Potassium-Switch Signaling Pathway Dictates Acute Blood Pressure Response to Dietary Potassium.

BACKGROUND: Potassium (K+)-deficient diets, typical of modern processed foods, increase blood pressure (BP) and NaCl sensitivity. A K+-dependent signaling pathway in the kidney distal convoluted tubule, coined the K+ switch, that couples extracellular K+ sensing to activation of the thiazide-sensitive NaCl cotransporter (NCC) and NaCl retention has been implicated, but causality has not been established. METHODS: To test the hypothesis that small, physiological changes in plasma K+ (PK+) are translated to BP through the switch pathway, a genetic approach was used to activate the downstream switch kinase, SPAK (SPS1-related proline/alanine-rich kinase), within the distal convoluted tubule. The CA-SPAK (constitutively active SPS1-related proline/alanine-rich kinase mice) were compared with control mice over a 4-day PK+ titration (3.8-5.1 mmol) induced by changes in dietary K+. Arterial BP was monitored using radiotelemetry, and renal function measurements, NCC abundance, phosphorylation, and activity were made. RESULTS: As PK+ decreased in control mice, BP progressively increased and became sensitive to dietary NaCl and hydrochlorothiazide, coincident with increased NCC phosphorylation and urinary sodium retention. By contrast, BP in CA-SPAK mice was elevated, resistant to the PK+ titration, and sensitive to hydrochlorothiazide and salt at all PK+ levels, concomitant with sustained and elevated urinary sodium retention and NCC phosphorylation and activity. Thus, genetically locking the switch on drives NaCl sensitivity and prevents the response of BP to potassium. CONCLUSIONS: Low K+, common in modern ultraprocessed diets, presses the K+-switch pathway to turn on NCC activity, increasing sodium retention, BP, and salt sensitivity.

Dietary Potassium Supplementation Reduces Chronic Kidney Lesions Independent of Blood Pressure in Deoxycorticosterone-Acetate and High Sodium Chloride-Treated Mice.

We have previously shown that an excess of deoxycorticosterone acetate and high sodium chloride intake (DOCA/salt) in one-renin gene mice induces a high urinary Na/K ratio, hypokalemia, and cardiac and renal hypertrophy in the absence of hypertension. Dietary potassium supplementation prevents DOCA/salt-induced pathological processes. In the present study, we further study whether DOCA/salt-treated mice progressively develop chronic inflammation and fibrosis in the kidney and whether dietary potassium supplementation can reduce the DOCA/salt-induced renal pathological process. Results showed that (1) long-term DOCA/salt-treated one-renin gene mice developed severe kidney injuries including tubular/vascular hypertrophy, mesangial/interstitial/perivascular fibrosis, inflammation (lymphocyte's immigration), proteinuria, and high serum creatinine in the absence of hypertension; (2) there were over-expressed mRNAs of plasminogen activator inhibitor-1 (PAI-1), fibronectin, collagen type I and III, interferon-inducible protein-10 (IP-10), monocyte chemotactic protein-1 (MCP1), transforming growth factor-ß (TGF-ß), tumor necrosis factor-alpha (TNF-α), osteopontin, Nuclear factor kappa B (NF-κB)/P65, and intercellular adhesion molecule (ICAM)-1; and (3) dietary potassium supplementation normalized urinary Na/K ratio, hypokalemia, proteinuria, and serum creatinine, reduced renal hypertrophy, inflammations, and fibrosis, and down-regulated mRNA expression of fibronectin, Col-I and III, TGF-ß, TNF-α, osteopontin, and ICAM without changes in the blood pressure. The results provide new evidence that potassium and sodium may modulate proinflammatory and fibrotic genes, leading to chronic renal lesions independent of blood pressure.

Dietary potassium stimulates Ppp1Ca-Ppp1r1a dephosphorylation of kidney NaCl cotransporter and reduces blood pressure.

Consumption of low dietary potassium, common with ultraprocessed foods, activates the thiazide-sensitive sodium chloride cotransporter (NCC) via the with no (K) lysine kinase/STE20/SPS1-related proline-alanine-rich protein kinase (WNK/SPAK) pathway to induce salt retention and elevate blood pressure (BP). However, it remains unclear how high-potassium "DASH-like" diets (dietary approaches to stop hypertension) inactivate the cotransporter and whether this decreases BP. A transcriptomics screen identified Ppp1Ca, encoding PP1A, as a potassium-upregulated gene, and its negative regulator Ppp1r1a, as a potassium-suppressed gene in the kidney. PP1A directly binds to and dephosphorylates NCC when extracellular potassium is elevated. Using mice genetically engineered to constitutively activate the NCC-regulatory kinase SPAK and thereby eliminate the effects of the WNK/SPAK kinase cascade, we confirmed that PP1A dephosphorylated NCC directly in a potassium-regulated manner. Prior adaptation to a high-potassium diet was required to maximally dephosphorylate NCC and lower BP in constitutively active SPAK mice, and this was associated with potassium-dependent suppression of Ppp1r1a and dephosphorylation of its cognate protein, inhibitory subunit 1 (I1). In conclusion, potassium-dependent activation of PP1A and inhibition of I1 drove NCC dephosphorylation, providing a mechanism to explain how high dietary K+ lowers BP. Shifting signaling of PP1A in favor of activation of WNK/SPAK may provide an improved therapeutic approach for treating salt-sensitive hypertension.

