Identification and distribution of some developmental isoforms of myosin heavy chains in avian muscle fibres

Summary Two monoclonal antibodies that react with all the slow skeletal myosin heavy chains in the mammalian skeletal muscles appeared to react with only SM1 myosin heavy chain in the post-hatch muscles of chicken. Further studies on the developing chicken showed one of these two antibodies to react with an additional myosin heavy chain in the early embryonic skeletal muscle as well as with the cardiac muscle. It is concluded that this antibody identified a slow muscle-like embryonic isoform of myosin heavy chain during earlier stages of development. While this embryonic isoform was more abundant during early development, the synthesis of SM1 myosin heavy chain was restricted to only presumptive slow muscle cells.     

Myosin isozymes in avian skeletal muscles. II. Fractionation of myosin isozymes from adult and embryonic chicken pectoralis muscle by immuno-affinity chromatography

Chicken pectoralis consists primarily of large white fibres, which react exclusively with antibodies prepared against adult fast myosin. There is, however, a small region of uniformly red fibres which responds to antibodies against adult slow myosin as well as adult fast myosin. The myosin extracted from this red region is also heterogeneous as shown by the presence of both slow and fast light chains. By means of immunoadsorbents, it has been possible to separate the 'red myosin' into a 'fast' component and a 'slow' component. These two fractions have been characterized with respect to their light and heavy chain content by one-dimensional and two-dimensional gel electrophoresis. The myosin heavy chain was reduced to the smaller fragments required for electrophoresis by proteolytic degradation. We conclude from the electrophoretic patterns that the 'fast' and 'slow' myosin components from the pectoralis red region closely resemble the myosin from the white region of the pectoralis and the myosin from the slow anterior latissimus dorsi (ALD) muscle. The demonstration of a 'slow myosin' in adult pectoralis muscle raises the possibility that the crossreactivity of embryonic pectoralis myosin with anti-slow (ALD) myosin antibodies might be due to the presence of such slow components in embryonic chicken muscle. Direct isolation of a slow component from embryonic pectoralis was achieved by immunoadsorption, as described for adult mixed muscle myosin. Analysis of the subunit composition by gel electrophoresis shows an enrichment in adult-type slow light chains, but the heavy chain pattern is quite distinct from that of adult slow heavy chain. These studies suggest that several myosin isozymes exist in embryonic chicken pectoralis, but that none is identical to those myosins found in the different fibres of the adult pectoralis muscle.     

Morphological,histochemical, and myosin isoform analysis of the diaphragm of adult horses,Equus caballus

The horse provides an interesting model for study of the structure and function of the mammalian diaphragm. Multiple regions of diaphragm from seven adult horses were prepared for histochemistry, immunocytochemistry, myosin heavy chain electrophoresis, and native myosin electrophoresis. Two additional adults were dissected to demonstrate myofiber and central tendon morphology and stained for acetylcholinesterase to demonstrate motor endplates. All regions of the adult diaphragm were histochemically characterized by a preponderance of type I fibers with some type IIa fibers. Type IIb fibers were absent in all adult specimens. Myosin heavy chain electrophoresis supported the histochemical study: two isoform bands were present on SDS gels that comigrated at the same rate as rat type I and IIa myosin heavy chain isoforms. No isoform was determined to comigrate with rat type IIb heavy chain isoforms. Native myosin isoform analysis revealed two isoforms that comigrated with rat FM-4 and FM-3 (FM = fast myosin) and two isoforms that comigrated with rat SM-1 and SM-2 (SM = slow myosin) isoforms. In some samples, a third slow native myosin isoform was observed that comigrated at the same rate as the SM-3 of the equine biceps brachii muscle. This doublet (or “triplet”) of slow isoforms is unique to some horse muscles compared with other adult animals studied. It is not known if these multiple slow native myosin isoforms confer some functional advantage to the equine muscles. The adult equine diaphragm also differs in its morphology by having a large central tendon compared to that in other mammals, and is predominantly slow in fiber type and myosin isoform composition. © 1994 Wiley-Liss, Inc.     

