Force-generating capacity and contractile protein content of arterial smooth muscle

Caldesmon ablation in mice causes umbilical herniation and alters contractility of fetal urinary bladder smooth muscle

The actin-, myosin-, and calmodulin-binding protein caldesmon (CaD) is expressed in two splice isoforms: h-CaD, which is an integral part of the actomyosin domain of smooth muscle cells, and l-CaD, which is widely expressed and is involved in many cellular functions. Despite extensive research for many years, CaD's in vivo function has remained elusive. To explore the role of CaD in smooth muscle contraction in vivo, we generated a mutant allele that ablates both isoforms. Heterozygous animals were viable and had a normal life span, but homozygous mutants died perinatally, likely because of a persistent umbilical hernia. The herniation was associated with hypoplastic and dysmorphic abdominal wall muscles. We assessed mechanical parameters in isometrically mounted longitudinal strips of E18.5 urinary bladders and in ring preparations from abdominal aorta using wire myography. Ca²⁺ sensitivity was higher and relaxation rate was slower in Cald1^−/− compared with Cald1^+/+ skinned bladder strips. However, we observed no change in the content and phosphorylation of regulatory proteins of the contractile apparatus and myosin isoforms known to affect these contractile parameters. Intact fibers showed no difference in actin and myosin content, regardless of genotype, although KCl-induced force tended to be lower in homozygous and higher in heterozygous mutants than in WTs. Conversely, in skinned fibers, myosin content and maximal force were significantly lower in Cald1^−/− than in WTs. In KO abdominal aortas, resting and U46619 elicited force were lower than in WTs. Our results are consistent with the notion that CaD impacts smooth muscle function dually by (1) acting as a molecular brake on contraction and (2) maintaining the structural integrity of the contractile machinery. Most importantly, CaD is essential for resolution of the physiological umbilical hernia and ventral body wall closure.

Structural limits on force production and shortening of smooth muscle

This study determined the factors that limit force production and shortening in two smooth muscles having very different relationships between active and passive force as a function of muscle length. The rat anococcygeus muscle develops active force over the range of lengths 0.2-2.0× the optimum length for force production (Lo). Passive tension due to extension of the resting muscle occurs only at lengths exceeding Lo. In contrast, the rabbit taenia coli develops force in the range of lengths 0.4-1.1 Lo, and passive force which is detectable at 0.56 Lo, increases to ~0.45 maximum active force at Lo, and increases sharply with further extension. The anococcygeus muscle can shorten to 0.2 Lo and the taenia coli to 0.4 Lo. Dynamic stiffness and energy usage at short muscle lengths suggest that the limit of shortening in the taenia coli, in contrast to the anococcygeus muscle, is not due to a failure of cross bridge interaction. Phosphorylation of the regulatory myosin light chains in intact muscles decreased to a small extent at short lengths compared to the decrease in force production. The differences in force production and the extent of shortening in the two muscles was maintained even when, following permeabilization, the myosin light chains were irreversibly phosphorylated with ATPγS, indicating that differences in activation played little, if any role. Ultrastructural studies on resting and activated muscles show that the taenia coli, which is rich in connective tissue (unlike the anococcygeus muscle) undergoes marked cellular twisting and contractile filament misalignment at short lengths with compression of the extracellular matrix. As a result, force is not transmitted in the longitudinal axis of the muscle, but is dissipated against an internal load provided by the compressed extracellular matrix. These observations on two very different normal smooth muscles reveal how differences in the relative contribution of active and passive structural elements determine their mechanical behavior, and how this is potentially modified by remodeling that occurs in disease and in response to changes in functional demand.

Deciphering actin cytoskeletal function in the contractile vascular smooth muscle cell

This review focuses on the vascular smooth muscle cells present in the medial layer of the blood vessels wall in the fully differentiated state (dVSMCs). The dVSMC contractile phenotype enables these cells to respond in a highly regulated manner to changes in extracellular stimuli. Through modulation of vascular contractile force and vascular compliance dVSMCs regulate blood pressure and blood flow. The cellular and molecular mechanisms by which vascular smooth muscle contractile functions are regulated are not completely elucidated. Recent studies have documented a critical role for actin polymerization and cytoskeletal dynamics in the regulation of contractile function. Here we will review the current understanding of actin cytoskeletal dynamics and focal adhesion function in dVSMCs in order to better understand actin cytoskeleton connections to the extracellular matrix and the effects of cytoskeletal remodelling on vascular contractility and vascular stiffness in health and disease.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Nonmuscle myosin is regulated during smooth muscle contraction

The participation of nonmuscle myosin in force maintenance is controversial. Furthermore, its regulation is difficult to examine in a cellular context, as the light chains of smooth muscle and nonmuscle myosin comigrate under native and denaturing electrophoresis techniques. Therefore, the regulatory light chains of smooth muscle myosin (SM-RLC) and nonmuscle myosin (NM-RLC) were purified, and these proteins were resolved by isoelectric focusing. Using this method, intact mouse aortic smooth muscle homogenates demonstrated four distinct RLC isoelectric variants. These spots were identified as phosphorylated NM-RLC (most acidic), nonphosphorylated NM-RLC, phosphorylated SM-RLC, and nonphosphorylated SM-RLC (most basic). During smooth muscle activation, NM-RLC phosphorylation increased. During depolarization, the increase in NM-RLC phosphorylation was unaffected by inhibition of either Rho kinase or PKC. However, inhibition of Rho kinase blocked the angiotensin II-induced increase in NM-RLC phosphorylation. Additionally, force for angiotensin II stimulation of aortic smooth muscle from heterozygous nonmuscle myosin IIB knockout mice was significantly less than that of wild-type littermates, suggesting that, in smooth muscle, activation of nonmuscle myosin is important for force maintenance. The data also demonstrate that, in smooth muscle, the activation of nonmuscle myosin is regulated by Ca(2+)-calmodulin-activated myosin light chain kinase during depolarization and a Rho kinase-dependent pathway during agonist stimulation.

