Association of cystic fibrosis transmembrane conductance regulator with epithelial sodium channel subunits carrying Liddle's syndrome mutations.

The association of the cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial sodium channel (ENaC) in the pathophysiology of cystic fibrosis (CF) is controversial. Previously, we demonstrated a close physical association between wild-type (WT) CFTR and WT ENaC. We have also shown that the F508del CFTR fails to associate with ENaC unless the mutant protein is rescued pharmacologically or by low temperature. In this study, we present the evidence for a direct physical association between WT CFTR and ENaC subunits carrying Liddle's syndrome mutations. We show that all three ENaC subunits bearing Liddle's syndrome mutations (both point mutations and the complete truncation of the carboxy terminus), could be coimmunoprecipitated with WT CFTR. The biochemical studies were complemented by fluorescence lifetime imaging microscopy (FLIM), a distance-dependent approach that monitors protein-protein interactions between fluorescently labeled molecules. Our measurements revealed significantly increased fluorescence resonance energy transfer between CFTR and all tested ENaC combinations as compared with controls (ECFP and EYFP cotransfected cells). Our findings are consistent with the notion that CFTR and ENaC are within reach of each other even in the setting of Liddle's syndrome mutations, suggestive of a direct intermolecular interaction between these two proteins.

Ubiquitination of renal ENaC subunits in vivo.

Ubiquitination of the epithelial Na+ channel (ENaC) in epithelial cells may influence trafficking and hormonal regulation of the channels. We assessed ENaC ubiquitination (ub-ENaC) in mouse and rat kidneys using affinity beads to capture ubiquitinated proteins from tissue homogenates and Western blot analysis with anti-ENaC antibodies. Ub-αENaC was observed primarily as a series of proteins of apparent molecular mass of 40-70 kDa, consistent with the addition of variable numbers of ubiquitin molecules primarily to the NH2-terminal cleaved fragment (~30 kDa) of the subunit. No significant Ub-ßENaC was detected, indicating that ubiquitination of this subunit is minimal. For γENaC, the protein eluted from the affinity beads had the same apparent molecular mass as the cleaved COOH-terminal fragment of the subunit (~65 kDa). This suggests that the ubiquitinated NH2 terminus remains attached to the COOH-terminal moiety during isolation through disulfide bonds. Consistent with this, under nonreducing conditions, eluates contained material with increased molecular mass (90-150 kDa). In mice with a Liddle syndrome mutation (ß566X) deleting a putative binding site for the ubiquitin ligase neural precursor cell expressed developmentally downregulated 4-2, the amount of ub-γENaC was reduced as expected. To assess aldosterone dependence of ubiquitination, we fed rats either control or low-Na+ diets for 7 days before kidney harvest. Na+ depletion increased the amounts of ub-αENaC and ub-γENaC by three- to fivefold, probably reflecting increased amounts of fully cleaved ENaC. We conclude that ubiquitination occurs after complete proteolytic processing of the subunits, contributing to retrieval and/or disposal of channels expressed at the cell surface. Diminished ubiquitination does not appear to be a major factor in aldosterone-dependent ENaC upregulation.

Monogenic Forms of Hypertension.

Essential hypertension is a highly prevalent disease in the general population. Secondary hypertension is characterized by a specific and potentially reversible cause of increased blood pressure levels. Some secondary endocrine forms of hypertension are common (caused by uncontrolled cortisol, aldosterone, or catecholamines production). This article describes rare monogenic forms of hypertension, characterized by electrolyte disorders and suppressed renin-aldosterone axis. They represent simple models for the physiology of renal control of sodium levels and plasma volume, thus reaching a high scientific interest. Furthermore, they could explain some features closer to the essential phenotype of hypertension, suggesting a mechanistically driven personalized treatment.

Resistant Hypertension: A Clinical Perspective.

