Myosin heavy-chain-based isomyosins in developing, adult fast-twitch and slow-twitch muscles.

A modified method of electrophoresis under nondenaturing conditions made it possible to separate rat muscle extracts of defined myosin heavy chain (HC) and light chain (LC) composition into subsets of developmental, fast and slow myosin heavy-chain-based isomyosins. The fastest migrating isomyosins were the neonatal isomyosins (nM1, nM2, nM3), followed by the slightly slower migrating embryonic isomyosins (eM1, eM2, eM3, eM4). Of the nine adult fast isomyosins, the HCIIb-based isomyosins (FM1b, FM2b, FM3b) were the fastest migrating. These were followed by the HCIId-based isomyosins (FM1d, FM2d, FM3d). The HCIIa-based isomyosins (FM1a, FM2a, FM3a) were the slowest. Our results suggest that FM3a is identical with the so-called intermediate isomyosin (IM) described in the literature. The slow myosin heavy-chain-based isomyosins (SM1, SM2, SM3) migrated far behind the fast isomyosins. Whereas the gross electrophoretic mobilities of each of these isomyosin triplets is determined by the specific heavy chain complement, the different mobilities of the bands within each triplet result from different alkali light chain combinations. Thus, the fastest triplet bands of the neonatal (nM1) and adult fast isomyosins (FM1b, FM1d, FM1a) represent the LC3f homodimers, the slowest (nM3, FM3b, FM3d, FM3a) the LC1f homodimers, and the intermediate bands (nM2, FM2b, FM2d, FM2a) the LC1f/LC3f heterodimers. Different proportions of the adult fast isomyosin triplet bands indicate that the affinity for LC3f decreases in the order HCIIb, HCIId, HCIIa. The three slow isomyosins represent LC1sa (SM1) and LC1sb (SM3) homodimers and a LC1sa/LC1sb heterodimer (SM2). Circumstantial evidence suggests an inverse order in rabbit muscle where SM1 and SM3 most likely represent LC1sb and LC1sa homodimers, respectively.     

Developmental changes in myosin isoforms from slow and fast latissimus dorsi muscles in the chicken

In the course of muscle differentiation, changes in fibre-type population and in myosin composition occur. In this work, the expression of native myosin isoforms in developing fast-twitch (posterior latissimus dorsi; PLD) and slow-tonic (anterior latissimus dorsi; ALD) muscles of the chick was examined using electrophoresis under nondissociating conditions. The major isomyosin of 11-day-old embryonic PLD comigrated with the adult fast myosin FM3. Two additional components indistinguishable from adult fast FM2 and FM1 isomyosins appeared successively during the embryonic development. The relative proportion of these latter isoforms increased with age, and the adult pattern was established by the end of the 1st month after hatching. Between day 11 and day 16 of embryonic development, PLD muscle fibres also contained small amounts of slow isomyosins SM1 and SM2. This synthesis of slow isoforms may be related to the presence of slow fibres within the muscle. At all embryonic and posthatch stages, ALD was composed essentially of slow isomyosins that comigrated with the two slow components SM1 and SM2 identified in adult. Several studies have reported that the SM1:SM2 ratio decreases progressively throughout embryonic and posthatching development, SM2 being predominant in the adult. In contrast, we observed a transient increase in SM1:SM2 ratio at the end of embryonic life. This could reflect a transitional neonatal stage in myosin expression. In addition, the presence in trace amounts of fast isomyosins in developing ALD muscle could be related to the presence of a population of fast fibres within this muscle.     

