The heterotrimeric Gi(3) protein acts in slow but not in fast exocytosis of rat melanotrophs

Besides having a role in signal transduction some trimeric G-proteins may b e involved in a late stage of exocytosis, Using immunocytochemistry and con focal microscopy we found that Gi(3)-protein resides mainly in the plasma m embrane, whereas Gi(1/2)-protein is preferentially associated with secretor y granules, To study the function of trimeric Gi(3)- and Gi(1/2)-proteins, secretory responses in single rat melanotrophs were monitored by patch-clam p membrane capacitance measurements. We report here that mastoparan, an act ivator of trimeric G-proteins, enhances calcium-induced secretory activity in rat melanotrophs. The introduction of synthetic peptides corresponding t o the C-terminal domain of the alpha-subunit of Gi(3)- and Gi(1/2)-proteins indicated that Gb peptide specifically blocked the mastoparan-stimulated s ecretory activity, which indicates an involvement of a trimeric Gi(3)-prote in in mastoparan-stimulated secretory activity. Flash photolysis of caged C a2+-elicited biphasic capacitance increases consisting of a fast and a slow er component, Injection of anti-Gi(3) antibodies selectively inhibited the slow but not the fast component of secretory activity in rat melanotrophs. We propose that the plasma membrane-bound Gi(3)-protein may be involved in regulated secretion by specifically controlling the slower kinetic componen t of exocytosis.