Myosin polymorphism and differential expression in adult human skeletal muscle

Sex-related differences in motor unit behavior are influenced by myosin heavy chain during high- but not moderate-intensity contractions

AIMS

Motor unit recruitment and firing rate patterns of the vastus lateralis (VL) have not been compared between sexes during moderate- and high-intensity contraction intensities. Additionally, the influence of fiber composition on potential sex-related differences remains unquantified.

METHODS

Eleven males and 11 females performed 40% and 70% maximal voluntary contractions (MVCs). Surface electromyographic (EMG) signals recorded from the VL were decomposed. Recruitment thresholds (RTs), MU action potential amplitudes (MUAPAMP ), initial firing rates (IFRs), mean firing rates (MFRs), and normalized EMG amplitude (N-EMGRMS ) at steady torque were analyzed. Y-intercepts and slopes were calculated for MUAPAMP , IFR, and MFR versus RT relationships. Type I myosin heavy chain isoform (MHC) was determined with muscle biopsies.

RESULTS

There were no sex-related differences in MU characteristics at 40% MVC. At 70% MVC, males exhibited greater slopes (p = 0.002) for the MUAPAMP , whereas females displayed greater slopes (p = 0.001-0.007) for the IFR and MFR versus RT relationships. N-EMGRMS at 70% MVC was greater for females (p < 0.001). Type I %MHC was greater for females (p = 0.006), and was correlated (p = 0.018-0.031) with the slopes for the MUAPAMP , IFR, and MFR versus RT relationships at 70% MVC (r = -0.599-0.585).

CONCLUSION

Both sexes exhibited an inverse relationship between MU firing rates and recruitment thresholds. However, the sex-related differences in MU recruitment and firing rate patterns and N-EMGRMS at 70% MVC were likely due to greater type I% MHC and smaller twitch forces of the higher threshold MUs for the females. Evidence is provided that muscle fiber composition may explain divergent MU behavior between sexes.

How to Build a Super Predator: From Genotype to Phenotype

We present a drawing discovery lab that crosscuts multiple disciplines in biology and links concepts in genetics and evolutionary thinking to enhance understanding of the genotype-to-phenotype transformation. These combined concepts are also linked to ecological frameworks in nature through the model of biological plasticity. Students and teachers explore drawing skills to flesh out the future of a predator while engaging with the computational software MEGA, which introduces students and teachers to nucleotide changes, mutations, variation, phylogenetics, and molecular evolution.

Defective sarcomere organization and reduced larval locomotion and fish survival in slow muscle heavy chain 1 (smyhc1) mutants

Zebrafish skeletal muscles are broadly divided into slow-twitch and fast-twitch muscle fibers. The slow fibers, which express a slow fiber-specific myosin heavy chain 1 (Smyhc1), are the first group of muscle fibers formed during myogenesis. To uncover Smyhc1 function in muscle growth, we generated three mutant alleles with reading frame shift mutations in the zebrafish smyhc1 gene using CRISPR. The mutants showed shortened sarcomeres with no thick filaments and M-lines in slow fibers of the mutant embryos. However, the formation of slow muscle precursors and expression of other slow muscle genes were not affected and fast muscles appeared normal. The smyhc1 mutant embryos and larvae showed reduced locomotion and food intake. The mutant larvae exhibited increased lethality of incomplete penetrance. Approximately 2/5 of the homozygous mutants were viable and grew into reproductive adults. These adult mutants displayed a typical pattern of slow and fast muscle fiber distribution, and regained normal slow muscle formation. Together, our studies indicate that Smyhc1 is essential for myogenesis in embryonic slow muscles, and loss of Smyhc1 results in defective sarcomere assembly, reduces larval motility and fish survival, but has no visible impact on muscle growth in juvenile and adult zebrafish that escape the larval lethality.

Sex-related differences in muscle size explained by amplitudes of higher-threshold motor unit action potentials and muscle fibre typing

AIM

To investigate the relationships between motor unit action potential amplitudes (MUAP_AMP ), muscle cross-sectional area (mCSA) and composition (mEI), per cent myosin heavy chain (%MHC) areas and sex in the vastus lateralis (VL).

METHODS

Ten males and 10 females performed a submaximal isometric trapezoid muscle action that included a linearly increasing, steady torque at 40% maximal voluntary contraction, and linearly decreasing segments. Surface electromyographic decomposition techniques were utilized to determine MUAP_AMPS in relation to recruitment thresholds (RT). Ultrasound images were taken to quantify muscle mCSA and mEI. Muscle biopsies were collected to calculate %MHC areas. Y-intercepts and slopes were calculated for the MUAP_AMP vs RT relationships for each subject. Independent-samples t tests and ANOVA models examined sex-related differences in mCSA, mEI, slopes and y-intercepts for the MUAP_AMP vs RT relationships and %MHC areas. Correlations were performed among type IIA and total type II %MHC area, mCSA and the slopes and y-intercepts for the MUAP_AMP vs RT relationships.

