Properties of non-junctional acetylcholine receptor channels on innervated muscle of Xenopus laevis

Abstract

Patch-clamp recordings of current through acetylcholine-activated channels were made from non-junctional membrane of innervated myotomal muscle from Xenopus laevis. Two classes of acetylcholine (ACh) receptor channels were identified on the basis of current amplitudes. Both amplitude classes exhibited current-voltage relations which deviated from linearity as the extrapolated reversal potential was approached (-5 to -12 mV). Over the range of greatest linearity the conductances of the two classes were 64 and 44 pS. Both event classes were blocked by alpha-bungarotoxin. At the normal resting membrane potential (approximately -95 mV) the larger conductance channel (gamma) exhibited an apparent mean channel open time of less than 1 ms, compared to approximately 2 ms for the smaller gamma class. The apparent open time was voltage-dependent, changing e-fold with a 63 mV hyperpolarization for the high gamma channel and 93 mV hyperpolarization for the low gamma channel. At low ACh concentrations (0.1-0.3 microM) both amplitude classes exhibited bursts of successive openings separated by brief closures of less than 0.5 ms. Bursts were separated by longer closed intervals of 1 to greater than 100 ms. Closed interval histograms revealed corresponding populations of brief and long closures, indicating that at least two kinetic processes are required to describe the distribution of closed intervals. In the absence of exogenous ACh, channels were observed in an occasional patch which showed a conductance and extrapolated reversal potential similar to ACh-activated channels. In such patches the event frequency could occasionally be altered by adjusting the negative pressure applied to the patch. The two main conductance classes of ACh activated channels were observed to coexist in most patches. However, the most frequent event observed in non-junctional membrane of innervated muscle corresponded to the high gamma class. In this respect, the non-junctional ACh receptors bore a greater similarity to junctional ACh receptors than to non-junctional receptors reported for denervated muscle.