Differential subcellular localization of ENaC subunits in mouse kidney in response to high- and low-Na diets

Mechanisms of renal control of potassium homeostasis in complete aldosterone deficiency

Aldosterone-independent mechanisms may contribute to K(+) homeostasis. We studied aldosterone synthase knockout (AS(-/-)) mice to define renal control mechanisms of K(+) homeostasis in complete aldosterone deficiency. AS(-/-) mice were normokalemic and tolerated a physiologic dietary K(+) load (2% K(+), 2 days) without signs of illness, except some degree of polyuria. With supraphysiologic K(+) intake (5% K(+)), AS(-/-) mice decompensated and became hyperkalemic. High-K(+) diets induced upregulation of the renal outer medullary K(+) channel in AS(-/-) mice, whereas upregulation of the epithelial sodium channel (ENaC) sufficient to increase the electrochemical driving force for K(+) excretion was detected only with a 2% K(+) diet. Phosphorylation of the thiazide-sensitive NaCl cotransporter was consistently lower in AS(-/-) mice than in AS(+/+) mice and was downregulated in mice of both genotypes in response to increased K(+) intake. Inhibition of the angiotensin II type 1 receptor reduced renal creatinine clearance and apical ENaC localization, and caused severe hyperkalemia in AS(-/-) mice. In contrast with the kidney, the distal colon of AS(-/-) mice did not respond to dietary K(+) loading, as indicated by Ussing-type chamber experiments. Thus, renal adaptation to a physiologic, but not supraphysiologic, K(+) load can be achieved in aldosterone deficiency by aldosterone-independent activation of the renal outer medullary K(+) channel and ENaC, to which angiotensin II may contribute. Enhanced urinary flow and reduced activity of the thiazide-sensitive NaCl cotransporter may support renal adaptation by activation of flow-dependent K(+) secretion and increased intratubular availability of Na(+) that can be reabsorbed in exchange for K(+) secreted.

Transient receptor potential vanilloid 1 activation by dietary capsaicin promotes urinary sodium excretion by inhibiting epithelial sodium channel α subunit-mediated sodium reabsorption

High salt (HS) intake contributes to the development of hypertension. Epithelial sodium channels play crucial roles in regulating renal sodium reabsorption and blood pressure. The renal transient receptor potential vanilloid 1 (TRPV1) cation channel can be activated by its agonist capsaicin. However, it is unknown whether dietary factors can act on urinary sodium excretion and renal epithelial sodium channel (ENaC) function. Here, we report that TRPV1 activation by dietary capsaicin increased urinary sodium excretion through reducing sodium reabsorption in wild-type (WT) mice on a HS diet but not in TRPV1(-/-) mice. The effect of capsaicin on urinary sodium excretion was involved in inhibiting αENaC and its related with-no-lysine kinase 1/serum- and glucocorticoid-inducible protein kinase 1 pathway in renal cortical collecting ducts of WT mice. Dietary capsaicin further reduced the increased αENaC activity in WT mice attributed to the HS diet. In contrast, this capsaicin effect was absent in TRPV1(-/-) mice. Immunoprecipitation study indicated αENaC specifically coexpressed and functionally interact with TRPV1 in renal cortical collecting ducts of WT mice. Additionally, ENaC activity and expression were suppressed by capsaicin-mediated TRPV1 activation in cultured M1-cortical collecting duct cells. Long-term dietary capsaicin prevented the development of high blood pressure in WT mice on a HS diet. It concludes that TRPV1 activation in the cortical collecting ducts by capsaicin increases urinary sodium excretion and avoids HS diet-induced hypertension through antagonizing αENaC-mediated urinary sodium reabsorption. Dietary capsaicin may represent a promising lifestyle intervention in populations exposed to a high dietary salt intake.

Novel mechanisms of Na+ retention in obesity: phosphorylation of NKCC2 and regulation of SPAK/OSR1 by AMPK

Enhanced tubular reabsorption of salt is important in the pathogenesis of obesity-related hypertension, but the mechanisms remain poorly defined. To identify changes in the regulation of salt transporters in the kidney, C57BL/6 mice were fed a 40% fat diet [high-fat diet (HFD)] or a 12% fat diet (control diet) for 14 wk. Compared with control diet-fed mice, HFD-fed mice had significantly greater elevations in weight, blood pressure, and serum insulin and leptin levels. When we examined Na(+) transporter expression, Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) was unchanged in whole kidney and reduced in the cortex, Na(+)-Cl(-) cotransporter (NCC) and α-epithelial Na(+) channel (ENaC) and γ-ENaC were unchanged, and β-ENaC was reduced. Phosphorylation of NCC was unaltered. Activating phosphorylation of NKCC2 at S126 was increased 2.5-fold. Activation of STE-20/SPS1-related proline-alanine-rich protein kinase (SPAK)/oxidative stress responsive 1 kinase (OSR1) was increased in kidneys from HFD-fed mice, and enhanced phosphorylation of NKCC2 at T96/T101 was evident in the cortex. Increased activity of NKCC2 in vivo was confirmed with diuretic experiments. HFD-fed mice had reduced activating phosphorylation of AMP-activated protein kinase (AMPK) in the renal cortex. In vitro, activation of AMPK led to a reduction in phospho-SPAK/phospho-OSR1 in AMPK(+/+) murine embryonic fibroblasts (MEFs), but no effect was seen in AMPK(-/-) MEFs, indicating an AMPK-mediated effect. Activation of the with no lysine kinase/SPAK/OSR1 pathway with low-NaCl solution invoked a greater elevation in phospho-SPAK/phospho-OSR1 in AMPK(-/-) MEFs than in AMPK(+/+) MEFs, consistent with a negative regulatory effect of AMPK on SPAK/OSR1 phosphorylation. In conclusion, this study identifies increased phosphorylation of NKCC2 on S126 as a hitherto-unrecognized mediator of enhanced Na(+) reabsorption in obesity and identifies a new role for AMPK in regulating the activity of SPAK/OSR1.

Anti-inflammatory effects of ω-3 polyunsaturated fatty acids and soluble epoxide hydrolase inhibitors in angiotensin-II-dependent hypertension

The mechanisms underlying the anti-inflammatory and antihypertensive effects of long-chain ω-3 polyunsaturated fatty acids (ω-3 PUFAs) are still unclear. The epoxides of an ω-6 fatty acid, arachidonic acid epoxyeicosatrienoic acids also exhibit antihypertensive and anti-inflammatory effects. Thus, we hypothesized that the major ω-3 PUFAs, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), may lower the blood pressure and attenuate renal markers of inflammation through their epoxide metabolites. Here, we supplemented mice with an ω-3 rich diet for 3 weeks in a murine model of angiotensin-II-dependent hypertension. Also, because EPA and DHA epoxides are metabolized by soluble epoxide hydrolase (sEH), we tested the combination of an sEH inhibitor and the ω-3 rich diet. Our results show that ω-3 rich diet in combination with the sEH inhibitor lowered Ang-II, increased the blood pressure, further increased the renal levels of EPA and DHA epoxides, reduced renal markers of inflammation (ie, prostaglandins and MCP-1), downregulated an epithelial sodium channel, and upregulated angiotensin-converting enzyme-2 message and significantly modulated cyclooxygenase and lipoxygenase metabolic pathways. Overall, our findings suggest that epoxides of the ω-3 PUFAs contribute to lowering systolic blood pressure and attenuating inflammation in part by reduced prostaglandins and MCP-1 and by upregulation of angiotensin-converting enzyme-2 in angiotensin-II-dependent hypertension.

