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This review highlights the molecular basis of salt sensitivity in hypertension, with a focus on the regulation of sodium transport in the distal nephron. Sodium reabsorption in this region is often linked to the actions of aldosterone, although in recent years numerous findings have been reported on the aldosterone-independent pathway of acquiring salt sensitivity by potassium deficiency or potassium loading. The key to this discussion is the interplay, through extracellular potassium concentration, between the first part of the tubules expressing the Na+-Cl− cotransporter (NCC) and the second part expressing the epithelial Na+ channel (ENaC). The molecular pathways such as with-no-lysine 1 (WNK)-STE20/SPS1-related proline-alanine-rich kinase (SPAK)/oxidative stress-responsive kinase 1 (OSR1) signaling, Kelch-like family member 3 (KLHL3)-cullin 3 (CUL3) complex, protein phosphatases, and mechanistic target of rapamycin complex 2 (mTORC2)-Nedd4L pathway are described as the mechanism by which salt sensitivity on blood pressure is acquired in response to changes in physiological conditions including potassium depletion or loading. This review highlights the potential for targeting these molecular pathways to develop novel therapeutic strategies for the treatment of salt-sensitive hypertension, the mechanism of which remains to be elucidated. [ABSTRACT FROM AUTHOR]

We have previously observed that prolonged administration of rapamycin, an inhibitor targeting the mammalian target of rapamycin complex (mTORC)1, partially reduced hypertension and alleviated kidney inflammation in Dahl salt-sensitive (SS) rats. In contrast, treatment with PP242, an inhibitor affecting both mTORC1/mTORC2, not only completely prevented hypertension but also provided substantial protection against kidney injury. Notably, PP242 exhibited potent natriuretic effects that were not evident with rapamycin. The primary objective of this study was to pinpoint the specific tubular sites responsible for the natriuretic effect of PP242 in SS rats subjected to either 0.4% NaCl (normal salt) or 4.0% NaCl (high salt) diet. Acute effects of PP242 on natriuretic, diuretic, and kaliuretic responses were determined in unanesthetized SS rats utilizing benzamil, furosemide, or hydrochlorothiazide [inhibitors of epithelial Na+ channel (ENaC), Na-K-2Cl cotransporter (NKCC2), or Na-Cl cotransporter (NCC), respectively] either administered alone or in combination. The findings indicate that the natriuretic effects of PP242 in SS rats stem predominantly from the inhibition of NCC and a reduction of ENaC open probability. Molecular analysis revealed that mTORC2 regulates NCC activity through protein phosphorylation and ENaC activity through proteolytic cleavage in vivo. Evidence also indicated that PP242 also prevents the loss of K+ associated with the inhibition of NCC. These findings suggest that PP242 may represent an improved therapeutic approach for antihypertensive intervention, potentially controlling blood pressure and mitigating kidney injury in salt-sensitive human subjects. NEW & NOTEWORTHY: This study explored mechanisms underlying the natriuretic effects of mammalian target of rapamycin protein complex 2 inhibition using PP242 and revealed both epithelial Na+ channel and Na-Cl cotransporter in the distal tubular segments were potentially inhibited. These observations, with prior lab evidence, indicate that PP242 prevents hypertension via its potent inhibitory effects on these specific sodium transporters and by reducing renal immune responses. This dual action, coupled with potassium sparing effects, suggests an improved approach for managing hypertension and associated kidney damage. [ABSTRACT FROM AUTHOR]

Aldosterone is responsible for maintaining volume and potassium homeostasis. Although high salt consumption should suppress aldosterone production, individuals with hyperaldosteronism lose this regulation, leading to a state of high aldosterone despite dietary sodium consumption. The present study examines the effects of elevated aldosterone, with or without high salt consumption, on the expression of key Na+ transporters and remodelling in the distal nephron. Epithelial sodium channel (ENaC) α‐subunit expression was increased with aldosterone regardless of Na+ intake. However, ENaC β‐ and γ‐subunits unexpectedly increased at both a transcript and protein level with aldosterone when high salt was present. Expression of total and phosphorylated Na+Cl− cotransporter (NCC) significantly increased with aldosterone, in association with decreased blood [K+], but the addition of high salt markedly attenuated the aldosterone‐dependent NCC increase, despite equally severe hypokalaemia. We hypothesized this was a result of differences in distal convoluted tubule length when salt was given with aldosterone. Imaging and measurement of the entire pNCC‐positive tubule revealed that aldosterone alone caused a shortening of this segment, although the tubule had a larger cross‐sectional diameter. This was not true when salt was given with aldosterone because the combination was associated with a lengthening of the tubule in addition to increased diameter, suggesting that differences in the pNCC‐positive area are not responsible for differences in NCC expression. Together, our results suggest the actions of aldosterone, and the subsequent changes related to hypokalaemia, are altered in the presence of high dietary Na+. Key points: Aldosterone regulates volume and potassium homeostasis through effects on transporters in the kidney; its production can be dysregulated, preventing its suppression by high dietary sodium intake.Here, we examined how chronic high sodium consumption affects aldosterone's regulation of sodium transporters in the distal nephron.Our results suggest that high sodium consumption with aldosterone is associated with increased expression of all three epithelial sodium channel subunits, rather than just the alpha subunit.Aldosterone and its associated decrease in blood [K+] lead to an increased expression of Na‐Cl cotransporter (NCC); the addition of high sodium consumption with aldosterone partially attenuates this NCC expression, despite similarly low blood [K+].Upstream kinase regulators and tubule remodelling do not explain these results. [ABSTRACT FROM AUTHOR]

