Aldosterone-independent regulation of the epithelial Na+ channel (ENaC) by vasopressin in adrenalectomized mice

Bile acid receptors and renal regulation of water homeostasis

The kidney is the key organ responsible for maintaining the body's water and electrolyte homeostasis. About 99% of the primary urine filtered from the Bowman's capsule is reabsorbed along various renal tubules every day, with only 1-2 L of urine excreted. Aquaporins (AQPs) play a vital role in water reabsorption in the kidney. Currently, a variety of molecules are found to be involved in the process of urine concentration by regulating the expression or activity of AQPs, such as antidiuretic hormone, renin-angiotensin-aldosterone system (RAAS), prostaglandin, and several nuclear receptors. As the main bile acid receptors, farnesoid X receptor (FXR) and membrane G protein-coupled bile acid receptor 1 (TGR5) play important roles in bile acid, glucose, lipid, and energy metabolism. In the kidney, FXR and TGR5 exhibit broad expression across all segments of renal tubules, and their activation holds significant therapeutic potential for numerous acute and chronic kidney diseases through alleviating renal lipid accumulation, inflammation, oxidative stress, and fibrosis. Emerging evidence has demonstrated that the genetic deletion of FXR or TGR5 exhibits increased basal urine output, suggesting that bile acid receptors play a critical role in urine concentration. Here, we briefly summarize the function of bile acid receptors in renal water reabsorption and urine concentration.

Sodium Homeostasis, a Balance Necessary for Life

Body sodium (Na) levels must be maintained within a narrow range for the correct functioning of the organism (Na homeostasis). Na disorders include not only elevated levels of this solute (hypernatremia), as in diabetes insipidus, but also reduced levels (hyponatremia), as in cerebral salt wasting syndrome. The balance in body Na levels therefore requires a delicate equilibrium to be maintained between the ingestion and excretion of Na. Salt (NaCl) intake is processed by receptors in the tongue and digestive system, which transmit the information to the nucleus of the solitary tract via a neural pathway (chorda tympani/vagus nerves) and to circumventricular organs, including the subfornical organ and area postrema, via a humoral pathway (blood/cerebrospinal fluid). Circuits are formed that stimulate or inhibit homeostatic Na intake involving participation of the parabrachial nucleus, pre-locus coeruleus, medial tuberomammillary nuclei, median eminence, paraventricular and supraoptic nuclei, and other structures with reward properties such as the bed nucleus of the stria terminalis, central amygdala, and ventral tegmental area. Finally, the kidney uses neural signals (e.g., renal sympathetic nerves) and vascular (e.g., renal perfusion pressure) and humoral (e.g., renin-angiotensin-aldosterone system, cardiac natriuretic peptides, antidiuretic hormone, and oxytocin) factors to promote Na excretion or retention and thereby maintain extracellular fluid volume. All these intake and excretion processes are modulated by chemical messengers, many of which (e.g., aldosterone, angiotensin II, and oxytocin) have effects that are coordinated at peripheral and central level to ensure Na homeostasis.

Blood pressure effects of sodium transport along the distal nephron

The mammalian distal nephron is a target of highly effective antihypertensive drugs. Genetic variants that alter its transport activity are also inherited causes of high or low blood pressure, clearly establishing its central role in human blood pressure regulation. Much has been learned during the past 25 years about salt transport along this nephron segment, spurred by the cloning of major transport proteins and the discovery of disease-causing genetic variants. Recognition is increasing that substantial cellular and segmental heterogeneity is present along this segment, with electroneutral sodium transport dominating more proximal segments and electrogenic sodium transport dominating more distal segments. Coupled with recent insights into factors that modulate transport along these segments, we now understand one important mechanism by which dietary potassium intake influences sodium excretion and blood pressure. This finding has solved the aldosterone paradox, by demonstrating how aldosterone can be both kaliuretic, when plasma potassium is elevated, and anti-natriuretic, when extracellular fluid volume is low. However, what also has become clear is that aldosterone itself only stimulates a portion of the mineralocorticoid receptors along this segment, with the others being activated by glucocorticoid hormones instead. These recent insights provide an increasingly clear picture of how this short nephron segment contributes to blood pressure homeostasis and have important implications for hypertension prevention and treatment.

Chronic activation of vasopressin-2 receptors induces hypertension in Liddle mice by promoting Na+ and water retention

The renin-angiotensin-aldosterone and arginine vasopressin-V2 receptor-aquaporin-2 (AQP2) systems converge on the epithelial Na+ channel (ENaC) to regulate blood pressure and plasma tonicity. Although it is established that V2 receptors initiate renal water reabsorption through AQP2, whether V2 receptors can also induce renal Na+ retention through ENaC and raise blood pressure remains an open question. We hypothesized that a specific increase in V2 receptor-mediated ENaC activity can lead to high blood pressure. Our approach was to test effects of chronic activation of V2 receptors in Liddle mice, a genetic mouse model of high ENaC activity, and compare differences in ENaC activity, urine Na+ excretion, and blood pressure with control mice. We found that ENaC activity was elevated in Liddle mice and could not be stimulated further by administration of desmopressin (dDAVP), a V2 receptor-specific agonist. In contrast, Liddle mice showed higher levels of expression of AQP2 and aquaporin-3, but they could still respond to dDAVP infusion by increasing phospho-AQP2 expression. With dDAVP infusion, Liddle mice excreted smaller urine volume and less urine Na+ and developed higher blood pressure compared with control mice; this hypertension was attenuated with administration of the ENaC inhibitor benzamil. We conclude that V2 receptors contribute to hypertension in the Liddle mouse model by promoting primary Na+ and concomitant water retention.NEW & NOTEWORTHY Liddle syndrome is a classic model for hypertension from high epithelial Na+ channel (ENaC) activity. In the Liddle mouse model, vasopressin-2 receptors stimulate both ENaC and aquaporin-2, which increases Na+ and water retention to such an extent that hypertension ensues. Liddle mice will preserve plasma tonicity at the expense of a higher blood pressure; these data highlight the inherent limitation in which the kidney must use ENaC as a pathway to regulate both plasma tonicity and blood pressure.

Directing two-way traffic in the kidney: A tale of two ions

The kidneys regulate levels of Na+ and K+ in the body by varying urinary excretion of the electrolytes. Since transport of each of the two ions can affect the other, controlling both at the same time is a complex task. The kidneys meet this challenge in two ways. Some tubular segments change the coupling between Na+ and K+ transport. In addition, transport of Na+ can shift between segments where it is coupled to K+ reabsorption and segments where it is coupled to K+ secretion. This permits the kidney to maintain electrolyte balance with large variations in dietary intake.

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-Sensitive Hypertension of the Renal Tubular Cell-Specific NFAT5 (Nuclear Factor of Activated T-Cells 5) Knockout Mice

[Figure: see text].

