Why are some muscles striated? A structural mechanism that amplifies shortening velocity.

Effects of speed and distance of muscle shortening on force depression during voluntary contractions

The effect of initial muscle length on the speed of shortening at different relative loads has been determined for the soleus and flexor digitorum longus (FDL) muscles of the cat. Isometric tetanic force-length relationships for both muscles were similar to those shown previously. The functional length range for FDL occurred at relatively long lengths, from optimum (100%) to 135% of optimum length; however, soleus worked at relatively short lengths from 60% to 110% of optimum length. In FDL the speed of shortening at any given load was relatively constant within the functional range, but at very short muscle lengths the speed of shortening declined. Soleus also showed a decline in the speed of shortening at all loads at short muscle lengths, which included the functional working range of the muscle. Speed of shortening at any given load was maximal at optimum length but tended to decline at low loads and long muscle lengths. It is concluded that in FDL even when the toes are at the extremity of their range, speed of muscle shortening is unaffected. Soleus may be relatively disadvantaged because its functional range extends over short muscle lengths. The results indicate that soleus is capable of making a significant contribution in standing and a slow walk, but that at faster gaits the contribution of soleus may be negligible.

Myosin was isolated from 14 different muscles (mammals, lower vertebrates, and invertebrates) of known maximal speed of shortening. These myosin preparations were homogeneous in the analytical ultracentrifuge or, in a few cases, showed, in addition to the main myosin peak, part of the myosin in aggregated form. Actin- and Ca++-activated ATPase activities of the myosins were generally proportional to the speed of shortening of their respective muscles; i.e. the greater the intrinsic speed, the higher the ATPase activity. This relation was found when the speed of shortening ranged from 0.1 to 24 muscle lengths/sec. The temperature coefficient of the Ca++-activated myosin ATPase was the same as that of the speed of shortening, Q10 about 2. Higher Q10 values were found for the actin-activated myosin ATPase, especially below 10°C. By using myofibrils instead of reconstituted actomyosin, Q10 values close to 2 could be obtained for the Mg++-activated myofibrillar ATPase at ionic strength of 0.014. In another series of experiments, myosin was isolated from 11 different muscles of known isometric twitch contraction time. The ATPase activity of these myosins was inversely proportional to the contraction time of the muscles. These results suggest a role for the ATPase activity of myosin in determining the speed of muscle contraction. In contrast to the ATPase activity of myosin, which varied according to the speed of contraction, the F-actin-binding ability of myosin from various muscles was rather constant.

1. Isotonic shortening of cat superficial lumbrical muscles was studied during maximal tetanic contractions of single motor units of identified types. For each motor unit, the maximal speed of contraction, Vmax, was determined by extrapolating to zero the hyperbolic relation between applied tension and speed of shortening. 2. The maximal speeds of shortening of motor units formed a continuum with the highest velocities observed for the fast fatigable motor units and the lowest for the slow motor units. 3. On average, the maximum speed of shortening increased with the tetanic tension developed by the motor units. 4. In motor units with isometric twitch contraction times less than 35 ms, these times showed a significant inverse correlation with Vmax. Progressively longer contraction times were associated with rather small changes in Vmax. 5. The implications of these findings on the speed of muscle shortening during motor-unit recruitment are discussed.

Body movement relies on skeletal muscles generating power to move limbs effectively. Power is defined as force multiplied by velocity: a muscle produces force at a specific velocity (the speed of muscle shortening) and this results in power. In diseases like Duchenne Muscular Dystrophy (DMD), the absence of dystrophin weakens muscles and impairs their shortening velocity, leading to decreased power and consequently, impaired movement. Additionally, the diaphragm and heart muscles are also affected in DMD, causing difficulty breathing and impaired cardiac function. Compromised cardiorespiratory function can ultimately lead to death. Given the complex etiology of DMD and the essential role of power in all affected muscles, it is crucial to assess potential treatments for their effectiveness in improving muscle function. This review focuses on fundamental physiological assays used to evaluate muscle function in skeletal and diaphragm muscles. Common assays include force-frequency, force-velocity, power, and eccentric protocols, which are conducted ex vivo , in situ , and in vivo in small rodents (such as mice and rats) and larger intermediate animal models such as the Golden Retriever Muscular Dystrophy dog. Existing data support the use of skeletal muscle contractile assays as objective tools for assessing the efficacy of treatments.

