Disturbances of Na/K balance: pseudohypoaldosteronism revisited

Prevalence of and factors associated with Na + /K + imbalances in a population of children hospitalized with febrile urinary tract infection

We aimed to assess the prevalence of and factors associated with Na + /K + imbalances in children hospitalized for febrile urinary tract infection (fUTI). This retrospective Italian multicenter study included children aged 18 years or younger (median age = 0.5 years) who were discharged with a primary diagnosis of fUTI. Na + /K + imbalances were classified as hyponatremia (sodium < 135 mEq/L), hypernatremia (sodium > 145 mEq/L), hypokalemia (potassium < 3.5 mEq/L), hyperkalemia (potassium > 5.5 mEq/L), and concurrent hyponatremia and hyperkalemia, in the absence of evidence of hemolyzed blood samples. Among the 849 enrolled children, 23% had hyponatremia, 6.4% had hyperkalemia, 2.9% had concurrent hyponatremia and hyperkalemia, 0.7% had hypokalemia, and 0.4% had hypernatremia. In the multiple logistic regression analysis, after applying the Bonferroni correction, only C-reactive protein (C-RP) levels were significantly associated with hyponatremia (OR = 1.04; 95% CI: 1.02-1.06; p < 0.001), only age was significantly associated with hyperkalemia (OR = 1.7; 95% CI: 1.1-2.7; p = 0.01), and only CAKUT was significantly associated with concurrent hyponatremia and hyperkalemia (OR = 4.3; 95% CI: 1.7-10.8; p = 0.002). Even after adjusting for the presence of kidney hypoplasia, abnormal renal echogenicity, pelvi-caliceal dilation, ureteral dilation, uroepithelial thickening of the renal pelvis, bladder abnormalities, pathogen other than E. coli, concurrent hyponatremia and hyperkalemia persisted significantly associated with CAKUT (OR = 3.6; 95% CI: 1.2-10.9; p = 0.02).

CONCLUSION

Hyponatremia was the most common Na + /K + imbalance in children hospitalized for fUTI, followed by hyperkalemia and concurrent hyponatremia and hyperkalemia. C-RP levels were most strongly associated with hyponatremia, age with hyperkalemia, and CAKUT with concurrent hyponatremia and hyperkalemia (suggestive of transient secondary pseudo-hypoaldosteronism). Therefore, in children who develop concurrent hyponatremia and hyperkalemia during the course of a fUTI, an underlying CAKUT could be suspected.

WHAT IS KNOWN

• Na+ and K+ abnormalities can occur in patients hospitalized for febrile urinary tract infection (fUTI). • Concurrent hyponatremia and hyperkalemia during fUTI may suggest transient secondary pseudo-hypoaldosteronism (TPHA), for which limited data on prevalence are available.

WHAT IS NEW

• The most common Na+/K+ imbalance in children hospitalized with fUTI was hyponatremia (23%), followed by hyperkalemia (6.4%), concurrent hyponatremia and hyperkalemia (2.9%), hypokalemia (0.7%), and hypernatremia (0.4%). • Concurrent hyponatremia and hyperkalemia were mainly associated with CAKUT, while hyponatremia alone correlated with high C-reactive protein and hyperkalemia alone with younger age. In cases of concurrent hyponatremia and hyperkalemia during fUTI, an underlying CAKUT should be suspected.

Transient secondary pseudo-hypoaldosteronism in infants with urinary tract infections: systematic literature review

Infants with a congenital anomaly of the kidney and urinary tract sometimes present with hyponatremia, hyperkalemia, and metabolic acidosis due to under-responsiveness to aldosterone, hereafter referred to as secondary pseudo-hypoaldosteronism. The purpose of this report is to investigate pseudo-hypoaldosteronism in infant urinary tract infection. A systematic review was conducted following PRISMA guidelines after PROSPERO (CRD42022364210) registration. The National Library of Medicine, Excerpta Medica, Web of Science, and Google Scholar without limitations were used. Inclusion criteria involved pediatric cases with documented overt pseudo-hypoaldosteronism linked to urinary tract infection. Data extraction included demographics, clinical features, laboratory parameters, management, and course. Fifty-seven reports were selected, detailing 124 cases: 95 boys and 29 girls, 10 months or less of age (80% of cases were 4 months or less of age). The cases exhibited hyponatremia, hyperkalemia, acidosis, and activated renin-angiotensin II-aldosterone system. An impaired kidney function was found in approximately every third case. Management included antibiotics, fluids, and, occasionally, emergency treatment of hyperkalemia, hyponatremia, or acidosis. The recovery time averaged 1 week for electrolyte, acid-base imbalance, and kidney function. Notably, anomalies of the kidney and urinary tract were identified in 105 (85%) cases.

CONCLUSIONS

This review expands the understanding of overt transient pseudo-hypoaldosteronism complicating urinary tract infection. Management involves antimicrobials, fluid replacement, and consideration of electrolyte imbalances. Raising awareness of this condition within pediatric hospitalists is desirable.

WHAT IS KNOWN

• Infants affected by a congenital anomaly of the kidney and urinary tract may present with clinical and laboratory features resembling primary pseudo-hypoaldosteronism. • Identical features occasionally occur in infant urinary tract infection.

WHAT IS NEW

• Most cases of secondary pseudo-hypoaldosteronism associated with a urinary tract infection are concurrently affected by a congenital anomaly of the kidney and urinary tract. • Treatment with antibiotics and parenteral fluids typically results in the normalization of sodium, potassium, bicarbonate, and creatinine within approximately 1 week.

Variables Associated With Hyperkalemic Renal Tubular Acidosis in Solid Organ Transplant Recipients

INTRODUCTION

The occurrence of hyperkalemic renal tubular acidosis (RTA) in the post-transplantation period is likely underestimated, and its identification remains important to offer adequate medical management. Transplant recipients frequently present with clinical and biological characteristics that may be associated with the occurrence of this complication.

METHODS

This was a single-center retrospective study that compared transplanted patients with hyperkalemic RTA and a control group to identify variables associated with the occurrence of this complication. Fisher's exact test and the Mann-Whitney test, followed by multivariate logistic regression, were applied to test whether there was a significant association between hyperkalemic RTA and different variables.

RESULTS

Kidney and heart transplant recipients were at greater risk of developing RTA than lung transplant recipients (p = 0.016). There was also a significant association between the development of RTA and kalemia (p < 0.01), chloremia (p < 0.01), and bicarbonatemia (p < 0.01). The significant impact of these last three variables was confirmed by the results of the multivariate logistic regression. Residual serum tacrolimus levels (p = 0.13) and creatinine levels (p = 0.17) of renal transplant patients were not significantly associated with hyperkalemic RTA.

