Kinetic behavior of cloned mouse acetylcholine receptors. A semi-autonomous, stepwise model of gating

Abstract

This model assumes that any conformational changes consequent to binding one ligand are kinetically undetectable (on the time scale of the instrumentation) and/or irrelevant to the gating process. We have analysed AChR activation kinetics according to an alternative scheme that explicitly incorporates agonist-induced conformational changes. In this stepwise model, each subunit undergoes an isomerization after binding an agonist, with ionic conduction occurring only after both subunits have activated: -100 mV). Kinetic analysis followed the Horn and Lange (1983) full maximum likelihood method, with a first-order correction for missed events (Roux and Sauve, 1985). After selecting a subset of bursts that was apparently homogeneous, the rates of either activation scheme were iteratively adjusted by a Simplex algorithm until the -log likelihood was a minimum. The significance of the log likelihood ratios (LLR) was examined both by resampling and simulation methods (Horn, 1987). At 1,000 ~M, the predominant component of gaps with bursts has a time constant of 64 ~s, indicating an effective opening rate at this concentration of 15,600 S-I. The results of the likelihood fit to a patch exposed to 10 ~M ACh are shown below (all rates are S-I or S-l ~M-l). The association and dissociation rates of the stepwise scheme were assumed to be equal and independent, so that each model had the same number of independent parameters (six).