The gamma-subunit of ENaC is more important for channel surface expression than the beta-subunit

Glucocorticoids Promote Na+ Excretion in the Renal Epithelia of Heart Failure Rats by Suppressing Transporter Proteins Involved in Acute Sodium Loading

ABSTRACT

Glucocorticoid receptors are essential for normal development and stress responses. Their role in H 2 O and Na + metabolism, especially in chronic heart failure (CHF), is not well defined. In a previous study, we found that glucocorticoids potentiate urination in CHF and promote H 2 O excretion by inhibiting the vasopressin receptor 2 pathway. The present study examines the effect of glucocorticoids on renal Na + excretion and the underlying mechanisms in CHF rats with acute sodium loading. CHF was induced by left coronary artery ligation for 8 weeks. Rats were randomly assigned to 5 groups: control, CHF, dexamethasone (DEX)-administered CHF, DEX-administered CHF treated with RU486 (mifepristone, a glucocorticoid receptor antagonist), and RU486-treated CHF. An acute sodium loading test was performed 6 hours after DEX administration. Blood and urine samples were collected, and hemodynamics were measured. The expression and localization of Na + transporter proteins were determined by immunoblotting and immunohistochemistry. DEX increased the urine volume and urinary sodium and improved cardiac function and the estimated glomerular filtration rate in CHF rats. The upregulation of the epithelial sodium channel β and γ subunits, Na-K-2Cl cotransporter, serum glucocorticoid-regulated kinase 1 (SGK1), and Na + /K + -ATPase in the renal epithelium of CHF rats was downregulated by DEX. These beneficial effects were abolished by RU486. The expression of natriuretic peptide receptor A was opposite that of the above proteins. Glucocorticoids might induce profound natriuresis in CHF rats during acute sodium loading, which is associated with downregulating some Na + transporter proteins in the renal epithelium and improving intrarenal hemodynamics.

Salt sensitivity of volume and blood pressure in a mouse with globally reduced ENaC γ-subunit expression

The epithelial Na+ channel (ENaC) promotes the absorption of Na+ in the aldosterone-sensitive distal nephron, colon, and respiratory epithelia. Deletion of genes encoding subunits of ENaC results in early postnatal mortality. Here, we present the initial characterization of a mouse with dramatically suppressed expression of the ENaC γ-subunit. We used this hypomorphic (γmt) allele to explore the importance of this subunit in homeostasis of electrolytes and body fluid volume. At baseline, γ-subunit expression in γmt/mt mice was markedly suppressed in the kidney and lung, whereas electrolytes resembled those of littermate controls. Aldosterone levels in γmt/mt mice exceeded those seen in littermate controls. Quantitative magnetic resonance measurement of body composition revealed similar baseline body water, lean tissue mass, and fat tissue mass in γmt/mt mice and controls. γmt/mt mice exhibited a more rapid decline in body water and lean tissue mass in response to a low-Na+ diet than the controls. Replacement of drinking water with 2% saline selectively and transiently increased body water and lean tissue mass in γmt/mt mice relative to the controls. Lower blood pressures were variably observed in γmt/mt mice on a high-salt diet compared with the controls. γmt/mt also exhibited reduced diurnal blood pressure variation, a "nondipping" phenotype, on a high-Na+ diet. Although ENaC in the renal tubules and colon works to prevent extracellular fluid volume depletion, our observations suggest that ENaC in other tissues may participate in regulating extracellular fluid volume and blood pressure.NEW & NOTEWORTHY A mouse with globally suppressed expression of the epithelial Na+ channel γ-subunit showed enhanced sensitivity to dietary salt, including a transient increase in total body fluid, reduced blood pressure, and reduced diurnal blood pressure variation when given a dietary NaCl challenge. These results point to a role for the epithelial Na+ channel in regulating body fluid and blood pressure beyond classical transepithelial Na+ transport mechanisms.

Timeline of Changes in Biomarkers Associated with Spinal Cord Injury-Induced Polyuria

Deficits in upper and lower urinary tract function, which include detrusor overactivity, urinary incontinence, detrusor-sphincter dyssynergia, and polyuria, are among the leading issues that arise after spinal cord injury (SCI) affecting quality of life. Given that overproduction of urine (polyuria) has been shown to be associated with an imbalance in key regulators of body fluid homeostasis, the current study examined the timing of changes in levels of various relevant hormones, peptides, receptors, and channels post-contusion injury in adult male Wistar rats. The results show significant up- or downregulation at various time points, beginning at 7 days post-injury, in levels of urinary atrial natriuretic peptide, serum arginine vasopressin (AVP), kidney natriuretic peptide receptor-A, kidney vasopressin-2 receptor, kidney aquaporin-2 channels, and kidney epithelial sodium channels (β- and γ-, but not α-, subunits). The number of AVP-labeled neurons in the hypothalamus (supraoptic and -chiasmatic, but not paraventricular, nuclei) was also significantly altered at one or more time points. These data show significant fluctuations in key biomarkers involved in body fluid homeostasis during the post-SCI secondary injury phase, suggesting that therapeutic interventions (e.g., desmopressin, a synthetic analogue of AVP) should be considered early post-SCI.

Segregated Expression of ENaC Subunits in Taste Cells

Salt taste is one of the 5 basic taste qualities. Depending on the concentration, table salt is perceived either as appetitive or aversive, suggesting the contribution of several mechanisms to salt taste, distinguishable by their sensitivity to the epithelial sodium channel (ENaC) blocker amiloride. A taste-specific knockout of the α-subunit of the ENaC revealed the relevance of this polypeptide for low-salt transduction, whereas the response to other taste qualities remained normal. The fully functional ENaC is composed of α-, β-, and γ-subunits. In taste tissue, however, the precise constitution of the channel and the cell population responsible for detecting table salt remain uncertain. In order to examine the cells and subunits building the ENaC, we generated mice carrying modified alleles allowing the synthesis of green and red fluorescent proteins in cells expressing the α- and β-subunit, respectively. Fluorescence signals were detected in all types of taste papillae and in taste buds of the soft palate and naso-incisor duct. However, the lingual expression patterns of the reporters differed depending on tongue topography. Additionally, immunohistochemistry for the γ-subunit of the ENaC revealed a lack of overlap between all potential subunits. The data suggest that amiloride-sensitive recognition of table salt is unlikely to depend on the classical ENaCs formed by α-, β-, and γ-subunits and ask for a careful investigation of the channel composition.

Does ENaC Work as Sodium Taste Receptor in Humans?

Taste reception is fundamental for the proper selection of food and beverages. Among the several chemicals recognized by the human taste system, sodium ions (Na⁺) are of particular relevance. Na⁺ represents the main extracellular cation and is a key factor in many physiological processes. Na⁺ elicits a specific sensation, called salty taste, and low-medium concentrations of table salt (NaCl, the common sodium-containing chemical we use to season foods) are perceived as pleasant and appetitive. How we detect this cation in foodstuffs is scarcely understood. In animal models, such as the mouse and the rat, the epithelial sodium channel (ENaC) has been proposed as a key protein for recognizing Na⁺ and for mediating preference responses to low-medium salt concentrations. Here, I will review our current understanding regarding the possible involvement of ENaC in the detection of food Na⁺ by the human taste system.

