EVIDENCE FOR PHYSIOLOGICAL FUNCTIONS OF PROTEIN PHOSPHATASES IN THE HEART - EVALUATION WITH OKADAIC ACID

Okadaic acid exerts a positive inotropic effect in cardiac preparation s. We studied whether the positive inotropic effect of okadaic acid in cardiac preparations could be due to phosphatase inhibition and wheth er this inhibition affects the phosphorylation of cardiac proteins. In papillary muscles from guinea pigs, 30 muM okadaic acid increased for ce of contraction to 175% of predrug value. In isolated guinea pig ven tricular cardiomyocytes, okadaic acid augmented single Ca2+-channel cu rrents by enhancing channel availability. In homogenates from ventricl es, 1 muM okadaic acid completely inhibited phosphorylase alpha phosph atase activity. In isolated P-32-labeled ventricular cardiomyocytes, 3 0 muM okadaic acid increased phosphorylation of phospholamban (PLB) an d troponin inhibitor (TnI) to 325 and 284% of control, respectively. F urthermore, 30 muM okadaic acid increased phosphorylation of a hithert o unknown 23-kDa protein to 352% of control. It is concluded that the effects of okadaic acid could be mediated by increasing the phosphoryl ation state of several proteins including PLB, a 23-kDa protein, and T nI.