The effect of calcium on cardiac phosphorylase activation, contractile force and cyclic AMP in euthyroid and hyperthyroid rat hearts

Abstract
CaCl2 injected into isolated perfused rat hearts produced a positive inotropic effect and increased the levels of phosphorylase a (EC 2.4.1.1). The increase in enzyme activity lagged behind the inotropic effect. Pretreatment of animals with thyroid hormone enhanced the ability of noradrenaline to activate phosphorylase but did not affect the inotropic or phosphorylase activating effect of Ca. Thyroid hormone pretreatment enhanced the chronotropic effect of Ca. Ca did not affect the cardiac levels of cyclic AMP. Calcium can activate phosphorylase by a mechanism other than cyclic AMP, and the enhancement of adrenergic amine-induced phosphorylase activation by thyroid hormone is not a Ca mediated event.