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.

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.

Sequential nephron blockade versus sequential renin-angiotensin system blockade in resistant hypertension: a prospective, randomized, open blinded endpoint study

OBJECTIVE

To compare two drug regimens to treat resistant hypertension.

METHODS

In a prospective, randomized, open blinded endpoint study, 167 patients with mean baseline daytime ambulatory blood pressure 135 mmHg or more and/or 85 mmHg or more, despite 4 weeks' treatment with irbesartan 300 mg/day, hydrochlorothiazide 12.5 mg/day and amlodipine 5 mg/day, were randomized to sequential nephron blockade (group 1, n = 85) or sequential renin-angiotensin system blockade (group 2, n = 82). First, spironolactone 25 mg/day in group 1 or ramipril 5 mg/day in group 2 were added for 4 weeks. Treatment was increased at weeks 4, 8 or 10 if home blood pressure was 135 mmHg or more and/or 85 mmHg or more by sequentially administering furosemide 20 mg/day, furosemide 40 mg/day and amiloride 5 mg/day in group 1, or ramipril 10 mg/day, bisoprolol 5 mg/day and bisoprolol 10 mg/day in group 2. The primary endpoint was change in systolic daytime ambulatory blood pressure at week 12.

RESULTS

At week 12, the mean between-group difference in daytime ambulatory blood pressure was 10/4 mmHg (95% confidence interval: 7-14/2-7; P < 0.001/P = 0.0014) in favour of the group 1. The blood pressure goal (daytime ambulatory blood pressure <135/85 mmHg) was achieved in 58% in the group 1 and 20% in the group 2 (P < 0.0001). Discontinuation for drug-related adverse events was low (group 1, n = 7; group 2, n = 6).

CONCLUSION

In patients with resistant hypertension, sequential nephron blockade induces a large and well tolerated reduction in blood pressure via a progressive increase in sodium depletion, and is more effective than sequential renin-angiotensin system blockade.

Heterogeneous nuclear ribonucleoprotein A2/B1 is a tissue-specific aldosterone target gene with prominent induction in the rat distal colon

The steroid hormone aldosterone enhances transepithelial Na(+) reabsorption in tight epithelia and is crucial to achieve extracellular volume homeostasis and control of blood pressure. One of the main transport pathways regulated by aldosterone involves the epithelial Na(+) channel (ENaC), which constitutes the rate-limiting step of Na(+) reabsorption in parts of the distal nephron and the collecting duct, the distal colon, and sweat and salivary glands. Although these epithelial tissues share the same receptor for aldosterone (mineralocorticoid receptor, MR), and the same transport system (ENaC), it has become clear that the molecular mechanisms involved in the modulation of channel activity are tissue-specific. Recent evidence suggests that aldosterone controls transcription and also translation of ENaC subunits in some cell types. A possible pathway for translational regulation is binding of regulatory proteins to ENaC subunit mRNAs, such as the heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNP A2/B1). In this study, we examined whether hnRNP A2/B1 is an aldosterone-target gene in vivo. Our data show that physiological levels of aldosterone markedly induce hnRNP A2/B1 expression in an early and sustained manner in the late distal colon epithelium but not in other aldosterone-target tissues. The effect depends on MR but not on glucocorticoid receptor activity. We also demonstrate that the genomic region upstream of hnRNP A2/B1 contains aldosterone-responsive elements involved in the control of gene expression. We hypothesize that hnRNP A2/B1 is involved in the tissue-specific regulation of ENaC biosynthesis and may coordinate the response of other genes relevant for transepithelial Na(+) reabsorption by aldosterone.

Aldosterone-dependent and -independent regulation of the epithelial sodium channel (ENaC) in mouse distal nephron

Aldosterone is thought to be the main hormone to stimulate the epithelial sodium channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN) comprising the late distal convoluted tubule (DCT2), the connecting tubule (CNT) and the entire collecting duct (CD). There is immunohistochemical evidence for an axial gradient of ENaC expression along the ASDN with highest expression in the DCT2 and CNT. However, most of our knowledge about renal ENaC function stems from studies in the cortical collecting duct (CCD). Here we investigated ENaC function in the transition zone of DCT2/CNT or CNT/CCD microdissected from mice maintained on different sodium diets to vary plasma aldosterone levels. Single-channel recordings demonstrated amiloride-sensitive Na+ channels in DCT2/CNT with biophysical properties typical for ENaC previously described in CNT/CCD. In animals maintained on a standard salt diet, the average ENaC-mediated whole cell current (Δ Iami) was higher in DCT2/CNT than in CNT/CCD. A low salt diet increased Δ Iami in CNT/CCD but had little effect on Δ Iami in DCT2/CNT. To investigate whether aldosterone is necessary for ENaC activity in the DCT2/CNT, we used aldosterone synthase knockout (AS−/−) mice that lack aldosterone. In CNT/CCD of AS−/− mice, Δ Iami was lower than that in wild-type (WT) animals and was not stimulated by a low salt diet. In contrast, in DCT2/CNT of AS−/− mice, Δ Iami was similar to that in DCT2/CNT of WT animals both on a standard and on a low salt diet. We conclude that ENaC function in the DCT2/CNT is largely independent of aldosterone which is in contrast to its known aldosterone sensitivity in CNT/CCD.

Identification of permissive insertion sites for generating functional fluorescent mineralocorticoid receptors

The mineralocorticoid receptor (MR), a member of the nuclear receptor superfamily of transcription factors, is activated by aldosterone and mediates its natriferic action in tight epithelia. MR is also expressed in nonepithelial tissues. Importantly, it mediates the deleterious effects of inappropriately high aldosterone levels in the heart, in which it induces the development of cardiac fibrosis. Antagonism of MR in humans is useful in the treatment of severe cardiac failure and some forms of hypertension. Despite the important pathophysiological and pharmacological role of this receptor, many important questions about its cellular biology and functional roles remain unanswered. A major challenge in the study of MR is the unavailability of fully functional fluorescent derivatives of the receptor. In this study we have created a library of MR mutants with insertions of the yellow fluorescent protein in various internal locations in the receptor using a random-insertion transposon-based technique. Screening of this library using a transactivation assay allowed us to identify several fluorescent constructs that retain functionality. Detailed characterization of one of these construct showed that it induces aldosterone-target genes such as the epithelial Na(+) channel subunits and the serum and glucocorticoid-induced kinase 1 at physiological concentrations of aldosterone to an equal extent than the wild-type receptor. Furthermore, aldosterone affinity, hormone-induced nuclear translocation, DNA binding and regulation of nongenomic pathways are all indistinguishable from the wild-type receptor. This new set of fluorescent MR derivatives provides a useful tool for studying the cell biology of the receptor.