BACKGROUND: KDM6A (Lysine-Specific Demethylase 6A) is a specific demethylase for histone 3 lysine (K) 27 trimethylation (H3K27me3). The purpose of this study is to investigate whether KDM6A in renal tubule cells plays a role in the regulation of kidney function and blood pressure. METHODS: We first crossed Ksp-Cre+/− and KDM6Aflox/flox mice for generating inducible kidney-specific deletion of KDM6A gene. RESULTS: Notably, conditional knockout of KDM6A gene in renal tubule cells (KDM6A-cKO) increased H3K27me3 levels which leads to a decrease in Na excretion and elevation of blood pressure. Further analysis showed that the expression of NKCC2 (Na-K-2Cl cotransporter 2) and NCC (Na-Cl cotransporters) was upregulated which contributes to impaired Na excretion in KDM6A-cKO mice. The expression of AQP2 (aquaporin 2) was also increased in KDM6A-cKO mice, which may facilitate water reabsorption in KDM6A-cKO mice. The expression of Klotho was downregulated while expression of aging markers including p53, p21, and p16 was upregulated in kidneys of KDM6A-cKO mice, indicating that deletion of KDM6A in the renal tubule cells promotes kidney aging. Interestingly, KDM6A-cKO mice developed salt-sensitive hypertension which can be rescued by treatment with Klotho. KDM6A deficiency induced salt-sensitive hypertension likely through downregulation of the Klotho/ERK (extracellular signal-regulated kinase) signaling and upregulation of the WNK (with-no-lysine kinase) signaling. CONCLUSIONS: This study provides the first evidence that KDM6A plays an essential role in maintaining normal tubular function and blood pressure. Renal tubule cell specific KDM6A deficiency causes hypertension due to increased H3K27me3 levels and the resultant downregulation of Klotho gene expression which disrupts the Klotho/ERK/NCC/NKCC2 signaling. [ABSTRACT FROM AUTHOR]

The with-no-lysine (WNK) kinase family, comprising four serine-threonine protein kinases (WNK1-4), were first linked to hypertension due to their mutations in association with pseudohypoaldosteronism type II (PHAII). WNK kinases regulate crucial blood pressure regulators, SPAK/OSR1, to mediate the post-translational modifications (PTMs) of their downstream ion channel substrates, such as sodium chloride co-transporter (NCC), epithelial sodium chloride (ENaC), renal outer medullary potassium channel (ROMK), and Na/K/2Cl co-transporters (NKCCs). In this review, we summarize the molecular pathways dysregulating the WNKs and their downstream target renal ion transporters. We summarize each of the genetic variants of WNK kinases and the small molecule inhibitors that have been discovered to regulate blood pressure via WNK-triggered PTM cascades. [ABSTRACT FROM AUTHOR]

Abstract Urinary K+ potassium excretion rapidly increases after a potassium‐rich meal. The early aldosterone‐induced sgk1 gene (encoding serum and glucocorticoid‐induced kinase 1), activates potassium clearance, but the role of this kinase in the early activation of K+ secretion has not been clearly defined. Here, we challenged inducible renal‐tubule‐specific Sgk1Pax8/LC1 knockout mice with an acute high‐potassium load (HK:5%K+) and compared the physiological and molecular responses to control mice. We observe that urinary excretion after a K+ load over the first 3 h is not dependent on SGK1 but is coincident with the rapid dephosphorylation of the Na+,Cl−‐cotransporter (NCC) to increase distal salt delivery. Molecular analyses indicate that whereas SGK1‐mediated phosphorylation of the ubiquitin‐protein ligase NEDD4‐2 begins to increase by 3h, SGK1‐dependent proteolytic activation of ENaC only becomes detectable after 6 h of HK intake. Consistent with SGK1‐dependent ENaC activation via inhibition of NEDD4‐2‐mediated ubiquitylation, Sgk1Pax8/LC1 mice are unable to efficiently inhibit NEDD4‐2 or increase ENaC cleavage after 6 h of HK. Nevertheless, no defect in acute K+ balance was detected in the mutant mice after 6 h of HK. Moreover, we found that Sgk1Pax8/LC1 mice reduce NCC phosphorylation and NCC‐mediated salt absorption to a greater extent than control mice after a K+ load, promoting increased amiloride‐sensitive Na+‐reabsorption via ENaC to maintain adequate kaliuresis. Together, these data indicate that: (a) during the early 3 h of HK intake, K+ excretion is SGK1‐independent even under an extreme K+ challenge, (b) shortly after, SGK1 inhibits NEDD4‐2 and activates ENaC to stimulate K+‐secretion, (c) SGK1‐dependent phosphorylation of NCC occurs, acting more likely as a brake pedal to prevent excessive K+ loss.

