Guanosine 5'-triphosphate (GTP) and its nonhydrolyzable analogs, such as guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S), induce several responses in platelets including secretion, production of inositol 1,4,5-triphosphate (IP3) and mobilization of Ca2+ from intracellular sites. Because IP3 is well established as a second messenger in mobilizing Ca2+ from intracellular stores it has been generally assumed that Ca2+ release by GTP/GTP gamma S in platelets is mediated by IP3. However, studies in neuronal, hepatic and smooth muscle cells have suggested that IP3 and GTP/GTP gamma S activate Ca2+ release by distinct mechanisms and that IP3-independent mechanisms mediate GTP/GTP gamma S-induced Ca2+ release. In several tissues heparin inhibits binding of IP3 and blocks IP3-stimulated Ca2+ release in a competitive and specific manner. In the present studies, IP3 and GTP gamma S induced Ca2+ release and their relationship was examined in human platelets using heparin as a probe. In saponin permeabilized platelets, IP3 (0.05-5 microM) induced a prompt, dose-dependent release of Ca2+ (EC50 0.5 microM). GTP gamma S (1-50 microM) released Ca2+ in a dose-dependent manner with EC50 of 2 microM but with a time lag of 30-90 seconds. Exposure of platelets to 1 microM IP3 following a submaximal response with GTP gamma S (1 microM) resulted in a further increase in Ca2+ release but no further increase was noted on adding 1 microM IP3 following a maximal response with GTP gamma S (10 microM); similar findings were noted on reversing the order of addition of GTP gamma S and IP3 suggesting that these effectors release Ca2+ from the same source. IP3 (0.5 microM) induced Ca2+ release was blocked by low molecular weight (4000-6000) heparin (IC50 30 micrograms/ml). More importantly, heparin abolished GTP gamma S (2.5 microM) induced Ca2+ release (IC50 10 micrograms/ml). These results indicate that, in contrast to the findings in some other cells, in human platelets GTP gamma S-induced Ca2+ release is mediated largely by a mechanism involving IP3.