A Ca(2+)-activated Cl- current in sheep parotid secretory cells.

Previous studies have shown that the whole-cell current-voltage (I-V) relation of unstimulated sheep parotid cells is dominated by two K+ conductances, one outwardly and the other inwardly rectifying. We now show that once these K+ conductances are blocked by replacement of pipette K+ with Na+ and by the addition of 5 mmol/liter CsCl to the bath, there remains an outwardly rectifying conductance with a reversal potential of 0 mV. Replacement of 120 mmol/liter NaCl in the pipette solution with an equimolar amount of Na-glutamate shifted the reversal potential of this residual current to -55 mV, indicating that the conductance was Cl- selective. The Cl- current was activated by increasing the free Ca2+ in the pipette solution from 10 to 100 nmol/liter. When the Ca2+ concentration in the pipette solution was 10 nmol/liter, the relaxations observed in response to membrane depolarization could be fitted with a single exponential, whose time constant increased from 81 to 183 ms as the pipette potential was increased from -30 to +60 mV. Relaxation analysis showed that the current was activated by membrane depolarization. Reversal potential measurements in experiments in which external Cl- was replaced with various anions, gave the following relative permeabilities: SCN- (1.80) > I- (1.09) > Cl- (1) > NO3- (0.92) > Br- (0.75). The relative conductances were: SCN- (2.18) > I- (1.07) > Cl- (1.00) > Br- (0.91) > NO3- (0.50). The Cl- current was blocked by NPPB (ID50 approximately 10 microM), DIDS (10 or 30 mumol/liter) and furosemide (100 mumol/liter).