Regulation of potassium (K) handling in the renal collecting duct

SARS-CoV-2 N protein induces hypokalemia in acute kidney injury mice via ENaC-dependent mechanism

Hypokalemia is a prevalent complication of COVID-19 patients with acute kidney injury (AKI); however, mechanisms have yet to be fully understood. By single-nucleus RNA sequencing, we found that COVID-19 patients with AKI were associated with a marked upregulation of the epithelial sodium channel (ENaC) in the renal tubular epithelial cells (TECs). By using a mouse model of AKI induced by kidney-specific overexpressing SARS-CoV-2 N protein, we detected that overexpression of renal SARS-CoV-2 N protein could induce hypokalemia and AKI, which was associated with the upregulation of ENaC, ROMK, and BK proteins. Functionally, a patch-clamp study revealed that the overexpression of SARS-CoV-2 N protein largely increased the ENaC current in the TECs. Mechanically, we uncovered that kidney-specific overexpressing SARS-CoV-2 N protein could activate ENaC to cause hypokalemia and AKI directly by binding to the ENaCα and ENaCγ subunits and indirectly by activating the p38 MAPK pathway. Importantly, treatment with an ENaC specific inhibitor could protect against SARS-CoV-2 N-induced hypokalemia and AKI, revealing a regulatory role and therapeutic target of ENaC in SARS-CoV-2 N-induced hypokalemia and AKI. In conclusion, hypokalemia in COVID-19 AKI is induced by SARS-CoV-2 N protein via the ENaC-dependent mechanism. Targeting ENaC may offer a novel therapy for COVID-19 patients with AKI.

Urinary dickkopf-3 as a predictor for postoperative acute kidney injury in the intensive care unit

BACKGROUND

As a life-threatening complication in patients undergoing surgery, acute kidney injury (AKI) is strongly associated with a worse prognosis. Urinary dickkopf-related protein 3 (DKK3) has been identified as a biomarker for predicting postoperative AKI in patients undergoing cardiac surgery.

OBJECTIVE

To investigate the predictive value of urinary DKK3 on postoperative AKI and develop a clinical model based on the predictor for predicting the development of AKI within seven days for patients undergoing noncardiac surgery.

METHODS

All patients who were admitted to the Intensive Care Unit (ICU) after noncardiac surgery from March 2023 to June 2023 were included in this study. The patients' baseline data on demographic characteristics, lifestyle risk factors, medical history, clinical features, and laboratory tests before surgery were collected at the time of admission. Besides, the blood samples for cystatin C and routine laboratory tests and the urine samples for DKK3 tests were simultaneously collected at the time of admission to the ICU. In addition, the independent predictors of postoperative AKI were identified by univariate, multivariate, and LASSO analyses. Moreover, a nomogram for predicting postoperative AKI was developed based on these independent predictors. Finally, the nomogram was evaluated through calibration and decision curve analyses.

RESULTS

A total of 166 patients with a median age of 67 years old were included in this study, including 94 (56.63 %) males. Among these patients, 47 patients (28.3 %) developed postoperative AKI. Additionally, 7 independent risk factors, including preoperative serum creatinine, preoperative eGFR, preoperative serum albumin, preoperative serum potassium ion, cystatin C, uDKK3/uCr, and SOFA score, were selected by univariate and multivariate regression analyses. Eventually, 4 independent risk factors (including preoperative eGFR, cystatin C, uDKK3/uCr, and SOFA score) identified in this study by LASSO analyses were used to establish the nomogram. The area under the receiver operating characteristic (ROC) curve (AUC) for the prediction model was 0.868. The calibration curve and decision curve analysis results demonstrated that the nomogram had good prediction performance.

CONCLUSIONS

Urinary DKK3/creatinine was independently associated with postoperative AKI for patients in the ICU after noncardiac surgery. The nomogram constructed based on uDKK3/uCr, preoperative eGFR, cystatin C, and SOFA score showed a higher accuracy in predicting postoperative AKI.

Changes in kidney functions following acute infusion of low molecular weight polyvinylpyrrolidone in male rats

Polyvinylpyrrolidone (PVP), a water-soluble homopolymer, has been widely used in food, beverage, medical, and experimental tissue preparations. However, the effect of PVP on renal functions remains unknown. We investigated the acute administration of low MW of PVP on renal functions and whether it produces a toxic effect on the kidney. Renal clearance experiments were performed in rats and showed PVP infusion elicited significant diuretic and natriuretic effects. Urine volume, absolute (ENa), and fractional (FENa) Na+ excretion were significantly increased. A relatively small kaliuretic effect was also observed. After 2 h of PVP infusion, blood urea nitrogen (BUN) and urinary concentrations of beta-N-glucosaminidase (NAG) did not change significantly. Alpha-1-microglobulin, an indicator of proximal tubule absorption ability, excretion increased 12-fold, indicating that a large portion of the fluid and Na+ loss is due to reduced absorption in the proximal tubule. The 24-fold increase in ENa and the 12-fold increase in α1-microglobulin excretion suggest that fluid and electrolyte absorption were also reduced in other nephron segments. We conclude that acute low molecular weight PVP infusion produces diuretic and natriuretic effects due to the osmotically induced reduction of proximal tubular absorption, and acute PVP infusion does not cause renal damage.

New strategies for the treatment of hyperkalemia

Renin-angiotensin-aldosterone system inhibitors (RAASi) and mineralocorticoid receptor antagonists (MRAs) are key drugs in the management of patients with cardiovascular diseases (CVD), particularly those with hypertension, diabetes, chronic kidney disease and heart failure (HF), given their demonstrated effectiveness in reducing the risk of both surrogate and hard endpoints. Despite their positive impact on the outcome, patients with RAASi and MRAs are particularly vulnerable to hyperkalaemia, with approximately 50 % of these individuals experiencing two or more recurrences annually. The common practice of reducing the dose or discontinuing the treatment with RAASi and MRAs in conditions of hyperkalaemia results in suboptimal management of these patients, with a potential impact on their mortality and morbidity risk. Recent guidelines from cardiovascular and renal international societies increasingly recognize the need for alternative strategies to manage the risk of hyperkalaemia, allowing the continuation of RAASi and MRA therapies. In this review, we summarise the new potential options available to manage hyperkalaemia in patients with CVD and the recommendations of the most recent guidelines on the topic.

Potassium Intake and Bone Health: A Narrative Review

Potassium is a cation involved in the resting phase of membrane potential. Diets rich in fresh fruit and vegetables, whole grains, dairy products, and coffee have high potassium content. The shift from a pre-agriculture diet to today's consumption has led to reduced potassium intake. Indeed, the Western diet pattern is characterized by a high daily intake of saturated fats, sugars, sodium, proteins from red meat, and refined carbohydrates with a low potassium intake. These reductions are also mirrored by high sodium intakes and a high consumption of acid-generating food, which promote a chronic state of low-grade metabolic acidosis. The low-grade metabolic acidosis is a cause of the bone-wasting effect. Therefore, a long-standing acidotic state brings into play the bone that contributes to the buffering process through an increase in osteoclastic resorption. In consideration of this background, we carried out a review that focused on the pathophysiological mechanisms of the relationship between dietary potassium intake and bone health, underlining the detrimental effects of the Western dietary patterns characterized by low potassium consumption.