Resistant hypertension is a common clinical entity, defined as suboptimal blood pressure response to multiple therapies after excluding medication nonadherence and secondary forms of hypertension. Patients with resistant hypertension generally share several comorbidities. Resistant hypertension is more common in individuals of African descent. Blood pressure should be optimized using multiple strategies, including lifestyle changes and single-pill combination therapies, with the aim of reducing cardiovascular events while reducing side effects from using antihypertensive therapy. A renin/aldosterone-based diagnostic and treatment approach will help tailor therapy. The use of mineralocorticoid receptor antagonists or amiloride as appropriate is favored.

Identification of glycyrrhizin metabolites in humans and of a potential biomarker of liquorice-induced pseudoaldosteronism: a multi-centre cross-sectional study.

Liquorice [main ingredient, glycyrrhizin (GL)] is widely used as a food sweetener and herbal medicine. Occasionally, liquorice consumption causes pseudoaldosteronism as a side effect which causes oedema, hypokalaemia, and hypertension due to hyperactivity of mineral corticoid receptor. We aimed to detect GL metabolites in human blood and urine samples and to determine the pathological relationship between GL metabolites and pseudoaldosteronism. For this multi-centre, retrospective, cross-sectional study, we recruited patients who had visited Center for Kampo Medicine in Keio University Hospital, Department of Japanese Oriental (Kampo) Medicine in Chiba University Hospital, Clinic of Japanese Oriental (Kampo) Medicine in Kanazawa University Hospital, and Department of Oriental Medicine in Kameda Medical Center from November 2011 to July 2018. We collected laboratory data including concentration of serum potassium, plasma activity of renin and aldosterone, and residual blood and/or urine samples of participants who had experienced symptoms/signs of pseudoaldosteronism in the form of increase in blood pressure and occurrence or aggregation of oedema while taking liquorice-containing herbal preparations, and measured GL metabolites using a highly selective liquid chromatography tandem mass spectrometer system. We registered 97 participants (mean age 60 ± 15 years; male:female 14:83). 18ß-glycyrrhetinic acid (GA) was detected in 67 serum samples (median 122 nM, range 5 nM-1.8 µM) and 18ß-glycyrrhetyl-3-O-sulfate (compound 3) in 68 samples (median 239 nM, range 2 nM-4.2 µM). 3-Monoglucuronyl 18ß-glycyrrhetinic acid, 22α-hydroxy-18ß-glycyrrhetyl-3-O-sulfate-30-glucuronide, 22α-hydroxy-18ß-glycyrrhetyl-3-O-sulfate, and GL itself were not or rarely detected. We could not find any correlation between blood pressure or peripheral oedema and serum concentration of GL metabolites. Sulfotransferase 2A1 catalysed the metabolic reaction of GA to compound 3, a major GL metabolite in human blood. High serum concentration of compound 3 was related to lower renin, aldosterone, and potassium levels, suggesting a pathological relationship between compound 3 and liquorice-induced pseudoaldosteronism. This is the first study to identify the association between a novel metabolite, compound 3, and the incidence of pseudoaldosteronism, highlighting it as a promising biomarker.

Diagnostic approach to low-renin hypertension.

Renin-angiotensin-aldosterone system (RAAS) plays a crucial role in maintaining water and electrolytes homoeostasis, and its deregulation contributes to the development of arterial hypertension. Since the historical description of the "classical" RAAS, a dramatic increase in our understanding of the molecular mechanisms underlying the development of both essential and secondary hypertension has occurred. Approximatively 25% of the patients affected by arterial hypertension display low-renin levels, a definition that is largely arbitrary and depends on the investigated population and the specific characteristics of the assay. Most often, low-renin levels are expression of a physiological response to sodium-volume overload, but also a significant number of secondary hereditary or acquired conditions falls within this category. In a context of suppressed renin status, the concomitant examination of plasma aldosterone levels (which can be inappropriately elevated, within the normal range or suppressed) and plasma potassium are essential to formulate a differential diagnosis. To distinguish between the different forms of low-renin hypertension is of fundamental importance to address the patient to the proper clinical management, as each subtype requires a specific and targeted therapy. The present review will discuss the differential diagnosis of the most common medical conditions manifesting with a clinical phenotype of low-renin hypertension, enlightening the novelties in genetics of the familial forms.

