Time-dependent changes in myosin heavy chain (MHC) isoform expression were investigated in rat soleus muscle unloaded by hindlimb suspension. Changes at the mRNA level were measured by RT-PCR and correlated with changes in the pattern of MHC protein isoforms. Protein analyses of whole muscle revealed that MHCI decreased after 7 days, when MHCIIa had increased, reaching a transient maximum by 15 days. Longer periods led to inductions and progressive increases of MHCIId(x) and MHCIIb. mRNA analyses of whole muscle showed that MHCIId(x) displayed the steepest increase after 4 days and continued to rise until 28 days, the longest time period investigated. MHCIIb mRNA followed a similar time course, although at lower levels. MHCIalpha mRNA, present at extremely low levels in control soleus, peaked after 4 days, stayed elevated until 15 days, and then decayed. Immunohistochemistry of 15-day unloaded muscles revealed that MHCIalpha was present in muscle spindles but at low amounts also in extrafusal fibers. The slow-to-fast transitions thus seem to proceed in the order MHCIbeta --> MHCIIa --> MHCIId(x) --> MHCIIb. Our findings indicate that MHCIalpha is transiently upregulated in some fibers as an intermediate step during the transition from MHCIbeta to MHCIIa.