Stimulation of the Epithelial Na+ Channel in Renal Principal Cells by Gs-Coupled Designer Receptors Exclusively Activated by Designer Drugs

The activity of the Epithelial Na⁺ Channel (ENaC) in renal principal cells (PC) fine-tunes sodium excretion and consequently, affects blood pressure. The Gs-adenylyl cyclase-cAMP signal transduction pathway is believed to play a central role in the normal control of ENaC activity in PCs. The current study quantifies the importance of this signaling pathway to the regulation of ENaC activity in vivo using a knock-in mouse that has conditional expression of Gs-DREADD (designer receptors exclusively activated by designer drugs; GsD) in renal PCs. The GsD mouse also contains a cAMP response element-luciferase reporter transgene for non-invasive bioluminescence monitoring of cAMP signaling. Clozapine N-oxide (CNO) was used to selectively and temporally stimulate GsD. Treatment with CNO significantly increased luciferase bioluminescence in the kidneys of PC-specific GsD but not control mice. CNO also significantly increased the activity of ENaC in principal cells in PC-specific GsD mice compared to untreated knock-in mice and CNO treated littermate controls. The cell permeable cAMP analog, 8-(4-chlorophenylthio)adenosine 3′,5′-cyclic monophosphate, significantly increased the activity and expression in the plasma membrane of recombinant ENaC expressed in CHO and COS-7 cells, respectively. Treatment of PC-specific GsD mice with CNO rapidly and significantly decreased urinary Na⁺ excretion compared to untreated PC-specific GsD mice and treated littermate controls. This decrease in Na⁺ excretion in response to CNO in PC-specific GsD mice was similar in magnitude and timing as that induced by the selective vasopressin receptor 2 agonist, desmopressin, in wild type mice. These findings demonstrate for the first time that targeted activation of Gs signaling exclusively in PCs is sufficient to increase ENaC activity and decrease dependent urinary Na⁺ excretion in live animals.

The physiological and molecular mechanisms to maintain water and salt homeostasis in response to high salt intake in Mongolian gerbils (Meriones unguiculatus)

Desert rodents are faced with many challenges such as high dietary salt in their natural habitats and they have evolved abilities to conserve water and tolerate salt. However, the physiological and molecular mechanisms involved in water and salt balances in desert rodents are unknown. We hypothesized that desert rodents regulated water and salt balances by altering the expression of AQP2 and α-ENaC in the kidney. Mongolian gerbils (Meriones unguiculatus), a desert species, were acclimated to drinking water with different salt contents: (0, control; 4% NaCl, moderate salt, MS; 8% NaCl, high salt, HS) for 4 weeks. The gerbils drinking salty water had lower body mass, food intake, water intake, metabolic water production and urine volume. The HS gerbils increased the expression of arginine vasopressin (AVP) in the hypothalamus, and also enhanced the expression of AQP2 and cAMP/PKA/CREB signaling pathway in the kidney. In addition, these gerbils reduced serum aldosterone levels and α-ENaC expression in the kidney. Creatinine clearance was lower in the HS group than that in the control group, but serum and urine creatinine levels did not change. These data indicate that desert rodents rely on AVP-dependent upregulation of AQP2 and aldosterone-dependent downregulation of α-ENaC in the kidney to promote water reabsorption and sodium excretion under high salt intake.

Interactive Actions of Aldosterone and Insulin on Epithelial Na+ Channel Trafficking

Epithelial Na⁺ channel (ENaC) participates in renal epithelial Na⁺ reabsorption, controlling blood pressure. Aldosterone and insulin elevate blood pressure by increasing the ENaC-mediated Na⁺ reabsorption. However, little information is available on the interactive action of aldosterone and insulin on the ENaC-mediated Na⁺ reabsorption. In the present study, we tried to clarify if insulin would modify the aldosterone action on the ENaC-mediated Na⁺ reabsorption from a viewpoint of intracellular ENaC trafficking. We measured the ENaC-mediated Na⁺ transport as short-circuit currents using a four-state mathematical ENaC trafficking model in renal A6 epithelial cells with or without aldosterone treatment under the insulin-stimulated and -unstimulated conditions. We found that: (A) under the insulin-stimulated condition, aldosterone treatment (1 µM for 20 h) significantly elevated the ENaC insertion rate to the apical membrane (kI) 3.3-fold and the ENaC recycling rate (kR) 2.0-fold, but diminished the ENaC degradation rate (kD) 0.7-fold without any significant effect on the ENaC endocytotic rate (kE); (B) under the insulin-unstimulated condition, aldosterone treatment decreased kE 0.5-fold and increased kR 1.4-fold, without any significant effect on kI or kD. Thus, the present study indicates that: (1) insulin masks the well-known inhibitory action of aldosterone on the ENaC endocytotic rate; (2) insulin induces a stimulatory action of aldosterone on ENaC apical insertion and an inhibitory action of aldosterone on ENaC degradation; (3) insulin enhances the aldosterone action on ENaC recycling; (4) insulin has a more effective action on diminution of ENaC endocytosis than aldosterone.

Effect of Angiotensin II on ENaC in the Distal Convoluted Tubule and in the Cortical Collecting Duct of Mineralocorticoid Receptor Deficient Mice

Background Angiotensin II stimulates epithelial Na+ channel (ENaC) by aldosterone-independent mechanism. We now test the effect of angiotensin II on ENaC in the distal convoluted tubule (DCT) and cortical collecting duct (CCD) of wild-type (WT) and kidney-specific mineralocorticoid receptor knockout mice (KS-MR-KO). Methods and Results We used electrophysiological, immunoblotting and renal-clearance methods to examine the effect of angiotensin II on ENaC in KS-MR-KO and wild-type mice. High K+ intake stimulated ENaC in the late DCT/early connecting tubule (DCT2/CNT) and in the CCD whereas low sodium intake stimulated ENaC in the CCD but not in the DCT2/CNT. The deletion of MR abolished the stimulatory effect of high K+ and low sodium intake on ENaC, partially inhibited ENaC in DCT2/CNT but almost abolished ENaC activity in the CCD. Application of losartan inhibited ENaC only in DCT2/CNT of both wild-type and KS-MR-KO mice but not in the CCD. Angiotensin II infusion for 3 days has a larger stimulatory effect on ENaC in the DCT2/CNT than in the CCD. Three lines of evidence indicate that angiotensin II can stimulate ENaC by MR-independent mechanism: (1) angiotensin II perfusion augmented ENaC expression in KS-MR-KO mice; (2) angiotensin II stimulated ENaC in the DCT2/CNT but to a lesser degree in the CCD in KS-MR-KO mice; (3) angiotensin II infusion augmented benzamil-induced natriuresis, increased the renal K+ excretion and corrected hyperkalemia of KS-MR-KO mice. Conclusions Angiotensin II-induced stimulation of ENaC occurs mainly in the DCT2/CNT and to a lesser degree in the CCD and MR plays a dominant role in determining ENaC activity in the CCD but to a lesser degree in the DCT2/CNT.

