Blockade of AT2R causes hyperkalemia by stimulating the basolateral Kir4.1 and apical NCC in the distal convoluted tubule (DCT)

Abstract

Angiotensin type 2 receptor (AT2R) is expressed in the aldosterone‐sensitive distal nephron (ASDN) and its expression is up‐regulated during increasing dietary K intake, suggesting a possible role of AT2R in stimulating K secretion in ASDN. Recent studies have indicated that distal convoluted tubule (DCT) plays an important role in the regulation of K homeostasis by controlling Na and volume delivery to the late ASDN through the regulation of Na‐Cl cotransporter (NCC). The aim of the present study is to explore the role of AT2R in the regulation of DCT function using electrophysiology, protein chemistry and renal clearance in Kcnj10+/+ (WT) and kidney‐specific Kcnj10−/− (KS‐Kir4.1KO) mice. We inhibited AT2R with continuous infusion of PD123319 through a mini‐osmotic pump for up to 4 days. PD123319 infusion for 4 days significantly increased the plasma potassium concentration from 4.4 ± 0.10 mM to 5.18 ± 0.33 mM. Also, the inhibition of AT2R with PD123319 significantly increased the expression of NCC and p‐NCC and augmented hydrochlorothiazide (HCTZ)‐induced natriuretic effect in comparison to vehicle‐treated WT mice. In contrast, PD123319 perfusion had no effect on the expression of Na‐K‐Cl cotransporter (NKCC2) and epithelial sodium channel (ENaC). These results strongly suggest that the inhibition of AT2R stimulates NCC function and that high NCC activity may be responsible for causing hyperkalemia in PD123319‐treated mice. We have previously demonstrated that the basolateral K conductance in the DCT determines the NCC activity such that high K conductance stimulates while low K conductance inhibits NCC. Thus, we next examined whether the stimulation of basolateral Kir4.1 in the DCT is responsible for increasing NCC activity in the WT mice receiving PD123319. Single channel recording showed that PD123319 perfusion for 4 days increased the channel open probability and the probability of finding the 40 pS K channel, a Kir4.1/Kir5.1 heterotetramer which is the only type of K channel in the basolateral membrane of the DCT. Whole‐cell recording also demonstrated that PD123319 infusion increased Ba2+‐sensitive K currents in the DCT of WT mice and the negativity of K reversal potential (an indication of membrane hyperpolarization). To confirm the role of Kir4.1 in mediating the effect of AT2R inhibition on NCC, we examined the expression of NCC in KS‐Kir4.1 KO mice treated with PD123319 for 4 days. The depletion of Kir4.1 completely abolished AT2R inhibition‐induced stimulation of NCC expression. Moreover, HCTZ‐induced natriuretic effect was absent in PD123319 treated or untreated KS‐Kir4.1 KO mice. These results strongly suggest that AT2R inhibition‐induced stimulation of NCC function is the results of the activation of the basolateral Kir4.1 channels in the DCT. In summary, inhibition of AT2R increases the basolateral Kir4.1 channel activity and hyperpolarizes the cell membrane in the DCT thereby stimulating the apical NCC activity. We conclude that AT2R in the DCT should play a role in stimulating K secretion by inhibiting the basolateral Kir4.1 and apical NCC in the DCT.Support or Funding InformationThe work is supported by National Natural Science Foundation of China Grant 31400993.