Changes in the expression of Na transport proteins were measured in the kidneys of mice with increased dietary K intake for 1 wk. The epithelial Na channel (ENaC) was upregulated, with enhanced expression of full-length and cleaved forms of α-ENaC and cleaved α-ENaC. At the same time, the amount of the NaCl cotransporter NCC and its phosphorylated form decreased by **50% and **80%, respectively. The expression of the phosphorylated form of the Na-K-2Cl cotransporter NKCC2 also decreased, despite an increase in overall protein content. The effect was stronger in males (80%) than in females (40%). This implies that less Na+ is reabsorbed in the thick ascending limb of Henle's loop and distal convoluted tubule along with Cl-, whereas more is reabsorbed in the aldosterone-sensitive distal nephron in exchange for secreted K+. The abundance of the proximal tubule Na/H exchanger NHE3 decreased by ~40%, with similar effects in males and females. Time-course studies indicated that NCC and NHE3 proteins decreased progressively over 7 days on a high-K diet. Expression of mRNA encoding these proteins increased, implying that the decreased protein levels resulted from decreased rates of synthesis or increased rates of degradation. The potential importance of changes in NHE3, NKCC2, and NCC in promoting K+ excretion was assessed with a mathematical model. Simulations indicated that decreased NHE3 produced the largest effect. Regulation of proximal tubule Na+ transport may play a significant role in achieving K homeostasis. [ABSTRACT FROM AUTHOR]

Obstruction of urine flow at the level of the pelvoureteric junction (UPJO) and subsequent development of hydronephrosis is one of the most common congenital renal malformations. UPJO is associated with development of salt-sensitive hypertension, which is set by the obstructed kidney, and with a stimulated reninangiotensin-aldosterone system (RAAS) in rodent models. This study aimed at investigating the hypothesis that 1) in pediatric patients with UPJO the RAAS is activated before surgical relief of the obstruction; 2) in rats with UPJO the RAAS activation is reflected by increased abundance of renal aldosterone-stimulated Na transporters; and 3) the injured UPJO kidney allows aberrant filtration of plasminogen, leading to proteolytic activation of the epithelial Na channel γ-subunit (γ-ENaC). Hydronephrosis resulting from UPJO in pediatric patients and rats was associated with increased urinary plasminogen-to-creatinine ratio. In pediatric patients, plasma renin, angiotensin II, urine and plasma aldosterone, and urine soluble prorenin receptor did not differ significantly before or after surgery, or compared with controls. Increased plasmin-to-plasminogen ratio was seen in UPJO rats. Intact γ-ENaC abundance was not changed in UPJO kidney, whereas lowmolecular cleavage product abundance increased. The Na-Cl cotransporter displayed significantly lower abundance in the UPJO kidney compared with the nonobstructed contralateral kidney. The Na-KATPase α-subunit was unaltered. Treatment with an angiotensinconverting enzyme inhibitor (8 days, captopril) significantly lowered blood pressure in UPJO rats. It is concluded that the RAAS contributes to hypertension following partial obstruction of urine flow at the pelvo-ureteric junction with potential contribution from proteolytic activation of ENaC. [ABSTRACT FROM AUTHOR]

We assessed effects of acute volume reductions induced by administration of diuretics in rats. Direct block of Na+ transport produced changes in urinary electrolyte excretion. Adaptations to these effects appeared as alterations in the expression of protein for the distal nephron Na+ transporters NCC and ENaC. Two hours after a single injection of furosemide (6 mg/kg) or hydrochlorothiazide (HCTZ; 30 mg/kg) Na+ and K+ excretion increased but no changes in the content of activated forms of NCC (phosphorylated on residue T53) or ENaC (cleaved γ-subunit) were detected. In contrast, amiloride (0.6 mg/kg) evoked a similar natriuresis that coincided with decreased pT53NCC and increased cleaved γENaC. Alterations in posttranslational membrane protein processing correlated with an increase in plasma K+ of 0.6-0.8 mM. Decreased pT53NCC occurred within 1 h after amiloride injection, whereas changes in γENaC were slower and were blocked by the mineralocorticoid receptor antagonist spironolactone. Increased γENaC cleavage correlated with elevation of the surface expression of the subunit as assessed by in situ biotinylation. Na depletion induced by 2 h of furosemide or HCTZ treatment increases total NCC expression without affecting ENaC protein. However, restriction of Na intake for 10 h (during the day) or 18 h (overnight) increased the abundance of both total NCC and of cleaved α- and γENaC. We conclude that the kidneys respond acutely to hyperkalemic challenges by decreasing the activity of NCC while increasing that of ENaC. They respond to hypovolemia more slowly, increasing Na+ reabsorptive capacities of both of these transporters. [ABSTRACT FROM AUTHOR]

