterone production and Ca²⁺ response at the single-cell level. Individual cells also differ in their response to different stimuli. Although the majority of the cells generate a Ca²⁺ signal in response to both K⁺ and ANG II, only a smaller percentage of the cells respond to vasopressin (AVP) (447) and still less to ACTH (551). Immunocell blot assay data suggest that 20–25% of the glomerulosa cells release ANG II (103).

This cellular heterogeneity is also reflected by the variable Ca²⁺ response of single cells to ANG II and K⁺ (119) (Fig. 1). However, some general characteristics of the response may be observed. Similar to other Ca²⁺-mobilizing agonists (47, 545), ANG II evokes an oscillating signal at physiological concentrations (usually below 300 pM). The hormone concentration affects the frequency rather than the amplitude of the single spikes. Gradual confluence of the spikes occurs at higher concentrations of the peptide, and the Ca²⁺ response becomes sustained in the nanomolar angiotensin II range (rat, Refs. 447–449, 466; bovine cells, Refs. 107, 269, 479). Also, the onset of the response exhibits a concentration-dependent delay (447, 466). Oscillatory Ca²⁺ signals show a gradual decrease in spiking frequency and/or amplitude after a few minutes (269, 433), which results in a gradually decreasing Ca²⁺ signal in cell suspension, where the response of 100,000 or more cells is averaged. (The underlying mechanism may also account for the declining [Ca²⁺] during the sustained phase of the Ca²⁺ signal in single cells exposed to higher concentration of ANG II.) Ca²⁺ spiking is maintained in the absence of extracellular Ca²⁺ for at least 20 min, indicating that oscillating Ca²⁺ release occurs from the IP₃-sensitive intracellular store (479). The frequency of Ca²⁺ oscillations is reduced by nifedipine, suggesting that voltage-activated Ca²⁺ channels have a supporting role during the process (479). Several models have been proposed for the mechanism of Ca²⁺ oscillation in other cell types (for review, see Refs. 178, 490), but none of these has been thoroughly tested for steroid-secreting cells.

View larger version (20K): [in this window] [in a new window]   FIG. 1. Effect of angiotensin II (ANG II) applied at a physiological concentration of 150 or 300 pM (37°C) on cytoplasmic and mitochondrial [Ca2+]. Cytoplasmic [Ca2+] was monitored with Fura Red (red), and changes in mitochondrial [Ca2+] were followed with rhod 2 (blue) by confocal microscopy in MitoTracker Green-preloaded single glomerulosa cells. The y-axis shows the fluorescence ratio of (Fo-F)/Fo for Fura Red and (F-Fo)/Fo for rhod 2, where F is the actual fluorescence intensity and Fo is the averaged fluorescence intesity of the control period. Note the heterogeneity of the Ca2+ response of individual cells. (Recordings courtesy of A. Spät.)

When cytoplasmic Ca²⁺ concentration ([Ca²⁺]_c) is examined in cell suspension, i.e., a statistical average of several hundred thousand cells is monitored, the Ca²⁺ signal induced by low concentration of ANG II is characterized by a slow increase followed by a gradual decrease, whereas that induced by supraphysiological concentration consists of an initial peak followed by a smaller plateau (24, 255, 301). If extracellular Ca²⁺ is chelated in a superfusion system after the onset of ANG II-induced aldosterone production, hormone output falls below resting levels (503). This phenomenon indicates that the maintenance of [Ca²⁺]_c at a suprabasal level is essential for maintaining hypersecretion of aldosterone (269, 300, 435, 449).

1. ANG II receptors

ANG II is the major physiological regulator of aldosterone secretion, cell growth, and proliferation of glomerulosa cells. The effects of ANG II in glomerulosa cells and other target tissues are mediated by binding to heptahelical, G protein-coupled receptors. Two different binding sites of ANG II, termed AT₁ and AT₂, were identified in the rat adrenal cortex, and the AT₁ receptor was found to be the predominant isoform (17, 140, 579), whereas the bovine cortex contains almost exclusively AT₁ receptors (17). Immunocytochemical localization of the receptors in the adult rat confirmed that glomerulosa cells contain AT₁ receptors, whereas AT₂ is expressed in the medulla (177, 501). Nonpeptide receptor antagonists have demonstrated that AT₁ receptors mediate the ANG II-induced enhanced formation of IP₃ and depolarization (217), inhibition of adenylyl cyclase (17), stimulation of aldosterone production (17, 217), and the growth-promoting effect of ANG II in glomerulosa cells (see sect. VIA). However, AT₂ receptors appear to activate protein phosphatases, the nitric oxide-cGMP system, and phospholipase A₂ (390)_. AT₂ receptors inhibit cell growth and stimulate apoptosis, and the expression of this receptor is increased during fetal growth and tissue regeneration (132, 360). In contrast to other cloned mammalian AT₁ receptors, rat and mouse AT₁ receptors exist as two distinct subtypes, termed AT_1A and AT_1B. They are 95% identical in their amino acid sequences and have very similar ligand binding and activation properties, but differ in tissue distribution and transcriptional regulation. Although most ANG II target tissues express predominantly AT_1A receptor mRNA, in the adrenal and the pituitary glands the AT_1B message is the major subtype (132, 192). In contrast to pituitary cells, in rat and murine adrenal cells the mRNA of AT_1A receptors can also be detected (144, 192).

Competition curves of ANG II in radioligand-binding assays are characterized by a slope factor significantly lower than 1. These curves were originally explained by the presence of high- and low-affinity AT₁ receptor-binding sites. Depending on the experimental conditions in rat and bovine glomerulosa cells, the high-affinity sites have dissociation constant (K_d) values around or below 1 nM, whereas the affinity of the low-affinity site is 1 order of magnitude lower (73, 133). Both uncoupling of the receptor from G proteins, by nonhydrolyzable GTP analogs (see references in Ref. 132) and desensitization of the receptor, markedly reduces the number of high-affinity sites (see sect. VC). Although the two-state model is now generally considered to be an oversimplification of the receptor activation process (257), the effects of G protein coupling and/or receptor phosphorylation on binding affinity are considered to be general features of many G protein-coupled receptors (GPCRs).

2. G proteins

Catt and co-workers (194) described the inhibition of the binding of ANG II by guanine nucleotides as early as 1974. The underlying mechanism of this phenomenon was elucidated by the discovery of heterotrimeric G proteins 6 years later (464). Of the different heterotrimeric G proteins in glomerulosa cells, G_i, the inhibitory G protein of adenylyl cyclase, was the first to be identified on the basis of pertussis toxin-induced ADP-ribosylation of a 41-kDa protein (162). Immunoblotting revealed G_i (and the related G_o) in bovine glomerulosa cells (344). These observations were in harmony with the previous finding that ANG II reduces, rather than enhances, ACTH-induced and serotonin-induced cAMP production in rat glomerulosa cells (43, 230, 463) and adenylyl cyclase activity of membrane preparations (587). G_i seems to couple the activated ANG II receptor with T-type voltage-activated Ca²⁺ channels in bovine and L-type Ca²⁺ channels in both rat and bovine glomerulosa cells (see sect. IIB).

The primary effect of calcium-mobilizing agonists, such as ANG II, is the hydrolysis of phosphoinositides by a phosphoinositide-specific phospholipase C (PLC). Although -subunits derived from G_i/o may activate the ₂-isoform of PLC (454), inhibition of G_i by pertussis toxin failed to influence the ANG II-induced and AVP-induced activation of phosphoinositide metabolism (36, 162, 204, 296) or aldosterone production (230). Therefore, it may be assumed that the ₂-isoform of PLC is not expressed in the glomerulosa cell; however, nonhydrolyzable GTP analogs increased the formation or potentiated the ANG II-induced formation of IP₃ by permeabilized glomerulosa cells and glomerulosa membrane preparations (36, 162, 473). These observations indicated the involvement of another G protein in the action of ANG II. It was demonstrated in 1992 that the two pertussis toxin-insensitive G proteins, G_q and G₁₁ (the G_q family of heterotrimeric G proteins), mediate the effects of ANG II and AVP on PLC (210). Few data are available on G_q in adrenocortical cells. It was found that the G_q/G₁₁ protein is associated with the cytoskeleton and that this association is essential for the translocation of cytosolic G protein to the plasma membrane (121). As shown with the application of a specific antibody, _q/11 mediates the inhibitory effect of ANG II on Ca²⁺-dependent K⁺ (BK) channels (408).

The third heterotrimeric G protein participating in the control of steroid-producing cell types is G_s, the stimulator of adenylyl cyclase. The presence of _s, the guanyl nucleotide binding subunit of G_s, in glomerulosa cells was demonstrated by Western blot analysis. This also showed that a 1-min exposure to ACTH induced a large but transient translocation of _s from the cytoskeleton to the plasma membrane (122).

The site of action of heterotrimeric G proteins in the glomerulosa cell is summarized in Figure 2.

View larger version (16K): [in this window] [in a new window]   FIG. 2. Site of action of heterotrimeric G proteins in the glomerulosa cell. Mcr2, melanocortin type 2 receptor. The red arrows indicate stimulatory actions, and the green arrows indicate inhibitory actions. (Inhibition of adenylyl cyclase by Gi may not be effective in bovine cells; see sect. IVB.)

3. Phosphoinositide metabolism and formation of inositol phosphates

Enhancement of phosphoinositide turnover by ANG II, but not by the two other physiological stimuli of the glomerulosa cell, K⁺ and ACTH, was described two decades ago (153, 245, 580). The effect of ANG II did not require extracellular Ca²⁺ (246), indicating that reduction of the radioactively labeled phosphatidylinositol pool was not secondary to ANG II-induced Ca²⁺ influx.

It was R. H. Michel who first presented data showing that agonist stimulation resulted in a rapid, phosphodiesterase-catalyzed breakdown of phosphatidylinositol, phosphatidylinositol 4-phosphate, and phosphatidylinositol 4,5-bisphosphate (PIP₂) that did not appear to be a response to changes in [Ca²⁺]_c (356). Subsequent work by Berridge, Irvine, Schulz, and co-workers (52, 522) definitely confirmed in 1983 that Ca²⁺-mobilizing agonists induce the breakdown of PIP₂, resulting in the formation of the water-soluble, Ca²⁺-releasing second messenger IP₃ and DAG. Shortly after this landmark discovery, the ANG II-induced rapid breakdown of PIP₂ (161, 172, 289) and the ensuing formation of IP₃ (18, 161, 188, 473) were observed in bovine and rat glomerulosa cells.

Whereas the ACTH-induced cAMP response does not evoke the formation of IP₃ (161, 172), vasopressin (22, 204, 589), endothelin (588), and acethylcholine (293), all acting in a paracrine mode within the adrenal gland, enhance the formation of IP₃. The hydrolyzing enzyme, the phosphoinositide-specific PLC, is activated not only by hormonal or neurotransmitter stimuli (through G_q), but also by Ca²⁺ (158, 366). The K⁺-induced moderate breakdown of PIP₂ (218) is compatible with such a Ca²⁺ action.

Although ANG II- (18, 161) and AVP-induced increases in IP₃ (204) peak within 10–15 s and decrease thereafter, a second phase of enhanced PIP₂ breakdown may last as long as stimulation is persisting. In the presence of lithium, an inhibitor of inositol polyphosphate 1-phosphatase and of inositol monophosphatase, labeled inositol phosphates, exhibit continuous accumulation, indicating the sustained activation of PLC (161, 289). In contrast to the initial formation of IP₃ (246, 473), the sustained phase of ANG II-stimulated IP₃ formation is dependent on Ca²⁺ influx (248, 590). This observation may be accounted for by the Ca²⁺ requirement of PLC activity (158, 375) and is consistent with the Ca²⁺ requirement of the sustained (but not the initial) phase of ANG II-stimulated aldosterone production (503).

The sustained phase of stimulation with ANG II is characterized not only by increased formation of inositol phosphates but also by a change in the ratio of different inositol phosphate fractions. Dephosphorylation by 5-phosphomonoesterase is the prevailing metabolic route of IP₃ in resting cells and is also a significant pathway in ANG II-stimulated cells (18). Another metabolic route is the sequential phosphorylation of IP₃ to inositol 1,3,4,5-tetrakisphosphate (IP₄) by IP₃ 3-kinase (262) and dephosphorylation to its biologically inactive product, inositol 1,3,4-trisphosphate. Ca²⁺ signaling may activate IP₃ -kinase (59), leading to increased formation of IP₄ and inositol 1,3,4-trisphosphate, as also observed in bovine glomerulosa cells (18, 23, 475). In the course of sustained stimulation, inositol 1,3,4-trisphosphate is consecutively converted into inositol 1,3,4,6-tetrakisphosphate (23, 250), inositol 1,3,4,5,6-pentakisphosphate, and inositol 1,4,5,6-tetrakisphosphate (25). An alternative pathway of the pentakisphosphate metabolism, which seems to be significant only after several hours of stimulation, is the formation of two further inositol tetrakisphosphate stereoisomers (1,4,5,6- and 3,4,5,6-P₄) (20).

Of these compounds, increased labeling of inositol 1,3,4-trisphosphate and inositol 1,3,4,5-tetrakisphosphate has been found in rat glomerulosa cells exposed to ANG II (19, 505) or AVP (204). The formation of an unspecified pentakisphosphate was also detected, and its labeling was not influenced by AVP within 30 min (204). The small number of glomerulosa cells that can be isolated from rat adrenal (a few hundred thousands per rat) has discouraged further physiologal studies on higher inositol polyphosphate metabolism in these cells.

4. Ca²⁺ release by IP₃

Before the availability of Ca²⁺-sensitive fluorescent dyes, Ca²⁺ release from an intracellular particle into the cytoplasm in response to hormones and neurotransmitters was revealed by the application of isotope flux techniques. Similar to the effect of acetylcholine on the salivary gland (387), ANG II was found to enhance Ca²⁺ efflux from rat glomerulosa cells, irrespective of the presence of extracellular Ca²⁺ (27). These observations indicated that these agonists induce intracellular Ca²⁺ release and that their primary action does not require Ca²⁺ influx. Moreover, in ANG II-stimulated glomerulosa cells the applied isotope flux technique revealed a nonmitochondrial origin of the released Ca²⁺ (27). IP₃-induced Ca²⁺ release from a nonmitochondrial intracellular store was first demonstrated in permeabilized pancreatic acinar cells, in 1983 (522). This observation was soon confirmed in permeabilized bovine glomerulosa cells (289) and in insulinoma microsomes (436). High-affinity IP₃-binding sites were described in 1986 by Spät and coworkers in permeabilized peritoneal polymorphonuclears and hepatocytes (507) and liver microsomes (510). The concentration dependence of binding and Ca²⁺ release were comparable, showing that the binding sites represented biologically active receptors. Whereas Scatchard analysis revealed both high-affinity and low-affinity binding compartments in liver microsomes (426, 510), only a single (high-affinity) compartment of binding sites was observed in adrenocortical microsomes (37, 94, 201, 403). It is possible that the lack of low-affinity binding sites in the adrenal cortex was caused by Ca²⁺-induced conversion of low-affinity to high-affinity binding sites (367, 427).

In glomerulosa cells, in harmony with other cell types, the K_d of IP₃ binding to the (high-affinity) receptor (10^–9 M) is about two orders of magnitude lower than the EC₅₀ of IP₃-induced Ca²⁺ release (201). This discrepancy may be accounted for by the different experimental conditions (temperature and absence or presence of Mg-ATP) of binding and transport studies. The redox state of thiol groups also influences the binding of the ligand (203). It has also been proposed that Ca²⁺ release by IP₃ in other cell types depends on the occupancy of low-affinity binding sites (426). It is possible that the IP₃ concentration in confined regions of stimulated cells might attain 10^–6 to 10^–5 M, which may induce maximal Ca²⁺ release also from low-affinity receptors (reviewed in Ref. 546).

Fractionation of liver homogenates (510) as well as electron microscopic studies on cerebellar Purkinje cells (486) confirmed the localization of the IP₃ receptor in the endoplasmic reticulum (ER). That IP₃ acted on the ER was supported later by several studies, among others on adrenocortical membrane fractions (94, 201, 403). Comparison of the effect of IP₃ and thapsigargin on glomerulosa cells indicated that only a subpopulation of ER vesicles can be regarded as IP₃ sensitive (159, 465). These data were in harmony with the results of subfractionation studies on liver that suggested that the receptor may not be evenly distributed in the whole reticulum but concentrated in specialized vesicles, marked with the calcium-binding protein calreticulin (166). In intact exocrine cells (421) and neurons (51), the ER appeared to be a continuous membrane system that has a number of regional specializations. Provided that also in intact glomerulosa cells ER is luminally continuous, IP₃ receptors may cluster in confined regions that could appear as a separate population of vesicles after homogenization.

IP₃ receptors have been identified in the plasma membrane of T lymphocytes (304). However, these receptors probably differ from the ER receptor, since activation of such nonselective cation channels in the plasma membrane would cause depolarization; that is not the case. IP₃ receptors in the plasma membrane were also reported in adrenocortical cells (474). Observations on adrenocortical cells (202) and liver homogenates (166, 471) strongly suggest that IP₃ receptor detected in the plasma membrane fraction is localized in ER vesicles, loosely attached to the plasma membrane by cytoskeletal elements. Additional IP₃ binding sites, described in the nuclear envelope and in the Golgi apparatus, as well as in secretory vesicles, (presently) have no known relevance in steroid-producing cells, and therefore we refer to a recent review (405).

The IP₃ receptor type cloned first (IP₃R1) contains 2,749 amino acid residues, corresponding to a molecular mass of 313 kDa. It consists of three functional domains, the cytosolic NH₂-terminal ligand-binding domain, a COOH-terminal channel-forming domain, and an interconnecting coupling domain. The COOH-terminal 550 amino acid residues contain six membrane-spanning regions, which form the ion-conducting pore. The receptor expressed in the brain may differ from that expressed in peripheral tissues by splice variance: the former containing, and the latter missing, a 40-amino acid segment (SII) in the coupling domain. IP₃ receptors and ryanodine receptors (RyR) share 40% amino acid sequence identity in their COOH-terminal, pore-forming domains and show some sequence identity in the NH₂-terminal, cytosolic ligand binding domains (for review, see Refs. 55, 542). Following the cloning and characterization of IP₃R1, two further types (IP₃R2 and IP₃R3) have been identified. The central nervous system contains almost exclusively IP₃R1 [SII(+) splice variant], while several peripheral cell types express more than one type of IP₃R (reviewed in Refs. 55, 405, 542). Simultaneous with the observation of the coexpression of more than one type of IP₃R in peripheral tissue, including the whole adrenal (containing both cortex and medulla) (384), we reported that rat glomerulosa cells express the mRNAs of all three types of IP₃R (165). About four times more IP₃R1 mRNA was expressed than IP₃R2 mRNA, and the SI segment was present in nearly 50% of the IP₃R1 receptors. Somewhat less than 5% of the total IP₃R was expressed as type 3. Interestingly, sodium restriction for 1 wk, a strong stimulus of aldosterone secretion by glomerulosa cells, failed to influence the ratio of expression of the different receptor types or spliced variants (165).

The receptor affinity for IP₃ has a rank order of IP₃R2 > IP₃R1 > IP₃R3 (364, 384). However, the membrane environment may modify the properties of the receptor (425). Different receptor types may exhibit different downregulation or rates of receptor degradation (498, 585). The receptor has a tetrameric structure of 1.2 kDa (for review, see Refs. 55, 174), and different receptor isoforms may constitute heterotetramers (272). Since the three receptor types differ in their affinity for IP₃ as well as in their sensitivity to controlling factors (see below), the diverse composition of these heterotetrameric receptors may provide more versatile control of IP₃ receptor function.

Physiological concentrations of Ca²⁺-mobilizing agonists usually evoke oscillatory Ca²⁺ signals (see sect. IIA). Although no uniformly valid model for different cell types has yet been presented, oscillations are obviously evoked by positive and negative feedback control of [Ca²⁺]_c by Ca²⁺ itself. Ca²⁺ variously activates different isoforms of PLC, protein kinase C (PKC), plasmalemmal Ca²⁺-ATPase, adenylyl cyclase, and cAMP phosphodiesterase, and cell type-specific expression of these isoforms contributes to the complexity of Ca²⁺ signal generation. Nevertheless, the major factor responsible for Ca²⁺ oscillations is probably the dual effect of Ca²⁺ on IP₃R itself. Elevation of [Ca²⁺]_c within the lower physiological range (up to 300 nM) increases, and further elevation, still in the physiological range, decreases the sensitivity of IP₃R1 to IP₃ (260). Gradual elevation of [IP₃] from 20 nM to 180 µM shifts the peak of the Ca²⁺-dependence curve of channel open probability from 100 nM to 1 µM (275). The biphasic effect of Ca²⁺ on channel activity is extremely important in determining the pattern of Ca²⁺ signaling. The Ca²⁺-dependent enhancement of IP₃R activity implies that Ca²⁺-induced Ca²⁺ release (CICR), similar to that occurring through RyRs, may also occur through the IP₃R provided that [Ca²⁺]_c is <300 nM (55). This process may have an essential role in the generation of cytoplasmic Ca²⁺ oscillations and waves (48). The dependence of the nadir of the bell-shaped Ca²⁺-dependence curve on [IP₃] ensures the coregulation of channel function by Ca²⁺ and IP₃, allowing the generation of short-lasting Ca²⁺ transients at low levels of IP₃, while maintaining high [Ca²⁺]_c during prolonged and intense stimulation of the cell. Ca²⁺ can directly facilitate IP₃-induced Ca²⁺ release (at Glu²¹⁰⁰ in IP₃R1) (56, 224, 365, 405). Calmodulin-dependent kinase II (CaMKII), on the other hand, phosphorylates the IP₃R and potentiates the Ca²⁺-releasing effect of IP₃, thus exerting positive feedback on IP₃R function (83, 598). High [Ca²⁺]_c may reduce IP₃ binding (510). This inhibition, which may be attributed to an increase in the K_d of IP₃ binding to the receptor (367), is independent of calmodulin. However, calmodulin probably exerts tonic inhibition on the IP₃R under physiological conditions (357, 389, 405, 406). In contrast to the bell-shaped Ca²⁺ sensitivity IP₃R1, peaking at 300 nM Ca²⁺, IP₃R2 displays a sigmoidal increase in open probability of the release channel up to 1 µM, and [Ca²⁺]_c exerts a moderate inhibitory effect only above 1 µM (452, but see Ref. 364). Observations on the Ca²⁺ dependence of IP₃R3 are conflicting, since increasing (212), unaltered (364), and decreasing open probability at high [Ca²⁺] were reported (333, 363). The predominant expression of IP₃R1 in the glomerulosa cell is compatible with the observed high affinity of IP₃ binding; moreover, the bell-shaped Ca²⁺ dependence of IP₃-induced Ca²⁺ release is to be expected.

Simultaneous knock-out of the three types of IP₃R in B lymphocytes abolishes agonist-induced Ca²⁺ signals. However, signal generation is maintained if any of the three isoforms can function (523). Nevertheless, when only a single isoform was expressed, different patterns of Ca²⁺ signals were observed. IP₃R2 was required for sustained Ca²⁺ oscillations, and IP₃R1 mediated less regular Ca²⁺ oscillations, while IP₃R3 generated monophasic Ca²⁺ transients (364).

The threshold and pattern of IP₃-induced Ca²⁺ signals are significantly modified by other signal transduction mechanisms. The receptor is phosphorylated in its coupling domain by protein kinase A (PKA), PKC, and CaMKII. The number, and even the site, of the phosphorylation consensus sites varies in the different receptor types (for review, see Refs. 405, 546). In the nonneural [SII(–)] splice variant of IP₃R1, that is predominantly expressed in glomerulosa cells (165), low cAMP concentrations preferentially phosphorylate Ser¹⁵⁸⁹ and therefore potentiate IP₃-induced Ca²⁺ release (80, 215). Substantial phosphorylation of the IP₃R with PKC, as well as augmentation of IP₃ potency in stimulating ⁴⁵ Ca release, were detected after pharmacological inhibition of the Ca²⁺/calmodulin-dependent phosphatase calcineurin, known to be anchored to IP₃R (83). Phosphorylation of IP₃R by PKC and dephosphorylation by calcineurin were reported also in bovine glomerulosa cells (430). The potentiation by PKC of IP₃ action at low [Ca²⁺]_c and the calcineurin-mediated inhibition of IP₃ action at higher [Ca²⁺]_c may contribute to the bell-shaped Ca²⁺-sensitive response to IP₃. However, in bovine glomerulosa cells a persistent high-conductance state was observed in the phosphorylated state of the release channel, suggesting that PKC-induced phosphorylation causes cessation of ANG II-induced Ca²⁺ oscillations (430). Tyrosine phosphorylation and cGMP-dependent phosphorylation of IP₃R were detected in stimulated T lymphocytes and vascular smooth muscle cells, respectively (reviewed in Ref. 405), but no data are available for glomerulosa cells.

ATP exerts a concentration-dependent effect on the function of IP₃R. Applied at micromolar concentrations, it enhances the binding of IP₃ (508, but see Ref. 583) and potentiates the open probability of the IP₃-gated Ca²⁺ channel (for review, see Refs. 55, 405). IP₃R1, the dominant receptor isoform in glomerulosa cells, exhibits greater affinity for ATP and greater ATP sensitivity of IP₃-induced Ca²⁺ release than IP₃R3 (331, 332). Competitive inhibition of the binding of IP₃ was observed with supramicromolar concentrations of ATP in several cell types (391, 512, 582), including adrenocortical cells (94, 474). This competition may be an additional and significant factor in the 10^–7 M EC₅₀ of IP₃-induced Ca²⁺ release (in ATP-containing media), as opposed to the 10^–9 M K_d of IP₃ binding. The question may be raised: how can the potentiating effect of low, and the inhibitory effect of high concentration of ATP on IP₃-induced Ca²⁺ release be reconciled? We assume that following Ca²⁺ release the activated Ca²⁺-ATPase would reduce ATP concentration around the ER. The ensuing disinhibition of IP₃R would facilitate the generation of Ca²⁺ signal.

When describing the control of the different types of IP₃R, emphasis was placed on that of IP₃R1, which is predominantly expressed in glomerulosa cells. IP₃R3 constitutes a minor fraction within the IP₃R pool (165), and its significance has yet to be studied. In this respect the Trp binding site (72) should be kept in mind, since it may be essential for capacitative Ca²⁺ influx (see sect. IIB).

Significant expression of IP₃R in rat adrenal fasciculata-reticularis cells (secreting the glucocorticoid corticosterone) could not be detected. The IP₃R mRNA content was <0.5 molecules/fasciculata cell, while it was 300 copies/glomerulosa cell (524). Since the rat fasciculata cell would be unique among nucleated cells in lacking IP₃R, this unexpected result certainly requires confirmation by other laboratories. Even so, it seems to account for data showing that ANG II is capable of increasing the formation of IP₃ in these cells (468, 581) but fails to evoke Ca²⁺ signals (77, 584) or increased production of corticosterone (581).

5. Actions of higher inositol phosphates and polyphosphoinositides

There is a notable contrast between the number of identified highly phosphorylated inositol phosphates and our knowledge of their role. There are sporadic observations on the effect of a given compound in a special cell type, but confirmation of these observations in other cell types is usually missing. The most studied compound is the first metabolite of IP₃, inositol 1,3,4,5-tetrakisphosphate. It potentiated the Ca²⁺-releasing effect of IP₃ in neuroblastoma (193) and L1210 lymphoma cells (126) as well as in pituitary microsomes (512). It was also found to synergize with IP₃ in inducing Ca²⁺ influx in sea urchin eggs (263) and in exocrine acinar cells (420). It was also proposed that capacitative Ca²⁺ influx is triggered by the interaction of IP₃Rs in the ER vesicle and the vicinal, plasmalemmal IP₄ receptors (261). Adrenocortical microsomes also contain high-affinity binding sites for IP₄ (168), but their function has not been examined. Anti-secretagogue (555) and hemodynamic effects of other inositol tetra- and pentakisphosphate analogs (556) have no relevance for our subject. Data on the modulation of chromatin-remodeling complexes by inositol polyphosphates (494) are not yet available for adrenocortical cells.

It was recognized only in recent years that PIP₂, in addition to being the precursor of the second messengers IP₃ and DAG, controls the function of several membrane-associated proteins. However, phosphoinositides also regulate protein targets through direct binding to specific phosphoinositide-binding domains (266). These domains include pleckstrin homology (PH) domains, Fab1P, YOTB, Vps27p, EEA1 (FYVE) domains, Phox homology (PX) domains and epsin NH₂-terminal homology (ENTH) domains, and other less well-defined lipid-binding structures.

The PH domain is a conserved region of 100–120 amino acids that was first identified in pleckstrin but also present in many other proteins. PH domains have a central role in the formation of molecular complexes involved in signaling and trafficking of receptors. These domains can be categorized into four groups, which preferentially bind phosphatidylinositol 3,4,5-P₃ [PI(3,4,5)P₃] (e.g., Bruton tyrosine kinase), PI(4,5)P₂ (e.g., PLC1, pleckstrin-N, spectrin), or PI(3,4)P₂ (e.g., Akt/PKB) or show no clear ligand specificity (e.g., dynamin) (see references in Ref. 266). The affinity of such domains for specific phosphoinositide ligands was sufficient to monitor ANG II-induced changes in membrane lipid composition using GFP-tagged PH domains (558, 561), and the lipid binding of the PH domain of dynamin is required for -arrestin-dependent AT₁ receptor endocytosis (see references in sect. VD). However, the latter effect cannot be explained by the role of the PH domain in the targeting of dynamin, since the lipid binding is not required for the proper targeting of dynamin to clathrin-coated structures (531). Therefore, the interaction with phosphoinositides can regulate the function or the submicroscopical orientation of PH domain-containing proteins.

FYVE domains bind PI(3)P and are present in proteins involved in the processing of endosomal vesicles (e.g., EEA1) and regulation of actin cytoskeleton (e.g., Fgd1). Inhibition of phosphatidylinositol (PI) 3-kinase has a marked effect on the trafficking of AT₁ receptors expressed in HEK 293 cells by interfering with the proper localization of FYVE domain-containing endosomal proteins (249). PX domains, which also bind 3-phosphoinositides, were first identified in two cytosolic components of NADPH oxidase (p47^phox and p40^phox), but they also occur in a variety of other proteins associated with signaling (e.g., class II PI 3-kinase, phospholipase D) and membrane trafficking (e.g., sorting nexins). Finally, the ENTH domain, which was first identified in epsin, is present in proteins that participate in endocytosis via clathrin-coated pits, and the intact phosphoinositide binding of this domain was shown to be required for clathrin-mediated endocytosis (see references in Ref. 266).

In addition to these well-defined protein folds, the lipid binding domains of plasmalemmal Ca²⁺ pumps, the Na⁺/Ca²⁺ exchanger, Na⁺/H⁺ exchangers, the RyR (for review, see Ref. 237) and the background K⁺ channel TASK (see sect. IIB) may also be relevant to the adrenocortical actions of ANG II. The role of lipid rafts in the plasma membrane in the colocalization of the receptor of the Ca²⁺-mobilizing hormone, PIP₂,G_q/11 and PLC, as well as the transport proteins, is currently being elucidated.

B. Ca²⁺ Transport Through the Plasma Membrane

ANG II-induced Ca²⁺ influx was one of the most contradictory subjects of aldosterone research. Species differences, inappropriately applied experimental techniques, and application of ANG II at concentrations one or two orders of magnitude higher than required for maximal aldosterone production have all delayed the recognition of the physiological role and mechanism of ANG II action.

Several early studies, applying the ⁴⁵Ca flux technique, failed to detect increased Ca²⁺ influx in response to ANG II. This failure may be due to neglecting the significance of measuring the initial rate of isotope uptake. Yet, this technique could reveal the ANG II-induced reduction of the exchangeable Ca²⁺ pool in bovine (107, 154, 289) and rat glomerulosa cells (27). The loss of cell Ca²⁺ is due to increased efflux of Ca²⁺, also detectable in a Ca²⁺-free medium (27, 157, 176, 301, 584). This Ca²⁺ efflux is due to the extrusion of Ca²⁺, released from the IP₃-sensitive store, by the Ca²⁺-activated plasmalemmal Ca²⁺-ATPase. The subsequently verified increase of Ca²⁺ influx in ANG II-stimulated cells is not in contradiction with the sustained efflux; together they perform continuous Ca²⁺ cycling across the plasma membrane. Although a high concentration of ANG II was applied in all the aforementioned studies, in a single report on the effect of ANG II at low concentration no significant change in the exchangeable Ca²⁺ pool could be revealed (107).

The frequently observed dependence of the ANG II-induced sustained Ca²⁺ signal on extracellular Ca²⁺ (89, 301, 467) has been regarded as evidence in favor of ANG II-induced Ca²⁺ influx. The successful demonstration of ANG II-induced increase in the initial influx rate of ⁴⁵Ca confirmed this interpretation in rat (95, 503) as well as bovine glomerulosa cells (159, 290, 301).

Calcium influx immediately following Ca²⁺ release is brought about by the depletion of IP₃-sensitive calcium stores and, subsequently, the slowly developing depolarization activates voltage-operated Ca²⁺ influx. Ca²⁺ is extruded from the cytoplasm by means of the plasmalemmal and ER Ca²⁺ pumps. In addition, Na⁺/Ca²⁺ exchange may modify [Ca²⁺]_c.

