Role of phospholipid molecular species containing poly unsaturated fatty acids

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We have indicated that several nanomolar thromboxane A_2 (TXA_2) originating from inositollipids may directly cause Ca^<2+> mobilization in human platelets during activation with collagen.The mechanism of arachidonic acid (AA) release in collagen-activated human platelets was studied. An arachidonic acid metabolite, thromboxane B_2, was formed in parallel with the formation of phosphatidic acid without fomation of lysophosphatidic acid or lysophosphatidylinositol in the absence of extracellular Ca^<2+>, suggesting that AA was released from PI via a PI-specific phospholipase C/diacylglycerol lipase/monoacylglycerol lipase pathway under the cytosolic low Ca^<2+> concentrations. Moreover, solubilized DG lipase and MG lipase could hydrolyze the substrates at basel cytosolic free Ca^<2+> concentrations. Subsequently, the relationship of cytosolic free Ca^2 concentrations and formation of AA metabolites was analyzed using Ca^<2+> ionophore, A23187. Collagen was able to induce a release of small amounts of AA under basal cytosolic Ca^<2+> conditions. However, a release of large amounts of AA was induced by phospholipase A_2 activated by both collagen-receptor occupancy and elevated Ca^<2+> levels. A TXA_2 mimetic agonist, STA_2 induced all the responses except for AA release. From these results, the mechanism of AA release and signal transduction in collagen-activated human platelets is discussed.