Muscle Protein Composition in Aerobically Trained Skeletal Muscle

Abstract

Aerobic exercise training uniquely alters skeletal muscle fiber contractile function. However, it is unclear if these alterations are due to changes in the general muscle protein composition or specific contractile elements (actin and myosin). PURPOSE: To examine skeletal muscle protein composition in aerobically trained skeletal muscle as it relates to myofiber contractile function. METHODS: Skeletal muscle biopsy samples were obtained from the lateral head of the gastrocnemius of seven collegiate (RUN; 71±1 ml·kg-1·min-1) and eight recreational (REC; 46±2 ml·kg-1·min-1) runners. Muscle water content and protein composition were determined through freeze-drying, separation of protein fractions, SDS-PAGE, staining and densitometry. RESULTS: MHC I and MHC IIa myofibers from RUN exhibited a higher (p<0.05) unloaded shortening velocity and a trend (p=0.07) for a lower specific tension (P0/CSA) compared to REC. Muscle water content was lower (p<0.05) in RUN (73±1%) compared to REC (75±1%). Relative to muscle wet weight, total (RUN, 165±4; REC, 147±4 μg·mg-1) and myofibrillar (RUN, 106±6; REC, 82±3 μg·mg-1) protein fractions were greater (p<0.05) in RUN as compared to REC. However, this pattern was not apparent after correcting for muscle dry weight in the total (RUN, 612±1; REC, 652±27 μg·mg-1) and myofibrillar (RUN, 395±21; REC, 357±21 μg·mg-1) protein fractions. Myosin concentration was not different between REC (0.990.08 AU) and RUN (1.010.07 AU). Additionally, actin concentration was similar between REC (14±1 μg·mg-1 muscle wet wt) and RUN (17±2 μg·mg-1 muscle wet wt). CONCLUSION: These data suggest that muscle water content, total and myofibrillar protein content and the concentration of the main contractile proteins actin and myosin do not explain the robust differences in myofiber function between these groups of runners. Therefore, it is likely that the arrangement, rather than the composition of skeletal muscle proteins, is more relevant to understanding myofiber contractile function in aerobically trained human skeletal muscle. Supported by NIH Grant AG032127