1. Membrane potential and Ca²⁺ transport

A major factor controlling Ca²⁺ transport through the plasma membrane is the membrane potential (E_m). Together with the concentration gradient it determines the driving force of Ca²⁺ transport and also regulates the activity of a set of Ca²⁺ channels. Considering that the two major physiological stimuli of aldosterone secretion, K⁺ and ANG II, depolarize the glomerulosa cell, we first discuss the control of E_m in these cells. E_m depends on the distribution of diffusible ions and their selective conductance accross the membrane. The Na⁺-K⁺ and Ca²⁺ pumps are responsible for the uneven distribution of the major cations. The greater the contribution of opened K⁺ channels to the total conductance of the cell membrane, the more the membrane potential approaches the equilibrium potential of K⁺ (E_K), which is in the range of 70–90 mV (inside negative) in excitable cells. Accordingly, a reduction of (the absolute value of) E_K following elevation of extracellular [K⁺] ([K⁺]_o), decrease of intracellular [K⁺], inhibition of Na⁺-K⁺-ATPase, or closure of K⁺ channels will depolarize the cell. This depolarization may, in turn, activate voltage-operated channels.

[K⁺]_o under resting conditions in the rat is 3.5–4 mM (75, 513). At such K⁺ levels E_m may exceed –80 mV, as measured with microelectrodes (445) or by patch-clamp technique (321, 324, 563). This highly negative E_m is due to a dominant K⁺ conductance (g_K), as confirmed by the perforated patch technique, which avoids the rundown of susceptible ionic currents. The linear relationship between log [K⁺]_o and E_m had a slope of 53.7 mV/10-fold change in [K⁺] (323) that almost attains the value predicted by the Nernst equation for a membrane with K⁺ conductance only (58 mV/decade). These findings are consistent with the lack of Na⁺ channel expression in the glomerulosa cell.

The primary step in the energy-dependent generation of membrane potential is the build-up of a transmembrane K⁺ concentration gradient by the Na⁺-K⁺-ATPase ("sodium pump"). Furthermore, the exchange of three cytoplasmic Na⁺ for two extracellular K⁺ also contributes to the generation of intracellular electronegativity ("pump potential"). Reducing the activity of the sodium pump dissipates the K⁺ gradient and results in depolarization. It was revealed a quarter of a century ago that ANG II decreased rather than increased the potassium content of rat glomerulosa cells (329, 526), suggesting that ANG II inhibits the pump. Although ANG II had no significant effect on ouabain-sensitive or potassium-stimulated ATPase activity in adrenal capsular membranes (139, 175), damage of the signal-transducing pathway in such membrane preparations cannot be ruled out. In fact, as shown in our laboratory (213), the uptake of ⁸⁶Rb, a surrogate for K⁺, is abolished by ouabain, the conventional inhibitor of the Na⁺-K⁺ pump. Half-maximal inhibition was attained at 80 µM, indicating that of the two forms of Na⁺-K⁺-ATPase present in rat tissues, the -isoenzyme is the predominant form in the rat glomerulosa cell. The isotope uptake is dependent on cytoplasmic [Na⁺], and it attains half-maximal rate at 10 mM, corresponding again to the -isoenzyme form. Maximal uptake rate is reached at 25 mM Na⁺, which corresponds to the measured physiological level of cytoplasmic Na⁺ (557) and decreases again above 50 mM. The transport activity is 4 and 20 times higher than that in fasciculata cells and hepatocytes, respectively. ANG II reduces the ouabain-sensitive ⁸⁶Rb uptake, i.e., the activity of Na⁺-K⁺-ATPase, and the observed IC₅₀ of ANG II (300 pM) suggests that the inhibitory effect of the peptide has physiological relevance.

ANG II exerts its inhibitory effect on Na⁺-K⁺-ATPase through AT₁ receptors (213). A recent observation suggests that ANG II inhibits the pump through activating a tyrosine phosphatase (596), whereas activation of the pump by Ca²⁺ through CaMKII (597) may serve as negative feedback during intense stimulation with ANG II. When ouabain is applied at submillimolar concentrations, the drug induces a continuous increase in [Ca²⁺]_c without any detectable lag time, and this increase can be abolished with nifedipine (213). At similar concentrations the drug also stimulates the production of aldosterone, again requiring extracellular Ca²⁺ and activable voltage-operated Ca²⁺ channels (213, 488, 527, 595). These data suggest that during ANG II-induced stimulation of aldosterone secretion, inhibition of Na⁺-K⁺-ATPase contributes to the depolarization and Ca²⁺ signal generation in glomerulosa cells.

In addition to Na⁺-K⁺-ATPase, K⁺ conductance is of great significance in setting E_m. Patch-clamp studies revealed inwardly rectifying, outwardly (delayed) rectifying, Ca²⁺-activated as well as background K⁺ current in glomerulosa cells. The ion channel through which the current is conducted has been identified in only a few cases. Of major concern is the estimation of the physiological significance of the observed currents/channels.

Only ion channels that are active in the range of the resting membrane potential may play a role in its fine setting. Members of the superfamily of inwardly rectifying K⁺ channels (386, 481) display a large inward current at potentials negative to E_K (occurring only under laboratory conditions) and a small and gradually diminishing outward current as E_m is shifted from E_K to more positive values. It is this latter K⁺ efflux by which E_m approaches the more negative E_K. Inwardly rectifying K⁺ current, with characteristics of Kir 2, has been observed in the membrane patch of rat and bovine glomerulosa cells (564). Kir 2 passes through the ubiquitous IRK channels and, in contrast to other families of inwardly rectifying channels, does not require G protein for activation and is insensitive to the ADP/ATP ratio. Nonetheless, the physiological significance of Kir 2 in glomerulosa cells has been questioned by the lack of inward rectification in macroscopic current under whole cell configuration (79, 321).

The previously mysterious background (or leak) K⁺ current, which, by definition, is also capable of permeating K⁺ in the range of resting E_m, would also be capable of generating highly negative E_m. Such a current was described in glomerulosa cells by Lotshaw in 1997 (322). Since that time the respective channel has been identified and termed TASK, an acronym for TWIK (tandem of P domains in a weak inward rectifying K⁺ channel)-like, acid-sensitive K⁺ channel (148). The expression of this two-pore domain K⁺ channel in rat glomerulosa cell has been demonstrated by Enyedi and co-workers in our laboratory (129). The zona glomerulosa exhibits the highest density of TASK mRNA among all tissues hitherto examined. In Xenopus oocytes injected with glomerulosa mRNA, the expressed K⁺ current had a pharmacology comparable with that of the authentic TASK current (129). More recently, the mRNA of the subsequently described TASK-3 (284, 451) was also found in rat glomerulosa cells (128). Interestingly, the glomerulosa cell is exclusive among several examined cell types in expressing TASK-3. TASK-1 and TASK-3 are coexpressed (the latter in larger amount) and form heterodimers. A fine-tuning of channel control may be achieved by the formation of heterodimers by the two subtypes (127).

Several K⁺ currents or channels, active in depolarized cells only, have also been described in the glomerulosa cell. Noninactivating delayed rectifier K⁺ current (inducing K⁺ efflux from depolarized cells) was described in rat, bovine, and human glomerulosa cells (79, 407, 409, 564). This current probably corresponds to KvLQT1, the slow component of the delayed outward K⁺ current (I_Ks) in the heart. It is conducted by the channel KCNQ1, the regulatory subunit of which (KCNE1) has been detected in glomerulosa cells (13). In view of the voltage dependence of their activation, the delayed rectifier channels may not modify the resting E_m, but their activation could limit the extent of depolarization by allowing the efflux of K⁺. In harmony with this prediction, knock-out of the KCNE1 gene resulted in enhanced responsiveness of the plasma aldosterone concentration to K⁺ loading (13).

Transient outward (A-type) K⁺ current was observed in rat, bovine, and human glomerulosa cells (407, 409, 564). Ca²⁺-dependent maxi-K⁺ (BK) channels were also described in the rat (321, 408, 564). In view of the voltage dependence of their activation, the A-type and the BK channels may not modify the resting E_m. On the other hand, ANG II, probably acting through G_q/11, inhibits the Ca²⁺-dependent K⁺ channel (408), an action that could augment the depolarization triggered by some other mechanism. ANP, probably acting via cGMP, increases K⁺ conductance by facilitating the function of BK channels (191). The ensuing hyperpolarization may represent one of the means by which the peptide antagonizes the secretagogue action of Ca²⁺-mobilizing hormones.

Any agent that decreases K⁺ conductance evokes the shift of E_m from the strongly negative E_K towards the positive equilibrium potential of Na⁺ and Ca²⁺. The first evidence that ANG II induces membrane depolarization was obtained with intracellular microelectrode recording of cat adrenocortical slices as early as the 1970s (383). Later studies provided evidence that a significant component of this depolarization is caused by the reduction of g_K. In microelectrode studies a biphasic E_m response to ANG II was observed in rat glomerulosa cells (446). A brief hyperpolarization was followed by a long-lasting depolarization, characterized by a large decrease in cell conductance. The reversal potential of the response suggested that E_m followed changes in g_K. Subsequent ⁸⁶Rb flux measurements confirmed that ANG II inhibits g_K in rat glomerulosa cells (217). The primary site of action of ANG II is the AT₁ receptor, and its effect is mimicked neither by ionomycin-induced elevation of [Ca²⁺]_c nor by pharmacological activation of PKC, suggesting that inhibition of K⁺ conductance by ANG II is a membrane-delimited process. An initial increase, followed by a prolonged decrease in g_K, was observed by means of ⁸⁶Rb flux studies also in bovine glomerulosa cells. [Surprisingly, the decrease in g_K was found to be Ca²⁺ dependent (320).] The significance of the ANG II-induced decrease in K⁺ conductance is shown by the observation that the K⁺ ionophore valinomycin significantly reduced ANG II-stimulated aldosterone production (495).

Subsequently, in the patch-clamp era, ANG II was found to inhibit each type of the hitherto described K⁺ channels. However, only inhibition of channels active at resting E_m may be responsible for triggering depolarization. Of the two channel types meeting this requirement in glomerulosa cell, Kir 2 displayed reduced single-channel activity in membrane patches exposed to ANG II (276). However, as previously mentioned, this current has not yet been found in whole cells. The background K⁺ current, corresponding to the later described TASK, was inhibited by physiological concentrations of ANG II in rat glomerulosa cells. At the moderately elevated physiological concentration of 10^–10 M, ANG II reduced the leak membrane conductance (measured between –127 and –87 mV) by 50% and shifted E_m by 6 mV to positive direction. The extent of depolarization was 18 and 32 mV on average at 10^–9 and 10^–8 M ANG II, respectively (323). A stable level of current inhibition required 10 min at concentrations below 100 pM ANG II and at least 5 min at higher concentrations (321).

ANG II also inhibits the TASK channels responsible for background K⁺ current. ANG II added to isolated glomerulosa cells or to oocytes injected with glomerulosa mRNA inhibited the K⁺ current. ANG II also inhibited the TASK current in oocytes, coexpressing TASK and the AT_1a angiotensin II receptor. These observations provided firm evidence that TASK exerts a significant role in the generation of the strongly negative resting membrane potential in glomerulosa cells and that ANG II may depolarize the cell by inhibiting these channels (129). Although the inhibition of TASK-3 current by ANG II is smaller than that of TASK-1 current (128), it still may have physiological significance, especially in view of the modified characteristics of the TASK-1/TASK-3 heterodimers (127). Maneuvers applied to activate phospholipase C, G_q, and protein kinase C, or to increase the concentration of cytoplasmic Ca²⁺ and IP₃, led to the conclusion that the action of ANG II (or other Ca²⁺-mobilizing hormones) is mediated by the breakdown of PIP₂ in the plasma membrane (130).

ANG II inhibits the delayed rectifier K⁺ current (79, 276). The Ca²⁺-dependent maxi K⁺ channels are also inhibited by ANG II, and in this case the effect is mediated by a pertussis toxin-insensitive G protein (408). Considering that depolarization induced by ANG II levels occurring in vivo is probably not sufficient for the activation of these channel types, their role in the physiological action of ANG II awaits confirmation.

Activation of nonselective cation channels also results in depolarization. Such a channel, with a nearly linear slope conductance between –80 and 0 mV under quasi-physiological ionic conditions, has been described by Lotshaw and Li (324). The channel was also permeable to Ca²⁺, exhibiting a P_Ca/P_Na of 4 (with 110 mM Ca²⁺ on the extracellular side). It would be worthwhile to examine the relation of this current with the dihydropyridine (DHP)-insensitive background Ca²⁺ currents observed in glomerulosa cells by applying the cell-attached mode of the patch-clamp technique, but with pipette solutions lacking monovalent cations (149). In cell-attached patches ANG II (1 nM) increased the opening probability of the nonselective cation channel, rendering cation influx and depolarization possible in the intact cell. In fact, using the nystatin-perforated patch technique, depolarization began in 2 min and was characterized by a large increase in input resistance, indicating the decrease in g_K. However, transient depolarizations of variable amplitude were superimposed on the slow depolarization, and membrane conductance during these voltage transients exhibited large increases. Considering that Ca²⁺ was omitted from the pipette solution, the increased conductance indicated the activation of nonselective cation channels. Summarizing, these channels may contribute to signal generation by ANG II by two means: cation influx contributes to the voltage-dependent activation of Ca²⁺ channels; moreover, Ca²⁺ influx per se may participate in Ca²⁺ signaling. (For further studies of channel structure, see Ref. 325.)

The participation of the Na⁺-K⁺-ATPase and ion channels in hormonally induced resetting of E_m is summarized in Figure 3.

View larger version (21K): [in this window] [in a new window]   FIG. 3. The participation of the Na+-K+-ATPase and ion channels in hormonally induced resetting of membrane potential (Em). Nonspecif., nonspecific cation channel; delayed rect. K+, delayed rectifier K+ channel; big K, Ca2+-activated maxi (BK) K+ channel. The red arrows indicate stimulatory actions, and the green arrows indicate inhibitory hormonal actions.

Because changes in Cl^– conductance are involved in the depolarization or repolarization of various cell types, their function in glomerulosa cells is also summarized. Apart from a Ras-dependent chloride current activated by ACTH (105), we are not aware of any report on Cl^– current in the glomerulosa cell. No significant Cl^– current was detected in early microelectrode studies (445). We examined Cl^– currents with the patch-clamp technique in rat glomerulosa cells. K⁺ currents were inhibited with CsCl-containing pipette solution and 10 mM Ba²⁺ and 10 mM tetraethylammonium in the extracellular bath. With the application of nearly symmetrical Cl^– concentration, the current at –100 mV was less than –20 pA. Membrane resistance in the voltage range of –100 and –120 mV reached 7 G on average, indicating the absence of any significant channel activity. Also, the current displayed ohmic change only during a slow ramp depolarization from –100 to –60 mV. However, in a significant fraction of the cells, the slow activation of a tiny inward current could be observed at strongly negative voltages. This was significantly accentuated by reducing extracellular pH from 7.4 to 6.9, attaining –3.5 pA/pF on average at –100 mV. Both the kinetic and pharmacological characteristics of this inwardly rectifying current suggest that it passes through ClC-2 Cl^– channels (J. K. Makara and A. Spät, unpublished observations). The role of this current in the glomerulosa cells remains to be elucidated.

2. Capacitative influx

The concept of capacitative Ca²⁺ influx, also termed store-operated or calcium release-activated Ca²⁺ influx, was introduced by Putney and co-workers in 1989 (534, 535). They proposed that agonist-induced Ca²⁺ uptake from the extracellular fluid is evoked by the depletion of IP₃-sensitive intracellular calcium stores. Store depletion can be experimentally elicited by pharmacological inhibition of the SERCA-type Ca²⁺-ATPase (e.g., with thapsigargin). Since that time, capacitative Ca²⁺ uptake has been demonstrated as a general and major Ca²⁺ uptake mechanism in nonexcitable cells and may also be present in excitable cells. The current flowing through capacitative Ca²⁺ entry channels was first characterized in mast cells and has been termed calcium-release activated Ca²⁺ current (I_CRAC) (243). It is highly Ca²⁺ selective, inwardly rectifying, and although its very low unitary conductance is compatible with a carrier mechanism; noise analysis indicates that I_CRAC passes through a channel. There are, however, several types of store-operated currents, different from I_CRAC, in cells that exhibit store-operated Ca²⁺ influx (404).

Three major hypotheses have been proposed to explain the activation of capacitative Ca²⁺ influx by store depletion. One of these hypotheses assumes that a small diffusible cytosolic factor links the IP₃-sensitive calcium store with the plasmalemmal channel. Another model attributes channel activation to the insertion of channel-containing vesicles into the plasma membrane. The third, and currently most accepted, hypothesis suggests a Ca²⁺-dependent protein-protein interaction between the release and influx channels. The multitude of models may indicate that there is no uniform mechanism responsible for channel gating in different cell types. Since no specific data are available for the mechanism in glomerulosa cell, we refer to general reviews on this topic (49, 64, 404, 440, 565). The structure and gating mechanism of the channel responsible for Ca²⁺ entry remain to be clarified. Mammalian homologs of Trp proteins (termed after Drosophila transient receptor potential), products of at least seven genes, are presumed to constitute the channel. The channel contains four subunits, and the great number of possible heterotetramers may explain the variable function of these channels in different cell types. In fact, the channels may display selective or nonselective Ca²⁺ conductance after activation of PLC. Store depletion evoked by thapsigargin opens some but not all Trp channels (reviewed in Ref. 361).

In 1989 we observed that, in contrast to K⁺-induced Ca²⁺ influx, ANG II-induced early Ca²⁺ influx was not sensitive to the DHP Ca²⁺ channel inhibitor nifedipine. This observation led us to postulate that the two agonists activate different Ca²⁺ entry mechanisms (504). A similar observation was subsequently reported in bovine glomerulosa cells (302). Further results have unambiguously demonstrated the significance of DHP-insensitive capacitative Ca²⁺ influx in ANG II-induced Ca²⁺ uptake. In rat glomerulosa cells, thapsigargin dose-dependently increases [Ca²⁺]_c and aldosterone production (219). The Ca²⁺ signal is DHP insensitive and transient only in Ca²⁺-free medium. Readdition of Ca²⁺ induces a steeper and higher rise in [Ca²⁺]_c than that observed in thapsigargin-untreated cells (465), indicating that influx rate depends on the state of the calcium store rather than on [Ca²⁺]_c. Similar results were obtained in bovine glomerulosa cells. Thapsigargin dose-dependently increases [Ca²⁺]_c (159) and stimulates aldosterone production (81).

Both mRNA and protein of Trp 4 are abundantly expressed in the bovine adrenal cortex, where the only other transcript found at a considerable level was Trp1. Expression of antisense trp4 cDNA reduced both the endogenous capacitative current and the amount of Trp4 protein, indicating the role of this protein species in the formation of store-operated Ca²⁺ channels in bovine adrenocortical cells (424). The Trp-binding site was described in the cytosolic NH₂ terminus of IP₃R3 (72). Therefore, the expression of IP₃R3 in glomerulosa cells (165) may have a special role in controlling Ca²⁺ influx in glomerulosa cells.

The role of capacitative Ca²⁺ influx in the action of ANG II has been supported by several observations. In bovine adrenocortical microsomes, the thapsigargin-releasable Ca²⁺ pool was larger than the IP₃-sensitive pool (159). Consistent with this observation, in rat glomerulosa cells ANG II did not induce any further elevation of [Ca²⁺]_c after exposure to thapsigargin, whereas a second rise in [Ca²⁺]_c could be obtained if the agonists were applied in the reverse order (159). These observations indicate that the IP₃ -sensitive Ca²⁺ pool constitutes a subcompartment within a larger, thapsigargin-sensitive compartment. In bovine cells the oscillatory Ca²⁺ signal induced by ANG II at physiological (100 pM) concentration was maintained for at least 20 min in a Ca²⁺-free medium, but was rapidly extinguished by adding thapsigargin. Nifedipine reduced the frequency of oscillations by only 30% in the Ca²⁺-restored state; moreover, it had no effect on the plateau level of the Ca²⁺ signal induced by a high dose of ANG II (479). The Ca²⁺ signal, as measured 5 min after adding ANG II, was little affected by blocking T-type Ca²⁺ and L-type Ca²⁺ current but was eliminated by thapsigargin (81). The relatively high expression of a capacitative Ca²⁺ channel homologous to Drosophila Trp channels in bovine (whole) adrenal (423) is also in agreement with the biological relevance of capacitative Ca²⁺ influx in this organ, although the respective cell type has not yet been specified (423).

The sustainment of capacitative Ca²⁺ influx obviously requires continuous depletion of the IP₃-sensitive calcium store, which seems to be due to the continuous production of IP₃. Although the postinitial level of IP₃ is small, it is suprabasal (161), and this moderately elevated level may be sufficient for the continuous flow of Ca²⁺ out of the ER vesicles. If Li⁺ or a high concentration of wortmannin were added to the incubation medium to prevent the replenishment of the PIP₂ pool, the sustained formation of IP₃ (18) and the sustained phase of the Ca²⁺ signal (376) were inhibited in bovine glomerulosa cells. Lithium ions also severely compromised the sustained phase of the aldosterone response to ANG II (but not to ACTH) in rat cells (21). The requirement for maintained activation of PLC, and the ensuing formation of IP₃, is compatible with the essential role of capacitative Ca²⁺ influx in sustaining ANG II-induced aldosterone production.

3. Voltage-activated Ca²⁺ currents and Ca²⁺ channels

Angiotensin II induces capacitative Ca²⁺ influx and depolarization, which, in turn, reduce both the Ca²⁺ concentration gradient and electrical gradient, the driving forces of Ca²⁺ flux from the extracellular space into the cytoplasm. However, depolarization also results in the activation of voltage-operated Ca²⁺ channels, and the ensuing increase in Ca²⁺ conductance may compensate for the reduced driving force. In this way, voltage-activated Ca²⁺ channels subserve the maintenance of Ca²⁺ influx.

The various members of the superfamily of voltage-activated Ca²⁺ channels give rise to Ca²⁺ currents of diverse electrophysiological and pharmacological properties. Voltage-activated Ca²⁺ currents are conventionally classified as low-voltage (or low-threshold) and high-voltage (or high-threshold) currents. In view of the voltage-activated Ca²⁺ currents that participate in the control of aldosterone secretion, this discussion will be confined to the low-voltage, T-type current and the high-voltage, L-type current. Calcium channels may exist in three distinct states, identified as the closed, activated, and inactivated states. The latter refers to a depolarization-induced state from which the opened state cannot be directly attained. T-type current is activated at more negative potentials than L-type current. In the presence of 10 mM Ca²⁺, the activation range of T-type channels is positive to –70 mV, and that of L-type channels is positive to –10 mV. T-type current reaches maximum amplitude and is inactivated at more negative potential than L-type current. T-current inactivates more rapidly, but deactivates (closes) and reactivates (reprimes) more slowly than L-type current. T-type channels exhibit comparable permeability for Ca²⁺ and Ba²⁺, whereas Ba²⁺ conductance through L-type channels exceeds that of Ca²⁺ conductance. The T-type channel has a unitary conductance of 8 pS versus the 25 pS of an L-type channel (when the charge carrier is 100 mM Ba²⁺). T-type current is generally more sensitive to blockade by Ni²⁺, amiloride, and mibefradil, and less sensitive to blocking by Cd²⁺ and nifedipine than L-type current. In contrast to L-current, T-current is not enhanced by the DHP agonist BAY K 8644 (for further details, see Refs. 41, 341, 352, 518, 552).

The first electrophysiological study on voltage-operated Ca²⁺ currents in rat glomerulosa cells was published in 1987 (445) and demonstrated that depolarizing current pulses elicited a regenerative response. The ionic mechanism underlying the regenerative response was voltage-operated Ca²⁺ current. Following these pioneering studies, three types of voltage-activated Ca²⁺ currents, T-, L- and N-type currents, have been identified by application of the patch-clamp technique to glomerulosa cells. T-type current has been observed in rat (150, 342, 562), bovine (113, 118, 259, 323, 343), and human glomerulosa cells (409). L-type current has also been described in rat (150, 323, 562), bovine (113, 343, 344), and human cells (409). N-type current was observed only in Long-Evans rats (150); no report on its expression in any other rat strain or other species has been published.

Patch-clamp studies usually employ Ba²⁺ as the charge carrier to avoid Ca²⁺-dependent inactivation of L-type channels, and few data have been obtained using Ca²⁺ as charge carrier. The low-threshold, T-type current was characterized in the presence of 2.5 mM Ca²⁺ in rat and bovine glomerulosa cells by Quinn et al. (444). Following the elimination of K⁺ currents, the activation threshold was between –80 and –70 mV, peak current occurred near –30 mV, and half-maximal steady-state inactivation was attained at –73 mV. In our studies (563), performed with 2 mM Ca²⁺, the average activation threshold of T-type current was –71 mV. The threshold of L-type current was determined with a very slow voltage ramp depolarization from –100 to +40 mV and also with ramp depolarization following inactivating T-type channels at –60 mV. The average threshold of L-current was –57 mV, applying either protocol.

Voltage-activated Ca²⁺ channels have heteromultimeric structures, and the electrophysiological characteristics of their ionic current are primarily determined by the pore-forming ₁-subunit. This subunit consists of four homologous repeats (I–IV) of six membrane-spanning segments (termed SI–SVI). Every third amino acid residue in segments IV is lysine or arginine. Changes in the electrical field induce motion of these positively charged segments, which therefore serve as voltage sensors. The intramembrane loops between SV and SVI line the channel pore, and four glutamate residues in this pore domain are responsible for the selective Ca²⁺ permeability (352, 359, 414, 537, 540).

Molecular biological studies have identified the poreforming ₁-subunits of the low- and high-voltage-activated channels. Three isoforms of T-type ₁-subunits are known, indicated as Ca_v3.1, 3.2, and 3.3 (_1G, _1H, and _1I, respectively). Their intracellular loops and COOH termini exhibit significant differences in amino acid sequence, which provide opportunity for their selective pharmacological inhibition (307, 412). Half-maximal activation in the presence of 2 mM Ca²⁺ was observed for the three subtypes at –29, –31, and –25 mV, respectively. The steady-state inactivation of _1I occurs at higher potentials. _1G activates and inactivates more rapidly. Their deactivation is relatively slow and is fastest for _1I. All have a tiny single-channel conductance, and _1H is the most sensitive to Ni²⁺ (286, 311, 413).

Substantial expression of the mRNA transcript for the _1H subunit was found in the rat and bovine zona glomerulosa by in situ hybridization. (A much weaker expression was detected also in bovine zona fasciculata.) An extremely low level of _1G mRNA was found, and _1I was virtually absent. In accordance with these data, the IC₅₀ ofNi²⁺ for inhibition of T-type Ca²⁺ current in these cells was 6 µM (489), corresponding to the Ni²⁺ sensitivity of the _1H subunit (7 µM) (413). _1H is also expressed in the H295R human glomerulosa cell line, and, unexpectedly, exposure to aldosterone for 24 h increases the expression of the channel protein as well as the density of T-type current (317). It is noteworthy that only the activity of the H isoform of the T-type Ca²⁺ channel is enhanced by Ca²⁺, through phosphorylation by CaMKII (586).

The diverse nature of L-type channels was suggested long ago by the different excitation-contraction coupling in skeletal muscle and cardiac muscle. In both cell types, activation of L-type Ca²⁺ channels results in Ca²⁺ release from the sarcoplasmic reticulum through ryanodine receptors. However, in skeletal muscle cells the L-channels (DHP receptors) in the t-tubule membrane activate type 1 RyRs through protein-protein interaction, whereas in cardiac muscle it is Ca²⁺ influx through L-channels that evokes Ca²⁺-induced Ca²⁺ release through type 2 RyRs. In addition to the structural differences in RyRs, this differential coupling is attributed to the molecular differences in the two types of L-type channels. In fact, molecular biological studies on different cell types have identified three, rather than two, isoforms of the pore-forming ₁-subunit of L-type channels. The ₁-subunit of the skeletal (S), cardiac (C), and neuroendocrine (D) isoforms of L-type channel are termed Ca_v1.1, Ca_v1.2, and Ca_v1.3, respectively (415).

RT-PCR analysis of single rat glomerulosa cells, free of other cell types, detected mRNA of Ca_v1.2 and Ca_v1.3 (_1C and _1D, respectively) (242). The dominant expression of Ca_v1.3 probably results in a depolarization shift of the activation threshold and reduced rate of Ca²⁺-dependent inactivation (298, 601). The expression of Ca_v1.2 and Ca_v1.3 was also observed in human H295R cells (317).

Voltage-operated Ca²⁺ channels contain several isoforms of additional (₂--, -, and -subunits), which modify the current characteristics and pharmacology of the ₁-subunit as well as its expression at the plasma membrane (137, 271, 414, 560). We are not aware of studies on these additional subunits in the glomerulosa cell.

Several laboratories have observed that cAMP, through the activation of PKA, induces marked increases in L-type current, whereas data on the effect of PKC on T-type and L-type currents are conflicting (reviewed in Refs. 93, 352). cGMP opposes the current-enhancing effect of cAMP (268). CaMKII phosphorylates _s and thus enhances L-type current (348). Enhancement of T-type current by CaMKII was first observed in bovine glomerulosa cells (34). Heterotrimeric G proteins, in addition to regulating second messenger systems, may also regulate ionic channels acting via a membrane-delimited pathway. L-type current is enhanced by the -subunit of G_s (352, 575), and the neuronal N-type and P/Q-type Ca²⁺ currents are inhibited by G_o or G_i via their released -subunits (235, 285). Also, Ras-related small G protein that binds CaM inhibits the L-type current (42).

4. Action of ANG II on voltage-activated Ca²⁺ channels

Aldosterone production stimulated with ANG II or K⁺ is inhibited by the Ca²⁺ channel inhibitors verapamil (26, 305, 488) and nifedipine (3, 504). The frequently observed dependence of the ANG II-induced sustained Ca²⁺ signal on extracellular Ca²⁺ (89, 301, 467) has been regarded as evidence in favor of ANG II-induced Ca²⁺ influx. In fact, data on the augmented initial influx rate of ⁴⁵Ca after adding ANG II confirmed this interpretation in rat (95, 503) and bovine cells (159, 290, 301). Several, although not all, of the studies examining the effect of channel drugs on ANG II-induced Ca²⁺ signal also supported the stimulation of voltage-activated Ca²⁺ influx by ANG II. The DHP antagonist nifedipine usually moderately reduced the Ca²⁺ signal in bovine (7, 89) and in rat cells (77, 255, 516), whereas BAY K 8644, an L-type specific agonist, enhanced ANG II-induced Ca²⁺ signal in rat (but not bovine) glomerulosa cells (229, 255). However, it should be noted that ANG II was applied at pharmacological concentration in all these experiments, giving no information on channel activity under physiological conditions. Moreover, the description of changes in [Ca²⁺]_c gives only an indirect indication of changes in unidirectional ion transport and the corresponding channel activity.

We observed in 1991 that the nifedipine-sensitive Ca²⁺ signal induced by 18 mM K⁺ is inhibited by physiological concentrations of ANG II in the rat glomerulosa cell (24). As later discussed, at this concentration of K⁺ the Ca²⁺ signal is evoked dominantly by L-type channels. Subsequent patch-clamp studies provided evidence that L-type Ca²⁺ current is in fact inhibited by ANG II (323). When applied at a pharmacological concentration (10 nM), ANG II also inhibited the K⁺-induced Ca²⁺ signal or current in bovine cells (107, 344). [In contrast to these observations, ANG II was found to enhance L-type Ca²⁺ current in the Y1 adrenocortical tumor cell line (236), and AVP was also reported to augment L-type current in rat glomerulosa cells (196).]

The inhibition of L-type current in the face of enhanced Ca²⁺ influx indicates that ANG II stimulates Ca²⁺ influx through T-type channels only. Furthermore, the comparable concentration dependence of inhibition of T-type current and ANG II-induced aldosterone production by mibefradil, a more-or-less specific inhibitor of T-channels, provides evidence in favor of the essential role of T-type channels in eliciting Ca²⁺ influx in the ANG II-stimulated rat glomerulosa cell (323). The same conclusion could be drawn from the report that mibefradil, applied at a concentration that inhibits T-type but not L-type Ca²⁺ channels, reduced the [Ca²⁺]_c response to ANG II in bovine cells (476). The inhibition of Ca²⁺ current and ANG II-induced aldosterone production by tetrandine, an herbal alkaloid, at concentrations that inhibit T-type channels but do not yet affect L-type channels (478), also supports the significance of T-type current in ANG II-stimulated bovine cells.

ANG II appears to enhance T-type current in rat and bovine glomerulosa cells by different mechanisms. ANG II failed to affect the function of T-type channels in Lotshaw's experiments on rat glomerulosa cells (323), leading to the conclusion that ANG II activates the low-voltage-operated Ca²⁺ channel exclusively by depolarizing the plasma membrane. The enhancement of T-channel activity, reported in bovine cells by Barrett's group, was explained with a hyperpolarizing shift in the voltage dependence of channel activation (97). This shift requires intracellular GTP, suggesting that the channel-activating action of ANG II is mediated by a G protein (349). Although the Ca²⁺ signal activates calmodulin-dependent kinase II, and this activation may also shift the voltage-current function to more negative values (34, 327), the ANG II-induced shift is basically due to activation of G_i/o (97, 326). This model is in agreement with previous reports on the pertussis toxin sensitivity of ANG II-induced Ca²⁺ influx (236, 296, 344). Activation of pertussis toxin-sensitive G protein(s) also seem(s) to account for inhibition of L-type current by ANG II (344) or AVP (204).

