Different P2-purinergic receptor subtypes of endothelium and smooth muscle in canine blood vessels

Abstract

P2-Purinergic agonists can induce direct contractions in saphenous veins and endothelium-dependent relaxations in coronary arteries of the dog. For contractions, the rank order of agonist potency was alpha, beta-methylene-ATP much greater than ATP greater than ADP. For endothelium-dependent relaxation, the order was 2-methylthio-ATP greater than ADP greater than ATP much greater than alpha, beta-methylene-ATP (the latter causing no relaxation), suggesting two different subtypes of the P2-purinoceptors in these tissues. alpha, beta-Methylene-ATP could be used like an antagonist in the saphenous vein because it desensitizes receptors; exposure to it prevented contractions to ADP or ATP, but did not inhibit relaxations to them in the coronary artery. Reactive blue 2, from 2 X 10(-6) to 2 X 10(-5) M, behaved as a competitive antagonist of relaxations of the coronary artery induced by ADP and 2-methylthio-ATP; it did not inhibit contraction of the saphenous vein by alpha, beta-methylene-ATP. It antagonized slightly the endothelium-dependent relaxation of the coronary artery to acetylcholine, but not that to the calcium ionophore A23187. In contrast, methylene blue, a functional antagonist of endothelium-dependent relaxations, caused noncompetitive antagonism of relaxation to the same agonists. Thus, different subtypes of P2-purinoceptors on canine vascular smooth muscle and endothelium can be distinguished on the basis not only of the potencies of a series of agonists, but by two antagonists which discriminate between them.