Altered expression of renal NHE3, TSC, BSC-1, and ENaC subunits in potassium-depleted rats

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Magnesium in hypertension: mechanisms and clinical implications

Hypertension is associated with increased risk of cardiovascular disease and death. Evidence suggests that Mg2+ depletion contributes to hypertension. It is estimated that 25% or more of the United States population experiences chronic, latent Mg2+ depletion. This review explores mechanisms by which Mg2+ influences blood pressure, modifying risk of hypertension and complicating its treatment. Mechanisms addressed include effects upon i) sympathetic tone, via the modulation of N-methyl-D-aspartate (NMDA) receptor and N-type Ca2+ channel activity, influencing catecholamine release from sympathetic nerve endings; ii) vascular tone, via alteration of L-type Ca2+ and endothelial nitric oxide synthase (eNOS) activity and prostacyclin release; iii) renal K+ handling, influencing systemic K+ balance and potentially indirectly influencing blood pressure; iv) aldosterone secretion from the adrenal cortex; and v) modulation of pro-hypertensive inflammatory processes in dendritic cells and macrophages, including activation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome and stimulation of isolevuglandin (IsoLG) production. Discovery of these mechanisms has furthered our understanding of the pathogenesis of hypertension, with implications for treatment and has highlighted the role of Mg2+ balance in hypertension and cardiovascular disease.

Role of paraoxonase 3 in regulating ENaC-mediated Na+ transport in the distal nephron

Paraoxonase 3 (PON3) is expressed in the aldosterone-sensitive distal nephron, where filtered Na+ is reabsorbed mainly via the epithelial Na+ channel (ENaC) and Na+ -coupled co-transporters. We previously showed that PON3 negatively regulates ENaC through a chaperone mechanism. The present study aimed to determine the physiological role of PON3 in renal Na+ and K+ homeostasis. Pon3 knockout (KO) mice had higher amiloride-induced natriuresis and lower plasma [K+ ] at baseline. Single channel recordings in split-open tubules showed that the number of active channels per patch was significantly higher in KO mice, resulting in a higher channel activity in the absence of PON3. Although whole kidney abundance of ENaC subunits was not altered in Pon3 KOs, ENaC gamma subunit was more apically distributed within the connecting tubules and cortical collecting ducts of Pon3 KO kidneys. Additionally, small interfering RNA-mediated knockdown of PON3 in cultured mouse cortical collecting duct cells led to an increased surface abundance of ENaC gamma subunit. As a result of lower plasma [K+ ], sodium chloride co-transporter phosphorylation was enhanced in the KO kidneys, a phenotype that was corrected by a high K+ diet. Finally, PON3 expression was upregulated in mouse kidneys under dietary K+ restriction, potentially providing a mechanism to dampen ENaC activity and associated K+ secretion. Taken together, our results show that PON3 has a role in renal Na+ and K+ homeostasis through regulating ENaC functional expression in the distal nephron. KEY POINTS: Paraoxonase 3 (PON3) is expressed in the distal nephron of mouse kidneys and functions as a molecular chaperone to reduce epithelial Na+ channel (ENaC) expression and activity in heterologous expression systems. We examined the physiological role of PON3 in renal Na+ and K+ handling using a Pon3 knockout (KO) mouse model. At baseline, Pon3 KO mice had lower blood [K+ ], more functional ENaC in connecting tubules/cortical collecting ducts, higher amiloride-induced natriuresis, and enhanced sodium chloride co-transporter (NCC) phosphorylation. Upon challenge with a high K+ diet, Pon3 KO mice had normalized blood [K+ ] and -NCC phosphorylation but lower circulating aldosterone levels compared to their littermate controls. Kidney PON3 abundance was altered in mice under dietary K+ loading or K+ restriction, providing a potential mechanism for regulating ENaC functional expression and renal Na+ and K+ homeostasis in the distal nephron.

Trimethoprim inhibits renal H+-K+-ATPase in states of K+ depletion

There is growing consensus that under physiological conditions, collecting duct H+ secretion is independent of epithelial Na+ channel (ENaC) activity. We have recently shown that the direct ENaC inhibitor benzamil acutely impairs H+ excretion by blocking renal H+-K+-ATPase. However, the question remains whether inhibition of ENaC per se causes alterations in renal H+ excretion. To revisit this question, we studied the effect of the antibiotic trimethoprim (TMP), which is well known to cause K+ retention by direct ENaC inhibition. The acute effect of TMP (5 µg/g body wt) was assessed in bladder-catheterized mice, allowing real-time measurement of urinary pH, electrolyte, and acid excretion. Dietary K+ depletion was used to increase renal H+-K+-ATPase activity. In addition, the effect of TMP was investigated in vitro using pig gastric H+-K+-ATPase-enriched membrane vesicles. TMP acutely increased natriuresis and decreased kaliuresis, confirming its ENaC-inhibiting property. Under control diet conditions, TMP had no effect on urinary pH or acid excretion. Interestingly, K+ depletion unmasked an acute urine alkalizing effect of TMP. This finding was corroborated by in vitro experiments showing that TMP inhibits H+-K+-ATPase activity, albeit at much higher concentrations than benzamil. In conclusion, under control diet conditions, TMP inhibited ENaC function without changing urinary H+ excretion. This finding further supports the hypothesis that the inhibition of ENaC per se does not impair H+ excretion in the collecting duct. Moreover, TMP-induced urinary alkalization in animals fed a low-K+ diet highlights the importance of renal H+-K+-ATPase-mediated H+ secretion in states of K+ depletion.NEW & NOTEWORTHY The antibiotic trimethoprim (TMP) often mediates K+ retention and metabolic acidosis. We suggest a revision of the underlying mechanism that causes metabolic acidosis. Our results indicate that TMP-induced metabolic acidosis is secondary to epithelial Na+ channel-dependent K+ retention. Under control dietary conditions, TMP does not per se inhibit collecting duct H+ secretion. These findings add further argument against a physiologically relevant voltage-dependent mechanism of collecting duct H+ excretion.

