The dissociation of acetylcholine from open nicotinic receptor channels

Ligand-gated ion channels bind agonists with higher affinity in the open th an in the closed state. The kinetic basis of this increased affinity has re mained unknown, because even though the rate constants of agonist associati on to and dissociation from closed receptors can be estimated with reasonab le certainty, the kinetics of the binding steps in open receptors have prov en to be elusive. To be able to measure the agonist-dissociation rate const ant from open muscle nicotinic receptors, we increased the probability of l igand unbinding from the open state by engineering a number of mutations th at speed up opening and slow down closing but leave the ligand-binding prop erties unchanged. Single-channel patch-clamp recordings from the wild-type and mutant constructs were performed at very low concentrations of acetylch oline (ACh). The durations of individual channel activations were analyzed assuming that "bursts" of fully liganded (diliganded) receptor openings can be terminated by ligand dissociation from the closed or open state (follow ed by fast closure) or by desensitization. This analysis revealed that ACh dissociates from diliganded open receptors at approximate to 24 s(-1), that is, approximate to2,500 times more slowly than from diliganded closed rece ptors. This change alone without a concomitant change in the association ra te constant to the open state quantitatively accounts for the increased equ ilibrium affinity of the open channel for ACh. Also, the results predict th at both desensitization and ACh dissociation from the open state frequently terminate bursts of openings in naturally occurring gain-of-function mutan ts (which cause slow-channel congenital myasthenia) and therefore would con tribute significantly to the time course of the endplate current decay in t hese disease conditions.