Greater natriuretic response to ENaC inhibition in male versus female Sprague-Dawley rats

Genes for the epithelial sodium channel (ENaC) subunits are expressed in a circadian manner, but whether this results in time-of-day differences in activity is not known. Recent data show that protein expression of ENaC subunits is higher in kidneys from female rats, yet females are more efficient in excreting an acute salt load. Thus, our in vivo study determined whether there is a time-of-day difference as well as a sex difference in the response to ENaC inhibition by benzamil. Our results showed that the natriuretic and diuretic responses to a single dose of benzamil were significantly greater in male compared with female rats whether given at the beginning of the inactive period [Zeitgeber time 0 (ZT0), 7 AM] or active period (ZT12, 7 PM). However, the response to benzamil was not significantly different between ZT0 and ZT12 dosing in either male or female rats. There was no difference in renal cortical α-ENaC protein abundance between ZT0 and ZT12 or males and females. Given previous reports of flow-induced stimulation of endothelin-1 (ET-1) production and sex differences in the renal endothelin system, we measured urinary ET-1 excretion to assess the effects of increased urine flow on intrarenal ET-1. ET-1 excretion was significantly increased following benzamil administration in both sexes, but this increase was significantly greater in females. These results support the hypothesis that ENaC activity is less prominent in maintaining Na⁺ balance in females independent of renal ET-1. Because ENaC subunit genes and protein expression vary by time of day and are greater in female rat kidneys, this suggests a clear disconnect between ENaC expression and channel activity.

Vasopressin inactivation: Role of insulin-regulated aminopeptidase

The physiological importance of vasopressin inactivation has long been appreciated, but the mechanisms and potential pathophysiologic roles of this process remain active subjects of research. Human Placental Leucine Aminopeptidase (P-LAP, encoded by the LNPEP gene) is an important determinant of vasopressinase activity during pregnancy and is associated with gestational diabetes insipidus and preeclampsia. Insulin-Regulated Aminopeptidase (IRAP), the rodent homologue of P-LAP, is coregulated with the insulin-responsive glucose transporter, GLUT4, in adipose and muscle cells. Recently, the Tether containing a UBX domain for GLUT4 (TUG) protein was shown to mediate the coordinated regulation of water and glucose homeostasis. TUG sequesters IRAP and GLUT4 intracellularly in the absence of insulin. Insulin and other stimuli cause the proteolytic cleavage of TUG to mobilize these proteins to the cell surface, where IRAP acts to terminate the activity of circulating vasopressin. Intriguingly, genetic variation in LNPEP is associated with the vasopressin response and mortality during sepsis, and increased copeptin, a marker of vasopressin secretion, is associated with cardiovascular and metabolic disease. We propose that in the setting of insulin resistance in muscle, increased cell-surface IRAP and accelerated vasopressin degradation cause a compensatory increase in vasopressin secretion. The increased vasopressin concentrations present at the kidneys then contribute to hypertension in the metabolic syndrome. Further analyses of metabolism and of vasopressin and copeptin may yield novel insights into a unified pathophysiologic mechanism linking insulin resistance and hypertension, and potentially other components of the metabolic syndrome, in humans.

Podocyte Injury Augments Intrarenal Angiotensin II Generation and Sodium Retention in a Megalin-Dependent Manner

We have previously shown that podocyte injury increases the glomerular filtration of liver-derived Agt (angiotensinogen) and the generation of intrarenal Ang II (angiotensin II) and that the filtered Agt is reabsorbed by proximal tubules in a manner dependent on megalin. In the present study, we aimed to study the role of megalin in the generation of renal Ang II and sodium handling during nephrotic syndrome. We generated proximal tubule-specific megalin KO (knockout) mice and crossed these animals with NEP25 mice, in which podocyte-specific injury can be induced by injection of the immunotoxin LMB2. Without podocyte injury, renal Agt staining was markedly diminished and urinary Agt increased in KO mice. However, renal Ang II was similar between KO and control mice on average: 117 (95% CI, 101-134) versus 101 (95% CI, 68-133) fmol/g tissue. We next tested the effect of megalin KO on intrarenal Ang II generation with podocyte injury. Control NEP25 mice showed markedly increased renal Agt staining and renal Ang II levels: 450 (336-565) fmol/g tissue. Megalin KO/NEP25 mice showed markedly diminished Agt reabsorption and attenuated renal Ang II: 199 (156-242) fmol/g tissue (P<0.001). Compared with control NEP25 mice, megalin KO/NEP25 mice excreted 5-fold more sodium in the urine. Western blot analysis showed that megalin KO decreased NHE3 and the cleaved α and γ forms of Epithelial Na Channel. These data indicate that Agt reabsorbed by proximal tubules via megalin in nephrotic syndrome is converted to Ang II, which may contribute to sodium retention and edema formation by activating NHE3 and Epithelial Na Channel.

Renal Na+ excretion consequent to pharmacogenetic activation of Gq-DREADD in principal cells

Stimulation of metabotropic Gq-coupled purinergic P2Y2 receptors decreases activity of the epithelial Na+ channel (ENaC) in renal principal cells of the distal nephron. The physiological consequences of P2Y2 receptor signaling disruption in the P2Y2 receptor knockout mouse are decreased Na+ excretion and increased arterial blood pressure. However, because of the global nature of this knockout model, the quantitative contribution of ENaC and distal nephron compared with that of upstream renal vascular and tubular elements to changes in urinary excretion and arterial blood pressure is obscure. Moreover, it is uncertain whether stimulation of P2Y2 receptor inhibition of ENaC is sufficient to drive renal (urinary) Na+ excretion (UNaV). Here, using a pharmacogenetic approach and selective agonism of the P2Y2 receptor, we test the sufficiency of targeted stimulation of Gq signaling in principal cells of the distal nephron and P2Y2 receptors to increase UNaV. Selective stimulation of the P2Y2 receptor with the ligand MRS2768 decreased ENaC activity in freshly isolated tubules (as assessed by patch-clamp electrophysiology) and increased UNaV (as assessed in metabolic cages). Similarly, selective agonism of hM3Dq-designer receptors exclusively activated by designer drugs (DREADD) restrictively expressed in principal cells of the distal nephron with clozapine- N-oxide decreased ENaC activity and, consequently, increased UNaV. Clozapine- N-oxide, when applied to control littermates, failed to affect ENaC and UNaV. This study represents the first use of pharmacogenetic (DREADD) technology in the renal tubule and demonstrated that selective activation of the P2Y2 receptor and Gq signaling in principal cells is sufficient to promote renal salt excretion.

Adenine acts in the kidney as a signaling factor and causes salt- and water-losing nephropathy: early mechanism of adenine-induced renal injury

Chronic adenine feeding is extensively used to develop animal models of chronic renal failure with metabolic features resembling those observed in humans. However, the mechanism by which adenine induces renal failure is poorly understood. In this study, we examined the early effects of adenine on water metabolism and salt balance in rats placed in metabolic cages and fed control or adenine-containing diets for 7 days. Molecular and functional studies demonstrated that adenine-fed rats exhibited a significant reduction in food intake, polyuria, polydipsia, decreased urine osmolality, and increased salt wasting. These effects are independent of changes in food intake and result from a coordinated downregulation of water channel aquaporin-2 (AQP2) and salt transporter (Na⁺-K⁺-Cl^- cotransporter 2; NKCC2) in the collecting duct and medullary thick ascending limb, respectively. As a result, adenine-fed rats exhibited massive volume depletion, as indicated by a significant body weight loss, increased blood urea nitrogen, and increased hematocrit and hemoglobin levels, all of which were significantly corrected with NaCl replacement. Adenine-induced urinary concentrating defect was not corrected by exogenous arginine vasopressin (AVP), and it correlated with reduced cAMP production in vivo and in vitro. In conclusion, adenine acts on renal tubules as a signaling molecule and causes nephrogenic diabetes insipidus with salt wasting, at least, by directly interfering with AVP V2 receptor signaling with subsequent downregulation of NKCC2 and AQP2 in the kidney. The combination of renal fluid loss and decreased food intake with subsequent massive volume depletion likely plays an important role in the development of early prerenal failure that progresses to chronic kidney disease in long-term adenine feeding.

