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

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 ($a_{{\text{K}}^{\text{ + }} }^{{\text{Cell}}}$) 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.