The mechanism of arachidonic acid release in collagen-activated human platelets

The mechanism of arachidonic acid (AA) release in collagen-activated human platelets was studied. An arachidonic acid metabolite, thromboxane B2 (TXB2), was formed in parallel with the formation of phosphatidic acid (PA) without formation of lysophosphatidic acid (lysoPA) or lysophosphatidylinositol (LysoPI) in the absence of extracellular Ca(2+), suggesting the AA was released from PI via a Pi-specific phospholipase C (PI-PLC)/diacylglycerol (DG) lipase/monoacylglycerol (MG) lipase pathway under the cytosolic low CA(2+) concentrations. Moreover, solubilized DG lipase and MG lipase could hydrolyze the substrates at basal 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 A2 activated by both collagen-receptor occupancy and elevated Ca(2+) levels. A TXA2 mimetic agonist, STA2 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