An autoradiographic study of muscarinic cholinoceptors in blood vessels: no localization on vascular endothelium

In vitro labelling and autoradiographic techniques were used to examine the localization of [³H]quinuclidinyl benzilate ([³H]QNB) and [¹²⁵I]4-iodo-QNB ([¹²⁵I]4IQNB) to slide-mounted sections of rabbit aorta and pulmonary artery, cat aorta, pulmonary and superior mesenteric arteries. These vessels all respond to acetylcholine (ACh) with endothelium-dependent relaxation, yet there was no evidence for endothelium-related binding of either [³H]QNB or [¹²⁵I]4IQNB. Muscarinic receptors were localized over the medial smooth muscle and, in the rabbit pulmonary artery, the density of binding increased towards the adventitia. Binding of either radioligand to sections of rabbit pulmonary artery was not affected by the muscarinic M₁ receptor antagonist pirenzepine (20 nM) but was markedly reduced by the muscarinic M₂ antagonist 4DAMP (4-diphenylacetoxy-N-methyl-piperidine methobromide) (1 nM). This study provides evidence for muscarinic receptors located directly on smooth muscle cells, indicating that endothelium-dependent relaxation to ACh results from an indirect mechanism involving smooth muscle muscarinic receptors.