Postnatal expression of myosin isoforms in an expiratory muscle--external abdominal oblique.

We studied the postnatal expression of heavy-chain (MHC) and native myosin isoforms in an expiratory abdominal muscle of the rat, the external abdominal oblique (EO). Moreover, we contrasted EO myosin expression with that of the costal diaphragm (DIA) to draw inspiratory vs. expiratory muscle comparisons during development. Examination of MHC gels demonstrated a mature phenotype of slow and adult fast myosin isoforms at an earlier age in the EO (day 60) than in the DIA [day > 115 (adult)]. The mature MHC phenotype of the EO was characterized by a preponderance of MHC 2B, whereas the DIA was characterized by approximately equal portions of MHC slow, MHC 2A, and MHC 2X. During early postnatal development, there was a delay in the expression of MHC 2A in the EO compared with the DIA. However, MHC 2B, expressed later in development in both muscles, was noted in the EO before the DIA. We conclude that 1) the EO mature myosin phenotype is characterized by a preponderance of fast myosin isoforms and 2) the EO and DIA muscles are subject to different temporal patterns of isoform expression during postnatal development.     

Ontogeny of the Pectoralis muscle in the little brown bat,Myotis lucifugus

The ontogeny of a primary flight muscle, the pectoralis, in the little brown bat (Myotis lucifugus: Vespertilionidae) was studied using histochemical, immunocytochemical, and electrophoretic techniques. In fetal and early neonatal (postnatal age 1–6 days) Myotis, histochemical techniques for myofibrillar ATPase (mATPase) and antibodies for slow and fast myosins demonstrated the presence of two fiber types, here called types I and IIa. These data correlated with multiple transitional myosin heavy chain isoforms and native myosin isoforms demonstrated with SDS-PAGE and 4% pyrophosphate PAGE. There was a decrease in the distribution and number of type I fibers with increasing postnatal age. At postnatal age 8–9 days, the adult phenotype was observed with regard to muscle fiber type (100% type IIa fibers) and myosin isoform profile (single adult MHC and native myosin isoforms). This “adult” fiber type profile and myosin isoform composition preceeded adult function by about 2 weeks. For example, little brown bats were incapable of sustained flight until approximately postnatal day 24, and myofiber size did not achieve adult size until approximately postnatal day 25. Although Myotis pectoralis is unique in being composed of 100% type IIa fibers, transitional fiber types and isoforms were present. These transitional forms had been observed previously in other mammals bearing mixed adult muscle fibers and which undergo transitional stages in muscle ontogeny. However, in Myotis pectoralis, this transition transpires relatively early in development. © 1994 Wiley-Liss, Inc.     

Developmentally regulated expression of three slow isoforms of myosin heavy chain: diversity among the first fibers to form in avian muscle.

At least three slow myosin heavy chain (MHC) isoforms were expressed in skeletal muscles of the developing chicken hindlimb, and differential expression of these slow MHC isoforms produced distinct fiber types from the outset of skeletal muscle myogenesis. Immunohistochemistry with isoform-specific monoclonal antibodies demonstrated differences in MHC content among the fibers of the dorsal and ventral premuscle masses and distinctions among fibers before splitting of the premuscle masses into individual muscles (Hamburger and Hamilton Stage 25). Immunoblot analyses by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of myosin extracted from the hindlimb demonstrated the presence throughout development of different mobility classes of MHCs with epitopes associated with slow MHC isoforms. Immunopeptide mapping showed that one of the MHCs expressed in the embryonic limb was the same slow MHC isoform, slow MHC1 (SMHC1), that is expressed in adult slow muscles. SMHC1 was expressed in the dorsal and ventral premuscle masses, embryonic, fetal, and some neonatal and adult hindlimb muscles. In the embryo and fetus SMHC1 was expressed in future fast, as well as future slow muscles, whereas in the adult only the slow muscles retained expression of SMHC1. Those embryonic muscles destined in the adult to contain slow fibers or mixed fast/slow fibers not only expressed SMHC1, but also an additional slow MHC not previously described, designated as slow MHC3 (SMHC3). Slow MHC3 was shown by immunopeptide mapping to contain a slow MHC epitope (reactive with mAb S58) and to be structurally similar to a MHC expressed in the atria of the adult chicken heart. SMHC3 was designated as a slow MHC isoform because (i) it was expressed only in those muscles destined to be of the slow type in the adult, (ii) it was expressed only in primary fibers of muscles that subsequently are of the slow type, and (iii) it had an epitope demonstrated to be present on other slow, but not fast, isoforms of avian MHC. This study demonstrates that a difference in phenotype between fibers is established very early in the chicken embryo and is based on the fiber type-specific expression of three slow MHC isoforms.     

