Characterization of porcine coronary muscarinic receptors

To determine the muscarinic receptor subtype involved in the contractile response of coronary smooth muscle, we investigated the profiles of various muscarinic receptor antagonists competing for [3H]N-methyl-scopolamine ([3H]NMS) binding to membrane preparations from porcine coronary arteries. [3H]NMS binds to a single population of muscarinic binding sites with a KD of 135 pM and a Bmax of 57 fmol/mg. The affinity profiles of AF-DX 116 [11-2((-((diethylamino)methyl)-1-piperidinyl)acetyl)-5,11-dihydro-6H- pyrido (2,3-b)(1,4)-benzo-diazepin-6-one], atropine, 4-DAMP [4-diphenylacetoxy-N-methylpiperidine methiodide], methoctramine [N,N'-bis(6-((2-methoxybenzyl)amino)hexyl)-1,8-octane-diamine tetrahydrochloride], HHSiD [hexahydrosiladifenidol] and pirenzepine are consistent with binding to a mixed population of muscarinic binding sites, namely of the M2 and M3 subtype. Binding curves for AF-DX 116 and methoctramine are shallow with Hill-coefficients significantly less than unity. Comparison of data from binding studies with results obtained in functional experiments, i.e. antagonism of methacholine induced contraction of porcine coronary artery rings, it was found that only the low-affinity pKi values of AF-DX 116 (6.26) and methoctramine (6.51) correlated well with functional pA2 values. It is concluded that a mixed population of the M2 and M3 muscarinic receptor subtypes is present in porcine coronary arteries. Functional experiments do not support the contribution of the M2 subtype to the contractile response. Cholinergic induced contractions of porcine coronary arteries appear to be evoked via stimulation of the muscarinic M3 receptor subtype. However, since the compounds investigated here do not markedly discriminate between cloned m3, m4 and m5 receptors the involvement of muscarinic receptors different from M1, M2 and M3 cannot be excluded.