Heterogeneous β2-Adrenoceptor Expression and Dilation in Coronary Arterioles Across the Left Ventricular Wall

Background— Previous in vivo studies have shown that β-adrenoceptor agonists cause a redistribution of coronary flow away from the subendocardium; however, the underlying mechanism remains uncertain. We tested the hypothesis that a heterogeneous distribution of β-adrenoceptors and their vasomotor responses exists in the coronary microcirculation across the left ventricular wall. Methods and Results— Porcine subepicardial and subendocardial arterioles (<100 μm) were isolated from the left ventricle and pressurized for in vitro study of vasodilation to the nonselective β-adrenoceptor agonist isoproterenol and the selective β2-adrenoceptor agonist procaterol. Both vessel types developed a similar level of basal tone and dilated to isoproterenol and procaterol. However, subepicardial arterioles exhibited a much higher sensitivity and greater dilation capacity to both agonists. The isoproterenol-induced vasodilations were inhibited by glibenclamide, an ATP-sensitive potassium (KATP) channel blocker. In contras...