Calcium and exocytosis.

Amine secretion from electropermeabilized bovine chromaffin cells and human platelets requires Ca2+ and MgATP. There appears to be little correlation between the pH or potential of the interior of the amine storage granules of the chromaffin cells and the Ca2+ sensitivity or extent of secretion. The Ca2+ sensitivities of secretion for both preparations are increased by activators of protein kinase C. In the platelet, thrombin also increases the Ca2+ sensitivity. The thrombin-induced response is further enhanced by micromolar levels of GTP. The non-hydrolysable analogue GTP gamma S also potentiates the Ca2+-dependent secretory response, but this effect is additive to that seen by thrombin rather than synergistic, as is the case with GTP. GTP gamma S inhibits catecholamine secretion from bovine chromaffin cells. In both preparations the effects of GTP gamma S are inhibited by 10 microM GTP, even though GTP concentrations up to 1 mM are without effect when added alone. These results are consistent with there being two sites of action for the guanine nucleotides, one at the level of the agonist receptor and activated by GTP or one of its breakdown products, and the other one activated by GTP gamma S--possibly at the level of protein kinase C itself.