A Possible Link between Cell Plasticity and Renin Expression in the Collecting Duct: A Narrative Review

The hormone renin is produced in the kidney by the juxtaglomerular cells. It is the rate-limiting factor in the circulating renin-angiotensin-aldosterone system (RAAS), which contributes to electrolyte, water, and blood pressure homeostasis. In the kidneys, the distal tubule and the collecting duct are the key target segments for RAAS. The collecting duct is important for urine production and also for salt, water, and acid-base homeostasis. The critical functional role of the collecting duct is mediated by the principal and the intercalated cells and is regulated by different hormones like aldosterone and vasopressin. The collecting duct is not only a target for hormones but also a place of hormone production. It is accepted that renin is produced in the collecting duct at a low level. Several studies have described that the cells in the collecting duct exhibit plasticity properties because the ratio of principal to intercalated cells can change under specific circumstances. This narrative review focuses on two aspects of the collecting duct that remain somehow aside from mainstream research, namely the cell plasticity and the renin expression. We discuss the link between these collecting duct features, which we see as a promising area for future research given recent findings.

Kaliuresis and Intracellular Uptake of Potassium with Potassium Citrate and Potassium Chloride Supplements: A Randomized Controlled Trial

BACKGROUND

A potassium replete diet is associated with lower cardiovascular risk but may increase the risk of hyperkalemia, particularly in people using renin-angiotensin-aldosterone system inhibitors. We investigated whether intracellular uptake and potassium excretion after an acute oral potassium load depend on the accompanying anion and/or aldosterone and whether this results in altered plasma potassium change.

METHODS

In this placebo-controlled interventional cross-over trial including 18 healthy individuals, we studied the acute effects of one oral load of potassium citrate (40 mmol), potassium chloride (40 mmol), and placebo in random order after overnight fasting. Supplements were administered after a 6-week period with and without lisinopril pretreatment. Linear mixed effect models were used to compare blood and urine values before and after supplementation and between the interventions. Univariable linear regression was used to determine the association between baseline variables and change in blood and urine values after supplementation.

RESULTS

During the 4-hour follow-up, the rise in plasma potassium was similar for all interventions. After potassium citrate, both red blood cell potassium-as measure of the intracellular potassium-and transtubular potassium gradient (TTKG)-reflecting potassium secretory capacity-were higher than after potassium chloride or potassium citrate with lisinopril pretreatment. Baseline aldosterone was significantly associated with TTKG after potassium citrate, but not after potassium chloride or potassium citrate with lisinopril pretreatment. The observed TTKG change after potassium citrate was significantly associated with urine pH change during this intervention ( R =0.60, P < 0.001).

CONCLUSIONS

With similar plasma potassium increase, red blood cell potassium uptake and kaliuresis were higher after an acute load of potassium citrate as compared with potassium chloride alone or pretreatment with lisinopril.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Potassium supplementation in patients with chronic kidney disease and healthy subjects: effects on potassium and sodium balance, NL7618.

Stress-Dose Steroids: A Potential Therapeutic Option for Refractory Hyperkalemia

Hyperkalemia refractory to standard temporization measures can be life-threatening, and urgent hemodialysis is often utilized as a final resort. Our patient presented with hyperkalemia that was multifactorial in etiology, with acute kidney injury complicated by adrenal insufficiency. Her hyperkalemia was refractory to temporization and excretion agents, and hemodialysis was being considered. Given a recent infection, surgery, and borderline hypotension with low adrenocorticotropic hormone, there was a concern for adrenal insufficiency. However, a full investigation for secondary adrenal insufficiency via magnetic resonance imaging could not be conducted as the patient suffered from claustrophobia. Continued concern for adrenal insufficiency prompted the initiation of intravenous hydrocortisone, and the patient's hyperkalemia resolved within 24 hours. While suspected adrenal insufficiency is already a basis for stress-dose steroids in the setting of pathologies such as severe sepsis, clinicians should have a low threshold for considering refractory hyperkalemia alone as an indication for stress-dose steroids. When dialysis is being considered as an option, this treatment modality should be given even more consideration. Adopting this practice may not only lead to improved mortality from hyperkalemia but also lead to fewer patients being exposed to the risks of dialysis.

Renal K+ retention in physiological circumstances: focus on adaptation of the distal nephron and cross-talk with Na+ transport systems

Consumption of salt (NaCl) and potassium (K+) has been completely modified, switching from a rich-K+/low-NaCl diet in the hunter-gatherer population to the opposite in the modern, westernized population. The ability to conserve K+ is crucial to maintain the plasma K+ concentration in a physiological range when dietary K+ intake is decreased. Moreover, a chronic reduction in the K+ intake is correlated with an increased blood pressure, an effect worsened by a high-Na+ diet. The renal adaptation to a low-K+ diet in order to maintain the plasma K+ level in the normal range is complex and interconnected with the mechanisms of the Na+ balance. In this short review, we will recapitulate the general mechanisms allowing the plasma K+ value to remain in the normal range, when there is a necessity to retain K+ (response to low-K+ diet and adaptation to gestation), by focusing on the processes occurring in the most distal part of the nephron. We will particularly outline the mechanisms of K+ reabsorption and discuss the consequences of its absence on the Na+ transport systems and the regulation of the extracellular compartment volume and blood pressure.

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

Regulation of distal tubule sodium transport: mechanisms and roles in homeostasis and pathophysiology

Regulated Na⁺ transport in the distal nephron is of fundamental importance to fluid and electrolyte homeostasis. Further upstream, Na⁺ is the principal driver of secondary active transport of numerous organic and inorganic solutes. In the distal nephron, Na⁺ continues to play a central role in controlling the body levels and concentrations of a more select group of ions, including K⁺, Ca⁺⁺, Mg⁺⁺, Cl^-, and HCO₃^-, as well as water. Also, of paramount importance are transport mechanisms aimed at controlling the total level of Na⁺ itself in the body, as well as its concentrations in intracellular and extracellular compartments. Over the last several decades, the transporters involved in moving Na⁺ in the distal nephron, and directly or indirectly coupling its movement to that of other ions have been identified, and their interrelationships brought into focus. Just as importantly, the signaling systems and their components-kinases, ubiquitin ligases, phosphatases, transcription factors, and others-have also been identified and many of their actions elucidated. This review will touch on selected aspects of ion transport regulation, and its impact on fluid and electrolyte homeostasis. A particular focus will be on emerging evidence for site-specific regulation of the epithelial sodium channel (ENaC) and its role in both Na⁺ and K⁺ homeostasis. In this context, the critical regulatory roles of aldosterone, the mineralocorticoid receptor (MR), and the kinases SGK1 and mTORC2 will be highlighted. This includes a discussion of the newly established concept that local K⁺ concentrations are involved in the reciprocal regulation of Na⁺-Cl^- cotransporter (NCC) and ENaC activity to adjust renal K⁺ secretion to dietary intake.

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.

Role of collecting duct principal cell NOS1β in sodium and potassium homeostasis

The nitric oxide (NO)-generating enzyme, NO synthase-1β (NOS1β), is essential for sodium (Na+ ) homeostasis and blood pressure control. We previously showed that collecting duct principal cell NOS1β is critical for inhibition of the epithelial sodium channel (ENaC) during high Na+ intake. Previous studies on freshly isolated cortical collecting ducts (CCD) demonstrated that exogenous NO promotes basolateral potassium (K+ ) conductance through basolateral channels, presumably Kir 4.1 (Kcnj10) and Kir 5.1 (Kcnj16). We, therefore, investigated the effects of NOS1β knockout on Kir 4.1/Kir 5.1 channel activity. Indeed, in CHO cells overexpressing NOS1β and Kir 4.1/Kir 5.1, the inhibition of NO signaling decreased channel activity. Male littermate control and principal cell NOS1β knockout mice (CDNOS1KO) on a 7-day, 4% NaCl diet (HSD) were used to detect changes in basolateral K+ conductance. We previously demonstrated that CDNOS1KO mice have high circulating aldosterone despite a high-salt diet and appropriately suppressed renin. We observed greater Kir 4.1 cortical abundance and significantly greater Kir 4.1/Kir 5.1 single-channel activity in the principal cells from CDNOS1KO mice. Moreover, blocking aldosterone action with in vivo spironolactone treatment resulted in lower Kir 4.1 abundance and greater plasma K+ in the CDNOS1KO mice compared to controls. Lowering K+ content in the HSD prevented the high aldosterone and greater plasma Na+ of CDNOS1KO mice and normalized Kir 4.1 abundance. We conclude that during chronic HSD, lack of NOS1β leads to increased plasma K+ , enhanced circulating aldosterone, and activation of ENaC and Kir 4.1/Kir 5.1 channels. Thus, principal cell NOS1β is required for the regulation of both Na+ and K+ by the kidney.