Epithelial Na+ Channel (ENaC) Formed by One or Two Subunits Forms Functional Channels That Respond to Shear Force

Canonical epithelial sodium channels (ENaCs) are heterotrimers formed by α, β, and γ ENaC subunits in vertebrates and belong to the Degenerin/ENaC family of proteins. Proteins from this family form mechanosensitive channels throughout the animal kingdom. Activity of canonical ENaC is regulated by shear force (SF) mediating Na⁺ absorption in the kidney and vascular tone of arteries. Expression analysis suggests that non-canonical ENaC, formed by single or only two subunits, exist in certain tissues, but it is unknown if these channels respond to SF. α, β, γ, and δ ENaC subunits were expressed either alone or in combinations of two subunits in Xenopus oocytes. Amiloride-sensitive currents and the responses to SF were assessed using two-electrode voltage clamp recordings. With the exception of γ ENaC, all homomeric channels provided amiloride-sensitive currents and responded to SF applied via a fluid stream directed onto the oocytes. Channels containing two subunits were also activated by SF. Here, the presence of the γ ENaC subunit when co-expressed with α or δ augmented the SF response in comparison to the αβγ/δβγ ENaC. Overall, we provide evidence that non-canonical ENaC can form channels that respond to SF. This supports a potential function of non-canonical ENaC as mechanosensors in epithelial, vascular, and sensory cells.

Ursodeoxycholic acid inhibits ENaC and Na/K pump activity to restore airway surface liquid height in cystic fibrosis bronchial epithelial cells

Cystic fibrosis (CF) is a disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) that in the airways result in reduced Cl^- secretion and increased Na⁺ absorption, airway surface liquid (ASL) dehydration, decreased mucociliary clearance, infection and inflammation leading to lung injury. Cystic fibrosis patients often present with bile acids in the lower airways, however the effects of bile acids on ASL and ion transport in CF airways are not known. Secondary bile acids, such as ursodeoxycholic acid (UDCA), have been shown to modulate immune responses and epithelial ion transport. Here we investigated the effects of UDCA in normal and CF airway epithelial cell models. NuLi-1 (normal genotype) and CuFi-1 (CF genotype, Δ508/Δ508) primary immortalized airway epithelial cells were grown under an air-liquid interface. Electrogenic transepithelial ion transport was measured by short-circuit current (Isc) across cell monolayers mounted in Ussing chambers. We observed that UDCA (500 μM, 60 min, bilateral) decreased the basal Isc and ENaC currents in both NuLi-1 and CuFi-1 cells. UDCA inhibited the amiloride-sensitive ENaC current by 44% in NulI-1 monolayers and by 30% in CuFi-1 cells. Interestingly, UDCA also inhibited currents through the basolateral Na/K pump in both Nuli-1 and CuFi-1 monolayers without alterting the expression of ENaC or Na⁺/K⁺-ATPase proteins. The airway surface liquid height is regulated by transpeithelial Na⁺ absorption (ENaC) and Cl^- secretion (CFTR) in normal airway but mainly by ENaC activity in CF epithelia when Cl^- secretion is compromised by CFTR mutations. UDCA increased ASL height by 50% in Nuli-1 and by 40% in CUFI-1 monolayers. In conclusion, we demonstrate a previously unknown effect of UDCA to inhibit ENaC activity and increase ASL height in normal and CF human airway epithelial cells suggesting a therapeutic potential for UDCA in CF lung disease.

Analyses of epithelial Na+ channel variants reveal that an extracellular β-ball domain critically regulates ENaC gating

Epithelial Na⁺ channel (ENaC)-mediated Na⁺ transport has a key role in the regulation of extracellular fluid volume, blood pressure, and extracellular [K⁺]. Among the thousands of human ENaC variants, only a few exist whose functional consequences have been experimentally tested. Here, we used the Xenopus oocyte expression system to investigate the functional roles of four nonsynonymous human ENaC variants located within the β7-strand and its adjacent loop of the α-subunit extracellular β-ball domain. αR350Wβγ and αG355Rβγ channels exhibited 2.5- and 1.8-fold greater amiloride-sensitive currents than WT αβγ human ENaCs, respectively, whereas αV351Aβγ channels conducted significantly less current than WT. Currents in αH354Rβγ-expressing oocytes were similar to those expressing WT. Surface expression levels of three mutants (αR350Wβγ, αV351Aβγ, and αG355Rβγ) were similar to that of WT. However, three mutant channels (αR350Wβγ, αH354Rβγ, and αG355Rβγ) exhibited a reduced Na⁺ self-inhibition response. Open probability of αR350Wβγ was significantly greater than that of WT. Moreover, other Arg-350 variants, including αR350G, αR350L, and αR350Q, also had significantly increased channel activity. A direct comparison of αR350W and two previously reported gain-of-function variants revealed that αR350W increases ENaC activity similarly to αW493R, but to a much greater degree than does αC479R. Our results indicate that αR350W along with αR350G, αR350L, and αR350Q, and αG355R are novel gain-of-function variants that function as gating modifiers. The location of these multiple functional variants suggests that the αENaC β-ball domain portion that interfaces with the palm domain of βENaC critically regulates ENaC gating.

Testosterone enhances expression and functional activity of epithelial sodium channel (ENaC), cystic fibrosis transmembrane regulator (CFTR) and sodium hydrogen exchanger (NHE) in vas deferens of sex-steroid deficient male rats

Effects of testosterone on expression and functional activity of ENaC, CFTR and NHE in vas deferens were investigated.

METHODS

Orchidectomized, adult male rats were given 125 and 250 μg/kg/day testosterone subcutaneously, with or without flutamide and finasteride for seven consecutive days. At the end of the treatment, rats were anesthetized and vas deferens were perfused. Changes in vas deferens fluid secretion rate, pH, HCO₃^-, Cl^- and Na⁺ concentrations were recorded in the presence of amiloride and Cftr inh-172. Rats were then sacrificed and vas deferens were harvested and subjected for molecular biological analysis.

RESULTS

Testosterone treatment caused the fluid pH and HCO₃^- concentrations to decrease but secretion rate, Cl^- and Na⁺ concentrations to increase, where upon amiloride administration, the pH and HCO₃^- concentration increased but Cl^- and Na⁺ concentrations further increased. In testosterone-treated rats, administration of Cftr inh-172 caused all fluid parameters to decrease. In testosterone-treated rats co-administered with flutamide or finasteride, pH and HCO₃^- concentration increased but fluid secretion rate, Cl^- and Na⁺ concentrations decreased and these parameters were not affected by amiloride or Cftr inh-172 administration. Under testosterone influence, CFTR and γ-ENaC were highly expressed at the apical membrane while NHE-1 and 4 were highly expressed at the basolateral membrane of vas deferens epithelium. Meanwhile, NHE-2 and 3 were highly expressed at the apical membrane.

CONCLUSIONS

Differential expression of ENaC, CFTR and NHE in vas deferens under testosterone influence indicated the important role of these transporters in creating optimal fluid microenvironment that is essential for preserving male fertility.

Antisense oligonucleotide targeting of mRNAs encoding ENaC subunits α, β, and γ improves cystic fibrosis-like disease in mice

BACKGROUND

The epithelial sodium channel ENaC consists of three subunits encoded by Scnn1a, Scnn1b, and Scnn1g and increased sodium absorption through this channel is hypothesized to lead to mucus dehydration and accumulation in cystic fibrosis (CF) patients.

METHODS

We identified potent and specific antisense oligonucleotides (ASOs) targeting mRNAs encoding the ENaC subunits and evaluated these ASOs in mouse models of CF-like lung disease.

RESULTS

ASOs designed to target mRNAs encoding each ENaC subunit or a control ASO were administered directly into the lungs of mice. The reductions in ENaC subunits correlated well with a reduction in amiloride sensitive channel conductance. In addition, levels of mucus markers Gob5, AGR2, Muc5ac, and Muc5b, periodic acid-Schiff's reagent (PAS) goblet cell staining, and neutrophil recruitment were reduced and lung function was improved when levels of any of the ENaC subunits were decreased.

CONCLUSIONS

Delivery of ASOs targeting mRNAs encoding each of the three ENaC subunits directly into the lung improved disease phenotypes in a mouse model of CF-like lung disease. These findings suggest that targeting ENaC subunits could be an effective approach for the treatment of CF.