The World Health Organization ranks hypertension the leading global risk factor for disease, specifically, cardiovascular disease. Blood pressure (BP) is higher in Westernized populations consuming Na+-rich processed foods than in isolated societies consuming K+-rich natural foods. Evidence suggests that lowering dietary Na+ is particularly beneficial in hypertensive individuals who consume a high-Na+ diet. Nonetheless, numerous population studies demonstrate a relationship between higher dietary K+, estimated from urinary excretion or dietary recall, and lower BP, regardless of Na+ intake. Interventional studies with K+ supplementation suggest that it provides a direct benefit; K+ may also be a marker for other beneficial components of a "natural" diet. Recent studies in rodent models indicate mechanisms for the K+ benefit: the distal tubule Na+-Cl- cotransporter (NCC) controls Na+ delivery downstream to the collecting duct, where Na+ reabsorbed by epithelial Na+ channels drives K+ secretion and excretion through K+ channels in the same region. High dietary K+ provokes a decrease in NCC activity to drive more K+ secretion (and Na+ excretion, analogous to the actions of a thiazide diuretic) whether Na+ intake is high or low; low dietary K+ provokes an increase in NCC activity and Na+ retention, also independent of dietary Na+. Together, the findings suggest that public health efforts directed toward increasing consumption of K+-rich natural foods would reduce BP and, thus, cardiovascular and kidney disease. [ABSTRACT FROM AUTHOR]

Sodium and its associated anions are the major determinant of extracellular fluid volume, and the reabsorption of Na+ by the kidney plays a crucial role in long-term blood pressure control. The goal of this study was to investigate the extent to which inhibitors of transepithelial N+ transport (TNa) along the nephron alter urinary solute excretion and TNa efficiency and how those effects may vary along different nephron segments. To accomplish that goal, we used the multinephron model developed in the companion study (28). That model represents detailed transcellular and paracellular transport processes along the nephrons of a rat kidney. We simulated the inhibition of the Na+/H+ exchanger (NHE3), the bumetanide-sensitive Na+-K+-2Cl- transporter (NKCC2), the Na+-Cl- cotransporter (NCC), and the amiloride-sensitive Na+ channel (ENaC). Under baseline conditions, NHE3, NKCC2, NCC, and ENaC reabsorb 36, 22, 4, and 7%, respectively, of filtered Na+. The model predicted that inhibition of NHE3 substantially reduced proximal tubule TNa and oxygen consumption (QO2). Whole-kidney TNa efficiency, as reflected by the number of moles of Na+ reabsorbed per moles of O2 consumed (denoted by the ratio TNa/QO2), decreased by ~20% with 80% inhibition of NHE3. NKCC2 inhibition simulations predicted a substantial reduction in thick ascending limb TNa and QO2; however, the effect on whole-kidney TNa/QO2 was minor. Tubular K+transport was also substantially impaired, resulting in elevated urinary K+ excretion. The most notable effect of NCC inhibition was to increase the excretion of Na+, K+, and Cl- its impact on whole-kidney TNa and its efficiency was minor. Inhibition of ENaC was predicted to have opposite effects on the excretion of Na+ (increased) and K+ (decreased) and to have only a minor impact on whole-kidney TNa and TNa/QO2. Overall, model predictions agree well with measured changes in Na+ and K+ excretion in response to diuretics and Na+ transporter mutations. [ABSTRACT FROM AUTHOR]

The discovery of four genes responsible for pseudohypoaldosteronism type II, or familial hyperkalemic hypertension, which features arterial hypertension with hyperkalemia and metabolic acidosis, unmasked a complex multiprotein system that regulates electrolyte transport in the distal nephron. Two of these genes encode the serine-threonine kinases WNK1 and WNK4. The other two genes [kelch-like 3 ( KLHL3) and cullin 3 ( CUL3)] form a RING-type E3-ubiquitin ligase complex that modulates WNK1 and WNK4 abundance. WNKs regulate the activity of the Na+:Cl− cotransporter (NCC), the epithelial sodium channel (ENaC), the renal outer medullary potassium channel (ROMK), and other transport pathways. Interestingly, the modulation of NCC occurs via the phosphorylation by WNKs of other serine-threonine kinases known as SPAK-OSR1. In contrast, the process of regulating the channels is independent of SPAK-OSR1. We present a review of the remarkable advances in this area in the past 10 years. [ABSTRACT FROM AUTHOR]

