Microelectrode assessment of chloride-conductive properties of cortical collecting duct

The chloride-conductive properties of the isolated rabbit cortical collecting duct were assessed with microelectrode techniques. The transepithelial, apical, and basolateral membrane potential differences, V(te), V^a, and V^b, respectively, were monitored continuously along with periodic measurements of the transepithelial conductance, G(te), and fractional resistance, fR^a (ratio of apical to apical plus basolateral membrane resistance). Active transport was eliminated in all experiments by luminal addition of 50 μM amiloride in HCO₃-free solutions. Upon reducing the chloride activity in the bath (gluconate replacement), there was a marked depolarization of V^b and decrease in G(te) and fR^a, demonstrating a major dependence of the basolateral membrane conductance on the bath chloride activity. However, a significant K⁺ conductance at that barrier was also apparent since raising the bath K⁺ concentration caused an increase in G(te) and fR^a and depolarization of V^b. Lowering the chloride activity of the perfusate caused a consistent decrease of G(te) but not of fR^a, 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 x 10^-5 cm.s^-1 for the tight junction, P(Cl)(tj), and 5 x 10^-5 cm.s^-1 for the basolateral cell membrane, P(Cl)^b. It is concluded that the paracellular pathway provides a major route for transepithelial Cl^- transport. Furthermore, since the isotopically measured Cl^- permeability is severalfold greater than P(Cl)(tj), 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.