The Angiotensin II Type 1 Receptor-Associated Protein Attenuates Angiotensin II-Mediated Inhibition of the Renal Outer Medullary Potassium Channel in Collecting Duct Cells

Adjustments in renal K⁺ excretion constitute a central mechanism for K⁺ homeostasis. The renal outer medullary potassium (ROMK) channel accounts for the major K⁺ secretory route in collecting ducts during basal conditions. Activation of the angiotensin II (Ang II) type 1 receptor (AT1R) by Ang II is known to inhibit ROMK activity under the setting of K⁺ dietary restriction, underscoring the role of the AT1R in K⁺ conservation. The present study aimed to investigate whether an AT1R binding partner, the AT1R-associated protein (ATRAP), impacts Ang II-mediated ROMK regulation in collecting duct cells and, if so, to gain insight into the potential underlying mechanisms. To this end, we overexpressed either ATRAP or β-galactosidase (LacZ; used as a control), in M-1 cells, a model line of cortical collecting duct cells. We then assessed ROMK channel activity by employing a novel fluorescence-based microplate assay. Experiments were performed in the presence of 10⁻¹⁰ M Ang II or vehicle for 40 min. We observed that Ang II-induced a significant inhibition of ROMK in LacZ, but not in ATRAP-overexpressed M-1 cells. Inhibition of ROMK-mediated K⁺ secretion by Ang II was accompanied by lower ROMK cell surface expression. Conversely, Ang II did not affect the ROMK-cell surface abundance in M-1 cells transfected with ATRAP. Additionally, diminished response to Ang II in M-1 cells overexpressing ATRAP was accompanied by decreased c-Src phosphorylation at the tyrosine 416. Unexpectedly, reduced phospho-c-Src levels were also found in M-1 cells, overexpressing ATRAP treated with vehicle, suggesting that ATRAP can also downregulate this kinase independently of Ang II-AT1R activation. Collectively, our data support that ATRAP attenuates inhibition of ROMK by Ang II in collecting duct cells, presumably by reducing c-Src activation and blocking ROMK internalization. The potential role of ATRAP in K⁺ homeostasis and/or disorders awaits further investigation.

Adrenal adaptation in potassium-depleted men: role of progesterone?

BACKGROUND

In rodents, the stimulation of adrenal progesterone is necessary for renal adaptation under potassium depletion. Here, we sought to determine the role of progesterone in adrenal adaptation in potassium-depleted healthy human volunteers and compared our findings with data collected in patients with Gitelman syndrome (GS), a salt-losing tubulopathy.

METHODS

Twelve healthy young men were given a potassium-depleted diet for 7 days at a tertiary referral medical centre (NCT02297048). We measured by liquid chromatography coupled to tandem mass spectroscopy plasma steroid concentrations at Days 0 and 7 before and 30 min after treatment with tetracosactide. We compared these data with data collected in 10 GS patients submitted to tetracosactide test.

RESULTS

The potassium-depleted diet decreased plasma potassium in healthy subjects by 0.3 ± 0.1 mmol/L, decreased plasma aldosterone concentration by 50% (P = 0.0332) and increased plasma 17-hydroxypregnenolone concentration by 45% (P = 0.0232) without affecting other steroids. CYP17 activity, as assessed by 17-hydroxypregnenolone/pregnenolone ratio, increased by 60% (P = 0.0389). As compared with healthy subjects, GS patients had 3-fold higher plasma concentrations of aldosterone, 11-deoxycortisol (+30%) and delta 4-androstenedione (+14%). Their post-tetracosactide progesterone concentration was 2-fold higher than that of healthy subjects and better correlated to plasma potassium than to plasma renin.

CONCLUSION

The increase in 17-hydroxypregnenolone concentration after mild potassium depletion in otherwise healthy human subjects suggests that 17 hydroxylation of pregnenolone prevents the increase in progesterone observed in potassium-depleted mice. The unexpected over-response of non-mineralocorticoid steroids to tetracosactide in GS subjects suggests that the adrenal system not only adapts to sodium depletion but may also respond to hypokalaemia.

The Renal Physiology of Pendrin-Positive Intercalated Cells

Intercalated cells (ICs) are found in the connecting tubule and the collecting duct. Of the three IC subtypes identified, type B intercalated cells are one of the best characterized and known to mediate Cl- absorption and HCO3- secretion, largely through the anion exchanger pendrin. This exchanger is thought to act in tandem with the Na+-dependent Cl-/HCO3- exchanger, NDCBE, to mediate net NaCl absorption. Pendrin is stimulated by angiotensin II and aldosterone administration via the angiotensin type 1a and the mineralocorticoid receptors, respectively. It is also stimulated in models of metabolic alkalosis, such as with NaHCO3 administration. In some rodent models, pendrin-mediated HCO3- secretion modulates acid-base balance. However, of probably more physiological or clinical significance is the role of these pendrin-positive ICs in blood pressure regulation, which occurs, at least in part, through pendrin-mediated renal Cl- absorption, as well as their effect on the epithelial Na+ channel, ENaC. Aldosterone stimulates ENaC directly through principal cell mineralocorticoid hormone receptor (ligand) binding and also indirectly through its effect on pendrin expression and function. In so doing, pendrin contributes to the aldosterone pressor response. Pendrin may also modulate blood pressure in part through its action in the adrenal medulla, where it modulates the release of catecholamines, or through an indirect effect on vascular contractile force. In addition to its role in Na+ and Cl- balance, pendrin affects the balance of other ions, such as K+ and I-. This review describes how aldosterone and angiotensin II-induced signaling regulate pendrin and the contribution of pendrin-positive ICs in the kidney to distal nephron function and blood pressure.

14-3-3γ, a novel regulator of the large-conductance Ca2+-activated K+ channel

14-3-3γ is a small protein regulating its target proteins through binding to phosphorylated serine/threonine residues. Sequence analysis of large-conductance Ca²⁺-activated K⁺ (BK) channels revealed a putative 14-3-3 binding site in the COOH-terminal region. Our previous data showed that 14-3-3γ is widely expressed in the mouse kidney. Therefore, we hypothesized that 14-3-3γ has a novel role in the regulation of BK channel activity and protein expression. We used electrophysiology, Western blot analysis, and coimmunoprecipitation to examine the effects of 14-3-3γ on BK channels both in vitro and in vivo. We demonstrated the interaction of 14-3-3γ with BK α-subunits (BKα) by coimmunoprecipitation. In human embryonic kidney-293 cells stably expressing BKα, overexpression of 14-3-3γ significantly decreased BK channel activity and channel open probability. 14-3-3γ inhibited both total and cell surface BKα protein expression while enhancing ERK1/2 phosphorylation in Cos-7 cells cotransfected with flag-14-3-3γ and myc-BK. Knockdown of 14-3-3γ by siRNA transfection markedly increased BKα expression. Blockade of the ERK1/2 pathway by incubation with the MEK-specific inhibitor U0126 partially abolished 14-3-3γ-mediated inhibition of BK protein expression. Similarly, pretreatment of the lysosomal inhibitor bafilomycin A1 reversed the inhibitory effects of 14-3-3γ on BK protein expression. Furthermore, overexpression of 14-3-3γ significantly increased BK protein ubiquitination in embryonic kidney-293 cells stably expressing BKα. Additionally, 3 days of dietary K⁺ challenge reduced 14-3-3γ expression and ERK1/2 phosphorylation while enhancing renal BK protein expression and K⁺ excretion. These data suggest that 14-3-3γ modulates BK channel activity and protein expression through an ERK1/2-mediated ubiquitin-lysosomal pathway.