Atrial natriuretic hormone, a physiological inhibitor of aldosterone secretion, attenuates agonist-induced Ca²⁺ influx (95, 186). In patch-clamp studies this inhibition was confined to T-type channels; Ca²⁺ current through L-type channels was even enhanced (33, 350). The inhibition of ANG II-induced aldosterone production may be due to the reduction of T-current, and this effect apparently is not counteracted by a concomitant enhancement of L-current because L-type channels are inhibited by a G_i-mediated mechanism in cells exposed to ANG II. The neurotransmitter dopamine, which may be released from local varicose axon terminals around glomerulosa cells (571), also inhibits T-type Ca²⁺ channel (143, 400) and thus inhibits aldosterone secretion (2).

As summarized in Figure 4, ANG II enhances Ca²⁺ influx through T-type channels. Depolarization as well as a negative shift in the voltage dependence of channel activation are the factors inducing enhanced Ca²⁺ influx, the latter shift being evoked by activation of a pertussis toxin-sensitive G protein, probably belonging to the G_i/o family. It is presently not clear whether the relative participation of these factors in current amplification depends on species or experimental conditions. ANG II, at the same time, may inhibit L-type current, probably also through the activation of G_i/o. This inhibition may protect the cell from Ca²⁺ overload.

View larger version (21K): [in this window] [in a new window]   FIG. 4. Generation of Ca2+ signal in angiotensin II-stimulated glomerulosa cells. SOC, store-operated Ca2+ channel. The red arrows indicate stimulatory actions, and the green arrows indicate inhibitory actions. Metabolic transformation and transport processes are shown by black arrows. No data are available for TASK channels in bovine glomerulosa cells. Gi-mediated activation of T-type Ca2+ channels was found only in bovine cells. The role of Na+-K+-ATPase is shown in Fig. 3.

5. DHP-sensitive Ca²⁺ channels and IP₃-induced Ca²⁺ release

In rat glomerulosa cells, the L-type channel-specific DHP agonist BAY K 8644 enhances ANG II-induced Ca²⁺ response and aldosterone production (229, 255). This suggests that even if ANG II-induced Ca²⁺ influx takes place via T-channels, some L-channels also contribute to Ca²⁺ signaling. This presumption, however, is in sharp contradiction to the previously described inhibition of L-type channels by ANG II. On the other hand, the actions of channel agonist and antagonists suggest an interaction between L-type channels and subplasmalemmal IP₃ receptors. Such a model (516) has been based on the following experimental observations.

ANG II-induced depolarization and Ca²⁺ influx do not become detectable until after the initial peak of Ca²⁺ signal has occurred. ANG II (10^–9 or 10^–8 M)-induced depolarization begins at 2 min in bovine (88) and after 30 s in rat glomerulosa cells (255). (The applied fluorimetric technique could record the immediate depolarizing effect of K⁺.) In patch-clamp studies on rat glomerulosa cells, using the whole cell configuration, 200 nM ANG II evoked depolarization after an average lag time of 24 s (408). In patch-clamp experiments using the perforated patch technique, depolarization induced by 10^–10 to 10^–8 M ANG II began later than 1 min (323). These data suggest that depolarization-triggered voltage-operated Ca²⁺ influx cannot account for the generation of the initial (approximately first 15 s) Ca²⁺ signal.

In fluorimetic studies, the Mn²⁺-quench technique is a sensitive method of detecting Ca²⁺ influx. In rat glomerulosa cells, in contrast to the rapid quenching effect of a physiological elevation of [K⁺], 25 nM ANG II had no quenching effect within 150 s. Mn²⁺ exerted a rapid quenching effect only if added several minutes after the application of ANG II (516). This observation is in accordance with measurements of the initial ⁴⁵Ca influx rate in rat (107, 504) and bovine glomerulosa cells (301), stimulated with nanomolar ANG II. Accordingly, these studies indicate that ANG II-induced Ca²⁺ influx has a lag time of >150 s. Nevertheless, the initial phase of ANG II-induced Ca²⁺ signal was modified by organic Ca²⁺ channel drugs in bovine (7, 81, 302), human (63), and rat glomerulosa cells (229). In our studies on rat glomerulosa cells, nifedipine and the benzothiazepine diltiazem reduced and BAY K 8644 augmented the amplitude of the ANG II-induced Ca²⁺ peak (attained at 13–15 s after adding the agonist) (255, 516). It is important to emphasize that nifedipine does not inhibit T-type Ca²⁺ current in these cells (562). The lack of any contribution of enhanced Ca²⁺ influx to the generation of the initial Ca²⁺ signal was also confirmed by examination of the signal when Ca²⁺ influx was blocked by 5 mM Ni²⁺. Nifedipine was capable of diminishing the ANG II-induced cytoplasmic Ca²⁺ signal even under such conditions (516).

These observations suggest that activation of L-type channels by a G protein (e.g., G_i, cf. Refs. 196, 236, 296, 344) modifies IP₃-induced Ca²⁺ release. Because nifedipine did not decrease ANG II-induced formation of IP₃ (516), an effect of L-type channels on the function of IP₃R had to be assumed (516). In this respect it should be recalled that a population of the IP₃-sensitive calcium-storing vesicles is cytoskeletally attached to the plasma membrane (166, 471). Moreover, considering the protein-protein interaction between type 1 RyRs and L-type Ca²⁺ channels (Ca_v 1.1) in skeletal muscle, as well as the significant amino acid identity between the cytoplasmic NH₂-terminal loops of RyR and IP₃ receptors (181, 536), coupling between L-type channels (Ca_v1.2 and 1.3) and subplasmalemmal IP₃Rs can be hypothesized.

The assumption of an interaction between L-type channels and IP₃ receptors is compatible with observations on both smooth and skeletal muscle cells. Ca²⁺ release from IP₃-sensitive stores in coronary myocytes, induced by acetylcholine (but not by supramaximal concentrations of IP₃), was potentiated by depolarization through a mechanism that did not require extracellular Ca²⁺ (190). Spontaneous membrane depolarizations (termed slow waves) in visceral smooth muscle also did not require Ca²⁺ influx, but were dependent on Ca²⁺ release from IP₃-sensitive stores. Nevertheless, their frequency increased at more depolarized potentials (559). The response of peeled skeletal muscle fibers to exogenous IP₃ was significantly larger when t tubules were in a depolarized state (138). Although the authors emphasized the role of E_m in controlling the effect of IP₃, it should be recalled that depolarization changes the conformation of L-type channels.

Sustained formation of IP₃, leading to prolonged IP₃-induced Ca²⁺ release, is a prerequisite for sustained enhancement of aldosterone production (21). Therefore, modulation of IP₃-dependent Ca²⁺ release by dihydropyridines should influence both Ca²⁺ signal and aldosterone production throughout the duration of stimulation with ANG II. Although our model still requires direct evidence, it presently offers an explanation for the effects of channel-regulating drugs on ANG II-induced Ca²⁺ and aldosterone responses in the face of an inhibited state of L-type channels.

6. Ca²⁺ influx induced by K⁺

As discussed in section IIB, the glomerulosa cell is characterized by a dominant K⁺ conductance. The high density of the background (TASK) K⁺ channel makes it an excellent sensor for [K⁺]_o. K⁺-induced depolarization, which almost perfectly follows the Nernst equation, activates voltage-operated Ca²⁺ channels. Raising [K⁺]_o gradually to 20 mM evokes a gradual increase of [Ca²⁺]_c (432, 444, 445, 450). The K⁺-induced Ca²⁺ signal is nonoscillating and long-lasting (28, 89, 448, 450, 466).

In rat glomerulosa cells, the activation threshold of T-type Ca²⁺ channels in the presence of 2–2.5 mM Ca²⁺ is between –70 and –80 mV, and half-maximal steady-state inactivation occurs at –73 mV (444, 563). At membrane potentials where T-type channels can be activated and inactivation is incomplete, as indicated by the intersection of peak current and steady-state inactivation curves (in function of E_m), a small but sustained Ca²⁺ current (termed window current) should occur. Because window currents were usually analyzed using a high concentration of Ca²⁺ or Ba²⁺ as charge carrier, no precise data are available for physiological conditions. Assuming that window current begins at –80 mV at physiological extracellular [Ca²⁺], T-type channels may participate in eliciting the Ca²⁺ signal during stimulation with physiologically elevated [K⁺].

The question still remains whether the high voltage-activated L-type channels also contribute to Ca²⁺ influx. Because Ca²⁺ response to 6.0 mM K⁺ (28, 33, 229) is enhanced by the L-type specific DHP agonist BAY K 8644, it may be assumed that a fraction of L-type channels is already open under moderately depolarized conditions. Consistent with this possibility are observations on the effect of mebafradil, which inhibits T-type channels much more effectively than L-type channels. The effect of mebafradil on K⁺-induced aldosterone production by rat glomerulosa cells was gradually attenuated as a function of [K⁺] (i.e., with increasing depolarization), indicating that above 6 mM [K⁺] a shift occurs from T-type to L-type channels in the generation of Ca²⁺ influx (323). Rossier et al. (472) examined the effect of nifedipine and calciseptin on Ca²⁺ currents, [Ca²⁺]_c, and aldosterone production in bovine glomerulosa cells. Their results support the previous data, indicating that in K⁺-stimulated cells aldosterone production correlates with T-type channel activity.

Barrett et al. (33) examined aldosterone production by bovine glomerulosa cells over a wide range of [K⁺]. From 3 to 20 mM K⁺ the concentration-response curve was similar to that described for rat glomerulosa cells, a biphasic function peaking at 10 mM. However, there was a second increase in aldosterone production between 20 and 30 mM. The voltage dependence of the aldosterone production rate and that of the predicted Ca²⁺ influx through T- and L-type channels were similar, suggesting that both types of Ca²⁺ channels may support steroid production. Within the potential range studied (–76 to –23 mV) no Ca²⁺ influx would be carried exclusively via one or the other channel population, but their contribution would depend on E_m. The calculations show that at 6 mM K⁺ T-type channel influx would be 90% of maximal, whereas that of L-type channel influx would attain 10% of maximal.

Rat glomerulosa cells may respond to an elevation of [K⁺] as small as 0.5 mM with Ca²⁺ signals (563) and increased activity of mitochondrial NAD(P)H formation (432). Raising [K⁺] from 3.6 to 4.6 mM may double aldosterone production (432). Although window current (see above) may account for the activation of T-type channels despite the tiny shift of E_m, the phenomenon still seems to be disproportionate and requires more complete elucidation. A possible mechanism for increasing the sensitivity of glomerulosa cells to K⁺ is an increase in cell volume. Hyposmosis-induced swelling enhances T-type Ca²⁺ current and, hence, K⁺-induced Ca²⁺ signal (334). The enhanced Ca²⁺ response is not sensitive either to cytochalasin D or colchicine (335). When [K⁺] is raised from 3.6 to 5 mM, a steep initial rise in [Ca²⁺]_c is followed by a slower further rise, and several cells display a second phase of rapid rise, superimposed on the already elevated [Ca²⁺]_c. This second phase coincides with the detectable onset of swelling. The postinitial elevation of [Ca²⁺]_c may be prevented by keeping cell volume constant, with application of KCl in a hyperosmotic medium (335). This observation indicates that increased cell volume sensitizes T-type channels to depolarization. Raising [K⁺]_o from 4 mM to the supraphysiological level of 7 mM also induces swelling in bovine glomerulosa cells (231). However, the mechanism sensitizing bovine cells to elevated [K⁺] seems to differ from that observed in rat glomerulosa cells. The Ca²⁺ signal evokes a CaMKII-mediated shift of the voltage dependence of the activation of T-type channels (34, 327), more specifically that of the poreforming subunit _1H (but not that of _1G) (586) to more negative potential values. Therefore, it appears that increased cell volume exerts a positive feed-forward effect in rat, and elevated [Ca²⁺]_c exerts a positive feedback effect on the voltage threshold of T-type channels in bovine glomerulosa cells, and these mechanisms contribute to the generation of Ca²⁺ responses to elevation of [K⁺].

Summarizing the mechanism of K⁺-induced Ca²⁺ signaling, increased K⁺ in the physiological concentration range enhances Ca²⁺ influx through T-type channels. This response may be amplified by K⁺-evoked cell swelling and/or the developing Ca²⁺ signal itself. At high [K⁺], which still occurs in vivo, activation of some L-type channels contributes to Ca²⁺ signaling, rendering Ca²⁺ signal generation and aldosterone secretion DHP sensitive. At pharmacological K⁺ concentrations, T-type channels are inactivated and continuous Ca²⁺ influx takes place through L-type channels. (The role of cAMP in K⁺-elicited cell response is discussed in section IVB.)

7. Na⁺/Ca²⁺ exchange

The sodium-calcium antiporter, which exchanges three Na⁺ for one Ca²⁺ and is therefore electrogenic, is an important regulator of [Ca²⁺]_c (for review, see Ref. 496). In most cell types, at least under resting conditions, it brings about net Ca²⁺ efflux. Its presence in rat glomerulosa cells was indicated by the Na⁺ dependence of Ca²⁺ transport. The efflux rate of ⁴⁵Ca was decreased when extracellular [Na⁺] was reduced, whereas ⁴⁵Ca influx was accelerated by ouabain-induced increase in intracellular [Na⁺] (253). [Ca²⁺]_c also depends on extracellular [Na⁺] in bovine glomerulosa cells (295).

Benzamil derivative inhibitors of the antiporter attenuated the effects of extracellular (295) and cytoplasmic [Na⁺] (253) on Ca²⁺ uptake and also inhibited ANG II-induced aldosterone production (253, 557). The drug reduced the initial influx rate of ⁴⁵Ca and the exchangeable (60-min) pool size of calcium in control cells and inhibited the stimulatory effect of 25 nM ANG II, added 10 min earlier, on ⁴⁵Ca uptake (504). The driving force for Ca²⁺ influx may be the high intracellular [Na⁺] of glomerulosa cells (131, 557). Depolarization and an increase in cytoplasmic [Na⁺] (557) may further enhance Ca²⁺ influx by the Na⁺/Ca²⁺ antiporter in cells stimulated with ANG II (Fig. 5). {PKC activates Na⁺/H⁺ exchange and thus raises [Na⁺]_c in several cell types, and such an effect of PKC has also been suggested in glomerulosa cells (117).} These observations indicate the participation of the antiporter in the calcium metabolism of glomerulosa cells, and also suggest that it contributes to Ca²⁺ influx during exposure to ANG II (382). Benzamil derivatives compete with Na⁺ for its binding site. However, since they accumulate inside the cell where they have to compete with Na⁺ present at low concentration, they may inhibit Ca²⁺ influx more efficiently than Ca²⁺ efflux. Therefore, although these drugs provide a means to reveal the participation of the antiporter in an examined mechanism, their effect on Ca²⁺ efflux may be masked by a more pronounced inhibition of Ca²⁺ influx. Thus the data on their effect on Ca²⁺ influx do not exclude the possibility that the Na⁺/Ca²⁺ exchanger mediates net Ca²⁺ efflux.

View larger version (18K): [in this window] [in a new window]   FIG. 5. Activation of Ca2+ influx by the sodium-calcium antiporter (NCX) in the angiotensin II-stimulated glomerulosa cells. The red arrows indicate stimulatory actions, and the green arrows indicate inhibitory actions. (The pathway shown with dotted lines has been shown in other cell types only.)

ANG II, through the activation of lipoxygenase and the ensuing formation of 12-HETE, activates p38 mitogen-activated protein kinase (MAPK) (382, 519) (see sect. IIC). The activation of p38 MAPK, in turn, was reported to inhibit Na⁺/Ca²⁺ exchange and to prolong the sustained phase of the Ca²⁺ signal (519). However, the fact that ANG II reduces the exchangeable Ca²⁺ pool (27, 107, 154, 289) in spite of the enhanced Ca²⁺ influx requires further consideration. Extrusion of Ca²⁺ from the cytoplasm by the plasmalemmal Ca²⁺-ATPase and the Na⁺-Ca²⁺ antiporter is enhanced by the increase in [Ca²⁺]_c and is partly opposed by reduction of the membrane PIP₂ pool, which attenuates the activity of the Ca²⁺ pump. The proposed inhibition of the antiporter by ANG II could mean a further blunting of Ca²⁺ loss. In view of the potential significance of such a mechanism, direct evidence in favor of the inhibition of the antiporter by the 12-HETE-p38 MAPK pathway should be provided by unidirectional radioflux measurements, at low ANG II concentrations.

8. Ca²⁺-eliminating mechanisms

Mechanisms eliminating Ca²⁺ from the cytoplasm include the plasma membrane Ca²⁺-ATPase (PMCA), the sarco/endoplasmatic Ca²⁺-ATPase (SERCA), and the Na⁺-Ca²⁺ antiporter. The function of the latter has been discussed in the previous subsection. In view of the current topic, it should be emphasized that PMCA is activated by acidic phospholipids (e.g., PIP₂) and CaM (91). For lack of studies on the glomerulosa cells, in all other respects we refer to general reviews (e.g., Refs. 200, 330).

C. Diacylglycerol-PKC and Lipoxygenase Pathways

Rasmussen and co-workers (294) proposed that sustained responses to Ca²⁺-mobilizing agonists would be initiated by Ca²⁺, while protein phosphorylation by PKC (or PKA) would account for the sustained phase of steroid production. Their postulation was based mainly on studies on bovine glomerulosa cells, in which stimulation of PKC with tetradecanoylphorbol acetate (TPA) slightly increased aldosterone production per se and efficiently potentiated the effect of a Ca²⁺ ionophore. The kinetics of hormone production stimulated by the combined application of TPA and the ionophore were similar to that of ANG II-induced production (294). In view of the cell-damaging effect of prolonged elevation of [Ca²⁺]_c, this dual system was considered to have advantages over a simple, Ca²⁺-mediated system (32, 289, 292). Occasionally, phosphorylation by PKA would serve a similar purpose (291).

PKC was originally regarded as a Ca²⁺ and phospholipid-dependent protein-phosphorylating enzyme. Today, the large family of PKCs is known to consist of at least a dozen isoenzymes with various activation characteristics and substrate specificities. While the classical forms are activated by DAG, high [Ca²⁺] may attenuate the DAG requirement of some isoforms and arachidonate may also be an efficient activator (135, 170, 388).

DAG is produced simultaneously with IP₃ from PIP₂ by the action of PLC. In rat glomerulosa cells ANG II evokes a biphasic increase in DAG production, with an early peak at 30 s and a later one at 10 min (378). In the bovine glomerulosa cell an early peak (at 30 s), detected if sampling is frequent enough (248, 297), is followed by a second phase, maintained for several tens of minutes (67, 248, 297). DAG may also derive from phosphatidylcholine by the combined activity of phospholipase D (PLD) and phosphatidic acid phosphohydrolase (60). It has been suggested by Bollag and co-workers (66, 68, 273) that, during the sustained phase of stimulation with ANG II, the activation of PLD contributes to the formation of DAG. It is worth mentioning that in K⁺-stimulated cells, weak activation of PLC by Ca²⁺, obviously through the formation of DAG, results in the moderate activation of PKC (218).

In spite of the convincing logic of the dual-control system, the role of PKC in the control of aldosterone secretion is highly controversial. The criteria for accepting such a role are the following: 1) translocation of PKC from the cytosol into the membrane fraction in response to the respective stimulus; 2) induction of the biological response with DAG analogs or an active phorbol ester; and 3) inhibition of the biological response with a specific inhibitor of the enzyme (e.g., staurosporine) or by means of depleting the cell of activable PKC (with several hours of exposure to a high concentration of an active phorbol ester). We will group the data in the literature according to these criteria.

ANG II brings about the translocation of PKC from the cytosol into the particulate fraction in bovine (226, 291, 308) as well as rat glomerulosa cells (377). The enzyme also translocates to the membrane fraction in response to high [Ca²⁺]_c (171), and ANG II-induced activation of PKC may depend on concomitant Ca²⁺ influx (288).

At variance with the reports of Rasmussen's group, TPA or 1-oleoyl-2-acetylglycerol, a DAG analog, added to bovine glomerulosa cells failed to increase basal aldosterone production (187). These drugs also inhibited the effect of ANG II and K⁺ on T-type current, [Ca²⁺]_c, and the aldosterone response (12, 187, 470). TPA augmented aldosterone production in freshly isolated human adrenocortical cells (305) but failed to do so in the H295R human adrenocortical cell line (110). In the rat, TPA-stimulated aldosterone production was observed exclusively in adrenal capsular tissue (569). In contrast, TPA inhibited basal as well as ANG II-stimulated, K⁺-stimulated, and ACTH-stimulated hormone production by isolated glomerulosa cells (218). TPA induced the phosphorylation of the cholesterol carrier protein StAR (see sect. VIIA) in bovine glomerulosa cells (54, 226) but not in rat cells (226). TPA, in contrast to ANG II, also failed to increase the steady-state StAR mRNA level in human (H295R) glomerulosa cells (110). PKC, at least in the hamster glomerulosa cell, also depresses the expression of aldosterone synthase (314, 315). Overall, these data are not compatible with the proposed secretagogue action of PKC.

In one report the PKC inhibitor Ro31–8220 reduced ANG II-stimulated aldosterone production in rat glomerulosa cells (277). On the other hand, in our studies the PKC antagonist staurosporine potentiated the aldosterone response to K⁺ and enhanced the initial (30-min) phase of ANG II-induced aldosterone production, but failed to influence the effect of ACTH. The enzyme inhibitor enhanced the initial (30 min) phase of the ANG II-induced aldosterone production (218).

A more straightforward means of preventing PKC effects is by depletion of the enzyme. When PKC activity in both the cytosol and the membrane fraction was reduced below the control level by long-lasting exposure to TPA, aldosterone production induced by physiological stimuli (300 pM ANG II or 5.4 mM K⁺) was significantly enhanced (218). [In K⁺-stimulated cells the activation of PKC was secondary to a weak activation of PLC by Ca²⁺ (218).] PKC depletion failed to alter ANG II-induced aldosterone production in the experiments of Aguilera and co-workers (377), who also refuted the involvement of PKC in the stimulation of aldosterone production by ANG II.

The contradictions in the PKC literature may be attributed to several factors. Determination of enzyme activity has several pitfalls, including lack of appropriate substrate specificity as well as limited proteolysis of the enzyme during homogenization, which leads to a Ca²⁺ and phospholipid-independent kinase activity. TPA or other DAG analogs may be anchored in the whole cell membrane rather than at confined domains around the (probably clustered) ANG II receptors. Therefore, PKC may translocate to nonspecific sites of the membrane, resulting in artifacts. In view of the moderate specificity of kinase C blockers, their inhibitory effects may be inconclusive about the role of PKC. The species-dependent and cell type-dependent expression of different isoforms of the enzyme, often exerting opposing effects (135), should also be kept in mind. In summary, the bulk of evidence indicates that PKC has an inhibitory, rather than stimulatory, role in the acute control of aldosterone secretion.

The hydrolysis of phosphoinositides by PLA₂ results in the release of arachidonate, a precursor of eicosanoids. Since PLA₂ is not activated in ANG II-stimulated rat glomerulosa cells (244), any arachidonate released should originate from DAG. This means that the role of DAG, formed by the hydrolysis of PIP₂, may not be confined to the activation of PKC, but may also serve as an inducer of additional signaling pathways. The arachidonate derivatives, prostaglandins E₂ and A₂, stimulate aldosterone production (511, 517). Nevertheless, cyclooxygenase inhibitors do not counteract the induction of aldosterone production by ANG II, K⁺, or ACTH. Therefore, we must reject the physiological role of prostaglandins in the control of aldosterone secretion (164). However, the observation showing that the aldosterone-stimulating effect of arachidonate cannot be blocked by cyclooxygenase inhibitors (164) suggests a role for some other arachidonate derivative in the control of steroid production (164).

Rat glomerulosa cells metabolized arachidonate to hydroxyeicosatetraenoic acids (HETEs) by lipoxygenases and epoxyeicosatrienoic acids (EETs) by a cytochrome P-450 epoxygenase, but no leukotrienes were detected (84). A DAG lipase inhibitor, which prevents the release of arachidonate, blocked ANG II-induced 12-HETE and aldosterone formation. In contrast, a DAG kinase inhibitor, which prevents the conversion of DAG to phosphatidic acid, potentiated 12-HETE and aldosterone production (378). 12-HETE itself induced Ca²⁺ release from intracellular stores as well as Ca²⁺ influx from the extracellular fluid, whereas lipoxygenase inhibitors inhibited the effect of ANG II on [Ca²⁺]_c and aldosterone production (520). 12-Lipoxygenase products activate p38 MAPK (382), which, in turn, was reported to reduce Ca²⁺ efflux (519) (see sect. IIB7). More recent experiments applying molecular biological techniques (199) gave strong support to these data, and also indicated that activation of p38 MAPK leads to increased expression of the CYP11B2 gene product, aldosterone synthase (cytochrome P-450_aldo). Overall, these data support the assumption that 12-HETE is required for a full response of the glomerulosa cells to ANG II. Nevertheless, it does not seem to be sufficient to evoke maximal aldosterone response, since 12-HETE production in cells overexpressing 12-lipoxygenase was severalfold higher than that in ANG II-exposed cells transfected with empty vector, yet aldosterone production attained comparable (submaximal) levels in the two groups. The participation of 12-HETE in ANG II-induced aldosterone secretion is shown in Figure 6.

View larger version (13K): [in this window] [in a new window]   FIG. 6. Proposed mechanism of the stimulation of aldosterone production by ANG II via the lipoxygenase product 12-HETE and p38 MAPK (based on Refs. 84, 199, 382, 519, 520). The red arrows indicate stimulatory actions, and the green arrows indicate inhibitory actions. Metabolic routes are shown by black arrows.

Whether the formation of 12-HETEs should be regarded as a consequence of PIP₂ hydrolysis only, or the release of arachidonate from DAG is activated by a separate mechanism, should be further studied.

In view of the presumably increased formation of arachidonate, another important issue worth examination is whether ANG II activates an arachidonic acid-dependent non-store-operated Ca²⁺ channel before the activation of store-operated (capacitative) Ca²⁺ channels, as recently observed in other cell types (reviewed in Ref. 69).

D. Vasopressin: A Transiently Acting Paracrine Agonist

Several neurotransmitters may reach the glomerulosa cells, either from splanchnic nerve endings (528, 571, 572) or from the chromaffin cells, located in medullary rays within the zona glomerulosa (185). The most studied Ca²⁺-mobilizing paracrine agonist, vasopressin, binds to the pressor (V₁) type receptor in glomerulosa cells. Activation of the V₁ receptor elicits the breakdown of PIP, induces Ca²²⁺ signaling, and stimulates aldosterone production in rat (22, 205, 589) and human glomerulosa cells (206).

In rat glomerulosa cells, ANG II and AVP, applied at concentrations equipotent in terms of the phosphoinositide response, stimulated initial aldosterone production to the same extent, but only ANG II elicited a sustained response. AVP does not maintain aldosterone production at a high level, in spite of activating phosphoinositide turnover for at least 2 h. The early decay in AVP-induced aldosterone production is not due to some form of inhibition, since AVP does not attenuate ANG II-stimulated aldosterone production (160). The Ca²⁺ signals, elicited by the two agonists, have different kinetics. AVP-induced oscillations decay within 15 min, while ANG II (evoking a similar phosphoinositide response) induces sustained Ca²⁺ oscillations as well as prolonged and significantly greater aldosterone production (447). In contrast to G_i-mediated inhibition of L-type Ca²⁺ channels in ANG II-stimulated rat glomerulosa cells (see sect. IIB), AVP was reported to stimulate pertussis toxin-sensitive (i.e., G_i/o-mediated) Ca²⁺ influx through such channels (196, 204). AVP exerts a weak inhibition on T-type Ca²⁺ channels (196). In view of the significance of T-type Ca²⁺ current in the stimulatory action of ANG II, the AVP-induced inhibition of T-type channels may be a factor responsible for the transient character of vasopressin action. Activation of PKC (183) may also result in a gradually developing inhibition of aldosterone secretion.

	III. EFFECT OF CYTOPLASMIC CALCIUM ON MITOCHONDRIAL FUNCTION

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The adrenal cortex and the gonads do not store steroids; they secrete de novo synthesized hormones. Side-chain cleavage of cholesterol and all except one (C-21) of hydroxylation steps take place in mitochondria which occupy 25–30% of the cytoplasmic volume in glomerulosa cells (392). The mitochondrial hydroxylations require NADPH, which is generated from NAD⁺ by the mitochondrial nicotinamide nucleotide transhydrogenase (222, 240). The role of Ca²⁺ in the control of mitochondrial function in glomerulosa cells has been a neglected area of research.

Following the earlier assumption that the role of mitochondria in intracellular calcium metabolism is confined to situations of calcium overload, it became firmly established that physiological changes in [Ca²⁺]_c also influence mitochondrial metabolism. The renaissance of mitochondrial studies (431) began by the observations of Denton, McCormack, and co-workers (351), who demonstrated in mitochondrial homogenates and suspensions that Ca²⁺ activates three mitochondrial dehydrogenases: the pyruvate, isocitrate, and oxoglutarate dehydrogenase. It was also found in suspended cardiac mitochondria that physiological fluctuations in their calcium content alters [Ca²⁺]_m in the range that regulates the matrix dehydrogenases (328). We provided evidence for the physiological significance of the cytoplasmic-mitochondrial Ca²⁺ relationship in intact cells in 1992, observing that in rat glomerulosa cells K⁺ stimulates the formation of mitochondrial pyridine nucleotides, NADH plus NADPH [designated as NAD(P)H] in a Ca²⁺-dependent manner (432). Activation of voltage-dependent Ca²⁺ channels in sensory neurons also increased the formation of NAD(P)H (145). These data suggested the transfer of cytoplasmic Ca²⁺ signal into the mitochondrial matrix, an assumption that was directly demonstrated in the same year following the successful targeting of the Ca²⁺-sensitive luminescent protein aequorin into the mitochondria (461). Since that time, the mitochondrial Ca²⁺ and/or NAD(P)H response to cytoplasmic Ca²⁺ signal has been described in several cell types (reviewed in Refs. 147, 214, 458). Mitochondria, by buffering the changes in [Ca²⁺]_c, often reshape the Ca²⁺ signal and may also modify the function of IP₃R (146). In glomerulosa cells the mitochondrial response to ANG II (78, 433, 466) and AVP (466) have also been reported, but no data on signal reshaping are available.

The question may be raised whether stimuli of physiological intensity, i.e., physiological increases in [Ca²⁺]_c, are also effective in eliciting the mitochondrial response. Elevating [K⁺] from 3.6 to 4.1 mM slowly induces a small cytoplasmic Ca²⁺ signal (563) and enhances the formation of NAD(P)H (432). However, this observation was in conflict with the low affinity of the mitochondrial Ca²⁺ uptake mechanism, which predicts that submicromolar Ca²⁺ signals would fail to affect net Ca²⁺ uptake by the mitochondria. Mitochondria contain calcium at millimolar concentration; however, its major fraction is complexed with phosphate and ATP or bound to matrix protein. Under resting conditions the concentration of ionized calcium in the matrix of glomerulosa cells is comparable to that in the cytoplasm (428). The mitochondrial outer membrane is assumed to be permeable to Ca²⁺ (but see Ref. 125), and the barrier of Ca²⁺ diffusion is the inner mitochondrial membrane. The transport of Ca²⁺ through this membrane has been reviewed by Gunter et al. (207). Briefly, Ca²⁺ uptake via the ruthenium-sensitive Ca²⁺ uniporter is driven by the 150–180 mV (inside negative) mitochondrial membrane potential. The half-maximal transport rate is attained in the range of 10^–6-10^–4 M Ca²⁺, and external Ca²⁺ exerts a positive cooperative effect with a Hill coefficient of 2. Efflux of Ca²⁺ occurs by means of a Na⁺-Ca²⁺ and/or a H⁺-Ca²⁺ antiporter, the former having been detected also in the adrenal cortex (123, 477). The capacity of the efflux mechanism is low, and it saturates under conditions when [Ca²⁺]_c reaches 0.5 µM. At this [Ca²⁺]_c the plasmalemmal Ca²⁺ pump also saturates, and therefore further elevation of [Ca²⁺]_c results in steeply increasing net Ca²⁺ flux into the mitochondria. These data led to the still-prevailing view that net mitochondrial Ca²⁺ uptake occurs only if [Ca²⁺]_c exceeds the micromolar, but at least the half-micromolar level. Excessive uptake of Ca²⁺ may activate the mitochondrial permeability transition pore, resulting in the, often fatal, loss of ions and molecules smaller than 1,600 Da (reviewed, e.g., in Ref. 46). Due to the lack of relevant data in regard to glomerulosa cells, we do not discuss this issue in more detail.

The global Ca²⁺ signal, as measured in the cytoplasm, is the average of highly focused, transient, elementary changes of Ca²⁺ (50), limited by the diffusion of Ca²⁺ in the cytoplasm (e.g., Ref. 6). Therefore, the uptake of Ca²⁺ by, and the ensuing activation of, mitochondria will depend primarily on the distance between the Ca²⁺ source and the mitochondrion. Since ER vesicles may form contacts with mitochondria (338, 457), the narrow space around such contact points could be presumed to be an optimal site for the activation of mitochondria. The discovery of rapidly forming but short-lived high-Ca²⁺ microdomains in the perimitochondrial cytoplasm near the IP₃-sensitive calcium stores (460) provided an explanation for the large mitochondrial Ca²⁺ signal (216, 459) evoked by IP₃-mediated agonists. In fact, activation (125) or overexpression (453) of the Ca²⁺-permeable voltage-dependent anion channel in the outer mitochondrial membrane enhances the IP₃-induced (but not the thapsigargin-induced) mitochondrial Ca²⁺ signal, indicating the special role of high-Ca²⁺ microdomains at the ER-mitochondrial contact sites. Similarly, the abrupt increase in subplasmalemmal [Ca²⁺] during voltage-activated Ca²⁺ influx may be responsible for the activation of mitochondrial metabolism in insulin-producing INS-1 cells (280) and neurons (429). The observation that in rat glomerulosa cells IP₃ -induced Ca²⁺ release more effectively induces the formation of mitochondrial NAD(P)H than Ca²⁺ influx (467) may also be attributed to the IP₃-induced formation of perimitochondrial high-Ca²⁺ microdomain. The low affinity of the Ca²⁺ uniporter, and the formation of high-Ca²⁺ microdomains, led to the general view that the rapid formation of micromolar [Ca²⁺] in the perimitochondrial space is essential for the activation of mitochondria.

