Effects of brazilin on the phospholipase A2 activity and changes of intracellular free calcium concentration in rat platelets

Brazilin {7,11 b-dihydrobenz[b]indeno[1,2-d]pyran-3,6a,9,10(6H)-tetrol} inhibited thrombin-, collagen-and ADP-induced aggregation of washed rat platelets. Thrombin- and collagen-induced ATP release were also inhibited by brazilin in a concentration-dependent manner. Brazilin inhibited the formation of platelet thromboxane A2 caused by thrombin, whereas it had no effect on the prostaglandin D2 formation. Brazilin inhibited [3H]-arachidonic acid liberation from membrane phospholipids of thrombin-stimulated platelets. Brazilin inhibited the rise of intracellular free calcium caused by thrombin. These results indicate that the inhibition of phospholipase (PLA2) activity and [Ca2+]i elevation might be at least a part of antiplatelet mechanism of brazilin.