Spatial and temporal patterns of myosin heavy chain expression in developing rat extraocular muscle

Summary The present study describes transitions in myosin heavy chain expression in the extraocular muscles of rats between the ages of E17 and adult. The unique phenotype of the extraocular muscle is reflected in its fibre type composition, which is comprised by six distinct profiles, each defined by location (orbital versus global layer) and innervation pattern (single versus multiple terminals). During extraocular muscle myogenesis, developmental myosin heavy chains were expressed in both primary and secondary fibres from embryonic day E17 through the first postnatal week. At this time, the downregulation of developmental myosin heavy chain isoforms began in the global layer in a fibre type-specific manner, reaching completion only after the first postnatal month. By contrast, developmental isoforms were retained in the overwhelming majority of orbital layer fibres into adulthood and expressed differentially along the length of these fibres. Fast myosin heavy chain was detected pre- and postnatally in developing secondary fibres and in all of the singly innervated fibre types and one of the multiply innervated fibre types in the adult. As many as four fast isoforms were detected in maturing extraocular muscle, including the extraocular muscle-specific myosin heavy chain. Slow myosin heavy chain was expressed in primary fibres throughout development and in one of the multiply innervated fibre types in the adult. In contrast, the pure fast-twitch retractor bulbi initially expressed slow myosin heavy chain in fibres destined to switch to the fast myosin heavy chain developmental programme. Based upon spatial and temporal patterns of myosin heavy chain isoform transitions, we suggest that epigenetic influences, rather than purely myogenic stage-specific factors, are critical in determining the unique extraocular muscle phenotype.     

Identification of alpha-cardiac myosin heavy chain mRNA and protein in extraocular muscle of the adult rabbit

Summary Extraocular muscles contain both fast-twitch and multiply-innervated, tonic-contracting fibres. In rat, these fibres collectively express numerous myosin heavy chain isoforms including fast-type embryonic and neonatal, adult slow twitch type I and fast twitch type II, and a fast isoform unique to extraocular muscle. Immunocytochemical and Western blotting results are presented which suggest that, in rabbit, an additional species, the -cardiac myosin heavy chain, is present. The immunoreactive species is found in all rabbit extraocular muscles and in the rotatory extraocular muscles is expressed in almost all fibres which do not contain a fast myosin heavy chain. Positive identification of this isoform as the -cardiac myosin heavy chain was obtained by sequencing a cloned PCR product derived from extraocular muscle mRNA unique to the 3-end of rabbit -cardiac myosin heavy chain mRNA. This is the first unequivocal demonstration of -cardiac myosin heavy chain expression in extraocular muscle.     

Nascent muscle fiber appearance in overloaded chicken slow-tonic muscle

The application of a weight overload to the humerus of chickens induces a hypertrophy of anterior latissimus dorsi (ALD) muscle fibers. This growth is accompanied by a rapid and almost complete replacement of one slow-tonic myosin isoform, SM-1, by another slow-tonic isoform, SM-2. In addition, a population of small fibers appears mainly in extrafascicular spaces and, concurrently, three additional myosin bands are detected by gel electrophoresis. Five antibodies against myosin heavy chain (MHC) isoforms were selected as immunocytochemical probes to determine the cellular location and nature of these myosins. The antibodies react with ventricular, fast skeletal muscle and either SM-1 or SM-2, or both the slow-tonic MHCs. The antifast and antiventricular antibodies react with myosin present in the 10-day embryonic ALD muscle but do not react with myosin in posthatch ALD muscle. The small fibers in overloaded muscle contain a myosin isoform characteristically expressed during the embryonic stage of ALD muscle development and therefore are named nascent myofibers. Some of the nascent myofibers do not react with the antibody to both slow-tonic MHCs, indicating the lack of the normal adult slow-tonic myosins which are expressed in 10-day embryos. In order to explore the origin of the nascent fibers, an electron microscopic study was performed. Stereological analysis of the existing fibers shows a stimulation of numbers and sizes of satellite cells. In addition, the volume occupied by nonmuscle and undifferentiated cells increases dramatically. Myotube formation with incipient myofibrils is seen in extrafascicular spaces. These data suggest that new muscle fiber formation accompanies hypertrophy in overloaded chicken ALD muscle, and the process may involve satellite cell migration.     

