Mechanisms of estrogen-induced vasodilation: In vivo studies in canine coronary conductance and resistance arteries

Objectives We sought to examine the immediate vasodilator effect of intracoronary estrogen on epicardial and resistance coronary arteries in 19 dogs. Background Although estrogen reportedly dilates coronary arteries in vitro, the site and mechanisms of its action have not been fully defined in vivo. Methods Epicardial coronary artery dimensions and coronary flow velocity were assessed using simultaneous intracoronary two-dimensional and Doppler ultrasound. Results Estrogen (0.1 and 1 μmol/liter) induced significant increase in coronary cross-sectional area, flow velocity and volumetric blood flow. Estrogen-induced vasodilation was not influenced either by pretreatment with Nω-nitro-l-arginine methyl ester (l-NAME) (100 μmol/liter intracoronary), indomethacin (5 mg/kg body weight intravenously), propranolol (0.75 mg/kg intravenously) or the classic estrogen receptor antagonist ICI 182,780 (10 μmol/liter). Balloon denudation of the endothelium did not attenuate estrogen-induced epicardial vasodilation. Pretreatment with glibenclamide (10 μmol/liter) attenuated estrogeninduced vasodilation only in epicardial arteries, as did verapamil (0.1 μmol/liter). Estrogen had no effect on phenylephrine dose-response curve in either epicardial coronary arteries or the microcirculation. Conclusions Acute estrogen-induced dilation in canine coronary arteries is endothelium independent and is not mediated by the classic intracellular estrogen receptor but through nongenomic mechanisms, presumably at the membrane level, which in epicardial arteries may include effects on adenosine triphosphate-sensitive potassium or calcium channels, or both.