HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (113) Citing Articles via Google Scholar Google Scholar Articles by Nerbonne, J. M. Articles by Kass, R. S. Search for Related Content PubMed PubMed Citation Articles by Nerbonne, J. M. Articles by Kass, R. S.

Molecular Physiology of Cardiac Repolarization

Department of Molecular Biology and Pharmacology, Washington University Medical School, St. Louis, Missouri; and Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, New York

ABSTRACT

I. INTRODUCTION

II. MYOCARDIAL ACTION POTENTIALS AND VOLTAGE-GATED INWARD SODIUM AND CALCIUM CURRENTS

A. Voltage-Gated Na+(Nav) Currents

B. Voltage-Gated Ca2+ (Cav) Currents

III. MYOCARDIAL ACTION POTENTIALS AND REPOLARIZING VOLTAGE-GATED POTASSIUM CURRENTS

A. Transient Outward Kv Currents

B. Delayed Rectifier Kv Currents

C. Regional Differences in Kv Current Expression and Properties

IV. OTHER MYOCARDIAL POTASSIUM CURRENTS CONTRIBUTING TO REPOLARIZATION

V. MOLECULAR COMPONENTS OF MYOCARDIAL NAV AND CAV CHANNELS

A. Nav Channel Pore-Forming {alpha}-Subunits

B. Nav Channel Accessory Subunits and Other Interacting Proteins

C. Cav Channel Pore-Forming {alpha}-Subunits

D. Cav Channel Accessory Subunits and Other Interacting Proteins

VI. MOLECULAR COMPONENTS OF MYOCARDIAL KV CHANNELS

A. Kv Channel Pore-Forming {alpha}-Subunits

B. Kv Channel Accessory Subunits

C. Molecular Correlates of Cardiac Transient Outward Kv Channels

D. Molecular Correlates of Cardiac Delayed Rectifier Kv Channels

VII. MOLECULAR COMPONENTS OF OTHER CARDIAC POTASSIUM CHANNELS

A. Inwardly Rectifying Cardiac K+ (Kir) Channel Pore-Forming {alpha}-Subunits

B. Two-Pore Domain K+ (K2P) Channel Pore-Forming {alpha}-Subunits

VIII. MYOCARDIAL POTASSIUM CHANNELS AND THE ACTIN CYTOSKELETON

IX. SUMMARY AND CONCLUSIONS

GRANTS

ACKNOWLEDGMENTS

REFERENCES

	ABSTRACT

Top Next References

	I. INTRODUCTION

Top Previous Next References

 
The normal mechanical (pump) functioning of the mammalian heart depends on proper electrical functioning (56, 173), reflected in the sequential activation of cells in specialized, "pacemaker" regions of the heart and the propagation of activity through the ventricles (Fig. 1). Myocardial electrical activity is attributed to the generation of action potentials in individual cardiac cells, and the normal coordinated electrical functioning of the whole heart is readily detected in surface electrocardiograms (Fig. 1). The propagation of activity and the coordination of the electromechanical functioning of the ventricles also depend on electrical coupling between cells, mediated by gap junctions (251, 435). The generation of myocardial action potentials reflects the sequential activation and inactivation of ion channels that conduct depolarizing, inward (Na⁺ and Ca²⁺), and repolarizing, outward (K⁺), currents (24, 375). The waveforms of action potentials in different regions of the heart are distinct (Fig. 1), owing to differences in the expression and/or the properties of the underlying ion channels (24, 374). These differences contribute to the normal unidirectional propagation of excitation through the myocardium and to the generation of normal cardiac rhythms (23, 24, 259, 374, 375). Changes in the properties or the functional expression of myocardial ion channels, resulting from inherited mutations in the genes encoding these channels (23, 36, 51, 102, 204, 243, 253) or from myocardial disease (34, 49, 67, 184, 365, 496, 501–503, 510), can lead to changes in action potential waveforms, synchronization, and/or propagation, thereby predisposing the heart to potentially life-threatening arrhythmias (13, 14, 16, 24, 127, 259, 436). There is, therefore, considerable interest in delineating the molecular, cellular, and systemic mechanisms contributing to the generation and maintenance of normal cardiac rhythms, as well as in understanding how these mechanisms are altered in the diseased myocardium.

View larger version (25K): [in this window] [in a new window]   FIG. 1. Electrical activity in the myocardium. Top: schematic of a human heart with illustration of typical action potential waveforms recorded in different regions. Bottom: schematic of a surface electrocardiogram; three sequential beats are displayed.

View larger version (15K): [in this window] [in a new window]   FIG. 2. Action potential waveforms and underlying ionic currents in adult human and ventricular (left) and atrial (right) myocytes. The time- and voltage-dependent properties of the voltage-gated inward Na+ (Nav) and Ca2+ (Cav) currents expressed in human atrial and ventricular myocytes are similar. In contrast, there are multiple types of K+ currents, particularly Kv currents, contributing to atrial and ventricular action potential repolarization. The properties of the various Kv currents are distinct, and in contrast to the inward currents, there are multiple Kv currents expressed in individual myocytes throughout the myocardium.

The driving force for K⁺ efflux is high during the plateau phase of the action potential in ventricular and atrial myocardium and, as the Cav channels inactivate, the outward K⁺ currents predominate, resulting in (phase 3) repolarization, bringing the membrane voltage back to the resting potential (Fig. 2). In contrast to Nav and Cav currents, however, there are multiple types of voltage-gated K⁺ (Kv) currents, as well as non-voltage-gated, inwardly rectifying K⁺ (Kir) currents (Table 1), that contribute to myocardial action potential repolarization. The greatest functional diversity is among Kv channels (Table 1). At least two types of transient outward currents, I_to,f and I_to,s, and several components of delayed rectification, including I_Kr [I_K(rapid)], I_Ks [I_K(slow)], and I_Kur [I_{K(ultrarapid)}], for example, have been distinguished (Table 1). The time- and voltage-dependent properties of the various Kv currents identified in myocytes isolated from different species and/or from different regions of the heart in the same species, however, are remarkably similar, suggesting that the same (or very similar) molecular entities contribute to the generation of each of the various types of Kv channels (Table 1) in different cells/species. The relative Kv channel expression levels vary in cardiac cells in different regions (i.e., atria, ventricles) of the heart, and this heterogeneity contributes importantly to the observed regional differences in action potential waveforms (24, 127, 373, 374). Changes in the properties or the functional expression of Kv channels, as occurs in a variety of myocardial diseases (34, 49, 67, 365, 496, 503, 510), can, therefore, have dramatic effects on action potential waveforms, propagation, and rhythmicity.

A large number of pore-forming () subunits, encoding Nav, Cav, Kv, and Kir channels, and a variety of channel accessory (, , and ) subunits have been identified (Tables 2–6), and considerable progress has been made in defining the expression patterns of these subunits in the heart and the roles of the individual subunits in the generation of functional cardiac (Nav, Cav, Kv, and Kir) channels (Tables 2–6). These studies have demonstrated that distinct molecular entities underlie the various cardiac ion channels/currents that have been distinguished electrophysiologically and shown to contribute to myocardial action potential repolarization. It also has now been shown that mutations in the genes encoding the subunits involved in the generation of functional cardiac Nav, Kv, Cav, and Kir channels underlie several inherited cardiac arrhythmias (23, 36, 51, 102, 204, 253, 479, 481). Although inherited rhythm disorders are rare, these mutations belong to an ever-increasing number of "channelopathies," i.e., diseases linked to genes encoding ion channels (35, 123, 204, 220, 234, 243, 267, 309, 359, 403, 442, 459). Based on the rapid progression of this field (249) and the growing molecular complexity of ion channels, it seems certain that the number of genes encoding ion channels or ion channel regulatory molecules linked to inherited and acquired disorders of the cardiovascular (and other) system will continue to increase, perhaps dramatically, in the future. The densities and the functional properties of myocardial Nav, Cav, Kv, and Kir currents also change in a number of acquired myocardial disease states (34, 49, 67, 365, 496, 503, 510), and these changes can lead to the generation of potentially life-threatening cardiac arrhythmias. At present, therefore, there is considerable interest in understanding the detailed molecular mechanisms controlling the properties and the functional cell surface expression of the various ion channels controlling myocardial action potential repolarization, as well as the impact of genetic and epigenetic factors, including cardiac and noncardiac disease, on the functioning of these channels.

View this table: [in this window] [in a new window]   TABLE 2. Diversity of voltage-gated Na+ (Nav) channel - and -subunits

	II. MYOCARDIAL ACTION POTENTIALS AND VOLTAGE-GATED INWARD SODIUM AND CALCIUM CURRENTS

Top Previous Next References

Voltage-gated cardiac Na⁺ (Nav) channels open rapidly on membrane depolarization (Fig. 2) and underlie the rapidly rising phases of the action potentials recorded in mammalian ventricular and atrial myocytes and in cardiac Purkinje fibers (93, 375). Although not evident in all cells, Nav channels with similar properties are also expressed in subsets of mammalian SAN and AVN cells, and differences in functional Nav channel expression likely contribute to action potential heterogeneity in pacemaker cells (262, 361, 579, 582). Although the properties of the Nav channels expressed in different cardiac cells are similar, the biophysical and pharmacological properties of these channels are distinct from Nav channels expressed in other excitable cells, such as neurons and skeletal muscle (93, 573). Cardiac Nav channels, for example, are remarkably insensitive to the Nav channel toxin tetrodotoxin (TTX), which binds with high (nM) affinity to neuronal and skeletal muscle Nav channels and blocks Na⁺ influx (93, 573). This observation was probably the first indication that the molecular identities of the Nav channels in cardiac myocytes, neurons, and skeletal muscle were distinct, and as detailed in section VA, this has now been demonstrated.

On membrane depolarization, cardiac Nav channels activate and inactivate rapidly (172, 174). The threshold for Nav channel activation is quite negative (approximately –55 mV), and the activation of these channels is steeply voltage dependent. Importantly, inactivation is also voltage dependent, and cardiac Nav channels can undergo voltage-dependent inactivation without ever opening (174). Nevertheless, persistent openings of cardiac Nav channels are occasionally observed, even at depolarized membrane potentials (437, 591). At potentials corresponding to the action potential plateau in ventricular myocytes, present estimates are that 99% of the Nav channels are in an inactivated, nonconducting state (423, 525). There is, therefore, a finite, albeit small (1%), probability of Nav channels being open at potentials corresponding to the action potential plateau (525). A slow component of Nav channel inactivation has indeed been described in normal human ventricular myocytes (323). This current is modulated by lysolipids (501) and appears to be upregulated in failing myocardium (501–503). Importantly, the inward (Na⁺) current through open Nav channels during the action potential plateau (phase 2) will counter the effects of the increased K⁺ efflux, thereby slowing or delaying repolarization and increasing action potential durations (23, 102). It follows, therefore, that changes in Nav channel open probability at voltages corresponding to the plateau (i.e., phase 2) could markedly affect action potential waveforms, particularly in ventricular cells.

The probability of Nav channel opening at depolarized potentials (i.e., during phase 2) is determined by the overlap of the curves describing the voltage dependences of channel activation and inactivation (30). At the molecular level, the fact that some channels are open over this voltage "window" implies that there is a finite probability that inactivation is reversible, i.e., that inactivated channels can reopen at depolarized potentials. Consistent with these predictions, electrophysiological studies reveal the presence of a sustained component of inward Nav current, i.e., a "persistent" Nav current, during prolonged membrane depolarizations. Although the persistent Nav channel ("window") current is small, this current could, in principle, contribute to determining action potential durations (438, 439). In this context, it is interesting to note that it has been reported that the expression level of the persistent Nav current component varies in different regions of the ventricles (438), differences that could contribute to the observed regional heterogeneities in ventricular action potential durations (24, 374). The concept that small inward (Na⁺) currents could profoundly affect action potential waveforms and excitability in the myocardium was suggested more than 50 years ago in the pioneering studies of Silvio Weidman (542). The impact of alterations in the "persistent" Nav channel window current on cardiac rhythms has now been definitively demonstrated with the identification of mutations in the gene, SCN5A, encoding the TTX-insensitive cardiac Nav channels (see sect. VA) in patients with an inherited form of long QT syndrome, LQT3 (52). A number of SCN5A mutations in different affected individuals/families have been identified (Fig. 3A) and linked to Brugada syndrome and to conduction defects, in addition to LQT3 (23, 36, 51, 60, 93, 102, 105, 204, 253, 422, 519, 525, 540). Interestingly, mutations in noncardiac Nav channel genes have also been linked to familial paroxysmal dysfunction in the skeletal and nervous systems (123, 204, 220, 359).

View larger version (46K): [in this window] [in a new window]   FIG. 3. Pore-forming () subunits of cardiac Nav (A) and Kv (B and C) channels linked to inherited arrhythmias. A: membrane topology of the four domains (I–IV) of SCN5A is illustrated with mutations linked to LQT3 and Brugada syndromes and to conduction disorders. Schematics illustrating the transmembrane topologies of KCNH2 (B) and KCNQ1 (C) subunits with the mutations linked to LQT2 and LQT1, respectively, depicted by the open and closed circles.

In contrast to skeletal muscle, it has long been recognized that Ca²⁺ entry from the extracellular space is required for excitation-contraction coupling in the mammalian myocardium (56, 57, 154, 173). The pathway for plasmalemmal Ca²⁺ entry was first revealed in voltage-clamp recordings from multicellular (frog) atrial preparations and was termed the "slow inward" current pathway (416–418, 429). Subsequent studies revealed that this "slow inward" current is carried by Ca²⁺ through a membrane conductance distinct from the voltage-dependent (Nav channel) pathway for Na⁺ movement (45, 332, 386, 416–418, 429). Further studies detailed the time- and voltage-dependent properties of voltage-gated cardiac Ca²⁺ (Cav) currents, first, in multicellular preparations and later, in isolated single cardiac cells (45, 57, 172, 416–418).

Although the presence of two functionally distinct types of Cav currents in single (starfish egg) cells was first reported in 1975 (201), it was not until the late 1980s that the import and the generality of these observations became clear. Two types of Cav currents/channels, for example, were clearly distinguished in (chick and rat) sensory neurons, based primarily on differences in the thresholds for channel activation (85, 86). These channels were termed high voltage-activated (HVA) and low voltage-activated (LVA) Cav channels. Cardiac HVA and LVA Cav channels were first described in isolated canine atrial cells (44). LVA Cav channels, also referred to as T-type Ca²⁺ channels (394), activate at relatively hyperpolarized membrane potentials, i.e., approximately –50 mV, and these channels activate and inactivate rapidly (85, 86, 382). HVA Cav channels, in contrast, open on depolarization to membrane potentials positive to approximately –20 mV, and these channels inactivate over a time course of several tens of milliseconds to seconds, depending on the preparation and the charge carrying ion (85, 326). Under physiological conditions, with Ca²⁺ as the charge carrier, HVA channels in most cells inactivate in <100 ms at depolarized voltages (44, 326).

The detailed kinetic, pharmacological, and voltage-dependent properties of HVA Cav channels in different cell types are distinct, suggesting that HVA Cav channels are heterogeneous, particularly compared with LVA Cav channels. Consistent with this view, multiple types of HVA channels have now been identified in different cell types, and these are referred to as L, N, P, Q, or R channels (276, 382, 394). Although all HVA Cav channels exhibit relatively large single-channel conductances (13–25 pS) and have similar permeation properties, the detailed biophysical properties and the pharmacological sensitivities of the various types of HVA Cav channels are distinct. In the mammalian heart, L-type HVA Cav currents appear to be ubiquitously expressed (44, 49, 326). In addition, the properties and the densities of L-type Cav channel currents in cells isolated from different regions of the myocardium, as well as in cardiac cells from different species, are quite similar, suggesting that the molecular compositions of the underlying channels and the molecular mechanisms controlling the functional expression of these channels are the same. Importantly, however, the time- and voltage-dependent properties of cardiac L-type HVA currents are distinct from the L-type HVA Cav currents expressed in skeletal muscle and in neurons (44, 326, 340), suggesting that, similar to the Nav channels, distinct molecular entities underlie the L-type HVA Cav channels in different tissues (see sect. VC).

The opening of cardiac L-type Cav channels in response to membrane depolarization is delayed relative to the Nav channels (Fig. 2), and these channels, therefore, contribute little to phase 0 depolarization in Purkinje, atrial and ventricular cells. Rather, the opening of HVA L-type Cav channels and the Ca²⁺ entry through these channels underlies the action potential plateau (phase 2), which is particularly prominent in ventricular and Purkinje cells (Fig. 2). In addition, Ca²⁺ influx through the L-type HVA Cav channels triggers Ca²⁺release from intracellular Ca²⁺stores and underlies excitation-contraction coupling in the working (ventricular) myocardium (56, 57, 154, 173). L-type HVA Cav channels are also expressed in SAN and AVN cells, where they play a role in action potential generation, as well as in regulating automaticity (49, 72, 262, 340, 361, 579). Cardiac L-type HVA Cav channels undergo rapid voltage- and Ca²⁺-dependent inactivation (166, 281, 326), processes that will also influence action potential waveforms (Fig. 2) by affecting the duration of the plateau (phase 2) and the time course of action potential repolarization.

In addition to the ubiquitously expressed HVA L-type cardiac Cav currents, LVA or T-type Cav channel currents have also been identified in voltage-clamp recordings from adult atrial myocytes and conducting tissues in several different species (44, 200, 340, 394). Although not evident in normal adult ventricular myocytes (394), T type Cav currents have also been recorded in neonatal rat and rabbit ventricular myocytes (547, 548). In addition, it has been demonstrated that T-type Cav currents are expressed in ventricular myocytes in several animal models of ventricular hypertrophy (328, 383), findings consistent with the view that substantial remodeling occurs in the hypertrophied myocardium, reflecting a reversion to a fetal/neonatal pattern of gene expression (486). It is certainly possible that similar remodeling occurs in the hypertrophied human heart (49). Nevertheless, it is important to note that, to date, T-type LVA Cav channels have not been detected in normal or diseased human myocardial cells (49, 394).

In addition to marked differences in biophysical properties, the physiological role(s) of cardiac T-type LVA Cav channels appears to be quite different from the L-type HVA Cav channels. As noted previously, for example, Ca²⁺ entry through L-type channels in cardiac cells results in Ca²⁺-induced Ca²⁺ release from intracellular (Ca²⁺) stores and is the main trigger for excitation-contraction coupling (57, 154). Although Ca²⁺ entry through T-type channels also triggers Ca²⁺ release from intracellular stores, the coupling is less efficient (589), and it seems unlikely that T channels contribute importantly to excitation-contraction coupling. This may simply reflect the fact that LVA channel densities are low and that, owing to rapid inactivation, very little Ca²⁺ actually enters cells on depolarization. Alternatively, these functional differences may reflect the fact that HVA and LVA cardiac Cav channels are differentially localized, i.e., L-type, but not T-type, Cav channels are highly localized in the t tubules near the storage sites for intracellular Ca²⁺ sequestration/release (394, 589). Further experiments will be necessary to determine the mechanistic basis for the distinct functional roles of L- and T-type Ca²⁺ channels in regulating excitation-contraction coupling.

The finding that T-type LVA Cav currents activate at relatively hyperpolarized potentials and that these channels are expressed preferentially in pacemaker and conducting cells in the heart suggests the interesting possibility that there is a role for LVA channels in pacemaking (200). Although some experimental support for this hypothesis has been provided, rigorous testing is complicated by the paucity of highly selective LVA Cav channel blockers (394). In addition, it has been reported that (rabbit) SAN cells express rapidly activating Na⁺-dependent inward currents that are blocked by Ca²⁺ channel blockers (582). These observations suggest that pacemaker cells may express additional novel inward currents that contribute to shaping action potential waveforms and to regulating normal cardiac rhythms. Additional studies, focused on further characterization of the properties of LVA Cav channels in SAN and AVN cells and determination of the functional roles of these channels in regulating pacemaking, will be needed to explore these possibilities directly.

	III. MYOCARDIAL ACTION POTENTIALS AND REPOLARIZING VOLTAGE-GATED POTASSIUM CURRENTS

Top Previous Next References

 
Voltage-gated K⁺ (Kv) channels are the primary determinants of action potential repolarization in the mammalian myocardium, and compared with cardiac Nav and Cav channels, there is considerable electrophysiological and functional cardiac Kv channel diversity (42, 373, 375). Based primarily on differences in time- and voltage-dependent properties and pharmacological sensitivities (42, 373, 375), two broad classes of repolarizing cardiac Kv currents have been distinguished (42): transient outward K⁺ currents (I_to) and delayed, outwardly rectifying K⁺ currents (I_K) (Table 1). The transient currents (I_to) activate and inactivate rapidly on membrane depolarizations to potentials positive to approximately –30 mV and underlie the early phase (phase 1) of repolarization in ventricular and atrial cells (Fig. 2). Cardiac delayed rectifiers (I_K) activate at similar membrane potentials and with variable kinetics, and these currents determine the latter phase (phase 3) of repolarization back to the diastolic potential (Fig. 2). Multiple types of myocardial I_to and I_K channels with distinct time- and voltage-dependent properties (Table 1), however, have been identified, and differences in the densities and the biophysical properties of these channels contribute to variations in the waveforms of action potentials recorded in different cardiac cell types (Fig. 1), as well as in different species (24, 374, 375, 436). The detailed pharmacological, time- and voltage-dependent properties of each of the various repolarizing Kv currents characterized in different cardiac cell types and species are quite similar, thereby allowing Kv channels to be classified based on these biophysical properties (Table 1). The observed similarities in Kv channel properties also suggest that the molecular correlates of the underlying Kv channels are also similar (42, 373), and considerable experimental evidence has now been provided to support this hypothesis (see sect. VI).

A. Transient Outward Kv Currents

Although cardiac transient outward currents were first described in (sheep) Purkinje fibers and thought to reflect Cl^– conductances (143, 175), subsequent work demonstrated the presence of two transient outward currents with distinct properties and referred to as I_to1 and I_to2 (112). Pharmacological studies revealed that I_to1 is blocked by 4-aminopyridine (4-AP) and unaffected by changes in extracellular Ca²⁺, whereas I_to2 is not blocked by 4-AP and is Ca²⁺ dependent (257, 256). In further studies, it was shown that the Ca²⁺-dependent I_to2 in Purkinje fibers and ventricular cells is a Cl^– (not a K⁺) current (593, 594). In contrast, the Ca²⁺-independent component, I_to1, was shown to be K⁺ selective (594), and transient outward K⁺ currents, referred to variably by different laboratories as I_to, I_to1, or I_t (42, 83, 436), have now been described in many cardiac cell types and in most species. Comparison of the detailed biophysical properties of the transient outward K⁺ currents described in various cell types/species, however, suggested there might actually be two types of transient outward K⁺ currents (42), and electrophysiological and pharmacological studies have now provided considerable support for this hypothesis. In adult mouse ventricular myocytes, for example, two transient K⁺ currents, termed I_to,fast (I_to,f) and I_to,slow (I_to,s), have been distinguished (562). On membrane depolarization, mouse ventricular I_to,f channels activate and inactivate rapidly, and on membrane repolarization, these (I_to,f) channels recover rapidly from steady-state inactivation (562). In the adult mouse, I_to,f channels contribute importantly to the rapid repolarization of action potentials (194, 562) that is likely necessary to maintain the very high resting heart rates (700 beats/min) in these animals. In humans and other larger mammals, I_to,f underlies the early phase (phase 1) of repolarization in ventricular and atrial cells (Fig. 2) and likely also contributes to determining the plateau (phase 2).

Similar to I_to,f, mouse ventricular I_to,s channels activate and inactivate rapidly (562). In contrast to I_to,f, however, I_to,s channels recover very slowly (time constants of seconds) from (steady-state) inactivation and are functionally distinct from I_to,f channels (196, 562). In addition, I_to,f is readily distinguished from other Kv currents, including I_to,s,(562), using the spider K⁺ channel toxins Heteropoda toxin-2 or -3 (449). The distinct properties of I_to,f and I_to,s suggested that these currents reflect the functioning of two molecularly distinct Kv channels, and considerable evidence has now been provided to support this hypothesis (see sect. VIC). Detailed comparisons of the properties of the transient outward K⁺ currents expressed in other species, whether termed I_to, I_to1, or I_t, suggest that, in each case, these currents could also be classified as I_to,s or I_to,f, based on the kinetics of current inactivation and recovery from steady-state inactivation, as well as by the differential sensitivities of the channels to the Heteropoda toxins. Although the properties of the transient K⁺ currents in different cell types and species are similar and are amenable to classification as either I_to,f or I_to,s (Table 1), there are differences in the detailed biophysical properties of the (I_to,f and I_to,s) currents in different cells/species (15). These observations suggest that there may well be subtle, albeit potentially important, molecular heterogeneity among I_to,f and I_to,s channels in different cell types and/or in different species (see sect. VIC).

Although originally identified in Purkinje fibers, I_to,f is a prominent repolarizing current in atrial and ventricular myocytes in most species (26, 58, 65, 69, 75, 79, 160, 178, 264, 294, 297, 500, 530, 545, 546, 578), including humans. Nevertheless, there are exceptions. In guinea pig ventricular cells, for example, I_to,f has not been detected except when extracellular Ca²⁺ is removed (224). In addition, I_to,f is not detected in rabbit atrial or ventricular cells (156, 160, 183, 530). Nevertheless, there are transient Kv currents in rabbit myocytes (typically referred to as I_t), which inactivate slowly and recover from (steady-state) inactivation very slowly (160, 530). The properties of these currents, therefore, more closely resemble mouse ventricular I_to,s than I_to,f (562). Similar to the mouse, however, two distinct transient outward Kv currents have been described in the ventricles of other mammals (77, 178, 294, 500), including humans (225, 264, 545, 546), and the properties of these currents are quite similar to those of mouse ventricular I_to,f and I_to,s (79, 196, 562), permitting their classification as such (Table 1). In ferret, the rates of inactivation and recovery (from steady-state inactivation) of the transient Kv currents in myocytes isolated from the left ventricular (LV) endocardium are significantly slower than the currents in cells from the epicardial surface of the LV, suggesting the presence of two distinct transient Kv currents that are differentially expressed (77). Examination of the reported biophysical properties (77) suggests that the endocardial and epicardial LV currents can be classified as I_to,s and I_to,f, respectively (Table 1). The distinct transient Kv currents in the epicardial, midmyocardial, and endocardial layers of canine (294, 500) and human (264, 545, 546) ventricles can also be appropriately referred to as I_to,f or I_to,s (Table 1).

Transient Kv currents that can be classified as I_to,f (Table 1) have also been shown to be expressed in (rabbit) SAN cells, although, similar to Nav currents, I_to,f densities vary markedly among (SAN) cells (213, 283). I_to,f densities are higher, for example, in the larger cells isolated from the periphery, compared with the smaller cells in the center, of the SAN (213, 283). In addition, when expressed, I_to,f appears to play a role in shaping action potential waveforms and in regulating automaticity in SAN cells (72, 213, 283). Cells isolated from the (rabbit) AVN also express I_to,f (349, 361, 371), and detailed kinetic analysis of the currents reveals the presence of two components with distinct rates of inactivation and recovery (349). It is unclear whether these findings reflect differences in the kinetic properties of a single type of I_to,f channel or if two distinct types of I_to channels are expressed in (rabbit) AVN cells. Similar to the (rabbit) SAN, there is considerable heterogeneity in I_to densities among (rabbit) AVN cells (349). In contrast to SAN cells (213, 283), however, the differences in I_to,f densities are not correlated with cell size in AVN cells (349). Interestingly, and similar to findings in guinea pig atrial and ventricular cells, I_to,f is not detected in guinea pig AVN cells (579). It is presently unclear, however, whether currents with properties similar to ventricular I_to,s are expressed in conducting tissues in guinea pig heart. Owing to the marked differences in inactivation and recovery kinetics of I_to,f and I_to,s channels, however, the differential expression of these two channel types would be expected to have profound functional effects on the regulation of rhythmicity in the normal heart, effects that will be augmented in the diseased myocardium.

B. Delayed Rectifier Kv Currents

Myocardial delayed rectifier Kv currents, I_K, also first described in (sheep) Purkinje fibers (379), have been characterized in atrial and ventricular myocytes, as well as in pacemaker cells, isolated from a variety of different species, and, in most cases, multiple components of I_K are coexpressed (Table 1). In guinea pig ventricular and atrial myocytes, for example, two prominent components of I_K, I_Kr (I_K,rapid) and I_Ks (I_K,slow), were first distinguished, based on marked differences in time- and voltage-dependent properties (216, 445, 446). Both I_Kr and I_Ks are also coexpressed in guinea pig AVN cells (579). Although I_Kr activates rapidly, inactivates very rapidly, and displays marked inward rectification, no inward rectification is evident for the slowly activating I_Ks (445, 446). These channels can also be distinguished at the microscopic level, as well as by their unique pharmacological profiles (37, 524). Similar to guinea pig, I_Kr and I_Ks are also reportedly coexpressed in human atrial and ventricular myocytes (290, 514, 532, 533), as well as in canine (296, 297, 513, 522, 578) and rabbit (440, 518) ventricles and in canine Purkinje fibers (513) and, in each case, are prominent repolarizing currents. In adult rodent ventricles, in contrast, I_Kr and I_Ks densities are very low (104) or the currents are undetectable (562).

The unique time- and voltage-dependent properties of I_Kr and I_Ks suggest that these currents play prominent roles in action potential repolarization, particularly in ventricular myocytes and Purkinje fibers. Nevertheless, in some cardiac cells, only I_Kr or I_Ks appears to be expressed. In isolated human (225), feline (171), and rat (404) ventricular myocytes and in rat atrial (404), mouse SAN (108), rabbit AVN and SAN (218, 233, 467) cells, for example, only I_Kr is detected. It has also been reported, however, that both I_Kr and I_Ks are coexpressed in rabbit SAN cells (284). In this study, the measured densities of I_Kr and I_Ks (in rabbit SAN cells) were quite variable, although, in general, I_Kr and I_Ks densities are highest in the larger cells found at the periphery of the node (284). It may be that the densities of I_Kr and/or I_Ks in some cells are too low to be resolved reliably or, alternatively, that the properties of I_Ks and I_Kr in each of these cell types are distinct from guinea pig ventricular and atrial I_Ks and I_Kr. The apparent absence of I_Kr and I_Ks in some cells, as well as the observation that I_Kr and I_Ks expression is heterogeneous and variable, might also reflect the fact that functional cardiac Kv channel expression is labile and might well be affected by the isolation methods, which typically involve the use of enzymes (578). Detailed studies focused on current characterizations in intact preparations, as well as on examining the effects of specific enzymes and cell isolation methods on Kv current densities and properties, will be needed to explore these various possibilities further.

