MHC IIa Fibers Research Articles

Effects of Aging and Fatigue on Human Skeletal Muscle Cellular Contractile Function

Aging is associated with a reduction in skeletal muscle mass and function, known as sarcopenia. While reductions in contractile force are typically explained by loss in muscle cellular cross sectional area (CSA) loss of contractile power during high velocity contractions exceed reductions in force and cannot be solely explained by loss of muscle size and are observed at the single fiber within the same myosin heavy chain isoform (MHC). These phenomenon may be explained by myosin binding protein C (MyBP-C), which is known to modulate contractile force and velocity in a phosphorylation-dependent manner in both cardiac and skeletal muscle. Few studies have investigated the inherent contractile performance of single fibers between old and young adults, and none have explored the effects of altered MyBP-C phosphorylation on single fiber function. Therefore, the PURPOSE of this study was to investigate the effects of age and fatigue-induced MyBP-C phosphorylation on single fiber contractile performance. We HYPOTHESIZED that isometric tension will be enhanced in older adults but velocity and power will be reduced, and fatigue will enhance velocity and power in single fibers. To test this hypothesis, we recruited 4 young (18-35) and 3 older (65-80) adult males to perform a single bout of unilateral knee extensions at 30% maximal voluntary isometric contraction (MVIC) until task failure. Immediately following, a bilateral biopsy of the vastus lateralis (VL) muscle was performed on the fatigued and non-fatigued control limbs. Tissue was either flash frozen for western blot or prepared for mechanical analysis of single fiber tension (Tmax), velocity (Vmax), and power (Pmax) under optimal intracellular metabolic conditions. Our analyses were limited to MHC IIA due to differences in fiber type distribution between the groups. Our RESULTS showed that neither age nor fatigue had an effect on contractile performance in MHC IIA fibers. However, independent analysis of fatigue within age groups suggest trends towards an increase in velocity (p=0.055) in young (MHC IIA) and reduction in tension (p=0.085) in old (MHC I) adult fibers. We CONCLUDE that fatigue may modulate contractile function in the single fiber in an age- and fiber-type dependent manner. Future studies will be aimed at collecting multiple fiber types and investigating the role of biological sex. Wu Tsai Human Performance Alliance. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Skeletal Muscle Function Is Altered in Male Mice on Low-Dose Androgen Receptor Antagonist or Estrogen Receptor Agonist.

In males, skeletal muscle function may be altered by shifts in either circulating testosterone or estrogen. We examined the effect of acute (2-week) exposures to 17α-ethinyl estradiol (EE2), an estrogen receptor (ER) agonist, or flutamide, an androgen receptor (AR) antagonist, on the contractile function of individual skeletal muscle fibers from slow-contracting soleus and fast-contracting extensor digitorum longus muscles from adult male mice. Single fiber specific tension (force divided by cross-sectional area) was decreased with flutamide treatment in all myosin heavy chain (MHC) fiber types examined (I, IIA, and IIB); similar effects were observed with EE2 treatment but only in the fastest-contracting MHC IIB fibers. The decreases in maximally Ca2+-activated specific tension were primarily a result of fewer strongly bound myosin-actin cross-bridges, with flutamide treatment also showing lower myofilament lattice stiffness. Myosin-actin cross-bridge kinetics were slower in MHC IIA fibers in flutamide-treated mice, but faster in EE2-treated mice, indicating that contractile velocity may be affected differently in this fiber type, which is commonly expressed in human skeletal muscle. Importantly, these effects were observed in the absence of outcomes previously used to evaluate ER agonists or AR antagonists in rodents including weight of reproductive organs or mammary gland morphology. Our findings indicate that substantial shifts in skeletal muscle function occur in male mice following acute exposures to low doses of a pharmacological ER agonist and an AR antagonist. These results suggest that countermeasures to maintain physical function may be needed early in situations that induce similar ER agonist and AR antagonist conditions.

Revisiting specific force loss in human permeabilised single skeletal muscle fibres obtained from older individuals.

