Rat mast cells, pretreated with metabolic inhibitors and permeabilized by streptolysin-O, secrete histamine when provided with Ca2+ (buffered in the micromolar range) and nucleoside triphosphates. We have surveyed the ability of various exogenous nucleotides to support or inhibit secretion. The preferred rank order in support of secretion is ITP greater than XTP greater than GTP much greater than ATP. Pyrimidine nucleotides (UTP and CTP) are without effect. Nucleoside diphosphates included alongside Ca2+ plus ITP inhibit secretion in the order 2'-deoxyGDP greater than GDP greater than o-GDP greater than ADP approximately equal to 2'deoxyADP approximately equal to IDP. Secretion from the metabolically inhibited and permeabilized cells can also be induced by stable analogues of GTP (GTP-gamma-S greater than GppNHp greater than GppCH2p) which synergize with Ca2+ to trigger secretion in the absence of phosphorylating nucleotides. ATP enhances the effective affinity for Ca2+ and GTP analogues in the exocytotic process but does not alter the maximum extent of secretion. The results suggest that the presence of Ca2+ combined with activation of events controlled by a GTP regulatory protein provide a sufficient stimulus to exocytotic secretion from mast cells.

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