Potassium modulates electrolyte balance and blood pressure through effects on distal cell voltage and chloride

Abstract

Dietary potassium deficiency, common in modern diets, raises blood pressure and enhances salt sensitivity. Potassium homeostasis requires a molecular switch in the distal convoluted tubule (DCT), which fails in familial hyperkalemic hypertension (pseudohypoaldosteronism type 2), activating the thiazide-sensitive NaCl cotransporter, NCC. Here, we show that dietary potassium deficiency activates NCC, even in the setting of high salt intake, thereby causing sodium retention and a rise in blood pressure. The effect is dependent on plasma potassium, which modulates DCT cell membrane voltage and, in turn, intracellular chloride. Low intracellular chloride stimulates WNK kinases to activate NCC, limiting potassium losses, even at the expense of increased blood pressure. These data show that DCT cells, like adrenal cells, sense potassium via membrane voltage. In the DCT, hyperpolarization activates NCC via WNK kinases, whereas in the adrenal gland, it inhibits aldosterone secretion. These effects work in concert to maintain potassium homeostasis.

Figure 1. Effects of Dietary Potassium Intake on NCC Abundance

(A) Western blot of kidney from mice consuming HS/NK and HS/LK diets. The HS/LK diet increased the abundance of total NCC, pNCC-T53, and pNCC-T58 significantly (p < 0.01 for each by unpaired t test). NKCC2 was unaffected. Actin is a loading control. (B) Plasma aldosterone and angiotensin II concentrations from the two groups. *p < 0.05 by unpaired t test. (C) Western blot of kidney from AT_1a^−/− mice treated as in (A). p < 0.01 for pNCC-T53 by unpaired t test. (D) Western blot of kidney from mice consuming NS/LK and HS/LK diets. Dietary salt loading increased the abundance of pNCC-T53 on a LK diet. p < 0.05 by unpaired t test. (E) Western blot for pNCC-T53 of urinary exosomes from mice consuming either HS/LK or HS/NK diets. The HS/LK diet increased pNCC-T53. p < 0.05 by unpaired t test. (F) Western blot for pNCC-T53 of urinary exosomes from volunteers consuming HS/LK and HS/NK diets. *p < 0.05 by paired t test. (G) Dietary Na⁺, K⁺, and calorie content of volunteers consuming HS/LK and HS/NK diets. Bars represent mean ± SEM. See Table S1 for densitometry.

Figure 2. Effects of Dietary Potassium Intake on Urine Sodium and Blood Pressure

(A) Urine sodium excretion (24 hr) from mice on HS/NK and HS/LK diets. *p < 0.05 by paired t test. (B) Mean arterial pressure (MAP) recording (24 hr) from mice on a HS/LK diet. Trace represents 2 hr averages from three animals. Mice were maintained on a HS/NK diet for 1 week until time 0, when they were switched to HS/LK. There was a statistically significant increase in MAP during the active period between day 0 and day 6 (Inset shows mean ± SEM). *p < 0.05 by paired t test. (C) Systolic blood pressure measurements for wild-type and Slc12a3^−/− mice on HS/NK and HS/LK diets. **p < 0.05 by unpaired t test for the difference between the group differences. (D) Urine calcium excretion (24 hr) from wild-type and Slc12a3^−/− mice on HS/NK and HS/LK diets. **p < 0.05 by unpaired t test for the difference between the group differences.