ENaC and ROMK activity are inhibited in the DCT2/CNT of TgWnk4PHAII mice.

Mice transgenic for genomic segments harboring PHAII (pseudohypoaldosteronism type II) mutant Wnk4 (with-No-Lysine kinase 4) (TgWnk4PHAII) have hyperkalemia which is currently believed to be the result of high activity of Na-Cl cotransporter (NCC). This leads to decreasing Na+ delivery to the distal nephron segment including late distal convoluted tubule (DCT) and connecting tubule (CNT). Since epithelial Na+ channel (ENaC) and renal outer medullary K+ channel (ROMK or Kir4.1) are expressed in the late DCT and play an important role in mediating K+ secretion, the aim of the present study is to test whether ROMK and ENaC activity in the DCT/CNT are also compromised in the mice expressing PHAII mutant Wnk4. Western blot analysis shows that the expression of ßENaC and γENaC subunits but not αENaC subunit was lower in TgWnk4PHAII mice than that in wild-type (WT) and TgWnk4WT mice. Patch-clamp experiments detected amiloride-sensitive Na+ currents and TPNQ-sensitive K+ currents in DCT2/CNT, suggesting the activity of ENaC and ROMK. However, both Na+ and ROMK currents in DCT2/CNT of TgWnk4PHAII mice were significantly smaller than those in WT and TgWnk4WT mice. In contrast, the basolateral K+ currents in the DCT were similar among three groups, despite higher NCC expression in TgWnk4PHAII mice than those of WT and TgWnk4WTmice. An increase in dietary K+ intake significantly increased both ENaC and ROMK currents in the DCT2/CNT of all three groups. However, high-K+ (HK) intake-induced stimulation of Na+ and K+ currents was smaller in TgWnk4PHAII mice than those in WT and TgWnk4WT mice. We conclude that ENaC and ROMK channel activity in DCT2/CNT are inhibited in TgWnk4PHAII mice and that Wnk4PHAII-induced inhibition of ENaC and ROMK may contribute to the suppression of K+ secretion in the DCT2/CNT in addition to increased NCC activity.

Whole-exome sequencing reveals an inherited R566X mutation of the epithelial sodium channel ß-subunit in a case of early-onset phenotype of Liddle syndrome.

To comprehensively evaluate a European-American child with severe hypertension, whole-exome sequencing (WES) was performed on the child and parents, which identified causal variation of the proband's early-onset disease. The proband's hypertension was resistant to treatment, requiring a multiple drug regimen including amiloride, spironolactone, and hydrochlorothiazide. We suspected a monogenic form of hypertension because of the persistent hypokalemia with low plasma levels of renin and aldosterone. To address this, we focused on rare functional variants and indels, and performed gene-based tests incorporating linkage scores and allele frequency and filtered on deleterious functional mutations. Drawing upon clinical presentation, 27 genes were selected evidenced to cause monogenic hypertension and matched to the gene-based results. This resulted in the identification of a stop-gain mutation in an epithelial sodium channel (ENaC), SCNN1B, an established Liddle syndrome gene, shared by the child and her father. Interestingly, the father also harbored a missense mutation (p.Trp552Arg) in the α-subunit of the ENaC trimer, SCNN1A, possibly pointing to pseudohypoaldosteronism type I. This case is unique in that we present the early-onset disease and treatment response caused by a canonical stop-gain mutation (p.Arg566*) as well as ENaC digenic hits in the father, emphasizing the utility of WES informing precision medicine.

Liddle syndrome in a Turkish family with heterogeneous phenotypes.

