Evidence of Ca2+ mobilizing action of arachidonic acid in human platelets

The addition of arachidonic acid induced a rapid release of 45Ca2+ from human platelet membrane vesicles which accumulated 45Ca2+ in the presence of ATP. Docosahexaenoic acid, eicosapentaenoic acid, linolenic acid and linoleic acid were less active than arachidonic acid. In contrast, oleic acid, myristic acid and palmitic acid were without effect. The thromboxane A2 analogue induced no 45Ca2+ release. The cyclooxygenase/lipoxygenase inhibitor failed to suppress arachidonic acid-induced 45Ca2+ release at the concentration which inhibited the production of lipid peroxides. These data indicate that the activity of arachidonic acid may be due to fatty acid itself and not to its metabolites. The combination of arachidonic acid and inositol 1,4,5-trisphosphate (IP3) resulted in a greater 45Ca2+ release from platelet membrane vesicles than either compound alone. When the intracellular free Ca2+ concentration ([Ca2+]i) was measured using fura-2, the thrombin-induced [Ca2+]i increase was reduced in platelets which had been treated with a phospholipase A2 inhibitor, ONO-RS-082 (2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid). These results provide evidence that arachidonic acid alone may cause Ca2+ increase and also may induce an additional Ca2+ mobilization to IP3-induced Ca2+ release in human platelets.