Chloride channels on epithelial cells cultured from human fetal epididymis

Using single-channel recording techniques, we have detected two types of outwardly rectifying chloride channel on epithelial cells cultured from human fetal epididymis. A small-conductance channel (2.8-5.0 pS) was spontaneously active in 29% of cell-attached patches but rapidly disappeared on patch excision. This channel often occurred in clusters and exhibited slow kinetics with open and closed times of the order of tens or hundreds of msec; an open-state probability that was essentially independent of voltage; and a very low permeability to bicarbonate relative to chloride. Exposing epididymal cells to either forskolin (3 microM) or adrenaline (1 microM) activated this channel (up to 350-fold), suggesting that it may be involved in cyclic AMP-mediated anion secretion by the male reproductive tract. The large-conductance channel (14 to 29 pS) was never detected in cell-attached patches but could be activated by depolarization (40 mV) in 3% of excised, inside-out patches. Once activated, opening of this 'large' channel was voltage independent, and it had a relatively high permeability to both gluconate (Pgluconate/Pchloride = 0.24) and bicarbonate (Pbicarbonate/Pchloride = 0.4). The proportion of excised patches that contained this channel was increased 2.5-fold by prior stimulation of the epididymal cells; however, because the channel was never observed in cell-attached patches its physiological role must remain uncertain.