Dietary sodium induces a redistribution of the tubular metabolic workload

Cyclosporin-induced hypertension is associated with the up-regulation of Na+-K+-2Cl- cotransporter (NKCC2)

BACKGROUND

The use of cyclosporin A (CsA) is hampered by the development of nephrotoxicity including hypertension, which is partially dependent on renal sodium retention. To address this issue, we have investigated in vivo sodium reabsorption in different nephron segments of CsA-treated rats through micropuncture study coupled to expression analyses of sodium transporters. To translate the findings in rats to human, kidney-transplanted patients having CsA treatment were enrolled in the study.

METHODS

Adult male Sprague-Dawley rats were treated with CsA (15 mg/kg/day) for 21 days, followed by micropuncture study and expression analyses of sodium transporters. CsA-treated kidney-transplanted patients with resistant hypertension were challenged with 50 mg furosemide.

RESULTS

CsA-treated rats developed hypertension associated with reduced glomerular filtration rate. In vivo microperfusion study demonstrated a significant decrease in rate of absolute fluid reabsorption in the proximal tubule but enhanced sodium reabsorption in the thick ascending limb of Henle's loop (TAL). Expression analyses of sodium transporters at the same nephron segments further revealed a reduction in Na+-H+ exchanger isoform 3 (NHE3) in the renal cortex, while TAL-specific, furosemide-sensitive Na+-K+-2Cl- cotransporter (NKCC2) and NHE3 were significantly upregulated in the inner stripe of outer medulla. CsA-treated patients had a larger excretion of urinary NKCC2 protein at basal condition, and higher diuretic response to furosemide, showing increased FeNa+, FeCl- and FeCa2+ compared with both healthy controls and FK506-treated transplanted patients.

CONCLUSION

Altogether, these findings suggest that up-regulation of NKCC2 along the TAL facilitates sodium retention and contributes to the development of CsA-induced hypertension.

Angiotensin II hypertension along the female rat tubule: predicted impact on coupled transport of Na+ and K. Am J Physiol Renal Physiol 2023;325:F733-F749. [PMID: 37823196 PMCID: PMC10878725 DOI: 10.1152/ajprenal.00232.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Chronic infusion of subpressor level of angiotensin II (ANG II) increases the abundance of Na+ transporters along the distal nephron, balanced by suppression of Na+ transporters along the proximal tubule and medullary thick ascending limb (defined as "proximal nephron"), which impacts K+ handling along the entire renal tubule. The objective of this study was to quantitatively assess the impact of chronic ANG II on the renal handling of Na+ and K+ in female rats, using a computational model of the female rat renal tubule. Our results indicate that the downregulation of proximal nephron Na+ reabsorption (TNa), which occurs in response to ANG II-triggered hypertension, involves changes in both transporter abundance and trafficking. Our model suggests that substantial (∼30%) downregulation of active NHE3 in proximal tubule (PT) microvilli is needed to reestablish the Na+ balance at 2 wk of ANG II infusion. The 35% decrease in SGLT2, a known NHE3 regulator, may contribute to this downregulation. Both depression of proximal nephron TNa and stimulation of distal ENaC raise urinary K+ excretion in ANG II-treated females, while K+ loss is slightly mitigated by cortical NKCC2 and NCC upregulation. Our model predicts that K+ excretion may be more significantly limited during ANG II infusion by ROMK inhibition in the distal nephron and/or KCC3 upregulation in the PT, which remain open questions for experimental validation. In summary, our analysis indicates that ANG II hypertension triggers a series of events from distal TNa stimulation followed by compensatory reduction in proximal nephron TNa and accompanying adjustments to limit excessive K+ secretion.NEW & NOTEWORTHY We used a computational model of the renal tubule to assess the impact of 2-wk angiotensin II (ANG II) infusion on the handling of Na+ and K+ in female rats. ANG II strongly stimulates distal Na+ reabsorption and K+ secretion. Simulations indicate that substantial downregulation of proximal tubule NHE3 is needed to reestablish Na+ balance at 2 wk. Proximal adaptations challenge K+ homeostasis, and regulation of distal NCC and specific K+ channels likely limit urinary K+ losses.

A low dietary sodium dose is associated with a more pronounced aldosterone response in normotensive than in hypertensive individuals

In this comprehensive meta-regression analysis encompassing 79 randomized controlled trials, we observed that in populations assigned to a high sodium intake level exceeding 94 mmol, there was no discernible link between plasma aldosterone levels and sodium intake. However, among populations with normal blood pressure subjected to a lower sodium intake, falling below 111 mmol (N = 1544), the association between sodium intake and plasma aldosterone levels manifested as a decrease of 192 pg/ml per 100 mmol of sodium (95% CI - 303 to - 81). In hypertensive populations (N = 1145), this association was less pronounced, with a reduction of 46 pg/ml per 100 mmol sodium, (95% CI - 112 to 20). Furthermore, in normotensive populations the plasma aldosterone increase associated with a decrease in sodium intake was 70 pg/ml per 100 mmol sodium (95% CI 27 to 113). In hypertensive populations, the observed increase was more modest, at 30 pg/ml per 100 mmol sodium, (95% CI 6.8 to 54). A limitation of this study lies in the absence of individual participant data. Our analysis included adjustments for potential effect-modifiers, encompassing bias estimation, which did not substantially alter these associations. One perspective of the present results may be to prompt a reconsideration of current sodium reduction recommendations.

