ABSTRACT
Although circulating substances obviously have an easy access to the endothelium, relatively few hormones evoke endothelium-dependent responses. This probably reflects that endothelium-dependent modulation of vascular tone is primarily a local regulatory mechanism. There are no indications that activation of α1-adrenergic receptors can lead to the release of endothelium-derived relaxing factor(s) (EDRFs), although a-adrenergic activation evokes the release of prostacyclin from the endothelium. n the canine and porcine coronary artery, in canine pulmonary and femoral arteries and veins, and in the aorta and tail artery of the rat norepinephrine and the selective α2-adrenergic agonist UK 14307 initiate endothelium-dependent relaxations. In the canine coronary artery and in the cerebral artery of the cat, no evidence could be obtained that acetylcholine released from cholinergic nerves activates endothelial receptors. Thus, although neurogenically released acetylcholine might evoke the release of EDRF at the microcirculatory level, it seems unlikely that this is the case in large conduit arteries.
