The binding of [3H]4-diphenylacetoxy-N-methylpiperidine methiodide to longitudinal ileal smooth muscle muscarinic receptors

Muscarinic receptors present in longitudinal ileum were characterized using the non-selective radioligand [3H]N-methylscopolamine [( 3H]NMS) and the M3 selective radioligand [3H]4-diphenylacetoxy-N-methylpiperidine methiodide [( 3H]4DAMP). In saturation studies, [3H]4DAMP, but not [3H]NMS, identified two populations of binding sites with 17% of the sites (155 fmol/mg protein) displaying high affinity (Kd = 0.39 nM) for [3H]4DAMP and the remaining sites displaying low affinity for the radioligand (Kd = 4.43 nM). In competition studies gallamine and methoctramine, but not AF-DX 116, identified two populations of [3H]NMS binding sites. Affinity estimates for gallamine and methoctramine indicated that 80% of the [3H]NMS binding sites were of the M2 subtype. The minor population of [3H]NMS binding sites could not be readily characterized, due partly to the low selectivity of the competing ligands and also to the relatively low density of the sites. In studies using the M3 muscarinic receptor selective radioligand [3H]4DAMP, the minor population of sites could be preferentially labeled by using a low concentration (0.4 nM) of [3H]4DAMP. Under these conditions, [3H]4DAMP labeled approximately equal levels of the two muscarinic receptor binding sites present in the ileum. Competition studies with AF-DX 116, gallamine and methoctramine indicated that the two [3H]4DAMP binding sites displayed the pharmacology expected of the M2 and M3 receptors, respectively. These results provide additional evidence that longitudinal ileal smooth muscle membranes contain both M2 and M3 muscarinic receptors and indicate that [3H]4DAMP is a useful ligand for identifying heterogeneity of muscarinic receptor subtypes.