The allosteric interaction of otenzepad (AF-DX 116) at muscarinic M-2 receptors in guinea pig atria

The effects of the muscarinic receptor antagonist, otenzepad, in combinatio n with the competitive antagonists N-methylscopolamine, dexetimide and atro pine, or the allosteric modulators, C-7/3 ' -path, gallamine and alcuronium , were measured in the guinea pig electrically driven left atrium using the agonists, carbachol or acetylcholine. Otenzepad, in combination with C-7/3 ' -phth or gallamine, gave concentration-ratios close to additive and in a greement with theoretical model predictions for combination of two alloster ic modulators acting at a common site. However, when otenzepad was combined with alcuronium, dexetimide or N-methylscopolamine, supra-additive effects were observed. For either competitive antagonist in combination with otenz epad, the degree of supra-additivity was more evident after 2-h equilibrati on than alter 40 min. When otenzepad was combined ru-im atropine, no supra- additivity was observed with carbachol as the agonist, but was evident with acetylcholine. Otenzepad was also unable to fully inhibit [H-3]N-methylsco polamine binding when the radioligand was employed at a concentration of si milar to 100 X K-D. It is concluded that the action of otenzepad involves a n allosteric site and a number of possibilities are discussed for its locat ion. (C) 2001 Elsevier Science B.V. All rights reserved.