Dietary anions control potassium excretion: it is more than a poorly absorbable anion effect.

The urinary potassium (K+) excretion machinery is upregulated with increasing dietary K+, but the role of accompanying dietary anions remains inadequately characterized. Poorly absorbable anions, including [Formula: see text], are thought to increase K+ secretion through a transepithelial voltage effect. Here, we tested if they also influence the K+ secretion machinery. Wild-type mice, aldosterone synthase (AS) knockout (KO) mice, or pendrin KO mice were randomized to control, high-KCl, or high-KHCO3 diets. The K+ secretory capacity was assessed in balance experiments. Protein abundance, modification, and localization of K+-secretory transporters were evaluated by Western blot analysis and confocal microscopy. Feeding the high-KHCO3 diet increased urinary K+ excretion and the transtubular K+ gradient significantly more than the high-KCl diet, coincident with more pronounced upregulation of epithelial Na+ channels (ENaC) and renal outer medullary K+ (ROMK) channels and apical localization in the distal nephron. Experiments in AS KO mice revealed that the enhanced effects of [Formula: see text] were aldosterone independent. The high-KHCO3 diet also uniquely increased the large-conductance Ca2+-activated K+ (BK) channel ß4-subunit, stabilizing BKα on the apical membrane, the Cl-/[Formula: see text] exchanger, pendrin, and the apical KCl cotransporter (KCC3a), all of which are expressed specifically in pendrin-positive intercalated cells. Experiments in pendrin KO mice revealed that pendrin was required to increase K+ excretion with the high-KHCO3 diet. In summary, [Formula: see text] stimulates K+ excretion beyond a poorly absorbable anion effect, upregulating ENaC and ROMK in principal cells and BK, pendrin, and KCC3a in pendrin-positive intercalated cells. The adaptive mechanism prevents hyperkalemia and alkalosis with the consumption of alkaline ash-rich diets but may drive K+ wasting and hypokalemia in alkalosis.NEW & NOTEWORTHY Dietary anions profoundly impact K+ homeostasis. Here, we found that a K+-rich diet, containing [Formula: see text] as the counteranion, enhances the electrogenic K+ excretory machinery, epithelial Na+ channels, and renal outer medullary K+ channels, much more than a high-KCl diet. It also uniquely induces KCC3a and pendrin, in B-intercalated cells, providing an electroneutral KHCO3 secretion pathway. These findings reveal new K+ balance mechanisms that drive adaption to alkaline and K+-rich foods, which should guide new treatment strategies for K+ disorders.

TRPV4 functional status in cystic cells regulates cystogenesis in autosomal recessive polycystic kidney disease during variations in dietary potassium.

Mechanosensitive TRPV4 channel plays a dominant role in maintaining [Ca2+ ]i homeostasis and flow-sensitive [Ca2+ ]i signaling in the renal tubule. Polycystic kidney disease (PKD) manifests as progressive cyst growth due to cAMP-dependent fluid secretion along with deficient mechanosensitivity and impaired TRPV4 activity. Here, we tested how regulation of renal TRPV4 function by dietary K+ intake modulates the rate of cystogenesis and mechanosensitive [Ca2+ ]i signaling in cystic cells of PCK453 rats, a homologous model of human autosomal recessive PKD (ARPKD). One month treatment with both high KCl (5% K+ ) and KB/C (5% K+ with bicarbonate/citrate) diets significantly increased TRPV4 levels when compared to control (0.9% K+ ). High KCl diet caused an increased TRPV4-dependent Ca2+ influx, and partial restoration of mechanosensitivity in freshly isolated monolayers of cystic cells. Unexpectedly, high KB/C diet induced an opposite effect by reducing TRPV4 activity and worsening [Ca2+ ]i homeostasis. Importantly, high KCl diet decreased cAMP, whereas high KB/C diet further increased cAMP levels in cystic cells (assessed as AQP2 distribution). At the systemic level, high KCl diet fed PCK453 rats had significantly lower kidney-to-bodyweight ratio and reduced cystic area. These beneficial effects were negated by a concomitant administration of an orally active TRPV4 antagonist, GSK2193874, resulting in greater kidney weight, accelerated cystogenesis, and augmented renal injury. High KB/C diet also exacerbated renal manifestations of ARPKD, consistent with deficient TRPV4 activity in cystic cells. Overall, we demonstrate that TRPV4 channel activity negatively regulates cAMP levels in cystic cells thus attenuating (high activity) or accelerating (low activity) ARPKD progression.