Nephrogenic diabetes insipidus: a comprehensive overview

Nephrogenic diabetes insipidus (NDI) is characterized by the inability to concentrate urine that results in polyuria and polydipsia, despite having normal or elevated plasma concentrations of arginine vasopressin (AVP). In this study, we review the clinical aspects and diagnosis of NDI, the various etiologies, current treatment options and potential future developments. NDI has different clinical manifestations and approaches according to the etiology. Hereditary forms of NDI are mainly caused by mutations in the genes that encode key proteins in the AVP signaling pathway, while acquired causes are normally associated with specific drug exposure, especially lithium, and hydroelectrolytic disorders. Clinical manifestations of the disease vary according to the degree of dehydration and hyperosmolality, being worse when renal water losses cannot be properly compensated by fluid intake. Regarding the diagnosis of NDI, it is important to consider the symptoms of the patient and the diagnostic tests, including the water deprivation test and the baseline plasma copeptin measurement, a stable surrogate biomarker of AVP release. Without proper treatment, patients may developcomplications leading to high morbidity and mortality, such as severe dehydration and hypernatremia. In that sense, the treatment of NDI consists in decreasing the urine output, while allowing appropriate fluid balance, normonatremia, and ensuring an acceptable quality of life. Therefore, therapeutic options include nonpharmacological interventions, including sufficient water intake and a low-sodium diet, and pharmacological treatment. The main medications used for NDI are thiazide diuretics, nonsteroidal anti-inflammatory drugs (NSAIDs), and amiloride, used isolated or in combination.

Dietary K+ acts as a genuine diuretic

K⁺ balance in mammals relies on regulated renal K⁺ excretion matching unregulated fluctuating K⁺ intake. Upon a K⁺ rich meal, rapid and powerful K⁺ excretion is needed. Renal K⁺ secretion is stimulated by the increased tubular flow. We speculated that high K⁺ intake acutely increases urinary flow to stimulate K⁺ excretion.

METHODS

Mice were K⁺ challenged through diets or gavage. Post K⁺ loading urinary output, osmolarity, [K⁺ ]_u , [Na⁺ ]_u , plasma osmolarity, [copeptin]_p , [K⁺ ]_p , and [Na⁺ ]_p were measured. To locate the mechanism of K⁺ -induced diuresis in the glomerular/tubular system we measured creatinine excretion and assessed functional transport in isolated perfused TALs and CDs during an acute [K⁺ ]_bl switch from 3.6 to 6.5 mM. Molecular adaptations of transport proteins involved in water reabsorption were investigated by immunoblotting.

RESULTS

(1) Mice switched from a 1% to 2% K⁺ diet increased diuresis within 12 hours and reciprocally reduced diuresis when switched from 1% to 0.01% K⁺ diet. (2) A single K⁺ gavage load, corresponding to 25%-50% of daily K⁺ intake, induced 100% increase in diuresis within 30 minutes. This occurred despite augmented plasma osmolarity and AVP synthesis. (3) K⁺ gavage did not change GFR. (4) In isolated perfused TALs, shifting [K⁺ ]_bl from 3.6 to 6.5 mM did not affect AVP-induced NaCl transport. (5) In sharp contrast, in isolated perfused CDs, shifting [K⁺ ]_bl from 3.6 to 6.5 mM markedly reduced CD AVP sensitivity, ie inhibited water absorption.

CONCLUSION

Dietary K⁺ loading induces a rapidly on-setting diuresis. The mechanism of K⁺ -induced diuresis involves desensitization of the CD to AVP.

Urinary Extracellular Vesicles for Renal Tubular Transporters Expression in Patients With Gitelman Syndrome

Background: The utility of urinary extracellular vesicles (uEVs) to faithfully represent the changes of renal tubular protein expression remains unclear. We aimed to evaluate renal tubular sodium (Na⁺) or potassium (K⁺) associated transporters expression from uEVs and kidney tissues in patients with Gitelman syndrome (GS) caused by inactivating mutations in SLC12A3.

Methods: uEVs were isolated by ultracentrifugation from 10 genetically-confirmed GS patients. Membrane transporters including Na⁺-hydrogen exchanger 3 (NHE3), Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2), NaCl cotransporter (NCC), phosphorylated NCC (p-NCC), epithelial Na⁺ channel β (ENaCβ), pendrin, renal outer medullary K1 channel (ROMK), and large-conductance, voltage-activated and Ca²⁺-sensitive K⁺ channel (Maxi-K) were examined by immunoblotting of uEVs and immunofluorescence of biopsied kidney tissues. Healthy and disease (bulimic patients) controls were also enrolled.

Results: Characterization of uEVs was confirmed by nanoparticle tracking analysis, transmission electron microscopy, and immunoblotting. Compared with healthy controls, uEVs from GS patients showed NCC and p-NCC abundance were markedly attenuated but NHE3, ENaCβ, and pendrin abundance significantly increased. ROMK and Maxi-K abundance were also significantly accentuated. Immunofluorescence of the representative kidney tissues from GS patients also demonstrated the similar findings to uEVs. uEVs from bulimic patients showed an increased abundance of NCC and p-NCC as well as NHE3, NKCC2, ENaCβ, pendrin, ROMK and Maxi-K, akin to that in immunofluorescence of their kidney tissues.

Conclusion: uEVs could be a non-invasive tool to diagnose and evaluate renal tubular transporter adaptation in patients with GS and may be applied to other renal tubular diseases.

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."

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.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

Impact of angiotensin II-mediated stimulation of sodium transporters in the nephron assessed by computational modeling

Angiotensin II (ANG II) raises blood pressure partly by stimulating tubular Na⁺ reabsorption. The effects of ANG II on tubular Na⁺ transporters (i.e., channels, pumps, cotransporters, and exchangers) vary between short-term and long-term exposure. To better understand the physiological impact, we used a computational model of transport along the rat nephron to predict the effects of short- and long-term ANG II-induced transporter activation on Na⁺ and K⁺ reabsorption/secretion, and to compare measured and calculated excretion rates. Three days of ANG II infusion at 200 ng·kg^-1·min^-1 is nonpressor, yet stimulates transporter accumulation. The increase in abundance of Na⁺/H⁺ exchanger 3 (NHE3) or activated Na⁺-K⁺-2Cl^- cotransporter-2 (NKCC2-P) predicted significant reductions in urinary Na⁺ excretion, yet there was no observed change in urine Na⁺. The lack of antinatriuresis, despite Na⁺ transporter accumulation, was supported by Li⁺ and creatinine clearance measurements, leading to the conclusion that 3-day nonpressor ANG II increases transporter abundance without proportional activation. Fourteen days of ANG II infusion at 400 ng·kg^-1·min^-1 raises blood pressure and increases Na⁺ transporter abundance along the distal nephron; proximal tubule and medullary loop transporters are decreased and urine Na⁺ and volume output are increased, evidence for pressure natriuresis. Simulations indicate that decreases in NHE3 and NKCC2-P contribute significantly to reducing Na⁺ reabsorption along the nephron and to pressure natriuresis. Our results also suggest that differential regulation of medullary (decrease) and cortical (increase) NKCC2-P is important to preserve K⁺ while minimizing Na⁺ retention during ANG II infusion. Lastly, our model indicates that accumulation of active Na⁺-Cl^- cotransporter counteracts epithelial Na⁺ channel-induced urinary K⁺ loss.

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.

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.

