DISTRIBUTION OF MYOSIN HEAVY-CHAIN ISOFORMS IN NON-WEIGHT-BEARING RATSOLEUS MUSCLE-FIBERS

The effects of 14 days of spaceflight (SF) or hindlimb suspension (HS) (Cosmos 2044) on myosin heavy chain (MHC) isoform content of the rat soleus muscle and single muscle fibers were determined. On the basis o f electrophoretic analyses, there was a de novo synthesis of type IIx MHC but no change in either type I or IIa MHC isoform proportions afte r either SF or HS compared with controls. The percentage of fibers con taining only type I MHC decreased by 26 and 23%, and the percentage of fibers with multiple MHCs increased from 6% in controls to 32% in HS and 34% in SF rats. Type IIx MHC was always found in combination with another MHC or combination of MHCs; i.e., no fibers contained type IIx MHC exclusively. These data suggest that the expression of the normal complement of MHC isoforms in the adult rat soleus muscle is dependen t, in part, on normal weight bearing and that the absence of weight be aring induces a shift toward type IIx MHC protein expression in the pr eexisting type I and IIa fibers of the soleus.