Angiotensin II (Ang II) has been implicated as a major cause of oxidative stress and vascular dysfunction in the peripheral circulation. Although chronic hypertension is a major risk factor for cerebral vascular disease, stroke and vascular cognitive impairment, very little is known regarding mechanisms of action of Ang II or effects of chronic Ang II-dependent hypertension in the cerebral circulation. In initial studies, we tested the hypothesis that Ang II produces constriction of cerebral arteries that is mediated by activation of the AT1a receptors and Rho kinase. Basilar arteries (baseline diameter ∼130 μm) from control (C57BL/6) mice were isolated, cannulated and pressurized in order to measure vessel diameter. Ang II constricted arteries from male mice beginning at 0.01 nM with a maximum decrease in diameter of 25±2% (mean±SE) at 1 nM Ang II. In contrast, Ang II had very modest effects in arteries from female mice (−3±1% at 1 nM Ang II). In male mice, vasoconstriction in response to Ang II was completely prevented by an inhibitor of Rho kinase (Y27632, 3 μM). In contrast, constriction of the basilar artery to KCl was similar in males and females and was not inhibited by Y27632. Vasoconstrictor responses to Ang II were almost eliminated in male mice deficient in expression of AT1a receptors (AT1a −/−). Recent pharmacological studies suggest that intact Ang II-mediated signaling is important in maintaining endothelial function in cerebral arteries. We found that responses of the basilar artery to acetylcholine (ACh, an endothelium-dependent agonist) were similar in AT1a +/+ and AT1a −/− mice. In a second series of studies, we examined effects of Ang II using a chronic model of hypertension, mice which overexpress human renin (R+) and human angiotensinogen (A+). ACh produced dilation of cerebral arteries in control mice that was eliminated by an inhibitor of nitric oxide synthase. Responses to ACh were markedly impaired in R+A+ mice: 1 μM ACh dilated the arteries by 41±6 vs 9±5%, (P<0.01) in control and R+A+ mice, respectively. A23187 (0.1 μM, another endothelium-dependent agonist), produced vasodilation in control mice (35±7%) but vasocontriction in R+A+ mice (−8±5%). Impaired responses to ACh in R+A+ mice were restored to normal by the superoxide scavenger PEG-superoxide dismutase. In contrast, dilation of the basilar artery in response to a NO donor (NONOate) was similar in R+A+ mice and controls (P>0.05). Thus, Ang II is a potent constrictor of cerebral arteries and this effect is mediated by AT1a receptors and activation of Rho kinase. There are marked, but selective, gender differences in cerebral vascular responses to Ang II. Our findings provide direct evidence that endothelial function is not affected by deletion of AT1a receptors. Finally, endothelial function is greatly impaired in mice made chronically hypertensive by expression of renin and angiotensinogen. The mechanism of this impairment involves increased oxidative stress.