Pendrin regulation is prioritized by anion in high-potassium diets.

The Cl-/[Formula: see text] exchanger pendrin in the kidney maintains acid-base balance and intravascular volume. Pendrin is upregulated in models associated with high circulating aldosterone concentration, such as dietary NaCl restriction or an aldosterone infusion. However, it has not been established if pendrin is similarly regulated by aldosterone with a high-K+ diet because the effects of accompanying anions have not been considered. Here, we explored how pendrin is modulated by different dietary potassium salts. Wild-type (WT) and aldosterone synthase (AS) knockout (KO) mice were randomized to control, high-KHCO3, or high-KCl diets. Dietary KCl and KHCO3 loading increased aldosterone in WT mice to the same extent but had opposite effects on pendrin abundance. KHCO3 loading increased pendrin protein and transcript abundance. Conversely, high-KCl diet feeding caused pendrin to decrease within 8 h of switching from the high-KHCO3 diet, coincident with an increase in plasma Cl- and a decrease in [Formula: see text]. In contrast, switching the high-KCl diet to the high-KHCO3 diet caused pendrin to increase in WT mice. Experiments in AS KO mice revealed that aldosterone is necessary to optimally upregulate pendrin protein in response to the high-KHCO3 diet but not to increase pendrin mRNA. We conclude that pendrin is differentially regulated by different dietary potassium salts and that its regulation is prioritized by the dietary anion, providing a mechanism to prevent metabolic alkalosis with high-K+ base diets and safeguard against hyperchloremic acidosis with consumption of high-KCl diets.NEW & NOTEWORTHY Regulation of the Cl-/[Formula: see text] exchanger pendrin has been suggested to explain the aldosterone paradox. A high-K+ diet has been proposed to downregulate a pendrin-mediated K+-sparing NaCl reabsorption pathway to maximize urinary K+ excretion. Here, we challenged the hypothesis, revealing that the accompanying anion, not K+, drives pendrin expression. Pendrin is downregulated with a high-KCl diet, preventing acidosis, and upregulated with an alkaline-rich high-K+ diet, preventing metabolic alkalosis. Pendrin regulation is prioritized for acid-base balance.

ROMK channels are inhibited in the aldosterone-sensitive distal nephron of renal tubule Nedd4-2-deficient mice.

We used whole cell recording to examine the renal outer medullary K+ channel (ROMK or Kir1.1) and epithelial Na+ channel (ENaC) in the late distal convoluted tubule (DCT2)/initial connecting tubule (iCNT) and in the cortical collecting duct (CCD) of kidney tubule-specific neural precursor cell-expressed developmentally downregulated protein 4-2 (Nedd4-2) knockout mice (Ks-Nedd4-2 KO) and floxed neural precursor cell-expressed developmentally downregulated 4-like (Nedd4l) mice (control). Tertiapin Q (TPNQ)-sensitive K+ currents (ROMK) were smaller in both the DCT2/iCNT and CCD of Ks-Nedd4-2 KO mice on a normal diet than in control mice. Neither high dietary salt intake nor low dietary salt intake had a significant effect on ROMK activity in the DCT2/iCNT and CCD of control and Ks-Nedd4-2 KO mice. In contrast, high dietary K+ intake (HK) increased, whereas low dietary K+ intake (LK) decreased TPNQ-sensitive K+ currents in floxed Nedd4l mice. However, the effects of dietary K+ intake on ROMK channel activity were absent in Ks-Nedd4-2 KO mice since neither HK nor LK significantly affected TPNQ-sensitive K+ currents in the DCT2/iCNT and CCD. Moreover, TPNQ-sensitive K+ currents in the DCT2/iCNT and CCD of Ks-Nedd4-2 KO mice on HK were similar to those of control mice on LK. Amiloride-sensitive Na+ currents in the DCT2/iCNT and CCD were significantly higher in Ks-Nedd4-2 KO mice than in floxed Nedd4l mice on a normal K+ diet. HK increased ENaC activity of the DCT2/iCNT only in control mice, but HK stimulated ENaC of the CCD in both control and Ks-Nedd4-2 KO mice. Moreover, the HK-induced increase in amiloride-sensitive Na+ currents was larger in Ks-Nedd4-2 KO mice than in control mice. Deletion of Nedd4-2 increased with no lysine kinase 1 expression and abolished HK-induced inhibition of with no lysine kinase 1. We conclude that deletion of Nedd4-2 increases ENaC activity but decreases ROMK activity in the aldosterone-sensitive distal nephron and that HK fails to stimulate ROMK, but robustly increases ENaC activity in the CCD of Nedd4-2-deficient mice.NEW & NOTEWORTHY We demonstrate that renal outer medullary K+ (ROMK) channel activity is inhibited in the late distal convoluted tubule/initial connecting tubule and cortical collecting duct of neural precursor cell-expressed developmentally downregulated protein 4-2 (Nedd4-2)-deficient mice. Also, deletion of Nedd4-2 abolishes the stimulatory effect of dietary K+ intake on ROMK. The lack of high K+-induced stimulation of ROMK is associated with the absence of high K+-induced inhibition of with no lysine kinase 1.

