Divalent cation transport by the distal nephron: insights from Bartter's and Gitelman's syndromes

Paradoxes in magnesium transport in type 1 Bartter's syndrome and Gitelman's syndrome: a modeling analysis

Type 1 Bartter's syndrome and Gitelman's syndrome are characterized by mutations in two key renal Na+ transporters, Na-K-2Cl cotransporter (NKCC2) and Na-Cl cotransporter (NCC). Since these two transporters play an important role in regulating magnesium (Mg2+) and calcium (Ca2+) transport in the kidney, significant alterations in the transport of these two electrolytes are observed in type 1 Bartter's syndrome and Gitelman's syndrome. In this study, we used our sex-specific computational models of renal electrolyte transport in rats to understand the complex compensatory mechanisms, in terms of alterations in tubular dimensions and ion transporter activities, that lead to Mg2+ and Ca2+ preservation or wasting in these two genetic disorders. Given the sexual dimorphism in renal transporter patterns, we also assessed how the magnitude of these alterations may differ between males and females. Model simulations showed that in type 1 Bartter's syndrome, nephron adaptations prevent salt wasting and favor Mg2+ preservation but not Ca2+, whereas in Gitelman's syndrome, those adaptations favor Ca2+ preservation over Mg2+. In addition, our models predicted that the compensatory alterations in tubular dimensions and ion transporter activities are stronger in females than in males.NEW & NOTEWORTHY Although changes in Ca2+ excretion in type 1 Bartter's syndrome and Gitelman's syndrome are well understood, Mg2+ excretion displays an interesting paradox. This computational modeling study provides insights into how renal adaptations in these two disorders impact Ca2+ and Mg2+ transport along different nephron segments. Model simulations showed that nephron adaptations favor Mg2+ preservation over Ca2+ in Bartter's syndrome and Ca2+ preservation over Mg2+ in Gitelman's syndrome and are stronger in females than in males.

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.

Kinase Scaffold Cab39 Is Necessary for Phospho-Activation of the Thiazide-Sensitive NCC

BACKGROUND

Potassium regulates the WNK (with no lysine kinase)-SPAK (STE20/SPS1-related proline/alanine-rich kinase) signaling axis, which in turn controls the phosphorylation and activation of the distal convoluted tubule thiazide-sensitive NCC (sodium-chloride cotransporter) for sodium-potassium balance. Although their roles in the kidney have not been investigated, it has been postulated that Cab39 (calcium-binding protein 39) or Cab39l (Cab39-like) is required for SPAK/OSR1 (oxidative stress response 1) activation. This study demonstrates how they control the WNK-SPAK/OSR1-NCC pathway.

METHODS

We created a global knockout of Cab39l and a tamoxifen-inducible, NCC-driven, Cab39 knockout. The 2 lines were crossed to generate Cab39-DKO (Cab39 double knockout) animals. Mice were studied under control and low-potassium diet, which activates WNK-SPAK/OSR1-NCC phosphorylation. Western blots were used to assess the expression and phosphorylation of proteins. Blood and urine electrolytes were measured to test for compromised NCC function. Immunofluorescence studies were conducted to localize SPAK and OSR1.

RESULTS

Both Cab39l and Cab39 are expressed in distal convoluted tubule, and only the elimination of both leads to a striking absence of NCC phosphorylation. Cab39-DKO mice exhibited a loss-of-NCC function, like in Gitelman syndrome. In contrast to the apical membrane colocalization of SPAK with NCC in wild-type mice, SPAK and OSR1 become confined to intracellular puncta in the Cab39-DKO mice.

CONCLUSIONS

In the absence of Cab39 proteins, NCC cannot be phosphorylated, resulting in a Gitelman-like phenotype. Cab39 proteins function to localize SPAK at the apical membrane with NCC, reminiscent of the Cab39 yeast homolog function, translocating kinases during cytokinesis.

Modus operandi of ClC-K2 Cl- Channel in the Collecting Duct Intercalated Cells

The renal collecting duct is known to play a critical role in many physiological processes, including systemic water-electrolyte homeostasis, acid-base balance, and the salt sensitivity of blood pressure. ClC-K2 (ClC-Kb in humans) is a Cl--permeable channel expressed on the basolateral membrane of several segments of the renal tubule, including the collecting duct intercalated cells. ClC-Kb mutations are causative for Bartters' syndrome type 3 manifested as hypotension, urinary salt wasting, and metabolic alkalosis. However, little is known about the significance of the channel in the collecting duct with respect to the normal physiology and pathology of Bartters' syndrome. In this review, we summarize the available experimental evidence about the signaling determinants of ClC-K2 function and the regulation by systemic and local factors as well as critically discuss the recent advances in understanding the collecting-duct-specific roles of ClC-K2 in adaptations to changes in dietary Cl- intake and maintaining systemic acid-base homeostasis.

Gitelman syndrome with normocalciuria - a case report

Background

Gitelman Syndrome (GS) is a hereditary tubulopathy associated with a biallelic inactivating mutations of the SLC12A3 gene encoding the thiazide-sensitive sodium-chloride cotransporter (NCCT). The typical clinical manifestation is a hypokalemic metabolic alkalosis with significant hypomagnesemia, and low urinary calcium excretion. Hypocalciuria is widely believed to be a hallmark of GS that distinguishes it from Barter’s syndrome, presenting as hypercalciuria. The pathomechanism of hypocalciuria in GS is not fully elucidated. Up to date, a clinical course of GS with normocalciuria has been reported only in men, while women have a milder course of the disease with typical hypocalciuria, which is believed as the result of sex hormone. Additionally, there is a growing evidence that calcium channels of the distal nephron could be regulated by a variety of hormones, including aldosterone (Aldo).

Case presentation

We present the case of a 28-year-old Caucasian woman with asymptomatic, chronic hypokalemia, hypomagnesemia, hypochloremic alkalosis and normal urinary calcium excretion. A high renin levels with normal concentration of Aldo in serum have also been found. The values of blood pressure were low. Based on genetic studies, two heterozygous mutations in the trans position were confirmed: c.2186G>T (p.Gly729Val) and c.1247G>C (p.Cys416Ser) in the SLC12A3 gene, which ultimately confirmed the diagnosis of GS.

Conclusions

We report here the first case of genetically confirmed GS manifested as normocalciuria in a Caucasian woman. Thus, our result does not confirm a role of sex hormones on the level of calciuria. Based on the results of normal Aldo concentration despite high renin level in our patient, we hypothesized that Aldo may be connecting with the level of urinary calcium excretion in patients with the GS.

The Extraglycemic Effect of SGLT-2is on Mineral and Bone Metabolism and Bone Fracture

Type 2 diabetes mellitus (T2DM) is a risk factor for osteoporosis. The effects of T2DM and anti-diabetic agents on bone and mineral metabolism have been observed. Sodium-glucose co-transporter 2 inhibitors (SGLT-2is) promote urinary glucose excretion, reduce blood glucose level, and improve the cardiovascular and diabetic nephropathy outcomes. In this review, we focused on the extraglycemic effect and physiological regulation of SGLT-2is on bone and mineral metabolism. SGLT-2is affect the bone turnover, microarchitecture, and bone strength indirectly. Clinical evidence of a meta-analysis showed that SGLT-2is might not increase the risk of bone fracture. The effect of SGLT-2is on bone fracture is controversial, and further investigation from a real-world study is needed. Based on its significant benefit on cardiovascular and chronic kidney disease (CKD) outcomes, SGLT-2is are an outstanding choice. Bone mineral density (BMD) and fracture risk evaluation should be considered for patients with a high risk of bone fracture.