In steroid-producing cells, physiological stimuli induce submicromolar Ca²⁺ signals. The activation of mitochondrial dehydrogenases is not, however, confined to the action of the IP₃ -induced Ca²⁺ release in glomerulosa (432, 433, 466) or luteal cells (525), capacitative Ca²⁺ influx also induces mitochondrial response in either cell type (467, 525). The increase in [Ca²⁺]_m and the enhanced formation of NAD(P)H occur in spite of the obvious absence of high-Ca²⁺ microdomains, since [Ca²⁺]_c increases gradually (giving time for Ca²⁺ diffusion from the plasma membrane to more remote sites in the cytoplasm) and does not attain 200 nM (219, 525). Therefore, the transfer of submicromolar Ca²⁺ signals into the mitochondrial matrix had to be assumed. We have measured the effect of buffered [Ca²⁺]_c on [Ca²⁺]_m with the fluorescent dye rhod 2 in permeabilized rat glomerulosa cells (428). [Ca²⁺]_m correlated with [Ca²⁺]_c in the examined range of 60–740 nM, and an increase in [Ca²⁺]_m could be detected already on raising [Ca²⁺]_c from 60 to only 140 nM. The mitochondrial response was small but reproducible and was associated with an increased level of NAD(P)H. In rat luteal cells [Ca²⁺]_m, as measured either with the fluorescent dye rhod 2 or with the mitochondrially targeted luminescent protein aequorin, showed similar dependence on [Ca²⁺]_c (428, 515). Moreover, the increased formation rate of NAD(P)H during capacitative Ca²⁺ influx has also been documented in this cell type (525). This unexpected responsiveness is not unique to steroid-producing cell types in which the formed NADPH is a cofactor of steroid hydroxylation. It also occurs in insulin-producing INS-1/EK-3 cells, where increased formation of NADH, via ATP, induces the specific biological response of the cell, the exocytosis of insulin (428).

The mitochondrial response to submicromolar increases in [Ca²⁺]_c seems to be a hitherto neglected, yet more-or-less general phenomenon. Our data are in harmony with the results of direct measurements of cytoplasmic-mitochondrial Ca²⁺ relationship in excitable cells such as chromaffin cells (15) and ventricular myocytes (602), as well as in the nonexcitable HeLa cells (115) and osteosarcoma cells (428). Indirect measurements of [Ca²⁺]_m (114, 179, 429) in chromaffin and neural cells also support the view that submicromolar Ca²⁺ signal can be transferred into the mitochondrial matrix. {The RBL-2H3 cell line with a response threshold of 2 µM [Ca²⁺]_c seems to be an exception; however, in this cell type even the outer mitochondrial membrane exhibits restricted permeability to Ca²⁺ (125).}

A comparison of data from different laboratories indicates that glomerulosa and luteal cells, the two hitherto examined steroid-producing cell types, exhibit the greatest mitochondrial sensitivity to cytoplasmic Ca²⁺ signals. Interestingly, this sensitivity is reflected not only by the response threshold (below 200 nM [Ca²⁺]_c) but also, at least in the case of glomerulosa cells, by kinetic data. Similar to other cell types, oscillating Ca²⁺ signals often evoke an oscillating mitochondrial Ca²⁺ (Fig. 1) and NAD(P)H response in glomerulosa cells (433, 466). What makes a difference from several other cell types is that the sustained cytoplasmic Ca²⁺ signal, induced by K⁺, elicits sustained mitochondrial Ca²⁺ (514) and NAD(P)H (432) level. Similarly, elevated [Ca²⁺]_c evoked by high concentrations of ANG II (466), or achieved by cell permeabilization (466), is associated with sustained rises in the mitochondrial NAD(P)H level. These observations contrast with the behavior of hepatocytes, in which only oscillatory Ca²⁺ signals, but not sustained rises in [Ca²⁺]_c, can induce sustained mitochondrial Ca²⁺ and NAD(P)H responses (216, 433, 462). The special coupling between [Ca²⁺]_c and mitochondrial metabolism in the glomerulosa cell reflects adaptation to physiological demands. Physiological stimuli of hepatocytes induce oscillating Ca²⁺ signals, whereas glomerulosa cells respond to physiological K⁺ stimuli with a sustained Ca²⁺ signal. HeLa cells may represent the other extreme of mitochondrial kinetics, since not even trains of repetitive Ca²⁺ oscillations can maintain elevated [Ca²⁺]_m (115). The physiological significance of the ability to sustain mitochondrial response in the glomerulosa cell is obvious, the adrenocortical responsiveness to sustained exposure to hyperkalemia is essential for vertebral homeostasis, and the sustained enhancement of mitochondrial NADPH formation may efficiently support the long-term hypersecretion of aldosterone under such a condition (see sect. VII). The significance of cell type-specific mitochondrial Ca²⁺ management has been recently demonstrated also in synaptic transmission (61).

	IV. CROSS-TALK BETWEEN CALCIUM AND cAMP-ACTIVATED PATHWAYS

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A. Ca²⁺ Influx Evoked by ACTH

Although ACTH acts primarily through cAMP, as originally observed in the adrenal fasciculata zone (232), it also stimulates Ca²⁺ influx in glomerulosa cells. Both ACTH and 8-bromo-cAMP induce a sustained Ca²⁺ response after a lag time of 10 min in rat glomerulosa cells (551). ACTH also elicits a Ca²⁺ signal, with a lag time of 2 min, in human glomerulosa cells (184). Such responses are caused by activation of L-type Ca²⁺ channels (151, 184). The ability of -adrenergic stimulation to enhance ANG II-induced Ca²⁺ influx and aldosterone secretion in rat adrenal capsules (434) may result from the same mechanism. The potentiation of ANG II-induced inositol phosphate formation by 8-bromo-cAMP in bovine glomerulosa cells (38) is also attributable to increased Ca²⁺ influx, which, in turn, may stimulate PLC. The cAMP-induced Ca²⁺ influx may exert positive feedback on the formation of cAMP itself and could also support stimulated steroidogenesis (Fig. 7) by the means discussed in section VII. cAMP, via PKA, may also potentiate IP₃ -induced Ca²⁺ release (80, 215), an effect that has not been examined in the glomerulosa cell.

View larger version (18K): [in this window] [in a new window]   FIG. 7. Interaction of the Ca2+-cAMP signaling pathways. The red arrows indicate stimulatory actions.

Serotonin, potentially released by neighboring chromaffin or mast cells, also stimulates aldosterone production (370) and enhances ANG II-induced Ca²⁺ influx (463). In rat glomerulosa cells, serotonin via the 5-HT₇ receptor increases both cAMP and Ca²⁺ levels. The primary action of serotonin is on the G_s-mediated activation of adenylyl cyclase, leading to cAMP formation and a slowly developing Ca²⁺ signal. T-type Ca²⁺ current was enhanced both by 8-bromo-cAMP and serotonin, and the latter effect was reduced by a PKA inhibitor. Serotonin also induced a slow and sustained high-threshold current, probably via L-type Ca²⁺ channels (Fig. 7A in Ref. 316). This activation resembles the L-channel activating action of ACTH. Although activation of T-type channels by cAMP is an unprecedented observation, it is noteworthy these studies were performed in freshly isolated cells, whereas other laboratories have examined glomerulosa cells cultured for 1–2 days.

B. Ca²⁺-Induced Formation of cAMP

Although the messenger role of cAMP has been well established, the steroidogenic action of ACTH also requires Ca²⁺. In its major target organ, the adrenal zona fasciculata, chelation of extracellular Ca²⁺ prevents the corticosterone response to even high doses of ACTH, but only reduces the response to dibutyryl cAMP. This disparity indicates that the Ca²⁺ requirement in the action of ACTH is more important for events before the formation of cAMP (220).

Studies on glomerulosa cells have also demonstrated the Ca²⁺ dependence of ACTH action. After an initial increase, the sustained phase of ACTH-stimulated aldosterone secretion by superfused rat glomerulosa cells is completely abolished in the absence of Ca²⁺ (503). The question can be raised again whether Ca²⁺ is required for the formation or the effect of cAMP. Studies with the organic Ca²⁺ channel inhibitor verapamil (26, 488) indicate that the activation of adenylyl cyclase has a higher requirement for Ca²⁺ than activation of steroid synthesis by cAMP. This conclusion was also supported by measurements showing that ACTH-induced cAMP production is highly Ca²⁺ dependent, and only a very small and transient increase in cAMP is observed in the absence of Ca²⁺ (184).

Although extracellular Ca²⁺ is essential for the binding of ACTH to its receptors (92), Ca²⁺ also exerts postreceptorial effects. Studies by Taits' group (258) revealed that stepwise elevation of K⁺ from 3.6 to 8.9 mM progressively increased cAMP secretion from rat glomerulosa cells. This response required extracellular Ca²⁺ and was prevented with nifedipine, indicating that adenylyl cyclase was stimulated by Ca²⁺ entering through voltage-operated Ca²⁺ channels (258). In contrast, K⁺ did not induce Ca²⁺ signaling and did not influence cAMP output in fasciculata cells (5). While these observations indicated an interaction between the cAMP and Ca²⁺ signaling pathways in glomerulosa cells, there were also conflicting reports on this proposal. More recently, Tait and Tait (532) provided a critical review of the literature, considering the methodological problems arising from the various biological preparations, sample for analysis, and artifacts related to phosphodiesterase inhibition by isobutyl methylxanthine. Their conclusion was consistent with the original concept that Ca²⁺, entering the cell in response to K⁺, activates adenylyl cyclase and cAMP formation that prolongs the stimulation of steroid production. Considering the slight but reproducible activation of phospholipase C by K⁺ (218), Tait and Tait also proposed that "the action of increased extracellular K⁺ can potentially involve all known mechanisms for the stimulation of steroidogenesis in endocrine cells."

Taits' observation suggests that a Ca²⁺-dependent isoform of adenylyl cylase is expressed in rat glomerulosa cells. Out of the nine well-characterized isoforms of adenylyl cyclase, types 1, 3, and 8 are stimulated by Ca²⁺, provided that G_s is present. The PKC-activated cyclases AC2 and AC7 may be also subject to Ca²⁺ control through Ca²⁺-dependent PKC (10). Type AC3 is expressed in bovine (82) and human glomerulosa cells (120). AC1 could not be detected (493), and no data are available on the expression of other Ca²⁺-dependent iosoforms in rat glomerulosa cells.

In rat glomerulosa cells, ANG II reduces, rather than enhances, cAMP production (43, 587), and inhibition of PKA by H-89 does not affect ANG II- and AVP-induced aldosterone production (184). The failure of the Ca²⁺ signal to activate adenylyl cylase may be due to activation of the inhibitory G protein, G_i, a phenomenon observed in bovine cells (97, 326, 344).

ANG II also inhibits adenylyl cyclase activity in purified membranes from the bovine zona glomerulosa (339), yet increases in basal and potentiation of ACTH-induced adenylyl cyclase activity by ANG II have been observed in intact cells (39, 82). Pharmacological data suggest that the enhancement of the cAMP response by ANG II involves the Ca²⁺-dependent and PKC-dependent phosphatase calcineurin (39). This finding may account for the positive interaction of ANG II and ACTH.

What is the significance of Ca²⁺-induced or Ca²⁺-amplified cAMP production in the glomerulosa cell? As shown in section IVA, cAMP-dependent phosphorylation may exert positive feedback on Ca²⁺ influx. Enhancement of cholesterol transport into mitochondria is discussed in section VIIA. Activation of cell metabolism by cAMP also supports increased hormone production, but the details of this effect are beyond the scope of this review.

The interaction of the cAMP and Ca²⁺ signaling pathways is shown in Figure 7.

	V. REGULATION AT THE LEVEL OF PLASMA MEMBRANE RECEPTORS

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Continuous stimulation of cells with hormones and other stimulators very rarely evokes a continuous biological response. Although some agonists cause increased sensitivity, most activate mechanisms that reduce the biological response of the cell and protect it from excessive stimulation. Desensitization is defined as the tendency of responses to reduce in intensity despite the continued presence of the stimulus (313). Agonist-induced cellular desensitization may occur at several different levels. In this section, changes in cellular responsiveness that occur at the level of plasma membrane receptors will be discussed. Mechanisms that regulate more distal elements of the signal transduction pathway have been discussed previously. After a brief introduction to the regulation of GPCRs, special attention will be given to the regulation of angiotensin receptors in adrenocortical cells.

A. Mechanisms for Regulation of GPCRs

The accepted paradigm of GPCR regulation involves their homologous and heterologous desensitization, receptor internalization, and downregulation. Desensitization and internalization are rapid phenomena that develop in minutes, or in some cases seconds, after the stimulation of GPCRs, whereas downregulation is a much slower decrease in the total cellular receptor pool.

Homologous desensitization of a GPCR involves the uncoupling of the receptor from its cognate G protein, after stimulation of the receptor with its agonist. This process is associated with agonist-induced phosphorylation of the receptor. In most cases, specific GPCR kinases (GRKs) appear to be responsible for this phosphorylation. Receptor phosphorylation promotes the binding of -arrestin molecules, rather than the respective G protein, to the receptor (108, 303). However, in some receptors other kinases, such as casein kinases or PKA, might serve as receptor kinase (16, 549, 574). Furthermore, recent studies on metabotropic glutamate receptors suggested that binding of GRKs to the activated receptor can cause phosphorylation-independent desensitization (136). After prolonged stimulation, significant accumulation of desensitized receptors may occur, depending on the kinetics of receptor internalization and recycling (see sect. VD), but pharmacological agonist concentrations are often required to detect cellular desensitization induced by this mechanism. However, the importance of homologous desensitization should not be underestimated since it selectively affects agonist-activated receptors, which undergo rapid uncoupling from G proteins by this mechanism limiting the duration of the signal generation after receptor activation. Thus this mechanism is more important in desensitizing signaling of the receptor in time than in desensitizing the hormonal responsiveness of the tissue. However, if continuous supply of plasma membrane receptors (e.g., via new receptor synthesis or receptor recycling) is not available, prolonged agonist stimulation may lead to depletion of the nondesensitized plasma membrane receptor population, and homologous desensitization of the tissue may occur.

Heterologous desensitization is caused by phosphorylation of GPCRs during stimulation of the cell with agonist(s) that target different receptors. This process is mediated by second messenger-induced stimulation of protein kinases, such as PKA or PKC. Heterologous desensitization typically occurs at lower hormone concentrations than homologous desensitization, since full activation of the second messenger generation may occur, when receptor occupancy is still low. Activated second messenger-regulated kinases can phosphorylate and effectively desensitize both active and inactive receptors, in which the consensus site for the kinase is present. Because desensitization of a large receptor population may occur at relatively low hormone concentrations, this mechanism is an important regulator of the agonist sensitivity of hormone target tissues (reviewed in Ref. 313).

Agonist binding also stimulates internalization of the receptor from the plasma membrane into intracellular vesicles. Although clathrin-independent mechanisms (e.g., via caveolae) have also been reported, endocytosis via clathrin-coated vesicles is the most well-defined mechanism of internalization of GPCRs. The process is preceded by agonist-induced phosphorylation and -arrestin binding to the activated receptor. -Arrestin, in addition to its role in homologous desensitization, serves as an adapter molecule to connect the receptor to clathrin and 2-adaptin, two main components of the clathrin coat (108, 252, 303). During this mechanism, internalization follows rather than precedes desensitization because the internalized receptors are already uncoupled from the G protein by -arrestins.

Intracellular trafficking of internalized receptors occurs via acidic endosomal compartments, in which the ligand can dissociate and the receptor can be dephosphorylated by intracellular phosphatases (312). After these steps the resensitized receptor may recycle to the plasma membrane. Some internalized receptors also move from the endosomes to the lysosomes for degradation. Whereas receptor recycling mediates receptor resensitization after homologous desensitization, lysosomal degradation causes downregulation of the receptor (see below). Some GPCRs, such as the AT₁ receptor, are preferentially targeted for recycling, whereas others, such as the thrombin receptor, are destined for degradation (550). Such targeting is determined by amino acid sequences, usually in the cytoplasmic tail of the receptor, that appear to interact with specific proteins that affect intracellular sorting (86, 550, 578).

Whereas the term internalization is applied to the translocation of cell surface receptors into intracellular compartments with no major change in total receptor number, downregulation signifies a decrease in the total cellular receptor pool. This decrease may result from both increased receptor degradation and reduced receptor synthesis, and usually develops over hours or days. Although receptor degradation, preceded by agonist-induced endocytosis, appears to be a major mechanism of downregulation, mutations that affect receptor internalization do not always interfere with the extent of downregulation (228). This finding indicates the potential importance of impaired receptor synthesis in the process. Long-term agonist stimulation frequently reduces receptor transcription, or induces mechanisms that destabilize receptor mRNA. These mechanisms cause internalization-independent downregulation by decreasing the steady-state level of the receptor population (reviewed in Ref. 487).

B. Regulation of Adrenal Angiotensin Receptors In Vivo

AT₁ receptors are present in very high density in glomerulosa cells. Because the biological response correlates with the formation of a defined number of hormone-receptor complexes, consistent with the law of mass action, the higher the number of surface receptors the lower the agonist concentration required for maximal stimulation. Thus, due to the high receptor density, maximal steroidogenic response occurs at ANG II concentrations that only partially saturate the available receptors. The higher the number of excess ("spare") receptors, the more left-shifted is the dose-response curve for the biological response in comparision to the receptor saturation curve. The presence of a high receptor reserve explains the finding that the EC₅₀ of ANG II-induced aldosterone response is about one order of magnitude lower than the K_d of the receptor in glomerulosa cells (411). This consideration is also valid for in vivo conditions. Because circulating levels of ANG II (see the first paragraph of sect. II) are well below the K_d of AT₁ receptors (see sect. IIA), physiological elevations of ANG II levels that only partially saturate the receptor can elicit full stimulation of steroid secretion. Regulation of ANG II receptors in the zona glomerulosa also frequently affects the magnitude of the receptor reserve. Upregulation of AT₁ receptors creates a larger receptor reserve, enhances the potency of ANG II to stimulate aldosterone secretion, consequently increasing the sensitivity of the tissue. However, a reduction of the number of functional surface receptors (e.g., by downregulation) decreases receptor reserve and causes a right-shift in the dose-response curve for steroidogenesis. In this situation, desensitization of the tissue does occur because aldosterone secretion will be reduced at any given circulating ANG II level.

In most ANG II target tissues, prolonged agonist activation causes desensitization of its biological response and downregulation of its receptors. However, in vivo studies in rats have shown that long-term (1- to 2-day) infusion of low doses of ANG II causes upregulation of adrenal ANG II receptors and increases the aldosterone response to ANG II (141, 227). The hormonal response was similarly enhanced by ANG II infusion in humans (395). It was also observed several decades ago that sodium depletion (known to stimulate renin secretion) enhances the aldosterone seceretory response to ANG II in humans, rats, and dogs (reviewed in Refs. 369, 570). Because low-sodium diet increases AT_1A and AT_1B receptor mRNAs in rat adrenal glands, and this increase is inhibited by an AT₁ receptor blocker (144), it is likely that ANG II induces upregulation of AT₁ receptors during sodium deficiency. Increased receptor binding of ANG II during sodium depletion and ANG II infusion was originally attributed to a positive feedback effect of ANG II on its receptors (227). Although molecular biological studies supported the receptor-inducing effect of ANG II (267), and this mechanism seems to explain upregulation of AT_1B receptors (573), the effect of sodium depletion is primarily induced via an ANG II-AT₁ receptor-independent mechanism (63, 573). Furthermore, in extra-adrenal tissues of other species, and even in the zona glomerulosa of dogs, prolonged low-dose infusion of ANG II reduces the responsiveness of the target tissue to ANG II (40). Downregulation of adrenal angiotensin receptors was also observed in rats following in vivo long-term administration of pharmacological doses of ANG II (416). Because ANG II also exerts extra-adrenal effects and, in the sodium-depleted rat, changes in potassium balance (75, 541) can also modulate adrenal ANG II responsiveness (140), the upregulation of adrenal AT₁ receptors in sodium deficiency may be predominantly due to an indirect effect of the hormone. In agreement with this, as detailed in section VC, studies with isolated glomerulosa and fasciculata cells found no evidence of ANG II-induced up-regulation of angiotensin receptors. Overall, these findings suggest that the in vivo upregulation of angiotensin receptors is an indirect effect of agonist treatment or requires the presence of cofactors, such as ACTH or other circulating hormones, which are absent in experiments with isolated cells.

C. Desensitization of Glomerulosa Cells

Desensitization of the receptor for several hormones, including ANG II, ACTH, bradykinin, and vasopressin, has been shown in mammalian glomerulosa cells (106, 167, 183), and similar mechanisms operate in the adrenal glands of lower vertebrates (70). Early studies showed that ANG II-induced steroid secretion is subject to homologous and heterologous regulatory mechanisms in glomerulosa (167) and fasciculata cells (411). These experiments demonstrated that ANG II directly reduces adrenal responsiveness and does not cause upregulation of ANG II-induced aldosterone responses, even in cells pretreated with low physiological concentrations of ANG II. In isolated rat glomerulosa cells, superfusion of ANG II for 6 h reduced the aldosterone responses to ANG II, potassium ions, and ACTH. This effect was maintained when superfusion with ANG II was performed in the presence of aminogluthetimide to prevent steroid synthesis, suggesting that the inhibitory effect was caused by factors other than substrate depletion or a metabolite of steroid synthesis (e.g., free radical). The ANG II-induced heterologous desensitization of the aldosterone response was attributed to inhibition of the late stage of aldosterone biosynthesis. Preincubation with ANG II evoked homologous desensitization of the ANG II-induced inositol phosphate generation, suggesting that this process involves mechanisms proximal to signal generation, such as receptor desensitization or downregulation. Phorbol ester treatment did not elicit such desensitization, suggesting that it was independent of PKC activation (163).

In other studies on the mechanism of ANG II-induced desensitization of bovine adrenocortical cells, short-term (30-min) pretreatment of glomerulosa cells with 10 nM ANG II caused rapid loss of ¹²⁵I-ANG II binding. Scatchard analysis revealed that exposure to ANG II induced the transformation of high-affinity ANG II binding sites (K_d 0.2 nM) to a low-affinity state (K_d 2 nM) with no change in the total number of extracellular binding sites (73). This homologous desensitization was suggested to be caused by PKC- and calmodulin kinase-independent uncoupling of AT₁ receptors from G proteins. ANG II binding to AT₂ receptors did not show similar regulation. In analogy to the ₂-adrenergic receptor and other GPCRs (313), the most likely mechanism for ANG II-induced rapid homologous desensitization of AT₁ receptors is their phosphorylation by receptor kinases, such as GRKs. Phosphorylation-induced uncoupling of the receptor from its cognate G protein(s) can explain the observed reduction in receptor affinity, since interaction with G proteins is believed to stabilize the high-affinity conformation of the receptor (484). In fact, ANG II rapidly induces phosphorylation of the AT₁ receptor in bovine glomerulosa cells. Such phosphorylation was correlated with the degree of agonist occupancy of the receptor and was not affected by tyrosine kinase inhibitors. Also, it was stimulated, rather than reduced, by PKC inhibitors (501). These data suggest that receptor phosphorylation, which possibly causes early homologous desensitization of the receptor and may be responsible for the above-mentioned effect of ANG II on the affinity of AT₁ receptors, is mediated by receptor kinase(s).

Although the receptor kinase that mediates this effect in glomerulosa cells has not been identified, studies with AT₁ receptors expressed in tumor cell lines suggest that GRKs have an important role in the ANG II-induced rapid phosphorylation of AT₁ receptors. Coexpression of a kinase-deficient GRK2 exerted dominant negative inhibitory effect on ANG II-induced receptor phosphorylation in HEK293 (399) and COS7 cells (398). Oppermann et al. (399) also observed that in HEK 293 cells the kinase-deficient GRK2 prevented homologous desensitization of the AT_1A receptor, whereas inhibition of PKC failed to affect this process (399). Nevertheless, studies in other cell types question the general role of GRK2 in homologous desensitization of AT₁ receptors. In COS7 cells, inhibition of receptor phosphorylation by coexpression of a kinase-deficient GRK2 did not affect the desensitization of the receptor (398). Also, a study in CHO cells suggested that homologous desensitization of AT_1A receptors is mediated by a heparin-sensitive kinase that is different from GRKs (538). In the latter cell line, desensitization of AT_1B receptors is mediated by PKC (539). These data indicate that the identification of the receptor kinase responsible for this process in glomerulosa cells requires additional studies. Nevertheless, it is likely that -arrestin binding to the phosphorylated receptor has an important role in the uncoupling of the receptor molecule from the G protein, since it has been firmly established that ANG II induces association of these molecules with the cytoplasmic tail of AT₁ receptors (8, 173, 443).

Much less is known about the desensitization of AT₂ receptors. It has been reported that no homologous desensitization of AT₂ receptors occurs in bovine adrenocortical cells, but heterologous desensitization of AT₂ receptors caused by stimulation of AT₁ receptors has been reported (402). The latter effect was mimicked by activation of PKC, which is consistent with the PKC-mediated stimulation of AT₂ receptor phosphorylation observed in transfected COS-7 cells (397).

D. Intracellular Trafficking of Angiotensin Receptors

ANG II-induced angiotensin receptor internalization in glomerulosa cells was first described by Bianchi et al. (58), who showed in the rat that injected ¹²⁵I-ANG II first accumulates on the cell surface, then clusters within coated pits, is internalized in coated vesicles, and appears in lysosomes within 20 min. Internalization of radiolabeled ANG II was also reported in cultured bovine adrenocortical cells (124). Immunocytochemical localization of AT₁ receptors using receptor specific antibodies has also been reported in glomerulosa cells (501, 567). Detailed analyses of the mechanism of intracellular trafficking of AT₁ receptors have been performed in expression systems using epitope-tagged (233) or green fluorescent protein (GFP)-labeled (96, 249, 362) receptors. Endocytosis of a GFP-tagged rat AT_1A receptor after stimulation with rhodamine-labeled ANG II is demonstrated in Figure 8. Endocytosis of surface-bound radiolabeled ANG II occurs very rapidly in bovine glomerulosa cells (254, 455), which express predominantly AT₁ receptors (see sect. IIA). Incubation of adrenocortical cells with ANG II causes progressive loss of surface angiotensin receptors (402, 411). In fasciculata cells, half-maximal loss of surface receptors occurred at an ANG II concentration (3 nM) similar to the K_d (2 nM) of the receptor, which resembles the dose dependence of homologous desensitization (see sect. VB). The EC₅₀ of ANG II to stimulate this response was much higher than that to induce steroidogenesis (411). ANG II is unable to induce internalization of AT₂ receptors in cells that selectively express AT₂ receptors (233, 251, 397, 438). In accordance with these findings, no internalization of AT₂ receptors was detected in bovine fasciculata cells (402).

View larger version (84K): [in this window] [in a new window]   FIG. 8. Endocytosis of rhodamine-ANG II (red) in HEK 293 cells stably expressing a GFP-tagged rat AT1A receptor (green). Confocal images of the distribution of the receptors after stimulation for 1 min (top panels) or 30 min (bottom panels) with rhodamine-ANG II at 37°C. The internalized receptors colocalize with rhodamine-ANG II in peripheral early endosomes and juxtanuclear recycling endosomes as described previously (249). (Images courtesy of L. Hunyady and T. Balla.)

Most of the available data suggest that the main mechanism of agonist-induced endocytosis of AT₁ receptors in glomerulosa cells is endocytosis via clathrin-coated vesicles. In rat glomerulosa cells, morphological studies detected internalized angiotensin receptors in coated pits (58). Disruption of the clathrin coat by potassium depletion or phenylarsine oxide treatment markedly inhibits AT₁ receptor endocytosis in bovine glomerulosa cells and other ANG II target tissues (252, 254). In COS-7 cells the kinetics of AT_1A receptors are somewhat faster than that of AT_1B receptors (256). Internalization of expressed AT₁ receptors is independent of G protein coupling (247) and appears to be regulated by agonist-induced phosphorylation of the receptor molecule (251, 502, 543, 544). The receptor kinase that regulates the internalization process has not been identified, since PKC inhibitors and/or coexpression of a kinase-deficient GRK markedly reduced agonist-induced phosphorylation of AT₁ receptors without affecting its internalization kinetics (30, 31, 398). Nevertheless, -arrestins associate with the ANG II-stimulated and phosphorylated receptors in tumor cells expressing AT₁ receptors (8, 443, 599). Coexpression of functionally deficient, mutant -arrestins and dynamins with the AT₁ receptor exerts dominant negative inhibitory effect on the internalization of the receptor at physiological hormone concentrations (182, 443), and internalization of AT₁ receptors is strongly impaired in cells derived from mice deficient in -arrestins (287). These data provide a mechanism for endocytosis of the AT₁ receptor via clathrin-coated pits, and it is likely that endocytosis of AT₁ receptors in glomerulosa cells is mediated by a similar mechanism. However, at higher ANG II concentrations, when the AT₁ receptor population becomes rapidly saturated with the agonist, this mechanism apparently becomes saturated and a -arrestin- and dynamin-independent mechanism becomes predominant (182, 600). Although caveolae have been suggested as another possible mechanism for AT₁ receptor endocytosis (265), this requires further studies, since endocytosis via caveolae is also dynamin dependent (234). The exact mechanism of -arrestin- and dynamin-independent endocytosis of AT₁ receptors and its importance in adrenal cells has yet to be elucidated.

Studies in HEK 293 cells using fluorescent markers have demonstrated that early endosomes, recycling endosomes, and multivesicular bodies related to late endosomes participate in the intracellular trafficking of internalized AT₁ receptors (249, 491). Acidification of the endosomal compartments is required for efficient recycling of adrenocortical and expressed AT₁ receptors to the cell surface (124, 233, 411, 455). Acidification causes agonist dissociation from many GPCRs, including angiotensin receptors, and the resulting conformational change is believed to promote the dissociation of -arrestins from internalized receptors and dephosphorylation of the receptor molecule (312). The rapid ANG II-induced loss of plasmalemmal AT₁ receptors in adrenocortical cells treated with monensin, an inhibitor of vesicular acidification, suggests that recycling of the receptor to the plasma membrane is a highly efficient process (411).

It was initially assumed that receptor internalization mediates agonist-induced desensitization of the cells by reducing the number of surface receptors. However, at physiological hormone concentrations many activated GPCRs, including AT₁ receptors, use -arrestins as adaptor proteins to connect with clathrin-coated structures (182, 303). Because -arrestins also uncouple the receptors from their G proteins, the internalized receptors are already desensitized. Thus endocytosis of AT₁ receptors, similar to that of other GPCRs, could serve to maintain signal generation, since it leads to intracellular dephosphorylation, resensitization, and recycling of receptors (312). This mechanism may explain the previously reported importance of receptor internalization for the sustained phase of ANG II-induced inositol phosphate and Ca²⁺ signal generation in bovine glomerulosa cells (254). Some of the internalized AT₁ receptors are also targeted to lysosomes for degradation, and during prolonged ANG II stimulation this process could eventually cause degradation of ANG II and downregulation of the AT₁ receptors in bovine adrenocortical cells (124, 411). It has also been proposed that endocytosis of AT₁ receptors is required for ANG II-induced PKC translocation (568). There is no evidence in adrenal cells for a role of -arrestins and/or receptor internalization in ANG II-induced MAPK activation (see also sect. VIC). Intracellular accumulation of ANG II also prolongs the half-life of the peptide, which is rapidly degraded in the circulation, and may be utilized for paracrine and autocrine actions, or could exert intracellular actions by stimulating cytoplasmic or nuclear angiotensin receptors. However, in adrenocortical cells the importance of these mechanisms has not been established. This topic was reviewed in more detail previously (252).

E. Receptor Downregulation

Downregulation of receptors is defined as a decrease of total number of receptors (313), which typically requires agonist exposure for at least 1 h and may involve modulation of the rate of receptor synthesis or degradation. Long-term incubation of adrenocortical cells with high concentrations of ANG II has been suggested to cause downregulation of angiotensin receptors (402, 411). However, since the available studies determined surface binding of the receptor, and the decrease during the first 3 h was caused by internalization of surface receptors, it is unclear whether true downregulation of total cellular binding sites occurred during this period. More prolonged (more than 3 h) stimulation with pharmacological concentration of ANG II reduces the level of AT₁ receptor mRNA (402, 456). This decrease in mRNA levels is caused by inhibition of transcription, without significant change in the half-life of the mRNA (402). Interestingly, in the same study stimulation of AT₁ receptors also reduced the binding sites and the mRNA of AT₂ receptors, but this effect was mediated by a decrease in AT₂ receptor mRNA stability. AT₁ receptor-mediated regulation of AT₂ receptors was also observed in expression systems, where it has been shown to cause PKC-mediated phosphorylation of AT₂ receptors (397).