Liddle syndrome (LS) is a familial disease characterized by early onset hypertension (HT). Although regarded as rare, its incidence may be greater than expected because the classical findings of hypokalemic metabolic alkalosis with suppressed renin and aldosterone levels are not consistently present. Herein, we present the case of an adolescent boy and maternal relatives who were followed up with misdiagnosis of essential HT for a long duration. Clinical diagnosis of LS was confirmed on genetic analysis. Despite carrying the same mutation, the index patient and the family members manifested heterogeneous phenotypes of the disease including age at presentation, degree of HT, presence of hypokalemia and renal/cardiac complications. LS should be considered in the differential diagnosis of HT in children with a strong family history of HT resistant to conventional treatment; and genetic screening should be performed in these circumstances.

Inherited forms of mineralocorticoid hypertension.

Aldosterone plays an essential role in the maintenance of fluid and electrolyte homeostasis in the distal nephron. Monogenic forms of mineralocorticoid hypertension result from genetic defects leading to excessive production of aldosterone (or other mineralocorticoids) from the adrenal cortex or to illegitimate mineralocorticoid effects in the kidney. They are characterized in the majority of cases by early onset, severe or resistant hypertension and associated with suppressed renin levels. Depending on their causes, these diseases are distinguished at the clinical and biochemical level and differently affect aldosterone levels and kalemia. The diagnosis is confirmed by genetic testing, which allows in many cases targeted treatment to prevent severe cardiovascular consequences of high blood pressure or aldosterone excess. In this review we describe the different forms of inherited mineralocorticoid hypertension, providing an overview of their clinical and biochemical features, their underlying genetic defects and specific therapeutic options.

Pendrin gene ablation alters ENaC subcellular distribution and open probability.

The present study explored whether the intercalated cell Cl(-)/HCO3(-) exchanger pendrin modulates epithelial Na(+) channel (ENaC) function by changing channel open probability and/or channel density. To do so, we measured ENaC subunit subcellular distribution by immunohistochemistry, single channel recordings in split open cortical collecting ducts (CCDs), as well as transepithelial voltage and Na(+) absorption in CCDs from aldosterone-treated wild-type and pendrin-null mice. Because pendrin gene ablation reduced 70-kDa more than 85-kDa γ-ENaC band density, we asked if pendrin gene ablation interferes with ENaC cleavage. We observed that ENaC-cleaving protease application (trypsin) increased the lumen-negative transepithelial voltage in pendrin-null mice but not in wild-type mice, which raised the possibility that pendrin gene ablation blunts ENaC cleavage, thereby reducing open probability. In mice harboring wild-type ENaC, pendrin gene ablation reduced ENaC-mediated Na(+) absorption by reducing channel open probability as well as by reducing channel density through changes in subunit total protein abundance and subcellular distribution. Further experiments used mice with blunted ENaC endocytosis and degradation (Liddle's syndrome) to explore the significance of pendrin-dependent changes in ENaC open probability. In mouse models of Liddle's syndrome, pendrin gene ablation did not change ENaC subunit total protein abundance, subcellular distribution, or channel density, but markedly reduced channel open probability. We conclude that in mice harboring wild-type ENaC, pendrin modulates ENaC function through changes in subunit abundance, subcellular distribution, and channel open probability. In a mouse model of Liddle's syndrome, however, pendrin gene ablation reduces channel activity mainly through changes in open probability.

A semi-physiologically based pharmacokinetic pharmacodynamic model for glycyrrhizin-induced pseudoaldosteronism and prediction of the dose limit causing hypokalemia in a virtual elderly population.

Glycyrrhizin (GL) is a widely used food additive which can cause severe pseudoaldosteronism at high doses or after a long period of consumption. The aim of the present study was to develop a physiologically based pharmacokinetic (PBPK) pharmacodynamic (PD) model for GL-induced pseudoaldosteronism to improve the safe use of GL. Since the major metabolite of GL, glycyrrhetic acid (GA), is largely responsible for pseudoaldosteronism via inhibition of the kidney enzyme 11ß-hydroxysteroiddehydrogenase 2 (11ß-HSD 2), a semi-PBPK model was first developed in rat to predict the systemic pharmacokinetics of and the kidney exposure to GA. A human PBPK model was then developed using parameters either from the rat model or from in vitro studies in combination with essential scaling factors. Kidney exposure to GA was further linked to an Imax model in the 11ß-HSD 2 module of the PD model to predict the urinary excretion of cortisol and cortisone. Subsequently, activation of the mineralocorticoid receptor in the renin-angiotensin-aldosterone-electrolyte system was associated with an increased cortisol level. Experimental data for various scenarios were used to optimize and validate the model which was finally able to predict the plasma levels of angiotensin II, aldosterone, potassium and sodium. The Monte Carlo method was applied to predict the probability distribution of the individual dose limits of GL causing pseudoaldosteronism in the elderly according to the distribution of sensitivity factors using serum potassium as an indicator. The critical value of the dose limit was found to be 101 mg with a probability of 3.07%.

