Dissociation between myosin phosphorylation and shortening velocity in canine trachea

Molecular-level evidence of force maintenance by smooth muscle myosin during LC20 dephosphorylation

Smooth muscle (SM) is found in most hollow organs of the body. Phasic SM, as found in the gut, contracts to propel content, whereas tonic SM, as found in most blood vessels, maintains tension. This force maintenance is referred to as the latch state and occurs at low levels of myosin activation (myosin light chain [LC20] phosphorylation). Molecular mechanisms have been proposed to explain the latch state but have been studied only at the whole-muscle level because of technological limitations. In the current study, an assay chamber was devised to allow injection of myosin light chain phosphatase (MLCP) during laser trap and in vitro motility assays, without creating bulk flow, to reproduce latch state conditions at the molecular level. Using the laser trap in a single-beam mode, an actin filament was brought in contact with several myosin molecules on a pedestal. Myosin pulled on the actin filament until a plateau force was reached, at which point, MLCP was injected. Force maintenance was observed during LC20 dephosphorylation, the level of which was assessed in a parallel in vitro motility assay performed in the same conditions. Force was maintained longer for myosin purified from tonic SM than from phasic SM. These data support the longstanding dogma of strong bonds caused by dephosphorylated, noncycling cross-bridges. Furthermore, MLCP injection in an in vitro motility mixture assay performed with SM and skeletal muscle myosin suggests that the maintenance of these strong bonds is possible only if no energy is provided by surrounding actively cycling myosin molecules.

Characterization of cystic fibrosis airway smooth muscle cell proliferative and contractile activities

Cystic fibrosis (CF) is a genetic disease that causes multiple airway abnormalities. Two major respiratory consequences of CF are airway hyperresponsiveness (AHR) and airway remodeling. Airway smooth muscle (ASM) is hypothesized to be responsible for the airway dysfunction, since their thickening is involved in remodeling, and excessive contraction by the ASM may cause AHR. It is unclear whether the ASM is intrinsically altered to favor increased contractility or proliferation or if microenvironmental influences induce pathological behavior in vivo. In this study, we examined the contractile and proliferative properties of ASM cells isolated from healthy donor and CF transplant lungs. Assays of proliferation showed that CF ASM proliferates at a higher rate than healthy cells. Through calcium analysis, no differences in contractile activation in response to histamine were found. However, CF ASM cells lagged in their reuptake of calcium in the sarcoplasmic reticulum. The combination CFTR corrector and potentiator, VX-809/770, used to restore CFTR function in CF ASM, resulted in a reduction in proliferation and in a normalization of calcium reuptake kinetics. These results show that impaired CFTR function in ASM cells causes intrinsic changes in their proliferative and contractile properties.

The Huxley crossbridge model as the basic mechanism for airway smooth muscle contraction

The cyclic interaction between myosin crossbridges and actin filaments underlies smooth muscle contraction. Phosphorylation of the 20-kDa myosin light chain (MLC20) is a crucial step in activating the crossbridge cycle. Our current understanding of smooth muscle contraction is based on observed correlations among MLC20 phosphorylation, maximal shortening velocity (Vmax), and isometric force over the time course of contraction. However, during contraction there are changes in the extent of phosphorylation of many additional proteins as well as changes in activation of enzymes associated with the signaling pathways. As a consequence, the mechanical manifestation of muscle contraction is likely to change with time. To simplify the study of these relationships, we measured the mechanical properties of airway smooth muscle at different levels of MLC20 phosphorylation at a fixed time during contraction. A simple correlation emerged when time-dependent variables were fixed. MLC20 phosphorylation was found to be directly and linearly correlated with the active stress, stiffness, and power of the muscle; the observed weak dependence of Vmax on MLC20 phosphorylation could be explained by the presence of an internal load in the muscle preparation. These results can be entirely explained by the Huxley crossbridge model. We conclude that when the influence of time-dependent events during contraction is held constant, the basic crossbridge mechanism in smooth muscle is the same as that in striated muscle.

