GTP gamma S stimulates exocytosis in patch-clamped rat melanotrophs

We investigated the molecular mechanisms regulating exocytosis in patch-clamped melanotrophs by measuring the membrane capacitance. Ca(2+)-dependent exocytosis could be induced by membrane depolarization or by including solutions containing 2 microM free Ca2+ in the patch pipette. Ca(2+)-dependent exocytosis was inhibited by GDP beta S, suggesting involvement of a GTP-binding protein. The hydrolysis-resistant GTP analogs, GTP gamma S and GppNHp, were able to stimulate exocytosis at low free Ca2+ concentrations. The stimulatory response to GTP gamma S was abolished by both GDP beta S and GTP. The latter suggests that a sustained activation of a GTP-binding protein is necessary for exocytosis. This behavior is similar to the stimulation of exocytosis by guanine nucleotides in mast cells and other nonexcitable cells and suggests a common regulatory mechanism.