Lumen-positive Chloride Transport Potential in the Early Distal Tubule of Triturus Kidney: Its Absolute Dependence on the Presence of Na⁺ and K⁺ in the Luminal Fluid

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Microperfusion experiments with Na⁺-free and/or K⁺-free solutions revealed that the generation of Cl^--dependent lumen-positive transtubular potentials of the early distal tubule of Triturus kidney was absolutely dependent on the presence of both Na⁺ and K⁺ in the lumen. Increases in luminal Na⁺ or K⁺ concentration increased the luminal positivity in a saturable manner, and the half saturation concentration was found to be about 1mM for both cations. The results strongly suggest that Cl^- is cotransported with Na⁺ and K⁺ across the luminal membrane.