Potassium activity in cells of isolated perfused cortical thick ascending limbs of rabbit kidney

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 cellK+) 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 (PDbl) as well as the PD sensed by the single barrelled K+-selective electrode (PDK+). PDbl was -64 ± 1 (n = 65) mV and PDK+ + 15 ± 1 (n = 32) mV under control conditions. The positive PDK+ value, significantly different from zero, indicates that a cellK+ is higher than predicted for passive distribution. The estimate for a cellK+ obtained from PDbl and PDK+ was 113 ± 8 mmol X l-1. Furosemide lead to the previously reported hyperpolarization of PDbl by 17 ± 4 (n = 13) mV and to a reduction of PDK+ from 15 ± 1 to 5 ± 1 (n = 20) mV. The a cellK+, 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 cellK+ is significantly above Nernst equilibrium under control condition