DIFFERENTIAL REGULATION OF PROTEIN-KINASE-C ISOFORMS IN ISOLATED NEONATAL AND ADULT-RAT CARDIOMYOCYTES

We have immunologically identified the isoforms of protein kinase C (P KC) present in neonatal and adult rat cardiomyocytes and examined thei r regulation by hormones and phorbol ester. Both cell types express th e Ca2+-dependent alpha-PKC and the Ca2+-independent epsilon- and delta -PKC isoforms. The atypical zeta-PKC isoform is also expressed in neon atal, but only weakly in adult cells. Stimulation of the alpha(1)-adre nergic or purinergic receptor with phenylephrine or ATP, respectively, increases membrane-associated immunoreactivity of both epsilon- and d elta-PKC in neonatal and adult cells; endothelin and carbachol are als o effective in adult cells. In contrast, none of the agonists leads to increases in membrane-associated alpha-PKC in cardiomyocytes. PKC zet a is also unaffected by receptor stimulation. The phorbol ester phorbo l 12-myristate 13-acetate causes redistribution and subsequently down- regulation of alpha-, epsilon-, and delta- but not zeta-PKC. The three isoforms are down-regulated at distinctively different rates, with al pha-PKC being the most rapid and epsilon-PKC the slowest. We used sele ctive down-regulation of alpha-, epsilon-, and delta-PKC to investigat e the role of these isoforms in PKC phosphorylation-dependent events i n neonatal myocytes. Our findings suggest that epsilon-PKC is responsi ble for the phenylephrine-induced phosphorylation of MARCKS, an endoge nous PKC-specific substrate. In contrast, agonist-induced c-fos expres sion is unlikely to be mediated by epsilon-PKC since the response is r apidly down-regulated and apparently Ca2+-dependent. Our finding that the PKC isoforms are differentially responsive to neurohormones sugges ts that they play distinct and specific roles in cardiac function.