Mechanics of Vascular Smooth Muscle

Vascular smooth muscle (VSM; see Table 1 for a list of abbreviations) is a heterogeneous biomaterial comprised of cells and extracellular matrix. By surrounding tubes of endothelial cells, VSM forms a regulated network, the vasculature, through which oxygenated blood supplies specialized organs, permitting the development of large multicellular organisms. VSM cells, the engine of the vasculature, house a set of regulated nanomotors that permit rapid stress-development, sustained stress-maintenance and vessel constriction. Viscoelastic materials within, surrounding and attached to VSM cells, comprised largely of polymeric proteins with complex mechanical characteristics, assist the engine with countering loads imposed by the heart pump, and with control of relengthening after constriction. The complexity of this smart material can be reduced by classical mechanical studies combined with circuit modeling using spring and dashpot elements. Evaluation of the mechanical characteristics of VSM requires a more complete understanding of the mechanics and regulation of its biochemical parts, and ultimately, an understanding of how these parts work together to form the machinery of the vascular tree. Current molecular studies provide detailed mechanical data about single polymeric molecules, revealing viscoelasticity and plasticity at the protein domain level, the unique biological slip-catch bond, and a regulated two-step actomyosin power stroke. At the tissue level, new insight into acutely dynamic stress-strain behavior reveals smooth muscle to exhibit adaptive plasticity. At its core, physiology aims to describe the complex interactions of molecular systems, clarifying structure-function relationships and regulation of biological machines. The intent of this review is to provide a comprehensive presentation of one biomachine, VSM.

Development and maintenance of force and stiffness in airway smooth muscle

Airway smooth muscle (ASM) plays a central role in the excessive narrowing of the airway that characterizes the primary functional impairment in asthma. This phenomenon is known as airway hyper-responsiveness (AHR). Emerging evidence suggests that the development and maintenance of ASM force involves dynamic reorganization of the subcellular filament network in both the cytoskeleton and the contractile apparatus. In this review, evidence is presented to support the view that regulation of ASM contraction extends beyond the classical actomyosin interaction and involves processes within the cytoskeleton and at the interfaces between the cytoskeleton, the contractile apparatus, and the extracellular matrix. These processes are initiated when the muscle is activated, and collectively they cause the cytoskeleton and the contractile apparatus to undergo structural transformation, resulting in a more connected and solid state that allows force generated by the contractile apparatus to be transmitted to the extracellular domain. Solidification of the cytoskeleton also serves to stiffen the muscle and hence the airway. Oscillatory strain from tidal breathing and deep inspiration is believed to be the counter balance that prevents hypercontraction and stiffening of ASM in vivo. Dysregulation of this balance could lead to AHR seen in asthma.

A multi-scale approach to airway hyperresponsiveness: from molecule to organ

Airway hyperresponsiveness (AHR), a characteristic of asthma that involves an excessive reduction in airway caliber, is a complex mechanism reflecting multiple processes that manifest over a large range of length and time scales. At one extreme, molecular interactions determine the force generated by airway smooth muscle (ASM). At the other, the spatially distributed constriction of the branching airways leads to breathing difficulties. Similarly, asthma therapies act at the molecular scale while clinical outcomes are determined by lung function. These extremes are linked by events operating over intermediate scales of length and time. Thus, AHR is an emergent phenomenon that limits our understanding of asthma and confounds the interpretation of studies that address physiological mechanisms over a limited range of scales. A solution is a modular computational model that integrates experimental and mathematical data from multiple scales. This includes, at the molecular scale, kinetics, and force production of actin-myosin contractile proteins during cross-bridge and latch-state cycling; at the cellular scale, Ca²⁺ signaling mechanisms that regulate ASM force production; at the tissue scale, forces acting between contracting ASM and opposing viscoelastic tissue that determine airway narrowing; at the organ scale, the topographic distribution of ASM contraction dynamics that determine mechanical impedance of the lung. At each scale, models are constructed with iterations between theory and experimentation to identify the parameters that link adjacent scales. This modular model establishes algorithms for modeling over a wide range of scales and provides a framework for the inclusion of other responses such as inflammation or therapeutic regimes. The goal is to develop this lung model so that it can make predictions about bronchoconstriction and identify the pathophysiologic mechanisms having the greatest impact on AHR and its therapy.