Use of Urine Electrolytes and Urine Osmolality in the Clinical Diagnosis of Fluid, Electrolytes, and Acid-Base Disorders

We discuss the use of urine electrolytes and urine osmolality in the clinical diagnosis of patients with fluid, electrolytes, and acid-base disorders, emphasizing their physiological basis, their utility, and the caveats and limitations in their use. While our focus is on information obtained from measurements in the urine, clinical diagnosis in these patients must integrate information obtained from the history, the physical examination, and other laboratory data.

Deletion of the transcription factor Prox-1 specifically in the renal distal convoluted tubule causes hypomagnesemia via reduced expression of TRPM6 and NCC

The renal distal convoluted tubule (DCT) is critical for the fine-tuning of urinary ion excretion and the control of blood pressure. Ion transport along the DCT is tightly controlled by posttranscriptional mechanisms including a complex interplay of kinases, phosphatases, and ubiquitin ligases. Previous work identified the transcription factor Prox-1 as a gene significantly enriched in the DCT of adult mice. To test if Prox-1 contributes to the transcriptional regulation of DCT function and structure, we developed a novel mouse model (NCC^cre:Prox-1^flox/flox) for an inducible deletion of Prox-1 specifically in the DCT. The deletion of Prox-1 had no obvious impact on DCT structure and growth independent whether the deletion was achieved in newborn or adult mice. Furthermore, DCT-specific Prox-1 deficiency did not alter DCT-proliferation in response to loop diuretic treatment. Likewise, the DCT-specific deletion of Prox-1 did not cause other gross phenotypic abnormalities. Body weight, urinary volume, Na⁺ and K⁺ excretion as well as plasma Na⁺, K⁺, and aldosterone levels were similar in Prox-1_DCT^KO and Prox-1_DCT^Ctrl mice. However, Prox-1_DCT^KO mice exhibited a significant hypomagnesemia with a profound downregulation of the DCT-specific apical Mg²⁺ channel TRPM6 and the NaCl cotransporter (NCC) at both mRNA and protein levels. The expression of other proteins involved in distal tubule Mg²⁺ and Na⁺ handling was not affected. Thus, Prox-1 is a DCT-enriched transcription factor that does not control DCT growth but contributes to the molecular control of DCT-dependent Mg²⁺ homeostasis in the adult kidney.

Thiazide but not loop diuretics is associated with hypomagnesaemia in the general population

PURPOSE

Hypomagnesaemia has been associated with various adverse outcomes. Loop and thiazide diuretics promote urinary magnesium excretion. However, it is unknown if this links to hypomagnesaemia. We study if loop or thiazide diuretic use affects serum magnesium levels and if it associates with hypomagnesaemia. In addition, we study the effect of combining a potassium-sparing diuretic with a thiazide diuretic on the presence of hypomagnesaemia.

METHODS

The study performed a cross-sectional analysis within 9820 participants from the prospective Rotterdam Study. Hypomagnesaemia was defined as a serum magnesium level ≤0.72 mmol/L. Participants were categorized by defined daily dose (DDD), and all analyses were adjusted for age, sex, BMI, eGFR, serum potassium levels, proton pump inhibitor use, and comorbidities.

RESULTS

Loop diuretic use was associated with higher serum magnesium levels (<1 DDD: 0.004 mmol/L 95% CI: -0.008; 0.017; 1 DDD: 0.023 mmol/L 95% CI: 0.013; 0.032; >1 DDD: 0.043 mmol/L 95% CI: 0.028; 0.057). Thiazide diuretic use was associated with lower serum magnesium levels (<1 DDD: -0.013 mmol/L 95% CI: -0.023; -0.002; ≥1 DDD: -0.018 mmol/L 95% CI: -0.028; -0.010), resulting in an increased odds ratio of hypomagnesaemia of 3.14 (95% CI: 1.67; 5.92) and 2.74 (95% CI: 1.57; 4.77), respectively. These effects were predominantly seen in participants using diuretics for more than 390 days. Combining thiazide diuretics with a potassium-sparing agent was not associated with lower serum magnesium levels or hypomagnesaemia.

CONCLUSIONS

Thiazide diuretic use is associated with lower serum magnesium levels and an increased risk of hypomagnesaemia. This increased risk is not seen in participants using a combination of thiazide diuretics with a potassium-sparing agent. The use of loop diuretics is not associated with an increased risk of hypomagnesaemia.

The importance of the thick ascending limb of Henle's loop in renal physiology and pathophysiology

The thick ascending limb (TAL) of Henle’s loop is a crucial segment for many tasks of the nephron. Indeed, the TAL is not only a mainstay for reabsorption of sodium (Na⁺), potassium (K⁺), and divalent cations such as calcium (Ca²⁺) and magnesium (Mg²⁺) from the luminal fluid, but also has an important role in urine concentration, overall acid–base homeostasis, and ammonia cycle. Transcellular Na⁺ transport along the TAL is a prerequisite for Na⁺, K⁺, Ca²⁺, Mg²⁺ homeostasis, and water reabsorption, the latter through its contribution in the generation of the cortico-medullar osmotic gradient. The role of this nephron site in acid–base balance, via bicarbonate reabsorption and acid secretion, is sometimes misunderstood by clinicians. This review describes in detail these functions, reporting in addition to the well-known molecular mechanisms, some novel findings from the current literature; moreover, the pathophysiology and the clinical relevance of primary or acquired conditions caused by TAL dysfunction are discussed. Knowing the physiology of the TAL is fundamental for clinicians, for a better understanding and management of rare and common conditions, such as tubulopathies, hypertension, and loop diuretics abuse.

SGLT2 inhibitors-induced electrolyte abnormalities: An analysis of the associated mechanisms

AIMS

Sodium-glucose co-transporter 2 (SGLT2) inhibitors are a new class of antidiabetic drugs that affect serum electrolytes levels. The aim of this review is the detailed presentation of the associated mechanisms of the SGLT2 inhibitors-induced electrolyte abnormalities.

MATERIALS AND METHODS

Eligible trials and relevant articles published in PubMed (last search in July 2017) are included in the review.

RESULTS

SGLT2 inhibitors induce small increases in serum concentrations of magnesium, potassium and phosphate. The small increase in serum phosphate concentration may result in reduced bone density and increased risk of bone fractures, mainly seen with canagliflozin, but recent meta-analyses did not show increased risk of bone fractures with SGLT2 inhibitors.

CONCLUSION

The increases in serum electrolytes levels may play a role in the cardiovascular protection that has been recently reported with empagliflozin and canagliflozin.

Gitelman syndrome: an analysis of the underlying pathophysiologic mechanisms of acid-base and electrolyte abnormalities

Gitelman syndrome is the most common inherited tubular disease resulting from mutations of the SLC12A3 gene that encodes the thiazide-sensitive sodium-chloride cotransporter in the early distal convoluted tubules. The review presents the underlying pathophysiologic mechanisms of acid-base and electrolyte abnormalities observed in patients with Gitelman syndrome. The syndrome is usually characterized by hypokalemic metabolic alkalosis in combination with hypomagnesemia and hypocalciuria. Additionally, increased chloride excretion and renin/aldosterone levels, hypophosphatemia (occasionally), hyponatremia (rarely) and glucose intolerance/insulin resistance have been reported. The knowledge of the pathophysiologic mechanisms is useful for the treatment of patients with Gitelman syndrome as well as for the understanding of other tubular diseases.

