Effects of nitrite and nitroxyl on human vascular and platelet function.

Abstract

The identification of Nitric oxide (NO) as an endothelium-derived relaxing factor stimulated research into the physiology of this most important biological messenger, which maintains a healthy vascular endothelium and an anti-thrombotic intravascular environment. Healthy endothelial cells constantly produce NO to create ‘basal’ vasorelaxation via the classical L-arginine/sGC/cGMP activation cascade. Under physiological conditions this NO pathway is the fundamental to maintenance of normal cardiovascular health, and conversely it is the substrate for development of many cardiovascular disease states, when the balance in this system becomes impaired. Endothelial dysfunction, with the closely associated phenomenon of “NO resistance”, can affect any NO-sensitive tissues including blood vessels and platelets, and is now believed to trigger atherogenesis and thrombogenesis. Treatment of cardiovascular diseases associated with this phenomenon utilizing NO donors often has proved to be ineffective. Furthermore, treatment with organic nitrates is subject to development of nitrate tolerance, limiting efficacy of this class of agents. Several agents can ameliorate NO resistance over days or weeks, but there remains a problem in circumventing NO resistance in cardiac emergencies. In this thesis we demonstrate for the first time in humans partial circumvention of NO resistance with nitroxyl, a structural analogue of NO. Additionally, another NO sibling nitrite (NO₂⁻) has been attracting substantial interest in the last decade. Evidence has been accumulating that effects of nitrite are increased during hypoxia: - nitrite becomes a potent vasodilator and anti-aggregant when compared to normoxic environment. This is especially important in the situation of chronic tissue hypoxia or in acute vascular emergencies. Key findings from the experiments in this thesis are: 1. Nitrite is a potent vasodilator compared to GTN: in general nitrite vasodilator effects are significantly potentiated in hypoxia in human saphenous veins. However, in human internal mammary arteries, nitrite-induced vasodilation is not potentiated under hypoxia. Prolonged exposure of human saphenous vein to nitrite does not cause tolerance or cross-tolerance to GTN. Nitrite effects in saphenous veins are substantially inhibited by ODQ, suggesting that they are largely mediated by soluble guanylate cyclase. Haemoglobin, myoglobin and red blood cells significantly increase hypoxic potentiation of nitrite vasodilator effects in human saphenous veins. Hypoxic potentiation of nitrite is diminished when saphenous vein intrinsic myoglobin is blocked by ferricyanide. 2. In platelets, the anti-aggregatory effects of nitrite are markedly and selectively potentiated under hypoxia. However, nitrite is subject to “NO resistance”. Antiaggregatory actions of nitrite are more potent in venous relative to arterial blood and correlate with (greater) deoxyhaemoglobin levels. Deoxyhaemoglobin is the primary nitrite reductase in blood. We have also presented evidence that continuous generation of NO from endogenous nitrite is important in homeostasis of platelet aggregability. 3. Nitroxyl is a more potent anti-aggregant than SNP. Anti-aggregatory effects of nitroxyl are partially sGC mediated. Nitroxyl partially circumvents the phenomenon of “NO resistance” in platelets. Nitroxyl is also a potent dilator of human saphenous veins. Its effects are not NO-mediated but partially sGCmediated.