Regulation of renal Na transporters in response to dietary K

Changes in the expression of Na transport proteins were measured in the kidneys of mice with increased dietary K intake for 1 wk. The epithelial Na channel (ENaC) was upregulated, with enhanced expression of full-length and cleaved forms of α-ENaC and cleaved γ-ENaC. At the same time, the amount of the NaCl cotransporter NCC and its phosphorylated form decreased by ~50% and ~80%, respectively. The expression of the phosphorylated form of the Na-K-2Cl cotransporter NKCC2 also decreased, despite an increase in overall protein content. The effect was stronger in males (80%) than in females (40%). This implies that less Na⁺ is reabsorbed in the thick ascending limb of Henle's loop and distal convoluted tubule along with Cl^-, whereas more is reabsorbed in the aldosterone-sensitive distal nephron in exchange for secreted K⁺. The abundance of the proximal tubule Na/H exchanger NHE3 decreased by ~40%, with similar effects in males and females. Time-course studies indicated that NCC and NHE3 proteins decreased progressively over 7 days on a high-K diet. Expression of mRNA encoding these proteins increased, implying that the decreased protein levels resulted from decreased rates of synthesis or increased rates of degradation. The potential importance of changes in NHE3, NKCC2, and NCC in promoting K⁺ excretion was assessed with a mathematical model. Simulations indicated that decreased NHE3 produced the largest effect. Regulation of proximal tubule Na⁺ transport may play a significant role in achieving K homeostasis.

Renal potassium handling in rats with subtotal nephrectomy: modeling and analysis

We sought to decipher the mechanisms underlying the kidney's response to changes in K⁺ load and intake, under physiological and pathophysiological conditions. To accomplish that goal, we applied a published computational model of epithelial transport along rat nephrons in a sham rat, an uninephrectomized (UNX) rat, and a 5/6-nephrectomized (5/6-NX) rat that also considers adaptations in glomerular filtration rate and tubular growth. Model simulations of an acute K⁺ load indicate that elevated expression levels and activities of Na⁺/K⁺-ATPase, epithelial sodium channels, large-conductance Ca²⁺-activated K⁺ channels, and renal outer medullary K⁺ channels, together with downregulation of sodium-chloride cotransporters (NCC), increase K⁺ secretion along the connecting tubule, resulting in a >6-fold increase in urinary K⁺ excretion in sham rats, which substantially exceeds the filtered K⁺ load. In the UNX and 5/6-NX models, the acute K⁺ load is predicted to increase K⁺ excretion, but at significantly reduced levels compared with sham. Acute K⁺ load is accompanied by natriuresis in sham rats. Model simulations suggest that the lesser natriuretic effect observed in the nephrectomized groups may be explained by impaired NCC downregulation in these kidneys. At a single-nephron level, a high K⁺ intake raises K⁺ secretion along the connecting tubule and reabsorption along the collecting duct in sham, and even more in UNX and 5/6-NX. However, the increased K⁺ secretion per tubule fails to sufficiently compensate for the reduction in nephron number, such that nephrectomized rats have an impaired ability to excrete an acute or chronic K⁺ load.

Ammonia Transporters and Their Role in Acid-Base Balance

Acid-base homeostasis is critical to maintenance of normal health. Renal ammonia excretion is the quantitatively predominant component of renal net acid excretion, both under basal conditions and in response to acid-base disturbances. Although titratable acid excretion also contributes to renal net acid excretion, the quantitative contribution of titratable acid excretion is less than that of ammonia under basal conditions and is only a minor component of the adaptive response to acid-base disturbances. In contrast to other urinary solutes, ammonia is produced in the kidney and then is selectively transported either into the urine or the renal vein. The proportion of ammonia that the kidney produces that is excreted in the urine varies dramatically in response to physiological stimuli, and only urinary ammonia excretion contributes to acid-base homeostasis. As a result, selective and regulated renal ammonia transport by renal epithelial cells is central to acid-base homeostasis. Both molecular forms of ammonia, NH₃ and NH₄⁺, are transported by specific proteins, and regulation of these transport processes determines the eventual fate of the ammonia produced. In this review, we discuss these issues, and then discuss in detail the specific proteins involved in renal epithelial cell ammonia transport.

Renal Tubular Ubiquitin-Protein Ligase NEDD4-2 Is Required for Renal Adaptation during Long-Term Potassium Depletion

Adaptation of the organism to potassium (K⁺) deficiency requires precise coordination among organs involved in K⁺ homeostasis, including muscle, liver, and kidney. How the latter performs functional and molecular changes to ensure K⁺ retention is not well understood. Here, we investigated the role of ubiquitin-protein ligase NEDD4-2, which negatively regulates the epithelial sodium channel (ENaC), Na⁺/Cl^- cotransporter (NCC), and with no-lysine-kinase 1 (WNK1). After dietary K⁺ restriction for 2 weeks, compared with control littermates, inducible renal tubular NEDD4-2 knockout (Nedd4L^Pax8/LC1 ) mice exhibited severe hypokalemia and urinary K⁺ wasting. Notably, expression of the ROMK K⁺ channel did not change in the distal convoluted tubule and decreased slightly in the cortical/medullary collecting duct, whereas BK channel abundance increased in principal cells of the connecting tubule/collecting ducts. However, K⁺ restriction also enhanced ENaC expression in Nedd4L^Pax8/LC1 mice, and treatment with the ENaC inhibitor, benzamil, reversed excessive K⁺ wasting. Moreover, K⁺ restriction increased WNK1 and WNK4 expression and enhanced SPAK-mediated NCC phosphorylation in Nedd4L^Pax8/LC1 mice, with no change in total NCC. We propose a mechanism in which NEDD4-2 deficiency exacerbates hypokalemia during dietary K⁺ restriction primarily through direct upregulation of ENaC, whereas increased BK channel expression has a less significant role. These changes outweigh the compensatory antikaliuretic effects of diminished ROMK expression, increased NCC phosphorylation, and enhanced WNK pathway activity in the distal convoluted tubule. Thus, NEDD4-2 has a crucial role in K⁺ conservation through direct and indirect effects on ENaC, distal nephron K⁺ channels, and WNK signaling.