CONCLUSION

The type of transplanted organ, kalemia, chloremia, and bicarbonatemia were significantly associated with the occurrence of hyperkalemic RTA. This study calls into question certain approaches to managing this complication proposed in a number of case reports, such as reducing the target serum residual of tacrolimus or discontinuing trimethoprim-sulfamethoxazole (TMP-SMX) in favor of another antibiotic prophylactic agent, potentially exposing patients to graft rejection and opportunistic infections.

A Five-Month-Old Boy With Hypotonia, Electrolyte Derangements, and Failure to Thrive

Failure to thrive in the setting of profound hypotonia and multiple electrolyte derangements is a challenging constellation of findings that offers a broad differential diagnosis for providers to consider. Initial management should focus on the stabilization of the patient and correction of potential life-threatening electrolyte derangements. Once completed, the diagnosis should be sought, and in this case, many were considered and ultimately ruled out with thorough history and physical examination. Laboratory abnormalities revealed the final diagnosis of pseudohypoaldosteronism and connected the case. With proper treatment, our patient had a resolution of laboratory anomalies along with improved growth and tone.

Importance of Micromilieu for Pathophysiologic Mineralocorticoid Receptor Activity-When the Mineralocorticoid Receptor Resides in the Wrong Neighborhood

The mineralocorticoid receptor (MR) is a member of the steroid receptor family and acts as a ligand-dependent transcription factor. In addition to its classical effects on water and electrolyte balance, its involvement in the pathogenesis of cardiovascular and renal diseases has been the subject of research for several years. The molecular basis of the latter has not been fully elucidated, but an isolated increase in the concentration of the MR ligand aldosterone or MR expression does not suffice to explain long-term pathologic actions of the receptor. Several studies suggest that MR activity and signal transduction are modulated by the surrounding microenvironment, which therefore plays an important role in MR pathophysiological effects. Local changes in micromilieu, including hypoxia, ischemia/reperfusion, inflammation, radical stress, and aberrant salt or glucose concentrations affect MR activation and therefore may influence the probability of unphysiological MR actions. The surrounding micromilieu may modulate genomic MR activity either by causing changes in MR expression or MR activity; for example, by inducing posttranslational modifications of the MR or novel interaction with coregulators, DNA-binding sites, or non-classical pathways. This should be considered when developing treatment options and strategies for prevention of MR-associated diseases.

Case Report: A Novel Compound Heterozygote Mutation of the SCNN1B Gene Identified in a Chinese Familial Pseudohypoaldosteronism Disease Type I With Persistent Hyperkalemia

BACKGROUND

Pseudohypoaldosteronism (PHA) diseases are difficult to diagnose because symptoms are often non-specific and an in-depth pathogenesis study is still lacking.

CASE PRESENTATION

We present the case of a 19-day-old neonate who presented with unexplained recurrent hyperkalaemia, hypovolemia and metabolic acidosis, whose parents did not have significant clinical disease characteristics. Whole-exome sequencing was performed to confirm the disease and genetic pattern of the neonate. Sanger sequencing was performed to identify the mutation sites. Secondary structure comparisons and 3D model construction were used to predict changes in protein structure. Two novel frameshift mutations in the SCNN1B gene were identified (c.1290delA and c.1348_1361del), which resulted in amino acid synthesis termination (p.Gln431ArgfsTer2 and p.Thr451AspfsTer6). Considering the clinical phenotype and genetic analysis, this case was finally identified as a PHA type I disease. Genetic analysis showed that the neonate suffered complex heterozygosity in the SCNN1B gene inherited from the parents, which is passed on in an autosomal recessive inheritance pattern. These two deleterious mutations resulted in an incomplete protein 3D structure.

CONCLUSIONS

Our results have confirmed the associations of mutations in the SCNN1B gene with recurrent hyperkalaemia, which can cause severe PHA type I disease, meanwhile suggested clinical attention should be paid when persistent recurrent hyperkalemia is accompanied by these types of mutations.

Short-term changes in dietary sodium intake influence sweat sodium concentration and muscle sodium content in healthy individuals

OBJECTIVE

There is increasing evidence that sodium can be stored in the skin and muscles without being osmotically active, yet whether acute changes in dietary sodium intake alter sweat and muscle sodium content has not been investigated previously.

METHODS

In a cross-over design, we assessed muscle sodium content by Na-MRI in 38 healthy normotensive volunteers (aged 33.5 ± 11.1 years, 76.3% women) after 5 days of high-sodium diet (6 g of salt added to their normal diet) and 5 days of a low-sodium diet. In a subgroup of 18 participants (72.2% women) we conducted quantitative pilocarpine iontophoretic sweat collections and measured the sodium concentration in sweat. Plasma aldosterone and plasma renin activity levels were measured in all participants.

RESULTS

Under high-sodium diet conditions urinary sodium excretion, muscle sodium content and sweat sodium concentration all increased significantly. Muscle sodium content (rm = 0.47, P = 0.03) and sodium sweat concentration (rm = 0.72, P < 0.001) correlated positively with salt intake as estimated by 24-h urine sodium excretion. Age, sex or the phase of the menstrual cycle did not influence muscle or sweat sodium concentrations or their changes. In contrast, plasma aldosterone levels were negatively associated with both muscle sodium (rs = -0.42, P = 0.0001) and sweat sodium content (rs = -0.52, P = 0.002). Plasma renin activity correlated negatively with sweat sodium (rs = -0.43, P = 0.012) and muscle sodium levels (rs = -0.42, P < 0.001).

CONCLUSION

Muscle and sweat sodium concentrations are significantly higher on a high-salt intake in healthy male and female individuals, suggesting that muscle and sweat play a role in regulating sodium balance in humans.

Trimethoprim-associated electrolyte and acid-base abnormalities

INTRODUCTION

The antimicrobial trimethoprim is structurally related to potassium-sparing diuretics and may consequently lead to derangements in electrolyte and acid-base balance. Since no report so far analyzed the literature documenting individual cases with electrolyte and acid-base derangements induced by trimethoprim, a systematic review was carried out.

EVIDENCE ACQUISITION

We retained 53 reports documenting 68 cases (42 males and 26 females 23 to 96 years of age) of electrolyte or acid-base derangements occurring on trimethoprim for about 5 days.