Poor phenotype-genotype association in a large series of patients with Type III Bartter syndrome

Introduction

Type III Bartter syndrome (BS) is an autosomal recessive renal tubule disorder caused by loss-of-function mutations in the CLCNKB gene, which encodes the chloride channel protein ClC-Kb. In this study, we carried out a complete clinical and genetic characterization in a cohort of 30 patients, one of the largest series described. By comparing with other published populations, and considering that 80% of our patients presented the p.Ala204Thr Spanish founder mutation presumably associated with a common phenotype, we aimed to test the hypothesis that allelic differences could explain the wide phenotypic variability observed in patients with type III BS.

Methods

Clinical data were retrieved from the referral centers. The exon regions and flanking intronic sequences of the CLCNKB gene were screened for mutations by polymerase chain reaction (PCR) followed by direct Sanger sequencing. Presence of gross deletions or duplications in the region was checked for by MLPA and QMPSF analyses.

Results

Polyuria, polydipsia and dehydration were the main common symptoms. Metabolic alkalosis and hypokalemia of renal origin were detected in all patients at diagnosis. Calciuria levels were variable: hypercalciuria was detected in 31% of patients, while 23% had hypocalciuria. Nephrocalcinosis was diagnosed in 20% of the cohort. Two novel CLCNKB mutations were identified: a small homozygous deletion (c.753delG) in one patient and a small deletion (c.1026delC) in another. The latter was present in compound heterozygosis with the already previously described p.Glu442Gly mutation. No phenotypic association was obtained regarding the genotype.

Conclusion

A poor correlation was found between a specific type of mutation in the CLCNKB gene and type III BS phenotype. Importantly, two CLCNKB mutations not previously described were found in our cohort.

Hydrochlorothiazide treatment increases the abundance of the NaCl cotransporter in urinary extracellular vesicles of essential hypertensive patients

The thiazide-sensitive NaCl cotransporter (NCC), located apically in distal convoluted tubule epithelia, regulates the fine-tuning of renal sodium excretion. Three isoforms of NCC are generated through alternative splicing of the transcript, of which the third isoform has been the most extensively investigated in pathophysiological conditions. The aim of this study was to investigate the effect of different anti-hypertensive treatments on the abundance and phosphorylation of all three NCC isoforms in urinary extracellular vesicles (uEVs) of essential hypertensive patients. In uEVs isolated from patients (n = 23) before and after hydrochlorothiazide or valsartan treatment, the abundance and phosphorylation of the NCC isoforms was determined. Additionally, clinical biochemistry and blood pressure of the patients was assessed. Our results show that NCC detected in human uEVs has a glycosylated and oligomeric structure, comparable to NCC present in human kidney membrane fractions. Despite the inhibitory action of hydrochlorothiazide on NCC activity, immunoblot analysis of uEVs showed significantly increased abundance of NCC isoforms 1 and 2 (NCC_1/2), total NCC (NCC_1-3), and the phosphorylated form of total NCC (pNCC_1-3-T55/T60) in essential hypertensive patients treated with hydrochlorothiazide but not with valsartan. This study highlights that NCC_1/2, NCC_1-3, and pNCC_1-3-T55/T60 are upregulated by hydrochlorothiazide, and the increase in NCC abundance in uEVs of essential hypertensive patients correlates with the blood pressure response to hydrochlorothiazide.

Basolateral Kir4.1 activity in the distal convoluted tubule regulates K secretion by determining NaCl cotransporter activity

PURPOSE OF REVIEW

Renal potassium (K) secretion plays a key role in maintaining K homeostasis. The classic mechanism of renal K secretion is focused on the connecting tubule and cortical collecting duct, in which K is uptaken by basolateral Na-K-ATPase and is secreted into the lumen by apical ROMK (Kir1.1) and Ca-activated big conductance K channel. Recently, genetic studies and animal models have indicated that inwardly rectifying K channel 4.1 (Kir4.1 or Kcnj10) in the distal convoluted tubule (DCT) may play a role in the regulation of K secretion in the aldosterone-sensitive distal nephron by targeting the NaCl cotransporter (NCC). This review summarizes recent progresses regarding the role of Kir4.1 in the regulation of NCC and K secretion.

RECENT FINDINGS

Kir4.1 is expressed in the basolateral membrane of the DCT, and plays a predominant role in contributing to the basolateral K conductance and in participating in the generation of negative membrane potential. Kir4.1 is also the substrate of src-family tyrosine kinase and the stimulation of src-family tyrosine kinase activates Kir4.1 activity in the DCT. The genetic deletion or functional inhibition of Kir4.1 depolarizes the membrane of the DCT, inhibits ste20-proline-alanine rich kinase, and suppresses NCC activity. Moreover, the downregulation of Kir4.1 increases epithelial Na channel expression in the collecting duct and urinary K excretion. Finally, mice with low Kir4.1 activity in the DCT are hypomagnesemia and hypokalemia.

SUMMARY

Recent progress in exploring the regulation and the function of Kir4.1 in the DCT strongly indicates that Kir4.1plays an important role in initiating the regulation of renal K secretion by targeting NCC and it may serves as a K sensor in the kidney.

New perspective of ClC-Kb/2 Cl- channel physiology in the distal renal tubule

Since its identification as the underlying molecular cause of Bartter's syndrome type 3, ClC-Kb (ClC-K2 in rodents, henceforth it will be referred as ClC-Kb/2) is proposed to play an important role in systemic electrolyte balance and blood pressure regulation by controlling basolateral Cl(-) exit in the distal renal tubular segments from the cortical thick ascending limb to the outer medullary collecting duct. Considerable experimental and clinical effort has been devoted to the identification and characterization of disease-causing mutations as well as control of the channel by its cofactor, barttin. However, we have only begun to unravel the role of ClC-Kb/2 in different tubular segments and to reveal the regulators of its expression and function, e.g., insulin and IGF-1. In this review we discuss recent experimental evidence in this regard and highlight unexplored questions critical to understanding ClC-Kb/2 physiology in the kidney.

The expression, regulation, and function of Kir4.1 (Kcnj10) in the mammalian kidney

Kir4.1 is an inwardly rectifying potassium (K(+)) channel and is expressed in the brain, inner ear, and kidney. In the kidney, Kir4.1 is expressed in the basolateral membrane of the late thick ascending limb (TAL), the distal convoluted tubule (DCT), and the connecting tubule (CNT)/cortical collecting duct (CCD). It plays a role in K(+) recycling across the basolateral membrane in corresponding nephron segments and in generating negative membrane potential. The renal phenotypes of the loss-function mutations of Kir4.1 include mild salt wasting, hypomagnesemia, hypokalemia, and metabolic alkalosis, suggesting that the disruption of Kir4.1 mainly impairs the transport in the DCT. Patch-clamp experiments and immunostaining demonstrate that Kir4.1 plays a predominant role in determining the basolateral K(+) conductance in the DCT. However, the function of Kir4.1 in the TAL and CNT/CCD is not essential, because K(+) channels other than Kir4.1 are also expressed. The downregulation of Kir4.1 in the DCT reduced basolateral chloride (Cl(-)) conductance, suppressed the expression of ste20 proline-alanine-rich kinase (SPAK), and decreased Na-Cl cotransporter (NCC) expression and activity. This suggests that Kir4.1 regulates NCC expression by the modulation of the Cl(-)-sensitive with-no-lysine kinase-SPAK pathway.

