Single fiber analyses of type IIA myosin heavy chain distribution in hyper- and hypothyroid soleus

The objectives of this study were to 1) examine the effect of hypo- and hyperthyroidism (triiodothyronine treatment) on the distribution of type IIA myosin heavy chain (MHC) in the soleus at the single fiber level and 2) correlate changes in the single fiber distribution of type IIA MHC with the maximal shortening velocity of whole skeletal muscle. The presence of the type IIA MHC in single fibers was determined using a monoclonal antibody reactive to the type IIA MHC and quantified with a Meridian ACAS 570 interactive laser cytometer. The findings of this study demonstrate that 1) hyperthyroidism significantly increases the relative number of muscle fibers that express type IIA MHC, 2) not all type I fibers are capable of expressing fast type IIA MHC under hyperthyroid conditions, and 3) there is a high correlation between maximal shortening velocity and the relative number of type IIA fibers. This latter observation suggests that the maximal shortening velocity of whole skeletal muscle may not be solely determined by its fastest fiber(s) but rather by the relative proportion of fibers expressing fast type IIA MHC.