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Urinary Bladder Contraction and Relaxation: Physiology and Pathophysiology

Departments of Clinical Pharmacology and Physiological Sciences, University of Lund, Lund, Sweden

ABSTRACT

I. INTRODUCTION

II. MORPHOLOGY OF THE LOWER URINARY TRACT

III. THE CONTRACTILE SYSTEM

A. Key Features of the Detrusor Smooth Muscle Cells

B. Structure of the Contractile Apparatus and Cytoskeleton

1. Contractile proteins and filaments

2. Cytoskeleton and intermediate filaments

C. Actin-Myosin Interaction

D. Energetics and Cell Metabolism

E. Detrusor Muscle Mechanics

F. Pathophysiological Adaptations

IV. EXCITATION-CONTRACTION COUPLING

A. Regulation of Contractile Proteins

V. MEMBRANE EXCITATION

A. Resting Membrane Potential and Action Potentials

B. Ca2+ Channels

C. K+ Channels

1. ATP-sensitive K+ channels

2. Ca2+-activated K+ channels

3. Kv channels

D. Stretch-Activated Channels

E. Ligand-Activated Channels

F. Myogenic Activity

G. Interstitial Cells

VI. NEURAL AND HORMONAL CONTROL

A. Cholinergic Mechanisms

1. Muscarinic receptors

B. Adrenergic Mechanisms

1. {alpha}-Adrenoceptors

2. {beta}-Adrenoceptors

C. NANC Mechanisms

1. ATP

2. Nitric oxide

3. Neuropeptides

A) VASOACTIVE INTESTINAL POLYPEPTIDE.

B) ENDOTHELINS.

C) TACHYKININS.

D) ANGIOTENSINS.

4. Prostanoids

VII. SUMMARY AND FUTURE PERSPECTIVES

ACKNOWLEDGMENTS

REFERENCES

	ABSTRACT

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	I. INTRODUCTION

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The urinary bladder has two important functions: storage of urine and emptying. Storage of urine occurs at low pressure, which implies that the bladder relaxes during the filling phase. Disturbances of the storage function may result in lower urinary tract symptoms (LUTS), such as urgency, frequency, and urge incontinence, the components of the overactive bladder syndrome (3). The overactive bladder syndrome, which may be due to involuntary contractions of the smooth muscle of the bladder (detrusor) during the storage phase, is a common and underreported problem, the prevalence of which has only recently been assessed (467).

Emptying requires a coordinated contraction of the bladder and relaxation of the urethra. Disturbances of the voiding function can lead to symptoms of hesitancy, weak stream, feeling of incomplete bladder emptying, and postmicturition dribble. LUTS is increasing markedly with age in both males and females and is a major problem in the elderly population. Partly due to an increasing awareness of the problem of LUTS, and the lack of effective treatment of the disorder, interest in the research of lower urinary tract physiology and pathophysiology has increased.

To find ways to control micturition, knowledge about the mechanisms of contraction and relaxation of detrusor smooth muscle under normal and pathological conditions is necessary. It is well known that smooth muscles exhibit an extreme variability, not only in ultrastructural details, but also in their contractile, regulatory, and electrophysiological properties and in their sensitivities to drugs and neurotransmitters. Nevertheless, most smooth muscles have properties in common. This fact may be of interest because therapeutic approaches based on findings in other types of smooth muscle may be applied also to those of the lower urinary tract.

To identify unique properties of the muscles of the lower urinary tract, possibly involved in pathological conditions, or as potential targets for therapeutic interventions, is a challenge to the research field. Much of our current knowledge is based on animal experimentation, and species differences may be a problem when extrapolating animal findings to the human situation.

Many factors, e.g., central and peripheral nervous control and the contribution of other components of the lower urinary tract, may influence micturition. In the last decade many reviews have focused on different aspects of the physiology and pathophysiology of the bladder (18–20, 147, 174, 203, 253, 325, 478, 506, 666, 675, 697).

The present overview is focused on the processes, from cellular receptors to the contractile machinery, involved in physiological contraction and relaxation of bladder smooth muscle (detrusor). Special attention has been given to the role of these processes in pathophysiological alterations in bladder function associated with, e.g., bladder outlet obstruction, detrusor hypertrophy, and detrusor overactivty. The size of the bladder varies over a large range between species (bladder capacity: mouse, 0.15 ml; rat, 1 ml; human, 500 ml). Also micturition patterns, contractile properties, and contractile regulation vary between species. When possible in this review, information on human detrusor is discussed. Because in several respects basic information on the properties of the human detrusor is fragmentary or missing, animal data will be presented, when appropriate.

	II. MORPHOLOGY OF THE LOWER URINARY TRACT

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The lower urinary tract consists of the urinary bladder and the urethra (Fig. 1). The urethra contains both smooth and striated muscles, and details on its structure can be found elsewhere (76, 717). The bladder can be divided into two main components: the bladder body, which is located above the ureteral orifices, and the base, consisting of the trigone, urethrovesical junction, deep detrusor, and the anterior bladder wall. The bladder is a hollow smooth muscle organ lined by a mucous membrane and covered on its outer aspect partly by peritoneal serosa and partly by fascia. Its muscular wall is formed of smooth muscle cells, which comprise the detrusor muscle. The detrusor is structurally and functionally different from, e.g., trigonal and urethral smooth muscle. As pointed out above, in this review, focus is on properties of the detrusor smooth muscle.

View larger version (49K): [in this window] [in a new window]   fig. 1. Schematic drawing of the bladder.

Within the main bundles, the smooth muscle cells may exist in groups of small functional units, or fascicles (see Ref. 159). The orientation and interaction between the smooth muscle cells in the bladder are important, since this will determine how the bladder wall behaves and what effect activity in the cells will have on its shape and intraluminal pressure. In smaller animals, e.g., rabbit, the muscle bundles are less complex and the patterns of arrangement simpler than in the human detrusor.

The individual smooth muscle cells in the detrusor are typical smooth muscle cells, similar to those in other muscular organs. They are long, spindle-shaped cells with a central nucleus. When fully relaxed, the cells are several hundred microns long, and the widest diameter is 5–6 µm. The cytoplasm is packed with the normal myofilaments, and the membranes contain regularly spaced dense bands, with membrane vesicles (caveoli) between them. There are also scattered dense bodies in the cytoplasm. Mitochondria and fairly sparse elements of sarcoplasmic reticulum (mostly near the nucleus) are also present (152).

	III. THE CONTRACTILE SYSTEM

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A. Key Features of the Detrusor Smooth Muscle Cells

The two main functions of the lower urinary tract, to store urine without leakage for longer periods of time and to rapidly expel it during micturition, occur naturally in normal life. They involve a very complex interaction between the structural/anatomic parts of the urinary tract and between nervous control systems. In addition to these demands on integrative control, both filling and emptying of the urinary bladder provide a challenge to the muscle components in the walls of the lower urinary tract. During filling of the urinary bladder, the smooth muscle cells have to relax, and to elongate and rearrange in the wall over a very large length interval. During micturition, force generation and shortening must be initiated comparatively fast, be synchronized, and occur over a large length range. These activities thus require both regulation of contraction and regulation of relaxation. To respond to the nervous and hormonal control systems, each part of the urinary tract muscles has to have specific receptors for the transmitters/modulators, released from nerves or generated locally, and the associated cellular pathways for initiating contraction and relaxation.

