Three allosteric modulators act at a common site, distinct from that of competitive antagonists, at muscarinic acetylcholine M₂ receptors

Functional studies were conducted on guinea pig atrial muscarinic acetylcholine M₂ receptors with the allosteric modulators heptane-1,7- bis(dimethyl-3'-phthalimidopropyl)ammonium bromide (C₇/3'-phth), gallamine and alcuronium to determine whether these ligands are able to recognize a common accessory site. The three modulators inhibited the negative inotropic response to carbachol in this tissue. When used in combination, C₇/3'-phth and gallamine or C₇/3'-phth and alcuronium gave dose ratios that were either additive or underadditive. In contrast, the combinations of C₇/3'-phth or alcuronium with the competitive antagonists, N-methylscopolamine or atropine, yielded supra-additive dose ratios. The data could be reconciled with a model involving a ternary complex between (1) the receptor, (2) carbachol, N- methylscopolamine or atropine acting at the orthosteric binding site and (3) C₇/3'-phth, alcuronium or gallamine acting at a common, allosteric site with varying degrees of heterotropic cooperativity.