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 ( \(a_{{\text{K}}^{\text{ + }} }^{{\text{Cell}}} \) ) under control conditions and after inhibition of the cotransport system by furosemide (50·10⁻⁶ mol·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 ( \({\text{PD}}_{{\text{K}}^{\text{ + }} } \) ). PD_bl was −64±1 (n=65) mV and \({\text{PD}}_{{\text{K}}^{\text{ + }} } \) +15±1 (n=32) mV under control conditions. The positive \({\text{PD}}_{{\text{K}}^{\text{ + }} } \) value, significantly different from zero, indicates that \(a_{{\text{K}}^{\text{ + }} }^{{\text{Cell}}} \) is higher than predicted for passive distribution. The estimate for \(a_{{\text{K}}^{\text{ + }} }^{{\text{Cell}}} \) obtained from PD_bl and \({\text{PD}}_{{\text{K}}^{\text{ + }} } \) was 113±8 mmol·l⁻¹. Furosemide lead to the previously reported hyperpolarization of PD_bl by 17±4 (n=13) mV and to a reduction of \({\text{PD}}_{{\text{K}}^{\text{ + }} } \) from 15±1 to 5±1 (n=20) mV. The \(a_{{\text{K}}^{\text{ + }} }^{{\text{Cell}}} \) , obtained from this set of data, was 117±9 mmol·l⁻¹, and was not different from the control value. The present data indicate that \(a_{{\text{K}}^{\text{ + }} }^{{\text{Cell}}} \) is significantly above Nernst equilibrium under control conditions. The source for this above equilibrium accumulation of K⁺ stems from the carrier mediated uptake of Na⁺2Cl⁻ and K⁺. Consequently, the electrochemical gradient for K⁺ is rapidly reduced when the carrier is blocked by furosemide. The electrochemical gradient for K⁺, under control conditions, energizes the back leak of K⁺ from cell to lumen. This K⁺ flux is one component responsible for the lumen positive transepithelial PD.