Myosin isoform transitions in regeneration of fast and slow muscles during postnatal development of the rat

Regeneration of rat fast (gastrocnemius medialis) and slow (soleus) muscles was examined after degeneration of myofibers had been achieved by injection of cardiotoxin into the hindleg during the first week after birth. Myogenesis in the regenerating muscles was compared to postnatal myogenesis in the contralateral and in control muscles. Synthesis of embryonic and neonatal myosin isoforms was initiated 3 days after injury. These forms were gradually replaced by the intermediate and fast adult isoforms (type II fiber myosins), whose synthesis followed the same curve in regenerating, contralateral, and control muscles. In contrast, synthesis of the slow myosin isoform (type I fiber myosin) was greatly delayed in injured muscles, but eventually became equal to its synthesis in contralateral and control muscles. It therefore appears that synthesis of type II fiber myosins is similarly regulated, probably by thyroid hormone, in developing regenerating and normal muscles, while synthesis of type I fiber myosin depends on other factor(s).     

Co-expression of multiple myosin heavy chain genes, in addition to a tissue-specific one, in extraocular musculature

We have investigated the developmental transitions of myosin heavy chain (MHC) gene expression in the rat extraocular musculature (EOM) at the mRNA level using S1-nuclease mapping techniques and at the protein level by polypeptide mapping and immunochemistry. We have isolated a genomic clone, designated lambda 10B3, corresponding to an MHC gene which is expressed in the EOM fibers (recti and oblique muscles) of the adult rat but not in hind limb muscles. Using cDNA and genomic probes for MHC genes expressed in skeletal (embryonic, neonatal, fast oxidative, fast glycolytic, and slow/cardiac beta-MHC), cardiac (alpha-MHC), and EOM (lambda 10B3) muscles, we demonstrate the concomitant expression at the mRNA level of at least six different MHC genes in adult EOM. Protein and immunochemical analyses confirm the presence of at least four different MHC types in EOM. Immunocytochemistry demonstrates that different myosin isozymes tend to segregate into individual myofibers, although some fibers seem to contain more than one MHC type. The results also show that the EOM fibers exhibit multiple patterns of MHC gene regulation. One set of fibers undergoes a sequence of isoform transitions similar to the one described for limb skeletal muscles, whereas other EOM myofiber populations arrest the MHC transition at the embryonic, neonatal/adult, or adult EOM-specific stage. Thus, the MHC gene family is not under the control of a strict developmental clock, but the individual genes can modify their expression by tissue-specific and/or environmental factors.     

