Effects of 84-days of bedrest and resistance training on single muscle fibre myosin heavy chain distribution in human vastus lateralis and soleus muscles

AIM: This investigation determined the effects of 84 days of bedrest on the composition of myosin heavy chain (MHC) in single skeletal muscle fibres with and without a resistance-training countermeasure programme. METHODS: Muscle biopsies were obtained from the m. vastus lateralis (VL) and m. soleus (SOL) before and after 84 days of bedrest. While control (BR) subjects (VL n = 9; SOL n = 3) refrained from exercise, BRE subjects (VL n = 8; SOL n = 3) performed knee extensor and plantar flexor resistance exercise every third day. Approximately 110 fibres per sample were analysed for MHC composition using SDS-PAGE. RESULTS: BR-VL had 16 and 14% decreases (P < 0.05) in MHC I and IIa fibres, respectively. There were 10% increases (P < 0.05) in MHC I/IIa, IIa/IIx, I/IIa/IIx, and a approximately 30% increase (P < 0.05) in total hybrid fibres. BRE-VL showed a 15% reduction (P < 0.05) in MHC I fibres, no change in MHC IIa fibres, and a 13% increase (P < 0.05) in total hybrids. BR-SOL had a 19% decrease (P < 0.05) in MHC I fibres with a 22% increase in total hybrids. BRE-SOL showed no change in MHC composition across all fibre types. CONCLUSION: These data suggest that the exercise countermeasures programme prevented MHC shifts in the SOL and mitigated MHC shifts in the VL. Furthermore, in the VL it appears that the resistance training programme employed in this investigation during bedrest, emphasized the use of MHC IIa phenotype muscle fibres.     

Human skeletal muscle structure and function preserved by vibration muscle exercise following 55 days of bed rest

Prolonged immobilization of the human body results in functional impairments and musculoskeletal system deconditioning that may be attenuated by adequate muscle exercise. In a 56-day horizontal bed rest campaign involving voluntary males we investigated the effects of vibration muscle exercise (RVE, 2×6 min daily) on the lower limb skeletal muscles using a newly designed foot plantar trainer (Galileo Space) for use at supine position during bed rest. The maximally voluntary isometric plantar flexion force was maintained following regular RVE bouts during bed rest (controls −18.6 %, P<0.05). At the start (BR2) and end of bed rest (BR55) muscle biopsies were taken from both mixed fast/slow-type vastus lateralis (VL) and mainly slow-type soleus muscle (SOL), each having n=10. RVE group: the size of myofiber types I and II was largely unchanged in VL, and increased in SOL. Ctrl group: the SOL depicted a disrupted pattern of myofibers I/II profiles (i.e., type II>140 % vs. preBR) suggesting a slow-to-fast muscle phenotype shift. In RVE-trained SOL, however, an overall conserved myofiber I/II pattern was documented. RVE training increased the activity-dependent expression of nitric oxide synthase type 1 immunofluorescence at SOL and VL myofiber membranes. These data provide evidence for the beneficial effects of RVE training on the deconditioned structure and function of the lower limb skeletal muscle. Daily short RVE should be employed as an effective atrophy countermeasure co-protocol preferentially addressing postural calf muscles during prolonged clinical immobilization or long-term human space missions.     

Myosin heavy chain isoforms in single fibres from m. vastus lateralis of sprinters: influence of training