Although I_Kr and I_Ks are not prominent repolarizing Kv currents in rodent atria or ventricles, there are other components of delayed rectification with time- and voltage-dependent properties distinct from I_Ksand I_Kr(Table 1) in myocytes from these (and other) species. In rat ventricular myocytes, for example, there are multiple delayed rectifier Kv currents that are coexpressed, and these are referred to as I_K, I_Klate, and I_ss (26, 210, 561). In adult mouse ventricular myocytes, three distinct delayed rectifier Kv currents have also been separated and characterized (168, 196, 263, 291, 303, 560, 562, 586, 587), and these are referred to as I_K,slow1, I_K,slow2, and I_ss (Table 1). Multiple components of delayed rectification have also been described in rodent atrial myocytes (68, 69, 74, 75, 497). In both rat and mouse, it has been demonstrated that all the various delayed rectifier Kv current components contribute, together with I_to,f channels, to (ventricular and atrial) action potential repolarization (291, 303, 560, 586, 587). It is interesting to note that a steady-state, noninactivating K⁺ current, which resembles I_ss in rodent atria and ventricles, has also been described in human atrial myocytes (58).

In rat (74, 75), canine (577), and human (531–533) atrial myocytes, a novel, very rapidly activating, and largely noninactivating, outward Kv current, now typically referred to as I_Kultrarapid or I_Kur (479), has been described (Table 1). In most species, I_Kur is not detected in ventricular cells, and it seems likely that the expression and the properties of I_Kur, together with I_to,f, contribute to determining the more rapid repolarization evident in atrial, compared with ventricular, myocytes (Fig. 2). Importantly, as in most other species, I_Kur is not expressed in human ventricular myocytes or in Purkinje fibers, suggesting that I_Kur channels might represent a therapeutic target for the treatment of atrial arrhythmias without complicating effects on impulse propagation, ventricular functioning, or cardiac output (444). The potential of this pharmacological strategy, however, will have to be determined by the atrial specificity/selectivity of the drugs that are developed. In contrast to rat, canine, and human, Kv currents have been described in guinea pig (576) and mouse (168, 291, 587) ventricular myocytes that have biophysical properties very similar to human (rat or canine) atrial I_Kur. Indeed, the properties of the rapidly activating, I_Kur-like, current in guinea pig ventricular myocytes, referred to as I_Kp (576), and the micromolar 4-AP-sensitive component of mouse ventricular I_K,slow, referred to as I_K,slow1 (69, 291, 302, 587), are indistinguishable from human (canine and rat) atrial I_Kur. These currents should, therefore, probably be renamed I_Kur (Table 1) to reflect the similarities in properties, as well as molecular identities of the channels underlying I_Kp and I_K,slow1 and I_Kur (see sect. VID).

C. Regional Differences in Kv Current Expression and Properties

Although the properties of I_to,f in different cardiac cells are similar (Table 1), there are marked regional differences in current densities. In humans (160, 367, 527, 542, 543) and in rats (26, 74, 75), for example, I_to,f densities are significantly higher in atrial, compared with ventricular, myocytes. Similarly, in the rabbit, I_to,s densities are higher in atrial myocytes and Purkinje fibers than in ventricular cells (75, 514). In the mouse, however, I_to,fdensity is significantly higher in ventricular (79, 562), than in atrial (69), myocytes. The density of I_to,f is also quite variable in sheep Purkinje fibers (520) and in different regions of the ventricles in canine (294, 297, 522), cat (178), ferret (77), human (367, 545, 546), mouse (79, 196, 562), and rat (107, 545) hearts. In canine (522) and mouse (79, 196, 562) heart, for example, I_to,f density is higher in the right ventricle (RV), compared with the left ventricle (LV), and I_to,f densities are lower in the base of the LV than in the LV apex (79). In addition, in canine and in human heart, I_to,f density varies throughout the thickness of the ventricular walls, being severalfold higher in the epicardial and midmyocardial, than in the endocardial, layers (297, 546). In large mammals, the regional and cellular heterogeneities in I_to,f densities are directly reflected in the differences in action potential waveforms in Purkinje, ventricular, and atrial cells (75, 107, 366, 520, 551). Within the ventricles, for example, the differences in I_to,f densities are revealed by the presence and the appearance and depth of the "notch" in the initial phase (phase 1) of action potential repolarization (24, 364, 374; see Fig. 2).

There are also marked regional differences in the expression/distribution of I_to,s in adult rat, mouse, human, and canine ventricles (77, 79, 196, 366, 367, 551, 562). In mouse RV and LV, for example, I_to,s is undetectable, whereas cells in the interventricular septum express only I_to,s or both I_to,f and I_to,s (79, 196, 562). Even when expressed, however, I_to,f density is significantly lower in septum, compared with ventricular (or atrial), cells (79, 562). The densities of the delayed rectifier Kv currents, I_K,slow1, I_K,slow2, and I_ss, in contrast, are similar throughout adult mouse ventricles (79, 196, 562). The main determinant of action potential heterogeneity in the mouse, therefore, appears to be the differential expression of I_to,f (79, 196).

In larger mammals, including humans, the differential expression of I_to,f is also a primary determinant of action potential heterogeneity (24, 374). In human heart, however, it is clear that differences in the expression levels of the various delayed rectifier Kv currents, as well as the persistent component of the Nav current (see sect. IIA), also play important roles in regulating action potential heterogeneity (24, 374). In canine heart, for example, I_Ks density is higher in cells in the RV, compared with the LV, whereas I_Kr densities are similar in both chambers (24, 522). I_Ks density is also higher in canine LV epicardial and endocardial cells than in M cells (24, 296). In guinea pig heart, I_Kr and I_Ks densities are approximately twofold higher in atrial, than in ventricular, myocytes (37, 216, 442, 524). There are also regional differences in functional I_Kr and I_Ks expression within the ventricles (80, 316). In cells isolated from the (guinea pig) LV free wall, for example, I_Kr density is higher in subepicardial, than in either midmyocardial or subendocardial, myocytes (316). At the base of the LV, however, the densities of both I_Kr and I_Ks are significantly lower in endocardial, than in either epicardial or midmyocardial, cells (80). These differences clearly contribute to the marked differences in action potential waveforms and frequency-dependent properties in cells through the thickness of the ventricular wall (24). In addition to having a major impact on action potential repolarization, it is now very clear that differences in functional I_Kr and I_Ks densities are also expected to influence the maintenance of normal cardiac rhythms and the susceptibility to rhythm disturbances (24).

	IV. OTHER MYOCARDIAL POTASSIUM CURRENTS CONTRIBUTING TO REPOLARIZATION

Top Previous Next References

 
In addition to the depolarization-activated Kv currents, non-voltage-gated inwardly rectifying K⁺(Kir) currents, through I_K1 channels, also contribute to myocardial action potential repolarization, particularly in ventricular cells (232, 306, 377, 380). There are also other types of Kir channels that are expressed and are important in the normal functioning of the heart, although these do not seem to play important roles in action potential repolarization under normal physiological conditions (306, 377). One example of a functionally important class of myocardial Kir channels is the I_KATP channels, which are inhibited by intracellular ATP, activated by nucleotide diphosphates, and thought to provide a link between cellular metabolism and membrane potential (232, 380). In the ventricular myocardium, the opening of I_KATP channels is thought to be important under conditions of metabolic stress, as occurs during ischemia or hypoxia, and to result in shortening action potential durations and minimizing K⁺ efflux (165, 232). The opening of I_KATP channels has also been suggested to contribute to the cardioprotection resulting from ischemic preconditioning (141, 188). Although I_KATP channels appear to be distributed uniformly in the RV and LV and through the thickness of the ventricular wall, these channels are expressed at much higher density than other sarcolemmal K⁺ channels, suggesting that action potentials could be shortened markedly when only very small numbers of I_KATP channels are activated (463).

Another important cardiac Kir channel type is the I_K(ACh) channels, which are gated through a G protein-coupled mechanism mediated by muscarinic acetylcholine receptor activation (275, 564). Physiologically, I_K(ACh) channels are activated by the binding of G protein -subunits in response to the acetylcholine released on vagal stimulation (414). Although I_K(ACh) channels are expressed in AVN, SAN, atrial, and Purkinje cells, and are activated by acetylcholine released on vagal stimulation, these channels are not thought to contribute appreciably to action potential repolarization under normal physiological conditions. Consistent with this hypothesis, targeted deletion of one of the Kir subunits (Table 6) encoding I_K(ACh) channels, Kir3.4, does not measurably affect resting heart rates (552). Interestingly, however, atrial fibrillation is not evident in Kir3.4 null mice exposed to the acetylcholine receptor agonist carbachol, suggesting that activation of I_K(ACh) channels is involved in the cholinergic induction of atrial fibrillation (269).

View this table: [in this window] [in a new window]   TABLE 6. Diversity of inwardly rectifying K+ (Kir) and two-pore domain K+ (K2P) channel -subunits

The expression of I_K1 is clearly reflected in the negative slope region (between approximately –50 and –10 mV) of the (total steady-state) myocyte conductance-voltage relation, which is prominent in ventricular myocytes, but is small or undetectable in atrial cells (183). The fact that the strongly inwardly rectifying I_K1channels conduct at negative membrane potentials suggests that these channels will play a role in establishing the resting membrane potentials of Purkinje fibers, as well as of atrial and ventricular myocytes. Direct experimental support for this hypothesis was provided with the demonstration that ventricular membrane potentials are depolarized in the presence of Ba²⁺ (377), which blocks I_K1 channels. In addition, action potentials are prolonged, and phase 3 repolarization is slowed in the presence of extracellular Ba²⁺(306), suggesting that I_K1 channels also contribute to repolarization, particularly in the ventricular myocardium. The voltage-dependent properties of I_K1 channels (306, 377), however, are such that the conductance is low at potentials positive to approximately –40 mV. Nevertheless, because the driving force on K⁺ is markedly increased at depolarized potentials, these channels should contribute outward K⁺current during the phase 2 plateau and during phase 3 repolarization (Fig. 2). In contrast to atrial, ventricular, and Purkinje cells, I_K1 density is low or undetectable in SAN and AVN cells (244, 381, 468). These observations, as well as the fact that pacemaker currents are expressed and functional in SAN and AVN cells, likely explain the findings that resting membrane potentials in these (SAN/AVN) cells are depolarized (significantly) and that the rising phases of the action potentials in these cells are less steep, relative to resting membrane potentials/action potentials in atrial and ventricular cells (Fig. 1).

Similar to the Kv channels, I_K1 densities and the detailed biophysical properties of the currents do vary in different myocardial cell types. In human heart, for example, I_K1 density is more than twofold higher in ventricular, than in atrial, cells (514). In guinea pig, the properties of the atrial and ventricular I_K1 currents are also distinct in that ventricular I_K1 inactivates during maintained depolarizations, whereas atrial I_K1 does not (133, 223). In addition, changes in extracellular K⁺ modulate the magnitude of ventricular I_K1, but have little effect on atrial I_K1 (223). At the microscopic level, (guinea pig) atrial and ventricular I_K1 channels are also distinct. Mean channel open times of ventricular I_K1 channels, for example, are approximately five times longer than those of atrial I_K1 channels, whereas the single atrial and ventricular I_K1 channel conductances are indistinguishable (223). Taken together, these observations suggested the interesting possibility that distinct molecular entities underlie ventricular and atrial I_K1 channels, and experimental support for this hypothesis has now been provided (133).

	V. MOLECULAR COMPONENTS OF MYOCARDIAL NAV AND CAV CHANNELS

Top Previous Next References

 
A. Nav Channel Pore-Forming -Subunits

Voltage-gated Na⁺ (Nav) channel pore-forming () subunits (Fig. 3A) belong to the "S4" superfamily of voltage-gated ion channel genes (93, 172, 174, 573). Nav -subunits have four homologous domains (I to IV), each of which contains six transmembrane-spanning regions (S1-S6), and these four domains come together to form the Na⁺-selective pore. Structure-function studies have revealed many of the important features of voltage-dependent Nav channel gating (91, 93). The cytoplasmic linker between domains III and IV, for example, has been shown to play a pivotal role in voltage-dependent Nav channel inactivation (392), and a critical isoleucine, phenylalanine, methionine (IFM) motif within this linker (91, 444) has been identified as an important molecular component of the inactivation gate (516, 517, 544). Voltage-dependent inactivation of Nav channels is attributed to the rapid block of the inner mouth of the channel pore by the cytoplasmic linker between domains III and IV that occurs within milliseconds of membrane depolarization (483). Consistent with the functional electrophysiological data, solution NMR analysis of this cytoplasmic linker peptide revealed a rigid helical structure positioned to block the pore (427).

Although there are a number of homologous Nav -subunits (Table 2), Nav1.5 (SCN5A) is the prominent Nav -subunit expressed in the mammalian myocardium, and this subunit encodes the rapidly activating and inactivating, tetrodotoxin (TTX)-insensitive Nav channels that underlie rapid (phase 0) depolarization in atrial and ventricular myocytes and in Purkinje fibers (Fig. 1). Nevertheless, several studies have demonstrated that mRNAs encoding other Nav -subunits, notably Nav1.1, Nav1.3 (120, 426, 471), and Nav1.4 (590), which are typically considered the Nav -subunits encoding brain and skeletal muscle Nav channels, respectively, are also expressed in the myocardium. In contrast to the Nav channels formed by Nav1.5, however, Nav1.1-, Nav1.3- and Nav1.4-encoded Nav channels are blocked by nanomolar concentrations of TTX (120, 426, 471, 590). In addition, although cardiac Nav currents are generally considered relatively TTX insensitive (174, 573), application of nanomolar concentrations of TTX has been reported to shorten canine Purkinje fiber action potential durations (113). These findings suggest a possible role for TTX-sensitive Nav channels in the generation of the persistent component of cardiac Nav currents, at least in canine Purkinje fibers. Nevertheless, there have been very few reports documenting the presence of TTX-sensitive inward Nav current components in cardiac cells, raising some concern about the functional significance of the expression data, in spite of the fact that the (message) expression levels of Nav1.1, Nav1.3, and Nav1.4 in the myocardium appear to be quite high (120, 426, 471, 590).

It has also been reported that there are several Nav1 -subunit proteins in addition to Nav1.5 in adult (mouse) myocardium (314). These include Nav1.1, Nav1.3, and Nav1.6 (314). The immunolocalization data also suggest that the Nav1.1, Nav1.3, and Nav1.6 -subunits are localized in the t tubules in adult mouse ventricles (314), whereas Nav1.5 appears to be localized preferentially to intercalated disks in mouse, as well as in rabbit and rat, hearts (270, 315, 400). The subcellular localization of Nav1.5-encoded myocardial Nav channels at the intercalated disks has been interpreted as suggesting that these (Nav) channels play a major role in regulating conduction (270). Although the functional role(s) of t-tubular Nav channels in cardiac functioning has not been established, voltage-clamp studies have clearly demonstrated that TTX-sensitive Nav currents can be measured in whole cell recordings from adult mouse ventricular myocytes treated with -scorpion toxin, which shifts the voltage dependence of activation of brain Nav channels, but does not affect cardiac (i.e., SCN5A-encoded) Nav channels (314). These observations have been interpreted as suggesting a distinct role for the neuronal Nav channels localized to the t tubules, i.e., linking depolarization of the sarcolemmal membrane with the t tubules, thereby coupling depolarization with excitation-contraction coupling (314). This hypothesis has important functional implications and certainly warrants further direct experimental testing.

Immunohistochemical studies have also provided evidence suggesting that Nav1.1 and Nav1.3, but not Nav1.5, are expressed in rat and mouse SAN (314). These findings suggest a substantive molecular difference between the SAN and the remainder of the myocardium in terms of Nav channel expression. Given the primary role of the SAN in regulating heart rate, it would seem certain that modulating the (TTX-sensitive) Na⁺ current in the SAN should impact heart rate. Nevertheless, exposure to TTX reportedly has no effect on heart rate in the mouse (314). In mice in which one copy of the SCN5A gene has been disrupted, however, conduction defects, as well as ventricular dysfunction, are evident (389), suggesting that SCN5A encodes most, if not all, of the cardiac Nav current. It seems reasonable to suggest, therefore, that additional studies focused on exploring the functioning of Nav1.1-, Nav1.3- and Nav1.6-encoded channels in the heart are warranted.

During the plateau phase of the action potential in human ventricular myocytes, 99% of the Nav channels are in an inactivated, nonconducting state with the inactivation gate occluding the inner mouth of the conducting pore through specific interactions with sites on either the S6 segment (345) or the S4-S5 loop (346) of domain IV. Mutations in the linker between domains III and IV in SCN5A, linked to the LQT3 syndrome (Fig. 3A), disrupt Nav channel inactivation (52, 346). These "gain of function" mutations lead to an increase in the amplitude of the sustained component of the Na⁺ current (Fig. 4B) and to action potential prolongation (Fig. 4A). The enhanced inward current can be measured during sustained depolarizations and appears to reflect a change in (Nav channel) gating that results in channel "bursting" (52). As illustrated in Figure 4A, the increase in late inward Na⁺ current (due to Nav channel bursting) prolongs "modeled" cardiac action potentials (103, 105). Interestingly, action potential prolongation is also evident in genetically modified mice expressing human LQT3-associated mutant SCN5A Nav channels (384). In mice heterozygous for an SCN5A deletion at residues (KPQ) 1505–1507, a modification linked to LQT3, premature ventricular beats and pacing-induced ventricular tachycardia are also evident (384). Mutations in SCN5A are also linked to another (rare) inherited rhythm disorder, the Brugada syndrome (23, 41) and, similar to LQT3 mutations, a number of Brugada mutations in SCN5A have been identified (Fig. 4A). In contrast to long QT3, however, Brugada syndrome mutations are "loss of function" mutations in Nav1.5 and result in reduced Nav current (Fig. 4D) and lead to slowing of the action potential upstroke (Fig. 4C). Reductions in Nav current can also influence action potential amplitudes (Fig. 4C), as well as phase 1 repolarization. In addition, owing to intrinsic electrical heterogeneity of the heart (Fig. 1), the impact of Brugada and LQT3 mutations in SCN5A would be expected to be variable in different regions/cell types, an effect which may contribute further to arrhythmogenesis.

View larger version (15K): [in this window] [in a new window]   FIG. 4. Simulated human ventricular action potentials reveal the impact of gain of function (LQT3) and loss of function (Brugada) mutations in SCN5A. Steady-state action potential waveforms (A and C) and inward Nav1.5 currents (B and D) were simulated (103, 105). Control action potential and current waveforms simulated for wild-type SCN5A-encoded Nav1.5 currents are depicted by the solid black lines in A–D. The corresponding voltage and current waveforms resulting from LQT3 and Brugada mutations in SCN5A (Fig. 3A) are represented by the dashed purple (LQT3) and red (Brugada) lines in A–D. "Gain of function" LQT3 mutations in SCN5A, resulting in an increase in the persistent component of the Nav current (B), lead to marked action potential prolongation (A). "Loss of function" Brugada mutations in SCN5A, in contrast, result in reduced Nav current (D) and changes in the action potential upstroke velocity (C). The impact of the loss of function Brugada mutations are more readily illustrated in the insets of C and D, in which the voltage (C) and current (D) deflections are displayed on an expanded time scale.

Analyses of additional mutations in SCN5A linked to LQT3, Brugada, and conduction system defects have provided further molecular insights into Nav channel functioning and arrhythmia mechanisms. One of the well-described LQT3 mutations, I1768V, for example, does not result in increased channel bursting, but rather, accelerates the rate of recovery of Nav channels from inactivation at diastolic membrane potentials (106). Computational analysis predicted that this mutation would have a substantial effect under nonequilibrium conditions, e.g., during action potential repolarization (106). Subsequent experiments confirmed this prediction, revealing a novel mechanism by which mutation-altered Nav channel gating can prolong cardiac action potentials (7). Interestingly, it has also been demonstrated that a common polymorphism (that results in an S/Y switch) at residue 1102 in SCN5A is associated with elevated arrhythmia risk in African Americans (481). Expression studies revealed that this variant results in very subtle changes in Nav1.5 channel activation and inactivation. Modeling studies suggest, however, that these changes are not likely to alter cellular electrical activity in carriers unless they are treated with drugs that block (cardiac) Kv channels (481). Additional polymorphisms in SCN5A have also been identified that affect, at least in heterologous expression systems, the trafficking of functional cell surface Nav channels (318, 570). In principle, the presence of these polymorphisms could, like the S1102Y polymorphism (481), impact arrhythmia susceptibility in the context of other factors (e.g., disease or drugs) that affect membrane excitability, action potential durations, and rhythmicity (318). Mutations in SCN5A and changes in Nav1.5-channel gating have also been linked to sudden infant death syndrome (538). In addition, mutations in two of the neuronal Nav channel -subunits, SCN1A and SCN2A, are associated with epilepsies (204, 220, 249, 359). It will be interesting to determine if the molecular mechanisms linking these mutations, as well as the mutations in the skeletal muscle Nav channel SCN4A, to disorders in membrane excitability are similar to those evident for SCN5A in the LQT3 and Brugada syndromes.

B. Nav Channel Accessory Subunits and Other Interacting Proteins

Although molecular and functional studies of cardiac Nav channels have focused primarily on the pore-forming Nav1.5 -subunit, it is now quite clear that functional Nav channels in cardiac (and other) cells reflect the assembly of multimeric protein complexes comprising accessory subunits, as well as a variety of other auxiliary, interacting and regulatory proteins. All available evidence suggests, for example, that functional Nav channels in cardiac (and other) cells are multisubunit proteins consisting of a central pore-forming Nav -subunit (Fig. 5) and one to two auxiliary Nav -subunits (229). In brain, the functional stoichiometry appears to be one Nav - to two Nav -subunits (229); the /-subunit composition of cardiac Nav changes is probably similar. Three different Nav -subunit genes, SCN1b (230, 320), SCN2b (231, 241), and SCN3b (354) encoding Nav1, Nav2 and Nav3 proteins, respectively, have been identified, and it appears that all three Nav -subunits are expressed in heart (Table 2). The functional role(s) of the Nav -subunits in the generation of cardiac Nav currents, however, is not well understood (20). Expression studies suggest that SCN2b plays a role in controlling the Ca²⁺ permeability of Nav channels (450). The targeted deletion of SCN2b (in Nav2 –/– mice) markedly affects neuronal Nav channel expression and properties and has profound neurological consequences (95). No cardiac phenotype, however, has been described in Nav2 –/– mice (95), suggesting that Nav1 or Nav3 more likely contributes to the formation of the SCN5A-encoded cardiac Nav channels. Consistent with this hypothesis, heterologous coexpression of Nav1, which markedly affects Nav1.4-encoded skeletal muscle Nav channels (319), alters the inactivation kinetics and the densities of Nav1.5-encoded (cardiac) Nav channels (20, 134, 155). Heterologous coexpression of SCN3b with SCN5A also reportedly increases the cell surface density and modifies the inactivation kinetics of Nav1.5-encoded currents (155).

View larger version (69K): [in this window] [in a new window]   FIG. 5. Molecular assembly of cardiac Cav (Nav), Kv, and Kir channels. Top: the four domains (I–IV) of individual Cav (and Nav) -subunits contribute to the formation of individual Cav (Nav) channels, whereas four Kv (or Kir) -subunits combine to form tetrameric Kv (or Kir) channels. Bottom: schematic illustrating functional cardiac Nav, Cav, and Kv channels, composed of the pore-forming -subunits and a variety of channel accessory subunits.

View larger version (57K): [in this window] [in a new window]   FIG. 6. Schematic illustrating the complexity of protein-protein interactions that likely are involved in regulating/modulating the expression, distribution, and functioning of myocardial ion channels. The - and -subunits of Nav channels interact with the actin cytoskeleton through syntrophin-dystrophin and ankyrin B and with the extracellular matrix through the sarcoglycan complex. Interactions between Kv channel (and/or ) subunits and the actin cytoskeleton are mediated by the actin binding proteins filamin and -actinin and through PDZ domain-containing scaffolding proteins.

An important functional role for the actin cytoskeleton in the regulation of Nav channel gating was directly revealed with the demonstration that mice heterozygous for a targeted deletion of ankyrin B, ankyrin B +/–, have abnormal cardiac electrical activity attributed to altered Nav channel gating and cell surface expression levels (40). Single-channel studies on ankyrin B +/– ventricular cells revealed increased channel bursting, consistent with a LQT channel phenotype (94). These observations were interpreted as suggesting that mutations in ankyrin, which would lead to Nav channel dysfunction, might well be important in familial LQT syndromes or other inherited cardiac rhythm disturbances (50). Consistent with this hypothesis, molecular genetic studies subsequently revealed a loss-of-function mutation in ankyrin B (E1425G) that is causally linked to variant 4 of familial long QT syndrome, LQT4 (351). In addition to providing fundamentally important new insights into the molecular defect underlying LQT4 (351), these findings demonstrate a novel, physiologically important, mechanism for regulating Nav functioning involving interactions between channel proteins and the cytoskeleton that are coordinated by the adaptor protein ankyrin B. It is possible that the ankyrin B mutations interfere directly with Nav1-Nav1 interactions, or perhaps indirectly, through other regulatory and/or signaling molecules. In this context, it is also interesting to note that the cell surface expression of the Na⁺-K⁺-ATPase, the Na⁺/Ca²⁺ exchanger, and inositol 1,4,5-trisphosphate (IP₃) receptors, each of which also interacts with ankyrin B, are all affected in ankyrin B +/– ventricular myocytes (351). It seems certain that the ankyrin B-mediated interactions between each of these molecules and the actin cytoskeleton are important for the normal functioning of each of these ion transport proteins, as well as the intracellular pathways coupled to these proteins. The physiological import of these interactions and the impact of the disruption of each of these interactions on the generation of normal cardiac rhythms is potentially staggering.

Extrapolating this concept further, it is interesting to note that there is now a growing body of evidence in the cardiovascular (and other) system that functional Nav (and other) channels interact directly and/or indirectly with the actin cytoskeleton and with a variety of regulatory and signaling molecules (Fig. 6) which might well play a role in the regulation of channel trafficking and channel expression and/or in the modulation of channel properties and functioning (418). It has, for example, been reported that syntrophins, proteins thought to provide a link between the actin cytoskeleton and other membrane-associated proteins, interact directly with Nav -subunits, including Nav1.5 and the skeletal muscle Nav channel -subunit Nav1.4 (182, 212). Because the syntrophins also interact directly with dystrophin and dystrobrevin (117) and, therefore, with the entire dystrophin-associated complex (151) in cardiac and skeletal muscle, it seems reasonable to suggest further that alterations in the properties of any of the protein components of this macromolecular complex (Fig. 5) could alter myocardial Nav channel functioning or expression. It is of further interest to note that cardiomyopathy is often evident in patients with congenital (skeletal) muscular dystrophies, attributed to mutations in the dystrophin gene (343), as well as to mutations in other genes that are part of the dystrophin-sarcoglycan protein complex (144). With the assumption that the model of functional cardiac Nav channels proposed in Figure 6 is at least qualitatively correct, it would seem reasonable to speculate that the mutations in any of the genes encoding any of the components of the depicted macromolecular complex could lead to altered Nav channel functioning and cardiac arrhythmias alone or in the background of other myocardial disease, particularly structural heart disease. If complexes such as those illustrated in Figure 6 are indeed shown to be the physiologically important units of cardiac Nav functioning, exploring the molecular mechanisms involved in mediating the many protein-protein interactions important in controlling the assembly, trafficking and functioning of these macromolecular channel protein complexes might well provide insights into the link between structural heart disease and the electrophysiological abnormalities that are linked to the generation of life-threatening cardiac arrhythmias in a wide variety of myocardial disease states.

Other potentially important signaling molecules in cardiac physiology and pathophysiology are also linked to syntrophin and, therefore, to Nav1.5 channels (Fig. 6). Nitric oxide synthase (NOS), for example, which is also part of the dystrophin-proteoglycan complex (189), and thought to play a role in myocardial ischemia (466), appears to bind directly to syntrophin, as well as to caveolin-3 (115, 207, 265), the muscle-specific caveolin in plasmalemmal caveoli. Nearly all of the endothelial NOS activity can be coimmunoprecipitated from cardiac muscle using an anti-caveolin-3 antibody (161, 162). The caveolins are also important regulators of NOS activity (161, 415), and it is of interest to note that overexpression of caveolin-3 results in cardiomyopathy and the downregulation of NOS and other components of the dystrophin-dystroglycan complex (27). Given that it has also been demonstrated that caveolin-3 binds directly to the cytoplasmic tail of -dystroglycan (477), these observations suggest additional links between cardiac Nav channels, the actin cytoskeleton, and the extracellular matrix (Fig. 6). Caveolin-3 is thought to play a direct role in regulating the interaction of caveoli with the sarcolemmal membrane and, therefore, to function to facilitate the transfer of (Nav or other) channels to the cell surface membrane from the intracellular compartment (161). It seems reasonable to suggest here that alterations in the interactions between any of the individual components of the proposed functional Nav channel complex (Fig. 6), through acquired or inherited disease, could have profound effects on the properties and/or the functional cell surface expression of Nav channels, effects that in turn will impact the generation of normal cardiac rhythms and the likelihood that rhythm disturbances will occur.

In cardiac myocytes, functional cell surface Nav channel expression is also regulated directly by -adrenergic receptor occupancy and the activation of the stimulatory G protein (G_s) pathway (310, 334). In addition, it has now been demonstrated that G_s functions through binding to caveolin-3, increasing the presentation of caveoli to the sarcolemmal membrane which could lead to increased cell surface expression of Nav channels (569). It has also been reported that caveolin-3 expression is increased and that nitric oxide signaling is augmented in a (canine) pacing-induced model of heart failure (203). The relationship between these biochemical changes and the observed structural and electrical changes evident in this model and the relevance of this model and these changes to human heart failure remain to be established. As for heterologously expressed Nav channels, there may well also be additional regulatory molecules, including growth factors (295, 555) and membrane lipid-anchoring proteins (460), that play a role in regulating the properties and the functional cell surface density of cardiac Nav channels. Clearly, this possibility warrants direct experimental testing.

