Mass Changes in Myoinositol Trisphosphate in Human Platelets Stimulated by Thrombin. Inhibitory Effects of Phorbol Ester

Myoinositol trisphosphate (IP3) is formed when phosphatidylinositol 4,5-bisphosphate (PIP2) is hydrolyzed by phospholipase C. At micromolar concentrations, IP3 is a stimulus for Ca2+ release in both platelet membranes and various permeabilized cells. We have utilized a combination of ion exchange and capillary gas chromatography to quantitate the mass of IP3 produced by human platelets stimulated by thrombin. Accumulations of IP3 are transient and detectable within 5 s of exposure to thrombin. Within 15 s, thrombin (1 unit/ml) promotes the formation of 134 pmol of IP3/10(9) platelets, the equivalent of an intracellular concentration of 13.4 microM. Incubation of platelets with a stimulus for protein kinase C, 12-O-tetradecanoyl phorbol 13-acetate, prior to the addition of thrombin impairs the hydrolysis of PIP2 and the increase in IP3, with 50% inhibition occurring at 60 nM TPA. We conclude that platelets produce sufficient quantities of IP3 to cause Ca2+ release from membrane stores. TPA inhibits the activation of phospholipase C and consequently the generation of IP3. The decreased accumulation of IP3 in platelets exposed to TPA may account for the inhibited rise in cytoplasmic Ca2+ which has been observed in such platelets.