The myosin heavy chain (MHC) composition of single fibres from m. vastus lateralis of a group of male sprint athletes (n= 6) was analysed, before and after a three months period of intensive strength- and interval-training, using a sensitive gel electrophoretic technique. Significant improvements were observed after training in almost all of a series of performance tests. After training the sprinters revealed a decrease in fibres containing only MHC isoform I (52.0 ±3.0% vs. 41.2 ±4.7% (mean±SE) (P < 0.05)) and an increase in the amount of fibres containing only MHC isoform IIA (34.7 ±6.1% vs. 52.3 ±3.6% (P < 0.05)). Fibres showing co-existence of MHC isoforms IIA and IIB decreased with training (12.9 ± 5.0% vs. 5.1 ±3.1%, (P < 0.05)). Only one out of 1000 fibres analysed contained only MHC isoform IIB. In contrast, a higher amount of type IIB fibres (18.8 ± 3.6% vs. 10.5 ± 3.9%, (P < 0.05)) was observed with myofibrillar ATPase histochemistry. The majority of histochemically determined type IIB fibres of sprinters seems therefore to contain both MHC isoforms IIA and IIB. Sprint-training appears to induce an increased expression of MHC isoform IIA in skeletal muscles. This seems related to a bi-directional transformation from both MHC isoforms I and IIB towards MHC isoform IIA.     

Intramyocellular lipid and glycogen content are reduced following resistance exercise in untrained healthy males

Resistance exercise has recently been shown to improve whole-body insulin sensitivity in healthy males. Whether this is accompanied by an exercise-induced decline in skeletal muscle glycogen and/or lipid content remains to be established. In the present study, we determined fibre-type-specific changes in skeletal muscle substrate content following a single resistance exercise session. After an overnight fast, eight untrained healthy lean males participated in a ~45 min resistance exercise session. Muscle biopsies were collected before, following cessation of exercise, and after 30 and 120 min of post-exercise recovery. Subjects remained fasted throughout the test. Conventional light and (immuno)fluorescence microscopy were applied to assess fibre-type-specific changes in intramyocellular triacylglycerol (IMTG) and glycogen content. A significant 27±7% net decline in IMTG content was observed in the type I muscle fibres (P<0.05), with no net changes in the type IIa and IIx fibres. Muscle glycogen content decreased with 23±6, 40±7 and 44±7% in the type I, IIa and IIx muscle fibres, respectively (P<0.05). Fibre-type-specific changes in intramyocellular lipid and/or glycogen content correlated well with muscle fibre-type oxidative capacity. During post-exercise recovery, type I muscle fibre lipid content returned to pre-exercise levels within 120 min. No changes in muscle glycogen content were observed during recovery. We conclude that intramyocellular lipid and glycogen stores are readily used during resistance exercise and this is likely associated with the reported increase in whole-body insulin sensitivity following resistance exercise.     

Knee extensor and plantar flexor muscle size and function following 90 days of bed rest with or without resistance exercise

Skeletal muscle atrophy and strength loss induced by short-term simulated spaceflight are offset or attenuated by resistance exercise (RE). This study compared the effects of plantar flexor and knee extensor RE on muscle size and function in 17 healthy men (aged 26–41years) subjected to 90 days 6° head-down-tilt bed rest with (BRE; n=8) or without (BR; n=9) RE. The RE program consisted of coupled maximal concentric and eccentric actions in the supine squat (4 sets of 7 repetitions) and calf press (4×14) every third day employing a gravity-independent flywheel ergometer (FW). Prior to, and following bed rest, muscle volume was assessed using magnetic resonance imaging. Similarly, muscle strength and power and surface electromyographic (EMG) activity were determined during maximal actions using FW or isokinetic dynamometry. In BR, knee extensor and plantar flexor muscle volume decreased (P<0.05) 18% and 29%, respectively. Torque or force and power decreased (P<0.05) 31–60% (knee extension) and 37–56% (plantar flexion) while knee extensor and plantar flexor EMG activity decreased 31–38% and 28–35%, respectively following BR. Muscle atrophy in BRE was prevented (P>0.05; knee extensors) or attenuated (–15%; plantar flexors). BRE maintained task-specific force, power and EMG activity. The decrease in non-task-specific torque was less (P<0.05) than in BR. The present data imply that the triceps surae and quadriceps muscles show different responsiveness to long-term bed rest with or without resistance exercise. The results also suggest that designing in-flight resistance exercise protocols for space travellers is complex and must extend beyond preserving muscle only.     