Loss of renal medullary endothelin B receptor function during salt deprivation is regulated by angiotensin II

We have recently demonstrated that chronic infusion of exogenous ANG II, which induces blood pressure elevation, attenuates renal medullary endothelin B (ET(B)) receptor function in rats. Moreover, this was associated with a reduction of ET(B) receptor expression in the renal inner medulla. The aim of this present work was to investigate the effect of a physiological increase in endogenous ANG II (low-salt diet) on the renal ET system, including ET(B) receptor function. We hypothesized that endogenous ANG II reduces renal medullary ET(B) receptor function during low-salt intake. Rats were placed on a low-salt diet (0.01-0.02% NaCl) for 2 wk to allow an increase in endogenous ANG II. In rats on normal-salt chow, the stimulation of renal medullary ET(B) receptor by ET(B) receptor agonist sarafotoxin 6c (S6c) causes an increase in water (3.6 ± 0.4 from baseline vs. 10.5 ± 1.3 μl/min following S6c infusion; P < 0.05) and sodium excretion (0.38 ± 0.06 vs. 1.23 ± 0.17 μmol/min; P < 0.05). The low-salt diet reduced the ET(B)-dependent diuresis (4.5 ± 0.5 vs. 6.1 ± 0.9 μl/min) and natriuresis (0.40 ± 0.11 vs. 0.46 ± 0.12 μmol/min) in response to acute intramedullary infusion of S6c. Chronic treatment with candesartan restored renal medullary ET(B) receptor function; urine flow was 7.1 ± 0.9 vs. 15.9 ± 1.7 μl/min (P < 0.05), and sodium excretion was 0.4 ± 0.1 vs. 1.1 ± 0.1 μmol/min (P < 0.05) before and after intramedullary S6c infusion, respectively. Receptor binding assays determined that the sodium-depleted diet resulted in a similar level of ET(B) receptor binding in renal inner medulla compared with rats on a normal-salt diet. Candesartan reduced renal inner medullary ET(B) receptor binding (1,414 ± 95 vs. 862 ± 50 fmol/mg; P < 0.05). We conclude that endogenous ANG II attenuates renal medullary ET(B) receptor function to conserve sodium during salt deprivation independently of receptor expression.

High salt intake down-regulates colonic mineralocorticoid receptors, epithelial sodium channels and 11β-hydroxysteroid dehydrogenase type 2

Besides the kidneys, the gastrointestinal tract is the principal organ responsible for sodium homeostasis. For sodium transport across the cell membranes the epithelial sodium channel (ENaC) is of pivotal relevance. The ENaC is mainly regulated by mineralocorticoid receptor mediated actions. The MR activation by endogenous 11β-hydroxy-glucocorticoids is modulated by the 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2). Here we present evidence for intestinal segment specific 11β-HSD2 expression and hypothesize that a high salt intake and/or uninephrectomy (UNX) affects colonic 11β-HSD2, MR and ENaC expression. The 11β-HSD2 activity was measured by means of 3H-corticosterone conversion into 3H-11-dehydrocorticosterone in Sprague Dawley rats on a normal and high salt diet. The activity increased steadily from the ileum to the distal colon by a factor of about 3, an observation in line with the relevance of the distal colon for sodium handling. High salt intake diminished mRNA and protein of 11β-HSD2 by about 50% (p<0.001) and reduced the expression of the MR (p<0.01). The functionally relevant ENaC-β and ENaC-γ expression, a measure of mineralocorticoid action, diminished by more than 50% by high salt intake (p<0.001). The observed changes were present in rats with and without UNX. Thus, colonic epithelial cells appear to contribute to the protective armamentarium of the mammalian body against salt overload, a mechanism not modulated by UNX.

The epithelial sodium channel δ-subunit: new notes for an old song

Amiloride-sensitive epithelial Na(+) channels (ENaCs) can be formed by different combinations of four homologous subunits, named α, β, γ, and δ. In addition to providing an apical entry pathway for transepithelial Na(+) reabsorption in tight epithelia such as the kidney distal tubule and collecting duct, ENaCs are also expressed in nonepithelial cells, where they may play different functional roles. The δ-subunit of ENaC was originally identified in humans and is able to form amiloride-sensitive Na(+) channels alone or in combination with β and γ, generally resembling the canonical kidney ENaC formed by α, β, and γ. However, δ differs from α in its tissue distribution and channel properties. Despite the low sequence conservation between α and δ (37% identity), their similar functional characteristics provide an excellent model for exploring structural correlates of specific ENaC biophysical and pharmacological properties. Moreover, the study of cellular mechanisms modulating the activity of different ENaC subunit combinations provides an opportunity to gain insight into the regulation of the channel. In this review, we examine the evolution of ENaC genes, channel subunit composition, the distinct functional and pharmacological features that δ confers to ENaC, and how this can be exploited to better understand this ion channel. Finally, we briefly consider possible functional roles of the ENaC δ-subunit.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

Renal and vascular mechanisms of thiazolidinedione-induced fluid retention

Thiazolidinediones (TZDs) are peroxisome proliferator-activated receptor subtype gamma (PPARgamma) activators that are clinically used as an insulin sensitizer for glycemic control in patients with type 2 diabetes. Additionally, TZDs exhibit novel anti-inflammatory, antioxidant, and antiproliferative properties, indicating therapeutic potential for a wide variety of diseases associated with diabetes and other conditions. The clinical applications of TZDs are limited by the common major side effect of fluid retention. A better understanding of the molecular mechanism of TZD-induced fluid retention is essential for the development of novel therapies with improved safety profiles. An important breakthrough in the field is the finding that the renal collecting duct is a major site for increased fluid reabsorption in response to rosiglitazone or pioglitazone. New evidence also indicates that increased vascular permeability in adipose tissues may contribute to edema formation and body weight gain. Future research should therefore be directed at achieving a better understanding of the detailed mechanisms of TZD-induced increases in renal sodium transport and in vascular permeability.

