Characterization of the Cl⁻ conductance in the granular duct cells of mouse mandibular glands

Abstract

We have previously shown that mouse mandibular granular ducts contain a hyperpolarization-activated Cl⁻ conductance. We now show that the instantaneous current/voltage (I/V) relation of this Cl⁻ conductance is inwardly rectifying with a slope conductance of 15.4±1.8 nS (n=4) at negative potentials and of 6.7±0.9 nS (n=4) at positive potentials. Thus, the inward rectification seen in the steady-state I/V relation is due, not only to voltage activation of the Cl⁻ conductance, but also to the intrinsic conductance properties of the channel. We show further that the ductal Cl⁻ conductance is not activated by including ATP (10 mmol/l) in the pipette solution. Finally, we show that the conductance is not blocked by the addition of any of the following compounds to the extracellular solution: anthracene-9-carboxylate (A9C, 1 mmol/l), diphenylamine-2-carboxylate (DPC, 1 mmol/l), 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 100 μmol/l), 4,4′-diisothiocyanato-stilbene-2,2′-disulphonate (DIDS, 100 μmol/l), indanyloxyacetic acid (IAA-94, 100 μmol/l), verapamil (100 μmol/l), glibenclamide (100 μmol/l) and Ba²⁺ (5 mmol/l). The properties of the ductal Cl⁻ conductance most nearly resemble those of the ClC-2 channel. Both channel types have instantaneous I/V relations that are slightly inwardly rectifying, are activated by hyperpolarization with a time-course in the order of hundreds of milliseconds, have a selectivity sequence of Br⁻>Cl⁻>I⁻, and are insensitive to DIDS. The only identified difference between the two is that the ClC-2 channel is 50% blocked both by DPC and A9C (1 mmol/l), whereas the ductal Cl⁻ conductance is insensitive to these compounds.