RESULTS

Males exhibited greater slopes for the MUAP_AMP vs RT relationships (P = .003), mCSA (P < .001) and type IIA %MHC (P = .011), whereas females had greater type I %MHC area (P = .010) and mEI (P = .024). The mCSA, type IIA and total II %MHC area variables were correlated (P < .001-.015, r = .596-.836) with the slopes from the MUAP_AMP vs RT relationships.

CONCLUSION

Sex-related differences in mCSA and MUAP_AMPS of the higher-threshold MUs were likely the result of larger muscle fibres expressing type II characteristics for males.

Chronic Adaptations to Eccentric Training: A Systematic Review

BACKGROUND

Resistance training is an integral component of physical preparation for athletes. A growing body of evidence indicates that eccentric strength training methods induce novel stimuli for neuromuscular adaptations.

OBJECTIVE

The purpose of this systematic review was to determine the effects of eccentric training in comparison to concentric-only or traditional (i.e. constrained by concentric strength) resistance training.

METHODS

Searches were performed using the electronic databases MEDLINE via EBSCO, PubMed and SPORTDiscus via EBSCO. Full journal articles investigating the long-term (≥4 weeks) effects of eccentric training in healthy (absence of injury or illness during the 4 weeks preceding the training intervention), adult (17-35 years), human participants were selected for the systematic review. A total of 40 studies conformed to these criteria.

RESULTS

Eccentric training elicits greater improvements in muscle strength, although in a largely mode-specific manner. Superior enhancements in power and stretch-shortening cycle (SSC) function have also been reported. Eccentric training is at least as effective as other modalities in increasing muscle cross-sectional area (CSA), while the pattern of hypertrophy appears nuanced and increased CSA may occur longitudinally within muscle (i.e. the addition of sarcomeres in series). There appears to be a preferential increase in the size of type II muscle fibres and the potential to exert a unique effect upon fibre type transitions. Qualitative and quantitative changes in tendon tissue that may be related to the magnitude of strain imposed have also been reported with eccentric training.

CONCLUSIONS

Eccentric training is a potent stimulus for enhancements in muscle mechanical function, and muscle-tendon unit (MTU) morphological and architectural adaptations. The inclusion of eccentric loads not constrained by concentric strength appears to be superior to traditional resistance training in improving variables associated with strength, power and speed performance.

Noninvasive Assessment of Skeletal Muscle Myosin Heavy Chain Expression in Trained and Untrained Men

Fry, AC, Housh, TJ, Cramer, JB, Weir, JP, Beck, TW, Schilling, BK, Miller, JD, and Nicoll, JX. Noninvasive assessment of skeletal muscle myosin heavy chain expression in trained and untrained men. J Strength Cond Res 31(9): 2355-2362, 2017-Numerous conditions and types of physical activity (e.g., exercise, aging, and muscle-related diseases) can influence muscle fiber types and the proteins expressed. To date, muscle fibers can only be characterized by actually obtaining a tissue sample using the invasive muscle biopsy procedure. Mechanomyography (MMG) is the assessment of the vibration properties of contracting skeletal muscle and has been proposed as a possible noninvasive method for muscle fiber analysis. Therefore, the purpose of this project was to examine the feasibility of using MMG and muscle performance measures to noninvasively assess muscle fiber characteristics. Fifteen men (5 endurance-trained, 5 weight-trained, and 5 sedentary) provided muscle samples from their vastus lateralis muscle. These samples were analyzed for relative myosin heavy chain (MHC) protein expression, which is highly correlated with % muscle fiber type areas. Additionally, each subject performed several muscle performance tests, and MMG of the quadriceps was assessed during a knee extension exercise. Multiple regression was used to develop prediction equations for determining relative muscle content of MHC types I, IIa, and IIx. A combination of MMG and knee extension performance variables estimated types I, IIa, and IIx MHCs with approximately 80% accuracy. Although preliminary, these data suggest that muscle performance tests in addition to MMG assessments during a simple muscle performance task (knee extension) can be used to estimate muscle fiber type composition in a healthy male population. Such methods could ultimately be used to noninvasively monitor muscle health and fitness.

Influence of the contractile properties of muscle on motor unit firing rates during a moderate-intensity contraction in vivo

It is suggested that firing rate characteristics of motor units (MUs) are influenced by the physical properties of the muscle. However, no study has correlated MU firing rates at recruitment, targeted force, or derecruitment with the contractile properties of the muscle in vivo. Twelve participants (age = 20.67 ± 2.35 yr) performed a 40% isometric maximal voluntary contraction of the leg extensors that included linearly increasing, steady force, and decreasing segments. Muscle biopsies were collected with myosin heavy chain (MHC) content quantified, and surface electromyography (EMG) was recorded from the vastus lateralis. The EMG signal was decomposed into the firing events of single MUs. Slopes and y-intercepts were calculated for 1) firing rates at recruitment vs. recruitment threshold, 2) mean firing rates at steady force vs. recruitment threshold, and 3) firing rates at derecruitment vs. derecruitment threshold relationships for each subject. Correlations among type I %MHC isoform content and the slopes and y-intercepts from the three relationships were examined. Type I %MHC isoform content was correlated with MU firing rates at recruitment (y-intercepts: r = -0.577; slopes: r = 0.741) and targeted force (slopes: r = 0.853) vs. recruitment threshold and MU firing rates at derecruitment (y-intercept: r = -0.597; slopes: r = 0.701) vs. derecruitment threshold relationships. However, the majority of the individual MU firing rates vs. recruitment and derecruitment relationships were not significant (P > 0.05) and, thus, revealed no systematic pattern. In contrast, MU firing rates during the steady force demonstrated a systematic pattern with higher firing rates for the lower- than higher-threshold MUs and were correlated with the physical properties of MUs in vivo.