EVIDENCE SYNTHESIS

One hundred five electrolyte imbalances were detected in the 68 patients: hyperkalemia (>5.0 mmol/L) in 62 (91%), hyponatremia (<135 mmol/L) in 29 (43%) and metabolic acidosis (pH<7.38 and bicarbonate <19 mmol/L) in 14 (21%) cases. Following possible predisposing factors for electrolyte and acid-base abnormalities were found in 54 (79%) patients: high-dose trimethoprim, comedication with drugs that have been associated with electrolyte and acid-base derangements, preexisting kidney disease, age ≥80 years and diabetes mellitus.

CONCLUSIONS

High-dose trimethoprim, comedicated with drugs that have been associated with electrolyte and acid-base derangements, poor kidney function, age ≥80 years and diabetes mellitus predispose to trimethoprim-associated electrolyte and acid-base abnormalities. Clinicians must recognize patients at risk, possibly avoid drug combinations that may worsen the problem and monitor the laboratory values.

Cofilin participates in regulating alpha-epithelial sodium channel by interaction with 14-3-3 isoforms

Renal epithelial sodium channel (ENaC) plays a crucial role in maintaining homeostasis and sodium absorption. While insulin participates in controlling sodium transport across the renal epithelium, the underlying molecular mechanism remain unclear. In this study, we found that insulin increased the expression and function of alpha-epithelial sodium channel (α-ENaC) as well as phosphorylation of cofilin, a family of actin-binding proteins which disassembles actin filaments, in mouse cortical collecting duct (mpkCCD_c14) cells. The wild-type (WT) cofilin and its constitutively phosphorylated form (S3D), but not its constitutively non-phosphorylable form (S3A), contributed to the elevated expression on α-ENaC. Overexpression of 14-3-3ε, β, or γ increased the expression of α-ENaC and cofilin phosphorylation, which was blunted by knockdown of 14-3-3ε, β, or γ. Moreover, it was found that insulin increased the interaction between cofilin and 14-3-3 isoforms, which indicated relevance of 14-3-3 isoforms with cofilin. Furthermore, LIMK1/SSH1 pathway was involved in regulation of cofilin and α-ENaC expression by insulin. The results from this work indicate that cofilin participates in the regulation of α-ENaC by interaction with 14-3-3 isoforms.

Hyperkalemia in the Hypertensive Patient

PURPOSE OF REVIEW

Hyperkalemia develops in a patient with systemic arterial hypertension (HTN) if one or more risk factors are present, namely chronic kidney disease (CKD) (especially severe stage 4-5 CKD), diabetes mellitus (DM), heart failure (HF), or pharmacological therapies that interfere with potassium homeostasis, mainly through renin-angiotensin-aldosterone inhibition (RAASi). Hyperkalemia is a considerable reason of morbidity (emergency department (ED) visits and hospitalizations) and portends a higher mortality risk in patients at risk; for instance, hyperkalemia increases the risk of mortality within 1 day of a hyperkalemic event. This review aims to identify the risk factors for high-serum potassium, highlight the risk versus benefit of RAASi in certain patient populations, and outline preventive as well as therapeutic strategies for hyperkalemia.

RECENT FINDINGS

A growing body of evidence supports the safety and efficacy of cation-exchange resins, patiromer, or sodium zirconium cyclosilicate, in patients with a compelling indication for RAASi, yet in whom such therapy was complicated by hyperkalemia, allowing these patients to benefit from continued RAASi therapy. In summary, novel cation exchange polymers present the clinician with a new and safe strategy to address hyperkalemia in patients with a compelling indication for ongoing RAASi therapy instead of withdrawal of such therapy.

TNF Lectin-Like Domain Restores Epithelial Sodium Channel Function in Frameshift Mutants Associated with Pseudohypoaldosteronism Type 1B

Previous in vitro studies have indicated that tumor necrosis factor (TNF) activates amiloride-sensitive epithelial sodium channel (ENaC) current through its lectin-like (TIP) domain, since cyclic peptides mimicking the TIP domain (e.g., solnatide), showed ENaC-activating properties. In the current study, the effects of TNF and solnatide on individual ENaC subunits or ENaC carrying mutated glycosylation sites in the α-ENaC subunit were compared, revealing a similar mode of action for TNF and solnatide and corroborating the previous assumption that the lectin-like domain of TNF is the relevant molecular structure for ENaC activation. Accordingly, TNF enhanced ENaC current by increasing open probability of the glycosylated channel, position N511 in the α-ENaC subunit being identified as the most important glycosylation site. TNF significantly increased Na⁺ current through ENaC comprising only the pore forming subunits α or δ, was less active in ENaC comprising only β-subunits, and showed no effect on ENaC comprising γ-subunits. TNF did not increase the membrane abundance of ENaC subunits to the extent observed with solnatide. Since the α-subunit is believed to play a prominent role in the ENaC current activating effect of TNF and TIP, we investigated whether TNF and solnatide can enhance αβγ-ENaC current in α-ENaC loss-of-function frameshift mutants. The efficacy of solnatide has been already proven in pathological conditions involving ENaC in phase II clinical trials. The frameshift mutations αI68fs, αT169fs, αP197fs, αE272fs, αF435fs, αR438fs, αY447fs, αR448fs, αS452fs, and αT482fs have been reported to cause pseudohypoaldosteronism type 1B (PHA1B), a rare, life-threatening, salt-wasting disease, which hitherto has been treated only symptomatically. In a heterologous expression system, all frameshift mutants showed significantly reduced amiloride-sensitive whole-cell current compared to wild type αβγ-ENaC, whereas membrane abundance varied between mutants. Solnatide restored function in α-ENaC frameshift mutants to current density levels of wild type ENaC or higher despite their lacking a binding site for solnatide, previously located to the region between TM2 and the C-terminus of the α-subunit. TNF similarly restored current density to wild type levels in the mutant αR448fs. Activation of βγ-ENaC may contribute to this moderate current enhancement, but whatever the mechanism, experimental data indicate that solnatide could be a new strategy to treat PHA1B.