Pkd2+/- vascular smooth muscles develop exaggerated vasocontraction in response to phenylephrine stimulation

Vascular complications are the leading cause of morbidity and mortality in autosomal dominant polycystic kidney disease. Although evidence suggests an abnormal vascular reactivity, contractile function in Pkd mutant vessels has not been studied previously. Contractile response to phenylephrine (PE; 10(-10) to 10(-4)M), an alpha1-adrenergic receptor agonist, was examined. De-endothelialized Pkd2(+/-) aortic rings generated a higher maximum force (F(max)) than that in wild-type (wt; 5.78 +/- 0.73 versus 2.69 +/- 0.43 mN; P < 0.001) and a significant left shift in PE dosage-response curve. On simultaneous recordings, Pkd2(+/-) aortic helical strips also responded to PE with a greater F(max) but a lesser [Ca(2+)](i) rise, resulting in a greatly enhanced Deltaforce/DeltaCa(2+) ratio than that in wt. At F(max), a higher elevation in the phosphorylated regulatory myosin light chain was observed in Pkd2(+/-) strips. Ca(2+)-dependent calmodulin/myosin light-chain kinase-mediated contraction was examined by direct Ca(2+) (pCa8-5) stimulation to beta-escin permeabilized aortic strips; the pCa-force curve in Pkd2(+/-) strips was not shifted, thereby indicating that PE induced dosage-response alteration that resulted from Ca(2+)-independent mechanisms. Quantitative analyses of contractile proteins demonstrated elevated expressions in smooth muscle alpha-actin and myosin heavy chain in Pkd2(+/-) arteries, changes that likely contribute to the higher F(max). Similar to those in aortas, de-endothelialized Pkd2(+/-) resistance (fourth-order mesenteric) arteries responded to PE with a stronger contraction but a lesser [Ca(2+)](i) rise than in wt. Taken together, the arterial vasculature in Pkd2(+/-) mice exhibits an exaggerated contractile response and increased sensitivity to PE. An enhanced Ca(2+)-independent force generation and elevated contractile protein expression likely contribute to these abnormalities.

The lord of the ring: mandatory role of the kidney in drug therapy of hypertension

Strong evidence supports the idea that total peripheral resistance (TPR) is increased in all forms of human and experimental hypertension. Although the etiological participation of TPR in the origin and long-term maintenance of hypertension has been extensively debated, it now seems clear that the renal, nonadaptive, infinite gain-working, pressure-sensitive natriuresis and diuresis is the main mechanism of blood pressure control in the long term. The tissue, cellular, biochemical, and genetic sensors and executors of this process have not been fully identified yet, but the role of the renal medulla has gained growing attention as the physiopathological scenario in which the key regulatory elements reside. Specifically, the functionality of the renomedullary vasculature seems to be highly responsible for blood pressure control. The vasculature of the renal medulla becomes a new and more specific target for the therapeutic intervention of hypertension. Recent data on the effect of baroreceptor-controlled renal sympathetic activity on the long-term regulation of blood pressure are integrated. The renomedullary effects of the main antihypertensive drugs are discussed, and new perspectives for the therapeutic intervention of hypertension are outlined. Comparison of the genetic program of the renal medulla before and after the development of hypertension in spontaneously hypertensive and experimentally induced animal models might provide a mechanism for identifying the key genes that become activated or suppressed in the development of high blood pressure. These genes, their encoded proteins, or other elements related to their signalling and genetic pathways might serve as new and more specific targets for the pharmacological treatment of abnormally elevated blood pressure. Besides, proteins specifically located to the luminal side of the renomedullary vascular endothelium may serve as potential targets for site-directed drug and gene therapy.

Cardiac myosin isoforms from different species have unique enzymatic and mechanical properties

The mammalian heart contains two cardiac myosin isoforms: beta-myosin heavy chain (MHC) is found predominantly in the ventricles of large mammals, and alpha-MHC is expressed in the atria. The sequence identity between these isoforms is approximately 93%, with nonidentical residues clustered in discrete, functionally important domains associated with actin binding and ATPase activity. It is well-established that rabbit alpha-cardiac myosin has a 2-fold greater unloaded shortening velocity than beta-cardiac myosin but a 2-fold lower average isometric force. Here, we test the generality of these relationships for another large mammal, the pig, as well as for a small rodent, the mouse, which expresses alpha-MHC in its ventricles throughout adulthood. Hydrophobic interaction chromatography (HIC) was used to purify myosin from mouse, rabbit, and pig hearts. The superior resolving power of HIC made it possible to prepare highly homogeneous, enzymatically active myosin from small amounts of tissue. The movement of actin filaments by myosin was measured in an in vitro motility assay. The same assay could be used to determine average isometric force by loading the actin filaments with increasing concentrations of alpha-actinin to stop filament motion. We conclude that myosin from the mouse has significantly higher velocities for both alpha and beta isoforms than myosin from rabbits and pigs, even though the 2-fold difference in velocity between isoforms is maintained. Unlike the larger mammals, however, the small rodent generates the same high isometric force for both alpha and beta isoforms. Thus, nature has adapted the function of cardiac myosin isoforms to optimize power output for hearts of a given species.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Cooperative attachment of cross bridges predicts regulation of smooth muscle force by myosin phosphorylation

Serine 19 phosphorylation of the myosin regulatory light chain (MRLC) appears to be the primary determinant of smooth muscle force development. The relationship between MRLC phosphorylation and force is nonlinear, showing that phosphorylation is not a simple switch regulating the number of cycling cross bridges. We reexamined the MRLC phosphorylation-force relationship in slow, tonic swine carotid media; fast, phasic rabbit urinary bladder detrusor; and very fast, tonic rat anococcygeus. We found a sigmoidal dependence of force on MRLC phosphorylation in all three tissues with a threshold for force development of approximately 0.15 mol P(i)/mol MRLC. This behavior suggests that force is regulated in a highly cooperative manner. We then determined whether a model that employs both the latch-bridge hypothesis and cooperative activation could reproduce the relationship between Ser(19)-MRLC phosphorylation and force without the need for a second regulatory system. We based this model on skeletal muscle in which attached cross bridges cooperatively activate thin filaments to facilitate cross-bridge attachment. We found that such a model describes both the steady-state and time-course relationship between Ser(19)-MRLC phosphorylation and force. The model required both cooperative activation and latch-bridge formation to predict force. The best fit of the model occurred when binding of a cross bridge cooperatively activated seven myosin binding sites on the thin filament. This result suggests cooperative mechanisms analogous to skeletal muscle that will require testing.

