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Abstract

Experiments in the amphibian early distal tubule have shown that Cl transport is secondarily active, coupled to the flux of Na and dependent on the presence of luminal K. Omission of luminal K results in a decrease of cellular Na, a finding that suggests inhibition of luminal Na entry. In the present study intracellular chloride activity (Cl_i) and peritubular cell membrane potentials (PD_pt) were evaluated before and after omission of luminal K. Furthermore, the effect of inhibition of the luminal K conductance by barium on the electrochemical gradient of Cl (E^Cl_te ) across the distal epithelium was determined at static head conditions. Experiments were performed in early distal segments of the isolated perfused kidney ofAmphiuma andRana esculenta. Cl_i and PD_pt were measured simultaneously in single cells by double barreled Cl sensitive microelectrodes in the presence and absence of luminal K. E^Cl_te was determined at zero net flux conditions with single barreled electrodes in control tubules, in the presence of barium (3·10⁻³ mol/l) and in the presence of furosemide (5·10⁻⁵ mol/l).

In 26 individual cellular impalements omission of luminal K hyperpolarized PD_pt from 72.5±1.2 to 90.0±1.9 mV (cell interior negative). Concomitantly, Cl_i fell from 8.5±0.4 to 5.4±0.3 mmol/l. Both effects occurred within seconds and were fully reversible. Addition of barium to the luminal fluid diminished E^Cl_te (directed lumen positive) from a control value of 39.5±1.4 mV to 28.5±2.5mV. E^Cl_te could be further diminished to 14.1±2.1 mV and to 1.3±0.5 mV after application of barium on both sides and after luminal application of furosemide, respectively.

The experiments indicate that active Cl uptake across the luminal cell membrane depends critically on the presence of luminal K. Omission of luminal K achieved either by perfusing the lumen with K-free solutions or by inhibition of K back flux from the cell interior into the lumen by barium reduces Cl reabsorption.

Together with previous data on the K dependence of the Na uptake the present experiments support the hypothesis of a common transport system for K, Na, Cl located in the luminal cell membrane.