Figure 3. Plasma Potassium Signals via WNK/SPAK Independent of Diet

(A) Plasma potassium levels from mice treated with vehicle (Veh), amiloride (Amil), and amiloride combined with LKdiet. *p<0.01 by unpaired t test corrected for multiple comparisons. (B) Hematocrit of mice treated with vehicle, amiloride, and amiloride combined with LK diet. *p < 0.01 by unpaired t test corrected for multiple comparisons. (C) Western blot of kidney from mice treated with vehicle, amiloride, and amiloride combined with LK diet. p < 0.01 for each by unpaired t test (Veh versus Amil) corrected for multiple comparisons. (D) Western blot of kidney from mice consuming HS/NK and HS/LK diets. p < 0.02 by unpaired t test for SPAK and WNK4. Actin loading control is same as Figure 1A. (E) Western blot of kidney from mice fed NS/NK and NS/LK diets. WNK4 abundance increased on NS/LK diet. p < 0.05 by unpaired t test. (F) WNK4 and pSPAK/pOxSR1 immunofluorescence of DCT sections (dotted lines) from mice fed HS/NK and HS/LK diets. (G) pWNK immunofluorescence of DCT sections (dotted lines) from mice fed HS/NK and HS/LK diets. (H) SPAK and OxSR1 immunofluorescence of DCT (dotted lines) from mice fed HS/NK and HS/LK diets. (I) Western blot of pNCC-T53 abundance from SPAK^−/− mice fed HS/NK and HS/LK diets. p < 0.01 by unpaired t test. (J) Western blot of pNCC-T53 abundance from KS OxSR1^−/− mice fed HS/NK and HS/LK diets. p < 0.01 by unpaired t test. (K) Western blot of pNCC-T53 abundance from SPAK^−/− and SPAK^−/−/KS OxSR1^−/− mice fed HS/NK and HS/LK diets. pNCC-T53 abundance increased on HS/LK in both genotypes (p < 0.01 by unpaired t test corrected for multiple comparisons) but increased less in SPAK^−/−/KS OxSR1^−/− mice. Two-way ANOVA indicated p < 0.01 for diet, genotype, and interaction. Total NCC abundance increased in SPAK^−/− mice (p < 0.01 by unpaired t test corrected for multiple comparisons), but not in SPAK^−/−/KS OxSR1^−/− mice. Two-way ANOVA indicated p < 0.01 for diet and interaction. Bars represent mean ± SEM. See Table S2 for densitometry.

Figure 4. Effects of Extracellular Potassium on NCC in HEK Cells

(A) Western blot of HEK cells cultured in NK or LK medium. LK increased pNCC-T53 and pNCC-T58 abundance (p < 0.01 for both by unpaired t test). (B) Western blot of HEK cells cultured in NK or LK medium. LK increased pSPAK/pOxSR1 abundance (p < 0.05 by unpaired t test). (C) Western blot of HEK cells cultured in NK or LK medium following WNK1 siRNA knockdown or treatment with a negative control (scramble) siRNA. WNK1 abundance was significantly decreased following knockdown (p <0.05 by unpaired t test). Cells inwhich WNK1 was knocked down exhibited a reduced increase in pNCC-T53 and pSPAK/pOxSR1 following culture in LK medium compared with cells that were transfected with a scramble siRNA. p <0.05 for interaction of K⁺ treatment and WNK1 abundance by two-way ANOVA. (D) Western blot of HEK cells cultured in NK or LK medium following WNK3 siRNA knockdown or treatment with a negative control (scramble) siRNA. WNK3 abundance was significantly decreased (p < 0.05 by unpaired t test). Cells in which WNK1 was knocked down exhibited a reduced increase in pNCC-T53 and pSPAK/pOxSR1 following culture in LK medium compared with cells that were transfected with a scramble siRNA. p < 0.05 for interaction of K⁺ treatment and WNK3 abundance by two-way ANOVA. (E) Western blot of HEK cells cultured in NK, LK, or LK medium with Rb⁺ added back. LK increased pNCC-T53 and pSPAK/pOxSR1 as before (p < 0.01 by unpaired t test corrected for multiple comparisons), whereas adding back Rb⁺ prevented these effects. (F) Western blot of HEK cells cultured in mannitol (30 mM) or BaCl₂ (10 mM). Treatment with BaCl₂ decreased pNCC-T53 and pSPAK/pOxSR1 (p < 0.01 by unpaired t test). See Table S3 for densitometry. Representative images are shown.