Specific force (SF) has been shown to be reduced in some but not all studies of human ageing using chemically skinned single muscle fibres. This may be due in part to the health status/physical activity levels of different older cohorts, but also from methodological differences in studying skinned fibres. The aim of the present study was to compare SF in fibres from older hip fracture patients (HFP), healthy master cyclists (MC) and healthy non-trained young adults (YA) using two different activating solutions. Quadriceps muscle samples and 316 fibres were obtained from HFPs (74.6±4 years, n=5), MCs (74.8±1, n=5) and YA (25.5±2, n=6). Fibres were activated (pCa 4.5, 15oC) in solutions containing either 60 mM TES or 20 mM imidazole. SF was determined by normalising force to fibre CSA assuming either an elliptical or circular shape and to fibre myosin heavy chain content. Activation in TES resulted in significantly higher MHC-I SF in all groups and YA MHC-IIA fibres, irrespective of normalisation method. Whilst there were no differences in SF between the participant groups, the ratio of SF between the TES and imidazole solutions was lower in HFPs compared to YAs (MHC-I p<0.05; MHC-IIA p=0.055). Activating solution composition, as opposed to donor characteristics, had a more notable effect on single fibre SF. However, this two-solution approach revealed an age-related difference in sensitivity in HFPs which was not shown in MCs. This suggests further novel approaches may be required to probe age/activity-related differences in muscle contractile quality.

β2-Adrenergic agonist salbutamol augments hypertrophy in MHCIIa fibers and sprint mean power output but not muscle force during 11 weeks of resistance training in young men.

In this study, we examined the effect of β2-agonist salbutamol at oral doses during a period of resistance training on sprint performance, quadriceps contractile function, skeletal muscle hypertrophy, fiber type composition, maximal activity of enzymes of importance for anaerobic energy turnover, and sarcoplasmic reticulum Ca2+ handling in young men. Twenty-six men (23 ± 2 yr; means ± SD) were randomized to daily intake of oral salbutamol (16 mg/day; RES+SAL) or placebo (RES) during 11 wk of full-body resistance training 3 times/wk. Mean power output during 10-s maximal cycling increased more (P = 0.027) in RES+SAL (+12%) than in RES (+7%), whereas peak power output increased similarly (RES+SAL: +8%; RES: +7%; P = 0.400). Quadriceps dynamic peak torque and maximal voluntary isometric torque increased by 13 and 14% (P ≤ 0.001) in RES+SAL and 13 and 13% (P ≤ 0.001) in RES, respectively. Myosin heavy-chain (MHC) isoform distribution transitioned from MHCI and MHCIIx toward MHCIIa in RES+SAL (P = 0.002), but not in RES (P = 0.323). MHCIIa cross-sectional-area increased more (P = 0.040) in RES+SAL (+35%) than RES (+21%). Sarcoplasmic reticulum Ca2+ release rate increased in both groups (RES+SAL: +9%, P = 0.048; RES: +13%, P = 0.008), whereas Ca2+-uptake rate increased only in RES (+12%, P = 0.022) but was not different from the nonsignificant change in RES+SAL (+2%, P = 0.484). Maximal activity of lactate dehydrogenase increased only in RES+SAL (+13%, P = 0.008). Muscle content of the dihydropyridine receptor, ryanodine receptor 1, and sarcoplasmic reticulum Ca2+-ATPase isoform 1 and 2 did not change with the intervention in either group (P ≥ 0.100). These observations indicate that the enhancement of sprint mean power output induced by salbutamol is at least partly attributed to greater hypertrophy of MHCIIa fibers and transition toward the MHCIIa isoform.NEW & NOTEWORTHY Here, we show that daily oral treatment with selective β2-agonist salbutamol induces muscle fiber isoform transition from myosin-heavy-chain (MHC)-I toward MHCIIa and augments hypertrophy of MHCIIa fibers during a period of resistance training. Compared with placebo, salbutamol enhanced sprint mean power output, whereas peak power output and measures of muscle strength increased similarly during the resistance training period despite augmented hypertrophy with salbutamol. Thus, salbutamol is a muscle anabolic drug that can enhance sprint ability adaptations to resistance training.