Myosin heavy chain isoform transformation in single fibres from m. vastus lateralis in spinal cord injured individuals: Effects of long-term functional electrical stimulation (FES)

The myosin heavy chain (MHC) composition of single fibres from m. vastus lateralis of five spinal-cord-injured (SCI) individuals was analysed by Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) before, and after 6 and 12 months of functional electrical stimulation (FES)-training, administrated for 30 min three times per week. Prior to FES training 37.2% of the fibres contained only MHC HB, 21.2% only MHC IIA, and 40.7% co-expressed MHC IIA and MHC IIB. After 6 months of FES-training the number of fibres containing only MHC IIB was reduced to 2.6% (P < 0.05), the number of fibres containing only MHC IIA was increased to 44.3 (P < 0.05), and the number of fibres co-expressing MHC IIA and MHC HB was 50.9% (ns). After 12 months almost all fibres (91.2%,P < 0.05) contained only MHC IIA. The number of fibres containing only MHC IIB was 2.3 % and the fibres co-expressing MHC HA and HB had decreased to 4.6% (P < 0.05). The amount of fibres containing only MHC I never exceeded 0.5%. Likewise, the number of fibres co-expressing MHC I and MHC IIA was below 2% throughout the study period. In total, the MHC composition of 1596 single fibres was determined. This study shows that FES-training of paralysed human skeletal muscle administrated over a prolonged period of time, can lead to a marked switch in MHC expression from about equal amounts of MHC HA and MHC HB to an almost total dominance of MHC HA.     

Clenbuterol induces expression of multiple myosin heavy chain isoforms in rat soleus fibres

Clenbuterol, a beta2-agonist, administration results in hypertrophy of fast fibres and an increase in the fast myosin heavy chain (MHC) composition of both fast and slow muscles. The present study was designed to determine the phenotypic response at the single fibre level. Clenbuterol was added to the drinking water (30 mg L(-1)) of adult male Wistar rats for 4 weeks. Single fibres from the soleus muscle of control (10 rats; 555 fibres) and clenbuterol-treated (10 rats; 577 fibres) were dissected and their MHC isoform composition was determined using sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis. Body, heart, and soleus weights were 9, 24, and 27% higher in clenbuterol-treated than control rats. The mean cross-sectional areas of fast and slow/fast hybrid fibres were approximately 64 and approximately 74% larger in the clenbuterol-treated than control rats, whereas the size of the slow fibres were similar in the two groups. Fibres from control soleus showed three MHC patterns: pure type I (84%), pure type IIa (4%), and type I + IIa (12%) MHC. Some fibres from clenbuterol-treated soleus showed a de novo expression of type IIx MHC resulting in the following fibre type proportions: pure type I (62%), pure type IIa (2%), type I + IIa (26%), type I + IIa + IIx (6%), and type IIa + IIx (1%). In those fibres containing multiple MHCs, there was a shift towards the faster MHC isoforms after clenbuterol treatment. These data indicate that clenbuterol results in muscle fibre hypertrophy, stimulates a de novo expression of type IIx MHC and increases the percentage of fibres containing multiple MHC isoforms in the rat soleus muscle. These phenotypic changes at the single fibre level are consistent with a clenbuterol-related shift in the functional properties of the soleus towards those observed in a faster muscle.     

Improving human skeletal muscle myosin heavy chain fiber typing efficiency

Single muscle fiber sodium dodecyl sulfate polyacrylamide gel-electrophoresis (SDS-PAGE) is a sensitive technique for determining skeletal muscle myosin heavy chain (MHC) composition of human biopsy samples. However, the number of fibers suitable to represent fiber type distribution via this method is undefined. Muscle biopsies were obtained from the vastus lateralis (VL) of nine resistance-trained males (25 ± 1 year, height = 179 ± 5 cm, mass = 82 ± 8 kg). Single fiber MHC composition was determined via SDS-PAGE. VL fiber type distribution [percent MHC I, I/IIa, IIa, IIa/IIx, and total “hybrids” (i.e. I/IIa + IIa/IIx)] was evaluated according to number of fibers analyzed per person (25 vs. 125). VL fiber type distribution did not differ according to number of fibers analyzed (P > 0.05). VL biopsy fiber type distribution of nine subjects is represented by analyzing 25 fibers per person. These data may help minimize cost, personnel-time, and materials associated with this technique, thereby improving fiber typing efficiency in humans.     

