CA2-DEPENDENT AND GTP-DEPENDENT EXOCYTOSIS IN MOUSE PANCREATIC BETA-CELLS INVOLVES BOTH COMMON AND DISTINCT STEPS()

1. The effects of GTP and Ca2+ secretion from single pancreatic beta-c ells were studied using capacitance measurements as an indicator of ex ocytosis. 2. GTP or GTP gamma S produced a concentration-dependent inc rease in cell capacitance in the absence of intracellular calcium. The re was no effect of cyclic AMP or BAPTA on GTP-induced secretion. 3. I n tile absence of GTP, the relationship between intracellular calcium concentration and the maximum rate of secretion was fitted by the Hill equation with a slope factor of 2.5 and half-maximal activation at 1. 6 mu M intracellular Ca2+. Similar values n ere obtained in the presen ce of GTP gamma S, suggesting GTP does not alter the sensitivity of th e secretory machinery to Ca2+. 4. GDP beta S alone had no effect on ce ll capacitance but caused a dose-dependent inhibition of exocytosis in duced by infusion of either GTP gamma S or Ca2+, suggesting both stimu li involve G-protein activation. GDP beta S was without effect on exoc ytosis evoked by depolarization-mediated Ca2+ entry. 5. The time cours e of exocytosis following rapid elevation of GTP gamma S by photolysis of a caged precursor was dependent on the intracellular Ca2+ and cycl ic AMP concentrations. 6. Our results are interpreted in terms of a mo del in which the secretory pathways stimulated by Ca2+ and GTP contain both common and separate parts.