Background: The major source of superoxide (.O2-) in vascular tissues is an NADH/NADPH-dependent, membrane-bound oxidase. We have previously shown that this oxidase is activated in angiotensin II-but not norepinephrine-induced hypertension. We hypothesized that hypertension associated with chronically elevated angiotensin II might be caused in part by vascular .O2- production.
Methods and results: We produced hypertension in rats by a 5-day infusion of angiotensin II or norepinephrine. Rats were also treated with liposome-encapsulated superoxide dismutase (SOD) or empty liposomes. Arterial pressure was measured in conscious rats under baseline conditions and during bolus injections of either acetylcholine or nitroprusside. Vascular .O2- production was assessed by lucigenin chemiluminescence. In vitro vascular relaxations were examined in organ chambers. Norepinephrine infusion increased blood pressure to a similar extent as angiotensin II infusion (179 +/- 5 and 189 +/- 4 mm Hg, respectively). In contrast, angiotensin II-induced hypertension was associated with increased vascular .O2- production, whereas norepinephrine-induced hypertension was not. Treatment with liposome-encapsulated SOD reduced blood pressure by 50 mm Hg in angiotensin II-infused rats while having no effect on blood pressure in control rats or rats with norepinephrine-induced hypertension. Similarly, liposome-encapsulated SOD enhanced in vivo hypotensive responses to acetylcholine and in vitro responses to endothelium-dependent vasodilators in angiotensin II-treated rats.
Conclusions: Hypertension caused by chronically elevated angiotensin II is mediated in part by .O2-, likely via degradation of endothelium-derived NO. Increased vascular .O2- may contribute to vascular disease in high renin/angiotensin II states.