Absence of complement factor H reduces physical performance in C57BL6 mice

Complement (C) system is a double edge sword acting as the first line of defense on the one hand and causing aggravation of disease on the other. C activation when unregulated affects different organs including muscle regeneration. However, the effect of factor H (FH), a critical regulator of the alternative C pathway in muscle remains to be studied. FH deficiency results in excessive C activation and generates proinflammatory fragments C5a and C3a as byproducts. C3a and C5a signal through their respective receptors, C5aR and C3aR. In this study, we investigated the role of FH and downstream C5a/C5aR signaling in muscle architecture and function. Using the FH knockout (fh-/-) and fh-/-/C5aR-/double knockout mice we explored the role of C, specifically the alternative C pathway in muscle dysfunction. Substantial C3 and C9 deposits occur along the walls of the fh-/- muscle fibers indicative of unrestricted C activation. Physical performance assessments of the fh-/- mice show reduced grip endurance (76 %), grip strength (14 %) and rotarod balance (36 %) compared to controls. Histological analysis revealed a shift in muscle fiber populations indicated by an increase in glycolytic MHC IIB fibers and reduction in oxidative MHC IIA fibers. Consistent with this finding, mitochondrial DNA (mtDNA) and citrate synthase (CS) expression were both reduced indicating possible reduction in mitochondrial biomass. In addition, our results showed a significant increase in TGFβ expression and altered TGFβ localization in this setting. The architecture of cytoskeletal proteins actin and vimentin in the fh-/- muscle was changed that could lead to contractile weakness and loss of skeletal muscle elasticity. The muscle pathology in fh-/- mice was reduced in fh-/-/C5aR-/- double knockout (DKO) mice, highlighting partial C5aR dependence. Our results for the first time demonstrate an important role of FH in physical performance and skeletal muscle health.

Muscle health and performance in monozygotic twins with 30 years of discordant exercise habits.

Physical health and function depend upon both genetic inheritance and environmental factors (e.g., exercise training). To enhance the understanding of heritability/adaptability, we explored the skeletal muscle health and physiological performance of monozygotic (MZ) twins with > 30 years of chronic endurance training vs. no specific/consistent exercise. One pair of male MZ twins (age = 52 years; Trained Twin, TT; Untrained Twin, UT) underwent analyses of: (1) anthropometric characteristics and blood profiles, (2) markers of cardiovascular and pulmonary health, and (3) skeletal muscle size, strength, and power and molecular markers of muscle health. This case study represents the most comprehensive physiological comparison of MZ twins with this length and magnitude of differing exercise history. TT exhibited: (1) lower body mass, body fat%, resting heart rate, blood pressure, cholesterol, triglycerides, and plasma glucose, (2) greater relative cycling power, anaerobic endurance, and aerobic capacity (VO2max), but lower muscle size/strength and poorer muscle quality, (3) more MHC I (slow-twitch) and fewer MHC IIa (fast-twitch) fibers, (4) greater AMPK protein expression, and (5) greater PAX7, IGF1Ec, IGF1Ea, and FN14 mRNA expression than UT. Several measured differences are the largest reported between MZ twins (TT expressed 55% more MHC I fibers, 12.4ml/kg/min greater VO2max, and 8.6% lower body fat% vs. UT). These data collectively (a) support utilizing chronic endurance training to improve body composition and cardiovascular health and (b) suggest the cardiovascular and skeletal muscle systems exhibit greater plasticity than previously thought, further highlighting the importance of studying MZ twins with large (long-term) differences in exposomes.

Skeletal muscle signature of a champion sprint runner.

We had the unique opportunity to study the skeletal muscle characteristics, at the single fiber level, of a world champion sprint runner who is the current indoor world record holder in the 60-m hurdles (7.30 s) and former world record holder in 110-m hurdles (12.91 s). Muscle biopsies were obtained from the vastus lateralis at rest and 4 h after a high-intensity exercise challenge (4 × 7 repetitions of resistance exercise). Single muscle fiber analyses were conducted for fiber type distribution (myosin heavy chain, MHC), fiber size, contractile function (strength, speed, and power) and mRNA expression (before and after the exercise bout). The world-class sprinter's leg muscle had a high abundance (24%) of the pure MHC IIx muscle fibers with a total fast-twitch fiber population of 71%. Power output of the MHC IIx fibers (35.1 ± 1.4 W/l) was 2-fold higher than MHC IIa fibers (17.1 ± 0.5 W/l) and 14-fold greater than MHC I fibers (2.5 ± 0.1 W/l). Additionally, the MHC IIx fibers were highly responsive to intense exercise at the transcriptional level for genes involved with muscle growth and remodeling (Fn14 and myostatin). To our knowledge, the abundance of pure MHC IIx muscle fibers is the highest observed in an elite sprinter. Further, the power output of the MHC IIa and MHC IIx muscle fibers was greater than any human values reported to date. These data provide a myocellular basis for the high level of sprinting success achieved by this individual.