C. Cav Channel Pore-Forming -Subunits

Similar to Nav channels, voltage-gated Ca²⁺(Cav) channel pore-forming -subunits belong to the "S4" superfamily of voltage-gated ion channel genes (92, 395), and functional voltage-gated Ca²⁺(Cav) channels reflect the multimeric assembly of one Cav -subunit (₁) and auxiliary Cav and Cav₂, as well, at least in some cases, as Cav, subunits (Fig. 5). Also similar to Nav channels, Cav -subunits comprise four homologous domains (domains I–IV), each of which is composed of six transmembrane segments (S1–S6), with an "S4" voltage-sensing domain and a Ca²⁺-selective pore region between S5 and S6 (92, 395). Four distinct subfamilies of Cav channel pore-forming ₁-subunits, Cav1, Cav2, Cav3, and Cav4 (47), each with many subfamily members (Table 3), and alternately spliced transcripts (476) have been identified. The Cav ₁-subunits are differentially expressed, and studies in heterologous expression systems have revealed that the various Cav ₁-subunit genes encode Cav channels with distinct time- and voltage-dependent properties and pharmacological sensitivities. Heterologous expression of any one of the four members of the Cav1 subfamily, Cav1.1, Cav1.2, Cav1.3, or Cav1.4 (Table 3), for example, reveals L-type HVA Cav channel currents (92, 395), whereas heterologous expression of Cav3 -subunits produces T-type LVA Cav channel currents (92, 395).

View this table: [in this window] [in a new window]   TABLE 3. Diversity of voltage-gated Ca2+ (Cav) channel - and -subunits

All available molecular and biochemical evidence suggests that Cav1.2, which encodes the _1C-subunit, is the prominent Cav -subunit expressed in the mammalian myocardium. The CACNA1C gene is large, comprising 44 invariant and 6 alternative exons (474), and the Cav1.2 message is widely expressed. From the many possible splice sites, a number of different variants of the _1C protein, including Cav1.2a, Cav1.2b, and Cav1.2c, are generated (6, 348, 474, 475). Interestingly, although nearly identical (>95%) in amino acid sequence, the various _1C splice variants appear to be expressed in different cells/tissues, and the cardiac specific isoform is Cav1.2a, which encodes cardiac L-type HVA Cav channels (6, 92). Thus similar to cardiac Nav channels and consistent with the similarities in the properties of the L-type cardiac Cav channel currents, it appears that a single pore-forming ₁-subunit (Cav1.2a) is responsible for the generation of HVA channels throughout the myocardium (92, 395). Interestingly, however, it has been reported that sinus node dysfunction is seen in mice with a targeted disruption (363) of the Cav1.3 -subunit gene, CACNAID (585), suggesting that SA nodal HVA Cav channels are encoded by Cav1.3, rather than by Cav1.2 (585). These observations raise the interesting possibility that, with the right tools, one would be able to manipulate selectively the functioning of atrial/ventricular (Cav1.2) and/or nodal (Cav1.3)-encoded myocardial Cav channels.

The recently identified linkage between Timothy syndrome and a point mutation in the CACNA1C gene encoding Cav1.2a (479) demonstrates, as likely would have been expected, that defective Cav channel (like defective Na and Kv channel) functioning can lead to cardiac arrhythmias. The Timothy syndrome mutation is a missense mutation that results in a single amino acid change, glycine (G) to arginine (R), at residue 406 (479), which is in the cytoplasmic loop between domains I and II, immediately C terminal to S6 transmembrane segment in domain I (Fig. 3). Heterologous expression studies reveal that the G406R mutation in Cav1.2a markedly reduces voltage-dependent channel inactivation, resulting in increased persistent inward Ca²⁺ current (479). The biophysical consequence of the Timothy syndrome mutation in Cav1.2a, therefore, is highly reminiscent of several Nav1.5 channel mutations associated with long QT and Brugada syndromes that result in increased persistent inward Na⁺ currents (and in action potential prolongation). Interestingly, however, in contrast to the LQT and Brugada syndromes, Timothy syndrome is a multisystem disorder (479). This latter observation is consistent with the hypothesis that Cav1.2a, unlike Nav1.5, is expressed widely and that mutations in Cav1.2a result in phenotypic consequences in many different organ systems.

D. Cav Channel Accessory Subunits and Other Interacting Proteins

There are a number of Cav accessory subunits that coassemble with Cav₁ (Fig. 5) and play a role in the generation of functional Cav channels in cardiac, as well as in other, cells. Three distinct types of Cav channel accessory subunits, Cav, Cav₂, and Cav (Table 3), for example, have been identified (28, 110). Of these subunits, only the accessory Cav and Cav₂ subunits appear to be expressed in the myocardium (Table 3) and to contribute to the formation of functional cardiac Cav channels (110). The accessory Cav -subunits are cytosolic proteins that assemble with Cav1 -subunits and regulate the expression of functional cell surface HVA Cav channels, including cardiac L-type Cav channels. Four different Cav subunit-encoding genes, CACNB1, CACNB2, CACNB3, and CACNB4, which encode the Cav₁(408, 434), Cav₂ (222, 396), Cav₃ (89, 222, 396), and Cav₄ (89, 505) proteins, respectively, have been identified (Table 3). It appears, however, that Cav₂ is most prominently expressed in the heart (222, 396). In each Cav subunit, there are three variable regions flanking two highly conserved domains (89, 222, 396, 408, 434, 505). The variable regions are in the COOH termini, the NH₂ termini, and a small (100 amino acids) region in the center of the linear protein sequence between the two conserved domains (89, 222, 396, 408, 434, 505). The conserved domains of the Cav -subunits mediate interactions with the pore-forming Cav ₁-subunits, whereas the variable domains influence the functional effects of Cav -subunit coexpression on the properties of the resulting Cav channels (407). In heterologous expression systems, coexpression of Cav -subunits with Cav ₁-subunits markedly increases Cav channel current amplitudes and densities (59, 187, 541, 565), effects which could reflect increased cell surface channel expression, increased channel open probability, and/or the stabilization of the Cav₁-Cav channel complexes in the cell membrane (98, 99, 565). In addition to increasing current amplitudes, coexpression of Cav -subunits also modifies the kinetics and the voltage dependences of Cav current activation and inactivation (70, 242, 279, 353).

A highly conserved sequence motif in Cav ₁-subunits, called the alpha subunit interaction domain, appears to mediate -subunit interaction(s) with accessory Cav -subunits (407). The Cav₁ interaction domain (QqxExxLxGYxxWIxxxE) is located in the cytoplasmic loop between domains I and II, exactly 24 amino acids from the S6 transmembrane region of domain I (63, 64, 132, 211). Interestingly, the Timothy syndrome mutation, G406R, is close to this subunit interaction domain (479), suggesting that the (G406R) mutations might disrupt Cav₁-Cav subunit-subunit interactions. Regions outside of this interaction domain, including low-affinity binding sites in the COOH termini of the Cav ₁-subunits, however, have also been suggested to participate in Cav-Cav₁ subunit-subunit interactions (412, 493, 523). Indeed, it now appears that these COOH-terminal regions in Cav ₁-subunits also interact specifically with a second, highly conserved domain in the Cav -subunits to produce the observed modulatory effects of accessory Cav -subunit coexpression (131, 181).

In addition to the Cav -subunits, another type of accessory subunit, referred to as Cav₂, has also been shown to be part of functional Cav channel complexes (146). Unlike Cav, the Cav₂ subunits are transmembrane accessory subunits (Fig. 5), the first of which, Cav₂-1, was cloned from skeletal muscle (146). At least four different Cav₂-1 subunit-encoding genes, CACNA2D1, CACNA2D2, CACNA2D3, and CACNA2D4, have been identified (Table 3), and all produce heavily glycosylated proteins that are cleaved posttranslationally to yield ₂ and proteins that then become linked via disulfide bridges (Fig. 5). In each of the Cav₂-1 complexes, the Cav₂ domain is located extracellularly, whereas the Cav domain, which has a large hydrophobic region, inserts into the membrane (Fig. 5) and serves as an anchor to secure the entire (Cav₂) complex (197, 198, 554). In contrast to the accessory Cav subunits, the functional roles of accessory Cav₂ subunits are variable and appear to depend, at least in part, on the identities of the coexpressed Cav₁and Cav subunits, as well as on the expression environment. In general, however, coexpression of Cav₂-1 shifts the voltage dependence of activation of Cav/Cav-encoded channels, accelerates current activation and inactivation, and increases current amplitudes, compared with the channels/currents produced on expression of the Cav₁and Cav subunits alone (38, 158, 197, 198, 260, 472). The increase in functional cell surface Cav current densities on coexpression of Cav₂-1 (and Cav) subunits appears to reflect improved targeting of Cav ₁-subunits to the plasma membrane (469). This effect (improved targeting) is produced through interactions with Cav₂, whereas the changes in channel kinetics are attributed to the presence of the Cav protein (469).

A distinct type of Cav channel accessory subunit was revealed with the identification of the Cav subunit, Cav1, that is expressed in mammalian skeletal muscle, and that contributes to the formation of functioning of skeletal muscle Cav channels (462). A number (seven) of additional Cav-encoding genes, CACNG1-CACNG8 (Table 3), have now been identified in skeletal muscle and in brain (250). All Cav subunits have four transmembrane spanning domains with the COOH and NH₂ termini predicted to be intracellular (462). Coexpression of -subunits with various combinations of Cav and Cav subunits has been shown to affect both the time- and the voltage-dependent properties of the resulting Cav currents (250). In addition, the Cav -subunits that are expressed in the nervous system, Cav₂, Cav₃, Cav₄, and Cav₈, all have COOH-terminal PDZ-binding domain motifs (250). These observations suggest the interesting possibility that the Cav -subunits play a role in controlling the localization and/or trafficking of functional Cav channels (250). Interestingly, it has also been reported that Cav₂ interacts with the AMPA subtype of neuronal glutamate receptors, suggesting that the Cav -subunits may, like other channel accessory subunits, be multifunctional proteins (96). Although Cav -subunits appear to be widely expressed in the nervous system (250), it is not clear at present whether one or more of the CACNG genes is expressed in the heart and/or if these subunits play a functional role(s) in the generation of cardiac L-type Cav channels. Clearly, further studies focused on exploring this topic and determining directly the role(s) of the various Cav channel accessory subunits in the generation of cardiac Cav channels are warranted.

As noted in section IIB, it is now very well documented that HVA myocardial Cav channels undergo rapid Ca²⁺- and voltage-dependent inactivation (44, 166, 281, 326). Fundamentally important insights into the likely molecular mechanism underlying the Ca²⁺-dependent component of inactivation were revealed with the demonstration that the EF-hand domain containing protein, calmodulin, that binds Ca²⁺ and modulates a variety of Ca²⁺-dependent processes, is associated with the COOH-terminal cytoplasmic domain of L-type HVA channels (398). Several subsequent studies have provided many of the molecular details of the calmodulin:Cav channel ₁-subunit association and interaction domains (17, 149, 293, 355, 428). In addition, the generality of the calmodulin-mediated mechanism of Ca²⁺-dependent inactivation of Cav channels was documented with the demonstration that P/Q-type neuronal HVA channels are also regulated by calmodulin binding (128).

In addition to the regulation of channel gating by Ca²⁺ and calmodulin, the properties and the functional expression of myocardial Cav channels are also regulated by a variety of extracellular signals and intracellular signaling pathways. Prominent among these are the rather well studied -adrenergic G protein-coupled receptor-mediated augmentation of cardiac L-type Cav channel currents, increased Ca²⁺ entry, and positive inotropy (255, 509). Considerable experimental evidence suggests that the pore-forming Cav1.2 -subunit and Cav -subunits are targets of posttranslational modifications by a variety of protein kinases that impact the functional cell surface expression and the properties of cardiac L-type Cav channels (255, 509). It has also been reported that cardiac HVA Cav channels are actually associated with -adrenergic receptors in macromolecular complexes that likely also include heterotrimeric G proteins, adenylate cyclase, protein kinases, phosphatases and protein kinase A binding proteins, or AKAPs (18, 122), which appear to subserve a scaffolding function (18). Unlike Nav channels, however, no direct links between cardiac Cav channel subunits and actin or actin-binding proteins have been demonstrated to date. Nevertheless, a number of Ca²⁺-binding proteins, including calmodulin, have been shown to be linked both directly and indirectly to the actin cytoskeleton (452). In addition, it has been reported that the time- and voltage-dependent properties of L-type HVA channels are altered in skeletal muscle from dystrophin-deficient animals, an effect interpreted as resulting from remodeling of the subcellular actin cytoskeleton (111). Similarly, neuronal HVA Cav channel inactivation is differentially affected by agents that stabilize and destabilize the actin cytoskeleton (239). In retinal ganglion neurons, for example, stabilization or disruption of the actin cytoskeleton affects functional cell surface Cav channel expression (454). It seems reasonable to suggest, therefore, that Cav channels interact directly or indirectly with the actin cytoskeleton (556) and that, similar to cardiac Nav channels, the functioning of myocardial Cav channels likely also depends importantly on interactions with the actin cytoskeleton. In support of this hypothesis, targeted deletion of endothelial NOS eliminates the muscarinic modulation of myocardial L-type Cav channels (202). Further investigations focused on delineating the molecular mechanism controlling functional Cav channel expression will be needed to define the role of the cytoskeleton in regulating myocardial Cav (and Nav) channel expression and functioning.

	VI. MOLECULAR COMPONENTS OF MYOCARDIAL KV CHANNELS

Top Previous Next References

 
A. Kv Channel Pore-Forming -Subunits

Similar to Nav and Cav -subunits, voltage-gated K⁺ channel (Kv) pore-forming () subunits (Fig. 3) belong to the "S4" superfamily of voltage-gated channels (405). In contrast to Nav and Cav channel -subunits, however, Kv -subunits are six transmembrane-spanning domain proteins (Fig. 3, B and C), and functional Kv channels comprise four -subunits (Fig. 5). A very large number of Kv -subunit genes have been identified, and a systematic terminology for naming these subunits (Table 4) has been developed (199). Heterologous expression of Kv -subunits in the Kv1-Kv4 subfamilies reveals functional Kv channels with distinct time- and voltage-dependent properties (116), whereas Kv -subunits of the Kv5–9 subfamilies (Table 4) are electrically silent (88, 142, 221, 441). Coexpression of Kv5-Kv9 subunits with Kv2 -subunits, however, attenuates the amplitudes of the Kv2-encoded K⁺ currents (441). Nevertheless, the functional roles of the "silent" Kv -subunits in the generation of myocardial Kv channels remains to be determined.

View this table: [in this window] [in a new window]   TABLE 4. Diversity of voltage-gated K+ (Kv) channel -subunits

Additional subfamilies of Kv -subunit genes in the KCNQ and KCNH subfamilies (Table 4) have been identified, and one member of each of these subfamilies, KCNQ1 and KCNH2, has been shown to be the loci of mutations leading to congenital long QT syndromes, LQT1 (Fig. 3C) and LQT2 (Fig. 3B), respectively (40, 119, 447, 448, 499, 528). Heterologous expression of human KCNH2, which encodes the ether-a-go-go-related protein ERG1, reveals Kv currents (448, 499) that are similar to cardiac I_Kr (Table 4). Similar to SCN5A mutations linked to the LQT3 and Brugada syndromes (Fig. 3A), LQT2 mutations in KCNH2 are found throughout the ERG1 protein sequence (Fig. 3B). These (LQT2) mutations are all "loss of function" and result in reduced functional I_Kr channel expression owing to dominant negative effects or to alterations in channel processing or trafficking (23, 126, 220, 246, 253, 273, 584). Interestingly, a novel "gain of function" mutation in KCNH2, which results in increased I_Kr channel densities, has recently been identified and linked to one form of short QT syndrome (78).

There are six additional members of the KCNH subfamily, KCNH3-KCNH8 (Table 4). Two of these, KCNH3 and KCNH4, appear to be nervous system specific (465), and it is presently unclear whether any of the others are present in the myocardium. Alternatively processed forms of KCNH2, with unique NH₂ and COOH termini, however, have been cloned from both mouse and human heart cDNA libraries and postulated to contribute to the generation of functional cardiac I_Kr channels (272, 282, 304). Indeed, coexpression of the NH₂-terminal splice variant ERG1b with the full-length ERG1a produces Kv currents that more closely resemble cardiac I_Kr than the currents produced on expression of ERG1a alone (304). Although it has been reported that only the full-length ERG1 protein(s) are detected in adult rat, mouse, and human hearts (404), raising some doubts about the functional significance of alternative splicing of KCNH2 transcripts, more recent studies have identified ERG1b protein in adult rat, human, and canine heart (240). Presumably, these disparate results reflect the fact that different anti-ERG1 antibodies were used (240, 404). Further studies will be needed to explore this and other possible explanations. Using antibodies targeting the specific ERG1 isoforms, it was also demonstrated that ERG1a and ERG1b coimmunoprecipitate from heart, suggesting that functional cardiac I_Kr channels reflect the heteromeric assembly of ERG1a and ERG1b subunits (240). The expression, distribution, and functioning of the COOH-terminal variant of ERG, ERG-USO (272), in contrast, remains to be explored.

As noted previously, mutations in the KCNQ1 gene have been linked to LQT1 (253, 329). Heterologous expression of KvLQT1 (KCNQ1) alone yields rapidly activating and noninactivating outward Kv currents, whereas coexpression with the Kv channel accessory subunit, minK (see sect. VB), produces slowly activating K⁺ currents that resemble the slow component of cardiac delayed rectification, I_Ks (40, 447). Similar to the SCN5A mutations linked to the LQT3 and Brugada syndromes (Fig. 3A) and KCNH2 mutations linked to LQT2 (Fig. 3B), KCNQ1 mutations linked to LQT1 have been identified throughout the protein sequence (Fig. 3C). Expression studies suggest that the various LQT1-associated mutations in KCNQ1 are all loss of function, resulting in reductions in functional I_Ks cell surface channel expression. Simulations demonstrate that reductions in I_Ks density (Fig. 7B) result in markedly prolonged ventricular action potential waveforms (Fig. 7A). Given the intrinsic heterogeneities in I_Ks (and other) channel densities and action potential waveforms throughout the myocardium (Fig. 1), the effects of LQT1 mutations in KCNQ1 might also be heterogeneous, further impacting the arrhythmogenic potential of these mutations. Importantly, a novel "gain of function" mutation in KCNQ1 (V307L) was identified and linked to short QT interval syndrome (46). Heterologous expression studies revealed that expression of KCNQ1 V307L, alone or with wild-type KCNQ1, in the presence of KCNE1, produces I_Ks-like currents with markedly altered activation kinetics and voltage-dependent properties (46) relative to the channels produced by wild-type KCNQ1 and KCNE1. A gain of function mutation (S140G) in KCNQ1 has also been identified in a family with hereditary persistent atrial fibrillation (97). Computer simulations incorporating the KCNQ1 short QT mutant channels reveal that I_Ks densities are increased (Fig. 7D) and that action potentials are shortened markedly (Fig. 7C). As noted above for LQT1 mutations, the impact of gain of function mutations in KCNQ1 on different cell types and regions of the heart will likely be heterogeneous, owing to the existing heterogeneity in I_Ks (and other current) densities and action potential waveforms, an effect which may acerbate the arrhythmogenic potential of alterations in I_Ks densities.

View larger version (14K): [in this window] [in a new window]   FIG. 7. Simulations reveal the effects of loss of function (LQT1) and gain of function (short QT) mutations in KCNQ1. Steady-state action potential waveforms (A and C) and outward IKs currents (B and D) were simulated (103, 105). Control action potential and current waveforms simulated for wild-type KCNQ1-encoded IKs currents are depicted as the solid black lines in A–D. The corresponding simulated voltage and current waveforms depicting the effects of KCNQ1 mutations are illustrated by the dashed purple (LQT1) and red (short QT) lines in A–D. "Loss of function" LQT1 mutations (Fig. 3C), resulting in a decrease in the maximum amplitude and a slowing of the time to peak of IKs (B), lead to marked action potential prolongation (A). In contrast, "gain of function" short QT mutations in KCNQ1 increase the maximal amplitude of IKs (D) and shorten action potential durations.

B. Kv Channel Accessory Subunits

Similar to the Nav and Cav channels, a number of different types of Kv channel accessory subunits have been identified (Table 5) and postulated to contribute to the generation of functional myocardial Kv channels. The first of these was cloned from human (362), and later from rat (170), heart and was referred to as minK, i.e., "minimal K^+" channel subunit. MinK, which is encoded by the KCNE1 gene on chromosome 21 in human (Table 5), is a small (130 amino acids) protein with a single transmembrane spanning domain (170, 288, 362). Although initial characterizations of minK in Xenopus oocytes suggested that this small protein could produce functional Kv channels when expressed alone (170, 362), subsequent studies demonstrated the presence of a KCNQ1 homolog in oocytes that combines with the heterologously expressed minK to generate Kv channels that very closely resemble cardiac I_Ks (447). As noted above, these observations have led to suggestions that minK coassembles with the KvLQT1 protein to produce functional cardiac I_Ks channels (40, 447). It has also been reported, however, that minK coassembles with the ERG1 protein in heterologous expression systems, observations interpreted as suggesting a role for minK in the generation of cardiac I_Kr channels (341). It has become increasingly clear, however, that accessory Kv subunits, such as minK, can, at least in heterologous expression systems, associate with multiple different Kv subunits. It is not a given, however, that these interactions occur in intact tissues. As a result, it is presently unclear whether minK subunits actually associate with both KvLQT1 and ERG1 (and other Kv?) -subunits in the myocardium and contribute to the function of both cardiac I_Ks and I_Kr (and other Kv?) channels. Biochemical studies focused on exploring these questions are clearly warranted. Although the details of minK functioning remain to be clarified, it is important to emphasize that the physiological significance of this subunit in the generation of cardiac membrane currents was clearly demonstrated with the identification of mutations in KCNE1 that are associated with one type of inherited long QT syndrome, LQT5 (62, 478, 481).

Another type of Kv channel accessory subunit was revealed with the biochemical identification (360) and subsequent cloning (419) of low molecular mass (<45 kDa) cytosolic Kv -subunits from brain. Three distinct, but homologous, Kv -subunits, Kv 1, Kv 2, and Kv 3, encoded on three different (KCNAB) genes (Table 5), as well as alternatively spliced transcripts (337), have now been identified (90, 124, 147, 148, 317, 352). Of these, Kv1.1, Kv1.2, Kv1.3, and Kv2 have been shown to be expressed in the heart (12, 90, 124, 147, 148, 278, 317, 352). The Kv -subunits share a conserved COOH-terminal "core" region, related in amino acid sequence to aldo-ketone reductases, members of the triose phosphate isomerase enzyme family (101, 338). The NH₂-terminal domains of the Kv -subunits are unique, and members of the Kv1 subfamily, as well as Kv3.1, have long NH₂-terminal sequences that are structurally similar to the Shaker (Kv1) channel inactivation gate that functions in a "ball and chain"-like mechanism (217) to accelerate Kv1 channel inactivation (300 301). Although the core region of the Kv -subunits contains an NAPH/NADPH binding site (101, 338) and Kv -subunits crystallize with NADPH bound (191), the role of this binding in regulating the functional interactions between Kv - and Kv -subunits and/or in controlling the expression/properties of the resulting Kv channels has not been defined.

Previous studies have demonstrated that the Kv -subunits interact with the intracellular T1 tetramerization domain of the Kv -subunits of the Kv1 subfamily, combining in a 1:1 stoichiometric ratio (190, 192). Heterologous coexpression studies have revealed that Kv -subunits affect the functional properties and the cell surface expression of Kv1 -subunit-encoded channels (9, 10, 90, 147, 148, 317, 352, 464). In some cases, the functional consequences of Kv coexpression have been shown to be Kv1 -subunit specific (10). Coexpression of Kv2, for example, increases the expression of Kv1.2-encoded channels and decreases the expression of Kv1.5-encoded channels (9, 10). Because Kv1 and Kv subunits coassemble in the endoplasmic reticulum (369), the observed increases in functional cell surface Kv1-encoded channel expression suggest that the Kv subunits affect Kv1 channel assembly, processing or stability or, possibly, function as chaperone proteins.

The facts that the Kv -subunits were originally identified in association with Kv1 -subunits (360) and that heterologous expression studies suggest that Kv1 and Kv2 interact only with the Kv1 -subunits (370, 458) have been interpreted as suggesting that the Kv -subunits function as specific accessory subunits in the generation of Kv channels encoded by Kv1 -subunits. In expression systems, however, Kv3 has also been shown to interact specifically with Kv2 subunits (167). Biochemical studies also suggest that Kv -subunits might well be functionally more diverse (370, 393, 458). Both Kv1.1 and Kv1.2, for example, coimmunoprecipitate with Kv4 -subunits following coexpression in COS-1 cells (370). Coexpression of Kv1 subunits has also been reported to modulate the cell surface expression of Kv4.3-encoded currents, an effect attributed to Kv1 binding to the COOH terminus of Kv4.3 (566). In addition, although without any detectable effect on current properties or densities, coexpression (in Xenopus oocytes) of Kv1.2 with Kv4.2 -subunits modulates the redox sensitivity of Kv4.2-encoded channels (393).

Similar to the KCNE subfamily of Kv accessory subunits, therefore, the functional roles assigned to the Kv -subunits in the generation of myocardial Kv channels have been largely a matter of speculation. Interestingly, however, recent studies, completed on ventricular myocytes isolated from mice with a targeted disruption in the Kv1 locus, reveal that the loss of Kv1 affects the functional cell surface densities of Kv4- and Kv2-encoded, but not Kv1-encoded, Kv channels (12). It has also been reported that Kv -subunits associate with -subunits of the eag (KCNH) subfamily (553). On the basis of all the available data, therefore, it seems reasonable to suggest that the Kv -subunits subserve multiple functions in the generation of myocardial Kv channels. Further studies focused on providing the molecular details of Kv -functioning are clearly warranted.

A novel Kv channel accessory protein, KChAP (K⁺ channel accessory protein) (Table 5), was identified in a yeast two-hybrid screen using the NH₂ terminus of Kv4.2 as the bait (550). Sequence analysis of KChAP revealed a 574-amino acid protein with no transmembrane domains and no homology to Kv - or Kv -subunits (550). Coexpression of KChAP with Kv2.1 (or Kv2.2) in Xenopus oocytes, however, markedly increases functional Kv2.x-induced current densities without measurably affecting the time- and/or voltage-dependent properties of the currents (550), suggesting that KChAP functions as a chaperone protein (277). Yeast two-hybrid assays also revealed that KChAP interacts with the NH₂ termini of -subunits in the Kv1 subfamily and with the COOH termini of Kv 1-subunits (278, 550). Interestingly, it was subsequently demonstrated that KChAP belongs to the protein inhibitor of the activated STAT family of proteins that interact with a variety of transcription factors and play a role in programmed cell death (549). The relationship between the apoptotic and chaperone functions of KChAP, as well as the functional role of KChAP in the generation/regulation of Kv channels in the normal and the diseased myocardium, however, remain to be determined.

With the use of the intracellular NH₂ terminus (amino acids 1–180) of Kv4.2 as the "bait" in a yeast two-hybrid screen, three novel Kv channel interacting proteins, KChIP1, KChIP2, and KChIP3 (Table 5), were identified (21). The KChIPs belong to the recoverin family of neuronal Ca²⁺-sensing (NCS) proteins, particularly in the "core" regions, which contain multiple EF-hand domains (81). Unlike other NCS-1 proteins, however, KChIP2 and KChIP3 lack NH₂-terminal myristoylation sites, and the NH₂ termini of each of the KChIP proteins are unique (21). Nevertheless, KChIP2 and KChIP3 have several potential palmitoylation (on cysteine residues) sites, and metabolic labeling studies suggest that these sites are palmitoylated in situ (491). It is now clear that KChIP3 is identical to the previously identified protein calsenilin, which is a Ca²⁺-binding protein that interacts with presenilin-1 and presenilin-2 and regulates the proteolytic processing of these two proteins (82). In addition, however, KChIP3 is also identical to another previously identified protein called DREAM, which is a Ca²⁺-regulated transcriptional repressor (87). The DREAM protein has been shown to bind to the downstream regulatory elements (DRE) of several genes in the absence of Ca²⁺ and to dissociate from the DRE sequence when Ca²⁺is elevated (87). Thus DREAM is thought to act as an activity-dependent regulator of gene expression (87). An additional member of the KChIP family, KChIP4, also referred to as calsenilin-like-protein or CALP, was subsequently identified in biochemical studies focused on identifying the binding partners of the presenilin proteins (356). The interactions between KChIP3 (calsenilin) and KChIP4 (CALP) and the presenilin proteins are also Ca²⁺ dependent (356).

Of the four KChIP genes, only KChIP2 appears to be expressed in the heart (21, 431). There are, however, numerous splice variants of KChIP2 that have now been identified (33, 125, 129, 390, 391, 430, 431). Studies in heterologous systems have revealed that coexpression of any one of the (full-length) KChIP proteins with Kv4 -subunits increases the functional cell surface expression of Kv4.x-encoded Kv channels, slows current inactivation, speeds recovery from inactivation and shifts the voltage dependence of channel activation (21, 193, 195). In contrast, KChIP expression reportedly does not affect the properties or the densities of the K⁺ currents produced on expression of other Kv -subunits, including Kv1.4 and Kv2.1 (21). These observations were interpreted as suggesting that the modulatory effects of the KChIP proteins are specific for -subunits of the Kv4 subfamily (21). In addition, although the binding of the KChIP proteins to Kv4 -subunits is not Ca²⁺ dependent, mutations in EF hand domains 2, 3, and 4 of KChIP1 reportedly eliminate the modulatory effects of KChIP1 on Kv4.2-encoded K⁺ currents in CHO cells (21). It has, however, also been reported that a splice variant of KChIP2, KChIP2d, which lacks three of the four EF hand domains of full-length KChIP2, modifies the inactivation kinetics of heterologously expressed Kv4.3-encoded K⁺ currents, but does not alter the kinetics of channel recovery from steady-state inactivation (390). Taken together, these findings suggest that distinct regions of the (full-length) KChIP proteins underlie the various modulatory effects of KChIPs on the properties and cell surface expression of Kv4-encoded channels. Structural analysis has revealed that Kv4 and KChIP accessory subunits assemble in a 4:4 stoichiometry and provide new insights into the intracellular interactions between the NH₂ termini of Kv4 subunits and the EF hand domains of the KChIPs (258, 588). In addition, it had been shown that myristoylation of KChIP1 appears necessary for the normal trafficking of newly synthesized (KChIP1) protein to the endoplasmic reticulum where the association with Kv4 -subunits occurs (385). It may be that palmitoylation of KChIP2 and KChIP3 plays a similar functional role. Mutagenesis and structural studies have also revealed that two regions in the NH₂ termini of Kv4 subunits are necessary for KChIPx interaction with (and modulation of) Kv4-encoded channels and that residues 71–90 (in Kv4.x) form a "contact loop" that mediates the interaction with the KChIP protein(s) (451).