Spatial Geometries of Self-Assembled Chitohexaose Monolayers Regulate Myoblast Fusion

Myoblast fusion into functionally-distinct myotubes to form in vitro skeletal muscle constructs under differentiation serum-free conditions still remains a challenge. Herein, we report that our microtopographical carbohydrate substrates composed of bioactive hexa-N-acetyl-d-glucosamine (GlcNAc6) modulated the efficiency of myoblast fusion without requiring horse serum or any differentiation medium during cell culture. Promotion of the differentiation of dissociated mononucleated skeletal myoblasts (C2C12; a mouse myoblast cell line) into robust myotubes was found only on GlcNAc6 micropatterns, whereas the myoblasts on control, non-patterned GlcNAc6 substrates or GlcNAc6-free patterns exhibited an undifferentiated form. We also examined the possible role of GlcNAc6 micropatterns with various widths in the behavior of C2C12 cells in early and late stages of myogenesis through mRNA expression of myosin heavy chain (MyHC) isoforms. The spontaneous contraction of myotubes was investigated via the regulation of glucose transporter type 4 (GLUT4), which is involved in stimulating glucose uptake during cellular contraction. Narrow patterns demonstrated enhanced glucose uptake rate and generated a fast-twitch muscle fiber type, whereas the slow-twitch muscle fiber type was dominant on wider patterns. Our findings indicated that GlcNAc6-mediated integrin interactions are responsible for guiding myoblast fusion forward along with myotube formation.

The change in motor unit firing rates at de-recruitment relative to recruitment is correlated with type I myosin heavy chain isoform content of the vastus lateralis in vivo

AIM

To investigate the change in motor unit (MU) firing rates (FR) at de-recruitment relative to recruitment and the relation to % type I myosin heavy chain isoform content (type I %MHC) of the vastus lateralis (VL) in vivo.

METHODS

Ten subjects performed a 22-s submaximal isometric trapezoid muscle action that included a linearly increasing, steady force at 50% maximal voluntary contraction, and linearly decreasing segments. Surface electromyographic signals were collected from the VL and were decomposed into constituent MU action potentials trains. A tissue sample from the VL was taken to calculate type I %MHC. The y-intercepts and slopes were calculated for the changes (Δ) in FR at de-recruitment (FRDEREC ) relative to FR at recruitment (FRREC ) vs. FRREC relationship for each subject. Correlations were performed between the y-intercepts and slopes with type I %MHC.

RESULTS

The majority of MUs had greater FRDEREC than FRREC . The y-intercepts (r = -0.600, P = 0.067) were not significantly correlated, but the slopes (r = -0.793, P = 0.006) were significantly correlated with type I %MHC.

CONCLUSION

The majority of the motoneuron pool had greater FRDEREC than FRREC , however, individuals with higher type I %MHC had a greater propensity to have MUs with FRREC > FRDEREC as indicated by the slope values. Overall, the contractile properties of the muscle (MHC) could partially explain the differences in MU firing rates at de-recruitment relative to recruitment. Thus, suggesting the fatigability of the muscle influences the alterations in MU firing rates from recruitment to de-recruitment.

Aerobic or Resistance Training and Pulse Wave Velocity in Kidney Transplant Recipients: A 12-Week Pilot Randomized Controlled Trial (the Exercise in Renal Transplant [ExeRT] Trial)

BACKGROUND

Cardiovascular disease remains the leading cause of death in kidney transplant recipients. This pilot study examined the potential effect of aerobic training or resistance training on vascular health and indexes of cardiovascular risk in kidney transplant recipients.

STUDY DESIGN

Single-blind, randomized, controlled, parallel trial.

SETTING & PARTICIPANTS

60 participants (mean age, 54 years; 34 men) were randomly assigned to aerobic training (n=20), resistance training (n=20), or usual care (n=20). Participants were included if they had a kidney transplant within 12 months prior to baseline assessment. Patients were excluded if they had unstable medical conditions or had recently started regular exercise.

INTERVENTION

Aerobic training and resistance training were delivered 3 days per week for a 12-week period. The usual-care group received standard care.

OUTCOMES & MEASUREMENTS

Pulse wave velocity, peak oxygen uptake (Vo2peak), sit-to-stand 60, isometric quadriceps force, and inflammatory biomarkers were assessed at 0 and 12 weeks.

