The time course of miniature endplate currents and its modification by receptor blockade and ethanol.

Abstract

Miniature endplate currents (MEPC) recorded from mouse diaphragms with a point voltage clamp, without inhibition of acetylcholinesterase (AChE) and in the absence of any drug, showed in their decay phase consistent deviations from an exponential time course, consisting of a curvature and a progressive increase of decay rate during most of the decay phase, followed by late tails. Both phenomena persisted when MEPC (and channel lifetime) were prolonged by ethanol. Curvature was increased by muscle fiber depolarization and decreased by hyperpolarization. Receptor blockade by (+)-tubocurarine, .alpha.-bungarotoxin, hexamethonium or myasthenic IgG accelerated the decay of the main part of MEPC and eliminated curvature; the time constant of MEPC became close to the channel time constant. Curvature arises from repeated action of ACh [acetylcholine] with cooperativity in ACh-receptor interaction; the voltage sensitivity of curvature follows from the voltage sensitivity of channel closing. Ethanol, in addition to its effect to prolong channel lifetime, enhances the tendency of ACh to act more than once to open channels before being lost to the system. Analysis of the rising phase of the MEPC, in terms of driving functions, also indicated that ethanol promotes channel opening by ACh; this action can account for a substantial increase of MEPC height by ethanol when MEPC are made small by receptor blockade. Driving functions were also voltage sensitive, in a manner indicating acceleration of channel opening, but reduction of channel conductance, with hyperpolarization. Poisoning or inhibition of AChE prolonged MEPC without altering the duration of ionic channels. Ethanol caused further prolongation of MEPC after poisoning of AChE, with little change in MEPC height suggesting that the extension of mean channel lifetime by ethanol is accompanied by a similar extension of ACh binding to receptors. After poisoning of AChE, MEPC became very variable in time course and the decay rate (.tau.-1) was correlated with MEPC height with a slope of log .tau. vs. log height of 0.77 for MEPCs of > 60% mean size. This slope is larger than expected from cooperativity in ACh-receptor interaction. Correlation of .tau. and height of MEPC also exists when AChE is intact; the slope of log .tau. vs. log height was 0.12 with or without prolongation of MEPC by ethanol.