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 m uscle cell of the parasite Ascaris suum contain a large conductance, v oltage-sensitive, calcium-activated chloride channel. The ability of t his channel to conduct a variety of carboxylic acids, a number of whic h are products of anaerobic respiration, was investigated using the pa tch-clamp technique and isolated inside-out patches of muscle membrane . The channel has a conductance of 140 pS in symmetrical 140 mM chlori de. Replacement of internal chloride with various carboxylic acids (14 0 mM) caused large hyperpolarizing shifts in the reversal potential. P ermeability ratios, relative to chloride, were calculated for each aci d. The relationship between permeability ratio and ionic size is inver se and linear predicting a pore diameter of 6.55 Angstrom. This simple relationship was not observed between ionic size and conductance. Cal culation of the transition state energy required to transfer a single methyl group from aqueous phase to the binding site afforded a value t hat was low but favorable, indicating a cationic binding site of low f ield strength. As the channel is able to open fully at the resting mem brane potential of Ascaris and is permeable to fatty acids produced by anaerobic respiration, the possible role of this channel in the remov al of metabolic products across the muscle membrane is discussed.