Epithelial Na+ channel (ENaC) expression in obstructive sialadenitis of the submandibular gland

BACKGROUND AND OBJECTIVES

The epithelial sodium channel (ENaC) has a critical role in the control of sodium balance, blood volume, blood pressure and viscosity of extracellular water. In submandibular gland disease, viscosity of the saliva plays an important role in the pathophysiology. However, there has been little study into the relationship between expression of ENaC and obstructive sialadenitis. The aim of this study was to elucidate the ENaC expression in excretory duct obstruction of the submandibular gland.

MATERIALS AND METHODS

Seven-week old male Sprague-Dawley 12 rats were enrolled in the study. The rats were decapitated and histological changes of the submandibular gland tissue examined on days 0, 1, 3, 7, 14 and 21 after submandibular gland duct ligation. The expression of ENaC mRNA in the submandibular gland tissue was tested by reverse transcriptase-polymerase chain reaction (RT-PCR). Quantitative analysis of the ENaC protein was performed through Western blotting and tissue localization of the protein was performed by immunohistochemical staining.

RESULTS

By real time RT-PCR, the expression of ENaC (α, β, γ) mRNA increased after ligation of the submandibular gland duct. α and γ ENaC mRNA expression began to increase after 1 day. But β ENaC mRNA expression began to increase significantly after 14 days. The increase of ENaC mRNA expression lasted for 3 weeks. The expression of ENaC (α, β, γ) protein was identified by Western blotting analysis, and ENaC protein was localized in ductal epithelial cells by immunohistochemistry.

CONCLUSION

The expression of ENaC (α, β, γ) was increased by excretory duct ligation of the submandibular gland in rats and ENaC was considered to have a certain role in the pathogenesis of obstructive sialadenitis.

Enhanced expression of epithelial sodium channels in the renal medulla of Dahl S rats

Inner medullary collecting duct (IMCD) cells from salt-sensitive (S) Dahl rats transport twice as much Na+ as cells from salt-resistant (R) rats, possibly related to dysregulation of the renal epithelial sodium channel (ENaC). The effect of a high-salt diet on ENaC expression in the inner medulla of S versus R rats has not yet been studied. Young, male S and R rats were placed on a regular-salt (0.3%) or high-salt (8%) diet for 2 or 4 weeks. mRNA and protein expression of ENaC subunits were studied by real-time PCR and immunoblotting. Intracellular distribution of the subunits in the IMCD was evaluated by immunohistochemistry. On regular salt, the abundance of the mRNA of β and γENaC was higher in the medulla of S rats than R rats. This was associated with a greater protein abundance of 90 kDa γENaC and higher immunoreactivity for both α and γ ENaC. High salt did not affect mRNA abundance in either strain and decreased apical staining of βENaC in IMCD of R rats. In contrast, high salt did not affect the higher apical localization of αENaC and increased the apical membrane staining for β and γENaC in the IMCD of S rats. Expression of ENaC subunits is enhanced in the medulla of S vs. R rats on regular salt, and further increased on high salt. The persistent high expression of αENaC and increase in apical localization of β and γENaC may contribute to greater retention of sodium in S rats on a high-salt diet.

Activation of the epithelial sodium channel by the metalloprotease meprin β subunit

The Epithelial Na(+) Channel (ENaC) is an apical heteromeric channel that mediates Na(+) entry into epithelial cells from the luminal cell surface. ENaC is activated by proteases that interact with the channel during biosynthesis or at the extracellular surface. Meprins are cell surface and secreted metalloproteinases of the kidney and intestine. We discovered by affinity chromatography that meprins bind γ-ENaC, a subunit of the ENaC hetero-oligomer. The physical interaction involves NH(2)-terminal cytoplasmic residues 37-54 of γ-ENaC, containing a critical gating domain immediately before the first transmembrane domain, and the cytoplasmic COOH-terminal tail of meprin β (residues 679-704). This potential association was confirmed by co-expression and co-immunoprecipitation studies. Functional assays revealed that meprins stimulate ENaC expressed exogenously in Xenopus oocytes and endogenously in epithelial cells. Co-expression of ENaC subunits and meprin β or α/β in Xenopus oocytes increased amiloride-sensitive Na(+) currents approximately two-fold. This increase was blocked by preincubation with an inhibitor of meprin activity, actinonin. The meprin-mediated increase in ENaC currents in oocytes and epithelial cell monolayers required meprin β, but not the α subunit. Meprin β promoted cleavage of α and γ-ENaC subunits at sites close to the second transmembrane domain in the extracellular domain of each channel subunit. Thus, meprin β regulates the activity of ENaC in a metalloprotease-dependent fashion.

In vivo nuclear translocation of mineralocorticoid and glucocorticoid receptors in rat kidney: differential effect of corticosteroids along the distal tubule

Aldosterone and corticosterone bind to mineralocorticoid (MR) and glucocorticoid receptors (GR), which, upon ligand binding, are thought to translocate to the cell nucleus to act as transcription factors. Mineralocorticoid selectivity is achieved by the 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) that inactivates 11β-hydroxy glucocorticoids. High expression levels of 11β-HSD2 characterize the aldosterone-sensitive distal nephron (ASDN), which comprises the segment-specific cells of late distal convoluted tubule (DCT2), connecting tubule (CNT), and collecting duct (CD). We used MR- and GR-specific antibodies to study localization and regulation of MR and GR in kidneys of rats with altered plasma aldosterone and corticosterone levels. In control rats, MR and GR were found in cell nuclei of thick ascending limb (TAL), DCT, CNT, CD cells, and intercalated cells (IC). GR was also abundant in cell nuclei and the subapical compartment of proximal tubule (PT) cells. Dietary NaCl loading, which lowers plasma aldosterone, caused a selective removal of GR from cell nuclei of 11β-HSD2-positive ASDN. The nuclear localization of MR was unaffected. Adrenalectomy (ADX) resulted in removal of MR and GR from the cell nuclei of all epithelial cells. Aldosterone replacement rapidly relocated the receptors in the cell nuclei. In ASDN cells, low-dose corticosterone replacement caused nuclear localization of MR, but not of GR. The GR was redistributed to the nucleus only in PT, TAL, early DCT, and IC that express no or very little 11β-HSD2. In ASDN cells, nuclear GR localization was only achieved when corticosterone was replaced at high doses. Thus ligand-induced nuclear translocation of MR and GR are part of MR and GR regulation in the kidney and show remarkable segment- and cell type-specific characteristics. Differential regulation of MR and GR may alter the level of heterodimerization of the receptors and hence may contribute to the complexity of corticosteroid effects on ASDN function.

alphaENaC-mediated lithium absorption promotes nephrogenic diabetes insipidus

Lithium-induced nephrogenic diabetes insipidus (NDI) is accompanied by polyuria, downregulation of aquaporin 2 (AQP2), and cellular remodeling of the collecting duct (CD). The amiloride-sensitive epithelial sodium channel (ENaC) is a likely candidate for lithium entry. Here, we subjected transgenic mice lacking αENaC specifically in the CD (knockout [KO] mice) and littermate controls to chronic lithium treatment. In contrast to control mice, KO mice did not markedly increase their water intake. Furthermore, KO mice did not demonstrate the polyuria and reduction in urine osmolality induced by lithium treatment in the control mice. Lithium treatment reduced AQP2 protein levels in the cortex/outer medulla and inner medulla (IM) of control mice but only partially reduced AQP2 levels in the IM of KO mice. Furthermore, lithium induced expression of H(+)-ATPase in the IM of control mice but not KO mice. In conclusion, the absence of functional ENaC in the CD protects mice from lithium-induced NDI. These data support the hypothesis that ENaC-mediated lithium entry into the CD principal cells contributes to the pathogenesis of lithium-induced NDI.