Myosin II self-assembles to form thick filaments that are attributed to its long coiled-coil tail domain. The present study has determined a region critical for filament formation of vertebrate smooth muscle and nonmuscle myosin II. A monoclonal antibody recognizing the 28 residues from the C-terminal end of the coiled-coil domain of smooth muscle myosin II completely inhibited filament formation, whereas other antibodies recognizing other parts of the coiled-coil did not. To determine the importance of this region in the filament assembly in vivo, green fluorescent protein (GFP)-tagged smooth muscle myosin was expressed in COS-7 cells, and the filamentous localization of the GFP signal was monitored by fluorescence microscopy. Wild type GFP-tagged smooth muscle myosin colocalized with F-actin during interphase and was also recruited into the contractile ring during cytokinesis. Myosin with the nonhelical tail piece deleted showed similar behavior, whereas deletion of the 28 residues at the C-terminal end of the coiled-coil domain abolished this localization. Deletion of the corresponding region of GFP-tagged nonmuscle myosin IIA also abolished this localization. We conclude that the C-terminal end of the coiled-coil domain, but not the nonhelical tail piece, of myosin II is critical for myosin filament formation both in vitro and in vivo.

Skeletal muscle in vertebrates is formed by two major routes, as illustrated by the mouse embryo. Somites give rise to myogenic progenitors that form all of the muscles of the trunk and limbs. The behavior of these cells and their entry into the myogenic program is controlled by gene regulatory networks, where paired box gene 3 (Pax3) plays a predominant role. Head and some neck muscles do not derive from somites, but mainly form from mesoderm in the pharyngeal region. Entry into the myogenic program also depends on the myogenic determination factor (MyoD) family of genes, but Pax3 is not expressed in these myogenic progenitors, where different gene regulatory networks function, with T-box factor 1 (Tbx1) and paired-like homeodomain factor 2 (Pitx2) as key upstream genes. The regulatory genes that underlie the formation of these muscles are also important players in cardiogenesis, expressed in the second heart field, which is a major source of myocardium and of the pharyngeal arch mesoderm that gives rise to skeletal muscles. The demonstration that both types of striated muscle derive from common progenitors comes from clonal analyses that have established a lineage tree for parts of the myocardium and different head and neck muscles. Evolutionary conservation of the two routes to skeletal muscle in vertebrates extends to chordates, to trunk muscles in the cephlochordate Amphioxus and to muscles derived from cardiopharyngeal mesoderm in the urochordate Ciona, where a related gene regulatory network determines cardiac or skeletal muscle cell fates. In conclusion, Eric Davidson’s visionary contribution to our understanding of gene regulatory networks and their evolution is acknowledged.

