Thyroid hormone regulation of MHC isoform composition and myofibrillar ATPase activity in rat skeletal muscles

Myosin heavy chain (MHC) isoform. composition and Ca2+ Mg2+ dependent ATPas e activity were determined in myofibrils prepared hom skeletal muscles (dia phragm, soleus, plantaris and tibialis anterior) of euthyroid (C), hypothyr oid (Tx) and hyperthyroid (T3) rats. Direct comparison between T3 and Tx gave an indication of the maximal effec t of thyroid hormones. Significant differences in MHC-1 and MHC-2B proporti ons and in ATPase activity were found in all muscles. The difference in MWC -2A/X proportion was significant only in soleus, diaphragm and plantaris. W hen T3 and C were compared, significant variations in MHC isoform compositi on were found only in plantaris and diaphragm. The comparison between Tx an d C showed significant differences in MHC isoform distribution and in ATPas e activity in mast muscles. The differences in ATPase activity among muscles and among thyroid states w ere consistent with these in MHC isoform distribution. From the correlation s between ATPase activity and MHC isoform distribution the enzymatic activi ties of individual MHC isoforms were calculated. The results indicate that MHC isoform distribution is controlled by thyroid state in all skeletal muscles and that changes in MHC isoforms distributio n are accompanied by proportional changes in ATPase activity.