Respiratory transition in the newborn: a three-phase process

We propose that the respiratory transition at birth passes through three distinct, but overlapping phases, which reflect different physiological states of the lung. Accordingly, respiratory support given to infants should be optimised to suit the underlying physiological state of the lung as it passes through each phase. During the first phase, the airways are liquid-filled and so no pulmonary gas exchange can occur. Respiratory support should, therefore, be focused on clearing the gas exchange regions of liquid. In the absence of gas exchange, little or no CO2will accumulate within the airways and, therefore, interrupting inflation pressures to allow the lung to deflate and exhale CO2is unnecessary. This is the primary rationale for administering a sustained inflation at birth. During the second phase, the gas exchange regions are mostly cleared of liquid, allowing pulmonary gas exchange to commence. However, the liquid cleared from the airways resides within the tissue during this phase, which increases perialveolar interstitial tissue pressures and the risk of liquid re-entry back into the airways. As a result, respiratory support should be optimised to minimise alveolar re-flooding during expiration, which can be achieved by applying an end-expiratory pressure. The third and final phase occurs when the liquid is eventually cleared from lung tissue. Although gas exchange may be restricted by lung immaturity, injury and inflammation during this phase, considerations of how fetal lung liquid can adversely affect lung function are no longer relevant.

Pseudohypoaldosteronism in a Neonate Presenting as Life-Threatening Hyperkalemia

Context. Pseudohypoaldosteronism type 1 (PHA1) is a life-threatening disease that causes severe hyperkalemia and cardiac arrest if not treated appropriately or if diagnosis is missed. Objective. To report a case of a newborn with vomiting and lethargy, ultimately diagnosed with pseudohypoaldosteronism. Patient. This case presented to the ED at an age of 14 days in hypovolemic shock. There was a family history of sudden infant death, her sister who was diagnosed with CAH and passed away at 3 months of age despite regular hormone replacement. Our patient had cardiac arrest in ED, due to hyperkalemia; while receiving fluid boluses, cardiopulmonary resuscitation was initiated. After stabilization, diagnostic workup demonstrated persistently low sodium, acidosis, and high potassium, which required peritoneal dialysis. Based on these findings, the patient was diagnosed with CAH. It turned out later that the patient had PHA1. Two years later, the patient had a new sibling with the same disease diagnosed at birth and started immediately on treatment without any complication. Conclusions and Outcome. This case highlights the significant diagnostic and therapeutic challenges in treating children with PHA1. Adrenal crisis is not always CAH; delayed diagnosis can lead to complication and even death. The presence of high plasma renin activity, aldosterone, and cortisol, along with the presence of hyponatremia and hyperkalemia, established the diagnosis of PHA type 1 and ruled out CAH.

Pathophysiology, diagnosis, and treatment of mineralocorticoid disorders

The renin-angiotensin-aldosterone system (RAAS) is a major regulator of blood pressure control, fluid, and electrolyte balance in humans. Chronic activation of mineralocorticoid production leads to dysregulation of the cardiovascular system and to hypertension. The key mineralocorticoid is aldosterone. Hyperaldosteronism causes sodium and fluid retention in the kidney. Combined with the actions of angiotensin II, chronic elevation in aldosterone leads to detrimental effects in the vasculature, heart, and brain. The adverse effects of excess aldosterone are heavily dependent on increased dietary salt intake as has been demonstrated in animal models and in humans. Hypertension develops due to complex genetic influences combined with environmental factors. In the last two decades, primary aldosteronism has been found to occur in 5% to 13% of subjects with hypertension. In addition, patients with hyperaldosteronism have more end organ manifestations such as left ventricular hypertrophy and have significant cardiovascular complications including higher rates of heart failure and atrial fibrillation compared to similarly matched patients with essential hypertension. The pathophysiology, diagnosis, and treatment of primary aldosteronism will be extensively reviewed. There are many pitfalls in the diagnosis and confirmation of the disorder that will be discussed. Other rare forms of hyper- and hypo-aldosteronism and unusual disorders of hypertension will also be reviewed in this article.

Genetic disorders of potassium homeostasis

Hereditary disorders of potassium homeostasis are an interesting group of disorders, affecting people from the newborn period to adults of all ages. The clinical presentation varies from severe hypotension at birth to uncontrolled hypertension in adults, often associated with abnormal potassium values, although many patients may have a normal serum potassium concentration despite being affected by the genetic disorder. A basic understanding of these disorders and their underlying mechanisms has significant clinical implications, especially in the few patients with subtle clinical signs and symptoms. We present a summary of these disorders, with emphasis on the clinical presentation and genetic mechanisms of these disorders.

Establishing functional residual capacity in the non-breathing infant

The transition to newborn life critically depends upon lung aeration and the onset of air-breathing, which triggers major cardiovascular changes required for postnatal life, including increases in pulmonary blood flow. Recent imaging studies indicate that lung aeration and functional residual capacity (FRC) recruitment results from inspiratory efforts, which create transpulmonary pressure gradients. During inspiration, these pressure gradients drive airway liquid movement through the conducting and into the distal airways where it crosses the pulmonary epithelium and enters the surrounding tissue. Although this process can occur rapidly (within 3-5 breaths), liquid clearance from lung tissue is much slower, resulting in oedema and increased interstitial tissue pressures, facilitating liquid re-entry into the airways at FRC. Whereas this liquid may be cleared during the next inspiration, liquid re-entry at FRC can be opposed by Na(+) reabsorption, oncotic pressures and expiratory braking manoeuvres. Recognition that transpulmonary pressure gradients mainly drive airway liquid clearance at birth has provided a clearer understanding of how this process may be facilitated in very preterm infants. In particular, it underpins the rationale for providing respiratory support that initially focuses on moving liquid through tubes (airways) rather than air. As the viscosity of liquid is much greater than air, the resistance to moving liquid is ≈ 100 times greater than air, necessitating the use of higher pressures or longer inflation times. Although it is unclear how this strategy could be safely applied clinically, it is clear that end-expiratory pressures are required to create and maintain FRC in preterm infants.

Pseudohypoaldosteronism type 1: clinical features and management in infancy

Type 1 pseudohypoaldosteronism (PHA) is a rare heterogeneous group of disorders characterised by resistance to aldosterone action. There is resultant salt wasting in the neonatal period, with hyperkalaemia and metabolic acidosis. Only after results confirm isolated resistance to aldosterone can the diagnosis of type 1 PHA be confidently made. Type 1 PHA can be further classified into i) renal type 1 (autosomal dominant (AD)) and ii) multiple target organ defect/systemic type 1 (autosomal recessive (AR)). The aim of this case series was to characterise the mode of presentation, management and short-term clinical outcomes of patients with PHA type 1. Case notes of newly diagnosed infants presenting with PHA type 1 were reviewed over a 5-year time period. Seven patients were diagnosed with PHA type 1. Initial presentation ranged from 4 to 28 days of age. Six had weight loss as a presenting feature. All subjects had hyperkalaemia, hyponatraemia, with elevated renin and aldosterone levels. Five patients have renal PHA type 1 and two patients have systemic PHA type, of whom one has had genetic testing to confirm the AR gene mutation on the SCNN1A gene. Renal PHA type 1 responds well to salt supplementation, whereas management of patients with systemic PHA type 1 proves more difficult as they are likely to get frequent episodes of electrolyte imbalance requiring urgent correction.

