ROLE OF WEIGHT-BEARING FUNCTION ON EXPRESSION OF MYOSIN ISOFORMS DURING REGENERATION OF RAT SOLEUS MUSCLES

The expression of myosin isoforms was studied in regenerated rat soleu s muscle during either normal or altered postural activity. Regenerati on was induced following injury by venom from the Notechis scutatus sc utatus snake. Immunohistochemical analysis showed that, in regeneratin g soleus muscle after 3 wk of hindlimb suspension, nearly all fibers r eacted positively with the myosin heavy chain (MHC) antibody associate d with fast-twitch muscle fibers (fast MHC). When 3 wk of recovery wit h normal weight-bearing activity followed hindlimb suspension, the reg enerated soleus muscle exhibited a nearly homogeneous staining with th e MHC antibody associated with slow-twitch muscle fibers (slow MHC). T hese findings were in accordance with quantitative analysis of the ele ctrophoretic separation of the native myosin isoforms. Immunohistochem ical data showed that removal of weight bearing in the 21-day-old rege nerated soleus muscles resulted in an increase in fast MHC expression. Together, the results of the present study clearly demonstrate that t he postural load is an important component in the induction of slow MH C in regenerating muscle and that the control of the expression of MHC in muscle comprising a homogeneous population of fibers deriving from satellite cells appears more homogeneous and more complete than in a nondegenerated one.