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

Abstract

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