Losartan Reduces Phenylephrine Constrictor Response in Aortic Rings From Spontaneously Hypertensive Rats

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Abstract
Nitric oxide seems to be involved in the mechanisms underlying the antihypertensive and renal responses of losartan in spontaneously hypertensive rats (SHR). We investigated the contribution of nitric oxide to the effect of this angiotensin II (Ang II) type 1 (AT 1 ) receptor antagonist on the constrictor response of phenylephrine in aortic rings from SHR. Furthermore, since it has been suggested that Ang II could bind to unblocked AT 2 receptors, during administration of an AT 1 receptor antagonist, we also studied the effect of the AT 2 receptor antagonist PD 123319 on the contractile response to phenylephrine in aortic rings from SHR. To this end, we studied dose-response curves of phenylephrine (10 −9 to 10 −5 mol/L) in the presence and absence of losartan (10 −9 , 10 −7 , and 10 −5 mol/L) in SHR aortic rings. Preincubation with losartan reduced the constrictor response to phenylephrine but not to KCl (10 to 120 mmol/L) in a dose-dependent manner. On the other hand, the presence of captopril (10 −5 mol/L) in the incubation medium did not alter the response to phenylephrine, even at the dose of 10 −3 mol/L. The reduced response to phenylephrine in the presence of losartan was abolished in both endothelium-denuded rings and rings treated with a nitric oxide synthesis inhibitor. A similar situation was observed in PD 123319–pretreated rings, in which the effect of losartan on the contractile response to phenylephrine was reversed. Losartan was not able to stimulate the production of aortic cGMP compared with the control group. Likewise, losartan did not modify the relaxing responses to either acetylcholine or sodium nitroprusside in phenylephrine-preconstricted aortic rings. Furthermore, losartan did not alter isometric tension in aortic rings in either basal or phenylephrine-preconstricted conditions. These data demonstrate that Ang II potentiates the vasoconstriction induced by phenylephrine through the stimulation of AT 1 receptors. Moreover, AT 2 receptors and nitric oxide appear to be involved in this effect.