Platelet desensitization induced by arachidonic acid is not due to cyclo-oxygenase inactivation and involves the endoperoxide receptor

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Abstract
1 Human platelets pre-exposed to arachidonic acid (AA) (0.1-1 mM) or to the endoperoxide analogue U46619 (1–3 μM) and then washed and resuspended, failed to respond with aggregation or secretion to a second challenge by either agonist. The response to thrombin at low (0.04-0.1 u ml−1) but not at high (2.5 u ml−1) concentrations was also inhibited by pre-exposure to AA and U46619. 2 The ability of platelets to synthesize thromboxane (Tx) B2 from AA or upon challenge with thrombin persisted despite platelet desensitization. 3 In the presence of the reversible cyclo-oxygenase (CO) inhibitors methyl salicylate (MS) or L8027, pre-exposure to AA had no effect on subsequent challenge by the same agonist or by U46619, whereas platelet desensitization by pre-exposure to U46619 persisted. However, platelet activation by, and desensitization to AA and U46619, was prevented by trimetoquinol and compound L636499, two thromboxane/endoperoxide receptor antagonists. 4 In contrast to the CO inhibitors, the thromboxane synthetase inhibitor dazoxiben, which in 3 ‘responders’ out of 5 subjects suppressed aggregation, secretion, and Tx formation induced by AA, failed to prevent AA-induced desensitization. 5 Compared to quiescent cells the distances between platelets desensitized after re-exposure to AA were reduced in electron microscopy, but the tight connections associated with aggregated cells were not observed. Degranulation was also not observed and cell morphology resembled that of normal quiescent platelets. 6 In conclusion, (a) AA and U46619 desensitize human platelets at a similar site sensitive to prostaglandin/thromboxane receptor antagonists, and show cross-desensitization; (b) desensitization by AA appears to be mediated by a CO-dependent metabolite, as CO inhibitors prevent desensitization by AA but not to U46619; (c) the failure of dazoxiben to prevent desensitization by AA suggests that a metabolite other than TxA2, possibly the endoperoxides, mediates the phenomenon; (d) desensitization does not involve inactivation of CO or thromboxane synthetase enzymes.