Downregulation of AT₁ receptors may also be caused by increased receptor degradation, which is preceded by internalization of the receptor into endosomal compartments. It has been shown in bovine adrenocortical cells that after internalization, cellular processing of the hormone-receptor complex leads to degradation of the ligand. The acidic pH of endosomal compartments, which facilitates hormone dissociation, is also essential for ligand degradation (124). Studies in HEK 293 cells have suggested that most internalized AT₁ receptors recycle to the plasma membrane, whereas the hormone dissociates in acidic compartments and enters lysosomes for degradation (233). However, it is likely that a small fraction of receptors undergoes lysosomal degradation, and after prolonged incubation with ANG II, this leads to increased receptor loss. It is presently unclear whether ubiquitylation of the AT₁ receptor has a role in its targeting to lysosomes, but recent studies indicated that elimination of the lysine residues that serve as potential ubiquitylation sites causes increased accumulation of labeled ANG II in CHO cells expressing AT_1A receptors (358). This finding raises the possibility that ubiquitylation has a role in the regulation of intracellular targeting of the receptor.

A recent study has questioned the importance of short-term AT₁ receptor desensitization and long-term receptor downregulation in ANG II-induced regulation of glomerulosa cells, because desensitization of the steroidogenic action of ANG II required 24-h preincubation with a pharmacological concentration (1 µM) of ANG II, whereas 10 nM ANG II had no effect on the steroidogenic response (456). However, the steroidogenic response was analyzed using only maximally effective concentrations of ANG II, which may not reveal changes in cellular responsiveness in cells with excess receptors. In fact, earlier studies showed that ANG II pretreatment had a much larger effect on ANG II-induced steroidogenesis in rat and bovine glomerulosa cells (167, 411). Also, rapid desensitization of second messenger production was observed after ANG II treatment in AT₁ receptor-transfected cells (399, 539). Thus the available data indicate that ANG II-induced desensitization or reduction of plasma membrane AT₁ receptors causes desensitization of adrenocortical cells.

	VI. LONG-TERM EFFECTS OF ANGIOTENSIN II

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Long-term effects of ANG II at the cellular level include regulation of cell growth, proliferation, differentiation and apoptosis, induction of steroidogenic enzymes, modulation of cell migration, and extracellular matrix deposition. These effects are mediated by complex and highly cell-specific signaling pathways, involving G proteins, receptor and nonreceptor tyrosine kinases, MAPKs, and cytokine-like signaling pathways (132). In this paper we focus on mechanisms that have been reported to be present in aldosterone-producing cells, and we refer to recent reviews for mechanisms identified in other ANG II target cells (132, 152, 483).

A. Activation of Growth Responses by ANG II in the Glomerulosa Cell

Rat glomerulosa cells respond to sodium deficiency with hypertrophy and hyperplasia and undergo regression in animals on a high-sodium diet. These changes are dependent on the level of circulating ANG II, which has proven to be a major determinant of trophic changes in the zona glomerulosa (see references in Refs. 132, 372). These studies provided the first evidence that ANG II stimulates cell growth and/or proliferation in its target tissues, a phenomenon which has since been shown in many other cell types, including vascular smooth muscle cells, cardiac myocytes and fibroblasts, and renal mesangial cells (132). Pharmacological studies have revealed that the growth-promoting actions of ANG II are mediated by the AT₁ receptor and that ANG II binding to AT₂ receptors has an opposing effect in many tissues (390).

The growth-promoting effects of ANG II are exerted directly by the hormone, because the stimulation observed in vivo can be demonstrated in isolated glomerulosa cells. However, lipoxygenase-mediated formation of arachidonic acid metabolites (see sect. IIC) has been suggested as a contributing factor in this process (379). In rat (345) and bovine (547) glomerulosa cells, ANG II stimulates cell growth and increases thymidine incorporation and cell proliferation. Activation of PKC, tyrosine kinases, and MAPKs, leading to increased expression of several early genes, including c-fos, c-jun, and c-myc, is believed to play a central role in these processes (29, 109).

B. Role of Receptor and Nonreceptor Tyrosine Kinases

In rat glomerulosa cells, parallel roles of tyrosine kinase and PKC activation in ANG II-mediated stimulation of DNA synthesis and cell proliferation have been reported and are independent of PLA₂, cyclooxygenase, and lipoxygenase activation (345). Endothelins, acting via the ET_A endothelin receptor, also stimulate proliferation of glomerulosa cells by activating tyrosine kinases and PKC (347). Endothelins, ghrelin, and proadrenomedullin-derived peptides exert proliferative actions on glomerulosa cells by stimulating tyrosine kinase-dependent activation of the p42/p44 MAPK pathway (9, 44, 346). The specific tyrosine kinases and their mode of action during ANG II-induced proliferation of adrenal glomerulosa cells have not yet been identified. It has also been proposed that ANG II promotes capacitative calcium entry and aldosterone biosynthesis in these cells via tyrosine kinases (11, 65, 277). Furthermore, tyrosine kinases have long-term effects on the synthesis of steroidogenic enzymes, and in the H295R cell line Src tyrosine kinase was reported to have a role in maintaining the glomerulosa-like phenotype (499).

In smooth muscle and other target cells, ANG II also promotes cell migration and induces changes in cell shape and volume by activating focal adhesion kinase. It stimulates the closely related tyrosine kinase, Pyk2, which is an important link between ANG II-mediated Ca²⁺ signal generation and the growth factor-regulated signaling pathways (152, 211). ANG II can also activate the Jak family of tyrosine kinases, which mediate the actions of cytokinetype receptors on transcription via phosphorylation of STAT (signal transducers and activators of transcription) proteins (57, 340). However, additional studies are required to demonstrate the roles of these mechanisms in adrenal glomerulosa cells.

Transactivation of receptor tyrosine kinases, such as the epidermal growth factor receptor and platelet-derived growth factor receptor or Axl, has been proposed to have a major role in ANG II-induced proliferation of vascular smooth muscle and other target cells (152, 482) (Fig. 9). EGF receptors are present in glomerulosa cells, as evidenced by the stimulatory effect of EGF on the aldosterone production of porcine glomerulosa cells (282), but the relevance of EGF receptor transactivation to the proliferative action of ANG II has not been elucidated in these cells.

View larger version (21K): [in this window] [in a new window]   FIG. 9. Mechanism of ERK1/2 activation in glomerulosa cells. The thickness of the arrows reflects the quantitative importance of the pathways in glomerulosa cells, as described in text. R, receptor.

C. ANG II-Induced Activation of MAPKs

MAPKs are serine/threonine kinases that have a central role in the regulation of gene expression by plasma membrane receptors. Receptor tyrosine kinases and GPCRs can stimulate phosphorylation cascades leading to concomitant phosphorylation of MAPKs on adjacent threonine and tyrosine residues. Activated MAPKs translocate to the nucleus and regulate transcription factors and gene expression, resulting in cell-specific long-term cellular responses such as cell growth, apoptosis, and differentiation (270). The functional outcome of MAPK activation is dependent on the availability of downstream substrates.

Mammalian subtypes of MAPKs are grouped into six major subfamilies, such as ERK1/2 (also known as p42/p44 MAPKs), c-Jun NH₂-terminal protein kinases (JNKs, or stress-activated protein kinases, SAPKs), p38 MAPKs, ERK6, ERK3, and ERK5 (or Big MAPK). Typically, receptor-mediated activation of ERK1/2 regulates cell growth and differentiation. JNKs and p38 MAPKs are activated by cellular stress and inflammatory cytokines, which regulate cellular processes leading to inhibition of cell proliferation and induction of apoptosis, whereas ERK5 has been proposed as a redox-sensitive MAPK (1). The ability of ANG II to activate ERK1/2, JNK, p38 MAPK and ERK5 is consistent with its growth-promoting, cytokine-like, and redox-sensitive actions (132).

In glomerulosa and H295R cells, ANG II-induced activation of the ERK, JNK, and the p38 MAPK pathways has been reported (199, 354, 548, 577). Activation of the ERK pathway is believed to be the major mediator of the effect of ANG II on the proliferation of these cells (Fig. 9). As in other cell types, ANG II causes tyrosine kinase-dependent activation of the Ras small G protein in bovine and rat glomerulosa cells, leading to stimulation of Raf-1, a MAPK kinase kinase, which phosphorylates MEK, a MAPK kinase, and to phosphorylation and activation of ERKs (354, 548). However, in contrast to other target cells (152), ANG II-stimulated ERK activation in bovine glomerulosa cells was not mediated by the increase in [Ca²⁺]_c, although Ca²⁺ played a permissive role in this process (548). The major mechanism leading to ERK activation was pertussis toxin insensitive, although a small contribution from pertussis toxin-sensitive G_i/o proteins could not be excluded. Although PKC depletion significantly reduced ANG II-induced ERK activation in these cells, ras- and raf-1 kinase-mediated ERK activation was independent of PKC. This difference suggests that ras/raf-1 activation represents a PKC-independent pathway, whereas the major pathway of ERK activation is mediated by PKC via an unidentified target downstream to raf-1 kinase, possibly MEK (548). More detailed studies on the mechanism of ANG II-induced raf-1 kinase activation have demonstrated that the PKC-independent stimulation of this kinase is mediated partly by pertussis toxin-sensitive G_i/o proteins and phosphatidylinositol 3-kinase (500). The proposed mechanisms of MAPK activation in the glomerulosa cell are shown in Figure 9.

In H295R cells, high concentrations of ANG II induced prolonged stimulation of ERK and JNK (SAPK) activity with a maximal response at 5 and 30 min, respectively (577). However, the effect of ANG II on JNK activation in these cells is controversial (199, 577). ANG II also caused ras and ERK-dependent increases in cyclin D1 promoter activity as well as enhanced cyclin D1 mRNA levels and cyclin D1 protein levels, and promoted cell cycle progression (577). c-Fos and c-Jun have a role in the stimulation of the cyclin D1 promoter activity, and the induction of JNK activity by ANG II may also contribute to the enhanced transactivation by c-Jun. Since the induction of cyclin D1 serine/threonine kinase activity promotes cell cycle progression and cellular proliferation, these effects participate in the mitogenic action of ANG II in aldosterone-producing cells.

ANG II also stimulates the transcription of the aldosterone synthase (CYP11B2) gene, and the details of this process are described in section VIIC.

	VII. THE FINAL ACTION: ROLE OF CALCIUM IN THE CONTROL OF STEROID PRODUCTION

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Neurons and skeletal muscle cells may exhibit Ca²⁺ signals that far exceed 10^–5 M in well-confined microdomains. Such a large Ca²⁺ signal may be required for the extremely rapid biological response, often occurring within milliseconds. The release of steroid hormones takes place by a much slower rate by diffusion across the plasma membrane. There are no data indicating the generation of such Ca²⁺ hotspots in steroid-producing cells, in which they are not required. Nevertheless, even without such hotspots, in this review ample evidence has been presented in favor of the messenger role of Ca²⁺ in the control of steroidogenesis by the glomerulosa cell; Ca²⁺ ionophores stimulate aldosterone production, secretagogue stimuli evoke a cytoplasmic Ca²⁺ signal, and prevention of this Ca²⁺ signal abolishes the hormonal response. There is an excellent correlation between K⁺-induced Ca²⁺ signal and aldosterone production (28, 89, 432); moreover, aldosterone stimulation by the cAMP-mediated agonist ACTH is also Ca²⁺ dependent (see sect. IVB).

The amplitude of the Ca²⁺ signal is an essential, but not the only, factor controlling Ca²⁺-induced steroid responses. Coapplication of ANG II or thapsigargin, on the one hand, and K⁺, on the other, stimulates aldosterone production at a rate that far exceeds that attained by simple additive effects. To stimulate aldosterone production with K⁺ alone, [Ca²⁺]_c has to attain a much higher value than that measured in the cells stimulated with two agonists simultaneously (81, 219). This observation led us to postulate (509) that it is not only the size, but also the subcellular localization of the Ca²⁺ signal that determines the rate of aldosterone production. It should be recalled in this regard that Ca²⁺ released from IP₃-sensitive stores is more efficient in activating mitochondrial dehydrogenases than that entering from the extracellular space (467). Also, Ca²⁺ entering through voltage-activated Ca²⁺ channels into the subplasmalemmal space may be more efficient in activating adenylyl cyclase (5, 258).

Ca²⁺ exerts a feedback effect on the initial steps of various signal-transducing pathways, but its major action is on the stimulation of steroidogenesis. Its best-elucidated effect is the stimulation of cholesterol transport into the mitochondria, the process that determines the rate of hormone secretion during acute stimulation in each steroid-producing cell type.

A. Cholesterol Transport

Steroid-secreting cells do not store hormones; the rate of hormone secretion depends on the de novo synthesis of the cell-specific steroid. The precursor of steroid hormones is cholesterol, which may be synthesized intracellularly from acetyl CoA. However, increased hormone secretion requires its uptake from plasma lipoproteins. Glomerulosa cells utilize both low-density lipoprotein (LDL) and high-density lipoprotein (HDL) as a source of cholesterol (374), and ANG II stimulates the uptake of HDL-cholesterol. This effect of ANG II is due to enhanced expression of scavenger receptor class B type 1 (SR-B1) (98). Cholesterol is stored in esterified form in cytoplasmic lipid droplets and released in response to hormonal activation of cholesterol ester hydrolase. ANG II activates the enzyme via the MAPK extracellular signal-regulated kinase (ERK 1/2)-mediated phosphorylation (87).

The rate-limiting step in steroidogenesis is the conversion of the precursor compound cholesterol to pregnenolone. This step, which results in the removal of a six-carbon unit from cholesterol by the cholesterol side-chain-cleaving enzyme, cytochrome P-450scc (the gene product of CYP11A1), takes place at the matrix side of the inner mitochondrial membrane. The transport of cholesterol from the cytoplasmic lipid droplets to the mitochondria requires an intact cytoskeleton and is facilitated by specific proteins. While the sterol carrier protein-2 (SCP2) (554) may transport cholesterol to the outer mitochondrial membrane, the limiting step of steroid synthesis is the rate of cholesterol transfer from the outer to the inner mitochondrial membrane through the aqueous phase of the intermembrane space. This transfer is dependent on a "cycloheximide-sensitive, highly labile protein," the level of which is raised by ACTH via cAMP-dependent phosphorylation (for reviews, see Refs. 497, 521). Ca²⁺ also activates the transport of cholesterol into mitochondria and its subsequent side-chain cleavage (223, 299). Elevation of [Ca²⁺]_c by either ANG II or via cell permeation results in increased cholesterol content in the inner mitochondrial membrane, and especially in the contact sites between the outer and inner mitochondrial membrane. This effect of Ca²⁺ is also sensitive to cycloheximide (100, 101).

Although the previously described "steroidogenesis activating polypeptide" (SAP) (410) has characteristics similar to those of the "labile protein," the 37-kDa protein (p37) termed steroidogenic acute regulating (StAR) protein (169) currently meets the requirements for being identical with the labile protein. StAR is expressed in all steroid-secreting cell types, and its mutation is the cause of congenital lipoid adrenal hyperplasia, an autosomal recessive disorder characterized by impaired synthesis of adrenal and gonadal steroid hormones (318). The half-life of StAR is 3–5 min, which corresponds to that of the labile protein. After its transport into the mitochondrial matrix, its mitochondrial leader sequence is cleaved off to yield the stable 30-kDa intramitrochondrial StAR (p30). StAR mRNA and protein are induced via a cAMP-mediated mechanism that causes the enhancement of StAR transcription and/or mRNA stability. ACTH, acting via PKA, phosphorylates and activates both the putative "labile protein" and StAR. Phosphorylation of StAR itself may not be required for mitochondrial import, but phosphorylation is directly linked to the steroidogenic response of the cell to stimulation (521). The crystal structure of its lipid transfer domain suggests that StAR acts by shuttling cholesterol molecules one at a time through the intermembrane space of the mitochondrion (553). However, the experimental data on its site of action are controversial. Some observations suggest that phosphorylated StAR (pp37) exerts its action at the outer mitochondrial membrane and that its further transport into the matrix and conversion to pp30 are irrelevant to cholesterol transport (71). Other observations suggest that phosphorylation of newly synthetized StAR p37, as well as its mitochondrial import and processing to pp30, are essential for enhanced steroidogenesis (14).

Expression and phosphorylation of StAR in glomerulosa cells are enhanced by ACTH as well as by Ca²⁺-mediated stimuli such as ANG II and K⁺ (54, 101, 111, 156, 226). Elevated [Ca²⁺]_c, induced by the Ca²⁺ channel agonist BAY K 8644 (111) or by application of a Ca²⁺ ionophore (101), also induced StAR expression. The transcriptional effect of ANG II involves the activation of MAPK ERK 1/2, which represses DAX-1 (401), a transcription factor known to repress the StAR protein (306). Atrial natriuretic hormone, a physiological antagonist of aldosterone secretion, in addition to increasing K⁺ conductance and inhibiting T-type Ca²⁺ channels (see sect. IIB), also inhibits the transcription of the StAR gene (99). Accordingly, ANF prevents the mitochondrial import of StAR protein and the accumulation of cholesterol in mitochondrial contact sites (99, 156).

B. Increased Reduction of Pyridine Nucleotides in Mitochondria

Whereas the action of CaM and CaMKII in supporting aldosterone production is confined to the transport of cholesterol to the inner mitochondrial membrane (422), the cytoplasmic Ca²⁺ signal is transferred into the mitochondrial matrix (see sect. III), and intramitochondrial Ca²⁺ contributes to the stimulation of hormone production. In cycloheximide-treated adrenocortical mitochondria (which are depleted of the cholesterol-transporting labile protein), the conversion of cholesterol (present in the inner mitochondrial membrane) to pregnenolone is stimulated by Ca²⁺. This effect is completely inhibited by ruthenium red, an inhibitor of mitochondrial Ca²⁺ uptake (299). In electropermeabilized bovine glomerulosa cells, the Ca²⁺-induced aldosterone response in the presence of NADP⁺ is also blocked by ruthenium red (90). The mitochondrial Na⁺/Ca²⁺ exchanger extrudes Ca²⁺ from the mitochondria, and its inhibition also augments Ca²⁺-induced steroid production (78). These observations indicate that Ca²⁺ has an intramitochondrial site of action, and this could suggest that the underlying mechanism of this action is the activation of cholesterol transport again. However, intramitochondrial Ca²⁺ does not affect the function of StAR transiently located in the outer mitochondrial membrane (71), and no data are available to support a role of intramitochondrial Ca²⁺ in the transport of cholesterol.

Intramitochondrial NADP⁺ is reduced by transhydrogenation at the expense of NADH. All of the major mitochondrial steps of aldosterone biosynthesis, namely, the side-chain cleavage of cholesterol (yielding pregnenolone), the hydroxylation of 11-deoxycorticosterone to corticosterone, and the conversion of the latter compound to aldosterone, require NADPH. Tricarboxylic acid cycle intermediates support not only the reduction of pyridine nucleotides (439) but also steroid hydroxylation in (mixed) adrenocortical cells (310, 319, 417), including glomerulosa cells (197). Inhibition of the electron transport chain at site 1 by rotenone stimulates the side-chain cleavage of cholesterol, indicating an effect of the accumulated NADPH (222). Both NAD(P)H level and aldosterone production rate exhibit a biphasic response to K⁺, with a maximum at 8.4 mM K⁺ (432). Increased aldosterone production is associated with increased utilization of NADPH, as shown by the finding that the reoxidation of NADPH, formed in response to K⁺, is delayed by the addition of aminoglutethimide, which inhibits the conversion of cholesterol to pregnenolone (466). Although stimulation of mitochondrial metabolism by Ca²⁺ may not be essential for supporting submaximal formation of pregnenolone (441), the bulk of the evidence supports the significance of enhanced NADPH formation during stimulation of hormone secretion.

Utilizing NADH for increased formation of ATP is, obviously, essential during cell stimulation with Ca²⁺-mobilizing agonists. ATP is required for protein phosphorylation by different protein kinases. Furthermore, Ca²⁺ signaling alters not only the Ca²⁺ content of the cytoplasm and intracellular Ca²⁺ stores, but also induces secondary alterations in sodium, potassium, and proton balance. The restoration of intracellular ionic composition by PMCA and SERCA as well as Na⁺-K⁺-ATPase may cause a rapidly developing and prolonged energy demand.

C. Induction of Aldosterone Synthase

Long-term stimulation of the adrenal cortex results in cell proliferation (see sect. VIA) and increased synthesis of steroidogenic enzymes. With regard to aldosterone secretion, the increased expression of the enzyme system converting deoxycorticosterone to aldosterone has special significance. In humans and rodents, this conversion is carried out by the zona glomerulosa-specific aldosterone synthase (CYP11B2, also termed cytochrome P-450_aldo or P-450c11AS), the gene product of CYP11B2. [An isoform of aldosterone synthase, 11-hydroxylase (P-450c11), the gene product of CYP11B1, is responsible for 11-hydroxylation in the zona fasciculata, and also acts as aldosterone synthase in bovine and porcine glomerulosa cells (533, 591).] It has been amply verified in rat, human (H295R), and hamster glomerulosa cells that increases in [Ca²⁺]_c, evoked by ANG II, or K⁺, or a Ca²⁺ ionophore, induce the expression of CYP11B2 (62, 112, 315, 485, 592). This effect of Ca²⁺ is mediated by calcium/calmodulin-dependent protein kinase I (CaMKI) (116). The lag time of Ca²⁺-induced expression of aldosterone synthase in H295R cells is <6 h, as examined by real-time RT-PCR (W. E. Rainey, personal communication). This lag time accounts for the observation of increased activity of the late stage of aldosterone biosynthesis (conversion of corticosterone to aldosterone) in the chronically, but not in the acutely, sodium-depleted rat (506).

Both CYP11B2, encoding aldosterone synthase in glomerulosa cells, and CYP11B1, encoding 11-hydroxylase in fasciculata cells, contain a cAMP-response element (CRE) in its promoter region. CaMKI and CaMKIV can phosphorylate the activating transcription factor ATF-1 and CRE-binding protein (CREB) that bind to CREs, but this binding is not sufficient to support CYP11B2 expression. CYP11B2 contains at least two additional cis-acting regulatory elements (Ad-5 and NBRE-1) that are not present in CYP11B1. The ability of these elements to bind two members of the NGFIB family of orphan nuclear receptors, NGFIB and NURR1, suggests that these proteins enhance the expression of CYP11B2 (35). The role of CaMKs in the activation of these factors is still to be elucidated.

PKC, which under acute conditions inhibits rather than stimulates aldosterone production (see sect. IIC), depresses the expression of CYP11B2, as observed in the hamster glomerulosa cell (314, 315).

The sites of steroidogenic Ca²⁺ action are shown summarized in Figure 10.

View larger version (52K): [in this window] [in a new window]   FIG. 10. Steroidogenic actions of Ca2+. The direct actions of Ca2+ are shown by red arrows, and the black arrows indicate metabolic transformations and transport processes. StAR, steroidogenic acute regulatory protein; CYP11B2, aldosterone synthase.

	ACKNOWLEDGMENTS

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We thank Dr. Kevin J. Catt for critical reading of this manuscript and for his valuable comments. The linguistic revision by Matthew Suff (Central European University, Budapest) is greatly appreciated.

This review honors the late Professor Sylvia A. S. Tait and her husband Professor James F. Tait on the occasion of the 50th anniversary of the discovery of aldosterone.

Experimental work performed in the authors' laboratories has been supported by the National Science Foundation (OTKA), the Hungarian Academy of Sciences, the (Hungarian) Council for Medical Science (ETT), the Wellcome Trust, and the European Union.

Address for reprint requests and other correspondence: A. Spät, Dept. of Physiology, Semmelweis University, Faculty of Medicine, PO Box 259, H-1444 Budapest, Hungary (E-mail: Spat{at}Puskin.SOTE.Hu).

	REFERENCES

Top

 

