ABSTRACT
Xanthine oxidoreductase (XOR) is generally known as the final enzyme in purine metabolism and as a source of reactive oxygen species (ROS). In addition, this enzyme has been suggested to mediate nitric oxide (NO) formation via reduction of inorganic nitrate and nitrite. This NO synthase (NOS)-independent pathway for NO generation is of particular importance during certain conditions when NO bioavailability is diminished due to reduced activity of endothelial NOS (eNOS) or increased oxidative stress, including aging and cardiovascular disease. The exact interplay between NOS- and XOR-derived NO generation is not fully elucidated yet. The aim of the present study was to investigate if eNOS deficiency is associated with changes in XOR expression and activity and the possible impact on nitrite, NO and ROS homeostasis. Plasma levels of nitrate and nitrite were similar between eNOS deficient (eNOS-/-) and wildtype (wt) mice. XOR activity was upregulated in eNOS-/- compared with wt, but not in nNOS-/-, iNOS-/- or wt mice treated with the non-selective NOS inhibitor L-NAME. Following an acute dose of nitrate, plasma nitrite increased more in eNOS-/- compared with wt, and this augmented response was abolished by the selective XOR inhibitor febuxostat. Livers from eNOS-/- displayed higher nitrite reducing capacity compared with wt, and this effect was attenuated by febuxostat. Dietary supplementation with nitrate increased XOR expression and activity, but concomitantly reduced superoxide generation. The latter effect was also seen in vitro after nitrite administration. Treatment with febuxostat elevated blood pressure in eNOS-/-, but not in wt mice. A high dose of dietary nitrate reduced blood pressure in naïve eNOS-/- mice, and again this effect was abolished by febuxostat. In conclusion, eNOS deficiency is associated with an upregulation of XOR facilitating the nitrate-nitrite-NO pathway and decreasing the generation of ROS. This interplay between XOR and eNOS is proposed to play a significant role in NO homeostasis and blood pressure regulation.
Subject(s)
Nitric Oxide Synthase Type III/genetics , Nitric Oxide Synthase Type II/genetics , Nitric Oxide Synthase Type I/genetics , Nitric Oxide/blood , Xanthine Dehydrogenase/genetics , Animals , Blood Pressure/drug effects , Enzyme Inhibitors/pharmacology , Febuxostat/pharmacology , Gene Expression Regulation , Male , Mice , Mice, Knockout , NG-Nitroarginine Methyl Ester/pharmacology , Nitrates/blood , Nitrates/pharmacology , Nitric Oxide Synthase Type I/antagonists & inhibitors , Nitric Oxide Synthase Type I/deficiency , Nitric Oxide Synthase Type II/antagonists & inhibitors , Nitric Oxide Synthase Type II/deficiency , Nitric Oxide Synthase Type III/antagonists & inhibitors , Nitric Oxide Synthase Type III/deficiency , Nitrites/blood , Nitrites/pharmacology , Oxidation-Reduction , Signal Transduction , Superoxides/metabolism , Xanthine Dehydrogenase/antagonists & inhibitors , Xanthine Dehydrogenase/metabolismABSTRACT
BACKGROUND AND PURPOSE: NO deficiency and oxidative stress are crucially involved in the development or progression of cardiovascular disease, including hypertension and stroke. We have previously demonstrated that acute treatment with the newly discovered organic nitrate, 2-nitrate-1,3-dibuthoxypropan (NDBP), is associated with NO-like effects in the vasculature. This study aimed to further characterize the mechanism(s) and to elucidate the therapeutic potential in a model of hypertension and oxidative stress. EXPERIMENTAL APPROACH: A combination of ex vivo, in vitro and in vivo approaches was used to assess the effects of NDBP on vascular reactivity, NO release, NADPH oxidase activity and in a model of hypertension. KEY RESULTS: Ex vivo vascular studies demonstrated NDBP-mediated vasorelaxation in mesenteric resistance arteries, which was devoid of tolerance. In vitro studies using liver and kidney homogenates revealed dose-dependent and sustained NO generation by NDBP, which was attenuated by the xanthine oxidase inhibitor febuxostat. In addition, NDBP reduced NADPH oxidase activity in the liver and prevented angiotensin II-induced activation of NADPH oxidase in the kidney. In vivo studies showed that NDBP halted the progression of hypertension in mice with chronic angiotensin II infusion. This was associated with attenuated cardiac hypertrophy, and reduced NADPH oxidase-derived oxidative stress and fibrosis in the kidney and heart. CONCLUSION AND IMPLICATIONS: The novel organic nitrate NDBP halts the progression of angiotensin II-mediated hypertension. Mechanistically, our findings suggest that NDBP treatment is associated with sustained NO release and attenuated activity of NADPH oxidase, which to some extent requires functional xanthine oxidase.