Molecular genetics of Liddle's syndrome.

Liddle's syndrome, an autosomal dominant form of monogenic hypertension, is characterized by salt-sensitive hypertension with early penetrance, hypokalemia, metabolic alkalosis, suppression of plasma rennin activity and aldosterone secretion, and a clear-cut response to epithelial sodium channel (ENaC) blockers but not spironolactone therapy. Our understanding of ENaCs and Na(+) transport defects has expanded greatly over the past two decades and provides detailed insight into the molecular basis of Liddle's syndrome. In this review, we offer an overview of recent advances in understanding the molecular genetics of Liddle's syndrome, involving mutation analysis, molecular mechanisms and genetic testing. The ENaC in the distal nephron is composed of α, ß and γ subunits that share similar structures. Mutations associated with Liddle's syndrome are positioned in either ß or γ subunits and disturb or truncate a conserved proline-rich sequence (i.e., PY motif), leading to constitutive activation of the ENaC. Genetic testing has made it possible to make accurate diagnoses and develop tailored therapies for mutation carriers.

Epithelial sodium channel stiffens the vascular endothelium in vitro and in Liddle mice.

Liddle syndrome, an inherited form of hypertension, is caused by gain-of-function mutations in the epithelial Na(+) channel (ENaC), the principal mediator of Na(+) reabsorption in the kidney. Accordingly, the disease pathology was ascribed to a primary renal mechanism. Whether this is the sole responsible mechanism, however, remains uncertain as dysregulation of ENaC in other tissues may also be involved. Previous work indicates that ENaC in the vascular endothelium is crucial for the regulation of cellular mechanics and thus vascular function. The hormone aldosterone has been shown to concomitantly increase ENaC surface expression and stiffness of the cell cortex in vascular endothelial cells. The latter entails a reduced release of the vasodilator nitric oxide, which eventually leads to an increase in vascular tone and blood pressure. Using atomic force microscopy, we have found a direct correlation between ENaC surface expression and the formation of cortical stiffness in endothelial cells. Stable knockdown of αENaC in endothelial cells evoked a reduced channel surface density and a lower cortical stiffness compared with the mock control. In turn, an increased αENaC expression induced an elevated cortical stiffness. More importantly, using ex vivo preparations from a mouse model for Liddle syndrome, we show that this disorder evokes enhanced ENaC expression and increased cortical stiffness in vascular endothelial cells in situ. We conclude that ENaC in the vascular endothelium determines cellular mechanics and hence might participate in the control of vascular function.

Salt-induced hypertension in a mouse model of Liddle syndrome is mediated by epithelial sodium channels in the brain.