Effect of anabolic steroids on skeletal muscle mass during hindlimb suspension

The efficacy of anabolic steroid treatment [0.3 or 0.9 mg nandrolone decanoate (Deca-Durabolin) per day] was examined in the context of sparing rodent fast-twitch plantaris and slow-twitch soleus muscle weight, sparing subcellular protein, and altering isomyosin expression in response to hindlimb suspension. Female rats were assigned to four groups (7 rats/group for 6 wk): 1) normal control (NC), 2) normal steroid (NS), 3) normal suspension (N-SUS), and 4) suspension steroid (SUS-S). Compared with control values for the plantaris and soleus muscles, suspension induced 1) smaller body and muscle weight (P less than 0.05), 2) losses in myofibril content (mg/muscle, P less than 0.05), and 3) shifts in the relative expression (expressed as %of total isomyosin) of isomyosins which favored lesser slow myosin and greater fast myosin isotypes (P less than 0.05). Steroid treatment of suspended animals (SUS-S vs. N-SUS) partially spared body and muscle weight (P less than 0.05) and spared plantaris but not soleus myofibril content (mg/muscle, P less than 0.05). However, steroid treatment did not modify the isomyosin pattern induced by suspension. In normal rats (NS vs. NC), steroid treatment enhanced body and plantaris muscle weight but not soleus weight (P less than 0.05) and did not alter isomyosin expression in either muscle type. Collectively these data suggest that in young female rats anabolic steroids 1) enhance the body weight and the weight of a fast-twitch ankle extensor in normal rats, 2) ameliorate the loss in body weight, fast-twitch muscle weight and protein content and slow-twitch muscle weight associated with hindlimb suspension.(ABSTRACT TRUNCATED AT 250 WORDS)     

Myosin structure and thyroidian control of myosin synthesis in urodelan amphibian skeletal muscle

Electrophoretic analysis in non-dissociating conditions reveals three types of myosin in adult urodelan amphibian skeletal muscles: 3 isoforms of fast myosin (FM), one isoform of intermediate myosin (IM) and one or two isoforms of slow myosin (SM). Each type is characterized by a specific heavy chain HCf (FM), HCi (IM) and HCs (SM), respectively. In all urodelan species, as in mammals, fast isomyosins associate HCf and the three fast light chains LC1f, LC2f, and LC3f. In most urodelan species the intermediate myosin contains LC1f and LC2f and can be considered as an homodimer of the alkali LC1f. However, in Euproctus asper, IM is characterized by the association of both slow and fast LC with HCi. Slow myosin is a hybrid molecule associating HCs with slow and fast LC. During metamorphosis, a myosin isoenzymic transition occurs consisting in the replacement of three larval myosins (LM) characterized by a specific heavy chain (HCI), by the adult isomyosins with lower electrophoretic mobilities. At the same time there is a change in the ATPase myofibrillar pattern, with the larval fiber types being replaced by adult fibers of types I, IIA and IIB. In the neotenic and perennibranchiate species, which do not undergo spontaneous metamorphosis, sexually mature larval animals present a change in the myosin isoenzymic profile, but no complete transition. The coexistence of larval and adult isomyosins and the persistence of transitional fibers of type IIC in the skeletal muscle are demonstrated. Experimental hypo- and hyperthyroidism indicate that thyroid hormone stimulates the regression of the larval isomyosins, possibly through indirect pathways. In contrast, the appearance and the persistence of the adult isomyosins seem to be independent of thyroid hormone. Thus, the control of the isoenzymic transition in the skeletal muscle of urodelan amphibians appears to imply indirect mechanisms, operating differently on each of the two phases of the complete transition.     

Characterization of myosin isoforms in satellite cell cultures from adult rat diaphragm, soleus and tibialis anterior muscles

Satellite cells were isolated by enzymatic dissociation and Percoll gradient centrifugation from adult rat diaphragm, soleus, and tibialis anterior muscles with fairly reproducible yields. Diaphragm and soleus muscle yielded approximately five times more satellite cells than tibialis anterior muscle. According to light microscopic criteria, no morphological differences existed between the satellite cell cultures of different origin. Contrary to the donor muscles, myotubes from the 10-day-cultured satellite cells contained a uniform myosin heavy chain (MHC) pattern with predominance of an immunochemically identified embryonic heavy chain. The three types of cultures displayed a typical embryonic light chain (LC) pattern with LC1emb, LC1f, LC2f, and traces of LC3f. The isomyosin pattern was characterized by four embryonic isomyosins, eM1-eM4, with similar distributions in the three cultures. In summary, these myosin analyses provide no evidence for the existence of satellite cell diversity among three rat muscles of different fiber-type composition, at least not under the applied in vitro conditions.     

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.     

Changes in skeletal-muscle myosin isoenzymes with hypertrophy and exercise.

