Carrageenan and thrombin trigger prostaglandin synthetase-independent aggregation of rabbit platelets: inhibition by phospholipase A2 inhibitors.

Acetylsalicylic acid, salicylic acid and indomethacin were equally effective in inhibiting aggregation of plasma-free rabbit platelets induced by carrageenan and by thrombin. In contrast, only acetylsalicylic acid and indomethacin suppressed the accompanying formation of prostaglandin-like activities. Blockade of aggregation required the presence of the inhibitors in the platelet preparation, whereas blockade of prostaglandin synthetase remained even when the inhibitors were washed out. Prostaglandin synthetase-derived products appear not to be involved in the development of aggregation by carrageenan or by thrombin. Such aggregation was inhibited by two phospholipase A2 inhibitors, bromophenacyl bromide and mepacrine. It is suggested that carrageenan and thrombin-induced aggregation are mediated by non-prostaglandin, phospholipase A2-derived products.