PROTEIN-KINASE C-MEDIATED PHOSPHOLIPASE-A(2) ACTIVATION, PLATELET-ACTIVATING-FACTOR GENERATION AND PROSTACYCLIN RELEASE IN SPONTANEOUSLY BEATING RAT CARDIOMYOCYTES

The expression of protein kinase C (PKC) isoenzymes and the effects of PKC activation on myocardial phospholipase A2 (PLA2) activity, platelet-activating factor (PAF) generation and eicosanoid release were studied in spontaneously beating cultured rat cardiomyocytes. Western blotting analysis indicated that these cells contain PKC alpha, beta, delta and xi, but not PKC gamma or epsilon. Stimulation of cardiomyocytes with 4beta-phorbol 12-myristate 13-acetate (PMA) led to a rapid increase in particulate-bound PKC activity, a response attributed to the activation of alpha-, delta- and xi-type PKCs but not beta-type PKC. Translocation of PKC alpha, delta and xi was accompanied by simultaneous increases in cellular lysophosphatidylcholine (lyso-PC), PAF, 15(S)-hydroxy-5,8,-11, 13-eicosatetraenoic acid (15-HETE), prostaglandin E2 (PGE2) and prostacyclin (PGI2) generation, suggesting that one or more of these isoenzymes directly or indirectly activates a PLA2 in these cells. Confirming this, 4beta-phorbol 12-monoacetate and 4alpha-phorbol had no effect on cellular eicosanoid formation, while the PMA-induced response was fully abolished both in the presence of the PKC inhibitors staurosporine and CGP 41251 and in PKC-down-regulated cells. PKC alpha, delta and/or xi therefore appear to play an important role in the PMA-mediated activation of cardiomyocyte PLA2, an event leading to subsequent production of PGI2, PGE2, 15-HETE, lyso-PC and PAF in this tissue.