EditorialEmerging roles for estrogen in regulating skeletal muscle physiologyBridget Coyle-Asbil, Leslie M. Ogilvie, and Jeremy A. SimpsonBridget Coyle-AsbilDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CanadaIMPART Investigator Team Canada, Saint John, New Brunswick, Canada, Leslie M. OgilvieDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CanadaIMPART Investigator Team Canada, Saint John, New Brunswick, Canada, and Jeremy A. SimpsonDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CanadaIMPART Investigator Team Canada, Saint John, New Brunswick, CanadaPublished Online:13 Feb 2023https://doi.org/10.1152/physiolgenomics.00158.2022This is the final version - click for previous versionMoreSectionsPDF (682 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations Skeletal muscle mass and strength decline with age, which contributes to impaired balance and reduced mobility, leading to falls. In fact, measures of muscle mass and strength are independent predictors of overall health in men and women (1). Throughout aging, there are sex-specific differences in skeletal muscle function, where declines in muscle strength occur at an earlier age in females than in males (2). Interestingly, this decline in muscle function in females is associated with reductions in circulating estrogen levels—a result of menopause (3–5). This observation is compelling and introduces sex hormones as a mechanism that directly regulates skeletal muscle physiology.Posttranslational modifications (PTMs) are chemical modifications to proteins that change the structure and subsequent function of proteins. There are over 200 different PTMs, which directly influence protein-protein interactions that subsequently drive the phenotype of the organ. One of the most common PTMs of muscle is phosphorylation, the addition of a phosphate group to either serine, or threonine, or tyrosine residues. Protein phosphorylation leads to the activation or inactivation of a protein and impacts homeostatic cell signaling. Modifications to the skeletal muscle phosphoproteome are involved in coordinating muscle growth, metabolism, repair, and contraction to generate force (6, 7). Throughout aging, there are modifications to the skeletal muscle phosphoproteome, indicating a connection between the muscle phosphoproteome and muscle function (8, 9). For example, phosphorylation levels of myosin light chain 2 and tropomyosin α, proteins in the contractile machinery, were increased in skeletal muscle of aged rats (8). Although several studies have investigated how protein phosphorylation impacts muscle function (10–13), these have been carried out largely in male subjects. Despite this, sex hormones are emerging as a novel mechanism regulating skeletal muscle function, which opens up new therapeutic avenues to target age-associated muscle dysfunction, enhance performance, and improve injury recovery. However, what remains to be discovered is the target protein and the specific amino acids that are directly phosphorylated by sex hormones (Fig. 1).Figure 1.Role of sex hormones in regulating skeletal muscle physiology in mice. Sex hormones (e.g., estrogen, testosterone, and progesterone) alter protein signaling through posttranslational modifications, isoform shifting, and de novo synthesis, which impacts skeletal muscle molecular signaling of sarcomeric proteins, transcription factors, calcium signaling, and metabolic proteins. However, the specific impact of each individual sex hormone on protein alterations and their functional impacts remains unknown. [Image created with BioRender.com and published with permission.]Download figureDownload PowerPointIn a recent issue of Physiological Genomics, Peyton and colleagues (14) performed a global phosphoproteomic analysis of tibialis anterior muscle in ovarian-hormone deficient mice to determine whether the decline of estrogen in females alters the skeletal muscle phosphoproteome. Using a mass spectrometric approach, they identified 22 proteins that are differentially phosphorylated in ovariectomized (OVX) versus sham mice, establishing that ovarian-derived sex hormones are involved in regulating the skeletal muscle phosphoproteome under resting conditions (i.e., noncontracting muscle). Pathway enrichment analysis, performed to gain further insight into the functional impact of the differentially regulated phosphopeptides in estrogen-deficient mice, showed an overrepresentation in proteins associated with calcium (e.g., calcium ion channel activity, myofilament calcium sensitivity, intracellular calcium release, and reuptake), metabolic signaling (e.g., insulin signaling, glycolysis, and gluconeogenesis), and cellular functions related to muscle maintenance and cytoskeletal integrity (e.g., sarcomeric organization and contractile function). As these signaling pathways are critical to maintaining normal muscle function, remodeling of the muscle phosphoproteome will have consequences on the protein’s function. These findings lay the molecular foundation linking sex hormones to muscle physiology and outline critical next steps to examining the functional impact of sex hormone depletion on skeletal muscle in females.Our current understanding of how sex hormones impact basic biological functions continues to expand. Sex hormones play important physiological roles that extend beyond their well-established reproductive functions. In fact, estradiol, the ovarian-derived form of estrogen, alters signaling pathways in the musculoskeletal, cardiovascular, and central nervous systems in both females and males (15–17). For example, in male rats, estrogen decreased eccentric cardiac hypertrophy following chronic volume overload, a model of cardiac stress (18). Thus, as estrogen levels are altered throughout aging, this has functional consequences on these biological systems (17, 19–22). As such, further investigation into how estrogen regulates pathways involved in skeletal muscle function may be of benefit to both sexes.The current issue follows their previous work, where Lai et al. (23) showed that OVX decreases phosphorylation of skeletal myosin regulatory light chain (RLC), which impairs muscle contractility in female mice. Furthermore, they demonstrated that muscle contractility and RLC protein phosphorylation are restored with the administration of exogenous estradiol, confirming that the effects of OVX on muscle function are estrogen-mediated rather than other ovarian-derived hormones (i.e., progesterone and testosterone). Although their previous work establishes that estrogen regulates muscle physiology at a sarcomeric level, a broader and unbiased investigation into the effects of estrogen on muscle proteins was lacking until now. Here, Lai et al. (23) present a comprehensive phosphoproteomic profiling of skeletal muscle and provide compelling evidence that sex hormones are involved in regulating signaling pathways such as energy metabolism, calcium signaling, and protein trafficking, which are critical for maintaining normal muscle function. Their results provide fundamental knowledge of female