In early work by Emil Bozler (71), two classes, "single-unit" and "multiunit," of smooth muscle were defined on the basis of contractile behavior. Bozler's class of single-unit smooth muscles is described to be arranged in sheets or bundles, and the cell membranes have many points of close contact, gap junctions. Gap junctions constitute low-resistance pathways, formed by connexin subunits, through which ions can flow from one cell to the other, and thereby an electrical signal can be spread rapidly throughout the tissue (cf. Ref. 82). Action potentials can thus be conducted from one area to another by direct electrical conduction. A minority of the fibers in a single-unit muscle spontaneously generates action potentials, pacemaker cells. Characteristically, in single-unit smooth muscles a contractile response can often be induced by stretching the muscle. Multiunit smooth muscle are thought to be composed of discrete muscle fibers or bundles of fibers that operate independently of each other. They are richly innervated by the autonomic nervous system and are controlled mainly by nerve signals. They rarely show spontaneous contractions. In practice, multiunit muscles would be very rare since the vast majority of smooth muscles have some degree of interconnection between cells. However, although the detrusor muscle exhibits several of the characteristics ascribed to a single-unit smooth muscle, it also shows several features ascribed to multiunit smooth muscles, being densly innervated, and functionally requiring nervous coordination to achieve voiding.

An alternative division of smooth muscles into "phasic" and "tonic" smooth muscle has been proposed (613). This division into these types is based on membrane properties and contractile behavior. As shown by Horiuti et al. (285), the phasic and tonic smooth muscle types have different kinetics of both the regulatory systems and of the contractile machinery. The contractile kinetics of different smooth muscles most likely reflect a continuum, and division into a fast phasic type and a slow tonic type might be difficult. As discussed below, the urinary bladder smooth muscle is a comparatively fast smooth muscle with characteristics of a "phasic" smooth muscle.

B. Structure of the Contractile Apparatus and Cytoskeleton

1. Contractile proteins and filaments

Contraction of smooth muscle is due to interaction between the contractile proteins actin and myosin in a similar manner as in other muscle types. Regular sarcomeric structures are lacking in smooth muscle, and the structure of the thin (actin-containing) and the thick (myosin-containing) filaments and their cellular organization in smooth muscle are at present not fully characterized. Recently, a large, megaDalton, structural protein, smitin, has been described to be present in smooth muscle and to interact with smooth muscle myosin filaments (344), which could suggest that large titinlike proteins are involved in the arrangement of the smooth muscle contractile apparatus. Cytoplasmic dense bodies, which contain -actinin, have been found to be mechanically connected and suggested to provide a framework for the attachment of the contractile structures (66, 223, 324, 665). The organization and length, or length distribution, of such "sarcomere equivalents" or "contractile units" in smooth muscle would be an important factor determining the force and shortening velocity of smooth muscle and the length interval over which a smooth muscle can generate force. The length of the contractile unit would be dependent on the length of the contractile filaments. The actin filament length has not been unequivocally determined in smooth muscle. Values similar to those in striated muscle have been presented, as well as higher values (160, 606). The thick filament length has been reported to be slightly longer in smooth (2.2 µm; Ref. 44) compared with skeletal muscle (1.6 µm). At present, no unique structural features of the contractile unit in the urinary bladder have been associated with the large volume and cell length span over which the urinary bladder muscle can operate. The arrangement of contractile filaments appears to be generally similar to that of other types of smooth muscle (153, 214). Longer contractile units could theoretically give a longer length interval over which the cells could contract. However, this would also lead to a lower maximal shortening velocity (562). Because the urinary bladder is a comparatively fast smooth muscle with a maximal shortening velocity similar to that of other smooth muscles with similar contractile protein isoform composition, the contractile units do not seem to be extensively long in the urinary bladder. The changes in bladder volume are directly coupled with changes in cell length (671, 677), also excluding sliding or rearrangement of the smooth muscle cells in the tissue during bladder filling and emptying.

The actin concentration in smooth muscle is similar to that of skeletal muscle and has been reported to be in the range of 20–50 mg/g smooth muscle cell for vascular tissue (125, 436, 486, 487). The value determined for rat and mouse urinary bladder, 40 mg/g smooth muscle cell, falls within this range (440, 598). Four different isoforms of actin are expressed in smooth muscle, -, -, and two forms of -actin (681). In bladder muscle, -, -, and -actin (comprising both smooth and nonmuscle -actin) are present in the relative proportions 33:25:42% for human (441), 41:19:40% for rat (440), and 44:10:46% for mouse (456). The functional roles of the actin isoforms and the consequences of isoform shifts are unclear at present. The cellular distribution of the actin isoforms has been reported to differ, -actin is predominantly found in the cytoskeletal domain of some smooth muscle cells (508), and it is possible that the different actin isoforms have different functions in the cytoskeleton. With regard to the contractile properties, it should be noted that actin is a highly conserved protein and the different isoforms have a significant degree of homology (549). The filament translocation velocity in the in vitro motility assay was similar under a variety of conditions for smooth and skeletal actin (259). Smooth muscle preparations with different isoactin composition appear functionally similar (160). It is thus unlikely that the actin isoform composition is a major factor determining the extent of force development or the shortening velocity of smooth muscle.

The amount of myosin in smooth muscle is approximately three to five times less than in skeletal muscle (10–16 mg/g smooth muscle cell, Refs. 436, 487) and the ratio of actin to myosin filaments is 15:1, compared with 2:1 in striated muscle (cf. Ref. 618). In bladder tissue, the myosin amount is in the range above (rat, 17 mg/g smooth muscle cell, Ref. 440; mouse, 26 mg/g smooth muscle cell, Ref. 598). Myosin molecules polymerize with the globular head regions, containing the nucleotide and actin binding sites, projecting at regular 14-nm intervals to form cross-bridges with actin. The detailed assembly of the smooth muscle thick filament is not resolved; both phase- or side-polar filaments with cross-bridges projecting in an antiparallel fashion on both sides of the filament (127, 133, 603, 730) and bipolar filaments, with a bare zone and with cross-bridges projecting in opposite directions at each end like in skeletal muscle thick filaments (44), have been proposed.

Smooth muscle myosin belongs to the myosin II superfamily of filament forming myosin motors (cf. Ref. 584). The myosin molecule is a hexamer formed by six polypeptide chains: two heavy chains and two pairs of light chains. The carboxy-terminal parts of the heavy chains dimerize into an -helical coiled tail region constituting the backbone of the thick filaments. Two light chains, one essential (molecular mass 17 kDa) and one regulatory (20 kDa), are associated with each of the two amino-terminal head parts of the heavy chains (cf. Ref. 5).

