LOCAL AMPLIFICATION OF PLATELET-FUNCTION BY 8-EPI PROSTAGLANDIN-F2-ALPHA IS NOT MEDIATED BY THROMBOXANE RECEPTOR ISOFORMS

8-epi-Prostaglandin (PG) F-2 alpha may be formed by cyclooxygenases 1 and 2 or by a free radical catalyzed process as an isoprostane. Concen trations of 8-epi-PGF(2 alpha) in the range 1 nM to 1 mu M induce a do se-dependent increase in platelet shape change, in calcium release fro m intracellular stores [Ca2+](i) and in inositol phosphates; it also c auses irreversible platelet aggregation, dependent on thromboxane gene ration, when incubated with subthreshold concentrations of ADP, thromb in, collagen, and arachidonic acid, Much higher concentrations of 8-ep i-PGF(2 alpha) (10-20 mu M) alone induce weak, reversible aggregation. Although these effects are prevented by pharmacological thromboxane r eceptor antagonists, they are unlikely to be mediated by thromboxane r eceptors, Thus, 8-epi-PGF(2 alpha) does not compete for binding at the stably expressed placental or endothelial isoforms of the thromboxane receptor or for binding of thromboxane ligands to human platelets, Fu rthermore, the response to 8-epi PGF(2 alpha) exhibits structural spec ificity versus 8-epi PGF(3 alpha) and PGF(2 alpha). Concentrations in the range that evoke its effects on platelets do not desensitize the a ggregation response stimulated by thromboxane or PGH(2) analogs, Unlik e primary prostaglandins, which are rapidly metabolized to inactive pr oducts, 8-epi PGF(2 alpha) circulates in plasma, However, the systemic concentrations found in healthy volunteers (median 48 pmol/liter) and in patients with hepatic cirrhosis (median 147 pmol/liter), a syndrom e of oxidant stress in vivo, fall well below those which modulate plat elet function. 8-Epi PGF(2 alpha) may amplify the response to platelet agonists in syndromes where oxidant stress and platelet activation co incide, Despite blockade by thromboxane antagonists, 8-epi PGF(2 alpha ) does not activate either of the thromboxane receptor isoforms descri bed in platelets, Activation of a distinct receptor would be consisten t with the enzymatic formation of 8-epi PGF(2 alpha) by cyclooxygenase s. However, incidental activation of such a receptor by systemic conce ntrations of 8-epi PGF(2 alpha) is unlikely to occur, even in syndrome s of excessive free radical generation in vivo.