Na+/HCO3- Cotransporter NBCn2 Mediates HCO3- Reclamation in the Apical Membrane of Renal Proximal Tubules

The kidney maintains systemic acid-base balance by reclaiming from the renal tubule lumen virtually all HCO₃^- filtered in glomeruli and by secreting additional H⁺ to titrate luminal buffers. For proximal tubules, which are responsible for about 80% of this activity, it is believed that HCO₃^- reclamation depends solely on H⁺ secretion, mediated by the apical Na⁺/H⁺ exchanger NHE₃ and the vacuolar proton pump. However, _NHE3 and the proton pump cannot account for all HCO₃^- reclamation. Here, we investigated the potential contribution of two variants of the electroneutral Na⁺/HCO₃^- cotransporter NBCn2, the amino termini of which start with the amino acids MCDL (MCDL-NBCn2) and MEIK (MEIK-NBCn2). Western blot analysis and immunocytochemistry revealed that MEIK-NBCn2 predominantly localizes at the basolateral membrane of medullary thick ascending limbs in the rat kidney, whereas MCDL-NBCn2 localizes at the apical membrane of proximal tubules. Notably, NH₄Cl-induced systemic metabolic acidosis or hypokalemic alkalosis downregulated the abundance of MCDL-NBCn2 and reciprocally upregulated NHE₃ Conversely, NaHCO₃-induced metabolic alkalosis upregulated MCDL-NBCn2 and reciprocally downregulated NHE₃ We propose that the apical membrane of the proximal tubules has two distinct strategies for HCO₃^- reclamation: the conventional indirect pathway, in which NHE₃ and the proton pump secrete H⁺ to titrate luminal HCO₃^-, and the novel direct pathway, in which NBCn2 removes HCO₃^- from the lumen. The reciprocal regulation of NBCn2 and NHE₃ under different physiologic conditions is consistent with our mathematical simulations, which suggest that HCO₃^- uptake and H⁺ secretion have reciprocal efficiencies for HCO₃^- reclamation versus titration of luminal buffers.

Potassium Homeostasis: The Knowns, the Unknowns, and the Health Benefits

Potassium homeostasis has a very high priority because of its importance for membrane potential. Although extracellular K+ is only 2% of total body K+, our physiology was evolutionarily tuned for a high-K+, low-Na+ diet. We review how multiple systems interface to accomplish fine K+ balance and the consequences for health and disease.

Electrolyte disturbances and risk factors of acute kidney injury patients receiving dialysis in exertional heat stroke

Background

Exertional heat stroke (EHS) is a life-threatening illness and leads to multi-organ dysfunction including acute kidney injury (AKI). The clinical significance of abnormal electrolytes and renal outcomes in ESH patients has been poorly documented. We aim to exhibit the electrolyte abnormalities, renal outcomes and risk factors of patients with AKI receiving dialysis in EHS.

Methods

A retrospective cohort study in EHS patients between 2003 and 2014 were conducted. Clinical and laboratory outcomes including serum and urine electrolytes, AKI and dialysis were assessed on admission, during hospitalization and at the time of their discharge from the hospital. A logistic regression analysis was performed for risk factors of acute dialysis.

Results

All 66 subjects with mean age 22.1 ± 4.3 years were included. On admission, the common electrolyte disturbances were hypokalemia (71.2 %), hypophosphatemia (59.1 %), hyponatremia (53.0 %), hypocalcemia (51.5 %), and hypomagnesemia (34.9 %). Electrolytes depletion was confirmed as renal loss (potassium loss; 54.2 %, phosphate loss; 86.7 %, sodium loss; 64.7 % and magnesium loss; 83.3 %). During hospitalization ranging from 2 to 209 days, 90.9 % patients suffered from AKI with 16.7 % receiving acute dialysis, and 3 % patients died. At discharge, AKI and electrolyte abnormalities had dramatically improved. The prognosis factors for AKI receiving dialysis were identified as neurological status, renal function and serum muscle enzyme at time of admission.

Conclusion

The study suggests that hypoelectrolytemia and AKI are frequently observed in patients with EHS. Neurological impairment, impaired renal function, and increased serum muscle enzyme should be considered risk factors of acute dialysis.

Mood Disorders in Uncontrolled Hypertension Despite Multiple Anti-Hypertensive Medications: Searching for a Link

INTRODUCTION

Resistant hypertension is a clinical condition in which blood pressure (BP) control is not achieved under a pharmacological therapy including a diuretic and at least two additional antihypertensive drug classes.

AIM

To discuss an unusual presentation of uncontrolled hypertension despite multiple anti-hypertensive medications.

METHODS AND RESULTS

A 46-year-old woman presented with resistant hypertension (HT) and with a long history of polydipsia, polyuria, weight loss and psychiatric symptoms (sudden onset of personality disorder with free anxiety, negativism and asthenia) unsuccessfully treated with antidepressant drugs. Tests for secondary HT showed a marked increase of serum renin and aldosterone both in clinostatic (342 pg/ml and 907 pmol/l, respectively) and orthostatic posture (351 pg/ml and 2845 pmol/l, respectively), hypokalemia (2.9 mmol/l) and macroalbuminuria (431 mg/day). Diagnostic examinations also revealed a focal stenosis of approximately 70 % of the proximal right renal artery with post-stenotic dilation. After percutaneous balloon angioplasty and stent implantation, BP was normalized with 5 mg/day amlodipine and psychiatric symptoms suddenly disappeared.

CONCLUSIONS

Psychopathological symptoms are rare at the onset of hyperaldosteronism, and their aetiology is not well defined. A proper diagnostic and therapeutic process is mandatory in order to get the recommended therapeutic targets in short-midterm improving long-term prognosis. We also suggest not considering depressed or treat with antidepressant agents a young hypertensive subject with uncontrolled hypertension despite multiple anti-hypertensive medications without having ruled out a secondary form of hypertension.

Autophagic degradation of aquaporin-2 is an early event in hypokalemia-induced nephrogenic diabetes insipidus

Hypokalemia (low serum potassium level) is a common electrolyte imbalance that can cause a defect in urinary concentrating ability, i.e., nephrogenic diabetes insipidus (NDI), but the molecular mechanism is unknown. We employed proteomic analysis of inner medullary collecting ducts (IMCD) from rats fed with a potassium-free diet for 1 day. IMCD protein quantification was performed by mass spectrometry using a label-free methodology. A total of 131 proteins, including the water channel AQP2, exhibited significant changes in abundance, most of which were decreased. Bioinformatic analysis revealed that many of the down-regulated proteins were associated with the biological processes of generation of precursor metabolites and energy, actin cytoskeleton organization, and cell-cell adhesion. Targeted LC-MS/MS and immunoblotting studies further confirmed the down regulation of 18 selected proteins. Electron microscopy showed autophagosomes/autophagolysosomes in the IMCD cells of rats deprived of potassium for only 1 day. An increased number of autophagosomes was also confirmed by immunofluorescence, demonstrating co-localization of LC3 and Lamp1 with AQP2 and several other down-regulated proteins in IMCD cells. AQP2 was also detected in autophagosomes in IMCD cells of potassium-deprived rats by immunogold electron microscopy. Thus, enhanced autophagic degradation of proteins, most notably including AQP2, is an early event in hypokalemia-induced NDI.