The patterns of myosin isoenzymes in fast- and slow-twitch muscles of the rat hindlimb were studied, by pyrophosphate/polyacrylamide-gel electrophoresis, with hypertrophy (induced by synergist removal) and with spontaneous running exercise of 4 and 11 weeks duration. At 11 weeks, changes with hypertrophy in the slow-twitch soleus, composed of greater than 95% SM2 (slow myosin 2) in normal muscles, were minor, and consisted of an increase in the SM1 and SM1', and a loss of intermediate myosin (IM), an isoenzyme characteristic of Type IIa fibres [Fitzsimons & Hoh (1983) J. Physiol. (London) 343, 539-550]. The changes were dramatic, however, in the fast-twitch plantaris muscle. There was a 3-fold increase in the proportion of SM. In addition, IM became the predominant isoenzyme in the profile of hypertrophied plantaris by 4 weeks. These increases were balanced by decreases in the proportion of FM2 (fast myosin 2), with FM1 completely absent from the profile at 11 weeks. The changes in the plantaris with exercise were similar in direction but not as extensive as those with hypertrophy, and FM1 remained present at control levels throughout the study. When hypertrophy and exercise were combined, the increase in slow myosin was equal to the sum of the increases with each treatment alone. Changes at 4 weeks were intermediate between those of control and 11-week muscles. Peptide mapping of individual myosin isoenzymes showed that the heavy chains of IM were different from either fast or slow heavy chains. Furthermore, IM was found to be composed of a mixture of fast and slow light chains. These changes suggest that a transformation of myosin from fast to slow isoforms was in progress in the plantaris in response to hypertrophy, via a Type-IIa-myosin (IM) intermediate stage, a phenomenon similar to that occurring in chronically stimulated fast muscles during fast-to-slow transformation [Brown, Salmons & Whalen (1983) J. Biol. Chem. 258, 14686-14692].     

Absence of the iron-containing superoxide dismutase in mitochondria from mustard (Brassica campestris).

Human myosin from different skeletal muscles was analysed in a non-denaturing gel system, and the isoenzyme composition correlated with the histochemical composition of the muscle. Two components (SM1 and SM2) were associated with type 1 (slow-twitch) fibres, and three (FM1, FM2 and FM3) with type 2 (fast-twitch) fibres. Light-chain analysis was performed in sodium dodecyl sulphate/polyacrylamide gels. There are three light chains (LCs1a, LCS1b and LCs2) in type 1 fibres, and three (LCf1, LCf2 and LCf3) in type 2 fibres. LCf1 and LCs1b co-migrate in sodium dodecyl sulphate gels. The ratio of LCf3/LCf2 is correlated with the distribution of the individual fast isoenzymes. These results explain apparent discrepancies in the literature concerning the light-chain distribution of human myosin.     

Endurance training-induced changes in alkali light chain patterns in type IIB fibers of the rat.

The effects of endurance training on the expression of myosin were electrophoretically analyzed in the deep portion of vastus lateralis muscle from the rat. A 10-wk running program led to increases (P < 0.01) in myosin heavy chain (MHC) 2a and 2d with a decrease (P < 0.01) in MHC(2b). Training also evoked a rearrangement of the isomyosin pattern with decreases in fast isomyosin (FM) 1 (P < 0.01) and FM2 (P < 0.05) and a rise in intermediate isomyosin (P < 0.01). These changes were accompanied by a 61% decrease (P < 0.01) in myosin light chain (MLC) 3F (11.8 +/- 2.7 vs. 4.6 +/- 4.2%). Two-dimensional electrophoresis made it possible to separate the triplet of isomyosins (FMb) consisting of MHC(2b). Training elicited a 26% decrease (P < 0.05) in the FM1b fraction within FMb, i.e., FM1b/(FM1b + FM2b + FM3b) (24.2 +/- 5.5 vs. 18.0 +/- 4.3%). These changes resulted in a 10% decrease (P < 0.05) in the MLC(3F) fraction, i.e., MLC(3F)/(MLC(1F) + MLC(3F)), in FMb (44.9 +/- 4.5 vs. 40.3 +/- 3.2%). These results suggest that endurance training may exert the depressive effect on the contractile velocity of type IIB fibers and that a training-induced decrease in the contractile velocity of whole muscle may be caused by alterations in fast alkali MLC complements within a given fiber type as well as by transitions in MHC-based fiber populations.     

Fractional synthesis rates in vivo of skeletal-muscle myosin isoenzymes.