The smooth muscle myosin heavy chain (SM-MHC) is encoded by a single gene (491, 737). Different myosin heavy chain isoforms can be formed by alternative splicing. With the use of gel electrophoresis (163, 561), two smooth muscle two heavy chain variants, the SM1 (molecular mass 204 kDa) and the SM2 (200 kDa), have been identified. These two isoforms are formed by alternative splicing (47, 490), where the larger SM1 isoform contains a unique sequence of 43 amino acids in the carboxy-terminal tail region comprising a phosphorylation site for casein kinase II (330, 332, 333) and the smaller SM2 isoform 9 unique amino acids. The relative expression of SM1 and SM2 varies between adult smooth muscle tissues and between cells in the tissue (462, 463, 569, 586). During fetal life the expression of SM1 is high and the ratio SM1/SM2 decreases with development (30, 31, 164, 165, 470). In fetal and neonatal bladders, SM1 is the predominant isoform (30, 400). In the adult urinary bladder the relative content of SM1 is 70% of the myosin heavy chain in the rat (440) and 40% in the rabbit (30). No major differences have been noted between the bladder parts (292). In urinary bladder of humans, the SM1 content is 40% (193, 441).

The functional consequences of changes in the SM1/SM2 ratio in smooth muscle tissue is unclear. A few studies have reported a correlation between tissue SM1/SM2 expression and the maximal shortening velocity, as an index of the kinetics of the actin-myosin interaction (280). However, it should be noted that the SM1 and SM2 differ in the tail region, which does not primarily interact with actin and other studies using in vitro motility assay, isolated cells or a comparative approach (334, 437, 463, 475, 586) have failed to show a correlation between shortening velocity and SM1/SM2 ratio. A correlation between SM1/SM2 ratio and the extent of cell shortening has been reported (462). The SM1 and SM2 exhibit differences in their filament assembly properties (559), and the SM1 contains a phosphorylation site in the tail region, which could suggest that the expression of these isoforms can be important for the structure of the contractile machinery.

Two additional myosin heavy chain isoforms, SM-A and SM-B, are formed by alternative splicing in the amino-terminal region. The SM-B has an extra seven-amino acid insert in the loop 1 region of myosin close to the ATPase site, coded by exon 5B (46, 48, 254, 335, 713). Both the SM1 and the SM2 can contain the insert, enabling four possible isoforms SM1-A, SM1-B, SM2-A, and SM2-B (cf. Ref. 49). The insert in loop 1 region at the 25/50-kDa domain junction in the head of the myosin heavy chain is close to the catalytic site and has been suggested to modulate actin-myosin kinetics (633). It was early recognized that this region of myosin influences the kinetics of its interaction with actin. With the use of the in vitro motility assay (335, 377, 560), it was shown that myosin with the insert (SM-B) propels actin at a higher velocity than myosin without. The relative SM-B expression varies between tissues, and comparative studies on muscle preparations have found a correlation between expression of the inserted (SM-B) isoform and the maximal shortening velocity (162, 409, 410, 600, 713). Ablation of the SM-B form in transgenic mice has been shown to result in a slower smooth muscle phenotype (48, 323, 323). Comparative studies and studies on transgenic animals cannot entirely exclude that alterations in other proteins contribute to the modulation of shortening velocity. The difference in velocity between the SM-B and SM-A forms in the in vitro motility assay is about twofold (560), which is less than the almost sevenfold difference between fast and slow smooth muscle preparations (437). This can suggest that other factors, in addition to the myosin heavy chain insert, influence velocity in the organized contractile system; such candidates include the essential myosin light chain and nonmuscle myosin heavy chain isoforms (see below), as well as thin filament-associated proteins, e.g., calponin (308, 456, 510, 510, 640). Urinary bladder tissue has a comparatively high expression of the inserted myosin isoform with 80–90% SM-B at the mRNA level (30, 713), which would be consistent with the urinary bladder being a comparatively fast smooth muscle type. Newborn bladders have slightly lower content of SM-B, and culture of bladder myocytes decreases the content further (30).

Smooth muscle expresses two types of essential light chains, the acidic form LC_17a and the basic, nonmuscle LC_17b, which randomly can combine on the myosin heavy chain (100, 264, 275, 327, 374). These two isoforms are formed by alternative splicing, are of equal size, and differ in the carboxy-terminal nine amino acids (489). A large variation in the relative content of LC_17a and LC_17b exists between smooth muscles (275, 437, 635). A correlation between smooth muscle expression of essential light chain variants, and both the maximal shortening velocity and the ATPase activity have been found; high LC_17b content correlated with low shortening velocity and low ATPase activity (275, 437, 600). Extraction/reconstitution of essential light chain isoforms in muscle fibers and overexpression in isolated cells have shown a slowing effect of LC_17b on contractile kinetics (288, 457). Other comparative studies on isolated cells (586), in vitro motility experiments using expressed myosin heavy meromyosin (HMM) fragments, and essential light chain exchange experiments on isolated myosin have failed to show effects of the essential light chain composition on cell shortening velocity, ATPase, and actin translocation velocity (335, 553). In view of the negative in vitro motility data regarding effects of essential light-chain exchange and the strong evidence for effects of myosin heavy chain insert, the essential light chain isoform expression does not seem to be the primary modulator of contractile kinetics in smooth muscle. Comparative studies of smooth muscles show a correlation between both the essential light chain and inserted heavy chain composition (cf. Refs. 37, 612), and it is possible that the expression of these two forms is coregulated in the smooth muscle. A high LC_17b content and low SM-B content correlate with a slow and economical smooth muscle phenotype. How these structures interact in the organized system is not known. The relative LC_17b content is low in the urinary bladder tissue, 10% in rat (600) and 20% in mouse (598), which is consistent with a comparatively fast smooth muscle phenotype.

In addition to the smooth muscle myosin heavy chain described above, smooth muscle can express type II filament-forming nonmuscle myosin isoforms (232, 366, 373). Two separate genes generate the two main nonmuscle myosin heavy chains, type A (NM-MHC-A, molecular mass 196 kDa) and type B (NM-MHC-B, molecular mass 198 kDa) (328, 331, 366, 595). The NM-MHC-A form is also expressed in other cell types, e.g., fibroblasts and platelets, and is upregulated in smooth muscle cell culture (329). The NM-MHC-B form, which is also denoted smooth muscle embryonic (SM_emb), is expressed in smooth muscle cells during development and in atherosclerotic plaques (366). The NM-MHC-B is expressed as an inserted larger isoform (molecular mass 229 kDa) in nervous tissue (641). Recently, the presence of a novel conventional myosin heavy chain with similarities to non- and smooth muscle heavy chain has been proposed on the basis of an analysis of the human genome (57); its tissue expression and cellular functions are currently not known.

