ALTERED EXPRESSION OF MYOSIN MESSENGER-RNA AND PROTEIN IN RAT SOLEUS AND TIBIALIS ANTERIOR FOLLOWING REINNERVATION

Myosin heavy chain (MHC) expression was studied in rat soleus and tibi alis anterior (TA) at the mRNA and protein levels following reinnervat ion 8 and 32 wk after sciatic nerve injury. A sciatic nerve crush or t ransection injury was produced in the midthigh region of adult female Sprague-Dasyley rats. A ribonuclease protection assay was developed to measure four of the adult MHCs (I, IIa, IIx, IIb) in a single sample. MHC mRNA and protein were measured and compared in the same muscles. Eight and thirty-two weeks after a crush injury, the MHC mRNA profiles were similar to control with the exception of soleus MHC IIa and TA M HC IIb, which were significantly less than control at both time points . In contrast, reinnervation of the soleus following a sciatic nerve t ransection injury resulted in an MHC isoform shift characterized by in creases in the relative amounts of fast myosin (IIa and IIx) and a dec rease in slow myosin. As expected, significant changes first occurred at the mRNA level followed by changes in protein expression. Thirty-tw o weeks after transection injury and repair, the primary MHC mRNA isof orm in the soleus was MHC IIx. Moreover, at 32 wk, MHC IIb mRNA was de tected in 50% of the reinnervated soleus following a transection injur y. Reinnervation of the TA following sciatic nerve transection led to replacement of the MHC IIb isoform with MHC IIx.