The synthesis rates of different myosin isoenzymes in a single muscle, and of the same isoenzymes in different muscles (soleus, masseter and plantaris), were measured. The rate of total protein synthesis was significantly higher in the soleus [greater than 95% slow myosin (SM)] than in the plantaris [greater than 95% fast myosin (FM)]. Two fast isoenzymes, FM2 and FM3, were synthesized at different rates in the masseter, and SM was synthesized at a faster rate than FM. Intermediate myosin had a synthesis rate similar to that of FM. There was a small but significant difference between the synthesis rates of the SM isoenzymes of the soleus and masseter muscles. FM3 was synthesized faster in the masseter than in the plantaris, whereas FM2 was synthesized faster in the plantaris than in the masseter.     

Effects of zero gravity on myofibril content and isomyosin distribution in rodent skeletal muscle

The purpose of this experiment was to investigate the effects of 12.5 days of zero gravity (0 g) exposure (Cosmos 1887 Biosputnik) on the enzymatic properties, protein content, and isomyosin distribution of the myofibril fraction of the slow-twitch vastus intermedius (VI) and the fast-twitch vastus lateralis (VL) muscles of adult male rats. Measurements were obtained on three experimental groups (n = 5 each group) designated as flight group (FG), vivarium control (VC), and synchronous control (SC). Body weight of the FG was significantly lower than that of the two control groups (P less than 0.05). Compared with the two control groups, VI weight was lower by 23% (P less than 0.10), whereas no such pattern was apparent for the VL muscle. Myofibril yields (mg protein/g muscle) in the VI were 35% lower in the FG than in controls (P less than 0.05), whereas no such pattern was apparent for the VL muscle. When myofibril yields were expressed on a muscle basis (mg/g x muscle weight), the loss of myofibril protein was more exaggerated and suggests that myofibril protein degradation is an early event in the muscle atrophy response to 0 g. Analysis of myosin isoforms indicated that slow myosin (Sm) was the primary isoform lost in the calculated degradation of total myosin. No evidence of loss of the fast isomyosins was apparent for either muscle following spaceflight. Myofibril ATPase activity of the VI was increased in the FG compared with controls, which is consistent with the observation of preferential Sm degradation. These data suggest that muscles containing a high percentage of slow-twitch fibers undergo greater degrees of myofibril protein degradation than muscles containing predominantly fast-twitch fibers in response to a relatively short period of 0 g exposure, and the primary target appears to be the Sm molecule.     

Comparative effects of hindlimb suspension and exercise on skeletal muscle myosin isozymes in rats

The purpose of this study was to ascertain the time course of changes, whilst suspending the hindlimb and physical exercise training, of myosin light chain (LC) isoform expression in rat soleus and vastus lateralis muscles. Two groups of six rats were suspended by their tails for 1 or 2 weeks, two other groups of ten rats each were subjected to exercise training on a treadmill for 9 weeks, one to an endurance training programme (1-h running at 20 m.min-1 5 days.week-1), and the other to a sprint programme (30-s bouts of running at 60 m.min-1 with rest periods of 5 min). At the end of these experimental procedures, soleus and vastus lateralis superficialis muscles were removed for myosin LC isoform determination by two-dimensional gel electrophoresis. Hindlimb suspension for 2 weeks significantly increased the proportion of fast myosin LC and decreased slow myosin LC expression in the soleus muscle. The pattern of myosin LC was unchanged in the vastus lateralis muscle. Sprint training or endurance training for 9 weeks increased the percentage of slow myosin LC in vastus lateralis muscle, whereas soleus muscle myosin LC was not modified. These data indicate that hindlimb suspension influences myosin LC expression in postural muscle, whereas physical training acts essentially on phasic muscle. There were no differences in myosin LC observed under the influence of sprint- or endurance-training programme.     

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.     

White muscle differentiation in the eel (Anguilla anguilla L.): changes in the myosin isoforms pattern and ATPase profile during post-metamorphic development

Myosin isoforms and their light and heavy chains subunits were studied in the white lateral muscle of the eel during the post metamorphic development, in relation with the myosin ATPase profile. At elver stage VI A1 the myosin isoforms pattern was characterized by at least two isoforms, FM3 and FM2. The fast isomyosin type 1 (FM1) appeared during subsequent development. It increased progressively in correlation with the increase in the level of the light chain LC3f. FM1 became predominant at stage VI A4. At the elver stage VI A1, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed at least two heavy chains, namely type II-1 and II-2. The type II-1 heavy chain disappeared in the yellow eel white muscle, and V8-protease peptide map showed the appearance of a minor heavy chain type II-3 as early as stage VI B. Comparison of myosin heavy chains and myosin isoforms patterns showed the comigration of different myosin isoforms during white muscle development. The myosin ATPase profile was characterized by a uniform pattern as far as stage VI A4. A mosaic aspect in white muscle was observed as early as stage VI B, showing the appearance of small acid labile fibers. This observation suggests that the type II-3 heavy chain is specific to the small fibers.     