Cirrhosis is a frequent and severe disease, complicated by renal sodium retention leading to ascites and oedema. A better understanding of the complex mechanisms responsible for renal sodium handling could improve clinical management of sodium retention. Our aim was to determine the importance of the amiloride-sensitive epithelial sodium channel (ENaC) in collecting ducts in compensate and decompensate cirrhosis. Bile duct ligation was performed in control mice (CTL) and collecting duct-specific αENaC knockout (KO) mice, and ascites development, aldosterone plasma concentration, urinary sodium/potassium ratio and sodium transporter expression were compared. Disruption of ENaC in collecting ducts (CDs) did not alter ascites development, urinary sodium/potassium ratio, plasma aldosterone concentrations or Na,K-ATPase abundance in CCDs. Total αENaC abundance in whole kidney increased in cirrhotic mice of both genotypes and cleaved forms of α and γ ENaC increased only in ascitic mice of both genotypes. The sodium chloride cotransporter (NCC) abundance was lower in non-ascitic KO, compared to non-ascitic CTL, and increased when ascites appeared. In ascitic mice, the lack of αENaC in CDs induced an upregulation of total ENaC and NCC and correlated with the cleavage of ENaC subunits. This revealed compensatory mechanisms which could also take place when treating the patients with diuretics. These compensatory mechanisms should be considered for future development of therapeutic strategies. [ABSTRACT FROM AUTHOR]

In Sprague Dawley rats, 2-week angiotensin II (Ang II) infusion increases Na+ transporter abundance and activation from cortical thick ascending loop of Henle ( TALH) to medullary collecting duct ( CD) and raises blood pressure associated with a pressure natriuresis, accompanied by depressed Na+ transporter abundance and activation from proximal tubule ( PT) through medullary TALH. This study tests the hypothesis that early during Ang II infusion, before blood pressure raises, Na+ transporters' abundance and activation increase all along the nephron. Male Sprague Dawley rats were infused via osmotic minipumps with a subpressor dose of Ang II (200 ng/kg/min) or vehicle for 3 days. Overnight urine was collected in metabolic cages and sodium transporters' abundance and phosphorylation were determined by immunoblotting homogenates of renal cortex and medulla. There were no significant differences in body weight gain, overnight urine volume, urinary Na+ and K+ excretion, or rate of excretion of a saline challenge between Ang II and vehicle infused rats. The 3-day nonpressor Ang II infusion significantly increased the abundance of PT Na+/H+ exchanger 3 ( NHE3), cortical TALH Na-K-2Cl cotransporter 2 ( NKCC2), distal convoluted tubule ( DCT) Na-Cl cotransporter ( NCC), and cortical CD ENaC subunits. Additionally, phosphorylation of cortical NKCC2, NCC, and STE20/ SPS1-related proline-alanine-rich kinase ( SPAK) were increased; medullary NKCC2 and SPAK were not altered. In conclusion, 3-day Ang II infusion provokes PT NHE3 accumulation as well as NKCC2, NCC, and SPAK accumulation and activation in a prehypertensive phase before evidence for intrarenal angiotensinogen accumulation. [ABSTRACT FROM AUTHOR]

During angiotensin II (ANG II)-dependent hypertension, ANG II stimulates, while hypertension inhibits, Na+ transporter activity to balance Na+ output to input. This study tests the hypothesis that ANG II infusion activates Na+ transporters in the distal nephron while inhibiting transporters along the proximal nephron. Male Sprague-Dawley rats were infused with ANG II (400 ng·kg-1·min-1) or vehicle for 2 wk. Kidneys were dissected (cortex vs. medulla) or fixed for immunohistochemistry (IHC). ANG II increased mean arterial pressure by 40 mmHg, urine Na+ by 1.67-fold, and urine volume by 3-fold, evidence for hypertension and pressure natriuresis. Na+ transporters' abundance and activation [assessed by phosphorylation (-P) or proteolytic cleavage] were measured by immunoblot. During ANG II infusion Na+/H+ exchanger 3 (NHE3) abundance decreased in both cortex and medulla; Na-K-2Cl cotransporter 2 (NKCC2) decreased in medullary thick ascending loop of Henle (TALH) and increased, along with NKCC2-P, in cortical TALH; Na-Cl cotransporter (NCC) and NCC-P increased in the distal convoluted tubule; and epithelial Na+ channel subunits and their cleaved forms were increased in both cortex and medulla. Like NKCC2, STE20/SPS1-related proline alanine-rich kinase (SPAK) and SPAK-P were decreased in medulla and increased in cortex. By IHC, during ANG II NHE3 remained localized to proximal tubule microvilli at lower abundance, and the differential regulation of NKCC2 and NKCC2-P in cortex versus medulla was evident. In summary, ANG II infusion increases Na+ transporter abundance and activation from cortical TALH to medullary collecting duct while the hypertension drives a natriuresis response evident as decreased Na+ transporter abundance and activation from proximal tubule through medullary TALH. [ABSTRACT FROM AUTHOR]

