Characterization of muscarinic cholinergic receptors on the endothelium and the smooth muscle of the rabbit thoracic aorta.

Pharmacodynamic and quantitative autoradiographic studies were performed to characterize the muscarinic receptors on the endothelium and smooth muscle of the rabbit thoracic aorta. The antagonistic effect of atropine and pirenzepine on the relaxation induced by acetylcholine showed that the relaxation was mediated by muscarinic receptors with pA2 values of 9.4 and 6.6, respectively, suggesting the presence of muscarinic receptors on the endothelium with low affinity for pirenzepine. The quantitative autoradiographic study using [3H]-propylbenzilylcholine mustard (3H-PrBCM) showed that the specific 3H-PrBCM binding was time dependent and saturable. Saturation times of the bindings were not significantly different between the receptors on the endothelium and those on the smooth muscle layer. The density of the receptors was higher in the smooth muscle layer than in the endothelium. The specific [3H]-quinuclidinyl benzilate (3H-QNB) binding was displaced by atropine or pirenzepine dose dependently. IC50 of pirenzepine for the receptors on the endothelium was not significantly different from that for the receptors on the smooth muscle layer. These findings suggest that there is a smaller density of muscarinic receptors on the endothelium than on the smooth muscle layer of the rabbit thoracic aorta. The muscarinic receptors on the endothelium can be subclassified into the same subtype as the receptors on the smooth muscle layer, which have low affinity for pirenzepine.