FRET analysis of actin–myosin interaction in contracting rat aortic smooth muscle

We examined the interaction of smooth muscle myosin with α-actin and β-actin isoforms during the contraction of A7r5 smooth muscle cells and rat aortic smooth muscle. The techniques of confocal microscopy and fluorescence resonance energy transfer (FRET) analysis were utilized in examining A7r5 cells and rat aortic rings contracted with phorbol 12,13-dibutyrate. Visual evaluation of confocal images of A7r5 smooth muscle cells contracted by phorbol 12,13-dibutyrate indicated significant disassociation of myosin from α-actin but not β-actin. Whole-cell FRET analysis confirmed these observations (α-actin–myosin –67%, β-actin–myosin –2%). Time course studies further showed that α-actin–myosin complex increased significantly (40%) within 1.5 min after the addition of phorbol 12,13-dibutyrate and then declined as contraction progressed. FRET analysis of rat aortic rings at different intervals of contraction indicated significant increases in α-actin–myosin at the initiation (79%) and plateau (67%) in force development, but not during the intermediate period of slowly developing tension (–4%). By comparison, β-actin–myosin complex was unchanged except during slow force development, in which the association was significantly decreased (–30%). Similar to that of α-actin–myosin, Alexa 488 – phalloidin staining fluorescence indicated increased tissue F-actin content at the initiation (21%) and plateau (62%) in force. FRET images indicated the development of thickened cables and patches of α-actin–myosin in tissue throughout the interval of contraction. The results provide direct evidence of dynamic remodeling of the contractile protein during vascular smooth muscle contraction and suggest that FRET analysis may be a powerful tool for assessment of tissue protein–protein associations.

Myosin, transgelin, and myosin light chain kinase: expression and function in asthma

RATIONALE

Airway smooth muscle (SM) of patients with asthma exhibits a greater velocity of shortening (Vmax) than that of normal subjects, and this is thought to contribute to airway hyperresponsiveness. A greater Vmax can result from increased myosin activation. This has been reported in sensitized human airway SM and in models of asthma. A faster Vmax can also result from the expression of specific contractile proteins that promote faster cross-bridge cycling. This possibility has never been addressed in asthma.

OBJECTIVES

We tested the hypothesis that the expression of genes coding for SM contractile proteins is altered in asthmatic airways and contributes to their increased Vmax.

METHODS

We quantified the expression of several genes that code for SM contractile proteins in mild allergic asthmatic and control human airway endobronchial biopsies. The function of these contractile proteins was tested using the in vitro motility assay.

MEASUREMENTS AND MAIN RESULTS

We observed an increased expression of the fast myosin heavy chain isoform, transgelin, and myosin light chain kinase in patients with asthma. Immunohistochemistry demonstrated the expression of these genes at the protein level. To address the functional significance of this overexpression, we purified tracheal myosin from the hyperresponsive Fisher rats, which also overexpress the fast myosin heavy chain isoform as compared with the normoresponsive Lewis rats, and found a faster rate of actin filament propulsion. Conversely, transgelin did not alter the rate of actin filament propulsion.

CONCLUSIONS

Selective overexpression of airway smooth muscle genes in asthmatic airways leads to increased Vmax, thus contributing to the airway hyperresponsiveness observed in asthma.

Smooth muscle molecular mechanics in airway hyperresponsiveness and asthma

Asthma is a respiratory disorder characterized by airway inflammation and hyperresponsiveness associated with reversible airway obstruction. The relative contributions of airway hyperresponsiveness and inflammation are still debated, but ultimately, airway narrowing mediated by airway smooth muscle contraction is the final pathway to asthma. Considerable effort has been devoted towards identifying the factors that lead to the airway smooth muscle hypercontractility observed in asthma, and this will be the focus of this review. Airway remodeling has been observed in severe and fatal asthma. However, it is unclear whether remodeling plays a protective role or worsens airway responsiveness. Smooth muscle plasticity is a mechanism likely implicated in asthma, whereby contractile filament rearrangements lead to maximal force production, independent of muscle length. Increased smooth muscle rate of shortening via altered signaling pathways or altered contractile protein expression has been demonstrated in asthma and in numerous models of airway hyperresponsiveness. Increased rate of shortening is implicated in counteracting the relaxing effect of tidal breathing and deep inspirations, thereby creating a contracted airway smooth muscle steady-state. Further studies are therefore required to understand the numerous mechanisms leading to the airway hyperresponsiveness observed in asthma as well as their multiple interactions.

