Hypoxia–Reoxygenation Triggers Coronary Vasospasm in Isolated Bovine Coronary Arteries via Tyrosine Nitration of Prostacyclin Synthase

The role of peroxynitrite in hypoxia–reoxygenation-induced coronary vasospasm was investigated in isolated bovine coronary arteries. Hypoxia–reoxygenation selectively blunted prostacyclin (PGI₂)-dependent vasorelaxation and elicited a sustained vasoconstriction that was blocked by a cyclooxygenase inhibitor, indomethacin, and SQ29548, a thromboxane (Tx)A₂/prostaglandin H₂ receptor antagonist, but not by CGS13080, a TxA₂ synthase blocker. The inactivation of PGI₂ synthase, as evidenced by suppressed 6-keto-PGF_1α release and a decreased conversion of ¹⁴C-prostaglandin H₂ into 6-keto-PGF_1α, was paralleled by an increased nitration in both vascular endothelium and smooth muscle of hypoxia–reoxygenation-exposed vessels. The administration of the nitric oxide (NO) synthase inhibitors as well as polyethylene-glycolated superoxide dismutase abolished the vasospasm by preventing the inactivation and nitration of PGI₂ synthase, suggesting that peroxynitrite was implicated. Moreover, concomitant administration to the organ baths of the two precursors of peroxynitrite, superoxide, and NO mimicked the effects of hypoxia–reoxygenation, although none of them were effective when given separately. We conclude that hypoxia–reoxygenation elicits the formation of superoxide, which causes loss of the vasodilatory action of NO and at the same time yields peroxynitrite. Subsequently, peroxynitrite nitrates and inactivates PGI₂ synthase, leaving unmetabolized prostaglandin H₂, which causes vasospasm, platelet aggregation, and thrombus formation via the TxA₂/prostaglandin H₂ receptor.