Abstract
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) > CI- (1) > NO -3 (0.92) > Br- (0.75). The relative conductances were: SCN- (2.18) > I- (1.07) > Cl− (1.00) > Br- (0.91) > NO -3 (0.50). The Cl− current was blocked by NPPB (ID50 ≈ 10 μm), DIDS (10 or 30 μmol/liter) and furosemide (100 μmol/liter).
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This project was supported by the Australian Research Council. We thank Professor J.A. Young for his critical reading of the manuscript, Professor R. Greger (Physiologisches Institut, Freiburg, FRG) for providing NPPB and Drs. N. Sangster and J. Rothwell and Mr. R. Murphy for giving us access to their sheep.
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Ishikawa, T., Cook, D.I. A Ca2+-activated Cl− current in sheep parotid secretory cells. J. Membarin Biol. 135, 261–271 (1993). https://doi.org/10.1007/BF00211098
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DOI: https://doi.org/10.1007/BF00211098