COORDINATED FAST-TO-SLOW TRANSITIONS OF MYOSIN AND SERCA ISOFORMS IN CHRONICALLY STIMULATED MUSCLES OF EUTHYROID AND HYPERTHYROID RABBITS

Changes in the patterns of myosin heavy chain (MHC) isoforms, isomyosi ns, and Ca2+-ATPase (SERCA) isoforms were studied in long-term (72 d) stimulated fast-twitch extensor digitorum longus (EDL) and tibialis an terior (TA) muscles of euthyroid and hyperthyroid rabbits. The chronic low-frequency stimulation-induced fast-to-slow transitions in MHC iso forms, isomyosins and SERCA isoforms were pronounced in muscles from e uthyroid rabbits, but less pronounced in muscles from hyperthyroid rab bits. Thus, hyperthyroidism counteracted to some extent the stimulatio n-induced fast-to-slow transition. Analyses of all parameters were per formed on the same individual muscles, providing information on the co -ordinated expression of SERCA and myosin isoforms. A high correlation (r = 0.97) was detected between relative concentrations of slow SERCA 2a and slow MHCI isoforms. This correlation persisted under all experi mental conditions, suggesting a co-ordinated expression of slow myosin and Ca2+-ATPase isoforms. Conversely, fast SERCA1a was correlated to fast myosin isoforms as a whole.