Claudins in the Renal Collecting Duct

The renal collecting duct fine-tunes urinary composition, and thereby, coordinates key physiological processes, such as volume/blood pressure regulation, electrolyte-free water reabsorption, and acid-base homeostasis. The collecting duct epithelium is comprised of a tight epithelial barrier resulting in a strict separation of intraluminal urine and the interstitium. Tight junctions are key players in enforcing this barrier and in regulating paracellular transport of solutes across the epithelium. The features of tight junctions across different epithelia are strongly determined by their molecular composition. Claudins are particularly important structural components of tight junctions because they confer barrier and transport properties. In the collecting duct, a specific set of claudins (Cldn-3, Cldn-4, Cldn-7, Cldn-8) is expressed, and each of these claudins has been implicated in mediating aspects of the specific properties of its tight junction. The functional disruption of individual claudins or of the overall barrier function results in defects of blood pressure and water homeostasis. In this concise review, we provide an overview of the current knowledge on the role of the collecting duct epithelial barrier and of claudins in collecting duct function and pathophysiology.

Hyperkalemia and blood pressure regulation

Hypertension is common in the general population. Management of hypertensive patients at risk of hyperkalemia is challenging due to potential life-threatening complications such as cardiac arrest. Chronic hyperkalemia is often associated with impaired renal ability to excrete excessive potassium ions (K+). This may refer to chronic kidney disease or certain pharmacological interventions, including broadly used renin-angiotensin-aldosterone system and calcineurin inhibitors. Understanding the intrinsic mechanisms permitting kidney adaptations to hyperkalemia is critical for choosing therapeutic strategies. Valuable insights were obtained from the analysis of familial hyperkalemic hypertension (FHHt) syndrome, which became a classic model for coincidence of high blood pressure and hyperkalemia. FHHt can be caused by mutations in several genes, all of them resulting in excessive activity of with-no-lysine kinases (WNKs) in the distal nephron of the kidney. WNKs have been increasingly recognized as key signalling enzymes in the regulation of renal sodium ions (Na+) and K+ handling, enabling adaptive responses to systemic shifts of potassium homoeostasis consequent to variations in dietary potassium intake or disease. The WNK signalling pathway recruits a complex protein network mediating catalytic and non-catalytic effects of distinct WNK isoforms on relevant Na+- or K+-transporting proteins. In this review article, we summarize recent progress in understanding WNK signalling. An update of available models for renal adaptation to hyperkalemic conditions is presented. Consequences for blood pressure regulation are discussed. Pharmacological targeting of WNKs or their substrates offers promising options to manage hypertension while preventing hyperkalemia.

Changes in Serum and Urinary Potassium Handling Associated with Renin-Angiotensin-Aldosterone System Inhibitors in Advanced Chronic Kidney Disease Patients

Objective This study aimed to (i) compare the extent of urinary potassium (K+) excretion in addition to the changes in serum K+ concentration: and (ii) clarify the association between changes in serum K+ concentration, urinary K+ excretion, and acid-base status with or without renin-angiotensin-aldosterone system (RAAS) inhibitors in patients with advanced chronic kidney disease (CKD) stages. Methods Six hundred and ninety-one patients with advanced CKD (CKD G3b, 161; G4, 271; G5, 259) were retrospectively evaluated. Differences in serum K+ concentration, urinary K+ excretion, and serum sodium and chloride differences (Na+-Cl-) were compared among patients with RAAS inhibitors, RAAS inhibitors and diuretic agents, and without either medication in each CKD stage. Results Serum K+ concentrations in patients with RAAS inhibitors were significantly higher than in those with RAAS inhibitors and diuretics in CKD stage G3b and the other two treatment groups in CKD stage G4. Urinary K+ excretion among the three groups did not differ significantly in each CKD stage. Serum Na+-Cl- differences in patients with RAAS inhibitors were significantly smaller than in those with RAAS inhibitors and diuretics in CKD stages G3b (p = 0.006) and the other two groups in CKD stage G4 (vs. RAAS inhibitors and diuretics, p <0.001; vs. without either medication, p = 0.008). Conclusion Our study demonstrated that RAAS inhibitor use might be associated with hyperkalemia via not decreased urinary K+ excretion but rather K+ redistribution from intracellular to extracellular fluid induced by the progression of metabolic acidosis in patients with advanced CKD, particularly stages G3b and G4.

Claudin-7 Modulates Cl- and Na+ Homeostasis and WNK4 Expression in Renal Collecting Duct Cells

Claudin-7 knockout (CLDN7^-/-) mice display renal salt wasting and dehydration phenotypes. To address the role of CLDN7 in kidneys, we established collecting duct (CD) cell lines from CLDN7^+/+ and CLDN7^-/- mouse kidneys. We found that deletion of CLDN7 increased the transepithelial resistance (TER) and decreased the paracellular permeability for Cl^- and Na⁺ in CLDN7^-/- CD cells. Inhibition of transcellular Cl^- and Na⁺ channels has no significant effect on TER or dilution potentials. Current-voltage curves were linear in both CLDN7^+/+ and CLDN7^-/- CD cells, indicating that the ion flux was through the paracellular pathway. The impairment of Cl^- and Na⁺ permeability phenotype can be rescued by CLDN7 re-expression. We also found that WNK4 (its mutations lead to hypertension) expression, but not WNK1, was significantly increased in CLDN7^-/- CD cell lines as well as in primary CLDN7^-/- CD cells, suggesting that the expression of WNK4 was modulated by CLDN7. In addition, deletion of CLDN7 upregulated the expression level of the apical epithelial sodium channel (ENaC), indicating a potential cross-talk between paracellular and transcellular transport systems. This study demonstrates that CLDN7 plays an important role in salt balance in renal CD cells and modulating WNK4 and ENaC expression levels that are vital in controlling salt-sensitive hypertension.

TRPV4 deletion protects against hypokalemia during systemic K+ deficiency

Tight regulation of K+ balance is fundamental for normal physiology. Reduced dietary K+ intake, which is common in Western diets, often leads to hypokalemia and associated cardiovascular- and kidney-related pathologies. The distal nephron, and, specifically, the collecting duct (CD), is the major site of controlled K+ reabsorption via H+-K+-ATPase in the state of dietary K+ deficiency. We (Mamenko MV, Boukelmoune N, Tomilin VN, Zaika OL, Jensen VB, O'Neil RG, Pochynyuk OM. Kidney Int 91: 1398-1409, 2017) have previously demonstrated that the transient receptor potential vanilloid type 4 (TRPV4) Ca2+ channel, abundantly expressed in the CD, contributes to renal K+ handling by promoting flow-induced K+ secretion. Here, we investigated a potential role of TRPV4 in controlling H+-K+-ATPase-dependent K+ reabsorption in the CD. Treatment with a K+-deficient diet (<0.01% K+) for 7 days reduced serum K+ levels in wild-type (WT) mice from 4.3 ± 0.2 to 3.3 ± 0.2 mM but not in TRPV4-/- mice (4.3 ± 0.1 and 4.2 ± 0.3 mM, respectively). Furthermore, we detected a significant reduction in 24-h urinary K+ levels in TRPV4-/- compared with WT mice upon switching to K+-deficient diet. TRPV4-/- animals also had significantly more acidic urine on a low-K+ diet, but not on a regular (0.9% K+) or high-K+ (5% K+) diet, which is consistent with increased H+-K+-ATPase activity. Moreover, we detected a greatly accelerated H+-K+-ATPase-dependent intracellular pH extrusion in freshly isolated CDs from TRPV4-/- compared with WT mice fed a K+-deficient diet. Overall, our results demonstrate a novel kaliuretic role of TRPV4 by inhibiting H+-K+-ATPase-dependent K+ reabsorption in the CD. We propose that TRPV4 inhibition could be a novel strategy to manage certain hypokalemic states in clinical settings.