Immunolocalization of hyperpolarization-activated cationic HCN1 and HCN3 channels in the rat nephron: regulation of HCN3 by potassium diets

Hyperpolarization-activated cationic and cyclic nucleotide-gated channels (HCN) comprise four homologous subunits (HCN1-HCN4). HCN channels are found in excitable and non-excitable tissues in mammals. We have previously shown that HCN2 may transport ammonium (NH4 (+)), besides sodium (Na(+)), in the rat distal nephron. In the present work, we identified HCN1 and HCN3 in the proximal tubule (PT) and HCN3 in the thick ascending limb of Henle (TALH) of the rat kidney. Immunoblot assays detected HCN1 (130 kDa) and HCN3 (90 KDa) and their truncated proteins C-terminal HCN1 (93 KDa) and N-terminal HCN3 (65 KDa) in enriched plasma membranes from cortex (CX) and outer medulla (OM), as well as in brush-border membrane vesicles. Immunofluorescence assays confirmed apical localization of HCN1 and HCN3 in the PT. HCN3 was also found at the basolateral membrane of TALH. We evaluated chronic changes in mineral dietary on HCN3 protein abundance. Animals were fed with three different diets: sodium-deficient (SD) diet, potassium-deficient (KD) diet, and high-potassium (HK) diet. Up-regulation of HCN3 was observed in OM by KD and in CX and OM by HK; the opposite effect occurred with the N-terminal truncated HCN3 in CX (KD) and OM (HK). SD diet did not produce any change. Since HCN channels activate with membrane hyperpolarization, our results suggest that HCN channels may play a role in the Na(+)-K(+)-ATPase activity, contributing to Na(+), K(+), and acid-base homeostasis in the rat kidney.

Dietary potassium and the renal control of salt balance and blood pressure

Dietary potassium (K(+)) intake has antihypertensive effects, prevents strokes, and improves cardiovascular outcomes. The underlying mechanism for these beneficial effects of high K(+) diets may include vasodilation, enhanced urine flow, reduced renal renin release, and negative sodium (Na(+)) balance. Indeed, several studies demonstrate that dietary K(+) intake induces renal Na(+) loss despite elevated plasma aldosterone. This review briefly highlights the epidemiological and experimental evidences for the effects of dietary K(+) on arterial blood pressure. It discusses the pivotal role of the renal distal tubule for the regulation of urinary K(+) and Na(+) excretion and blood pressure and highlights that it depends on the coordinated interaction of different nephron portions, epithelial cell types, and various ion channels, transporters, and ATPases. Moreover, we discuss the relevance of aldosterone and aldosterone-independent factors in mediating the effects of an altered K(+) intake on renal K(+) and Na(+) handling. Particular focus is given to findings suggesting that an aldosterone-independent downregulation of the thiazide-sensitive NaCl cotransporter significantly contributes to the natriuretic and antihypertensive effect of a K(+)-rich diet. Last but not least, we refer to the complex signaling pathways enabling the kidney to adapt its function to the homeostatic needs in response to an altered K(+) intake. Future work will have to further address the underlying cellular and molecular mechanism and to elucidate, among others, how an altered dietary K(+) intake is sensed and how this signal is transmitted to the different epithelial cells lining the distal tubule.

Renal aquaporins and water balance disorders

BACKGROUND

Aquaporins (AQPs) are a family of proteins that can act as water channels. Regulation of AQPs is critical to osmoregulation and the maintenance of body water homeostasis. Eight AQPs are expressed in the kidney of which five have been shown to play a role in body water balance; AQP1, AQP2, AQP3, AQP4 and AQP7. AQP2 in particular is regulated by vasopressin.

SCOPE OF REVIEW

This review summarizes our current knowledge of the underlying mechanisms of various water balance disorders and their treatment strategies.

MAJOR CONCLUSIONS

Dysfunctions of AQPs are involved in disorders associated with disturbed water homeostasis. Hyponatremia with increased AQP levels can be caused by diseases with low effective circulating blood volume, such as congestive heart failure, or osmoregulation disorders such as the syndrome of inappropriate secretion of antidiuretic hormone. Treatment consists of fluid restriction, demeclocycline and vasopressin type-2 receptor antagonists. Decreased AQP levels can lead to diabetes insipidus (DI), characterized by polyuria and polydipsia. In central DI, vasopressin production is impaired, while in gestational DI, levels of the vasopressin-degrading enzyme vasopressinase are abnormally increased. Treatment consists of the vasopressin analogue dDAVP. Nephrogenic DI is caused by the inability of the kidney to respond to vasopressin and can be congenital, but is most commonly acquired, usually due to lithium therapy. Treatment consists of sufficient fluid supply, low-solute diet and diuretics.

GENERAL SIGNIFICANCE

In recent years, our understanding of the underlying mechanisms of water balance disorders has increased enormously, which has opened up several possible new treatment strategies. This article is part of a Special Issue entitled Aquaporins.

Nephrogenic diabetes insipidus: essential insights into the molecular background and potential therapies for treatment

The water channel aquaporin-2 (AQP2), expressed in the kidney collecting ducts, plays a pivotal role in maintaining body water balance. The channel is regulated by the peptide hormone arginine vasopressin (AVP), which exerts its effects through the type 2 vasopressin receptor (AVPR2). Disrupted function or regulation of AQP2 or the AVPR2 results in nephrogenic diabetes insipidus (NDI), a common clinical condition of renal origin characterized by polydipsia and polyuria. Over several years, major research efforts have advanced our understanding of NDI at the genetic, cellular, molecular, and biological levels. NDI is commonly characterized as hereditary (congenital) NDI, arising from genetic mutations in the AVPR2 or AQP2; or acquired NDI, due to for exmple medical treatment or electrolyte disturbances. In this article, we provide a comprehensive overview of the genetic, cell biological, and pathophysiological causes of NDI, with emphasis on the congenital forms and the acquired forms arising from lithium and other drug therapies, acute and chronic renal failure, and disturbed levels of calcium and potassium. Additionally, we provide an overview of the exciting new treatment strategies that have been recently proposed for alleviating the symptoms of some forms of the disease and for bypassing G protein-coupled receptor signaling.