Dysregulation of the Renin-Angiotensin System and the Vasopressinergic System Interactions in Cardiovascular Disorders

Purpose of Review

In many instances, the renin-angiotensin system (RAS) and the vasopressinergic system (VPS) are jointly activated by the same stimuli and engaged in the regulation of the same processes.

Recent Findings

Angiotensin II (Ang II) and arginine vasopressin (AVP), which are the main active compounds of the RAS and the VPS, interact at several levels. Firstly, Ang II, acting on AT1 receptors (AT1R), plays a significant role in the release of AVP from vasopressinergic neurons and AVP, stimulating V1a receptors (V1aR), regulates the release of renin in the kidney. Secondly, Ang II and AVP, acting on AT1R and V1aR, respectively, exert vasoconstriction, increase cardiac contractility, stimulate the sympathoadrenal system, and elevate blood pressure. At the same time, they act antagonistically in the regulation of blood pressure by baroreflex. Thirdly, the cooperative action of Ang II acting on AT1R and AVP stimulating both V1aR and V2 receptors in the kidney is necessary for the appropriate regulation of renal blood flow and the efficient resorption of sodium and water. Furthermore, both peptides enhance the release of aldosterone and potentiate its action in the renal tubules.

Summary

In this review, we (1) point attention to the role of the cooperative action of Ang II and AVP for the regulation of blood pressure and the water-electrolyte balance under physiological conditions, (2) present the subcellular mechanisms underlying interactions of these two peptides, and (3) provide evidence that dysregulation of the cooperative action of Ang II and AVP significantly contributes to the development of disturbances in the regulation of blood pressure and the water-electrolyte balance in cardiovascular diseases.

Genomic and rapid effects of aldosterone: what we know and do not know thus far

Aldosterone is the most known mineralocorticoid hormone synthesized by the adrenal cortex. The genomic pathway displayed by aldosterone is attributed to the mineralocorticoid receptor (MR) signaling. Even though the rapid effects displayed by aldosterone are long known, our knowledge regarding the receptor responsible for such event is still poor. It is intense that the debate whether the MR or another receptor-the "unknown receptor"-is the receptor responsible for the rapid effects of aldosterone. Recently, G protein-coupled estrogen receptor-1 (GPER-1) was elegantly shown to mediate some aldosterone-induced rapid effects in several tissues, a fact that strongly places GPER-1 as the unknown receptor. It has also been suggested that angiotensin receptor type 1 (AT1) also participates in the aldosterone-induced rapid effects. Despite this open question, the relevance of the beneficial effects of aldosterone is clear in the kidneys, colon, and CNS as aldosterone controls the important water reabsorption process; on the other hand, detrimental effects displayed by aldosterone have been reported in the cardiovascular system and in the kidneys. In this line, the MR antagonists are well-known drugs that display beneficial effects in patients with heart failure and hypertension; it has been proposed that MR antagonists could also play an important role in vascular disease, obesity, obesity-related hypertension, and metabolic syndrome. Taken altogether, our goal here was to (1) bring a historical perspective of both genomic and rapid effects of aldosterone in several tissues, and the receptors and signaling pathways involved in such processes; and (2) critically address the controversial points within the literature as regarding which receptor participates in the rapid pathway display by aldosterone.

Thirst Increases Chorda Tympani Responses to Sodium Chloride

In nature, water is present as a low-salt solution, thus we hypothesized that thirst would increase taste responses to low-salt solutions. We investigated the effect of thirst on the 2 different salt detection mechanisms present in the rat chorda tympani (CT) nerve. The first mechanism is dependent upon the epithelial sodium channel (ENaC), is blocked by benzamil, and is specific to the cation sodium. The second mechanism, while undefined, is independent of ENaC, and detects multiple cations. We expected thirst to increase benzamil-sensitive sodium responses due to mechanistically increasing the benzamil-sensitive ENaC. We recorded CT whole-nerve electrophysiological responses to lingual application of NaCl, KCl (30, 75, 150, 300, 500, and 600 mM), and imitation rainwater in both control and 24-h water-restricted male rats. NaCl solutions were presented in artificial saliva before and after lingual application of 5µM benzamil. Water restriction significantly increased the integrated CT responses to NaCl but not to KCl or imitation rainwater. Consistent with our hypothesis, only the benzamil-sensitive, and not the benzamil-insensitive, CT sodium response significantly increased. Additionally, CT responses to salt were recorded following induction of either osmotic or volemic thirst. Both thirsts significantly enhanced the integrated CT responses to NaCl and KCl, but not imitation rainwater. Interestingly, osmotic and volemic thirsts increased CT responses by increasing both the benzamil-sensitive and benzamil-insensitive CT sodium responses. We propose that thirst increases the sensitivity of the CT nerve to sodium.

Cardiorenal Syndrome: Role of Arginine Vasopressin and Vaptans in Heart Failure

Heart and kidney failure continued to be of increasing prevalence in today's society, and their comorbidity has synergistic effect on the morbidity and mortality of patients. Cardiorenal syndrome (CRS) is a complex disease with multifactorial pathophysiology. Better understanding of this pathophysiological network is crucial for the successful intervention to prevent advancement of the disease process. One of the major factors in this process is neurohormonal activation, predominantly involving renin-angiotensin-aldosterone system (RAAS) and arginine vasopressin (AVP). Heart failure causes reduced cardiac output/cardiac index (CO/CI) and fall in renal perfusion pressures resulting in activation of baroreceptors and RAAS, respectively. Activated baroreceptors and RAAS stimulate the release of AVP (non-osmotic pathway), which acts on V₂ receptors located in the renal collecting ducts, causing fluid retention and deterioration of heart failure. Effective blockade of AVP action on V₂ receptors has emerged as a potential treatment option in volume overload conditions especially in the setting of hyponatremia. Vasopressin receptor antagonists (VRAs), such as vaptans, are potent aquaretics causing electrolyte-free water diuresis without significant electrolyte abnormalities. Vaptans are useful in hypervolemic hyponatremic conditions like heart failure and liver cirrhosis, and euvolemic hyponatremic conditions like syndrome of inappropriate anti-diuretic hormone secretion. Tolvaptan and conivaptan are pharmaceutical agents that are available for the treatment of these conditions.

