Direct binding and functional studies on muscarinic cholinoceptors in porcine coronary artery.

The muscarinic cholinoceptors in porcine coronary artery were identified and characterized by a binding assay using (-)-[3H]quinuclidinyl benzilate (QNB) and also by pharmacological method. Specific (-)-[3H]QNB binding in the coronary artery was saturable and of high affinity (Kd = 0.08 nM), and it showed a pharmacological specificity as well as stereoselectivity which characterized muscarinic receptors. Muscarinic antagonists competed with the (-)-[3H]QNB binding in order: nonlabeled QNB greater than dexetimide greater than atropine greater than pirenzepine greater than AF-DX 116 greater than levetimide greater than gallamine. Dexetimide was approximately 2000 times as potent as levetimide. The potencies (pKi) of these muscarinic antagonists in competing for (-)-[3H]QNB binding sites in porcine coronary artery correlated well with their pharmacological potencies (pA2 for antagonistic effect of acetylcholine-induced contraction of coronary artery). The decrease in the (-)-[3H]QNB binding by atropine and pirenzepine was due to a reduction in the apparent affinity with little change in the number of maximal binding sites, suggesting a competitive antagonism. Specific (-)-[3H]QNB binding (Kd and maximal number of binding sites) in porcine coronary artery was not changed by the removal of endothelium. We conclude: 1) (-)-[3H]QNB selectively labels the physiologically relevant muscarinic receptors in porcine coronary artery and 2) the majority of these receptors is localized on vascular smooth muscles and the receptors mediate the acetylcholine-induced contractile response of coronary artery.