Protein dephosphorylation rates in myocytes after isoproterenol withdrawal

Dephosphorylation of substrates for cyclic AMP-dependent protein kinase is essential for reversing the effects of this enzyme. It has been proposed that the relevant phosphatase(s) is stimulated by muscarinic cholinergic agonists, thereby accentuating cholinergic antagonism of beta-adrenergic agonists in the heart. To test this hypothesis, dephosphorylation of the three major substrates of cardiac cyclic AMP-dependent protein kinase (phospholamban, troponin-I, and C-protein) was examined. In isolated myocytes, isoproterenol caused concentration-dependent phosphorylation of these three proteins. Simultaneous exposure to acetylcholine with the isoproterenol caused a rightward shift in the concentration-response curve that was similar for protein phosphorylation in myocytes and for the inotropic response of the intact heart. The addition of propranolol after exposure to isoproterenol resulted in protein dephosphorylation, the onset of which was accelerated by acetylcholine. However, acetylcholine did not affect the rate of dephosphorylation for any of the proteins, indicating that phosphatase activity in cardiac muscle is not enhanced by acetylcholine.