RESULTS

The anticipated 60 participants were recruited within 12 months. 46 participants completed the study (aerobic training, n=13; resistance training, n=13; and usual care, n=20), resulting in a 23% attrition rate. Analyses of covariance, adjusted for baseline values, age, and dialysis vintage pretransplantation, revealed significant mean differences between aerobic training and usual care in pulse wave velocity of -2.2±0.4 (95% CI, -3.1 to -1.3) m/s (P<0.001) and between resistance training and usual care of -2.6±0.4 (95% CI, -3.4 to -1.7) m/s (P<0.001) at 12 weeks. Secondary analyses indicated significant improvements in Vo2peak in the aerobic training group and in Vo2peak, sit-to-stand 60, and isometric muscle force in the resistance training group compared with usual care at 12 weeks. There were no reported adverse events, cardiovascular events, or hospitalizations as a result of the intervention.

LIMITATIONS

Pilot study, small sample size, no measure of endothelial function.

CONCLUSIONS

Both aerobic training and resistance training interventions appear to be feasible and clinically beneficial in this patient population.

Interrelationship between bone substitution materials and skeletal muscle tissue

Bone density and quantity are primary conditions for the insertion and stability of dental implants. In cases of a lack of adequate maxillary or mandibulary bone, bone augmentation will be necessary. The use of synthetic bioactive bone substitution materials is of increasing importance as alternatives to autogenously bone grafts. It is well known that bone can influence muscle function and muscle function can influence bone structures. Muscles have a considerable potential of adaptation and muscle tissue surrounding an inserted implant or bone surrogate can integrate changes in mechanical load of the muscle and hereupon induce signaling cascades with protein synthesis and arrangement of the cytoskeleton. The Musculus latissimus dorsi is very often used for the analyses of the in vivo biocompatibility of newly designed biomaterials. Beside macroscopically and histologically examination, biocompatibility can be assessed by analyses of the biomaterial influence of gene expression. This review discusses changes in the fiber type distribution, myosin heavy chain isoform composition, histological appearance and vascularization of the skeletal muscle after implantation of bone substitution materials. Especially, the effects of bone surrogates should be described at the molecular-biological and cellular level.

Action of GH on skeletal muscle function: molecular and metabolic mechanisms

Skeletal muscle is a target tissue of GH. Based on its anabolic properties, it is widely accepted that GH enhances muscle performance in sports and muscle function in the elderly. This paper critically reviews information on the effects of GH on muscle function covering structure, protein metabolism, the role of IGF1 mediation, bioenergetics and performance drawn from molecular, cellular and physiological studies on animals and humans. GH increases muscle strength by enhancing muscle mass without affecting contractile force or fibre composition type. GH stimulates whole-body protein accretion with protein synthesis occurring in muscular and extra-muscular sites. The energy required to power muscle function is derived from a continuum of anaerobic and aerobic sources. Molecular and functional studies provide evidence that GH stimulates the anaerobic and suppresses the aerobic energy system, in turn affecting power-based functional measures in a time-dependent manner. GH exerts complex multi-system effects on skeletal muscle function in part mediated by the IGF system.

Selection for lean meat yield in lambs reduces indicators of oxidative metabolism in the longissimus muscle

Selection for increased lean meat yield using Australian Sheep Breeding Values for reduced post-weaning c-site fat depth (PFAT) and increased post-weaning eye muscle depth (PEMD) reduces the oxidative capacity of muscle. Isocitrate dehydrogenase (ICDH) activity and myoglobin concentration were measured in 3178 and 5580 lambs, respectively, to indicate oxidative capacity. In the progeny of sires with a reduced PFAT, ICDH activity and myoglobin concentration were reduced by 0.46 μmol/min/g tissue and 0.67 mg/g tissue across the 5 and 6mm PFAT ranges respectively. In the progeny of sires with an increased PEMD, ICDH activity and myoglobin concentration were reduced by 0.50 μmol/min/g tissue and 0.49 mg/g tissue across the 7 and 6 mm PEMD ranges respectively. However, the sites at which the lambs were raised had a larger impact on oxidative capacity than genetic or other production factors.

Evaluation of functional erythropoietin receptor status in skeletal muscle in vivo: acute and prolonged studies in healthy human subjects

BACKGROUND

Erythropoietin receptors have been identified in human skeletal muscle tissue, but downstream signal transduction has not been investigated. We therefore studied in vivo effects of systemic erythropoietin exposure in human skeletal muscle.