Fetal Reprogramming of Nutrient Surplus Signaling, O-GlcNAcylation, and the Evolution of CKD

ABSTRACT

Fetal kidney development is characterized by increased uptake of glucose, ATP production by glycolysis, and upregulation of mammalian target of rapamycin (mTOR) and hypoxia-inducible factor-1 alpha (HIF-1 α ), which (acting in concert) promote nephrogenesis in a hypoxic low-tubular-workload environment. By contrast, the healthy adult kidney is characterized by upregulation of sirtuin-1 and adenosine monophosphate-activated protein kinase, which enhances ATP production through fatty acid oxidation to fulfill the needs of a normoxic high-tubular-workload environment. During stress or injury, the kidney reverts to a fetal signaling program, which is adaptive in the short term, but is deleterious if sustained for prolonged periods when both oxygen tension and tubular workload are heightened. Prolonged increases in glucose uptake in glomerular and proximal tubular cells lead to enhanced flux through the hexosamine biosynthesis pathway; its end product-uridine diphosphate N -acetylglucosamine-drives the rapid and reversible O-GlcNAcylation of thousands of intracellular proteins, typically those that are not membrane-bound or secreted. Both O-GlcNAcylation and phosphorylation act at serine/threonine residues, but whereas phosphorylation is regulated by hundreds of specific kinases and phosphatases, O-GlcNAcylation is regulated only by O-GlcNAc transferase and O-GlcNAcase, which adds or removes N-acetylglucosamine, respectively, from target proteins. Diabetic and nondiabetic CKD is characterized by fetal reprogramming (with upregulation of mTOR and HIF-1 α ) and increased O-GlcNAcylation, both experimentally and clinically. Augmentation of O-GlcNAcylation in the adult kidney enhances oxidative stress, cell cycle entry, apoptosis, and activation of proinflammatory and profibrotic pathways, and it inhibits megalin-mediated albumin endocytosis in glomerular mesangial and proximal tubular cells-effects that can be aggravated and attenuated by augmentation and muting of O-GlcNAcylation, respectively. In addition, drugs with known nephroprotective effects-angiotensin receptor blockers, mineralocorticoid receptor antagonists, and sodium-glucose cotransporter 2 inhibitors-are accompanied by diminished O-GlcNAcylation in the kidney, although the role of such suppression in mediating their benefits has not been explored. The available evidence supports further work on the role of uridine diphosphate N -acetylglucosamine as a critical nutrient surplus sensor (acting in concert with upregulated mTOR and HIF-1 α signaling) in the development of diabetic and nondiabetic CKD.

Metabolic Responses of Normal Rat Kidneys to a High Salt Intake

In this study, novel methods were developed, which allowed continuous (24/7) measurement of arterial blood pressure and renal blood flow in freely moving rats and the intermittent collection of arterial and renal venous blood to estimate kidney metabolic fluxes of O2 and metabolites. Specifically, the study determined the effects of a high salt (HS; 4.0% NaCl) diet upon whole kidney O2 consumption and arterial and renal venous plasma metabolomic profiles of normal Sprague-Dawley rats. A separate group of rats was studied to determine changes in the cortex and outer medulla tissue metabolomic and mRNAseq profiles before and following the switch from a 0.4% to 4.0% NaCl diet. In addition, targeted mRNA expression analysis of cortical segments was performed. Significant changes in the metabolomic and transcriptomic profiles occurred with feeding of the HS diet. A progressive increase of kidney O2 consumption was found despite a reduction in expression of most of the mRNA encoding enzymes of TCA cycle. A novel finding was the increased expression of glycolysis-related genes in Cx and isolated proximal tubular segments in response to an HS diet, consistent with increased release of pyruvate and lactate from the kidney to the renal venous blood. Data suggests that aerobic glycolysis (eg, Warburg effect) may contribute to energy production under these circumstances. The study provides evidence that kidney metabolism responds to an HS diet enabling enhanced energy production while protecting from oxidative stress and injury. Metabolomic and transcriptomic analysis of kidneys of Sprague-Dawley rats fed a high salt diet.

Molecular Physiological Evidence for the Role of Na+-Cl− Co-Transporter in Branchial Na+ Uptake in Freshwater Teleosts

The gills are the major organ for Na+ uptake in teleosts. It was proposed that freshwater (FW) teleosts adopt Na+/H+ exchanger 3 (Nhe3) as the primary transporter for Na+ uptake and Na+-Cl− co-transporter (Ncc) as the backup transporter. However, convincing molecular physiological evidence to support the role of Ncc in branchial Na+ uptake is still lacking due to the limitations of functional assays in the gills. Thus, this study aimed to reveal the role of branchial Ncc in Na+ uptake with an in vivo detection platform (scanning ion-selective electrode technique, SIET) that has been recently established in fish gills. First, we identified that Ncc2-expressing cells in zebrafish gills are a specific subtype of ionocyte (NCC ionocytes) by using single-cell transcriptome analysis and immunofluorescence. After a long-term low-Na+ FW exposure, zebrafish increased branchial Ncc2 expression and the number of NCC ionocytes and enhanced gill Na+ uptake capacity. Pharmacological treatments further suggested that Na+ is indeed taken up by Ncc, in addition to Nhe, in the gills. These findings reveal the uptake roles of both branchial Ncc and Nhe under FW and shed light on osmoregulatory physiology in adult fish.

miRNA-23a modulates sodium-hydrogen exchanger 1 expression: studies in medullary thick ascending limb of salt-induced hypertensive rats

BACKGROUND

The kidney is the main organ in the pathophysiology of essential hypertension. Although most bicarbonate reabsorption occurs in the proximal tubule, the medullary thick ascending limb (mTAL) of the nephron also maintains acid-base balance by contributing to 25% of bicarbonate reabsorption. A crucial element in this regulation is the sodium-hydrogen exchanger 1 (NHE1), a ubiquitous membrane protein controlling intracellular pH, where proton extrusion is driven by the inward sodium flux. MicroRNA (miRNA) expression of hypertensive patients significantly differs from that of normotensive subjects. The aim of this study was to determine the functional role of miRNA alterations at the mTAL level.

METHODS

By miRNA microarray analysis, we identified miRNA expression profiles in isolated mTALs from high sodium intake-induced hypertensive rats (HSD) versus their normotensive counterparts (NSD). In vitro validation was carried out in rat mTAL cells.

RESULTS

Five miRNAs involved in the onset of salt-sensitive hypertension were identified, including miR-23a, which was bioinformatically predicted to target NHE1 mRNA. Data demonstrated that miRNA-23a is downregulated in the mTAL of HSD rats while NHE1 is upregulated. Consistently, transfection of an miRNA-23a mimic in an mTAL cell line, using a viral vector, resulted in NHE1 downregulation.

CONCLUSION

NHE1, a protein involved in sodium reabsorption at the mTAL level and blood pressure regulation, is upregulated in our model. This was due to a downregulation of miRNA-23a. Expression levels of this miRNA are influenced by high sodium intake in the mTALs of rats. The downregulation of miRNA-23a in humans affected by essential hypertension corroborate our data and point to the potential role of miRNA-23a in the regulation of mTAL function following high salt intake.

