Persistent Expression of Developmental Myosin Heavy Chain Isoforms in the Tapered Ends of Adult Pigeon Pectoralis Muscle Fibres

We have shown previously that in addition to the adult myosin heavy chain (MyHC) isoform present throughout the length of each fast-twitch glycolytic muscle fibre within the pectoralis of the mature chicken, the neonatal isoform is retained in the tapered ends of these fibres. This work, however, has been the only published report of this phenomenon. Here, we tested the hypothesis that similar to the chicken, the ends of mature pigeon pectoralis muscle fibres contain developmental MyHC isoform(s). A histological stain was used to visualize endomysium to assist in the analysis of transverse sections of pectoralis muscle from four mature pigeons. Immunocytochemical techniques were used to localize MyHC isoform(s) characteristic of pigeon pectoralis development. We show that within mature pigeon pectoralis, the ends of both fast-twitch glycolytic and fast-twitch oxidative-glycolytic fibre types express MyHC isoform(s) characteristic of their earlier development. Thus, we extend our findings on chicken to another species and an additional muscle fibre type. Retention of developmental MyHC isoform(s) within the tapered ends of mature muscle fibres may be more widespread than is currently appreciated.     

New insights into skeletal muscle fibre types in the dog with particular focus towards hybrid myosin phenotypes

Electrophoresis, immunoblots, immunohistochemistry and image analysis methods were applied to characterise canine trunk and appendicular muscle fibres according to their myosin heavy chain (MyHC) composition and to determine, on a fibre-to-fibre basis, the correlation between contractile [MyHC (s), myofibrillar ATPase (mATPase) and sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) isoforms], metabolic [succinate dehydrogenase (SDH) and glycerol-3-phosphate dehydrogenase (GPDH) activities and glycogen and phospholamban (PLB) content] and morphological (cross-sectional area and capillary and nuclear densities) features of individual myofibres. An accurate delineation of MyHC-based fibre types was obtained with the developed immunohistochemical method, which showed high sensitivity and objectivity to delineate hybrid fibres with overwhelming dominance of one MyHC isoform. Phenotypic differences in contractile, metabolic and morphological properties seen between fibre types were related to MyHC content. All canine skeletal muscle fibre types had a relatively high histochemical SDH activity but significant differences existed in the order IIA>I>IIX. Mean GPDH was ranked according to fibre type such that I<IIA<IIX. Type IIA fibres were the smallest, type IIX fibres the largest and type I of intermediate size. Capillary and nuclear density decreased in the order IIA>I>IIX. Hybrid fibres, which represented nearly one third of the whole pool of skeletal muscle fibres analysed, had mean values intermediate between their respective pure phenotypes. Slow fibres expressed the slow SERCA isoform and PLB, whereas type II fibres expressed the fast SERCA isoform. Discrimination of myofibres according to their MyHC content was possible on the basis of their contractile, metabolic and morphological features. These intrafibre interrelationships suggest that myofibres of control dogs exhibit a high degree of co-ordination in their physiological, biochemical and morphological characteristics. This study demonstrates that canine skeletal muscle fibres have been misclassified in numerous previous studies and offers useful baseline data and new prospects for future work on muscle-fibre-typing in canine experimental studies.     

Regenerative potential of human skeletal muscle during aging

In this study, we have investigated the consequences of aging on the regenerative capacity of human skeletal muscle by evaluating two parameters: (i) variation in telomere length which was used to evaluate the in vivo turn-over and (ii) the proportion of satellite cells calculated as compared to the total number of nuclei in a muscle fibre. Two skeletal muscles which have different types of innervation were analysed: the biceps brachii, a limb muscle, and the masseter, a masticatory muscle. The biopsies were obtained from two groups: young adults (23 +/- 1.15 years old) and aged adults (74 +/- 4.25 years old). Our results showed that during adult life, minimum telomere lengths and mean telomere lengths remained stable in the two muscles. The mean number of myonuclei per fibre was lower in the biceps brachii than in the masseter but no significant change was observed in either muscle with increasing age. However, the number of satellite cells, expressed as a proportion of myonuclei, decreased with age in both muscles. Therefore, normal aging of skeletal muscle in vivo is reflected by the number of satellite cells available for regeneration, but not by the mean number of myonuclei per fibre or by telomere lengths. We conclude that a decrease in regenerative capacity with age may be partially explained by a reduced availability of satellite cells.     

