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

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Abstract
Patch-clamp recordings of current through acetylcholine[ACh]-activated channels were made from non-junctional membrane of innervated myotomal muscle from X. laevis. Two classes of 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 2 classes were 64 and 44 pS [siemens]. Both event classes were blocked by .alpha.-bungarotoxin. At the normal resting membrane potential (.apprx. -95 mV) the larger conductance channel (.gamma.) exhibited an apparent mean channel open time of less than 1 ms, compared to .apprx. 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 .mu.M) 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 > 100 ms. Closed interval histograms revealed corresponding populations of brief and long closures, indicating that at least 2 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 occsasionally be altered by adjusting the negative pressure applied to the patch. The 2 main conductance classes of ACh activated channels were observed to coexist in most patches. 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.