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. Ca2+-dependent exocytosis could be induced by membrane depolarization or by including solutions containing 2 muM free Ca2+ in the patch pipette. Ca2+-dependent exocytosis was inhibited by GDPbetaS, suggesting involvement of a GTP-binding protein. The hydrolysis-resistant GTP analogs, GTPgammaS and GppNHp, were able to stimulate exocytosis at low free Ca2+ concentrations. The stimulatory response to GTPgammaS was abolished by both GDPbetaS 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.