Metabolic responses of normal rat kidneys to a high salt intake

In the present study, novel methods were developed which allowed continuous (24/7) measurement of blood pressure (BP) and renal blood flow (RBF) in freely moving rats and the intermittent collection of arterial and renal venous blood to estimate kidney metabolic fluxes of O 2 and metabolites. The study determined the effects of a high salt (HS) diet upon whole kidney O 2 consumption and the metabolomic profiles of normal Sprague Dawley (SD) rats. A separate group of rats was studied to determine changes in the cortex (Cx) and outer medulla (OM) tissue metabolomic and mRNAseq profiles before and following the switch from a 0.4% to a 4.0% NaCl diet. Significant changes in the metabolomic and transcriptomic profiles occurred with feeding of the HS diet. A progressive increase of kidney O 2 consumption was found despite a reduction in expression of most of the mRNA encoding enzymes of TCA cycle. Increased glycolysis was evident with the elevation of mRNA expression encoding key glycolytic enzymes and release of pyruvate and lactate from the kidney in the renal venous blood. Glycolytic production of NADH is used in either the production of lactate or oxidized via the malate aspartate shuttle. Aerobic glycolysis (e.g., Warburg-effect) may account for the needed increase in cellular energy. The study provides evidence that kidney metabolism responds to a HS diet enabling enhanced energy production while protecting from oxidate stress and injury.

Diuretic resistance in patients with kidney disease: Challenges and opportunities

Edema caused by kidney disease is called renal edema. Edema is a common symptom of many human kidney diseases. Patients with renal edema often need to take diuretics.However, After taking diuretics, patients with kidney diseases are prone to kidney congestion, decreased renal perfusion, decreased diuretics secreted by renal tubules, neuroendocrine system abnormalities, abnormal ion transporter transport, drug interaction, electrolyte disorder, and hypoproteinemia, which lead to ineffective or weakened diuretic use and increase readmission rate and mortality. The main causes and coping strategies of diuretic resistance in patients with kidney diseases were described in detail in this report. The common causes of DR included poor diet (electrolyte disturbance and hypoproteinemia due to patients' failure to limit diet according to correct sodium, chlorine, potassium, and protein level) and poor drug compliance (the patient did not take adequate doses of diuretics. true resistance occurs only if the patient takes adequate doses of diuretics, but they are not effective), changes in pharmacokinetics and pharmacodynamics, electrolyte disorders, changes in renal adaptation, functional nephron reduction, and decreased renal blood flow. Common treatment measures include increasing in the diuretic dose and/or frequency, sequential nephron blockade,using new diuretics, ultrafiltration treatment, etc. In clinical work, measures should be taken to prevent or delay the occurrence and development of DR in patients with kidney diseases according to the actual situation of patients and the mechanism of various causes. Currently, there are many studies on DR in patients with heart diseases. Although the phenomenon of DR in patients with kidney diseases is common, there is a relatively little overview of the mechanism and treatment strategy of DR in patients with kidney diseases. Therefore, this paper hopes to show the information on DR in patients with kidney diseases to clinicians and researchers and broaden the research direction and ideas to a certain extent.

Current understanding on pathogenesis and effective treatment of glycogen storage disease type Ib with empagliflozin: new insights coming from diabetes for its potential implications in other metabolic disorders

Glycogen storage type Ib (GSDIb) is a rare inborn error of metabolism caused by glucose-6-phosphate transporter (G6PT, SLC37A4) deficiency. G6PT defect results in excessive accumulation of glycogen and fat in the liver, kidney, and intestinal mucosa and into both glycogenolysis and gluconeogenesis impairment. Clinical features include hepatomegaly, hypoglycemia, lactic acidemia, hyperuricemia, hyperlipidemia, and growth retardation. Long-term complications are liver adenoma, hepatocarcinoma, nephropathy and osteoporosis. The hallmark of GSDIb is neutropenia, with impaired neutrophil function, recurrent infections and inflammatory bowel disease. Alongside classical nutritional therapy with carbohydrates supplementation and immunological therapy with granulocyte colony-stimulating factor, the emerging role of 1,5-anhydroglucitol in the pathogenesis of neutrophil dysfunction led to repurpose empagliflozin, an inhibitor of the renal glucose transporter SGLT2: the current literature of its off-label use in GSDIb patients reports beneficial effects on neutrophil dysfunction and its clinical consequences. Surprisingly, this glucose-lowering drug ameliorated the glycemic and metabolic control in GSDIb patients. Furthermore, numerous studies from big cohorts of type 2 diabetes patients showed the efficacy of empagliflozin in reducing the cardiovascular risk, the progression of kidney disease, the NAFLD and the metabolic syndrome. Beneficial effects have also been described on peripheral neuropathy in a prediabetic rat model. Increasing evidences highlight the role of empagliflozin in regulating the cellular energy sensors SIRT1/AMPK and Akt/mTOR, which leads to improvement of mitochondrial structure and function, stimulation of autophagy, decrease of oxidative stress and suppression of inflammation. Modulation of these pathways shift the oxidative metabolism from carbohydrates to lipids oxidation and results crucial in reducing insulin levels, insulin resistance, glucotoxicity and lipotoxicity. For its pleiotropic effects, empagliflozin appears to be a good candidate for drug repurposing also in other metabolic diseases presenting with hypoglycemia, organ damage, mitochondrial dysfunction and defective autophagy.

Inverse Salt Sensitivity of Blood Pressure: Mechanisms and Potential Relevance for Prevention of Cardiovascular Disease

PURPOSE OF REVIEW

To review the etiology of inverse salt sensitivity of blood pressure (BP).

RECENT FINDINGS

Both high and low sodium (Na+) intake can be associated with increased BP and cardiovascular morbidity and mortality. However, little is known regarding the mechanisms involved in the increase in BP in response to low Na+ intake, a condition termed inverse salt sensitivity of BP, which affects approximately 15% of the adult population. The renal proximal tubule is important in regulating up to 70% of renal Na+ transport. The renin-angiotensin and renal dopaminergic systems play both synergistic and opposing roles in the regulation of Na+ transport in this nephron segment. Clinical studies have demonstrated that individuals express a "personal salt index" (PSI) that marks whether they are salt-resistant, salt-sensitive, or inverse salt-sensitive. Inverse salt sensitivity results in part from genetic polymorphisms in various Na+ regulatory genes leading to a decrease in natriuretic activity and an increase in renal tubular Na+ reabsorption leading to an increase in BP. This article reviews the potential mechanisms of a new pathophysiologic entity, inverse salt sensitivity of BP, which affects approximately 15% of the general adult population.