Sodium and potassium balance depends on αENaC expression in connecting tubule

Mutations in α, β, or γ subunits of the epithelial sodium channel (ENaC) can downregulate ENaC activity and cause a severe salt-losing syndrome with hyperkalemia and metabolic acidosis, designated pseudohypoaldosteronism type 1 in humans. In contrast, mice with selective inactivation of αENaC in the collecting duct (CD) maintain sodium and potassium balance, suggesting that the late distal convoluted tubule (DCT2) and/or the connecting tubule (CNT) participates in sodium homeostasis. To investigate the relative importance of ENaC-mediated sodium absorption in the CNT, we used Cre-lox technology to generate mice lacking αENaC in the aquaporin 2-expressing CNT and CD. Western blot analysis of microdissected cortical CD (CCD) and CNT revealed absence of αENaC in the CCD and weak αENaC expression in the CNT. These mice exhibited a significantly higher urinary sodium excretion, a lower urine osmolality, and an increased urine volume compared with control mice. Furthermore, serum sodium was lower and potassium levels were higher in the genetically modified mice. With dietary sodium restriction, these mice experienced significant weight loss, increased urinary sodium excretion, and hyperkalemia. Plasma aldosterone levels were significantly elevated under both standard and sodium-restricted diets. In summary, αENaC expression within the CNT/CD is crucial for sodium and potassium homeostasis and causes signs and symptoms of pseudohypoaldosteronism type 1 if missing.

AS160 modulates aldosterone-stimulated epithelial sodium channel forward trafficking

AS160 assists in defining an intracellular compartment in which ENaC accumulates under basal conditions, and this compartment is accessed by aldosterone, via SGK-mediated phosphorylation of AS160, to permit the forward trafficking of ENaC to the apical membrane.

Aldosterone-induced increases in apical membrane epithelial sodium channel (ENaC) density and Na transport involve the induction of 14-3-3 protein expression and their association with Nedd4-2, a substrate of serum- and glucocorticoid-induced kinase (SGK1)-mediated phosphorylation. A search for other 14-3-3 binding proteins in aldosterone-treated cortical collecting duct (CCD) cells identified the Rab-GAP, AS160, an Akt/PKB substrate whose phosphorylation contributes to the recruitment of GLUT4 transporters to adipocyte plasma membranes in response to insulin. In CCD epithelia, aldosterone (10 nM, 24 h) increased AS160 protein expression threefold, with a time-course similar to increases in SGK1 expression. In the absence of aldosterone, AS160 overexpression increased total ENaC expression 2.5-fold but did not increase apical membrane ENaC or amiloride-sensitive Na current (I_sc). In AS160 overexpressing epithelia, however, aldosterone increased apical ENaC and I_sc 2.5-fold relative to aldosterone alone, thus recruiting the accumulated ENaC to the apical membrane. Conversely, AS160 knockdown increased apical membrane ENaC and I_sc under basal conditions to ∼80% of aldosterone-stimulated values, attenuating further steroid effects. Aldosterone induced AS160 phosphorylation at five sites, predominantly at the SGK1 sites T568 and S751, and evoked AS160 binding to the steroid-induced 14-3-3 isoforms, β and ε. AS160 mutations at SGK1 phospho-sites blocked its selective interaction with 14-3-3β and ε and suppressed the ability of expressed AS160 to augment aldosterone action. These findings indicate that the Rab protein regulator, AS160, stabilizes ENaC in a regulated intracellular compartment under basal conditions, and that aldosterone/SGK1-dependent AS160 phosphorylation permits ENaC forward trafficking to the apical membrane to augment Na absorption.

Are sodium transporters in urinary exosomes reliable markers of tubular sodium reabsorption in hypertensive patients?

BACKGROUND

Altered renal sodium handling has a major pathogenic role in salt-sensitive hypertension. Renal sodium transporters are present in urinary exosomes. We hypothesized that sodium transporters would be excreted into the urine in different amounts in response to sodium intake in salt-sensitive versus salt-resistant patients.

METHODS

Urinary exosomes were isolated by ultracentrifugation, and their content of Na-K-2Cl cotransporter (NKCC2) and Na-Cl cotransporter (NCC) was analyzed by immunoblotting. Animal studies: NKCC2 and NCC excretion was measured in 2 rat models to test whether changes in sodium transporter excretion are indicative of regulated changes in the kidney tissue. Human studies: in hypertensive patients (n = 41), we investigated: (1) a possible correlation between sodium reabsorption and urinary exosomal excretion of sodium transporters, and (2) the profile of sodium transporter excretion related to blood pressure (BP) changes with salt intake. A 24-hour ambulatory BP monitoring and a 24-hour urine collection were performed after 1 week on a low- and 1 week on a high-salt diet.

RESULTS

Animal studies: urinary NKCC2 and NCC excretion rates correlated well with their abundance in the kidney. Human studies: 6 patients (15%) were classified as salt sensitive. The NKCC2 and NCC abundance did not decrease after the high-salt period, when the urinary sodium reabsorption decreased from 99.7 to 99.0%. In addition, the changes in BP with salt intake were not associated with a specific profile of exosomal excretion.

CONCLUSIONS

Our results do not support the idea that excretion levels of NKCC2 and NCC via urinary exosomes are markers of tubular sodium reabsorption in hypertensive patients.