1. Abe J, Kusuhara M, Ulevitch RJ, Berk BC, and Lee JD. Big mitogen-activated protein kinase 1 (BMK1) is a redox-sensitive kinase. J Biol Chem 271: 16586–16590, 1996.[Abstract/Free Full Text]
2. Aguilera G and Catt KJ. Dopaminergic modulation of aldosterone secretion in the rat. Endocrinology 114: 176–181, 1984.[Abstract/Free Full Text]
3. Aguilera G and Catt KJ. Participation of voltage-dependent calcium channels in the regulation of adrenal glomerulosa function by angiotensin II and potassium. Endocrinology 118: 112–118, 1986.[Abstract/Free Full Text]
4. Aguilera G, Harwood JP, and Catt KJ. Somatostatin modulates effects of angiotensin II in adrenal glomerulosa zone. Nature 292: 262–263, 1981.[CrossRef][Medline]
5. Albano JDM, Brown BL, Ekins RP, Tait SAS, and Tait JF. The effects of potassium, 5-hydroxytryptamine, adrenocorticotrophin and angiotensin II on the concentration of adenosine 3':5'-cyclic monophosphate in suspensions of dispersed rat adrenal zona glomerulosa and zona fasciculata cells. Biochem J 142: 391–400, 1974.[Medline]
6. Allbritton NL, Meyer T, and Stryer L. Range of messenger action of calcium ion and inositol 1,4,5-trisphosphate. Science 258: 1812–1815, 1992.[Abstract/Free Full Text]
7. Ambroz C and Catt KJ. Angiotensin II receptor-mediated calcium influx in bovine adrenal glomerulosa cells. Endocrinology 131: 408–414, 1992.[Abstract/Free Full Text]
8. Anborgh PH, Seachrist JL, Dale LB, and Ferguson SSG. Receptor/beta-arrestin complex formation and the differential trafficking and resensitization of beta2-adrenergic and angiotensin II type 1A receptors. Mol Endocrinol 14: 2040–2053, 2000.[Abstract/Free Full Text]
9. Andreis PG, Malendowicz LK, Trejter M, Neri G, Spinazzi R, Rossi GP, and Nussdorfer GG. Ghrelin and growth hormone secretagogue receptor are expressed in the rat adrenal cortex: evidence that ghrelin stimulates the growth, but not the secretory activity of adrenal cells. FEBS Lett 536: 173–179, 2003.[CrossRef][Web of Science][Medline]
10. Antoni FA. Calcium regulation of adenylyl cyclase. Relevance for endocrine control. Trends Endocrinol Metabol 8: 7–14, 1997.[CrossRef][Medline]
11. Aptel HB, Burnay MM, Rossier MF, and Capponi AM. The role of tyrosine kinases in capacitative calcium influx-mediated aldosterone production in bovine adrenal zona glomerulosa cells. J Endocrinol 163: 131–138, 1999.[Abstract]
12. Aptel HB, Johnson EIM, Vallotton MB, Rossier MF, and Capponi AM. Demonstration of an angiotensin II-induced negative feedback effect on aldosterone synthesis in isolated rat adrenal zona glomerulosa cells. Mol Cell Endocrinol 119: 105–111, 1996.[CrossRef][Medline]
13. Arrighi I, Bloch-Faure M, Grahammer F, Bleich M, Warth R, Mengual R, Drici MD, Barhanin J, and Meneton P. Altered potassium balance and aldosterone secretion in a mouse model of human congenital long QT syndrome. Proc Natl Acad Sci USA 98: 8792–8797, 2001.[Abstract/Free Full Text]
14. Artemenko IP, Zhao D, Hales DB, Hales KH, and Jefcoate CR. Mitochondrial processing of newly synthesized steroidogenic acute regulatory protein (StAR), but not total StAR, mediates cholesterol transfer to cytochrome P450 side chain cleavage enzyme in adrenal cells. J Biol Chem 276: 46583–46596, 2001.[Abstract/Free Full Text]
15. Babcock DF, Herrington J, Goodwin PC, Park YB, and Hille B. Mitochondrial participation in the intracellular Ca²⁺ network. J Cell Biol 136: 833–844, 1997.[Abstract/Free Full Text]
16. Baig AH, Swords FM, Noon LA, King PJ, Hunyady L, and Clark AJL. Desensitization of the Y1 cell adrenocorticotropin receptor: evidence for a restricted heterologous mechanism implying a role for receptor-effector complexes. J Biol Chem 276: 44792–44797, 2001.[Abstract/Free Full Text]
17. Balla T, Baukal AJ, Eng S, and Catt KJ. Angiotensin II receptor subtypes and biological responses in the adrenal cortex and medulla. Mol Pharmacol 40: 401–406, 1991.[Abstract]
18. Balla T, Baukal AJ, Guillemette G, and Catt KJ. Multiple pathways of inositol polyphosphate metabolism in angiotensin-stimulated adrenal glomerulosa cells. J Biol Chem 263: 4083–4091, 1988.[Abstract/Free Full Text]
19. Balla T, Baukal AJ, Guillemette G, Morgan RO, and Catt KJ. Angiotensin-stimulated production of inositol trisphosphate isomers and rapid metabolism through inositol 4-monophosphate in adrenal glomerulosa cells. Proc Natl Acad Sci USA 83: 9323–9327, 1986.[Abstract/Free Full Text]
20. Balla T, Baukal AJ, Hunyady L, and Catt KJ. Agonist-induced regulation of inositol tetrakisphosphate isomers and inositol pentakisphosphate in adrenal glomerulosa cells. J Biol Chem 264: 13605–13611, 1989.[Abstract/Free Full Text]
21. Balla T, Enyedi P, Hunyady L, and SpätA. Effects of lithium on angiotensin-stimulated phosphatidylinositol turnover and aldosterone production in adrenal glomerulosa cells: a possible causal relationship. FEBS Lett 171: 179–182, 1984.[CrossRef][Medline]
22. Balla T, Enyedi P, Spät A, and Antoni FA. Pressor-type vasopressin receptors in the adrenal cortex: properties of binding, effects on phosphoinositide metabolism and aldosterone secretion. Endocrinology 117: 421–423, 1985.[Abstract/Free Full Text]
23. Balla T, Guillemette G, Baukal AJ, and Catt KJ. Formation of inositol 1,3,4,6-tetrakisphosphate during angiotensin II action in bovine adrenal glomerulosa cells. Biochem Biophys Res Commun 148: 199–205, 1987.[Medline]
24. Balla T, Holló Z, Várnai P, and Spät A. Angiotensin II inhibits potassium-induced calcium signal generation in rat adrenal glomerulosa cells. Biochem J 273: 399–404, 1991.[Medline]
25. Balla T, Hunyady L, Baukal AJ, and Catt KJ. Structures and metabolism of inositol tetrakisphosphates and inositol pentakisphosphate in bovine adrenal glomerulosa cells. J Biol Chem 264: 9386–9390, 1989.[Abstract/Free Full Text]
26. Balla T, Hunyady L, and SpätA. Possible role of calcium uptake and calmodulin in adrenal glomerulosa cells: effects of verapamil and trifluoperazine. Biochem Pharmacol 31: 1267–1271, 1982.[CrossRef][Web of Science][Medline]
27. Balla T, Szebeny M, Kanyár B, and Spät A. Angiotensin II and FCCP mobilizes calcium from different intracellular pools in adrenal glomerulosa cells: analysis of calcium fluxes. Cell Calcium 6: 327–342, 1985.[CrossRef][Web of Science][Medline]
28. Balla T, Várnai P, Holló Z, and SpätA. Effects of high potassium concentration and dihydropyridine Ca²⁺-channel agonists on cytoplasmic Ca²⁺ and aldosterone production in rat adrenal glomerulosa cells. Endocrinology 127: 815–822, 1990.[Abstract/Free Full Text]
29. Balla T, Varnai P, Tian Y, and Smith RD. Signaling events activated by angiotensin II receptors: what goes before and after the calcium signals. Endocr Res 24: 335–344, 1998.[Web of Science][Medline]
30. Balmforth AJ, Shepherd FH, Warburton P, and Ball SG. Evidence of an important and direct role for protein kinase C in agonist-induced phosphorylation leading to desensitization of the angiotensin AT1A receptor. Br J Pharmacol 122: 1469–1477, 1997.[CrossRef][Web of Science][Medline]
31. Barker S, Kapas S, Fluck RJ, and Clark AJL. Effects of the selective protein kinase C inhibitor Ro 31–7549 on human angiotensin II receptor desensitisation and intracellular calcium release. FEBS Lett 369: 263–266, 1995.[CrossRef][Web of Science][Medline]
32. Barrett PQ, Bollag WB, Isales CM, McCarthy RT, and Rasmussen H. Role of calcium in angiotensin II-mediated aldosterone secretion. Endocr Rev 10: 496–518, 1989.[Abstract/Free Full Text]
33. Barrett PQ, Isales CM, Bollag WB, and McCarthy RT. Ca²⁺ channels and aldosterone secretion: modulation by K⁺ and atrial natriuretic peptide. Am J Physiol Renal Fluid Electrolyte Physiol 261: F706–F719, 1991.[Abstract/Free Full Text]
34. Barrett PQ, Lu HK, Colbran R, Czernik A, and Pancrazio JJ. Stimulation of unitary T-type Ca²⁺ channel currents by calmodulin-dependent protein kinase II. Am J Physiol Cell Physiol 279: C1694–C1703, 2000.[Abstract/Free Full Text]
35. Bassett MH, White PC, and Rainey WE. The regulation of aldosterone synthase expression. Mol Cell Endocrinol. In press.
36. Baukal AJ, Balla T, Hunyady L, Hausdorff W, Guillemette G, and Catt KJ. Angiotensin II and guanine nucleotides stimulate formation of inositol 1,4,5-trisphosphate and its metabolites in permeabilized adrenal glomerulosa cells. J Biol Chem 263: 6087–6092, 1988.[Abstract/Free Full Text]
37. Baukal AJ, Guillemette G, Rubin R, Spät A, and Catt KJ. Binding sites for inositol trisphosphate in the bovine adrenal cortex. Biochem Biophys Res Commun 133: 532–538, 1985.[CrossRef][Web of Science][Medline]
38. Baukal AJ, Hunyady L, Balla T, Ely JA, and Catt KJ. Modulation of agonist-induced inositol phosphate metabolism by cyclic adenosine 3',5'-monophosphate in adrenal glomerulosa cells. Mol Endocrinol 4: 1712–1719, 1990.[Abstract/Free Full Text]
39. Baukal AJ, Hunyady L, Catt KJ, and Balla T. Evidence for participation of calcineurin in potentiation of agonist-stimulated cyclic AMP formation by the calcium-mobilizing hormone, angiotensin II. J Biol Chem 269: 24546–24549, 1994.[Abstract/Free Full Text]
40. Bean BL, Brown JJ, Casals-Stenzel J, Fraser R, Lever AF, Millar JA, Morton JJ, Petch B, Riegger AJ, Robertson JI, and Tree M. The relation of arterial pressure and plasma angiotensin II concentration. A change produced by prolonged infusion of angiotensin II in the conscious dog. Circ Res 44: 452–458, 1979.[Free Full Text]
41. Bean BP. Classes of calcium channels in vertebrate cells. Annu Rev Physiol 51: 367–384, 1989.[CrossRef][Web of Science][Medline]
42. Beguin P, Nagashima K, Gonoi T, Shibasaki T, Takahashi K, Kashima Y, Ozaki N, Geering K, Iwanaga T, and Seino S. Regulation of Ca²⁺ channel expression at the cell surface by the small G protein kir/Gem. Nature 411: 701–706, 2001.[CrossRef][Medline]
43. Bell JBG, Tait JF, Tait SAS, Barnes GD, and Brown BL. Lack of effect of angiotensin on levels of cyclic AMP in isolated adrenal zona glomerulosa cells from the rat. J Endocrinol 91: 145–154, 1981.[Abstract/Free Full Text]
44. Belloni AS, Albertin G, Forneris ML, and Nussdorfer GG. Proadrenomedullin-derived peptides as autocrine-paracrine regulators of cell growth. Histol Histopathol 16: 1263–1274, 2001.[Web of Science][Medline]
45. Berka JL, Kelly DJ, Robinson DB, Alcorn D, Marley PD, Fernley RT, and Skinner SL. Adrenaline cells of the rat adrenal cortex and medulla contain renin and prorenin. Mol Cell Endocrinol 119: 175–184, 1996.[CrossRef][Web of Science][Medline]
46. Bernardi P. Mitochondrial transport of cations: channels, exchangers, and permeability transition. Physiol Rev 79: 1127–1155, 1999.[Abstract/Free Full Text]
47. Berridge MJ. Calcium oscillations. J Biol Chem 265: 9583–9586, 1990.[Free Full Text]
48. Berridge MJ. Inositol trisphosphate and calcium signalling. Nature 361: 315–325, 1993.[CrossRef][Medline]
49. Berridge MJ. Capacitative calcium entry. Biochem J 312: 1–11, 1995.[Web of Science][Medline]
50. Berridge MJ. Elementary and global aspects of calcium signalling. J Physiol 499: 291–306, 1997.[Free Full Text]
51. Berridge MJ. Neuronal calcium signaling. Neuron 21: 13–26, 1998.[Medline]
52. Berridge MJ, Dawson RMC, Downes CP, Heslop JP, and Irvine RF. Changes in the levels of inositol phosphates after agonist-dependent hydrolysis of membrane phosphoinositides. Biochem J 212: 473–482, 1983.[Web of Science][Medline]
53. Berridge MJ and Irvine RF. Inositol trisphosphate, a novel second messenger in cellular signal transduction. Nature 312: 315–321, 1984.[CrossRef][Medline]
54. Betancourt-Calle S, Calle RA, Isales CM, White S, Rasmussen H, and Bollag WB. Differential effects of agonists of aldosterone secretion on steroidogenic acute regulatory phosphorylation. Mol Cell Endocrinol 173: 87–94, 2001.[CrossRef][Web of Science][Medline]
55. Bezprozvanny I and Ehrlich BE. The inositol 1,4,5-trisphosphate (InsP₃) receptor. J Membr Biol 145: 205–216, 1995.[Web of Science][Medline]
56. Bezprozvanny I, Watras J, and Ehrlich BE. Bell-shaped calcium-response curves of Ins(1,4,5)P_3- and calcium-gated channels from endoplasmic reticulum of cerebellum. Nature 351: 751–754, 1991.[CrossRef][Medline]
57. Bhat GJ, Thekkumkara TJ, Thomas WG, Conrad KM, and Baker KM. Activation of the STAT pathway by angiotensin II in T3CHO/AT_1A cells. Cross-talk between angiotensin II and interleukin-6 nuclear signaling. J Biol Chem 270: 19059–19065, 1995.[Abstract/Free Full Text]
58. Bianchi C, Gutkowska J, De Léan A, Ballak M, Anand-Srivastava MB, Genest J, and Cantin M. Fate of [¹²⁵I]angiotensin II in adrenal zona glomerulosa cells. Endocrinology 118: 2605–2607, 1986.[Abstract/Free Full Text]
59. Biden TJ and Wollheim CB. Ca²⁺ regulates the inositol tris/tetrakisphosphate pathway in intact and broken preparations of insulin-secreting RINm5F cells. J Biol Chem 261: 11931–11934, 1986.[Abstract/Free Full Text]
60. Billah MM, Eckel S, Mullmann TJ, Egan RW, and Siegel MI. Phosphatidylcholine hydrolysis by phospholipase D determines phosphatidate and diglyceride levels in chemotactic peptide-stimulated human neutrophils. Involvement of phosphatidate phosphohydrolase in signal transduction. J Biol Chem 264: 17069–17077, 1989.[Abstract/Free Full Text]
61. Billups B and Forsythe ID. Presynaptic mitochondrial calcium sequestration influences transmission at mammalian central synapses. J Neurosci 22: 5840–5847, 2002.[Abstract/Free Full Text]
62. Bird IM, Mason JI, and Rainey WE. Battle of the kinases: integration of adrenal responses to cAMP, DG and Ca²⁺ at the level of steroidogenic cytochromes P450 and 3betaHSD expression in H295R cells. Endocr Res 24: 345–354, 1998.[Web of Science][Medline]
63. Bird IM, Word RA, Clyne C, Mason JI, and Rainey WE. Potassium negatively regulates angiotensin II type 1 receptor expression in human adrenocortical H295R cells. Hypertension 25: 1129–1134, 1995.[Abstract/Free Full Text]
64. Birnbaumer L, Boulay G, Brown D, Jiang M, Dietrich A, Mikoshiba K, Zhu X, and Qin N. Interaction between IP₃ receptor and TRP links the internal calcium storage compartment to plasma membrane CCE channels. Recent Prog Horm Res 55: 127–161, 2000.[Web of Science][Medline]
65. Bodart V, Ong H, and De Léan A. A role for protein tyrosine kinase in the steroidogenic pathway of angiotensin II in bovine zona glomerulosa cells. J Steroid Biochem Mol Biol 54: 55–62, 1995.[CrossRef][Web of Science][Medline]
66. Bollag WB, Barrett PQ, Isales CM, Liscovitch M, and Rasmussen H. A potential role for phospholipase-D in the angiotensin-II-induced stimulation of aldosterone secretion from bovine adrenal glomerulosa cells. Endocrinology 127: 1436–1443, 1990.[Abstract/Free Full Text]
67. Bollag WB, Barrett PQ, Isales CM, and Rasmussen H. Angiotensin-II-induced changes in diacylglycerol levels and their potential role in modulating the steroidogenic response. Endocrinology 128: 231–241, 1991.[Abstract/Free Full Text]
68. Bollag WB, Jung EM, and Calle RA. Mechanism of angiotensin II-induced phospholipase D activation in bovine adrenal glomerulosa cells. Mol Cell Endocrinol 192: 7–16, 2002.[CrossRef][Web of Science][Medline]
69. Bootman MD, Berridge MJ, and Roderick HL. Calcium signalling: more messengers, more channels, more complexity. Curr Biol 12: R563–R565, 2002.[CrossRef][Web of Science][Medline]
70. Bornstein SR and Vaudry H. Paracrine and neuroendocrine regulation of the adrenal gland— basic and clinical aspects. Horm Metab Res 30: 292–296, 1998.[Medline]
71. Bose HS, Lingappa VR, and Miller WL. Rapid regulation of steroidogenesis by mitochondrial protein import. Nature 417: 87–91, 2002.[CrossRef][Medline]
72. Boulay G, Brown DM, Qin N, Jiang M, Dietrich A, Zhu MX, Chen Z, Birnbaumer M, Mikoshiba K, and Birnbaumer L. Modulation of Ca²⁺ entry by polypeptides of the inositol 1,4,5-trisphosphate receptor (IP₃R) that bind transient receptor potential (TRP): evidence for roles of TRP and IP₃R in store depletion-activated Ca²⁺ entry. Proc Natl Acad Sci USA 96: 14955–14960, 1999.[Abstract/Free Full Text]
73. Boulay G, Chrétien L, Richard DE, and Guillemette G. Shortterm desensitization of the angiotensin II receptor of bovine adrenal glomerulosa cells corresponds to a shift from a high to a low affinity state. Endocrinology 135: 2130–2136, 1994.[Abstract]
74. Boyd JE and Mulrow PJ. Further studies of the influence of potassium upon aldosterone production in the rat. Endocrinology 90: 299–301, 1972.[Abstract/Free Full Text]
75. Boyd JE, Mulrow PJ, Palmore WP, and Silva P. Importance of potassium in the regulation of aldosterone production. Circ Res 32/33: I-39–I-45, 1973.
76. Boyd JE, Palmore WP, and Mulrow PJ. Role of potassium in the control of aldosterone secretion in the rat. Endocrinology 88: 556–565, 1971.[Abstract/Free Full Text]
77. Braley LM, Menachery AI, Brown EM, and Williams GH. Comparative effect of angiotensin II, potassium, adrenocorticotropin, and cyclic adenosine 3',5'-monophosphate on cytosolic calcium in rat adrenal cells. Endocrinology 119: 1010–1019, 1986.[Abstract/Free Full Text]
78. Brandenburger Y, Kennedy ED, Python CP, Rossier MF, Vallotton MB, Wollheim CB, and Capponi AM. Possible role for mitochondrial calcium in angiotensin II- and potassium-stimulated steroidogenesis in bovine adrenal glomerulosa cells. Endocrinology 137: 5544–5551, 1996.[Abstract]
79. Brauneis U, Vassilev PM, Quinn SJ, Williams GH, and Tillotson DL. ANG II blocks potassium currents in zona glomerulosa cells from rat, bovine, and human adrenals. Am J Physiol Endocrinol Metab 260: E772–E779, 1991.[Abstract/Free Full Text]
80. Burgess GM, Bird GSJ, Obie JF, and Putney JW Jr. The mechanism for synergism between phospholipase C- and adenylyl cyclase-linked hormones in liver. Cyclic AMP-dependent kinase augments inositol trisphosphate-mediated Ca²⁺ mobilization without increasing the cellular levels of inositol polyphosphates. J Biol Chem 266: 4772–4781, 1991.[Abstract/Free Full Text]
81. Burnay MM, Python CP, Vallotton MB, Capponi AM, and Rossier MF. Role of the capacitative calcium influx in the activation of steroidogenesis by angiotensin-II in adrenal glomerulosa cells. Endocrinology 135: 751–758, 1994.[Abstract]
82. Burnay MM, Vallotton MB, Capponi AM, and Rossier MF. Angiotensin II potentiates adrenocorticotrophic hormone-induced cAMP formation in bovine adrenal glomerulosa cells through a capacitative calcium influx. Biochem J 330: 21–27, 1998.[Web of Science][Medline]
83. Cameron AM, Steiner JP, Roskams AJ, Ali SM, Ronnett GV, and Snyder SH. Calcineurin associated with the inositol 1,4,5-trisphosphate receptor-FKBP12 complex modulates Ca²⁺ flux. Cell 83: 463–472, 1995.[CrossRef][Web of Science][Medline]
84. Campbell WB, Brady MT, Rosolowsky LJ, and Falck JR. Metabolism of arachidonic acid by rat adrenal glomerulosa cells: synthesis of hydroxyeicosatetraenoic acids and epoxyeicosatrienoic acids. Endocrinology 128: 2183–2194, 1991.[Abstract/Free Full Text]
85. Campbell WB, Currie MG, and Needleman P. Inhibition of aldosterone biosynthesis by atriopeptins in rat adrenal cells. Circ Res 57: 113–118, 1985.[Abstract/Free Full Text]
86. Cao TT, Deacon HW, Reczek D, Bretscher A, and Von Zastrow M. A kinase-regulated PDZ-domain interaction controls endocytic sorting of the beta2-adrenergic receptor. Nature 401: 286–290, 1999.[CrossRef][Medline]
87. Capponi AM. The control of cholesterol supply for aldosterone biosynthesis. In: International Symposium on Aldosterone London UK 2003, p. S15.
88. Capponi AM, Lew PD, Jornot L, and Vallotton MB. Correlation between cytosolic free Ca²⁺ and aldosterone production in bovine adrenal glomerulosa cells. Evidence for a difference in the mode of action of angiotensin II and potassium. J Biol Chem 259: 8863–8869, 1984.[Abstract/Free Full Text]
89. Capponi AM, Lew PD, and Vallotton MB. Quantitative analysis of the cytosolic-free-Ca²⁺-dependency of aldosterone production in bovine adrenal glomerulosa cells. Different requirements for angiotensin II and K⁺. Biochem J 247: 335–340, 1987.[Web of Science][Medline]
90. Capponi AM, Rossier MF, Davies E, and Valloton MB. Calcium stimulates steroidogenesis in permeabilized bovine adrenal cortical cells. J Biol Chem 263: 16113–16117, 1988.[Abstract/Free Full Text]
91. Carafoli E. Calcium pump of the plasma membrane. Physiol Rev 71: 129–153, 1991.[Free Full Text]
92. Catalano RD, Stuve L, and Ramachandran J. Characterization of corticotropin receptors in human adrenocortical cells. J Clin Endocrinol Metab 62: 300–304, 1986.[Abstract/Free Full Text]
93. Catterall WA, De Jongh K, Rotman E, Hell J, Westenbroek R, Dubel SJ, and Snutch TP. Molecular properties of calcium channels in skeletal muscle and neurons. Ann NY Acad Sci 681: 342–355, 1993.[Web of Science][Medline]
94. Challiss RA, Chilvers ER, Willcocks AL, and Nahorski SR. Heterogeneity of [³H]inositol 1,4,5-trisphosphate binding sites in adrenal-cortical membranes. Characterization and validation of a radioreceptor assay. Biochem J 265: 421–427, 1990.[Web of Science][Medline]
95. Chartier L and Schiffrin EL. Role of calcium in effects of atrial natriuretic peptide on aldosterone production in adrenal glomerulosa cells. Am J Physiol Endocrinol Metab 252: E485–E491, 1987.[Abstract/Free Full Text]
96. Chen R, Mukhin YV, Garnovskaya MN, Thielen TE, Iijima Y, Huang C, Raymond JR, Ullian ME, and Paul RV. A functional angiotensin II receptor-GFP fusion protein: evidence for agonist-dependent nuclear translocation. Am J Physiol Renal Physiol 279: F440–F448, 2000.[Abstract/Free Full Text]
97. Chen XL, Bayliss DA, Fern RJ, and Barrett PQ. A role for T-type Ca²⁺ channels in the synergistic control of aldosterone production by angiotensin II and K⁺. Am J Physiol Renal Physiol 276: F674–F683, 1999.[Abstract/Free Full Text]
98. Cherradi N, Bideau M, Arnaudeau S, Demaurex N, James RW, Azhar S, and Capponi AM. Angiotensin II promotes selective uptake of high density lipoprotein cholesterol esters in bovine adrenal glomerulosa and human adrenocortical carcinoma cells through induction of scavenger receptor class B type I. Endocrinology 142: 4540–4549, 2001.[Abstract/Free Full Text]
99. Cherradi N, Brandenburger Y, Rossier MF, Vallotton MB, Stocco DM, and Capponi AM. Atrial natriuretic peptide inhibits calcium-induced steroidogenic acute regulatory protein gene transcription in adrenal glomerulosa cells. Mol Endocrinol 12: 962–972, 1998.[Abstract/Free Full Text]
100. Cherradi N, Rossier MF, Vallotton MB, and Capponi AM. Calcium stimulates intramitochondrial cholesterol transfer in bovine adrenal glomerulosa cells. J Biol Chem 271: 25971–25975, 1996.[Abstract/Free Full Text]
101. Cherradi N, Rossier MF, Vallotton MB, Timberg R, Friedberg I, Orly J, Wang XJ, Stocco DM, and Capponi AM. Submitochondrial distribution of three key steroidogenic proteins (steroidogenic acute regulatory protein and cytochrome p450_scc and 3-hydroxysteroid dehydrogenase isomerase enzymes) upon stimulation by intracellular calcium in adrenal glomerulosa cells. J Biol Chem 272: 7899–7907, 1997.[Abstract/Free Full Text]
102. Chiou CY, Kifor I, Moore TJ, and Williams GH. The effect of losartan on potassium-stimulated aldosterone secretion in vitro. Endocrinology 134: 2371–2375, 1994.[Abstract/Free Full Text]
103. Chiou CY, Williams GH, and Kifor I. Study of the rat adrenal renin-angiotensin system at a cellular level. J Clin Invest 96: 1375–1381, 1995.[Web of Science][Medline]
104. Chiu AT, Herblin WF, McCall DE, Ardecky RJ, Carini DJ, Duncia JV, Pease LJ, Wong PC, Wexler RR, Johnson AL, and Timmermans PBMWM. Identification of angiotensin II receptor subtypes. Biochem Biophys Res Commun 165: 196–203, 1989.[CrossRef][Web of Science][Medline]
105. Chorvatova A, Gendron L, Bilodeau L, Gallo-Payet N, and Payet MD. A Ras-dependent chloride current activated by adrenocorticotropin in rat adrenal zona glomerulosa cells. Endocrinology 141: 684–692, 2000.[Abstract/Free Full Text]
106. Chretien L, Richard DE, Poirier SN, Poitras M, and Guillemette G. Bovine adrenal glomerulosa cells express such a low level of functional B2 receptors that bradykinin does not significantly increase their aldosterone production. J Endocrinol 156: 449–460, 1998.[Abstract]
107. Cirillo M, Quinn SJ, and Canessa ML. Early and late effects of angiotensin-II on Ca²⁺ fluxes in bovine adrenal zona glomerulosa cells. Endocrinology 132: 1921–1930, 1993.[Abstract/Free Full Text]
108. Claing A, Laporte SA, Caron MG, and Lefkowitz RJ. Endocytosis of G protein-coupled receptors: roles of G protein-coupled receptor kinases and ss-arrestin proteins. Prog Neurobiol 66: 61–79, 2002.[CrossRef][Web of Science][Medline]
109. Clark AJL, Balla T, Jones MR, and Catt KJ. Stimulation of early gene expression by angiotensin II in bovine adrenal glomerulosa cells: roles of calcium and protein kinase C. Mol Endocrinol 6: 1889–1898, 1992.[Abstract/Free Full Text]
110. Clark BJ and Combs R. Angiotensin II and cyclic adenosine 3',5'-monophosphate induce human steroidogenic acute regulatory protein transcription through a common steroidogenic factor-1 element. Endocrinology 140: 4390–4398, 1999.[Abstract/Free Full Text]
111. Clark BJ, Pezzi V, Stocco DM, and Rainey WE. The steroidogenic acute regulatory protein is induced by angiotensin II and K⁺ in H295R adrenocortical cells. Mol Cell Endocrinol 115: 215–219, 1995.[CrossRef][Web of Science][Medline]
112. Clyne CD, White PC, and Rainey WE. Calcium regulates human CYP11B2 transcription. Endocr Res 22: 485–492, 1996.[Medline]
113. Cohen CJ, McCarthy RT, Barrett PQ, and Rasmussen H. Ca channels in adrenal glomerulosa cells: K⁺ and angiotensin II increase T-type Ca channel current. Proc Natl Acad Sci USA 85: 2412–2416, 1988.[Abstract/Free Full Text]
114. Colegrove SL, Albrecht MA, and Friel DD. Dissection of mitochondrial Ca²⁺ uptake and release fluxes in situ after depolarization-evoked [Ca²⁺]_i elevations in sympathetic neurons. J Gen Physiol 115: 351–369, 2000.[Abstract/Free Full Text]
115. Collins TJ, Lipp P, Berridge MJ, and Bootman MD. Mitochondrial Ca²⁺ uptake depends on the spatial and temporal profile of cytosolic Ca²⁺ signals. J Biol Chem 276: 26411–26420, 2001.[Abstract/Free Full Text]
116. Condon JC, Pezzi V, Drummond BM, Yin S, and Rainey WE. Calmodulin-dependent kinase I regulates adrenal cell expression of aldosterone synthase. Endocrinology 143: 3651–3657, 2002.[Abstract/Free Full Text]
117. Conlin PR, Williams GH, and Canessa ML. Angiotensin II-induced activation of Na⁺-H⁺ exchange in adrenal glomerulosa cells is mediated by protein kinase C. Endocrinology 129: 1861–1868, 1991.[Abstract/Free Full Text]
118. Connolly T, Rapiejko PJ, and Gilmore R. Requirement of GTP hydrolysis for dissociation of the signal recognition particle from its receptor. Science 252: 1171–1173, 1991.[Free Full Text]
119. Connor JA, Cornwall MC, and Williams GH. Spatially resolved cytosolic calcium response to angiotensin II and potassium in rat glomerulosa cells measured by digital imaging techniques. J Biol Chem 262: 2919–2927, 1987.[Abstract/Free Full Text]
120. Cote M, Guillon G, Payet MD, and Gallo-Payet N. Expression and regulation of adenylyl cyclase isoforms in the human adrenal gland. J Clin Endocrinol Metab 86: 4495–4503, 2001.[Abstract/Free Full Text]
121. Coté M, Payet MD, Dufour MN, Guillon G, and Gallo-Payet N. Association of the G protein _q11-subunit with cytoskeleton in adrenal glomerulosa cells: role in receptor-effector coupling. Endocrinology 138: 3299–3307, 1997.[Abstract/Free Full Text]
122. Côté M, Payet MD, and Gallo-Payet N. Association of _s-subunit of the G_s protein with microfilaments and microtubules: implication during adrenocorticotropin stimulation in rat adrenal glomerulosa cells. Endocrinology 138: 69–78, 1997.[Abstract/Free Full Text]
123. Crompton M, Moser R, Ludi H, and Carafoli E. The interrelations between the transport of sodium and calcium in mitochondria of various mammalian tissues. Eur J Biochem 82: 25–31, 1978.[Web of Science][Medline]
124. Crozat A, Penhoat A, and Saez JM. Processing of angiotensin II (A-II) and (Sar1,Ala8)A-II by cultured bovine adrenocortical cells. Endocrinology 118: 2312–2318, 1986.[Abstract/Free Full Text]
125. Csordás G, Madesh M, Antonsson B, and Hajnóczky G. tcBid promotes Ca²⁺ signal propagation to the mitochondria: control of Ca²⁺ permeation through the outer mitochondrial membrane. EMBO J 21: 2198–2206, 2002.[CrossRef][Web of Science][Medline]
126. Cullen PJ, Irvine RF, and Dawson AP. Synergistic control of Ca²⁺ mobilization in permeabilized mouse L1210 lymphoma cells by inositol 2,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. Biochem J 271: 549–553, 1990.[Medline]
127. Czirják G and Enyedi P. Formation of functional heterodimers between the TASK-1 and TASK-3 two-pore domain potassium channel subunits. J Biol Chem 277: 5426–5432, 2002.[Abstract/Free Full Text]
128. Czirják G and Enyedi P. TASK-3 dominates the background potassium conductance in rat adrenal glomerulosa cells. Mol Endocrinol 16: 621–629, 2002.[Abstract/Free Full Text]
129. Czirják G, Fischer T, Spät A, Lesage F, and Enyedi P. TASK (TWIK-related acid-sensitive K⁺ channel) is expressed in glomerulosa cells of rat adrenal cortex and inhibited by angiotensin II. Mol Endocrinol 14: 863–874, 2000.[Abstract/Free Full Text]
130. Czirják G, Petheö GL, Spät A, and Enyedi P. Inhibition of TASK-1 potassium channel by phospholipase C. Am J Physiol Cell Physiol 281: C700–C708, 2001.[Abstract/Free Full Text]
131. Decorzant C, Riondel AM, Philippe MJ, Bertrand J, and Vallotton MB. Detection of Na⁺ and K⁺ in the rat adrenal cortex with the electron microprobe. Clin Sci Mol Med 53: 423–430, 1977.[Medline]
132. De Gasparo M, Catt KJ, Inagami T, Wright JW, and Unger T. International union of pharmacology. XXIII. The angiotensin II receptors. Pharmacol Rev 52: 415–472, 2000.[Abstract/Free Full Text]
133. De Lean A, Ong H, Gutkowska J, Schiller PW, and McNicoll N. Evidence for agonist-induced interaction of angiotensin receptor with a guanine nucleotide-binding protein in bovine adrenal zona glomerulosa. Mol Pharmacol 26: 498–508, 1984.[Abstract]
134. De Léan A, Racz K, McNicoll N, and Desrosiers ML. Direct -adrenergic stimulation of aldosterone secretion in cultured bovine adrenal subcapsular cells. Endocrinology 115: 485–492, 1984.[Abstract/Free Full Text]
135. Dempsey EC, Newton AC, Mochly-Rosen D, Fields AP, Reyland ME, Insel PA, and Messing RO. Protein kinase C isozymes and the regulation of diverse cell responses. Am J Physiol Lung Cell Mol Physiol 279: L429–L438, 2000.[Abstract/Free Full Text]
136. Dhami GK, Anborgh PH, Dale LB, Sterne-Marr R, and Ferguson SS. Phosphorylation-independent regulation of metabotropic glutamate receptor signaling by G protein-coupled receptor kinase 2. J Biol Chem 277: 25266–25272, 2002.[Abstract/Free Full Text]
137. Dolphin AC, Wyatt CN, Richards J, Beattie RE, Craig P, Lee JH, Cribbs LL, Volsen SG, and Perez-Reyes E. The effect of 2- and other accessory subunits on expression and properties cf the calcium channel 1G. J Physiol 519: 35–45, 1999.[Abstract/Free Full Text]
138. Donaldson SK, Goldberg ND, Walseth TF, and Huetteman DA. Voltage dependence of inositol 1,4,5-trisphosphate-induced Ca²⁺ release in peeled skeletal muscle fibers. Proc Natl Acad Sci USA 85: 5749–5753, 1988.[Abstract/Free Full Text]