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.     

Aberrant post‐translational modifications compromise human myosin motor function in old age

Novel experimental methods, including a modified single fiber in vitro motility assay, X‐ray diffraction experiments, and mass spectrometry analyses, have been performed to unravel the molecular events underlying the aging‐related impairment in human skeletal muscle function at the motor protein level. The effects of old age on the function of specific myosin isoforms extracted from single human muscle fiber segments, demonstrated a significant slowing of motility speed (P < 0.001) in old age in both type I and IIa myosin heavy chain (MyHC) isoforms. The force‐generating capacity of the type I and IIa MyHC isoforms was, on the other hand, not affected by old age. Similar effects were also observed when the myosin molecules extracted from muscle fibers were exposed to oxidative stress. X‐ray diffraction experiments did not show any myofilament lattice spacing changes, but unraveled a more disordered filament organization in old age as shown by the greater widths of the 1, 0 equatorial reflections. Mass spectrometry (MS) analyses revealed eight age‐specific myosin post‐translational modifications (PTMs), in which two were located in the motor domain (carbonylation of Pro79 and Asn81) and six in the tail region (carbonylation of Asp900, Asp904, and Arg908; methylation of Glu1166; deamidation of Gln1164 and Asn1168). However, PTMs in the motor domain were only observed in the IIx MyHC isoform, suggesting PTMs in the rod region contributed to the observed disordering of myosin filaments and the slowing of motility speed. Hence, interventions that would specifically target these PTMs are warranted to reverse myosin dysfunction in old age.     

Fiber type and myosin heavy chain compositions of adult pretarsal orbicularis oculi muscle

Summary Pretarsal orbicularis oculi muscle (POOM) is an important structure of eyelid movement in human. The aim of this study was to investigate fiber histomorphology and myosin heavy chain (MyHC) isoform composition of adult POOM, and to clarify their age-related changes. Eyelid specimens from 58 subjects (age range, 21 to 91 years) were collected during upper blepharoplasty procedures. Serial cross sections of POOM were ATPase-stained and examined under miscroscope. Quantitative measures of muscle fiber size and fiber type distribution were obtained in 35 subjects with adequate fiber cross sections. Relative MyHC isoform contents of POOM were retrieved by gel electrophoresis in all 58 subjects. Examination of the histochemical staining revealed an abundance of type II fiber ( >85%) in human POOM, with more type IIX than IIA fibers. Decreased mean area of all fibers and type IIA fibers were noted in the old group when compared to the young. As for MyHC analysis, the relative content of MyHC isoforms exhibited an order of IIX > IIA > I, and the relative MyHC IIA content showed a negative correlation with age. Comparing with previous studies of limb or masticatory muscles, adult POOM exhibits a unique fiber and MyHC composition, as well as a different aging pattern.     

Intrafusal myosin heavy chain expression of human masseter and biceps muscles at young age shows fundamental similarities but also marked differences

Muscle spindles are skeletal muscle mechanoreceptors that provide proprioceptive information to the central nervous system. The human adult masseter muscle has greater number, larger and more complex muscle spindles than the adult biceps. For a better knowledge of muscle diversity and physiological properties, this study examined the myosin heavy chain (MyHC) expression of muscle spindle intrafusal fibres in the human young masseter and young biceps muscles by using a panel of monoclonal antibodies (mAbs) against different MyHC isoforms. Eight MyHC isoforms were detected in both muscles-slow-tonic, I, IIa, IIx, foetal, embryonic, α-cardiac and an isoform not previously reported in intrafusal fibres, termed IIx′. Individual fibres co-expressed 2–6 isoforms. MyHC-slow tonic separated bag₁, AS-bag₁ and bag₂ fibres from chain fibres. Typically, bag fibres also expressed MyHC-I and α-cardiac, whereas chain fibres expressed IIa and foetal. In the young masseter 98 % of bag₁ showed MyHC-α cardiac versus 30 % in the young biceps, 35 % of bag₂ showed MyHC-IIx′ versus none in biceps, 17 % of the chain fibres showed MyHC-I versus 61 % in the biceps. In conclusion, the result showed fundamental similarities in intrafusal MyHC expression between young masseter and biceps, but also marked differences implying muscle-specific proprioceptive control, probably related to diverse evolutionary and developmental origins. Finding of similarities in MyHC expression between young and adult masseter and biceps muscle spindles, respectively, in accordance with previously reported similarities in mATPase fibre type composition suggest early maturation of muscle spindles, preceding extrafusal fibres in growth and maturation.     