Effect of dietary salt intake on epithelial Na+ channels (ENaCs) in the hypothalamus of Dahl salt-sensitive rats

All three epithelial Na+ channel (ENaC) subunits (α, β, and γ) and the mineralocorticoid receptor (MR), a known regulator of ENaC, are located in vasopressin (VP) synthesizing magnocellular neurons in the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei. Our previous study showed that ENaC mediates a Na+ leak current that affects the steady-state membrane potential of VP neurons. This study was conducted in Dahl salt-sensitive (Dahl-SS) rats to determine if any abnormal responses in the expression of ENaC subunits and MR occur in the hypothalamus and kidney in response to a high dietary salt intake. After 21 days of high salt consumption, Dahl-SS rat resulted in a significant increase in γENaC expression and exhibited proteolytic cleavage of this subunit compared to Sprague-Dawley (SD) rats. Additionally, Dahl-SS rats had dense somato-dendritic γENaC immunoreactivity in VP neurons, which was absent in SD rats. In contrast, SD rats fed a high salt diet had significantly decreased αENaC subunit expression in the kidney and MR expression in the hypothalamus. Plasma osmolality measured daily for 22 days demonstrated that Dahl-SS rats fed a high salt diet had a steady increase in plasma osmolality, whereas SD rats had an initial increase that decreased to baseline levels. Findings from this study demonstrate that Dahl-SS rats lack a compensatory mechanism to down regulate ENaC during high dietary salt consumption, which may contribute to the development of hypertension.

Environmentally relevant levels of bisphenol A affect uterine decidualization and embryo implantation through the estrogen receptor/serum and glucocorticoid-regulated kinase 1/epithelial sodium ion channel α-subunit pathway in a mouse model

OBJECTIVE

To investigate whether bisphenol A (BPA) exposure is associated with uterine decidualization and embryo implantation failure in mice.

DESIGN

Experimental animal study and in vitro study.

SETTING

University-based infertility center.

ANIMAL(S)

ICR mice.

INTERVENTION(S)

Mice treated with different doses of BPA; Ishikawa cells cultured in medium of different concentrations of BPA.

MAIN OUTCOME MEASURE(S)

Embryo implantation sites, uterine weight, quantitative real-time reverse transcriptase-polymerase chain reaction, Western blot analysis, hematoxylin and eosin staining, and immunohistochemical, cell proliferation, and statistical analyses.

RESULT(S)

In the experiment of mouse model, administration of 1-100 μg/kg/day of BPA by gavage led to reduction of the number of embryo implantation sites in a dose-dependent manner; 100 μg/kg/day of BPA statistically significantly reduced the number of implantation sites compared with the control group. The uterine weight change (the wet weight of the decidualized uterine horn divided by the wet weight of the undecidualized uterine horn of the mouse) in groups exposed to BPA (100-10,000 μg/kg/day) were statistically significantly lower compared with the control group. Immunohistochemical analysis demonstrated that administration of 100, 1,000, or 10,000 μg/kg/day of BPA by gavage statistically significantly down-regulated the expression of epithelial Na⁺ channel α-subunit (ENaCα) in the luminal epithelial cells and desmin in decidual cells of the oil-induced decidualized uterine horns. Administration of 100 μg/kg/day BPA on embryo days 0.5-3.5 by gavage statistically significantly decreased the level of uterine serum and glucocorticoid-regulated kinase 1 (SGK1) protein expression on embryo days 4 and 6. After treatment with 0.001, 0.01, 0.1, or 1.0 μg/mL of BPA for 48 hours, the SGK1, ENaCα, and phospho-SGK1 protein expression of Ishikawa cells was down-regulated, and the effect of BPA on SGK1 could be abrogated by fulvestrant.

CONCLUSION(S)

Our study provides the first indication that BPA exposure at levels as low as 100 μg/kg/day can impair embryo implantation in mice and BPA can affect decidualization of the uterus in mouse model. Our results suggest that BPA can down-regulate SGK1 and ENaCα protein expression through estrogen receptors in Ishikawa cells.

Plasma Potassium Determines NCC Abundance in Adult Kidney-Specific γENaC Knockout

The amiloride-sensitive epithelial sodium channel (ENaC) and the thiazide-sensitive sodium chloride cotransporter (NCC) are key regulators of sodium and potassium and colocalize in the late distal convoluted tubule of the kidney. Loss of the αENaC subunit leads to a perinatal lethal phenotype characterized by sodium loss and hyperkalemia resembling the human syndrome pseudohypoaldosteronism type 1 (PHA-I). In adulthood, inducible nephron-specific deletion of αENaC in mice mimics the lethal phenotype observed in neonates, and as in humans, this phenotype is prevented by a high sodium (HNa⁺)/low potassium (LK⁺) rescue diet. Rescue reflects activation of NCC, which is suppressed at baseline by elevated plasma potassium concentration. In this study, we investigated the role of the γENaC subunit in the PHA-I phenotype. Nephron-specific γENaC knockout mice also presented with salt-wasting syndrome and severe hyperkalemia. Unlike mice lacking αENaC or βΕΝaC, an HNa⁺/LK⁺ diet did not normalize plasma potassium (K⁺) concentration or increase NCC activation. However, when K⁺ was eliminated from the diet at the time that γENaC was deleted, plasma K⁺ concentration and NCC activity remained normal, and progressive weight loss was prevented. Loss of the late distal convoluted tubule, as well as overall reduced βENaC subunit expression, may be responsible for the more severe hyperkalemia. We conclude that plasma K⁺ concentration becomes the determining and limiting factor in regulating NCC activity, regardless of Na⁺ balance in γENaC-deficient mice.

Expression and activity of epithelial sodium channel in hyperoxia-induced bronchopulmonary dysplasia in neonatal rats

BACKGROUND

The aim of the present study was to investigate the expression and activity of epithelial sodium channel (ENaC) in hyperoxia-induced bronchopulmonary dysplasia (BPD) in neonatal rats.

METHODS

Neonatal rats were exposed to hyperoxia to establish BPD models (control group was exposed to air), lung water was measured and Western blot was applied to detect the expression of three homologous subunits: α-, β- and γ-ENaC in the lung tissues. Furthermore, ATII cells were isolated from neonatal rats, and primarily cultured under normoxic or hyperoxic conditions. The ENaC expression was also examined in these cells. In addition, the amiloride-sensitive Na(+) currents induced by hyperoxia were recorded using the whole-cell patch clamp technique.

RESULTS

The α-ENaC expression was increased after 5 days of hyperoxia in rat lung tissues, whereas not after 1, 3 and 7 days. ATII cells showed α-ENaC expression was reduced after 1 and 2 days' hyperoxia, but no change after 3 days. In contrast, β- and γ-ENaC expression was increased after hyperoxia in both in vivo and in vitro experiments. The amiloride-sensitive Na(+) currents in hyperoxia-exposed ATII cells were also increased, which was consistent with the upregulated expression of β- and γ-ENaC.

CONCLUSION

Hyperoxia upregulates the expression of ENaC, especially β- and γ-ENaC subunits, in both neonatal rat lung tissues and ATII cells. Hyperoxia also enhanced the activity of ENaC in neonatal rat ATII cells. Dysfunctional transport of Na(+) may not be a key factor involving pulmonary edema at the early stage of BPD.

