FEEDFORWARD CONTROL OF CORONARY BLOOD-FLOW VIA CORONARY BETA-RECEPTOR STIMULATION

It is usually assumed that the increase in coronary blood flow observed with norepinephrine occurs through local metabolic vasodilation secondary to cardiac beta-receptor activation. However, direct feedforward beta-receptor-mediated coronary vasodilation is also a possibility. In dogs with alpha-receptor blockade, the left circumflex artery was perfused at constant pressure. The vasodilator effect of intracoronary norepinephrine injections was determined during prolonged diastoles to avoid the chronotropic and inotropic effects of norepinephrine. Norepinephrine caused a dose-dependent increase in coronary blood flow that was attenuated by both the selective beta1-antagonist practolol and the selective beta2-antagonist ICI 118 551. These data indicate that norepinephrine activates beta1- and beta2-receptors in coronary resistance vessels to cause vasodilation independent of inotropic and chronotropic effects. The physiological significance of coronary beta-receptor-mediated vasodilation was investigated in the beating heart. The coronary blood flow response and coronary venous oxygen tension response were compared when myocardial oxygen consumption was increased over the same range by one of three positive inotropic interventions: (1) norepinephrine, (2) paired-pulse stimulation, or (3) norepinephrine after alpha-blockade. During norepinephrine infusion (intervention 1), coronary venous oxygen tension decreased, indicating that the match between myocardial oxygen consumption and oxygen delivery is not maintained when coronary blood flow is controlled by alpha- and beta-receptors in addition to local metabolic factors. Paired-pulse stimulation (intervention 2) also resulted in a decrease in coronary venous oxygen tension, demonstrating that the balance between oxygen consumption and delivery is not maintained when blood flow is controlled by local metabolic factors alone. However, when coronary beta-receptor-mediated vasodilation was unmasked by alpha-blockade, norepinephrine infusion (intervention 3) produced no change in coronary venous oxygen tension. Therefore, coronary beta-receptor vasodilation helps maintain the balance between flow and metabolism in a feedforward manner in the beating heart.