Genetic dissection of sodium and potassium transport along the aldosterone-sensitive distal nephron: importance in the control of blood pressure and hypertension

In this review, we discuss genetic evidence supporting Guyton's hypothesis stating that blood pressure control is critically depending on fluid handling by the kidney. The review is focused on the genetic dissection of sodium and potassium transport in the distal nephron and the collecting duct that are the most important sites for the control of sodium and potassium balance by aldosterone and angiotensin II. Thanks to the study of Mendelian forms of hypertension and their corresponding transgenic mouse models, three main classes of diuretic receptors (furosemide, thiazide, amiloride) and the main components of the aldosterone- and angiotensin-dependent signaling pathways were molecularly identified over the past 20 years. This will allow to design rational strategies for the treatment of hypertension and for the development of the next generation of diuretics.

Failure to thrive, hyponatremia, and hyperkalemia in a neonate

CME EDUCATIONAL OBJECTIVES: 1.Describe the varying clinical presentations of pseudohypoaldosteronism in the neonatal period.2.Review the physiology of aldosterone production and pathophysiology of pseudohypoaldosteronism.3.Identify treatment options for pseudohypoaldosteronism when identified in the neonatal period. Pseudohypoaldosteronism type I (PHA1) is a rare disease of mineralocorticoid resistance caused by defects in sodium transport in the distal tubule of the kidney. It presents in the neonate with life-threatening dehydration due to salt wasting, accompanied by hyperkalemia, acidosis, and, frequently, failure to thrive. Patients with PHA1 are often initially diagnosed with congenital adrenal hyperplasia, but their electrolyte abnormalities are resistant to treatment with glucocorticoids and mineralocorticoids. In these patients, an astute clinician will broaden his or her differential, resulting in life-saving treatment.

Five novel mutations in the SCNN1A gene causing autosomal recessive pseudohypoaldosteronism type 1

BACKGROUND

Pseudohypoaldosteronism type 1 (PHA1) is a monogenic disease caused by mutations in the genes encoding the human mineralocorticoid receptor (MR) or the α (SCNN1A), β (SCNN1B) or γ (SCNN1G) subunit of the epithelial Na(+) channel (ENaC). While autosomal dominant mutation of the MR cause renal PHA1, autosomal recessive mutations of the ENaC lead to systemic PHA1. In the latter, affected children suffer from neonatal onset of multi-organ salt loss and often exhibit cystic fibrosis-like pulmonary symptoms.

OBJECTIVE

We searched for underlying mutations in seven unrelated children with systemic PHA1, all offsprings of healthy consanguineous parents.

METHODS AND RESULTS

Amplification of the SCNN1A gene and sequencing of all 13 coding exons unraveled mutations in all of our patients. We found five novel homozygous mutations (c.587_588insC in two patients, c.1342_1343insTACA, c.742delG, c.189C>A, c.1361-2A>G) and one known mutation (c.1474C>T) leading to truncation of the αENaC protein. All parents were asymptomatic heterozygous carriers of the respective mutations, confirming the autosomal recessive mode of inheritance. Five out of seven patients exhibited pulmonary symptoms in the neonatal period.

CONCLUSION

The α subunit is essential for ENaC function and mutations truncating the pore-forming part of the protein leading to systemic PHA1. Based on current knowledge, the pulmonary phenotype cannot be satisfactorily predicted.

Regulation of paracellular transport in the distal nephron

PURPOSE OF REVIEW

Claudins play a major role in the regulation of paracellular electrolyte reabsorption in the kidney. This review describes the recent findings of the physiological function of claudins underlying the paracellular transport mechanisms for Cl(-) reabsorption in the collecting duct.

RECENT FINDINGS

There are two parallel mechanisms for transepithelial Cl(-) reabsorption in the collecting duct that utilize the Na-driven Cl-bicarbonate exchanger (NDCBE) and the claudin-based paracellular channel. Histological studies have demonstrated the renal localization of claudin-3, claudin-4, claudin-7, and claudin-8 in the collecting duct. Molecular analyses using several collecting duct cell models have come to the conclusion that claudin-4 functions as a paracellular Cl(-) channel. The channel function of claudin-4 is conferred by a charged lysine residue (K65) in its extracellular loop. Claudin-8 is required for paracellular Cl(-) permeation through its interaction with and recruitment of claudin-4 during tight junction assembly. Claudin-7 provides the basic barrier function to the collecting duct. Genetic ablation of claudin-7 in animals results in systemic dehydration owing to the loss of extracellular ions and fluid in the kidney.

SUMMARY

The paracellular pathway in the collecting duct is an important route for transepithelial Cl(-) reabsorption that determines the extracellular NaCl content and the blood pressure. In the collecting duct cells, claudin-4 and claudin-8 interact to form a paracellular Cl(-) channel, whereas claudin-7 maintains the transepithelial resistance. Different subsets of the claudin family proteins fulfill diverse aspects of the tight junction function that will be fundamental to understanding the physiology of the paracellular pathway.

A case of systemic pseudohypoaldosteronism with a novel mutation in the SCNN1A gene

We report a neonatal case of systemic pseudohypoaldosteronism type 1 caused by a novel mutation in the SCNN1A gene (homozygous c.1052+2dupT in intron 3) in which the patient presented with life-threatening hyperkalemia, hyponatremia and metabolic acidosis. It remains uncertain if there is genotype-phenotype correlation, due to the rarity of the disease. This mutation, which to our best knowledge has not been described before, was associated with a very severe phenotype requiring aggressive therapy.

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.

Revealing a subclinical salt-losing phenotype in heterozygous carriers of the novel S562P mutation in the alpha subunit of the epithelial sodium channel

OBJECTIVE

Pseudohypoaldosteronism type I (PHA1) is a rare inborn disease causing severe salt loss. Mutations in the three coding genes of the epithelial sodium channel (ENaC) are responsible for the systemic autosomal recessive form. So far, no phenotype has been reported in heterozygous carriers.

PATIENTS

A consanguineous family from Somalia giving birth to a neonate suffering from PHA1 was studied including clinical and hormonal characteristics of the family, mutational analysis of the SCNN1A, SCNN1B, SCNN1G and CFTR genes and in vitro analysis of the functional consequences of a mutant ENaC channel.