Altered regulation of renal sodium transporters in salt-sensitive hypertensive rats induced by uninephrectomy

Uninephrectomy (uNx) in young rats causes salt-sensitive hypertension (SSH). Alterations of sodium handling in residual nephrons may play a role in the pathogenesis. Therefore, we evaluated the adaptive alterations of renal sodium transporters according to salt intake in uNx-SSH rats. uNx or sham operations were performed in male Sprague-Dawley rats, and normal-salt diet was fed for 4 weeks. Four experimental groups were used: sham-operated rats raised on a high-salt diet for 2 weeks (CHH) or on a low-salt diet for 1 week after 1 week's high-salt diet (CHL) and uNx rats fed on the same diet (NHH, NHL) as the sham-operated rats were fed. Expression of major renal sodium transporters were determined by semiquantitative immunoblotting. Systolic blood pressure was increased in NHH and NHL groups, compared with CHH and CHL, respectively. Protein abundances of Na(+)/K(+)/2Cl(-) cotransporter (NKCC2) and Na(+)/Cl(-) cotransporter (NCC) in the CHH group were lower than the CHL group. Expression of epithelial sodium channel (ENaC)-γ increased in the CHH group. In contrast, expressions of NKCC2 and NCC in the NHH group didn't show any significant alterations, compared to the NHL group. Expressions of ENaC-α and ENaC-β in the NHH group were higher than the CHH group. Adaptive alterations of NKCC2 and NCC to changes of salt intake were different in the uNx group, and changes in ENaC-α and ENaC-β were also different. These altered regulations of sodium transporters may be involved in the pathogenesis of SSH in the uNx rat model.

E Prostanoid-1 receptor regulates renal medullary alphaENaC in rats infused with angiotensin II

E Prostanoid (EP) receptors play an important role in urinary Na(+) excretion. In the kidney, the epithelial sodium channel (ENaC) is the rate-limiting-step for Na(+) reabsorption. We hypothesized that activation of EP1/EP3 regulates the expression of ENaC in the face of renin-angiotensin-aldosterone-system (RAAS) activation. In primary cultures of inner medullary collecting duct (IMCD) cells, sulprostone (EP1>EP3 agonist, 1 microM) and 17 Phenyl trinor (17 Pt, EP1 agonist, 10 microM) prevented the up-regulation of alphaENaC mRNA induced by aldosterone (10 nM). In Sprague-Dawley rats infused with angiotensin II (0.4 microg/kg/min), alphaENaC expression was up-regulated in renal cortex and medulla coincidently with high plasma aldosterone levels. Sulprostone and/or 17 Pt prevented this effect in renal medulla but not in cortex. Immunocytochemistry demonstrated that IMCD cells express EP1. Our results suggest that specific activation of EP1 receptor during RAAS activation antagonizes the action of aldosterone on alphaENaC expression in the renal medulla.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

Dysregulation of renal aquaporins and epithelial sodium channel in lithium-induced nephrogenic diabetes insipidus

Lithium is used commonly to treat bipolar mood disorders. In addition to its primary therapeutic effects in the central nervous system lithium has a number of side effects in the kidney. The side effects include nephrogenic diabetes insipidus with polyuria, mild sodium wasting, and changes in acid/base balance. These functional changes are associated with marked structural changes in collecting duct cell composition and morphology, likely contributing to the functional changes. Over the past few years, investigations of lithium-induced renal changes have provided novel insight into the molecular mechanisms that are responsible for the disturbances in water, sodium, and acid/base metabolism. This includes dysregulation of renal aquaporins, epithelial sodium channel, and acid/base transporters. This review focuses on these issues with the aim to present this in context with clinically relevant features.

Effects of dietary salt on renal Na+ transporter subcellular distribution, abundance, and phosphorylation status

During high-salt (HS) diet the kidney increases urinary Na+ and volume excretion to match intake. We recently reported that HS provokes a redistribution of distal convoluted tubule Na+-Cl- cotransporter (NCC) from apical to subapical vesicles and decreases NCC abundance. This study aimed to test the hypothesis that the other renal Na+ transporters' abundance and or subcellular distribution is decreased by HS diet. Six-week-old Sprague-Dawley rats were fed a normal (NS) 0.4% NaCl diet or a HS 4% NaCl diet for 3 wk or overnight. Kidneys excised from anesthetized rats were fractionated on density gradients or analyzed by microscopy; transporters and associated regulators were detected with specific antibodies. Three-week HS doubled Na+/H+ exchanger (NHE)3 phosphorylation at serine 552 and provoked a redistribution of NHE3, dipeptidyl peptidase IV (DPPIV), myosin VI, Na+-Pi cotransporter (NaPi)-2, ANG II type 2 receptor (AT2R), aminopeptidase N (APN), Na+-K+-2Cl- cotransporter (NKCC2), epithelial Na+ channel (ENaC) beta-subunit, and Na+-K+-ATPase (NKA) alpha1- and beta1-subunits from low-density plasma membrane-enriched fractions to higher-density intracellular membrane-enriched fractions. NHE3, myosin VI, and AT2R retraction to the base of the microvilli (MV) during HS was evident by confocal microscopy. HS did not change abundance of NHE3, NKCC, or NKA alpha1- or beta1-subunits but increased ENaC-beta in high-density intracellular enriched membranes. Responses to HS were fully apparent after just 18 h. We propose that retraction of NHE3 to the base of the MV, driven by myosin VI and NHE3 phosphorylation and accompanied by redistribution of the NHE3 regulator DPPIV, contributes to a decrease in proximal tubule Na+ reabsorption during HS and that redistribution of transporters out of low-density plasma membrane-enriched fractions in the thick ascending limb of the loop of Henle and distal nephron may also contribute to the homeostatic natriuretic response to HS diet.

Surface expression of epithelial Na channel protein in rat kidney

Expression of epithelial Na channel (ENaC) protein in the apical membrane of rat kidney tubules was assessed by biotinylation of the extracellular surfaces of renal cells and by membrane fractionation. Rat kidneys were perfused in situ with solutions containing NHS-biotin, a cell-impermeant biotin derivative that attaches covalently to free amino groups on lysines. Membranes were solubilized and labeled proteins were isolated using neutravidin beads, and surface β and γENaC subunits were assayed by immunoblot. Surface αENaC was assessed by membrane fractionation. Most of the γENaC at the surface was smaller in molecular mass than the full-length subunit, consistent with cleavage of this subunit in the extracellular moiety close to the first transmembrane domains. Insensitivity of the channels to trypsin, measured in principal cells of the cortical collecting duct by whole-cell patch-clamp recording, corroborated this finding. ENaC subunits could be detected at the surface under all physiological conditions. However increasing the levels of aldosterone in the animals by feeding a low-Na diet or infusing them directly with hormone via osmotic minipumps for 1 wk before surface labeling increased the expression of the subunits at the surface by two- to fivefold. Salt repletion of Na-deprived animals for 5 h decreased surface expression. Changes in the surface density of ENaC subunits contribute significantly to the regulation of Na transport in renal cells by mineralocorticoid hormone, but do not fully account for increased channel activity.

Alpha2-Antiplasmin Is a Critical Regulator of Angiotensin II–Mediated Vascular Remodeling

Objective— Alpha2-antiplasmin (α2-AP) is the major circulating inhibitor of plasmin, which plays a determining role in the regulation of intravascular fibrinolysis. We investigated the role of α 2 -AP on vascular remodeling in response to angiotensin II (Ang II).

