α- and β-Adrenergic Control of Large Coronary Arteries in Conscious Calves

Summary

Large and small coronary arteries are subject to control by α- and β-adrenergic mechanisms. However, controversy exists as to the distribution and physiological effects of α- and β-adrenergic receptor subtypes in large coronary arteries. Studies in our laboratory have addressed these questions in conscious calves, chronically instrumented to measure large coronary artery diameter and coronary blood flow. Additionally, adrenergic receptor subtype distribution was determined using ligand binding assays in membrane preparations isolated from large coronary arteries of calves. Physiological results demonstrate, in contrast to the results of most previous studies, that both α₁- and α₂-adrenergic receptors elicit constriction of the large coronary artery. Studies with ganglionic blockade indicate that the constriction was unaltered by autonomic reflexes or presynaptic release of neurotransmitters. Selective β-adrenergic receptor activation demonstrated that both β₁- and β₂-adrenergic receptors elicit dilation of large coronary arteries, and that the vasodilation was direct, i.e., it was not mediated by increases in coronary blood flow. Biochemical characterization of adrenergic subtype density indicated the presence of both α₁ - and α₂-, as well as β₁ - and β₂-adrenergic receptor subtypes. Thus, both biochemical and physiological data support the concept that large coronary arteries are regulated by both α₁ - and α₂-, as well as β₁ - and β₂-adrenergic receptor subtypes.