Dietary l-arginine and α-tocopherol reduce vascular oxidative stress and preserve endothelial function in hypercholesterolemic rabbits via different mechanisms

Abstract

Vascular oxidative stress brought about by superoxide radicals and oxidized low-density lipoproteins (oxLDL) is a major factor contributing to decreased NO-dependent vasodilator function in hypercholesterolemia and atherosclerosis. We investigated whether chronic administration of l-arginine (2% in drinking water) or of α-tocopherol (300 mg/day) improves endothelium-dependent vasodilator function and systemic NO production, reduces vascular oxidative stress, and reduces the progression of atherosclerosis in cholesterol-fed rabbits with pre-existing hypercholesterolemia. Systemic NO production was assessed as urinary nitrate excretion; oxidative stress was measured by urinary 8-iso-PGF 2 α excretion in vivo, by lucigenin-enhanced chemiluminescence of isolated aortic rings ex vivo, and by copper-mediated LDL oxidation in vitro. Endothelium-dependent relaxation was almost completely abrogated in cholesterol-fed rabbits. Urinary nitrate excretion was reduced by 46±10%, and 8-iso-PGF 2 α excretion was increased by 61±18% as compared to controls (each P<0.05). Vascular superoxide radical release stimulated by PMA ex vivo was increased by 273±93% in this group, and the lag time of LDL oxidation was reduced by 35±6% (each P<0.05). Treatment with l-arginine and α-tocopherol reduced intimal lesion formation (by 68±6 and 41±11%, respectively; P<0.05) and improved endothelium-dependent relaxation. Both treatments also normalized urinary 8-iso-PGF 2 α excretion. l-Arginine increased urinary nitrate excretion by 43±13% ( P<0.05) and reduced superoxide radical release by isolated aortic rings to control levels, which was unaffected by vitamin E treatment. By contrast, vitamin E dramatically increased the resistance of isolated LDL to copper-mediated oxidation in vitro by 178±7% ( P<0.05), which was only marginally prolonged by l-arginine. Intimal thickening was reduced by both treatments. We conclude that both l-arginine and α-tocopherol reduce the progression of atherosclerotic plaques in cholesterol-fed rabbits. However, while l-arginine increases NO formation and reduces superoxide release, α-tocopherol antagonizes mainly oxLDL-related events in atherogenesis. Thus, both treatments reduce urinary isoprostane excretion and improve endothelium-dependent vasodilation via different mechanisms.