Molecular regulation of NKCC2 in the thick ascending limb

The kidney plays an essential role in blood pressure regulation by controlling short-term and long-term NaCl and water balance. The thick ascending limb of the loop of Henle (TAL) reabsorbs 25-30% of the NaCl filtered by the glomeruli in a process mediated by the apical Na(+)-K(+)-2Cl(-) cotransporter NKCC2, which allows Na(+) and Cl(-) entry from the tubule lumen into TAL cells. In humans, mutations in the gene coding for NKCC2 result in decreased or absent activity characterized by severe salt and volume loss and decreased blood pressure (Bartter syndrome type 1). Opposite to Bartter's syndrome, enhanced NaCl absorption by the TAL is associated with human hypertension and animal models of salt-sensitive hypertension. TAL NaCl reabsorption is subject to exquisite control by hormones like vasopressin, parathyroid, glucagon, and adrenergic agonists (epinephrine and norepinephrine) that stimulate NaCl reabsorption. Atrial natriuretic peptides or autacoids like nitric oxide and prostaglandins inhibit NaCl reabsorption, promoting salt excretion. In general, the mechanism by which hormones control NaCl reabsorption is mediated directly or indirectly by altering the activity of NKCC2 in the TAL. Despite the importance of NKCC2 in renal physiology, the molecular mechanisms by which hormones, autacoids, physical factors, and intracellular ions regulate NKCC2 activity are largely unknown. During the last 5 years, it has become apparent that at least three molecular mechanisms determine NKCC2 activity. As such, membrane trafficking, phosphorylation, and protein-protein interactions have recently been described in TALs and heterologous expression systems as mechanisms that modulate NKCC2 activity. The focus of this review is to summarize recent data regarding NKCC2 regulation and discuss their potential implications in physiological control of TAL function, renal physiology, and blood pressure regulation.

Effects of antenatal, postpartum and post-weaning melatonin supplementation on blood pressure and renal antioxidant enzyme activities in spontaneously hypertensive rats

Although melatonin lowers blood pressure in spontaneously hypertensive rats (SHR), its effect following antenatal and postpartum supplementation on the subsequent development of hypertension in SHR pups remains unknown. To investigate this, SHR dams were given melatonin in drinking water (10 mg/kg body weight/day) from day 1 of pregnancy until day 21 postpartum. After weaning, a group of male pups continued to receive melatonin till the age of 16 weeks (Mel-SHR), while no further melatonin was given to another group of male pups (Maternal-Mel-SHR). Controls received plain drinking water. Systolic blood pressure (SBP) was measured at 4, 6, 8, 12 and 16 weeks of age, after which the kidneys were collected for analysis of antioxidant enzyme profiles. SBP was significantly lower till the age of 8 weeks in Maternal-Mel-SHR and Mel-SHR than that in the controls, after which no significant difference was evident in SBP between the controls and Maternal-Mel-SHR. SBP in Mel-SHR was lower than that in controls and Maternal-Mel-SHR at 12 and 16 weeks of age. Renal glutathione peroxidase (GPx) and glutathione s-transferase (GST) activities, levels of total glutathione and relative GPx-1 protein were significantly higher in Mel-SHR. GPx protein was however significantly higher in Mel-SHR. No significant differences were evident between the three groups in the activities of superoxide dismutase, catalase and glutathione reductase. In conclusion, it appears that while antenatal and postpartum melatonin supplementation decreases the rate of rise in blood pressure in SHR offspring, it however does not alter the tendency of offspring of SHR to develop hypertension.

Role of melatonin in metabolic regulation

Although the human genome has remained unchanged over the last 10,000 years, our lifestyle has become progressively more divergent from those of our ancient ancestors. This maladaptive change became apparent with the Industrial Revolution and has been accelerating in recent decades. Socially, we are people of the 21st century, but genetically we remain similar to our early ancestors. In conjunction with this discordance between our ancient, genetically-determined biology and the nutritional, cultural and activity patterns in contemporary Western populations, many diseases have emerged. Only a century ago infectious disease was a major cause of mortality, whereas today non-infectious chronic diseases are the greatest cause of death in the world. Epidemics of metabolic diseases (e.g., cardiovascular diseases, type 2 diabetes, obesity, metabolic syndrome and certain cancers) have become major contributors to the burden of poor health and they are presently emerging or accelerating, in most developing countries. One major lifestyle consequence is light at night and subsequent disrupted circadian rhythms commonly referred to as circadian disruption or chronodisruption. Mounting evidence reveals that particularly melatonin rhythmicity has crucial roles in a variety of metabolic functions as an anti-oxidant, anti-inflammatory chronobiotic and possibly as an epigenetic regulator. This paper provides a brief outline about metabolic dysregulation in conjunction with a disrupted melatonin rhythm.

PPARs: the vasculature, inflammation and hypertension

PURPOSE OF REVIEW

Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear receptors activated by nutrient molecules and their derivatives. Their role has been increasingly recognized to be important in hypertension, metabolic disorders and cardiovascular disease, including atherosclerosis. Control of innate inflammatory processes mostly through alteration of monocyte/macrophage phenotype promises to be a unifying paradigm in understanding the pleiotropic effects of PPAR agonists.

RECENT FINDINGS

Although PPAR-gamma was the first to be described as an anti-inflammatory agent, both PPAR-alpha and PPAR-delta are now known to have similar effects as well. Inflammation is an important part of the damage caused by hypertensive diseases. PPARs have now been recognized as important determinants of macrophage polarization. Monocyte precursors of classical and alternatively activated macrophages are being defined as important changes in progression of cardiovascular disease associated with metabolic syndrome including hypertension, hyperlipidemia and obesity.

SUMMARY

A major unifying role for PPARs in hypertension and its complications through modification of the innate immune system and inflammation is increasingly likely. PPAR agonists may be beneficial, alone or in combination with other drugs that modify the inflammatory response, in treating hypertension, atherosclerosis and metabolic derangements associated with obesity.

Expression of ENaC subunits, chloride channels, and aquaporins in ovine fetal lung: ontogeny of expression and effects of altered fetal cortisol concentrations

Transition of the epithelium of the fetal lung from fluid secretion to fluid reabsorption requires changes in the expression of ion channels. Corticosteroids regulate expression of several of these channels, including the epithelium sodium channel (ENaC) subunits and aquaporins (AQP). We investigated the ontogenetic changes in these ion channels in the ovine fetal lung during the last half of gestation, a time of increasing adrenal maturation. Expression of the mRNAs for the chloride channels, cystic fibrosis transmembrane conductance regulator (CFTR), and chloride channel 2 (CLCN2) decreased with age. Expression of mRNAs for AQP1, AQP5, and for subunits of ENaC (alpha, beta, gamma) increased with age. In the fetal sheep the expression of ENaCbeta mRNA was dramatically higher than the expression of ENaCalpha or ENaCgamma, but expression of ENaCbeta protein decreased with maturation, although the ratio of the mature (112 kDa) to immature (102 kDa) ENaCbeta protein increased with age, particularly in the membrane fraction. In contrast, ENaCalpha mRNA and protein both increase with maturation, and the mature form of ENaCalpha (68 kDa) predominates at all ages. A modest increase in fetal cortisol, within the range expected to occur naturally in late gestation but prior to active labor, increased ENaCalpha mRNA but not ENaCbeta, ENaCgamma, or AQP mRNAs. We conclude that in the ovine fetal lung, appearance of functional sodium channels is associated with induction of ENACalpha and ENaCgamma, and that ENaCalpha expression may be induced by even small, preterm increases in fetal cortisol.

alpha-ENaC is a functional element of the hypertonicity-induced cation channel in HepG2 cells and it mediates proliferation