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.

Renal blood flow and oxygenation

Our kidneys receive about one-fifth of the cardiac output at rest and have a low oxygen extraction ratio, but may sustain, under some conditions, hypoxic injuries that might lead to chronic kidney disease. This is due to large regional variations in renal blood flow and oxygenation, which are the prerequisite for some and the consequence of other kidney functions. The concurrent operation of these functions is reliant on a multitude of neuro-hormonal signaling cascades and feedback loops that also include the regulation of renal blood flow and tissue oxygenation. Starting with open questions on regulatory processes and disease mechanisms, we review herein the literature on renal blood flow and oxygenation. We assess the current understanding of renal blood flow regulation, reasons for disparities in oxygen delivery and consumption, and the consequences of disbalance between O2 delivery, consumption, and removal. We further consider methods for measuring and computing blood velocity, flow rate, oxygen partial pressure, and related parameters and point out how limitations of these methods constitute important hurdles in this area of research. We conclude that to obtain an integrated understanding of the relation between renal function and renal blood flow and oxygenation, combined experimental and computational modeling studies will be needed.

Activation of the Hypoxia-Inducible Factor Pathway Inhibits Epithelial Sodium Channel-Mediated Sodium Transport in Collecting Duct Principal Cells

BACKGROUND

Active sodium reabsorption is the major factor influencing renal oxygen consumption and production of reactive oxygen species (ROS). Increased sodium reabsorption uses more oxygen, which may worsen medullary hypoxia and produce more ROS via enhanced mitochondrial ATP synthesis. Both mechanisms may activate the hypoxia-inducible factor (HIF) pathway. Because the collecting duct is exposed to low oxygen pressure and variations of active sodium transport, we assessed whether the HIF pathway controls epithelial sodium channel (ENaC)-dependent sodium transport.

METHODS

We investigated HIF's effect on ENaC expression in mpkCCD cl4 cells (a model of collecting duct principal cells) using real-time PCR and western blot and ENaC activity by measuring amiloride-sensitive current. We also assessed the effect of hypoxia and sodium intake on abundance of kidney sodium transporters in wild-type and inducible kidney tubule-specific Hif1α knockout mice.

RESULTS

In cultured cells, activation of the HIF pathway by dimethyloxalylglycine or hypoxia inhibited sodium transport and decreased expression of β ENaC and γ ENaC, as well as of Na,K-ATPase. HIF1 α silencing increased β ENaC and γ ENaC expression and stimulated sodium transport. A constitutively active mutant of HIF1 α produced the opposite effect. Aldosterone and inhibition of the mitochondrial respiratory chain slowly activated the HIF pathway, suggesting that ROS may also activate HIF. Decreased γ ENaC abundance induced by hypoxia in normal mice was abolished in Hif1α knockout mice. Similarly, Hif1α knockout led to increased γ ENaC abundance under high sodium intake.

CONCLUSIONS

This study reveals that γ ENaC expression and activity are physiologically controlled by the HIF pathway, which may represent a negative feedback mechanism to preserve oxygenation and/or prevent excessive ROS generation under increased sodium transport.

Sex-specific adaptations to high-salt diet preserve electrolyte homeostasis with distinct sodium transporter profiles

Kidneys continuously filter an enormous amount of sodium and adapt kidney Na+ reabsorption to match Na+ intake to maintain circulatory volume and electrolyte homeostasis. Males (M) respond to high-salt (HS) diet by translocating proximal tubule Na+/H+ exchanger isoform 3 (NHE3) to the base of the microvilli, reducing activated forms of the distal NaCl cotransporter (NCC) and epithelial Na+ channel (ENaC). Males (M) and females (F) on normal-salt (NS) diet present sex-specific profiles of "transporters" (cotransporters, channels, pumps, and claudins) along the nephron, e.g., F exhibit 40% lower NHE3 and 200% higher NCC abundance than M. We tested the hypothesis that adaptations to HS diet along the nephron will, likewise, exhibit sexual dimorphisms. C57BL/6J mice were fed for 15 days with 4% NaCl diet (HS) versus 0.26% NaCl diet (NS). On HS, M and F exhibited normal plasma [Na+] and [K+], similar urine volume, Na+, K+, and osmolal excretion rates normalized to body weight. In F, like M, HS lowered abundance of distal NCC, phosphorylated NCC, and cleaved (activated) forms of ENaC. The adaptations associated with achieving electrolyte homeostasis exhibit sex-dependent and independent mechanisms. Sex differences in baseline "transporters" abundance persist during HS diet, yet the fold changes during HS diet (normalized to NS) are similar along the distal nephron and collecting duct. Sex-dependent differences observed along the proximal tubule during HS show that female kidneys adapt differently from patterns reported in males, yet achieve and maintain fluid and electrolyte homeostasis.

High salt diet-induced proximal tubular phenotypic changes and sodium-glucose cotransporter-2 expression are coordinated by cold shock Y-box binding protein-1

High salt diet (HSD) is a hallmark of blood pressure elevations, weight gain and diabetes onset in the metabolic syndrome. In kidney, compensatory mechanisms are activated to balance salt turnover and maintain homeostasis. Data on the long-term effects of HSD with respect to tubular cell functions and kidney architecture that exclude confounding indirect blood pressure effects are scarce. Additionally we focus on cold shock Y-box binding protein-1 as a tubular cell protective factor. A HSD model (4% NaCl in chow; 1% NaCl in water) was compared to normal salt diet (NSD, standard chow) over 16 months using wild type mice and an inducible conditional whole body knockout for cold shock Y-box binding protein-1 (BL6J/N, Ybx1). HSD induced no difference in blood pressure over 16 months, comparing NSD/HSD and Ybx1 wild type/knockout. Nevertheless, marked phenotypic changes were detected. Glucosuria and subnephrotic albuminuria ensued in wild type animals under HSD, which subsided in Ybx1-deficient animals. At the same time megalin receptors were upregulated. The sodium-glucose cotransporter-2 (SGLT2) was completely downregulated in wild type HSD animals that developed glucosuria. In Ybx1 knockouts, expression of AQP1 and SGLT2 was maintained under HSD; proximal tubular widening and glomerular tubularization developed. Concurrently, amino aciduria of neutral and hydrophobic amino acids was seen. In vitro translation confirmed that YB-1 translationally represses Sglt2 transcripts. Our data reveal profound effects of HSD primarily within glomeruli and proximal tubular segments. YB-1 is regulated by HSD and orchestrates HSD-dependent changes; notably, sets reabsorption thresholds for amino acids, proteins and glucose.

