The role of protein kinase C in regulating Ca2 + channel activity was investigated using the whole-cell patch-clamp technique in the mouse pituitary tumor cell line AtT-20. The Ca2 + current was activated by depolarizing voltage steps from a holding potential of –80 mV. Extracellular application of the protein kinase C activator 1-oleoyl-2-acetylglycerol (OAG) reduced voltage-dependent Ca2 + current. This effect was reversible and dose dependent (10–100 µM). Pertussis toxin did not block the effect of OAG on Ca2 + current, suggesting that OAG does not affect Ca2 + channels via a pertussis toxin sensitive guanosine triphosphate binding protein. Na + -free solutions did not block the effect of OAG on Ca2 + channels, suggesting that this effect of OAG does not involve the Na+/H+ antiporter. The phorbol esters 12-deoxyphorbol-13-isobutyrate (10 µM) and phorbol-12,13-diacetate (100 µM) also reduced Ca2 + current. The results suggest that protein kinase C may be an inhibitory regulator of voltage-dependent Ca2 + ch