Abstract
Previous in vivo studies have shown that beta-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 beta-adrenoceptors and their vasomotor responses exists in the coronary microcirculation across the left ventricular wall. Porcine subepicardial and subendocardial arterioles (<100 microm) were isolated from the left ventricle and pressurized for in vitro study of vasodilation to the nonselective beta-adrenoceptor agonist isoproterenol and the selective beta2-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 (K(ATP)) channel blocker. In contrast to isoproterenol, dilations of subepicardial and subendocardial arterioles to pinacidil, a K(ATP) channel opener, were similar. In both vessel types, isoproterenol-induced dilation was inhibited by the beta2-adrenoceptor blocker ICI-118,551...Continue Reading
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