Comparison of myosin and creatine kinase isoforms in left ventricles of young and senescent Fischer 344 rats after treatment with triiodothyronine.

The effects of perturbation of thyroid hormone levels on expression of myosin and creatine kinase isoforms were examined in maturing and senescent rats. Whereas LV of maturing rats contain only alpha-MHC, LV of senescent rats contain nearly equal amounts of both alpha-MHC and beta-MHC. When maturing rats were made hypothyroid by treatment for 14 days with the antithyroid agent, propylthiouracil (PTU), beta-MHC and beta-MHC mRNA levels increased significantly. Administration of T3 to senescent rats, or maturing rats made hypothyroid by PTU treatment produced similar decreases in levels of both beta-MHC and beta-MHC mRNA. In contrast, treatment with T3 produced little change in creatine kinase isoform distribution. Thus, thyroid hormone appears to play a critical role in regulating expression of the isoforms of myosin but not of creatine kinase.