Regulation of magnesium balance: lessons learned from human genetic disease

Magnesium (Mg(2+)) is the fourth most abundant cation in the body. Thus, magnesium homeostasis needs to be tightly regulated, and this is facilitated by intestinal absorption and renal excretion. Magnesium absorption is dependent on two concomitant pathways found in both in the intestine and the kidneys: passive paracellular transport via claudins facilitates bulk magnesium absorption, whereas active transcellular pathways mediate the fine-tuning of magnesium absorption. The identification of genes responsible for diseases associated with hypomagnesaemia resulted in the discovery of several magnesiotropic proteins. Claudins 16 and 19 form the tight junction pore necessary for mass magnesium transport. However, most of the causes of genetic hypomagnesaemia can be tracked down to transcellular magnesium transport in the distal convoluted tubule. Within the distal convoluted tubule, magnesium reabsorption is a tightly regulated process that determines the final urine magnesium concentration. Therefore, insufficient magnesium transport in the distal convoluted tubule owing to mutated magnesiotropic proteins inevitably leads to magnesium loss, which cannot be compensated for in downstream tubule segments. Better understanding of the molecular mechanism regulating magnesium reabsorption will give new opportunities for better therapies, perhaps including therapies for patients with chronic renal failure.

Molecular pathophysiology of Bartter's and Gitelman's syndromes

BACKGROUND

In the last two decades, progress in cytogenetic and genome research has enabled investigators to unravel the underlying molecular mechanisms of inherited tubulopathies such as Bartter's and Gitelman's syndromes and helped physicians to better understand not only these two pathologic entities but also renal pathophysiology and salt sensitive hypertension.

DATA SOURCES

Articles collected from PubMed and open access journals included original articles, research articles, and comprehensive reviews. They were evaluated by the authors with an special emphasis on originality and up to date information about molecular pathophysiology.

RESULTS

Bartter's and Gitelman's syndromes are two different inherited salt loosing tubulopathies. They are characterized by various inability of distal nephron to reabsorb sodium chloride with resultant extarcellular volume contraction and increased activity of the renin angiotensin aldosterone system. Hypokalemic metabolic alkalosis is a common feature of these two forms of tubulopathies. Hypercalciuria characterizes the majority of Bartter's syndrome, and hypomagnesemia with hypocalciuria characterizes Gitelman's syndrome. Low blood pressure is a common feature among patients who suffered from these tubulopathies. Bartter's syndromes encompass a heterogeneous group of ion channels defects localized at the thick ascending limp of Henle's loop with resultant loss of function of sodium-potassium-2 chloride cotransporter. These defects result in the impairment of the countercurrent multiplication system of the kidney as well as calcium, potassium and acid base disturbances which in the majority of cases are proved lethal especially in the antenatal and/or immediate postnatal life period. The underlying pathology in Gitelman's syndrome is defined to the distal convoluted tubule and is related to loss of function of the sodium-chloride cotransporter. The results of this defect encompass the inability of extracellular volume homeostasis, magnesium and potassium conservation, and acid base disturbances which are generally mild and in the majority of cases are not life-threatening.

CONCLUSIONS

Recent advances in molecular pathophysiology of Bartter's and Gitelman's syndromes have helped physicians to better understand the underlying mechanisms of these pathologic entities which remain obscure. Data collected from experiments among genetically manipulated animals enable us to better understand the pathophysiology of mammalian kidney and the underlying mechanisms of salt sensitive hypertension and to lay a foundation for the future development of new drugs, especially diuretics and antihypertensive drugs.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Structural determinants for the ouabain-stimulated increase in Na-K ATPase activity

Recent studies suggest that at low concentrations, ouabain increases Na-K ATPase and NHE1 activity and activates the Src signaling cascade in proximal tubule cells. Our laboratory demonstrated that low concentrations of ouabain increase blood pressure in rats. We hypothesize that ouabain-induced increase in blood pressure and Na-K ATPase activity requires NHE1 activity and association. To test this hypothesis we treated rats with ouabain (1μgkg body wt(-1)day(-1)) for 9days in the presence or absence of the NHE1 inhibitor, zoniporide. Ouabain stimulated a significant increase in blood pressure which was prevented by zoniporide. Using NHE1-expressing Human Kidney cells 2 (HK2), 8 (HK8) and 11 (HK11) and Mouse Kidney cells from Wild type (WT) and NHE1 knock-out mice (SWE) cell lines, we show that ouabain stimulated Na-K ATPase activity and surface expression in a Src-dependent manner in NHE1-expressing cells but not in NHE1-deplete cells. Zoniporide prevented ouabain-induced stimulation of (86)Rb uptake in the NHE1-expressing cells. FRET and TIRF microscopy showed that ouabain increased association between GFP-NHE1 and mCherry-Na-K ATPase transfected into NHE1-deficient SWE cells. Mutational analysis demonstrated that the caveolin binding motif (CBM) of Na-K ATPase α1 is required for translocation of both Na-K ATPase α1 and NHE1 to the basolateral membrane. Mutations in activity or scaffold domains of NHE1 resulted in loss of ouabain-mediated regulation of Na-K ATPase. These results support that NHE1 is required for the ouabain-induced increase in blood pressure, and that the caveolin binding motif of Na-K ATPase α1 as well as the activity and scaffolding domains of NHE1 are required for their functional association.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

A molecular update on pseudohypoaldosteronism type II

The DCT (distal convoluted tubule) is the site of microregulation of water reabsorption and ion handling in the kidneys, which is mainly under the control of aldosterone. Aldosterone binds to and activates mineralocorticoid receptors, which ultimately lead to increased sodium reabsorption in the distal part of the nephron. Impairment of mineralocorticoid signal transduction results in resistance to aldosterone and mineralocorticoids, and, therefore, causes disturbances in electrolyte balance. Pseudohypoaldosteronism type II (PHAII) or familial hyperkalemic hypertension (FHHt) is a rare, autosomal dominant syndrome characterized by hypertension, hyperkalemia, metabolic acidosis, elevated or low aldosterone levels, and decreased plasma renin activity. PHAII is caused by mutations in the WNK isoforms (with no lysine kinase), which regulate the Na-Cl and Na-K-Cl cotransporters (NCC and NKCC2, respectively) and the renal outer medullary potassium (ROMK) channel in the DCT. This review focuses on new candidate genes such as KLHL3 and Cullin3, which are instrumental to unraveling novel signal transductions pathways involving NCC, to better understand the cause of PHAII along with the molecular mechanisms governing the pathophysiology of PHAII and its clinical manifestations.