RESULTS

CFTR mutations have been excluded. SCNN1A gene analysis revealed a novel homozygous c.1684T > C mutation resulting in a S562P substitution in the alphaENaC protein of the patient. Functional analysis showed a significantly reduced S562P channel function compared to ENaC wild type. Protein synthesis and channel subunit assembly were not altered by the S562P mutation. Co-expression of mutant and wild-type channels revealed a dominant negative effect. In heterozygote carriers, sweat sodium and chloride concentrations were increased without additional hormonal or clinical phenotypes.

CONCLUSION

Hence, the novel mutation S562P is causing systemic PHA1 in the homozygous state. A thorough clinical investigation of the heterozygote SCNN1A mutation carriers revealed increased sweat sodium and chloride levels consistent with a dominant effect of the mutant S562P allele. Whether this subclinical phenotype is of any consequence for the otherwise asymptomatic heterozygous carriers has to be elucidated.

Conditional gene targeting of the ENaC subunit genes Scnn1b and Scnn1g

Epithelial sodium channels (ENaC) are members of the degenerin/ENaC superfamily of non-voltage-gated, highly amiloride-sensitive cation channels that are composed of three subunits (alpha-, beta-, and gamma-ENaC). Since complete gene inactivation of the beta- and gamma-ENaC subunit genes (Scnn1b and Scnn1g) leads to early postnatal death, we generated conditional alleles and obtained mice harboring floxed and null alleles for both gene loci. Using quantitative RT-PCR analysis, we showed that the introduction of the loxP sites did not interfere with the mRNA transcript expression level of the Scnn1b and Scnn1g gene locus, respectively. Upon a regular and salt-deficient diet, both beta- and gamma-ENaC floxed mice showed no difference in their mRNA transcript expression levels, plasma electrolytes, and aldosterone concentrations as well as weight changes compared with control animals. These mice can now be utilized to dissect the role of ENaC function in classical and nonclassic target organs/tissues.

An obligatory heterodimer of 14-3-3beta and 14-3-3epsilon is required for aldosterone regulation of the epithelial sodium channel

Increased distal nephron sodium absorption in response to aldosterone involves Nedd4-2 phosphorylation, which blocks its ability to ubiquitylate ENaC and increases apical membrane channel density by reducing its endocytosis. Our prior work (Liang, X., Peters, K. W., Butterworth, M. B., and Frizzell, R. A. (2006) J. Biol. Chem. 281, 16323-16332) showed that aldosterone selectively increased 14-3-3 protein isoform expression and that the association of 14-3-3beta with phospho-Nedd4-2 was required for sodium transport stimulation. The knockdown of 14-3-3beta alone nearly eliminated the response to aldosterone, despite the expression of other 14-3-3 isoforms in cortical collecting duct (CCD) cells. To further examine this marked effect of 14-3-3beta knockdown, we evaluated the hypothesis that phospho-Nedd4-2 binding prefers a heterodimer composed of two different 14-3-3 isoforms. We tested this concept in polarized CCD cells using RNA interference and assays of sodium transport and of the interaction of Nedd4-2 with 14-3-3epsilon, a second aldosterone-induced isoform. As observed previously for 14-3-3beta knockdown, small interfering RNA-induced reduction of 14-3-3epsilon markedly attenuated aldosterone-stimulated ENaC expression and sodium transport and increased the interaction of Nedd4-2 with ENaC toward prealdosterone levels. After aldosterone induction, 14-3-3beta and 14-3-3epsilon were quantitatively co-immunoprecipitated from CCD cell lysates, and the association of both isoforms with Nedd4-2 increased. Finally, the knockdown of either 14-3-3beta or 14-3-3epsilon reduced the association of Nedd4-2 with the other isoform. We conclude that the two aldosterone-induced 14-3-3 isoforms, beta and epsilon, interact with phospho-Nedd4-2 as an obligatory heterodimer, blocking its interaction with ENaC and thereby increasing apical ENaC density and sodium transport.

Selective Genotyping Reveals Association Between the Epithelial Sodium Channel γ-Subunit and Systolic Blood Pressure

Systolic blood pressure is determined in large part by genes. Six independent studies have reported evidence of linkage between systolic pressure and chromosome 16p12 that incorporatesSCNN1G, the gene encoding the γ-subunit of the epithelial sodium channel. We undertook the first comprehensive association analysis ofSCNN1Gand systolic pressure. To achieve genetic contrast, we sampled unrelated subjects within the upper (mean: 166 mm Hg; n=96) and lower (mean: 98 mm Hg; n=94) 10% of the systolic pressure distribution of 2911 subjects from the Victorian Family Heart Study. We examined genotypes and haplotypes related to 26 single nucleotide polymorphisms acrossSCNN1Gand its promoter. Each of 3 single nucleotide polymorphisms (rs13331086,rs11074553, andrs4299163) in introns 5 and 6 showed evidence of association with systolic pressure in logistic regression analyses adjusted for age, sex, and body mass index. Considered as a haplotype block, these single nucleotide polymorphisms were significantly associated with systolic pressure (haplo.score global:P=0.0001). In permutation analyses to account for multiple testing, a result such as this was observed only once in 10 000 permutations. The estimated frequency of 1 haplotype (TGC) was substantially greater in high (13.3%) than low (0.6%) systolic pressure subjects (P=0.0001). Three other haplotypes (TGG, TAC, and GGC) showed associations with high or low systolic pressure consistent with the observed associations of their composite alleles. These findings identify relatively common polymorphisms in theSCNN1Ggene that are associated with high systolic blood pressure in the general Australian white population.

Dietary electrolyte-driven responses in the renal WNK kinase pathway in vivo

WNK1 and WNK4 are unusual serine/threonine kinases with atypical positioning of the catalytic active-site lysine (WNK: With-No-K[lysine]). Mutations in these WNK kinase genes can cause familial hyperkalemic hypertension (FHHt), an autosomal dominant, hypertensive, hyperkalemic disorder, implicating this novel WNK pathway in normal regulation of BP and electrolyte balance. Full-length (WNK1-L) and short (WNK1-S) kinase-deficient WNK1 isoforms previously have been identified. Importantly, WNK1-S is overwhelmingly predominant in kidney. Recent Xenopus oocyte studies implicate WNK4 in inhibition of both thiazide-sensitive co-transporter-mediated Na+ reabsorption and K+ secretion via renal outer medullary K+ channel and now suggest that WNK4 is inhibited by WNK1-L, itself inhibited by WNK1-S. This study examined WNK pathway gene expression in mouse kidney and its regulation in vivo. Expression of WNK1-S and WNK4 is strongest in distal tubule, dropping sharply in collecting duct and with WNK4 also expressed in thick ascending limb and the macula densa. These nephron segments that express WNK1-S and WNK4 mRNA have major influence on long-term NaCl reabsorption, BP, K+, and acid-base balance, processes that all are disrupted in FHHt. In vivo, this novel WNK pathway responds with significant upregulation of WNK1-S and WNK4 with high K+ intake and reduction in WNK1-S on chronic lowering of K+ or Na+ intake. A two-compartment distal nephron model explains these in vivo findings and the pathophysiology of FHHt well, with WNK and classic aldosterone pathways responding to drivers from K+ balance, extracellular volume, and aldosterone and cross-talk through distal Na+ delivery regulating electrolyte balance and BP.