Differential effects of muscle fibre length and insulin on muscle-specific mRNA content in isolated mature muscle fibres during long-term culture

The aims of this study were (1) to determine the relationship between muscle fibre cross-sectional area and cytoplasmic density of myonuclei in high- and low-oxidative Xenopus muscle fibres and (2) to test whether insulin and long-term high fibre length caused an increase in the number of myonuclei and in the expression of α-skeletal actin and of myogenic regulatory factors (myogenin and MyoD) in these muscle fibres. In high- and low-oxidative muscle fibres from freshly frozen iliofibularis muscles, the number of myonuclei per millimetre fibre length was proportional to muscle fibre cross-sectional area. The in vivo myonuclear density thus seemed to be strictly regulated, suggesting that the induction of hypertrophy required the activation of satellite cells. The effects of muscle fibre length and insulin on myonuclear density and myonuclear mRNA content were investigated on high-oxidative single muscle fibres cultured for 4–5 days. Muscle fibres were kept at a low length (~15% below passive slack length) in culture medium with a high insulin concentration (~6 nmol/l: “high insulin medium”) or without insulin, and at a high length (~5% above passive slack length) in high insulin medium. High fibre length and high insulin medium did not change the myonuclear density of isolated muscle fibres during culture. High insulin increased the myonuclear α-skeletal actin mRNA content, whereas fibre length had no effect on α-skeletal actin mRNA content. After culture at high fibre length in high insulin medium, the myonuclear myogenin mRNA content was 2.5-fold higher than that of fibres cultured at low length in high insulin medium or in medium without insulin. Myonuclear MyoD mRNA content was not affected by fibre length or insulin. These in vitro experiments indicate that high muscle fibre length and insulin enhance muscle gene expression but that other critical factors are required to induce adaptation of muscle fibre size and performance.This work was partially supported by a research grant from the Haak Bastiaanse Kuneman Stichting.     

Co-ordinated expression of contractile and non-contractile features of control equine muscle fibre types characterised by immunostaining of myosin heavy chains

Combined methodologies of immunohistochemistry, histochemistry and photometric image analysis were applied: (1) to characterise control equine skeletal muscle fibres according to their myosin heavy chain (MyHC) composition and (2) to determine on a fibre-to-fibre basis the correlation between contractile [i.e. MyHC(s), myofibrillar ATPase (mATPase) and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) isoforms], metabolic [i.e. succinate dehydrogenase (SDH) and alpha-glycerophosphate dehydrogenase (GPD) activities, glycogen and phospholamban (PLB) contents], and morphological [i.e. cross-sectional area (CSA), capillary and nuclear densities] features of individual myofibres. An accurate delineation of MyHC-based fibre types was obtained with the immunohistochemical method developed. This protocol showed a high sensitivity and objectivity to delineate hybrid fibres with overwhelming dominance of one MyHC isoform and, furthermore, it allowed a semiquantitative delineation of fast hybrid fibres according to the predominant MyHC isoform expressed. The phenotypic differences in contractile, metabolic and morphological properties seen between fibre types were related to MyHC content. Slow fibres had the lowest mATPase activity (related to shortening velocity), the highest SDH activity (oxidative capacity), the lowest GPD activity (glycolytic metabolism) and glycogen content, the smallest CSA, the greatest capillary and nuclear densities, and expressed slow SERCA isoform and PLB, but not the fast SERCA isoform. The reverse pattern was true for pure IID/X fibres, and type IIA fibres had intermediate properties. Hybrid IIAD/X fibres had mean values intermediate to those of their respective pure phenotypes. Discrimination of fibres according to their MyHC content was possible on the basis of their contractile and non-contractile profiles. These intrafibre interdependencies suggest that, even when controlled by different mechanisms, myofibres of control horses exhibit a high degree of co-ordination in their physiological, biochemical and anatomical features.     