Aerobic Exercise Training Prevents Insulin Resistance and Hepatic Lipid Accumulation in LDL Receptor Knockout Mice Chronically Fed a Low-Sodium Diet

Background: A low-sodium (LS) diet reduces blood pressure, contributing to the prevention of cardiovascular diseases. However, intense dietary sodium restriction impairs insulin sensitivity and worsens lipid profile. Considering the benefits of aerobic exercise training (AET), the effect of LS diet and AET in hepatic lipid content and gene expression was investigated in LDL receptor knockout (LDLr-KO) mice. Methods: Twelve-week-old male LDLr-KO mice fed a normal sodium (NS) or LS diet were kept sedentary (S) or trained (T) for 90 days. Body mass, plasma lipids, insulin tolerance testing, hepatic triglyceride (TG) content, gene expression, and citrate synthase (CS) activity were determined. Results were compared by 2-way ANOVA and Tukey’s post-test. Results: Compared to NS, LS increased body mass and plasma TG, and impaired insulin sensitivity, which was prevented by AET. The LS-S group, but not the LS-T group, presented greater hepatic TG than the NS-S group. The LS diet increased the expression of genes related to insulin resistance (ApocIII, G6pc, Pck1) and reduced those involved in oxidative capacity (Prkaa1, Prkaa2, Ppara, Lipe) and lipoprotein assembly (Mttp). Conclusion: AET prevented the LS-diet-induced TG accumulation in the liver by improving insulin sensitivity and the expression of insulin-regulated genes and oxidative capacity.

Local and downstream actions of proximal tubule angiotensin II signaling on Na+ transporters in the mouse nephron

The renal nephron consists of a series of distinct cell types that function in concert to maintain fluid and electrolyte balance and blood pressure. The renin-angiotensin system (RAS) is central to Na⁺ and volume balance. We aimed to determine how loss of angiotensin II signaling in the proximal tubule (PT), which reabsorbs the bulk of filtered Na⁺ and volume, impacts solute transport throughout the nephron. We hypothesized that PT renin-angiotensin system disruption would not only depress PT Na⁺ transporters but also impact downstream Na⁺ transporters. Using a mouse model in which the angiotensin type 1a receptor (AT_1aR) is deleted specifically within the PT (AT_1aR PTKO), we profiled the abundance of Na⁺ transporters, channels, and claudins along the nephron. Absence of PT AT_1aR signaling was associated with lower abundance of PT transporters (Na⁺/H⁺ exchanger isoform 3, electrogenic Na⁺-bicarbonate cotransporter 1, and claudin 2) as well as lower abundance of downstream transporters (total and phosphorylated Na⁺-K⁺-2Cl^- cotransporter, medullary Na⁺-K⁺-ATPase, phosphorylated NaCl cotransporter, and claudin 7) versus controls. However, transport activities of Na⁺-K⁺-2Cl^- cotransporter and NaCl cotransporter (assessed with diuretics) were similar between groups in order to maintain electrolyte balance. Together, these results demonstrate the primary impact of angiotensin II regulation on Na⁺ reabsorption in the PT at baseline and the associated influence on downstream Na⁺ transporters, highlighting the ability of the nephron to integrate Na⁺ transport along the nephron to maintain homeostasis.NEW & NOTEWORTHY Our study defines a novel role for proximal tubule angiotensin receptors in regulating the abundance of Na⁺ transporters throughout the nephron, thereby contributing to the integrated control of fluid balance in vivo.

Renal Inflammation Induces Salt Sensitivity in Male db/db Mice through Dysregulation of ENaC

BACKGROUND

Hypertension is considered a major risk factor for the progression of diabetic kidney disease. Type 2 diabetes is associated with increased renal sodium reabsorption and salt-sensitive hypertension. Clinical studies show that men have higher risk than premenopausal women for the development of diabetic kidney disease. However, the renal mechanisms that predispose to salt sensitivity during diabetes and whether sexual dimorphism is associated with these mechanisms remains unknown.

METHODS

Female and male db/db mice exposed to a high-salt diet were used to analyze the progression of diabetic kidney disease and the development of hypertension.

RESULTS

Male, 34-week-old, db/db mice display hypertension when exposed to a 4-week high-salt treatment, whereas equivalently treated female db/db mice remain normotensive. Salt-sensitive hypertension in male mice was associated with no suppression of the epithelial sodium channel (ENaC) in response to a high-salt diet, despite downregulation of several components of the intrarenal renin-angiotensin system. Male db/db mice show higher levels of proinflammatory cytokines and more immune-cell infiltration in the kidney than do female db/db mice. Blocking inflammation, with either mycophenolate mofetil or by reducing IL-6 levels with a neutralizing anti-IL-6 antibody, prevented the development of salt sensitivity in male db/db mice.

CONCLUSIONS

The inflammatory response observed in male, but not in female, db/db mice induces salt-sensitive hypertension by impairing ENaC downregulation in response to high salt. These data provide a mechanistic explanation for the sexual dimorphism associated with the development of diabetic kidney disease and salt sensitivity.

Dietary sodium intake does not alter renal potassium handling and blood pressure in healthy young males

Background

The effects of sodium (Na⁺) intakes on renal handling of potassium (K⁺) are insufficiently studied.

Methods

We assessed the effect of Na⁺ on renal K⁺ handling in 16 healthy males assigned to three 7-day periods on low salt diet [LSD, 3 g sodium chloride (NaCl)/day], normal salt diet (NSD, 6 g NaCl/day) and high salt diet (HSD, 15 g NaCl/day), with constant K⁺ intake. Contributions of distal NaCl co-transporter and epithelial Na⁺ channel in the collecting system on K⁺ and Na⁺ handling were assessed at steady state by acute response to 100 mg oral hydrochlorothiazide and with addition of 10 mg of amiloride to hydrochlorothiazide, respectively.

