In an earlier study we reported the effect of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in releasing Ca2+ from highly purified human platelet intracellular membrane vesicles. [Authi & Crawford (1985) Biochem. J. 230, 247-253]. We have now investigated the metabolic and functional consequences of introducing Ins(1,4,5)P3 into saponin-permeabilized platelets. Washed human platelets when resuspended in a suitable medium were permeabilized with saponin (10-14 micrograms/ml) to allow entry of low-Mr water-soluble molecules without significant release of the cytoplasmic marker enzyme protein lactate dehydrogenase. Saponin-permeabilized platelets show identical platelet responses (shape change, aggregation and release of 5-hydroxy[14C]tryptamine) to both collagen (5 micrograms/ml) and thrombin (0.1 unit/ml) as obtained with intact cells, indicating that there is minimal disturbance to the surface membrane receptor topography for these two agonists. Ins(1,4,5)P3 (1-10 microM) added to saponin-treated platelets (but not to intact platelets) induced dose-related shape change, aggregation and release of 5-hydroxy[14C]tryptamine which at maximal doses was comparable with responses obtained with thrombin or collagen. The cyclo-oxygenase inhibitors indomethacin and aspirin, if added prior to saponization and Ins(1,4,5)P3 addition, completely inhibited both aggregation and release of 5-hydroxy[14C]tryptamine (EC50 for indomethacin, 50 nM; for aspirin, 30 microM). We believe that Ins(1,4,5)P3 induces the release of Ca2+ from intracellular storages sites which stimulates the Ca2+-dependent phospholipase A2 releasing arachidonic acid from membrane phospholipids. Arachidonic acid is then converted to the aggregatory prostanoids (prostaglandin H2 and thromboxane A2) resulting in the observed responses. This concept is supported by the use of the thromboxane receptor antagonists EPO 45 and EPO 92, both of which also completely inhibit Ins(1,4,5)P3-induced responses in saponin-permeabilized platelets. Electron microscopy of the platelet preparations revealed that thrombin- and collagen-induced platelet aggregates of intact and saponized cells were identical, showing extensive pseudopod formation and dense granule release. The Ins(1,4,5)P3-induced aggregates also showed similar dense granule release but an almost total absence of pseudopod formation. These results are discussed in the light of the second messenger role of Ins(1,4,5)P3 in stimulus-response coupling in platelets.

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