Cyclic-AMP regulates postnatal development of neural and behavioral responses to NaCl in rats

During postnatal development rats demonstrate an age-dependent increase in NaCl chorda tympani (CT) responses and the number of functional apical amiloride-sensitive epithelial Na+ channels (ENaCs) in salt sensing fungiform (FF) taste receptor cells (TRCs). Currently, the intracellular signals that regulate the postnatal development of salt taste have not been identified. We investigated the effect of cAMP, a downstream signal for arginine vasopressin (AVP) action, on the postnatal development of NaCl responses in 19-23 day old rats. ENaC-dependent NaCl CT responses were monitored after lingual application of 8-chlorophenylthio-cAMP (8-CPT-cAMP) under open-circuit conditions and under ±60 mV lingual voltage clamp. Behavioral responses were tested using 2 bottle/24h NaCl preference tests. The effect of [deamino-Cys1, D-Arg8]-vasopressin (dDAVP, a specific V2R agonist) was investigated on ENaC subunit trafficking in rat FF TRCs and on cAMP generation in cultured adult human FF taste cells (HBO cells). Our results show that in 19-23 day old rats, the ENaC-dependent maximum NaCl CT response was a saturating sigmoidal function of 8-CPT-cAMP concentration. 8-CPT-cAMP increased the voltage-sensitivity of the NaCl CT response and the apical Na+ response conductance. Intravenous injections of dDAVP increased ENaC expression and γ-ENaC trafficking from cytosolic compartment to the apical compartment in rat FF TRCs. In HBO cells dDAVP increased intracellular cAMP and cAMP increased trafficking of γ- and δ-ENaC from cytosolic compartment to the apical compartment 10 min post-cAMP treatment. Control 19-23 day old rats were indifferent to NaCl, but showed clear preference for appetitive NaCl concentrations after 8-CPT-cAMP treatment. Relative to adult rats, 14 day old rats demonstrated significantly less V2R antibody binding in circumvallate TRCs. We conclude that an age-dependent increase in V2R expression produces an AVP-induced incremental increase in cAMP that modulates the postnatal increase in TRC ENaC and the neural and behavioral responses to NaCl.

ENaC activity in the cortical collecting duct of HKα1 H+,K+-ATPase knockout mice is uncoupled from Na+ intake

Modulation of the epithelial Na⁺ channel (ENaC) activity in the collecting duct (CD) is an important mechanism for normal Na⁺ homeostasis. ENaC activity is inversely related to dietary Na⁺ intake, in part due to inhibitory paracrine purinergic regulation. Evidence suggests that H⁺,K⁺-ATPase activity in the CD also influences Na⁺ excretion. We hypothesized that renal H⁺,K⁺-ATPases affect Na⁺ reabsorption by the CD by modulating ENaC activity. ENaC activity in HKα₁ H⁺,K⁺-ATPase knockout (HKα₁^-/-) mice was uncoupled from Na⁺ intake. ENaC activity on a high-Na⁺ diet was greater in the HKα₁^-/- mice than in WT mice. Moreover, dietary Na⁺ content did not modulate ENaC activity in the HKα₁^-/- mice as it did in WT mice. Purinergic regulation of ENaC was abnormal in HKα₁^-/- mice. In contrast to WT mice, where urinary [ATP] was proportional to dietary Na⁺ intake, urinary [ATP] did not increase in response to a high-Na⁺ diet in the HKα₁^-/- mice and was significantly lower than in the WT mice. HKα₁^-/- mice fed a high-Na⁺ diet had greater Na⁺ retention than WT mice and had an impaired dipsogenic response. These results suggest an important role for the HKα₁ subunit in the regulation of purinergic signaling in the CD. They are also consistent with HKα₁-containing H⁺,K⁺-ATPases as important components for the proper regulation of Na⁺ balance and the dipsogenic response to a high-salt diet. Such observations suggest a previously unrecognized element in Na⁺ regulation in the CD.

Secretin is involved in sodium conservation through the renin-angiotensin-aldosterone system

Secretin (SCT) and its receptor (SCTR) are important in fluid regulation at multiple levels via the modulation of expression and translocation of renal aquaporin 2 and functions of central angiotensin II (ANGII). The functional interaction of SCT with peripheral ANGII, however, remains unknown. As the ANGII-aldosterone axis dominates the regulation of renal epithelial sodium channel (ENaC) function, we therefore tested whether SCT/SCTR can regulate sodium homeostasis via the renin-angiotensin-aldosterone system. SCTR-knockout (SCTR^-/-) mice showed impaired aldosterone synthase (CYP11B2) expression and, consequently, aldosterone release upon intraperitoneal injection of ANGII. Endogenous ANGII production induced by dietary sodium restriction was higher in SCTR^-/- than in C57BL/6N [wild-type (WT)] mice, but CYP11B2 and aldosterone synthesis were not elevated. Reduced accumulation of cholesteryl ester-the precursor of aldosterone-was observed in adrenal glands of SCTR^-/- mice that were fed a low-sodium diet. Absence of SCTR resulted in elevated basal transcript levels of adrenal CYP11B2 and renal ENaCs. Although transcript and protein levels of ENaCs were similar in WT and SCTR^-/- mice under sodium restriction, ENaCs in SCTR^-/- mice were less sensitive to amiloride hydrochloride. In summary, the SCT/SCTR axis is involved in aldosterone precursor uptake, and the knockout of SCTR results in defective aldosterone biosynthesis/release and altered sensitivity of ENaCs to amiloride.-Bai, J., Chow, B. K. C. Secretin is involved in sodium conservation through the renin-angiotensin-aldosterone system.

Involvement of ENaC in the development of salt-sensitive hypertension

Salt-sensitive hypertension is associated with renal and vascular dysfunctions, which lead to impaired fluid excretion, increased cardiac output, and total peripheral resistance. It is commonly accepted that increased renal sodium handling and plasma volume expansion are necessary factors for the development of salt-induced hypertension. The epithelial sodium channel (ENaC) is a trimeric ion channel expressed in the distal nephron that plays a critical role in the regulation of sodium reabsorption in both normal and pathological conditions. In this mini-review, we summarize recent studies investigating the role of ENaC in the development of salt-sensitive hypertension. On the basis of experimental data obtained from the Dahl salt-sensitive rats, we and others have demonstrated that abnormal ENaC activation in response to a dietary NaCl load contributes to the development of high blood pressure in this model. The role of different humoral factors, such as the components of the renin-angiotensin-aldosterone system, members of the epidermal growth factors family, arginine vasopressin, and oxidative stress mediating the effects of dietary salt on ENaC are discussed in this review to highlight future research directions and to determine potential molecular targets for drug development.

The function and regulation of acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC): IUPHAR Review 19

Acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC) are both members of the ENaC/degenerin family of amiloride-sensitive Na(+) channels. ASICs act as proton sensors in the nervous system where they contribute, besides other roles, to fear behaviour, learning and pain sensation. ENaC mediates Na(+) reabsorption across epithelia of the distal kidney and colon and of the airways. ENaC is a clinically used drug target in the context of hypertension and cystic fibrosis, while ASIC is an interesting potential target. Following a brief introduction, here we will review selected aspects of ASIC and ENaC function. We discuss the origin and nature of pH changes in the brain and the involvement of ASICs in synaptic signalling. We expose how in the peripheral nervous system, ASICs cover together with other ion channels a wide pH range as proton sensors. We introduce the mechanisms of aldosterone-dependent ENaC regulation and the evidence for an aldosterone-independent control of ENaC activity, such as regulation by dietary K(+) . We then provide an overview of the regulation of ENaC by proteases, a topic of increasing interest over the past few years. In spite of the profound differences in the physiological and pathological roles of ASICs and ENaC, these channels share many basic functional and structural properties. It is likely that further research will identify physiological contexts in which ASICs and ENaC have similar or overlapping roles.

