The addition of 12-O-tetradecanoylphorbol-13-acetate (TPA) markedly enhanced cAMP formation induced by carbacyclin, a stable prostacyclin analogue, in cultured mast cells (IC2 cells), but did not enhance basal or NaF plus AlCl3-induced cAMP formation. On the other hand, W-7, a calmodulin (CaM) inhibitor, almost completely suppressed the enhancing activity of TPA, suggesting the involvement of CaM in the enhancement by TPA of carbacyclin-induced cAMP formation. The enhancing activity of TPA disappeared in TPA-treated cells permeabilized with saponin in the presence of Ca2+, but reconstitution with CaM in the permeable cells resulted in remarkable restoration of the action of TPA. On the other hand, TPA treatment induced the phosphorylation and translocation of myristoylated alanine-rich C kinase substrate (MARCKS) from the membrane to the cytosol. Exogenously added protein kinase C (PKC) also phosphorylated MARCKS and induced its translocation in the cells permeabilized with saponin. Whereas the addition of CaM did not enhance the carbacyclin-stimulated GTPase activity and adenylate cyclase activity in the control permeable cells, in which MARCKS bound to the membrane, CaM markedly enhanced those activities in the PKC-treated permeable cells, which lost endogenous membrane-bound MARCKS. When MARCKS was added to the PKC-treated permeable cells, MARCKS bound to the membrane and inhibited the effects of CaM. These results suggest that activation of PKC enhances the prostacyclin-activated adenylate cyclase through a CaM/MARCKS system.