The molecular correlate of hypertonicity-induced cation channels (HICCs) and their role in proliferation vs. apoptosis is a matter of debate. We report in this paper that, in whole-cell patch-clamp recordings, hypertonic stress (340→450 mosM) reversibly increased the Na⁺ conductance of HepG2 cells from 0.8 to 5.8 nS. The effect was dose-dependently inhibited by flufenamate and amiloride, known blockers of HICCs, with some 50% efficiency at 300 μM. In parallel, both drugs decreased HepG2 cell proliferation [in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and with automatic cell counting]. Small interfering RNA (siRNA) silencing of the α-subunit of the epithelial Na⁺ channel (ENaC) reduced hypertonicity-induced Na⁺ currents to 60%, whereas the rate of HepG2 cell proliferation was approximately half of that of the control. Moreover, α-ENaC siRNA inhibited the regulatory volume increase of HepG2 cells (measured with scanning acoustic microscopy) by 60%. In florescence-activated cell sorting measurements, silencing of α-ENaC led to a significant decrease in the G1 and an increase in the G2/M phase of the cell cycle, whereas the S phase was not changing. Finally (determined by a caspase 3/7 assay), HICC inhibition by flufenamate and silencing of α-ENaC increased the rate of apoptosis in HepG2 cells. It is concluded that α-ENaC is one functional element of the HICC in HepG2 cells and that the channel is an important mediator of cell proliferation; likewise, HICC blockage shifts the system from a proliferative into a rather apoptotic one. This is the first report of a role of α-ENaC in cell proliferation.

Melatonin and ulcerative colitis: evidence, biological mechanisms, and future research

Ulcerative colitis (UC) is an inflammatory bowel disease that afflicts up to 1 million people in the US. Current treatments for UC are mostly nonspecific, not always effective, and often accompanied by serious side effects. Therefore, there is considerable interest in finding alternative and more tolerable treatments for this disease. Physiologic data suggest that melatonin is an important regulator of both inflammation and motility in the gastrointestinal tract, and data from in vitro studies, animal experiments, and limited studies in humans suggest that supplemental melatonin may have an ameliorative effect on colitis. In this review we summarize the evidence regarding melatonin as a possible therapeutic agent in UC and discuss possible biological mechanisms and directions for future research.

Cleavage in the {gamma}-subunit of the epithelial sodium channel (ENaC) plays an important role in the proteolytic activation of near-silent channels

The mechanisms by which proteases activate the epithelial sodium channel (ENaC) are not yet fully understood. We investigated the effect of extracellular proteases on rat ENaC heterologously expressed in Xenopus laevis oocytes. Application of trypsin increased ENaC whole-oocyte currents by about 8-fold without a concomitant increase in channel surface expression. The stimulatory effect of trypsin was preserved in oocytes expressing alphagamma-ENaC, but was abolished in oocytes expressing alphabeta-ENaC. Thus, the gamma-subunit appears to be essential for channel activation by extracellular proteases. Site-directed mutagenesis of a putative prostasin cleavage site in the extracellular loop of the gamma-subunit revealed that mutating the 181Lys residue to alanine (gammaK181A) increases ENaC baseline whole-oocyte currents, decreases channel surface expression, and largely reduces the stimulatory effect of extracellular proteases (trypsin, chymotrypsin and human neutrophil elastase). In single-channel recordings from outside-out patches we demonstrated that the gammaK181A mutation essentially abolishes the activation of near-silent channels by trypsin, while a stimulatory effect of trypsin on channel gating is preserved. This apparent dual effect of trypsin on channel gating and on the recruitment of near-silent channels was confirmed by experiments using the beta518C mutant ENaC which can be converted to a channel with an open probability of nearly one by exposure to a sulfhydryl reagent. Interestingly, the gammaK181A mutation results in the spontaneous appearance of a 67 kDa fragment of the gamma-subunit in the plasma membrane which can be prevented by a furin inhibitor and also occurs after channel activation by extracellular trypsin. This suggests that the mutation promotes channel cleavage and activation by endogenous proteases. This would lower the pool of near-silent channels and explain the constitutive activation and reduced responsiveness of the mutant channel to extracellular proteases. We conclude that the mutated site (K181A) affects a region in the gamma-subunit of ENaC that is functionally important for the activation of near-silent channels by extracellular proteases.

Protective effect of melatonin on Ca2+ homeostasis and contractility in acute cholecystitis

Impaired Ca2+ homeostasis and smooth muscle contractility co-exist in acute cholecystitis (AC) leading to gallbladder dysfunction. There is no pharmacological treatment for this pathological condition. Our aim was to evaluate the effects of melatonin treatment on Ca2+ signaling pathways and contractility altered by cholecystitis. [Ca2+]i was determined by epifluorescence microscopy in fura-2 loaded isolated gallbladder smooth muscle cells, and isometric tension was recorded from gallbladder muscle strips. Malondialdehyde (MDA) and reduced glutathione (GSH) contents were determined by spectrophotometry and cycloxygenase-2 (COX-2) expression was quantified by western blot. Melatonin was tested in two experimental groups, one of which underwent common bile duct ligation for 2 days and another that was later de-ligated for 2 days. Inflammation-induced impairment of Ca2+ responses to cholecystokinin and caffeine were recovered by melatonin treatment (30 mg/kg). This treatment also ameliorated the detrimental effects of AC on Ca2+ influx through both L-type and capacitative Ca2+ channels, and it was effective in preserving the pharmacological phenotype of these channels. Despite its effects on Ca2+ homeostasis, melatonin did not improve contractility. After de-ligation, Ca2+ influx and contractility were still impaired, but both were recovered by melatonin. These effects of melatonin were associated to a reduction of MDA levels, an increase in GSH content and a decrease in COX-2 expression. These findings indicate that melatonin restores Ca2+ homeostasis during AC and resolves inflammation. In addition, this indoleamine helps in the subsequent recovery of functionality.

Cardiovascular diseases: protective effects of melatonin

This brief review considers some of the cardiac diseases and conditions where free radicals and related reactants are believed to be causative. The report also describes the beneficial actions of melatonin against oxidative cardiovascular disorders. Based on the data available, melatonin seems to have cardioprotective properties via its direct free radical scavenger and its indirect antioxidant activity. Melatonin efficiently interacts with various reactive oxygen and reactive nitrogen species (receptor independent actions) and it also upregulates antioxidant enzymes and downregulates pro-oxidant enzymes (receptor-dependent actions). Moreover, melatonin enters all cells and subcellular compartments and crosses morphophysiologic barriers. These findings have implications for the protective effects of melatonin against cardiac diseases induced by oxidative stress. Melatonin attenuates molecular and cellular damages resulting from cardiac ischemia/reperfusion in which destructive free radicals are involved. Anti-inflammatory and antioxidative properties of melatonin are also involved in the protection against a chronic vascular disease, atherosclerosis. The administration of melatonin, as a result of its antioxidant features, has been reported to reduce hypertension and cardiotoxicity induced by clinically used drugs. The results described herein help to clarify the beneficial effects of melatonin against these conditions and define the potential clinical applicability of melatonin in cardiovascular diseases.

Melatonin ameliorates oxidative stress, inflammation, proteinuria, and progression of renal damage in rats with renal mass reduction

The progressive deterioration of renal function and structure resulting from renal mass reduction are mediated by a variety of mechanisms, including oxidative stress and inflammation. Melatonin, the major product of the pineal gland, has potent_antioxidant and anti-inflammatory properties, and its production is impaired in chronic renal failure. We therefore investigated if melatonin treatment would modify the course of chronic renal failure in the remnant kidney model. We studied rats followed 12 wk after renal ablation untreated (Nx group, n = 7) and treated with melatonin administered in the drinking water (10 mg/100 ml) (Nx + MEL group, n = 8). Sham-operated rats (n = 10) were used as controls. Melatonin administration increased 13-15 times the endogenous hormone levels. Rats in the Nx + MEL group had reduced oxidative stress (malondialdehyde levels in plasma and in the remnant kidney as well as nitrotyrosine renal abundance) and renal inflammation (p65 nuclear factor-kappaB-positive renal interstitial cells and infiltration of lymphocytes and macrophages). Collagen, alpha-smooth muscle actin, and transforming growth factor-beta renal abundance were all increased in the remnant kidney of the untreated rats and were reduced significantly by melatonin treatment. Deterioration of renal function (plasma creatinine and proteinuria) and structure (glomerulosclerosis and tubulointerstitial damage) resulting from renal ablation were ameliorated significantly with melatonin treatment. In conclusion, melatonin administration improves the course of chronic renal failure in rats with renal mass reduction. Further studies are necessary to define the potential usefulness of this treatment in other animal models and in patients with chronic renal disease.

