Vol. 282, Issue 1, 278-285, 1997

Three Allosteric Modulators Act at a Common Site, Distinct from that of Competitive Antagonists, at Muscarinic Acetylcholine M₂ Receptors¹

Alfred Lanzafame, Arthur Christopoulos and Fred Mitchelson

Department of Pharmaceutical Biology and Pharmacology, Victorian College of Pharmacy (Monash University), Parkville, Victoria, Australia, 3052

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.