Smooth muscle myosin isoform expression and LC20 phosphorylation in innate rat airway hyperresponsiveness

Four smooth muscle myosin heavy chain (SMMHC) isoforms are generated by alternative mRNA splicing of a single gene. Two of these isoforms differ by the presence [(+)insert] or absence [(-)insert] of a 7-amino acid insert in the motor domain. The rate of actin filament propulsion of the (+)insert SMMHC isoform, as measured in the in vitro motility assay, is twofold greater than that of the (-)insert isoform. We hypothesized that a greater expression of the (+)insert SMMHC isoform and greater regulatory light chain (LC(20)) phosphorylation contribute to airway hyperresponsiveness. We measured airway responsiveness to methacholine in Fischer hyperresponsive and Lewis normoresponsive rats and determined SMMHC isoform mRNA and protein expression, as well as essential light chain (LC(17)) isoforms, h-caldesmon, and alpha-actin protein expression in their tracheae. We also measured tracheal muscle strip contractility in response to methacholine and corresponding LC(20) phosphorylation. We found Fischer rats have more (+)insert mRNA (69.4 +/- 2.0%) (mean +/- SE) than Lewis rats (53.0 +/- 2.4%; P < 0.05) and a 44% greater content of (+)insert isoform relative to total myosin protein. No difference was found for LC(17) isoform, h-caldesmon, and alpha-actin expression. The contractility experiments revealed a greater isometric force for Fischer trachealis segments (4.2 +/- 0.8 mN) than Lewis (1.9 +/- 0.4 mN; P < 0.05) and greater LC(20) phosphorylation level in Fischer (55.1 +/- 6.4) than in Lewis (41.4 +/- 6.1; P < 0.05) rats. These results further support the contention that innate airway hyperresponsiveness is a multifactorial disorder in which increased expression of the fast (+)insert SMMHC isoform and greater activation of LC(20) lead to smooth muscle hypercontractility.

Mediation of acetylcholine and substance P induced contractions by myosin light chain phosphorylation in feline colonic smooth muscle

OBJECTIVES

To determine the role of myosin light chain phosphorylation in feline colonic smooth muscle contraction.

SAMPLE POPULATION

Colonic tissue was obtained from eight 12- to 24-month-old cats.

PROCEDURE

Colonic longitudinal smooth muscle strips were attached to isometric force transducers for measurements of isometric stress. Myosin light chain phosphorylation was determined by isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Stress and phosphorylation were determined following stimulation with ACh or SP, in the absence or presence of a calmodulin antagonist (W-7; 0.1 to 1.0 mM), myosin light chain kinase inhibitor (ML-9; 1 to 10 microM), or extracellular calcium free solutions.

RESULTS

Unstimulated longitudinal colonic smooth muscle contained low amounts (6.9+/-3.2%) of phosphorylated myosin light chain. Phosphorylation of the myosin light chains was dose and time dependent with maximal values of 58.5% at 30 seconds of stimulation with 100 microM Ach and 60.2% at 45 seconds of stimulation with 100 nM SP Active isometric stress development closely paralleled phosphorylation of the myosin light chains in ACh- or SP-stimulated muscle. W-7 and ML-9 dose dependently inhibited myosin light chain phosphorylation and isometric stress development associated with ACh or SP stimulation. Removal of extracellular calcium inhibited myosin light chain phosphorylation and isometric stress development in ACh-stimulated smooth muscle.

CONCLUSIONS AND CLINICAL RELEVANCE

Feline longitudinal colonic smooth muscle contraction is calcium-, calmodulin-, and myosin light chain kinase-dependent. Myosin light chain phosphorylation is necessary for the initiation of contraction in feline longitudinal colonic smooth muscle. These findings may prove useful in determining the biochemical and molecular defects that accompany feline colonic motility disorders.