Figure 5. Effects of Kir4.1 Mutant Channels on Whole-Cell Reversal Potential and NCC

(A) Reversal potential of HEK cells expressing WT Kir4.1 or mutant Kir4.1 channels. All mutants depolarized cells compared with WT Kir4.1. *p < 0.01 by unpaired t test corrected for multiple comparisons. (B) Western blot of HEK cells expressing WT or EAST syndrome mutant Kir4.1 channels. EAST syndrome mutants decreased pNCC-T53 compared with WT Kir4.1. p < 0.01 by unpaired t test corrected for multiple comparisons. The original image used for the first two panels is shown in Figure S4D. (C) Western blot of HEK cells expressing WT or selectivity filter mutant Kir4.1 G130R. Expression of the mutant decreased pNCC-T53 compared with WT Kir4.1. p < 0.05 by unpaired t test. Bars represent SEM. See Table S4 for densitometry. Representative images are shown.

Figure 6. Role of Intracellular Chloride in Mediating K⁺ Effects on NCC

(A) Intracellular [Cl⁻] in HEK cells in NK or LK medium. *p < 0.05 by unpaired t test. (B) Relative intracellular [Cr] in HEK cells expressing WT Kir4.1 or Kir4.1 mutants. *p < 0.01 by unpaired t test corrected for multiple comparisons. (C) Western blot of HEK cells cultured in normal [Cl⁻] with mannitol or high [Cl⁻] and also treated with NK or LK medium. p< 0.05 for the interaction between K⁺ treatment and Cl⁻ treatment by two-way ANOVA. (D) Western blot of HEK cells treated with vehicle (DMSO) or DIDS and also treated with NK or LK medium. p < 0.05 for the interaction between K⁺ treatment and drug treatment by two-way ANOVA. (E) Western blot of HEK cells transfected with WTCLC-K2 or Bartter syndrome type III mutant CLC-K2 P124L channels and then cultured in NK or LK medium. All cells were also transfected with NCC and barttin. p < 0.05 for the interaction between K⁺ treatment and CLC-K2 genotype by two-way ANOVA. (F) Western blot of HEK cells transfected with WT CLC-K2 or Bartter syndrome type III mutant CLC-K2 H357Q channels and then cultured in NK or LK medium. All cells were also transfected with NCC and barttin. p < 0.05 for the interaction between K⁺ treatment and CLC-K2 genotype by two-way ANOVA. (G) Western blot of HEK cells transfected with WT or WNK1 L369FL371F(WNK1 Cl⁻ mutant) and then cultured in LK or NK medium. Cells expressing WTWNK1, but not WNK1 Cl⁻ mutant, exhibited substantially lower pNCC-T53 abundance when cultured in NK compared with LK medium. p < 0.05 for the interaction between K⁺ treatment and WNK1 genotype by two-way ANOVA. WNK1 was detected with anti-myc antibody. See Figure S6C for the original image. (H) Western blot of HEK cells transfected with WTWNK1 cultured in NK or LK medium and WNK1 L369F L371F in NK medium. pWNK1 abundance increased in both the WT WNK1 LK group and the WNK1 L369F L371F group. p < 0.05 for unpaired t test corrected for multiple comparisons. Total WNK1 abundance obtained before adding non-phospho-WNK1 peptide to block antibodies recognizing unphosphorylated WNK1. pWNK1 S382 abundance was obtained after addition of the non-phospho-WNK1 peptide. Bars represent mean ± SEM. See Table S5 for densitometry.

Figure 7. Effects of Cl⁻ Sensitivity on Modeled DCT Function

(A–C) Membrane voltage (A), intracellular chloride concentration (B), and transepithelial chloride flux (J_Cl) (C) as functions of peritubular [K⁺] concentration. Circled are values at [K⁺]_P, as shown in the cartoon. (D) The results of peritubular [K+] concentration on sodium delivery to the CNT. The black lines are taken from Weinstein (2005). The red lines show results modified by the inclusion of NCC sensitivity to intracellular [Cl⁻]. (E) Cartoon showing the effects of plasma [K⁺] on cells from the adrenal zona glomerulosa and renal distal convoluted tubule. Directionally similar changes in membrane voltage elicit opposite effects to activate or inhibit cell activity. Note that, for simplicity, effects of angiotensin II have been omitted. In this case, angiotensin II may have similar effects to stimulate both cell populations.