The oxime HGG-12 as a muscarinic acetylcholine receptor antagonist without intrinsic activity in cardiac membranes

Direct interactions of the bispyridinium oxime HGG-12 with muscarinic acetylcholine receptors were investigated in porcine cardiac atrial membranes. Competition binding experiments using the radiolabeled muscarinic receptor antagonist (³H)QNB revealed specific binding of HGG-12 to muscarinic acetylcholine receptors of porcine atrial membranes with a dissociation constant of 3.8×10^–7 mol/l. Muscarinic acetylcholine receptor-stimulated binding of the radiolabeled GTP analog (³⁵S)GTPS] to guanine nucleotide binding proteins (G-proteins) was used to study antagonistic and possible agonistic effects of HGG-12 at muscarinic acetylcholine receptors. HGG-12 completely inhibited carbachol- and oxotremorine-stimulated (³⁵S)GTPS] binding to pertussis toxin sensitive and insensitive G-proteins in a competitive manner. Inhibition constants (K_I) of HGG-12 for blockade of carbachol- and oxotremorine-stimulated GTPS]-binding (9.7×10^–7 mol/l and 1.7×10^–6 mol/l, respectively) were higher by about a factor of 100 than those of the muscarinic acetylcholine receptor antagonist atropine. In the absence of muscarinic acetylcholine receptor agonists, HGG-12 by itself had no stimulatory effect on (³⁵S)GTPS] binding in porcine atrial membranes. The results of this study show that the oxime HGG-12 is a competitive antagonist without intrinsic activity at porcine atrial muscarinic acetylcholine receptors. The stimulatory action of HGG-12 on muscarinic acetylcholine receptors which has been described by several authors is, therefore, suggested to be due to partial inhibition of acetylcholinesterase by the oxime rather than to direct agonism at muscarinic acetylcholine receptors.