Differences in myosin composition between human oro-facial,masticatory and limb muscles: enzyme-, immunohisto-and biochemical studies

Summary Immunohistochemistry was used to determine the myosin composition of defined fibre types of three embryologically different adult muscles, the oro-facial, masseter and limb muscles. In addition, the myosin composition in whole muscle specimens was analysed with biochemical methods. Both similarities and differences between muscles in the content of myosin heavy chains and myosin light chains were found. Nevertheless, each muscle had its own distinct identity. Our results indicated the presence of a previously undetected fast myosin heavy chain isoform in the oro-facial type II fibre population, tentatively termed fast F. The masseter contained aberrant myosin isoforms, such as foetal myosin heavy chain and -cardiac myosin heavy chain and unique combinations of myosin heavy chain isoforms which were not found in the limb or oro-facial muscles. The type IM and IIC fibres coexpressed slow and fast A myosin heavy chains in the oro-facial and limb muscles but slow and a fast B like myosin heavy chain in the masseter. While single oro-facial and limb muscle fibres contained one or two myosin heavy chain types, single masseter fibres coexpressed up to four different myosin heavy chain isoforms. Describing the fibres according to their expression of myosin heavy chain isozymes, up to five fibre types could be distinguished in the oro-facial and limb muscles and eight in the masseter. Oro-facial and limb muscles expressed five myosin light chains, MLC_1S, MLC_2S, MLC_1F, MLC_2F and MLC_3F, and the masseter four, MLC_1S, MLC_2S, MLC_1F, and, in addition, an embryonic myosin light chain, MLC_temb, which is usually not present in normal adult skeletal muscle. These results probably reflect the way the muscles have evolved to meet the specialized functional requirements imposed upon them and are in agreement with the previously proposed concept that jaw and limb muscles belong to two distinct allotypes.     

Myosin isozymes in avian skeletal muscles. I. Sequential expression of myosin isozymes in developing chicken pectoralis muscles

Myosin has been purified from chicken pectoralis muscle at various stages of development, from 10 days' incubation to approximately 10 months after hatching. Embryonic myosin from the earliest stage showed a high level of ATPase activity, similar to that obtained for adult pectoralis myosin. Two-dimensional peptide mapping of partial chymotryptic digests showed, however, that is heavy chain is quite different from that of adult fast myosin. The immunological crossreactivity observed between embryonic myosin and adult fast (pectoralis) myosin is therefore due to shared antigenic determinants rather than the presence of any adult isoforms. In an accompanying paper we will show that embryonic myosin at 10 days' incubation is not a single species, but consists of at least two heavy chain isozymes. The minor fraction binds slow light chains preferentially, and appears to be largely responsible for the observed crossreactivity with slow (ALD) myosin. None of the embryonic myosins is equivalent to the adult forms. Prior to hatching, LC3f is present only in very small amounts (less than 5%), and the adult light chain pattern, containing LC1f and LC3f in equimolar amounts, is not generated until after one week post-hatching. At about that time a new heavy chain population is detected, different from either the embryonic heavy chain or the adult heavy chain. The adult heavy chain peptide pattern appears from about three weeks' post-hatching, but a map indistinguishable from that of adult myosin is not observed until about 26 weeks. None of the observed differences in peptide maps can be related to different strains of chicken; pectoralis myosin from adult White Rock gave an identical map to that from White Leghorn. Unexpectedly, posterior latissimus dorsi (PLD) myosin from White Leghorn appears to be different from pectoralis myosin from the same strain, despite the histochemical and immunocytochemical similarity of the two muscles. We conclude that myosin polymorphism is widespread in muscle tissue, and that the expression of myosin isozymes and their subunits is under developmental regulation.     

