Levamisole‐activated single‐channel currents from muscle of the nematode parasite Ascaris suum

1 The patch‐clamp technique was used to examine levamisole‐activated channels in muscle vesicles from Ascaris suum . Cell‐attached and isolated inside‐out patches were used. 2 Levamisole (1–90 μ m ), applied to the extracellular surface, activated channels which had apparent mean open‐times in the range 0.80–2.85 ms and linear I/V relationships with conductances in the range 19–46 pS. Ion‐replacement experiments showed the channels to be cation selective. 3 The kinetics of the channels were analysed. Generally open‐ and closed‐time distributions were best fitted by two, and three expotentials respectively, indicating the presence of at least two open states and at least three closed states. The distributions of burst‐times were best‐fitted by two exponentials. 4 Channel open‐ and burst‐times were voltage‐sensitive: at low levamisole concentrations (1–10 μ m ), they increased with hyperpolarization. At higher concentrations of levamisole (30 μ m and 90 μ m ) flickering channel‐block was observed at hyperpolarized potentials. Using a simple channel‐block model, values for the blocking dissociation constant, K B were determined as 123 μ m at −50 mV, 46 μ m at −75 mV and 9.4 μ m at −100 mV. 5 At the higher concentration of levamisole (30 μ m and 90 μ m ) long closed‐times separating ‘clusters' of bursts were observed, at both hyperpolarized and depolarized membrane potentials and this was interpreted as desensitization.