Potassium acts through mTOR to regulate its own secretion

Potassium (K+) secretion by kidney tubule cells is central to electrolyte homeostasis in mammals. In the K+ secretory "principal" cells of the distal nephron, electrogenic Na+ transport by the epithelial sodium channel (ENaC) generates the electrical driving force for K+ transport across the apical membrane. Regulation of this process is attributable in part to aldosterone, which stimulates the gene transcription of the ENaC-regulatory kinase, SGK1. However, a wide range of evidence supports the conclusion that an unidentified aldosterone-independent pathway exists. We show here that in principal cells, K+ itself acts through the type 2 mTOR complex (mTORC2) to activate SGK1, which stimulates ENaC to enhance K+ excretion. The effect depends on changes in K+ concentration on the blood side of the cells, and requires basolateral membrane K+-channel activity. However, it does not depend on changes in aldosterone, or on enhanced distal delivery of Na+ from upstream nephron segments. These data strongly support the idea that K+ is sensed directly by principal cells to stimulate its own secretion by activating the mTORC2-SGK1 signaling module, and stimulate ENaC. We propose that this local effect acts in concert with aldosterone and increased Na+ delivery from upstream nephron segments to sustain K+ homeostasis.

Angiotensin II Signal Transduction: An Update on Mechanisms of Physiology and Pathophysiology

The renin-angiotensin-aldosterone system plays crucial roles in cardiovascular physiology and pathophysiology. However, many of the signaling mechanisms have been unclear. The angiotensin II (ANG II) type 1 receptor (AT₁R) is believed to mediate most functions of ANG II in the system. AT₁R utilizes various signal transduction cascades causing hypertension, cardiovascular remodeling, and end organ damage. Moreover, functional cross-talk between AT₁R signaling pathways and other signaling pathways have been recognized. Accumulating evidence reveals the complexity of ANG II signal transduction in pathophysiology of the vasculature, heart, kidney, and brain, as well as several pathophysiological features, including inflammation, metabolic dysfunction, and aging. In this review, we provide a comprehensive update of the ANG II receptor signaling events and their functional significances for potential translation into therapeutic strategies. AT₁R remains central to the system in mediating physiological and pathophysiological functions of ANG II, and participation of specific signaling pathways becomes much clearer. There are still certain limitations and many controversies, and several noteworthy new concepts require further support. However, it is expected that rigorous translational research of the ANG II signaling pathways including those in large animals and humans will contribute to establishing effective new therapies against various diseases.

Renal potassium physiology: integration of the renal response to dietary potassium depletion

We summarize the current understanding of the physiology of the renal handling of potassium (K⁺), and present an integrative view of the renal response to K⁺ depletion caused by dietary K⁺ restriction. This renal response involves contributions from different nephron segments, and aims to diminish the rate of excretion of K⁺ as a result of: decreasing the rate of electrogenic (and increasing the rate of electroneutral) reabsorption of sodium in the aldosterone-sensitive distal nephron (ASDN), decreasing the abundance of renal outer medullary K⁺ channels in the luminal membrane of principal cells in the ASDN, decreasing the flow rate in the ASDN, and increasing the reabsorption of K⁺ in the cortical and medullary collecting ducts. The implications of this physiology for the association between K⁺ depletion and hypertension, and K⁺ depletion and formation of calcium kidney stones are discussed.

Distal tubule basolateral potassium channels: cellular and molecular mechanisms of regulation

PURPOSE OF REVIEW

Multiple clinical and translational evidence support benefits of high potassium diet; however, there many uncertainties underlying the molecular and cellular mechanisms determining effects of dietary potassium. Kir4.1 and Kir5.1 proteins form a functional heteromer (Kir4.1/Kir5.1), which is the primary inwardly rectifying potassium channel on the basolateral membrane of both distal convoluted tubule (DCT) and the collecting duct principal cells. The purpose of this mini-review is to summarize latest advances in our understanding of the evolution, physiological relevance and mechanisms controlling these channels.

RECENT FINDINGS

Kir4.1 and Kir5.1 channels play a critical role in determining electrolyte homeostasis in the kidney and blood pressure, respectively. It was reported that Kir4.1/Kir5.1 serves as potassium sensors in the distal nephron responding to variations in dietary intake and hormonal stimuli. Global and kidney specific knockouts of either channel resulted in hypokalemia and severe cardiorenal phenotypes. Furthermore, knock out of Kir5.1 in Dahl salt-sensitive rat background revealed the crucial role of the Kir4.1/Kir5.1 channel in salt-induced hypertension.

SUMMARY

Here, we focus on reviewing novel experimental evidence of the physiological function, expression and hormonal regulation of renal basolateral inwardly rectifying potassium channels. Further investigation of molecular and cellular mechanisms controlling Kir4.1 and Kir4.1/Kir5.1-mediating pathways and development of specific compounds targeting these channels function is essential for proper control of electrolyte homeostasis and blood pressure.

G protein pathway suppressor 2 enhanced the renal large-conductance Ca2+-activated potassium channel expression via inhibiting ERK1/2 signaling pathway

G protein pathway suppressor 2 (GPS2) is a multifunctional protein and transcriptional regulation factor that is involved in the G protein MAPK signaling pathway. It has been shown that the MAPK signaling pathway plays an important role in the regulation of renal large-conductance Ca²⁺-activated potassium (BK) channels. In this study, we investigated the effects of GPS2 on BK channel activity and protein expression. In human embryonic kidney (HEK) BK stably expressing cells transfected with either GPS2 or its vector control, a single-cell recording showed that GPS2 significantly increased BK channel activity ( NP_o), increasing BK open probability ( P_o), and channel number ( N) compared with the control. In Cos-7 cells and HEK 293 T cells, GPS2 overexpression significantly enhanced the total protein expression of BK in a dose-dependent manner. Knockdown of GPS2 expression significantly decreased BK protein expression, while increasing ERK1/2 phosphorylation. Knockdown of ERK1/2 expression reversed the GPS2 siRNA-mediated inhibition of BK protein expression in Cos-7 cells. Pretreatments of Cos-7 cells with either the lysosomal inhibitor bafilomycin A1 or the proteasomal inhibitor MG132 partially reversed the inhibitory effects of GPS2 siRNA on BK protein expression. In addition, feeding a high-potassium diet significantly increased both GPS2 and BK protein abundance in mice. These data suggest that GPS2 enhances BK channel activity and its protein expression by reducing ERK1/2 signaling-mediated degradation of the channel.

