Abstract 72: Vascular and Cardiac Effects of Inducible Nitric Oxide Synthase in Hyperhomocysteinemic Mice

Background Diet-induced hyperhomocysteinemia (HHcy) produces endothelial and cardiac dysfunction and promotes thrombosis through a mechanism proposed to involve oxidative stress. Inducible nitric oxide synthase (iNOS) is upregulated in HHcy and can generate oxidants such as superoxide. We tested the hypothesis that iNOS mediates the adverse oxidative vascular, thrombotic, and cardiac effects of HHcy. Methods Mice deficient in iNOS ( Nos2-/ -) and their wild-type (WT) littermates were fed either a control (C) diet or a high methionine/low folate (HM/LF) diet for 3-6 months. Vasomotor function was assessed in cerebral arterioles in response to the endothelium-dependent dilator acetylcholine or the endothelium-independent dilator nitroprusside. Thrombotic occlusion of the carotid artery was measured in response to photochemical injury (rose bengal). Regional myocardial injury was measured after 30 minutes of ischemia and 2 hours of reperfusion of the left coronary artery. Results Nos2 -/- and WT mice fed the HM/LF diet developed similar levels of HHcy, with a 2-fold increase in plasma total homocysteine (P<0.001 vs. C diet). As expected, WT mice with HHcy exhibited endothelial dysfunction, with impaired dilatation to acetylcholine but not nitroprusside, and enhanced susceptibility to thrombosis, with shortened times to occlusion of the carotid artery (P<0.05 vs. C diet). Interestingly, deficiency of iNOS resulted in decreased, rather than increased dilatation responses to acetylcholine (P<0.05 vs. WT mice). Nos2 -/- mice also exhibited shortened times to thrombotic occlusion in mice fed the C diet (P<0.05 vs. WT mice) and failed to protect from endothelial dysfunction or accelerated thrombosis in mice fed the HM/LF diet. Deficiency of iNOS did not alter myocardial infarct size in mice fed the C diet but significantly increased infarct size and cardiac superoxide production in mice fed the HM/LF diet (P<0.05 vs. WT mice). Conclusions These findings suggest that endogenous iNOS protects from, rather than exacerbates, endothelial dysfunction, thrombosis, and myocardial ischemia-reperfusion injury. In the setting of HHcy, iNOS appears to function to blunt cardiac oxidative stress rather than as a source of superoxide.