A role for inositol triphosphate in intracellular Ca2+ mobilization and granule secretion in platelets

Two of the earliest known events in platelet activation include formation of inositol 1,4,5-triphosphate (IP3) and Ca2+ release from the platelet dense tubular system. Although the mechanism which triggers Ca2+ release from the dense tubular system is unknown, recent evidence suggests that the IP3 plays a role. In the present studies, human platelets permeabilized with saponin were used to examine Ca2+ movements and dense granule secretion in response to IP3. At low concentrations, saponin caused complete loss of the cytosolic enzyme lactate dehydrogenase without liberating enzyme markers for the dense tubular system. In the presence of ATP and Mg2+, the permeabilized platelets were able to sequester 45Ca in both mitochondrial and nonmitochondrial pools. IP3 released up to 70% of the nonmitochondrial 45Ca in less than 5 s. Half-maximal Ca2+ release occurred at 1 microM IP3, a concentration compatible with the amount of IP3 produced during platelet activation. The absolute quantity of Ca2+ released, approximately 400 pmol/10(8) platelets, is similar to the Ca2+ content of the platelet dense tubular system estimated from steady-state exchange studies in intact platelets. Increases in pH, but not Na+ or Ca2+, comparable to those thought to occur during platelet activation potentiated the effect of IP3. IP3 was also able to cause serotonin release from the saponin-treated platelets. This effect was inhibited by EGTA and had a dose-response curve identical to that for IP3-induced Ca2+ release, which suggests that the IP3-induced secretion is mediated by the released Ca2+ and not directly by the IP3. Therefore, these data demonstrate: 1) that IP3 is able to trigger the rapid release of large amounts of Ca2+ from the platelet dense tubular system, 2) that other early events in platelet activation can affect this process, and 3) that the Ca2+ released by IP3 can serve as a trigger for subsequent events including secretion of the contents of the platelet dense granules.