Microsomes were prepared from cultured neonatal rat cardiomyocytes. Incubation of microsomes in buffer containing 5 microM CaCl2, 5 mM cholate and 100 nM [3H-]Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5) P2) resulted in the formation of [3H-]InsP3. GTP-gamma-S (125 microM) stimulated the production of [3H-]InsP3. Microsomes prepared from phorbol ester-treated (100 nM phorbol 12-myristate 13-acetate, PMA) cardiomyocytes showed decreased activities of basal as well as GTP-gamma-S-stimulated [3H-]PtdIns(4,5)P2 hydrolysis. In the microsomes a 15 kD protein was demonstrated to be the major substrate phosphorylated by intrinsic protein kinase C, which was activated by 0.5 mM Ca2+. Addition of phorbol ester (100 nM PMA) enhanced the 32P-incorporation into the 15 kD protein. Protein kinase C, purified from rat brain, in the presence of Ca2+, diglyceride, and phosphatidylserine did not change the phosphorylation pattern any further. In conclusion, it was shown that phorbol ester pretreatment of neonatal rat cardiomyocytes reduces microsomal GTP-gamma-S-stimulated PtdIns(4,5)P2-specific phospholipase C activity, as estimated with exogenous substrate, and that in cardiomyocyte microsomes phorbol ester activates protein kinase C-induced 15 kD protein phosphorylation. The results indicate that phorbol ester may down-regulate alpha 1-adrenoceptor mediated PtdIns(4,5)P2 hydrolysis by activation of protein kinase C-induced 15 kD protein phosphorylation.