Prominence of myosin heavy chain hybrid fibers in soleus muscle of spinal cord-transected rats.

The effect of a midthoracic spinal cord transection (ST) on myosin heavy chain (MHC) isoform expression in the rat soleus muscle was studied. Electrophoretic analyses demonstrated that 15 days after ST there were significant proportional increases in type IIx, decreases in type IIa, and no change in type I MHC composition. Thirty days after ST, some type IIb MHC was expressed, there were further proportional increases in type IIx, an increase in type IIa (compared with 15-day ST), and a decrease in type I MHC. At both time periods after ST, many fibers expressed multiple MHCs, as demonstrated by immunohistochemistry where a battery of monoclonal antibodies specific to MHC isoforms was used. Fibers were observed containing types I and II together or multiple type II MHC isoforms. These data suggest that the expression of the normal complement of MHC isoforms in the adult rat soleus muscle is dependent, in part, on normal neuromuscular activation.