Angiotensin II type 1 receptor blocker irbesartan ameliorates vascular function in spontaneously hypertensive rats regardless of oestrogen status

BACKGROUND: Angiotensin II type 1 (AT1) receptor overexpression may play a decisive role in endothelial dysfunction during oestrogen deficiency in spontaneously hypertensive rats (SHRs). Similarly, exaggerated production of angiotensin II and enhanced expression of AT1 receptor have been reported in vessels of SHRs compared with normotensive rats. OBJECTIVE: To test the hypothesis that antihypertensive treatment with the AT1 receptor antagonist, irbesartan, could not only decrease blood pressure but also ameliorate endothelial dysfunction associated with both hypertension and oestrogen deficiency. METHODS: Ovariectomized and sham-ovariectomized SHRs were treated with 50 mg/kg irbesartan per day, administered with chow for 30 weeks. Sham-ovariectomized and ovariectomized rats receiving no treatment were used as control groups. At the end of the treatment period, the vascular reactivity of aortic rings was studied. RESULTS: In the irbesartan-treated groups, vasoconstriction induced by Nomega-nitro-l-arginine methyl ester (l-NAME) was increased and the response to phenylephrine exhibited greater potentiation in the presence of l-NAME, demonstrating a greater availability of basal nitric oxide in these groups. In addition, chronic treatment with irbesartan similarly enhanced the responsiveness of aortic rings from ovariectomized or sham-ovariectomized rats to acetylcholine and sodium nitroprusside. Incubation with indomethacin did not significantly alter acetylcholine- and sodium nitroprusside-induced relaxations in the irbesartan-treated rats. However, relaxations induced by acetylcholine and sodium nitroprusside in aortic rings from non-treated rats were significantly greater in the presence of indomethacin. CONCLUSION: Our data suggest that irbesartan enhances basal nitric oxide availability and ameliorates vascular relaxations in SHRs, by decreasing the production of cyclooxygenase-dependent contracting factors in smooth muscle cells, regardless of oestrogen status.