A continuum of myofibers in adult rabbit extraocular muscle: force, shortening velocity, and patterns of myosin heavy chain colocalization

Extraocular muscle (EOM) myofibers do not fit the traditional fiber typing classifications normally used in noncranial skeletal muscle, in part, due to the complexity of their individual myofibers. With single skinned myofibers isolated from rectus muscles of normal adult rabbits, force and shortening velocity were determined for 220 fibers. Each fiber was examined for myosin heavy chain (MyHC) isoform composition by densitometric analysis of electrophoresis gels. Rectus muscle serial sections were examined for coexpression of eight MyHC isoforms. A continuum was seen in single myofiber shortening velocities as well as force generation, both in absolute force (g) and specific tension (kN/m(2)). Shortening velocity correlated with MyHCIIB, IIA, and I content, the more abundant MyHC isoforms expressed within individual myofibers. Importantly, single fibers with similar or identical shortening velocities expressed significantly different ratios of MyHC isoforms. The vast majority of myofibers in both the orbital and global layers expressed more than one MyHC isoform, with up to six isoforms in single fiber segments. MyHC expression varied significantly and unpredictably along the length of single myofibers. Thus EOM myofibers represent a continuum in their histological and physiological characteristics. This continuum would facilitate fine motor control of eye position, speed, and direction of movement in all positions of gaze and with all types of eye movements-from slow vergence movements to fast saccades. To fully understand how the brain controls eye position and movements, it is critical that this significant EOM myofiber heterogeneity be integrated into hypotheses of oculomotor control.     

Changes in actomyosin ATP consumption rate in rat diaphragm muscle fibers during postnatal development.

Early postnatal development of rat diaphragm muscle (Dia(m)) is marked by dramatic transitions in myosin heavy chain (MHC) isoform expression. We hypothesized that the transition from the neonatal isoform of MHC (MHC(Neo)) to adult fast MHC isoform expression in Dia(m) fibers is accompanied by an increase in both the maximum velocity of the actomyosin ATPase reaction (V(max) ATPase) and the ATP consumption rate during maximum isometric activation (ATP(iso)). Rat Dia(m) fibers were evaluated at postnatal days 0, 14, and 28 and in adults (day 84). Across all ages, V(max) ATPase of fibers was significantly higher than ATP(iso). The reserve capacity for ATP consumption [1 - (ratio of ATP(iso) to V(max) ATP(ase))] was remarkably constant ( approximately 55-60%) across age groups, although at day 28 and in adults the reserve capacity for ATP consumption was slightly higher for fibers expressing MHC(Slow) compared with fast MHC isoforms. At day 28 and in adults, both V(max) ATPase and ATP(iso) were lower in fibers expressing MHC(Slow) followed in rank order by fibers expressing MHC(2A), MHC(2X), and MHC(2B). For fibers expressing MHC(Neo), V(max) ATPase, and ATP(iso) were comparable to values for adult fibers expressing MHC(Slow) but significantly lower than values for fibers expressing fast MHC isoforms. We conclude that postnatal transitions from MHC(Neo) to adult fast MHC isoform expression in Dia(m) fibers are associated with corresponding but disproportionate changes in V(max) ATPase and ATP(iso).     

Expression of unique and developmental myosin heavy chain isoforms in adult human digastric muscle.

Digastric muscle (DGM) is a powerful jaw-opening muscle that participates in chewing, swallowing, breathing, and speech. For better understanding of its contractile properties, five pairs of adult human DGMs were obtained from autopsies and processed with immunocytochemistry and/or immunoblotting. Monoclonal antibodies against alpha-cardiac, slow tonic, neonatal, and embryonic myosin heavy chain (MHC) isoforms were employed to determine whether the DGM fibers contain these MHC isoforms, which have previously been demonstrated in restricted specialized craniocervical skeletal muscles but have not been reported in normal adult human trunk and limb muscles. The results showed expression of all these MHC isoforms in adult human DGMs. About half of the fibers reacted positively to the antibody specific for the alpha-cardiac MHC isoform in DGMs, and the number of these fibers decreased with age. Slow tonic MHC isoform containing fibers accounted for 19% of the total fiber population. Both the alpha-cardiac and slow tonic MHC isoforms were found to coexist mainly with the slow twitch MHC isoform in a fiber. A few DGM fibers expressed the embryonic or neonatal MHC isoform. The findings suggest that human DGM fibers may be specialized to facilitate performance of complex motor behaviors in the upper airway and digestive tract.     