Polymorphism of myofibrillar proteins of rabbit skeletal-muscle fibres. An electrophoretic study of single fibres.

Rabbit predominantly fast-twitch-fibre and predominantly slow-twitch-fibre skeletal muscles of the hind limbs, the psoas, the diaphragm and the masseter muscles were fibre-typed by one-dimensional polyacrylamide-gel electrophoresis of the myofibrillar proteins of chemically skinned single fibres. Investigation of the distribution of fast-twitch-fibre and slow-twitch-fibre isoforms of myosin light chains and the type of myosin heavy chains, based on peptide ''maps'' published in Cleveland. Fischer, Kirschner & Laemmli [(1977) J. Biol. Chem. 252, 1102-1106], allowed a classification of muscle fibres into four classes, corresponding to histochemical types I, IIA, IIB and IIC. Type I fibres with a pure slow-twitch-type of myosin were found to be characterized by a unique set of isoforms of troponins I, C and T, in agreement with the immunological data of Dhoot & Perry [(1979) Nature (London) 278, 714-718], by predominance of the beta-tropomyosin subunit and by the presence of a small amount of an additional tropomyosin subunit, apparently dissimilar from fast-twitch-fibre alpha-tropomyosin subunit. The myofibrillar composition of type IIB fast-twitch white fibres was the mirror image of that found for slow-twitch fibres in that the fast-twitch-fibre isoforms only of the troponin subunits were present and the alpha-tropomyosin subunit predominated. Type IIA fast-twitch red fibres showed a troponin subunit composition identical with that of type IIB fast-twitch white fibres. On the other hand, a unique type of myosin heavy chains was found to be associated with type IIA fibres. Furthermore, the myosin light-chain composition of these fibres was invariably characterized by a small amount of LC3F light chain and by a pattern that was either a pure fast-twitch-fibre light-chain pattern or a hybrid LC1F/LC2F/LC3F/LC1Sb light-chain pattern. By these criteria type IIA fibres could be distinguished from type IIC intermediate fibres, which showed coexistence of fast-twitch-fibre and slow-twitch-fibre forms of myosin light chains and of troponin subunits.     

Comparison of myosin heavy chains in atria and ventricles from hyperthyroid, hypothyroid, and euthyroid rabbits

The structural similarities of the heavy chains (HC) of myosin isolated from atria and ventricles of hyper-, hypo-, and euthyroid rabbits were compared by immunological analysis, by one- and two-dimensional peptide mapping, and by electrophoresis under nondenaturing conditions. Monoclonal and polyclonal antibodies, which are specific for HC alpha of ventricular myosin, cross-reacted equally with the HCs of euthyroid atrial myosin. Our immunological analysis identified multiple epitopes common to euthyroid atrial HC and ventricular HC alpha. One- and two-dimensional gel electrophoretic analysis of peptides produced by partial proteolytic digestion of each type of HC reveal no differences between euthyroid atrial HCs and ventricular HC alpha, whereas marked differences are apparent between atrial HC and ventricular HC beta. Nondenaturing gel electrophoresis separates ventricular myosin from hyper- and hypothyroid rabbits into two forms, V1 and V3, respectively. In euthyroid atria, two isomyosins, A1 and A2, were resolved; with the slower moving A2 isomyosin having the same mobility as that of the V1 isomyosin. After cross-hybridization of light chains of ventricular myosin with euthyroid atrial HCs, only a single band having a mobility identical with that of the V1 isomyosin was seen. Furthermore, atrial myosin HCs isolated from hyper- and hypothyroid rabbits were indistinguishable from euthyroid atrial HC and from ventricular HC alpha by these procedures. We conclude that ventricular HC alpha and atrial HC are indistinguishable proteins, and that the type of HC expressed in the atria is unaffected by the thyroid state of the rabbit.     