Renal ammonia metabolism and transport

Renal ammonia metabolism and transport mediates a central role in acid-base homeostasis. In contrast to most renal solutes, the majority of renal ammonia excretion derives from intrarenal production, not from glomerular filtration. Renal ammoniagenesis predominantly results from glutamine metabolism, which produces 2 NH4(+) and 2 HCO3(-) for each glutamine metabolized. The proximal tubule is the primary site for ammoniagenesis, but there is evidence for ammoniagenesis by most renal epithelial cells. Ammonia produced in the kidney is either excreted into the urine or returned to the systemic circulation through the renal veins. Ammonia excreted in the urine promotes acid excretion; ammonia returned to the systemic circulation is metabolized in the liver in a HCO3(-)-consuming process, resulting in no net benefit to acid-base homeostasis. Highly regulated ammonia transport by renal epithelial cells determines the proportion of ammonia excreted in the urine versus returned to the systemic circulation. The traditional paradigm of ammonia transport involving passive NH3 diffusion, protonation in the lumen and NH4(+) trapping due to an inability to cross plasma membranes is being replaced by the recognition of limited plasma membrane NH3 permeability in combination with the presence of specific NH3-transporting and NH4(+)-transporting proteins in specific renal epithelial cells. Ammonia production and transport are regulated by a variety of factors, including extracellular pH and K(+), and by several hormones, such as mineralocorticoids, glucocorticoids and angiotensin II. This coordinated process of regulated ammonia production and transport is critical for the effective maintenance of acid-base homeostasis.

Congenital nephrogenic diabetes insipidus: the current state of affairs

The anti-diuretic hormone arginine vasopressin (AVP) is released from the pituitary upon hypovolemia or hypernatremia, and regulates water reabsorption in the renal collecting duct principal cells. Binding of AVP to the arginine vasopressin receptor type 2 (AVPR2) in the basolateral membrane leads to translocation of aquaporin 2 (AQP2) water channels to the apical membrane of the collecting duct principal cells, inducing water permeability of the membrane. This results in water reabsorption from the pro-urine into the medullary interstitium following an osmotic gradient. Congenital nephrogenic diabetes insipidus (NDI) is a disorder associated with mutations in either the AVPR2 or AQP2 gene, causing the inability of patients to concentrate their pro-urine, which leads to a high risk of dehydration. This review focuses on the current knowledge regarding the cell biological aspects of congenital X-linked, autosomal-recessive and autosomal-dominant NDI while specifically addressing the latest developments in the field. Based on deepened mechanistic understanding, new therapeutic strategies are currently being explored, which we also discuss here.

Effects of K+-deficient diets with and without NaCl supplementation on Na+, K+, and H2O transporters' abundance along the nephron

Dietary potassium (K(+)) restriction and hypokalemia have been reported to change the abundance of most renal Na(+) and K(+) transporters and aquaporin-2 isoform, but results have not been consistent. The aim of this study was to reexamine Na(+), K(+) and H(2)O transporters' pool size regulation in response to removing K(+) from a diet containing 0.74% NaCl, as well as from a diet containing 2% NaCl (as found in American diets) to blunt reducing total diet electrolytes. Sprague-Dawley rats (n = 5-6) were fed for 6 days with one of these diets: 2% KCl, 0.74% NaCl (2K1Na, control chow) compared with 0.03% KCl, 0.74% NaCl (0K1Na); or 2% KCl, 2%NaCl (2K2Na) compared with 0.03% KCl, 2% NaCl (0K2Na, Na(+) replete). In both 0K1Na and 0K2Na there were significant decreases in: 1) plasma [K(+)] (<2.5 mM); 2) urinary K(+) excretion (<5% of control); 3) urine osmolality and plasma [aldosterone], as well as 4) an increase in urine volume and medullary hypertrophy. The 0K2Na group had the lowest [aldosterone] (172.0 ± 17.4 pg/ml) and lower blood pressure (93.2 ± 4.9 vs. 112.0 ± 3.1 mmHg in 2K2Na). Transporter pool size regulation was determined by quantitative immunoblotting of renal cortex and medulla homogenates. The only differences measured in both 0K1Na and 0K2Na groups were a 20-30% decrease in cortical β-ENaC, 30-40% increases in kidney-specific Ste20/SPS1-related proline/alanine-rich kinase, and a 40% increase in medullary sodium pump abundance. The following proteins were not significantly changed in both the 0 K groups: Na(+)/H(+) exchanger isoform 3; Na(+)-K(+)-Cl(-) cotransporter; Na(+)-Cl(-) cotransporter, oxidative stress response kinase-1; renal outer medullary K(+) channel; autosomal recessive hypercholesterolemia; c-Src, aquaporin 2 isoform; or renin. Thus, despite profound hypokalemia and renal K(+) conservation, we did not confirm many of the changes that were previously reported. We predict that changes in transporter distribution and activity are likely more important for conserving K(+) than changes in total abundance.

Mechanisms of the effects of acidosis and hypokalemia on renal ammonia metabolism

Renal ammonia metabolism is the predominant component of net acid excretion and new bicarbonate generation. Renal ammonia metabolism is regulated by acid-base balance. Both acute and chronic acid loads enhance ammonia production in the proximal tubule and secretion into the urine. In contrast, alkalosis reduces ammoniagenesis. Hypokalemia is a common electrolyte disorder that significantly increases renal ammonia production and excretion, despite causing metabolic alkalosis. Although the net effects of hypokalemia are similar to metabolic acidosis, molecular mechanisms of renal ammonia production and transport have not been well understood. This mini review summarizes recent findings regarding renal ammonia metabolism in response to chronic hypokalemia.

Does hypokalaemia cause nephropathy? An observational study of renal function in patients with Bartter or Gitelman syndrome

BACKGROUND

Hypokalaemic nephropathy has been described in patients with chronic potassium depletion; it is a condition in which proximal tubular vacuolization and interstitial fibrosis occur, resulting in a decline in glomerular filtration rate (GFR) and, in some cases, renal failure. It has been described in patients with chronic diarrhoea, eating disorders, laxative abuse and primary hyperaldosteronism; also occasionally in Bartter syndrome (BS), in which severe hypokalaemia accompanies significant renal sodium and water losses, though rarely in Gitelman syndrome (GS), in which there is equally severe hypokalaemia, but only modest sodium losses.

AIM

We hypothesized that hypokalaemic nephropathy may not be due to potassium depletion per se, but persistently elevated circulating levels of aldosterone, possibly with superimposed episodes of renal hypoperfusion.

DESIGN AND METHODS

We searched UK and European data sets to retrospectively compare serum and urinary parameters in patients with GS and BS.

RESULTS

The patients with GS often had lower serum potassium concentrations than patients with BS, but the BS patients had significantly higher serum creatinine concentrations and lower estimated GFRs (eGFR). BS patients had significantly higher fractional excretions of sodium compared with GS patients, as well as higher plasma renin activities and serum aldosterone levels.

CONCLUSION

These findings show that in genetically confirmed cases of BS and GS, the degree of hypokalaemia (as an index of chronic potassium depletion) does not correlate with GFR, and that on-going sodium and water losses, and consequent secondary hyperaldosteronism, may play a more important role in the aetiology of hypokalaemic nephropathy.