Neural precursor cell expressed and developmentally downregulated 4-2 protein (Nedd4-2) facilitates the endocytosis of epithelial Na channels (ENaCs). Both mice and humans with a loss of regulation of ENaC by Nedd4-2 have salt-induced hypertension. ENaC is also expressed in the brain, where it is critical for hypertension on a high-salt diet in salt-sensitive rats. In the present studies we assessed whether Nedd4-2 knockout (-/-) mice have the following: (1) increased brain ENaC; (2) elevated cerebrospinal fluid (CSF) sodium on a high-salt diet; and (3) enhanced pressor responses to CSF sodium and hypertension on a high-salt diet, both mediated by brain ENaC. Prominent choroid plexus and neuronal ENaC staining was present in -/- but not in wild-type mice. In chronically instrumented mice, ICV infusion of Na-rich artificial CSF increased mean arterial pressure 3-fold higher in -/- than in wild-type mice. ICV infusion of the ENaC blocker benzamil abolished this enhancement. In telemetered -/- mice on a high-salt diet (8% NaCl), CSF [Na(+)], mean arterial pressure, and heart rate increased significantly, mean arterial pressure by 30 to 35 mmHg. These mean arterial pressure and heart rate responses were largely prevented by ICV benzamil but only to a minor extent by SC benzamil at the ICV rate. We conclude that increased ENaC expression in the brain of Nedd4-2 -/- mice mediates their hypertensive response to a high-salt diet by causing increased sodium levels in the CSF, as well as hyperresponsiveness to CSF sodium. These findings highlight the possible causative contribution of central nervous system ENaC in the etiology of salt-induced hypertension.

3-Monoglucuronyl-glycyrrhretinic acid is a substrate of organic anion transporters expressed in tubular epithelial cells and plays important roles in licorice-induced pseudoaldosteronism by inhibiting 11ß-hydroxysteroid dehydrogenase 2.

Licorice (glycyrrhiza root) has been used as a herbal medicine worldwide with its main active constituent being glycyrrhizin (GL). Licorice sometimes causes adverse effects such as inducing pseudoaldosteronism by inhibiting type 2 11ß-hydroxysteroid dehydrogenase (11ß-HSD2) caused by glycyrrhetinic acid (GA), a major metabolite of GL. In this study we compared the inhibitory effects of GA, GL, and 3-monoglucuronyl-glycyrrhetinic acid (3MGA), another metabolite of GL, on 11ß-HSD2 activity by using microsomes and rat kidney tissue slices. GA, 3MGA, and GL inhibited 11ß-HSD2 in rat kidney microsomes, with IC(50) values of 0.32, 0.26, and 2.2 µM, respectively. However, the inhibitory activity of these compounds was reduced markedly, in the slices, in a medium containing 5% bovine serum albumin. Assays using human embryonic kidney 293 cells with transient transformation in transporter genes showed that 3MGA is a substrate of human organic anion transporter (OAT) 1, human OAT3, and human organic anion-transporting peptide 4C1, whereas GA is not. When GA (100 mg/kg/day) was administered orally for 16 days to Eisai hyperbilirubinemic rats, plasma concentrations and urinary excretion of 3MGA were significantly higher, whereas the activity of 11ß-HSD2 in kidney microsomes was significantly lower compared with Sprague Dawley rats. These results suggest that 3MGA is actively transported into tubules through OATs, resulting in the inhibition of 11ß-HSD2. Because the plasma level of 3MGA depends on the function of hepatic transporters, monitoring 3MGA levels in plasma or urine may be useful for preventing pseudoaldosteronism when licorice or GL is prescribed to patients.

Inherited forms of mineralocorticoid hypertension.

PURPOSE OF REVIEW: Inherited forms of mineralocorticoid hypertension are a group of monogenic disorders that, although rare, have enlightened our understanding of normal physiology, and subsequent processes implicated in the pathogenesis of 'essential' hypertension. They often present in early life and can be a cause of major morbidity and mortality that can be effectively treated with simple but targeted pharmacological therapy. Interestingly, all the conditions centre on the regulation of sodium transport through its epithelial channel, either directly or through mediators that act via the mineralocorticoid receptor. RECENT FINDINGS: In recent years, molecular mechanisms of these conditions and their functional consequences have been elucidated. Diagnosis has been facilitated by plasma and urinary biomarkers. SUMMARY: We provide an overview and diagnostic approach to apparent mineralocorticoid excess, glucocorticoid remediable aldosteronism, familial hyperaldosteronism type 2, Liddle's syndrome, Gordon's syndrome, activating mutations of the mineralocorticoid receptor, generalized glucocorticoid resistance and hypertensive forms of congenital adrenal hyperplasia.

