Roles of amino acids and subunits in determining the inhibition of nicotinic acetylcholine receptors by competitive antagonists

Abstract

BACKGROUND: Binding sites for agonists and competitive antagonists (nondepolarizing neuromuscular blocking agents) are located at the α-δ and α-ε subunit interfaces of adult nicotinic acetylcholine receptors. Most information about the amino acids that participate in antagonist binding comes from binding studies with (+)-tubocurarine and metocurine. These bind selectively to the α-ε interface but are differentially sensitive to mutations. To test the generality of this observation, the authors measured current inhibition by five competitive antagonists on wild-type and mutant acetylcholine receptors. METHODS: HEK293 cells were transfected with wild-type or mutant (αY198F, εD59A, εD59N, εD173A, εD173N, δD180K) mouse muscle acetylcholine receptor complementary DNA. Outside-out patches were excised and perfused with acetylcholine in the absence and presence of antagonist. Concentration-response curves were constructed to determine antagonist IC50. An antagonist-removal protocol was used to determine dissociation and association rates. RESULTS: Effects of mutations were antagonist specific. αY198F decreased the IC50 of (+)-tubocurarine 10-fold, increased the IC50 of vecuronium 5-fold, and had smaller effects on other antagonists. (+)-Tubocurarine was the most sensitive antagonist to εD173 mutations. εD59 mutations had large effects on metocurine and cisatracurium. δD180K decreased inhibition by pancuronium, vecuronium, and cisatracurium. Inhibition by these antagonists was increased for receptors containing two δ subunits but no ε subunit. Differences in IC50 arose from differences in both dissociation and association rates. CONCLUSION: Competitive antagonists exhibited different patterns of sensitivity to mutations. Except for pancuronium, the antagonists were sensitive to mutations at the α-ε interface. Pancuronium, vecuronium, and cisatracurium were selective for the α-δ interface. This suggests the possibility of synergistic inhibition by pairs of antagonists. © 2007 American Society of Anesthesiologists, Inc.