PT  - JOURNAL ARTICLE
AU  - Man Liu
AU  - James P. Dilger
TI  - Site Selectivity of Competitive Antagonists for the Mouse Adult Muscle Nicotinic Acetylcholine Receptor
AID  - 10.1124/mol.108.051060
DP  - 2009 Jan 01
TA  - Molecular Pharmacology
PG  - 166--173
VI  - 75
IP  - 1
4099  - http://molpharm.aspetjournals.org/content/75/1/166.short
4100  - http://molpharm.aspetjournals.org/content/75/1/166.full
SO  - Mol Pharmacol2009 Jan 01; 75
AB  - The muscle-type nicotinic acetylcholine receptor has two nonidentical binding sites for ligands. The selectivity of acetylcholine and the competitive antagonists (+)-tubocurarine and metocurine for adult mouse receptors is known. Here, we examine the site selectivity for four other competitive antagonists: cisatracurium, pancuronium, vecuronium, and rocuronium. We rapidly applied acetylcholine to outside-out patches from transfected BOSC23 cells and measured macroscopic currents. We have reported the IC50 of the antagonists individually in prior publications. Here, we determined inhibition by pairs of competitive antagonists. At least one antagonist was present at a concentration producing ≥67% receptor inhibition. Metocurine shifted the apparent IC50 of (+)-tubocurarine in quantitative agreement with complete competitive antagonism. The same was observed for pancuronium competing with vecuronium. However, pancuronium and vecuronium each shifted the apparent IC50 of (+)-tubocurarine less than expected for complete competition but more than expected for independent binding. The situation was similar for cisatracurium and (+)-tubocurarine or metocurine. Cisatracurium did not shift the apparent IC50 of pancuronium or vecuronium, indicating independent binding of these two pairs. The data were fit to a two-site, two-antagonist model to determine the antagonist binding constants for each site, Lαϵ and Lαδ. We found Lαϵ/Lαδ = 0.22 (range, 0.14-0.34), 20 (9-29), 21 (4-36), and 1.5 (0.3-2.9) for cisatracurium, pancuronium, vecuronium, and rocuronium, respectively. The wide range of Lαϵ/Lαδ for some antagonists may reflect experimental uncertainties in the low affinity site, relatively poor selectivity (rocuronium), or possibly that the binding of an antagonist at one site affects the affinity of the second site. The American Society for Pharmacology and Experimental Therapeutics