Biochemical methods were exploited in efforts that led to the identification of another Kv channel accessory subunit, DPPX or DPP6, that also appears to interact specifically with Kv4 -subunits (368). A novel protein of previously unknown function, DPP6 is structurally related to CD26, which is a dipeptidyl aminotransferase and a cell adhesion protein (368). Interestingly, DPP6 actually belongs to a family of nonclassical serine proteases, although DPP6 itself has no enzymatic activity (368). In contrast to the KChIPs, DPP6 is an integral membrane glycoprotein with a rather large extracellular COOH-terminal domain (368). Coexpression of DPP6 with Kv4 -subunits affects the trafficking and the membrane targeting of Kv4 -subunits and modifies the kinetic properties of expressed cell surface Kv4-encoded channels (368). Although expressed in brain and thought to function in the generation of neuronal Kv4-encoded transient outward Kv currents, DPP6 does not appear to be expressed in heart and, therefore, cannot contribute to the formation of functional cardiac Kv channels. Another member of the dipeptidyl transferase family, DPP10, has also been shown to associate with Kv4 -subunits in heterologous expression systems and to modify the biophysical properties of Kv4.x-encoded channels (235). The effects of DPP10 on Kv4 channels are qualitatively similar to the effects of DPP6 (235), although, like DPP6, DPP10 also appears to be expressed predominantly in the brain (235). It also seems unlikely, therefore, that DPP10 plays a role in the generation of cardiac Kv channels. It is certainly possible, however, that there are additional members of this family that are expressed in the myocardium and that remain to be identified and characterized.

C. Molecular Correlates of Cardiac Transient Outward Kv Channels

All available evidence suggests that Kv -subunits of the Kv4 subfamily encode rapidly activating, inactivating, and recovering cardiac transient outward Kv channels referred to as I_to,f (Table 1). In rat and mouse ventricular myocytes exposed to antisense oligodeoxynucleotides (AsODNs) targeted against Kv4.2 or Kv4.3, for example, I_to,f density is reduced by 50% (169, 193). Significant reductions in rat ventricular I_to,f density are also seen in cells exposed to an adenoviral construct encoding a truncated Kv4.2 subunit (Kv4.2ST) that functions as a dominant negative (238). In addition, it has been reported that I_to,f is eliminated in ventricular myocytes isolated from transgenic mice expressing a pore mutant of Kv4.2, Kv4.2W362F, that functions as a dominant negative, Kv4.2DN (43). Taken together, these results demonstrate that members of the Kv4 subfamily underlie I_to,f in mouse and rat ventricles. Biochemical studies have also shown that Kv4.2 and Kv4.3 are associated in adult mouse ventricles, suggesting that functional mouse ventricular I_to,f channels reflect the heteromeric assembly of the Kv4.2 and Kv4.3 -subunits (193). Given that the properties of the currents classified as I_to,f in other species (Table 1) are very similar to mouse (and rat) I_to,f, it seems reasonable to suggest that Kv4 -subunits also underlie I_to,f in other species. In dog and human myocardium, however, Kv4.2 appears not to be expressed (266), suggesting that only Kv4.3 contributes to I_to,f in larger mammals. Direct biochemical and/or molecular evidence to support this hypothesis, however, has not been provided to date. In addition, multiple splice variants of Kv4.3 have been identified in human (387) and rat (490) heart, although the functional roles of these variants in the generation of cardiac I_to,f channels remain to be determined.

It has been demonstrated that KChIP2 coimmunoprecipitates with Kv4.2 and Kv4.3 -subunits from adult mouse ventricles, consistent with a role for this subunit in the generation of functional Kv4-encoded mouse ventricular I_to,f channels (193). An important structural role of KChIP2 in the generation of myocardial I_to,f channels is suggested by the observation that I_to,f is eliminated in ventricular myocytes isolated from mice with a targeted disruption of the KChIP2 locus (271). In both canine and human heart, it has been demonstrated that there is a gradient in KChIP2 message expression across the thickness of the (left and right) ventricular walls (429, 431), observations interpreted as suggesting that KChIP2 underlies the observed differences in I_to,f densities in myocytes isolated from the epicardial, midmyocardial, and endocardial layers of the (human and canine) ventricles (429, 431). Although this point remains somewhat controversial (129), it has been reported that KChIP2 protein expression in canine ventricles parallels KChIP2 message expression (429), lending further support to the hypothesis that KChIP2, not Kv4.3, underlies the gradient in canine (and human) ventricular I_to,f densities. In rat and mouse, however, there is no detectable gradient in KChIP2 message or protein expression in the ventricles (193, 431), and it appears that differences in Kv4.2 expression underlie the regional variations in I_to,f densities in rodents (137, 193). Thus there seem to be two potentially important differences between I_to,f in rodents and I_to,f in large mammals, including humans. In rat and mouse, I_to,f channels reflect the heteromeric assembly of Kv4.2, Kv4.3, and KChIP2, and differences in Kv4.2 expression underlie regional differences in I_to,f densities. In large mammals, however, I_to,f channels appear to be produced by the coassembly of Kv4.3 and KChIP2, and KChIP2 appears to be the primary determinant of the observed regional differences in I_to,f densities.

Although the Kv accessory subunits were originally identified based on association with Kv1 -subunits and have been considered to be Kv1 -subunit specific, recent studies suggest a functional role for Kv1 subunits in the generation of cardiac I_to,f channels (12). Electrophysiological studies, for example, have revealed that I_to,f densities are decreased in ventricular myocytes isolated from mice bearing a targeted deletion of the KCNAB1 gene, which encodes Kv1 subunits (12). In addition, biochemical studies revealed that Kv4.2 and Kv4.3 coimmunoprecipitate with the Kv1 splice variants, Kv1.1 and Kv1.2, from adult mouse ventricles (12). Taken together, these observations suggest that (mouse) ventricular I_to,f channels function as multimeric protein complexes comprising the Kv4.2 and Kv4.3 pore-forming -subunits and the accessory Kv1.1, Kv1.2, and KChIP2 subunits (Fig. 5). The targeted disruption of Kv1 reduces the cell surface membrane expression of Kv4 -subunits (12), further suggesting that Kv1 functions to regulate the assembly and/or the trafficking of mouse ventricular I_to,f channels from the endoplasmic reticulum to the cell surface (12).

The Kv1 COOH-terminal "core" domain has been shown to interact with NH₂-terminal tetramerization (T1) domains in Kv1 -subunits (191), and recent studies suggest that Kv4 -subunit NH₂-terminal domains structurally resemble Kv1 T1 domains (451). It seems reasonable to suggest, therefore, that Kv4 NH₂ termini are likely involved in mediating the interaction with Kv1 subunits. As noted above, however, it has also previously been demonstrated that Kv4 -subunit NH₂-terminal domains are also important in mediating the interactions with KChIPs (21, 451). Taken together, these observations suggest that Kv4 NH₂-terminal domains are multifunctional, mediating -subunit/-subunit interactions, as well as the associations with the accessory KChIP2 and Kv1 subunits. It has also been reported, however, that Kv1 subunits regulate the cell surface expression of Kv4.3 subunits in heterologous expression systems through interactions with the COOH, not the NH₂, terminus (566). It is not clear if Kv1 subunits play a role in the generation of I_to,f (and/or other) channels in large mammals, including humans, primarily because this possibility has not been explored directly. Given the heterogeneity of subunits that can affect Kv4 channel properties in heterologous expression systems (130), it seems reasonable to suggest that additional accessory subunits or regulatory proteins might be involved in mediating the interaction(s) between Kv4 and Kv1 subunits. It is certainly also possible that the interactions between Kv4 and Kv1 subunits are indirect, mediated, for example, by other accessory subunits, such as KChIP2 or KChAP (278) or through scaffolding proteins (39) or components of the actin cytoskeleton (401, 529). In addition, there could well be further complexity in the subunit composition of I_to,f channels, as well as in I_to,f channel regulation and posttranslational processing in some cell types/species. Further studies focused on defining all of the molecular components of functional myocardial I_to,f (and other) channels are needed to provide insights into the detailed molecular mechanisms involved in the regulation and modulation of these channels in the normal and in the diseased myocardium.

Electrophysiological studies on atrial myocytes isolated from Kv4.2DN mice revealed that, similar to the findings in ventricular cells (43), I_to,f is eliminated (563). There are some differences, however, in the properties of mouse (and rat) ventricular and atrial I_to,f (26, 43, 68, 69, 75, 79, 194, 562, 563), differences that may reflect variations in the subunit composition of the channels and/or in posttranslational processing of these subunits. Further studies focused on detailing the molecular compositions and the mechanisms controlling the expression and functioning of I_to,f channels in other cell types, particularly atrial, nodal, and Purkinje cells, in rodents and in large animals, are needed to define definitively the similarities/differences in the molecular compositions of I_to,f channels in different cell types/species.

The kinetic and pharmacological properties of slow transient outward myocardial Kv currents, referred to as I_to,s (Table 1), are different from I_to,f, observations interpreted as suggesting that the molecular correlates of I_to,s and I_to,f channels are also distinct. Direct support for this hypothesis was provided in studies (196) completed on myocytes isolated from (Kv1.4 –/–) mice with a targeted disruption in the KCNA4 (Kv1.4) locus (305). The waveforms of the outward currents in cells isolated from the ventricles of Kv1.4 –/– animals are indistinguishable from those recorded in wild-type ventricular cells (196). In cells isolated from the interventricular septum of Kv1.4 –/– animals, however, I_to,s is undetectable, thereby demonstrating directly that Kv1.4 underlies I_to,s (196). Given the similarities in the time- and voltage-dependent properties of I_to,s (Table 1) in other species (77, 83, 178, 294, 545, 546), it seems reasonable to suggest that Kv1.4 also encodes I_to,s in ferret, rabbit, canine, and human atrial and ventricular myocytes.

Interestingly, it has also been reported that I_to,s and the Kv1.4 protein are upregulated in the right and left ventricles of Kv4.2DN-expressing transgenic animals (43, 194, 196), suggesting that electrical remodeling occurs in the myocardium when I_to,f is eliminated. When the Kv4.2DN transgene is expressed in the Kv1.4 –/– null background, however, both I_to,f and I_to,s are eliminated and, interestingly, no further electrical remodeling is evident (194). Indeed, electrophysiological recordings from Kv4.2DN-expressing Kv1.4 –/– cells revealed that the waveforms of the Kv currents in RV, LV, and interventricular septum cells are indistinguishable (194). Although these observations suggest that the molecular mechanisms underlying the observed electrical remodeling in Kv4.2DN ventricles is highly specific for Kv1.4, it is presently unclear which of the many possible transcriptional, translational, and/or posttranslational mechanisms (430) might be operative. Future studies focused on delineating the molecular mechanisms involved in the regulation of ion channel remodeling in this and other mouse models will likely provide important new mechanistic insights.

D. Molecular Correlates of Cardiac Delayed Rectifier Kv Channels

As noted above, KCNH2 has been identified as the locus of mutations underlying one form of familial long QT syndrome, LQT2 (119). Heterologous expression of KCNH2 cRNA in Xenopus oocytes reveals voltage-gated, inwardly rectifying K⁺-selective channels with properties similar to cardiac I_Kr channels (448, 499), observations interpreted as suggesting that KCNH2 encodes I_Kr (448). Subsequent studies identified NH₂- and COOH-terminal splice variants of the ERG1 protein (272, 282, 304), and recent biochemical studies suggest that an NH₂-terminal ERG1 splice variant, ERG1b, coassembles with the full-length ERG1a protein to form heteromeric I_Krchannels in rat, human, and canine heart (240). The role of the COOH-terminal variants of ERG1, ERG1-USO (272) in the generation of functional cardiac I_Kr channels, however, is presently unclear. It has been reported that heterologously expressed KCNH₂ and minK (KCNE1) coimmunoprecipitate (341) and that antisense oligodeoxynucleotides targeted against minK attenuate I_Kr amplitudes in AT-1 (an atrial tumor line) cells (567). It has also been reported that heterologous coexpression of another member of the KCNE subfamily of accessory subunits, MiRP1 (KCNE2), modifies the properties of KCNH2-encoded Kv currents (5). It is presently unclear, however, whether the minK or MiRP1 (or both) accessory subunits associate with ERG1a and/or ERG1b in adult human heart and contribute to the generation of functional cardiac I_Kr channels. The availability of specific anti-ERG1 antibodies that can be exploited to immunoprecipitate ERG1 proteins from heart (240) should make it possible to explore directly the association between the minK/MiRP accessory subunits and the pore-forming ERG1 subunits and the functional roles of these interactions in the generation of myocardial I_Kr channels.

Biochemical studies have now revealed that the heat shock proteins, Hsp70 and Hsp90, coimmunoprecipitate with heterologously expressed ERG1 and that geldanamycin, a specific inhibitor of Hsp90, prevents the maturation (posttranslational processing) and increases the proteosomal degradation of the ERG1 protein (163). Interestingly, the interactions between the ERG1 protein and Hsp70/Hsp90 are increased in LQT2 trafficking deficient KCNH2 mutants, such as ERG1G601S (180). In addition, the mutant ERG1G601S protein is retained in the endoplasmic reticulum (163). Importantly, it has also been demonstrated that inhibition of Hsp90 decreases functional I_Krdensities in isolated ventricular myocytes (163). Taken together, these results suggest that Hsp70 and Hsp90 function as chaperone proteins, bringing mature ERG1 protein complexes to the cell surface to generate functional I_Kr channels (163). It is certainly possible that there are additional components of myocardial I_Kr channels that influence the properties and/or the functional cell surface expression of these channels, and further studies are needed to test these hypotheses directly.

Heterologous expression of KCNQ1, the locus of mutations in LQT1 (522), reveals rapidly activating, noninactivating Kv currents, whereas coexpression with KCNE1 (minK) produces slowly activating Kv currents similar to cardiac I_Ks (40, 447). These observations, together with biochemical data demonstrating that heterologously expressed KvLQT1 and minK proteins associate (447), have been interpreted as suggesting that minK coassembles with KvLQT1 to form functional cardiac I_Ks channels (40, 447). In addition, the finding that mutations in the transmembrane domain of minK alter the properties of the KCNQ-1 encoded Kv channels was interpreted as suggesting that the transmembrane segment of minK contributes to the channel pore (185, 487, 492, 527). Nevertheless, and similar to the suggested interaction between ERG1 and MiRP1 (and/or minK), there is presently no direct biochemical/molecular evidence demonstrating a functional interaction between the minK and KvLQT1 proteins and/or that minK/KvLQT1 interactions play a role in the generation of cardiac I_Kschannels.

A yeast two-hybrid screen, using the intracellular cytoplasmic COOH terminus of minK as the bait, led to the identification of a novel LIM-domain-containing protein, fh12 (274). Heterologous expression studies further suggest that fh12 is required for the generation of functional cell surface KvLQT1/minK (I_Ks) channels (274). These observations suggest that fh12 is required for the proper assembly of the KvLQT1 and minK subunits, the trafficking of assembled channels, and/or the cell surface expression of functional KvLQT1/minK (I_Ks) channels. Nevertheless, direct biochemical evidence for coassembly of the KvLQT1 protein with minK, with fh12 and/or with any other Kv channel accessory subunits (Table 5) in the myocardium has not been provided, and the subunit stoichiometry of functional myocardial I_Ks channels remains to be determined. Similar to KCNH₂, splice variants of KCNQ1, which exert a dominant negative effect when coexpressed with the full-length KvLQT1 protein (236), have also been described, although the roles of these variants in the generation of I_Ks channels in vivo remain to be determined.

A variety of experimental strategies, primarily in mice, have been exploited in studies focused on defining the molecular correlates of several of the other types of cardiac delayed rectifier Kv currents (Table 1). A role for Kv1 -subunits in the generation of mouse ventricular I_K,slow, for example, was revealed with the demonstration that I_K,slow is selectively attenuated in ventricular myocytes isolated from transgenic mice expressing a truncated Kv1.1 -subunit, Kv1.1N206Tag, that functions as a dominant negative (303). It was subsequently shown, however, that I_K,slow is also reduced in ventricular myocytes expressing a dominant negative mutant of Kv 2.1, Kv2.1N216 (560). Further analyses revealed that there are actually two distinct components of wild-type mouse ventricular I_K,slow: one that is sensitive to micromolar concentrations of 4-AP and encoded by Kv1 -subunits, and another that is sensitive to TEA and encoded by Kv2 -subunits (263, 560, 587). These currents are now referred to as I_K,slow1 and I_K,slow2, respectively (263, 291, 587). Subsequent studies revealed that I_K,slow1 is selectively eliminated in ventricular myocytes isolated from mice in which Kv1.5 has been deleted, suggesting that Kv1.5 encodes the micromolar 4-AP-sensitive mouse ventricular I_K,slow1 (302). These findings, together with the previous results obtained on cells isolated from Kv1.4 –/– animals (305), in which I_to,s is eliminated (196), suggest that, in contrast to the Kv4 -subunits (193), the Kv1 -subunits, Kv1.4 and Kv1.5, do not associate in adult mouse ventricles in situ. Rather, functional Kv1 -subunit-encoded Kv channels in mouse ventricular myocytes appear to be homomeric, composed of Kv1.4 -subunits (I_to,s) or Kv1.5 -subunits (I_K,slow1). The roles of Kv channel accessory subunits in the generation of these myocardial Kv1 -subunit-encoded Kv channels, however, remain to be defined.

Electrophysiological studies completed on isolated rat atrial myocytes (74, 75), and later on canine (577), human (19, 531), and mouse (69) atrial myocytes, demonstrated the presence of a novel component of delayed rectification, referred to as I_Kur (I_Kultrarapid), with time- and voltage-dependent properties that are quite distinct from I_Kr and I_Ks (479). Although I_Kur appears to be an atrial specific current in large mammals, the properties of I_K,slow1 in mouse ventricular myocytes are indistinguishable from (rat, human, and canine) atrial I_Kur (168, 562, 586). Human, rat, and canine atrial I_Kur, like mouse ventricular I_K,slow1, activates rapidly and undergoes little or no inactivation during brief depolarizations, properties similar to those seen on heterologous expression of several different Kv -subunits, including Kv1.2, Kv1.5, Kv3.1, and others. In addition, I_Kur, like I_K,slow1, is sensitive to micromolar concentrations of 4-AP (168, 562, 586). The later finding led to the hypothesis that Kv1.5 likely encodes human and rat atrial I_Kur (75, 533). Direct experimental support for this hypothesis was provided with the demonstration that exposure to antisense oligodeoxynucleotides targeted against Kv1.5 selectively attenuates I_Kur in isolated adult human (159) and rat (68) atrial myocytes. The important physiological role for Kv1.5 in human atria is suggested by the finding that I_Kur densities and Kv1.5 protein expression are reduced markedly in the atria of patients with chronic atrial fibrillation (511).

Although it was reported that Kv3.1, rather than Kv1.5, functions in canine atria to encode I_Kur (479), subsequent work demonstrated that Kv3.1 is not detectable in canine atria, whereas Kv1.5 (message and protein) is robustly expressed (157). It seems reasonable to conclude, therefore, that, similar to other Kv channels, the molecular correlate of cardiac I_Kur (Kv1.5) is similar across species. At present, it is unclear if Kv accessory subunits play a role in the generation of atrial (mouse, rat, canine, or human) I_Kur. Unexpectedly, however, it has now been demonstrated that Kv1 subunits do not associate with Kv1.5 in adult mouse ventricles and that the targeted deletion of Kv1 has no detectable effect on mouse ventricular I_Kur (I_K,slow1) (12). It may well be, however, that other Kv channel accessory subunits contribute to the generation of I_Kur channels. Further studies, focused on defining the molecular composition of I_Kur channels and the roles of accessory subunits, are needed to define the underlying molecular mechanisms involved in the regulation of I_Kur channels in the normal and diseased myocardium.

	VII. MOLECULAR COMPONENTS OF OTHER CARDIAC POTASSIUM CHANNELS

Top Previous Next References

 
A. Inwardly Rectifying Cardiac K⁺ (Kir) Channel Pore-Forming -Subunits

Similar to the Kv channels, functionally distinct types of myocardial inwardly rectifying K⁺ channels (378) are formed by the association of diverse inward rectifier K⁺ (Kir) channel pore-forming -subunit genes (140). Several Kir subunit subfamilies, Kir1 through Kir6, most with several members, have been identified (Table 6) and, like Kv -subunits, Kir -subunits also assemble as tetramers to form functional K⁺ selective channels (Fig. 5). Also similar to Kv channel -subunits, a unifying terminology has been developed for naming the Kir -subunit proteins (Kir1.x–Kir6.x) and the genes (KCNJ1–KCNJ15) encoding these proteins (199). In contrast to the Kv -subunits, however, the Kir -subunits have two (not six) transmembrane domains (Fig. 5).

Based on the properties of heterologously expressed Kir subunits, it was suggested that -subunits of the Kir2 subfamily likely encode the strongly inwardly rectifying Kir channels, I_K1, in cardiac cells (140, 378), and all (three) members of the Kir2 subfamily (Table 6) are expressed in the myocardium (298, 488). Interestingly, the KCNJ2 gene, which encodes Kir2.1, has been identified as the locus of mutations in Andersen's syndrome (243, 403), an inherited disorder that is often life-threatening owing to QT prolongation and cardiac (ventricular) arrhythmias. Similar to Timothy's syndrome (479), however, Andersen's syndrome is actually a multisystem disorder involving the cardiovascular, skeletomuscular, and other systems, and typically, Andersen's syndrome patients present initially with developmental abnormalities (494). The mutations in KCNJ2 that are associated with Andersen's syndrome that have been described to date appear to result in mutant Kir2.1 proteins that function in a dominant negative fashion to suppress Kv2.x-encoded I_K1 currents (11, 280, 409). Individuals carrying Andersen's syndrome mutations in KCNJ2 can display QT prolongation (Long QT7), periodic paralysis, as well as craniofacial malformations (11, 22, 494, 498), alone or in combination. Because only the KCNJ2 gene appears to be affected in Andersen's syndrome, the multisystem nature of this disorder likely reflects the fact that Kir2.x-encoded channels are expressed and are functional in a variety of cells/tissues. Myocardial I_K1 (and other K⁺channels) have been shown to be regulated directly by phosphatidylinositol bisphosphate (PIP₂) (209, 219, 470, 489). Interestingly, many of the Andersen's mutations are in the PIP₂ binding region of Kir2.1 (139), suggesting that the regulation/modulation of I_K1 channels by PIP₂ is altered and that it is the alterations in the modulatory effects of PIP₂ that underlie the phenotypic consequences of the Andersen's syndrome mutations.

The first direct molecular evidence that Kir2 -subunits encode cardiac I_K1 channels was provided in studies completed on myocytes isolated from mice bearing a targeted disruption of the coding region of Kir2.1 (Kir2.1 –/–) or Kir 2.2 (Kir2.2 –/–) (580, 581). Although the Kir2.1 –/– mice have cleft palate and die shortly after birth, thereby precluding electrophysiological studies on adult cells (580), experiments completed on isolated newborn Kir2.1 –/– ventricular myocytes revealed that I_K1 is absent (581). Interestingly, however, an inwardly rectifying current, with properties distinct from the wild-type I_K1, is evident in Kir2.1 –/– myocytes (581), suggesting either that an additional Kir current component is present, but difficult to resolve in wild-type cells in the presence of I_K1 or, alternatively, that a novel I_K1 is upregulated in Kir2.1 –/– hearts. In contrast to the findings in Kir2.1 –/– cells, voltage-clamp recordings from adult Kir2.2 –/– ventricular myocytes revealed that I_K1 densities are reduced, compared with I_K1 densities in wild-type cells, and that the properties of the residual I_K1 currents (in Kir2.2 –/– cells) are indistinguishable from wild-type I_K1 (581). These results were interpreted as suggesting that both Kir2.1 and Kir2.2 contribute to (mouse) ventricular I_K1 channels, and subsequent studies provided biochemical and molecular evidence to support this hypothesis (342, 592). The observation that the inwardly rectifying channels remaining in the absence of Kir2.1 have properties distinct from the endogenous I_K1 channels further suggests that functional cardiac I_K1 channels are heteromeric. Consistent with this hypothesis, detailed comparisons of the properties of heterologously expressed Kir2.1, Kir2.2, and Kir2.3 -subunits and endogenous guinea pig and sheep atrial and ventricular myocytes suggests marked regional and cell type specific differences in the molecular composition of I_K1 channels (133). Further studies focused on defining the molecular compositions of myocardial I_K1 channels in different cell types and in different species, including humans, are needed to define the molecular diversity and the functioning of these channels.

In the heart, weakly inwardly rectifying I_KATP channels are thought to play a role in both myocardial ischemia and preconditioning (232, 377, 380). In heterologous systems, I_KATP channels can be reconstituted by coexpression of Kir6.x subunits with ATP-binding cassette proteins that encode the sulfonylurea receptors, SURx (31, 456). Although previous pharmacological and molecular studies suggest that cardiac sarcolemmal I_KATP channels reflect the heteromeric assembly of Kir6.2 and SUR2A subunits, Kir6.1 is also expressed in the heart (406), and exposure of isolated (rat neonatal) ventricular myocytes to antisense oligodeoxynucleotides against SUR1 reduces I_KATP channel densities (572). These observations suggest the interesting possibility that there may be some molecular heterogeneity among cardiac I_KATP channels. The absolute requirement for the Kir6.2 subunit in the generation of cardiac I_KATP channels, however, was unequivocally demonstrated in studies completed on mice in which the Kir6.2 gene was disrupted by homologous recombination (292, 456, 484). Voltage-clamp recordings from ventricular myocytes isolated from these (Kir6.2 –/–) animals revealed no detectable I_KATP channel activity (292, 484). These findings clearly suggest that Kir6.1 alone (i.e., in the absence of Kir6.2) cannot generate functional myocardial I_KATP channels. Nevertheless, it is certainly still possible that Kir6.1 coassembles with Kir6.2 to form Kir6.1/Kir 6.2 heteromeric cardiac I_KATP channels.

The suggestion that SUR2 plays a pivotal role in the generation of cardiac I_KATP channels is supported by the finding that I_KATP channel density is reduced in myocytes from animals (SUR2 –/–) in which SUR2 has been deleted (411). In contrast, there are no measurable cardiac effects of the targeted disruption of SUR1 (455, 456). Nevertheless, it is interesting to note that I_KATP channel density is reduced, i.e., the channels are not eliminated, in SUR2 –/– ventricular myocytes, and the properties of the residual I_KATP channels in SUR2 –/– ventricular myocytes are similar to those of the channels produced on heterologous coexpression of Kir6.2 and SUR1 (411). These findings strongly suggest that SUR1 likely also coassembles with Kir 6.2 in ventricular myocytes to produce functional I_KATP channels (at least in the absence of SUR2). Immunohistochemical studies suggest that Kir6.2 and SUR2A assemble to form plasmalemmal cardiac I_KATP channels, whereas Kir6.1, Kir6.2 and SUR2A are expressed in mitochondria, observations interpreted as suggesting that the molecular compositions of functional I_Kr channels in different cellular compartments are distinct (473). Similar to I_K1 channels, myocardial I_KATP channels are also modulated by the binding of PIP₂ and other membrane lipids (209, 470).

The waveforms of action potentials recorded from isolated Kir6.2 –/– ventricular myocytes are indistinguishable from those recorded from wild-type cells (484). These observations clearly suggest that I_KATP channels do not play a role in shaping action potential waveforms (in mouse ventricles) under normal physiological conditions. The action potential shortening typically observed in wild-type ventricular cells during ischemia or metabolic blockade, however, is abolished in Kir6.2 –/– ventricular cells (484). In addition, the protective effect of ischemic preconditioning is abolished in Kir6.2 –/– hearts (456, 485), and infarct size in Kir6.2 –/– animals, with and without preconditioning, is the same (485). These observations are consistent with the hypothesis that cardiac I_KATP channels play an important role under pathophysiological conditions, particularly those involving metabolic stress (74, 577). Interestingly, however, it has been demonstrated that action potential durations are also largely unaffected in transgenic animals expressing mutant I_KATP channels with markedly (40-fold) reduced ATP sensitivity (268). The mutant I_KATP channels would be expected to be open (owing to the reduced sensitivity to closure by ATP) at rest and to markedly affect cardiac membrane excitability. The fact that action potentials are unaffected in ventricular myocytes expressing mutant I_KATPchannels clearly suggests that additional inhibitory regulatory mechanisms play a role in the physiological control of cardiac I_KATP channel activity in vivo (268). Further studies focused on defining and characterizing these regulatory mechanisms will be of considerable interest.

B. Two-Pore Domain K⁺ (K2P) Channel Pore-Forming -Subunits

In addition to the many Kv (Table 4) and Kir (Table 6) channel -subunits, a novel type of K⁺pore-forming -subunit with four transmembrane spanning regions and two pore domains (K2P) was identified with the cloning of TWI_K-1 (289), now referred to as KCNK1 (199). Studies in heterologous systems suggest that functional K2P channels, unlike Kv and Kir channels that assemble as tetramers, reflect the dimeric assembly of (two) K2P -subunits and that each of the (two) pore domains in each -subunit contributes to the formation of the K⁺-selective pore (286). Subsequent to the identification of TWI_K-1, a rather large number of K2P -subunit genes, KCNK1–KCNK17, were identified, and a subset of these appears to be expressed in the myocardium (Table 6). Similar to the Kv and Kir channels, a systematic terminology has been developed for naming the (KCNK) genes encoding K2P -subunits.

Heterologous expression studies have demonstrated that the members of various K2P subunit subfamilies give rise to K⁺-selective currents with distinct time- and voltage-dependent properties and differential sensitivities to a variety of modulators, including pH, fatty acids, and anesthetics (286, 287). It seems likely, therefore, that K2P subunit-encoded K⁺ channels could be important in regulating the normal physiological functioning of the adult mammalian heart, as well, perhaps, as influencing myocardial responses to pathophysiological stimuli. Direct experimental support for this hypothesis was provided with the demonstration that the pathophysiological effects of platelet activating factor on the myocardium are directly linked to inhibition of KCNK3 (TASK-1)-encoded (or closely related) K⁺ channels in ventricular myocytes (39). Interestingly, the effect of platelet activating factor is dependent on protein kinase C (39), although the underlying molecular mechanisms have not been detailed. Although it has been demonstrated that different KCNK-encoded K2P -subunits coassemble to form heteromeric channels (55), it is presently unclear whether heteromeric K2P -subunit assembly is physiologically relevant in the myocardium. Similarly, there is very little information presently available about the role(s) of accessory subunits and/or other regulatory molecules in the generation of K2P -subunit-encoded myocardial channels.