Expression of the epithelial sodium channel in airway epithelium of newborn infants depends on gestational age

OBJECTIVE

In the newborn infant, removal of fetal lung liquid from the airways depends on ion transport through the airway epithelium. The epithelial sodium channel is considered rate limiting for the postnatal clearance of lung liquid, but it is unknown whether during the early postnatal period the expression of epithelial sodium channel is associated with maturity. Our objective was to study the relationship between gestational age and epithelial sodium channel expression in airway epithelium.

METHODS

In 90 newborn infants (preterm [gestational age < 37]: n = 29; term [gestational age > or = 37]: n = 61), we measured the expression of epithelial sodium channel (reported as attomoles of subunit expression normalized to femtomoles of expression of cytokeratin 18) in nasal epithelium at 1 to 5 and 22 to 28 hours after birth.

RESULTS

At 1 to 5 hours postnatally, airway expression of alpha-, beta-, and gamma-subunits of epithelial sodium channel was lower in preterm than in term infants. At this time point, significant correlations existed between gestational age and airway expression of alpha- and beta-epithelial sodium channel. By 22 to 28 hours after birth, only the expression of beta-epithelial sodium channel had decreased significantly in the preterm infants, whereas the expression of all epithelial sodium channel subunits had decreased significantly in the term infants. At this time point, no difference in expression of any of the subunits was found between preterm and term infants.

CONCLUSIONS

Airway expression of epithelial sodium channel at 1 to 5 hours of age is significantly lower in preterm than in term infants. Low postnatal expression of alpha-, beta-, and gamma-epithelial sodium channel subunits in the airway epithelium may contribute to the development of respiratory distress in the preterm infant.

Subunits alpha, beta and gamma of the epithelial Na+ channel (ENaC) are functionally related to the hypertonicity-induced cation channel (HICC) in rat hepatocytes

Specific small interfering RNA (siRNA) constructs were used to test for the functional relation of subunits alpha, beta, and gamma of the epithelial Na(+) channel (ENaC) to the hypertonicity-induced cation channel (HICC) in confluent rat hepatocytes. In current-clamp recordings, hypertonic stress (300 --> 400 mosM) increased membrane conductance from 75.4 +/- 9.4 to 91.1 +/- 11.2 pS (p < 0.001). The effect was completely blocked by 100 microM amiloride and reduced to 46, 30, and 45% of the control value by anti-alpha-, anti-beta-, and anti-gamma-rENaC siRNA, respectively. Scanning acoustic microscopy revealed an initial shrinkage of cells from 6.98 +/- 0.45 to 6.03 +/- 0.43 pl within 2 min. This passive response was then followed by a regulatory volume increase (RVI) by 0.42 +/- 0.05 pl (p < 0.001). With anti-alpha-, anti-beta-, and anti-gamma-rENaC siRNA, the volume response was reduced to 31, 31, and 36% of the reference level, respectively. It is concluded that all three subunits of the ENaC are functionally related to RVI and HICC activation in rat hepatocytes.

Mycophenolate mofetil administration reduces renal inflammation, oxidative stress, and arterial pressure in rats with lead-induced hypertension

Hypertension is a likely consequence of chronic lead exposure in humans, especially in association with reduced renal function and in high risk populations. Numerous studies have demonstrated that oxidative stress plays an important role in the pathogenesis of experimental lead-induced hypertension and we have shown recently that tubulointerstitial immune cell infiltration is a feature of chronic low-dose lead exposure. Since oxidative stress, renal inflammation, and angiotensin activity are closely linked characteristics in experimental models of hypertension, we decided to investigate whether lead-induced hypertension would be ameliorated by suppressing renal inflammation with the immunosuppressive drug mycophenolate mofetil (MMF). We studied rats exposed for 14 wk to lead acetate (100 ppm in the drinking water) that, in addition, received either MMF, 20 mg.kg(-1).day(-1) by gastric gavage (Pb.MMF group, n = 12) or vehicle (Pb group, n = 12). Control rats received MMF alone (n = 5) or neither lead nor MMF (n = 6). All rats were killed at the end of the experiment. Low-dose lead exposure resulted in mild to moderate tubular cell damage and a progressive increment in blood pressure, oxidative stress, interstitial accumulation of lymphocytes and macrophages, NF-kappaB activation, and increased renal angiotensin II level. The administration of MMF suppressed the tubulointerstitial accumulation of lymphocytes and macrophages and prevented the hypertension, oxidative stress, and NF-kappaB activation and reduced the heightened renal angiotensin content associated with chronic lead exposure. We conclude that interstitial inflammation plays an important role in lead-induced hypertension.

Melatonin as a potential antihypertensive treatment

The number of patients with well-controlled hypertension is alarmingly low worldwide and new approaches to treatment of increased blood pressure (BP) are being sought. Melatonin has a role in blood pressure regulation. The nighttime production of melatonin is found to be reduced in hypertensive individuals. Administration of melatonin decreased BP in several animal models of hypertension, in healthy men and women, and in patients with arterial hypertension. Most promising results were achieved in patients with non-dipping nighttime pressure, in which the circadian rhythm of BP variation is disturbed. Several potential mechanisms of BP reduction are considered. Melatonin can, via its scavenging and antioxidant nature, improve endothelial function with increased availability of nitric oxide exerting vasodilatory and hypotensive effects. Melatonin seems to interfere with peripheral and central autonomic system, with a subsequent decrease in the tone of the adrenergic system and an increase of the cholinergic system. Melatonin may act on BP also via specific melatonin receptors localized in peripheral vessels or in parts of central nervous system participating in BP control. With a large clinical trial using melatonin in hypertension treatment, many important questions could be answered, such as the dose of melatonin and regimen of its application, the choice of patients with greatest possible benefit from melatonin treatment, the potential of anti-remodeling effect of melatonin and the interaction of melatonin with other antihypertensive drugs.

Renal NF-κB activation and TNF-α upregulation correlate with salt-sensitive hypertension in Dahl salt-sensitive rats

Molecular mechanisms of salt-sensitive (SS) hypertension related to renal inflammation have not been defined. We seek to determine whether a high-salt (HS) diet induces renal activation of NF-κB and upregulation of TNF-α related to the development of hypertension in Dahl SS rats. Six 8-wk-old male Dahl SS rats received a HS diet (4%), and six Dahl SS rats received a low-sodium diet (LS, 0.3%) for 5 wk. In the end, mean arterial pressure was determined in conscious rats by continuous monitoring through a catheter placed in the carotid artery. Mean arterial pressure was significantly higher in the HS than the LS group (177.9 ± 3.7 vs. 109.4 ± 2.9 mmHg, P < 0.001). There was a significant increase in urinary albumin secretion in the HS group compared with the LS group (22.3 ± 2.6 vs. 6.1 ± 0.7 mg/day; P < 0.001). Electrophoretic mobility shift assay demonstrated that the binding activity of NF-κB p65 proteins in the kidneys of Dahl SS rats was significantly increased by 53% in the HS group compared with the LS group ( P = 0.007). ELISA indicated that renal protein levels of TNF-α, but not IL-6, interferon-γ, and CCL28, were significantly higher in the HS than the LS group (2.3 ± 0.8 vs. 0.7 ± 0.2 pg/mg; P = 0.036). We demonstrated that plasma levels of TNF-α were significantly increased by fivefold in Dahl SS rats on a HS diet compared with a LS diet. Also, we found that increased physiologically relevant sodium concentration (10 mmol/l) directly stimulated NF-κB activation in cultured human renal proximal tubular epithelial cells. These findings support the hypothesis that activation of NF-κB and upregulation of TNF-α are the important renal mechanisms linking proinflammatory response to SS hypertension.