METHODOLOGY/PRINCIPAL FINDINGS

The protocols involved 1) acute effects of a single bolus injection of erythropoietin followed by consecutive muscle biopsies for 1-10 hours, and 2) a separate study with prolonged administration for 16 days with biopsies obtained before and after. The presence of erythropoietin receptors in muscle tissue as well as activation of Epo signalling pathways (STAT5, MAPK, Akt, IKK) were analysed by western blotting. Changes in muscle protein profiles after prolonged erythropoietin treatment were evaluated by 2D gel-electrophoresis and mass spectrometry. The presence of the erythropoietin receptor in skeletal muscle was confirmed, by the M20 but not the C20 antibody. However, no significant changes in phosphorylation of the Epo-R, STAT5, MAPK, Akt, Lyn, IKK, and p70S6K after erythropoietin administration were detected. The level of 8 protein spots were significantly altered after 16 days of rHuEpo treatment; one isoform of myosin light chain 3 and one of desmin/actin were decreased, while three isoforms of creatine kinase and two of glyceraldehyd-3-phosphate dehydrogenase were increased.

CONCLUSIONS/SIGNIFICANCE

Acute exposure to recombinant human erythropoietin is not associated by detectable activation of the Epo-R or downstream signalling targets in human skeletal muscle in the resting situation, whereas more prolonged exposure induces significant changes in the skeletal muscle proteome. The absence of functional Epo receptor activity in human skeletal muscle indicates that the long-term effects are indirect and probably related to an increased oxidative capacity in this tissue.

A noninvasive, log-transform method for fiber type discrimination using mechanomyography

This study examined the log-transformed mechanomyographic (MMG(RMS)) and electromyographic (EMG(RMS)) amplitude vs. force relationships for aerobically-trained (AT), resistance-trained (RT), and sedentary (SED) individuals. Subjects performed isometric ramp contractions from 5% to 90% maximal voluntary contraction. Muscle biopsies were collected and thigh skinfolds, MMG and EMG were recorded from the vastus lateralis muscle. Linear regression models were fit to the log-transformed EMG(RMS) and MMG(RMS) vs. force relationships. The slope (b coefficient) and the antilog of the y-intercept (a coefficient) were calculated. The AT group had the highest percentage of type I fiber area, the RT group had the highest percentage of type IIa fiber area, and the SED group had the highest percentage of type IIx fiber area. The a coefficients were higher for the AT group than the RT and SED groups in both the MMG(RMS) and EMG(RMS) vs. force relationships, whereas the b coefficients were lower for the AT group than the RT and SED groups only in the MMG(RMS) vs. force relationship. The group differences among the a coefficients may have reflected subcutaneous fat acting as a filter thereby reducing EMG(RMS) and MMG(RMS). The lower b coefficients for the AT group in the MMG(RMS) patterns may have reflected fiber area-related differences in motor unit activation strategies.

β2-Adrenergic receptor downregulation and performance decrements during high-intensity resistance exercise overtraining

Previous research on overtraining due to excessive use of maximal resistance exercise loads [100% 1 repetition maximum (1 RM)] indicates that peripheral muscle maladaptation contributes to overtraining-induced performance decrements. This study examined the cellular and molecular responses of skeletal muscle to performance decrements due to high-relative-intensity (%1 RM) resistance exercise overtraining. Weight-trained men were divided into overtrained (OT, n = 8) and control (Con, n = 8) groups. The OT group performed 10 x 1 at 100% 1 RM daily for 2 wk, whereas the Con group performed normal training 2 days/wk. Muscle biopsies from the vastus lateralis muscle, voluntary static and dynamic muscle performances, and nocturnal urinary epinephrine were assessed before (pre) and after (post) overtraining. Overtraining occurred as indicated by a decrease in 1-RM strength for the OT group (mean +/- SE; OT pre = 159.3 +/- 10.1 kg, OT post = 151.4 +/- 9.9 kg, Con pre = 146.0 +/- 12.9 kg, Con post = 144.9 +/- 13.3 kg), as well as a 36.3% decrease in mean power at 100% 1-RM loads. Normal training could be resumed only after 2-8 wk of training cessation. Muscle beta(2)-adrenergic receptor (beta(2)-AR; fmol/mg protein) density significantly decreased by 37.0% for the OT group and was unchanged for the Con group (-1.8%). Nocturnal urinary epinephrine for the OT group increased by 49%, although this was not significant (effect size = 0.42). The ratio of nocturnal urinary epinephrine to beta(2)-AR density suggested a decreased beta(2)-AR sensitivity for the OT group (2.4-fold increase). Overtraining occurred based on decreased muscular force and power. Desensitization of the beta(2)-AR system suggests that this may be an important contributor to performance decrements due to excessive use of maximal resistance exercise loads.

Specific isomyosin proportions in hyperexcitable and physiologically denervated mouse muscle

We show here, by high resolution sodium dodecyl sulfate gel electrophoresis, that the proportions of myosin heavy chain (MyHC) isoforms of mouse muscles are specifically shifted by hereditary neuromuscular diseases. In wild-type and dystrophic MDX anterior tibial muscle (TA) about 60% of the MyHC is IIB, 30% IIX, at most 10% IIA and <2% type I (slow). In myotonic fast muscles, hyperexcitability leads to a drastic reduction of MyHC IIB which is compensated by IIA. Slow muscles, like soleus and diaphragm, were only marginally changed by myotonia. The MyHC pattern of TA of spinal muscular atrophy (SMA) 'wobbler' mice is shifted to a faster phenotype, with nearly 90% IIB. In the SMA mutant 'muscle deficient', all four adult isomyosins are expressed in the TA. These findings may be relevant for the future diagnosis of neurological disorders both in mouse disease models and in human patients.

