| Abstract: | The effects of a37-day period of bed rest on myosin heavy chain (MHC) expression onboth mRNA and protein level in human skeletal muscle fibers werestudied. Muscle biopsies from vastus lateralis muscle were obtainedfrom seven healthy young male subjects before and after the bed-restperiod. Combined in situ hybridization, immunocytochemistry, and ATPasehistochemistry analysis of serial sections of the muscle biopsiesdemonstrated that fibers showing a mismatch between MHC isoforms at themRNA and protein level increased significantly after the bed-restperiod, suggesting an increase in the amount of muscle fibers in atransitional state. Accordingly, fibers showing a match in expressionof MHC-1 and of MHC-2A at the mRNA and protein level decreased, whereasfibers showing a match between MHC-2X mRNA and protein increased after bed rest. Overall, there was an increase in fibers in a transitional state from phenotypic type 1  2A and 2A 2X.Furthermore, a number of fibers with unusual MHC mRNA and isoproteincombinations were observed after bed rest (e.g., type 1 fibers withonly mRNA for 2X and type 1 fibers negative for mRNA for MHC- /slow,2A, and 2X). In contrast, no changes were revealed after an examination at the protein level alone. These data suggest that the reduced load-bearing activity imposed on the skeletal muscles through bed restwill alter MHC gene expression, resulting in combinations of mRNA andMHC isoforms normally not (or only rarely) observed in musclessubjected to load-bearing activity. On the other hand, the present dataalso show that 37 days of bed rest are not a sufficient stimulus toinduce a similar change at the protein level, as was observed at thegene level. |
