New Estimates for the ‘Allosteric’ Constant of Neuromuscular Acetylcholine Receptor-Channels

Abstract

Nicotinic acetylcholine receptors (AChRs) open rarely in the absence of agonists but many different mutations substantially increase the unliganded gating equilibrium constant (the ‘allosteric’ constant, E0). In the adult mouse neuromuscular AChR a single mutation, ‘Special character’ A96H (in both ‘Special character’ subunits), increased E0 by >105-fold and caused spontaneous openings to occur in clusters arising from individual receptors. We measured E0 for three different sets of mutant combinations and by extrapolation estimated E0 for wild type AChRs. The estimates were E0wt=6.0x10−7 and 7.4 x 10−7 in adult-type AChRs and 4.3x10−7 in fetal-type AChRs (−100 mV, 23 oC). The adult value is in excellent agreement with one obtained previously by using a completely different method (6.6x10−7, from monoliganded gating). We found that E0wt decreased e-fold with a ∼58 mV depolarization, and that Na+ and K+ in the extracellular solution had no effect on E0. However, Cs+ acted as a very weak ‘partial’ agonist. The results are discussed with regard to the energetics of receptor activation and the competitive antagonism of ions with regard to agonist binding.