physiology and the impact of female sex hormones on skeletal muscle at a molecular level. These findings present a critical first step in identifying the molecular intermediates involved in driving specific muscle phenotypes. It is intriguing to consider the consequences of estrogen deficiency in older females and the impact this has on their strength, mobility, and quality of life. It will be of great interest to investigate the effects of sex hormones on other tissues such as cardiac and respiratory muscles.As estrogen is involved in regulating a variety of physiological mechanisms, it is imperative to investigate how the loss of ovarian-derived estrogen alters the regulation of normal physiology. Currently, there are two primary animal models used to evaluate the effects of estrogen loss in females. OVX, the surgical removal of one or both ovaries, has been the gold standard menopause model used to evaluate the effects of ovarian hormone loss, including estrogen, on various biological systems (24). However, when the ovaries are removed, the levels of other sex hormones (e.g., progesterone, testosterone, follicle-stimulating hormone, and luteinizing hormone) are also altered, which may influence the interpretation of how estrogen loss affects physiology. An emerging rodent model of menopause is produced by administering the chemical, 4-vinylcyclohexene diepoxide (VCD). VCD selectively targets and depletes the ovarian follicles, resulting in a hormone profile that is more similar to the natural menopause transition in women. However, VCD is toxic at high doses, which may present confounding effects on other organs if the appropriate administration doses are exceeded. Overall, the OVX and VCD models of menopause both provide the opportunity to study how ovarian-derived hormones influence biological systems. In both cases, it is important to consider the advantages of each model when designing research experiments and to understand the limitations to ensure that findings are interpreted in the appropriate biological context. Furthermore, how male sex hormones affect the phosphoproteome is an intriguing comparison that requires further consideration where orchiectomy, the surgical removal of the gonads and spermatic cord, may be used as a valuable model to examine changes in testosterone in humans with age.Throughout their manuscript, Peyton et al. (14) highlight insightful similarities between cardiac and skeletal muscle physiology and discuss the implications of how alterations in the phosphoproteome are the basis for various pathologies in both muscle types. Indeed, phosphoproteomic modifications in cardiac muscle are key features of several cardiomyopathies. For example, cardiac troponin I (cTnI) and RLC are sarcomeric proteins that play important roles in calcium regulation and mediating actin-myosin interactions to generate force. In heart failure, both of these proteins are dephosphorylated, leading to severe impairments in cardiac muscle contraction and relaxation (25–27). This demonstrates the importance of regulating protein phosphorylation to maintain normal muscle function. Furthermore, estrogen acts directly on the heart through various estrogen receptors localized in different cardiac cells (e.g., cardiomyocytes, endothelial cells, and cardiac fibroblasts) suggesting that sex hormones are also involved in regulating cardiac function. Estrogen has a protective effect on the heart contributing to a lower incidence of heart disease in females premenopause compared with aged-matched males and postmenopausal females when estrogen levels decline (28). The identification of altered protein phosphorylation patterns and the role of sex hormones in muscle physiology may reveal targets for therapeutic intervention to improve muscle weakness and dysfunction in both cardiac and skeletal muscle pathologies.Here, Peyton and colleagues (14) have presented the first global phosphoproteomic analysis in skeletal muscle examining phosphorylation modifications following OVX in female mice. Their results show that with OVX, the phosphorylation of proteins associated with calcium signaling, metabolic regulation, and sarcomere organization is altered, all of which are important for regulating normal muscle function. With the identification of these phosphoproteins that are regulated by sex hormones, research into the receptors, signaling pathways, and kinases involved is required along with which sex hormones are driving each specific change. We are eager to learn the functional impact of these phosphoproteomic changes on skeletal muscle. These data provide fundamental insight into how female sex hormones regulate pathways important for maintaining normal muscle function. This information will ultimately provide novel therapeutic pathways for targeting muscle weakness and improving muscle function. Overall, given the importance of integrating sex and gender into research, a specific focus on improving our understanding of female physiology provides new insight into how estrogen deficiency impacts the skeletal muscle phosphoproteome and the consequences for muscle strength and function in aging females.GRANTSThis work was supported by the Canadian Institutes of Health Research (CIHR), Natural Sciences and Engineering Research Council of Canada (NSERC), and Heart and Stroke Foundation of Canada grants to J. A. Simpson. B. Coyle-Asbil was supported by a Canada Graduate Scholarship-Master’s NSERC and L. M. Ogilvie was supported by an Alexander Graham Bell Canada Graduate Scholarship-Doctoral NSERC.DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the authors.AUTHOR CONTRIBUTIONSB.C.-A., L.M.O., and J.A.S. prepared figures; drafted manuscript; edited and revised manuscript; and approved final version of manuscript.REFERENCES1. Santilli V, Bernetti A, Mangone M, Paoloni M. Clinical definition of sarcopenia. Clin Cases Miner Bone Metab 11: 177–180, 2014. PubMed | Google Scholar2. Haynes EMK, Neubauer NA, Cornett KMD, O'Connor BP, Jones GR, Jakobi JM. Age and sex-related decline of muscle strength across the adult lifespan: a scoping review of aggregated data. Appl Physiol Nutr Metab 45: 1185–1196, 2020. doi:10.1139/apnm-2020-0081. Crossref | PubMed | ISI | Google Scholar3. Greising SM, Baltgalvis KA, Lowe DA, Warren GL. Hormone therapy and skeletal muscle strength: a meta-analysis. J Gerontol A Biol Sci Med Sci 64: 1071–1081, 2009. doi:10.1093/gerona/glp082. 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Download PDF Back to Top Next FiguresReferencesRelatedInformation Related ArticlesGlobal phosphoproteomic profiling of skeletal muscle in ovarian hormone-deficient mice 24 Oct 2022Physiological Genomics More from this issue > Volume 55Issue 2February 2023Pages 75-78 Crossmark Copyright & PermissionsCopyright © 2023 the American Physiological Society.https://doi.org/10.1152/physiolgenomics.00158.2022PubMed36622080History Received 8 November 2022 Accepted 5 January 2023 Published online 13 February 2023 Published in print 1 February 2023 Keywordsestrogenphosphoproteomesex hormonesskeletal muscle Metrics