Genetics of type III Bartter syndrome in Spain, proposed diagnostic algorithm

The p.Ala204Thr mutation (exon 7) of the CLCNKB gene is a "founder" mutation that causes most of type III Bartter syndrome cases in Spain. We performed genetic analysis of the CLCNKB gene, which encodes for the chloride channel protein ClC-Kb, in a cohort of 26 affected patients from 23 families. The diagnostic algorithm was: first, detection of the p.Ala204Thr mutation; second, detecting large deletions or duplications by Multiplex Ligation-dependent Probe Amplification and Quantitative Multiplex PCR of Short Fluorescent Fragments; and third, sequencing of the coding and flanking regions of the whole CLCNKB gene. In our genetic diagnosis, 20 families presented with the p.Ala204Thr mutation. Of those, 15 patients (15 families) were homozygous (57.7% of overall patients). Another 8 patients (5 families) were compound heterozygous for the founder mutation together with a second one. Thus, 3 patients (2 siblings) presented with the c. -19-?_2053+? del deletion (comprising the entire gene); one patient carried the p.Val170Met mutation (exon 6); and 4 patients (3 siblings) presented with the novel p.Glu442Gly mutation (exon 14). On the other hand, another two patients carried two novel mutations in compound heterozygosis: one presented the p.Ile398_Thr401del mutation (exon 12) associated with the c. -19-?_2053+? del deletion, and the other one carried the c.1756+1G>A splice-site mutation (exon 16) as well as the already described p.Ala210Val change (exon 7). One case turned out to be negative in our genetic screening. In addition, 51 relatives were found to be heterozygous carriers of the described CLCNKB mutations. In conclusion, different mutations cause type III Bartter syndrome in Spain. The high prevalence of the p.Ala204Thr in Spanish families thus justifies an initial screen for this mutation. However, should it not be detected further investigation of the CLCNKB gene is warranted in clinically diagnosed families.

Risk factors for pseudogout in the general population

Objective. To evaluate the association between the purported risk factors for chondrocalcinosis and gout and the risk of pseudogout in the general population. Methods. We conducted a case-control study nested within a UK general practice database (The Health Improvement Network) by identifying incident cases of pseudogout between 1986 and 2007 and up to 10 control subjects matched to each case, based on age, sex and follow-up time. We evaluated the purported risk factors for chondrocalcinosis (i.e. OA, RA, hyperparathyroidism and diuretics) and established risk factors for gout (as comparison exposures) using conditional logistic regression analysis. Results. We identified 795 cases of pseudogout and 7770 matched control subjects. The risk of pseudogout was associated with hyperparathyroidism [odds ratio (OR) 4.87; 95% CI 2.10, 11.3], OA (OR 2.91; 95% CI 2.48, 3.43) and loop diuretic use (OR 1.35; 95% CI 1.09, 1.67). RA, thiazide diuretic use, BMI and other gout risk factors were not associated with the risk of pseudogout, except for chronic renal failure (OR 2.29; 95% CI 1.30, 4.01). Conclusion. This general population study based on physician-recorded pseudogout suggests that most of the previously observed associations with chondrocalcinosis are replicable with the risk of pseudogout, but there are notable differences, such as thiazide diuretics, RA and chronic renal failure, highlighting the need to study the clinical outcome, pseudogout. Avoiding loop diuretics may help individuals with recurrent pseudogout.

A comparison of the natriuretic and kaliuretic effects of cicletanine and hydrochlorothiazide in prehypertensive and hypertensive humans

OBJECTIVES

The aim of this study was to compare the single-dose effects of thiazide-type diuretics cicletanine and hydrochlorothiazide (HCTZ), on natriuresis and kaliuresis in prehypertensive and treatment-naïve, stage 1 hypertensive patients and to explore the impact of GRK4 gene polymorphisms on thiazide-induced urinary electrolyte excretion.

METHODS

The study was a randomized, double-blind, placebo-controlled, three-period, four-treatment, balanced incomplete block, cross-over study in male patients assigned to treatment sequences consisting of placebo, cicletanine 50 mg, cicletanine 150 mg, and HCTZ 25 mg, doses used to treat hypertension. Cumulative urine samples were collected predosing and over 24 h after dosing in each period to compare urine electrolyte excretion profiles of potassium (UKV), sodium (UNaV), magnesium, calcium, phosphate, chloride, and pH among groups. Each treatment was administered to 18 different patients in each period, and an equal number of patients had less than and at least three GRK4 allele variants.

RESULTS

Compared with placebo, mean UKV was significantly increased with HCTZ 25 mg (12.7 mmol/day; P ≤ 0.001), cicletanine 50 mg (4.6 mmol/day; P = 0.026), and cicletanine 150 mg (5.5 mmol/day; P = 0.011), and mean UNaV was significantly increased with HCTZ 25 mg (102.2 mmol/day; P ≤ 0.001), cicletanine 50 mg (21.7 mmol/day; P = 0.005), and cicletanine 150 mg (57.9 mmol/day; P ≤ 0.001).

CONCLUSION

All treatments had more natriuresis, diuresis, and kaliuresis than placebo, and both doses of cicletanine had less kaliuresis than HCTZ. These findings suggest that cicletanine is a favorable and well tolerated option for the treatment of hypertension with an improved safety profile compared with HCTZ.

Syndromes of impaired ion handling in the distal nephron: pseudohypoaldosteronism and familial hyperkalemic hypertension

The distal nephron, which is the site of the micro-regulation of water absorption and ion handling in the kidneys, is under the control of aldosterone. Impairment of the mineralocorticoid signal transduction pathway results in resistance to the action of aldosterone and of mineralocorticoids in general. Herein, we review two syndromes in which ion handling in the distal nephron is impaired: pseudohypoaldosteronism (PHA) and familial hyperkalemic hypertension (FHH). PHA is a rare inherited syndrome characterized by mineralocorticoid resistance, which leads to salt loss, hypotension, hyperkalemia and metabolic acidosis. There are two types of this syndrome: a renal (autosomal dominant) type due to mutations of the mineralocorticoid receptor (MR), and a systemic (autosomal recessive) type due to mutations of the epithelial sodium channel (ENaC). There is also a transient form of PHA, which may be due to urinary tract infections, obstructive uropathy or several medications. FHH is a rare autosomal dominant syndrome, characterized by salt retention, hypertension, hyperkalemia and metabolic acidosis. In FHH, mutations of WNK (with-no-lysine kinase) 4 and 1 alter the activity of several ion transportation systems in the distal nephron. The study of the pathophysiology of PHA and FHH greatly elucidated our understanding of the renin-angiotensin-aldosterone system function and ion handling in the distal nephron. The physiological role of the distal nephron and the pathophysiology of diseases in which the renal tubule is implicated may hence be better understood and, based on this understanding, new drugs can be developed.

The Sweet Pee model for Sglt2 mutation

Inhibiting renal glucose transport is a potential pharmacologic approach to treat diabetes. The renal tubular sodium-glucose transporter 2 (SGLT2) reabsorbs approximately 90% of the filtered glucose load. An animal model with sglt2 dysfunction could provide information regarding the potential long-term safety and efficacy of SGLT2 inhibitors, which are currently under clinical investigation. Here, we describe Sweet Pee, a mouse model that carries a nonsense mutation in the Slc5a2 gene, which results in the loss of sglt2 protein function. The phenotype of Sweet Pee mutants was remarkably similar to patients with mutations in the Scl5a2 gene. The Sweet Pee mutants had improved glucose tolerance, higher urinary excretion of calcium and magnesium, and growth retardation. Renal physiologic studies demonstrated a prominent distal osmotic diuresis without enhanced natriuresis. Sweet Pee mutants did not exhibit increased KIM-1 or NGAL, markers of acute tubular injury. After induction of diabetes, Sweet Pee mice had better overall glycemic control than wild-type control mice, but had a higher risk for infection and an increased mortality rate (70% in homozygous mutants versus 10% in controls at 20 weeks). In summary, the Sweet Pee model allows study of the long-term benefits and risks associated with inhibition of SGLT2 for the management of diabetes. Our model suggests that inhibiting SGLT2 may improve glucose control but may confer increased risks for infection, malnutrition, volume contraction, and mortality.