Influence of corticosteroids on myonuclear domain size in the rat diaphragm muscle.

Skeletal muscle fibers are multinucleated. Each myonucleus regulates gene products and protein expression in only a restricted portion of the muscle fiber, the myonuclear domain (MND). In the rat diaphragm muscle (DIAm), corticosteroid (CoS) treatment causes atrophy of fibers containing myosin heavy chain (MHC): MHC2X and/or MHC2B. We hypothesized that DIAm fiber MND size is maintained during CoS-induced atrophy. Adult male rats received methylprednisolone for 11 days at 1 (CoS-Low, n = 8) or 8 mg x kg(-1) x day(-1) (CoS-High, n = 8). Age-matched (CTL-AgeM, n = 8), sham-operated (SHAM-AgeM, n = 8), and weight-matched (CTL-WtM, n = 8) animals served as controls. In single DIAm fibers, cross-sectional area (CSA), MND size, and MHC expression were determined. Fiber CSA and MND size were similar in CTL-AgeM and SHAM-AgeM groups. Only fibers containing MHCslow or MHC2A displayed smaller CSA in CTL-WtM than in CTL-AgeM and SHAM-AgeM groups, and MND size was reduced in all fibers. Thus fibers containing MHCslow and MHC2A maintain the number of myonuclei, whereas MHC2X or MHC2B fibers show loss of myonuclei during normal muscle growth. Both CoS groups displayed smaller CSA and MND size than CTL-AgeM and SHAM-AgeM groups. However, compared with CTL-WtM DIAm fibers, only fibers containing MHC2X or MHC2B displayed reduced CSA and MND size after CoS treatment. Thus little, if any, loss of myonuclei was associated with CoS-induced atrophy of MHC2X or MHC2B DIAm fibers. In summary, MND size does not appear to be regulated during CoS-induced DIAm atrophy.     

Drastic increase of myosin light chain MLC-2 in senescent skeletal muscle indicates fast-to-slow fibre transition in sarcopenia of old age

The age-dependent decline in skeletal muscle mass and function is believed to be due to a multi-factorial pathology and represents a major factor that blocks healthy aging by increasing physical disability, frailty and loss of independence in the elderly. This study has focused on the comparative proteomic analysis of contractile elements and revealed that the most striking age-related changes seem to occur in the protein family representing myosin light chains (MLCs). Comparative screening of total muscle extracts suggests a fast-to-slow transition in the aged MLC population. The mass spectrometric analysis of the myofibril-enriched fraction identified the MLC2 isoform of the slow-type MLC as the contractile protein with the most drastically changed expression during aging. Immunoblotting confirmed an increased abundance of slow MLC2, concomitant with a switch in fast versus slow myosin heavy chains. Staining of two-dimensional gels of crude extracts with the phospho-specific fluorescent dye ProQ-Diamond identified the increased MLC2 spot as a muscle protein with a drastically enhanced phosphorylation level in aged fibres. Comparative immunofluorescence microscopy, using antibodies to fast and slow myosin isoforms, confirmed a fast-to-slow transformation process during muscle aging. Interestingly, the dramatic increase in slow MLC2 expression was restricted to individual senescent fibres. These findings agree with the idea that aged skeletal muscles undergo a shift to more aerobic-oxidative metabolism in a slower-twitching fibre population and suggest the slow MLC2 isoform as a potential biomarker for fibre type shifting in sarcopenia of old age.     

SIRT1 regulates nuclear number and domain size in skeletal muscle fibers

Skeletal muscle fibers are giant multinucleated cells wherein individual nuclei govern the protein synthesis in a finite volume of cytoplasm; this is termed the myonuclear domain (MND). The factors that control MND size remain to be defined. In the present study, we studied the contribution of the NAD⁺‐dependent deacetylase, sirtuin 1 (SIRT1), to the regulation of nuclear number and MND size. For this, we isolated myofibers from mice with tissue‐specific inactivation (mKO) or inducible overexpression (imOX) of SIRT1 and analyzed the 3D organisation of myonuclei. In imOX mice, the number of nuclei was increased whilst the average MND size was decreased as compared to littermate controls. Our findings were the opposite in mKO mice. Muscle stem cell (satellite cell) numbers were reduced in mKO muscles, a possible explanation for the lower density of myonuclei in these mice; however, no change was observed in imOX mice, suggesting that other factors might also be involved, such as the functional regulation of stem cells/muscle precursors. Interestingly, however, the changes in the MND volume did not impact the force‐generating capacity of muscle fibers. Taken together, our results demonstrate that SIRT1 is a key regulator of MND sizes, although the underlying molecular mechanisms and the cause‐effect relationship between MND and muscle function remain to be fully defined.     