Tuning smooth muscle contraction by molecular motors

As in striated muscle, smooth muscle cells (SMC) contract by Ca2+ activated cyclic interaction between actin and type II myosin. However, smooth muscle maintains tone at basal activating Ca2+ and low energetic cost during sustained activation. This review analyzes the regulation of phasic and tonic contraction of SMC on the molecular level. Type II myosin is the molecular motor also of smooth muscle contraction. Six myosin heavy chain (MHC) isoenzymes (four smooth muscle, two nonmuscle) and five myosin light chain (MLC) isoforms (two 17 kDa, two 20 kDa, one 23 kDa) are expressed in SMC. These myosin subunits could be generated by alternative splicing or by differential gene expression. Thus different myosin isoenzymes are generated which may be modified posttranslationally by phosphorylation, affecting the contractile state of the SMC. Furthermore, they may be part of distinct contractile systems which are targeted by different second messenger cascades and are recruited differentially during activation, electromechanical, and pharmacomechanical coupling. Low energy consumption, shortening velocity, and MLC20 phosphorylation at low Ca2+ activation levels during tone maintenance ("latch") could be explained by a switch from smooth muscle myosin to nonmuscle myosin activation upon prolonged activation.

Biochemical and morphological effects of bladder pumping on the urinary bladder in rats

AIMS

To study the influence of bladder pumping on the urinary bladder in 44 female rats.

METHODS

Under halothane anesthesia, a urethral catheter was inserted into the bladder of 27 rats, and air (0.4-0.8 mL) was pumped in and out of the bladder at 0.5 cycles/second for a period of 5 minutes. Twenty-four hours after pumping, the bladder was harvested for measurement of the tissue levels of myosin, actin, and nerve growth factor, as well as for electron microscopy. In nine of the 27 rats, cystometry was performed without anesthesia before and 1, 7, 30, and 90 days after bladder pumping. The remaining 17 rats that did not undergo pumping were anesthetized and their bladders were harvested as a control.

RESULTS

Bladder pumping increased the bladder capacity and decreased the maximum bladder contraction pressure, but did not increase the residual volume. Bladder pumping also increased the tissue level of nerve growth factor and decreased the levels of myosin and actin. Electron microscopy showed degeneration of bladder smooth muscle cells and nerve fibers after bladder pumping, as well as derangement and disruption of collagen fiber bundles in the bladder wall. These functional and morphological effects of pumping disappeared within 90 days.

CONCLUSIONS

Bladder pumping therapy appears to have various effects on the bladder wall collagen fiber bundles, smooth muscle cells, and nerves.

Myosin cross-bridge kinetics in airway smooth muscle: a comparative study of humans, rats, and rabbits

To analyze the kinetics and unitary force of cross bridges (CBs) in airway smooth muscle (ASM), we proposed a new formalism of Huxley's equations adapted to nonsarcomeric muscles (Huxley AF. Prog Biophys Biophys Chem 7: 255-318, 1957). These equations were applied to ASM from rabbits, rats, and humans (n = 12/group). We tested the hypothesis that species differences in whole ASM mechanics were related to differences in CB mechanics. We calculated the total CB number per square millimeter at peak isometric tension (Psi x10(9)), CB unitary force (Pi), and the rate constants for CB attachment (f(1)) and detachment (g(1) and g(2)). Total tension, Psi, and Pi were significantly higher in rabbits than in humans and rats. Values of Pi were 8.6 +/- 0.1 pN in rabbits, 7.6 +/- 0.3 pN in humans, and 7.7 +/- 0.2 pN in rats. Values of Psi were 4.0 +/- 0.5 in rabbits, 1.2 +/- 0.1 in humans, and 1.9 +/- 0.2 in rats; f(1) was lower in humans than in rabbits and rats; g(2) was higher in rabbits than in rats and in rats than in humans. In conclusion, ASM mechanical behavior of different species was characterized by specific CB kinetics and CB unitary force.

Protein kinase C and cerebral vasospasm

Twenty-five years after the discovery of protein kinase C (PKC), the physiologic function of PKC, and especially its role in pathologic conditions, remains a subject of great interest with 30,000 studies published on these aspects. In the cerebral circulation, PKC plays a role in the regulation of myogenic tone by sensitization of myofilaments to calcium. Protein kinase C phosphorylates various ion channels including augmenting voltage-dependent Ca2+ channels and inhibiting K+ channels, which both lead to vessel contraction. These actions of PKC amplify vascular reactivity to different agonists and may be critical in the regulation of cerebral artery tone during vasospasm. Evidence accumulated during at least the last decade suggest that activation of PKC in cerebral vasospasm results in a delayed but prolonged contraction of major arteries after subarachnoid hemorrhage. Most of the experimental results in vitro or in animal models support the view that PKC is involved in cerebral vasospasm. Implication of PKC in cerebral vasospasm helps explain increased arterial narrowing at the signal transduction level and alters current perceptions that the pathophysiology is caused by a combination of multiple receptor activation, hemoglobin toxicity, and damaged neurogenic control. Activation of protein kinase C also interacts with other signaling pathways such as myosin light chain kinase, nitric oxide, intracellular Ca2+, protein tyrosine kinase, and its substrates such as mitogen-activated protein kinase. Even though identifying PKC revolutionized the understanding of cerebral vasospasm, clinical advances are hampered by the lack of clinical trials using selective PKC inhibitors.

