Qualitative and quantitative adaptations of muscle fibers andmuscle protein pattern to 35-days bed rest

Ten healthy sedentary subjects underwent 35-days bed rest (BR) and needle biopsy samples of the vastus lateralis muscle were collected pre-BR and post-BR. One portion of the biopsy was glycerinated and used to dissect individual muscle fibers, which were studied for measuring fiber size, myosin content and myosin actin ratio by quantitative electrophoresis. Another portion of the biopsy was immediately frozen and used to determine myosin heavy chain (MHC) isoform distribution and to perform proteomic analysis by two-dimensional gel electrophoresis. As expected on the basis of previous findings, muscle fibers were found to go through a significant degree of atrophy. Myosin concentration was found to be lower post-BR than pre-BR in individual muscle fibers, whereas in the same fibers myosin actin ratio was unchanged. The latter findings indicate a disproportionate loss of myosin with respect to fiber CSA and a proportional loss of myosin and actin, suggesting a decrease in myofibrillar density within the fibers. MHC isoform distribution showed a shift in the direction MHC-1 MHC-2A MHC-2X as expected mainly on the basis of previous findings in rat models of disuse. The proteomic analysis identified several differentially expressed proteins post-BR, which mainly belonged to antioxidant defense systems and energy metabolism. The antioxidant defense systems were down-regulated suggesting that oxidative stress could occur in disused human muscle as previously showed in rat models. Both an oxidative and four glycolytic enzymes were down-regulated post-BR suggesting a general downsizing of energy metabolism.