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Abstract

Previous data suggest the cotransport of Na⁺, Cl⁻, and K⁺ across the lumen membrane of the cortical thick ascending limb (cTAL) of rabbit nephron. For this cotransporter to operate K⁺ recycling across the lumen membrane has to be postulated. The present data focus on the conductivity properties of the lumen membrane. Methods for impaling individual cells of in vitro perfused cTAL segments are described. The mean PD across the lumen membrane (PD₁) is 76 mV (lumen positive). Rapid increase in lumen perfusate K⁺ concentration (3.6→18.6 mmol · l⁻¹ leads to a depolarization of PD₁ by 20 mV. Ba²⁺ (3 mmol · l⁻¹ added to the lumen perfusate inhibits the K⁺ conductive pathway of the lumen membrane, and consequently increases the voltage divider ratio (current pulse induced voltage deflection across the lumen membrane divided by that across the basolateral membrane) from 2 to 36 as well as transepithelial resistance from 34 to 46 Ω cm². From these changes, and with the use of simultaneous equations of the VDR and of Kirchhoff's law, the resistances of the lumen membrane (88 Ω cm²), of the basolateral membrane (47 Ω cm²), and of the paracellular shunt pathway (47 Ω cm²) can be calculated. Using these estimates of the individual resistances and using the observed change in PD₁ in the K⁺ concentration step experiments (29±3 mV per decade in K⁺ concentration change) an apparent transference number of the lumen membrane for K⁺ in the order of 0.9–1.0 can be calculated. This indicates that the lumen membrane is essentially K⁺ conductive. This conclusion is strengthened further by the results of another series in which no evidence for a Cl⁻ conductive pathway in the lumen was obtained. The data of this study can be used to calculate the intracellular K⁺ activity of some 90–100 mmol ·l⁻¹. For this K⁺ activity the K⁺ diffusion from cell to lumen equals the carrier mediated uptake from lumen to cell. This indicates that an essentially complete recycling of K⁺ across the lumen membrane of the cTAL segment is feasible.