Effects of Growth hormone on rat skeletal muscle after hindlimb suspension

To examine the effects of growth hormone (GH) on the preferential atrophy of the soleus muscle (SOL) occurring after hindlimb suspension (HS), two groups of male rats received daily injections of 2 IU · kg ^–1 body mass of recombinant human growth hormone (rhGH). Rats were either suspended by the tail for 21 days (HS-GH, n = 5) or nonsuspended (CGH, n=5). The effects of rhGH treatment on SOL and extensor digitorum longus muscles (EDL) were compared in two groups of animals receiving daily injections of saline, either suspended by the tail (HS-SA, n = 5) or nonsuspended (C-SA, n = 5). The results showed that the SOL hypertrophy in response to rhGH administration was mostly observed in C rats (+33%, P<0.01). This increase in muscle mass was correlated with a concomitant increase in the size of type I fibres (+21%, P<0.05). Although SOL mass decreased during HS in rhGH treated animals (–44%, P<0.001), the mean normalized mass of this muscle did not significantly differ between C-SA and HS-GH groups. A statistically significant increase in the absolute mass of EDL occurred with rhGH treatment in CGH (+12%, P<0.05). The HS-induced decrease in the percentage distribution of type I fibres in SOL was unaffected by the rhGH treatment. In addition, a decrease in the citrate synthase activity in the whole SOL was observed in the two groups of tail-suspended rats (–31%, P<0.05; –21%, P<0.05 in SA and GH animals, respectively). The activity of 3-hydroxyacyl coenzyme A dehydrogenase was enhanced by the rhGH treatment (P<0.05) with similar magnitude in both C (+25%) and HS rats (+24%). Therefore, GH prevented only slightly the atrophy of SOL, occurring after 21 days of HS. The effects of rhGH treatment appeared most effective in C rats, suggesting that HS impaired the growth-promoting effects of this hormone on skeletal muscle.     

Changes in electromyographic activity,muscle fibre and force production characteristics during heavy resistance/power strength training in middle‐aged and older men and women

The effects of a 6‐month resistance training (2 day/week) designed to develop both strength and power on neural activation by electromyographic activity (EMG) of the agonist and antagonist knee extensors, muscle fibre proportion and areas of type I, IIa, and IIb of the vastus lateralis (VL) as well as maximal concentric one repetition maximum (1 RM) strength and maximal and explosive isometric strength of the knee extensors were examined. A total of 10 middle‐aged men (M40; 42 ± 2), 11 middle‐aged women (W40; 39 ± 3), 11 elderly men (M70; 72 ± 3) and 10 elderly women (W70; 67 ± 3) served as subjects. Maximal and explosive strength values remained unaltered during a 1‐month control period. After the 6‐month training maximal isometric and 1RM strength values increased in M40 by 28 ± 14 and 27 ± 7% (P < 0.001), in M70 by 27 ± 17 and 21 ± 9% (P < 0.001), in W40 by 27 ± 19 and 35 ± 14% (P < 0.001) and in W70 by 26 ± 14 and 31 ± 14% (P < 0.001), respectively. Explosive strength improved in M40 by 21 ± 41% (P < 0.05), in M70 by 21 ± 24% (P < 0.05), in W40 by 32 ± 45% (NS) and in W70 by 22 ± 28% (P < 0.05). The iEMGs of the VL and vastus medialis (VM) muscles increased during the training in M40 (P < 0.001 and 0.05), in M70 (P < 0.001 and 0.05), in W40 (P < 0.001 and 0.05) and in W70 (P < 0.001 and 0.05). The antagonist biceps femoris (BF) activity during the isometric knee extension remained unaltered in M40, in W40, and in M70 but decreased in W70 (from 42 ± 34 to 32 ± 26%; P < 0.05) during the first 2 months of training. Significant increases occurred during the training in the mean fibre areas of type I in W70 (P < 0.05) and of overall type II along with a specific increase in IIa in both W40 (P < 0.05) and in W70 (P < 0.05), while the changes in the male groups were not statistically significant. The individual percentage values for type II fibres at pretraining correlated with the individual values for 1 RM strength in both W70 (r=0.80; P < 0.05) and M70 (r=0.61; P < 0.05) and also at post‐training for maximal isometric torque in W70 (r=0.77, P < 0.05). The findings support the concept of the important role of neural adaptations in strength and power development in middle‐aged and older men and women. The muscle fibre distribution (percentage type II fibres) seems to be an important contributor on muscle strength in older people, especially older women. Women of both age groups appear to be hypertrophically responsive to the total body strength training protocol performed two times a week including heavier and lower (for fast movements) loads designed for both maximal strength and power development, while such a programme has limited effects on muscle hypertrophy in men.     