Methods and Results— α2-AP–deficient mice were performed. Ang II and N ω -nitro- L-arginine methyl ester (L-NAME)–induced perivascular fibrosis was significantly decreased in α2-AP −/− mice compared with wild-type mice. In situ gelatinolytic activity analysis shows that perivascular gelatinolytic activity was increased in α2-AP −/− mice, which was responsible for decreased perivascular fibrosis in response to Ang II and L-NAME. Ang II–induced arterial wall thickening, vascular cell proliferation, apoptosis, c-Myc, and collagen Ι expression were significantly decreased in α2-AP −/− mice compared with wild-type mice. Further analysis shows that increased p53 and p21 expression were responsible for inhibition of Ang II–induced vascular remodeling in α2-AP −/− mice.

Conclusion— The results show that α2-AP is a critical regulator for vascular remodeling by inhibiting p53/p21 pathway, suggesting that α2-AP is proposed to be a potential therapeutic target for vascular remodeling.

Effect of aldosterone on BK channel expression in mammalian cortical collecting duct

Apical large-conductance Ca(2+)-activated K(+) (BK) channels in the cortical collecting duct (CCD) mediate flow-stimulated K(+) secretion. Dietary K(+) loading for 10-14 days leads to an increase in BK channel mRNA abundance, enhanced flow-stimulated K(+) secretion in microperfused CCDs, and a redistribution of immunodetectable channels from an intracellular pool to the apical membrane (Najjar F, Zhou H, Morimoto T, Bruns JB, Li HS, Liu W, Kleyman TR, Satlin LM. Am J Physiol Renal Physiol 289: F922-F932, 2005). To test whether this adaptation was mediated by a K(+)-induced increase in aldosterone, New Zealand White rabbits were fed a low-Na(+) (LS) or high-Na(+) (HS) diet for 7-10 days to alter circulating levels of aldosterone but not serum K(+) concentration. Single CCDs were isolated for quantitation of BK channel subunit (total, alpha-splice variants, beta-isoforms) mRNA abundance by real-time PCR and measurement of net transepithelial Na(+) (J(Na)) and K(+) (J(K)) transport by microperfusion; kidneys were processed for immunolocalization of BK alpha-subunit by immunofluorescence microscopy. At the time of death, LS rabbits excreted no urinary Na(+) and had higher circulating levels of aldosterone than HS animals. The relative abundance of BK alpha-, beta(2)-, and beta(4)-subunit mRNA and localization of immunodetectable alpha-subunit were similar in CCDs from LS and HS animals. In response to an increase in tubular flow rate from approximately 1 to 5 nl.min(-1).mm(-1), the increase in J(Na) was greater in LS vs. HS rabbits, yet the flow-stimulated increase in J(K) was similar in both groups. These data suggest that aldosterone does not contribute to the regulation of BK channel expression/activity in response to dietary K(+) loading.

Mouse model of type II Bartter's syndrome. II. Altered expression of renal sodium- and water-transporting proteins

Bartter's syndrome represents a group of hereditary salt- and water-losing renal tubulopathies caused by loss-of-function mutations in proteins mediating or regulating salt transport in the thick ascending limb (TAL) of Henle's loop. Mutations in the ROMK channel cause type II antenatal Bartter's syndrome that presents with maternal polyhydramnios and postnatal life-threatening volume depletion. We have developed a colony of Romk null mice showing a Bartter-like phenotype and with increased survival to adulthood, suggesting the activation of compensatory mechanisms. To test the hypothesis that upregulation of Na+-transporting proteins in segments distal to the TAL contributes to compensation, we studied expression of salt-transporting proteins in ROMK-deficient ( Romk−/−) mice. Plasma aldosterone was 40% higher and urinary PGE2 excretion was 1.5-fold higher in Romk−/− compared with wild-type littermates. Semiquantitative immunoblotting of kidney homogenates revealed decreased abundances of proximal tubule Na+/H+ exchanger (NHE3) and Na+-Pi cotransporter (NaPi-IIa) and TAL-specific Na+-K+-2Cl−-cotransporter (NKCC2/BSC1) in Romk−/− mice, while the distal convoluted tubule (DCT)-specific Na+-Cl− cotransporter (NCC/TSC) was markedly increased. The abundance of the α-,β-, and γ-subunits of the epithelial Na+ channel (ENaC) was slightly increased, although only differences for γ-ENaC reached statistical significance. Morphometry revealed a fourfold increase in the fractional volume of DCT but not of connecting tubule (CNT) and collecting duct (CCD). Consistently, CNT and CD of Romk−/− mice revealed no apparent increase in the luminal abundance of the ENaC compared with those of wild-type mice. These data suggest that the loss of ROMK-dependent Na+ absorption in the TAL is compensated predominately by upregulation of Na+ transport in downstream DCT cells. These adaptive changes in Romk−/− mice may help to limit renal Na+ loss, and thereby, contribute to survival of these mice.

Nedd4-2 isoforms ubiquitinate individual epithelial sodium channel subunits and reduce surface expression and function of the epithelial sodium channel

We previously reported the existence of multiple isoforms of human Nedd4-2 (Am J Physiol Renal Physiol 285: F916-F929, 2003). When overexpressed in M-1 collecting duct epithelia, full-length Nedd4-2 (Nedd4-2), Nedd4-2 lacking the NH(2)-terminal C2 domain (Nedd4-2DeltaC2), and Nedd4-2 lacking WW domains 2 and 3 (Nedd4-2DeltaWW2,3) variably reduce benzamil-sensitive Na(+) transport. We investigated the effect of each of the Nedd4-2 isoforms on cell surface expression and ubiquitination of ENaC subunits. We find that alphaENaC when transfected alone or with beta and gammaENaC is expressed at the cell surface and this membrane expression is variably reduced by coexpression with each of the Nedd4-2 isoforms. Nedd4-2 reduces the half-life of ENaC subunits and enhances the ubiquitination of alpha, beta, and gammaENaC subunits when expressed alone or together suggesting that each subunit is a target for Nedd4-2-mediated ubiquitination. As has been reported recently, we confirm that the surface-expressed pool of ENaC is multi-ubiquitinated. Inhibitors of the proteasome increase ubiquitination of ENaC subunits and stimulate Na(+) transport in M-1 cells consistent with a role for the ubiquitin-proteasome pathway in regulating Na(+) transport in the collecting duct.