Plasticity in Skeletal, Cardiac, and Smooth Muscle: Selected Contribution: Mechanisms underlying increased force generation by rat diaphragm muscle fibers during development

It has beenfound that maximum specific force (F_max; force percross-sectional area) of rat diaphragm muscle doubles from birth to 84 days (adult). We hypothesize that this developmental change inF_max reflects an increase in myosin heavy chain (MHC) content per half-sarcomere (an estimate of the number of cross bridgesin parallel) and/or a greater force per cross bridge in fibersexpressing fast MHC isoforms compared with slow and neonatal MHCisoforms (MHC_slow and MHC_neo, respectively).Single Triton 100-X-permeabilized fibers were activated at a pCa of4.0. MHC isoform expression was determined by SDS-PAGE. MHC content per half-sarcomere was determined by densitometric analysis and comparison to a standard curve of known MHC concentrations. MHC content per half-sarcomere progressively increased during early postnatal development. When normalized for MHC content per half-sarcomere, fibersexpressing MHC_slow and coexpressing MHC_neoproduced less force than fibers expressing fast MHC isoforms. Weconclude that lower force per cross bridge in fibers expressingMHC_slow and MHC_neo contributes to the lowerF_max seen in early postnatal development.

     

Origin of intrafusal muscle fibers in the rat

The expression of several isoforms of myosin heavy chain (MHC) by intrafusal and extrafusal fibers of the rat soleus muscle at different stages of development was compared by immunocytochemistry. The first intrafusal myotube to form, the bag2 fiber, expressed a slow-twitch MHC isoform identical to that expressed by the primary extrafusal myotubes. The second intrafusal myotube to form, the bag1 fiber, expressed a fast-twitch MHC similar to that initially expressed by the secondary extrafusal myotubes. At subsequent stages of development, the equatorial and juxtaequatorial regions of bag2 and bag1 intrafusal myofibers began to express a slow-tonic myosin isoform not expressed by extrafusal fibers, and ceased to express some of the MHC isoforms present initially. Myotubes which eventually matured into chain fibers expressed initially both the slow-twitch and fast-twitch MHC isoforms similar to some secondary extrafusal myotubes. In contrast, adult chain fibers expressed the fast-twitch MHC isoform only. Hence intrafusal myotubes initially expressed no unique MHCs, but rather expressed MHCs similar to those expressed by extrafusal myotubes at the same chronological stage of muscle development. These observations suggest that both intrafusal and extrafusal fibers develop from common pools of bipotential myotubes. Differences in MHC expression observed between intrafusal and extrafusal fibers of rat muscle might then result from a morphogenetic effect of afferent innervation on intrafusal myotubes.     

Fiber-type proportions in mammalian soleus muscle during postnatal development

We analyzed the fiber-type composition of the soleus muscle in rats and mice to determine whether the adult proportion of fiber types is fixed soon after birth or whether it changes during postnatal maturation. We examined muscles from animals varying in age from 1 week to 1 year using monoclonal antibodies that distinguish between fast and slow isoforms of myosin heavy chains. In cross sections of unfixed muscle containing profiles of all myofibers in the muscle, we counted the fibers that stained with antibodies to fast myosin, and in adjacent sections, those that stained positive with an antibody to slow myosin. We also counted the total number of fibers in each section. Rat soleus contained about 2500 myofibers, and mouse about 1000 at all ages studied, suggesting that myogenesis ceases in soleus by 1 week after birth or sooner. In mouse soleus, the relative proportions of fibers staining positive with fast and slow myosin antibodies were similar at all ages studied, about 60%–70% being fast and 30%–40% slow. In rat soleus, however, the proportions of fast antibody-positive and slow antibody-positive fibers changed dramatically during postnatal maturation. At 1 week after birth, about 50% of rat soleus fibers stained with fast myosin antibodies, whereas between 1 and 2 months this value fell to about 10%. In mouse, about 10% of fibers at 1 week, but none at 1 year, reacted with both fast and slow antibodies, whereas in rat, fewer than 3% bound both antibodies to a significant degree at 1 week. It is puzzling why, in rat soleus, the majority of apparently fast fibers present at 1 week is converted to a slow phenotype, whereas in mouse soleus the predominant change appears to be the suppression of fast myosin expression in a subset of fibers that expresses both myosin types at 1 week. It is possible that this may be related to differences in size and the amount of body growth between these two species.     