Remodeling of the actin cytoskeleton in the contracting A7r5 smooth muscle cell

It has been proposed that the reorganization of components of the actin cytomatrix could contribute to force development and the low energy cost of sustained contraction in contractile cells which lack a structured sarcomere (A.S. Battistella-Patterson, S. Wang and G.L. Wright (1997) Can J Physiol Pharmacol 75: 1287-1299). However, there has been no direct evidence of an apropos actin reorganization specifically linked to the contractile response in cells of this type. Remodeling of the alpha- and beta-actin domains was studied in A7r5 smooth muscle cells during phorbol 12,13 dibutyrate (PDB)-induced contraction using immunohistologic staining and beta-actin-green fluorescent protein (beta-actin-GFP) fusion protein expression. Cell stained with phalloidin as well as cells expressing beta-actin-GFP showed densely packed actin stress cables, arranged in parallel and extending across the cell body. PDB caused approximately 85% of cells to contract with evidence of forcible detachment from peripheral adhesion sites seen in many cells. The contraction of the cell body was not uniform but occurred along a principal axis parallel to the system of densely packed beta-actin cables. During the interval of contraction, the beta-actin cables shortened without evidence of disassembly or new cable formation. The use of cytochalasin to inhibit actin polymerization resulted in the dissolution of the actin cables at the central region of the cell and caused the elongation of precontracted cells. In unstimulated cells, alpha-actin formed cables similar in arrangement to the cell spanning beta-actin cables. Within a short interval after PDB addition; however, the majority of alpha-actin cables disassembled and reformed into intensely fluorescing column-like structures extending vertically from the cell base at the center of clusters of alpha-actin filaments. The alpha-actin columns of contracting cells showed strong colocalization of alpha-actinin suggesting they could be structurally analogous to the dense bodies of highly differentiated smooth muscle cells. The results indicate that the alpha- and beta-actin domains of A7r5 cells undergo a highly structured reorganization during PDB-induced contraction. The extent and nature of this restructuring suggest that remodeling could play a role in contractile function.

ATP hydrolysis during contraction of permeabilized airway smooth muscle

This study determined whether the time-dependent decline in the rate of ATP hydrolysis by actomyosin ATPase during sustained isometric force can occur in the absence of a time-dependent decline in regulatory myosin light chain (rMLC) phosphorylation in Triton X-100-permeabilized canine tracheal smooth muscle. Maximal activation with 10 microM Ca(2+) induced sustained increases in isometric force, stiffness, and rMLC phosphorylation; however, the increase in the ATP hydrolysis rate was initially high but then declined to a steady-state level above that of the unstimulated muscle (basal 31.8 +/- 5.8 nmol. cm(-3). s(-1); peak 81.4 +/- 11.3 nmol. cm(-3). s(-1); steady-state 62.2 +/- 9.1 nmol. cm(-3). s(-1)). Activation of strips in which the rMLC was irreversibly and maximally thiophosphorylated with adenosine 5'-O-(3-thiotriphosphate) also induced sustained increases in isometric force and stiffness but a nonsustained increase in ATP hydrolysis rate. There was no significant difference in the peak or steady-state isometric force, stiffness, or ATP hydrolysis rate or in the steady-state maximum unloaded shortening velocity between strips activated by 10 microM Ca(2+) or rMLC thiophosphorylation (0.058 +/- 0.016 and 0.047 +/- 0.011 muscle lengths/s, respectively). Mechanisms other than changes in rMLC phosphorylation contribute to the time-dependent decline in actomyosin ATPase activity during sustained activation of canine tracheal smooth muscle.

