Effect of thyroid hormone on the myosin heavy chain isoforms in slow and fast muscles of the rat

The myosin heavy chain (MHC) was studied by biochemical methods in the slow -twitch (soleus) and two fast-twitch leg muscles of the triiodothyronine tr eated (hyperthyroid), thyreoidectomized (hypothyroid) and euthyroid (contro l) rats. The changes in the contents of individual MHC isoforms (MHC-1, MHC -2A, MHC-2B and MHC-2X) were evaluated in relation to the muscle mass and t he total MHC content. The MHC-1 content decreased in hyperthyreosis, while it increased in hypothyreosis in the soleus and in the fast muscles. The MH C-2A content increased in hyperthyreosis and it decreased in hypothyreosis in the soleus muscle. In the fast muscles hyperthyreosis did not affect the MHC-2A content, whereas hypothyreosis caused an increase in this MHC isofo rm content. The MHC-2X, present only in traces or undetected in the control soleus muscle, was synthesised in considerable amount in hyperthyreosis; i n hypothyreosis the MHC-2X was not detected in the soleus. In the fast musc les the content of MHC-2X was not affected by any changes in the thyroid ho rmone level. The MHC-2B seemed to be not influenced by hyperthyreosis in th e fast muscles, whereas the hypothyreosis caused a decrease of its content. In the soleus muscle the MHC-2B was not detected in any groups of rats. The results suggest that the amount of each of the four MHC isoforms expres sed in the mature rat leg muscles is influenced by the thyroid hormone in a different way. The MHC-2A and the MHC-2X are differently regulated in the soleus and in the fast muscles; thyroid hormone seems to be necessary for e xpression of those isoforms in the soleus muscle.