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

Okadaic acid exerts a positive inotropic effect in cardiac preparations. We studied whether the positive inotropic effect of okadaic acid in cardiac preparations could be due to phosphatase inhibition and whether this inhibition affects the phosphorylation of cardiac proteins. In papillary muscles from guinea pigs, 30 muM okadaic acid increased force of contraction to 175% of predrug value. In isolated guinea pig ventricular cardiomyocytes, okadaic acid augmented single Ca2+-channel currents by enhancing channel availability. In homogenates from ventricles, 1 muM okadaic acid completely inhibited phosphorylase alpha phosphatase activity. In isolated P-32-labeled ventricular cardiomyocytes, 30 muM okadaic acid increased phosphorylation of phospholamban (PLB) and troponin inhibitor (TnI) to 325 and 284% of control, respectively. Furthermore, 30 muM okadaic acid increased phosphorylation of a hitherto unknown 23-kDa protein to 352% of control. It is concluded that the effects of okadaic acid could be mediated by increasing the phosphorylation state of several proteins including PLB, a 23-kDa protein, and TnI.