Fibre type composition of soleus and extensor digitorum longus muscles in normal female inbred Lewis rats

We have analysed the fibre type composition of soleus and extensor digitorum longus (EDL) muscles of normal female 4-6-month-old inbred Lewis rats. This rat strain is used in our ongoing study of the effects of thyroid hormone on myosin heavy chain (MyHC) isoform expression. On the basis of the mATPase reaction, soleus muscles contained 96.1 +/- 2.9% of type 1 fibres supplemented by 2A fibres. EDL muscles contained type 1 (5.5 +/- 1.0%), type 2A (18.8 +/- 1.7%) and type 2B (75.7 +/- 2.2%) fibres. Immunohistochemical analysis and SDS gel electrophoresis confirmed that most fibres in the soleus muscle expressed the type 1 (slow) MyHC isoform and that only a small proportion of fibres expressed the fast 2a MyHC isoform. Immunohistochemical analysis and SDS gel electrophoresis demonstrated that almost half of the 2B fibres of EDL muscles expressed the 2x/d MyHC isoform. In both muscle types, many fibres expressed more than one MyHC isoform. The content of slow fibres in the soleus muscle of female inbred Lewis rats was slightly higher than that reported for Wistar rats, but was considerably higher than that of Sprague-Dawley rats, whereas substantial differences were not found in the proportion of slow and fast fibre types in EDL muscles in these strains.     

Slow-to-fast transformation of denervated soleus muscle of the rat,in the presence of an antifibrillatory drug

The myofibrillar changes of rat denervated soleus muscle were studied in the presence and in the absence of an antifibrillatory drug. After bilateral sciaticotomy, a concentrated solution of procainamide hydrochloride was steadily released, by way of a miniosmotic pump, in the space between the soleus and the gastrocnemius muscles of one leg. Fibrillation activity of soleus muscles was checked electromyografically at 3- to 5-day intervals. On the 21st day following denervation the muscles were excised, stained for adenosine triphosphatase activity and analysed for myosin heavy chain (MHC) isoforms. In the denervated-procainamidetreated muscles fibrillation was consistently (–75% on average) depressed in comparison to the contralateral denervated muscles. Type 1 (slow) fibres and MHC isoform were also significantly reduced, to the advantage of type 2A (fast) fibres and MHC isoform. The results support the view that denervation inactivity, like other kinds of muscle inactivity, favours the expression of fast type myofibrillar isoforms, and that this effect is counteracted, at least partially, by the spontaneous activity of the denervated muscle.     

Expression of myosin heavy chain and of myogenic regulatory factor genes in fast or slow rabbit muscle satellite cell cultures

Summary We investigated the myogenic properties of rabbit fast or slow muscle satellite cells during their differentiation in culture, with a particular attention to the expression of myosin heavy chain and myogenic regulatory factor genes. Satellite cells were isolated from Semimembranosus proprius (slow-twitch muscle; 100% type I fibres) and Semimembranosus accessorius (fast-twitch muscle; almost 100% type II fibres) muscles of 3-month-old rabbits. Satellite cells in culture possess different behaviours according to their origin. Cells isolated from slow muscle proliferate faster, fuse earlier into more numerous myotubes and mature more rapidly into striated contractile fibres than do cells isolated from fast muscle. This pattern of proliferation and differentiation is also seen in the expression of myogenic regulatory factor genes. Myf5 is detected in both fast or slow 6-day-old cell cultures, when satellite cells are in the exponential stage of proliferation. MyoD and myogenin are subsequently detected in slow satellite cell cultures, but their expression in fast cell cultures is delayed by 2 and 4 days respectively. MRF4 is detected in both types of cultures when they contain striated and contractile myofibres. Muscle-specific myosin heavy chains are expressed earlier in slow satellite cell cultures. No adult myosin heavy chain isoforms are detected in fast cell cultures for 13 days, whereas cultures from slow cells express neonatal, adult slow and adult fast myosin heavy chain isoforms at that time. In both fast and slow satellite cell cultures containing striated contractile fibres, neonatal and adult myosin heavy chain isoforms are coexpressed. However, cultures made from satellite cells derived from slow muscles express the slow myosin heavy chain isoform, in addition to the neonatal and the fast isoforms. These results are further supported by the expression of the mRNA encoding the adult myosin heavy chain isoforms. These data provide further evidence for the existence of satellite cell diversity between two rabbit muscles of different fibre-type composition, and also suggest the existence of differently preprogrammed satellite cells.     

