Abstract

The interaction of the allosteric muscarine receptor antagonist heptane-1,7-bis(dimethyl-3'-phthalimidopropyl)ammonium bromide (C7/3-phth) with M1 muscarine receptors in rat cerebral cortex and rabbit vas deferens and M2 muscarine receptors in guinea pig atria was investigated. In atria, C7/3-phth completely inhibited the dissociation of N-[3H]methylscopolamine ([3H]NMS) in the presence of excess unlabeled NMS and slowed the washout of NMS in functional experiments. C7/3-phth also produced supra-additive inhibition of the negative inotropic effects of carbachol when combined with NMS. This latter phenomenon was less pronounced when pirenzepine (PZP) was used in place of NMS. Cooperativity factors for the interaction of C7/3-phth with other antagonists were obtained by fitting the data to a theoretical model for interaction between an agonist, a competitive antagonist, and an allosteric antagonist. The values obtained indicate that C7/3-phth exhibits a greater degree of negative heterotropic cooperativity with PZP than with NMS at the M2 muscarine receptor. In the rat cerebral cortex, C7/3-phth slowed the dissociation of [3H]NMS and [3H]quinuclidinyl benzilate from the M1 receptor to the same extent but appeared not to affect the dissociation of [3H]PZP. In rabbit vas deferens, the inhibitory effect of the combination of C7/3-phth and atropine on the responses to McN-A-343 at the M1 receptor was more pronounced than that of the combination of C7/3-phth and PZP. Comparison of the findings for both central and peripheral M1 receptors with those obtained for the cardiac M2 receptor suggests that the allosteric interaction of C7/3-phth is less evident at the M1 receptor, particularly in the case of PZP.