The expression of nonmuscle myosins is low in adult urinary bladder [10% of total heavy chain in the rat (440) and low also in other species (118, 408)]. Nonmuscle myosin is found in nonsmooth muscle cells in the serosa and urothelium and in a few cells in the interstitium between muscle bundles. In smooth muscle of newborn bladders, NM-MHC-A and low levels of NM-MHC-B are expressed, whereas in the adult smooth muscle cells only NM-MHC-A is found (30, 116, 408). The functions of the nonmuscle myosins in the urinary bladder are not known in detail. The NM-MHC-B is present in several organ systems during development, and ablation of the NM-MHC-B gene results in high prenatal lethality and severe cardiac and neurological disorders, suggesting that myosin form is important for development. The NM-MHC-A and NM-MHC-B are upregulated in urinary bladder smooth muscle cells in cell culture, suggesting that they can be important for smooth muscle cell migration and proliferation (30). In a recent study by Morano et al. (474), a mouse model was introduced where the smooth muscle myosin was ablated. The animals are born alive but die a few days after birth. Urinary bladder muscle from such animals can contract by the action of the nonmuscle myosins and provides a unique model for analysis of nonmuscle myosin function. The nonmuscle myosin forms filaments in these smooth muscle cells and supports a contraction with a slow onset and a low shortening velocity (408, 474). These results show that nonmuscle myosin also can have a contractile function in smooth muscle, which can be important during fetal life or in adult tissue with high content of nonmuscle myosins, e.g., large elastic arteries. In view of the low nonmuscle myosin content in the adult urinary bladder, it seems however unlikely that this myosin form contributes significantly to force or shortening of the adult urinary bladder. Nonmuscle myosin might also be a marker for nonsmooth muscle cells in the urinary bladder which have been suggested to differentiate into smooth muscle cells or have special functions during urinary bladder hypertrophy, as discussed below.

2. Cytoskeleton and intermediate filaments

The cytoskeleton in smooth muscle provides a structural framework for the cell as well as membrane attachments (cf. review in Ref. 605). The dense bodies are associated with a network of the cytoskeletal (10 nm) intermediate filaments (66, 665). In addition to the cellular contractile domain, composed of myosin and smooth muscle -actin, a cytoskeletal domain composed of the intermediate filaments, nonmuscle - and - actins, filamin, and calponin has been distinguished (418, 419, 508, 604). In the cell membrane, the cytoskeleton forms sites for contact with the cell environment. The dense bands in smooth muscle are cell adhesion complexes and are associated with several structural proteins, including, e.g., -actinin, actin, filamin, calponin, vinculin, tensin, and integrins (cf. Ref. 605). These very complex and dynamic structures have mechanical roles in transmitting force from the contractile machinery to the surrounding cells and matrix, but also to receive and generate signaling information, e.g., for gene expression, cell migration, cell growth, and adaptation. The adhesion complexes, their links to the cytoskeleton, and role in signaling will not be covered further in this review but have been reviewed elsewhere (224, 303, 731, 744).

The intermediate filaments in smooth muscle are mainly composed of the proteins desmin and vimentin, although other intermediate filament proteins, e.g., cytokeratins, have been found. Vimentin is mainly found in smooth muscle of large arteries, and it is also present in mesenchymal nonmuscle cells, e.g., fibroblasts, whereas desmin is mainly found in intestinal smooth muscle and in the striated muscles (199, 212, 306, 378). Desmin and vimentin can coexist in the same smooth muscle cell (517, 663), and an interesting gradient in expression exists in the vascular tree, from mainly vimentin in the large arteries to more desmin in microarterial vessels (313, 518, 705).

The urinary bladder of rat and human contains predominantly desmin intermediate filaments (440, 441). Vimentin-positive nonmuscle mesenchymal cells are found at the serosal and mucosal surfaces and in a limited number in the interstitium between the muscle bundles (86, 158, 599). The concentration of desmin in the mouse urinary bladder smooth muscle is more than 20-fold higher than in skeletal muscles (50). The ratio of desmin to actin contents is 0.16 in the rat urinary bladder (440), which would correspond to a concentration 6.7 mg/g smooth muscle cell.

The generation of desmin-deficient (Des –/–) mice (397, 466) has introduced new possibilities to study the function of the desmin intermediate filaments. These animals develop normally, showing that desmin is not required for normal muscle development. However, a cardiomyopathy with degeneration, calcification, and impaired cardiac function (51, 654) has been described. The structure of smooth muscle is essentially normal, except for the absence of intermediate filaments, showing that the desmin intermediate filaments are not required for development of a normal smooth muscle contractile phenotype (598). No compensatory upregulation of vimentin is observed. In the urinary bladder, passive tension in the muscle layer at optimal length for active force generation was slightly lower, suggesting that the intermediate filaments might have a small role in the support of passive bladder wall tension. Similar results have been reported for the passive wall tension in microarteries (705). However, passive tension could be well maintained over a large length interval showing that the intermediate filaments in the smooth muscle cells are not the only structures responsible for passive wall tension in the urinary bladder. The active force in the bladder wall was decreased to 50% of normal, a change that could not be due to alterations in the content of contractile proteins or in the contractile activation systems. Similar results have been reported for cardiac muscle from desmin-deficient mice (51). These results suggest that the desmin intermediate filaments are responsible for transmission of active force in the smooth muscle cells, possibly by anchoring the contractile apparatus to the cell membrane or by coupling the dense body structures during active contraction.

C. Actin-Myosin Interaction

During force generation and shortening of muscle, the myosin cross-bridges interact with actin and hydrolyze MgATP to the products MgADP and P_i. The energy is supplied by the cell metabolism keeping the MgATP high while lowering the product concentrations, thereby shifting the MgATP hydrolysis reaction from its equilibrium. The energy is released in a multistep enzymatic process, where the myosin (M) cross-bridges bind and attach to actin (A) in a cyclic manner. The structural and biochemical events of this process in skeletal muscle have been reviewed in detail elsewhere (128). In smooth muscle the cross-bridge interaction is considered to follow the same general scheme of reactions as that proposed for skeletal muscle, although the rates of some reactions are different (cf. Refs. 39, 447).

In the absence of substrate, myosin binds strongly to actin, forming the rigor (A.M) complex in skeletal and smooth muscles. In living smooth muscle cells, the relative population in the rigor state is low, the A.M complex is rapidly dissociated by MgATP. This cross-bridge dissociation reaction is slightly slower in smooth compared with skeletal muscle, the second-order rate constant being 1 x 10⁵ M^–1 · s^–1 in the smooth muscle from rabbit urinary bladder (342), compared with 5 x 10⁵ M^–1 · s^–1 in the rabbit psoas muscle (235). The rate is slower in the slower, tonic, smooth muscle type of the femoral artery (342) compared with the faster phasic bladder smooth muscle. However, the MgATP-induced dissociation is rapid enough not to limit the rate of cross-bridge turnover or the shortening velocity in smooth muscle at normal MgATP concentrations (cf. Ref. 411), and it is not likely that this reaction is primarily responsible for the difference in contractile kinetics between skeletal and smooth muscles and between fast and slow smooth muscle types.

