To investigate the role of inositol phospholipid turnover in the atrial natriuretic peptide (ANP) secretion, the secretory responses from isolated perfused rat hearts to the ionophore, A23187, and the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), alone or in combination, were studied. A23187 induced a sharp increase in ANP secretion, whereas TPA caused a slowly progressive increase in secretion rate. 4 alpha-phorbol-12,13-didecanoate, which lacks the ability to activate protein kinase C, had no effect on ANP secretion. The combination of A23187 and TPA stimulated ANP secretion higher than the calculated additive value for each agent. The synergistic effect of the agents suggests a role of calcium-activated protein kinase C in ANP secretion from atrial cardiocytes.