Results

Diurnal blood pressure slightly increased from 119.30 ± 7.95 mmHg under LSD to 123.00 ± 7.50 mmHg (P = 0.02) under HSD, while estimated glomerular filtration rate increased from 133.20 ± 34.68 mL/min under LSD to 187.00 ± 49.10 under HSD (P = 0.005). The 24-h K⁺ excretion remained stable on all Na⁺ intakes (66.28 ± 19.12 mmol/24 h under LSD; 55.91 ± 21.17 mmol/24 h under NSD; and 66.81 ± 20.72 under HSD, P = 0.9). The hydrochlorothiazide-induced natriuresis was the highest under HSD (30.22 ± 12.53 mmol/h) and the lowest under LSD (15.38 ± 8.94 mmol/h, P = 0.02). Hydrochlorothiazide increased kaliuresis and amiloride decreased kaliuresis similarly on all three diets.

Conclusions

Neither spontaneous nor diuretic-induced K⁺ excretion was influenced by Na⁺ intake in healthy male subjects. However, the respective contribution of the distal convoluted tubule and the collecting duct to renal Na⁺ handling was dependent on dietary Na⁺ intake.

ENaC expression correlates with the acute furosemide-induced K+ excretion

BACKGROUND

In the aldosterone-sensitive distal nephron (ASDN), epithelial sodium channel (ENaC)-mediated Na+ absorption drives K+ excretion. K+ excretion depends on the delivery of Na+ to the ASDN and molecularly activated ENaC. Furosemide is known as a K+ wasting diuretic as it greatly enhances Na+ delivery to the ASDN. Here, we studied the magnitude of acute furosemide-induced kaliuresis under various states of basal molecular ENaC activity.

METHODS

C57/Bl6J mice were subjected to different dietary regimens that regulate molecular ENaC expression and activity levels. The animals were anesthetized and bladder-catheterized. Diuresis was continuously measured before and after administration of furosemide (2 µg/g BW) or benzamil (0.2 µg/g BW). Flame photometry was used to measure urinary [Na+ ] and [K+ ]. The kidneys were harvested and, subsequently, ENaC expression and cleavage activation were determined by semiquantitative western blotting.

RESULTS

A low K+ and a high Na+ diet markedly suppressed ENaC protein expression, cleavage activation, and furosemide-induced kaliuresis. In contrast, furosemide-induced kaliuresis was greatly enhanced in animals fed a high K+ or low Na+ diet, conditions with increased ENaC expression. The furosemide-induced diuresis was similar in all dietary groups.

CONCLUSION

Acute furosemide-induced kaliuresis differs greatly and depends on the a priori molecular expression level of ENaC. Remarkably, it can be even absent in animals fed a high Na+ diet, despite a marked increase of tubular flow and urinary Na+ excretion. This study provides auxiliary evidence that acute ENaC-dependent K+ excretion requires both Na+ as substrate and molecular activation of ENaC.

Activation of the Sympathetic Nervous System Promotes Blood Pressure Salt-Sensitivity in C57BL6/J Mice

Global salt intake averages >8 g/person per day, over twice the limit advocated by the American Heart Association. Dietary salt excess leads to hypertension, and this partly mediates its poor health outcomes. In ≈30% of people, the hypertensive response to salt is exaggerated. This salt-sensitivity increases cardiovascular risk. Mechanistic cardiovascular research relies heavily on rodent models and the C57BL6/J mouse is the most widely used reference strain. We examined the effects of high salt intake on blood pressure, renal, and vascular function in the most commonly used and commercially available C57BL6/J mouse strain. Changing from control (0.3% Na⁺) to high salt (3% Na⁺) diet increased systolic blood pressure in male mice by ≈10 mm Hg within 4 days of dietary switch. This hypertensive response was maintained over the 3-week study period. Returning to control diet gradually reduced blood pressure back to baseline. High-salt diet caused a rapid and sustained downregulation in mRNA encoding renal NHE3 (sodium-hydrogen-exchanger 3) and EnaC (epithelial sodium channel), although we did not observe a suppression in aldosterone until ≈7 days. During the development of salt-sensitivity, the acute pressure natriuresis relationship was augmented and neutral sodium balance was maintained throughout. High-salt diet increased ex vivo sensitivity of the renal artery to phenylephrine and increased urinary excretion of adrenaline, but not noradrenaline. The acute blood pressure-depressor effect of hexamethonium, a ganglionic blocker, was enhanced by high salt. Salt-sensitivity in commercially sourced C57BL6/J mice is attributable to sympathetic overactivity, increased adrenaline, and enhanced vascular sensitivity to alpha-adrenoreceptor activation and not sodium retention or attenuation of the acute pressure natriuresis response.

Determinants of renal oxygen metabolism during low Na+ diet: effect of angiotensin II AT1 and aldosterone receptor blockade

KEY POINTS

Reducing Na⁺ intake reduces the partial pressure of oxygen in the renal cortex and activates the renin-angiotensin-aldosterone system. In the absence of high blood pressure, these consequences of dietary Na⁺ reduction may be detrimental for the kidney. In a normotensive animal experimental model, reducing Na⁺ intake for 2 weeks increased renal oxygen consumption, which was normalized by mineralocorticoid receptor blockade. Furthermore, blockade of the angiotensin II AT₁ receptor restored cortical partial pressure of oxygen by improving oxygen delivery. This shows that increased activity of the renin-angiotensin-aldosterone system contributes to increased oxygen metabolism in the kidney after 2 weeks of a low Na⁺ diet. The results provide insights into dietary Na⁺ restriction in the absence of high blood pressure, and its consequences for the kidney.