The cross-bridge power stroke is considered to be associated with release of P_i from the actin-myosin-ADP-P_i (A.M.ADP.P_i) state in skeletal and smooth muscle (281, 520). The active force of smooth muscle can thus be inhibited by high P_i concentrations and by the compound 2,3-butanedione monoxime (BDM), which interferes with the P_i release reaction (520, 521). Interestingly the effects of added P_i and BDM are lower in smooth compared with skeletal muscle and lower in slow compared with fast smooth muscles, possibly suggesting that the A.M.ADP state immediately after P_i release is less populated or that the binding constant for P_i is lower in muscles with slower myosins. This relationship seems to extend also to the nonmuscle myosins (408), where force is little affected by addition of P_i. The P_i release reaction is not rate-limiting for the maximal shortening velocity, since addition of P_i does not influence velocity (411, 520). Smooth muscle has been shown to generate higher force per myosin head than skeletal muscle (487). However, the cross-bridge power stroke of smooth muscle myosin generates 10 nm unitary displacement or a unitary force of 1 pN (246, 377), values which are similar to those found in skeletal muscle myosin. Thus the difference in force-generating ability between smooth and skeletal muscles does not reside in the molecular mechanics. Instead, the higher force per myosin head in smooth muscle is due to the kinetics of the interaction (246). The relative time spent in attached force-generating states (duty cycle) is longer for smooth muscle cross-bridges. Also when fast and slow smooth muscle HMM, i.e., with and without the seven-amino acid insert in the myosin head region, are compared with an optical trap method, similar unitary forces are found, but the duty cycle was longer in the noninserted myosin (377).

The maximal shortening velocity (V_max) of muscle reflects the maximal rate at which the myosin can propel the actin filaments under unloaded conditions. In general, shortening velocity of the comparatively fast urinary bladder smooth muscle [e.g., rat detrusor: 0.2 muscle lengths (ML)/s at 22°C (600)] is more than 10-fold slower than that of the fast skeletal muscle (129). A large span in shortening velocity also exists within the smooth muscle group, where the slow aorta muscle is about fivefold slower than the fast smooth muscles (cf. Ref. 437). This maximal shortening velocity is considered to be rate-limited by the cross-bridge dissociation after the power stroke, and a strong correlation exists between the rate of the ADP release reaction and the maximal shortening velocity (593, 707). In smooth muscle preparations with actively cycling phosphorylated myosin, MgADP inhibits shortening velocity (411), showing that kinetics of the MgADP reaction can limit shortening velocity of smooth muscle. In vitro the MgADP binding to smooth muscle myosin in the presence of actin is strong compared with that of skeletal muscle myosin (139). In optical trap measurements and in in vitro motility assays, the rate of ADP release is approximately two- to fivefold slower in smooth muscle myosin without the heavy chain insert compared with the inserted myosin (377). In smooth muscle fiber preparations a strong binding of ADP to rigor cross-bridges has been reported (32, 502), with a binding constant in the micromolar range. The comparatively fast (phasic) urinary bladder smooth muscle has a lower binding of ADP in rigor (206) and in the dephosphorylated state during relaxation compared with the slower (tonic) arterial muscle (340). A markedly stronger MgADP binding is also observed during active cross-bridge cycling in slow compared with fast smooth muscles (411). Thus, in general, MgADP binding is stronger in slow compared with fast smooth muscles and in smooth compared with skeletal muscle. The kinetics of the MgADP reaction can be responsible for the modulation of shortening velocity of the organized contractile system in smooth muscle. Interestingly, isolated smooth muscle myosin exhibits unique structural changes associated with MgADP binding (236, 714). These properties have implications for analysis of the conversion energy in the myosin cross-bridge (cf. Ref. 222), although analysis of the MgADP binding reaction in vitro suggests that the MgADP release is not likely to be coupled to the force generation (139). The mechanical effects of MgADP binding to smooth muscle preparations in rigor are controversial. No mechanical effects were noted by Dantzig et al. (144), whereas a decrease in rigor force upon MgADP binding was observed by Khromov et al. (341) in both arterial and urinary bladder smooth muscle.

The kinetics of the urinary bladder contractile system thus reflect a comparatively fast smooth muscle phenotype. The maximal shortening velocity of adult urinary bladder is 80% of the fastest mammalian smooth muscle found (rabbit rectococcygeus, Refs. 408, 437, 600). In relation to the slower smooth muscle phenotypes, e.g., in arteries, the urinary bladder muscle has a comparatively low affinity for MgADP (206, 340), high affinity for MgATP (342), and high phosphate sensitivity of active force (408, 411). These properties of the reaction(s) determining the force generation and the shortening velocity are correlated with the myosin expression (cf. sect. IIIB1), with low essential light chain LC_17b content and high content of inserted myosin heavy chain (SM-B).

D. Energetics and Cell Metabolism

ATP is the immediate substrate for the different processes involved in contraction and relaxation of the urinary bladder muscle, from membrane pump activity, Ca²⁺ handling, phosphorylation processes, to cross-bridge cycling. The cellular ATP concentration is maintained by mitochondrial respiration, glycolysis, and conversion of the high-energy compound phosphocreatine (PCr). Reviews regarding high-energy phosphates and cellular energy metabolism in smooth muscle have been presented (91, 531, 675, 676). The cellular concentrations of ATP and PCr in smooth muscle are generally low compared with skeletal muscle. In particular, the PCr is lower in smooth compared with skeletal muscle (531). The contents in urinary bladder tissue determined by biochemical assays are within the range observed for different smooth muscle tissues [relaxed rabbit urinary bladder (in µmol/g): ATP, 0.79; ADP, 0.15; P_i, 1.3; PCr, 1.76; Ref. 274], although some species-dependent differences might exist (386). Data from urinary bladders have also been obtained using ³¹P-nuclear magnetic resonance (NMR) measurements (274, 368). Hellstrand and Vogel (274) reported that NMR measurements gave similar PCr/ATP ratios, but lower values for ADP and P_i, compared with biochemical measurements, which might reflect an intracellular compartmentalization of the latter compounds.

The cellular concentration of ATP is well maintained during sustained contractions of urinary bladder muscle although PCr is decreased by 10–30% (274). In uterine tissue it has been reported that contractions are associated with decresaed ATP and PCr and incresaed P_i levels (371). A pronounced decrease in ATP and PCr and increase ADP was reported after metabolic inhibition of the urinary bladder tissue (274). Lowered cellular MgATP concentration would influence several processes in the bladder muscle cell. The actin myosin interaction in the cross-bridge cycle can operate at very low [MgATP], and the primary effect of lowered [MgATP] in severe ischemia or during impaired metabolic activity would rather be a reduced myosin light-chain phosphorylation and decreased activation of the contractile system (273). Increased concentrations of phosphate could theoretically inhibit force; however, it is unlikely that the cellular concentration of phosphate can reach high enough levels to have an effect on force in fast smooth muscle (cf. Ref. 520). The [MgADP] in smooth muscle increases during prolonged contraction and in ischemia (192, 365). The binding of MgADP to the smooth muscle actin-myosin complex is very strong, and shortening velocity of smooth muscle is inhibited at low MgADP concentrations as discussed above. A physiological effect would be that shortening velocity and possibly also relaxation, rather than active force, are inhibited at the cross-bridge level by small increases in [MgADP]. This effect would be stronger in hypertrophic urinary bladder smooth muscle where the detrusor muscle changes toward a slower contractile phenotype, in which MgADP binding is stronger (see sect. IIIB1). Whether the cellular ADP can inhibit shortening velocity at unchanged force also during sustained contractions under conditions with normal energy supply is not clear.

