ENaC and ROMK activity are inhibited in the DCT2/CNT of TgWnk4^PHAII mice

Abstract

Mice transgenic for genomic segments harboring PHAII (pseudohypoaldosteronism type II) mutant Wnk4 (with-No-Lysine kinase 4) (TgWnk4) have hyperkalemia which is currently believed to be the result of high activity of Na-Cl cotransporter (NCC). This leads to decreasing Na delivery to the distal nephron segment including late distal convoluted tubule (DCT) and connecting tubule (CNT). Since epithelial Na channel (ENaC) and renal outer medullary K channel (ROMK or Kir4.1) are expressed in the late DCT and play an important role in mediating K secretion, the aim of the present study is to test whether ROMK and ENaC activity in the DCT/CNT are also compromised in the mice expressing PHAII mutant Wnk4. Western blot analysis shows that the expression of βENaC and γENaC subunits but not αENaC subunit was lower in TgWnk4 mice than that in wild-type (WT) and TgWnk4 mice. Patch-clamp experiments detected amiloride-sensitive Na currents and TPNQ-sensitive K currents in DCT2/CNT, suggesting the activity of ENaC and ROMK. However, both Na and ROMK currents in DCT2/CNT of TgWnk4 mice were significantly smaller than those in WT and TgWnk4 mice. In contrast, the basolateral K currents in the DCT were similar among three groups, despite higher NCC expression in TgWnk4 mice than those of WT and TgWnk4mice. An increase in dietary K intake significantly increased both ENaC and ROMK currents in the DCT2/CNT of all three groups. However, high-K (HK) intake-induced stimulation of Na and K currents was smaller in TgWnk4 mice than those in WT and TgWnk4 mice. We conclude that ENaC and ROMK channel activity in DCT2/CNT are inhibited in TgWnk4 mice and that Wnk4-induced inhibition of ENaC and ROMK may contribute to the suppression of K secretion in the DCT2/CNT in addition to increased NCC activity.

A mouse model of pseudohypoaldosteronism type II reveals a novel mechanism of renal tubular acidosis

“…We observed an increase in CCD mass that is paralleled by an increase in pendrin activity with only a modest decrease in CNT mass (CNT fractional volumes were 5.16% AE 0.54% vs. 3.41% AE 0.36% of kidney cortex in control and TgWnk4 PHAII mice, respectively (n ¼ 4 for each group, P ¼ 0.036), which likely explains the decrease in ENaC and ROMK currents seen in the DCT2/early CNT of TgWnk4 PHAII mice. 34 However, the observation that pendrin deletion normalized blood K þ concentration in TgWnk4 PHAII mice indicates that remodeling of the CNT is not the primary cause of K þ retention in our model. In summary, both models (CA-SPAK and WNK4-PHAII) are characterized by the combination of NCC activation along with a second mechanism that differs from a model to another but that results ultimately in a decrease in K þ secretion.…”

Constitutively Active SPAK Causes Hyperkalemia by Activating NCC and Remodeling Distal Tubules

“…Altered regulation of ROMK and ENaC has been reported, but the phenotypes are surprisingly divergent manners depending on the WNK FFHt model. Recently, Zhang and coworkers 51 found that ROMK and ENaC activities are suppressed in the late DCT/early CNT of mice transgenic for WNK4 FFHt, identical to CA-SPAK mice. By contrast, WNK4 FFHt mice were reported to have increased ENaC activity and no change in ROMK protein abundance, although ROMK was misidentified with a nonspecific antibody.…”

Potassium intake modulates the thiazide-sensitive sodium-chloride cotransporter (NCC) activity via the Kir4.1 potassium channel

“…We demonstrated previously that the basolateral Kir4.1 activity is closely related to the NCC activity in the DCT because deletion of Kir4.1 reduced NCC expression and function [22]. In contrast, the basolateral K + channel activity in the DCT in NCC null mice (Wang’s unpublished observation) or in Wnk4 PHAII transgenic mice with high NCC was similar to WT mice[43]. Here, we show that Kir4.1 activity plays a key role in mediating the effect of dietary K + intake on NCC activity, because the effect of dietary K + intake on NCC activity is completely absent in KS-Kir4.1 KO mice.…”