Evidence for a new cardiac myosin species in thyrotoxic rabbit

Thyroid hormone may modulate the contractile behavior of the heart by influencing the ATPase activity of myosin. The velocity of myocardial contraction is increased in thyrotoxic animals [l-4] and the ATPase activity of myosin [5-81 and actomyosin are elevated [9,10]. In contrast, the ATPase activity of myosin is depressed after hypophysectomy and can be restored to normal by administration of thyroxine [ 111. The mechanism whereby thyroxine exerts these effects is obscure, but a plausible explanation would be the induction of a myosin isozyme within the heart which has a higher ATPase activity. Recently, changes in myosin isozyme have been implicated in the modification of contraction velocities for skeletal muscle under different experimental conditions [ 12,131. Further support of this concept is found in the observation that the rate of cardiac myosin replacement may be accelerated by thyroxine administration [ 141. To date however, there has been no conclusive evidence that thyroxine induces a change in myosin structure. Although Thyrum et al. [S] found changes in the amino acid composition and the helix content of cardiac myosin after thyroxine administration, subsequent investigations have failed to find abnormalities either in the amino acid composition or the electrophoretic mobility of the light and heavy subunits of cardiac myosin from thyrotoxic animals [6,8]. Also, the changes in ATPase activity induced by thyroxine