Single muscle fiber gene expression with run taper.

This study evaluated gene expression changes in gastrocnemius slow-twitch myosin heavy chain I (MHC I) and fast-twitch (MHC IIa) muscle fibers of collegiate cross-country runners (n = 6, 20±1 y, VO2max = 70±1 ml•kg−1•min−1) during two distinct training phases. In a controlled environment, runners performed identical 8 kilometer runs (30∶18±0∶30 min:s, 89±1% HRmax) while in heavy training (∼72 km/wk) and following a 3 wk taper. Training volume during the taper leading into peak competition was reduced ∼50% which resulted in improved race times and greater cross-section and improved function of MHC IIa fibers. Single muscle fibers were isolated from pre and 4 hour post run biopsies in heavily trained and tapered states to examine the dynamic acute exercise response of the growth-related genes Fibroblast growth factor-inducible 14 (FN14), Myostatin (MSTN), Heat shock protein 72 (HSP72), Muscle ring-finger protein-1 (MURF1), Myogenic factor 6 (MRF4), and Insulin-like growth factor 1 (IGF1) via qPCR. FN14 increased 4.3-fold in MHC IIa fibers with exercise in the tapered state (P<0.05). MSTN was suppressed with exercise in both fiber types and training states (P<0.05) while MURF1 and HSP72 responded to running in MHC IIa and I fibers, respectively, regardless of training state (P<0.05). Robust induction of FN14 (previously shown to strongly correlate with hypertrophy) and greater overall transcriptional flexibility with exercise in the tapered state provides an initial molecular basis for fast-twitch muscle fiber performance gains previously observed after taper in competitive endurance athletes.

Four days of muscle disuse impairs single fiber contractile function in young and old healthy men

The purpose of the study was to investigate the effects of 4days of disuse (knee brace) on contractile function of isolated vastus lateralis fibers (n=486) from 11 young (24.3±0.9yrs) and 11 old (67.2±1.0yrs) healthy men having comparable levels of physical activity. Prior to disuse single fiber specific force (maximal Ca2+-activated force per cross-sectional area) was lower in MHC I vs. IIa fibers (p<0.05) both in young (44%) and old (32%), and specific force of MHC IIa fibers was lower in old vs. young (19%, p<0.05). Further, Ca2+ sensitivity was higher in MHC I vs. IIa fibers (p<0.05) in both age groups. Following disuse single fiber specific force decreased (p<0.05) in MHC I fibers in young (−19%) and old (−17%), and in MHC IIa fibers in young (−21%) but not in old (−11%; ns). The extent of these decreases did not differ between young and old. Ca2+ sensitivity decreased (p<0.05) in MHC I fibers in old (−0.11 pCa units) but not in young (−0.08 pCa units; ns). The extent of these decreases was greater in old vs. young (p<0.05). In conclusion, 4days of lower limb disuse led to marked impairments in single muscle fiber specific force independently of age, while changes in Ca2+ sensitivity were dependent on age and MHC isoform composition. The present findings stress the importance of determining and implementing effective preventive and rehabilitative approaches for old individuals exposed to as little as 4days of disuse.

Novel Application of Western Blotting for Human Skeletal Muscle Fiber Type Specific Protein Content Assessment

Little is known about protein profiles in slow-twitch (MHC I) and fast-twitch (MHC IIa and MHC IIx) muscle fiber types from human skeletal muscle. PURPOSE:To develop a method of assessing fiber type specific protein content with low amounts of protein via Western blotting following single fiber SDS-PAGE MHC identification. METHODS:Two hundred and sixty four individual muscle fibers from a muscle biopsy of the vastus lateralis were isolated. Approximately ¼ of each single fiber was clipped and fiber-typed based on MHC composition using SDS-PAGE combined with silver staining. The remaining ¾ was stored in individually labeled wells of an RNALater filled 96-well plate. Following MHC identification, the preserved portions were combined into pools of 20 like fibers. Homogenized pools of known fiber types were loaded onto Western blots and probed for GAPDH, Actin, and total p38 to establish reliable protein detection using this methodology. RESULTS:At low amounts of protein, good reliability of this method was shown as MHC I and MHC IIa pools loaded in quintuplicate displayed 11% and 4% variance in GAPDH expression, respectively. When the amount of sample loaded on the blot doubled, GAPDH expression doubled in both MHC I and MHC IIa samples, implying consistent transferring and equal protein loading. Actin content did not differ between fiber types and was therefore found to be an appropriate loading control. GAPDH content was 67% (MHC IIa) and 84% (MHC IIx) more abundant when compared to MHC I pools. Similarly, total p38 expression was 40% greater in MHC IIa fibers when compared to MHC I fibers. CONCLUSIONS:These data establish a novel application of Western blotting for fiber type specific protein quantification in human skeletal muscle. These initial results show content of specific proteins are 40-84% higher in fast-twitch muscle fibers as compared to slow-twitch muscle fibers (MHC IIx>MHC IIa>MHC I). Application of these methods can enhance our knowledge on fiber type specific differences in relation to skeletal muscle health profiles among physically active and clinically based populations.