Circadian regulation of renal function

The kidneys regulate many vital functions that require precise control throughout the day. These functions, such as maintaining sodium balance or regulating arterial pressure, rely on an intrinsic clock mechanism that was commonly believed to be controlled by the central nervous system. Mounting evidence in recent years has unveiled previously underappreciated depth of influence by circadian rhythms and clock genes on renal function, at the molecular and physiological level, independent of other external factors. The impact of circadian rhythms in the kidney also affects individuals from a clinical standpoint, as the loss of rhythmic activity or clock gene expression have been documented in various cardiovascular diseases. Fortunately, the prognostic value of examining circadian rhythms may prove useful in determining the progression of a kidney-related disease, and chronotherapy is a clinical intervention that requires consideration of circadian and diurnal rhythms in the kidney. In this review, we discuss evidence of circadian regulation in the kidney from basic and clinical research in order to provide a foundation on which a great deal of future research is needed to expand our understanding of circadian relevant biology.

Hypokalemia: a clinical update

Hypokalemia is a common electrolyte disturbance, especially in hospitalized patients. It can have various causes, including endocrine ones. Sometimes, hypokalemia requires urgent medical attention. The aim of this review is to present updated information regarding: (1) the definition and prevalence of hypokalemia, (2) the physiology of potassium homeostasis, (3) the various causes leading to hypokalemia, (4) the diagnostic steps for the assessment of hypokalemia and (5) the appropriate treatment of hypokalemia depending on the cause. Practical algorithms for the optimal diagnostic, treatment and follow-up strategy are presented, while an individualized approach is emphasized.

Dietary K+ and Cl- independently regulate basolateral conductance in principal and intercalated cells of the collecting duct

The renal collecting duct contains two distinct cell types, principal and intercalated cells, expressing potassium Kir4.1/5.1 (KCNJ10/16) and chloride ClC-K2 (ClC-Kb in humans) channels on their basolateral membrane, respectively. Both channels are thought to play important roles in controlling systemic water-electrolyte balance and blood pressure. However, little is known about mechanisms regulating activity of Kir4.1/5.1 and ClC-K2/b. Here, we employed patch clamp analysis at the single channel and whole cell levels in freshly isolated mouse collecting ducts to investigate regulation of Kir4.1/5.1 and ClC-K2/b by dietary K+ and Cl- intake. Treatment of mice with high K+ and high Cl- diet (6% K+, 5% Cl-) for 1 week significantly increased basolateral K+-selective current, single channel Kir4.1/5.1 activity and induced hyperpolarization of basolateral membrane potential in principal cells when compared to values in mice on a regular diet (0.9% K+, 0.5% Cl-). In contrast, basolateral Cl--selective current and single channel ClC-K2/b activity was markedly decreased in intercalated cells under this condition. Substitution of dietary K+ to Na+ in the presence of high Cl- exerted a similar inhibiting action of ClC-K2/b suggesting that the channel is sensitive to variations in dietary Cl- per se. Cl--sensitive with-no-lysine kinase (WNK) cascade has been recently proposed to orchestrate electrolyte transport in the distal tubule during variations of dietary K+. However, co-expression of WNK1 or its major downstream effector Ste20-related proline-alanine-rich kinase (SPAK) had no effect on ClC-Kb over-expressed in Chinese hamster ovary (CHO) cells. Treatment of mice with high K+ diet without concomitant elevations in dietary Cl- (6% K+, 0.5% Cl-) elicited a comparable increase in basolateral K+-selective current, single channel Kir4.1/5.1 activity in principal cells, but had no significant effect on ClC-K2/b activity in intercalated cells. Furthermore, stimulation of aldosterone signaling by Deoxycorticosterone acetate (DOCA) recapitulated the stimulatory actions of high K+ intake on Kir4.1/5.1 channels in principal cells but was ineffective to alter ClC-K2/b activity and basolateral Cl- conductance in intercalated cells. In summary, we report that variations of dietary K+ and Cl- independently regulate basolateral potassium and chloride conductance in principal and intercalated cells. We propose that such discrete mechanism might contribute to fine-tuning of urinary excretion of electrolytes depending on dietary intake.

Renal intercalated cells and blood pressure regulation

Type B and non-A, non-B intercalated cells are found within the connecting tubule and the cortical collecting duct. Of these cell types, type B intercalated cells are known to mediate Cl- absorption and HCO3- secretion largely through pendrin-dependent Cl-/HCO3- exchange. This exchange is stimulated by angiotensin II administration and is also stimulated in models of metabolic alkalosis, for instance after aldosterone or NaHCO3 administration. In some rodent models, pendrin-mediated HCO3- secretion modulates acid-base balance. However, the role of pendrin in blood pressure regulation is likely of more physiological or clinical significance. Pendrin regulates blood pressure not only by mediating aldosterone-sensitive Cl- absorption, but also by modulating the aldosterone response for epithelial Na+ channel (ENaC)-mediated Na+ absorption. Pendrin regulates ENaC through changes in open channel of probability, channel surface density, and channels subunit total protein abundance. Thus, aldosterone stimulates ENaC activity through both direct and indirect effects, the latter occurring through its stimulation of pendrin expression and function. Therefore, pendrin contributes to the aldosterone pressor response. Pendrin may also modulate blood pressure in part through its action in the adrenal medulla, where it modulates the release of catecholamines, or through an indirect effect on vascular contractile force. This review describes how aldosterone and angiotensin II-induced signaling regulate pendrin and the contributory role of pendrin in distal nephron function and blood pressure.

Potassium regulation in the neonate

Potassium, the major cation in intracelluar fluids, is essential for vital biological functions. Neonates maintain a net positive potassium balance, which is fundamental to ensure somatic growth but places these infants, especially those born prematurely, at risk for life-threatening disturbances in potassium concentration [K⁺] in the extracellular fluid compartment. Potassium conservation is achieved by maximizing gastrointestinal absorption and minimizing renal losses. A markedly low glomerular filtration rate, plus adaptations in tubular transport along the nephron, result in low potassium excretion in the urine of neonates. Careful evaluation of clinical data using reference values that are normal for the neonate's postmenstrual age is critical to avoid over-treating infants with laboratory results that represent physiologic values for their developmental stage. The treatment should be aimed at correcting the primary cause when possible. Alterations in the levels or sensitivity to aldosterone are common in neonates. In symptomatic patients, the disturbances in [K⁺] should be corrected promptly, with close electrocardiographic monitoring. Plasma [K⁺] should be monitored during the first 72 h of life in all premature infants born before 30 weeks of postmenstrual age as these infants are prone to develop non-oliguric hyperkalemia with potential serious complications.

Lipidomic and proteomic analysis of exosomes from mouse cortical collecting duct cells

Exosomes are endosome-derived nanovesicles that are involved in cellular communication and signaling. Exosomes are produced by epithelial cells and are found in biologic fluids including blood and urine. The packaged material within exosomes includes proteins and lipids, but the molecular comparison within exosome subtypes is largely unknown. The purpose of this study was to investigate differences between exosomes derived from the apical plasma membrane and basolateral plasma membrane of polarized murine cortical collecting duct principal cells. Nanoparticle tracking analysis showed that the size and concentration of apical and basolateral exosomes remained relatively stable across 3 different temperatures (23, 37, and 42°C). Liquid chromatography-tandem mass spectrometry analysis revealed marked differences between the proteins packaged within the two types of exosomes from the same cells. Several proteins expressed at the inner leaflet of the plasma membrane, including α-actinin-1, moesin, 14-3-3 protein ζ/δ, annexin A1/A3/A4/A5/A6, clathrin heavy chain 1, glyceraldehyde-3-phosphate dehydrogenase, α-enolase, filamin-A, and heat shock protein 90, were identified in samples of apical plasma membrane-derived exosomes, but not in basolateral plasma membrane exosomes from mouse cortical collecting duct cells. In addition to differences at the protein level, mass spectrometry-based shotgun lipidomics analysis showed significant differences in the lipid classes and fatty acid composition of the two types of exosomes. We found higher levels of sphingomyelin and lower levels of cardiolipin, among other phospholipids in the apical plasma membrane compared to the basolateral plasma membrane exosomes. The molecular analyses of exosome subtypes presented herein will contribute to our understanding of exosome biogenesis, and the results may have potential implications for biomarker discovery.-Dang, V. D., Jella, K. K., Ragheb, R. R. T., Denslow, N. D., Alli, A. A. Lipidomic and proteomic analysis of exosomes from mouse cortical collecting duct cells.