Myosin heavy chain isoforms in single fibres from m. vastus lateralis of soccer players: effects of strength-training

The myosin heavy chain (MHC) composition of single fibres (n= 2171) was analysed with an electrophoretic technique in biopsy material from m. vastus lateralis of two groups of soccer players before and after a 3-month period of either strength- (n= 8) or non-training (control) (n= 6). Traditional myofibrillar ATPase histochemistry demonstrated a decrease in type IIA fibres with strength-training (35.4 ± 2.1 vs. 26.7 ± 2.4% (P < 0.05)). This was not observed in the non-training group (25.7 ± 4.6 vs. 23.8 ± 1.7%). One-dimensional electrophoresis on muscle homogenates showed no significant change in the amount of MHC isoforms in either of the two groups. The MHC isoform I IB was undetectable in all but three samples. No changes in the proportions of fibres containing any of the MHC isoforms were observed. Fibres containing only MHC isoform I IB were found in very small numbers (only 11 out of 2171). Before the experimental period, between 6 and 10% histochemical type IIB fibres were found in both groups. This was identical with the proportion of fibres showing co-existence of MHC isoforms IIA and IIB, but in contrast to the very few fibres containing only MHC isoform IIB. This suggests that nearly all histochemical type IIB fibres of the soccer players display co-existence of both MHC isoform IIA and IIB. No major change in the muscle fibre area of the two groups was observed. The strength-training group increased mean power output in a short-term dynamic knee-extensor test after strength-training (119 ± 6.6 vs. 136.5 ± 4.7 W (P < 0.01)), whereas no change was observed in the non-training group (151.2 ± 4.5 vs. 148.4 ± 6.5 W).     

Lack of coordinated changes in metabolic enzymes and myosin heavy chain isoforms in regenerated muscles of trained rats

We investigated training-induced changes in biochemical properties and myosin heavy chain (MHC) composition of regenerated (cardiotoxin-injected) plantaris muscles (PLA) in rats either maintained sedentary (S, n = 9) or endurance trained on a treadmill over a 8-week period (T, n = 7). Both endurance training and regeneration altered the pattern of fast MHC expression. An analysis of the two-way interaction between training and regeneration showed that the relative content of type IIa MHC was affected (P < 0.05). The 140% increase in type IIa MHC observed in regenerated PLA from T rats compared with nontreated muscle of S rats, exceeded the 102% increase resulting from the combination of regeneration alone (26%) and training alone (61%). A similar interaction between training and regeneration was shown for the percentage of fibres expressing either type IIa or type IIb MHC (P < 0.05). In contrast, a significant increase in the citrate synthase (CS) activity was shown in PLA as a result of endurance training, without specific effect of regeneration. Furthermore, training-induced changes in CK and LDH isoenzyme distribution occurred to a similar extent in regenerated and non-treated PLA muscles, and thus did not follow the changes in MHC isoforms. An increase in the mitochondrial CK isozyme activity (mi-CK) was shown in both non-treated and previously degenerated PLA muscles (123 and 117%, P < 0.01, respectively), without specific effect of regeneration. The ratio of mi-CK to CS activity, an estimate of the mitochondrial specific activity of mi-CK was significantly increased by training (P < 0.02) and decreased by regeneration (P < 0.05). Taken together, these data suggest that while training and regeneration have cumulative effects on the pattern of fast MHC expression, the training-induced changes in the energy metabolism shown in mature non-treated myofibres are similar to those observed in regenerated fibres.     