139. Douglas J, Saltman S, Williams C, Bartley P, Kondo T, and Catt KJ. An examination of possible mechanisms of angiotensin II stimulated steroidogenesis. Endocr Res Commun 5: 173–188, 1978.[Medline]
140. Douglas JG. Potassium ion as a regulator of adrenal angiotensin II receptors. Am J Physiol Endocrinol Metab 239: E317–E321, 1980.[Abstract/Free Full Text]
141. Douglas JG and Brown GP. Effect of prolonged low dose infusion of angiotensin II and aldosterone on rat smooth muscle and adrenal angiotensin II receptors. Endocrinology 111: 988–992, 1982.[Abstract/Free Full Text]
142. Douglas WW and Rubin RP. The role of calcium in the secretory response of the adrenal medulla to acetylcholine. J Physiol 159: 40–57, 1961.[Free Full Text]
143. Drolet P, Bilodeau L, Chorvatova A, Laflamme L, Gallo-Payet N, and Payet MD. Inhibition of the T-type Ca²⁺ current by the dopamine D1 receptor in rat adrenal glomerulosa cells: requirement of the combined action of the G protein subunit and cyclic adenosine 3',5'-monophosphate. Mol Endocrinol 11: 503–514, 1997.[Abstract/Free Full Text]
144. Du Y, Guo DF, Inagami T, Speth RC, and Wang DH. Regulation of ANG II-receptor subtype and its gene expression in adrenal gland. Am J Physiol Heart Circ Physiol 271: H440–H446, 1996.[Abstract/Free Full Text]
145. Duchen MR. Ca²⁺-dependent changes in the mitochondrial energetics in single dissociated mouse sensory neurons. Biochem J 283: 41–50, 1992.[Web of Science][Medline]
146. Duchen MR. Contributions of mitochondria to animal physiology: from homeostatic sensor to calcium signalling and cell death. J Physiol 516: 1–17, 1999.[Abstract/Free Full Text]
147. Duchen MR. Mitochondria and Ca²⁺ in cell physiology and pathophysiology. Cell Calcium 28: 339–348, 2000.[CrossRef][Web of Science][Medline]
148. Duprat F, Lesage F, Fink M, Reyes R, Heurteaux C, and Lazdunski M. TASK, a human background K⁺ channel to sense external pH variations near physiological pH. EMBO J 16: 5464–5471, 1997.[CrossRef][Web of Science][Medline]
149. Durroux T, Gallo-Payet N, Bilodeau L, and Payet MD. Background calcium permeable channels in glomerulosa cells from adrenal gland. J Membr Biol 129: 145–153, 1992.[Medline]
150. Durroux T, Gallo-Payet N, and Payet MD. Three components of the calcium current in cultured glomerulosa cells from rat adrenal gland. J Physiol 404: 713–729, 1988.[Abstract/Free Full Text]
151. Durroux T, Gallo-Payet N, and Payet MD. Effects of adrenocorticotropin on action potential and calcium currents in cultured rat and bovine glomerulosa cells. Endocrinology 129: 2139–2147, 1991.[Abstract/Free Full Text]
152. Eguchi S and Inagami T. Signal transduction of angiotensin II type 1 receptor through receptor tyrosine kinase. Regul Pept 91: 13–20, 2000.[CrossRef][Web of Science][Medline]
153. Elliott ME, Alexander RC, and Goodfriend TL. Aspects of angiotensin action in the adrenal. Key roles for calcium and phosphatidyl inositol. Hypertension 4 Suppl 2: II-52–II-58, 1982.
154. Elliott ME and Goodfriend TL. Angiotensin alters ⁴⁵Ca²⁺ fluxes in bovine adrenal glomerulosa cells. Proc Natl Acad Sci USA 78: 3044–3048, 1981.[Abstract/Free Full Text]
155. Elliott ME and Goodfriend TL. Mechanism of fatty acid inhibition of aldosterone synthesis by bovine adrenal glomerulosa cells. Endocrinology 132: 2453–2460, 1993.[Abstract/Free Full Text]
156. Elliott ME, Goodfriend TL, and Jefcoate CR. Bovine adrenal glomerulosa and fasciculata cells exhibit 28.5-kilodalton proteins sensitive to angiotensin, other agonists, and atrial natriuretic peptide. Endocrinology 133: 1669–1677, 1993.[Abstract/Free Full Text]
157. Elliott ME, Siegel FL, Hadjokas NE, and Goodfriend TL. Angiotensin effects on calcium and steroidogenesis in adrenal glomerulosa cells. Endocrinology 116: 1051–1059, 1985.[Abstract/Free Full Text]
158. Ellis MV, James SR, Perisic O, Downes CP, Williams RL, and Katan M. Catalytic domain of phosphoinositide-specific phospholipase C (PLC). J Biol Chem 273: 11650–11659, 1998.[Abstract/Free Full Text]
159. Ely JA, Ambroz C, Baukal AJ, Christensen SB, Balla T, and Catt KJ. Relationship between agonist- and thapsigargin-sensitive calcium pools in adrenal glomerulosa cells. Thapsigargin-induced Ca²⁺ mobilization and entry. J Biol Chem 266: 18635–18641, 1991.[Abstract/Free Full Text]
160. Enyedi P, Balla T, Antoni FA, and SpätA. Effect of angiotensin II and vasopressin on aldosterone production and phosphoinositide turnover in rat adrenal glomerulosa cells. J Mol Endocrinol 1: 117–124, 1988.[Abstract/Free Full Text]
161. Enyedi P, Büki B, Mucsi I, and Spät A. Polyphosphoinositide metabolism in adrenal glomerulosa cells. Mol Cell Endocrinol 41: 105–112, 1985.[CrossRef][Web of Science][Medline]
162. Enyedi P, Mucsi I, Hunyady L, Catt KJ, and SpätA. The role of guanyl nucleotide binding proteins in the formation of inositol phosphates in adrenal glomerulosa cells. Biochem Biophys Res Commun 140: 941–947, 1986.[CrossRef][Medline]
163. Enyedi P and Spät A. The mechanism of angiotensin-induced desensitization of adrenal glomerulosa cells. Mol Cell Endocrinol 51: 83–86, 1987.[Medline]
164. Enyedi P, Spät A, and Antoni FA. Role of prostaglandins in the control of the function of adrenal glomerulosa cells. J Endocrinol 91: 427–437, 1981.[Abstract/Free Full Text]
165. Enyedi P, Szabadkai GY, Horváth A, Szilágyi L, Gráf L, and Spät A. Inositol 1,4,5-trisphosphate receptor subtypes in adrenal glomerulosa cells. Endocrinology 134: 2354–2359, 1994.[Abstract/Free Full Text]
166. Enyedi P, Szabadkai GY, Krause KH, Lew DP, and Spät A. Inositol 1,4,5-trisphosphate binding sites copurify with putative Ca-storage protein calreticulin in rat liver. Cell Calcium 14: 485–492, 1993.[CrossRef][Web of Science][Medline]
167. Enyedi P, Szabó B, and Spät A. Reduced responsiveness of glomerulosa cells after prolonged stimulation with angiotensin II. Am J Physiol Endocrinol Metab 248: E209–E214, 1985.[Abstract/Free Full Text]
168. Enyedi P and Williams GH. Heterogeneous inositol tetrakisphosphate binding sites in the adrenal cortex. J Biol Chem 263: 7940–7942, 1988.[Abstract/Free Full Text]
169. Epstein LF and Orme-Johnson NR. Regulation of steroid hormone biosynthesis. J Biol Chem 266: 19739–19745, 1991.[Abstract/Free Full Text]
170. Faragó A and Nishizuka Y. Protein kinase C in transmembrane signalling. FEBS Lett 268: 350–354, 1990.[CrossRef][Web of Science][Medline]
171. Faragó A, Seprödi J, and Spät A. Subcellular distribution of protein kinase C in rat adrenal glomerulosa cells. Biochem Biophys Res Commun 156: 628–633, 1988.[Medline]
172. Farese RV, Larson RE, and Davis JS. Rapid effects of angiotensin-II on polyphosphoinositide metabolism in the rat adrenal glomerulosa. Endocrinology 114: 302–304, 1984.[Abstract/Free Full Text]
173. Feng X, Zhang J, Barak LS, Meyer T, Caron MG, and Hannun YA. Visualization of dynamic trafficking of a protein kinase C betaII/green fluorescent protein conjugate reveals differences in G protein-coupled receptor activation and desensitization. J Biol Chem 273: 10755–10762, 1998.[Abstract/Free Full Text]
174. Ferris CD and Snyder SH. Inositol 1,4,5-trisphosphate-activated calcium channels. Annu Rev Physiol 54: 469–488, 1992.[CrossRef][Web of Science][Medline]
175. Foster R, Lobo MV, and Marusic ET. Studies of relationship between angiotensin II and potassium ions on aldosterone release. Am J Physiol Endocrinol Metab Gastrointest Physiol 237: E363–E366, 1979.[Free Full Text]
176. Foster R and Rasmussen H. Angiotensin-mediated calcium efflux from adrenal glomerulosa cells. Am J Physiol Endocrinol Metab 245: E281–E287, 1983.[Abstract/Free Full Text]
177. Frei N, Weissenberger J, Beck-Sickinger AG, Hofliger M, Weis J, and Imboden H. Immunocytochemical localization of angiotensin II receptor subtypes and angiotensin II with monoclonal antibodies in the rat adrenal gland. Regul Pept 101: 149–155, 2001.[CrossRef][Web of Science][Medline]
178. Friel DD. Calcium oscillations in neurons. Ciba Found Symp 188: 210–223, 1995.[Medline]
179. Friel DD and Tsien RW. An FCCP-sensitive Ca²⁺ store in bullfrog sympathetic neurons and its participation in stimulus-evoked changes in [Ca²⁺]_i. J Neurosci 14: 4007–4024, 1994.[Abstract]
180. Funder JW, Blair-West JR, Coghlan JP, Denton DA, Scoggins BA, and Wright RD. Effect of plasma [K⁺] on the secretion of aldosterone. Endocrinology 85: 381–384, 1969.[Abstract/Free Full Text]
181. Furuichi T, Yoshikawa S, Miyawaki A, Wada K, Maeda N, and Mikoshiba K. Primary structure and functional expression of the inositol 1,4,5-trisphosphate-binding protein P₄₀₀. Nature 342: 32–38, 1989.[CrossRef][Medline]
182. Gáborik Z, Szaszák M, Szidonya L, Balla B, Paku S, Catt KJ, Clark AJL, and Hunyady L. Beta-arrestin- and dynamin-dependent endocytosis of the AT1 angiotensin receptor. Mol Pharmacol 59: 239–247, 2001.[Abstract/Free Full Text]
183. Gallo-Payet N, Chouinard L, Balestre MN, and Guillon G. Involvement of protein kinase C in the coupling between the V₁ vasopressin receptor and phospholipase C in rat glomerulosa cells: effects on aldosterone secretion. Endocrinology 129: 623–634, 1991.[Abstract/Free Full Text]
184. Gallo-Payet N, Grazzini E, Côté M, Chouinard L, Chorvatova A, Bilodeau L, Payet MD, and Guillon G. Role of Ca²⁺ in the action of adrenocorticotropin in cultured human adrenal glomerulosa cells. J Clin Invest 98: 460–466, 1996.[Web of Science][Medline]
185. Gallo-Payet N, Pothier P, and Isler H. On the presence of chromaffin cells in the adrenal cortex: their possible role in adrenocortical function. Biochem Cell Biol 65: 588–592, 1987.[Web of Science][Medline]
186. Ganguly A. Atrial natriuretic peptide-induced inhibition of aldosterone secretion: a quest for mediator(s). Am J Physiol Endocrinol Metab 263: E181–E194, 1992.[Abstract/Free Full Text]
187. Ganguly A, Chiou S, Fineberg NS, and Davis JS. Greater importance of Ca²⁺-calmodulin in maintenance of Ang II- and K⁺-mediated aldosterone secretion: lesser role of protein kinase C. Biochem Biophys Res Commun 182: 254–261, 1992.[CrossRef][Medline]
188. Ganguly A, Chiou S, West LA, and Davis JS. Aldosterone secretagogues and inositol trisphosphate as intracellular mediator. J Hypertens 4: s361–s363, 1986.
189. Ganguly A and Davis JS. Role of calcium and other mediators in aldosterone secretion from the adrenal glomerulosa cells. Pharmacol Rev 46: 417–448, 1994.[Web of Science][Medline]
190. Ganitkevich VY and Isenberg G. Membrane potential modulates inositol 1,4,5-trisphosphate-mediated Ca²⁺ transients in guinea-pig coronary myocytes. J Physiol 470: 35–44, 1993.[Abstract/Free Full Text]
191. Ganz MB, Nee JJ, Isales CM, and Barrett PQ. Atrial natriuretic peptide enhances activity of potassium conductance in adrenal glomerulosa cells. Am J Physiol Cell Physiol 266: C1357–C1365, 1994.[Abstract/Free Full Text]
192. Gasc JM, Shanmugam S, Sibony M, and Corvol P. Tissue-specific expression of type 1 angiotensin II receptor subtypes. An in situ hybridization study. Hypertension 24: 531–537, 1994.[Abstract/Free Full Text]
193. Gawler DJ, Potter BVL, and Nahorski SR. Inositol 1,3,4,5-tetrakisphosphate-induced release of intracellular Ca²⁺ in SH-SY5Y neuroblastoma cells. Biochem J 272: 519–524, 1990.[Medline]
194. Glossmann H, Baukal AJ, and Catt KJ. Properties of angiotensin II receptors in the bovine and rat adrenal cortex. J Biol Chem 249: 825–834, 1974.[Abstract/Free Full Text]
195. Gomez-Sanchez CE, Cozza EN, Foecking MF, Chiou S, and Ferris MW. Endothelin receptor subtypes and stimulation of aldosterone secretion. Hypertension 15: 744–747, 1990.[Abstract/Free Full Text]
196. Grazzini E, Durroux T, Payet MD, Bilodeau L, Gallo-Payet N, and Guillon G. Membrane-delimited G protein-mediated coupling between V_1a vasopressin receptor and dihydropyridine binding sites in rat glomerulosa cells. Mol Pharmacol 50: 1273–1283, 1996.[Abstract]
197. Greengard P, Tallan HH, and Psychoyos S. Biosynthesis of aldosterone by cell-free systems. In: Functions of the Adrenal Cortex, edited by McKerns KW. Amsterdam: North Holland, 1968, p. 233–258.
198. Gross F. Renin und Hypertensin, physiologische oder pathologische Wirkstoffe? Klinische Wochenschrift 36: 693–706, 1958.[Medline]
199. Gu J, Wen Y, Mison A, and Nadler JL. 12-Lipoxygenase pathway increases aldosterone production, 3',5'-cyclic adenosine monophosphate response element-binding protein phosphorylation, and p38 mitogen-activated protein kinase activation in H295R human adrenocortical cells. Endocrinology 144: 534–543, 2003.[Abstract/Free Full Text]
200. Guerini D. The significance of the isoforms of plasma membrane calcium ATPase. Cell Tissue Res 292: 191–197, 1998.[CrossRef][Web of Science][Medline]
201. Guillemette G, Balla T, Baukal AJ, Spät A, and Catt KJ. Intracellular receptors for inositol 1,4,5-trisphosphate in angiotensin II target tissues. J Biol Chem 262: 1010–1015, 1987.[Abstract/Free Full Text]
202. Guillemette G, Favreau I, Boulay G, and Potier M. Solubilization and partial characterization of inositol 1,4,5-trisphosphate receptor of bovine adrenal cortex reveal similarities with the receptor of rat cerebellum. Mol Pharmacol 38: 841–847, 1990.[Abstract]
203. Guillemette G and Segui JA. Effects of pH, reducing and alkylating reagents on the binding and Ca²⁺ release activities of inositol 1,4,5-triphosphate in the bovine adrenal cortex. Mol Endocrinol 2: 1249–1255, 1988.[Abstract/Free Full Text]
204. Guillon G, Balestre MN, Chouinard L, and Gallo-Payet N. Involvement of distinct G proteins in the action of vasopressin on rat glomerulosa cells. Endocrinology 126: 1699–1708, 1990.[Abstract/Free Full Text]
205. Guillon G and Gallo-Payet N. Specific vasopressin binding to rat adrenal glomerulosa cells. Relationship to inositol lipid breakdown. Biochem J 235: 209–214, 1986.[Medline]
206. Guillon G, Trueba M, Joubert D, Grazzini E, Chouinard L, Côté M, Payet MD, Manzoni O, Barberis C, Robert M, and Gallo-Payet N. Vasopressin stimulates steroid secretion in human adrenal glands: comparison with angiotensin-II effect. Endocrinology 136: 1285–1295, 1995.[Abstract]
207. Gunter TE and Pfeiffer DR. Mechanisms by which mitochondria transport calcium. Am J Physiol Cell Physiol 258: C755–C786, 1990.[Abstract/Free Full Text]
208. Gupta P, Franco-Saenz R, and Mulrow PJ. Transforming growth factor-1 inhibits aldosterone biosynthesis in cultured bovine zona glomerulosa cells. Endocrinology 132: 1184–1188, 1993.[Abstract/Free Full Text]
209. Gupta P, Franco-Saenz R, and Mulrow PJ. Locally generated angiotensin II in the adrenal gland regulates basal, corticotropin-, and potassium-stimulated aldosterone secretion. Hypertension 25: 443–448, 1995.[Abstract/Free Full Text]
210. Gutowski S, Smrcka A, Nowak L, Wu D, Simon M, and Sternweis PC. Antibodies to the _q subfamily of guanine nucleotide-binding regulatory protein subunits attenuate activation of phosphatidylinositol 4,5-bisphosphate hydrolysis by hormones. J Biol Chem 266: 20519–20524, 1991.[Abstract/Free Full Text]
211. Haendeler J and Berk BC. Angiotensin II mediated signal transduction: important role of tyrosine kinases. Regul Pept 95: 1–7, 2000.[CrossRef][Web of Science][Medline]
212. Hagar RE, Burgstahler AD, Nathanson MH, and Ehrlich BE. Type III InsP₃ receptor channel stays open in the presence of increased calcium. Nature 396: 81–84, 1998.[CrossRef][Medline]
213. Hajnóczky G, Csordás G, Hunyady L, Kalapos MP, Balla T, Enyedi P, and SpätA. Angiotensin-II inhibits Na⁺/K⁺ pump in rat adrenal glomerulosa cells: possible contribution to stimulation of aldosterone production. Endocrinology 130: 1637–1644, 1992.[Abstract/Free Full Text]
214. Hajnóczky G, Csordás G, Madesh M, and Pacher P. The machinery of local Ca²⁺ signalling between sarcoendoplasmic reticulum and mitochondria. J Physiol 529: 69–81, 2000.[Abstract/Free Full Text]
215. Hajnóczky G, Gao E, Nomura T, Hoek JB, and Thomas AP. Multiple mechanisms by which protein kinase A potentiates inositol 1,4,5-trisphosphate-induced Ca²⁺ mobilization in permeabilized hepatocytes. Biochem J 293: 413–422, 1993.[Web of Science][Medline]
216. Hajnóczky G, Robb-Gaspers LD, Seitz MB, and Thomas AP. Decoding of cytosolic calcium oscillations in the mitochondria. Cell 82: 415–424, 1995.[CrossRef][Web of Science][Medline]
217. Hajnóczky G, Csordás GY, Bagó A, Chiu AT, and Spät A. Angiotensin II exerts its effect on aldosterone production and potassium permeability through receptor subtype AT₁ in rat adrenal glomerulosa cells. Biochem Pharmacol 43: 1009–1012, 1992.[CrossRef][Medline]
218. Hajnóczky G, Várnai P, Buday L, Faragó A, and Spät A. The role of protein kinase-C in control of aldosterone production by rat adrenal glomerulosa cells: activation of protein kinase-C by stimulation with potassium. Endocrinology 130: 2230–2236, 1992.[Abstract/Free Full Text]
219. Hajnóczky G, Várnai P, Holló Z, Christensen SB, Balla T, Enyedi P, and SpätA. Thapsigargin-induced increase in cytoplasmic Ca²⁺ concentration and aldosterone production in rat adrenal glomerulosa cells: interaction with potassium and angiotensin-II. Endocrinology 128: 2639–2644, 1991.[Abstract/Free Full Text]
220. Haksar A and Péron FG. The role of calcium in the steroidogenic response of rat adrenal cells to adrenocorticotropic hormone. Biochim Biophys Acta 313: 363–371, 1973.[Medline]
221. Halder SK, Takemori H, Hatano O, Nonaka Y, Wada A, and Okamoto M. Cloning of a membrane-spanning protein with epidermal growth factor-like repeat motifs from adrenal glomerulosa cells. Endocrinology 139: 3316–3328, 1998.[Abstract/Free Full Text]
222. Hall PF. A possible role for transhydrogenation in side-chain cleavage of cholesterol. Biochemistry 11: 2891–2897, 1972.[Medline]
223. Hall PF, Osawa S, and Thomasson CL. A role for calmodulin in the regulation of steroidogenesis. J Cell Biol 90: 402–407, 1981.[Abstract/Free Full Text]
224. Hamada K, Miyata T, Mayanagi K, Hirota J, and Mikoshiba K. Two-state conformational changes in inositol 1,4,5-trisphosphate receptor regulated by calcium. J Biol Chem 277: 21115–21118, 2002.[Abstract/Free Full Text]
225. Haning R, Tait SAS, and Tait JF. In vitro effects of ACTH, angiotensins, serotonin and potassium on steroid output and conversion of corticosterone to aldosterone by isolated adrenal cells. Endocrinology 87: 1147–1167, 1970.[Abstract/Free Full Text]
226. Hartigan JA, Green EG, Mortensen RM, Menachery A, Williams GH, and Orme-Johnson NR. Comparison of protein phosphorylation patterns produced in adrenal cells by activation of cAMP-dependent protein kinase and Ca-dependent protein kinase. J Steroid Biochem Mol Biol 53: 95–101, 1995.[CrossRef][Medline]
227. Hauger RL, Aguilera G, and Catt KJ. Angiotensin II regulates its receptor sites in the adrenal glomerulosa zone. Nature 271: 176–178, 1978.[CrossRef][Medline]
228. Hausdorff WP, Campbell PT, Ostrowski J, Yu SS, Caron MG, and Lefkowitz RJ. A small region of the -adrenergic receptor is selectively involved in its rapid regulation. Proc Natl Acad Sci USA 88: 2979–2983, 1991.[Abstract/Free Full Text]
229. Hausdorff WP and Catt KJ. Activation of dihydropyridine-sensitive calcium channels and biphasic cytosolic calcium responses by angiotensin II in rat adrenal glomerulosa cells. Endocrinology 123: 2818–2826, 1988.[Abstract/Free Full Text]
230. Hausdorff WP, Sekura RD, Aguilera G, and Catt KJ. Control of aldosterone production by angiotensin II is mediated by two guanine nucleotide regulatory proteins. Endocrinology 120: 1668–1678, 1987.[Abstract/Free Full Text]
231. Hayama N, Wang W, Robinson TV, Kramer RE, and Schneider EG. Osmolality and potassium cause alterations in the volume of glomerulosa cells. Endocrinology 132: 1230–1234, 1993.[Abstract/Free Full Text]
232. Haynes RC, Koritz SB, and Peron FG. Influence of adenosine 3',5'-monophosphate on corticoid production by rat adrenal glands. J Biol Chem 234: 1421–1423, 1959.[Free Full Text]
233. Hein L, Meinel L, Pratt RE, Dzau VJ, and Kobilka BK. Intracellular trafficking of angiotensin II and its AT₁ and AT₂ receptors: evidence for selective sorting of receptor and ligand. Mol Endocrinol 11: 1266–1277, 1997.[Abstract/Free Full Text]
234. Henley JR, Krueger EW, Oswald BJ, and McNiven MA. Dynamin-mediated internalization of caveolae. J Cell Biol 141: 85–99, 1998.[Abstract/Free Full Text]
235. Herlitze S, Garcia DE, Mackie K, Hille B, Scheuer T, and Catterall WA. Modulation of Ca²⁺ channels by G protein beta gamma subunits. Nature 380: 258–262, 1996.[CrossRef][Medline]
236. Hescheler J, Rosenthal W, Hinsch KD, Wulfern M, Trautwein W, and Schultz G. Angiotensin II-induced stimulation of voltage-dependent Ca²⁺ currents in an adrenal cortical cell line. EMBO J 7: 619–624, 1988.[Web of Science][Medline]
237. Hilgemann DW, Feng S, and Nasuhoglu C. The complex and intriguing lives of PIP₂ with ion channels and transporters. Sci STKE 2001: RE19, 2001.[Medline]
238. Himathonghkam T, Dluhy RG, and Williams GH. Potassiumaldosterone-renin interrelationships. J Clin Endocrinol Metab 41: 153–159, 1975.[Abstract/Free Full Text]
239. Hinson JP, Cameron LA, Purbrick A, and Kapas S. The role of neuropeptides in the regulation of adrenal zona glomerulosa function: effects of substance P, neuropeptide Y, neurotensin, Metenkephalin, Leu-enkephalin and corticotrophin-releasing hormone on aldosterone secretion in the intact perfused rat adrenal. J Endocrinol 140: 91–96, 1994.[Abstract/Free Full Text]
240. Hoek JB and Rydström J. Physiological roles of nicotinamide nucleotide transhydrogenase. Biochem J 254: 1–10, 1988.[Web of Science][Medline]
241. Hokin LE and Hokin MR. Effects of acetylcholine on the turnover of phosphoryl units in individual phospholipids of pancreas slices and brain cortex slices. Biochim Biophys Acta 18: 102–110, 1955.[Medline]
242. Horváth A, Szabadkai G, Várnai P, Arányi T, Wollheim CB, Spät A, and Enyedi P. Voltage dependent calcium channels in adrenal glomerulosa cells and in insulin producing cells. Cell Calcium 23: 33–42, 1998.[CrossRef][Web of Science][Medline]
243. Hoth M and Penner R. Depletion of intracellular calcium stores activates a calcium current in mast cells. Nature 355: 353–356, 1992.[CrossRef][Medline]
244. Hunyady L, Balla T, Enyedi P, and Spät A. The effect of angiotensin II on arachidonate metabolism in adrenal glomerulosa cells. Biochem Pharmacol 34: 3439–3444, 1985.[Medline]
245. Hunyady L, Balla T, Nagy K, and Spät A. Control of phosphatidylinositol turnover in adrenal glomerulosa cells. Biochim Biophys Acta 713: 352–357, 1982.[Medline]
246. Hunyady L, Balla T, and Spät A. Angiotensin II stimulates phosphatidylinositol turnover in adrenal glomerulosa cells by a calcium-independent mechanism. Biochim Biophys Acta 753: 133–135, 1983.[Medline]
247. Hunyady L, Baukal AJ, Balla T, and Catt KJ. Independence of type I angiotensin II receptor endocytosis from G protein coupling and signal transduction. J Biol Chem 269: 24798–24804, 1994.[Abstract/Free Full Text]
248. Hunyady L, Baukal AJ, Bor M, Ely JA, and Catt KJ. Regulation of 1,2-diacylglycerol production by angiotensin-II in bovine adrenal glomerulosa cells. Endocrinology 126: 1001–1008, 1990.[Abstract/Free Full Text]
249. Hunyady L, Baukal AJ, Gáborik Z, Olivares-Reyes JA, Bor M, Szaszák M, Lodge R, Catt KJ, and Balla T. Differential PI 3-kinase dependence of early and late phases of recycling of the internalized AT₁ angiotensin receptor. J Cell Biol 157: 1211–1222, 2002.[Abstract/Free Full Text]
250. Hunyady L, Baukal AJ, Guillemette G, Balla T, and Catt KJ. Metabolism of inositol-1,3,4,6-tetrakisphosphate to inositol pentakisphosphate in adrenal glomerulosa cells. Biochem Biophys Res Commun 157: 1247–1252, 1988.[Medline]
251. Hunyady L, Bor M, Balla T, and Catt KJ. Identification of a cytoplasmic Ser-Thr-Leu motif that determines agonist-induced internalization of the AT₁ angiotensin receptor. J Biol Chem 269: 31378–31382, 1994.[Abstract/Free Full Text]
252. Hunyady L, Catt KJ, Clark AJL, and Gáborik Z. Mechanisms and functions of AT₁ angiotensin receptor internalization. Regul Pept 91: 29–44, 2000.[CrossRef][Web of Science][Medline]
253. Hunyady L, Kayser S, Cragoe EJ Jr, Balla I, Balla T, and Spät A. Na⁺-H⁺ and Na⁺-Ca²⁺ exchange in glomerulosa cells: possible role in control of aldosterone production. Am J Physiol Cell Physiol 254: C744–C750, 1988.[Abstract/Free Full Text]
254. Hunyady L, Merelli F, Baukal AJ, Balla T, and Catt KJ. Agonist-induced endocytosis and signal generation in adrenal glomerulosa cells. A potential mechanism for receptor-operated calcium entry. J Biol Chem 266: 2783–2788, 1991.[Abstract/Free Full Text]
255. Hunyady L, Rohács T, Bagó A, Deák F, and Spät A. Dihydropyridine-sensitive initial component of the ANG II-induced Ca²⁺ response in rat adrenal glomerulosa cells. Am J Physiol Cell Physiol 266: C67–C72, 1994.[Abstract/Free Full Text]
256. Hunyady L, Tian Y, Sandberg K, Balla T, and Catt KJ. Divergent conformational requirements for angiotensin II receptor internalization and signaling. Kidney Int 46: 1496–1498, 1994.[Web of Science][Medline]
257. Hunyady L, Vauquelin G, and Vanderheyden P. Agonist induction and conformational selection during activation of a G protein-coupled receptor. Trends Pharmacol Sci 24: 81–86, 2003.[CrossRef][Medline]
258. Hyatt PJ, Tait JF, and Tait SAS. The mechanism of the effect of K⁺ on the steroidogenesis of rat zona glomerulosa cells of the adrenal cortex: role of cyclic AMP. Proc R Soc Lond B Biol Sci 227: 21–42, 1986.[Medline]
259. Iijima K, Lin L, Nasjletti A, and Goligorsky MS. Intracellular ramification of endothelin signal. Am J Physiol Cell Physiol 260: C982–C992, 1991.[Abstract/Free Full Text]
260. Iino M. Biphasic Ca²⁺ dependence of inositol 1,4,5-trisphosphate-induced Ca release in smooth muscle cells of the guinea pig taenia caeci. J Gen Physiol 95: 1103–1122, 1990.[Abstract/Free Full Text]
261. Irvine RF. Inositol phosphates and Ca²⁺ entry: toward a proliferation or a simplification. FASEB J 6: 3085–3091, 1992.[Abstract]
262. Irvine RF, Letcher AJ, Heslop JP, and Berridge MJ. The inositol tris/tetrakisphosphate pathway—demonstration of Ins(1,4,5)P₃ 3-kinase activity in animal tissues. Nature 320: 631–634, 1986.[CrossRef][Medline]
263. Irvine RF and Moor RM. Micro-injection of inositol 1,3,4,5-tetrakisphospahte activates sea urchin eggs by a mechanism dependent on external Ca²⁺. Biochem J 240: 917–920, 1986.[Web of Science][Medline]
264. Isales CM, Barrett PQ, Brines M, Bollag W, and Rasmussen H. Parathyroid hormone modulates angiotensin II-induced aldosterone secretion from the adrenal glomerulosa cell. Endocrinology 129: 489–495, 1991.[Abstract/Free Full Text]
265. Ishizaka N, Griendling KK, Lassegue B, and Alexander RW. Angiotensin II type 1 receptor: relationship with caveolae and caveolin after initial agonist stimulation. Hypertension 32: 459–466, 1998.[Abstract/Free Full Text]
266. Itoh T and Takenawa T. Phosphoinositide-binding domains: functional units for temporal and spatial regulation of intracellular signalling. Cell Signal 14: 733–743, 2002.[CrossRef][Web of Science][Medline]
267. Iwai N and Inagami T. Regulation of the expression of the rat angiotensin II receptor mRNA. Biochem Biophys Res Commun 182: 1094–1099, 1992.[CrossRef][Web of Science][Medline]
268. Jiang LH, Gawler DJ, Hodson N, Milligan CJ, Pearson HA, Porter V, and Wray D. Regulation of cloned cardiac L-type calcium channels by cGMP-dependent protein kinase. J Biol Chem 275: 6135–6143, 2000.[Abstract/Free Full Text]
269. Johnson EIM, Capponi AM, and Valloton MB. Cytosolic free calcium oscillates in single bovine adrenal glomerulosa cells in response to angiotensin II stimulation. J Endocrinol 122: 391–402, 1989.[Abstract/Free Full Text]
270. Johnson GL and Lapadat R. Mitogen-activated protein kinase pathways mediated by ERK, JNK, and p38 protein kinases. Science 298: 1911–1912, 2002.[Abstract/Free Full Text]
271. Jones SW. Overview of voltage-dependent calcium channels. J Bioenerg Biomembr 30: 299–312, 1998.[CrossRef][Web of Science][Medline]
272. Joseph SK, Lin C, Pierson S, Thomas AP, and Maranto AR. Heteroligomers of type-I and type-III inositol trisphosphate receptors in WB rat liver epithelial cells. J Biol Chem 270: 23310–23316, 1995.[Abstract/Free Full Text]
273. Jung EM, Betancourt-Calle S, Mann-Blakeney R, Foushee T, Isales CM, and Bollag WB. Sustained phospholipase D activation in response to angiotensin II but not carbachol in bovine adrenal glomerulosa cells. Biochem J 330: 445–451, 1998.[Web of Science][Medline]
274. Jurányi Z, Orsó E, Jánossy A, Szalay KS, Sperlágh B, Windisch K, Vinson GP, and Vizi ES. ATP and [³H]noradrenaline release and the presence of ecto-Ca²⁺-ATPases in the capsuleglomerulosa fraction of the rat adrenal gland. J Endocrinol 153: 105–114, 1997.[Abstract/Free Full Text]
275. Kaftan EJ, Ehrlich BE, and Watras J. Inositol 1,4,5-trisphosphate (InsP₃) and calcium interact to increase the dynamic range of InsP₃ receptor-dependent calcium signaling. J Gen Physiol 110: 529–538, 1997.[Abstract/Free Full Text]
276. Kanazirska MV, Vassilev PM, Quinn SJ, Tillotson DL, and Williams GH. Single K⁺ channels in adrenal zona glomerulosa cells. II. Inhibition by angiotensin II. Am J Physiol Endocrinol Metab 263: E760–E765, 1992.[Abstract/Free Full Text]
277. Kapas S, Purbrick A, and Hinson JP. Role of tyrosine kinase and protein kinase C in the steroidogenic actions of angiotensin II, -melanocyte-stimulating hormone and corticotropin in the rat adrenal cortex. Biochem J 305: 433–438, 1995.[Web of Science][Medline]
278. Kaplan NM and Bartter FC. The effect of ACTH, renin, angiotensin II, and various precursors on biosynthesis of aldosterone by adrenal slices. J Clin Invest 41: 715–724, 1962.[Web of Science][Medline]
279. Kau MM, Lo MJ, Tsai SC, Chen JJ, Pu HF, Chien EJ, Chang LL, and Wang PS. Effects of prolactin on aldosterone secretion in rat zona glomerulosa cells. J Cell Biochem 72: 286–293, 1999.[Medline]
280. Kennedy ED, Rizzuto R, Theler JM, Pralong WF, Bastianutto C, Pozzan T, and Wollheim CB. Glucose-stimulated insulin secretion correlates with changes in mitochondrial and cytosolic Ca²⁺ in aequorin-expressing INS-1 cells. J Clin Invest 98: 2524–2538, 1996.[Web of Science][Medline]
281. Kifor I, Moore TJ, Fallo F, Sperling E, Chiou CY, Menachery A, and Williams GH. Potassium-stimulated angiotensin release from superfused adrenal capsules and enzymatically dispersed cells of the zona glomerulosa. Endocrinology 129: 823–831, 1991.[Abstract/Free Full Text]
282. Kim SY, Park DJ, and Lee HK. EGF-stimulated aldosterone secretion is mediated by tyrosine phosphorylation but not by phospholipase C in cultured porcine adrenal glomerulosa cells. J Korean Med Sci 13: 629–637, 1998.[Medline]
284. Kim Y, Bang H, and Kim D. TASK-3, a new member of the tandem pore K⁺ channel family. J Biol Chem 275: 9340–9347, 2000.[Abstract/Free Full Text]
285. Kleuss C, Hescheler J, Ewel C, Rosenthal W, Schultz G, and Wittig B. Assignment of G protein subtypes to specific receptors inducing inhibition of calcium currents. Nature 353: 43–48, 1991.[CrossRef][Medline]