The facts that there are so many K2P -subunits (Table 6), that many of them are ubiquitously expressed, and that the properties of K2P -subunit-encoded channels are regulated by a variety of potentially relevant physiological (and pathophysiological) stimuli suggest that channels encoded by K2P -subunits likely subserve a variety of important physiological functions. Experimental support for this hypothesis was provided with the demonstration that mice bearing a targeted disruption of the KCNK2 gene (which encodes TREK-1, Table 6) display increased sensitivity to epilepsy and ischemia (208). It is unclear, however, whether there is a cardiac phenotype in the KCNK2 –/– mice, primarily because this possibility appears not to have been addressed (208). The physiological roles of TREK-1 and of each of the other K2P channel -subunits expressed in the myocardium, as well as in other cell types, therefore, remain largely unknown. Both TREK-1 and TASK-1 are expressed in the heart, and heterologous expression of either of these subunits alone gives rise to instantaneous, noninactivating K⁺currents that display little or no voltage dependence (286). These observations have led to suggestions that these subunits contribute to myocardial "background" or "leak" K⁺ currents (32), although presently, there is no direct experimental evidence to support this hypothesis. Interestingly, however, the properties of the currents produced on expression of TREK-1 or TASK-1 are similar to those of the current referred to as I_Kp identified in guinea pig ventricular myocytes (576), as well as to I_ss in mouse ventricular myocytes (79, 562). Further studies focused on defining the roles of each of the K2P -subunits in the generation of myocardial K⁺ channels, and the roles of these channels in the physiological, as well as the pathophysiological, functioning of the heart, are needed to clarify these issues.

	VIII. MYOCARDIAL POTASSIUM CHANNELS AND THE ACTIN CYTOSKELETON

Top Previous Next References

 
Similar to myocardial Nav channels, considerable evidence has now accumulated to suggest that several different types of plasmalemmal K⁺ channels in cardiac cells also interact with components of the actin cytoskeleton and that these interactions play important roles in regulating the properties, the trafficking, and/or the anchoring of these channels. It has been shown, for example, that myocardial I_KATP channels likely are linked to, and regulated by, the actin cytoskeleton (179, 336). Exposure to cytochalasin D, which disrupts/destabilizes actin filaments, for example, accelerates the rundown of cardiac I_KATP channels, whereas actin filament stabilizers inhibit channel rundown (179). It appears that cytochalasin D exerts its effects by interfering with the interaction between SUR2A and Kir6.2 subunits, thereby modifying SUR-mediated regulation of the I_KATP channels (76, 571). These observations suggest that the biophysical properties, as well as the cell surface expression of I_KATP channels, are affected by cytoskeletal interactions. Similarly, the biophysical properties, including rectification and Ca²⁺ (but not Mg²⁺) sensitivity, of myocardial I_K1 channels are affected by treatment with cytochalasin D (336).

There is also experimental evidence suggesting that the functioning of Kv channels in myocardial (and other) cells is regulated and/or modulated through interactions with the actin cytoskeleton. It has been demonstrated, for example, that exposure to phalloidin, which stabilizes actin filaments, markedly reduces action potential durations, whereas treatment with cytochalasin D (or cytochalasin B) prolongs action potential durations, in hypertrophied rat ventricular myocytes (568). Voltage-clamp studies revealed that the cytochalasin- and phalloidin-mediated effects on action potentials reflect the specific attenuation or augmentation, respectively, of I_to,f (568). These observations suggest that functional cardiac Kv4 -subunit-encoded I_to,fchannels are regulated/modulated directly or indirectly through interactions with the actin cytoskeleton. Interestingly, experiments in heterologous expression systems also suggest that the modulation of Kv1.5, which encodes cardiac I_Kur channels, by protein kinases and phosphatases, requires an intact cytoskeleton (333).

Similar to cardiac Nav channels, the interactions between functional myocardial Kir and Kv channels and the actin cytoskeleton are assumed to be mediated through association with actin-binding proteins and/or other scaffolding proteins, suggesting that Kir and Kv channels also function as multimeric protein complexes (Fig. 6). Consistent with this hypothesis, it was recently demonstrated that Kir2.x -subunits associate with several scaffolding proteins, including CASK, veli-3, mint-1, and SAP-97 (285). Interestingly, Kir subunits in brain also interact with a very large number and variety of PDZ domain-containing proteins (285, 372). In addition, it has been reported that Kir6.x subunits interact directly with the 14–3-3 protein and that this interaction is requisite for the functional cell surface expression of assembled Kir6.x-SUR-encoded I_KATP channels (575). It has also been reported that Kv -subunits in several subfamilies bind to PDZ-containing proteins, including PSD-95 and SAP-97 (145, 237, 557, 558), although the physiological significance of these observations in terms of the expression and/or the functioning of cardiac Kv channels has not been determined. The interactions between Kv1 and Kv4 -subunits and PSD-95 impact the recruitment of Kv1- and Kv4-encoded channels into lipid rafts (558). This finding may explain early observations suggesting that the expression of Kv4 -subunits alone fails to reveal targeting to lipid rafts (327). More importantly, these observations suggest that specific associations between Kv -subunits and PDZ-containing scaffolding proteins play an important role in the targeting of functional Kv channels to specific subcellular domains. The targeting of Kv channels to specific subcellular compartments would be expected to have rather profound effects on the regulation of membrane excitability and conduction through the myocardium (270, 463). In addition, Kv channel targeting could facilitate specific interactions between Kv channels and modulatory/regulatory proteins, including protein kinases and phosphatases, as has been clearly demonstrated in the protein kinase A-mediated regulation of cardiac I_Ks channels, that appears to be mediated by an A-kinase anchoring protein or AKAP (330, 331).

Interestingly, it has also been demonstrated that Kv1 -subunits contain PDZ-binding domains (145) and that Kv1.5 binds directly to the actin-binding protein -actinin-2 (329). Further studies revealed that members of three subfamilies of Kv -subunits, Kv1.5, Kv2.1, and Kv4.2, bind to -actinin-2, interactions that affect the properties and the functional expression of the resulting Kv -subunit-encoded K⁺ currents (329). These observations suggest that several Kv -encoded Kv channels likely also interact directly with the actin cytoskeleton via -actinin-2 (Fig. 6). It has also been reported that Kv4 -subunits interact directly with filamin (401) and that this interaction regulates the functional cell surface expression and the localization of Kv4-encoded channels (401). Subsequent studies revealed that actin depolymerization modulates the cell surface expression of heterologously expressed Kv4-encoded channels (529). Similar to Nav channels, these observations suggest the interesting and potentially important hypothesis that myocardial Kv channels also function as components of macromolecular complexes containing the channel components and a variety of scaffolding and regulatory proteins linked to the actin cytoskeleton (Fig. 6).

Recently, it was also suggested that Kir3-encoded channels interact with integrin (344), suggesting that myocardial K⁺ channel expression and functioning may also be linked to the extracellular matrix. Although clearly in the very early stages, it seems reasonable to suggest that the link between the cytoskeleton (as well, perhaps, as the extracellular matrix) in the regulation of myocardial K⁺ channel expression, localization, and functioning has been made. Further studies, aimed at exploring the roles of the cytoskeleton and the extracellular matrix in regulating myocardial K⁺ channel expression, distribution, and functioning, as well as those focused on probing the underlying molecular mechanisms, will likely provide important new insights into the physiological and pathophysiological roles of these interactions.

	IX. SUMMARY AND CONCLUSIONS

Top Previous Next References

 
Electrophysiological studies have identified multiple types of voltage-gated inward and outward currents expressed in cardiac cells (Table 1). The outward K⁺ currents are more numerous and more diverse than the inward (Na⁺ and Ca²⁺) currents, and most cardiac myocytes express multiple voltage-gated, as well as inwardly rectifying, K⁺ channels (Table 1). These (K⁺) channels are the primary determinants of myocardial action potential repolarization, and regional differences in K⁺ channel densities and properties underlie observed variations in action potential waveforms and contribute to the generation of normal cardiac rhythms. Voltage-gated inward Ca²⁺ channel currents and the Na⁺ channel "window" current, however, also contribute to myocardial action potential repolarization. The pivotal role played by the Nav channel "window" current, for example, has been elegantly demonstrated in electrophysiological studies characterizing mutations in SCN5A that underlie long QT3, as well as in computer-based simulations (Fig. 4) of cellular electrical activity (73, 90, 141, 188).

Molecular cloning has revealed an unexpected diversity of ion channel pore-forming -subunits (Tables 2, 4, and 6) and accessory subunits (Tables 3 and 5) that contribute to the formation of the various inward and outward current-carrying channels (Table 1) identified electrophysiologically in myocardial cells. Similar to the electrophysiological diversity of myocardial K⁺ channels (Table 1), the molecular analysis has revealed that multiple voltage-gated (Kv) (Table 4) and inwardly rectifying (Kir) (Table 6) K⁺ channel pore-forming -subunits, as well as a number of accessory subunits of these (Kir and Kv) channels (Table 5), are expressed in the myocardium. A variety of in vitro and in vivo experimental approaches have been exploited to probe the relationship(s) between these subunits and functional myocardial K⁺ channels, and important insights have been provided through molecular genetics and the application of techniques that allow functional channel expression to be manipulated directly. In contrast to the progress made in defining the roles of the various Kv and the Kir -subunits in the generation of functional myocardial Kv and Kir channels, there is very little known about the functional roles of the K2P -subunits (Table 6). In addition, the specific and/or multiple functional roles of most of the known K⁺ channel accessory subunits (Table 5) remain to be clarified. Defining the molecular correlates/compositions of the various myocardial K⁺ channels will facilitate future efforts focused on delineating the molecular mechanisms controlling the properties and the functional expression of these channels.

In addition to the diversity of pore-forming and accessory channel subunits (Tables 2–6), several recent studies, exploiting a combination of molecular, biochemical, and electrophysiological approaches, have revealed a rather staggering array of proteins that seem likely to contribute to regulating the properties, the cell surface expression, and/or the subcellular localization of functional myocardial membrane ion channels (Fig. 6). Taken together, the results of these studies suggest that myocardial ion channels function as macromolecular protein complexes. Interestingly, several components of these macromolecular protein channel complexes provide links to the actin cytoskeleton and the extracellular matrix (Fig. 6), suggesting important functional links between different ion channel complexes in cardiac cells, as well as between myocardial structure and electrical functioning. Understanding this molecular complexity clearly demands that novel experimental approaches, such as proteomics and genomics, be exploited to identify the various components of functional ion channel complexes, the sites of protein-protein interactions, and the underlying mechanisms involved in mediating these interactions. These areas represent important areas for further research focus in efforts directed towards defining the detailed mechanisms involved in the regulation/modulation of myocardial membrane excitability and the generation of normal cardiac rhythms.

Numerous studies have documented changes in functional ion channel expression during normal cardiac development, as well as in damaged or diseased heart. It has also been demonstrated that electrical remodeling occurs in the heart in response to changes in electrical activity or cardiac output, and this remodeling is directly attributed to changes in the functional expression and/or the properties of the various ion channels that underlie myocardial action potential generation. Although there are numerous possible (transcriptional, translational, and posttranslational) mechanisms that could be involved in regulating the functional expression and the properties of these channels, very little is presently known about the underlying molecular mechanisms that are important in mediating the changes in channel expression evident during normal development, as well as in conjunction with myocardial damage, disease, and/or electrical remodeling. An important focus of future research efforts will almost certainly be on exploring these mechanisms in detail.

	GRANTS

Top Previous Next References

 
We gratefully acknowledge the long-standing and continued financial support for our research endeavors provided by the National Heart, Lung, and Blood Institute of the National Institutes of Health.

	ACKNOWLEDGMENTS

Top References

 
We thank past and present members of our laboratories for their contributions to the understanding of the molecular basis of cardiac repolarization. In addition, we are indebted to Rick Wilson for his expert assistance with the generation of the tables and figures presented in this review and to Dr. Kevin Sampson for conducting the simulations illustrated in Figures 4 and 7.

Address for reprint requests and other correspondence: J. M. Nerbonne, Dept. of Molecular Biology and Pharmacology, Washington University Medical School, 660 South Euclid Ave., St. Louis, MO 63110 (E-mail: jnerbonne{at}msnotes.wustl.edu).

	REFERENCES

Top

 