Age and sex affect human muscle fibre adaptations to heavy-resistance strength training

This study assessed age and sex effects on muscle fibre adaptations to heavy-resistance strength training (ST). Twenty-two young men and women (20-30 years old) and 18 older men and women (65-75 years old) completed 9 weeks of heavy-resistance knee extension exercises with the dominant leg 3 days week(-1); the non-dominant leg served as a within-subject, untrained control. Bilateral vastus lateralis muscle biopsies were obtained before and after ST for analysis of type I, IIa and IIx muscle fibre cross-sectional area (CSA) and fibre type distribution. One-repetition maximum (1-RM) strength was also assessed before and after ST. ST resulted in increased CSA of type I, IIa and IIx muscle fibres in the trained leg of young men, type I and IIa fibres in young women, type IIa fibres in older men, and type IIx fibres in older women (all P<0.05). Analysis of fibre type distribution revealed a significant increase in the percentage of type I fibres (P<0.05) along with a decrease in type IIx fibres (P=0.054) after ST only in young women. There were no significant changes in muscle fibre CSA or fibre type distribution in the untrained leg for any group. All groups displayed significant increases in 1-RM (27-39%; all P<0.01). In summary, ST led to significant increases in 1-RM and type II fibre CSA in all groups; however, age and sex influence specific muscle fibre subtype responses to ST.     

Co-existence of myosin heavy chain I and IIa isoforms in human skeletal muscle fibres with endurance training

The myosin heavy chain (MHC) composition of single fibres from m. vastus lateralis was analysed by one-dimensional electrophoresis and immunoblotting in three groups of young men with distinct difference in physical activity patterns. No major co-existence of MHC isoforms was found in the group with some daily physical activity. In the very sedentary group, however, 19±5% (P<0.05) of the fibres exhibited coexistence of MHC type IIa and IIb. Further, in the endurance trained group co-existence of MHC type I and IIa was manifested in 36±4% (P<0.05) of the fibres. Disuse and extreme usage of muscle both give rise to an elevation in co-expression of MHC isoforms in single muscle fibres but of markedly different combination of isoforms.     

Clenbuterol-induced fiber type transition in the soleus of adult rats

This study examined the effects of 6 weeks of treatment with theβ ₍₂₎-adrenoceptor agonist, clenbuterol, on the soleus muscle of adult female Sprague-Dawley rats. Animals (4 months old) were divided into two groups: clenbuterol treated (CL,n=7) (2 mg kg⁻¹ body mass injected subcutaneously every other day), and control (CON,n=7) (injected with isotonic saline). Post-treatment body weights were ≈ 5% greater in the CL group compared to CON (P<0.05). Polyacrylamide gel electrophoresis (SDS-PAGE) of soleus myofibrillar protein indicated a clenbuterol-induced decrease (P<0.05) in the relative percentage of type I myosin heavy chain (MHC) with a concomitant increase (P<0.05) in type IIdx MHC, while the proportion of type IIa MHC was unaffected. ATPase fiber typing revealed increases (P<0.05) in the proportion of type II fibers expressed both as a percentage of total fiber number and total cross-sectional area (CSA). Finally, mean type II fiber CSA was ≈25% greater (P<0.05) in the CL groups as compared to the CON group. These data indicate that clenbuterol treatment results in alterations in the MHC phenotype and an increased proportion of type II fiber CSA in the soleus of adult rats. These observations were due to an increase in the total number of type II fibers, as well as hypertrophy of these fibers. Thus, the relative increase in the number of histochemically determined type II fibers and the emergence of the normally unexpressed type IIdx MHC isoform in the soleus suggest a clenbuterol-induced transition of muscle fiber phenotype as well as selective hypertrophy of the type 11 fibers.     