Renal sodium transport in renin-deficient Dahl salt-sensitive rats

Objective:

The Dahl salt-sensitive rat is a well-established model of salt-sensitive hypertension. The goal of this study was to assess the expression and activity of renal sodium channels and transporters in the renin-deficient salt-sensitive rat.

Methods:

Renin knockout (Ren^−/−) rats created on the salt-sensitive rat background were used to investigate the role of renin in the regulation of ion transport in salt-sensitive hypertension. Western blotting and patch-clamp analyses were utilized to assess the expression level and activity of Na⁺ transporters.

Results:

It has been described previously that Ren^−/− rats exhibit severe kidney underdevelopment, polyuria, and lower body weight and blood pressure compared to their wild-type littermates. Here we found that renin deficiency led to decreased expression of sodium-hydrogen antiporter (NHE3), the Na⁺/H⁺ exchanger involved in Na⁺ absorption in the proximal tubules, but did not affect the expression of Na-K-Cl cotransporter (NKCC2), the main transporter in the loop of Henle. In the distal nephron, the expression of sodium chloride cotransporter (NCC) was lower in Ren^−/− rats. Single-channel patch clamp analysis detected decreased ENaC activity in Ren^−/− rats which was mediated via changes in the channel open probability.

Conclusion:

These data illustrate that renin deficiency leads to significant dysregulation of ion transporters.

Prenatal Testosterone Exposure Decreases Aldosterone Production but Maintains Normal Plasma Volume and Increases Blood Pressure in Adult Female Rats

Plasma testosterone levels are elevated in pregnant women with preeclampsia and polycystic ovaries; their offspring are at increased risk for hypertension during adult life. We tested the hypothesis that prenatal testosterone exposure induces dysregulation of the renin-angiotensin-aldosterone system, which is known to play an important role in water and electrolyte balance and blood pressure regulation. Female rats (6 mo old) prenatally exposed to testosterone were examined for adrenal expression of steroidogenic genes, telemetric blood pressure, blood volume and Na⁺ and K⁺ levels, plasma aldosterone, angiotensin II and vasopressin levels, and vascular responses to angiotensin II and arg⁸-vasopressin. The levels of Cyp11b2 (aldosterone synthase), but not the other adrenal steroidogenic genes, were decreased in testosterone females. Accordingly, plasma aldosterone levels were lower in testosterone females. Plasma volume and serum and urine Na⁺ and K⁺ levels were not significantly different between control and testosterone females; however, prenatal testosterone exposure significantly increased plasma vasopressin and angiotensin II levels and arterial pressure in adult females. In testosterone females, mesenteric artery contractile responses to angiotensin II were significantly greater, while contractile responses to vasopressin were unaffected. Angiotensin II type-1 receptor expression was increased, while angiotensin II type-2 receptor was decreased in testosterone arteries. These results suggest that prenatal testosterone exposure downregulates adrenal Cyp11b2 expression, leading to decreased plasma aldosterone levels. Elevated angiotensin II and vasopressin levels along with enhanced vascular responsiveness to angiotensin II may serve as an underlying mechanism to maintain plasma volume and Na⁺ and K⁺ levels and mediate hypertension in adult testosterone females.

Coordinated Control of ENaC and Na+,K+-ATPase in Renal Collecting Duct

Tubular reabsorption of filtered sodium is tightly controlled to maintain body volume homeostasis. The rate of sodium transport by collecting duct (CD) cells varies widely in response to dietary sodium intake, GFR, circulating hormones, neural signals, and local regulatory factors. Reabsorption of filtered sodium by CD cells occurs via a two-step process. First, luminal sodium crosses the apical plasma membrane along its electrochemical gradient through epithelial sodium channels (ENaC). Intracellular sodium is then actively extruded into the interstitial space by the Na(+),K(+)-ATPase located along the basolateral membrane. Mismatch between sodium entry and exit induces variations in sodium intracellular concentration and cell volume that must be maintained within narrow ranges for control of vital cell functions. Therefore, renal epithelial cells display highly coordinated apical and basolateral sodium transport rates. We review evidence from experiments conducted in vivo and in cultured cells that indicates aldosterone and vasopressin, the two major hormones regulating sodium reabsorption by CD, generate a coordinated stimulation of apical ENaC and basolateral Na(+),K(+)-ATPase. Moreover, we discuss evidence suggesting that variations in sodium entry per se induce a coordinated change in Na(+),K(+)-ATPase activity through the signaling of protein kinases such as protein kinase A and p38 mitogen-activated protein kinase.

Severe Salt-Losing Syndrome and Hyperkalemia Induced by Adult Nephron-Specific Knockout of the Epithelial Sodium Channel α-Subunit

Systemic pseudohypoaldosteronism type 1 (PHA-1) is a severe salt-losing syndrome caused by loss-of-function mutations of the amiloride-sensitive epithelial sodium channel (ENaC) and characterized by neonatal life-threatening hypovolemia and hyperkalemia. The very high plasma aldosterone levels detected under hypovolemic or hyperkalemic challenge can lead to increased or decreased sodium reabsorption, respectively, through the Na(+)/Cl(-) cotransporter (NCC). However, the role of ENaC deficiency remains incompletely defined, because constitutive inactivation of individual ENaC subunits is neonatally lethal in mice. We generated adult inducible nephron-specific αENaC-knockout mice (Scnn1a(Pax8/LC1)) that exhibit hyperkalemia and body weight loss when kept on a regular-salt diet, thus mimicking PHA-1. Compared with control mice fed a regular-salt diet, knockout mice fed a regular-salt diet exhibited downregulated expression and phosphorylation of NCC protein, despite high plasma aldosterone levels. In knockout mice fed a high-sodium and reduced-potassium diet (rescue diet), although plasma aldosterone levels remained significantly increased, NCC expression returned to control levels, and body weight, plasma and urinary electrolyte concentrations, and excretion normalized. Finally, shift to a regular diet after the rescue diet reinstated the symptoms of severe PHA-1 syndrome and significantly reduced NCC phosphorylation. In conclusion, lack of ENaC-mediated sodium transport along the nephron cannot be compensated for by other sodium channels and/or transporters, only by a high-sodium and reduced-potassium diet. We further conclude that hyperkalemia becomes the determining factor in regulating NCC activity, regardless of sodium loss, in the ENaC-mediated salt-losing PHA-1 phenotype.

