Kir4.1 is Critical for Regulation of ENaC and ROMK Activity in the Late Distal Nephron

Abstract

The extent of NaCl reabsorption in the early distal convoluted tubule (DCT1) via the apical Na+‐Cl− cotransporter (NCC) defines the sodium load of the ensuing late DCT (DCT2) and connecting tubule (CNT). DCT2 and CNT express the epithelial sodium channel (ENaC) and potassium Kir1.1 channel (ROMK) to perform electrogenic sodium‐dependent potassium secretion into the urine. Kidney‐specific deletion of Kir4.1 (Ks‐Kir4.1−/−) causes profound hypokalemia in mice, which may result from reduced DCT function and increased sodium load of DCT2/CNT, because potassium sensing via Kir4.1 is a critical step for NCC activation. Since Kir4.1 expression extends throughout DCT2 and CNT, we hypothesized that the deletion of Kir4.1 may also affect the expression and activity of ENaC and ROMK. To address this hypothesis we examined ENaC and ROMK functions by measuring the amiloride‐sensitive Na+ currents and TPNQ‐sensitive K+ currents in the DCT2/CNT of wild‐type (WT) and Ks‐Kir4.1−/− mice fed regular or K+‐deficient diets for three days. Cellular distribution of ENaC and ROMK was studied by immunofluorescence. K+‐deficient diet decreased amiloride‐sensitive Na currents from 205 pA to 120 pA at −60 mV and decreased TPNQ‐sensitive K currents from 1100 pA to 600 pA at −40 mV in WT animals. Ks‐Kir4.1−/− mice showed increased baseline Na+ (520 pA at −60 mV) and K+ currents (1600 pA at −40 mV). Feeding Ks‐Kir4.1−/− mice with K+‐deficient diet failed to inhibit ENaC and ROMK. Localization experiments revealed substantially increased ROMK signal intensities along DCT2 and CNT, which were identified by double immunostaining for parvalbumin, calbindin, or aquaporin 2. Moreover, DCT2 exhibited significant elongation in Ks‐Kir4.1−/− mice indicating nephron remodeling. We conclude that Kir4.1 regulates ENaC and ROMK functions in the DCT2 and CNT. Their upregulation in Ks‐Kir4.1−/− mice likely contributes to renal K wasting. Support or Funding Information Deutsche Forschungsgemeinschaft grants MU2924/2‐2, BA700/22‐2, SFB 1365; National Institute of Health grants DK 54983 and DK 115366; Fondation LeDucq and NIDDK R0151496; Russian Academic Excellence project “5–100”.