A dilution immunoassay to measure myosin regulatory light chain phosphorylation

We examined the quantitation of myosin regulatory light chain phosphorylation (MRLCP) by Western blot and found both offset and saturation errors. The desirable characteristics of an MRLCP assay are that the dynamic range be 60- to 100-fold and that the detection threshold be known and preferably very small relative to total MRLC concentration. No technique examined provided all these characteristics. However, accurate measurements can be obtained by including serial dilutions of the sample to provide a fractional calibration scale in terms of the dephosphorylated light chain and by using interpolation of the phosphorylated band signal intensity to provide values for the relative phosphorylation ratio. We found that this method offers several advantages over methods that rely on signal ratios from single samples: The dilution ratio method is less subject to errors from differences in protein load, it offers estimates of the error in the individual measurement, and has some redundancy that increases the likelihood of obtaining a valid measurement despite gel or membrane artifacts.

cGMP-mediated phosphorylation of heat shock protein 20 may cause smooth muscle relaxation without myosin light chain dephosphorylation in swine carotid artery

Nitrovasodilators such as nitroglycerine, via production of nitric oxide and an increase in [cGMP], can induce arterial smooth muscle relaxation without proportional reduction in myosin light chain (MLC) phosphorylation or myoplasmic [Ca2+]. These findings suggest that regulatory systems, other than MLC phosphorylation and Ca2+, partially mediate nitroglycerine-induced relaxation. In swine carotid artery, we found that a membrane-permeant cGMP analogue induced relaxation without MLC dephosphorylation, suggesting that cGMP mediated the relaxation. Nitroglycerine-induced relaxation was associated with a reduction in O2 consumption, suggesting that the interaction between phosphorylated myosin and the thin filament was inhibited. Nitroglycerine-induced relaxation was associated with a 10-fold increase in the phosphorylation of a protein on Ser16. We identified this protein as heat shock protein 20 (HSP20), a member of a family of proteins known to bind to thin filaments. When homogenates of nitroglycerine-relaxed tissues were centrifuged at 6000 g, phosphorylated HSP20 preferentially sedimented in the pellet, suggesting that phosphorylation of HSP20 may increase its affinity for the thin filament. We noted that a domain of HSP20 is partially homologous to the 'minimum inhibitory sequence' of skeletal troponin I. The peptide HSP20110-121, which contains this domain, bound to actin-containing filaments only in the presence of tropomyosin, a characteristic of troponin I. High concentrations of HSP20110-121 abolished Ca2+-activated force in skinned swine carotid artery. HSP20110-121 also partially decreased actin-activated myosin S1 ATPase activity. These data suggest that cGMP-mediated phosphorylation of HSP20 on Ser16 may have a role in smooth muscle relaxation without MLC dephosphorylation. HSP20 contains an actin-binding sequence at amino acid residues 110-121 that inhibited force production in skinned carotid artery. We hypothesize that phosphorylation of HSP20 regulates force independent of MLC phosphorylation via binding of HSP20 to thin filaments and inhibition of cross-bridge cycling.

Maturational differences between vascular and bladder smooth muscle during ovine development

Maturation rates of vascular and visceral smooth muscle (SM) during ovine development were compared by quantifying contractile protein, myosin heavy chain (MHC) isoform contents, and contractile properties of aortas and bladders from female fetal (n = 19) and postnatal (n = 21) sheep. Actin, myosin, and protein contents rose progressively throughout development in both tissues (P </= 0.003); however, expression patterns differed. During the last trimester, i. e., 101-130 days (term approximately 145 days), bladder actin and MHC contents were approximately twofold greater (P < 0.04) than those in the aorta. Although the fractional content of 204-kDa SM1 MHC in the bladder decreased from 74 +/- 3% at midgestation to 48 +/- 2% 3 mo postnatal, the aorta exhibited an increase from 30 +/- 2% to 65 +/- 2%. Bladder MHC (MHC-B) migrating at 200 kDa contained only SM2 throughout development. In contrast, 200-kDa MHC in the aorta was predominantly nonmuscle MHC-B at midgestation, which was gradually replaced by SM2 as development progressed. Along with its early expression of SM2, bladder muscle obtained maximal stress generating capacity (1.7 x 10(5) N/m(2)) by term gestation, whereas the aorta exhibited no contractions until after birth. We conclude that whereas aortic SM maturation is delayed until after birth, bladder SM matures biochemically and functionally during prenatal development, thus supporting early requirements for micturition.

Cross-bridge cycling in smooth muscle: a short review

This review is focused on the cross-bridge interaction of the organized contractile system of smooth muscle fibres. By using chemically skinned preparations the different enzymatic reactions of actin-myosin interaction have been associated with mechanical events. A rigor state has been identified in smooth muscle and the binding of ATP causes dissociation of rigor cross-bridges at rates slightly slower than those in skeletal muscle, but fast enough not to be rate-limiting for cross-bridge turn over in the muscle fibre. The release of inorganic phosphate (Pi) is associated with force generation, and this process is not rate-limiting for maximal shortening velocity (Vmax) in the fully activated muscle. The binding of ADP to myosin is strong in the smooth muscle contractile system, a property that might be associated with the generally slow cross-bridge turn over. Both force and Vmax are modulated by the extent of myosin light chain phosphorylation. Low levels of activation are considered to be associated with the recruitment of slowly cycling dephosphorylated cross-bridges which reduces shortening velocity. The attachment of these cross-bridge states in skinned smooth muscles can be regulated by cooperative mechanisms and thin filament associated systems. Smooth muscles exhibit a large diversity in their Vmax and the individual smooth muscle tissue can alter its Vmax under physiological conditions. The diversity and the long-term modulation of phenotype are associated with changes in myosin heavy and light chain isoform expression.

