Postsynaptic end-plate ionic currents in fast and slow chick muscle fibers. Effect of cholinesterase inhibition

Abstract

Miniature end-plate potentials (MEPPs) were recorded in fast and slow chick muscle fibers extracellularly (focally) and under voltage clamp conditions. The duration of the MEPPs in synapses of slow fibers was on average 2.5 times longer than their duration in synapses of fast fibers. Inhibition of acetylcholinesterase (AChE) lengthened MEPPs by a varied degree: by 1.5 times in synapses of slow fibers and by 3.5 times in those of fast fibers. As a result the difference in the decay time of MEPPs in synapses of these types disappeared almost completely, and only a small difference remained in the rise time of the MEPPs. The MEPP decay time during hyperpolarization in the slow fiber synapse was rather less dependent on potential than in synapses of fast fibers; after inhibition of AChE it became even less dependent. Similar changes in potential dependence were found after lengthening of the MEPP by the action of ethanol. The functional significance of differences in AChE activity and in the activating effect of the mediator for the kinetics of MEPPs in synapses of fast and slow fibers is discussed.