Chloride channels in the luminal membrane of the rectal gland of the dogfish (Squalus acanthias)

The rectal gland of the dogfish (Squalus acanthias) secretes chloride via a chloride channel present in the apical cell membrane. Using the patch clamp technique in isolated perfused rectal gland tubules [7], two types of chloride channels are demonstrable in the apical membrane of cyclic AMP treated tubule segments. A small channel of about 11 pS and another channel of 40–50 pS are present. The small channel is described in the succeeding report. With NaCl on both sides (excised patches) the current amplitude of the larger channel is an almost linear function of the voltage (±50 mV). However, the open probability of this channel is grossly reduced at negative clamp potentials (corresponding to cell hyperpolarization). Therefore, the macroscopic Cl^– current through this channel is reduced with hyperpolarization on the cytosolic side. An analysis of time constants of this channel reveals that at depolarized voltages two open and two closed time constants of about 1 ms and of about 10 ms, respectively, are demonstrable. With hyperpolarized voltages the larger open state time constant is reduced significantly. This type of chloride channel is blocked reversibly by diphenylamine-2-carboxylate (10^–4 mol/l) and by 5-nitro-2-(3-phenylpropylamino)-benzoate (10^–5 mol/l). The channel is selective for Cl^– over Na^– and K^– as well as over Br^–. It is, however, permeable for NO₃^-. Since this channel is very rare or absent in nonstimulated rectal gland tubules, it is very likely that this type of channel is responsible for hormone and cAMP dependent chloride secretion in this organ.Supported by Deutsche Forschungsgemeinschaft Gr 480 and by NSF and NIH grants to the MDIBL