Effects of endurance exercise on isomyosin patterns in fast- and slow-twitch skeletal muscles

Although endurance training has been shown to profoundly affect the oxidative capacity of skeletal muscle, little information is available concerning the impact of endurance training on skeletal muscle isomyosin expression across a variety of muscle fiber types. Therefore, a 10-wk running program (1 h/day, 5 days/wk, 20% grade, 1 mile/h) was conducted to ascertain the effects of endurance training on isomyosin expression in the soleus, vastus intermedius (VI), plantaris (PLAN), red and white medial gastrocnemius (RMG and WMG), and red and white vastus lateralis muscles (RVL and WVL). Evidences of training were noted by the presence of a resting and a submaximal exercise bradycardia, as well as an enhancement in peak O2 consumption in the trained rodents relative to the nontrained controls. No evidence for skeletal muscle hypertrophy was observed subsequent to training when muscle weight was normalized to body weight. Shifts in the isomyosin profile of the trained VI, RMG, RVL, and PLAN were seen relative to the nontrained controls. Specifically, training affected the slow myosin (SM) composition of the VI by decreasing the relative content of the SM2 isoform by 14% while increasing that of the SM1 isoform (P less than 0.05). In addition, training elicited various degrees of a fast to slower myosin transformation in the RMG, RVL, and PLAN. All three muscles showed a significant reduction in the fast myosin 2 isoform (P less than 0.05), with significant increases in intermediate myosin in the RVL and PLAN along with elevations in SM2 in the RMG and PLAN (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Polymorphism of myosin light chains. An electrophoretic and immunological study of rabbit skeletal-muscle myosins.

Antibodies specific for rabbit fast-twitch-muscle myosin LCIF light chain were purified by affinity chromatography and characterized by both non-competitive and competitive enzyme-linked immunosorbent assay (ELISA) and a gel-electrophoresis-derived assay (GEDELISA). The antibodies did not cross-react with myosin heavy chains, and were weakly cross-reactive with the LC2F [5,5'-dithio-(2-nitrobenzoic acid)-dissociated] light chain and with all classes of dissociated light chains (LC1Sa, LC1Sb and LC2S), as well as with the whole myosin, from hind-limb slow-twitch muscle. The immunoreactivity of myosins with a truly mixed light-chain pattern (e.g. vastus lateralis and gastrocnemius) correlated with percentage content of fast-twitch-muscle-type light chains. A more extensive immunoreactivity was observed with diaphragm and masseter myosins, which were also characterized, respectively, by a relative or absolute deficiency of LC1Sa light chain. Furthermore, it was found that the LC1Sb light chain of masseter myosin is antigenically different from its slow-twitch-muscle myosin analogue, and is immunologically related to the LC1F light chain. Rabbit masseter muscle from its metabolic and physiological properties and the content, activity and immunological properties of sarcoplasmic-reticulum adenosine triphosphatase, is classified as a red, predominantly fast-twitch, muscle. Therefore our results suggest that the two antigenically different iso-forms of LC1Sb light chain are associated with the myosins of fast-twitch red and slow-twitch red fibres respectively.     

Myosin light chain patterns of individual fast and slow-twitch fibres of rabbit muscles

Summary Single muscle fibres were isolated by microdissection from freeze-dried samples of rabbit psoas and soleus muscles. The individual fibres were typed according to qualitative histochemical reactions for succinate dehydrogenase or NADH-tetrazolium reductase and for alkaline Ca²⁺-activated myofibrillar myosin ATPase after acid or alkaline preincubation. Methods are described for electrophoretic analysis by means of polyacrylamide disc electrophoresis in the presence of SDS of total myofibrillar proteins in single fibres after pre-extraction of soluble proteins. Fast-twitch white fibres revealed a myosin light chain pattern characteristic of fast-type myosin with three light chains of apparent molecular weights of 22,300 (LC₁), 18,400 (LC₂) and 16,000 (LC₃). Fast-twitch red fibres were indistinguishable in this respect from fast-twitch white fibres and showed an identical pattern of myosin light chains. Slow-twitch fibres could be characterized by a myosin light chain pattern typical of myosin of slow-twitch muscles with peptides of the apparent molecular weights of 23,500 (LC_1Sa), 23,000 (LC_1Sb) and 18,500 (LS_2S). Slow-twitch fibres isolated from soleus as well as from psoas muscle were indistinguishable with regard to their myosin light chain patterns, thus suggesting that fibres of the same histochemical type correspond in their myosin light chain patterns irrespective of their origin from different muscles.Dedicated to the memory of Ernest Gutmann who has contributed so much to our knowledge on differentiation of muscle and who died on August 6, 1977