High baseline ROMK activity in the mouse late distal convoluted and early connecting tubule probably contributes to aldosterone-independent K+ secretion.

The renal outer medullary K+ channel (ROMK) is colocalized with the epithelial Na+ channel (ENaC) in the late distal convoluted tubule (DCT2), connecting tubule (CNT), and cortical collecting duct (CCD). ENaC-mediated Na+ absorption generates the electrical driving force for ROMK-mediated tubular K+ secretion, which is critically important for maintaining renal K+ homeostasis. ENaC activity is aldosterone dependent in the late CNT and early CCD (CNT/CCD) but aldosterone independent in the DCT2 and early CNT (DCT2/CNT). This suggests that under baseline conditions with low plasma aldosterone, ROMK-mediated K+ secretion mainly occurs in the DCT2/CNT. Therefore, we hypothesized that baseline ROMK activity is higher in the DCT2/CNT than in the CNT/CCD. To test this hypothesis, patch-clamp experiments were performed in the DCT2/CNT and CNT/CCD microdissected from mice maintained on a standard diet. In single-channel recordings from outside-out patches, we detected typical ROMK channel activity in both the DCT2/CNT and CNT/CCD and confirmed that ROMK is the predominant K+ channel in the apical membrane. Amiloride-sensitive and tertiapin-sensitive whole-cell currents were determined to assess ENaC and ROMK activity, respectively. As expected, baseline amiloride-sensitive current was high in the DCT2/CNT (∼370 pA) but low in the CNT/CCD (∼60 pA). Importantly, tertiapin-sensitive current was significantly higher in the DCT2/CNT than in the CNT/CCD (∼810 vs. ∼350 pA). We conclude that high ROMK activity in the DCT2/CNT is critical for aldosterone-independent renal K+ secretion under baseline conditions. A low-K+ diet significantly reduced ENaC but not ROMK activity in the DCT2/CNT. This suggests that modifying ENaC activity in the DCT2/CNT plays a key regulatory role in adjusting renal K+ excretion to dietary K+ intake.NEW & NOTEWORTHY ROMK-mediated renal K+ secretion is essential for maintaining K+ balance and requires a lumen negative transepithelial potential critically dependent on ENaC activity. Using microdissected distal mouse tubules, we demonstrated that baseline apical ROMK activity is high in the DCT2/CNT. Aldosterone-independent baseline ENaC activity is also high in the DCT2/CNT and downregulated by a low-K+ diet, which highlights the important role of the DCT2/CNT in regulating K+ secretion in an aldosterone-independent manner.

Rapid development of vasopressin resistance in dietary K+ deficiency.

The association between diabetes insipidus (DI) and chronic dietary K+ deprivation is well known, but it remains uncertain how the disorder develops and whether it is influenced by the sexual dimorphism in K+ handling. Here, we determined the plasma K+ (PK) threshold for DI in male and female mice and ascertained if DI is initiated by polydipsia or by a central or nephrogenic defect. C57BL6J mice were randomized to a control diet or to graded reductions in dietary K+ for 8 days, and kidney function and transporters involved in water balance were characterized. We found that male and female mice develop polyuria and secondary polydipsia. Altered water balance coincided with a decrease in aquaporin-2 (AQP2) phosphorylation and apical localization despite increased levels of the vasopressin surrogate marker copeptin. No change in the protein abundance of urea transporter-A1 was observed. The Na+-K+-2Cl- cotransporter decreased only in males. Desmopressin treatment failed to reverse water diuresis in K+-restricted mice. These findings indicate that even a small fall in PK is associated with nephrogenic DI (NDI), coincident with the development of altered AQP2 regulation, implicating low PK as a causal trigger of NDI. We found that PK decreased more in females, and, consequently, females were more prone to develop NDI. Together, these data indicate that AQP2 regulation is disrupted by a small decrease in PK and that the response is influenced by sexual dimorphism in K+ handling. These findings provide new insights into the mechanisms linking water and K+ balances and support defining the disorder as "potassium-dependent NDI."NEW & NOTEWORTHY This study shows that aquaporin-2 regulation is disrupted by a small fall in plasma potassium levels and the response is influenced by sexual dimorphism in renal potassium handling. The findings provided new insights into the mechanisms by which water balance is altered in dietary potassium deficiency and support defining the disorder as "potassium-dependent nephrogenic diabetes insipidus."