1. Abbott GW and Goldstein SA. A superfamily of small potassium channel subunits: form and function of the MinK-related peptides (MiRPs). Q Rev Biophys 31: 357–398, 1998.[CrossRef][Web of Science][Medline]
2. Abbott GW and Goldstein S. Potassium channel subunits encoded by the KCNE gene family: physiology and pathophysiology of the minK-related peptides (MiRPs). Mol Interventions 1: 95–107, 2001.[Abstract/Free Full Text]
3. Abbott GW, Goldstein S, and Sesti F. Do all voltage-gated potassium channels use MiRPs? Circ Res 88: 981–983, 2001.[Free Full Text]
4. Abbott GW, Butler MH, Bendahhou S, Dalakas MC, Ptacek LJ, and Goldstein SA. MiRP2 forms potassium channels in skeletal muscle with Kv3.4 and is associated with periodic paralysis. Cell 104: 217–231, 2001.[CrossRef][Web of Science][Medline]
5. Abbott GW, Sesti F, Splawski I, Buck ME, Lehmann MH, Timothy KW, Keating MT, and Goldstein SA. MiRP1 forms I_Kr potassium channels with HERG and is associated with cardiac arrhythmia. Cell 97: 175–187, 1999.[CrossRef][Web of Science][Medline]
6. Abernethy DR and Soldatov NM. Structure-functional diversity of human L-type Ca²⁺ channel: perspectives for new pharmacological targets. J Pharmacol Exp Ther 300: 724–728, 2002.[Abstract/Free Full Text]
7. Abriel H, Cabo C, Wehrens XH, Rivolta I, Motoike HK, Memmi M, Napolitano C, Priori SG, and Kass RS. Novel arrhythmogenic mechanism revealed by a long QT syndrome mutation in the cardiac Na⁺ channel. Circ Res 88: 740–745, 2001.[Abstract/Free Full Text]
8. Abriel H, Wehrens XH, Benhorin J, Kerem B, and Kass RS. Molecular pharmacology of the sodium channel mutation D1790G linked to the long QT syndrome. Circulation 102: 921–925, 2000.[Abstract/Free Full Text]
9. Accili EA, Kiehn J, Wible BA, and Brown AM. Interactions among inactivating and noninactivating Kvbeta subunits, and Kvalpha1.2, produce potassium currents with intermediate inactivation. J Biol Chem 272: 28232–28236, 1997.[Abstract/Free Full Text]
10. Accili EA, Kiehn J, Yang Q, Wang Z, Brown AM, and Wible BA. Separable Kvbeta subunit domains alter expression and gating of potassium channels. J Biol Chem 272: 25824–25831, 1997.[Abstract/Free Full Text]
11. Ai T, Fujiwara Y, Tsuji K, Otani H, Nakano S, Kubo Y, and Horie M. Novel KCNJ2 mutation in familial periodic paralysis with ventricular dysrhythmia. Circulation 105: 2592–2594, 2002.[Abstract/Free Full Text]
12. Aimond F, Kwak SP, Rhodes KJ, and Nerbonne JM. The accessory Kv1 subunit differentially modulates the functional expression of voltage-gated K⁺ channels in adult rat ventricular myocytes. Circ Res 96: 451–458, 2005.[Abstract/Free Full Text]
13. Akar FG and Rosenbaum DS. Transmural electrophysiological heterogeneities underlying arrhythmogenesis in heart failure. Circ Res 93: 638–645, 2003.[Abstract/Free Full Text]
14. Akar FG, Laurita KR, and Rosenbaum DS. Cellular basis for dispersion of repolarization underlying reentrant arrhythmias. J Electrocardiol 33: 23–31, 2000.[CrossRef][Medline]
15. Akar FG, Wu RC, Deschênes I, Armoundas V, Piacentino V, Houser SR, and Tomaselli GF. Phenotypic differences in the transient outward K⁺ current of human and canine ventricular myocytes: insights into the molecular composition of I_to. Am J Physiol Heart Circ Physiol 286: H602–H609, 2004.[Abstract/Free Full Text]
16. Akar FG, Yan GX, Antzelevitch C, and Rosenbaum DS. Unique topographical distribution of M cells underlies reentrant mechanism of torsade de pointes in the long QT syndrome. Circulation 105: 1247–1253, 2002.[Abstract/Free Full Text]
17. Alseikhan BA, DeMaria CD, Colecraft HM, and Yue DT. Engineered calmodulins reveal the unexpected eminence of Ca²⁺ channel inactivation in controlling heart excitation. Proc Natl Acad Sci USA 99: 17185–17190, 2002.[Abstract/Free Full Text]
18. Altier C, Dubel SJ, Barrère C, Jarvis SE, Stotz SC, Spaetgens RL, Scott JD, Cornet V, DeWaard M, Zamponi GW, Nargeot J, and Bourinet E. Trafficking of L-type calcium channels mediated by the postsynaptic scaffolding protein AKAP79. J Biol Chem 277: 33598–33603, 2002.[Abstract/Free Full Text]
19. Amos GJ, Wettwer E, Metzger F, Li Q, Himmel HM, and Ravens U. Differences between outward currents of human atrial and subepicardial ventricular myocytes. J Physiol 491: 31–50, 1996.[Abstract/Free Full Text]
20. An RH, Wang XL, Kerem B, Benhorin J, Medina A, Goldmit M, and Kass RS. Novel LQT-3 mutation affects Na⁺ channel activity through interactions between alpha- and beta1-subunits. Circ Res 83: 141–146, 1998.[Abstract/Free Full Text]
21. An WF, Bowlby MR, Betty M, Cao J, Ling HP, Mendoza G, Hinson JW, Mattsson KI, Strassle BW, Trimmer JS, and Rhodes KJ. Modulation of A-type potassium channels by a family of calcium sensors. Nature 403: 553–556, 2000.[CrossRef][Medline]
22. Andelfinger G, Tapper AR, Welch RC, Vanoye CG, George AL Jr., and Benson DW. KCNJ2 mutation results in Andersen syndrome with sex-specific cardiac and skeletal muscle phenotypes. Am J Hum Genet 71: 663–668, 2002.[CrossRef][Web of Science][Medline]
23. Antzelevitch C. Molecular genetics of arrhythmias and cardiovascular conditions associated with arrhythmias. J Cardiovasc Electrophysiol 14: 1259–1272, 2003.[CrossRef][Web of Science][Medline]
24. Antzelevitch C and Dumaine R. Electrical heterogeneity in the heart: physiological, pharmacological and clinical implications. In: Handbook of Physiology. The Cardiovascular System. The Heart. Bethesda, MD: Am. Physiol. Soc., 2002, sect. 2, vol. I, p. 654–692.
25. Anumonwo JMB, Freeman LC, Kwok WM, and Kass RS. Delayed rectification in single cells isolated from guinea pig sinoatrial node. Am J Physiol Heart Circ Physiol 262: H921–H925, 1992.[Abstract/Free Full Text]
26. Apkon M and Nerbonne JM. Characterization of two distinct depolarization-activated K⁺ currents in isolated adult rat ventricular myocytes. J Gen Physiol 97: 973–1011, 1991.[Abstract/Free Full Text]
27. Aravamudan B, Volonte D, Ramani R, Gursoy E, Lisanti MP, London B, and Galbiati F. Transgenic overexpression of caveolin-3 in the heart induces a cardiomyopathic phenotype. Hum Mol Genet 12: 2777–2788, 2003.[Abstract/Free Full Text]
28. Arikkath J and Campbell KP. Auxiliary subunits: essential components of the voltage-gated calcium channel complex. Curr Opin Neurobiol 13: 298–307, 2003.[CrossRef][Web of Science][Medline]
29. Attali B, Lesage F, Ziliani P, Guillemare E, Honoré E, Waldmann R, Hugnot J, Mattéi M, Lazdunski M, and Barhanin J. Multiple mRNA isoforms encoding the mouse cardiac Kv1.5 delayed rectifier K⁺ channel. J Biol Chem 268: 24283–24289, 1993.[Abstract/Free Full Text]
30. Attwell D, Cohen I, Eisner D, Ohba M, and Ojeda C. The steady state TTX-sensitive ("window") sodium current in cardiac Purkinje fibres. Pflügers Arch 379: 137–142, 1979.[CrossRef][Web of Science][Medline]
31. Babenko AP, Aguilar-Bryan L, and Bryan J. A view of sur/KIR6.x, K_ATP channels. Annu Rev Physiol 60: 667–687, 1998.[CrossRef][Web of Science][Medline]
32. Backx PH and Marban E. Background potassium current active during the plateau of the action potential in guinea pig ventricular myocytes. Circ Res 72: 890–900, 1993.[Abstract/Free Full Text]
33. Bahring R, Dannenberg J, Peters HC, Leicher T, Pongs O, and Isbrandt D. Conserved Kv4 N-terminal domain critical for effects of Kv channel-interacting protein 2.2 on channel expression and gating. J Biol Chem 276: 23888–23894, 2001.[Abstract/Free Full Text]
34. Bailly P, Benitah JP, Mouchoniere M, Vassort G, and Lorente P. Regional alteration of the transient outward current in human left ventricular septum during compensated hypertrophy. Circulation 96: 1266–1274, 1997.[Abstract/Free Full Text]
35. Baloh RW and Jen JC. Genetics of familial episodic vertigo and ataxia. Ann NY Acad Sci 956: 338–345, 2002.[Web of Science][Medline]
36. Balser JR. Inherited sodium channelopathies: models for acquired arrhythmias? Am J Physiol Heart Circ Physiol 282: H1175–H1180, 2002.[Free Full Text]
37. Balser JR, Bennett PB, and Roden DM. Time-dependent outward current in guinea pig ventricular myocytes. Gating kinetics of the delayed rectifier. J Gen Physiol 96: 835–863, 1990.[Abstract/Free Full Text]
38. Bangalore R, Mehrke G, Gingrich K, Hofmann F, and Kass RS. Influence of L-type Ca channel alpha 2/delta-subunit on ionic and gating current in transiently transfected HEK 293 cells. Am J Physiol Heart Circ Physiol 270: H1521–H1528, 1996.[Abstract/Free Full Text]
39. Barbuti A, Ishii S, Shimizu T, Robinson RB, and Feinmark SJ. Block of the background K⁺ channel TASK-1 contributes to arrhythmogenic effects of platelet-activating factor. Am J Physiol Heart Circ Physiol 282: H2024–H2030, 2002.[Abstract/Free Full Text]
40. Barhanin J, Lesage F, Guillemare E, Fink M, Lazdunski M, and Romey G. KvLQT1 and lsK (minK) proteins associate to form the I_Ks cardiac potassium current. Nature 384: 78–80, 1996.[CrossRef][Web of Science][Medline]
41. Baroudi G and Chahine M. Biophysical phenotypes of SCN5A mutations causing long QT and Brugada syndromes. FEBS Lett 487: 224–228, 2000.[CrossRef][Web of Science][Medline]
42. Barry DM and Nerbonne JM. Myocardial potassium channels: electrophysiological and molecular diversity. Annu Rev Physiol 58: 363–394, 1996.[CrossRef][Web of Science][Medline]
43. Barry DM, Xu H, Schuessler RB, and Nerbonne JM. Functional knockout of the transient outward current, long QT syndrome, and cardiac remodeling in mice expressing a dominant-negative Kv4 alpha subunit. Circ Res 83: 560–567, 1998.[Abstract/Free Full Text]
44. Bean BP. Two kinds of calcium channels in canine atrial cells. Differences in kinetics, selectivity, and pharmacology. J Gen Physiol 86: 1–30, 1985.[Abstract/Free Full Text]
45. Beeler GW and Reuter H. The relation between membrane potential, membrane currents and activation of contraction in ventricular muscle fibers. J Physiol 207: 211–229, 1970.[Abstract/Free Full Text]
46. Bellocq C, van Ginneken ACG, Bezzina CR, Alders M, Escande D, Mannens MMAM, Baró I, and Wilde AAM. Mutation in the KCNQ1 gene leading to the short QT-interval syndrome. Circulation 109: 2394–2397, 2004.[Abstract/Free Full Text]
47. Benhorin J, Goldmit M, MacCluer JW, Blangero J, Goffen R, Leibovitch A, Rahat A, Wang Q, Medina A, Towbin J, and Kerem B. Identification of a new SCN5A mutation, D1840G, associated with the long QT syndrome. Mutations in brief no 153. Hum Mutat 12: 72, 1998.[Medline]
48. Benhorin J, Taub R, Goldmit M, Kerem B, Kass RS, Windman I, and Medina A. Effects of flecainide in patients with new SCN5A mutation: mutation-specific therapy for long QT syndrome? Circulation 101: 1698–1706, 2000.[Abstract/Free Full Text]
49. Bénitah JP, Gómez AM, Fauconnier J, Kerfant BG, Perrier E, Vassort G, and Richard S. Voltage-gated Ca²⁺ channels in the human pathophysiological heart: a review. Basic Res Cardiol 97 Suppl 1: 1/11–1/18, 2002.
50. Bennett PB. Anchors aweigh. Ion channels, cytoskeletal proteins and cellular excitability. Circ Res 86: 367–368, 2000.[Free Full Text]
51. Bennett PB. Long QT syndrome: biophysical and pharmacologic mechanisms in LQT3. J Cardiovasc Electrophysiol 11: 819–822, 2000.[Medline]
52. Bennett PB, Yazawa K, Makita N, and George AL. Molecular mechanism for an inherited cardiac arrhythmia. Nature 376: 683–685, 1995.[CrossRef][Medline]
53. Bennett V and Baines AJ. Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues. Physiol Rev 81: 1353–1392, 2001.[Abstract/Free Full Text]
54. Bennett V and Chen L. Ankyrins and cellular targeting of diverse membrane proteins to physiological sites. Curr Opin Cell Biol 13: 61–67, 2001.[CrossRef][Web of Science][Medline]
55. Berg AP, Talley EM, Manger JP, and Bayliss DA. Motoneurons express heteromeric TWI_K-related acid-sensitive K⁺ (TASK) channels containing TASK-1 (KCNK3) and TASK-3 (KCNK9) subunits. J Neurosci 24: 6693–6702, 2004.[Abstract/Free Full Text]
56. Bers DM. Excitation-Contraction Coupling and Contractile Force (2nd ed.). The Netherlands: Kluwer, 2001.
57. Bers DM and Perez-Reyes E. Ca channels in cardiac myocytes: structure and function in Ca influx and intracellular Ca release. Cardiovasc Res 42: 339–360, 1999.[Abstract/Free Full Text]
58. Bertaso F, Sharpe CC, Hendry BM, and James AF. Expression of voltage-gated K⁺ channels in human atrium. Basic Res Cardiol 97: 424–433, 2002.[CrossRef][Web of Science][Medline]
59. Beurg M, Sukhareva M, Strube C, Powers PA, Gregg RG, and Coronado R. Recovery of Ca²⁺ current, charge movements, and Ca²⁺ transients in myotubes deficient in dihydropyridine receptor beta 1 subunit transfected with beta 1 cDNA. Biophys J 73: 807–818, 1997.[Medline]
60. Bezzina C, Veldkamp MW, van Den Berg MP, Postma AV, Rook MB, Viersma JW, van Langen IM, Tan-Sindhunata G, Bink-Boelkens MT, van Der Hout AH, Mannens MM, and Wilde AA. A single Na⁺ channel mutation causing both long QT and Brugada syndromes. Circ Res 85: 1206–1213, 1999.[Abstract/Free Full Text]
61. Bhave G and Gereau RW. Growing pains: the cytoskeleton as a critical regulator of pain plasticity. Neuron 39: 577–583, 2003.[CrossRef][Medline]
62. Bianchi L, Shen Z, Dennis AT, Priori SG, Napolitano C, Ronchetti E, Bryskin R, Schwartz PJ, and Brown AM. Cellular dysfunction of LQT5-minK mutants: abnormalities of I_Ks, I_Kr and trafficking in long QT syndrome. Hum Mol Genet 8: 1499–1507, 1999.[Abstract/Free Full Text]
63. Bichet D, Lecomte C, Sabatier JM, Felix R, and De Waard M. Reversibility of the Ca²⁺ channel alpha1-beta subunit interaction. Biochem Biophys Res Commun 277: 729–735, 2000.[CrossRef][Web of Science][Medline]
64. Biel M, Ruth P, Bosse E, Hullin R, Stuhmer W, Flockerzi V, and Hofmann F. Primary structure and functional expression of a high voltage activated calcium channel from rabbit lung. FEBS Lett 269: 409–412, 1990.[CrossRef][Web of Science][Medline]
65. Biervert C, Schroeder BC, Kubisch C, Berkovic SF, Propping P, Jentsch TJ, and Steinlein OK. A potassium channel mutation in neonatal human epilepsy. Science 279: 403–406, 1998.[Abstract/Free Full Text]
66. Blackstone C and Sheng M. Protein targeting and calcium signaling microdomains in neuronal cells. Cell Calcium 26: 181–192, 1999.[CrossRef][Web of Science][Medline]
67. Bolli R and Marbán E. Molecular and cellular mechanisms of myocardial stunning. Physiol Rev 79: 609–634, 1999.[Abstract/Free Full Text]
68. Bou-Abboud E and Nerbonne JM. Molecular correlates of the calcium-independent, depolarization-activated K⁺ currents in rat atrial myocytes. J Physiol 517: 407–420, 1999.[Abstract/Free Full Text]
69. Bou-Abboud E, Li H, and Nerbonne JM. Molecular diversity of the repolarizing voltage-gated K⁺ currents in mouse atrial myocytes. J Physiol 529: 345–358, 2000.[Abstract/Free Full Text]
70. Bourinet E, Charnet P, Tomlinson WJ, Stea A, Snutch TP, and Nargeot J. Voltage-dependent facilitation of a neuronal alpha 1C L-type calcium channel. EMBO J 13: 5032–5039, 1994.[Web of Science][Medline]
71. Boyett MR, Dobrzynski H, Lancaster MK, Jones SA, Honjo H, and Kodama I. Sophisticated architecture is required for the sinoatrial node to perform its normal pacemaker function. J Cardiovasc Electrophysiol 14: 104–106, 2003.[CrossRef][Web of Science][Medline]
72. Boyett MR, Honjo H, and Kodama I. The sinoatrial node, a heterogeneous pacemaker structure. Cardiovasc Res 47: 658–687, 2000.[Abstract/Free Full Text]
73. Boyett MR, Honjo H, Yamamoto M, Nikmaram MR, Niwa R, and Kodama I. Regional differences in the effects of 4-aminopyridine within the sinoatrial node. Am J Physiol 275: H1158–H1168, 1998.[Web of Science][Medline]
74. Boyle WA and Nerbonne JM. A novel type of depolarization-activated K⁺ current in isolated adult rat atrial myocytes. Am J Physiol Heart Circ Physiol 260: H1236–H1247, 1991.[Abstract/Free Full Text]
75. Boyle WA and Nerbonne JM. Two functionally distinct 4-aminopyridine-sensitive outward K⁺ currents in rat atrial myocytes. J Gen Physiol 100: 1041–1067, 1992.[Abstract/Free Full Text]
76. Brady PA, Alekseev AE, Aleksandrova LA, Gomez LA, and Terzic A. A disrupter of actin microfilaments impairs sulfonylurea-inhibitory gating of cardiac K_ATP channels. Am J Physiol Heart Circ Physiol 271: H2710–H2716, 1996.[Abstract/Free Full Text]
77. Brahmajothi MV, Campbell DL, Rasmusson RL, Morales MJ, Trimmer JS, Nerbonne JM, and Strauss HC. Distinct transient outward potassium current I_to phenotypes and distribution of fast-inactivating potassium channel alpha subunits in ferret left ventricular myocytes. J Gen Physiol 113: 581–600, 1999.[Abstract/Free Full Text]
78. Brugada R, Hong K, Dumaine R, Cordeiro J, Gaita F, Borggrefe M, Menendez TM, Brugada J, Pollevick GD, Wolpert C, Burashnikov E, Matsuo K, Wu YS, Guerchicoff A, Bianchi F, Giustetto C, Schimpf R, Brugada P, and Antzelevitch C. Sudden death associated with short-QT syndrome linked to mutations in HERG. Circulation 109: 30–35, 2004.[Abstract/Free Full Text]
79. Brunet S, Aimond F, Li H, Guo W, Eldstrom J, Fedida D, Yamada KA, and Nerbonne JM. Heterogeneous expression of repolarizing voltage-gated K⁺ currents in adult mouse ventricles. J Physiol 559: 103–120, 2004.[Abstract/Free Full Text]
80. Bryant SM, Wan X, Shipsey SJ, and Hart G. Regional differences in the delayed rectifier current (I_Kr and I_Ks) contribute to the differences in action potential duration in basal left ventricular myocytes in guinea-pig. Cardiovasc Res 40: 322–331, 1998.[Abstract/Free Full Text]
81. Burgoyne RD and Weiss JL. The neuronal calcium sensor family of Ca²⁺-binding proteins. Biochem J 353: 1–12, 2001.[CrossRef][Web of Science][Medline]
82. Buxbaum JD, Choi EK, Luo Y, Lilliehook C, Crowley AC, Merriam DE, and Wasco W. Calsenilin: a calcium-binding protein that interacts with the presenilins and regulates the levels of a presenilin fragment. Nat Med 4: 1177–1181, 1998.[CrossRef][Web of Science][Medline]
83. Campbell DL, Rasmusson RL, Qu Y, and Strauss HC. The calcium-independent transient outward potassium current in isolated ferret right ventricular myocytes. I. Basic characterization and kinetic analysis. J Gen Physiol 101: 571–601, 1993.[Abstract/Free Full Text]
84. Cantiello HF. Role of actin filament organization in cell volume and ion channel regulation. J Exp Zool 279: 425–435, 1997.[CrossRef][Web of Science][Medline]
85. Carbone E and Lux HD. Kinetics and selectivity of a low-voltage-activated calcium current in chick and rat sensory neurones. J Physiol 386: 547–570, 1987.[Abstract/Free Full Text]
86. Carbone E and Lux HD. Single low-voltage-activated calcium channels in chick and rat sensory neurones. J Physiol 386: 571–601, 1987.[Abstract/Free Full Text]
87. Carrion AM, Link WA, Ledo F, Mellstrom B, and Naranjo JR. DREAM is a Ca²⁺-regulated transcriptional repressor. Nature 398: 80–84, 1999.[CrossRef][Medline]
88. Castellano A, Chiara MD, Mellstrom B, Molina A, Monje F, Naranjo JR, and Lopez-Barneo J. Identification and functional characterization of a K⁺ channel alpha-subunit with regulatory properties specific to brain. J Neurosci 17: 4652–4661, 1997.[Abstract/Free Full Text]
89. Castellano A, Wei X, Birnbaumer L, and Perez-Reyes E. Cloning and expression of a neuronal calcium channel beta subunit. J Biol Chem 268: 12359–12366, 1993.[Abstract/Free Full Text]
90. Castellino RC, Morales MJ, Strauss HC, and Rasmusson RL. Time- and voltage-dependent modulation of a Kv1.4 channel by a beta-subunit (Kv beta 3) cloned from ferret ventricle. Am J Physiol Heart Circ Physiol 269: H385–H391, 1995.[Abstract/Free Full Text]
91. Catterall WA. Molecular mechanisms of inactivation and modulation of sodium channels. Renal Physiol Biochem 17: 121–125, 1994.[Web of Science][Medline]
92. Catterall WA. Structure and regulation of voltage-gated Ca²⁺ channels. Annu Rev Cell Dev Biol 16: 521–555, 2000.[CrossRef][Web of Science][Medline]
93. Catterall WA. From ionic currents to molecular mechanisms: the structure and function of voltage-gated sodium channels. Neuron 26: 13–25, 2000.[CrossRef][Web of Science][Medline]
94. Chauhan VS, Tuvia S, Buhusi M, Bennett V, and Grant AO. Abnormal cardiac Na⁺ channel properties and AT heart rate adaptation in neonatal ankyrin B knockout mice. Circ Res 86: 441–447, 2000.[Abstract/Free Full Text]
95. Chen C, Bharucha V, Chen Y, Westenbroek RE, Brown A, Malhotra JD, Jones D, Avery C, Gillespie PJ, Kazen-Gillespie KA, Kazarinova-Noyes K, Shrager P, Saunders TL, MacDonald RL, Ransom BR, Scheur T, Catterall WA, and Isom LL. Reduced sodium channel density, altered voltage dependence of inactivation, and increased susceptibility to seizures in mice lacking sodium channel subunits. Proc Natl Acad Sci USA 99: 17072–17077, 2002.[Abstract/Free Full Text]
96. Chen L, Chetkovich DM, Petralia RS, Sweeney NT, Kawasaki Y, Wenthold RJ, Bredt DS, and Nicoll RA. Stargazin regulates synaptic targeting of AMPA receptors by two distinct mechanisms. Nature 408: 936–943, 2000.[CrossRef][Medline]
97. Chen YH, Xu SJ, Bendahhou S, Wang XL, Wang Y, Xu WY, Jin HW, Sun H, Su XY, Zhuang QN, Yang YQ, Li YB, Liu Y, Xu HJ, Li XF, Ma N, Mou CP, Chen Z, Barhanin J, and Huang W. KCNQ1 gain-of-function mutation in familial atrial fibrillation. Science 299: 251–254, 2003.[Abstract/Free Full Text]
98. Chien AJ, Gao T, Perez-Reyes E, and Hosey MM. Membrane targeting of L-type calcium channels. Role of palmitoylation in the subcellular localization of the beta2a subunit. J Biol Chem 273: 23590–23597, 1998.[Abstract/Free Full Text]
99. Chien AJ, Zhao X, Shirokov RE, Puri TS, Chang CF, Sun D, Rios E, and Hosey MM. Roles of a membrane-localized beta subunit in the formation and targeting of functional L-type Ca²⁺ channels. J Biol Chem 270: 30036–30044, 1995.[Abstract/Free Full Text]
100. Choe S and Roosild T. Regulation of the K channels by cytoplasmic domains. Biopolymers 66: 294–299, 2002.[Medline]
101. Chouinard SW, Wilson GF, Schlimgen AK, and Ganetzky B. A potassium channel beta subunit related to the aldo-keto reductase superfamily is encoded by the Drosophila hyperkinetic locus. Proc Natl Acad Sci USA 92: 6763–6767, 1995.[Abstract/Free Full Text]
102. Clancy CE and Kass RS. Defective cardiac ion channels: from mutations to clinical syndromes. J Clin Invest 110: 1075–1077, 2002.[CrossRef][Web of Science][Medline]
103. Clancy CE and Rudy Y. Linking a genetic defect to its cellular phenotype in a cardiac arrhythmia. Nature 400: 566–569, 1999.[CrossRef][Medline]
104. Clancy CE and Rudy Y. Na⁺ channel mutation that causes both Brugada and long QT syndrome phenotypes: a simulation study of mechanism. Circulation 105: 1208–1213, 2002.[Abstract/Free Full Text]
105. Clancy CE, Tateyama M, and Kass RS. Insights into the molecular mechanisms of bradycardia-triggered arrhythmias in long QT-3 syndrome. J Clin Invest 110: 1251–1262, 2002.[CrossRef][Web of Science][Medline]
106. Clancy CE, Tateyama M, Liu H, Wehrens XH, and Kass RS. Non-equilibrium gating in cardiac Na⁺ channels: an original mechanism of arrhythmia. Circulation 107: 2233–2237, 2003.[Abstract/Free Full Text]
107. Clark RB, Bouchard RA, Salinas-Stefanon E, Sanchez-Chapula J, and Giles WR. Heterogeneity of action potential waveforms and potassium currents in rat ventricle. Cardiovasc Res 27: 1795–1799, 1993.[Abstract/Free Full Text]
108. Clark RB, Mangoni ME, Lueger A, Couette B, Nargeot J, and Giles WR. A rapidly activating delayed rectifier K⁺ current regulates pacemaker activity in adult mouse sinoatrial node cells. Am J Physiol Heart Circ Physiol 286: H1757–H1766, 2004.[Abstract/Free Full Text]
109. Cohen SA. Immunocytochemical localization of rH1 sodium channel in adult rat heart atria and ventricle. Presence in terminal intercalated discs. Circulation 94: 3083–3086, 1994.
110. Colecraft HM, Alseikhan B, Takahashi SX, Chaudhuri D, Mittman S, Yegnasubramanian V, Alvania RS, Johns DC, Marban E, and Yue DT. Novel functional properties of Ca²⁺ channel beta subunits revealed by their expression in adult rat heart cells. J Physiol 541: 435–452, 2002.[Abstract/Free Full Text]
111. Collet C, Csernoch L, and Jacquemond V. Intramembrane charge movement and L-type calcium current in skeletal muscle fibers isolated from control and mdx mice. Biophys J 84: 251–265, 2003.[Medline]
112. Coraboeuf E and Carmeliet E. Existence of two transient outward currents in sheep cardiac Purkinje fibers. Pflügers Arch 392: 352–359, 1982.[CrossRef][Web of Science][Medline]
113. Coraboeuf E, Deroubaix E, and Coulombe A. Effect of tetrodotoxin on action potentials of the conducting system in the dog heart. Am J Physiol Heart Circ Physiol 271: H561–H567, 1979.
114. Cormier JW, Rivolta I, Tateyama M, Yang AS, and Kass RS. Secondary structure of the human cardiac Na⁺ channel C terminus. Evidence for a role of helical structures in modulation of channel inactivation. J Biol Chem 277: 9233–9241, 2002.[Abstract/Free Full Text]
115. Côté PD, Moukhle H, and Carbonetto S. Dystroglycan is not required for localization of dystrophin, syntrophin and neuronal nitric-oxide synthase at the sarcolemma but regulates integrin _7B expression and caveolin-3 distribution. J Biol Chem 277: 4672–4679, 2002.[Abstract/Free Full Text]
116. Covarrubias M, Wei AA, and Salkoff L. Shaker, Shal, Shab, and Shaw express independent K⁺ current systems. Neuron 7: 763–773, 1991.[CrossRef][Web of Science][Medline]
117. Crawford GE, Faulkner JA, Crossbie RH, Campbell KP, Froehner SC, and Chamberlain JS. Assembly of the dystrophin-associated protein complex does not require the dystrophin COOH terminal domain. J Cell Biol 150: 1399–1409, 2000.[Abstract/Free Full Text]
118. Cukovic D, Lu GW, Wible B, Steele DF, and Fedida D. A discrete amino terminal domain of Kv1.5 and Kv1.4 potassium channels interacts with the spectrin repeats of alpha-actinin-2. FEBS Lett 498: 87–92, 2001.[CrossRef][Web of Science][Medline]
119. Curran ME, Splawski I, Timothy KW, Vincent GM, Green ED, and Keating MT. A molecular basis for cardiac arrhythmia: HERG mutations cause long QT syndrome. Cell 80: 795–803, 1995.[CrossRef][Web of Science][Medline]
120. Dahr Malhotra J, Chen C, Rivolta I, Abriel H, Malhotra R, Mattei LN, Brosius FC, Kass RS, and Isom LL. Characterization of sodium channel - and -subunits in rat and mouse cardiac myocytes. Circulation 103: 1303–1310, 2001.[Abstract/Free Full Text]
121. Dalakas MC, Park K, Semino-Mora C, Lee HS, Sivakumar K, and Goldfarb LG. Desmin myopathy, a skeletal myopathy with cardiomyopathy caused by mutations in the desmin gene. N Engl J Med 342: 770–780, 2000.[Abstract/Free Full Text]
122. Davare MA, Avdonin V, Hall DD, Peden EM, Burette A, Weinberg RJ, Horne MC, Hoshi T, and Hell JW. A ₂ adrenergic receptor signaling complex assembled with the Ca²⁺ channel Ca_v1.2. Science 293: 98–101, 2001.[Abstract/Free Full Text]
123. Davies NP and Hanna MG. The skeletal muscle channelopathies: distinct entities and overlapping syndromes. Curr Opin Neurol 16: 559–568, 2003.[Medline]
124. Deal KK, England SK, and Tamkun MM. Molecular physiology of cardiac potassium channels. Physiol Rev 76: 49–67, 1996.[Abstract/Free Full Text]
125. Decher N, Barth A, Gonzalez T, Steinmeyer K, and Sanguinetti MC. Novel KChIP2 isoforms increase functional diversity of transient outward potassium currents. J Physiol 557: 761–772, 2004.[Abstract/Free Full Text]
126. Delisle BP, Anson BD, Rajamani S, and January CT. Biology of cardiac arrhythmias: ion channel protein trafficking. Circ Res 94: 1418–1428, 2004.[Abstract/Free Full Text]
127. Delmar M. Role of potassium currents on cell excitability in cardiac ventricular myocytes. J Cardiovasc Electrophysiol 3: 474–486, 1992.[CrossRef][Web of Science]
128. DeMaria CD, Soong TW, Alseikhan BA, Alvania RS, and Yue DT. Calmodulin bifurcates the local Ca²⁺ signal that modulates P/Q-type Ca²⁺ channels. Nature 411: 484–489, 2001.[CrossRef][Medline]
129. Deschenes I, DiSilvestre D, Juang J, Wu RC, An WF, and Tomaselli GF. Regulation of Kv4.3 current by KChIP2 splice variants: a component of native cardiac I_to? Circulation 106: 423–429, 2002.[Abstract/Free Full Text]
130. Deschenes I and Tomaselli GF. Modulation of Kv4.3 current by accessory subunits. FEBS Lett 528: 183–188, 2002.[CrossRef][Web of Science][Medline]
131. De Waard M, Pragnell M, and Campbell KP. Ca²⁺ channel regulation by a conserved beta subunit domain. Neuron 13: 495–503, 1994.[CrossRef][Web of Science][Medline]
132. De Waard M, Witcher DR, Pragnell M, Liu H, and Campbell KP. Properties of the alpha 1-beta anchoring site in voltage-dependent Ca²⁺ channels. J Biol Chem 270: 12056–12064, 1995.[Abstract/Free Full Text]
133. Dhamoon AS, Pandit SV, Sarmast F, Parisian KR, Guha P, Li Y, Bagwe S, Taffet SM, and Anumonwo JM. Unique Kir2.x properties determine regional and species differences in the cardiac inward rectifier K⁺ current. Circ Res 94: 1332–1339, 2004.[Abstract/Free Full Text]
134. Dhar Malhotra J, Chen C, Rivolta I, Abriel H, Malhotra R, Mattei LN, Brosius FC, Kass RS, and Isom LL. Characterization of sodium channel alpha- and beta-subunits in rat and mouse cardiac myocytes. Circulation 103: 1303–1310, 2001.[Abstract/Free Full Text]
135. Dhar Malhotra J, Koopmann MC, Kazen-Gillespie KA, Fettman N, Hortsch M, and Isom LL. Structural requirements for interaction of sodium channel 1 subunits with ankyrin. J Biol Chem 277: 26681–26688, 2002.[Abstract/Free Full Text]
136. Dina OA, McCarter GC, deCoupade C, and Levine JD. Role of the sensory neuron cytoskeleton in second messenger signaling for inflammatory pain. Neuron 39: 613–624, 2003.[CrossRef][Medline]
137. Dixon JE and McKinnon D. Quantitative analysis of potassium channel mRNA expression in atrial and ventricular muscle of rats. Circ Res 75: 252–260, 1994.[Abstract/Free Full Text]
138. Dixon JE, Shi W, Wang HS, McDonald C, Yu H, Wymore RS, Cohen IS, and McKinnon D. Role of the Kv4.3 K⁺ channel in ventricular muscle. A molecular correlate for the transient outward current. Circ Res 79: 659–668, 1996.[Abstract/Free Full Text]
139. Donaldson MR, Jensen JL, Tristani-Firouzi M, Tawil R, Bendahhou S, Suarez WA, Cobo AM, Poza JJ, Behr E, Wagstaff J, Szepetowski P, Pereira S, Mozaffar T, Escolar DM, Fu YH, and Ptacek LJ. PIP₂ binding residues of Kir2.1 are common targets of mutations causing Andersen syndrome. Neurology 60: 1811–1816, 2003.[Abstract/Free Full Text]
140. Doupnik CA, Davidson N, and Lester HA. The inward rectifier potassium channel family. Curr Opin Neurobiol 5: 268–277, 1995.[CrossRef][Web of Science][Medline]
141. Downey JM. Ischemic preconditioning: nature's own cardio-protective intervention. Trends Cardiovasc Med 2: 170–176, 1992.[CrossRef][Web of Science]
142. Drewe JA, Verma S, Frech G, and Joho RH. Distinct spatial and temporal expression patterns of K⁺ channel mRNAs from different subfamilies. J Neurosci 12: 538–548, 1992.[Abstract]
143. Dudel J, Peper K, Rudel R, and Trautwein W. The dynamic chloride component of membrane current in Purkinje fibers. Pflügers Arch 295: 197–212, 1967.[CrossRef][Web of Science]
144. Durbeej M and Campbell KP. Muscular dystrophies involving the dystrophin-glycoprotein complex: an overview of current mouse models. Curr Opin Genet Dev 12: 349–361, 2002.[CrossRef][Web of Science][Medline]
145. Eldstrom J, Doerksen KW, Steele DF, and Fedida D. N-terminal PDZ-binding domain in Kv1 potassium channels. FEBS Lett 531: 529–537, 2002.[CrossRef][Medline]