Assessment of the protein quality of the smooth muscle myofibrillar and connective tissue proteins of chicken gizzard

The objective of this study was to assess the protein quality of the myofibrillar and connective tissue proteins of chicken gizzard. Protein fractions were isolated from White Leghorn chicken gizzards and quantified by detailed amino acid analysis. This quantification involved repeated extractions of ground gizzards first with Triton X-100, then with low ionic strength imidazole-buffered saline (pH 7.1), followed by either 2% SDS or by 5 M guanidine hydrochloride. The total soluble intracellular protein fraction averaged 86.3% of the total protein and the insoluble extracellular connective tissue proteins comprised the remaining 13.7%. These fractions differed significantly in their essential amino acid (EAA) profiles, with the soluble intracellular fraction having the highest percentage EAA9 (48.6 to 49.0%) and the insoluble connective tissue fraction varying from 20.8 to 23%, compared to the FAO/WHO reference pattern value of 33.9% for a 2- to 5-yr-old child. Calculated protein efficiency ratios (PER) for intracellular proteins averaged 3.02 compared with a value of 1.65 for the extracellular matrix proteins. These results provide an accurate assessment of the protein quality of smooth muscle proteins of chicken gizzard and may prove valuable for industrial control of the amount of connective tissue added to formulations of meats and poultry products.

Myosin heavy chain isoforms and dynamic contractile properties: skeletal versus smooth muscle

Myosin, one of the primary contractile muscle proteins, displays molecular, enzymatic, structural, functional and regulatory variability. This variability has been shown to account for a significant amount of the functional uniqueness of skeletal and smooth muscle. However, the universal generation of force and/or shortening by these two muscle types belies the ever-increasing number of known distinct differences that bring this about. Thus, the notion that the functional roles of skeletal and smooth muscle, their development and regulation, all appear to be uniquely applicable for their physiological purpose no longer appears heretical. This manuscript presents a cursory overview of the numerous ways in which these two types of muscle use a host of myosin molecules to bring about a common result, force generation and/or shortening.

The molecular mechanics of smooth muscle myosin

Smooth muscle cells are capable of generating forces comparable to those of skeletal muscle cells but with far less myosin, the molecular motor that powers muscle contraction. This unique capability may be inherent to the myosin molecule. We have directly characterized the molecular mechanics of smooth muscle myosin using new technologies developed to measure the forces generated by these proteins. The data help explain the differences in force and velocity in whole smooth and skeletal muscles.

Critical review of the swinging crossbridge theory and of the cardinal active role of water in muscle contraction

A critical analysis is presented of the experimental findings that led to the sliding filament model and to its offspring--the swinging (by rotating or tilting) crossbridge theory of muscle contraction (SCBT). Several principles that have been taken for granted implicitly and explicitly by the creators of these dogmas are discussed. The failure of numerous efforts to verify predictions of the SCBT, particularly the idea that the myosin molecules undergo a major conformational change, is critically reviewed. Analysis of various experimental data suggests that water may play an active role in muscular contraction. Examination of both the experiments that do not fulfill the expectations of the SCBT and the measurements of water liberation during the "contractile" process suggests a new outlook according to which tension development and movement are not due to major conformational changes but rather to restructuring of the hydration shells of actin and myosin.

Energetic cost of activation processes during contraction of swine arterial smooth muscle

1. The objective of this study was to partition the increase in ATP consumption during contraction of swine carotid arterial smooth muscle estimated from suprabasal oxygen consumption (suprabasal JO2) and lactate release (Jlactate) into a component associated with cross-bridge cycling (JX) and one reflecting activation (JA). 2. Two experimental approaches-varying length under constant activation, and varying activation at a long length (1.8 times the optimal length for force development (Lo)) where force generation is minimal-revealed a linear dependence of JO2 and activation energy (JA) on cross-bridge phosphorylation. Protocols inducing a large increase in myosin regulatory light chain (MRLC) phosphorylation at 1.8 Lo resulted in significant elevations of JO2 and marked reductions in the economy of force maintenance. Our evidence suggests that this is primarily due to the increased cost of cross-bridge phosphorylation. 3. The extrapolated estimate of JA during maximal K(+)-induced depolarization made by varying length was 16%, while at 1.8 Lo it was 33% of the suprabasal JO2 at Lo. Calculated activation energies ranged from 17 to 45% of the suprabasal JO2 at Lo and from 72 to 87% of the suprabasal JO2 at 1.8 Lo under stimulation conditions that varied steady-state MRLC phosphorylation from 15 to 50%. 4. The results suggest that the kinetics of cross-bridge phosphorylation-dephosphorylation can rival those of cross-bridge cycling during isometric contractions in swine arterial smooth muscle.

Smooth muscle and skeletal muscle myosins produce similar unitary forces and displacements in the laser trap

Purified smooth muscle myosin in the in vitro motility assay propels actin filaments at 1/10 the velocity, yet produces 3-4 times more force than skeletal muscle myosin. At the level of a single myosin molecule, these differences in force and actin filament velocity may be reflected in the size and duration of single motion and force-generating events, or in the kinetics of the cross-bridge cycle. Specifically, an increase in either unitary force or duty cycle may explain the enhanced force-generating capacity of smooth muscle myosin. Similarly, an increase in attached time or decrease in unitary displacement may explain the reduced actin filament velocity of smooth muscle myosin. To discriminate between these possibilities, we used a laser trap to measure unitary forces and displacements from single smooth and skeletal muscle myosin molecules. We analyzed our data using mean-variance analysis, which does not rely on scoring individual events by eye, and emphasizes periods in the data with constant properties. Both myosins demonstrated multiple but similar event populations with discrete peaks at approximately +11 and -11 nm in displacement, and 1.5 and 3.5 pN in force. Mean attached times for smooth muscle myosin were longer than for skeletal-muscle myosin. These results explain much of the difference in actin filament velocity between these myosins, and suggest that an increased duty cycle is responsible for the enhanced force-generating capacity of smooth over skeletal-muscle myosin.

