Circulating plasma xanthine oxidase contributes to vascular dysfunction in hypercholesterolemic rabbits.

Abstract

Reactive oxygen species play a central role in vascular inflammation and atherogenesis, with enhanced superoxide (02-) production contributing significantly to impairment of nitric oxide (NO)-dependent relaxation of vessels from cholesterol-fed rabbits. We investigated potential sources of 02-production, which contribute to this loss of endothelium-dependent vascular responses. The vasorelaxation elicited by acetylcholine (ACh) in phenylephrinecontracted, aortic ring segments was impaired by cholesterol feeding. Pretreatment of aortic vessels with either heparin, which competes with xanthine oxidase (XO) for binding to sulfated glycosaminoglycans, or the XO inhibitor allopurinol resulted in a partial restoration (36-40o at 1 ,uM ACh) of ACh-dependent relaxation. Furthermore, 027-dependent lucigenin chemiluminescence, measured in intact ring segments from hypercholesterolemic rabbits, was decreased by addition of heparin, allopurinol or a chimeric, heparin-binding superoxide dismutase. XO activity was elevated more than two-fold in plasma of hypercholesterolemic rabbits. Incubation of vascular rings from rabbits on a normal diet with purified XO (10 milliunits/ml) also impaired 'NO-dependent relaxation but only in the presence of purine substrate. As with vessels from hypercholesterolemic rabbits, this effect was prevented by heparin and allopurinol treatment. We hypothesize that increases in plasma cholesterol induce the release of XO into the circulation, where it binds to endothelial cell glycosaminoglycans. Only in hypercholesterolemic vessels is sufficient substrate available to sustain the production of 02-and impair NO-dependent vasorelaxation. Chronically, the continued production of peroxynitrite, (ONOO-) which the simultaneous generation of NO and 02 -implies, may irreversibly impair vessel function.In recent years, numerous studies have shown that vascular function is compromised in a number of pathological conditions including atherosclerosis and hypertension (1-3). In atherosclerosis, blood vessels undergo marked changes in both structure and function that may predispose for angina and myocardial infarction. Defects in lipoprotein metabolism and vascular reactivity are fundamental pathological responses to hypercholesterolemia, with reactive oxygen species playing an important role in the initiation and progression of these lesions (4, 5). Isolated blood vessels from atherosclerotic patients (6, 7) and hypercholesterolemic animals (8-11) exhibit impaired, endothelium-dependent vascular relaxation. In these models, superoxide (02-) is generated at greater rates in both intracellular and extracellular compartments, ren.cting with 'NO to inhibit endothelium-dependent relaxation ard yielding peroxynitrite (ONOO-) (12). Since ONOO-is capable of oxidizing lipids and proteins, it may further contribute to foam cell formation in the atherosclerotic lesion (13-15).While oxidative injury plays a pivotal role in these processes, the cellular mechanisms of 0i-production and sites of reaction remain to be...

Oxygen radical inhibition of nitric oxide-dependent vascular function in sickle cell disease

“…The immunohistochemical localization of increased XO in the vessel wall of SCD mice could be a manifestation of either the binding and uptake of circulating XO by vascular endothelial cells (33,50,51) or the increased expression of XOR by activated vascular endothelium (52). Considering the high association constant of XO for binding to vascular endothelium (K d ϭ 6 nM; ref.…”

“…A precedent for these phenomena exists in animal models and clinical studies of atherosclerosis. In a hypercholesterolemic rabbit model of atherosclerosis, a 2.5-fold increase in plasma XO severely impairs ACh-mediated relaxation of the thoracic aorta (33,59). Heparin-induced dissociation of XO from vessel wall binding sites and allopurinol-mediated XO inhibition partially restores ACh-dependent relaxation and decreases vessel O 2 ⅐Ϫ production (33).…”

“…Isometric tension was measured in aortic segments from mice as described (33). Aortic segments were exposed in some cases to various mediators and inhibitors for 1 h, submaximally contracted with phenylephrine (10 Ϫ8 -10 Ϫ7 M), and acetylcholine (ACh; 1 ϫ 10 Ϫ9 to 3 ϫ 10 Ϫ6 M) was added in a cumulative manner to induce relaxation.…”

Superoxide as a Messenger of Endothelial Function

“…This enzyme can be converted to its oxidase form either by thiol oxidation or proteolytic attack (Harrison, 1997b). Xanthine oxidase may also be attached to the endothelial membrane after release from liver and transport in the blood stream (White et al, 1996;Radi et al, 1997 (Zou et al, 2002a;Xia et al, 1998). Thus there are four mechanisms known to date which could provide superoxide anions in a defined way and in quantities sufficient for PN formation when NO production has been increased in the presence of calcium or the MAP38 kinase phosphorylation cascade.…”

Protective effect of arginine on oxidative stress in transgenic sickle mouse models

“…However, there is no definitive data on the status of antioxidant enzyme and lipid peroxidation according to the disease severity, as well as on the effect of NO replenishment. Among the likely sources of O 2 •− in SCD are plasma membrane NADPH oxidase [12,44], enzyme NO synthase due to depleted substrate arginine [40] and increased plasma xanthine oxidase (XO) activity [3,41].…”