Salt-sensitive hypertension is closely related with natriuretic capacity of the kidney. Besides several genome-wide research reported candidate gene or gene polymorphism responsible for salt-sensitive hypertension, recently, several new factors for acquired salt-sensitive hypertension are reported. Among them, we have identified that rac1, a small GTPase, activates mineralocorticoid receptor in aldosterone-independent fashion and induces salt-sensitive hypertension in several rodent model. On the other hand, sympathoactivation in the brain and/or kidney regulate sodium handlings in the kidney. Recently it is reported that oxidative stress in the brain or in the kidney may modulate sympathetic tone. Moreover, we reported that β2 adrenoceptor alters histone acetylation and further regulates sodium resorption at distal tubules via activating glucocorticoid receptor. These regulations are to be confirmed in humans and the future, and may open a new door for diagnosis and treatment of salt-sensitive hypertension or moreover preventing development of salt-sensitive hypertension. [ABSTRACT FROM AUTHOR]

Nedd4-2 is a ubiquitin ligase previously demonstrated to regulate endocytosis and lyso-somal degradation of the epithelial Na+ channel (ENaC) and other ion channels and transporters. Recent studies using Nedd4-2 knockout mice specifically in kidney or lung epithelia has revealed a critical role for this E3 ubiquitin ligase in regulating salt and fluid transport in these tissues/organs and in maintaining homeostasis of body blood pressure. Interestingly, the primary targets for Nedd4-2 may differ in these two organs: in the lung Nedd4-2 targets ENaC, and loss of Nedd4-2 leads to excessive ENaC function and to cystic fibro-sis - like lung disease, whereas in the kidney, Nedd4-2 targets the Na+/Cl- cotransporter (NCC) in addition to targeting ENaC. In accord, loss of Nedd4-2 in the distal nephron leads to increased NCC abundance and function. The aldosterone-responsive kinase, Sgk1, appears to be involved in the regulation of NCC by Nedd4-2 in the kidney, similar to its regulation of ENaC. Collectively, these new findings underscore the physiological importance of Nedd4-2 in regulating epithelial salt and fluid transport and balance. [ABSTRACT FROM AUTHOR]

The renal outer medullary potassium channel (ROMK) is an adenosine triphosphate-sensitive inward-rectifier potassium channel (Kir1.1 or KCNJ1) highly expressed in the cortical and medullary thick ascending limbs (TAL), connecting segment (CNT) and cortical collecting duct (CCD) in the mammalian kidney, where it serves to recycle potassium (K) across the apical membrane in TAL and to secrete K in the CNT and CCD. ROMK channel mutations cause type II Bartter's syndrome with salt wasting and dehydration, and ROMK knockout mice display a similar phenotype of Bartter's syndrome in humans. Studies from ROMK null mice indicate that ROMK is required to form both the small-conductance (30pS, SK) K channels and the 70pS (IK) K channels in the TAL. The availability of ROMK mice has made it possible to study electrolyte transport along the nephron in order to understand the TAL function under physiological conditions and the compensatory mechanisms of salt and water transport under the conditions of TAL dysfunction. This review summarizes previous progress in the study of K channel activity in the TAL and CCD, ion transporter expression and activities along the nephron, and renal functions under physiological and pathophysiological conditions using ROMK mice. [ABSTRACT FROM AUTHOR]

Regulation of transport by principal cells of the distal nephron contributes to maintenance of Na+ and K+ homeostasis. To assess which of these ions is given a higher priority by these cells, we investigated the upregulation of epithelial Na+ channels (ENaC) in the rat cortical collecting duct (CCD) during Na depletion with and without simultaneous K depletion. ENaC activity, assessed as whole cell amiloride-sensitive current in split-open tubules, was 260 ± 40 pA/cell in K-repleted but virtually undetectable (3 ± 1 pA/cell) in K-depleted animals. This difference was confirmed biochemically by the reduced amounts of the cleaved forms of both the a-ENaC and γ-ENaC subunits measured in immunoblots. In contrast, in K-depleted rats, simultaneously reducing Na intake did not affect the activity of ROMK channels, assessed as tertiapin-Q-sensitive whole cell currents, in the CCDs. The lack of Na current in K-depleted animals was the result of reduced levels of aldosterone in plasma, rather than a reduced sensitivity to the hormone. However, rats on a low-Na, low-K diet for 1 wk did not excrete more Na than those on a low-Na, control-K diet for the same period of time. Immunoblot analysis indicated increased levels of the thiazide-sensitive NaCl cotransporter and the apical Na-H exchanger NHE3. This suggests that with reduced K intake, Na balance is maintained despite reduced aldosterone and Na+ channel activity by upregulation of Na+ transport in upstream segments. Under these conditions, Na+ transport by the aldosterone-sensitive distal nephron is reduced, despite the low-Na intake to minimize K+ secretion and urinary K losses. [ABSTRACT FROM AUTHOR]