Myosin light chain phosphorylation in contraction of gastric antral smooth muscle from neonate and adult rabbits

The decreased contractility of gastric antral smooth muscle in the neonate has been attributed to reduced levels of activator calcium. It is generally accepted that calcium-dependent myosin light chain phosphorylation (MLCP) is the key step in the initiation of force development in smooth muscle. In this study, we investigated the relationship between MLCP and force development in gastric antral smooth muscle from neonatal (4-6 d old) and adult rabbits. We tested the hypothesis that the reduced force development of circular smooth muscle from the neonate would be accompanied by decreased levels of MLCP, as compared with data from adult animals. Full thickness muscle strips oriented parallel to the circular muscle layer were examined for their contractile response to acetylcholine (ACh) (10(-8) M to 10(-3) M) or 10(-4) M ACh only. In the latter study, tissues were rapidly frozen in a dry ice-acetone slurry for subsequent MLCP determination. MLCP was determined at times corresponding to 5, 10, 15, 30, and 60 s of stimulation. For each age group, maximal active force developed at an ACh concentration of 10(-4) M and was significantly greater in tissues from adults (1.86 +/- 0.24 N/m2, adult; 0.95 +/- 0.05 N/m2, neonate; p < 0.05). In contrast, no significant differences were observed with respect to basal or agonist-stimulated levels of MLCP. The data suggest that factors other than levels of MLCP contribute to the reduced force-generating capacity of antral smooth muscle from the neonate.

Myosin phosphorylation and the control of myometrial contraction/relaxation

In summary, phosphorylation of the regulatory light chain of myosin by Ca2+/CaM-dependent MLCK plays an important role in smooth muscle contraction. Although there have been major advances in our understanding of the regulation and physiological functions of contractile proteins in smooth muscle in recent years, very little information exists on the functional status of these proteins in human myometrium during pregnancy. The simple view that contractile force in smooth muscle is proportionate to cytoplasmic Ca2+ concentrations (Ca2+i) and myosin light chain phosphorylation is now more complex as more experiments provide insights into mechanisms of regulation of the contractile elements. MLCK can be phosphorylated, which desensitizes its activation by Ca2+/CaM, and protein phosphatase activity toward myosin may also be regulated. Examples in smooth muscle tissue are sparse, and the different mechanisms by which these processes may be adapted in uterine smooth muscle during pregnancy are not well-defined. Much research is needed to define further the cellular, biochemical, and molecular basis for these physiological processes involved in the regulation of uterine smooth muscle contraction and relaxation.

Effect of unphosphorylated smooth muscle myosin on caldesmon-mediated regulation of actin filament velocity

The effect of smooth muscle myosin at different levels of light chain phosphorylation on caldesmon-mediated movement of actin filaments was investigated using an in vitro motility assay. Myosin at different levels of phosphorylation was obtained by mixing different proportions of fully phosphorylated and unphosphorylated myosin in monomeric form, while keeping the total myosin concentration constant. The average velocity of actin filaments containing tropomyosin was 1.20 +/- 0.046 microns s-1 at 30 degrees C with fully phosphorylated myosin. This velocity was not altered when the percentage of unphosphorylated myosin coated on the nitrocellulose surface was increased to 80%; further increases lowered the velocity. When the actin filaments with caldesmon bound at stoichiometric levels were used, filament velocity was unaffected until 50% of the myosin was unphosphorylated, but further increases in the percentage of unphosphorylated myosin induced a decrease in the velocity, and at 95% unphosphorylated myosin, filament movement had ceased. The decreased filament velocity in the presence of caldesmon was also observed when phosphorylated myosin was mixed with myosin rod instead of unphosphorylated myosin, but was not observed when the 38 kDa caldesmon C-terminal fragment, which lacks the myosin-binding domain, was used instead of intact caldesmon. These data indicate that the decreased filament velocity in the presence of caldesmon reflects the mechanical load produced by the tethering of actin to myosin through the interaction of the caldesmon N-terminal domain and the myosin S-2 region. The tethering effect mediated by caldesmon may play a role in smooth muscle contraction when a large number of myosin heads are dephosphorylated, as in force maintenance.