Differences in sodium voltage-gated channel properties according to myosin heavy chain isoform expression in single muscle fibres

The myosin heavy chain (MHC) isoform determines the characteristics and shortening velocity of muscle fibres. The functional properties of the muscle fibre are also conditioned by its membrane excitability through the electrophysiological properties of sodium voltage-gated channels. Macropatch-clamp is used to study sodium channels in fibres from peroneus longus (PL) and soleus (Sol) muscles (Wistar rats, n = 8). After patch-clamp recordings, single fibres are identified by SDS-PAGE electrophoresis according to their myosin heavy chain isoform (slow type I and the three fast types IIa, IIx, IIb). Characteristics of sodium currents are compared (Student's t test) between fibres exhibiting only one MHC isoform. Four MHC isoforms are identified in PL and only type I in Sol single fibres. In PL, maximal sodium current ( I _max), maximal sodium conductance ( g _Na,max) and time constants of activation and inactivation (τ _m and τ _h ) increase according to the scheme I→IIa→IIx→IIb ( P < 0.05). τ _m values related to sodium channel type and/or function, are similar in Sol I and PL IIb fibres ( P = 0.97) despite different contractile properties. The voltage dependence of activation ( V _a,1/2) shows a shift towards positive potentials from Sol type I to IIa, IIx and finally IIb fibres from PL ( P < 0.05). These data are consistent with the earlier recruitment of slow fibres in a fast-mixed muscle like PL, while slow fibres of postural muscle such as soleus could be recruited in the same ways as IIb fibres in a fast muscle.     

Velocity recovery cycles of C fibres innervating human skin

The purpose of this study was to determine whether induced expression of the Ca²⁺ buffering protein parvalbumin (PV) in slow-twitch fibres would lead to alterations in physiological, biochemical and molecular properties reflective of a fast fibre phenotype. Transgenic (TG) mice were generated that overexpressed PV in slow (type I) muscle fibres. In soleus muscle (SOL; 58 % type I fibres) total PV expression was 2- to 6-fold higher in TG compared to wild-type (WT) mice. Maximum twitch and tetanic tensions were similar in WT and TG but force at subtetanic frequencies (30 and 50 Hz) was reduced in TG SOL. Twitch time-to-peak tension and half-relaxation time were significantly decreased in TG SOL (time-to-peak tension: 39.3 ± 2.6 vs. 55.1 ± 4.7 ms; half-relaxation time: 42.1 ± 3.5 vs. 68.1 ± 9.6 ms,   P < 0.05  for TG vs. WT, respectively;   n = 8  –10). There was a significant increase in expression of type IIa myosin heavy chain (MHC) and ryanodine receptor at the mRNA level in TG SOL but there were no differences in MHC expression at the protein level and thus no difference in fibre type. Whole muscle succinate dehydrogenase activity was reduced by 12 ± 0.4 % in TG SOL and single fibre glycerol-3-phosphate dehydrogenase activity was decreased in a subset of type IIa fibres. These differences were associated with a 64 % reduction in calcineurin activity in TG SOL. These data show that overexpression of PV, resulting in decreased calcineurin activity, can alter the functional and metabolic profile of muscle and influence the expression of key marker genes in a predominantly slow-twitch muscle with minimal effects on the expression of muscle contractile proteins.     

