Effect of epinephrine and norepinephrine on coronary vascular resistance in dogs

The effects of intracoronary injection and infusion of epinephrine and norepinephrine have been studied in the beating and fibrillating dog heart. The heart performed no external work and coronary perfusion pressure was measured with the rate of blood flow through the coronary vascular bed held constant. Injection of both agents most often produced a fleeting rise in pressure followed by a prolonged fall in pressure. In the beating heart, the pressure rise preceded rise in heart rate. Slow infusion of norepinephrine often produced a small pressure rise whereas rapid infusion always produced a large pressure fall. Epinephrine sometimes produced similar changes but more often the pressure fell progressively as a function of the rate of infusion. The pressure rise frequently was unassociated with a change in heart rate or the proportion of a minute spent in systole. Both parameters, as well as oxygen consumption, increased with high rates of infusion. These findings suggest that the direct effect of these agents on coronary vessels is active constriction, whereas the indirect effect is active dilatation. In low concentrations the direct effect may predominate, resulting in a resistance increase, whereas in high concentrations the indirect effect always predominates, resulting in a decrease of resistance.