Injection of protein kinase inhibitor into cultured heart cells blocks calcium slow channels.

An inhibitor of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase was injected into cultured heart cells (reaggregates) by the phosphatidylcholine liposome method. The inhibitor blocked both the isoproterenol-induced slow action potentials and the spontaneously occurring slow action potentials of these cells. The inhibitory effect of protein kinase inhibitor was prevented by denaturation or reversed by the injection of the catalytic subunit of cAMP-dependent protein kinase. These findings support the phosphorylation hypothesis for the myocardial slow channels, namely that the slow channel protein must be phosphorylated for the channel to be available for voltage activation.