14-3-3 isoforms are induced by aldosterone and participate in its regulation of epithelial sodium channels

Aldosterone increases sodium absorption across renal collecting duct cells primarily by increasing the apical membrane expression of ENaC, the sodium entry channel. Nedd4-2, a ubiquitin-protein isopeptide ligase, tags ENaC with ubiquitin for internalization and degradation, but when it is phosphorylated by the aldosterone-induced kinase, SGK1, Nedd4-2 is inhibited and apical ENaC density and sodium absorption increase. We evaluated the hypothesis that 14-3-3 proteins participate in the aldosterone-mediated regulation of ENaC by associating with phosphorylated Nedd4-2. Mouse cortical collecting duct (mCCD) epithelia cultured on filters expressed several 14-3-3 isoforms; this study focused on an isoform whose expression was induced 3-fold by aldosterone, 14-3-3beta. In polarized mCCD epithelia, aldosterone elicited significant, time-dependent increases in the expression of alpha-ENaC, SGK1, phospho-Nedd4-2, and 14-3-3beta without altering total Nedd4-2. Aldosterone decreased the interaction of alpha-ENaC with Nedd4-2, and with similar kinetics increased the association of 14-3-3beta with phospho-Nedd4-2. Short interfering RNA-induced knockdown of 14-3-3beta blunted the aldosterone-induced increase in alpha-ENaC expression, returned alpha-ENaC-Nedd4-2 binding toward prealdosterone levels, and blocked the aldosterone-stimulated increase in transepithelial sodium transport. Incubation of cell extracts with a selective phospho-Nedd4-2 antibody blocked the aldosterone-induced association of 14-3-3beta with Nedd4-2, implicating SGK1 phosphorylation at Ser-328 as the primary site of 14-3-3beta binding. Our studies show that aldosterone increases the expression of 14-3-3beta, which interacts with phospho-Nedd4-2 to block its interaction with ENaC, thus enhancing sodium absorption by increasing apical membrane ENaC density.

Serum- and glucocorticoid-regulated kinase 1 regulates ubiquitin ligase neural precursor cell-expressed, developmentally down-regulated protein 4-2 by inducing interaction with 14-3-3

Serum- and glucocorticoid-regulated kinase 1 (SGK1) is an aldosterone-regulated early response gene product that regulates the activity of several ion transport proteins, most notably that of the epithelial sodium channel (ENaC). Recent evidence has established that SGK1 phosphorylates and inhibits Nedd4-2 (neural precursor cell-expressed, developmentally down-regulated protein 4-2), a ubiquitin ligase that decreases cell surface expression of the channel and possibly stimulates its degradation. The mechanistic basis for this SGK1-induced Nedd4-2 inhibition is currently unknown. In this study we show that SGK1-mediated phosphorylation of Nedd4-2 induces its interaction with members of the 14-3-3 family of regulatory proteins. Through functional characterization of Nedd4-2-mutant proteins, we demonstrate that this interaction is required for SGK1-mediated inhibition of Nedd4-2. The concerted action of SGK1 and 14-3-3 appears to disrupt Nedd4-2-mediated ubiquitination of ENaC, thus providing a mechanism by which SGK1 modulates the ENaC-mediated Na(+) current. Finally, the expression pattern of 14-3-3 is also consistent with a functional role in distal nephron Na(+) transport. These results demonstrate a novel, physiologically significant role for 14-3-3 proteins in modulating ubiquitin ligase-dependent pathways in the control of epithelial ion transport.

Chronic hyperaldosteronism in a transgenic mouse model fails to induce cardiac remodeling and fibrosis under a normal-salt diet

Primary aldosteronism causes severe hypertension in humans (Conn's syndrome) with cardiac hypertrophy, characterized by a fibrosis more severe than the one observed in patients with essential hypertension. This suggests that aldosterone by itself may have specific and direct effects on cardiac remodeling through the activation of the cardiac mineralocorticoid receptor. Experimental evidence obtained in studying uninephrectomized rats treated with aldosterone or deoxycorticosterone (DOC) together with salt loading has led to similar conclusions. To examine the direct consequences of chronically elevated aldosterone levels on cardiac pathophysiology, we analyzed a mouse model (α-epithelial Na channel −/−Tg) that is normotensive under normal-salt diet but exhibits chronic hyperaldosteronism. Sixteen-month-old transgenic rescue mice that were kept under a regular salt diet that contains a small amount of sodium (0.3% Na+) displayed a compensated PHA-1 phenotype with normal body weight, normal kidney index, normal blood pressure, but 6.3-fold elevated plasma aldosterone levels compared with the age-matched control group. Peripheral resistance of distal colon to aldosterone was shown by a significant decrease of the amiloride-sensitive rectal potential difference, and its diurnal cyclicity was blunted. Despite chronically high plasma aldosterone levels, these animals do not show any evidence of cardiac hypertrophy, remodeling, or fibrosis, using collagen staining and anti-α-skeletal and α-smooth actin immunochemical labeling of heart sections. Cardiac fibrosis as seen in DOC- or aldosterone/salt-treated animal models is therefore likely to be due to the synergistic effect of salt, aldosterone, and other confounding factors rather than to the elevated circulating aldosterone levels alone.

The epithelial sodium channel: from molecule to disease

Genetic analysis has demonstrated that Na absorption in the aldosterone-sensitive distal nephron (ASDN) critically determines extracellular blood volume and blood pressure variations. The epithelial sodium channel (ENaC) represents the main transport pathway for Na+ absorption in the ASDN, in particular in the connecting tubule (CNT), which shows the highest capacity for ENaC-mediated Na+ absorption. Gain-of-function mutations of ENaC causing hypertension target an intracellular proline-rich sequence involved in the control of ENaC activity at the cell surface. In animal models, these ENaC mutations exacerbate Na+ transport in response to aldosterone, an effect that likely plays an important role in the development of volume expansion and hypertension. Recent studies of the functional consequences of mutations in genes controlling Na+ absorption in the ASDN provide a new understanding of the molecular and cellular mechanisms underlying the pathogenesis of salt-sensitive hypertension.