A novel myosin present in cat jaw-closing muscles

Summary Cat jaw-closing muscles (masseter, posterior and anterior temporalis) are physiologically much faster than fast-twitch muscles of the hind limb. They have a highly oxidative metabolism and contain an alkali-stable ATPase as shown by histochemical staining. These observations have been extended here using immunocytochemical, enzymatic and biochemical analyses of myosin from posterior temporalis muscles. Antisera against posterior temporalis myosin do not precipitate gastrocnemius or soleus muscle actomyosin in Ouchterlony double diffusion. Using enzyme-linked immunoadsorbant, antisera to posterior temporalis myosin do not cross-react with heavy chains from flexor hallucis longus (FHL) or soleus myosin at dilutions where the reaction with posterior temporalis heavy chain is easily detected. Myosin isolated from posterior temporalis has an ATPase activity in the presence of EDTA (K⁺-ATPase) or calcium ions, that is, more than twice the corresponding value for FHL myosin. The myosin is unusual in its high degree of lability on storage at 0° C: it loses over 25% of the K⁺-ATPase per day and aggregates readily. Although the myosin is activated over 80-fold by actin, the maximum velocity obtained by extrapolation to saturating actin concentrations is considerably below that obtained for FHL myosin. This probably reflects the marked instability of the myosin rather than its true actin-activated ATPase. An alternative method of comparing actomyosin ATPase activity is by using myofibril preparations. Using conventional procedures it has not been possible to prepare myofibrils from posterior temporalis because of the extensive network of connective tissue surrounding the fibres. More drastic techniques, for example, the mini cell disruption chamber, will disrupt the fibres, but the myofibrils so produced are very small and often highly aggregated. Analysis of the myosin by gel electrophoresis shows that the LC1 and LC2 light chains of posterior temporalis myosin can be distinguished from those of soleus and FHL myosins. Polypeptide mapping of the various heavy chains also shows that posterior temporalis heavy chains are chemically different from those of the hind-limb muscle myosins.Fast-twitch oxidative fibres have been identified in hind limb and body muscles of the rat, cat and chicken; posterior temporalis myosin differs from these myosins also on the basis of its histochemical properties. We conclude that the myosin of jaw-closing muscles is phenotypically distinct from both fast-twitch oxidative and fast-twitch glycolytic muscles of the body and limbs, though similar super-fast fibres may occur elsewhere than in the jaw.     

Myosin heavy chain isoform transition in ageing fast and slow muscles of the rat.

Using gradient sodium dodecyl sulphate-polyacrylamide gel electrophoresis, myosin heavy chain (MHC) isoforms were studied in the extensor digitorum longus (EDL) and the soleus muscles of male Wistar rats at different ages (5, 10, 20 weeks, 1 and 2 years). In the EDL muscle, four types of MHC isoforms were observed in all age groups. There was an increase in the percentage of HCIId and a concomitant decrease in the percentage of HCIIb with increasing age. No significant difference was observed in the percentages of HCI and HCIIa isoforms in all the age groups. In contrast, the soleus muscle contained two MHC isoforms, HCI and HCIIa. There was an increase in the percentage of HCI and a concomitant decrease in the percentage of HCIIa with increasing age. These results suggest that age-related changes in the MHC isoforms in both the fast-twitch EDL and the slow-twitch soleus muscles are one factor underlying the age-related decrease in the speed of muscle contraction.     

