Role for peroxynitrite in the inhibition of prostacyclin synthase in nitrate tolerance

Objectives We tested whether in vivo nitroglycerin (NTG) treatment causes tyrosine nitration of prostacyclin synthase (PGI2-S), one of the nitration targets of peroxynitrite, and whether this may contribute to nitrate tolerance. Background Long-term NTG therapy causes tolerance secondary to increased vasoconstrictor sensitivity and increased vascular formation of reactive oxygen species. Because NTG releases nitric oxide (NO), NTG-induced stimulation of superoxide production should increase vascular nitrotyrosine levels, compatible with increased formation of peroxynitrite, the reaction product from NO and superoxide. Methods New Zealand White rabbits and Wistar rats were treated with NTG (0.4 mg/h for 3 days). Tolerance was assessed with isometric tension studies. Vascular peroxynitrite levels were quantified with luminol-derived chemiluminescence (LDCL) and peroxynitrite scavengers, such as uric acid and ebselen. As a surrogate parameter for the assessment of the activity of cyclic guanosine monophosphate-dependent kinase-I (cGK-I; the final signaling pathway for NO), the phosphorylation of the vasodilator-stimulated phosphoprotein (P-VASP) at serine 239 was analyzed. Results Nitroglycerin treatment increased LDCL, and the inhibitory effect of uric acid and ebselen on LDCL was augmented in tolerant rings. Immunoprecipitation of 3-nitrotyrosine–containing proteins and immunohistochemistry analysis identified PGI2-S as a tyrosine-nitrated protein. Accordingly, conversion of (14C)-PGH2 into 6-keto-PGF1α (=PGI2-S activity) was strongly inhibited. In vitro incubation of tolerant rings with ebselen and uric acid markedly increased the depressed P-VASP levels and improved NTG sensitivity of the tolerant vasculature. Conclusions Nitroglycerin-induced vascular peroxynitrite formation inhibits the activity of PGI2-S as well as NO, cGMP, and cGK-I signaling, which may contribute to vascular dysfunction in the setting of tolerance.