Low expression of human epithelial sodium channel in airway epithelium of preterm infants with respiratory distress

OBJECTIVE

Active ion transport is critical to postnatal clearance of lung fluid. The importance of epithelial sodium channel (ENaC) in this clearance has been demonstrated in animal studies in which alpha-ENaC knockout mice died postnatally as a result of respiratory insufficiency. In animals, the expression of alpha-ENaC in respiratory epithelium is dependent on gestational age, but when assessed by in situ hybridization in the human (h), the mRNA is present from the earliest stages of pulmonary development. Therefore, the purpose of the present investigation was to quantify mRNA of the alpha-, beta-, and gamma-hENaC subunits of newborn preterm infants with respiratory distress and compare the gene expression data against those detected in healthy term infants. In addition, the effect of systemic dexamethasone therapy on the 3 hENaC subunits was studied in 4 preterm infants who received prolonged assisted ventilation.

METHODS

The expression of subunits of hENaC was determined in samples taken from nasal respiratory epithelium of 7 healthy term infants (gestation age: 39.3 +/- 0.9 weeks [mean +/- standard deviation) and 5 preterm infants (gestational age: 27.2 +/- 0.9 weeks) with respiratory distress syndrome within 5 hours of birth. Betamethasone had been given to all mothers of preterm infants. In 4 additional preterm infants who still required assisted ventilation at 43 +/- 6 days postnatal age, the expression of alpha-hENaC was determined in samples taken before and during treatment with dexamethasone.

RESULTS

Preterm infants with respiratory distress syndrome had low expression of all hENaC subunits relative to healthy term infants (alpha-hENaC: 5.38 +/- 2.01 [amol/fmol cytokeratin 18] vs 9.13 +/- 2.26; beta-hENaC: 2.44 +/- 1.43 vs 4.25 +/- 1.10; gamma-hENaC: 2.43 +/- 0.11 vs 6.81 +/- 3.24). Each of the 4 preterm infants who were treated with dexamethasone at approximately 1 month of age showed an increase in expression of alpha-hENaC and beta-hENaC subunit normalized to cytokeratin 18.

CONCLUSION

All 3 subunits of the hENaC are low in preterm relative to full-term infants. alpha-hENaC mRNA in respiratory epithelium is increased by therapeutic doses of glucocorticosteroid. Low expression of alpha-hENaC in human respiratory epithelium may play a role in the pathogenesis of respiratory distress in preterm infants.

Pharmacogenomics of diuretic drugs: data on rare monogenic disorders and on polymorphisms and requirements for further research

This review summarizes the current status of our knowledge about the role of pharmacogenetic variation in response to diuretics and suggests future research topics for the field. Genes with a role in the pharmacokinetics of most diuretics are renal drug transporters, especially OAT1, OAT3 and OCT2 (genes SLC22A6, SLC22A8 and SLC22A2) whereas variants in carbonic anhydrase (CA), cytochrome P450 enzymes and sulfotransferases are relevant only for specific substances. Genes on the pharmacodynamic side include the primary targets of thiazide, loop, K+-sparing and aldosterone antagonistic diuretics: NCC, NKCC2, ENaC and the mineralocorticoid receptor (genes SLC12A3, SLC12A1, SCNN1A, B, G and NR3C2). Rare variants of these proteins cause Gitelman’s syndrome, Bartter’s syndrome, Liddle’s syndrome or pregnancy-induced hypertension. Polymorphisms in these and in associated proteins such as GNB3, α-adducin and angiotensin-converting enzyme (ACE) seem to be clinically relevant. In conclusion, first knowledge has evolved that efficacy of diuretic drugs may be determined by genetic polymorphisms in genes determining pharmacokinetics and pharmacodynamics of this drug class. In the future, the selection of a diuretic drug or the dosing schedules may be individually chosen based on pharmacogenetic parameters, however, many questions remain to be answered before this fantasy becomes reality.

WNK1, a Gene within a Novel Blood Pressure Control Pathway, Tissue-Specifically Generates Radically Different Isoforms with and without a Kinase Domain

ABSTRACT. WNK1 is a member of a novel serine/threonine kinase family, With-No-K, (lysine). Intronic deletions in the encoding gene cause Gordon syndrome, an autosomal dominant, hypertensive, hyperkalemic disorder particularly responsive to thiazide diuretics, a first-line treatment in essential hypertension. To elucidate the novel WNK1 BP control pathway active in distal nephron, WNK1 expression in mouse was studied. It was found that WNK1 is highly expressed in testis > heart, lung, kidney, placenta > skeletal muscle, brain, and widely at low levels. Several WNK1 transcript classes are demonstrated, showing tissue-, developmental-, and nephron-segment–specific expression. Importantly, in kidney, the most prominent transcripts are smaller than elsewhere, having the first four exons replaced by an alternative 5′-exon, deleting the kinase domain, and showing strong distal nephron expression, whereas larger transcripts show low-level widespread distribution. Alternative splicing of exons 11 and 12 is prominent—for example, transcripts containing exon 11 are abundant in neural tissues, testis, and secondary renal transcripts but are predominantly absent in placenta. The transcriptional diversity generated by these events would produce proteins greatly differing in both structure and function. These findings help further define and clarify the role of WNK1 and the thiazide-responsive pathway relevant to essential hypertension in which it participates. E-mail: Roger.Brown@ed.ac.uk

Epithelial sodium channel, salt intake, and hypertension

The epithelial sodium channel (ENaC) is a membrane protein made of three different but homologous subunits (a, b, and g) present in the apical membrane of epithelial cells of, for example, the distal nephron. This channel is responsible for salt reabsorption in the kidney and can cause human diseases by increasing channel function in Liddle's syndrome, a form of hereditary hypertension, or by decreasing channel function in pseudohypoaldosteronism type I, a salt-wasting disease in infancy. This review briefly discusses recent advances in understanding the implication of ENaC in Liddle's syndrome and in pseudohypoaldosteronism type I, both caused by mutations in the SCNN1 (ENaC) genes. Furthermore, it is still an open question to which extent SCNN1 genes coding for ENaC might be implicated in essential hypertension. The development of Scnn1 genetically engineered mouse models will provide the opportunity to test the effect of environmental factors, like salt intake, on the development of this kind of salt- sensitive hypertension.