CHARACTERIZATION OF THE CL- CONDUCTANCE IN THE GRANULAR DUCT CELLS OF MOUSE MANDIBULAR GLANDS

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 mu mol/l), 4,4'diisothiocyanato-stilbene-2,2'-disulphonate (DIDS, 100 mu mol/l), indanyloxyacetic acid (IAA-94, 100 mu mol/l), verapamil(100 mu mol/l), glibenclamide (100 mu mol/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.