Myosin heavy chain isoform transition in ageing fast and slow muscles of the rat

Sugiura , T., Matoba , H., Miyata , H., Kawai , Y. & Murakami , N. 1992. Myosin heavy chain isoform transition in ageing fast and slow muscles of the rat. Acta Physiol Scand 144 , 419423. Received 26 August 1 991 , accepted 3 December 1991. ISSN 0001–6772. Laboratory of Biomechanics and Physiology, Faculty of Liberal Arts, Yamaguchi University, Yamaguchi, Faculty of Integrated Arts and Sciences, Tokushima University, Tokushima, and Department of Physiology, Yamaguchi University School of Medicine, Ube, Yamaguchi, Japan. Using gradient sodium dodecyl sulphate-polyacrylamide gel electrophoresis, myosin heavy chain (MHC) isoforms were studied in the extensor digitorum longus (EDI,) and the soleus muscles of male Wistar rats at different ages ( 5, 10 , 20 weeks, 1 and 2 years). In the EDL muscle, four types of MHC isoforms were observed in all age groups. There was an increase in the percentage of HCIId and a concomitant decrease in the percentage of HCIIb with increasing age. No significant difference was observed in the percentages of HCI and HCIIa isoforms in all the age groups. In contrast, the soleus muscle contained two MHC isoforms, HCI and HCIIa. There was an increase in the percentage of HCI and a concomitant decrease in the percentage of HCIIa with increasing age. These results suggest that age-related changes in the MHC isoforms in both the fast-twitch EDI, and the slow-twitch soleus muscles are one factor underlying the age-related decrease in the speed of muscle contraction.     

Myosin heavy chain composition of the human lateral pterygoid and digastric muscles in young adults and elderly

The myosin heavy chain (MyHC) content in different parts of, two jaw opening muscle, the human lateral pterygoid and the digastric muscles of five young adult and five elderly subjects (mean age 22 and 73 years, respectively) was determined, using gel electrophoresis and immunohistochemical methods. The lateral pterygoid of both young and elderly contained predominantly slow MyHC, and fast A MyHC was the major fast isoform. In contrast, the digastric was composed of slow, fast A and fast X MyHCs in about equal proportions in both age groups. About half of the lateral pterygoid fibres contained mixtures of slow and fast MyHCs, often together with -cardiac MyHC. In the digastric, co-existence of slow and fast MyHCs was rare, and -cardiac MyHC was lacking. On the other hand, co-expression of fast A and fast X MyHCs was found more often in the digastric than in the lateral pterygoid. In both age groups about half of the digastric IIB fibres contained solely fast X MyHC. In the lateral pterygoid, type IIB fibres with pure fast X MyHC was found in only one subject. The lateral pterygoid in elderly showed a significant amount of fibres with solely fast A MyHC, which were occasionally found in young adults. In the digastric, no significant differences were found between young and elderly, although the muscles of elderly contained lower mean value of slow MyHC, as compared to that of young muscles. It is concluded that the lateral pterygoid and the digastric muscles differ not only in the MyHC composition but also in modifications of the MyHC phenotypes during aging, suggesting that they have separate roles in jaw opening function.     

Superfluousness of motor innervation for the formation of muscle spindles in neonatal rats

Summary Muscle spindles form de novo in reinnervated muscles of neonatal rats treated with nerve growth factor. Whether the spindles can also form in muscle reinnervated only by afferents was investigated by removing the lumbosacral segment of the spinal cord immediately after crushing the nerve to the medial gastrocnemius muscle at birth, and administering nerve growth factor for 10 days afterwards. As predicted, the medial gastrocnemius muscles were reinnervated by afferents, but not efferents. No motor endplates were visible on any muscle fibers, and extrafusal fibers were atrophied. The reinnervated muscles contained spindle-like encapsulations of one to four fibers at 5, 7, 9 and 30 days after the nerve crush. The number of spindles as well as encapsulated fibers exceeded that of normal medial gastrocnemius muscles. The encapsulated fibers resembled typical intrafusal fibers. They had normal sensory-muscle contacts, but no motor endings. The fibers displayed equatorial clusters of myonuclei and expressed the spindle-specific slow-tonic myosin heavy chain isoform at postnatal day 30. Thus, efferents are not essential for the formation and differentiation of muscle spindles in reinnervated muscles of neonatal rats.     

