Characterization of vascular muscarinic receptors: rabbit ear artery and bovine coronary artery.

The goal of this study was to use a functional bioassay to compare muscarinic receptors in two vascular preparations: the rabbit ear artery, which manifests an endothelial-dependent relaxation to cholinergic stimulation, and the bovine coronary artery, in which contraction is elicited independently of the endothelium. Four antagonists were selected, 4-diphenylacetoxy-N-methylpiperidine methobromide, dicyclomine, methoctramine and hexahydrosiladifenidol, to complement previous studies using pirenzepine and 11,2- ([2-(diethylaminomethyl)-1-piperidinyl]acetyl)-5,11-dihydro-6H- pyrido(2,3-b)(1,4)benzodiazepine-6-one, in order to distinguish M1, M2 and M3 receptors. All the antagonists produced competitive inhibition of responses to methacholine, with Schild plot slopes not different from one, with one exception: for 4-diphenylacetoxy-N-methylpiperidine methobromide, Schild plot slopes were greater than one in both tissues. Comparison of antagonist affinities suggests that the muscarinic receptor subtype is not different between the two tissues, despite the different cell types that mediate these responses. Furthermore, the pattern of antagonist affinity agrees most closely with that found previously for the M3 receptor, with high affinity for 4-diphenylacetoxy-N-methylpiperidine methobromide, hexahydrosiladifenidol and dicyclomine, intermediate affinity for pirenzepine, and low affinity for 11,2- ([2-(diethylaminomethyl)-1-piperidinyl]acetyl)-5,11-dihydro-6H- pyrido(2,3-b)(1,4)benzodiazepine-6-one and methoctramine. These findings demonstrate that currently available muscarinic antagonists do not discriminate between receptors found on endothelial and vascular smooth muscle cells, even though the ultimate responses produced are opposed.