Control of Ca2+ channel current and exocytosis in rat lactotrophs by basally active protein kinase C and calcineurin

Modulation of voltage-activated Ca2+ channel activity by phosphorylation was studied in metabolically intact voltage-clamped rat lactotrophs. Experiments using Ba2+ as a charge carrier indicated that a phorbol ester protein kinase C activator stimulates high-voltage-activated Ca2+ channel currents, but has no effect on low-voltage-activated currents. Extracellular application of structurally and mechanistically distinct protein kinase C inhibitors (staurosporin, H7, calphostin C, chelerythrine and Ro 31-8220) preferentially inhibited the high-voltage-activated Ba2+ current. This suggests that protein kinase C is required for maintainance of Ca2+ channel activity even in the absence of modulators. Cyclosporin A, an inhibitor of the Ca2+/calmodulin-dependent protein phosphatase calcineurin, increased the high-voltage-activated Ca2+ channel current, and staurosporin reversed this effect. Thus, dephosphosphorylation by calcineurin may limit basal Ca2+ channel activity. Time-domain monitoring of cellular capacitance changes demonstrated that cyclosporin A and 12-O-tetradecanoyl-phorbol-13-acetate do not affect exocytosis at a hyperpolarized potential, but each enhances depolarization-induced exocytosis. Facilitation of exocytosis by cyclosporin A differed from 12-O-tetradecanoyl-phorbol-13-acetate in that it was biphasic. The delayed facilitation induced by cyclosporin A could be accounted for by stimulation of the voltage-gated Ca2+ current. These results suggest that the high-voltage activated Ca2+ channel current in rat lactotrophs is determined by the opposing basal activities of protein kinase C and calcineurin. Furthermore, it is concluded that the regulation of Ca2+ channels by protein kinase C and calcineurin affects depolarization-induced exocytosis.