Comparative evolution of muscular dystrophy in diaphragm,gastrocnemius and masseter muscles from old male mdx mice

X chromosome-linked muscular dystrophic mdx mouse lacks the sarcolemmal protein dystrophin and represents a genetic homologue of human Duchenne muscular dystrophy (DMD). The present study analysed some aspects of pathological processes such as fibrosis, frequency of centralized nuclei, presence of degenerative or regenerative fibres, expression of utrophin and associated protein complexes, and myosin heavy chain isoforms in three muscles [diaphragm (DIA), gastrocnemius (GTC) and masseter (MAS)] from old male mdx mice. All parameters investigated comparatively in these pathological muscles provided evidence that the MAS mdx muscle presents a slight deterioration pattern in comparison to that of DIA and GTC muscles. Utrophin and associated proteins are present in many cell clusters with continuous membrane labelling in MAS muscle. Respective proportions of myosin heavy chain isoforms, measured by electrophoresis/densitometry, showed only slight change in GTC muscle, significant evolution in DIA muscle but drastic isoform conversions in MAS muscle. These results highlighted the difference in deterioration susceptibility of various muscles to muscular dystrophy. The reason why this occurs in MAS muscles is still obscure and discussed in terms of the comparative developmental origins of these muscles.     

Myosin heavy chain expression in developing rat intrafusal muscle fibers

The immunocytochemical expression of several isoforms of myosin heavy chains (MHC) was determined in developing intrafusal and extrafusal fibers of the soleus muscle of prenatal and postnatal rats. At the onset of spindle assembly, both bag2 intrafusal myotubes and primary extrafusal myotubes bound a slow-twitch MHC antibody, whereas the bag1 and chain myotubes expressed a fast-twitch MHC isoform identical to that expressed by secondary extrafusal myotubes. Subsequently, developing intrafusal fibers began to express unique myosin isoforms, and ceased to express some of the myosin isoforms present initially. The initial similarity in MHC composition of intrafusal and extrafusal fibers suggests that these two kinds of mammalian muscle cell originate from a common pool of bipotential myotubes. Differences in MHC expression by intrafusal and extrafusal fibers in adult muscles might result from the effect of sensory neurons on the developing intrafusal myotubes.     

Effects of 14 days of microgravity on fast hindlimb and diaphragm muscles of the rat

The purpose of the present study was to determine the effects of 14 days of microgravity on specific rat fast-twitch muscles, and to compare these data with previous data from rat fast-twitch muscles exposed to microgravity for 10 days (Kraemer et al. 2000). Hindlimb muscles containing predominately fast fibers [extensor digitorum longus (EDL), superficial “white” (GSW) and deep “red” (GDR) gastrocnemius] and the diaphragm (DIA) were removed from flight and ground-based control animals and analyzed for: muscle mass, fiber type distribution, cross-sectional area, and myosin heavy chain (MHC) isoform content. Gravitational unloading for 14 days caused significant decreases in muscle mass (8–9%) and cross-sectional area of almost all fiber types (10–35%) from both EDL and gastrocnemius muscles. However, microgravity had little effect on fiber type composition in these muscles with significant changes occurring only in the EDL type IID fiber population (9.5% decrease). Similarly, relative MHC isoform content was only slightly altered by exposure to microgravity (increased content of MHCIIa in flight EDL). No changes in area, fiber type percentages, or MHC isoform content were detected in the DIA following the 14-day spaceflight. Similar to data gathered following a 10-day spaceflight (Kraemer et al. 2000), the 14-day flight did not appear to cause significant slow-to-fast (I → IIA) or fast-to-faster (IIA → IID → IIB) transformations in hindlimb muscles containing predominantly fast-twitch fibers. However, the longer period of gravitational unloading did result in additional loss in muscle fiber cross-sectional area with involvement of more major fiber types.