Skeletal muscle myosin light chain composition of highly‐trained endurance runners

Individual slow (MHC I) and fast (MHC IIa) twitch muscle fibers from highly-trained competitive runners display a greater contractile velocity (Vo) compared to moderately-active individuals. Myosin light chains (MLC) have been implicated in altering myofiber Vo, but this relationship is ambiguous in human skeletal muscle. Therefore, the purpose of this study was to examine MLC composition and its association with single muscle fiber contractile function in competitive (RUN; 71±1 ml·kg−1·min−1, 95±2 km/wk running) and recreational runners (REC; 47±2 ml·kg−1·min−1, <25 km/wk). SDS-PAGE, silver staining, and densitometry were used to assess MLC composition in mixed muscle homogenates and individual myofibers (MHC I and IIa) obtained from skeletal muscle biopsy samples. At the mixed muscle level, MLC 2f was lower (p<0.05) in RUN compared to REC with trends (p<0.08) for greater 1s, 2s and lower 3f ratios. The MLC profile of MHC I fibers was similar between RUN and REC. However, the MLC profile of the MHC IIa fibers from RUN contained less MLC 1s and greater amounts of 1f and 2s (p<0.05) compared to REC. These data suggest that only subtle differences exist in the MLC profile between RUN and REC. Therefore, it is likely that MLC composition contributes only a minor, if any, role in the robust differences in contractile function between competitive and recreational runners. Supported by NIH Grant AG032127

A Method To Study In Vivo Protein Synthesis In Slow- And Fast-twitch Human Muscle Fibers

PURPOSE: To develop an approach to directly assess protein fractional synthesis rate (FSR) in isolated human muscle fibers and to examine potential differences in FSR between slow (MHC I) and fast twitch (MHC IIa) fibers. METHODS: Individual muscle fibers were isolated from biopsies of the vastus lateralis (VL) and soleus (SOL) obtained from eight young men during a primed, continuous infusion of 2H3 leucine performed under basal conditions. To determine mixed protein FSR, a portion of each fiber was used to identify fiber type, fibers of the same fiber type were pooled, and the 2H3 leucine enrichment determined via GC-MS. RESULTS: In the VL, MHC I fibers exhibited a 33% faster (p<0.05) mixed protein FSR compared to MHC IIa fibers (0.0900.005 vs. 0.0680.002 %/hr). Similar (p>0.05) mixed protein FSR values were observed between MHC I fibers from the SOL (0.0820.006 %/hr) and MHC I fibers from the VL. Feasibility of processing isolated fibers for myofibrillar (MF) protein FSR was also determined in non-fiber typed pooled fibers from the VL, resulting in a MF protein FSR of 0.0730.004 %/hr. CONCLUSION: These methods can be applied to the study of fiber type specific responses in human skeletal muscle, and the need for this level of investigation is underscored by the different basal protein synthesis rates in MHC I and MHC IIa fibers originating from the same muscle. Supported by NIH R01 AG025032; NASA NNJ06HF59G

Aerobic exercise training improves whole muscle and single myofiber size and function in older women