Application of skinned single muscle fibres to determine myofilament function in ageing and disease

The chemically skinned fibre is a suitable preparation to determine whether alterations in myofilament function contribute to muscle dysfunction during ageing and disorders such as chronic obstructive pulmonary disease (COPD). In this preparation the sarcolemma is chemically permeabilized and the myofilament lattice kept intact, functioning under controlled near-physiological conditions. As force generating capacity is an important determinant of muscle function and is related to fibre crosssectional area (FCSA), we compared several methods employed by researchers to determine FCSA. Specific tension, force divided by FCSA, has a co-efficient of variation of 27%, 37%, or 30% when the FCSA was measured from the width and depth assuming an elliptical circumference, the width assuming a circular circumference, and the width while the fibre was suspended in the air, respectively. The last method showed the closest relation with the FCSA in histological sections. The velocity of maximal unloaded shortening (V(0)) varied with fibre type, with fibres expressing the Beta/slow (type I) myosin heavy chain (MyHC) isoform being the slowest and fibres expressing the IIb MyHC isoform the fastest. While muscle weakness experienced after surgery could not be explained by changes in specific tension or FCSA of individual fibres, the preparation revealed significant changes in myofilament function during ageing and COPD.     

Cellular adaptation of the trapezius muscle in strength-trained athletes

 The aim of this study was to elucidate the cellular events that occur in the trapezius muscle following several years of strength training. In muscle biopsies from ten elite power lifters (PL) and six control subjects (C), several parameters were studied: cross-sectional area of muscle fibres, myosin heavy chain composition (MHC) and capillary supply [capillaries around fibres (CAF) and CAF/fibre area]. A method was also developed for counting the number of myonuclei and satellite cell nuclei. The proportion of fibres expressing MHC IIA, the cross-sectional area of each fibre type and the number of myonuclei, satellite cells and fibres expressing markers for early myogenesis were significantly higher in PL than in C (P<0.05). A significant correlation between the myonuclear number and the cross-sectional area was observed. Since myonuclei in mature muscle fibres are not able to divide, we suggest that the incorporation of satellite cell nuclei into muscle fibres resulted in the maintenance of a constant nuclear to cytoplasmic ratio. The presence of small diameter fibres expressing markers for early myogenesis indicates the formation of new muscle fibres. Accepted: 17 November 1998     

Potential sources of early-postnatal increase in myofibre number in pig skeletal muscle

In pigs, myogenesis is a biphasic phenomenon with the formation of primary and secondary fibres. Hyperplasia was reported to be accomplished around 90 days of gestation. However, some studies suggest a substantial increase in the total fibre number (TFN) from birth to weaning by counting fibre number in the muscle cross sections. The aim of this study was to establish in which way TFN increases after birth and whether this increase is imputable to new (tertiary) myofibres and/or fibre elongation. The semitendinosus muscle of 128 piglets was examined at days 1 (n = 63), 7 (n = 12), 21 (n = 12), and 28 (n = 41) of age. TFN was increased at days 7, 21 and 28 of age when compared with day 1 (P < 0.01). From day 1 to 28, TFN increased from 463 × 10³ to 825 × 10³. Microscopy of longitudinal and transversal serial sections revealed that at day 7 of age very small fibres expressing the embryonic myosin heavy chain (MyHC) isoform were apparent all over the muscle. In addition, intrafascicular terminations of normal-sized fibres expressed the embryonic MyHC isoform. These data suggest that the TFN in the pig muscle is not fixed at birth and its postnatal increase may be related to both elongation of existing muscle fibres and genesis of tertiary myofibres, mainly between birth and 3 weeks of age.     