Effects of strength, endurance and combined training on myosin heavy chain content and fibre-type distribution in humans

This study investigated the effect of strength training, endurance training, and combined strength plus endurance training on fibre-type transitions, fibre cross-sectional area (CSA) and MHC isoform content of the vastus lateralis muscle. Forty volunteers (24 males and 16 females) were randomly assigned to one of four groups: control (C), endurance training (E), strength training (S), or concurrent strength and endurance training (SE). The S and E groups each trained three times a week for 12 weeks; the SE group performed the same S and E training on alternate days. The development of knee extensor muscle strength was S>SE>E (P<0.05) and has been reported elsewhere. The reduction in knee extensor strength development in SE as compared to S corresponded to a 6% increase in MHCIIa content (P<0.05) in SE at the expense of the faster MHCIId(x) isoform (P<0.05), as determined by electrophoretic analyses; reductions in MHCIId/x content after S or E training were attenuated by comparison. Both S and SE induced three- to fourfold reductions (P<0.05) in the proportion of type IIA/IID(X) hybrid fibres. S also induced fourfold increases in the proportion of type I/IIA hybrid fibres within both genders, and in a population of fibres expressing a type I/IID(X) hybrid phenotype within the male subjects. Type I/IIA hybrid fibres were not detected after SE. Both S and SE training paradigms induced similar increases (16–19%, P<0.05) in the CSA of type IIA fibres. In contrast, the increase in CSA of type I fibres was 2.9-fold greater (P<0.05) in S as compared to SE after 12 weeks. We conclude that the interference of knee extensor strength development in SE versus S was related to greater fast-to-slow fibre-type transitions and attenuated hypertrophy of type I fibres. Data are given as mean (SEM) unless otherwise stated.     

Sarcoplasmic reticulum of human skeletal muscle: age-related changes and effect of trainirig

The effect of ageing on human skeletal muscle was investigated using needle biopsies from young and aged subjects and from aged subjects trained with different activity patterns. Histochemical staining for myofibrillar ATPase of ageing m. vastus lateralis demonstrated an unchanged fibre type distribution but a selective atrophy of type IIa and type IIb fibres. Analysis of myosin heavy chain (MHC) composition showed that type I MHC increased with ageing (P< 0.05). The relative content of the MHC isoforms correlated with the relative area of the respective fibre types. Sarcoplasmic reticulum (SR) proteins were investigated in muscle extracts by electrophoretic and immunoblotting techniques. When compared to a young control group (28 0.1 years old, n = 7) blots of post-myofibrillar supernatant proteins probed with polyclonal antibodies to the rabbit fast SR Ca-ATPase, a marker of extrajunctional SR, showed that the content of Ca-ATPase was significantly lower (P < 0.05) in the old control group (68 ± 0.5 years old, n= 8). On the other hand the content of calsequestrin (CS), the major intraluminal protein of SR terminal cisternae (TC), and of the 350-kDa ryanodine-binding protein, which is localized in the junctional regions of TC, did not show a concomitant decrease. These results suggest that ageing differentially affects extrajunctional and junctional SR of human skeletal muscle. These age-related changes were not observed within a group of old strength-trained subjects.     

Regional specialization of rat quadriceps myosin heavy chain isoforms occurring in distal to proximal parts of middle and deep regions is not mirrored by citrate synthase activity

Myosin heavy chain (MHC) isoform content and citrate synthase (CS) activities were measured in the Quadriceps femoris (QF) muscle of 18 female rats. The muscle group was divided into superficial, middle and deep, distal, central and proximal parts. MHC IIb and IIx were more abundant in superficial regions (P<0.05) with low CS activities compared with deeper parts. The deeper parts expressed all four isoforms (MHC IIb, MHC IIx, MHC IIa and MHC I), with a concomitantly higher CS activity. MHC I, MHC IIa and MHC IIb isoform content varied significantly along the length of the deep regions. Only MHC IIb and CS activity in the proximal middle part correlated (negatively) with each other. This study showed that the QF has regional specialization and that standardization of sampling site is important. Furthermore, CS activity and MHC isoforms are only loosely associated, or not at all.