Single-fiber myosin heavy chain polymorphism: how many patterns and what proportions?

Previous studies have reported the existence of skeletal muscle fibers that coexpress multiple myosin heavy chain isoforms. These surveys have usually been limited to studying the polymorphic profiles of skeletal muscle fibers from a limited number of muscles (i.e., usually <4). Additionally, few studies have considered the functional implications of polymorphism. Hence, the primary objective of this study was to survey a relatively large number of rat skeletal muscle/muscle regions and muscle fibers (n approximately 5,000) to test the hypothesis that polymorphic fibers represent a larger fraction of the total pool of fibers than do so-called monomorphic fibers, which express only one myosin heavy chain isoform. Additionally, we used Hill's statistical model of the force-velocity relationship to differentiate the functional consequences of single-fiber myosin heavy chain isoform distributions found in these muscles. The results demonstrate that most muscles and regions of rodent skeletal muscles contain large proportions of polymorphic fibers, with the exception of muscles such as the slow soleus muscle and white regions of fast muscles. Several muscles were also found to have polymorphic profiles that are not consistent with the I<-->IIA<-->IIX<-->IIB scheme of muscle plasticity. For instance, it was found that the diaphragm muscle normally contains I/IIX fibers. Functionally, the high degree of polymorphism may 1) represent a strategy for producing a spectrum of contractile properties that far exceeds that simply defined by the presence of four myosin heavy chain isoforms and 2) result in relatively small differences in function as defined by the force-velocity relationship.

Relationship between in vivo muscle force at different speeds of isokinetic movements and myosin isoform expression in men and women

In an attempt to explore the relationship between force production during voluntary contractions at different speeds of isokinetic movement and the myofibrillar protein isoform expression in humans, an improved isokinetic dynamometer that corrects for gravitation, controls for acceleration and deceleration, and identifies a maximum voluntary activation was used. Muscle torque recordings were compared at the same muscle length (knee angle) and the torque was calculated as the average torque at each angle over a large knee angle range (75 degrees -25 degrees ) to reduce the influence of small torque oscillation on the calculated torque. Muscle torque at fast (240 degrees s(-1)) versus slow (30 degrees s(-1)) speeds of movement, torque normalized to muscle cross-sectional area (specific tension), and absolute torque at fast speeds of movement were measured in 34 young healthy male and female short-, middle-, and long-distance runners. The relationship between the different measures of muscle function and the expression of myosin heavy chain (MyHC) isoforms using enzyme-histochemical and electrophoretic protein separation techniques were investigated. A significant correlation between the 240 degrees s(-1) vs 30 degrees s(-1) torque ratio and the relative area of the type II fibers and type II MyHC isoforms were observed in both the men (r=0.74; P<0.001) and the women (r=0.81; P<0.05). Thus, the present results confirm a significant relationship between in vivo human muscle function and the MyHC isoform expression in the contracting muscle.

Validation of a simple, rapid, and economical technique for distinguishing type 1 and 2 fibres in fixed and frozen skeletal muscle

AIMS

To produce a method of distinguishing between type 1 and 2 skeletal muscle fibres that would be more economical and reproducible than the standard ATPase method and be applicable to both fixed and frozen tissue. Because the ATPase method has been accepted as the basis for fibre identification for the past 50 years, the new method should not give significantly different results.

METHODS

Isoforms of myosin correlate with isoforms of myofibrillar ATPase and an immunohistochemical (IHC) double labelling protocol was devised using monoclonal antibodies to fast and slow myosin. This required one tissue section rather than four. The results of the two methods were compared by means of morphometric analysis of skeletal muscle biopsies from 20 normal healthy volunteers.

RESULTS

There were no significant differences (p = 0.57) in the percentages of type 1 (46% using the IHC method v 48% using ATPase) or type 2 fibres (54% v 52%, respectively). The 2a and 2b subtypes were distinguished easily. Analysis of variance revealed that cross sectional area (mu m(2)), diameter (mu m), form factor, and density of fibre staining (a measure of substrate-enzyme or protein) were all similar. The method worked equally well on fixed material.

CONCLUSION

An IHC method based on the fast and slow isoforms of myosin shows no significant differences in fibre type analysis from the standard ATPase method although it provides important advantages because it is applicable to fixed (including archival) material, is economical and reproducible, and yields a permanent preparation.