The metabolic substrates utilized by the urinary bladder smooth muscle in vivo are not known. For vascular smooth muscles, measurements of the respiratory quotient suggest that glucose is an important source of energy, although several other substrates may also be metabolized (531). The uptake of glucose in smooth muscle is considered to involve the GLUT1 glucose transporter, which is different from the GLUT4 type present in the insulin-sensitive striated muscle and adipose tissue (cf. Ref. 444). However, some insulin sensitivity has also been observed for the GLUT1 type. A small effect of insulin on the glucose uptake in bladder tissue has been described (269). An interesting aspect of smooth muscle glucose metabolism is that a large fraction of the glucose is metabolized to lactate also under aerobic conditions, possibly to supply membrane pump activities (cf. Refs. 98, 417). In the rabbit urinary bladder in vitro, 81% of the glucose is metabolized to lactate, whereas 11% is oxidized to CO₂ and 4.7% converted to glycogen (269). Since the ATP yield is severalfold higher for aerobic metabolism of glucose, the oxidative metabolism is the main source of ATP under normal conditions. In the relaxed rat urinary bladder in vitro in the presence of glucose, the oxygen consumption is 1.5 nmol · min^–1 · mm^–3, and lactate production is 0.5 nmol · min^–1 · mm^–3. During active contractions these rates increased about two- and threefold, respectively (41). With the use of these values, it can be calculated that 5–10% of the ATP in the urinary bladder is derived from the aerobic glycolysis to lactate (41).

The formation of lactate in muscle is catalyzed by lactate dehydrogenase (LDH). This enzyme exists as a tetramer with different combinations of the two M and H polypeptide chains (184), thus creating five different LDH isoforms. LDH with high M content is more directed toward formation of lactate and is found in fast skeletal muscle, whereas LDH with more of the H form is product inhibited by lactate and associated with slower aerobic striated muscles, e.g., the soleus and the heart (184, 283, 542). The rat urinary bladder smooth muscle has an LDH isoform pattern with less of the H form compared with the slow aorta smooth muscle (442), which suggests, in analogy with the situation in striated muscle, that the expression of enzymes in the cellular metabolism are correlated with the contractile properties. However, this is not a general correlation for all smooth muscles, since the different layers of the rabbit urethra, which have markedly different shortening velocities, exhibit similar LDH isoform patterns (42). Also under pathophysiological conditions the shortening velocity and LDH isoform expression pattern change in opposite directions, with more of the M-form being associated with lower shortening velocity (442, 600).

The intracellular pH has been measured to be 7 in isolated human detrusor muscle cells (201). During hypoxia in the detrusor active force is lowered (e.g., Ref. 41). In a study by Thomas and Fry (653a) it was shown that the lower detrusor force in hypoxia was associated with a transient initial intracellular alkalosis and a decreased extracellular pH. Extracellular acidosis is associated with decreased force generation of the detrusor in contrast to intracellular acidosis, which increases force (202, 406). The force generation, shortening properties, and myofilament Ca²⁺ sensitivity of the contractile system is little affected by variations in pH (33, 725). The lower force in extracellular acidosis has been attributed to attenuating effects on Ca²⁺ influx through L-type channels, which also affects release from the intracellular stores (202). Intracellular acidification has been shown to increase force via improved Ca²⁺ uptake and and release from intracellular stores (724).

E. Detrusor Muscle Mechanics

The bladder wall undergoes large changes in extension during normal filling and emptying. Isolated urinary strips of the urinary bladder wall can be examined in vitro to determine the relation between length and wall tension (674). These data for the relation between length and force can be converted to volume and pressure data using the law of Laplace and assuming a model for the bladder shape and for how the wall stretch is distributed in the bladder wall. An assumption of a spherical bladder shape, with an incompressible wall and isotropic homogeneous stretch, can give a good description of the bladder mechanics during filling (cf. Ref. 140).

The bladder muscle wall exhibits a nonlinear relation between stretch and passive tension. At lengths above that where maximal active force is recorded, the passive tension increases steeply (598, 671). Some differences between species in the properties of the length-tension relations have been reported (413). The length-tension relations determined in vitro are usually performed using slow stretch or longer equilibration periods at each length, which gives data reflecting the passive behavior of the bladder during slow filling in vivo. During fast stretch or extensive deformation, the bladder wall also exhibits viscous and plastic behavior (9, 131, 191, 360). These viscous properties would be involved in "stress relaxation" phenomena observed in the whole urinary bladder or isolated muscle strips. A rapid increase in length, or volume, results in a fast rise in force and pressure and a subsequent slow return to the original levels. A rapid decrease in length or volume would give an immediate drop in force and pressure followed by a gradual increase.

The cytoskeleton of the smooth muscle cells might contribute with a small component of the passive tension in the range of wall stretch where active tension is near maximal (598). It should also be noted that a fully relaxed state with a complete absence of cross-bridge interaction might not be attained in the living smooth muscle cells, and thus passive or relaxed properties of the bladder wall might contain a small contribution of cross-bridge interaction. However, the passive viscoelastic properties are considered to be mainly due to properties of the extracellular matrix in the bladder wall. Main extracellular components in the urinary bladder are elastic fibers and collagen fibrils, which are present in the serosa, between the muscle bundles, and in small amounts between the smooth muscle cells in the muscle bundles (214). The collagen fibrils in the urinary bladder are formed by collagen type I and III (185, 343) and the elastin fibers of elastin from the soluble precursor tropoelastin (383).

The active force of the bladder muscle is dependent on the wall stretch. The relation between muscle length and active force is comparatively broad, and bladder muscle from experimental animals and humans can generate force over a large length interval (413, 443, 672). The different extents of wall stretch are coupled with corresponding changes in cell length, which indicates that slippage of the cells in the bladder wall does not occur during the length changes (677); however, at short lengths, the cells might not be aligned along the long axis of the preparation (674). The length dependence of the active force most likely reflects that filament overlap and cross-bridge interaction is dependent on muscle length. In addition, the muscle stretch can influence the excitation-contraction coupling resulting in a less optimal activation at short lengths (36, 251). To obtain a measure for the maximal active force generation of the smooth muscle component, the preparations have to be examined at optimal length and corrections performed for the content of smooth muscle in the preparations. At optimal length the detrusor muscle has been reported to generate 200 mN/mm² smooth muscle area in human detrusor (443), 590 mN/mm² smooth muscle area or 5.5 µN/cell in guinea pig detrusor (677), 80 mN/mm² in rat detrusor (674), and 60 mN/mm² in mouse detrusor (598). For comparisons of cellular force generation of detrusor muscle from control and pathophysiologically altered bladders, several parameters, including the extent of stretch, the content of smooth muscle cells, and the degree of activation, have to be considered. The cellular force generation is the result of the number of cross-bridges acting in parallel and the intrinsic force generation of the cross-bridge. As discussed in section IIIC, a longer duty cycle seems to be one mechanism responsible for the comparatively high force output per amount of myosin in smooth muscle.

