Activation of permeabilized platelets by inositol-1,4,5-trisphosphate.

The effect of inositol 1,4,5-trisphosphate (IP3) was studied using human platelets permeabilized with saponin and suspended in a high potassium, Ca2+-free buffer containing 40 microM EGTA and 1.2 mM magnesium. Under these conditions IP3 stimulated aggregation at a concentration of 0.5 microM with maximum aggregation at 5.0 microM. Aggregation was associated with phosphorylation of myosin light chain and a 47,000 dalton protein, and with a change in platelet shape including granule centralization and pseudopod formation similar to changes seen when cytoplasmic calcium is raised by other means. IP3 stimulated [14C]-serotonin release from platelet dense granules, [14C]-arachidonic acid release from platelet phospholipids and production of thromboxane B2. Preincubation of platelets with aspirin which blocked thromboxane formation also inhibited protein phosphorylation, serotonin secretion and partially inhibited aggregation. These results support the concept that IP3 is a major intracellular messenger in platelets and suggests that its effects are mediated both through Ca2+ flux and thromboxane formation.