Effect of thromboxane A 2 synthetase inhibition, singly and combined with thromboxane A 2/prostaglandin endoperoxide receptor antagonism, on inositol phospholipid turnover and on 5-HT release by washed human platelets

Abstract

Differential effects on human platelet function of thromboxane A 2 (TXA 2) synthetase inhibition singly and of TXA 2 synthetase inhibition combined with TXA 2/prostaglandin endoperoxide receptor antagonism were revealed. using ridogrel as a probe. Ridogrel combines selective TXA 2 synthetase inhibition with TXA 2/prostaglandin receptor antagonism in one molecule; in washed human platelets, the compound reduces the production of TXB 2 (IC 50=1.3 × 10 −8 M) and increases that of PGF 2α, PGE 2, PGD 2 from [ 14C]arachidonic acid. Additionally, at higher concentrations (K i = 0.52 × 10 −6 M), it selectively antagonizes the breakdown of inositol phospholipids, subsequent to stimulation of TXA 2/prostaglandin endoperoxide receptors with U 46619. The latter happens in a competitive way with fast receptor association-dissociation characteristics. At low concentrations (1 × 10 −9 − × 10 −7 M) producing single TXA 2 synthetase inhibition, ridogrel reduces the collagen-induced formation of TXB 2 by washed platelets, but enhances [ 32P]phosphatidic acid (PA) accumulation and [ 3H]5-hydroxytryptamine (5-HT) release. At higher concentrations (1 × 10 −6 −1 × 10 −5 M) which additionally block U 46619-induced [ 32P]PA accumulation, ridogrel inhibits the [ 32P]PA accumulation and release of [ 3H]5-HT by human platelets stimulated with collagen. These observations, corroborated by results obtained with OKY 1581, sulotroban, indomethacin and human serum albumin, suggest a causal role for prostaglandin endoperoxides in the stimulation by TXA 2 synthetase inhibition of platelet reactions to collagen. They reinforce the concept that TXA 2 synthetase inhibition-induced reorientation of cyclic endoperoxide metabolism, away from TXA 2 into inhibitory prostanoids, requires additional TXA 2/prostaglandin endoperoxide receptor antagonism to achieve optimal anti-platelet effects.