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

Abstract

1Human platelets pre-exposed to arachidonic acid (AA) (0.1-1 mM) or to the endoperoxide analogue U46619 (1-3 tLM) 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-') but not at high (2.5 u ml ') 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 U466 19, 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 U466 19; (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.

Distinct platelet thromboxane A2/prostaglandin H2 receptor subtypes. A radioligand binding study of human platelets.

“…This schema is consistent with the notion that there are separate platelet TXA2 and endoperoxide (PGH2) receptors (3, 4) with distinct functions. As suggested by Carmo et al, endoperoxides may be responsible for irreversible platelet aggregation and secretion whereas TXA2 may cause initial platelet shape change and cellular approximation (3 (13). Antagonists bind with less specificity than do agonists and, in general, with greater capacity.…”

“…Due to the indistinguishable effects of these agents upon platelets, a common notation has been to label their combined putative receptor as TXA2/PGH2 (2). However, some studies have suggested that separate platelet receptors exist for the endoperoxide PGH2 and TXA2 (3,4). The thromboxane receptor has been postulated to mediate platelet shape change and the endoperoxide receptor has been postulated to mediate aggregation (3).…”

Signal transduction involved in the platelet adenylate cyclase sensitization associated with PGH2/TxA2 receptor desensitization

“…As described (Carmo et al, 1985), preincubation of washed platelets with U46619 (0 . 3 mM) led to platelet desensitization to U46619 itself, whereas the response to thrombin (1 IU/ ml) remained similar to control platelets (Fig 1).…”

“…3 mM) led to platelet desensitization to U46619 itself, whereas the response to thrombin (1 IU/ ml) remained similar to control platelets (Fig 1). This homologous desensitization by and to U46619 occurred at a site sensitive to PGH 2 /TxA 2 receptor antagonists (Carmo et al, 1985). The cAMP levels in control and desensitized platelets were evaluated in parallel at different times of cell incubation with PGI 2 (0 .…”

“…Arachidonic acid derivatives, thromboxane A2 (TxA 2 ) prostaglandin H 2 (PGH 2 ), and their stable mimetic U46619, desensitize human platelets at a common membrane site (Rao et al, 1981;Carmo et al, 1985). This desensitization, characterized by a biochemical and a functional modification of the PGH 2 /TxA 2 receptor, is prevented by the increase of intracellular cyclic adenosine 3 0 ,5 0 -monophosphate (cAMP) levels (Hatmi et al, 1986).…”