Smooth muscle myosin: a high force-generating molecular motor

Smooth muscle generates as much force per cross sectional area of muscle as skeletal muscle with only one-fifth the myosin content. Although this apparent difference could be explained at the tissue or cellular level, it is possible that at the molecular level smooth muscle cross-bridges generate greater average force than skeletal muscle cross-bridges. To test this hypothesis, we used an in vitro motility assay (VanBuren et al., 1994) in which either chicken thiophosphorylated gizzard smooth or pectoralis skeletal muscle monomeric myosin is adhered to a nitrocellulose surface. A fluorescently labeled actin filament, attached to an ultracompliant (50-200 nm/pN) glass microneedle, is brought in contact with the myosin surface. Isometric force, being generated by myosin cross-bridges pulling on the attached actin filament, is calculated from the extent to which the calibrated microneedle is deflected. By measuring the density of myosin adhered to the surface, we estimated the number of myosin cross-bridges that are able to interact with a length of actin filament in contact with the myosin surface. In a direct comparison between smooth and skeletal muscle myosin, the average force per cross-bridge was 0.8 and 0.2 pN, respectively. Surprisingly, smooth muscle myosin generates approximately 4 times greater average force per cross-bridge head than skeletal muscle myosin. Because average isometric force is the product of the cross-bridge unitary force and duty cycle, we are presently using a laser optical trap in an attempt to measure unitary events from single myosin molecules. This approach should allow us to determine whether an increase in unitary force, duty cycle, or both contribute to smooth muscle myosin's enhanced force-generating capacity compared with skeletal muscle myosin.

Smooth, cardiac and skeletal muscle myosin force and motion generation assessed by cross-bridge mechanical interactions in vitro

Differences in the mechanical properties of mammalian smooth, skeletal, and cardiac muscle have led to the proposal that the myosin isozymes expressed by these tissues may differ in their molecular mechanics. To test this hypothesis, mixtures of fast skeletal, V1 cardiac, V3 cardiac and smooth muscle (phosphorylated and unphosphorylated) myosin were studied in an in vitro motility assay in which fluorescently-labelled actin filaments are observed moving over a myosin coated surface. Pure populations of each myosin produced actin filament velocities proportional to their actin-activated ATPase rates. Mixtures of two myosin species produced actin filament velocities between those of the faster and slower myosin alone. However, the shapes of the myosin mixture curves depended upon the types of myosins present. Analysis of myosin mixtures data suggest that: (1) the two myosins in the mixture interact mechanically and (2) the same force-velocity relationship describes a myosin's ability to operate over both positive and negative forces. These data also allow us to rank order the myosins by their average force per cross-bridge and ability to resist motion (phosphorylated smooth > skeletal = V3 cardiac > V1 cardiac). The results of our study may reflect the mechanical consequence of multiple myosin isozyme expression in a single muscle cell.

Enhanced force generation by smooth muscle myosin in vitro

To determine whether the apparent enhanced force-generating capabilities of smooth muscle relative to skeletal muscle are inherent to the myosin cross-bridge, the isometric steady-state force produced by myosin in the in vitro motility assay was measured. In this assay, myosin adhered to a glass surface pulls on an actin filament that is attached to an ultracompliant (50-200 nm/pN) glass microneedle. The number of myosin cross-bridge heads able to interact with a length of actin filament was estimated by measuring the density of biochemically active myosin adhered to the surface; with this estimate, the average force per cross-bridge head of smooth and skeletal muscle myosins is 0.6 pN and 0.2 pN, respectively. Surprisingly, smooth muscle myosin generates approximately three times greater average force per cross-bridge head than does skeletal muscle myosin.

Extracellular Ca(2+)-dependent potentiation by cocaine of serotonin- and norepinephrine-induced contractions in rat vascular smooth muscle

Using front-surface fluorometry, we determined the effects of cocaine on force and cytosolic Ca2+ concentration ([Ca2+]i) in the rat aorta. We also examined the effects of cocaine on 45Ca2+ influx. Cocaine (10(-7) to 10(-4) M) alone did not alter the resting level of [Ca2+]i and force. Cocaine (< 10(-4) M), in a concentration-dependent manner, potentiated the 10(-6) M serotonin (5-HT)-induced or 10(-8) M norepinephrine (NE)-induced sustained increase in [Ca2+]i and force in the presence of extracellular Ca2+, whereas it had no potentiating effects in Ca(2+)-free solution. Similar potentiating effects of cocaine were observed in pharmacologically denervated strips. Cocaine (10(-5) M) produced a leftward shift of concentration-response curves for both 5-HT- and NE-induced increases in [Ca2+]i and force with no effect on the maximal response or the relations between [Ca2+]i and force. Cocaine (10(-5) M also accelerated the 45Ca2+ influx during activation by 10(-6) M 5-HT or by 10(-8) M NE. Cocaine (> 10(-3) M) inhibited 5-HT-, NE-, and high-K+ depolarization-induced contractions accompanied by decreases in [Ca2+]i in normal physiological salt solution and 5-HT- or NE-induced transient increase in [Ca2+]i and force in Ca(2+)-free physiological salt solution. Thus, low concentrations of cocaine potentiate NE- or 5-HT-induced contraction by augmenting the increase in [Ca2+]i. These potentiating effects may derive from either an increase in the affinity of the receptors to agonists or an increase in the Ca2+ influx. On the other hand, high concentrations of cocaine (> 10(-3) M) have a relaxant effect on vascular smooth muscle, as a result of a decrease in [Ca2+]i.

Role of extracellular and intracellular sources of Ca2+ in sarafotoxin S6b-induced contraction of strips of the rat aorta