To comprehensively assess the influence of aerobic training on muscle size and function, we examined seven older women (71 +/- 2 yr) before and after 12 wk of cycle ergometer training. The training program increased (P < 0.05) aerobic capacity by 30 +/- 6%. Quadriceps muscle volume, determined by magnetic resonance imaging (MRI), was 12 +/- 2% greater (P < 0.05) after training and knee extensor power increased 55 +/- 7% (P < 0.05). Muscle biopsies were obtained from the vastus lateralis to determine size and contractile properties of individual slow (MHC I) and fast (MHC IIa) myofibers, myosin light chain (MLC) composition, and muscle protein concentration. Aerobic training increased (P < 0.05) MHC I fiber size 16 +/- 5%, while MHC IIa fiber size was unchanged. MHC I peak power was elevated 21 +/- 8% (P < 0.05) after training, while MHC IIa peak power was unaltered. Peak force (Po) was unchanged in both fiber types, while normalized force (Po/cross-sectional area) was 10% lower (P < 0.05) for both MHC I and MHC IIa fibers after training. The decrease in normalized force was likely related to a reduction (P < 0.05) in myofibrillar protein concentration after training. In the absence of an increase in Po, the increase in MHC I peak power was mediated through an increased (P < 0.05) maximum contraction velocity (Vo) of MHC I fibers only. The relative proportion of MLC(1s) (Pre: 0.62 +/- 0.01; Post: 0.58 +/- 0.01) was lower (P < 0.05) in MHC I myofibers after training, while no differences were present for MLC(2s) and MLC(3f) isoforms. These data indicate that aerobic exercise training improves muscle function through remodeling the contractile properties at the myofiber level, in addition to pronounced muscle hypertrophy. Progressive aerobic exercise training should be considered a viable exercise modality to combat sarcopenia in the elderly population.

Human vastus lateralis and soleus muscles display divergent cellular contractile properties.

The purpose of this study was to investigate potential differences in single-fiber contractile physiology of fibers with the same myosin heavy chain isoform (MHC I and MHC IIa) originating from different muscles. Vastus lateralis (VL) and soleus biopsies were obtained from 27 recreationally active females (31 +/- 1 yr, 59 +/- 1 kg). A total of 943 single fibers (MHC I = 562; MHC IIa = 301) were isolated and examined for diameter, peak tension (Po), shortening velocity (Vo), and power. The soleus had larger (P < 0.05) fibers (MHC I +18%; MHC IIa +19%), higher MHC I Vo (+13%), and higher MHC I Po (+18%) compared with fibers from the VL. In contrast, fibers from the VL had higher (P < 0.05) specific tension (MHC I +18%; MHC IIa +20%), and MHC I normalized power (+25%) compared with the soleus. There was a trend for MHC IIa soleus fibers to have higher Vo [MHC IIa +13% (P = 0.058)], whereas VL MHC IIa fibers showed a trend for higher normalized power compared with soleus fibers [MHC IIa +33% (P = 0.079)]. No differences in absolute power were detected between muscles. These data highlight muscle-specific differences in single-fiber contractile function that should serve as a scientific basis for consideration when extending observations of skeletal muscle tissue from one muscle of interest to other muscles of origin. This is important when examining skeletal muscle adaptation to physical states such as aging, unloading, and training.

Peak power is elevated in slow and fast‐twitch muscle fibers from competitive distance runners

The purpose of this study was to characterize the influence of a long‐term (8‐yr) history of high‐intensity endurance training on the contractile properties of slow‐ and fast‐twitch myofibers. Muscle biopsies were obtained from 8 competitive distance runners (RUN) and 8 recreationally active individuals (REC). Slow‐twitch (MHC I) and fast‐twitch (MHC IIa) myofibers were analyzed for diameter, peak force (Po), contractile velocity (Vo), and power. MHC I and MHC IIa fibers from RUN subjects were ~20% larger (P&lt;0.05) than REC. Peak force of the MHC IIa fibers was 31% higher (P&lt;0.05) in RUN while Po of MHC I fibers was not different between groups. No differences for specific tension (Po/CSA) were present between groups for either fiber type. Vo was higher (P&lt;0.05) in MHC I (+70%) and MHC IIa (+18%) fibers from RUN subjects. Peak absolute power of both fiber types was greater (P&lt;0.05) in RUN. Normalized power of the MHC I fibers was 64% higher in RUN while no differences were noted for MHC IIa fibers. These data suggest that performing long‐duration endurance training modifies myofiber contractile function, resulting in higher peak power production of slow‐ and fast‐twitch fibers. Furthermore, these data suggest that long‐term training may result in adaptations in contractile physiology that are more pronounced than alterations previously observed in short‐term exercise interventions.