Primary Sjögren's disease and its complications presenting with progressive paralysis

A 24-year-old female presented with generalised weakness, lethargy and aches in legs. She was subsequently found to be markedly hypokalaemic and have a metabolic acidosis. A diagnosis of distal renal tubular acidosis (RTA) was made. In addition to this failure to alkalinise her urine, she was unable to concentrate it and so a diagnosis of nephrogenic diabetes insipidus was reached. Further questioning revealed previous investigation of a connective tissue disorder thought to be primary Sjögren's syndrome. RTA is a recognised but rare complication of Sjögren's syndrome. Urinary alkalinisation using potassium bicarbonate was instituted; the patient responded well to treatment and is having outpatient follow-up.

Mineralocorticoid and Glucocorticoid Receptors Stimulate Epithelial Sodium Channel Activity in a Mouse Model of Cushing Syndrome

Experiments in Cushing patients and healthy control subjects receiving adrenocorticotropic hormone (ACTH) indicate that transient renal sodium retention may contribute to the generation of hypertension. Here we have investigated the effect of chronic ACTH infusion on renal sodium handling in adult male C57BL/6J mice using selective antagonists to dissect mineralocorticoid and glucocorticoid receptor–mediated pathways. Mice were infused via osmotic minipump with ACTH (2.5 μg/d) or saline for 2 weeks before being anesthetized for renal function experiments. ACTH caused an increase in blood pressure and a reduction in fractional sodium excretion associated with enhanced activity of the epithelial sodium channel. Given separately, spironolactone and RU38486 blunted the pressor response to ACTH and the increased epithelial sodium channel activity; combined mineralocorticoid and glucocorticoid receptor blockade was required to resolve the response to ACTH excess. Dietary sodium depletion also prevented ACTH-induced hypertension. The effect of increased sodium reabsorption in the distal nephron is offset by downregulation of Na-K-Cl cotransport in the loop of Henle. Sodium excretion is normalized chronically, but blood pressure remains high; acute blockade of V1 receptors and α1 adrenoceptors in combination restored blood pressure to control values. In summary, ACTH excess promotes renal sodium reabsorption, contributing to the increased blood pressure; both glucocorticoid and mineralocorticoid receptor pathways are involved. These data are relevant to conditions associated with overactivity of the hypothalamic-pituitary-adrenal axis, such as obesity and chronic stress.

Changes of renal AQP2, ENaC, and NHE3 in experimentally induced heart failure: response to angiotensin II AT1 receptor blockade

Heart failure (HF) was induced by ligation of the left anterior descending artery (LAD). Left ventricular end-diastolic pressure (LVEDP) >25 mmHg (at day 23 after LAD ligation) was the inclusion criterion. The rats were divided into three groups: sham-operated (Sham, n = 23, LVEDP: 5.6 +/- 0.6 mmHg), HF (n = 14, LVEDP: 29.4 +/- 1.4 mmHg), and candesartan (1 mg.kg(-1).day(-1) sc)-treated HF (HF + Can, n = 9, LVEDP: 29.2 +/- 1.2 mmHg). After 7 days (i.e., 29 days after LAD ligation) semiquantitative immunoblotting revealed increased abundance of inner medulla aquaporin-2 (AQP2) and AQP2 phosphorylated at Ser(256) (p-AQP2) in HF. There was also markedly enhanced apical targeting of AQP2 and p-AQP2 in inner medullary collecting duct (IMCD) in HF compared with Sham rats, shown by immunocytochemistry. Candesartan treatment significantly reversed the increases in both AQP2 and p-AQP2 expression and targeting. In contrast, there were only modest changes in other collecting duct segments. Semiquantitative immunoblots revealed increased expression of type 3 Na(+)/H(+) exchanger (NHE3) and Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) in kidneys from HF compared with Sham rats: both effects were reversed or prevented by candesartan treatment. The protein abundance of alpha-epithelial sodium channel (alpha-ENaC) was increased while beta-ENaC and gamma-ENaC expression was decreased in the cortex and outer stripe of the outer medulla in HF compared with Sham rats, which was partially reversed by candesartan treatment. These findings strongly support an important role of angiotensin II in the pathophysiology of renal water and sodium retention associated with HF.

Electrolyte disturbances and acute kidney injury induced by imatinib therapy

Imatinib mesylate is an anticancer agent that selectively inhibits protein kinases involved in the pathophysiology of cancer. It is now the first-line therapy for patients with chronic myeloid leukaemia (CML) and is generally well tolerated. Here, we describe a case of a patient receiving imatinib for CML. The patient developed renal failure accompanied by severe hypophosphataemia, hypokalaemia and hypomagnesaemia. We discuss the pathophysiological characteristics of imatinib-induced renal injury, and we demonstrate that these electrolyte disturbances were caused by increased urinary excretion of phosphate and potassium. Early diagnosis and correction of imatinib-induced renal injury and electrolyte disorders can improve clinical outcomes.

Combined renal tubular acidosis and diabetes insipidus in hematological disease

BACKGROUND

A 39-year-old male with multiple myeloma was admitted for treatment with melphalan and autologous stem cell reinfusion. He presented with hypokalemia and hyperchloremic non-anion-gap metabolic acidosis with a high urinary pH. He also had hypomagnesemia, hypophosphatemia, hypouricemia, proteinuria and glucosuria. The patient subsequently developed polyuria with a low urine osmolality, hypernatremia and, finally, acute renal failure.

INVESTIGATIONS

Physical examination, blood and urine analyses, kidney biopsy and tonicity balance.

DIAGNOSIS

Fanconi syndrome with proximal (type II) renal tubular acidosis caused by myeloma kidney. Renal tubular acidosis was complicated by probable nephrogenic diabetes insipidus and acute renal failure.

MANAGEMENT

Potassium supplementation, sodium bicarbonate therapy, intravenous fluid therapy and dialysis.

Molecular mechanisms of renal ammonia transport

Acid-base homeostasis to a great extent relies on renal ammonia metabolism. In the past several years, seminal studies have generated important new insights into the mechanisms of renal ammonia transport. In particular, the theory that ammonia transport occurs almost exclusively through nonionic NH(3) diffusion and NH(4)(+) trapping has given way to a model postulating that a variety of proteins specifically transport NH(3) and NH(4)(+) and that this transport is critical for normal ammonia metabolism. Many of these proteins transport primarily H(+) or K(+) but also transport NH(4)(+). Nonerythroid Rh glycoproteins transport ammonia and may represent critical facilitators of ammonia transport in the kidney. This review discusses the underlying aspects of renal ammonia transport as well as specific proteins with important roles in renal ammonia transport.

