Myosin and actin content of human skeletal muscle fibers following 35 days bed rest

Biopsy samples were taken from the vastus lateralis muscle of seven male subjects pre‐ and post‐35 days bed rest (BR). The myosin heavy chain (MHC) isoform distribution of the samples was determined by densitometry of MHC bands separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS‐PAGE). Individual muscle fibers were dissected from biopsy samples pre‐BR (n=143) and post‐BR (n=144). They were studied as regards cross‐sectional area (CSA), myosin content by quantitative electrophoresis and myosin actin (M/A) ratio by densitometry of myosin and actin bands of individual muscle fibers. All fibers were typed according to their MHC isoform content determined by SDS‐PAGE. A decrease in MHC‐1 relative content and an increase in MHC‐2X content of whole muscle samples were found, suggesting a slow to fast shift in muscle phenotype. Consistently, fiber type distribution was shifted toward type 2X and 2AX fibers. Muscle fiber atrophy occurred at variable extent among fiber types. Myosin concentration was significantly lower in type 1 and type 2A muscle fibers post‐BR than pre‐BR, whereas M/A ratio did not vary. The latter findings indicate a disproportionate loss of myosin compared with fiber CSA and a proportional loss of myosin and actin.