Characterization of an outwardly rectifying chloride channel in a human submandibular gland duct cell line (HSG)

We have used the single-channel patch-clamp technique to study ion channels in the plasma membrane of the HSG human submandibular gland duct cell line. In cell-attached and excised inside-out patches, at least six channel types were observed. When the pipette contained an isotonic KCl-rich solution and the bath an isotonic NaCl-rich solution, the predominant channel type seen in excised inside-out patches was a Cl- channel with an outwardly rectifying current/voltage (I/V) relation that had a conductance of 12 pS at positive pipette potentials and 43 pS at negative pipette potentials. The channel was only seen in excised patches and its open probability was not significantly increased by membrane depolarization. The channel selectivity sequence (relative to Cl-) was estimated from reversal potential measurements to be: SCN- (1.8) > NO3- (1.4) > I- (1.1) approximately Cl- (1) approximately Br- (0.8) > acetate (0.35). In inside-out patches the channel was blocked by addition of 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) (100 mumol/l) to the bath but not by 9-anthracene carboxylic acid (9-AC) (100 mumol/l). The channel was not activated by increases in the free Ca2+ concentration on the cytosolic surface. This is the first report of an outwardly rectifying Cl- channel in a salivary epithelium. The properties of this channel are not in accordance with the properties of the Cl- conductances in the acinar or duct tissues which have been studied so far and its physiological role is unclear.