WNK4 kinase regulates surface expression of the human sodium chloride cotransporter in mammalian cells

Pseudohypoaldosteronism type II (PHA II) is caused by mutations of two members of WNK ((with no lysine (k)) kinase family. WNK4 wild type (WT) has been shown to inhibit the activity and surface expression of sodium chloride cotransporter (NCC) when expressed in Xenopus oocytes. Here, we have studied NCC protein processing in mammalian cells in the presence or absence of WNK4 WT and its mutants, E562K and R1185C, by surface biotinylation, Western blot, co-immunoprecipitation (Co-IP) and immunostaining. WNK4 WT significantly reduced NCC surface expression in Cos-7 cells (58.9+/-6.8% vs 100% in control, P<0.001, n=6), whereas its mutant E562K has no significant effect on NCC surface expression (92.9+/-5.3% vs 100%, P=NS, n=6). Another mutant R1185C still partially reduces surface expression of NCC (76.2+/-11.8% vs 100%, P<0.05, n=6). The reduction of NCC surface expression by WNK4 WT (62.9+/-3.3% of control group) is not altered by WT dynamin ((61.8+/-3.7% (P=NS)) or its mutant K44A ((65.4+/-14.1% (P=NS)). A Co-IP study showed that both WNK4 WT and WNK4 E562K interact with NCC. Furthermore, a proton pump inhibitor, bafilomycin A1, partially reverses the inhibitory effect of WNK4 WT on NCC expression. Our data suggest that WNK4 WT significantly inhibits NCC surface expression, which is not owing to an increase in clathrin-mediated endocytosis of NCC, but likely results from enhanced degradation of NCC through a lysosomal pathway.

Angiotensin II AT1 receptor blockade changes expression of renal sodium transporters in rats with chronic renal failure

We aimed to examine the effects of angiotensin II AT(1) receptor blocker on the expression of major renal sodium transporters and aquaporin-2 (AQP2) in rats with chronic renal failure (CRF). During 2 wks after 5/6 nephrectomy or sham operation, both CRF rats (n=10) and sham-operated control rats (n=7) received a fixed amount of low sodium diet and had free access to water. CRF rats (n=10) were divided into two groups which were either candesartan-treated (CRF-C, n=4) or vehicle-treated (CRF-V, n=6). Both CRF-C and CRF-V demonstrated azotemia, decreased GFR, polyuria, and decreased urine osmolality compared with sham-operated rats. When compared with CRF-V, CRF-C was associated with significantly higher BUN levels and lower remnant kidney weight. Semiquantitative immunoblotting demonstrated decreased AQP2 expression in both CRF-C (54% of control levels) and CRF-V (57%), whereas BSC-1 expression was increased in both CRF groups. Particularly, CRF-C was associated with higher BSC-1 expression (611%) compared with CRF-V (289%). In contrast, the expression of NHE3 (25%) and TSC (27%) was decreased in CRF-C, whereas no changes were observed in CRF-V. In conclusion, 1) candesartan treatment in an early phase of CRF is associated with decreased renal hypertrophy and increased BUN level; 2) decreased AQP2 level in CRF is likely to play a role in the decreased urine concentration, and the downregulation is not altered in response to candesartan treatment; 3) candesartan treatment decreases NHE3 and TSC expression; and 4) an increase of BSC-1 is prominent in candesartan-treated CRF rats, which could be associated with the increased delivery of sodium and water to the thick ascending limb.

Functional up-regulation of basolateral Na+-dependent HCO3- transporter NBCn1 in medullary thick ascending limb of K+-depleted rats

The electroneutral, Na(+)-coupled HCO(3)(-) co-transporter (NBCn1) is expressed in the basolateral membrane of rat medullary thick ascending limb (mTAL) segments and is up-regulated strongly during metabolic acidosis. We have suggested previously that NBCn1-mediated HCO(3)(-) uptake across the basolateral cell membrane is an important buffering mechanism during the transcellular transport of NH(4)(+) in the mTAL. To investigate this further we treated rats with a low-K(+) diet for 4 days, a manoeuvre known to increase proximal tubular ammoniagenesis and delivery of luminal NH(4)(+) to the mTAL. Hypokalaemia strongly increased immunolabelling of basolateral NBCn1 in the mTAL of the inner stripe of the outer medulla. Immunoblotting revealed an eightfold up-regulation of NBCn1 protein in K(+)-depleted rats. Subsequently we used in vitro perfusion of isolated mTAL and BCECF imaging to investigate Na(+)-coupled HCO(3)(-) influx in normal and K(+)-depleted rats. Hypokalaemia induced a threefold up-regulation of Na(+)-coupled HCO(3)(-) influx (1.64+/-0.28 vs. 0.47+/-0.05 pH units/minute, n=8). The Na(+)-dependent alkalinization was also significantly larger in K(+)-depleted rats (0.38+/-0.04 vs. 0.23+/-0.03 pH units). These data indicate that K(+)-depleted rats respond with a strong up-regulation of NBCn1 protein and function, probably to cope with the higher tubular load of NH(4)(+), and strengthen our previous suggestion that NBCn1 is an important player in the excretion of NH(4)(+). They also indicate that it is the delivery of luminal NH(4)(+) rather than systemic changes of pH that determine the expression of NBCn1.

Molecular physiology of cation-coupled Cl- cotransport: the SLC12 family

The electroneutral cation-chloride-coupled cotransporter gene family ( SLC12) was identified initially at the molecular level in fish and then in mammals. This nine-member gene family encompasses two major branches, one including two bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporters and the thiazide-sensitive Na(+):Cl(-) cotransporter. Two of the genes in this branch ( SLC12A1 and SLC12A3), exhibit kidney-specific expression and function in renal salt reabsorption, whereas the third gene ( SLC12A2) is expressed ubiquitously and plays a key role in epithelial salt secretion and cell volume regulation. The functional characterization of both alternatively-spliced mammalian Na(+)-K(+)-2Cl(-) cotransporter isoforms and orthologs from distantly related species has generated important structure-function data. The second branch includes four genes ( SLC12A4- 7) encoding electroneutral K(+)-Cl(-) cotransporters. The relative expression level of the neuron-specific SLC12A5 and the Na(+)-K(+)-2Cl(-) cotransporter SLC12A2 appears to determine whether neurons respond to GABA with a depolarizing, excitatory response or with a hyperpolarizing, inhibitory response. The four K(+)-Cl(-) cotransporter genes are co-expressed to varying degrees in most tissues, with further roles in cell volume regulation, transepithelial salt transport, hearing, and function of the peripheral nervous system. The transported substrates of the remaining two SLC12 family members, SLC12A8 and SLC12A9, are as yet unknown. Inactivating mutations in three members of the SLC12 gene family result in Mendelian disease; Bartter syndrome type I in the case of SLC12A1, Gitelman syndrome for SLC12A3, and peripheral neuropathy in the case of SLC12A6. In addition, knockout mice for many members of this family have generated important new information regarding their respective physiological roles.