Changes in apical surface expression of ion channels and transporters in the superficial rat renal cortex were assessed using biotinylation and immunoblotting during alterations in dietary K intake. A high-K diet increased, and a low-K diet decreased, both the overall and surface abundance of the β- and γ-subunits of the epithelial Na channel (ENaC). In the case of γ-ENaC, the effect was specific for the 65-kDa cleaved form of the protein. The overall amount of α-ENAC was also increased with increasing K intake. The total expression of the secretory K+ channels (ROMK) increased with a high-K diet and decreased with a low-K diet. The surface expression of ROMK increased with high K intake but was not significantly altered by a low-K diet. In contrast, the amounts of total and surface protein representing the thiazide-sensitive NaCl cotransporter (NCC) decreased with increasing K+ intake. We conclude that modulation of K+ secretion in response to changes in dietary K intake involves changes in apical K+ permeabili through regulation of K+ channels and in driving force subsequent to alterations in both Na delivery to the distal nephron and Na+ uptake across the apical membrane of the K+ secretory cells. [ABSTRACT FROM AUTHOR]

Gordon’s syndrome, also known as pseudohypoaldosteronism type II (PHA II) or familial hypertension with hyperkalemia, is an autosomal-dominant disease characterized by hypertension, hyperkalemia, hyperchloremic metabolic acidosis, and normal glomerular filtration rate. Recent positional cloning has linked mutations of WNK1 and WNK4 to Gordon’s syndrome. With-no-lysine [K] (WNK) kinases are a new family of large serine–threonine protein kinases with an atypical placement of the catalytic lysine. Here, we review the pathogenesis of PHA II based on current understanding of the actions of WNK1 and WNK4 on Na+ and K+ handling in the renal distal tubule. [ABSTRACT FROM AUTHOR]

With-No-Lysine [K] (WNKs) are a recently discovered family of serine/threonine protein kinases that contain a uniquely structured catalytic domain. Mutations in the genes encoding two family members, WNK1 and WNK4, cause a chloride-dependent, thiazide-sensitive inherited syndrome of hypertension and hyperkalemia. Over the past 5 years, physiologic studies have demonstrated that these proteins regulate transcellular and paracellular epithelial ion flux. In this mini review, we discuss WNK1 and WNK4 gene products and their regulatory effects on sodium chloride and potassium handling in the aldosterone-sensitive distal nephron. Experimental observations regarding the effects of these proteins on transport processes mediated by the thiazide-sensitive Na–Cl co-transporter, the epithelial sodium channel, the renal outer medullary potassium channel, and the paracellular pathway integrate into a model that suggests an essential role for WNKs in coordinating renal Na–Cl reabsorption and K+ secretion.Kidney International (2006) 70, 630–634. doi:10.1038/sj.ki.5001634; published online 5 July 2006 [ABSTRACT FROM AUTHOR]

Hypertensive inbred rats (ISIAH; inherited stress-induced arterial hypertension) present with baseline hypertension (>170 mmHg in adult rats), but attain substantially higher values upon mild emotional stress. We aimed to characterize key parameters related to hypertension in ISIAH. Kidneys, adrenals, and systemic endocrine parameters were studied in ISIAH of different ages and compared to normotensive Wistar albino Glaxo (WAG) rats. Native organs were obtained for Western and PCR analysis. Perfusion-fixed organs were prepared for histopathology and quantitative histochemistry. Plasma renin and adrenal hormones were measured. Renal morphology was unaltered in ISIAH. The hypothalamic-pituitary-adrenocortical (HPA) axis was constitutively upregulated with enlarged adrenal cortices and enhanced plasma corticosterone levels. Plasma renin activity was not different between groups, whereas aldosterone levels were in part reduced. Juxtaglomerular NO synthase type 1, cyclooxygenase type 2, and renin expression were significantly reduced, whereas tubular gene products related to sodium transport (bumetanide-sensitive Na, K, 2Cl cotransporter type 2; thiazide-sensitive Na, Cl cotransporter; epithelial Na channel-α; 11β-hydroxysteroid dehydrogenase type 2) were increased. These data suggest enhanced volume conservation by the kidney. Our data define ISIAH as an attractive model for the renal components determining salt and water homeostasis in hypertension. The specific condition of a basally stimulated HPA axis is highlighted, including the option to study effects superimposed by emotional stress. [ABSTRACT FROM AUTHOR]

Aldosterone is the body’s major hormone involved in volume homeostasis because of its effects on sodium reabsorption in the distal nephron. Our comprehension of the signaling pathways that this mineralocorticoid unleashes has been enhanced through the convergence of bedside physiologic observations with advances in medical genetics and molecular biology. This overview updates our current understanding of the aldosterone-initiated pathways throughout the distal nephron to promote sodium retention. Three essential features of the pathways are explored: how the mineralocorticoid gains specificity and targets gene transcription in distal tubular cells; how the key endpoints of aldosterone action in these cells—the epithelial sodium channel, the thiazide-sensitive sodium chloride cotransporter, and Na,K,ATPase—are regulated; and how 3 kinases, directly or indirectly, are activated by aldosterone and serve as critical intermediaries in regulating the sodium transporters. Remarkably, perturbations in many genes integral to aldosterone-induced pathways result in blood-pressure abnormalities. The familial disorders of hypertension and hypotension that follow from these mutated genes are presented with their molecular and physiologic consequences. The clustering of so many genetic disorders within the aldosterone-sensitive distal nephron supports the hypothesis that renal sodium regulation plays a pivotal role in long-term blood-pressure control. Identifying and characterizing other components of the pathways that modulate these sodium transporters represent the core challenges in this scientific field. It is posited that meeting these challenges will help elucidate the pathogenesis of human hypertension and provide new therapeutic options for its treatment. [ABSTRACT FROM AUTHOR]