Calcium and smooth muscle contraction

The fact that smooth muscle exists in almost every hollow organ and is involved in a large number of disease states has led to a vast increase in smooth muscle research, covering areas from testing response to antagonists and agonists to measuring the molecular force generated by a single actin filament. Yet, the exact mechanisms regulating contractile response of smooth muscle remain unsolved. Calcium has been a central player in mediating smooth muscle contraction through binding with calmodulin, although there is evidence showing that under special circumstances smooth muscle can contract without change in intracellular Ca2+. In addition to the major regulatory pathway of Ca(2+)-calmodulin-myosin light chain kinase, there are other thin filament linked regulatory mechanisms in which Ca(2+)-calmodulin dependent phosphorylation of calponin and caldesmon may be involved. Ca2+ sensitivity of smooth muscle contraction may vary under different situations and this has recently been recognized as an important regulatory mechanism. Examples are protein kinase C (PKC) dependent phosphorylation of myosin light chain kinase which results in partial inhibition of contraction, and activation of myosin light chain phosphatase. There is new evidence showing that not only does Ca2+ regulate contraction by regulating the interaction of contractile proteins in smooth muscle, but also that shortening of smooth muscle itself reduces intracellular Ca2+ concentration, via a negative feedback.

Energetics of crossbridge phosphorylation and contraction in vascular smooth muscle

Ca(2+)-dependent crossbridge phosphorylation is the primary mechanism governing crossbridge cycling in smooth muscle. A four-state crossbridge model in which phosphorylation is the only proposed regulatory mechanism was successful in predicting the mechanical properties of the swine carotid media including latch (sustained force with reduced crossbridge cycling). This model also predicts that the ATP consumption of crossbridge phosphorylation is approximately equal to that of crossbridge cycling and that ATP consumption will rise hyperbolically with increases in steady-state force. This review shows these predictions to be consistent with the available energetics data for the carotid media. The absolute energetic cost of covalent regulation is modest and less than the energy savings associated with latch. However, covalent regulation should reduce the total mechanical efficiency of smooth muscle relative to striated muscle.

[Cellular mechanism of muscle contraction of bronchial smooth muscle]

Airway smooth muscle is one of the main effector of bronchial reactivity. The understanding of the cellular mechanisms involved in the contraction of this muscle has advanced in the recent past since isolated cells in culture can now be studied. Extracellular messengers (neurotransmitters and mediators) as well as their specific membrane receptors have been analyzed in some details. The membrane transduction of extracellular messengers brings about the formation (or the increase in the concentration) of the intracellular second messenger which, in airway smooth muscle, is the cytosolic calcium (Ca2+i) via activation of calcium channels which depend on surface membrane potential changes (electromechanical coupling) on the one hand and mainly via mechanisms independent of surface membrane potential changes-so-called the pharmacomechanical coupling--which involves membrane phosphoinositides metabolism. Changes in Ca2+i activate contractile proteins leading the muscle to shorten and to develop force via several controlled steps such as phosphorylation of myosin or changes in the sensitivity to Ca2+ of the contractile elements. Experimental techniques that enable to simultaneously study different aspects of the cellular response are being developed in airway smooth muscle and are likely to provide complementary information about the cellular physiology and pathophysiology of this muscle.

Force-velocity relation and myosin light chain phosphorylation in bovine coronary arterial smooth muscle

We have investigated in bovine left ventricular coronary arteries the relation between the biochemical regulatory event of myosin light chain phosphorylation and the mechanical events of isometric stress and isotonic shortening, under conditions of stimulation by depolarization (65 mM KCl) or receptor occupancy (2 microM 5-hydroxytryptamine [5-HT]). At rest, levels of light chain phosphorylation were 0.07 +/- 0.01 mol phosphate/mol light chain. Maximal values were significantly different for KCl (0.42 +/- 0.02 mol phosphate/mol light chain at 1 minute) and 5-HT stimulation (0.58 +/- 0.01 mol phosphate/mol light chain at 30 seconds). Increases in light chain phosphorylation preceded isometric stress development, and values remained elevated at approximately 0.35 mol phosphate/mol light chain for up to 2 hours with both KCl and 5-HT. The sites of phosphorylation were identical for KCl and 5-HT at 2 hours. Maximal stresses for each stimulus were also maintained for 2 hours. Values of maximum velocity of shortening (Vo in muscle lengths [ML]/sec), obtained from the force-velocity relation, did not change significantly between 1 minute and 2 hours with KCl (0.070 +/- 0.008 ML/sec at 1 minute and 0.056 +/- 0.007 ML/sec at 2 hours, p greater than 0.2). However, during 5-HT stimulation, Vo declined significantly (0.053 +/- 0.006 ML/sec at 1 minute and 0.032 +/- 0.003 ML/sec at 2 hours, p less than 0.025). The relation between Vo and light chain phosphorylation was different for KCl and 5-HT, indicating that factors in addition to myosin light chain phosphorylation may modulate smooth muscle shortening velocity.