Improvement of lung compliance during postnatal adaptation correlates with airway sodium transport

RATIONALE

Fetal lung liquid secretion is coupled with chloride transport into the lung lumen. The postnatal clearance of lung liquid is dependent on osmotic force generated by active sodium absorption.

OBJECTIVE

To study the interaction between airway epithelial sodium transport and postnatal lung function.

METHODS

We determined lung compliance and nasal transepithelial potential difference as a measure of airway ion transport and epithelial sodium channel gene expression in 41 healthy newborn infants during the first 50 h after birth.

MEASUREMENTS AND MAIN RESULTS

Lung compliance improved significantly during the study period, whereas nasal potential difference remained constant. There was a significant decrease in the expressions of beta and gamma subunits of the epithelial sodium channel. A positive correlation existed between amiloride-sensitive nasal potential difference measured at 1-4 h of age and lung compliance at 21-27 h of age. We found no correlation between the molecular data and functional measurements.

CONCLUSIONS

An important part of pulmonary adaptation takes place during the first hour after birth. The improvement of lung compliance continues over the first postnatal days and coincides with down-regulation of epithelial sodium channel beta and gamma subunit expression.

cAMP increases surface expression of NKCC2 in rat thick ascending limbs: role of VAMP

NaCl absorption by the thick ascending limb of Henle's loop (TAL) is mediated by the apical Na-K-2Cl cotransporter NKCC2. cAMP increases NaCl absorption in the TAL by stimulating NKCC2. In oocytes, cAMP increases NKCC2 activity by regulating its trafficking. However, the mechanism by which cAMP stimulates NKCC2 in TALs is not clear. We hypothesized that cAMP increases surface expression of NKCC2 and NaCl absorption in TALs and that vesicle-associated membrane protein (VAMP) is involved in this mechanism. We used surface biotinylation of rat medullary TALs (mTAL) to examine surface and total NKCC2 levels. When mTAL suspensions were treated with dibutyryl cAMP (db-cAMP) or forskolin plus IBMX for 20 min, surface NKCC2 expression increased by 126 +/- 23 and 92 +/- 17% above basal, respectively (P < 0.03). No changes in total NKCC2 expression were observed, suggesting that cAMP increased translocation of NKCC2. We studied the role of VAMP in NKCC2 translocation and found that incubating mTALs with tetanus toxin (30 nM), which inhibits vesicle trafficking by inactivating VAMP-2 and -3, completely blocked the stimulatory effect of db-cAMP on surface NKCC2 expression (tetanus toxin = 100% vs. tetanus toxin + db-cAMP = 102 +/- 21% of control; not significant). We studied VAMP-2 and -3 expression and localization in isolated perfused TALs by confocal microscopy and found that both of them were located in the subapical space of the TAL. Finally, in isolated perfused mTALs, db-cAMP increased net Cl absorption by 95.0 +/- 34.8% (P < 0.03), and pretreatment of TALs with tetanus toxin blocked the stimulation of Cl absorption (from 110.9 +/- 15.9 to 109.7 +/- 15.6 pmol.min(-1).mm(-1); not significant). We concluded that cAMP increases NKCC2 surface expression by a mechanism involving VAMP and that NKCC2 trafficking to the apical membrane is involved in the stimulation of TAL NaCl absorption by cAMP.

Thiazolidinediones expand body fluid volume through PPARgamma stimulation of ENaC-mediated renal salt absorption

Thiazolidinediones (TZDs) are widely used to treat type 2 diabetes mellitus; however, their use is complicated by systemic fluid retention. Along the nephron, the pharmacological target of TZDs, peroxisome proliferator-activated receptor-gamma (PPARgamma, encoded by Pparg), is most abundant in the collecting duct. Here we show that mice treated with TZDs experience early weight gain from increased total body water. Weight gain was blocked by the collecting duct-specific diuretic amiloride and was also prevented by deletion of Pparg from the collecting duct, using Pparg (flox/flox) mice. Deletion of collecting duct Pparg decreased renal Na(+) avidity and increased plasma aldosterone. Treating cultured collecting ducts with TZDs increased amiloride-sensitive Na(+) absorption and Scnn1g mRNA (encoding the epithelial Na(+) channel ENaCgamma) expression through a PPARgamma-dependent pathway. These studies identify Scnn1g as a PPARgamma target gene in the collecting duct. Activation of this pathway mediates fluid retention associated with TZDs, and suggests amiloride might provide a specific therapy.

Subunits of the epithelial sodium channel family are differentially expressed in the retina of mice with ocular hypertension

Glaucoma is a prevalent cause of blindness, resulting in the apoptotic death of retinal ganglion cells and optic nerve degeneration. The disease is often associated with elevated intraocular pressure, however, molecular mechanisms involved in ganglion cell death are poorly understood. To identify proteins contributing to this pathological process, we analysed the retinal gene expression of DBA/2J mice that develop an elevated intraocular pressure by the age of 6 months with subsequent ganglion cell loss. In this study, we identified subunits of the epithelial sodium channel (ENaC) family that are specifically expressed under elevated intraocular pressure. Using reverse transcriptase polymerase chain reaction we observed a significant increase of alpha-ENaC in the neuronal retina of DBA/2J mice when compared with control animals, while beta-ENaC and gamma-ENaC were not detectable in this tissue. Specific immune sera to ENaC subunits showed up-regulation of alpha-ENaC in synaptic and nuclear layers of the retina, and in the retinal pigment epithelium. Consistent with our polymerase chain reaction data, beta-ENaC was not detected by specific antibodies in the retina, while gamma-ENaC was only present in the retinal pigment epithelium under ocular hypertension. Finally, the increase of alpha-ENaC gene expression in the neuronal retina and the retinal pigment epithelium was not observed in other tissues of DBA/2J mice. Since the intraocular pressure is regulated by the transport of aqueous humour across epithelial structures of the eye that in turn is associated with ion flux, the specific up-regulation of ENaC proteins could serve as a protecting mechanism against elevated intraocular pressure.

Inhibition of TRPC5 channels by Ca2+-binding protein 1 in Xenopus oocytes

The transient receptor potential canonical type 5 (TRPC5) channel is a member of the channels that has been implicated in neurite extension and growth cone morphology of hippocampal neurons. Although homomeric TRPC5 channels are activated following stimulation of G(q/11)-coupled receptors, the exact mechanism for this activation remains unresolved. Using two-electrode voltage clamp recordings, we show that the activity of TRPC5 channels expressed in Xenopus oocytes is dependent on the presence of Ca2+ at the extracellular as well as the cytoplasmic side of the plasma membrane. TRPC5 was activated by the stimulation of coexpressed M5 muscarinic receptors or by ionomycin. The TRPC5 activity was detectable with the presence of submillimolar levels of extracellular Ca2+, but it was eliminated by the injection of 5 mM 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid into the oocytes. Lanthanum could substitute for extracellular Ca2+ to support TRPC5 activity. Coexpression of Ca2+-binding protein 1 (CaBP1), but not calmodulin (CaM), inhibited the TRPC5 activity, without affecting the cell surface expression of TRPC5 proteins. Using in vitro binding assays, we demonstrated direction interactions between CaBP1 and TRPC5. The CaBP1-binding sites at the C terminus of TRPC5 are closely localized, but not identical, to CaM-binding sites. We conclude that TRPC5 is a Ca2+-regulated channel, and its activity is negatively controlled by CaBP1.