Hereditary tubular transport disorders: implications for renal handling of Ca2+ and Mg2+

The kidney plays an important role in maintaining the systemic Ca2+ and Mg2+ balance. Thus the renal reabsorptive capacity of these cations can be amended to adapt to disturbances in plasma Ca2+ and Mg2+ concentrations. The reabsorption of Ca2+ and Mg2+ is driven by transport of other electrolytes, sometimes through selective channels and often supported by hormonal stimuli. It is, therefore, not surprising that monogenic disorders affecting such renal processes may impose a shift in, or even completely blunt, the reabsorptive capacity of these divalent cations within the kidney. Accordingly, in Dent's disease, a disorder with defective proximal tubular transport, hypercalciuria is frequently observed. Dysfunctional thick ascending limb transport in Bartter's syndrome, familial hypomagnesaemia with hypercalciuria and nephrocalcinosis, and diseases associated with Ca2+-sensing receptor defects, markedly change tubular transport of Ca2+ and Mg2+. In the distal convolutions, several proteins involved in Mg2+ transport have been identified [TRPM6 (transient receptor potential melastatin 6), proEGF (pro-epidermal growth factor) and FXYD2 (Na+/K+-ATPase gamma-subunit)]. In addition, conditions such as Gitelman's syndrome, distal renal tubular acidosis and pseudohypoaldosteronism type II, as well as a mitochondrial defect associated with hypomagnesaemia, all change the renal handling of divalent cations. These hereditary disorders have, in many cases, substantially increased our understanding of the complex transport processes in the kidney and their contribution to the regulation of overall Ca2+ and Mg2+ balance.

Analysis of claudin genes in pediatric patients with Bartter's syndrome

Bartter's syndrome is a constellation of symptoms characterized by hyper-reninemic hypokalemia, metabolic alkalosis, elevated renin and aldosterone, low or normal blood pressure, and hyperplasia of the juxtaglomerular apparatus. So far, five gene mutations in proteins regulating the sodium chloride transport in the thick ascending limb of Henle's loop have been described. However, the molecular mechanisms underlying the presentation of hypomagnesemia in some of these patients remains unclear. Claudins are a family of transmembranous proteins within the tight junctions that have been shown to be important for the paracellular movement of ions. Mutations in claudin-16 have been identified in patients with familial hypomagnesemia with hypercalciuria and nephrocalcinosis. To test the hypothesis that mutations in claudin genes may be involved in the altered magnesium and calcium transport in Bartter's syndrome, we began to examine the genes of claudins known to be present in renal tubules in four pediatric patients with Bartter's syndrome. All four patients were African Americans with hypomagnesemia and hypercalciuria. In this study, we did not find any mutation in the coding regions of claudin-2, -3, -4, -7, -8, -10, -11, or -16 genes in these patients. However, all patients had a single nucleotide substitution of C for T at the position of 451 of claudin-8 gene sequence that changes amino acid residue from serine to proline at the position of 151 in the second extracellular domain of claudin-8 protein. The significance of this known single nucleotide polymorphism remains to be determined.

The voltage-gated K+ channel subunit Kv1.1 links kidney and brain

Analysis of Mendelian Mg2+ wasting disorders helps us to unravel the mechanisms of Mg2+ homeostasis. In this issue of the JCI, Glaudemans andcolleagues show that mutations in voltage-gated K+ channel subtype 1.1(Kv1.1) cause autosomal dominant hypomagnesemia in humans (see the related article beginning on page 936). Interestingly, other mutations in the same protein cause the neurological disease episodic ataxia type 1. The authors show, using cells with heterologous expression of the wild-type and mutant channels, that the mutant channel is dysfunctional and speculate that Mg2+ wasting results from changes in apical membrane voltage along the nephron. Mechanisms by which the apical voltage is generated and howKv1.1 fits within this context are discussed herein.

Influence of drugs and comorbidity on serum potassium in 15 000 consecutive hospital admissions

BACKGROUND

Drug trials often exclude subjects with relevant comorbidity or comedication. Nevertheless, after approval, these drugs will be prescribed to a much broader collective. Our goal was to quantify the impact of drugs and comorbidity on serum potassium in unselected patients admitted to the hospital.

METHODS

This was a retrospective pharmacoepidemiologic study in 15 000 consecutive patients admitted to the medical department of the Kantonsspital St. Gallen, a 700-bed tertiary hospital in eastern Switzerland. Patients with 'haemolytic' plasma and patients on dialysis or with an estimated glomerular filtration rate (GFR) <10 mL/min/1.73 m(2) were excluded. For the remaining 14 146 patients, drug history on admission, age, sex, body weight, physical findings, comorbidity (ICD-10 diagnoses) and laboratory information (potassium and creatinine) were extracted from electronic sources.

RESULTS

Estimated GFR was the strongest predictor of serum potassium (P < 0.0001). Angiotensin-converting enzyme inhibitors, cyclosporine, loop diuretics and potassium-sparing diuretics all showed a significant effect modification with decreasing GFR (P < 0.001). Similarly, in patients with liver cirrhosis a significantly stronger effect on potassium was found for angiotensin receptor blockers, betablockers and loop diuretics (P < 0.01). Several significant drug-drug interactions were identified. Diabetes, male sex, older age, lower blood pressure and higher body weight were all independently associated with higher serum potassium levels (P < 0.001). The model explained 14% of the variation of serum potassium.

CONCLUSIONS

The effects of various drugs on serum potassium are highly influenced by comorbidity and comedication. Although the presented model cannot be used to predict potassium in individual patients, we demonstrate that clinical databases could evolve as a powerful tool for industry-independent analysis of postmarketing drug safety.

Hypocalciuria in patients with Gitelman syndrome: role of blood volume

BACKGROUND

Hypocalciuria is common in patients with Gitelman syndrome (GS), and its cause primarily is enhanced renal reabsorption of calcium in the proximal tubule in response to hypovolemia, judged by recent studies in animals.

STUDY DESIGN

Uncontrolled trial in cases and controls to evaluate the effect of acute reexpansion of extracellular fluid volume (ECFV) on urine calcium excretion in patients with GS.

SETTING & PARTICIPANTS

8 patients with GS and 8 sex- and age-matched healthy control subjects (CSs) were enrolled in an academic medical center.

PREDICTOR

ECFV expansion with isotonic saline at 1 L/h for 3 hours.

OUTCOMES & MEASUREMENTS

Urinary calcium excretion was measured hourly for 6 hours, and subsequent 18-hour urine was analyzed as a single collection; hormones and electrolytes were measured.

RESULTS

Patients with GS had hypokalemia, metabolic alkalosis, hypomagnesemia, severe hypocalciuria (urine calcium-creatinine ratio, 0.006 +/- 0.002 versus 0.08 +/- 0.02 mg/mg [0.02 +/- 0.01 versus 0.22 +/- 0.05 mmol/mmol]; P < 0.005), and a mild degree of ECFV contraction. Sodium excretion and creatinine clearance rates were similar to those in CSs. In patients with GS, saline infusion increased ECFV, which caused a significantly greater sodium excretion rate, but there was only a small increase in calcium excretion rate, in both the first 6 hours (0.04 +/- 0.02 mg/min [1.0 +/- 0.6 micromol/min]) and subsequent 18-hour period (0.02 +/- 0.01 mg/min [0.4 +/- 0.2 micromol/min]), as in CSs. Notwithstanding, their calcium excretion rate was still much less than that in CSs before volume repletion (0.13 +/- 0.04 mg/min [3.2 +/- 1.0 micromol/min]).