Hrs controls sorting of the epithelial Na+ channel between endosomal degradation and recycling pathways.

Epithelial Na(+) absorption is regulated by Nedd4-2, an E3 ubiquitin ligase that reduces expression of the epithelial Na(+) channel (ENaC) at the cell surface. Defects in this regulation cause Liddle syndrome, an inherited form of hypertension. Previous work found that Nedd4-2 functions through two distinct effects on trafficking, enhancing both ENaC endocytosis and ENaC degradation in lysosomes. To investigate the mechanism by which Nedd4-2 targets ENaC to lysosomes, we tested the role of hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs), a component of the endosomal sorting complexes required for transport (ESCRT)-0 complex. We found that α-, ß-, and γENaC each interact with Hrs. These interactions were enhanced by Nedd4-2 and were dependent on the catalytic function of Nedd4-2 as well as its WW domains. Mutation of ENaC PY motifs, responsible for inherited hypertension (Liddle syndrome), decreased Hrs binding to ENaC. Moreover, binding of ENaC to Hrs was reduced by dexamethasone/serum- and glucocorticoid-inducible kinase and cAMP, which are signaling pathways that inhibit Nedd4-2. Nedd4-2 bound to Hrs and catalyzed Hrs ubiquitination but did not alter Hrs protein levels. Expression of a dominant negative Hrs lacking its ubiquitin-interacting motif (Hrs-ΔUIM) increased ENaC surface expression and current. This occurred through reduced degradation of the cell surface pool of proteolytically activated ENaC, which enhanced its recycling to the cell surface. In contrast, Hrs-ΔUIM had no effect on degradation of uncleaved inactive channels. The data support a model in which Nedd4-2 induces binding of ENaC to Hrs, which mediates the sorting decision between ENaC degradation and recycling.

Airway surface liquid volume regulation determines different airway phenotypes in liddle compared with betaENaC-overexpressing mice.

Studies in cystic fibrosis patients and mice overexpressing the epithelial Na(+) channel beta-subunit (betaENaC-Tg) suggest that raised airway Na(+) transport and airway surface liquid (ASL) depletion are central to the pathogenesis of cystic fibrosis lung disease. However, patients or mice with Liddle gain-of-function betaENaC mutations exhibit hypertension but no lung disease. To investigate this apparent paradox, we compared the airway phenotype (nasal versus tracheal) of Liddle with CFTR-null, betaENaC-Tg, and double mutant mice. In mouse nasal epithelium, the region that functionally mimics human airways, high levels of CFTR expression inhibited Liddle epithelial Nat channel (ENaC) hyperfunction. Conversely, in mouse trachea, low levels of CFTR failed to suppress Liddle ENaC hyperfunction. Indeed, Na(+) transport measured in Ussing chambers ("flooded" conditions) was raised in both Liddle and betaENaC-Tg mice. Because enhanced Na(+) transport did not correlate with lung disease in these mutant mice, measurements in tracheal cultures under physiologic "thin film" conditions and in vivo were performed. Regulation of ASL volume and ENaC-mediated Na(+) absorption were intact in Liddle but defective in betaENaC-Tg mice. We conclude that the capacity to regulate Na(+) transport and ASL volume, not absolute Na(+) transport rates in Ussing chambers, is the key physiologic function protecting airways from dehydration-induced lung disease.

A physiologic-based approach to the evaluation of a patient with hyperkalemia.

Hyperkalemia generally is attributable to cell shifts or abnormal renal potassium excretion. Cell shifts account for transient increases in serum potassium levels, whereas sustained hyperkalemia generally is caused by decreased renal potassium excretion. Impaired renal potassium excretion can be caused by a primary decrease in distal sodium delivery, a primary decrease in mineralocorticoid level or activity, or abnormal cortical collecting duct function. Excessive potassium intake is an infrequent cause of hyperkalemia by itself, but can worsen the severity of hyperkalemia when renal excretion is impaired. Before concluding that a cell shift or renal defect in potassium excretion is present, pseudohyperkalemia should be excluded.