Role of KLHL3 and dietary K+ in regulating KS-WNK1 expression.

The physiological role of the shorter isoform of with no lysine kinase (WNK)1 that is exclusively expressed in the kidney (KS-WNK1), with particular abundance in the distal convoluted tubule, remains elusive. KS-WNK1, despite lacking the kinase domain, is nevertheless capable of stimulating the NaCl cotransporter, apparently through activation of WNK4. It has recently been shown that a less severe form of familial hyperkalemic hypertension featuring only hyperkalemia is caused by missense mutations in the WNK1 acidic domain that preferentially affect cullin 3 (CUL3)-Kelch-like protein 3 (KLHL3) E3-induced degradation of KS-WNK1 rather than that of full-length WNK1. Here, we show that full-length WNK1 is indeed less impacted by the CUL3-KLHL3 E3 ligase complex compared with KS-WNK1. We demonstrated that the unique 30-amino acid NH2-terminal fragment of KS-WNK1 is essential for its activating effect on the NaCl cotransporter and recognition by KLHL3. We identified specific amino acid residues in this region critical for the functional effect of KS-WNK1 and KLHL3 sensitivity. To further explore this, we generated KLHL3-R528H knockin mice that mimic human mutations causing familial hyperkalemic hypertension. These mice revealed that the KLHL3 mutation specifically increased expression of KS-WNK1 in the kidney. We also observed that in wild-type mice, the expression of KS-WNK1 was only detectable after exposure to a low-K+ diet. These findings provide new insights into the regulation and function of KS-WNK1 by the CUL3-KLHL3 complex in the distal convoluted tubule and indicate that this pathway is regulated by dietary K+ levels.NEW & NOTEWORTHY In this work, we demonstrated that the kidney-specific isoform of with no lysine kinase 1 (KS-WNK1) in the kidney is modulated by dietary K+ and activity of the ubiquitin ligase protein Kelch-like protein 3. We analyzed the role of different amino acid residues of KS-WNK1 in its activity against the NaCl cotransporter and sensitivity to Kelch-like protein 3.

Roles of WNK4 and SPAK in K+-mediated dephosphorylation of the NaCl cotransporter.

Phosphorylation of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule (DCT) is altered rapidly in response to changes in extracellular K+ concentration ([K+]). High extracellular [K+] is believed to activate specific phosphatases to dephosphorylate NCC, thereby reducing its activity. This process is defective in the human disease familial hyperkalemic hypertension, in which extracellular [K+] fails to dephosphorylate NCC, suggesting an interplay between NCC-activating and NCC-inactivating switches. Here, we explored the role of STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and intracellular Cl- concentration in the rapid effects of extracellular K+ on NCC phosphorylation. SPAK was found to be rapidly dephosphorylated in vitro in human embryonic kidney cells and ex vivo in kidney slices by high [K+]. Acute high-K+ challenge resulted in DCT1-specific SPAK dephosphorylation in vivo and dissolution of SPAK puncta. In line with the postulate of interplay between activating and inactivating switches, we found that the "on" switch, represented by with no lysine kinase 4 (WNK4)-SPAK, must be turned off for rapid NCC dephosphorylation by high [K+]. Longer-term WNK-SPAK-mediated stimulation, however, altered the sensitivity of the system, as it attenuated rapid NCC dephosphorylation due to acute K+ loading. Although blockade of protein phosphatase (PP)1 increased NCC phosphorylation at baseline, neither PP1 nor PP3, singly or in combination, was essential for NCC dephosphorylation. Overall, our data suggest that NCC phosphorylation is regulated by a dynamic equilibrium between activating kinases and inactivating phosphatases, with kinase inactivation playing a key role in the rapid NCC dephosphorylation by high extracellular K+.NEW & NOTEWORTHY Although a great deal is known about mechanisms by which thiazide-sensitive NaCl cotransporter is phosphorylated and activated, much less is known about dephosphorylation. Here, we show that rapid dephosphorylation by high K+ depends on the Cl- sensitivity of with no lysine kinase 4 and the rapid dephosphorylation of STE20/SPS1-related proline-alanine-rich protein kinase, primarily along the early distal convoluted tubule.

Dietary Potassium Intake and 20-Year All-Cause Mortality in Older Adults: The Rancho Bernardo Study.