LIMITATION

Patients with GS did not become euvolemic on a long-term sodium chloride supplementation because they excreted sodium chloride so rapidly.

CONCLUSION

Hypovolemia is not the sole cause of hypocalciuria in patients with GS.

Cotransporters, WNKs and hypertension: important leads from the study of monogenetic disorders of blood pressure regulation

Major advances are being made in identifying the structure and behaviour of regulatory cascades that control the activity of cation-Cl(-) cotransporters and certain Na(+), K(+) and Cl(-) channels. These transporters play key roles in regulating arterial blood pressure as they are not only responsible for NaCl reabsorption in the thick ascending limb and distal tubule of the kidney, but are also involved in regulating smooth muscle Ca(2+) levels. It is now apparent that defects in these transporters, and particularly in the regulatory cascades, cause some monogenetic forms of hypertension and may contribute to essential hypertension and problems with K(+) homoeostasis. Two families of kinases are prominent in these processes: the Ste-20-related kinases [OSR1 (oxidative stress-responsive kinase 1) and SPAK (Ste20/SPS1-related proline/alanine-rich kinase)] and the WNKs [with no lysine kinases]. These kinases affect the behaviour of their targets through both phosphorylation and by acting as scaffolding proteins, bringing together regulatory complexes. This review analyses how these kinases affect transport by activating or inhibiting individual transporters at the cell surface, or by changing the surface density of transporters by altering the rate of insertion or removal of transporters from the cell surface, and perhaps through controlling the rate of transporter degradation. This new knowledge should not only help us target antihypertensive therapy more appropriately, but could also provide the basis for developing new therapeutic approaches to essential hypertension.

TRP channels in kidney disease

Mammalian TRP channel proteins form six-transmembrane cation-permeable channels that may be grouped into six subfamilies on the basis of amino acid sequence homology (TRPC, TRPV, TRPM, TRPA, TRPP, and TRPML). Recent studies of TRP channels indicate that they are involved in numerous fundamental cell functions and are considered to play an important role in the pathophysiology of many diseases. Many TRPs are expressed in kidney along different parts of the nephron and growing evidence suggest that these channels are involved in hereditary, as well as acquired kidney disorders. TRPC6, TRPM6, and TRPP2 have been implicated in hereditary focal segmental glomerulosclerosis (FSGS), hypomagnesemia with secondary hypocalcemia (HSH), and polycystic kidney disease (PKD), respectively. In addition, the highly Ca(2+)-selective channel, TRPV5, contributes to several acquired mineral (dys)regulation, such as diabetes mellitus (DM), acid-base disorders, diuretics, immunosuppressant agents, and vitamin D analogues-associated Ca(2+) imbalance whereas TRPV4 may function as an osmoreceptor in kidney and participate in the regulation of sodium and water balance. This review presents an overview of the current knowledge concerning the distribution of TRP channels in kidney and their possible roles in renal physiology and kidney diseases.

Recent advances in renal tubular calcium reabsorption

PURPOSE OF REVIEW

Knowledge of renal Ca2+ reabsorption has evolved greatly in recent years. This review focuses on two recent discoveries concerning passive and active Ca2+ reabsorption.

RECENT FINDINGS

The thiazide diuretics are known for their hypocalciuric effect. Recently, it has been demonstrated that TRPV5-knockout mice, in which active Ca2+ reabsorption in the distal convoluted tubule is completely abolished, show the same sensitivity towards thiazides as wild-type mice. This indicates that thiazide affects Ca2+ reabsorption indirectly via contraction of the extracellular volume, independent of active Ca2+ reabsorption in the distal convoluted tubule, thereby increasing passive paracellular Ca2+ transport in the proximal tubule. Moreover, the antiaging hormone Klotho regulates Ca2+ reabsorption in the distal convoluted tubule via a novel molecular mechanism. Klotho stabilizes the TRPV5 Ca2+ channel in the plasma membrane by deglycosylation of the protein.

SUMMARY

By showing that thiazide-induced hypercalciuria is due to increased passive Ca2+ reabsorption in the proximal tubule, a long-standing issue has been solved, underlining the importance of proximal paracellular Ca2+ reabsorption. Moreover, the molecular mechanism by which the antiaging hormone Klotho regulates TRPV5 activity may prove to be generally applicable in Klotho-mediated prevention of aging.

TRPV5, the gateway to Ca2+ homeostasis

Ca2+ homeostasis in the body is tightly controlled, and is a balance between absorption in the intestine, excretion via the urine, and exchange from bone. Recently, the epithelial Ca2+ channel (TRPV5) has been identified as the gene responsible for the Ca2+ influx in epithelial cells of the renal distal convoluted tubule. TRPV5 is unique within the family of transient receptor potential (TRP) channels due to its high Ca2+ selectivity. Ca2+ flux through TRPV5 is controlled in three ways. First, TRPV5 gene expression is regulated by calciotropic hormones such as vitamin D3 and parathyroid hormone. Second, Ca2+ transport through TRPV5 is controlled by modulating channel activity. Intracellular Ca2+, for example, regulates channel activity by feedback inhibition. Third, TRPV5 is controlled by mobilization of the channel through trafficking toward the plasma membrane. The newly identified anti-aging hormone Klotho regulates TRPV5 by cleaving off sugar residues from the extracellular domain of the protein, resulting in a prolonged expression of TRPV5 at the plasma membrane. Inactivation of TRPV5 in mice leads to severe hypercalciuria, which is compensated by increased intestinal Ca2+ absorption due to augmented vitamin D3 levels. Furthermore, TRPV5 deficiency in mice is associated with polyuria, urine acidification, and reduced bone thickness. Some pharmaceutical compounds, such as the immunosuppressant FK506, affect the Ca2+ balance by modulating TRPV5 gene expression. This underlines the importance of elucidating the role of TRPV5 in Ca(2+)-related disorders, thereby enhancing the possibilities for pharmacological intervention. This chapter describes a unique TRP channel and highlights its regulation and function in renal Ca2+ reabsorption and overall Ca2+ homeostasis.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

Role of with-no-lysine [K] kinases in the pathogenesis of Gordon's syndrome

Gordon's syndrome, also known as pseudohypoaldosteronism type II (PHA II) or familial hypertension with hyperkalemia, is an autosomal-dominant disease characterized by hypertension, hyperkalemia, hyperchloremic metabolic acidosis, and normal glomerular filtration rate. Recent positional cloning has linked mutations of WNK1 and WNK4 to Gordon's syndrome. With-no-lysine [K] (WNK) kinases are a new family of large serine-threonine protein kinases with an atypical placement of the catalytic lysine. Here, we review the pathogenesis of PHA II based on current understanding of the actions of WNK1 and WNK4 on Na+ and K+ handling in the renal distal tubule.

The epithelial calcium channels TRPV5 and TRPV6: regulation and implications for disease

The epithelial Ca(2+) channels TRPV5 and TRPV6 represent a new family of Ca(2+) channels that belongs to the superfamily of transient receptor potential channels. TRPV5 and TRPV6 constitute the apical Ca(2+) entry mechanism in active Ca(2+) transport in kidney and intestine. The central role of TRPV5 and TRPV6 in active Ca(2+) (re)absorption makes it a prime target for regulation to maintain Ca(2+) balance. This review covers the hormonal regulation, interaction with accessory proteins and (patho)physiological implications of these epithelial Ca(2+) channels.