Coordinated Regulation of Vasopressin Inactivation and Glucose Uptake by Action of TUG Protein in Muscle

In adipose and muscle cells, insulin stimulates the exocytic translocation of vesicles containing GLUT4, a glucose transporter, and insulin-regulated aminopeptidase (IRAP), a transmembrane aminopeptidase. A substrate of IRAP is vasopressin, which controls water homeostasis. The physiological importance of IRAP translocation to inactivate vasopressin remains uncertain. We previously showed that in skeletal muscle, insulin stimulates proteolytic processing of the GLUT4 retention protein, TUG, to promote GLUT4 translocation and glucose uptake. Here we show that TUG proteolysis also controls IRAP targeting and regulates vasopressin action in vivo. Transgenic mice with constitutive TUG proteolysis in muscle consumed much more water than wild-type control mice. The transgenic mice lost more body weight during water restriction, and the abundance of renal AQP2 water channels was reduced, implying that vasopressin activity is decreased. To compensate for accelerated vasopressin degradation, vasopressin secretion was increased, as assessed by the cosecreted protein copeptin. IRAP abundance was increased in T-tubule fractions of fasting transgenic mice, when compared with controls. Recombinant IRAP bound to TUG, and this interaction was mapped to a short peptide in IRAP that was previously shown to be critical for GLUT4 intracellular retention. In cultured 3T3-L1 adipocytes, IRAP was present in TUG-bound membranes and was released by insulin stimulation. Together with previous results, these data support a model in which TUG controls vesicle translocation by interacting with IRAP as well as GLUT4. Furthermore, the effect of IRAP to reduce vasopressin activity is a physiologically important consequence of vesicle translocation, which is coordinated with the stimulation of glucose uptake.

Optimum AT1 receptor-neprilysin inhibition has superior cardioprotective effects compared with AT1 receptor blockade alone in hypertensive rats

Neprilysin inhibitors prevent the breakdown of bradykinin and natriuretic peptides, promoting vasodilation and natriuresis. However, they also increase angiotensin II and endothelin-1. Here we studied the effects of a low and a high dose of the neprilysin inhibitor thiorphan on top of AT1 receptor blockade with irbesartan versus vehicle in TGR(mREN2)27 rats with high renin hypertension. Mean arterial blood pressure was unaffected by vehicle or thiorphan alone. Irbesartan lowered blood pressure, but after 7 days pressure started to increase again. Low- but not high-dose thiorphan prevented this rise. Only during exposure to low-dose thiorphan plus irbesartan did heart weight/body weight ratio, cardiac atrial natriuretic peptide expression, and myocyte size decrease significantly. Circulating endothelin-1 was not affected by low-dose thiorphan with or without irbesartan, but increased after treatment with high-dose thiorphan plus irbesartan. This endothelin-1 rise was accompanied by an increase in renal sodium-hydrogen exchanger 3 protein abundance, and an upregulation of constrictor vascular endothelin type B receptors. Consequently, the endothelin type B receptor antagonist BQ788 no longer enhanced endothelin-1-induced vasoconstriction (indicative of endothelin type B receptor-mediated vasodilation), but prevented it. Thus, optimal neprilysin inhibitor dosing reveals additional cardioprotective effects on top of AT1 receptor blockade in renin-dependent hypertension.

Inhibition of ENaC by endothelin-1

The amiloride-sensitive epithelial Na(+) channel (ENaC) is a key player in the regulation of Na(+) homeostasis. Its functional activity is under continuous control by a variety of signaling molecules, including bioactive peptides of endothelin family. Since ENaC dysfunction is causative for disturbances in total body Na(+) levels associated with the abnormal regulation of blood volume, blood pressure, and lung fluid balance, uncovering the molecular mechanisms of inhibitory modulation or inappropriate activation of ENaC is crucial for the successful treatment of a variety of human diseases including hypertension. The precise regulation of ENaC is particularly important for normal Na(+) and fluid homeostasis in organs where endothelins are known to act: the kidneys, lung, and colon. Inhibition of ENaC by endothelin-1 (ET-1) has been established in renal cells, and several molecular mechanisms of inhibition of ENaC by ET-1 are proposed and will be reviewed in this chapter.

Activation of ENaC by AVP contributes to the urinary concentrating mechanism and dilution of plasma

Arginine vasopressin (AVP) activates the epithelial Na(+) channel (ENaC). The physiological significance of this activation is unknown. The present study tested if activation of ENaC contributes to AVP-sensitive urinary concentration. Consumption of a 3% NaCl solution induced hypernatremia and plasma hypertonicity in mice. Plasma AVP concentration and urine osmolality increased in hypernatremic mice in an attempt to compensate for increases in plasma tonicity. ENaC activity was elevated in mice that consumed 3% NaCl solution compared with mice that consumed a diet enriched in Na(+) with ad libitum tap water; the latter diet does not cause hypernatremia. To determine whether the increase in ENaC activity in mice that consumed 3% NaCl solution served to compensate for hypernatremia, mice were treated with the ENaC inhibitor benzamil. Coadministration of benzamil with 3% NaCl solution decreased urinary osmolality and increased urine flow so that urinary Na(+) excretion increased with no effect on urinary Na(+) concentration. This decrease in urinary concentration further increased plasma Na(+) concentration, osmolality, and AVP concentration in these already hypernatremic mice. Benzamil similarly compromised urinary concentration in water-deprived mice and in mice treated with desmopressin. These results demonstrate that stimulation of ENaC by AVP plays a critical role in water homeostasis by facilitating urinary concentration, which can compensate for hypernatremia or exacerbate hyponatremia. The present findings are consistent with ENaC in addition to serving as a final effector of the renin-angiotensin-aldosterone system and blood pressure homeostasis, also playing a key role in water homeostasis by regulating urine concentration and dilution of plasma.

Renal neurohormonal regulation in heart failure decompensation

Decompensation in heart failure occurs when the heart fails to balance venous return with cardiac output, leading to fluid congestion and contributing to mortality. Decompensated heart failure can cause acute kidney injury (AKI), which further increases mortality. Heart failure activates signaling systems that are deleterious to kidneys such as renal sympathetic nerve activity (RSNA), renin-angiotensin-aldosterone system, and vasopressin secretion. All three reduce renal blood flow (RBF) and increase tubular sodium reabsorption, which may increase renal oxygen consumption causing AKI through renal tissue hypoxia. Vasopressin contributes to venous congestion through aquaporin-mediated water retention. Additional water retention may be mediated through vasopressin-induced medullary urea transport and hyaluronan but needs further study. In addition, there are several systems that could protect the kidneys and reduce fluid retention such as natriuretic peptides, prostaglandins, and nitric oxide. However, the effect of natriuretic peptides and nitric oxide are blunted in decompensation, partly due to oxidative stress. This review considers how neurohormonal signaling in heart failure drives fluid retention by the kidneys and thus exacerbates decompensation. It further identifies areas where there is limited data, such as signaling systems 20-HETE, purines, endothelin, the role of renal water retention mechanisms for congestion, and renal hypoxia in AKI during heart failure.

Nephrotic syndrome complicated by idiopathic central diabetes insipidus

BACKGROUND

There is ongoing discussion regarding the mechanisms underlying edema formation in nephrotic syndrome (NS). Many studies published in the last decade reported that primary renal sodium retention was a major factor in edema formation. However, many of the factors influencing edema formation in NS remain unclear, including the role of arginine vasopressin (AVP).