Skeletal muscle fiber type switching and atrophy in diabetes mellitus

This study demonstrates that switching of mammalian skeletal muscle fibers, identified as to Myosin Heavy Chain (MHC) type, from MHCI to MHCII occurs along with selective MHCII fiber atrophy in Diabetes Mellitus (DM). DM characterized by reduced plasma insulin level was induced by streptozotocin (40 mg/kg body wt STZ in buffer) injection of 6 wk old male Sprague‐Dawley rats; no exogenous insulin was administered. Rats, matched as to birth date and weight, were randomly assigned to test STZ‐DM or Control (injected with buffer only) group; after 3 wks the STZ‐DM (2) and Control rats (2) were sacrificed and their white gastrocnemius (GAST) and soleus (SOL) muscles excised. A 3–5 mm transverse section of each excised muscle was immediately cut and flash frozen (−80°C). The frozen tissue was cut into 12 μm cross‐sections in a cryostat; fibers in cross‐section were stained and identified as to type using 3 different MHC antibodies in serial cross‐sections. For each SOL and GAST cross‐section, 100 contiguous fibers were MHC typed. The proportions and CSA of combined MHCIIa, x/d,b and MHCI (nonhybrid) fibers were analyzed for GAST and SOL, comparing STZ‐DM to Control. MHCI to MHCII type switching in STZ‐DM was significant (p&lt;0.05) only for SOL, although it was evident in GAST. CSA of only the STZ‐DM MHCII fibers was significantly (p&lt;0.05) decreased in both SOL and GAST. Supported by Emory University (SKD) and R01 DK50740‐09 (SRP).

Fiber type specific myogenic and proteolytic gene expression after acute resistance exercise in very old women

The purpose of this investigation was to analyze fiber type specific gene expression (GE) of select myogenic [myogenic differentiation factor (MyoD), muscle regulatory factor 4 (MRF4), and myogenin] and proteolytic markers [atrogin-1, muscle RING finger-1 (MuRF-1), Forkhead box 3A (FOXO3A), cysteine-dependent aspartate protease (caspase-3)] and myostatin before and after resistance exercise (RE). Six old women (85 ± 1 y, 67 ± 4 kg) performed 3 × 10 knee extensions at 70% of 1-repetition maximum. Muscle biopsies were taken from vastus lateralis pre and 4-h post RE. Using real-time RT PCR, mRNA from pooled myosin heavy chain (MHC)-I and IIa fibers was amplified and normalized to GAPDH. Myogenic GE was not different between fiber types at baseline. With RE, only MHC-I fibers (time × type) increased (p<0.05) myogenic GE; MyoD (125%), myogenin (278%), and MRF4 (100%). Both fiber types had a decrease (p<0.05) in myostatin mRNA (I: 61% IIa: 73%). Proteolytic GE was higher (p<0.05) in MHC-I vs MHC-IIa fibers at rest and 4-h post RE for atrogin-1, FOXO3A, and caspase-3. With RE, both fiber types induced (p<0.05) atrogin-1 mRNA (I: 143%, IIa: 118%), while only MHC-IIa fibers (time × type) increased (p<0.05) MuRF-1 expression (184%). In summary, after a bout of RE MHC-I fibers increased myogenic mRNA levels, while the proteolytic mRNA induction after RE was more pronounced in MHC-IIa fibers. These findings highlight intrinsic differences between MHC-I and MHC-IIa fibers at rest and in response to RE, which may contribute to MHC-IIa selective atrophy with old age. Supported by NIH grant AG18409.