We examined the association between dietary potassium intake and all-cause and cause-specific mortality among community-dwelling older adults. Potassium intake was assessed with a food frequency questionnaire administered to 1,363 older adults (mean age 71.0 ± 10.6 years). Cox proportional hazard regressions estimated hazard ratios for sex-specific quintiles of calorie-adjusted potassium in relation to all-cause and cause-specific (cardiovascular disease, CVD, and stroke) mortality, adjusting for numerous covariates. There were 855 deaths (63% mortality) during the 20-year follow-up. Relative to the third quintile, potassium intake in the lowest quintile only was associated with increased risk of all-cause mortality (fully-adjusted hazard ratio 1.33; 95% CI 1.06, 1.67). Potassium intake was not significantly associated with CVD or stroke mortality. These results suggest that low potassium intake is associated with increased risk of mortality independent of overall health status. Ensuring adequate potassium in the diet may be an important strategy for reducing risk of earlier mortality among older adults.

MST3 Involvement in Na+ and K+ Homeostasis with Increasing Dietary Potassium Intake.

K+ loading inhibits NKCC2 (Na-K-Cl cotransporter) and NCC (Na-Cl cotransporter) in the early distal tubules, resulting in Na+ delivery to the late distal convoluted tubules (DCTs). In the DCTs, Na+ entry through ENaC (epithelial Na channel) drives K+ secretion through ROMK (renal outer medullary potassium channel). WNK4 (with-no-lysine 4) regulates the NCC/NKCC2 through SAPK (Ste20-related proline-alanine-rich kinase)/OSR1 (oxidative stress responsive). K+ loading increases intracellular Cl-, which binds to the WNK4, thereby inhibiting autophosphorylation and downstream signals. Acute K+ loading-deactivated NCC was not observed in Cl--insensitive WNK4 mice, indicating that WNK4 was involved in K+ loading-inhibited NCC activity. However, chronic K+ loading deactivated NCC in Cl--insensitive WNK4 mice, indicating that other mechanisms may be involved. We previously reported that mammalian Ste20-like protein kinase 3 (MST3/STK24) was expressed mainly in the medullary TAL (thick ascending tubule) and at lower levels in the DCTs. MST3 -/- mice exhibited higher ENaC activity, causing hypernatremia and hypertension. To investigate MST3 function in maintaining Na+/K+ homeostasis in kidneys, mice were fed diets containing various concentrations of Na+ and K+. The 2% KCl diets induced less MST3 expression in MST3 -/- mice than that in wild-type (WT) mice. The MST3 -/- mice had higher WNK4, NKCC2-S130 phosphorylation, and ENaC expression, resulting in lower urinary Na+ and K+ excretion than those of WT mice. Lower urinary Na+ excretion was associated with elevated plasma [Na+] and hypertension. These results suggest that MST3 maintains Na+/K+ homeostasis in response to K+ loading by regulation of WNK4 expression and NKCC2 and ENaC activity.

Potassium deficiency decreases the capacity for urea synthesis and markedly increases ammonia in rats.

Our study provides novel findings of experimental hypokalemia reducing urea cycle functionality and thereby severely increasing plasma ammonia. This is pathophysiologically interesting because plasma ammonia increases during hypokalemia by a hitherto unknown mechanism, which may be particular important in relation to the unexplained link between hypokalemia and hepatic encephalopathy. Potassium deficiency decreases gene expression, protein synthesis, and growth. The urea cycle maintains body nitrogen homeostasis including removal of toxic ammonia. Hyperammonemia is an obligatory trait of liver failure, increasing the risk for hepatic encephalopathy, and hypokalemia is reported to increase ammonia. We aimed to clarify the effects of experimental hypokalemia on the in vivo capacity of the urea cycle, on the genes of the enzymes involved, and on ammonia concentrations. Female Wistar rats were fed a potassium-free diet for 13 days. Half of the rats were then potassium repleted. Both groups were compared with pair- and free-fed controls. The following were measured: in vivo capacity of urea-nitrogen synthesis (CUNS); gene expression (mRNA) of urea cycle enzymes; plasma potassium, sodium, and ammonia; intracellular potassium, sodium, and magnesium in liver, kidney, and muscle tissues; and liver sodium/potassium pumps. Liver histology was assessed. The diet induced hypokalemia of 1.9 ± 0.4 mmol/L. Compared with pair-fed controls, the in vivo CUNS was reduced by 34% (P < 0.01), gene expression of argininosuccinate synthetase 1 (ASS1) was decreased by 33% (P < 0.05), and plasma ammonia concentrations were eightfold elevated (P < 0.001). Kidney and muscle tissue potassium contents were markedly decreased but unchanged in liver tissue. Protein expressions of liver sodium/potassium pumps were unchanged. Repletion of potassium reverted all the changes. Hypokalemia decreased the capacity for urea synthesis via gene effects. The intervention led to marked hyperammonemia, quantitatively explainable by the compromised urea cycle. Our findings motivate clinical studies of patients with liver disease.