Diabetes mellitus is associated with natriuresis, whereas estrogen has been shown to be renoprotective in diabetic nephropathy and may independently regulate renal sodium reabsorption. The aim of this study was to determine the effects of 17-β estradiol (E2) replacement to diabetic, ovariectomized (OVX) female rats on the expression of major renal sodium transporters. Female, Sprague–Dawley rats (210 g) were randomized into four groups: (1) OVX; (2) OVX+E2; (3) diabetic+ovariectomized (D+OVX); and (4) diabetic+ovariectomized+estrogen (D+OVX+E2). Diabetes was induced by a single intraperitoneal injection of streptozotocin (55 mg/kg·body weight (bw)). Rats received phytoestrogen-free diet and water ad libitum for 12 weeks. E2 attenuated hyperglycemia, hyperalbuminuria, and hyperaldosteronism in D rats, as well as the diabetes-induced changes in renal protein abundances for the bumetanide-sensitive Na–K–2Cl cotransporter (NKCC2), and the α- and β-subunits of the epithelial sodium channel (ENaC), that is, E2 decreased NKCC2, but increased α- and β-ENaC abundances. In nondiabetic rats, E2 decreased plasma K+ and increased urine K+/Na+ ratio, as well as decreased the abundance of NKCC2, β-ENaC, and α-1-Na–K–adenosine triphosphate (ATP)ase in the outer medulla. Finally, the diabetic, E2 rats had measurably lower final circulating levels of E2 than the nondiabetic E2 rats, despite an identical replacement protocol, suggesting a shorter biological half-life of E2 with diabetes. Therefore, E2 attenuated diabetes and preserved renal sodium handling and related transporter expression levels. In addition, E2 had diabetes-independent effects on renal electrolyte handling and associated proteins.Kidney International (2006) 69, 471–480. doi:10.1038/sj.ki.5000140; published online 6 January 2006 [ABSTRACT FROM AUTHOR]

Sodium-sensitive hypertension is thought to be dependent on primary alterations in renal tubular sodium reabsorption. The major apical plasma membrane Na+ transporters include the proximal tubular Na+-H+ exchanger, the thick ascending limb Na+-K+-2Cl− cotransport system, the distal tubular Na+-Cl− cotransporter, and the collecting duct epithelial sodium channel (ENaC). This article explores the role of each transporter in the pathogenesis of hypertension. Although the contribution of the proximal tubule Na+-H+ exchanger is not yet defined completely, more convincing data have been generated about the importance of the Na+-K+-2Cl−. Indeed at least 2 forms of hypertension appear to be related to the up-regulation of the transporter: the so-called programmed hypertension induced by low-protein diet during pregnancy and the early phase of hypertension in the Milan strain of rats. With respect to the Na+-Cl− cotransporter this may be overactive caused by inactivating mutation of WNK4 as in the Gordon syndrome, although it is the main actor for the maintenance phase of the hypertension found in the Milan strain of rats. Finally, the contribution of the ENaC has been established clearly; indeed, in the Liddle syndrome the mutation of the ENaC gene leads to a longer retention of the channel on the cell surface of collecting duct principal cells, thus inducing stronger sodium reabsorption along this segment. All these examples clearly indicate that renal sodium transporters may be responsible for various types of sodium-sensitive hypertension. [ABSTRACT FROM AUTHOR]

Background: Liver cirrhosis with ascites is associated with a decrease in renal sodium excretion and therefore sodium retention.Methods: In this paper, we utilize transporter-specific antibodies to address the hypothesis that dysregulation of one or more sodium transporters or channels is associated with sodium chloride (NaCl) retention in a rat model of cirrhosis induced by repeated exposure to carbon tetrachloride. Age-matched controls and cirrhotic rats were pair fed to ensure identical NaCl and water intake for 4 days prior to euthanasia for quantitative immunoblotting studies.Results and Conclusion: The rats manifested marked extracellular fluid volume expansion with massive ascites. Plasma aldosterone levels were markedly elevated. Analysis of immunoblots revealed marked increases in the abundances of both of the major aldosterone-sensitive apical transport proteins of the renal tubule, namely the thiazide-sensitive NaCl cotransporter NCC and the epithelial sodium channel alpha subunit (alpha-ENaC). These results are consistent with an important role for hyperaldosteronism in the pathogenesis of sodium retention and ascites formation in cirrhosis. In addition, we observed a large decrease in cortical NHE3 abundance (proximal tubule) and a large increase in NKCC2 abundance (thick ascending limb), potentially shifting premacula densa sodium absorption from proximal tubule to loop of Henle (which powers urinary concentration and dilution). [ABSTRACT FROM AUTHOR]