Blocking platelet aggregation inhibits thromboxane A(2) formation by low dose agonists but does not inhibit phosphorylation and activation of cytosolic phospholipase A(2).

Inhibition of aggregation by Ro 44-9883, a potent and selective non-peptide GPIIb/IIIa antagonist, resulted in inhibition of serotonin secretion induced by weak agonists such as ADP or low doses of either thrombin receptor agonist peptide (TRAP) or collagen. In contrast, alpha granule secretion was inhibited to different extents dependent on donor, averaging 60% inhibition. Inhibition of serotonin secretion correlated with an inhibition of thromboxane A(2) (TxA(2)) formation, both of which were overcome by higher doses of TRAP or collagen. Ro 44-9883 had no effect on the already reduced serotonin secretion and TxA(2) formation in Glanzmann's thrombasthenic platelets. Restoration of serotonin secretion in the absence of aggregation requires both TxA(2) and lysophosphatidic acid. In addition, Ro 44-9883 inhibition of TxA(2) formation was not due to a lack of phospholipase A(2) (PLA(2)) phosphorylation and activation as assayed in vitro. These results suggest that aggregation is required for weak or low dose agonist induced in vivo activity of PLA(2), possibly by either regulating phospholipid substrate availability or interaction of PLA(2) with platelet membranes. (C) 1997 Elsevier Science Ltd.