Regulation of phosphorylase kinase in rat ventricular myocardium. Role of calmodulin.

Conversion of phosphorylase b to a which is catalyzed by the enzyme phosphorylase kinase is known to require Ca++. Trifluoperazine, an inhibitor of calmodulin-dependent enzymes, was utilized in the present study to clarify the role in vivo of calcium-calmodulin regulation of phosphorylase kinase. Twenty-minute preperfusion of isolated rat ventricles with 10(-5) M trifluoperazine had no effect on basal levels of phosphorylase a but significantly attenuated phosphorylase activation induced by either calcium (3.75 mM) or isoproterenol (3 x 10(-9) M, 3 x 10(-8) M). The positive inotropic effect of both agents and cyclic adenosine 3',5'-monophosphate (cAMP) levels were not altered by trifluoperazine in the perfused hearts. In addition, no effects of 10(-5) M trifluoperazine were noted on beta-adrenergic receptor binding of 3Hcarazolol or on adenylate cyclase activity. In vitro studies with partially purified rat cardiac phosphorylase kinase demonstrated 1.5- to 3-fold stimulation by exogenous calmodulin. The addition of 10(-5) M trifluoperazine prevented calmodulin stimulation but had little effect on activity in the absence of exogenous calmodulin. The present results suggest that reversible binding of calcium-calmodulin may represent a physiological means for regulating phosphorylase kinase activity in rat cardiac muscle.