Single muscle fiber adaptations with marathon training

The purpose of this investigation was to characterize the effects of marathon training on single muscle fiber contractile function in a group of recreational runners. Muscle biopsies were obtained from the gastrocnemius muscle of seven individuals (22 +/- 1 yr, 177 +/- 3 cm, and 68 +/- 2 kg) before, after 13 wk of run training, and after 3 wk of taper. Slow-twitch myosin heavy chain [(MHC) I] and fast-twitch (MHC IIa) muscle fibers were analyzed for size, strength (P(o)), speed (V(o)), and power. The run training program led to the successful completion of a marathon (range 3 h 56 min to 5 h 35 min). Oxygen uptake during submaximal running and citrate synthase activity were improved (P < 0.05) with the training program. Muscle fiber size declined (P < 0.05) by approximately 20% in both fiber types after training. P(o) was maintained in both fiber types with training and increased (P < 0.05) by 18% in the MHC IIa fibers after taper. This resulted in >60% increase (P < 0.05) in force per cross-sectional area in both fiber types. Fiber V(o) increased (P < 0.05) by 28% in MHC I fibers with training and was unchanged in MHC IIa fibers. Peak power increased (P < 0.05) in MHC I and IIa fibers after training with a further increase (P < 0.05) in MHC IIa fiber power after taper. These data show that marathon training decreased slow-twitch and fast-twitch muscle fiber size but that it maintained or improved the functional profile of these fibers. A taper period before the marathon further improved the functional profile of the muscle, which was targeted to the fast-twitch muscle fibers.

Influence of exercise and nutrition countermeasures during 60 d of bedrest in women: Vastus lateralis single muscle fiber function

Twenty-four women underwent 60 d of bedrest (BR, n=8) to examine the effect of a concurrent aerobic (27 sessions) and resistance (19 sessions) exercise (BRE, n=8) or a leucine enriched high protein nutrition (BRN, n=8) countermeasure. Muscle biopsies were obtained before and day 59 of bedrest. Slow (MHC I) and fast (MHC IIa) muscle fibers were studied for diameter (Dia), peak force (Po), normalized peak force (Po/CSA), contractile velocity (Vo), and MHC composition. BR had a decrease (P<0.05) in MHC I Dia (−13%), Po (−36%), and Po/CSA (−17%). A similar trend was observed in BR MHC IIa fibers for Dia (−15%; P=0.09) and Po (−25%; P=0.14). BRN had a decrease (P<0.05) in MHC I Dia (−18%) and Po (−38%) and MHC IIa Dia (−26%). BRN had a trend for a decrease in MHC IIa Po (−41%; P=0.10). BR and BRN MHC I and IIa Vo was unchanged. BRE MHC I and IIa fiber Dia, Po, Po/CSA, and Vo were unchanged with bedrest. These data show that the MHC I fibers were more affected than the MHC IIa fibers with bedrest. The concurrent exercise-training program preserved MHC I and IIa diameter and contractile function during bedrest. The nutrition countermeasure did not provide any benefit for myofiber diameter or peak tension. Sponsors: NASA, ESA, CNES

Resistance training improves single muscle fiber contractile function in older women.

The purpose of this study was to 1) examine single cell contractile mechanics of skeletal muscle before and after 12 wk of progressive resistance training (PRT) in older women (n = 7; 74 +/- 2 yr) and 2) to compare these results to our previously completed single cell PRT work with older men (n = 7; 74 +/- 2 yr) (Trappe S, Williamson D, Godard M, Porter D, Rowden G, and Costill D. J Applied Physiol 89:143--152, 2000). Knee extensor PRT was performed 3 days/wk at 80% of one-repetition maximum. Muscle biopsies were obtained from the vastus lateralis before and after the PRT. Chemically skinned single muscle fibers (n = 313) were studied at 15 degrees C for peak tension (P(o)), unloaded shortening velocity (V(o)), and power. Due to the low number of hybrid fibers identified post-PRT, direct comparisons were limited to MHC I and IIa fibers. Muscle fiber diameter increased 24% (90 +/- 2 to 112 +/- 6 microm; P < 0.05) in MHC I fibers with no change in MHC IIa fibers. P(o) increased (P < 0.05) 33% in MHC I (0.76 +/- 0.04 to 1.01 +/- 0.09 mN) and 14% in MHC IIa (0.73 +/- 0.04 to 0.83 +/- 0.05 mN) fibers. Muscle fiber V(o) was unaltered in both fiber types with PRT. MHC I and IIa fiber power increased (P < 0.05) 50% [11 +/- 2 to 17 +/- 2 microN. fiber length (FL). s(-1)] and 25% (40 +/- 8 to 51 +/- 6 microN. FL. s(-1)), respectively. However, when peak power was normalized to cell size, no pre- to postimprovements were observed. These data indicate that PRT in elderly women increases muscle cell size, strength, and peak power in both slow and fast muscle fibers, which was similar to the older men. However, in contrast to the older men, no change in fiber V(o) or normalized power was observed in the older women. These data suggest that older men and women respond differently at the muscle cell level to the same resistance-training stimulus.