The Ca-activated chloride channel of Ascaris suum conducts volatile fatty acids produced by anaerobic respiration: a patch-clamp study.

Plasma membrane vesicles prepared from the bag region of the somatic muscle cell of the parasite Ascaris suum contain a large conductance, voltage-sensitive, calcium-activated chloride channel. The ability of this channel to conduct a variety of carboxylic acids, a number of which are products of anaerobic respiration, was investigated using the patch-clamp technique and isolated inside-out patches of muscle membrane. The channel has a conductance of 140 pS in symmetrical 140 mM chloride. Replacement of internal chloride with various carboxylic acids (140 mM) caused large hyperpolarizing shifts in the reversal potential. Permeability ratios, relative to chloride, were calculated for each acid. The relationship between permeability ratio and ionic size is inverse and linear predicting a pore diameter of 6.55 A. This simple relationship was not observed between ionic size and conductance. Calculation of the transition state energy required to transfer a single methyl group from aqueous phase to the binding site afforded a value that was low but favorable, indicating a cationic binding site of low field strength. As the channel is able to open fully at the resting membrane potential of Ascaris and is permeable to fatty acids produced by anaerobic respiration, the possible role of this channel in the removal of metabolic products across the muscle membrane is discussed.