Regulation of cholesterol metabolism in cultured cells and in the liver is dependent on actions of the LDL receptor. However, nonhepatic tissues have multiple pathways of cholesterol uptake. One possible pathway is mediated by LPL, an enzyme that primarily hydrolyzes plasma triglyceride into fatty acids. In this study, LDL uptake and tissue cholesterol levels in heart and skeletal muscle of wild-type and transgenic mice with alterations in LPL expression were assessed. Overexpression of a myocyte-anchored form of LPL in heart muscle led to increased uptake of LDL and greater heart cholesterol levels. Loss of LDL receptors did not alter LDL uptake into heart or skeletal muscle. To induce LDL receptors, mice were treated with simvastatin. Statin treatment increased LDL receptor expression and LDL uptake by liver and skeletal muscle but not heart muscle. Plasma creatinine phosphokinase as well as muscle mitochondria, cholesterol, and lipid droplet levels were increased in statin-treated mice overexpressing LPL in skeletal muscle. Thus, pathways affecting cholesterol balance in heart and skeletal muscle differ.

C2C12 cells offer a useful model to study the differentiation of non-muscle cells to skeletal muscle cells. Myosin phosphorylation and changes in related enzymes, with an emphasis on myosin phosphatase (MP) were analyzed over the first 6 days of C2C12 differentiation. There was a transition from myosin phosphatase target subunit 1 (MYPT1), predominant in the non-muscle cells to increased expression of MYPT2. Levels of MYPT1/2 were estimated, and both isoforms were higher in non- or partially differentiated cells compared to the concentrations in the differentiated isolated myotubes from day 6. A similar profile of expression was estimated for the type 1 protein phosphatase catalytic subunit, delta isoform (PP1c delta). Phosphatase activities, using phosphorylated smooth and skeletal muscle myosins, were estimated for total cell lysates and isolated myotubes. In general, smooth muscle myosin was the preferred substrate. Although the expression of MYPT1/2 and PP1c delta was considerably reduced in isolated myotubes the phosphatase activities were not reduced to corresponding levels. Most of the MP activity was due to PP1c, as indicated by okadaic acid. In spite of relatively high expression of MYPT1/2 and PP1c delta, marked phosphorylation of non-muscle myosin (over 50% of total myosin) was observed at day 2 (onset of expression of muscle-specific proteins) and both mono- and diphosphorylated light chains were observed. Partial inhibition of MLCK by 1-(5-chloronaphthalene-1-sulphonyl)-1H-hexahydro-1,4-diazepine HCl (ML-9) or by a construct designed from the autoinhibitory domain of MLCK, resulted in an increase in small myotubes (3-5 nuclei) after 3 days of differentiation and a decrease in larger myotubes (compared to control). The effect of ML-9 was not due to a reduction in intracellular Ca2+ levels. These results suggest that phosphorylation of non-muscle myosin is important in growth of myotubes, either in the fusion process to form larger myotubes or indirectly, by its role in sarcomere organization.

