Abstract: | Single-fiber(n = 3,818 fibers) electrophoreticanalyses were used to delineate the separate and combined effects ofhyperthyroidism (T3) andhindlimb suspension (HS) on the myosin heavy chain (MHC) isoformcomposition (1-, 2-, and 4-wk time points) of the rat soleus muscle.The key findings of this study are as follows. First,T3 and HS both altered thedistribution of MHC isoforms at the single-fiber level; however, thepopulations of fibers produced by these two interventions were clearlydifferent from one another. Second,T3 + HS rapidly converted thesoleus into a fast muscle, producing large increases in the relativecontents of the fast type IIx and IIb MHC isoforms which were primarily expressed in several populations of hybrid fibers (e.g., types I/IIa/IIx, I/IIx/IIb, I/IIa/IIx/IIb). Finally,T3 + HS produced uniquepopulations of hybrid fibers that did not adhere to the I IIa IIx IIb sequential scheme of MHC plasticity. Collectively, the findings of this study demonstrate that the intervention of T3 + HS is a powerful model formanipulating and studying MHC isoform plasticity in slow skeletal muscle. |