Fiber-type proportions in mammalian soleus muscle during postnatal development.

We analyzed the fiber-type composition of the soleus muscle in rats and mice to determine whether the adult proportion of fiber types is fixed soon after birth or whether it changes during postnatal maturation. We examined muscles from animals varying in age from 1 week to 1 year using monoclonal antibodies that distinguish between fast and slow isoforms of myosin heavy chains. In cross sections of unfixed muscle containing profiles of all myofibers in the muscle, we counted the fibers that stained with antibodies to fast myosin, and in adjacent sections, those that stained positive with an antibody to slow myosin. We also counted the total number of fibers in each section. Rat soleus contained about 2500 myofibers, and mouse about 1000 at all ages studied, suggesting that myogenesis ceases in soleus by 1 week after birth or sooner. In mouse soleus, the relative proportions of fibers staining positive with fast and slow myosin antibodies were similar at all ages studied, about 60%-70% being fast and 30%-40% slow. In rat soleus, however, the proportions of fast antibody-positive and slow antibody-positive fibers changed dramatically during postnatal maturation. At 1 week after birth, about 50% of rat soleus fibers stained with fast myosin antibodies, whereas between 1 and 2 months this value fell to about 10%. In mouse, about 10% of fibers at 1 week, but none at 1 year, reacted with both fast and slow antibodies, whereas in rat, fewer than 3% bound both antibodies to a significant degree at 1 week. It is puzzling why, in rat soleus, the majority of apparently fast fibers present at 1 week is converted to a slow phenotype, whereas in mouse soleus the predominant change appears to be the suppression of fast myosin expression in a subset of fibers that expresses both myosin types at 1 week. It is possible that this may be related to differences in size and the amount of body growth between these two species.     

Myosin heavy chain isoform composition in striated muscle after denervation and self-reinnervation

The total content of myosin heavy chains (MHC) and their isoform pattern were studied by biochemical methods in the slow-twitch (soleus) and fast-twitch (extensor digitorum longus) muscles of adult rat during atrophy after denervation and recovery after self-reinnervation. The pattern of fibre types, in terms of ultrastructure, was studied in parallel. After denervation, total MHC content decreased sooner in the slow-twitch muscle than in the fast-twitch. The ratio of MHC-1 and the MHC-2B isoforms to the MHC-2A isoform decreased in the slow and the fast denervated muscles, respectively. After reinnervation of the slow muscle, the normal pattern of MHC recovered within 10 days and the type 1 isoform increased above the normal. In the reinnervated fast muscle, the 2B/2A isoform ratio continued to decrease. Traces of the embryonic MHC isoform, identified by immunochemistry, were found in both denervated and reinnervated slow and fast muscles. A shift in fibre types was similar to that found in the MHC isoforms. Within 2 months of recovery a tendency to normalization was observed. The results show that (a) MHC-2B isoform and the morphological characteristics of the 2B-type muscle fibres are susceptible to lack of innervation, similar to those of type 1, (b) during muscle recovery induced by reinnervation the MHC isoforms and muscle fibres shift transiently to type 1 in the soleus and to type 2A in the extensor digitorum longus muscles, and (c) the embryonic isoform of MHC may appear in the adult skeletal muscles if innervation is disturbed.     