The effect of glucocorticoids on the myosin heavy chain isoforms' turnover in skeletal muscle

The purpose of this study was to find the effect of dexamethasone on the myosin heavy chain (MyHC) isoforms' composition in different skeletal muscles and glycolytic (G) fibres in relation with their synthesis rate and degradation of MyHC isoforms by alkaline proteinases. Eighteen-week-old male rats of the Wistar strain were treated with dexamethasone (100 microg/100 g bwt) during 10 days. The forelimb strength decreased from 9.52 to 6.19 N (P<0.001) and hindlimb strength from 15.54 to 8.55 N (P<0.001). Daily motor activity decreased (total activity from 933 to 559 and ambulatory activity from 482 to 226 movements/h, P<0.001). The degradation rate of muscle contractile proteins increased from 2.0 to 5.9% per day (P<0.001), as well as the myosin heavy chain IIB isoform degradation with alkaline proteinase in fast-twitch (F-T) muscles (12 +/- 0.9%; P<0.05) and glycolytic muscle fibres (15 +/- 1.1%; P<0.001). The synthesis rate of MyHC type II isoforms decreased in Pla muscles (P<0.05) and MyHC IIA (P<0.05) and IIB in EDL muscle and G fibres (P<0.001). The relative content of MyHC IIB isoform decreased in F-T muscles (P<0.001) and in G fibres (P<0.01), and the relative content of IIA and IID isoforms increased simultaneously. Dexamethasone decreased the MyHC IIB isoform synthesis rate and increased the sensibility of MyHC IIB isoform to alkaline proteinase, which in its turn led to the decrease of MyHC IIB isoform relative content in F-T muscles with low oxidative potential and G muscle fibres.     

Effects of hypothyroidism on myosin heavy chain composition and fibre types of fast skeletal muscles in a small marsupial, Antechinus flavipes

Effects of drug-induced hypothyroidism on myosin heavy chain (MyHC) content and fibre types of fast skeletal muscles were studied in a small marsupial, Antechinus flavipes. SDS-PAGE of MyHCs from the tibialis anterior and gastrocnemius revealed four isoforms, 2B, 2X, 2A and slow, in that order of decreasing abundance. After 5 weeks treatment with methimazole, the functionally fastest 2B MyHC significantly decreased, while 2X, 2A and slow MyHCs increased. Immunohistochemistry using monospecific antibodies to each of the four MyHCs revealed decreased 2b and 2x fibres, and increased 2a and hybrid fibres co-expressing two or three MyHCs. In the normally homogeneously fast superficial regions of these muscles, evenly distributed slow-staining fibres appeared, resembling the distribution of slow primary myotubes in fast muscles during development. Hybrid fibres containing 2A and slow MyHCs were virtually absent. These results are more detailed but broadly similar to the earlier studies on eutherians. We hypothesize that hypothyroidism essentially reverses the effects of thyroid hormone on MyHC gene expression of muscle fibres during myogenesis, which differ according to the developmental origin of the fibre: it induces slow MyHC expression in 2b fibres derived from fast primary myotubes, and shifts fast MyHC expression in fibres of secondary origin towards 2A, but not slow, MyHC.     

Training affects myosin heavy chain phenotype in the trapezius muscle of women

The aim of this investigation was to determine whether 10 weeks of three different types of training can alter the myosin heavy chain (MyHC) composition of the trapezius muscle. Twenty-one women were randomly assigned to three training groups that performed strength (n=9), endurance (n=7) or coordination training (n=5). Pre and post biopsies were taken from the upper part of the descending trapezius muscle and were analysed for MyHC isoform content using 5% gel electrophoresis. In addition, we have studied the expression of embryonic and neonatal MyHCs using double-immunofluorescence staining. In the strength-trained group, there was a significant increase in the amount of MyHC IIA and a significant decrease in the amount of MyHC IIB and MyHC I. In the endurance group, there was a significant decrease in the amount of MyHC IIB. MyHC composition in the coordination group was not altered. Following the training period, myotubes and individual small-sized muscle fibres were observed in the strength and endurance trained groups. These structures were stained with the markers for early myogenesis (MyHC embryonic and neonatal). These data suggest that specific shifts in MyHC isoforms occur in the trapezius muscle following strength and endurance training. The presence of small-sized muscle fibres expressing the developmental isoforms of MyHC suggests that strength and endurance training induced the formation of new muscle fibres. Accepted: 31 March 1999