Rapid Kinetics of Agonist Binding and Permeability Response Analyzed in Parallel on Acetylcholine Receptor Rich Membranes from Torpedo marmorata

Excitable acetylcholine receptor rich membrane fragments from Torpedo marmorata have been used to measure, in parallel, (1) the permeability response to the fluorescent cholinergic agonist Dns-C₆-Cho (in the 0.1 µM to millimolar concentration range) characterized by both the initial rate of Li⁺ transport and the rate of channel closure using the rapid-mixing quench-flow technique and (2) the kinetics of interaction of Dns-C₆-Cho with the acetylcholine receptor sites using the rapid-mixing stopped-flow technique. Analysis of the kinetics of Dns-C₆-Cho binding in the millisecond to minute time scale leads to the identification of at least three conformational states of the acetylcholine receptor: a “low-affinity” one (∼50 µM) that can be interconverted in the fraction of a second to a transient state of “intermediate affinity” (∼1 µM), followed by the final stabilization, in the second to minute time range, of a state of “high affinity” (∼3 nM). Comparison of Dns-C₆-Cho binding data with the permeability response to the same agonist demonstrates that the binding to the low-affinity conformation(s) of the acetylcholine receptor sites coincides with the triggering of the permeability increase—or “activation”—and the transitions to the intermediate- and high-affinity states with the two-step process of channel closing—or “desensitization”. The data are interpreted in terms of a minimum four-state “allosteric” model for the acetylcholine receptor.