Nitric oxide-mediated cyclooxygenase activation. A key event in the antiplatelet effects of nitrovasodilators

We have evaluated the contributions of nitric oxide (NO) and prostacyclin (PGI₂) in the in vivo antiplatelet effects of clinically useful nitrovasodilators. In rats, intravenous infusion of three NO donors, glyceryl trinitrate, sodium nitroprusside, or 3'-morpholinosydnonimine, the stable metabolite of molsidomine, released 6-keto PGF1α (the stable metabolite of PGI₂) and inhibited ex vivo human platelet aggregation to adenosine diphosphate by at least 80%. In in vitro studies, glyceryl trinitrate, sodium nitroprusside, and 3'-morpholinosydnonimine, at clinically attainable concentrations, increased cyclooxygenase activity in endothelial cells (EC), which resulted in a four- to sixfold release of 6-keto PGF1α. Pretreatment of the EC with hemoglobin which binds to and inactivates the biological actions of NO, but not by methylene blue (MeB), attenuated the NO-mediated PGI₂ from the EC by at least 70%. Release of 6-keto PGFIα by the NO donors increased the ability of these compounds to inhibit thrombin-induced human platelet aggregation by at least 10 times; this potentiation was inhibited by hemoglobin but not by MeB. MeB blocked the direct anti-platelet effect of the NO donors in the absence of EC, in summary, we have demonstrated that NO, directly as well as together with an NO-driven cyclooxygenase activation (and hence PGI₂), release contributes to the marked anti-platelet effects observed after the in vivo administration of clinically used nitrovasodilators.