Tempol improves vascular function in the mesenteric vascular bed of senescent rats

Ageing is associated with structural and functional alterations of the vasculature. The nature of age-related vascular disorders is not completely understood. Oxidative stress is hypothesized to play a crucial role in the pathophysiology of vascular complications. We investigated the effects of chronic treatment with the superoxide dismutase mimetic tempol (4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl) on vascular function in the mesenteric vasculature of aged rats. Young (3 weeks) and old (40 weeks) Sprague-Dawley rats were treated with tempol (1 mM in drinking water) or vehicle for 3 weeks. Arterial blood pressure was slightly, but significantly, higher in old than in young rats. Tempol had no effect on arterial blood pressure. The vasoconstrictor responses to norepinephrine (NE) and serotonin (5-HT) were exaggerated in the mesenteric vascular bed (MVB) removed from old rats. Vasodilator responses to acetylcholine (ACh), papaverine (PPV), and isoprenaline (ISO) were reduced in the MVB of old rats in comparison with young rats. Chronic treatment of old rats with tempol normalized their responses to NE and 5-HT. The dilator responses to ACh, PPV, and ISO were similar between old rats receiving tempol and young rats. The present findings suggest that oxidative stress contributes to vascular dysfunction in the mesentery of old rats. The vasculoprotective effects of tempol remain to be elucidated.

Evolution of renal interstitial inflammation and NF-kappaB activation in spontaneously hypertensive rats

Lymphocytes and macrophages infiltrate the kidney of spontaneously hypertensive rats (SHR) and interventions leading to their reduction are associated with improvement of the hypertension. The present studies examined the evolution of the interstitial inflammation in the natural course of the SHR to gain insight on the potential role of interstitial immune cell accumulation in the development of hypertension. We studied SHR and control WKY rats at 3 weeks (SHR-3 wk group, n = 11 and WKY-3 wk group, n = 10), 11 weeks (SHR-11 wk group, n = 5 and WKY-11 wk group, n = 5) and 24 weeks (SHR-24 wk group, n = 10 and WKY-24 wk group, n = 10). The SHR-3 wk group was normotensive and older SHR developed hypertension that was severe in the SHR-24 wk group. Tubulointerstitial accumulation of lymphocytes, macrophages, angiotensin II-positive cells, cells expressing the p65 DNA-binding subunit of NF-kappaB and activation of NF-kappaB in the kidney were all significantly increased (p < 0.01) in the prehypertensive SHR-3 wk group and augmented progressively, with the highest values in the SHR-24 wk group. The SHR-24 wk group showed increased (p < 0.001) helper (CD4) T cell infiltration and a high CD4/CD8 ratio. These findings are consistent with the possibility that activation of NF-kappaB and renal interstitial infiltration of immune cells may be part of the pathophysiologic process that drives hypertension in the SHR.

Roles of oxidative stress and antioxidant therapy in chronic kidney disease and hypertension

PURPOSE OF REVIEW

Oxidative stress is frequently associated with, and is partly involved in, the pathogenesis of chronic renal failure, hypertension and their complications. In the past few years, considerable progress has been made in deciphering the impact and the molecular mechanism of oxidative stress in these disorders. This article is intended to provide an overview of oxidative stress in hypertension and chronic renal failure.

RECENT FINDINGS

Recent studies have provided irrefutable evidence that oxidative stress can cause hypertension and hypertension can cause oxidative stress. The upregulation of nicotinamide adenine dinucleotide phosphate (reduced form) oxidase and the tubulointerstitial accumulation of activated T cells, macrophages and superoxide-producing cells are partly responsible for oxidative stress in several models of hypertension. Antioxidant therapy alleviates hypertension, averts nuclear factor kappa B activation, and mitigates tubulointerstitial inflammation in hypertensive animals. Oxidative stress contributes to hypertension, endothelial dysfunction and brain disorders in chronic renal failure animals, and is partly caused by the upregulation of nicotinamide adenine dinucleotide phosphate (reduced form) oxidase and the downregulation of superoxide dismutase.

SUMMARY

Oxidative stress, hypertension and inflammation are closely interrelated and involve a spiralling vicious cycle that can lead to progressive deterioration of hypertension and target organ damage.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Oxidative stress, renal infiltration of immune cells, and salt-sensitive hypertension: all for one and one for all

Recent evidence indicates that interstitial infiltration of T cells and macrophages plays a role in the pathogenesis of salt-sensitive hypertension. The present review examines this evidence and summarizes the investigations linking the renal accumulation of immune cells and oxidative stress in the development of hypertension. The mechanisms involved in the hypertensive effects of oxidant stress and tubulointerstitial inflammation, in particular intrarenal ANG II activity, are discussed, focusing on their potential for sodium retention. The possibility of autoimmune reactivity in hypertension is raised in the light of the proinflammatory and immunogenic pathways stimulated by the interrelationship between oxidant stress and inflammatory response. Finally, we present some clinical considerations derived from the recognition of this interrelationship.

Mutations in the extracellular loop of alpha-rENaC alter sensitivity to amiloride and reactive species

We studied the effects of two mutations of the extracellular loop of the alpha-subunit of the (ENaC) on amiloride-sensitive current in Xenopus laevis oocytes and the inhibition of this current by 3-morpholinosydnonimine (SIN-1). Injection of oocytes with wild-type (wt) alpha-,beta-,gamma-rENaC cRNA (8.3 ng/subunit) resulted 48-72 h later in inward Na(+) currents (-5.5 +/- 0.8 microA; means +/- SE at -100 mV; n = 21), which were completely inhibited by amiloride. Oocytes injected with either alpha(Y279A)- or alpha(Y283A)- and beta-,gamma-rENaC cRNAs had significantly lower Na(+) currents. Furthermore, alpha(Y279A)-,beta-,gamma-rENaC-injected oocytes had a higher K(i) for amiloride (0.54 +/- 0.97 vs. 0.10 +/- 0.04 microM; P < 0.01). Exposure of oocytes to SIN-1 (1 mM) for 5 min decreased both total Na(+) and amiloride-sensitive currents across wt and alpha(Y279A)- but not alpha(Y283A)-,beta-,gamma-rENaC. Furthermore, exposure to SIN-1 increased the K(i) for amiloride across wt but not alpha(Y279A)-,beta-,gamma-rENaC-injected oocytes. These data indicate that both tyrosines are important for proper ENaC function and their oxidative modifications contribute to altered ENaC function.

Noncoordinate regulation of ENaC: paradigm lost?

The epithelial sodium channel (ENaC) is composed of the three homologous subunits α, β, and γ. The basic oligomerization process inferred from all studies in heterologous systems is preferential assembly of the three subunits into a single oligomeric form. However, there is also considerable evidence that channels composed of only α-, αβ-, or αγ-subunits can form under some circumstances and that individual subunits expressed in heterologous systems can traffic to the cell membrane. In cells that express endogenous ENaC, the three subunits are often synthesized in a differential fashion, with one or two subunits expressed constitutively while the other(s) are induced by different physiological stimuli in parallel with increased ENaC activity. This phenomenon, which we term noncoordinate regulation, has been observed for both whole cell and apical membrane ENaC subunit expression. Several other heteromeric membrane proteins have also been observed to have differential rates of either turnover or trafficking of individual subunits after biosynthesis and membrane localization. Here, we examine the possibility that noncoordinate regulation of ENaC subunits may represent another mechanism in the arsenal of physiological responses to diverse stimuli.