1. The effect of sarafotoxin S6b (sarafotoxin), a vasoconstrictor peptide, on cytosolic Ca2+ concentration ([Ca2+]i) and force in rat aortic strips loaded with fura-2 was determined by front-surface fluorometry. The objective was to elucidate the role of extracellular and intracellular Ca2+ in the mechanism of action of this peptide. 2. In the presence of extracellular 1.25 mM Ca2+, sarafotoxin induced a biphasic response consisting of an initial rapid increase in [Ca2+]i followed by a secondary sustained increase. Tension developed slowly but was sustained during the application of sarafotoxin. Diltiazem (10 nM-0.1 mM) partially inhibited both the increases in [Ca2+]i and tension. 3. In the presence of extracellular Ca2+, the force developed in relation to the increase in [Ca2+]i ([Ca2+]i-force relationship) observed with sarafotoxin was much greater than that observed upon K+ depolarization. In the presence of diltiazem the sarafotoxin-induced [Ca2+]i-force relationship was shifted even further to the left. 4. In the absence of extracellular Ca2+, sarafotoxin induced a transient increase in [Ca2+]i and a sustained contraction. Extending the incubation time in Ca(2+)-free physiological solution, resulted in smaller responses. However, after 60 min in Ca(2+)-free solution, sarafotoxin induced a sustained contraction but no change in [Ca2+]i. This residual contraction was inhibited by H-7, which is known to inhibit protein kinase C. 5. After treatment with caffeine to reduce intracellular stored Ca2+, sarafotoxin could still elicit increases in [Ca2+]i and in tension, showing that the caffeine-sensitive intracellular Ca2+ store partially overlaps with the sarafotoxin-sensitive store. 6. We conclude that, in addition to those components of contraction dependent on extracellular- and on intracellularly stored Ca2 , sarafotoxin can also induce contraction without increasing [Ca2+],. This component may be partially linked to the activation of protein kinase C and may contribute, in part, to the leftward shift of the [Ca2+]i-force relationship in the presence of sarafotoxin.

Smooth and skeletal muscle actin are mechanically indistinguishable in the in vitro motility assay

Smooth muscle produces as much stress as skeletal muscle with less myosin. To determine if the actin isoforms specific to smooth muscle contribute to the enhanced force generation, the motility of actin filaments from smooth and skeletal muscle were compared in an in vitro assay in which single fluorescently labeled actin filaments slide over a myosin-coated coverslip. No difference was observed between the velocity of smooth versus skeletal muscle actin filaments over either smooth or skeletal muscle myosin over a large range of assay conditions (changes in pH, ionic strength, and [ATP]). Similarly, no difference was observed between the two actins when the filaments moved under load over mixtures of phosphorylated smooth and skeletal muscle myosin. Thus, it appears that the actin isoforms of smooth and skeletal muscle are mechanically indistinguishable in the motility assay and that smooth muscle's enhanced force generation may originate within the myosin molecule specific to smooth muscle.

Involvement of calponin and caldesmon in sustained contraction of arterial smooth muscle

The molecular mechanism of smooth muscle contraction was approached by a novel method, covalent 14C-labeling. Intra- and intermolecular protein interactions during contractile activity are reflected by changed reactivity of protein side chains; these can be detected by reagents which readily permeate through the muscle membrane without affecting the contractility and form covalent bonds with proteins in the muscle. The incorporation of 14CH2ICONH2 into proteins of 1-hour histamine contracted versus resting porcine carotid arterial muscles was determined. Out of fourteen 14C-labeled proteins analyzed, only two showed a change in reactivity during sustained contraction. The incorporation of 14CH2ICONH2 into calponin and caldesmon in contracted muscles was about 66% of that into these same proteins in resting muscles. A transformation of calponin and caldesmon molecules from an extended to a more compact conformation explains the decreased reactivity.

Adenosine 5'-triphosphate consumption by smooth muscle as predicted by the coupled four-state crossbridge model

We have proposed a four-state crossbridge model to explain contraction and the latch state in arterial smooth muscle. Ca(2+)-dependent crossbridge phosphorylation was the only postulated regulatory mechanism and the latchbridge (a dephosphorylated, attached crossbridge) was the only novel element in the model. In this study, we used the model to predict rates of ATP consumption by crossbridge phosphorylation (JPhos) and cycling (JCycle) during isometric and isotonic contractions in arterial smooth muscle; then we compared model predictions with experimental data. The model predicted that JPhos and JCycle were similar in magnitude in isometric contractions, and both increased almost linearly with myosin phosphorylation. The predicted relationship between isometric stress and ATP consumption was quasihyperbolic, but approximately linear when myosin phosphorylation was below 35%, in agreement with most of the available data. Muscle shortening increased the predicted values of JCycle up to 3.7-fold depending on shortening velocity and the level of myosin phosphorylation. The predicted maximum work output per ATP was 7.4-7.8 kJ/mol ATP and was relatively insensitive to changes in myosin phosphorylation. The predicted increase in JCycle with shortening was in agreement with available data, but the model prediction that work output per ATP was insensitive to changes in myosin phosphorylation was unexpected and remains to be tested in future experiments.

Effects of glibenclamide on cytosolic calcium concentrations and on contraction of the rabbit aorta

1. Using fluorometry of fura-2 and rabbit aortic strips, we studied the effects of glibenclamide (GLB), a sulphonylurea anti-diabetic drug and an inhibitor of opening of K+ channels, on cytosolic calcium concentrations ([Ca2+]i) and on force development. 2. Both high K(+)-depolarization and noradrenaline (NA) increased [Ca2+]i and force, in a concentration-dependent manner, in the presence of extracellular Ca2+ (1.25 mM). However, force development in relation to [Ca2+]i ([Ca2+]i-force relationship) observed with NA was much greater than that observed with K(+)-depolarization. 3. Pretreatment with GLB (10(-6)-10(-4) M) for 10 min partially inhibited, in a concentration-dependent manner, both [Ca2+]i elevation and the force development induced by 118 mM K(+)-depolarization or NA 10(-5) M in the presence of extracellular Ca2+. The [Ca2+]i-force relationship induced by both 118 mM K+ physiological salt solutions and by NA 10(-5) M in the GLB-treated strips overlapped that obtained in the non-treated strips, thereby suggesting that GLB has no effect on the Ca2(+)-sensitivity of the intracellular contractile apparatus. Only high concentrations (10(-4) M) of GLB decreased [Ca2+]i and the force, when applied after the force induced by 118 mM K+ PSS or NA 10(-5) M reached the maximum level. 4. In the absence of extracellular Ca2+, NA induced a transient increase in [Ca2+]i and in the force and these increases were inhibited when the vascular strips were pretreated with GLB for 10 min. The [Ca2+]i-force relationship obtained in the GLB-treated strips overlapped that in the non-treated ones. 5. An ATP-sensitive K+ channel opener, cromakalim (10-5M) reduced the increased [Ca2 + ]i and force induced by 25mm K+-depolarization and NA 10-SM. Subsequent application of GLB concentrationdependently reversed this relaxant effect of cromakalim on the NA-induced contraction (IC50 = 2x 10 7 M). Complete reversal of the effect was observed with 10IsM GLB. 6. We suggest that GLB inhibits both high K+-depolarization- and NA-induced contraction of the rabbit aorta, by decreasing [Ca2+]i and with no effect on the [Ca2+]i-force relationship. However, when NA-induced contractions were inhibited by a K+-channel opener, GLB reversed this inhibitory effect by inhibiting K+-channel opening and increasing [Ca2 +].

