Angiotensin-(1-7): a novel vasodilator of the coronary circulation

Angiotensin-(1-7) [Ang-(1-7)] possesses novel biological functions that are distinct from angiotensin II (Ang II). In coronary arteries, the octapeptide Ang II and the heptapeptide Ang-(1-7) exert opposing actions. Ang II elicits vasoconstriction and Ang-(1-7) is a vasodilator. Ang-(1-7) elicits vasodilation by an endothelium-dependent release of nitric oxide. Further, the vasorelaxant activity is markedly attenuated by the bradykinin (BK) B2 receptor antagonist icatibant and does not appear to be associated with the synthesis and release of prostaglandins. Ang-(1-7) vasodilation is mediated by a non-AT1/AT2 receptor, since [Sar1Thr8]-Ang II, but neither candesartan, an AT1 receptor antagonist, nor PD123319, an AT2 receptor antagonist, blocked the response. Specific and high affinity binding of 125I-Ang-(1-7) to the endothelial layer of canine coronary arteries was demonstrated using in vitro emulsion autoradiography. Binding was effectively competed for by either unlabeled Ang-(1-7) or the specific Ang-(1-7) antagonist [D-Ala7]-Ang-(1-7). Additionally, Ang-(1-7) potentiated synergistically BK-induced vasodilation. The EC50 of BK vasodilation (2.45 +/- 0.51 nmol/L vs 0.37 +/- 0.08 nmol/L) was shifted 6.6-fold left-ward in the presence of 2 mumol/L concentration of Ang-(1-7). The potentiated response was specific for BK, since Ang-(1-7) did not augment the vasodilation produced by either acetylcholine or sodium nitroprusside; further, it was specific for Ang-(1-7), since neither Ang I nor Ang II augmented the BK response. In contrast to the vasodilator actions of Ang-(1-7), the potentiated response was not blocked by candesartan, PD123319 or [Sar1Thr8]-Ang II. Novel studies from our group demonstrate that Ang-(1-7) is both a substrate and inhibitor for angiotensin converting enzyme (ACE). Ang-(1-7) was shown to retard the degradation of 125I-[Tyr0]-BK in coronary rings. These studies describe novel actions of Ang-(1-7) as a vasodilator and a local synergistic modulator of kinin-induced vasodilation in coronary arteries.