Fast and slow mammalian muscle myosins differ in the heavy chain sequences (MHC-2, MHC-1) and muscles expressing the two isoforms contract at markedly different velocities. One role of slow skeletal muscles is to maintain posture with low ATP turnover, and MHC-1 expressed in these muscles is identical to heavy chain of the beta-myosin of cardiac muscle. Few studies have addressed the biochemical kinetic properties of the slow MHC-1 isoform. We report here a detailed analysis of the MHC-1 isoform of the rabbit compared with MHC-2 and focus on the mechanism of ADP release. We show that MHC-1, like some non-muscle myosins, shows a biphasic dissociation of actin-myosin by ATP. Most of the actin-myosin dissociates at up to approximately 1000 s(-1), a very similar rate constant to MHC-2, but 10-15% of the complex must go through a slow isomerization (approximately 20 s(-1)) before ATP can dissociate it. Similar slow isomerizations were seen in the displacement of ADP from actin-myosin.ADP and provide evidence of three closely related actin-myosin.ADP complexes, a complex in rapid equilibrium with free ADP, a complex from which ADP is released at the rate required to define the maximum shortening velocity of slow muscle fibers (approximately 20 s(-1)), and a third complex that releases ADP too slowly (approximately 6 s(-1)) to be on the main ATPase pathway. The role of these actin-myosin.ADP complexes in the mechanochemistry of slow muscle contraction is discussed in relation to the load dependence of ADP release.