Isoform distribution and tissue contents of contractile and cytoskeletal proteins in hypertrophied smooth muscle from rat portal vein

Growth of the smooth muscle in the rat portal vein was initiated by an increased transmural pressure. After 7 days, the cross-sectional area of the vessel wall and the maximal active force of the longitudinal muscle layer had increased twofold. Electron microscopy showed that the cell cross-sectional area was increased, suggesting cellular hypertrophy. Increased amounts of intermediate (10 nm) filaments were observed in the hypertrophied cells. The hypertrophied vessels had decreased DNA content per unit wet weight compared with the control vessels (hypertrophied, 1.5 +/- 0.1; control, 1.9 +/- 0.1 micrograms/mg; p less than 0.01). Protein composition was studied with electrophoretic methods. Compared with control preparations the hypertrophied veins had similar myosin and actin contents per unit wet weight (myosin: hypertrophied, 4.4 +/- 0.8; control, 5.9 +/- 0.9; actin: hypertrophied 12.2 +/- 0.6; control, 11.8 +/- 1.0 mg/g). Two different forms of the myosin heavy chain were detected with 5% sodium dodecyl sulfate-polyacrylamide gels. The proportion of the lower molecular weight heavy chain relative to total heavy chain content was about 30% and similar in both preparations. The relation filamin/myosin was increased in the hypertrophied vessels. Pyrophosphate gel electrophoresis revealed two protein bands, with an increase in the slower migrating band in the hypertrophied vessels possibly reflecting an increase in filamin content in the extracts. In the control portal vein alpha-actin is the dominating isoform constituting about 55% of total actin. In hypertrophied vessels, alpha-actin decreased (by 15%) and gamma-actin increased (by 20%). The portal vein contained desmin and vimentin in a ratio of about 6:1. The hypertrophied vessels showed a marked increase in the amount of these proteins (desmin/actin: hypertrophied, 0.32; control, 0.14). In conclusion, during pressure-induced growth of the portal vein, contractile protein contents increase in proportion to the increase in weight. A change in isoforms of actin occurs but no evidence for a change in myosin isoforms was found. The structural proteins increase relative to tissue weight, possibly associated with the increased number of intermediate filaments demonstrated with electron microscopy.

Smooth muscle energetics and theories of cross-bridge regulation

The energetics of smooth muscle is characterized by low tension cost (rate of ATP utilization per isometric force/cross-section area), ranging from 100- to 500-fold less than skeletal muscle. The efficiency (ATP usage per work) of smooth muscle, although less well documented, is also somewhat (4-fold) less than skeletal muscle. Another well-known characteristic of smooth muscle is the linear relation between the steady-state of ATP utilization (JATP) and isometric force. Recently, Murphy and colleagues [C.-M. Hai and R. A. Murphy. Am. J. Physiol. 254 (Cell Physiol. 23) C99-C106, 1988] have put forth a kinetic model of cross-bridge regulation that predicts the time course of stress and myosin light chain phosphorylation (MLC-Pi). The energetics consequences of this model, in brief, are that the low tension cost is partly attributed to a slow detachment rate of the myosin cross bridge when dephosphorylated when attached to actin ("latch state"), whereas the lower efficiency is ascribed to a high rate of myosin phosphorylation-dephosphorylation inherent to a fit of data to this kinetic scheme. This latter corollary is somewhat controversial in light of current interpretations of smooth muscle energetics data. Using SCoP software (National Biomedical Simulation Resource, Duke University), we tested this model in terms of fitting existing data with respect to 1) is a high myosin-dephosphorylation adenosine triphosphatase (ATPase) necessary to fit the available data on the time course of stress and MLC-Pi?; and 2) can this model predict the observed linear relation between the steady-state rate of ATP hydrolysis (JATP) and isometric force?(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of okadaic acid on cytosolic calcium concentrations and on contractions of the porcine coronary artery

1. We investigated the effects of okadaic acid (OA), a phosphatase inhibitor derived from a 38-carbon fatty acid and isolated from the black sponge, genus Halichondria, on cytosolic Ca2+ concentration ([Ca2+]i) and tension developed in porcine coronary arterial strips loaded with fura-2. 2. Both in the presence (1.25 mM) and absence of extracellular Ca2+, OA (over 10(-6) M) induced a concentration-dependent, slow and progressive increase in tension. Calcium removal had no effect on the maximum level of tension, time between application of the drug and the onset of tension, or the time required to reach the maximum tension. However, there was a slight concentration-dependent increase in [Ca2+]i, only in the presence of extracellular Ca2+. 3. At a lower concentration that did not cause contraction or increase [Ca2+]i, OA (10(-6) M) inhibited tension development but not the Ca2+ transient on readmission of Ca2+ in 118 mM K+-depolarizing solution. OA inhibited the maximum levels of the developed tension, without affecting the KD value (598 +/- 204 nM for control vs 678 +/- 464 nM after OA treatment) or the Hill coefficient (1.78 +/- 0.10 for control vs 1.98 +/- 0.47 for OA treatment). 4. It is concluded that high concentrations of OA induce a contraction independent of extracellular Ca2+ and without any changes in [Ca2+]i. Lower concentrations of OA inhibit the Ca2+-dependent contractions. The lack of effect on KD values suggests that the [Ca2+]i-sensitivity of the contractile apparatus is not affected by this inhibition of contraction.