This study examined vascular function and the role of superoxide in mice that chronically express human renin (R+) and human angiotensinogen (A+). Responses of aortas from R+/A+ mice and from their normotensive littermates (RA- mice) were examined in vitro. Endothelium-dependent relaxation to acetylcholine was impaired in vessels from R+/A+ mice (e.g., maximal relaxation to 100 microM acetylcholine was 45 +/- 5% and 65 +/- 3% in R+/A+ and RA- mice, respectively; P < 0.05). Relaxation was also impaired to the endothelium-independent dilators authentic nitric oxide and nitroprusside in vessels from R+/A+ mice. Maximal vasorelaxation to the endothelium-independent, non-nitric oxide dilator papaverine was similar in R+/A+ and RA- mice. Incubation of vessels from R+/A+ mice with Tiron (1 mM), a superoxide scavenger, improved relaxation to acetylcholine, nitric oxide, and nitroprusside. In contrast, incubation with diethyldithiocarbamate (1 mM), an inhibitor of copper-containing SODs, reduced acetylcholine- and nitroprusside-induced relaxation in vessels from both R+/A+ and RA- mice. Basal superoxide levels, measured with lucigenin-enhanced chemiluminescence (5 microM lucigenin) and hydroethidine-based fluorescent confocal microscopy, were higher in vessels from R+/A+ mice and were Tiron and polyethylene glycol-SOD sensitive. These results suggest that increased superoxide contributes to impaired nitric oxide-mediated relaxation in this genetic model of chronic angiotensin II-dependent hypertension.