Roles of renal ammonia metabolism other than in acid-base homeostasis

The importance of renal ammonia metabolism in acid-base homeostasis is well known. However, the effects of renal ammonia metabolism other than in acid-base homeostasis are not as widely recognized. First, ammonia differs from almost all other solutes in the urine in that it does not result from arterial delivery. Instead, ammonia is produced by the kidney, and only a portion of the ammonia produced is excreted in the urine, with the remainder returned to the systemic circulation through the renal veins. In normal individuals, systemic ammonia addition is metabolized efficiently by the liver, but in patients with either acute or chronic liver disease, conditions that increase the addition of ammonia of renal origin to the systemic circulation can result in precipitation and/or worsening of hyperammonemia. Second, ammonia appears to serve as an intrarenal paracrine signaling molecule. Hypokalemia increases proximal tubule ammonia production and secretion as well as reabsorption in the thick ascending limb of the loop of Henle, thereby increasing delivery to the renal interstitium and the collecting duct. In the collecting duct, ammonia decreases potassium secretion and stimulates potassium reabsorption, thereby decreasing urinary potassium excretion and enabling feedback correction of the initiating hypokalemia. Finally, the stimulation of renal ammonia metabolism by hypokalemia may contribute to the development of metabolic alkalosis, which in turn can stimulate NaCl reabsorption and contribute to the intravascular volume expansion, increased blood pressure and diuretic resistance that can develop with hypokalemia. The evidence supporting these novel non-acid-base roles of renal ammonia metabolism is discussed in this review.

The renal TRPV4 channel is essential for adaptation to increased dietary potassium

To maintain potassium homeostasis, kidneys exert flow-dependent potassium secretion to facilitate kaliuresis in response to elevated dietary potassium intake. This process involves stimulation of calcium-activated large conductance maxi-K (BK) channels in the distal nephron, namely the connecting tubule and the collecting duct. Recent evidence suggests that the TRPV4 channel is a critical determinant of flow-dependent intracellular calcium elevations in these segments of the renal tubule. Here, we demonstrate that elevated dietary potassium intake (five percent potassium) increases renal TRPV4 mRNA and protein levels in an aldosterone-dependent manner and causes redistribution of the channel to the apical plasma membrane in native collecting duct cells. This, in turn, leads to augmented TRPV4-mediated flow-dependent calcium ion responses in freshly isolated split-opened collecting ducts from mice fed the high potassium diet. Genetic TRPV4 ablation greatly diminished BK channel activity in collecting duct cells pointing to a reduced capacity to excrete potassium. Consistently, elevated potassium intake induced hyperkalemia in TRPV4 knockout mice due to deficient renal potassium excretion. Thus, regulation of TRPV4 activity in the distal nephron by dietary potassium is an indispensable component of whole body potassium balance.

Severe hyponatremia in a patient with schizophrenia associated with prolonged consciousness disturbance

Hyponatremia presents with various central nervous system symptoms during its course and treatment. We treated a patient who presented with a prolonged consciousness disorder and was suspected of having complications of neuroleptic malignant syndrome and osmotic demyelination syndrome (ODS) during the treatment for his hyponatremia, which was caused by syndrome of inappropriate secretion of antidiuretic hormone (SIADH). The patient was a 30-year-old Japanese man who had been under treatment for schizophrenia. He presented with profound hyponatremia (96 mEq/L) and a consciousness disorder. Because he was taking a number of antipsychotic drugs and since psychogenic polydipsia was present along with laboratory findings, the patient was diagnosed with SIADH. However, the consciousness disorder reappeared after his serum sodium concentrations were normalized, and it persisted over a long period. Although ODS was suspected from the clinical course and imaging findings, there were several inconsistencies, such as the lack of quadriplegia. The patient also showed muscular rigidity and fever, and we, therefore, diagnosed complications of malignant hyperthermia syndrome caused by the discontinuation of all antipsychotic drugs at the time of onset. There have been no reports of complications of these two conditions, and we report this case for its clinically valuable information.

Dynamic coupling between TRPV4 and Ca2+-activated SK1/3 and IK1 K+ channels plays a critical role in regulating the K+-secretory BK channel in kidney collecting duct cells

The large-conductance Ca²⁺-activated K⁺ channel, BK (KCNMA1), is expressed along the connecting tubule (CNT) and cortical collecting duct (CCD) where it underlies flow- and Ca²⁺-dependent K⁺ secretion. Its activity is partially under the control of the mechanosensitive transient receptor potential vanilloid type 4 (TRPV4) Ca²⁺-permeable channel. Recently, we identified three small-/intermediate-conductance Ca²⁺-activated K⁺ channels, SK1 (KCNN1), SK3 (KCNN3), and IK1 (KCNN4), with notably high Ca²⁺-binding affinities, that are expressed in CNT/CCD and may be regulated by TRPV4-mediated Ca²⁺ influx. The K⁺-secreting CCD mCCDcl1 cells, which express these channels, were used to determine whether SK1/3 and IK1 are activated on TRPV4 stimulation and whether they contribute to Ca²⁺ influx and activation of BK. Activation of TRPV4 (GSK1016790A) modestly depolarized the membrane potential and robustly increased intracellular Ca²⁺, [Ca²⁺]_i Inhibition of both SK1/3 and IK1 by application of apamin and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), respectively, further depolarized the membrane potential and markedly suppressed the TRPV4-mediated rise in [Ca²⁺]_i Application of BK inhibitor iberiotoxin after activation of TRPV4 without apamin/TRAM-34 also reduced [Ca²⁺]_i and further intensified membrane depolarization, demonstrating BK involvement. However, the BK-dependent effects on [Ca²⁺]_i and membrane potential were largely abolished by pretreatment with apamin and TRAM-34, identical to that observed by separately suppressing TRPV4-mediated Ca²⁺ influx, demonstrating that SK1/3-IK1 channels potently contribute to TRPV4-mediated BK activation. Our data indicate a direct correlation between TRPV4-mediated Ca²⁺ signal and BK activation but where early activation of SK1/3 and IK1 channels are critical to sufficiently enhanced Ca²⁺ entry and [Ca²⁺]_i levels required for activation of BK.

Role and mechanisms of regulation of the basolateral Kir 4.1/Kir 5.1K+ channels in the distal tubules

Epithelial K+ channels are essential for maintaining electrolyte and fluid homeostasis in the kidney. It is recognized that basolateral inward-rectifying K+ (Kir ) channels play an important role in the control of resting membrane potential and transepithelial voltage, thereby modulating water and electrolyte transport in the distal part of nephron and collecting duct. Monomeric Kir 4.1 (encoded by Kcnj10 gene) and heteromeric Kir 4.1/Kir 5.1 (Kir 4.1 together with Kir 5.1 (Kcnj16)) channels are abundantly expressed at the basolateral membranes of the distal convoluted tubule and the cortical collecting duct cells. Loss-of-function mutations in KCNJ10 cause EAST/SeSAME tubulopathy in humans associated with salt wasting, hypomagnesaemia, metabolic alkalosis and hypokalaemia. In contrast, mice lacking Kir 5.1 have severe renal phenotype that, apart from hypokalaemia, is the opposite of the phenotype seen in EAST/SeSAME syndrome. Experimental advances using genetic animal models provided critical insights into the physiological role of these channels in electrolyte homeostasis and the control of kidney function. Here, we discuss current knowledge about K+ channels at the basolateral membrane of the distal tubules with specific focus on the homomeric Kir 4.1 and heteromeric Kir 4.1/Kir 5.1 channels. Recently identified molecular mechanisms regulating expression and activity of these channels, such as cell acidification, dopamine, insulin and insulin-like growth factor-1, Src family protein tyrosine kinases, as well as the role of these channels in NCC-mediated transport in the distal convoluted tubules, are also described.