146. Ellis SB, Williams ME, Ways NR, Brenner R, Sharp AH, Leung AT, Campbell KP, McKenna E, Koch WJ, and Hui A. Sequence and expression of mRNAs encoding the alpha 1 and alpha 2 subunits of a DHP-sensitive calcium channel. Science 241: 1661–1664, 1988.[Abstract/Free Full Text]
147. England SK, Uebele VN, Kodali J, Bennett PB, and Tamkun MM. A novel K⁺ channel beta-subunit (hKv beta 1.3) is produced via alternative mRNA splicing. J Biol Chem 270: 28531–28534, 1995.[Abstract/Free Full Text]
148. England SK, Uebele VN, Shear H, Kodali J, Bennett PB, and Tamkun MM. Characterization of a voltage-gated K⁺ channel beta subunit expressed in human heart. Proc Natl Acad Sci USA 92: 6309–6313, 1995.[Abstract/Free Full Text]
149. Erickson MG, Liang H, Mori MX, and Yue DT. FRET two-hybrid mapping reveals function and location of L-type Ca²⁺ channel CaM preassociation. Neuron 39: 97–107, 2003.[CrossRef][Web of Science][Medline]
150. Ertel EA, Campbell KP, Harpold MM, Hofmann F, Mori Y, Perez-Reyes E, Schwartz A, Snutch TP, Tanabe T, Birnbaumer L, Tsien RW, and Catterall WA. Nomenclature of voltage-gated calcium channels. Neuron 25: 533–535, 2000.[CrossRef][Web of Science][Medline]
151. Ervasti JM and Campbell KP. Membrane organization of the dystrophin-glycoprotein complex. Cell 66: 1121–1131, 1991.[CrossRef][Web of Science][Medline]
152. Ervasti JM and Campbell KP. A role for the dystrophin glycoprotein complex as a transmembrane linker between laminin and actin. J Cell Biol 122: 809–823, 1993.[Abstract/Free Full Text]
153. Escayg A, De Waard M, Lee DD, Bichet D, Wolf P, Mayer T, Johnston J, Baloh R, Sander T, and Meisler MH. Coding and noncoding variation of the human calcium-channel beta4-subunit gene CACNB4 in patients with idiopathic generalized epilepsy and episodic ataxia. Am J Hum Genet 66: 1531–1539, 2000.[CrossRef][Web of Science][Medline]
154. Fabiato A and Fabiato F. Calcium and cardiac excitation-secretion coupling. Annu Rev Physiol 41: 473–484, 1979.[CrossRef][Web of Science][Medline]
155. Fahmi AI, Patel M, Stevens EB, Fowden AL, John JE III, Lee K, Pinnock R, Morgan K, Jackson AP, and Vandenberg JI. The sodium channel beta-subunit SCN3B modulates the kinetics of SCN5A and is expressed heterogeneously in sheep heart. J Physiol 537: 693–700, 2001.[Abstract/Free Full Text]
156. Fedida D and Giles WR. Regional variations in action potentials and transient outward current in myocytes isolated from rabbit left ventricle. J Physiol 442: 191–209, 1991.[Abstract/Free Full Text]
157. Fedida D, Eldstrom J, Hesketh JC, Lamorgese M, Castel L, Steele DF, and Van Wagoner DR. Kv1.5 is an important component of repolarizing K⁺ current in canine atrial myocytes. Circ Res 93: 744–751, 2003.[Abstract/Free Full Text]
158. Felix R, Gurnett CA, De Waard M, and Campbell KP. Dissection of functional domains of the voltage-dependent Ca²⁺ channel alpha2delta subunit. J Neurosci 17: 6884–6891, 1997.[Abstract/Free Full Text]
159. Feng J, Wible B, Li GR, Wang Z, and Nattel S. Antisense oligonucleotides directed against Kv1.5 mRNA specifically inhibit ultrarapid delayed rectifier K⁺ current in cultured adult human atrial myocytes. Circ Res 80: 572–579, 1997.[Abstract/Free Full Text]
160. Fermini B, Wang Z, Duan D, and Nattel S. Differences in rate dependence of transient outward current in rabbit and human atrium. Am J Physiol Heart Circ Physiol 263: H1747–H1754, 1992.[Abstract/Free Full Text]
161. Feron O and Kelly RA. The caveolar paradox: suppressing, inducing, and terminating eNOS signaling. Circ Res 88: 129–131, 2001.[Free Full Text]
162. Feron O, Belhassen L, Kobzik L, Smith TW, Kellly RA, and Michel T. Endothelial nitric oxide synthase targeting to caveolae. Specific interactions with caveolin isoforms in cardiac myocytes and endothelial cells. J Biol Chem 271: 22810–22814, 1996.[Abstract/Free Full Text]
163. Ficker E, Dennis AT, Wang L, and Brown AM. Role of the cytosolic chaperones Hsp70 and Hsp90 in maturation of the cardiac potassium channel HERG. Circ Res 92: 87–100, 2003.[CrossRef]
164. Ficker E, Taglialatela M, Wible BA, Henley CM, and Brown AM. Spermine and spermidine as gating molecules for inward rectifier K⁺ channels. Science 266: 1068–1072, 1994.[Abstract/Free Full Text]
165. Findlay I. The ATP sensitive potassium channel of cardiac muscle and action potential shortening during metabolic stress. Cardiovasc Res 28: 760–761, 1994.[Free Full Text]
166. Findlay I. Physiological modulation of inactivation in L-type Ca²⁺ channels: one switch. J Physiol 554: 275–283, 2003.[CrossRef][Medline]
167. Fink M, Duprat F, Lesage F, Heurteaux C, Romey G, Barhanin J, and Lazdunski M. A new K⁺ channel beta subunit to specifically enhance Kv2.2 (CDRK) expression. J Biol Chem 271: 26341–26348, 1996.[Abstract/Free Full Text]
168. Fiset C, Clark RB, Larsen TS, and Giles WR. A rapidly activating sustained K⁺ current modulates repolarization and excitation-contraction coupling in adult mouse ventricle. J Physiol 504: 557–563, 1997.[Abstract/Free Full Text]
169. Fiset C, Clark RB, Shimoni Y, and Giles WR. Shal-type channels contribute to the Ca²⁺-independent transient outward K⁺ current in rat ventricle. J Physiol 500: 51–64, 1997.[Abstract/Free Full Text]
170. Folander K, Smith JS, Antanavage J, Bennett C, Stein RB, and Swanson R. Cloning and expression of the delayed-rectifier IsK channel from neonatal rat heart and diethylstilbestrol-primed rat uterus. Proc Natl Acad Sci USA 87: 2975–2979, 1990.[Abstract/Free Full Text]
171. Follmer CH and Colatsky TJ. Block of delayed rectifier K⁺ current, I_K, by flecainide and E-4031 in cat ventricular myocytes. Circulation 82: 289–293, 1990.[Abstract/Free Full Text]
172. Fozzard HA. Cardiac sodium and calcium channels: a history of excitatory currents. Cardiovasc Res 55: 1–8, 2002.[Abstract/Free Full Text]
173. Fozzard HA. Excitation-contraction coupling in the heart. Adv Exp Med Biol 308: 135–142, 1991.[Medline]
174. Fozzard HA and Hanck DA. Structure and function of voltage-dependent sodium channels: comparison of brain II and cardiac isoforms. Physiol Rev 76: 887–926, 1996.[Abstract/Free Full Text]
175. Fozzard HA and Hiroaka M. The positive dynamic current and its inactivation properties in cardiac Purkinje fibres. J Physiol 234: 569–586, 1973.[Abstract/Free Full Text]
176. Franco D, Demolombe S, Kuperschmidt S, Dumaine R, Dominguez JN, Roden D, Antzelevitch C, Escande D, and Moorman A. Divergent expression of delayed rectifier K⁺ channel subunits during mouse heart development. Cardiovasc Res 52: 65–75, 2001.[Abstract/Free Full Text]
177. Furukawa T, Kimura S, Furukawa N, Bassett AL, and Myerburg RJ. Potassium rectifier currents differ in myocytes of endocardial and epicardial origin. Circ Res 70: 91–103, 1992.[Abstract/Free Full Text]
178. Furukawa T, Myerburg RJ, Furukawa N, Bassett AL, and Kimura S. Differences in transient outward currents of feline endocardial and epicardial myocytes. Circ Res 67: 1287–1291, 1990.[Abstract/Free Full Text]
179. Furukawa T, Yamane T, Terai T, Katayama Y, and Hiraoka M. Functional linkage of the cardiac ATP-sensitive K⁺ channel to the actin cytoskeleton. Pflügers Arch 431: 504–512, 1996.[Web of Science][Medline]
180. Furutani M, Trudeau MC, Hagiwara N, Seki A, Gong Q, Zhou Z, Imamura S, Nagashima H, Kasanuki H, Takao A, Momma K, January CT, Robertson GA, and Matsuoka R. Novel mechanism associated with an inherited cardiac arrhythmia: defective protein trafficking by the mutant HERG G601S potassium channel. Circulation 99: 2290–2294, 1999.[Abstract/Free Full Text]
181. Gao T, Chien AJ, and Hosey MM. Complexes of the alpha1C and beta subunits generate the necessary signal for membrane targeting of class C L-type calcium channels. J Biol Chem 274: 2137–2144, 1999.[Abstract/Free Full Text]
182. Gee SH, Madhavan R, Levinson SR, Caldwell JH, Sealock R, and Froehner SC. Interaction of muscle and brain sodium channels with multiple members of the syntrophin family of dystrophin-associated proteins. J Neurosci 18: 128–137, 1998.[Abstract/Free Full Text]
183. Giles WR and Imaizumi Y. Comparison of potassium currents in rabbit atrial and ventricular cells. J Physiol 405: 123–145, 1988.[Abstract/Free Full Text]
184. Goldman L and Balke CW. Do defects in the late sodium current in human ventricular cells cause heart failure? J Mol Cell Cardiol 34: 1473–1476, 2002.[CrossRef][Medline]
185. Goldstein SA and Miller C. Site-specific mutations in a minimal voltage-dependent K⁺channel alter ion selectivity and open-channel block. Neuron 7: 403–408, 1991.[CrossRef][Web of Science][Medline]
186. Goldstein SA, Bockenhauer D, O'Kelly I, and Zilberberg N. Potassium leak channels and the KCNK family of two-P-domain subunits. Nat Rev Neurosci 2: 175–184, 2001.[Web of Science][Medline]
187. Gregg RG, Messing A, Strube C, Beurg M, Moss R, Behan M, Sukhareva M, Haynes S, Powell JA, Coronado R, and Powers PA. Absence of the beta subunit (cchb1) of the skeletal muscle dihydropyridine receptor alters expression of the alpha 1 subunit and eliminates excitation-contraction coupling. Proc Natl Acad Sci USA 93: 13961–13966, 1996.[Abstract/Free Full Text]
188. Grover GJ and Garlid KD. ATP-sensitive potassium channels: a review of their cardioprotective pharmacology. J Mol Cell Cardiol 32: 677–695, 2000.[CrossRef][Web of Science][Medline]
189. Grozdanovic Z and Baumgarten HG. Nitric oxide synthase in skeletal muscle fibers: a signaling component of the dystrophin-glycoprotein complex. Histol Histopathol 14: 243–256, 1999.[Web of Science][Medline]
190. Gulbis JM. The beta subunit of Kv1 channels: voltage-gated enzyme or safety switch? Novartis Found Symp 245: 127–145, 2002.[Web of Science][Medline]
191. Gulbis JM, Mann S, and MacKinnon R. Structure of a voltage-dependent K⁺ channel beta subunit. Cell 97: 943–952, 1999.[CrossRef][Web of Science][Medline]
192. Gulbis JM, Zhou M, Mann S, and MacKinnon R. Structure of the cytoplasmic beta subunit-T1 assembly of voltage-dependent K⁺ channels. Science 289: 123–127, 2000.[Abstract/Free Full Text]
193. Guo W, Li H, Aimond F, Johns DC, Rhodes KJ, Trimmer JS, and Nerbonne JM. Role of heteromultimers in the generation of myocardial transient outward K⁺ currents. Circ Res 90: 586–593, 2002.[Abstract/Free Full Text]
194. Guo W, Li H, London B, and Nerbonne JM. Functional consequences of elimination of I_to,f and I_to,s: early afterdepolarizations, atrioventricular block, and ventricular arrhythmias in mice lacking Kv1.4 and expressing a dominant-negative Kv4 alpha subunit. Circ Res 87: 73–79, 2000.[Abstract/Free Full Text]
195. Guo W, Malin SA, Johns DC, Jeromin A, and Nerbonne JM. Modulation of Kv4-encoded K⁺ currents in the mammalian myocardium by neuronal calcium sensor-1. J Biol Chem 277: 26436–26443, 2002.[Abstract/Free Full Text]
196. Guo W, Xu H, London B, and Nerbonne JM. Molecular basis of transient outward K⁺ current diversity in mouse ventricular myocytes. J Physiol 521: 587–599, 1999.[Abstract/Free Full Text]
197. Gurnett CA, De Waard M, and Campbell KP. Dual function of the voltage-dependent Ca²⁺channel alpha 2 delta subunit in current stimulation and subunit interaction. Neuron 16: 431–440, 1996.[CrossRef][Web of Science][Medline]
198. Gurnett CA, Felix R, and Campbell KP. Extracellular interaction of the voltage-dependent Ca²⁺ channel alpha2delta and alpha1 subunits. J Biol Chem 272: 18508–18512, 1997.[Abstract/Free Full Text]
199. Gutman GA, Chandy G, Adelman JP, Aiyar J, Bayliss DA, Clapham DE, Covarriubias M, Desir GV, Furuichi K, Banetzky B, Garcia M, Grissmer S, Jan LY, Karschin A, Kim D, Kuperschmidt S, Kurachi Y, Lazdunski M, Lesage F, Lester HA, McKinnon D, Nichols CG, O'Kelly I, Robbins J, Robertson GA, Rudy B, Sanguinetti M, Seino S, Stuehmer W, Tamkun MM, Vandenberg CA, Wei A, Wulff H, and Wymore RS. International Union of Pharmacology. XLI. Compendium of voltage-gated ion channels: potassium channels. Pharmacol Rev 55: 583–586, 2003.[Abstract/Free Full Text]
200. Hagiwara N, Irisawa H, and Kameyama M. Contribution of two types of calcium currents to the pacemaker potentials of rabbit sinoatrial node cells. J Physiol 395: 233–253, 1988.[Abstract/Free Full Text]
201. Hagiwara N, Ozawa S, and Sano O. Voltage clamp analysis of two inward current mechanisms in the egg cell membrane of a starfish. J Gen Physiol 65: 617–644, 1975.[Abstract/Free Full Text]
202. Han X, Kubota I, Feron O, Opel DJ, Arstall MA, Zhao YY, Huang P, Fishman MC, Michel T, and Kelly RA. Muscarininc cholinergic regulation of cardiac myocyte ICa-L is absent in mice with targeted disruption of endothelial nitric oxide synthase. Proc Natl Acad Sci USA 95: 6510–6515, 1998.[Abstract/Free Full Text]
203. Hare JM, Lofthouse RA, Juang GJ, Colman L, Ricker KM, Kim B, Senzaki H, Cao S, Tunin RS, and Kass DA. Contributions of caveolin protein abundance to augmented nitric oxide signaling in conscious dogs with pacing-induced heart failure. Circ Res 86: 1085–1092, 2000.[Abstract/Free Full Text]
204. Head C and Gardiner M. Paroxysms of excitement: sodium channel dysfunction in heart and brain. Bioessays 25: 981–999, 2003.[CrossRef][Medline]
205. Hebao Y, Michelsen K, and Schwappach B. 14–3-3 Dimers probe the assembly status of multimeric membrane proteins. Curr Biol 13: 638–646, 2003.[CrossRef][Web of Science][Medline]
206. Heidbüchel H, Vereecke J, and Carmeliet E. Three different potassium channels in human atria. Contribution to the basal potassium conductance. Circ Res 66: 1277–1286, 1990.[Abstract/Free Full Text]
207. Hermann R, Straub V, Blank M, Kutzick C, Franke N, Jacob EN, Lenard HG, Kröger S, and Voit T. Dissociation of the dystroglycan complex in caveolin-3-deficient limb girdle muscular dystrophy. Hum Mol Genet 9: 2335–2340, 2000.[Abstract/Free Full Text]
208. Heurteaux C, Guy N, Laigle C, Blondeau N, Duprat F, Mazzuca M, Lang-Lazdunski L, Widmann C, Zanzouri M, Romey G, and Lazdunski M. TREK-1, a K⁺ channel involved in neuroprotection and general anesthesia. EMBO J 23: 2684–2695, 2004.[CrossRef][Web of Science][Medline]
209. Hilgemann DW, Feng S, and Nasuhoglu C. The complex and intriguing lives of PI2 with ion channels and transporters. Science's Stke: Signal Transduction Knowledge Environ 2001: RE19, 2001.
210. Himmel HM, Wettwer E, Li Q, and Ravens U. Four different components contribute to outward current in rat ventricular myocytes. Am J Physiol Heart Circ Physiol 277: H107–H113, 1999.[Abstract/Free Full Text]
211. Hohaus A, Poteser M, Romanin C, Klugbauer N, Hofmann F, Morano I, Haase H, and Groschner K. Modulation of the smooth-muscle L-type Ca²⁺ channel alpha1 subunit (alpha1C-b) by the beta2a subunit: a peptide which inhibits binding of beta to the I-II linker of alpha1 induces functional uncoupling. Biochem J 348: 657–665, 2000.[CrossRef][Medline]
212. Holm AN, Rich A, Sarr MG, and Farrugia G. Whole cell current and membrane potential regulation by a human smooth muscle mechanosensitive calcium channel. Am J Physiol Gastrointest Liver Physiol 279: G1155–G1161, 2000.[Abstract/Free Full Text]
213. Honjo H, Boyett MR, and Kodama I. Heterogeneity of 4-aminopyridine-sensitive current in rabbit sinoatrial node cells. Am J Physiol Heart Circ Physiol 276: H1295–H1304, 1999.[Abstract/Free Full Text]
214. Honjo H, Inada S, Lancaster NMK, Yamamoto M, Niwa R, Jones SA, Shibata N, Mitsui K, Horiuchi T, Kamiya K, Kodama I, and Boyett MR. Sarcoplasmic reticulum Ca²⁺ release is not a dominating factor in sinoatrial node pacemaker activity. Circ Res 92: e41–e44, 2003.[Abstract/Free Full Text]
215. Honjo H, Lei M, Boyett MR, and Kodama I. Heterogeneity of 4-aminopyridine-sensitive current in rabbit sinoatrial node cells. Am J Physiol Heart Circ Physiol 276: H1295–H1304, 1999.[Abstract/Free Full Text]
216. Horie M, Hayashi S, and Kawai C. Two types of delayed rectifying K⁺channels in atrial cells of guinea pig heart. Jpn J Physiol 40: 479–490, 1990.[CrossRef][Web of Science][Medline]
217. Hoshi T, Zagotta WN, and Aldrich RW. Biophysical and molecular mechanisms of Shaker potassium channel inactivation. Science 250: 533–538, 1990.[Abstract/Free Full Text]
218. Howarth FC, Levi AJ, and Hancox JC. Characteristics of the delayed rectifier K⁺ current compared in myocytes isolated from the atrioventricular node and ventricle of the rabbit heart. Pflügers Arch 431: 713–722, 1996.[Web of Science][Medline]
219. Huang CL, Feng S, and Hilgemann DW. Direct activation of inward rectifier potassium channels by PIP₂ and its stabilization by Gbetagamma. Nature 391: 803–806, 1998.[CrossRef][Medline]
220. Hübner CA and Jentsch TJ. Ion channel diseases. Hum Mol Genet 11: 2435–2445, 2002.[Abstract/Free Full Text]
221. Hugnot JP, Salinas M, Lesage F, Guillemare E, de Weille J, Heurteaux C, Mattei MG, and Lazdunski M. Kv8.1, a new neuronal potassium channel subunit with specific inhibitory properties towards Shab and Shaw channels. EMBO J 15: 3322–3331, 1996.[Web of Science][Medline]
222. Hullin R, Singer-Lahat D, Freichel M, Biel M, Dascal N, Hofmann F, and Flockerzi V. Calcium channel beta subunit heterogeneity: functional expression of cloned cDNA from heart, aorta and brain. EMBO J 11: 885–890, 1992.[Web of Science][Medline]
223. Hume JR and Uehara A. Ionic basis of the different action potential configurations of single guinea pig atrial and ventricular myocytes. J Physiol 368: 525–544, 1985.[Abstract/Free Full Text]
224. Inoue M and Imanaga I. Masking of A-type K⁺ channel in guinea pig cardiac cells by extracellular Ca²⁺. Am J Physiol Cell Physiol 264: C1434–C1438, 1993.[Abstract/Free Full Text]
225. Iost N, Virag L, Opincariu M, Szecsi J, Varro A, and Papp JG. Delayed rectifier potassium current in undiseased human ventricular myocytes. Cardiovasc Res 40: 508–515, 1998.[Abstract/Free Full Text]
226. Irisawa H and Giles WR. Sinus and atrioventricular node cells: cellular electrophysiology. In: Cardiac Electrophysiology: From Cell to Bedside, edited by D. P. Zipes and J. Jalife. Philadelphia, PA: Saunders, 1990, p. 93–102.
227. Isbrandt D, Friederich P, Solth A, Haverkamp W, Ebneth A, Borggrefe M, Funke H, Sauter K, Breithardt G, Pongs O, and Schulze-Bahr E. Identification and functional characterization of a novel KCNE2 (MiRP1) mutation that alters HERG channel kinetics. J Mol Med 80: 524–532, 2002.[CrossRef][Web of Science][Medline]
228. Isom LL. Sodium channel beta subunits: anything but auxiliary. Neuroscientist 7: 42–54, 2001.[Abstract/Free Full Text]
229. Isom LL, De Jongh KS, and Catterall WA. Auxiliary subunits of voltage-gated ion channels. Neuron 12: 1183–1194, 1994.[CrossRef][Web of Science][Medline]
230. Isom LL, De Jongh KS, Patton DE, Reber BF, Offord J, Charbonneau H, Walsh K, Goldin AL, and Catterall WA. Primary structure and functional expression of the beta 1 subunit of the rat brain sodium channel. Science 256: 839–842, 1992.[Abstract/Free Full Text]
231. Isom LL, Ragsdale DS, De Jongh KS, Westenbroek RE, Reber BF, Scheuer T, and Catterall WA. Structure and function of the beta 2 subunit of brain sodium channels, a transmembrane glycoprotein with a CAM motif. Cell 83: 433–442, 1995.[CrossRef][Web of Science][Medline]
232. Isomoto S and Kurachi Y. Function, regulation, pharmacology, and molecular structure of ATP-sensitive K⁺ channels in the cardiovascular system. J Cardiovasc Electrophysiol 8: 1431–1446, 1997.[Web of Science][Medline]
233. Ito H and Ono K. A rapidly activating delayed rectifier K⁺ channel in rabbit sinoatrial node cells. Am J Physiol Heart Circ Physiol 269: H443–H452, 1995.[Abstract/Free Full Text]
234. Jeck N, Derst C, Wischmeyer E, Ott H, Weber S, Rudin C, Seyberth HW, Daut J, Karschin A, and Konrad M. Functional heterogeneity of ROMK mutations linked to hyperprostaglandin E syndrome. Kidney Int 59: 1803–1811, 2001.[CrossRef][Web of Science][Medline]
235. Jerng HH, Qian Y, and Pfaffinger PJ. Modulation of Kv4.2 channel expression and gating by dipeptidyl peptidase 10 (DPP10). Biophys J 87: 2380–2396, 2004.[CrossRef][Web of Science][Medline]
236. Jiang M, Tseng-Crank J, and Tseng GN. Suppression of slow delayed rectifier current by a truncated isoform of KvLQT1 cloned from normal human heart. J Biol Chem 272: 24109–24112, 1997.[Abstract/Free Full Text]
237. Jing J, Peretz T, Singer-Lahat D, Chikvashvili D, Thornhill WB, and Lotan I. Inactivation of a voltage-dependent K⁺ channel by subunit: modulation by a phosphorylation-dependent interaction between the distal C terminus of subunit and cytoskeleton. J Biol Chem 272: 14021–14024, 1997.[Abstract/Free Full Text]
238. Johns DC, Nuss HB, and Marban E. Suppression of neuronal and cardiac transient outward currents by viral gene transfer of dominant-negative Kv4.2 constructs. J Biol Chem 272: 31598–31603, 1997.[Abstract/Free Full Text]
239. Johnson BD and Byerly L. Ca²⁺ channel Ca²⁺-dependent inactivation in a mammalian central neuron involves the cytoskeleton. Pflügers Arch 429: 14–21, 1994.[Web of Science][Medline]
240. Jones E, Roti Roti EC, Wang J, Delfosse SA, and Robertson GA. Cardiac I_Kr channels minimally comprise hERG 1a and 1b subunits. J Biol Chem 279: 44690–44694, 2004.[Abstract/Free Full Text]
241. Jones JM, Meisler MH, and Isom LL. SCN2B, a voltage-gated sodium channel beta2 gene on mouse chromosome 9. Genomics 34: 258–259, 1996.[CrossRef][Medline]
242. Jones LP, Wei SK, and Yue DT. Mechanism of auxiliary subunit modulation of neuronal alpha1E calcium channels. J Gen Physiol 112: 125–143, 1998.[Abstract/Free Full Text]
243. Jongsma HJ and Wilders R. Channelopathies: Kir2.1 mutations jeopardize many cell functions. Curr Biol 11: R747–R750, 2001.[CrossRef][Web of Science][Medline]
244. Josephson IR and Brown AM. Inwardly rectifying single channel and whole cell K⁺ currents in rat ventricular myocytes. J Membr Biol 94: 19–35, 1986.[CrossRef][Web of Science][Medline]
245. Jouvenceau A, Eunson LH, Spauschus A, Ramesh V, Zuberi SM, Kullmann DM, and Hanna MG. Human epilepsy associated with dysfunction of the brain P/Q-type calcium channel. Lancet 358: 801–807, 2001.[CrossRef][Web of Science][Medline]
246. Kagan A, Yu Z, Fishman GI, and McDonald TV. The dominant negative LQT2 mutation A561V reduces wild-type HERG expression. J Biol Chem 275: 11241–11248, 2000.[Abstract/Free Full Text]
247. Kakulas BA. The differential diagnosis of the human dystrophinopathies and related disorders. Curr Opin Neurol 9: 380–388, 1996.[Medline]
248. Kambouris NG, Nuss HB, Johns DC, Marban E, Tomaselli G, and Balser JR. A revised view of cardiac sodium channel "blockade" in the long QT syndrome. J Clin Invest 105: 1133–1140, 2000.[Web of Science][Medline]
249. Kamiya K, Kaneda M, Sugawara T, Mazaki E, Okamura N, Montal M, Makita N, Tanaka M, Fukushima K, Fujwara T, Inoue Y, and Yamakawa K. A nonsense mutation of the sodium channel gene SCN2A in a patient with intractable epilepsy and mental decline. J Neurosci 24: 2690–2698, 2004.[Abstract/Free Full Text]
250. Kang MG and Campbell KP. Subunit of voltage-activated calcium channels. J Biol Chem 278: 21315–21318, 2003.[Free Full Text]
251. Kanno S and Saffitz JE. The role of myocardial gap junctions in electrical conduction and arrhythmogenesis. Cardiovasc Pathol 10: 169–177, 2001.[CrossRef][Web of Science][Medline]
252. Kazarinova-Noyes K, Malhotra JD, McEwen DP, Mattei LN, Berglund EO, Ranscht B, Levinson SR, Schacner M, Shrager P, Isom LL, and Xiao ZC. Contactin associates with Na⁺ channels and increases their functional expression. J Neurosci 21: 7517–7525, 2001.[Abstract/Free Full Text]
253. Keating MT and Sanguinetti MC. Molecular and cellular mechanisms of cardiac arrhythmias. Cell 104: 569–580, 2001.[CrossRef][Web of Science][Medline]
254. Keating MT, Atkinson D, Dunn C, Timothy K, Vincent GM, and Leppert M. Evidence of genetic heterogeneity in the long QT syndrome. Science 260: 1960–1961, 1993.[Free Full Text]
255. Keef KD, Hume JR, and Zhong J. Regulation of cardiac and smooth muscle Ca²⁺ channels [Ca(V)1.2a,b] by protein kinases. Am J Physiol Cell Physiol 281: C1743–C1756, 2001.[Abstract/Free Full Text]
256. Kenyon JL and Gibbons WR. 4-Aminopyridine and the early outward current in sheep cardiac Purkinje fibers. J Gen Physiol 73: 139–157, 1979.[Abstract/Free Full Text]
257. Kenyon JL and Gibbons WR. Influence of chloride, potassium, and tetraethylammonium on the early outward current of sheep cardiac Purkinje fibers. J Gen Physiol 73: 117–138, 1979.[Abstract/Free Full Text]
258. Kim LA, Furst J, Gutierrez D, Butler MH, Xu S, Goldstein S, and Grigorieff N. Three-dimensional structure of I_to: Kv4.2-KChIP2 ion channels by electron microscopy at 21 resolution. Neuron 41: 513–519, 2004.[CrossRef][Web of Science][Medline]
259. Kléber AG and Rudy Y. Basic mechanisms of cardiac impulse propagation and associated arrhythmias. Physiol Rev 84: 431–488, 2004.[Abstract/Free Full Text]
260. Klugbauer N, Marais E, Lacinova L, and Hofmann F. A T-type calcium channel from mouse brain. Pflügers Arch 437: 710–715, 1999.[CrossRef][Web of Science][Medline]
261. Kodama I, Boyett MR, Nikmaram MR, Yamamoto M, Honjo H, and Niwa R. Regional differences in effects of E-4031 within the sinoatrial node. Am J Physiol Heart Circ Physiol 276: H793–H802, 1999.[Abstract/Free Full Text]
262. Kodama I, Nikmaram MR, Boyett MR, Suzuki R, Honjo H, and Owen JM. Regional differences in the role of the Ca²⁺ and Na⁺ currents in pacemaker activity in the sinoatrial node. Am J Physiol Heart Circ Physiol 272: H2793–H2806, 1997.[Abstract/Free Full Text]
263. Kodirov S, Brunner M, Busconi L, Nerbonne JM, Buckett P, Mitchell G, and Koren G. Attenuation of I_K,slow1 and I_K,slow2 in Kv1 DN mice prolongs the APD and QT intervals but does not prevent spontaneous or inducible arrhythmias. Am J Physiol Heart Circ Physiol 286: H368–H374, 2004.[Abstract/Free Full Text]
264. Konarzewska H, Peeters GA, and Sanguinetti MC. Repolarizing K⁺ currents in nonfailing human hearts. Similarities between right septal subendocardial and left subepicardial ventricular myocytes. Circulation 92: 1179–1187, 1995.[Abstract/Free Full Text]
265. Kone BC. Protein-protein interactions controlling nitric oxide synthases. Acta Physiol Scand 168: 27–31, 2000.[CrossRef][Web of Science][Medline]
266. Kong W, Po S, Yamagishi T, Ashen MD, Stetten G, and Tomaselli GF. Isolation and characterization of the human gene encoding I_to: further diversity by alternative mRNA splicing. Am J Physiol Heart Circ Physiol 275: H1963–H1970, 1998.[Abstract/Free Full Text]
267. Kors EE, Van Den Maagdenberg AMJM, Plomp JJ, Frants RR, and Ferrari MD. Calcium channel mutations and migraine. Curr Opin Neurol 15: 311–316, 2002.[Medline]
268. Koster JC, Knopp A, Flagg TP, Markova KP, Sha Q, EnKvetchakul D, Betsuyaku T, Yamada KA, and Nichols CG. Tolerance for ATP-insensitive K(ATP) channels in transgenic mice. Circ Res 89: 1022–1029, 2001.[Abstract/Free Full Text]
269. Kovoor P, Wickman K, Maguire CT, Pu W, Gehrmann J, Berul CI, and Clampham DE. Evaluation of the role of I_KAch in atrial fibrillation using a mouse knockout model. J Am Coll Cardiol 37: 2136–2143, 2001.[Abstract/Free Full Text]
270. Kucera JP, Rohr S, and Rudy Y. Localization of sodium channels in intercalated disks modulates cardiac conduction. Circ Res 91: 1176–1182, 2002.[Abstract/Free Full Text]
271. Kuo HC, Cheng CF, Clark RB, Lin JJ, Lin JL, Hoshijima M, Nguyen-Tran VT, Gu Y, Ikeda Y, Chu PH, Ross J, Giles WR, and Chien KR. A defect in the Kv channel-interacting protein 2 (KChIP2) gene leads to a complete loss of I_to and confers susceptibility to ventricular tachycardia. Cell 107: 801–813, 2001.[CrossRef][Web of Science][Medline]
272. Kupershmidt S, Snyders DJ, Raes A, and Roden DM. A K⁺ channel splice variant common in human heart lacks a C-terminal domain required for expression of rapidly activating delayed rectifier current. J Biol Chem 273: 27231–27235, 1998.[Abstract/Free Full Text]
273. Kupershmidt S, Yang T, Chanthaphaychith S, Wang Z, Towbin JA, and Roden DM. Defective human Ether-a-go-go related gene trafficking linked to an endoplasmic reticulum retention signal in the C terminus. J Biol Chem 277: 27442–27448, 2002.[Abstract/Free Full Text]
274. Kupershmidt S, Yang IC, Sutherland M, Wells KS, Yang T, Yang P, Balser JR, and Roden DM. Cardiac-enriched LIM domain protein fh12 is required to generate I_Ks in a heterologous system. Cardiovasc Res 56: 93–103, 2002.[Abstract/Free Full Text]
275. Kurachi Y. G protein regulation of cardiac muscarinic potassium channels. Am J Physiol Cell Physiol 269: C821–C830, 1995.[Abstract/Free Full Text]
276. Kurihara T and Tanabe T. N type Ca²⁺ channel. Nippon Yakurigaku Zasshi-Folia Pharmacol Japonica 121: 211–222, 2003.
277. Kuryshev YA, Gudz TI, Brown AM, and Wible BA. KChAP as a chaperone for specific K⁺ channels. Am J Physiol Cell Physiol 278: C863–C864, 2000.[Free Full Text]
278. Kuryshev YA, Wible BA, Gudz TI, Ramirez AN, and Brown AM. KChAP/Kvbeta1.2 interactions and their effects on cardiac Kv channel expression. Am J Physiol Cell Physiol 281: C290–C299, 2001.[Abstract/Free Full Text]
279. Lacerda AE, Kim HS, Ruth P, Perez-Reyes E, Flockerzi V, Hofmann F, Birnbaumer L, and Brown AM. Normalization of current kinetics by interaction between the alpha 1 and beta subunits of the skeletal muscle dihydropyridine-sensitive Ca²⁺ channel. Nature 352: 527–530, 1991.[CrossRef][Medline]
280. Lange PS, Er F, Gassanov N, and Hoppe UC. Andersen mutations of KCNJ2 suppress the native inward rectifier current I_K1 in a dominant-negative fashion. Cardiovasc Res 59: 321–327, 2003.[Abstract/Free Full Text]
281. Lee K, Marbán E, and Tsien RW. Inactivation of calcium channels in mammalian heart cells: joint dependence on membrane potential and intracellular calcium. J Physiol 364: 395–411, 1985.[Abstract/Free Full Text]
282. Lees-Miller JP, Kondo C, Wang L, and Duff HJ. Electrophysiological characterization of an alternatively processed ERG K⁺ channel in mouse and human hearts. Circ Res 81: 719–726, 1997.[Abstract/Free Full Text]
283. Lei M, Honjo H, Kodama I, and Boyett MR. Characterisation of the transient outward K⁺ current in rabbit sinoatrial node cells. Cardiovasc Res 46: 433–441, 2000.[Abstract/Free Full Text]
284. Lei M, Honjo H, Kodama I, and Boyett MR. Heterogeneous expression of the delayed-rectifier K⁺ currents I_Kr and I_Ks in rabbit sinoatrial node cells. J Physiol 535: 703–714, 2001.[Abstract/Free Full Text]
285. Leonoudakis D, Conti LR, Anderson S, Radeke CM, McGuire LM, Adams ME, Froehner SC, Yates JR III, and Vandenberg CA. Protein trafficking and anchoring complexes revealed by proteomic analysis of inward rectifier potassium channel (Kir2.x)-associated proteins. J Biol Chem 279: 22331–22346, 2004.[Abstract/Free Full Text]
286. Lesage F and Lazdunski M. Potassium channels with two P domains. In: Current Topics in Membranes, edited by L. Y. Jan. San Diego, CA: Academic, 1999, vol. 46, p. 199–222.
287. Lesage F and Lazdunski M. Molecular and functional properties of two-pore-domain potassium channels. Am J Physiol Renal Physiol 279: F793–F801, 2000.[Abstract/Free Full Text]