ABSTRACT

Reduced Na⁺ intake reduces the P O 2 (partial pressure of oxygen) in the renal cortex. Upon reduced Na⁺ intake, reabsorption along the nephron is adjusted with activation of the renin-angiotensin-aldosterone system (RAAS). Thus, we studied the effect of reduced Na⁺ intake on renal oxygen homeostasis and function in rats, and the impact of intrarenal angiotensin II AT₁ receptor blockade using candesartan and mineralocorticoid receptor blockade using canrenoic acid potassium salt (CAP). Male Sprague-Dawley rats were fed standard rat chow containing normal (0.25%) and low (0.025%) Na⁺ for 2 weeks. The animals were anaesthetized (thiobutabarbital 120 mg kg^-1 ) and surgically prepared for kidney oxygen metabolism and function studies before and after acute intrarenal arterial infusion of candesartan (4.2 μg kg^-1 ) or intravenous infusion of CAP (20 mg kg^-1 ). Baseline mean arterial pressure and renal blood flow were similar in both dietary groups. Fractional Na⁺ excretion and cortical oxygen tension were lower and renal oxygen consumption was higher in low Na⁺ groups. Neither candesartan nor CAP affected arterial pressure. Renal blood flow and cortical oxygen tension increased in both groups after candesartan in the low Na⁺ group. Fractional Na⁺ excretion was increased and oxygen consumption reduced in the low Na⁺ group after CAP. These results suggest that blockade of angiotensin II AT₁ receptors has a major impact upon oxygen delivery during normal and low Na⁺ conditions, while aldosterone receptors mainly affect oxygen metabolism following 2 weeks of a low Na⁺ diet.

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.

Multiple acid-base and electrolyte disturbances upregulate NBCn1, NBCn2, IRBIT and L-IRBIT in the mTAL

KEY POINTS

The roles of the Na⁺ /HCO₃ ^- cotransporters NBCn1 and NBCn2 as well as their activators IRBIT and L-IRBIT in the regulation of the mTAL transport of NH₄ ⁺ , HCO₃ ^- , and NaCl are investigated. Dietary challenges of NH₄ Cl, NaHCO₃ or NaCl all increase the abundance of NBCn1 and NBCn2 in the outer medulla. The three challenges generally produce parallel increases in the abundance of IRBIT and L-IRBIT in the outer medulla. Both IRBIT and L-IRBIT powerfully stimulate the activities of the mTAL isoforms of NBCn1 and NBCn2 as expressed in Xenopus oocytes. Our findings support the hypothesis that NBCn1, NBCn2, IRBIT and L-IRBIT appropriately promote NH₄ ⁺ shunting but oppose HCO₃ ^- and NaCl reabsorption in the mTAL, and thus are at the nexus of the regulation pathways for multiple renal transport processes.

ABSTRACT

The medullary thick ascending limb (mTAL) plays a key role in urinary acid and NaCl excretion. NBCn1 and NBCn2 are present in the basolateral mTAL, where NBCn1 promotes NH₄ ⁺ shunting. IRBIT and L-IRBIT (the IRBITs) are two powerful activators of certain acid-base transporters. Here we use western blotting and immunofluorescence to examine the effects of multiple acid-base and electrolyte disturbances on expression of NBCn1, NBCn2 and the IRBITs in rat kidney. We also use electrophysiology to examine the functional effects of IRBITs on NBCn1 and NBCn2 in Xenopus oocytes. NH₄ Cl-induced metabolic acidosis (MAc) substantially increases protein expression of NBCn1 and NBCn2 in the outer medulla (OM) of rat kidney. Surprisingly, NaHCO₃ -induced metabolic alkalosis (MAlk) and high-salt diet (HSD) also increase expression of NBCn1 and NBCn2 (effect of NaHCO₃  > HSD). Moreover, all three challenges generally increase OM expression of the IRBITs. In Xenopus oocytes, the IRBITs substantially increase the activities of NBCn1 and NBCn2. We propose that upregulation of basolateral NBCn1 and NBCn2 plus the IRBITs in the mTAL: (1) promotes NH₄ ⁺ shunting by increasing basolateral HCO₃ ^- uptake to neutralize apical NH₄ ⁺ uptake during MAc; (2) inhibits HCO₃ ^- reabsorption during MAlk by opposing HCO₃ ^- efflux via the basolateral anion exchanger AE2; and (3) inhibits NaCl reabsorption by mediating (with AE2) net NaCl backflux into the mTAL cell during HSD. Thus, NBCn1, NBCn2 and the IRBITs are at the nexus of the regulatory pathways for multiple renal transport processes.

Sodium-hydrogen exchanger regulatory factor-1 (NHERF1) confers salt sensitivity in both male and female models of hypertension in aging

Hypertension is a risk factor for premature death and roughly 50% of hypertensive patients are salt-sensitive. The incidence of salt-sensitive hypertension increases with age. However, the mechanisms of salt-sensitive hypertension are not well understood. We had demonstrated decreased renal sodium‑hydrogen exchanger regulatory factor 1 (NHERF1) expression in old salt-resistant F344 rats. Based on those studies we hypothesized that NHERF1 expression is required for the development of some forms of salt-sensitive hypertension. To address this hypothesis, we measured blood pressure in NHERF1 expressing salt-sensitive 4-mo and 24-mo-old male and female Fischer Brown Norway (FBN) rats male and female 18-mo-old NHERF1 knock-out (NHERF1-/-) mice and wild-type (WT) littermates on C57BL/6J background after feeding high salt (8% NaCl) diet for 7 days. Our data demonstrate that 8% salt diet increased blood pressure in both male and female 24-mo-old FBN rats but not in 4-mo-old FBN rats and in 18-mo-old male and female WT mice but not in NHERF1-/- mice. Renal dopamine 1 receptor (D1R) expression was decreased in 24-mo-old rats, compared with 4-mo-old FBN rats. However, sodium chloride cotransporter (NCC) expression increased in 24-mo-old FBN rats. In FBN rats, age had no effect on NaK ATPase α1 and NKCC2 expression. By contrast, high salt diet increased the renal expressions of NKCC2, and NCC in 24-mo-old FBN rats. High salt diet also increased NKCC2 and NCC expression in WT mice but not NHERF1-/- mice. Our data suggest that renal NHERF1 expression confers salt sensitivity with aging, associated with increased expression of sodium transporters.