A Pilot Study Exploring the Plasma Potassium Variation in Dogs Undergoing Steroid Therapy and Its Clinical Importance

In most situations in veterinary medicine, glucocorticoids are the drugs of choice used, that is, to reduce the inflammatory response or limit an inappropriate immune response. Their use in long-term therapy may cause side effects that may weaken the patient. The aim of the study was to evaluate possible variations in the plasma potassium concentrations and their clinical relevance in dogs undergoing steroid therapy with methylprednisolone in anti-inflammatory doses. The study used a sample of 21 dogs (n = 21) presented for consultation, with a clinical condition requiring a corticosteroid therapeutic protocol with an anti-inflammatory dose of methylprednisolone. All the individuals were submitted to a corticosteroid therapeutic protocol administered orally during 18 days. During this period, 3 time points were considered: T0 (the day the prescription was first given), T1 (3 days later), and T2 (8 days later). Blood samples were collected from a peripheral vein to measure plasma potassium concentrations in T0, T1, and T2. Corticosteroid therapy on an outpatient basis statistically significantly decreased plasma potassium levels, especially between T1 and T2 (P = .03). The plasma potassium levels decreased in 12.5% of the males, compared with a decrease of 23.1% in the females. No statistically significant relationships were observe between the decreased plasma potassium levels and age, clinical condition, and patient׳s body weight. However, we found a statistically significant association between decreased plasma potassium levels and sex. The study results may justify the need for the systematic prescription of potassium supplements in patients undergoing steroid therapy, similar to what already occurs in human medicine.

Expression of a Diverse Array of Ca2+-Activated K+ Channels (SK1/3, IK1, BK) that Functionally Couple to the Mechanosensitive TRPV4 Channel in the Collecting Duct System of Kidney

The voltage- and Ca²⁺-activated, large conductance K⁺ channel (BK, maxi-K) is expressed in the collecting duct system of kidney where it underlies flow- and Ca²⁺-dependent K⁺ excretion. To determine if other Ca²⁺-activated K⁺ channels (KCa) may participate in this process, mouse kidney and the K⁺-secreting mouse cortical collecting duct (CCD) cell line, mCCDcl1, were assessed for TRPV4 and KCa channel expression and cross-talk. qPCR mRNA analysis and immunocytochemical staining demonstrated TRPV4 and KCa expression in mCCDcl1 cells and kidney connecting tubule (CNT) and CCD. Three subfamilies of KCa channels were revealed: the high Ca²⁺-binding affinity small-conductance SK channels, SK1and SK3, the intermediate conductance channel, IK1, and the low Ca²⁺-binding affinity, BK channel (BKα subunit). Apparent expression levels varied in CNT/CCD where analysis of CCD principal cells (PC) and intercalated cells (IC) demonstrated differential staining: SK1:PC<IC, and SK3:PC>IC, IK1:PC>IC, BKα:PC = IC, and TRPV4:PC>IC. Patch clamp analysis and fluorescence Ca²⁺ imaging of mCCDcl1 cells demonstrated potent TRPV4-mediated Ca²⁺ entry and strong functional cross-talk between TRPV4 and KCa channels. TRPV4-mediated Ca²⁺ influx activated each KCa channel, as evidenced by selective inhibition of KCa channels, with each active KCa channel enhancing Ca²⁺ entry (due to membrane hyperpolarization). Transepithelial electrical resistance (TEER) analysis of confluent mCCDcl1 cells grown on permeable supports further demonstrated this cross-talk where TRPV4 activation induce a decrease in TEER which was partially restored upon selective inhibition of each KCa channel. It is concluded that SK1/SK3 and IK1 are highly expressed along with BKα in CNT and CCD and are closely coupled to TRPV4 activation as observed in mCCDcl1 cells. The data support a model in CNT/CCD segments where strong cross talk between TRPV4-mediated Ca²⁺ influx and each KCa channel leads to enhance Ca²⁺ entry which will support activation of the low Ca²⁺-binding affinity BK channel to promote BK-mediated K⁺ secretion.

mTORC2 critically regulates renal potassium handling

The mTOR pathway orchestrates cellular homeostasis. The rapamycin-sensitive mTOR complex (mTORC1) in the kidney has been widely studied; however, mTORC2 function in renal tubules is poorly characterized. Here, we generated mice lacking mTORC2 in the distal tubule (Rictorfl/fl Ksp-Cre mice), which were viable and had no obvious phenotype, except for a 2.5-fold increase in plasma aldosterone. Challenged with a low-Na+ diet, these mice adequately reduced Na+ excretion; however, Rictorfl/fl Ksp-Cre mice rapidly developed hyperkalemia on a high-K+ diet, despite a 10-fold increase in serum aldosterone levels, implying that mTORC2 regulates kaliuresis. Phosphorylation of serum- and glucocorticoid-inducible kinase 1 (SGK1) and PKC-α was absent in Rictorfl/fl Ksp-Cre mice, indicating a functional block in K+ secretion activation via ROMK channels. Indeed, patch-clamp experiments on split-open tubular segments from the transition zone of the late connecting tubule and early cortical collecting duct demonstrated that Ba2+-sensitive apical K+ currents were barely detectable in the majority of Rictorfl/fl Ksp-Cre mice. Conversely, epithelial sodium channel (ENaC) activity was largely preserved, suggesting that the reduced ability to maintain K+ homeostasis is the result of impaired apical K+ conductance and not a reduced electrical driving force for K+ secretion. Thus, these data unravel a vital and nonredundant role of mTORC2 for distal tubular K+ handling.

Autoantibodies Targeting a Collecting Duct-Specific Water Channel in Tubulointerstitial Nephritis

Tubulointerstitial nephritis is a common cause of kidney failure and may have diverse etiologies. This form of nephritis is sometimes associated with autoimmune disease, but the role of autoimmune mechanisms in disease development is not well understood. Here, we present the cases of three patients with autoimmune polyendocrine syndrome type 1 who developed tubulointerstitial nephritis and ESRD in association with autoantibodies against kidney collecting duct cells. One of the patients developed autoantibodies targeting the collecting duct-specific water channel aquaporin 2, whereas autoantibodies of the two other patients reacted against the HOXB7 or NFAT5 transcription factors, which regulate the aquaporin 2 promoter. Our findings suggest that tubulointerstitial nephritis developed in these patients as a result of an autoimmune insult on the kidney collecting duct cells.

Maintaining K+ balance on the low-Na+, high-K+ diet

A low-Na⁺, high-K⁺ diet (LNaHK) is considered a healthier alternative to the "Western" high-Na⁺ diet. Because the mechanism for K⁺ secretion involves Na⁺ reabsorptive exchange for secreted K⁺ in the distal nephron, it is not understood how K⁺ is eliminated with such low Na⁺ intake. Animals on a LNaHK diet produce an alkaline load, high urinary flows, and markedly elevated plasma ANG II and aldosterone levels to maintain their K⁺ balance. Recent studies have revealed a potential mechanism involving the actions of alkalosis, urinary flow, elevated ANG II, and aldosterone on two types of K⁺ channels, renal outer medullary K⁺ and large-conductance K⁺ channels, located in principal and intercalated cells. Here, we review these recent advances.