Coupled expression of troponin T and troponin I isoforms in single skeletal muscle fibers correlates with contractility

Striated muscle contraction is powered by actin-activated myosin ATPase. This process is regulated by Ca(2+) via the troponin complex. Slow- and fast-twitch fibers of vertebrate skeletal muscle express type I and type II myosin, respectively, and these myosin isoenzymes confer different ATPase activities, contractile velocities, and force. Skeletal muscle troponin has also diverged into fast and slow isoforms, but their functional significance is not fully understood. To investigate the expression of troponin isoforms in mammalian skeletal muscle and their functional relationship to that of the myosin isoforms, we concomitantly studied myosin, troponin T (TnT), and troponin I (TnI) isoform contents and isometric contractile properties in single fibers of rat skeletal muscle. We characterized a large number of Triton X-100-skinned single fibers from soleus, diaphragm, gastrocnemius, and extensor digitorum longus muscles and selected fibers with combinations of a single myosin isoform and a single class (slow or fast) of the TnT and TnI isoforms to investigate their role in determining contractility. Types IIa, IIx, and IIb myosin fibers produced higher isometric force than that of type I fibers. Despite the polyploidy of adult skeletal muscle fibers, the expression of fast or slow isoforms of TnT and TnI is tightly coupled. Fibers containing slow troponin had higher Ca(2+) sensitivity than that of the fast troponin fibers, whereas fibers containing fast troponin showed a higher cooperativity of Ca(2+) activation than that of the slow troponin fibers. These results demonstrate distinct but coordinated regulation of troponin and myosin isoform expression in skeletal muscle and their contribution to the contractile properties of muscle.     

Origin of intrafusal muscle fibers in the rat

Summary The expression of several isoforms of myosin heavy chain (MHC) by intrafusal and extrafusal fibers of the rat soleus muscle at different stages of development was compared by immunocytochemistry. The first intrafusal myotube to form, the bag₂ fiber, expressed a slow-twitch MHC isoform identical to that expressed by the primary extrafusal myotubes. The second intrafusal myotube to form, the bag₁ fiber, expressed a fast-twitch MHC similar to that initially expressed by the secondary extrafusal myotubes. At subsequent stages of development, the equatorial and juxtaequatorial regions of bag₂ and bag₁ intrafusal myofibers began to express a slow-tonic myosin isoform not expressed by extrafusal fibers, and ceased to express some of the MHC isoforms present initially. Myotubes which eventually matured into chain fibers expressed initially both the slow-twitch and fast-twitch MHC isoforms similar to some secondary extrafusal myotubes. In contrast, adult chain fibers expressed the fast-twitch MHC isoform only. Hence intrafusal myotubes initially expressed no unique MHCs, but rather expressed MHCs similar to those expressed by extrafusal myotubes at the same chronological stage of muscle development. These observations suggest that both intrafusal and extrafusal fibers develop from common pools of bipotential myotubes. Differences in MHC expression observed between intrafusal and extrafusal fibers of rat muscle might then result from a morphogenetic effect of afferent innervation on intrafusal myotubes.     

Developmental effects on myonuclear domain size of rat diaphragm fibers.

During early postnatal development in rat diaphragm muscle (Diam), significant fiber growth and transitions in myosin heavy chain (MHC) isoform expression occur. Similar to other skeletal muscles, Diam fibers are multinucleated, and each myonucleus regulates the gene products within a finite volume: the myonuclear domain (MND). We hypothesized that postnatal changes in fiber cross-sectional area (CSA) are associated with increased number of myonuclei so that the MND size is maintained. The Diam was removed at postnatal days 14 (P-14) and 28 (P-28). MHC isoform expression was determined by SDS-PAGE. Fiber CSA, myonuclear number, and MND size were measured using confocal microscopy. By P-14, significant coexpression of MHC isoforms was present with no fiber displaying singular expression of MHCNeo. By P-28, singular expression was predominant. MND size was not different across fiber types at P-14. Significant fiber growth was evident by P-28 at all fiber types (fiber CSA increased by 61, 93, and 147% at fibers expressing MHCSlow, MHC2A, and MHC2X, respectively). The number of myonuclei per unit of fiber length was similar across fibers at P-14, but it was greater at fibers expressing MHC2X at P-28. The total number of myonuclei per fiber also increased between P-14 and P-28 at all fiber types. Accordingly, MND size increased significantly by P-28 at all fiber types, and it became larger at fibers expressing MHC2X compared with fibers expressing MHCSlow or MHC2A. These results suggest that MND size is not maintained during the considerable fiber growth associated with postnatal development of the Diam.     