As discussed in section IIIC, the V_max gives information regarding the rate of filament sliding. If the muscle is maximally activated, the V_max is considered to reflect the kinetics of the myosin cross-bridge interaction. Thus V_max varies between smooth muscles and can change during physiological adaptation. However, the maximal shortening velocity is also modulated by several factors in the living tissue. The velocity is dependent on the mode of activation, the time after stimulation, and muscle length (cf. Ref. 43), showing that an individual smooth muscle can modulate the cross-bridge turnover. Experiments on permeabilized smooth muscle have shown that both [Ca²⁺] and myosin light-chain phosphorylation can alter both V_max and force (34, 438). One important concept in this context is the "latch" phenomenon (149), where cross-bridges are suggested to attach in nonphosphorylated state and thereby lower velocity and maintain tension at low ATP turnover. Several mechanisms have been proposed to be involved in the latch state, a dephosphorylation of attached cross-bridges, additional regulatory systems, or possibly metabolic factors (cf. Ref. 43).

The velocity of filament sliding in striated skeletal muscle has been shown to be independent of the filament overlap (166). Experiments of fully activated human permeabilized urinary bladder strips (443) and electrically activated intact pig urinary bladders (469) suggest that the maximal shortening velocity is not length dependent. These results would fit with the view that cross-bridge kinetics under unloaded conditions are not dependent on the number of cross-bridges acting in parallel. In an early study Uvelius (672) reported, however, that the V_max of high-K⁺ activated intact rabbit urinary bladder is dependent on the length. This finding suggests that in the intact bladder wall velocity can be influenced by passive components or length dependence of the activation systems.

The V_max of intact urinary bladder smooth muscle preparations has been estimated to be in the range 0.3–0.4 ML/s at 37°C in different species (rabbit, Refs. 672, 678; guinea pig, Ref. 243). It was early recognized in urinary bladder muscle that the mode of activation influences shortening velocity (678). The velocity was higher after electrical stimulation compared with high-K⁺ activation. This is most likely a reflection of the activation-dependent modulation of V_max discussed above. To determine the V_max of the fully activated contractile machinery, experiments have been performed on permeabilized urinary bladder smooth preparations where the environment of the contractile proteins can be held constant and a maximal myosin light-chain phosphorylation can be achieved. Under these conditions at 22°C, V_max is reported to be 0.2 ML/s in the human urinary bladder (443), in the rat (600, 602), and in the mouse (598).

The relation between active force, or the load on the muscle, and the shortening velocity is described by a hyperbolic relationship (282), where V_max is the velocity at zero load and the isometric force (P_o) is the force at zero velocity. Since both P_o and V_max are regulated, different force velocity curves, with different V_max and P_o, can be obtained depending on the contractile activation (cf. Ref. 685). In the contracting urinary bladder wall during emptying, the detrusor muscle wall will thus operate along these force-velocity relationships depending on luminal pressure, bladder volume, and state of activation.

F. Pathophysiological Adaptations

Hypertrophy and hyperplasia of the smooth muscle in the urinary bladder can occur in response to urinary outlet obstruction, e.g., in benign prostatic hypertrophy, or after decentralization, e.g., in spinal cord injuries. The primary factor for induction of the growth seems to be stretch of the bladder wall components, although the cellular signals mediating the growth response are not fully characterized. Several receptors, signaling pathways, and growth factors might be involved, e.g., insulin-like growth factor I (IGF-I) or its binding proteins (1, 2, 109–111), epidermal growth factor (EGF) (690), heparin-binding EGF-like growth factor (HB-EGF) (501), angiotensin II receptors (527), basic fibroblast growth factor (bFGF) (107, 108), or altered Ca²⁺ handling (367).

Hypertrophic growth of detrusor muscle in response to urinary outlet obstruction is well documented in humans (229) and has been extensively studied in several animal models where a partial obstruction is applied to the urethra (80, 389, 460, 483, 591, 621, 718). Urinary bladder distension and growth is also found in animals with other modes of distension or increased urine volume load, e.g., rats with hereditary diabetes insipidus (435), rats with pharmacologically induced diabetes mellitus (40, 111, 402, 673), osmotic diuresis (412), application of a paraffin bolus in the bladder (539), and preganglionic denervation or removal of pelvic ganglia (58, 171, 172). Some of these conditions can influence the bladder structure via metabolic pathways. However, because denervated urinary bladders can hypertrophy and because this growth can be prevented by manually emptying the bladder (58, 431), the results clearly show that trophic influences of parasympathetic nerves or active contractions by the bladder muscle are not required for hypertrophic growth and are consistent with wall stress being the primary initiator of growth.

The growth of the urinary bladder in response outflow obstruction can be quite dramatic, e.g., in the rat the urinary bladder weight increases from the normal 80 mg to 140 mg in 3 days, 170 mg in 10 days, 640 mg in 42 days, and 1,000 mg in 90 days (550). The increase in bladder mass in response to outflow obstruction is reversible; when the obstruction is removed in experimental animals, the bladder regains almost normal weight and protein composition is normalized (405, 441, 702), although the deobstructed urinary bladders show differences in cellular and intercellular structure compared with both control and hypertrophic urinary bladders (215).

In a study by Gabella and Uvelius (214), the fine structure of normal and hypertrophic rat urinary bladder was extensively characterized. It was shown that the muscle bundles became larger and longer and that the transverse area of the smooth muscle cells increased, suggesting hypertrophy of the cells. No mitoses were found, although cells with two nuclei were present. Gap junctions were very few or absent both in control and hypertrophied tissues.

The bladder hypertrophy can also be associated with alterations in extracellular materials. In the trabeculated bladder from patients with prostatic enlargement, Gosling and Dixon (238) found an increase in extracellular material and collagen. In the hypertrophic rat urinary bladder, total collagen increases although the concentration of collagen appears to decrease (679).

Hypertrophy of smooth muscle cells seems to be a major cause for the growth of the urinary bladder wall in response to urinary outflow obstruction (214), although measurements of DNA content suggest some contribution of smooth muscle hyperplasia (680). Proliferation of mesenchymal nonmuscle myosin heavy chain A (NM-MHC-A, see above), vimentin, and smooth muscle -actin positive cells of the serosa has been described (86, 118). These cells have been suggested to mature towards adult smooth muscle cells in hypertrophic growth and after bladder wall injury (186, 557). These results suggest that bladder muscle has the ability to generate new smooth muscle cells that can be important in some pathophysiological conditions, e.g., in regeneration bladder wall after injury (186), after insertion of acellular grafts (547, 548), or after partial cystectomy (200), although such smooth muscle hyperplasia is most likely not the major cause for the growth of the detrusor muscle in urinary outflow obstruction.