Neuromuscular fatigue during repeated exhaustive submaximal static contractions of knee extensor muscles in endurance-trained, power-trained and untrained men

The neural and muscular changes during fatigue produced in repeated submaximal static contractions of knee extensors were measured. Three groups of differently adapted male subjects (power-trained, endurance-trained and untrained, 15 in each) performed the exercise that consisted of 10 trials of submaximal static contractions at the level of 40% of maximal voluntary contraction (MVC) force till exhaustion with the inter-trial rest intervals of 1 min. MVC force, reaction time and patellar reflex time components before and after the fatiguing exercise and following 5, 10 and 15 min of recovery were recorded. Endurance-trained athletes had a significantly longer holding times for all the 10 trials compared with power-trained athletes and untrained subjects. However, no significant differences in static endurance between power-trained athletes and untrained subjects were noted. The fatigue test significantly prolonged the time between onset of electrical and mechanical activity (electromechanical delay) in voluntary and reflex contractions. The electromechanical delay in voluntary contraction condition for power-trained and untrained subjects and in reflex condition for endurance-trained subjects had not recovered 15 min after cessation of exercise. No significant changes in the central component of visual reaction time (premotor time of MVC) and latency of patellar reflex were noted after fatiguing static exercise. It is concluded, that in this type of exercise the fatigue development may be largely owing to muscle contractile failure.

Kinetic properties of myosin heavy chain isoforms in single fibers from human skeletal muscle

The head portion of the myosin heavy chain is essential in force generation. As previously shown, Ca2+-activated muscle fibers from rat and rabbit display a strong correlation between their myosin heavy chain isoform composition and the kinetics of stretch activation, corresponding to an order of velocity: myosin heavy chain Ib > myosin heavy chain IId(x) > myosin heavy chain IIa >> myosin heavy chain I. Here, we show a similar correlation for human muscle fibers (myosin heavy chain IIb > myosin heavy chain IIa >> myosin heavy chain I), suggesting isoform-specific differences between the kinetics of force-generating power strokes. The kinetics of myosin heavy chain I are similar in human and rodents. This holds also true for myosin heavy chain IIa, but human myosin heavy chain IIb is slower than rodent myosin heavy chain IIb. It is similar to rodent myosin heavy chain IId(x).

Characterization of the carp myosin heavy chain multigene family

We isolated partial coding sequences for 29 carp myosin heavy chain genes (MyoHCs) and determined the nucleotide sequences around the region encoding the loop 2 of the myosin molecule. The predicted amino acid sequences from the isolated genes all showed very high similarity to those of skeletal and cardiac muscles from higher vertebrates, but not to those of smooth and non-muscle counterparts. Among all clones isolated, carp MyoHC10, MyoHCI-1-3 and MyoHC30 showed exon-nucleotide sequences identical to those of cDNAs encoding the loop 2 region of the 10 degrees C-, intermediate- and 30 degrees C-type fast skeletal isoforms [Hirayama and Watabe, Euro. J. Biochem. 246 (1997) 380-387]. The loop 2 of 28 types of carp MyoHCs was encoded by two exons separated by an intron corresponding to that of the 16th in higher vertebrate MyoHCs, whilst this intron was not found in carp MyoHC30. Although carp MyoHC30 had a gene organization different from those of higher vertebrates and other carp MyoHCs, its predicted amino acid sequence for loop 2 showed the highest homology to those of higher vertebrates among carp MyoHCs. In the 28 carp MyoHCs containing the intron, a combination of different nucleotide sequences for the two resulted in 14 distinct series for the combined coding sequence. These different nucleotide sequences encoded nine distinct amino acid sequences. Phylogenetic analysis for the present loop 2 and light meromyosin previously reported for carp MyoHCs [Imai et al., J. Exp. Biol. 200 (1997) 27-34] revealed that carp MyoHCs have recently diverged and are more closely related to each other than to MyoHCs from other species.

Myosin heavy chains IIa and IId are functionally distinct in the mouse

Myosin in adult murine skeletal muscle is composed primarily of three adult fast myosin heavy chain (MyHC) isoforms. These isoforms, MyHC-IIa, -IId, and -IIb, are >93% identical at the amino acid level and are broadly expressed in numerous muscles, and their genes are tightly linked. Mice with a null mutation in the MyHC-IId gene have phenotypes that include growth inhibition, muscle weakness, histological abnormalities, kyphosis (spinal curvature), and aberrant kinetics of muscle contraction and relaxation. Despite the lack of MyHC-IId, IId null mice have normal amounts of myosin in their muscles because of compensation by the MyHC-IIa gene. In each muscle examined from IId null mice, there was an increase in MyHC-IIa- containing fibers. MyHC-IIb content was unaffected in all muscles except the masseter, where its expression was extinguished in the IId null mice. Cross-sectional fiber areas, total muscle cross-sectional area, and total fiber number were affected in ways particular to each muscle. Developmental expression of adult MyHC genes remained unchanged in IId null mice. Despite this universal compensation of MyHC-IIa expression, IId null mice have severe phenotypes. We conclude that despite the similarity in sequence, MyHC-IIa and -IId have unique roles in the development and function of skeletal muscle.

