Direct binding studies on ileal and cardiac muscarinic receptors.

1 Functional studies have indicated that muscarinic receptors in cardiac tissue differ from those in the ileum. In the present study ileal and cardiac muscarinic receptors identified by [3H]-N-methyl scopolamine ([3H]-NMS) were characterized and the selectivity of currently available ileal and atrial selective antagonists determined. 2 In terms of the current functional classification of muscarinic receptors both ileal and cardiac muscarinic receptors were of the M2 subtype based upon their low affinity for pirenzepine. 3 Cyclohexylphenyl(2-piperidinoethyl)silanol (CPPS), a highly ileal selective antagonist in functional studies, was unable to distinguish between ileal and atrial muscarinic receptors identified in binding studies. Furthermore, although AF-DX 116 and dicyclomine were able to differentiate atrial and ileal muscarinic receptors, neither compound was more than 2 fold selective. These data indicate that it is not possible to subclassify ileal and atrial muscarinic receptors using direct ligand binding studies with these antagonists. 4 In circular ileal smooth muscle, apparent heterogeneity of the M2 muscarinic receptor population was observed. Thus AF-DX 116 identified two populations of sites with affinities differing by 30 fold. These two populations of M2 muscarinic receptors may represent the typical M2 muscarinic receptors identified in cardiac tissue and the more recently discovered 'gland type' M2 muscarinic receptors. 5 The circular ileal smooth muscle tissue homogenate was able to decrease dramatically the apparent affinity of adiphenine. This activity, which appeared to result from a phenylmethylsulphonylfluoride (PMSF) sensitive protease effect, should be considered when conducting studies using this tissue preparation and compounds of similar structure to adiphenine.