Subunit composition of rodent isomyosins and their distribution in hindlimb skeletal muscles

Three adult skeletal muscle sarcomeric myosin heavy chain (MHC) genes have been identified in the rat, suggesting that the expressed native myosin isoforms can be differentiated, in part, on the basis of their MHC composition. This study was undertaken to ascertain whether the five major native isomyosins [3 fast (Fm1, Fm2, Fm3), 1 slow (Sm), and 1 intermediate (Im)], typically expressed in the spectrum of adult rat skeletal muscles comprising the hindlimb, could be further differentiated on the basis of their MHC profiles in addition to their light chain composition. Results show that in muscles comprised exclusively of fast-twitch glycolytic (FG) fibers and consisting of Fm1, Fm2, and Fm3, such as the tensor fasciae latae, only one MHC, designated as fast type IIb, could be resolved. In soleus muscle, comprised of both slow-twitch oxidative and fast-twitch oxidative-glycolytic fibers and expressing Sm and Im, two MHC bands were resolved and designated as slow/cardiac beta-MHC and fast type IIa MHC. In muscles expressing a mixture of all three fiber types and a full complement of isomyosins, as seen in the plantaris, the MHC could be resolved into three bands. Light chain profiles were characterized for each muscle type, as well as for the purified isomyosins. These data suggest that Im (IIa) consists of a mixture of fast and slow light chains, whereas Fm (IIb) and Sm (beta) isoforms consist solely of fast- and slow-type light chains, respectively. Polypeptide mapping of denatured myosin extracted from muscles expressing contrasting isoform phenotypes suggests differences in the MHC primary structure between slow, intermediate, and fast myosin isotypes. These findings demonstrate that 1) Fm, Im, and Sm isoforms are differentiated on the bases of both their heavy and light chain components and 2) each isomyosin is distributed in a characteristic fashion among rat hindlimb skeletal muscles. Furthermore, these data suggest that the ratio of isomyosins in a given muscle or muscle region is of physiological importance to the function of that muscle during muscular activity.     

Regeneration after cardiotoxin injury of innervated and denervated slow and fast muscles of mammals. Myosin isoform analysis

The regeneration of adult rat and mouse slow (soleus) and fast (sternomastoid) muscles was examined after the degeneration of myofibers had been achieved by a snake venom cardiotoxin, under experimental conditions devised to spare as far as possible the satellite cells, the nerves, and the blood vessels of the muscles. Three days after the injury, no myosin was detectable in selected portions of the muscles. New myosins of embryonic, neonatal, and adult types started to be synthesized during the following two days. Adult myosins thus appeared more precociously than in development, which implies that the synthesis of myosin isoforms during regeneration does not entirely 'recapitulate' the sequence of myosin transitions observed during normal development. Two weeks after the injury, the isomyosin electrophoretic pattern displayed by regenerated muscles was already the same as that of control muscles; the normal adult pattern was therefore expressed more rapidly in regenerating than in developing muscles. Except for the synthesis of the slow isoform which was generally inhibited in denervated muscles, the same types of myosins were expressed during the early stages of regeneration in denervated as in innervated muscles; long-term denervation prevented however the qualitative and quantitative recovery of the normal myosin pattern.