The hypertrophic growth of the smooth muscle cells in the urinary bladder in response to outflow obstruction is associated with increased total concentrations of the contractile proteins (58, 440). In the hypertrophying rat urinary bladder, the synthesis of myosin appears, however, at some stages of growth not to be in pace with the increase on smooth muscle volume, resulting in a lower concentration of the contractile protein, which is correlated with a decreased active force per muscle area in hypertrophic tissue (41, 440). The myosin isoform pattern is changed during hypertrophic growth of the urinary bladder. A decrease in the SM2/SM1 ratio has been reported for obstructed urinary bladder of rabbit and rat (88, 118, 440, 441, 571, 702), whereas in biopsy samples from patients with bladder hypertrophy an increase has been reported (441). The actin isoform expression pattern has been reported to change towards a distribution with less smooth muscle -actin and more -actin in hypertrophic rat bladder (440) and towards less -actin more -actin at unchanged -actin in hypertrophic rabbit bladder (345). In obstructed urinary bladder of patients a significant increase in -actin and a decrease in -actin has been found compared with control bladders (441). The functional consequences of a change in actin isoform distribution and myosin SM2/SM1 ratio in smooth muscle are not clear at present. However, these isoforms do not seem to be major determinants of contractile kinetics as discussed in section IIIB1. The 17-kDa essential light-chain expression changes towards more of the LC_17b form and the amount of the inserted myosin heavy chain (SM-B) decreases in hypertrophic urinary bladder (600), changes which would be associated with a slower, more economical contractile phenotype. Early studies of actin-activated myosin ATPase in vitro (571) or tension-associated ATP turnover in bladder muscle preparations (41) suggested that actin-myosin cross-bridge turnover was not altered. However, recent determinations of the force-velocity relationship show that the maximal shortening velocity is decreased in hypertrophic urinary bladder (600, 626), which suggests that the changes in myosin isoform expression pattern result in a slower, more economical muscle.

A striking feature in several forms of hypertrophy in visceral smooth muscle is an increase in intermediate filaments (213). This is also evident in the hypertrophying urinary bladder of experimental animals and in humans, where the amount of the intermediate filament protein desmin increases relative to other contractile and cytoskeletal proteins (58, 440, 441, 690). The intermediate filament protein vimentin is much less abundant in the urinary bladder and is mainly found in cells at the mucosal and serosal surfaces and in the interstitium (e.g., Ref. 599). Vimentin in the urinary bladder has been reported to increase also in hypertrophy (118, 440). The mechanical function of the increased number of intermediate filaments and desmin in hypertrophying urinary bladder is unknown. The presence of, and increase in, intermediate filaments are not required for the hypertrophic growth, but possibly these structures are important for maintenance of cellular structure, as cell size increase (R. Sjuve and A. Arner, unpublished data).

The hypertrophy of the urinary bladder wall involves a thickening of both epithelium, muscle layer, and serosa. The tissue is well vascularized (214), suggesting formation of new blood vessels in the vascular wall. Microarterial vessels supplying the bladder also grow in size (64). Blood flow has been reported to increase to the rabbit bladder initially during hypertrophy (398). However, after 2 wk of obstruction, microcirculation has been found to be impaired (659) and in chronic decompensated hypertrophic urinary bladders blood flow is decreased (576). The content of mitochondrial enzymes and oxidative metabolism have been reported to decrease in hypertrophic rabbit urinary bladder and in urinary bladder from men with benign prostatic hyperplasia (60, 268, 388, 399). Decreased aerobic metabolism (326) and lowered cellular ATP and PCr levels have been reported (387). It has been suggested that altered mitochondrial function is involved in the bladder pathology associated with benign prostatic enlargement (499) and that structural and functional changes can reflect hypoxia in the wall of the urinary bladder (241). In contrast, in the rat urinary bladder the number of mitochondria in the hypertrophic cells increase, keeping the relative mitochondrial volume and the amount of mitochondrial enzymes per unit weight unchanged (141, 214). These results thus show that hypertrophy of the detrusor muscle can occur without major mitochondrial dysfunction. It is likely that the responses of the bladder wall are very complex and that the extent of structural changes and wall hypoxia/ischemia varies between species and with time and severity of the obstruction. In the hypertrophied rat urinary bladder active force is better maintained under hypoxic conditions (41). The LDH enzyme pattern changes towards more of the M-form, which is more directed toward formation of lactate (442) and the contractile system changes towards a more economical phenotype (see sect. IIID). These changes possibly reflect adaptations to impaired energy supply in the hypertrophying urinary bladder wall. A summary of the changes in contractile properties induced in the detrusor by adaptive growth is given in Figure 2. Changes in receptors and activation systems are presented in the subsequent sections.

View larger version (58K): [in this window] [in a new window]   FIG. 2. Changes induced in the detrusor by adaptive growth. SR, sarcoplasmic reticulum; LDH, lactate dehydrogenase.

	IV. EXCITATION-CONTRACTION COUPLING

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A. Regulation of Contractile Proteins

Smooth muscle contraction is initiated by an increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) (cf. Ref. 563). In principle, Ca²⁺ can enter the cytoplasm through the cell membrane, via Ca²⁺ channels, or be released from the sarcoplasmic reticulum (SR). These pathways for Ca²⁺ translocation have been the topic of several recent excellent reviews (65, 73, 217, 362, 572, 615).

The release of Ca²⁺ from the SR is an important step in activation of the detrusor muscle. This is evident from studies using blockers of SR function showing that both nerve- and agonist-induced contractions are dependent on a functional SR (142, 395, 485). The release of Ca²⁺ is triggered by inositol trisphosphate (IP₃) via IP₃ receptors and by Ca²⁺ (Ca²⁺-induced Ca²⁺ release) via ryanodine receptors (cf. Ref. 73). Interestingly, stretch of rabbit urinary bladder smooth muscle cells has been shown to activate Ca²⁺ release via gating of the ryanodine receptor, which suggests that stretch-induced Ca²⁺ release can be a further mechanism influencing Ca²⁺ release (312). The activity of the SR Ca²⁺-ATPase in the [SERCA] is inhibited by the associated protein phospholamban, and depletion of this protein leads to altered bladder contractility (504), suggesting that the content of phospholamban can be a factor modulating bladder contractility. In addition to releasing activator Ca²⁺, the SR can influence bladder contractility via modulating the K⁺ channel activity and thereby promote relaxation (cf. sect. VC2) and by introducing a Ca²⁺ sink buffering the Ca²⁺ influx (741). The mechanisms of Ca²⁺ influx through Ca²⁺ channels are described in section VB.

The Ca²⁺ activation of the contractile proteins is considered to occur via a phosphorylation pathway where Ca²⁺ binds to calmodulin, and the Ca²⁺/calmodulin complex activates the myosin light-chain kinase (MLCK) which catalyzes the phosphorylation of the 20-kDa myosin regulatory light chains on serine at position 19 (cf. Refs. 43, 216, 286, 625). Dephosphorylation of the regulatory light chain is performed by a myosin light-chain phosphatase (MLCP). The main pathways of cellular contractile activation are also shown in Figure 5 in context of the muscarinic receptor signaling. Several subtypes of the protein phosphatase exist (cf. Ref. 126). The phosphatase responsible for light-chain dephosphorylation in smooth muscle seems to be of type protein phosphatase (PP)1 or PP2A. PP1 has been isolated from smooth muscles including pig urinary bladder (588). The urinary bladder phosphatase (SMPP-1M, Ref. 588) is a trimeric protein composed of a 37-kDa catalytic PP1 subunit (PP1C) and two additional subunits of molecular mass 110–130 and 20 kDa, which is similar to the composition of the avian isoform (8, 262, 587). The larger 110- to 130-kDa subunit, which exists in different isoforms, is considered to have a regulatory function and is generally referred to as the m