Vascular smooth muscle contractile elements. Cellular regulation

For many years the simple view was held that contractile force in smooth muscle was proportional to cytosolic Ca2+ concentrations ([Ca2+]i). With the discovery that phosphorylation of myosin light chain by Ca2+/calmodulin-dependent myosin light chain kinase initiated contraction, regulation of the contractile elements developed more complex properties. Molecular and biochemical investigations have identified important domains of myosin light chain kinase: light chain binding sites, catalytic core, pseudosubstrate prototope, and calmodulin-binding domain. New protein phosphatase inhibitors such as okadaic acid and calyculin A should help in the identification of the physiologically important phosphatase and potential modes of regulation. The proposal of an attached, dephosphorylated myosin cross bridge (latch bridge) that can maintain force has evoked considerable controversy about the detailed functions of the myosin phosphorylation system. The latch bridge has been defined by a model based on physiological properties but has not been identified biochemically. Thin-filament proteins have been proposed as secondary sites of regulation of contractile elements, but additional studies are needed to establish physiological roles. Changes in the Ca2+ sensitivity of smooth muscle contractile elements with different modes of cellular stimulation may be related to inactivation of myosin light chain kinase or activation of protein phosphatase activities. Thus, contractile elements in smooth muscle cells are not dependent solely on [Ca2+]i but use additional regulatory mechanisms. The immediate challenge is to define their relative importance and to describe molecular-biochemical properties that provide insights into proposed physiological functions.

Calcium dependent regulation of vascular smooth muscle contraction

The experimental results discussed from our laboratory as well as from numerous other laboratories investigating the regulation of smooth muscle contraction have, in our opinion, clearly demonstrated that a simple Ca2+ dependent switch (MLC phosphorylation) cannot completely explain all of the mechanical and energetic findings. We and others have demonstrated that stress can be developed in the complete absence of increases in MLC phosphorylation, that crossbridge cycling rate can be regulated independent of changes in MLC phosphorylation, that Ca2+ can directly influence both stress and crossbridge cycling rate, and that protein kinase C can, apparently, directly initiate the development of stress supported by a specific population of crossbridges characterized by unphosphorylated MLC, low cycling rates, and weak binding characteristics. This information combined with the wealth of material demonstrating the important function played by the Ca2+ and calmodulin dependent MLC kinase is consistent with the hypothesis that there are two Ca2+ dependent regulatory systems acting in parallel in smooth muscle. One of these is the Ca2+ dependent MLC phosphorylation-dephosphorylation system responsible for the rapid development of stress and the second is a hypothesized Ca2+ dependent system responsible for the slow development of stress as well as the maintenance of previously developed stress. This second system has a higher Ca2+ sensitivity than that for MLC phosphorylation and may be activated by protein kinase C. The total stress attained by smooth muscle is activated by protein kinase C. The total stress attained by smooth muscle is the result of these two regulatory systems acting in concert. Although we believe the available information is consistent with this hypothesis of two regulatory systems functioning in parallel, it is by no means the only possibility. Early work from our laboratory and the recent work by the Somlyos and their colleagues and Kubota et al. suggest the possibility of a regulated MLC phosphatase which might functionally alter the Ca2+ sensitivity of the contractile filaments. Kerrick and Hoar and Nishimura and van Breemen have published data which imply a role for MgADP in latchbridge kinetics. These findings, as well as the discovery of several thin filament protein components which have been proposed as regulatory units, must all be taken into account in the final answer to the question: How does Ca2+ contract smooth muscle?