TRPV5 and TRPV6 in Ca(2+) (re)absorption: regulating Ca(2+) entry at the gate

Many physiological functions rely on the exact maintenance of body Ca(2+) balance. Therefore, the extracellular Ca(2+) concentration is tightly regulated by the concerted actions of intestinal Ca(2+) absorption, exchange of Ca(2+) to and from bone, and renal Ca(2+) reabsorption. Renal distal convoluted and connecting tubular cells as well as duodenal epithelial cells are unique in their ability to mediate transcellular (re)absorption of Ca(2+) at large and highly variable rates. Two members of the transient receptor potential (TRP) superfamily, TRP vanilloid (TRPV)5 and TRPV6, are specialized epithelial Ca(2+) channels responsible for the critical Ca(2+) entry step in transcellular Ca(2+) (re)absorption in intestine and kidney, respectively. Because transcellular Ca(2+) transport is fine-tuned to the body's specific requirements, regulation of the transmembrane Ca(2+) flux through TRPV5/6 is of particular importance and has, therefore, to be conspicuously controlled. We present an overview of the current knowledge and recent advances concerning the coordinated regulation of Ca(2+) influx through the epithelial Ca(2+) channels TRPV5 and TRPV6 in transcellular Ca(2+) (re)absorption.

TRPV5 and TRPV6 in Ca(2+) (re)absorption: regulating Ca(2+) entry at the gate

Many physiological functions rely on the exact maintenance of body Ca(2+) balance. Therefore, the extracellular Ca(2+) concentration is tightly regulated by the concerted actions of intestinal Ca(2+) absorption, exchange of Ca(2+) to and from bone, and renal Ca(2+) reabsorption. Renal distal convoluted and connecting tubular cells as well as duodenal epithelial cells are unique in their ability to mediate transcellular (re)absorption of Ca(2+) at large and highly variable rates. Two members of the transient receptor potential (TRP) superfamily, TRP vanilloid (TRPV)5 and TRPV6, are specialized epithelial Ca(2+) channels responsible for the critical Ca(2+) entry step in transcellular Ca(2+) (re)absorption in intestine and kidney, respectively. Because transcellular Ca(2+) transport is fine-tuned to the body's specific requirements, regulation of the transmembrane Ca(2+) flux through TRPV5/6 is of particular importance and has, therefore, to be conspicuously controlled. We present an overview of the current knowledge and recent advances concerning the coordinated regulation of Ca(2+) influx through the epithelial Ca(2+) channels TRPV5 and TRPV6 in transcellular Ca(2+) (re)absorption.

Effects of Bartter's syndrome on dentition and dental treatment: A clinical report

Bartter's syndrome is an autosomal recessive form of severe volume depletion due to renal salt wasting. This clinical report describes the prosthodontic treatment for a 24-year-old man who suffers from Bartter's syndrome. The treatment plan included endodontic treatment of the maxillary anterior incisors and placement of cast dowel-and-core restorations because of reduced crown height. The patient's remaining teeth were restored with metal-ceramic crowns.

Enhanced passive Ca2+ reabsorption and reduced Mg2+ channel abundance explains thiazide-induced hypocalciuria and hypomagnesemia

Thiazide diuretics enhance renal Na+ excretion by blocking the Na+-Cl- cotransporter (NCC), and mutations in NCC result in Gitelman syndrome. The mechanisms underlying the accompanying hypocalciuria and hypomagnesemia remain debated. Here, we show that enhanced passive Ca2+ transport in the proximal tubule rather than active Ca2+ transport in distal convolution explains thiazide-induced hypocalciuria. First, micropuncture experiments in mice demonstrated increased reabsorption of Na+ and Ca2+ in the proximal tubule during chronic hydrochlorothiazide (HCTZ) treatment, whereas Ca2+ reabsorption in distal convolution appeared unaffected. Second, HCTZ administration still induced hypocalciuria in transient receptor potential channel subfamily V, member 5-knockout (Trpv5-knockout) mice, in which active distal Ca2+ reabsorption is abolished due to inactivation of the epithelial Ca2+ channel Trpv5. Third, HCTZ upregulated the Na+/H+ exchanger, responsible for the majority of Na+ and, consequently, Ca2+ reabsorption in the proximal tubule, while the expression of proteins involved in active Ca2+ transport was unaltered. Fourth, experiments addressing the time-dependent effect of a single dose of HCTZ showed that the development of hypocalciuria parallels a compensatory increase in Na+ reabsorption secondary to an initial natriuresis. Hypomagnesemia developed during chronic HCTZ administration and in NCC-knockout mice, an animal model of Gitelman syndrome, accompanied by downregulation of the epithelial Mg2+ channel transient receptor potential channel subfamily M, member 6 (Trpm6). Thus, Trpm6 downregulation may represent a general mechanism involved in the pathogenesis of hypomagnesemia accompanying NCC inhibition or inactivation.

Salt handling in the distal nephron: lessons learned from inherited human disorders

The molecular basis of inherited salt-losing tubular disorders with secondary hypokalemia has become much clearer in the past two decades. Two distinct segments along the nephron turned out to be affected, the thick ascending limb of Henle's loop and the distal convoluted tubule, accounting for two major clinical phenotypes, hyperprostaglandin E syndrome and Bartter-Gitelman syndrome. To date, inactivating mutations have been detected in six different genes encoding for proteins involved in renal transepithelial salt transport. Careful examination of genetically defined patients (“human knockouts”) allowed us to determine the individual role of a specific protein and its contribution to the overall process of renal salt reabsorption. The recent generation of several genetically engineered mouse models that are deficient in orthologous genes further enabled us to compare the human phenotype with the animal models, revealing some unexpected interspecies differences. As the first line treatment in hyperprostaglandin E syndrome includes cyclooxygenase inhibitors, we propose some hypotheses about the mysterious role of PGE2in the etiology of renal salt-losing disorders.

Renal effects of Tamm-Horsfall protein (uromodulin) deficiency in mice

The Tamm-Horsfall protein (THP; uromodulin), the dominant protein in normal urine, is produced exclusively in the thick ascending limb of Henle's loop. THP mutations are associated with disease; however, the physiological role of THP remains obscure. We generated THP gene-deficient mice (THP −/−) and compared them with wild-type (WT) mice. THP −/− mice displayed anatomically normal kidneys. Steady-state electrolyte handling was not different between strains. Creatinine clearance was 63% lower in THP −/− than in WT mice ( P < 0.05). Sucrose loading induced no changes between strains. However, water deprivation for 24 h decreased urine volume from 58 ± 9 to 28 ± 4 μl·g body wt−1·24 h−1 in WT mice ( P < 0.05), whereas in THP −/− mice this decrease was less pronounced (57 ± 4 to 41 ± 5 μl·g body wt−1·24 h−1; P < 0.05), revealing significant interstrain difference ( P < 0.05). We further used RT-PCR, Northern and Western blotting, and histochemistry to study renal transporters, channels, and regulatory systems under steady-state conditions. We found that major distal transporters were upregulated in THP −/− mice, whereas juxtaglomerular immunoreactive cyclooxygenase-2 (COX-2) and renin mRNA expression were both decreased in THP −/− compared with WT mice. These observations suggest that THP influences transporters in Henle's loop. The decreased COX-2 and renin levels may be related to an altered tubular salt load at the macula densa, whereas the increased expression of distal transporters may reflect compensatory mechanisms. Our data raise the hypothesis that THP plays an important regulatory role in the kidney.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.