288. Lesage F, Attali B, Lazdunski M, and Barhanin J. IsK, a slowly activating voltage-sensitive K⁺ channel. Characterization of multiple cDNAs and gene organization in the mouse. FEBS Lett 301: 168–172, 1992.[CrossRef][Web of Science][Medline]
289. Lesage F, Guillemare E, Fink M, Duprat F, Lazdunski M, Romey G, and Barhanin J. TWI_K-1, a ubiquitous human weakly inward rectifying K⁺ channel with a novel structure. EMBO J 15: 1004–1011, 1996.[Web of Science][Medline]
290. Li GR, Feng J, Yue L, Carrier M, and Nattel S. Evidence for two components of delayed rectifier K⁺ current in human ventricular myocytes. Circ Res 78: 689–696, 1996.[Abstract/Free Full Text]
291. Li H, Guo W, Yamada KA, and Nerbonne JM. Selective elimination of one component of delayed rectification, I_K,slow1, in mouse ventricular myocytes expressing a dominant negative Kv1.5 subunit. Am J Physiol Heart Circ Physiol 286: H319–H328, 2004.[Abstract/Free Full Text]
292. Li RA, Leppo M, Miki T, Seino S, and Marban E. Molecular basis of electrocardiographic ST-segment elevation. Circ Res 87: 837–839, 2000.[Abstract/Free Full Text]
293. Liang H, DeMaria CD, Erickson MG, Mori MX, Alseikhan BA, and Yue DT. Unified mechanisms of Ca²⁺ regulation across the Ca²⁺ channel family. Neuron 39: 951–960, 2003.[CrossRef][Web of Science][Medline]
294. Litovsky SH and Antzelevitch C. Transient outward current prominent in canine ventricular epicardium but not endocardium. Circ Res 62: 116–126, 1988.[Abstract/Free Full Text]
295. Liu CH, Dib-Hajj SD, Renganathan M, Cummins TR, and Waxman SG. Modulation of the cardiac sodium channel Nav1.5 by fibroblast growth factor homologous factor 1B. J Biol Chem 278: 1029–1036, 2003.[Abstract/Free Full Text]
296. Liu DW and Antzelevitch C. Characteristics of the delayed rectifier current (I_Kr and I_Ks) in canine ventricular epicardial, midmyocardial, and endocardial myocytes. A weaker I_Ks contributes to the longer action potential of the M cell. Circ Res 76: 351–365, 1995.[Abstract/Free Full Text]
297. Liu DW, Gintant GA, and Antzelevitch C. Ionic basis for electrophysiological distinctions among epicardial, midmyocardial, and endocardial myocytes from the free wall of the canine left ventricle. Circ Res 72: 671–687, 1993.[Abstract/Free Full Text]
298. Liu GX, Derst C, Schlichthorl G, Heinen S, Seebohm G, Bruggemann A, Kummer W, Veh RW, Daut J, and Preisig-Muller R. Comparison of cloned Kir2 channels with native inward rectifier K⁺ channels from guinea-pig cardiomyocytes. J Physiol 532: 115–126, 2001.[Abstract/Free Full Text]
299. Liu H, Tateyama M, Clancy CE, Abriel H, and Kass RS. Channel openings are necessary but not sufficient for use-dependent block of cardiac Na⁺ channels by flecainide: evidence from the analysis of disease-linked mutations. J Gen Physiol 120: 39–51, 2002.[Abstract/Free Full Text]
300. Lombardi SJ, Truong A, Spence P, Rhodes KJ, and Jones PG. Structure-activity relationships of the Kvbeta1 inactivation domain and its putative receptor probed using peptide analogs of voltage-gated potassium channel alpha- and beta-subunits. J Biol Chem 273: 30092–30096, 1998.[Abstract/Free Full Text]
301. Lombardi SJ, Truong A, Spence P, Rhodes KJ, and Jones PG. Probing the potassium channel Kv beta 1/Kv1.1 interaction using a random peptide display library. Ann NY Acad Sci 868: 427–430, 1999.[Medline]
302. London B, Guo W, Pan Xh Lee JS, Shusterman V, Rocco CJ, Logothetis DA, Nerbonne JM, and Hill JA. Targeted replacement of Kv1.5 in the mouse leads to loss of the 4-aminopyridine-sensitive component of I_K,slow and resistance to drug-induced QT prolongation. Circ Res 88: 940–946, 2001.[Abstract/Free Full Text]
303. London B, Jeron A, Zhou J, Buckett P, Han X, Mitchell GF, and Koren G. Long QT and ventricular arrhythmias in transgenic mice expressing the N terminus and first transmembrane segment of a voltage-gated potassium channel. Proc Natl Acad Sci USA 95: 2926–2931, 1998.[Abstract/Free Full Text]
304. London B, Trudeau MC, Newton KP, Beyer AK, Copeland NG, Gilbert DJ, Jenkins NA, Satler CA, and Robertson GA. Two isoforms of the mouse ether-a-go-go-related gene coassemble to form channels with properties similar to the rapidly activating component of the cardiac delayed rectifier K⁺current. Circ Res 81: 870–878, 1997.[Abstract/Free Full Text]
305. London B, Wang DW, Hill JA, and Bennett PB. The transient outward current in mice lacking the potassium channel gene Kv1.4. J Physiol 509: 171–182, 1998.[Abstract/Free Full Text]
306. Lopatin AN and Nichols CG. Inward rectifiers in heart: an update on I_K1. J Mol Cell Cardiol 33: 625–638, 2001.[CrossRef][Web of Science][Medline]
307. Lopatin AN, Makhina EN, and Nichols CG. Potassium channel block by cytoplasmic polyamines as the mechanism of intrinsic rectification. Nature 372: 366–369, 1994.[CrossRef][Medline]
308. Lopatin AN, Makhina EN, and Nichols CG. The mechanism of inward rectification of potassium channels: "long-pore plugging" by cytoplasmic polyamines. J Gen Physiol 106: 923–955, 1995.[Abstract/Free Full Text]
309. Lossin C, Rhodes TH, Desai RR, Vanoye CG, Wang D, Carnicui S, Devinsky O, and George AL. Epilepsy-associated channel dysfunction in the voltage-gated neuronal sodium channel SCN1A. J Neurosci 23: 11289–11295, 2003.[Abstract/Free Full Text]
310. Lu T, Lee HC, Kabat JA, and Shibata EF. Modulation of rat cardiac sodium channels by the stimulatory G protein alpha subunit. J Physiol 518: 371–384, 1999.[Abstract/Free Full Text]
311. Luo CH and Rudy Y. A dynamic model of the cardiac ventricular action potential. I. Simulations of ionic currents and concentration changes. Circ Res 74: 1071–1096, 1994.[Abstract/Free Full Text]
312. Luo CH and Rudy Y. A dynamic model of the cardiac ventricular action potential. II. After depolarizations, triggered activity, and potentiation. Circ Res 74: 1097–1113, 1994.[Abstract/Free Full Text]
313. Maier LS and Bers DM. Calcium, calmodulin, and calcium-calmodulin kinase II: heartbeat to heartbeat and beyond. J Mol Cell Cardiol 34: 919–939, 2002.[CrossRef][Web of Science][Medline]
314. Maier SKG, Westenbroek RE, Schenkman KA, Feigl EO, Scheuer T, and Catterall WA. An unexpected role for brain-type sodium channels in coupling of cell surface depolarization to contraction in the heart. Proc Natl Acad Sci USA 99: 4073–4078, 2002.[Abstract/Free Full Text]
315. Maier SKG, Westenbroek RE, Yamanushi TT, Dobrzynski H, Boyett MR, Catterall WA, and Scheuer T. An unexpected requirement for brain-type sodium channels for control of heart rate in the mouse sinoatrial node. Proc Natl Acad Sci USA 100: 3507–3512, 2003.[Abstract/Free Full Text]
316. Main MC, Bryant SM, and Hart G. Regional differences in action potential characteristics and membrane currents of guinea-pig left ventricular myocytes. Exp Physiol 83: 747–761, 1998.[Abstract]
317. Majumder K, De Biasi M, Wang Z, and Wible BA. Molecular cloning and functional expression of a novel potassium channel beta-subunit from human atrium. FEBS Lett 361: 13–16, 1995.[CrossRef][Web of Science][Medline]
318. Makielski JC, Ye B, Valdivia CR, Pagel MD, Pu J, Tester DJ, and Ackerman MJ. A ubiquitous splice variant and a common polymorphism affect heterologous expression of recombinant human SCN5A heart sodium channels. Circ Res 93: 821–828, 2003.[Abstract/Free Full Text]
319. Makita N, Bennett PB, and George AL Jr. Molecular determinants of beta 1 subunit-induced gating modulation in voltage-dependent Na⁺ channels. J Neurosci 16: 7117–7127, 1996.[Abstract/Free Full Text]
320. Makita N, Sloan-Brown K, Weghuis DO, Ropers HH, and George AL Jr. Genomic organization and chromosomal assignment of the human voltage-gated Na⁺ channel beta 1 subunit gene (SCN1B). Genomics 23: 628–634, 1994.[CrossRef][Web of Science][Medline]
321. Malhotra JD, Kazen-Gillespie K, Hortsch M, and Isom LL. Sodium channel beta subunits mediate homophilic cell adhesion and recruit ankyrin to points of cell-cell contact. J Biol Chem 275: 11383–11388, 2000.[Abstract/Free Full Text]
322. Malhotra JD, Koopman MC, Kazen-Gillespie KA, Fettman N, Hortsch M, and Isom LL. Structure requirements for interaction of sodium channel 1 subunits with ankyrin. J Biol Chem 277: 26681–26688, 2002.[Abstract/Free Full Text]
323. Maltsev VA, Sabbah HN, Higgins RS, Silverman N, Lesh M, and Undrovinas AI. Novel, ultraslow inactivating sodium current in human ventricular myocytes. Circulation 98: 2546–2552, 1998.
324. Maltsev VA and Undrovinas AI. Cytoskeleton modulates coupling between availability and activation of cardiac sodium channel. Am J Physiol Heart Circ Physiol 273: H1832–H1840, 1997.[Abstract/Free Full Text]
325. Mangoni ME, Couette B, Bourinet E, Platzer J, Reimer D, Striessnig J, and Nargeot J. Functional role of L-type Cav1.3 Ca²⁺ channels in cardiac pacemaker activity. Proc Natl Acad Sci USA 100: 5543–5548, 2003.[Abstract/Free Full Text]
326. Marbán E and O'Rourke B. Calcium channels: structure function and regulation. In: Cardiac Electrophysiology: From Cell to Bedside, edited by D. P. Zipes and J. Jalife. Philadelphia, PA: Sauders, 1995, p. 11–21.
327. Martens JR, Navarro-Polanco R, Coppock EA, Nishiyama A, Parshley L, Grobaski TD, and Tamkun MM. Differential targeting of Shaker-like potassium channels to lipid rafts. J Biol Chem 275: 7443–7446, 2000.[Abstract/Free Full Text]
328. Martínez ML, Heredia MP, and Delgado C. Expression of Ca²⁺ channels in ventricular cells from hypertrophied rat hearts. J Mol Cell Cardiol 31: 1617–1625, 1999.[CrossRef][Web of Science][Medline]
329. Maruoka ND, Steele DF, Au BP, Dan P, Zhang X, Moore ED, and Fedida D. Alpha-actinin-2 couples to cardiac Kv1.5 channels, regulating current density and channel localization in HEK cells. FEBS Lett 473: 188–194, 2000.[CrossRef][Web of Science][Medline]
330. Marx S. Ion channel macromolecular complexes in the heart. J Mol Cell Cardiol 35: 37–44, 2003.[CrossRef][Web of Science][Medline]
331. Marx SO, Kurokawa J, Reiken S, Motoike H, D'Armiento J, Marks AR, and Kass RS. Requirement of a macromolecular signaling complex for adrenergic receptor modulation of the KCNQ1-KCNE1 potassium channel. Science 295: 496–499, 2002.[Abstract/Free Full Text]
332. Mascher D and Peper K. Two components of inward current in myocardial muscle fibers. Pflügers Arch 307: 190–203, 1969.[Medline]
333. Mason HS, Latten MJ, Godoy LD, Horowitz B, and Kenyon JL. Modulation of Kv1.5 currents by protein kinase A, tyrosine kinase, and protein tyrosine phosphatase requires an intact cytoskeleton. Mol Pharmacol 61: 285–293, 2002.[Abstract/Free Full Text]
334. Matsuda JJ, Lee H, and Shibata EF. Enhancement of rabbit cardiac sodium channels by beta-adrenergic receptor stimulation. Circ Res 70: 199–207, 1992.[Abstract/Free Full Text]
335. Mazzanti M and DiFrancesco D. Intracellular Ca modulates K-inward rectification in cardiac myocytes. Pflügers Arch 413: 322–324, 1989.[CrossRef][Web of Science][Medline]
336. Mazzanti M, Assandri R, Ferroni A, and DiFrancesco D. Cytoskeletal control of rectification and expression of four substates in cardiac inward rectifier K⁺ channels. FASEB J 10: 357–361, 1996.[Abstract]
337. McCormack K, McCormack T, Tanouye M, Rudy B, and Stuhmer W. Alternative splicing of the human Shaker K⁺ channel beta 1 gene and functional expression of the beta 2 gene product. FEBS Lett 370: 32–36, 1995.[CrossRef][Web of Science][Medline]
338. McCormack T and McCormack K. Shaker K⁺ channel beta subunits belong to an NAD(P)H-dependent oxidoreductase superfamily. Cell 79: 1133–1135, 1994.[CrossRef][Web of Science][Medline]
339. McCrossan ZA and Abbott GW. The minK-related peptides. Neuropharmacology 47: 787–821, 2004.[CrossRef][Web of Science][Medline]
340. 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: 365–407, 1994.[Free Full Text]
341. McDonald TV, Yu Z, Ming Z, Palma E, Meyers MB, Wang KW, Goldstein SA, and Fishman GI. A minK-HERG complex regulates the cardiac potassium current I(Kr). Nature 388: 289–292, 1997.[CrossRef][Web of Science][Medline]
342. McLerie M and Lopatin A. Dominant-negative suppression of I_K1 in the mouse heart leads to altered cardiac excitability. J Mol Cell Cardiol 35: 367–378, 2003.[CrossRef][Web of Science][Medline]
343. McNally E, Allikian M, Wheeler MT, Mislow JM, and Heydemann A. Cytoskeletal defects in cardiomyopathy. J Mol Cell Cardiol 35: 231–241, 2003.[CrossRef][Web of Science][Medline]
344. McPhee JC, Dang YL, Davidson N, and Lester HA. Evidence for a functional interaction between integrins and G protein-activated inward rectifier K⁺ channels. J Biol Chem 273: 34696–34702, 1998.[Abstract/Free Full Text]
345. McPhee JC, Ragsdale DS, Scheuer T, and Catterall WA. A critical role for transmembrane segment IVS6 of the sodium channel alpha subunit in fast inactivation. J Biol Chem 270: 12025–12034, 1995.[Abstract/Free Full Text]
346. McPhee JC, Ragsdale DS, Scheuer T, and Catterall WA. A critical role for the S4–S5 intracellular loop in domain IV of the sodium channel alpha-subunit in fast inactivation. J Biol Chem 273: 1121–1129, 1998.[Abstract/Free Full Text]
347. Meadows L, Malhotra JD, Stetzer A, Isom LL, and Ragsdale DS. The intracellular segment of the sodium channel 1 subunit is required for its efficient association with the channel subunit. J Neurochem 76: 1871–1878, 2001.[CrossRef][Web of Science][Medline]
348. 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]
349. Mitcheson JS and Hancox JC. Characteristics of a transient outward current (sensitive to 4-aminopyridine) in Ca²⁺ tolerant myocytes isolated from the rabbit atrioventricular node. Pflügers Arch 438: 68–78, 1999.[CrossRef][Web of Science][Medline]
350. Mohler PJ, Gramolini AO, and Bennett V. Ankyrins. J Cell Sci 115: 1565–1566, 2002.[Free Full Text]
351. Mohler PJ, Schott JJ, Gramolini AO, Dilly KW, Guatimosim S, duBell WH, Song LS, Haurogne K, Kyndt F, Ali ME, Rogers TB, Lederer WJ, Escande D, Le Marec H, and Bennett V. Ankyrin-B mutation causes type 4 long QT cardiac arrhythmia and sudden cardiac death. Nature 421: 634–639, 2003.[CrossRef][Medline]
352. Morales MJ, Castellino RC, Crews AL, Rasmusson RL, and Strauss HC. A novel beta subunit increases rate of inactivation of specific voltage-gated potassium channel alpha subunits. J Biol Chem 270: 6272–6277, 1995.[Abstract/Free Full Text]
353. Moreno H, Rudy B, and Llinas R. Subunits influence the biophysical and pharmacological differences between P- and Q-type calcium currents expressed in mammalian cell lines. Proc Natl Acad Sci 94: 14042–14047, 1997.[Abstract/Free Full Text]
354. Morgan K, Stevens EB, Shah B, Cox PJ, Dixon AK, Lee K, Pinnock RD, Hughes J, Richardson PJ, Mizuguchi K, and Jackson AP. Beta 3: an additional auxiliary subunit of the voltage-sensitive sodium channel that modulates channel gating with distinct kinetics. Proc Natl Acad Sci USA 97: 2308–2313, 2000.[Abstract/Free Full Text]
355. Mori MX, Erickson MG, and Yue DT. Functional stoichiometry and local enrichment of calmodulin interacting with Ca²⁺ channels. Science 304: 432–435, 2004.[Abstract/Free Full Text]
356. Morohashi Y, Hatano N, Ohya S, Takikawa R, Watabiki T, Takasugi N, Imaizumi Y, Tomita T, and Iwatsubo T. Molecular cloning and characterization of CALP/KChIP4, a novel EF-hand protein interacting with presenilin 2 and voltage-gated potassium channel subunit Kv4. J Biol Chem 277: 14965–14975, 2002.[Abstract/Free Full Text]
357. Moss AJ and Robinson JL. The long QT syndrome: genetic considerations. Trends Cardiovasc Med 2: 81–83, 1993.[Medline]
358. Motoike HK, Liu H, Glaaser IW, Yang AS, Tateyama M, and Kass RS. The Na⁺ channel inactivation gate is a molecular complex: a novel role for the COOH-terminal domain. J Gen Physiol 123: 155–165, 2004.[Abstract/Free Full Text]
359. Mulley JC, Scheffer IE, Petrou S, and Berkovic SF. Channelopathies as a genetic cause of epilepsy. Curr Opin Neurol 16: 171–176, 2003.[CrossRef][Web of Science][Medline]
360. Muniz ZM, Parcej DN, and Dolly JO. Characterization of monoclonal antibodies against voltage-dependent K⁺channels raised using alpha-dendrotoxin acceptors purified from bovine brain. Biochemistry 31: 12297–12303, 1992.[CrossRef][Medline]
361. Munk AA, Adjemian RA, Zhao J, Ogbagherriel A, and Shrier A. Electro-physiological properties of morphologically distinct cells isolated from the rabbit atrio-ventricular node. J Physiol 493: 801–818, 1996.[Abstract/Free Full Text]
362. Murai T, Kakizuka A, Takumi T, Ohkubo H, and Nakanishi S. Molecular cloning and sequence analysis of human genomic DNA encoding a novel membrane protein which exhibits a slowly activating potassium channel activity. Biochem Biophys Res Commun 161: 176–181, 1989.[CrossRef][Web of Science][Medline]
363. Muth JN, Varadi G, and Schwartz A. Use of transgenic mice to study voltage-dependent Ca²⁺ channels. Trends Pharmacol Sci 22: 526–532, 2001.[CrossRef][Medline]
364. Näbauer M. Electrical heterogeneity in the ventricular wall and in the M cell. Cardiovasc Res 40: 248–250, 1998.[Free Full Text]
365. Näbauer M and Käab S. Potassium channel down-regulation in heart failure. Cardiovasc Res 37: 324–334, 1998.[CrossRef][Web of Science][Medline]
366. Näbauer M, Barth A, and Kääb S. A second calcium-independent transient outward current present in human ventricular myocardium. Circulation 98: I-231, 1998.
367. Näbauer M, Beuckelmann DJ, überfuhr P, and Steinbeck G. Regional differences in current density and rate dependent properties of the transient outward current in subepicardial and subendocardial myocytes of human left ventricle. Circulation 93: 168–177, 1996.[Abstract/Free Full Text]
368. Nadal MS, Ozaita A, Amarillo Y, Vega-Saenz de Miera E, Ma Y, Mo W, Goldberg EM, Misumi Y, Ikehara Y, Neubert TA, and Rudy B. The CD26-related dipeptidyl aminopeptidase-like protein DPPX is a critical component of neuronal A-type K⁺ channels. Neuron 37: 449–461, 2003.[CrossRef][Web of Science][Medline]
369. Nagaya N and Papazian DM. Potassium channel alpha and beta subunits assemble in the endoplasmic reticulum. J Biol Chem 272: 3022–3027, 1997.[Abstract/Free Full Text]
370. Nakahira K, Shi G, Rhodes KJ, and Trimmer JS. Selective interaction of voltage-gated K⁺ channel beta-subunits with alpha-subunits. J Biol Chem 271: 7084–7089, 1996.[Abstract/Free Full Text]
371. Nakayama T and Irisawa H. Transient outward current carried by potassium and sodium in quiescent atrioventricular node cells of rabbits. Circ Res 57: 65–73, 1985.[Abstract/Free Full Text]
372. Nehring RB, Wischmeyer E, Doring F, Veh RW, Sheng M, and Karschin A. Neuronal inwardly rectifying K⁺ channels differentially couple to PDZ proteins of the PSD-95/SAP90 family. J Neurosci 20: 156–162, 2000.[Abstract/Free Full Text]
373. Nerbonne JM. Molecular analysis of voltage-gated K⁺ channel diversity and functioning in the mammalian heart. In: Handbook of Physiology. The Cardiovascular System. The Heart. Bethesda, MD: Am. Physiol. Soc., 2002, sect. 2, vol. I, p. 568–594.
374. Nerbonne JM and Guo W. Heterogeneous expression of voltage-gated potassium channels in the heart: roles in normal excitation and arrhythmias. J Cardiovasc Electrophysiol 13: 406–409, 2002.[CrossRef][Web of Science][Medline]
375. Nerbonne JM and Kass RS. Physiology and molecular biology of ion channels contributing to ventricular repolarization. In: Contemporary Cardiology: Cardiac Repolarization: Bridging Basic and Clinical Science, edited by I. Gussak and C. Antzelevitch. Totowa, NJ: Humana, 2003, chapt. 3, p. 25–62.
376. Nerbonne JM, Nichols CG, Schwarz TL, and Escande D. Genetic manipulation of cardiac K⁺ channel function in mice. What have we learned, and where do we go from here? Circ Res 89: 944–956, 2001.[Abstract/Free Full Text]
377. Nichols CG and Lopatin AN. Inward rectifier potassium channels. Annu Rev Physiol 59: 171–191, 1997.[CrossRef][Web of Science][Medline]
378. Nichols CG, Makhina EN, Pearson WL, Sha Q, and Lopatin AN. Inward rectification and implications for cardiac excitability. Circ Res 78: 1–7, 1996.[Abstract/Free Full Text]
379. Noble D and Tsien RW. Outward membrane currents activated in the plateau range of potentials in cardiac Purkinje fibres. J Physiol 200: 205–231, 1969.[Abstract/Free Full Text]
380. Noma A. ATP-regulated K⁺ channels in cardiac muscle. Nature 305: 147–148, 1983.[CrossRef][Medline]
381. Noma A, Nakayama T, Kurachi Y, and Irisawa H. Resting K conductances in pacemaker and non-pacemaker heart cells of the rabbit. Jpn J Physiol 34: 245–254, 1984.[Web of Science][Medline]
382. Nudler S, Piriz J, Urbano FJ, Rosato-Siri MD, Renteria ES, and Uchitel OD. Ca²⁺ channels and synaptic transmission at the adult, neonatal, and P/Q-type deficient neuromuscular junction. Ann NY Acad Sci 998: 11–17, 2003.[CrossRef][Medline]
383. Nuss HB and Houser SR. T-type Ca²⁺ current is expressed in hypertrophied adult feline left ventricular myocytes. Circ Res 73: 777–782, 1993.[Abstract/Free Full Text]
384. Nuyens D, Stengl M, Dugarmaa S, Rossenbacker T, Compernolle V, Rudy Y, Smits JF, Flameng W, Clancy CE, Moons L, Vos MA, Dewerchin M, Benndorf K, Collen D, Carmeliet E, and Carmeliet P. Abrupt rate accelerations or premature beats cause life-threatening arrhythmias in mice with long QT3 syndrome. Nat Med 7: 1021–1027, 2001.[CrossRef][Web of Science][Medline]
385. O'Callaghan DW, Hasdemir B, Leighton M, and Burgoyne RD. Residues within the myristoylation motif determine intracellular targeting of the neuronal Ca²⁺ sensor protein KChIP1 to post-ER transport vesicles and traffic of Kv4 K⁺ channels. J Cell Sci 116: 4833–4845, 2003.[Abstract/Free Full Text]
386. Ochi R. The slow inward current and the action of manganese ions in guinea pig myocardium. Pflügers Arch 316: 81–84, 1970.[Medline]
387. Ohya S, Tanaka M, Oku T, Asai Y, Watanabe M, Giles WR, and Imaizumi Y. Molecular cloning and tissue distribution of an alternatively spliced variant of an A-type K⁺ channel alpha-subunit, Kv4.3 in the rat. FEBS Lett 420: 47–53, 1997.[CrossRef][Web of Science][Medline]
388. Ophoff RA, Terwindt GM, Vergouwe MN, van Eijk R, Oefner PJ, Hoffman SM, Lamerdin JE, Mohrenweiser HW, Bulman DE, Ferrari M, Haan J, Lindhout D, van Ommen GJ, Hofker MH, Ferrari MD, and Frants RR. Familial hemiplegic migraine and episodic ataxia type-2 are caused by mutations in the Ca²⁺ channel gene CACNL1A4. Cell 87: 543–552, 1996.[CrossRef][Web of Science][Medline]
389. Papadatos GA, Wallerstein PMR, Head CEG, Ratcliff R, Brady PA, Benndorf K, Saumarez RC, Trezise AEO, Huang CLH, Vandenberg JI, Colledge WH, and Grace AA. Slowed conduction and ventricular tachycardia after targeted disruption of the cardiac sodium channel gene, SCN5A. Proc Natl Acad Sci USA 99: 6210–6215, 2002.[Abstract/Free Full Text]
390. Patel SP, Campbell DL, and Strauss HC. Elucidating KChIP effects on Kv4.3 inactivation and recovery kinetics with a minimal KChIP2 isoform. J Physiol 545: 5–11, 2002.[Abstract/Free Full Text]
391. Patel SP, Campbell DL, Morales MJ, and Strauss HC. Heterogeneous expression of KChIP2 isoforms in the ferret heart. J Physiol 539: 649–656, 2002.[Abstract/Free Full Text]
392. Patton DE, West JW, Catterall WA, and Goldin AL. Amino acid residues required for fast Na⁺-channel inactivation: charge neutralizations and deletions in the III-IV linker. Proc Natl Acad Sci USA 89: 10905–10909, 1992.[Abstract/Free Full Text]
393. Perez-Garcia MT, Lopez-Lopez JR, and Gonzalez C. Kv beta 1.2 subunit coexpression in HEK293 cells confers O₂ sensitivity to Kv4.2 but not to Shaker channels. J Gen Physiol 113: 897–907, 1999.[Abstract/Free Full Text]
394. Perez-Reyes E. Molecular physiology of low voltage-activated T-type calcium channels. Physiol Rev 83: 117–161, 2002.[Web of Science]
395. Perez-Reyes E and Schneider T. Calcium channels: structure function and classification. Drug Dev Res 33: 295–318, 1994.[CrossRef][Web of Science]
396. Perez-Reyes E, Castellano A, Kim HS, Bertrand P, Baggstrom E, Lacerda AE, Wei XY, and Birnbaumer L. Cloning and expression of a cardiac/brain beta subunit of the L-type calcium channel. J Biol Chem 267: 1792–1797, 1992.[Abstract/Free Full Text]
397. Petersen KR and Nerbonne JM. Expression environment determines K⁺ current properties: Kv1 and Kv4 alpha-subunit-induced K⁺ currents in mammalian cell lines and cardiac myocytes. Pflügers Arch 437: 381–392, 1999.[CrossRef][Web of Science][Medline]
398. Peterson BZ, DeMaria CD, Adelman JP, and Yue DT. Calmodulin is the Ca²⁺ sensor for Ca²⁺-dependent inactivation of L-type calcium channels. Neuron 22: 549–558, 1999.[CrossRef][Web of Science][Medline]
399. Petrecca K and Shrier A. Spatial distribution of ion channels, receptors and innervation in the AV node. In: Atrial-AV Nodal Electrophysiology: A View From the Millenium, edited By T. N. Mazgalev and P. J. Tehou. New York: Futura, 2000, p. 89–105.
400. Petrecca K, Amellal F, Laird DW, Cohen SA, and Shrier A. Sodium channel distribution within the rabbit atrioventricular node as analyzed by confocal microscopy. J Physiol 501: 263–274, 1997.[Abstract/Free Full Text]
401. Petrecca K, Miller DM, and Shrier A. Localization and enhanced current density of the Kv4.2 potassium channel by interaction with the actin-binding protein filamin. J Neurosci 20: 8736–8744, 2000.[Abstract/Free Full Text]
402. Pietrobon D. Calcium channels and channelopathies of the central nervous system. Mol Neurobiol 25: 31–50, 2002.[CrossRef][Web of Science][Medline]
403. Plaster NM, Tawil R, Tristani-Firouzi M, Canun S, Bendahhou S, Tsunoda A, Donaldson MR, Iannaccone ST, Brunt E, Barohn R, Clark J, Deymeer F, George AL Jr, Fish FA, Hahn A, Nitu A, Ozdemir C, Serdaroglu P, Subramony SH, Wolfe G, Fu YH, and Ptacek LJ. Mutations in Kir2.1 cause the developmental and episodic electrical phenotypes of Andersen's syndrome. Cell 105: 511–519, 2001.[CrossRef][Web of Science][Medline]
404. Pond AL, Scheve BK, Benedict AT, Petrecca K, Van Wagoner DR, Shrier A, and Nerbonne JM. Expression of distinct ERG proteins in rat, mouse, and human heart. Relation to functional I_Kr channels. J Biol Chem 275: 5997–6006, 2000.[Abstract/Free Full Text]
405. Pongs O. Molecular biology of voltage-dependent potassium channels. Physiol Rev 72 Suppl: S69–S88, 1992.[Web of Science][Medline]
406. Pountney DJ, Sun ZQ, Porter LM, Nitabach MN, Nakamura TY, Holmes D, Rosner E, Kaneko M, Manaris T, Holmes TC, and Coetzee WA. Is the molecular composition of K(ATP) channels more complex than originally thought? J Mol Cell Cardiol 33: 1541–1546, 2001.[CrossRef][Web of Science][Medline]
407. Pragnell M, De Waard M, Mori Y, Tanabe T, Snutch TP, and Campbell KP. Calcium channel beta-subunit binds to a conserved motif in the I-II cytoplasmic linker of the alpha 1-subunit. Nature 368: 67–70, 1994.[CrossRef][Medline]
408. Pragnell M, Sakamoto J, Jay SD, and Campbell KP. Cloning and tissue-specific expression of the brain calcium channel beta-subunit. FEBS Lett 291: 253–258, 1991.[CrossRef][Web of Science][Medline]
409. Preisig-Muller R, Schlichthorl G, George T, Heinen S, Bruggemann A, Rajan S, Derst C, Veh RW, and Daut J. Heteromerization of Kir2.x potassium channels contributes to the phenotype of Andersen's syndrome. Proc Natl Acad Sci USA 99: 7774–7779, 2002.[Abstract/Free Full Text]
410. Ptacek LJ, Tawil R, Griggs RC, Engel AG, Layzer RB, Kwiecinski H, McManis PG, Santiago L, Moore M, and Fouad G. Dihydropyridine receptor mutations cause hypokalemic periodic paralysis. Cell 77: 863–868, 1994.[CrossRef][Web of Science][Medline]
411. Pu J, Wada T, Valdivia C, Chutkow WA, Burant CF, and Makielski JC. Evidence of K_ATP channels in native cardiac cells without SUR. Biophys J 80: 625–626, 2001.
412. Qin N, Platano D, Olcese R, Stefani E, and Birnbaumer L. Direct interaction of Gbetagamma with a C-terminal Gbetagamma-binding domain of the Ca²⁺ channel alpha1 subunit is responsible for channel inhibition by G protein-coupled receptors. Proc Natl Acad Sci USA 94: 8866–8871, 1997.[Abstract/Free Full Text]
413. Ratcliffe CF, Westenbroek RE, Curtis R, and Catterall WA. Sodium channel beta1 and beta3 subunits associate with neurofascin through their extracellular immunoglobulin-like domain. J Cell Biol 154: 427–434, 2001.[Abstract/Free Full Text]
414. Ravens U and Dobrev D. Cardiac sympathetic innervation and control of potassium channel function. J Mol Cell Cardiol 35: 137–139, 2003.[Medline]
415. Razani B and Lisanti MP. Caveolin-deficient mice: insights into caveolin function in human disease. J Clin Invest 108: 1553–1561, 2001.[CrossRef][Web of Science][Medline]
416. Reuter H. The dependence of slow inward current in Purkinje fibres on the extracellular calcium-concentration. J Physiol 192: 479–492, 1967.[Abstract/Free Full Text]
417. Reuter H. Slow inactivation of currents in cardiac Purkinje fibres. J Physiol 197: 233–253, 1968.[Abstract/Free Full Text]
418. Reuter H and Beeler GW Jr. Calcium current and activation of contraction in ventricular myocardial fibers. Science 163: 399–401, 1969.[Abstract/Free Full Text]
419. Rettig J, Heinemann SH, Wunder F, Lorra C, Parcej DN, Dolly JO, and Pongs O. Inactivation properties of voltage-gated K⁺channels altered by presence of beta-subunit. Nature 369: 289–294, 1994.[CrossRef][Medline]
420. Rhodes KJ, Keilbaugh SA, Barrezueta NX, Lopez KL, and Trimmer JS. Association and colocalization of K⁺ channel alpha- and beta-subunit polypeptides in rat brain. J Neurosci 15: 5360–5371, 1995.[Abstract]
421. Ribauux P, Bleicher F, Couble ML, Amsellem J, Cohen SA, Berthier C, and Blaineu S. Voltage-gated sodium channel (SkM1) content in dystrophin-deficient muscle. Pflügers Arch 441: 746–755, 2001.[CrossRef][Web of Science][Medline]
422. Rivolta I, Abriel H, Tateyama M, Liu H, Memmi M, Vardas P, Napolitano C, Priori SG, and Kass RS. Inherited Brugada and long QT-3 syndrome mutations of a single residue of the cardiac sodium channel confer distinct channel and clinical phenotypes. J Biol Chem 276: 30623–30630, 2001.[Abstract/Free Full Text]
423. Rivolta I, Clancy CE, Tateyama M, Liu H, Priori SG, and Kass RS. A novel SCN5A mutation associated with long QT-3: altered inactivation kinetics and channel dysfunction. Physiol Gen 10: 191–197, 2002.
424. Robbins J. KCNQ potassium channels: physiology, pathophysiology, and pharmacology. Pharmacol Ther 90: 1–19, 2001.[CrossRef][Web of Science][Medline]
425. Roden DM and George AL. Structure and function of cardiac sodium and potassium channels. Am J Physiol Heart Circ Physiol 273: H511–H525, 1997.[Abstract/Free Full Text]
426. Rogart RB, Cribbs LL, Muglia LK, Kephart DD, and Kaiser MW. Molecular cloning of a putative tetrodotoxin-resistant rat heart Na⁺ channel isoform. Proc Natl Acad Sci USA 86: 8170–8174, 1989.[Abstract/Free Full Text]
427. Rohl CA, Boeckman FA, Baker C, Scheuer T, Catterall WA, and Klevit RE. Solution structure of the sodium channel inactivation gate. Biochemistry 38: 855–861, 1999.[CrossRef][Medline]
428. Romanin C, Gamsjaeger R, Kahr H, Schaufler D, Carlson O, Abernethy DR, and Soldatov NM. Ca²⁺ sensors of L-type Ca²⁺ channel. FEBS Lett 487: 301–306, 2000.[CrossRef][Web of Science][Medline]
429. Rosati B, Grau F, Rodriguez S, Li H, Nerbonne JM, and McKinnon D. Concordant expression of KChIP2 mRNA, protein and transient outward current throughout the canine ventricle. J Physiol 548: 815–822, 2003.[