The serum of patients with idiopathic dilated cardiomyopathy contains agonistic autoantibodies that bind to beta 1-adrenoceptor and exert a positive chronotropic effect in cultured neonatal rat cardiomyocytes. In this study it is shown that these antibodies like isoproterenol increase the activity of cyclic AMP-dependent protein kinase (PKA) in the challenged myocytes. Cytosolic and particulate PKA activity was found to be increased as indicated by a rise in the PKA activity ratio from control values of 0.20 +/- 0.02 to 0.33 +/- 0.03 (p < 0.001), respectively, after exposure to the autoantibodies. Increases in PKA activity ratio by the autoantibodies as well as by isoproterenol were blocked in the presence of (-) propranolol. These findings complete the pathway by which the autoantibodies realize their chronotropic effects via beta-adrenoceptor-binding, cyclic AMP generation and activation of particulate and cytosolic cyclic AMP-dependent protein kinase.