Hormonal control of heart function and myosin atpase activity

Peak intraventricular pressure and rate of pressure development were lower in hearts from hypophysectomized as compared to normal rats. Treatment of hypophysectomized rats with growth hormone (GH) and thyroxine (T 4) increased heart size, but only T 4 restored hemodynamic function to normal. Ca ++-activated ATPase of myosin from hearts of hypophysectomized rats was reduced as compared to myosin from hearts of normal rats. Treatment of hormone-deficient animals with T 4 restored ATPase to normal, but GH was without effect. K + and NH 4 +-activated ATPase of myosin did not vary with hormonal status of the animals. These findings indicated that hormonal regulation of cardiac contractility involved changes in the catalytic activity of myosin.