Rise in Potassium Deficiency in the US Population Linked to Agriculture Practices and Dietary Potassium Deficits.

This paper, for the first time, provides evidence that current practices that lead to agricultural crop removal of potassium are unsustainable and likely contributed to the decline in dietary potassium intake and rise in hypokalemia prevalence in the US population. Potassium concentrations in beef, pork, turkey, fruit, vegetables, cereal crops, and so forth decreased between 1999 and 2015 based on the examination of potassium values of food items of USDA standard reference. Ratios of potassium input to removal by crops between 1987 and 2014, potassium in topsoil, and crop-available soil potassium in US farms all declined in recent years. Reported reductions in dietary potassium intake correspond to these decreases in the food supply and to increases in hypokalemia prevalence in the US population. Results of this paper provide new understanding on links between potassium management in agricultural practices and potassium intake deficits, which is needed for combating increasing hypokalemia prevalence in the US population.

Effects of extreme potassium stress on blood pressure and renal tubular sodium transport.

We characterized mouse blood pressure and ion transport in the setting of commonly used rodent diets that drive K+ intake to the extremes of deficiency and excess. Male 129S2/Sv mice were fed either K+-deficient, control, high-K+ basic, or high-KCl diets for 10 days. Mice maintained on a K+-deficient diet exhibited no change in blood pressure, whereas K+-loaded mice developed an ~10-mmHg blood pressure increase. Following challenge with NaCl, K+-deficient mice developed a salt-sensitive 8 mmHg increase in blood pressure, whereas blood pressure was unchanged in mice fed high-K+ diets. Notably, 10 days of K+ depletion induced diabetes insipidus and upregulation of phosphorylated NaCl cotransporter, proximal Na+ transporters, and pendrin, likely contributing to the K+-deficient NaCl sensitivity. While the anionic content with high-K+ diets had distinct effects on transporter expression along the nephron, both K+ basic and KCl diets had a similar increase in blood pressure. The blood pressure elevation on high-K+ diets correlated with increased Na+-K+-2Cl- cotransporter and γ-epithelial Na+ channel expression and increased urinary response to furosemide and amiloride. We conclude that the dietary K+ maneuvers used here did not recapitulate the inverse effects of K+ on blood pressure observed in human epidemiological studies. This may be due to the extreme degree of K+ stress, the low-Na+-to-K+ ratio, the duration of treatment, and the development of other coinciding events, such as diabetes insipidus. These factors must be taken into consideration when studying the physiological effects of dietary K+ loading and depletion.

Differences in renal BMAL1 contribution to Na+ homeostasis and blood pressure control in male and female mice.

The renal circadian clock has a major influence on the function of the kidney. Aryl hydrocarbon receptor nuclear translocator-like protein 1 [ARNTL; also known as brain and muscle ARNT-like 1 (BMAL1)] is a core clock protein and transcription factor that regulates the expression of nearly half of all genes. Using male and female kidney-specific cadherin BMAL1 knockout (KS-BMAL1 KO) mice, we examined the role of renal distal segment BMAL1 in blood pressure control and solute handling. We confirmed that this mouse model does not express BMAL1 in thick ascending limb, distal convoluted tubule, and collecting duct cells, which are the final locations for solute and fluid regulation. Male KS-BMAL1 KO mice displayed a substantially lower basal systolic blood pressure compared with littermate control mice, yet their circadian rhythm in pressure remained unchanged [male control mice: 127 ± 0.7 mmHg (n = 4) vs. male KS-BMAL KO mice: 119 ± 2.3 mmHg (n = 5), P < 0.05]. Female mice, however, did not display a genotype difference in basal systolic blood pressure [female control mice: 120 ± 1.6 mmHg (n = 5) vs. female KS-BMAL1 KO mice: 119 ± 1.5 mmHg (n = 7), P = 0.4]. In addition, male KS-BMAL1 KO mice had less Na+ retention compared with control mice in response to a K+-restricted diet (15% less following 5 days of treatment). However, there was no genotype difference in Na+ handling after a K+-restricted diet in female mice. Furthermore, there was evidence indicating a sex-specific response to K+ restriction where female mice reabsorbed less Na+ in response to this dietary challenge compared with male mice. We propose that BMAL1 in the distal nephron and collecting duct contributes to blood pressure regulation and Na+ handling in a sex-specific manner.

The Association of Potassium Status with Parameters of Glucose Metabolism is influenced by Age in Adults.