Insulin and insulin-like growth factor (IGF-I) receptor binding and tyrosine kinase activity were characterized in cardiac and skeletal muscles of several vertebrates. Specific insulin binding per unit weight of skeletal muscle was clearly higher in pigeon and rat than in ectothermic vertebrates (32 +/- 5 and 25 +/- 2.7%/100 mg initial tissue in pigeon and rat, respectively, vs. 4.4 +/- 0.2%/100 mg in carp samples). Insulin binding clearly predominated over IGF-I binding in skeletal muscle of endotherms (IGF-I binding was 7.7 +/- 0.5%/100 mg in rat). In ectothermic vertebrates the situation was reversed, and IGF-I binding was higher than insulin binding. In cardiac muscle, specific binding of both insulin and especially IGF-I was higher than the values found in skeletal muscle of the same species (IGF-I binding was 60 +/- 4, 103 +/- 2, and 20 +/- 3%/100 mg in carp, turtle, and rat, respectively). The tyrosine kinase activity of insulin and IGF-I receptors of all species studied presented basal phosphotransferase rates (250-1,600 fmol P.micrograms protein-1.30 min-1) and percentage of stimulation (150-520%) with clear differences between species. The present data suggest that insulin and IGF-I binding to skeletal and cardiac muscles change through the vertebrate scale in both quantity and activity.

Smooth muscle contraction is regulated by the phosphorylation of myosin. It is well known that tonic smooth muscles can maintain force with low energy consumption (latch state); however, the molecular mechanism underlying this phenomenon is unresolved. Here we show that single-head phosphorylated smooth myosin (SHPMII) exhibits fast ( approximately 24 s(-1)) and slow prolonged ( approximately 1 s(-1)) actin interactions, whereas double-head phosphorylated myosin (DHPMII) predominantly exhibits the fast ( approximately 29 s(-1)) interaction, suggesting that the phosphorylated head of SHPMII is mechanically as active as that of DHPMII. Both the fast and the slow actin interactions of SHPMII support the positive net mechanical displacement of actin. The actin translocating velocity of SHPMII was much slower than that of DHPMII, which is consistent with the slow actin interaction of SHPMII. We propose that the "latch state" can be explained by the motor characteristics of SHPMII that is present during the sustained phase of contraction.

The filament lattice of striated muscle is an overlapping hexagonal array of thick and thin filaments within which muscle contraction takes place. Its structure can be studied by electron microscopy or X-ray diffraction. With the latter technique, structural changes can be monitored during contraction and other physiological conditions. The lattice of intact muscle fibers can change size through osmotic swelling or shrinking or by changing the sarcomere length of the muscle. Similarly, muscle fibers that have been chemically or mechanically skinned can be compressed with bathing solutions containing very large inert polymeric molecules. The effects of lattice change on muscle contraction in vertebrate skeletal and cardiac muscle and in invertebrate striated muscle are reviewed. The force developed, the speed of shortening, and stiffness are compared with structural changes occurring within the lattice. Radial forces between the filaments in the lattice, which can include electrostatic, Van der Waals, entropic, structural, and cross bridge, are assessed for their contributions to lattice stability and to the contraction process.

Myosin and F-actin were prepared from bovine carotid arterial smooth muscle and the properties of the binding of myosin to F-actin were compared with those of the binding of skeletal muscle myosin to F-actin. The following differences were observed between skeletal and smooth muscle myosins. 1. The rate of ATP-induced dissociation of arterial actomyosin was equal to that of hybrid actomyosin reconstituted from arterial myosin and skeletal muscle F-actin, but was much lower than those of skeletal muscle actomyosin and of hybrid actomyosin reconstituted from skeletal muscle myosin and arterial F-actin. 2. The amount of ATP necessary for complete dissociation of arterial actomyosin was 2 mol/mol of myosin, although it is well known that skeletal muscle actomyosin is dissociated completely by the addition of 1 mol ATP per mol of myosin. 3. Arterial actomyosin and hybrid actomyosin reconstituted from arterial myosin and skeletal muscle F-actin did not dissociate upon addition of 0.1 mM PPi, while skeletal muscle actomyosin dissociated completely. 4. In the absence of Mg2+, neither dissociation by ATP nor ATPase [EC 3.6.1.3] activity was observed with arterial actomyosin and hybrid actomyosin reconstituted from arterial myosin and skeletal muscle F-actin. On the other hand, skeletal muscle actomyosin dissociated almost completely upon addition of ATP and showed a considerably high ATPase activity. These observations reveal marked differences between myosins from skeletal and smooth muscles in their binding properties to F-actin.