Microelectrode assessment of chloride-conductive properties of cortical collecting duct

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Abstract
The Cl-conductive properties of the isolated rabbit cortical collecting duct were assessed with microelectrode techniques. The transepithelial, apical and basolateral membrane potential differences, Vte, Va and Vb, respectively, were monitored continuously along with periodic measurements of the transepithelial conductance, Gte, and fractional resistance, fRa (ratio of apical to apical plus basolateral membrane resistance). Active transport was eliminated in all experiments by luminal addition of 50 .mu.M amiloride in HCO3-free solutions. Upon reducing the Cl activity in the bath (gluconate replacement), there was a marked depolarization of Vb and decrease in Gte and fRa, demonstrating a major dependence of the basolateral membrane conductance on the bath Cl activity. A significant K+ conductance at that barrier was also apparent since raising the bath K+ concentration caused an increase in Gte and fRa and depolarization of Vb. Lowering the Cl activity of the perfusate caused a consistent decrease of Gte but not of fRa, effects consistent with a high Cl- conductance of the tight junction and little, if any, apical membrane Cl- conductance. By use of the Cl- dependent conductances, the Cl- permeabilities at equilibrium were estimated to be near 1.0 .times. 10-5 cm/s for the tight junction, .**GRAPHIC**. and 5 .times. 10-5 cm/s for the basolateral cell membrane, .**GRAPHIC**. The paracellular pathway provides a major route for transepithelial Cl- transport. Since the isotypically measured Cl- permeability is severalfold greater than .**GRAPHIC**. a significant transcellular flux of Cl- must exist, implicating a neutral exchange mechanism at the apical cell membrane in series with the high basolateral membrane Cl- conductance.