CASE-DIAGNOSIS/TREATMENT

We report a 12-year-old boy with steroid-dependent NS complicated by idiopathic central diabetes insipidus (CDI). He did not develop edema during his first relapse of NS after developing CDI, despite having hypoalbuminemia. He had polydipsia, polyuria, low urine osmolality, and a low serum arginine AVP level. His fractional excretion of sodium was only slightly low. Endocrinological testing and magnetic resonance imaging revealed idiopathic CDI. After starting desmopressin therapy, he developed edema when his NS relapsed.

CONCLUSIONS

This is the first known reported case of NS in a patient with CDI. The findings suggest that appropriate AVP secretion in response to an increase in serum osmolality caused by renal sodium retention is necessary for excess extracellular fluid accumulation in NS. Further investigation is needed to more fully understand the role of AVP in edema formation in NS.

Blood pressure regulation via the epithelial sodium channel: from gene to kidney and beyond

The epithelial sodium channel (ENaC) has long been recognized as playing a vital role in blood pressure (BP) regulation due to its involvement in fluid balance. The genes encoding the three ENaC subunits are likewise important contributors to hypertension, both in rare monogenic diseases and in the general population. The unusually high numbers of genetic variants associated with complex traits, including BP, that are located in non-coding areas suggest an involvement of these variants in regulatory functions. This may involve differential regulation of expression in different tissues. Emerging evidence indicates that the ENaC plays an important role in BP determination not only via its actions in the kidney, but also in other tissues commonly involved in BP regulation. The ENaC in the central nervous system is proposed to regulate BP via sympathetic nervous system activity. Recent evidence suggests that the ENaC contributes to vascular function and the myogenic response. Additional roles potentially include initiation of the baroreceptor reflex via ENaC in the baroreceptors and driving high salt intake with a 'taste for salt' via ENaC in the tongue. The present review describes the involvement of the ENaC in the determination of BP at a genetic and physiological level, detailing recent evidence for its role in the kidney and in other pertinent tissues.

Direct activation of ENaC by angiotensin II: recent advances and new insights

Angiotensin II (Ang II) is the principal effector of the renin-angiotensin-aldosterone system (RAAS). It initiates myriad processes in multiple organs integrated to increase circulating volume and elevate systemic blood pressure. In the kidney, Ang II stimulates renal tubular water and salt reabsorption causing antinatriuresis and antidiuresis. Activation of the RAAS is known to enhance activity of the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. In addition to its well described stimulatory actions on aldosterone secretion, Ang II is also capable of directly increasing ENaC activity. In this brief review, we discuss recent findings about non-classical Ang II actions on ENaC and speculate about its relevance for renal sodium handling.

Adenylyl cyclase VI mediates vasopressin-stimulated ENaC activity

Vasopressin modulates sodium reabsorption in the collecting duct through adenylyl cyclase-stimulated cyclic AMP, which exists as multiple isoforms; the specific isoform involved in vasopressin-stimulated sodium transport is unknown. To assess this, we studied mice deficient in adenylyl cyclase type VI specifically in the principal cells of the collecting duct. Knockout mice had increased urine volume and reduced urine sodium concentration, but regardless of the level of sodium intake, they did not exhibit significant alterations in urinary sodium excretion, arterial pressure, or pulse rate. Plasma renin concentration was elevated in knockout mice, however, suggesting a compensatory response. Valsartan significantly reduced arterial pressure in knockout mice but not in controls. Knockout mice had decreased renal cortical mRNA content of all three epithelial sodium channel (ENaC) isoforms, and total cell sodium channel isoforms α and γ were reduced in these animals. Patch-clamp analysis of split-open cortical collecting ducts revealed no difference in baseline activity of sodium channels, but knockout mice had abolished vasopressin-stimulated ENaC open probability and apical membrane channel number. In summary, these data suggest that adenylyl cyclase VI mediates vasopressin-stimulated ENaC activity in the kidney.

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.

The PGE(2)-EP4 receptor is necessary for stimulation of the renin-angiotensin-aldosterone system in response to low dietary salt intake in vivo

Increased cyclooxygenase-2 (COX-2) expression and PGE(2) synthesis have been shown to be prerequisites for renal renin release after Na(+) deprivation. To answer the question of whether EP4 receptor type of PGE(2) mediates renin regulation under a low-salt diet, we examined renin regulation in EP4(+/+), EP4(-/-), and in wild-type mice treated with EP4 receptor antagonist. After 2 wk of a low-salt diet (0.02% wt/wt NaCl), EP4(+/+) mice showed diminished Na(+) excretion, unchanged K(+) excretion, and reduced Ca(2+) excretion. Diuresis and plasma electrolytes remained unchanged. EP4(-/-) exhibited a similar attenuation of Na(+) excretion; however, diuresis and K(+) excretion were enhanced, and plasma Na(+) concentration was higher, whereas plasma K(+) concentration was lower compared with control diet. There were no significant differences between EP4(+/+) and EP4(-/-) mice in blood pressure, creatinine clearance, and plasma antidiuretic hormone (ADH) concentration. Following salt restriction, plasma renin and aldosterone concentrations and kidney renin mRNA level rose significantly in EP4(+/+) but not in EP4(-/-) and in wild-type mice treated with EP4 antagonist ONO-AE3-208. In the latter two groups, the low-salt diet caused a significantly greater rise in PGE(2) excretion. Furthermore, mRNA expression for COX-2 and PGE(2) synthetic activity was significantly greater in EP4(-/-) than in EP4(+/+) mice. We conclude that low dietary salt intake induces expression of COX-2 followed by enhanced renal PGE(2) synthesis, which stimulates the renin-angiotensin-aldosterone system by activation of EP4 receptor. Most likely, defects at the step of EP4 receptor block negative feedback mechanisms on the renal COX system, leading to persistently high PGE(2) levels, diuresis, and K(+) loss.

The role of the epithelial Na(+) channel (ENaC) in high AVP but low aldosterone states

Due to the abundance of seminal discoveries establishing a strong causal relation between changes in aldosterone signaling, the activity of the epithelial Na(+) channel (ENaC) and blood pressure, the role of ENaC in health and disease is understood almost exclusively through the concept that this channel functions (in the distal nephron) as a key end-effector controlling renal sodium excretion during feedback regulation of blood pressure by the renin-angiotensin-aldosterone system (RAAS). Recent findings of aldosterone-independent stimulation of ENaC by vasopressin challenge the completeness of dogmatic understanding where ENaC serves solely as an end-effector of the RAAS important for control of sodium balance. Rather the consequences of activating ENaC in the distal nephron appear to depend on whether the channel is activated in the absence (by aldosterone) or presence [by vasopressin (AVP)] of simultaneous activation of aquaporin 2 water channels. Thus, a unifying paradigm has ENaC at the junction of two signaling systems that sometimes must compete: one controlling and responding to changes in sodium balance, perceived as mean arterial pressure, and the other water balance, perceived as plasma osmolality.