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Abstract

The Na⁺2Cl^-K⁺ cotransporter in the apical membrane of the cortical thick ascending limb of the Henle's loop (cTAL) of rabbit nephron utilizes the electrochemical gradient for Na⁺ to transport K⁺ and Cl^- against an unfavorable electrochemical gradient from lumen to cell interior. In the present study attempts are made to measure intracellular K⁺ activity (a ^cell_K⁺) under control conditions and after inhibition of the cotransport system by furosemide (50 X 10^-6 mol X l^-1). 70 cTAL segments of 55 rabbits were perfused in vitro. Conventional Ling-Gerard and K⁺-selective microelectrodes were used to measure the PD across the basolateral membrane (PD_bl) as well as the PD sensed by the single barrelled K⁺-selective electrode (PD_K⁺). PD_bl was -64 ± 1 (n = 65) mV and PD_K⁺ + 15 ± 1 (n = 32) mV under control conditions. The positive PD_K⁺ value, significantly different from zero, indicates that a ^cell_K⁺ is higher than predicted for passive distribution. The estimate for a ^cell_K⁺ obtained from PD_bl and PD_K⁺ was 113 ± 8 mmol X l_-1. Furosemide lead to the previously reported hyperpolarization of PD_bl by 17 ± 4 (n = 13) mV and to a reduction of PD_K⁺ from 15 ± 1 to 5 ± 1 (n = 20) mV. The a ^cell_K⁺, obtained from this set of data, was 117 ± 9 mmol X l, and was not different from the control value. The present data indicate that a ^cell_K⁺ is significantly above Nernst equilibrium under control conditions