The actions of the anthelmintics pyrantel and levamisole, at the single channel level, on somatic muscle of the nematode parasite Ascaris suum

Abstract

The patch-clamp technique was used to examine single channel currents activated by the anthelmintics levamisole and pyrantel in muscle vesicles from Ascaris suum. Cellattached and isolated inside-out patches were used. Levamisole (1-90 μM), applied to the extracellular surface, activated channels which had apparent mean open-times in the range 0.12-2.23 ms and linear current / voltage relationships with conductances in the range 19-46 pS. Pyrantel (0.03-100 μM) activated cation selective channels with linear current/voltage plots and mean open times in the range 0.22-4.14 ms. With pyrantel there were at least two conductance levels: main conductance 41± 2.04 pS (mean±s.e., n=28), smaller conductance 22.4 ± 0.34 pS (mean±s.e., n=8). Ion-replacement experiments for both agonists showed the channels to be cation selective. The kinetics of the channels were analysed. Generally open- and closed-time distributions were best fitted by two and three exponentials respectively, indicating the presence of at least two open states and at least three closed states. The distributions of burst-times were bestfitted by two exponentials. Channel open- and burst-times were voltage-sensitive: at low levamisole (1-10 μM) or pyrantel concentrations (0.1-10 μM), they increased with hyperpolarisation. At higher concentrations of levamisole (30 μM & 90 μM) and pyrantel (100 μM), flickering channel-block was observed at hyperpolarised potentials. Using a simple channel-block model, values for the blocking dissociation constant, KΒ were determined as: 123 μM at -50 mV, 46 μM at -75 mV and 9.4 μM at -100 mV for levamisole; and 37 μM at -50 mV and 20 μM at -75 mV for pyrantel. At the higher concentrations of levamisole (30 μM & 90 μM) and pyrantel (100 μM) long closedtimes separating "clusters" of bursts were observed, at both hyperpolarised and depolarised membrane potentials and this was interpreted as desensitisation.

Although the simple channel-block model was used to describe the data, there were limitations in the use of this model: for example, the burst durations did not increase with concentration as forecast by the simple channel-block model. Limitations of this model are discussed

It was concluded that levamisole and pyrantel act in Ascaris suum by opening nicotinic ACh channels but in addition they produced open channel-block. The patch-clamp technique was also used to examine the effect of intracellular levamisole on nicotinic ACh receptors in muscle vesicle preparations from Ascaris suum. Initial experiments were performed where channels were activated with levamisole (2 μM) applied in the patch-pipette, isolated inside-out patches were used. In addition, levamisole (30-926 μM) was added to the cytoplasmic membrane surface via the bath solution. In 8 out of 9 experiments, addition of levamisole to the bath solution resulted in an increase in channel activity; a voltage-sensitive open channelblock and desensitisation. The open channel-block occurred at hyperpolarised potentials, an observation consistent with levamisole (a cationic substance) blocking the channel from the extracellular surface. Thus it was concluded that levamisole crossed from the cytoplasmic side of the membrane, via the lipid phase, to the extracellular surface of the patch. In the presence of high cytoplasmic concentrations of levamisole open channel-block was not observed at depolarised potentials suggesting channel asymmetry.