286. Klockner U, Lee JH, Cribbs LL, Daud A, Hescheler J, Pereverzev A, Perez-Reyes E, and Schneider T. Comparison of the Ca²⁺ currents induced by expression of three cloned alpha1 subunits, alpha1G, alpha1H and alpha1I, of low-voltage-activated T-type Ca²⁺ channels. Eur J Neurosci 11: 4171–4178, 1999.[CrossRef][Web of Science][Medline]
287. Kohout TA, Lin FS, Perry SJ, Conner DA, and Lefkowitz RJ. -Arrestin 1 and 2 differentially regulate heptahelical receptor signaling and trafficking. Proc Natl Acad Sci USA 98: 1601–1606, 2001.[Abstract/Free Full Text]
288. Kojima I, Kawamura N, and Shibata H. Rate of calcium entry determines the rapid changes in protein kinase C activity in angiotensin II-stimulated adrenal glomerulosa cells. Biochem J 297: 523–528, 1994.[Web of Science][Medline]
289. Kojima I, Kojima K, Kreutter D, and Rasmussen H. The temporal integration of the aldosterone secretory response to angiotensin occurs via two intracellular pathways. J Biol Chem 259: 14448–14457, 1984.[Abstract/Free Full Text]
290. Kojima I, Kojima K, and Rasmussen H. Characteristics of angiotensin II-, K⁺- and ACTH-induced calcium influx in adrenal glomerulosa cells. J Biol Chem 260: 9171–9176, 1985.[Abstract/Free Full Text]
291. Kojima I, Kojima K, and Rasmussen H. Intracellular calcium and adenosine 3',5'-cyclic monophosphate as mediators of potassium-induced aldosterone secretion. Biochem J 228: 69–76, 1985.[Web of Science][Medline]
292. Kojima I, Kojima K, and Rasmussen H. Role of calcium fluxes in the sustained phase of angiotensin II-mediated aldosterone secretion from adrenal glomerulosa cells. J Biol Chem 260: 9177–9184, 1985.[Abstract/Free Full Text]
293. Kojima I, Kojima K, Shibata H, and Ogata E. Mechanism of cholinergic stimulation of aldosterone secretion in bovine adrenal glomerulosa cells. Endocrinology 119: 284–291, 1986.[Abstract/Free Full Text]
294. Kojima I, Lippes H, Kojima K, and Rasmussen H. Aldosterone secretion: effect of phorbol ester and A23187. Biochem Biophys Res Commun 116: 555–562, 1983.[Web of Science][Medline]
295. Kojima I and Ogata E. Na-Ca exchanger as a calcium influx pathway in adrenal glomerulosa cells. Biochem Biophys Res Commun 158: 1005–1012, 1989.[CrossRef][Web of Science][Medline]
296. Kojima I, Shibata H, and Ogata E. Pertussis toxin blocks angiotensin II-induced calcium influx but not inositol trisphosphate production in adrenal glomerulosa cell. FEBS Lett 204: 347–351, 1986.[CrossRef][Web of Science][Medline]
297. Kojima I, Shibata H, and Ogata E. Phorbol ester inhibits angiotensin-induced activation of phospholipase C in adrenal glomerulosa cells. Its implication in the sustained action of angiotensin. Biochem J 237: 253–258, 1986.[Medline]
298. Koschak A, Reimer D, Huber I, Grabner M, Glossmann H, Engel J, and Striessnig J. _1D (Cav1.3) subunits can form L-type Ca²⁺ channels activating at negative voltages. J Biol Chem 276: 22100–22106, 2001.[Abstract/Free Full Text]
299. Kowluru R, Yamazaki T, McNamara BC, and Jefcoate CR. Metabolism of exogenous cholesterol by rat adrenal mitochondria is stimulated equally by physiological levels of free Ca²⁺ and by GTP. Mol Cell Endocrinol 107: 181–188, 1995.[CrossRef][Medline]
300. Kramer RE. Angiotensin II causes sustained elevations in cytosolic calcium in glomerulosa cells. Am J Physiol Endocrinol Metab 255: E338–E346, 1988.[Abstract/Free Full Text]
301. Kramer RE. Angiotensin II-stimulated changes in calcium metabolism in cultured glomerulosa cells. Mol Cell Endocrinol 60: 199–210, 1988.[CrossRef][Web of Science][Medline]
302. Kramer RE. Effects of diltiazem on calcium metabolism in cultured bovine glomerulosa cells: relationships to the actions of angiotensin II and potassium. J Pharmacol Exp Ther 266: 374–384, 1993.[Abstract/Free Full Text]
303. Krupnick JG and Benovic JL. The role of receptor kinases and arrestins in G protein-coupled receptor regulation. Annu Rev Pharmacol Toxicol 38: 289–319, 1998.[CrossRef][Web of Science][Medline]
304. Kuno M and Gardner P. Ion channels activated by inositol 1,4,5-trisphosphate in plasma membrane of human T-lymphocytes. Nature 326: 301–304, 1987.[CrossRef][Medline]
305. Laird SM, Hinson JP, Vinson GP, Mallick N, Kaps S, and Teja R. Control of steroidogenesis by the calcium messenger system in human adrenocortical cells. J Mol Endocrinol 6: 45–51, 1991.[Abstract/Free Full Text]
306. Lalli E, Bardoni B, Zazopoulos E, Wurtz JM, Strom TM, Moras D, and Sassone-Corsi P. A transcriptional silencing domain in DAX-1 whose mutation causes adrenal hypoplasia congenita. Mol Endocrinol 11: 1950–1960, 1997.[Abstract/Free Full Text]
307. Lambert RC, McKenna F, Maulet Y, Talley EM, Bayliss DA, Cribbs LL, Lee JH, Perez-Reyes E, and Feltz A. Low-voltage-activated Ca²⁺ currents are generated by members of the Ca_vT subunit family (alfa1G/H) in rat primary sensory neurons. J Neurosci 18: 8605–8613, 1998.[Abstract/Free Full Text]
308. Lang U and Vallotton MB. Angiotensin II but not potassium induces subcellular redistribution of protein kinase C in bovine adrenal glomerulosa cells. J Biol Chem 262: 8047–8050, 1987.[Abstract/Free Full Text]
309. Larsen PR, Kronenberg HM, Melmed S, and Polonsky K. Williams Textbook of Endocrinology. Philadelphia, PA: Saunders, 2003.
310. Laury LW and McCarthy JL. In vitro adrenal mitochondrial 11-hydroxylation following in vivo adrenal stimulation or inhibition: enhanced subtrate utilization. Endocrinology 87: 1380–1385, 1970.[Abstract/Free Full Text]
311. Lee JH, Daud AN, Cribbs LL, Lacerda AE, Pereverzev A, Klockner U, Schneider T, and Perez-Reyes E. Cloning and expression of a novel member of the low voltage-activated T-type calcium channel family. J Neurosci 19: 1912–1921, 1999.[Abstract/Free Full Text]
312. Lefkowitz RJ. G protein-coupled receptors. III. New roles for receptor kinases and beta-arrestins in receptor siganaling and desensitization. J Biol Chem 273: 18677–18680, 1998.[Free Full Text]
313. Lefkowitz RJ, Hausdorff WP, and Caron MG. Role of phosphorylation in desensitization of the beta-adrenoceptor. Trends Pharmacol Sci 11: 190–194, 1990.[CrossRef][Medline]
314. Lehoux JG, Dupuis G, and Lefebvre A. Regulation of CYP11B2 gene expression by protein kinase C. Endocr Res 26: 1027–1031, 2000.[Medline]
315. Lehoux JG and Lefebvre A. Transcriptional activity of the hamster CYP11B2 promoter in NCI-H295 cells stimulated by angiotensin II, potassium, forskolin and bisindolylmaleimide. J Mol Endocrinol 20: 183–191, 1998.[Abstract]
316. Lenglet S, Louiset E, Delarue C, Vaudry H, and Contesse V. Activation of 5-HT(7) receptor in rat glomerulosa cells is associated with an increase in adenylyl cyclase activity and calcium influx through T-type calcium channels. Endocrinology 143: 1748–1760, 2002.[Abstract/Free Full Text]
317. Lesouhaitier O, Chiappe A, and Rossier MF. Aldosterone increases T-type calcium currents in human adrenocarcinoma (H295R) cells by inducing channel expression. Endocrinology 142: 4320–4330, 2001.[Abstract/Free Full Text]
318. Lin D, Sugawara T, Strauss JF III, Clark BJ, Stocco DM, Saenger P, and Rogol AMW. Role of steroidogenic acute regulatory protein in adrenal and gonadal steroidogenesis. Science 267: 1828–1831, 1995.[Abstract/Free Full Text]
319. Lin MT, Haksar A, and Peron FG. The role of the Krebs cycle in the generation of intramitochondrial reducing equivalents for the 11-hydroxylation of deoxycorticosterone in isolated adrenal cells. Arch Biochem Biophys 164: 429–439, 1974.[Medline]
320. Lobo MV and Marusic ET. Angiotensin II causes a dual effect on potassium permeability in adrenal glomerulosa cells. Am J Physiol Endocrinol Metab 254: E144–E149, 1988.[Abstract/Free Full Text]
321. Lotshaw DP. Characterization of angiotensin II-regulated K⁺ conductance in rat adrenal glomerulosa cells. J Membr Biol 156: 261–277, 1997.[CrossRef][Web of Science][Medline]
322. Lotshaw DP. Effects of K⁺ channel blockers on K⁺ channels, membrane potential, and aldosterone secretion in rat adrenal zona glomerulosa cells. Endocrinology 138: 4167–4175, 1997.[Abstract/Free Full Text]
323. Lotshaw DP. Role of membrane depolarization and T-type Ca²⁺ channels in angiotensin II and K⁺ stimulated aldosterone secretion. Mol Cell Endocrinol 175: 157–171, 2001.[CrossRef][Web of Science][Medline]
324. Lotshaw DP and Li F. Angiotensin II activation of Ca²⁺-permeant nonselective cation channels in rat adrenal glomerulosa cells. Am J Physiol Cell Physiol 271: C1705–C1715, 1996.[Abstract/Free Full Text]
325. Lotshaw DP and Sheehan KA. Divalent cation permeability and blockade of Ca²⁺-permeant non-selective cation channels in rat adrenal zona glomerulosa cells. J Physiol 514: 397–411, 1999.[Abstract/Free Full Text]
326. Lu HK, Fern RJ, Luthin D, Linden J, Liu LP, Cohen CJ, and Barrett PQ. Angiotensin II stimulates T-type Ca²⁺ channel currents via activation of a G protein, G_i. Am J Physiol Cell Physiol 271: C1340–C1349, 1996.[Abstract/Free Full Text]
327. Lu HK, Fern RJ, Nee JJ, and Barrett PQ. Ca²⁺-dependent activation of T-type Ca²⁺ channels by calmodulin-dependent protein kinase II. Am J Physiol Renal Fluid Electrolyte Physiol 267: F183–F189, 1994.[Abstract/Free Full Text]
328. Lukács GL and Kapus A. Measurement of the matrix free Ca²⁺ concentration in heart mitochondria by entrapped fura-2 and quin2. Biochem J 248: 609–613, 1987.[Web of Science][Medline]
329. Mackie CM, Simpson ER, Mee MSR, Tait SAS, and Tait JF. Intracellular potassium and steroidogenesis of isolated rat adrenal cells: effects of potassium ions and angiotensin II on purified zona glomerulosa cells. Clin Sci Mol Med 53: 289–296, 1977.[Medline]
330. MacLennan DH, Rice WJ, and Green NM. The mechanism of Ca²⁺ transport by sarco(endo)plasmic reticulum Ca²⁺-ATPases. J Biol Chem 272: 28815–28818, 1997.[Free Full Text]
331. Maeda N, Kawasaki T, Nakade S, Yokota N, Taguchi T, Kasai M, and Mikoshiba K. Structural and functional characterization of inositol 1,4,5-trisphosphate receptor channel from mouse cerebellum. J Biol Chem 266: 1109–1116, 1991.[Abstract/Free Full Text]
332. Maes K, Missiaen L, De Smet P, Vanlingen S, Callewaert G, Parys JB, and De Smedt H. Differential modulation of inositol 1,4,5-trisphosphate receptor type 1 and type 3 by ATP. Cell Calcium 27: 257–267, 2000.[CrossRef][Web of Science][Medline]
333. Mak DO, McBride S, and Foskett JK. ATP regulation of recombinant type 3 inositol 1,4,5-trisphosphate receptor gating. J Gen Physiol 117: 447–456, 2001.[Abstract/Free Full Text]
334. Makara JK, Petheö GL, Tóth A, and Spät A. Effect of osmolarity on aldosterone production by rat adrenal glomerulosa cells. Endocrinology 141: 1705–1710, 2000.[Abstract/Free Full Text]
335. Makara JK, Koncz P, Petheö GL, and Spät A. Role of cell volume in K⁺-induced Ca²⁺ signaling by rat adrenal glomerulosa cells. Endocrinology. 144: 4916–4922, 2003.[Abstract/Free Full Text]
336. Malendowicz LK, Nowak M, Gottardo L, Tortorella C, Majchrzak MA, and Nussdorfer GG. Cholecystokinin stimulates aldosterone secretion from dispersed rat zona glomerulosa cells, acting through cholecystokinin receptors 1 and 2 coupled with the adenylate cyclase-dependent cascade. Endocrinology 142: 4251–4255, 2001.[Abstract/Free Full Text]
337. Mann JF, Johnson AK, and Ganten D. Plasma angiotensin II: dipsogenic levels and angiotensin-generating capacity of renin. Am J Physiol Regul Integr Comp Physiol 238: R372–R377, 1980.[Abstract/Free Full Text]
338. Mannella CA, Buttle K, Rath BK, and Marko M. Electron microscopic tomography of rat-liver mitochondria and their interaction with the endoplasmic reticulum. BioFactors 8: 225–228, 1998.[Web of Science][Medline]
339. Marie J and Jard S. Angiotensin II inhibits adenylate cyclase from adrenal cortex glomerulosa zone. FEBS Lett 159: 97–101, 1983.[CrossRef][Web of Science][Medline]
340. Marrero MB, Schieffer B, Paxton WG, Heerdt L, Berk BC, Delafontaine P, and Bernstein-KE. Direct stimulation of Jak/STAT pathway by the angiotensin II AT1 receptor. Nature 375: 247–250, 1995.[CrossRef][Medline]
341. Martin RL, Lee JH, Cribbs LL, Perez-Reyes E, and Hanck DA. Mibefradil block of cloned T-type calcium channels. J Pharmacol Exp Ther 295: 302–308, 2000.[Abstract/Free Full Text]
342. Matsunaga H, Maruyama Y, Kojima I, and Hoshi T. Transient Ca²⁺-channel current characterized by low-threshold voltage in zona glomerulosa cells of rat adrenal cortex. Pflügers Arch 408: 351–355, 1987.[CrossRef][Web of Science][Medline]
343. Matsunaga H, Yamashita N, Maruyama Y, Kojima I, and Kurokawa K. Evidence for two distinct voltage-gated calcium channel currents in bovine adrenal glomerulosa cells. Biochem Biophys Res Commun 149: 1049–1054, 1987.[CrossRef][Web of Science][Medline]
344. Maturana AD, Casal AJ, Demaurex N, Vallotton MB, Capponi AM, and Rossier MF. Angiotensin II negatively modulates L-type calcium channels through a pertussis toxin-sensitive G protein in adrenal glomerulosa cells. J Biol Chem 274: 19943–19948, 1999.[Abstract/Free Full Text]
345. Mazzocchi G, Malendowicz LK, Gottardo G, Rebuffat P, and Nussdorfer GG. Angiotensin-II stimulates DNA synthesis in rat adrenal zona glomerulosa cells: receptor subtypes involved and possible signal transduction mechanism. Endocr Res 23: 191–203, 1997.[Medline]
346. Mazzocchi G, Rossi GP, Malendowicz LK, Champion HC, and Nussdorfer GG. Endothelin-1[1–31], acting as an ET_A-receptor selective agonist, stimulates proliferation of cultured rat zona glomerulosa cells. FEBS Lett 487: 194–198, 2000.[CrossRef][Web of Science][Medline]
347. Mazzocchi G, Rossi GP, Rebuffat P, Malendowicz LK, Markowska A, and Nussdorfer GG. Endothelins stimulate deoxyribonucleic acid synthesis and cell proliferation in rat adrenal zona glomerulosa, acting through an endothelin a receptor coupled with protein kinase C- and tyrosine kinase-dependent signaling pathways. Endocrinology 138: 2333–2337, 1997.[Abstract/Free Full Text]
348. McCarron JG, McGeown JG, Reardon S, Ikebe M, Fay FS, and Walsh JV Jr. Calcium-dependent enhancement of calcium current in smooth muscle by calmodulin-dependent protein kinase II. Nature 357: 74–77, 1992.[CrossRef][Medline]
349. McCarthy RT, Isales C, and Rasmussen H. T-type calcium channels in adrenal glomerulosa cells: GTP-dependent modulation by angiotensin II. Proc Natl Acad Sci USA 90: 3260–3264, 1993.[Abstract/Free Full Text]
350. McCarthy RT, Isales CM, Bollag WB, Rasmussen H, and Barrett PQ. Atrial natriuretic peptide differentially modulates T- and L-type calcium channels. Am J Physiol Renal Fluid Electrolyte Physiol 258: F473–F478, 1990.[Abstract/Free Full Text]
351. McCormack JG, Halestrap AP, and Denton RM. Role of calcium ions in regulation of mammalian intramitochondrial metabolism. Physiol Rev 70: 391–425, 1990.[Free Full Text]
352. McDonald TF, Pelzer S, Trautwein W, and Pelzer DJ. Regulation and modulation of calcium channels in cardiac, skeletal, and smooth muscle cells. Physiol Rev 74: 365–507, 1994.[Free Full Text]
353. McKenna TJ, Island DP, Nicholson WE, and Liddle GW. Dopamine inhibits angiotensin-stimulated aldosterone biosynthesis in bovine adrenal cells. J Clin Invest 84: 287–291, 1979.
354. McNeill H, Puddefoot JR, and Vinson GP. MAP kinase in the rat adrenal gland. Endocr Res 24: 373–380, 1998.[Web of Science][Medline]
355. Michell RH. Inositol phospholipids and cell surface receptor function. Biochim Biophys Acta 415: 81–147, 1975.[Medline]
356. Michell RH, Kirk CJ, Jones LM, Downes CP, and Creba JA. The stimulation of inositol lipid-metabolism that accompanies calcium mobilization in stimulated cells: defined characteristics and unanswered questions. Philos Trans R Soc Lond B Biol Sci 296: 123–137, 1981.[Medline]
357. Michikawa T, Hirota J, Kawano S, Hiraoka M, Yamada M, Furuichi T, and Mikoshiba K. Calmodulin mediates calcium-dependent inactivation of the cerebellar type 1 inositol 1,4,5-trisphosphate receptor. Neuron 23: 799–808, 1999.[CrossRef][Web of Science][Medline]
358. Mihalik B, Gáborik Z, Várnai P, Clark AJL, Catt KJ, and Hunyady L. Endocytosis of the AT_1A angiotensin receptor is independent of ubiquitylation of its cytoplasmic serine/threonine-rich region. Int J Biochem Cell Biol 35: 992–1002, 2003.[CrossRef][Medline]
359. Mikami A, Imoto K, Tanabe T, Niidome T, Mori Y, Takeshima H, Narumiya S, and Numa S. Primary structure and functional expression of the cardiac dihydropyridine-sensitive calcium channel. Nature 340: 230–233, 1989.[CrossRef][Medline]
360. Millan MA, Carvallo P, Izumi S, Zemel S, Catt KJ, and Aguilera G. Novel sites of expression of functional angiotensin II receptors in the late gestation fetus. Science 244: 1340–1342, 1989.[Abstract/Free Full Text]
361. Minke B and Cook B. TRP channel proteins and signal transduction. Physiol Rev 82: 429–472, 2002.[Abstract/Free Full Text]
362. Miserey-Lenkei S, Lenkei Z, Parnot C, Corvol P, and Clauser E. A functional enhanced green fluorescent protein (EGFP)-tagged angiotensin II AT_1a receptor recruits the endogenous G_q/11 protein to the membrane and induces its specific internalization independently of receptor-G protein coupling in HEK-293 cells. Mol Endocrinol 15: 294–307, 2001.[Abstract/Free Full Text]
363. Missiaen L, Parys JB, Sienaert I, Maes K, Kunzelmann K, Takahashi M, Tanzawa K, and De Smedt H. Functional properties of the type-3 InsP3 receptor in 16HBE14o- bronchial mucosal cells. J Biol Chem 273: 8983–8986, 1998.[Abstract/Free Full Text]
364. Miyakawa T, Maeda A, Yamazawa T, Hirose K, Kurosaki T, and Iino M. Encoding of Ca²⁺ signals by differential expression of IP₃ receptor subtypes. EMBO J 18: 1303–1308, 1999.[CrossRef][Web of Science][Medline]
365. Miyakawa T, Mizushima A, Hirose K, Yamazawa T, Bezprozvanny I, Kurosaki T, and Iino M. Ca²⁺-sensor region of IP₃ receptor controls intracellular Ca²⁺ signaling. EMBO J 20: 1674–1680, 2001.[CrossRef][Web of Science][Medline]
366. Mogami H, Zhang H, Suzuki Y, Urano T, Saito N, Kojima I, and Petersen OH. Decoding of short-lived Ca²⁺ influx signals into long term substrate phosphorylation through activation of two distinct classes of protein kinase C. J Biol Chem 278: 9896–9904, 2003.[Abstract/Free Full Text]
367. Moraru II, Kaftan EJ, Ehrlich BE, and Watras J. Regulation of type 1 inositol 1,4,5-trisphosphate-gated calcium channels by InsP₃ and calcium: simulation of single channel kinetics based on ligand binding and electrophysiological analysis. J Gen Physiol 113: 837–849, 1999.[Abstract/Free Full Text]
368. Müller J. Aldosterone stimulation in vitro. II. Stimulation of aldosterone production by monovalent cations. Acta Endocrinol 50: 301–309, 1965.[Medline]
369. Müller J. Regulation of Aldosterone Biosynthesis. Berlin: Springer-Verlag, 1988.
370. Müller J and Ziegler WH. Stimulation of aldosterone biosynthesis in vitro by serotonin. Acta Endocrinol 59: 23–35, 1968.[Medline]
371. Mulrow PJ. Adrenal renin: a possible local regulator of aldosterone production. Yale J Biol Med 62: 503–510, 1989.[Medline]
372. Mulrow PJ. Angiotensin II and aldosterone regulation. Regul Pept 80: 27–32, 1999.[CrossRef][Web of Science][Medline]
373. Murakami M, Suzuki H, Nakajima S, Nakamoto H, Kageyama Y, and Saruta T. Calcitonin gene-related peptide is an inhibitor of aldosterone secretion. Endocrinology 125: 2227–2229, 1989.[Abstract/Free Full Text]
374. Nagy K, Koroknai L, and Spät A. Effect of lipoproteins on aldosterone production by isolated glomerulosa cells. J Steroid Biochem 20: 789–791, 1984.[Medline]
375. Nakanishi O, Homma Y, Kawasaki H, Emori Y, Suzuki K, and Takenawa T. Purification of two distinct types of phosphoinositide-specific phospholipase C from rat liver. Enzymological and structural studies. Biochem J 256: 453–459, 1988.[Web of Science][Medline]
376. Nakanishi S, Catt KJ, and Balla T. Inhibition of agonist-stimulated inositol 1,4,5-trisphosphate production and calcium signaling by the myosin light chain kinase inhibitor, wortmannin. J Biol Chem 269: 6528–6535, 1994.[Abstract/Free Full Text]
377. Nakano S, Carvallo P, Rocco S, and Aguilera G. Role of protein kinase C on the steroidogenic effect of angiotensin II in the rat adrenal glomerulosa cell. Endocrinology 126: 125–133, 1990.[Abstract/Free Full Text]
378. Natarajan R, Dunn WD, Stern N, and Nadler J. Key role of diacylglycerol-mediated 12-lipoxygenase product formation in angiotensin II-induced aldosterone synthesis. Mol Cell Endocrinol 72: 73–80, 1990.[CrossRef][Web of Science][Medline]
379. Natarajan R, Gonzales N, Hornsby PJ, and Nadler J. Mechanism of angiotensin II-induced proliferation in bovine adrenocortical cells. Endocrinology 131: 1174–1180, 1992.[Abstract/Free Full Text]
380. Natarajan R, Lanting L, Bai W, Bravo EL, and Nadler J. The role of nitric oxide in the regulation of aldosterone synthesis by adrenal glomerulosa cells. J Steroid Biochem Mol Biol 61: 47–53, 1997.[CrossRef][Web of Science][Medline]
381. Natarajan R and Nadler J. Platelet-derived growth factor is a potent inhibitor of angiotensin II-induced aldosterone synthesis. Mol Cell Endocrinol 83: 57–63, 1992.[Medline]
382. Natarajan R, Yang DC, Lanting L, and Nadler JL. Key role of p38 mitogen-activated protein kinase and the lipoxygenase pathway in angiotensin II actions in H295R adrenocortical cells. Endocrine 18: 295–301, 2002.[CrossRef][Web of Science][Medline]
383. Natke E Jr and Kabela E. Electrical responses in cat adrenal cortex: possible relation to aldosterone response. Am J Physiol Endocrinol Metab Gastrointest Physiol 237: E158–E162, 1979.[Abstract/Free Full Text]
384. Newton CL, Mignery GA, and Südhof TC. Co-expression in vertebrate tissues and cell lines of multiple inositol 1,4,5-trisphosphate (InsP₃) receptors with distinct affinities for InsP₃. J Biol Chem 269: 28613–28619, 1994.[Abstract/Free Full Text]
385. Nguyen G, Delarue F, Burcklé C, Bouzhir L, Giller T, and Sraer JD. Pivotal role of the renin/prorenin receptor in angiotensin II production and cellular responses to renin. J Clin Invest 109: 1417–1427, 2002.[CrossRef][Web of Science][Medline]
386. Nichols CG and Lopatin AN. Inward rectifier potassium channels. Annu Rev Physiol 59: 171–191, 1997.[CrossRef][Web of Science][Medline]
387. Nielsen SP and Petersen OH. Transport of calcium in the perfused submandibular gland of the cat. J Physiol 223: 685–697, 1972.[Abstract/Free Full Text]
388. Nishizuka Y. The family of protein kinase C for signal transduction. JAMA 262: 1826–1833, 1989.[Abstract/Free Full Text]
389. Nosyreva E, Miyakawa T, Wang ZN, Glouchankova L, Mizushima A, Iino M, and Bezprozvanny I. The high-affinity calcium-calmodulin-binding site does not play a role in the modulation of type 1 inositol 1,4,5-trisphosphate receptor function by calcium and calmodulin. Biochem J 365: 659–667, 2002.[Web of Science][Medline]
390. Nouet I and Nahmias I. Signal transduction from the angiotensin II AT2 receptor. Trends Endocrinol Metab 11: 1–6, 2000.[CrossRef][Web of Science][Medline]
391. Nunn DL and Taylor CW. Liver inositol 1,4,5-trisphosphate-binding sites are the Ca²⁺-mobilizing receptors. Biochem J 270: 227–232, 1990.[Web of Science][Medline]
392. Nussdorfer GG. Cytophysiology of the adrenal zona glomerulosa. Int Rev Cytol 64: 307–368, 1980.
393. Nussdorfer GG, Malendowicz LK, Belloni AS, Mazzocchi G, and Rebuffat P. Effects of substance P on the rat adrenal zona glomerulosa in vivo. Peptides 9: 1145–1149, 1988.[CrossRef][Medline]
394. Nussdorfer GG and Mazzocchi G. Vasoactive intestinal peptide (VIP) stimulates aldosterone secretion by rat adrenal glands in vivo. J Steroid Biochem 26: 203–205, 1987.[Medline]
395. Oelkers W, Schoneshofer M, Schultze G, Brown JJ, Fraser R, Morton JJ, Lever AF, and Robertson JI. Effect of prolonged low-dose angiotensin II infusion on the sensitivity of adrenal cortex in man. Circ Res 36: 49–56, 1975.[Abstract/Free Full Text]
396. Ogishima T, Suzuki H, Hata JI, Mitani F, and Ishimura Y. Zone-specific expression of aldosterone synthase cytochrome P-450 and cytochrome P-45011 in rat adrenal cortex: histochemical basis for the functional zonation. Endocrinology 130: 2971–2977, 1992.[Abstract/Free Full Text]
397. Olivares-Reyes JA, Jayadev S, Hunyady L, Catt KJ, and Smith RD. Homologous and heterologous phosphorylation of the AT₂ angiotensin receptor by protein kinase C. Mol Pharmacol 58: 1156–1161, 2000.[Abstract/Free Full Text]
398. Olivares-Reyes JA, Smith RD, Hunyady L, Shah BH, and Catt KJ. Agonist-induced signaling, desensitization, and internalization of a phosphorylation-deficient AT_1A angiotensin receptor. J Biol Chem 276: 37761–37768, 2001.[Abstract/Free Full Text]
399. Oppermann M, Freedman NJ, Alexander RW, and Lefkowitz RJ. Phosphorylation of the type 1A angiotensin II receptor by G protein-coupled receptor kinases and protein kinase C. J Biol Chem 271: 13266–13272, 1996.[Abstract/Free Full Text]
400. Osipenko ON, Várnai P, Mike A, Spät A, and Vizi ES. Dopamine blocks T-type calcium channels in cultured rat adrenal glomerulosa cells. Endocrinology 134: 511–514, 1994.[Abstract/Free Full Text]
401. Osman H, Murigande C, Nadakal A, and Capponi AM. Repression of DAX-1 and induction of SF-1 expression: two mechanisms contributing to the activation of aldosterone biosynthesis in adrenal glomerulosa cells. J Biol Chem 277: 41259–41267, 2002.[Abstract/Free Full Text]
402. Ouali R, Berthelon MC, Bégeot M, and Saez JM. Angiotensin II receptor subtypes AT1 and AT2 are down-regulated by angiotensin II through AT1 receptor by different mechanisms. Endocrinology 138: 725–733, 1997.[Abstract/Free Full Text]
403. Palmer S and Wakelam MJ. The Ins(1,4,5)P₃ binding site of bovine adrenocortical microsomes: function and regulation. Biochem J 260: 593–596, 1989.[Medline]
404. Parekh AB and Penner R. Store depletion and calcium influx. Physiol Rev 77: 901–930, 1997.[Abstract/Free Full Text]
405. Patel S, Joseph SK, and Thomas AP. Molecular properties of inositol 1,4,5-trisphosphate receptors. Cell Calcium 25: 247–264, 1999.[CrossRef][Web of Science][Medline]
406. Patel S, Morris SA, Adkins CE, O'Beirne G, and Taylor CW. Ca²⁺-independent inhibition of inositol trisphosphate receptors by calmodulin: redistribution of calmodulin as a possible means of regulating Ca²⁺ mobilization. Proc Natl Acad Sci USA 94: 11627–11632, 1997.[Abstract/Free Full Text]
407. Payet MD, Benabderrazik M, and Gallo-Payet N. Excitation-secretion coupling: ionic currents in glomerulosa cells: effects of adrenocorticotropin and K⁺ channel blockers. Endocrinology 121: 875–882, 1987.[Abstract/Free Full Text]
408. Payet MD, Bilodeau L, Drolet P, Ibarrondo J, Guillon G, and Gallo-Payet N. Modulation of a Ca²⁺-activated K⁺ channel by angiotensin II in rat adrenal glomerulosa cells: involvement of a G protein. Mol Endocrinol 9: 935–947, 1995.[Abstract/Free Full Text]
409. Payet MD, Durroux T, Bilodeau L, Guillon G, and Gallo-Payet N. Characterization of K⁺ and Ca²⁺ ionic currents in glomerulosa cells from human adrenal glands. Endocrinology 134: 2589–2598, 1994.[Abstract/Free Full Text]
410. Pedersen RC and Brownie AC. Steroidogenesis-activator polypeptide isolated from a rat Leydig cell tumor. Science 236: 188–190, 1987.[Abstract/Free Full Text]
411. Penhoat A, Jaillard C, Crozat A, and Saez JM. Regulation of angiotensin II receptors and steroidogenic responsiveness in cultured bovine fasciculata and glomerulosa adrenal cells. Eur J Biochem 172: 247–254, 1988.[Web of Science][Medline]
412. Perez-Reyes E. Three for T: molecular analysis of the low voltage-activated calcium channel family. Cell Mol Life Sci 56: 660–669, 1999.[CrossRef][Web of Science][Medline]
413. Perez-Reyes E, Lee JH, and Cribbs LL. Molecular characterization of two members of the T-type calcium channel family. Ann NY Acad Sci 868: 131–143, 1999.[CrossRef][Web of Science][Medline]
414. Perez-Reyes E and Schneider T. Molecular biology of calcium channels. Kidney Int 48: 1111–1124, 1995.[Web of Science][Medline]
415. Perez-Reyes E, Wei XY, Castellano A, and Birnbaumer L. Molecular diversity of L-type calcium channels. Evidence for alternative splicing of the transcripts of three non-allelic genes. J Biol Chem 265: 20430–20436, 1990.[Abstract/Free Full Text]
416. Pernollet MG, Devynck MA, Matthews PG, and Meyer P. Postnephrectomy changes in adrenal angiotensin II receptors in the rat: influence of exogenous angiotensin and a competitive inhibitor. Eur J Pharmacol 43: 361–372, 1977.[CrossRef][Medline]
417. Péron FG and McCarthy JL. Corticosteroidogenesis in the rat adrenal gland. In: Functions of the Adrenal Cortex, edited by McKerns KW. Amsterdam: North Holland, 1968, vol. 1, p. 261–338.
418. Peters B, Clausmeyer S, Obermüller N, Woyth A, Kränzlin B, Gretz N, and Peters J. Specific regulation of StAR expression in the rat adrenal zona glomerulosa: an in situ hybridization study. J Histochem Cytochem 46: 1215–1221, 1998.[Abstract/Free Full Text]
419. Peters J and Ganten D. Adrenal renin expression and its role in ren-2 transgenic rats TGR(mREN2)27. Horm Metab Res 30: 350–354, 1998.[Medline]
420. Petersen OH. Does inositol tetrakisphosphate play a role in the receptor-mediated control of calcium mobilization? Cell Calcium 10: 375–383, 1989.[Medline]
421. Petersen OH, Tepikin A, and Park MK. The endoplasmic reticulum: one continuous or several separate Ca²⁺ stores? Trends Neurosci 24: 271–276, 2001.[CrossRef][Web of Science][Medline]
422. Pezzi V, Clark BJ, Ando S, Stocco DM, and Rainey WE. Role of calmodulin-dependent protein kinase II in the acute stimulation of aldosterone production. J Steroid Biochem Mol Biol 58: 417–424, 1996.[CrossRef][Web of Science][Medline]
423. Philipp S, Cavalié A, Freichel M, Wissenbach U, Zimmer S, Trost C, Marquart A, Murakami M, and Flockerzi V. A mammalian capacitative calcium entry channel homologous to Drosophila TRP and TRPL. EMBO J 15: 6166–6171, 1996.[Web of Science][Medline]
424. Philipp S, Trost C, Warnat J, Rautmann J, Himmerkus N, Schroth G, Kretz O, Nastainczyk W, Cavalie A, Hoth M, and Flockerzi V. TRP4 (CCE1) protein is part of native calcium release-activated Ca²⁺-like channels in adrenal cells. J Biol Chem 275: 23965–23972, 2000.[Abstract/Free Full Text]
425. Picard L, Ibarrondo J, Coquil JF, Hilly M, and Mauger JP. Ligand-binding affinity of the type 1 and 2 inositol 1,4,5-trisphosphate receptors: effect of the membrane environment. Biochem Pharmacol 59: 131–139, 2000.[CrossRef][Medline]
426. Pietri F, Hilly M, Claret M, and Mauger JP. Characterization of two forms of inositol 1,4,5-trisphosphate receptor in rat liver. Cell Signal 2: 253–263, 1990.[CrossRef][Medline]
427. Pietri F, Hilly M, and Mauger JP. Calcium mediates the interconversion between two states of the liver inositol 1,4,5-trisphosphate receptor. J Biol Chem 265: 17478–17485, 1990.[Abstract/Free Full Text]
428. Pitter JG, Maechler P, Wollheim CB, and SpätA. Mitochondria respond to Ca²⁺ already in the submicromolar range: correlation with redox state. Cell Calcium 31: 97–104, 2002.[CrossRef][Web of Science][Medline]
429. Pivovarova NB, Hongpaisan J, Andrews SB, and Friel DD. Depolarization-induced mitochondrial Ca accumulation in sympathetic neurons: spatial and temporal characteristics. J Neurosci 19: 6372–6384, 1999.[Abstract/Free Full Text]
430. Poirier SN, Poitras M, Chorvatova A, Payet MD, and Guillemette G. FK506 blocks intracellular Ca²⁺ oscillations in bovine adrenal glomerulosa cells. Biochemistry 40: 6486–6492, 2001.[CrossRef][Medline]
431. Pozzan T and R