Time-course of sodium transport along the nephron in nephrotic syndrome: The role of potassium

The mechanism of sodium retention and its location in kidney tubules may vary with time in nephrotic syndrome (NS). We studied the mechanisms of sodium retention in transgenic POD-ATTAC mice, which display an inducible podocyte-specific apoptosis. At day 2 after the induction of NS, the increased abundance of NHE3 and phosphorylated NCC in nephrotic mice compared with controls suggest that early sodium retention occurs mainly in the proximal and distal tubules. At day 3, the abundance of NHE3 normalized, phosphorylated NCC levels decreased, and cleavage and apical localization of γ-ENaC increased in nephrotic mice. These findings indicate that sodium retention shifted from the proximal and distal tubules to the collecting system. Increased cleavage and apical localization of γ-ENaC persisted at day 5 in nephrotic mice when hypovolemia resolved and steady-state was reached. Sodium retention and γ-ENaC cleavage were independent of the increased plasma levels of aldosterone. Nephrotic mice displayed decreased glomerular filtration rate and urinary potassium excretion associated with hyperkaliemia at day 3. Feeding nephrotic mice with a low potassium diet prevented hyperkaliemia, γ-ENaC cleavage, and led to persistent increased phosphorylation of NCC. These results suggest that potassium homeostasis is a major determinant of the tubular site of sodium retention in nephrotic mice.

Dietary sodium modulates nephropathy in Nedd4-2-deficient mice

Salt homeostasis is maintained by tight control of Na⁺ filtration and reabsorption. In the distal part of the nephron the ubiquitin protein ligase Nedd4-2 regulates membrane abundance and thus activity of the epithelial Na⁺ channel (ENaC), which is rate-limiting for Na⁺ reabsorption. Nedd4-2 deficiency in mouse results in elevated ENaC and nephropathy, however the contribution of dietary salt to this has not been characterized. In this study we show that high dietary Na⁺ exacerbated kidney injury in Nedd4-2-deficient mice, significantly perturbing normal postnatal nephrogenesis and resulting in multifocal areas of renal dysplasia, increased markers of kidney injury and a decline in renal function. In control mice, high dietary Na⁺ resulted in reduced levels of ENaC. However, Nedd4-2-deficient kidneys maintained elevated ENaC even after high dietary Na⁺, suggesting that the inability to efficiently downregulate ENaC is responsible for the salt-sensitivity of disease. Importantly, low dietary Na⁺ significantly ameliorated nephropathy in Nedd4-2-deficient mice. Our results demonstrate that due to dysregulation of ENaC, kidney injury in Nedd4-2-deficient mice is sensitive to dietary Na⁺, which may have implications in the management of disease in patients with kidney disease.

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

Renal and Blood Pressure Response to a High-Salt Diet in Mice With Reduced Global Expression of the Glucocorticoid Receptor

Salt-sensitive hypertension is common in glucocorticoid excess. Glucocorticoid resistance also presents with hypercortisolemia and hypertension but the relationship between salt intake and blood pressure (BP) is not well defined. GR^βgeo/+ mice have global glucocorticoid receptor (GR) haploinsufficiency and increased BP. Here we examined the effect of high salt diet on BP, salt excretion and renal blood flow in GR^βgeo/+mice. Basal BP was ∼10 mmHg higher in male GR^βgeo/+ mice than in GR^+/+ littermates. This modest increase was amplified by ∼10 mmHg following a high-salt diet in GR^βgeo/+ mice. High salt reduced urinary aldosterone excretion but increased renal mineralocorticoid receptor expression in both genotypes. Corticosterone, and to a lesser extent deoxycorticosterone, excretion was increased in GR^βgeo/+ mice following a high-salt challenge, consistent with enhanced 24 h production. GR^+/+ mice increased fractional sodium excretion and reduced renal vascular resistance during the high salt challenge, retaining neutral sodium balance. In contrast, sodium excretion and renal vascular resistance did not adapt to high salt in GR^βgeo/+ mice, resulting in transient sodium retention and sustained hypertension. With high-salt diet, Slc12a3 and Scnn1a mRNAs were higher in GR^βgeo/+ than controls, and this was reflected in an exaggerated natriuretic response to thiazide and benzamil, inhibitors of NCC and ENaC, respectively. Reduction in GR expression causes salt-sensitivity and an adaptive failure of the renal vasculature and tubule, most likely reflecting sustained mineralocorticoid receptor activation. This provides a mechanistic basis to understand the hypertension associated with loss-of-function polymorphisms in GR in the context of habitually high salt intake.

Na restriction activates epithelial Na channels in rat kidney through two mechanisms and decreases distal Na+ delivery

KEY POINTS

Dietary Na restriction, through the mineralocorticoid aldosterone, acts on epithelial Na channels via both fast (24 h) and slow (5-7 days) mechanisms in the kidney. The fast effect entails increased proteolytic processing and trafficking of channel protein to the apical membrane. It is rapidly reversible by the mineralocorticoid receptor antagonist eplerenone and is largely lost when tubules are studied ex vivo. The slow effect does not require increased processing or surface expression, is refractory to acute eplerenone treatment, and is preserved ex vivo. Both slow and fast effects contribute to Na retention in vivo. Increased Na⁺ reabsorption in the proximal tubule also promotes Na conservation under conditions of chronic dietary Na restriction, reducing Na⁺ delivery to the distal nephron.

ABSTRACT

Changes in the activity of the epithelial Na channel (ENaC) help to conserve extracellular fluid volume. In rats fed a low-salt diet, proteolytic processing of ENaC increased within 1 day, and was almost maximal after 3 days. The rapid increase in the abundance of cleaved αENaC and γENaC correlated with decreased urinary Na⁺ excretion and with increased ENaC surface expression. By contrast, ENaC activity, measured ex vivo in isolated cortical collecting ducts, increased modestly after 3 days and required 5 days to reach maximal levels. The mineralocorticoid receptor antagonist eplerenone reversed the increase in cleaved γENaC and induced natriuresis after 1 or 3 days but failed to alter either ENaC currents or Na⁺ excretion after 7 days of Na restriction. We conclude that Na depletion, through aldosterone, stimulates ENaC via independent fast and slow mechanisms. In vivo, amiloride-induced natriuresis increased after 1 day of Na depletion. By contrast, hydrochlorothiazide (HCTZ)-induced natriuresis decreased gradually over 7 days, consistent with increased ability of ENaC activity to compensate for decreased Na⁺ reabsorption in the distal convoluted tubule. Administration of amiloride and HCTZ together increased Na⁺ excretion less in Na-depleted compared to control animals, indicating decreased delivery of Na⁺ to the distal nephron when dietary Na is restricted. Measurements of creatinine and Li⁺ clearances indicated that increased Na reabsorption by the proximal tubules is responsible for the decreased delivery. Thus, Na conservation during chronic dietary salt restriction entails enhanced transport by both proximal and distal nephron segments.