A high-conductance calcium-dependent chloride channel in Ascaris suum muscle

The properties of a high-conductance Ca-dependent Cl channel in the body muscle membrane of the nematode parasite Ascaris suum were examined with the patch-clamp technique. In Ascaris Ringer solution the current-voltage relationships of the channel appeared linear during hyperpolarization with a slope conductance of 200 pS (in symmetrical 175 mM-Cl): during depolarization the slope conductance reduced. Cation and anion replacement experiments showed this channel to be permeable to Cl ions. In artificial perienteric fluid (in symmetrical 78 mM-Cl) the channel conductance was found to be 114 pS. Subconductance states were seen when the patch was depolarized. The channel conductance showed saturation with increasing Cl concentrations. The channel was voltage sensitive: the probability of the channel being open and the mean open time of the channel reduced when the patch was depolarized. Mean channel open times between 2.4 and 215 ms were observed. Frequency histograms of the distributions of open and closed times were fitted with two and three exponentials respectively; this suggests complex channel kinetics with multiple open and closed states. Ca applied to the intracellular surface of the patch increased in a dose-dependent manner the number and probability of the channels being open. These channels were seen in about 20% of all patches and occasionally up to 15 channels were seen in a single patch. The distributions of the probabilities of seeing N channels open in multichannel patch records were not not always well fitted by the binomial distribution: it is suggested that adjacent channels could have different probabilities of being open. The possible functions of this channel are discussed.