A switch in the mechanism of hypertension in the syndrome of apparent mineralocorticoid excess

The syndrome of apparent mineralocorticoid excess arises from nonfunctional mutations in 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2), an enzyme that inactivates cortisol and confers aldosterone specificity on the mineralocorticoid receptor. Loss of 11betaHSD2 permits glucocorticoids to activate the mineralocorticoid receptor, and the hypertension in the syndrome is presumed to arise from volume expansion secondary to renal sodium retention. An 11betaHSD2 null mouse was generated on an inbred C57BL/6J genetic background, allowing survival to adulthood. 11betaHSD2(-/-) mice had BP approximately 20 mmHg higher on average compared with wild-type mice but were volume contracted, not volume expanded as expected. Initially, impaired sodium excretion associated with increased activity of the epithelial sodium channel was observed. By 80 days of age, however, channel activity was abolished and 11betaHSD2(-/-) mice lost salt. Despite the natriuresis, hypertension remained but was not attributable to intrinsic vascular dysfunction. Instead, urinary catecholamine levels in 11betaHSD2(-/-) mice were double those in wild-type mice, and alpha1-adrenergic receptor blockade rescued the hypertensive phenotype, suggesting that vasoconstriction contributes to the sustained hypertension in this model. In summary, it is proposed that renal sodium retention remains a key event in apparent mineralocorticoid excess but that the accompanying hypertension changes from a renal to a vascular etiology over time.

Dietary salt enhances benzamil-sensitive component of myogenic constriction in mesenteric arteries

Recent work from our laboratory indicates that epithelial Na(+) channel (ENaC) function plays an important role in modulating myogenic vascular reactivity. Increases in dietary sodium are known to affect vascular reactivity. Although previous studies have demonstrated that dietary salt intake regulates ENaC expression and activity in epithelial tissue, the importance of dietary salt on ENaC expression in vascular smooth muscle cells (VSMCs) and its role in myogenic constriction is unknown. Therefore, the goal of the present study was to determine whether dietary salt modulates ENaC expression and function in myogenic vasoconstriction. To accomplish this goal, we examined ENaC expression in freshly dispersed VSMCs and pressure-induced vasoconstrictor responses in isolated mesenteric resistance arteries from normotensive Sprague-Dawley rats fed a normal-salt (NS; 0.4% NaCl) or high-salt (HS; 8% NaCl for 2 wk) diet. VSMCs from the mesenteric arteries of NS-fed animals express alpha-, beta-, and gamma-ENaC. The HS diet reduced whole cell alpha- and gamma-ENaC and induced a pronounced translocation of beta-ENaC from intracellular regions toward the VSMC membrane (approximately 336 nm). Associated with this change in expression was a change in the importance of ENaC in pressure-induced constriction. Pressure-induced constriction in NS-fed animals was insensitive to ENaC inhibition with 1 microM benzamil, suggesting that ENaC proteins do not contribute to myogenic constriction in mesenteric arteries under NS intake. In contrast, ENaC inhibition blocked pressure-induced constriction in HS-fed animals. These data suggest that dietary sodium regulates ENaC expression and the quantitative importance of the vascular ENaC signaling pathway contributing to myogenic constriction.

Na+ and K+ transport by the renal connecting tubule

PURPOSE OF REVIEW

The connecting tubule is emerging as a nephron segment critical to the regulation of Na+ and K+ excretion and the maintenance of homeostasis for these ions. The segment is difficult to study, however, and much of the available information we have concerning its functions is indirect. Here, we review the major transport mechanisms and transporters found in this segment and outline several unsolved problems in the field.

RECENT FINDINGS

Recent electrophysiological and immunohistochemical measurements together with theoretical studies provide a more comprehensive view of ion transport in the connecting tubule. New signaling pathways governing Na+ and K+ transport have also been described.

SUMMARY

Key questions about how Na+ and K+ transport are regulated remain unanswered. Is the connecting tubule the site of final regulation of both Na+ and K+ excretion? If so, how are the transport rates of these two ions independently controlled?

Impairment of sodium balance in mice deficient in renal principal cell mineralocorticoid receptor

Germline inactivation of the mineralocorticoid receptor (MR) gene in mice results in postnatal lethality as a result of massive loss of sodium and water. The knockout mice show impaired epithelial sodium channel (ENaC) activity in kidney and colon. For determination of the role of renal MR in aldosterone-driven ENaC-mediated sodium reabsorption, mice with principal cell MR deficiency were generated using the Cre-loxP system. For driving Cre recombinase expression in principal cells, the regulatory elements of the mouse aquaporin 2 (AQP2) gene were used. Mutant mice (MR(AQP2Cre)) were obtained by crossing AQP2Cre mice with mice that carried a conditional MR allele. Under standard diet, MR(AQP2Cre) mice develop normally and exhibit unaltered renal sodium excretion but show strongly elevated aldosterone levels. Increased renal sodium and water excretion, resulting in continuous loss of body weight, occur under low-sodium diet. Immunofluorescence revealed that the loss of MR and apical ENaC staining is restricted to principal cells of the collecting duct (CD) and late connecting tubule (CNT) and that MR is crucial for ENaC trafficking to the apical membrane. These results demonstrate that inactivation of MR in CD and late CNT can be compensated under standard diet but no longer when sodium supply is limited. Because the mutant mice show preserved renal ENaC activity, this study provides evidence that the late distal convoluted tubule and early CNT can compensate to a large extent deficient ENaC-mediated sodium reabsorption in late CNT and CD.

Immunolocalization and mRNA expression of the epithelial Na+ channel alpha-subunit in the kidney and urinary bladder of the marine toad, Bufo marinus, under hyperosmotic conditions

The amiloride-sensitive epithelial sodium channel (ENaC) has previously been shown to be involved in the maintenance of body fluid volume and in Na(+) absorption across the skin and urinary bladder in amphibians. However, the function and distribution of ENaC have not been clearly described in amphibian kidney. We therefore cloned the ENaC alpha-subunit cDNA from kidney of the marine toad, Bufo marinus. The ENaC mRNA and protein were abundantly expressed in the kidney and in the urinary bladder and ventral pelvic skin. In an immunohistochemical study, the ENaC alpha-subunit protein was specifically localized to the apical membrane of the principal cells but not the intercalated cells from the late distal tubule to the collecting duct in the kidney or in the apical area of cells of urinary bladder epithelia. When toads were acclimated to dry and hyper-saline environments, the levels of ENaC mRNA expression in the kidney and urinary bladder decreased under hyper-saline acclimation, but not under dry conditions. Immunohistochemical observations indicated that the levels of ENaC protein expression were much lower in the apical area of renal distal tubules and urinary bladder epithelia of hyper-saline acclimated toad compared with controls. The present study suggests that Bufo ENaC is significantly expressed and functions during Na(+) reabsorption in the apical membrane domain in the distal nephron of normal and desiccated toads. Natriuresis may be caused by decreases in ENaC expression and its trafficking to the cell surface in the distal nephron, a response to prevent excessive Na(+) reabsorption in hyper-saline-acclimated toads.