Structural requirements for muscarinic receptor occupation and receptor activation by oxotremorine analogs in the guinea-pig ileum.

The muscarinic activities in the isolated guinea-pig ileum of nine analogs of oxotremorine, modified only in the amino group, were resolved into affinity and efficacy components. The method used involved analysis of dose-response data before and after fractional inactivation of receptors with propylbenzilylcholine mustard. The dissociation constants (KA) thus obtained for oxotremorine (6.79 X 10(-7) M), oxotremorine methiodide (6.74 X 10(-6) M) and oxotremorine-M (2.92 X 10(-6) M) agreed well with their reported low-affinity dissociation constants (KL) determined in receptor binding studies. There was no correlation between relative affinities and efficacies of the nine agonists studied, suggesting different structural requirements for occupation and activation of muscarinic receptors in the guinea-pig ileum. Although oxotremorine had higher affinity than its analogs, some of the latter had substantially greater efficacy than oxotremorine. Thus, replacement of the pyrrolidine ring of oxotremorine by azetidino, dimethylamino or trimethylammonium groups was accompanied by a 4- to 7-fold increase in efficacy. A diethylamino group in place of pyrrolidine gave an 18-fold decrease in efficacy and a triethylammonium group abolished efficacy. The relative efficacies of the nine agonists were inversely correlated with the size of the amino or ammonium group. No significant correlation was observed between relative affinities for the receptor and size of the cationic head.