Prejunctional Muscarinic Receptors in the Deep Muscular Plexus of Canine Ileum: Comparison with Smooth Muscle Receptors

Prejunctional muscarinic receptors from the deep muscular plexus of canine ileum were studied, and their properties were compared with those of the postjunctional receptors of the circular smooth muscle. In the purified synaptosomal fraction a fraction containing primarily the axonal varicosities of deep muscular plexus, the muscarinic ligand N-3Hmethylscopolamine labeled an apparently homogenous population of receptors nH 1 with a Kd of 2.7 nM and a Bmax of 195 - 44 fmolmg protein mean - S. D., n 4. These receptors showed a high affinity for the M3Mi-selective antagonist 4- diphenylacetoxy-N-methylpiperidine methiodide pKi 7.41 in contrast, the pKi values of pirenzepine 5.60, methoctramine 5.65 and AF-DX 116 5.21 implied little selectivity for these subtypes. The binding properties of muscarinic receptors in the synaptosomal fraction were different from the binding properties of muscarinic receptors in the purified circular smooth muscle plasma membranes. Most notably, the circular smooth muscle receptors had significantly lower affinity for N-3Hmethylscopolamine Kd 16 nM with a Bmax value of 2088 - 276 fmolmg. The affinities of the M2 subtype-selective muscarinic antagonists methoctramine and AF-DX 116 were similar in both membrane preparations. The receptor population associated with the deep muscular plexus synaptosomal fraction was linked to the inhibition of adenylate cyclase activity, as demonstrated by a concentration-dependent, atropine-sensitive inhibition of the forskolin-stimulated enzyme in the presence of muscarinic agonists carbachol and oxotremorine.