BACKGROUND: Potassium status has been found to affect glucose homeostasis. OBJECTIVE: This study therefore aimed at investigating relationships between potassium status or dietary intake and fasting plasma glucose (FPG) or glycated haemoglobin (HbA1c) in a sample of Austrian adults (18-80 years, n = 421, 61% women) from the Austrian Study on Nutritional Status 2012. METHODS: Dietary potassium intake was obtained by two 24 h recalls. FPG, plasma K+, and urinary K+ were determined photometrically, HbA1c by HPLC. Associations between the parameters were studied using multiple regression analysis after controlling for confounders and after age stratification of the sample (18-64 y vs. 65-80 y). RESULTS: Most of the participants had a potassium intake of less than the estimated adequate daily intake of 4000 mg/d. In the multiple regression analyses in the whole sample plasma K+ had a statistically significant positive effect on FPG only in the crude model (ß = 0.128, p < 0.01) and on HbA1c also in the fully adjusted model (ß = 0.129, p < 0.05). The small effects on HbA1c were also detected in the younger age group but were absent in the older population. However, in this latter, a reverse association of urinary K+ on HbA1c was observed as well as of dietary potassium intake on FPG with no effects in the younger sample. CONCLUSION: We suggest that age dependent differences in the association between parameters of potassium status and blood glucose regulation should also be taken into account.

Deletion of the serine protease CAP2/Tmprss4 leads to dysregulated renal water handling upon dietary potassium depletion.

The kidney needs to adapt daily to variable dietary K+ contents via various mechanisms including diuretic, acid-base and hormonal changes that are still not fully understood. In this study, we demonstrate that following a K+-deficient diet in wildtype mice, the serine protease CAP2/Tmprss4 is upregulated in connecting tubule and cortical collecting duct and also localizes to the medulla and transitional epithelium of the papilla and minor calyx. Male CAP2/Tmprss4 knockout mice display altered water handling and urine osmolality, enhanced vasopressin response leading to upregulated adenylate cyclase 6 expression and cAMP overproduction, and subsequently greater aquaporin 2 (AQP2) and Na+-K+-2Cl- cotransporter 2 (NKCC2) expression following K+-deficient diet. Urinary acidification coincides with significantly increased H+,K+-ATPase type 2 (HKA2) mRNA and protein expression, and decreased calcium and phosphate excretion. This is accompanied by increased glucocorticoid receptor (GR) protein levels and reduced 11ß-hydroxysteroid dehydrogenase 2 activity in knockout mice. Strikingly, genetic nephron-specific deletion of GR leads to the mirrored phenotype of CAP2/Tmprss4 knockouts, including increased water intake and urine output, urinary alkalinisation, downregulation of HKA2, AQP2 and NKCC2. Collectively, our data unveil a novel role of the serine protease CAP2/Tmprss4 and GR on renal water handling upon dietary K+ depletion.

Evaluation of measurement error in 24-hour dietary recall for assessing sodium and potassium intake among US adults - National Health and Nutrition Examination Survey (NHANES), 2014.

BACKGROUND: Understanding measurement error in sodium and potassium intake is essential for assessing population intake and studying associations with health outcomes. OBJECTIVE: The aim of this study was to compare sodium and potassium intake derived from 24-h dietary recall (24HDR) with intake derived from 24-h urinary excretion (24HUE). DESIGN: Data were analyzed from 776 nonpregnant, noninstitutionalized US adults aged 20-69 y who completed 1-to-2 24HUE and 24HDR measures in the 2014 NHANES. A total of 1190 urine specimens and 1414 dietary recalls were analyzed. Mean bias was estimated as mean of the differences between individual mean 24HDR and 24HUE measurements. Correlations and attenuation factors were estimated using the Kipnis joint-mixed effects model accounting for within-person day-to-day variability in sodium excretion. The attenuation factor reflects the degree to which true associations between long-term intake (estimated using 24HUEs) and a hypothetical health outcome would be approximated using a single 24HDR: values near 1 indicate close approximation and near 0 indicate bias toward null. Estimates are reported for sodium, potassium, and the sodium: potassium (Na/K) ratio. Model parameters can be used to estimate correlations/attenuation factors when multiple 24HDRs are available. RESULTS: Overall, mean bias for sodium was -452 mg (95% CI: -646, -259), for potassium -315 mg (CI: -450, -179), and for the Na/K ratio -0.04 (CI: -0.15, 0.07, NS). Using 1 24HDR, the attenuation factor for sodium was 0.16 (CI: 0.09, 0.21), for potassium 0.25 (CI:0.16, 0.36), and for the Na/K ratio 0.20 (CI: 0.10, 0.25). The correlation for sodium was 0.27 (CI: 0.16, 0.37), for potassium 0.35 (CI: 0.26, 0.55), and for the Na/K ratio 0.27 (CI: 0.13, 0.32). CONCLUSIONS: Compared with 24HUE, using 24HDR underestimates mean sodium and potassium intake but is unbiased for the Na/K ratio. Additionally, using 24HDR as a measure of exposure in observational studies attenuates the true associations of sodium and potassium intake with health outcomes.