Swimming exercise, thyroid state, and the distribution of myosin isoenzymes in rat heart.

We tested whether changes in cardiac myosin ATPase activity induced by swimming exercise in male rats are due to a redistribution of existing isoenzymic forms of ventricular myosin. The isoenzymic profiles were analyzed by nondissociating gel electrophoresis of ventricular samples and compared with ATPase activities of myofibrils prepared from the same ventricle. Myofibrils prepared from hearts of rats in the control sedentary group or from hearts of rats in the groups of 8- or 12-wk swimmers had the same actomyosin Mg2+-ATPase activities measured between pCa 8 and 5. However, the myosin Ca2+-ATPase activity of myofibrils prepared from hearts of 8- or 12-wk swimmers was 20% higher than the activity of control preparations. This increase in activity was in proportion to an increase in the relative amount of V1, the myosin isoenzyme with the highest Ca2+-ATPase activity. Thyroidectomized rats, whose hearts had no detectable V1, were also subjected to the swimming program. In the case of the hypothyroid rats, myofibrillar preparations from controls and 8- or 12-wk swimmers had the same actomyosin Mg2+-ATPase activity, myosin Ca2+-ATPase activity, and the same isoenzyme profiles. Co-electrophoresis of ventricular samples from the euthyroid and hypothyroid controls and swimmers showed no evidence for new variants of myosin. We conclude that the increase in myosin Ca2+-ATPase activity in the ventricles of euthyroid swimmers is due to a redistribution of existing isoforms of myosin and that the redistribution process may require thyroid hormone for its expression.