Quantitative analyses of myosin heavy-chain mRNA and protein isoforms in single fibers reveal a pronounced fiber heterogeneity in normal rabbit muscles

A highly sensitive method of reverse-transcriptase polymerase chain reaction (RT-PCR) was established to study myosin heavy-chain (MHC) mRNA isoform expression in single fibers of rabbit limb muscles. In combination with myofibrillar adenosine triphosphatase histochemistry and electrophoretic separation of MHC protein isoforms in fragments of the same fibers, the direct RT-PCR method identified the pMHC20-40 and pMHC24-79 cDNA sequences as being specific to MHCIIb and MHCIId/x isoforms, respectively. In addition, a direct RT-PCR was established for determining relative amounts of MHC mRNA isoforms by using a sequence specific to alpha-skeletal actin as an endogenous reference. Analyses of large amounts of single fibers revealed an unexpected heterogeneity of the fast fiber population with regard to numerous fibers coexpressing MHCIIb and MHCIId/x. Based on quantitative RT-PCR, the percentages of MHCIIb/MHCIId hybrid fibers amounted to approximately 55% in the deep portion of gastrocnemius, to 43% in the adductor magnus, and to 12% in psoas muscle. Moreover, the two MHC mRNA isoforms were nonuniformly distributed along the fiber length. Qualitative RT-PCR detected even higher amounts of hybrid fibers in the three muscles. The percentages of hybrid fibers identified at the protein level were smaller in adductor magnus muscle (25%) and psoas muscle (5%), but equaled that of the mRNA analysis in gastrocnemius muscle (61%). The detection of high amounts of IIBD and IIDB fibers suggested that hybrid fibers represent functional elements within the fiber spectrum of normal muscles. Our observations on hybrid fibers reveal a heterogeneity within the fiber population of normal muscles that has not been realized to date.

Mismatch between myosin heavy chain mRNA and protein distribution in human skeletal muscle fibers

The distribution of myosin heavy chain (MHC) isoforms was analyzed at the protein and mRNA levels in human skeletal muscle biopsies from young normal adult subjects. Using ATPase histochemical reactions, antibodies to fast- and slow-type MHCs, and in situ hybridization with probes specific for MHC-beta/slow, MHC-2A, and MHC-2X, we confirmed our previous results showing that most fibers contain either a single mRNA and isoprotein or a mixed 1/2A or 2A/2X phenotype with coexistence of two mRNAs and isoproteins. However, we also found a minor proportion of fibers showing a mismatch in the relative proportion of mRNA and protein, e.g., fibers containing MHC-2A mRNA but not the corresponding protein or fibers containing MHC-2A protein but not the corresponding transcript. These fibers were more frequent in biopsies obtained after a training or detraining period than before the training period. We propose that these fibers represent transitional fibers and that the relative content of each mRNA and isoprotein gives a clue as to the direction of change in MHC gene expression.

Chronic congestive heart failure elicits adaptations of endurance exercise in diaphragmatic muscle

BACKGROUND

During rest and exercise, patients with heart failure hyperventilate; therefore, the diaphragm can be viewed as undergoing constant moderate-intensity exercise. Accordingly, we hypothesized that heart failure elicits adaptations in the diaphragm similar to those elicited by endurance exercise in the limb muscles of normal subjects.

METHODS AND RESULTS

Costal diaphragmatic biopsy samples were obtained from 7 normal subjects (age, 36 +/- 20 years) and 10 patients (age, 50 +/- 6 years; left ventricular ejection fraction, 18 +/- 8%) at the time of transplant or left ventricular assist-device placement. We measured the distribution of myosin heavy chain isoforms I, IIa, and IIb by SDS gel electrophoresis. We also measured the activities of the following enzymes: citrate synthase, a marker of oxidative metabolism; beta-hydroxyacyl-CoA dehydrogenase, a marker of lipolytic metabolism; and lactate dehydrogenase, a marker of glycolytic metabolism. In normal subjects, the distribution of myosin heavy chain isoforms I, IIa, and IIb was 43 +/- 2%, 40 +/- 2%, and 17 +/- 1%, respectively. In contrast, in heart failure subjects, the fiber distribution was 55 +/- 2%, 38 +/- 2%, and 7 +/- 2% for types I, IIa, and IIb, respectively. Therefore, in heart failure, myosin heavy chain I is increased (P < .0001) and myosin heavy chain IIb decreased from normal levels (P < .001). Additionally, citrate synthase activity (normal, 0.33 +/- 0.14; heart failure, 0.54 +/- 0.21 mumol.min-1.mg protein-1; P < .05) and beta-hydroxyacyl-CoA dehydrogenase activity (normal, 0.27 +/- 0.04; heart failure, 0.38 +/- 0.02 mumol.min-1.mg protein-1; P < .05) were greater in heart failure patients than in normal subjects, whereas lactate dehydrogenase activity was significantly less in heart failure patients than in normal subjects (normal, 11.6 +/- 4.6; heart failure,: 4.3 +/- 2.2 mumol.min-1.mg protein-1; P < .01).

CONCLUSIONS

In the diaphragm in heart failure, there is a shift from fast to slow myosin heavy chain isoforms with an increase in oxidative capacity and a decrease in glycolytic capacity. These diaphragmatic muscle changes are consistent with those elicited by endurance training of the limb muscles in normal subjects.