Myosin heavy chains in fibers of TTX-paralyzed rat soleus and medial gastrocnemius muscles

The expression of five myosin heavy chain (MHC) isoforms was analyzed in th e rat soleus (Sol) and the deep and superficial medial gastrocnemius (dGM, sGM) muscle after 2 and 4 wk of TTX paralysis by using immunohistochemical techniques. In Sol, after 4 wk of paralysis, fibers containing type I MHC w ere either pure type I (14%) or also contained developmental (D; 76%), IIa (26%), or IIx (18%) MHC. Values for corresponding fibers in dGM were 8.5, 6 5, 38, and 22%. Also, by 4 wk an increase was seen in the proportions of fi bers expressing IIa MHC in Sol (from 16 to 38%) and dGM (from 24 to 74%). I n a region of sGM in control muscles containing pure IIb fibers, a major pr oportion (86%) remained pure after 4 wk of paralysis, with the remainder co expressing IIb and IIx. The results indicate that TTX-induced muscle paraly sis results in an increase in fibers containing multiple MHC isoforms and t hat the D isoform appears in a major proportion of these hybrid fibers.