SINGLE-FIBER ANALYSES OF TYPE-IIA MYOSIN HEAVY-CHAIN DISTRIBUTION IN HYPERTHYROID AND HYPOTHYROID SOLEUS

The objectives of this study were to 1) examine the effect of hypo- an d hyperthyroidism (triiodothyronine treatment) on the distribution of type IIA myosin heavy chain (MHC) in the soleus at the single fiber le vel 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 usin g a monoclonal antibody reactive to the type IIA MHC and quantified wi th a Meridian ACAS 570 interactive laser cytometer. The findings of th is study demonstrate that 1) hyperthyroidism significantly increases t he 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 hy perthyroid conditions, and 3) there is a high correlation between maxi mal shortening velocity and the relative number of type IIA fibers. Th is latter observation suggests that the maximal shortening velocity of whole skeletal muscle may not be solely determined by its fastest fib er(s) but rather by the relative proportion of fibers expressing fast type IIA MHC.