A patch-clamp study of acetylcholine-activated ion channels in Ascaris suum muscle.

Acetylcholine-activated single-channel currents were recorded from cell-attached and inside-out patches of isolated muscle vesicles from Ascaris suum. Acetylcholine (1-10 mumols l-1) activated cation-selective channels of two amplitudes: 40-50 pS and 25-35 pS. Both channels had linear I/V relationships and mean open durations independent of voltage. The larger conductance was analysed in detail to determine its open-, closed- and burst-time kinetics; the open and burst durations were composed of two components (short and long), while closed durations had at least three components (short, intermediate and long). The data were then corrected to allow for missing short events in order to estimate various parameters including corrected mean open time (1.26 + 0.11 ms, mean +/- S.E.). Values were also derived for the efficacy (beta/alpha = 4.9) and affinity [1/KD = 147 x 10(3) (mol l-1) -1] of acetylcholine at this receptor. Larger concentrations of acetylcholine (25-100 mumols l-1) were shown to produce desensitization and caused single-channel currents to occur in clusters with long closed times (mean 171 s) between clusters. It was concluded that the extrasynaptic muscle of Ascaris suum contains two types of acetylcholine-activated ion channel and these are possible sites of action of various anthelmintic drugs. This paper is the first to describe acetylcholine-activated single-channel currents in invertebrate muscle.