Dietary sodium alters aldosterone's effect on renal sodium transporter expression and distal convoluted tubule remodelling.

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 ß- 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.

Effect of Dietary Sodium on Blood Pressure: A Crossover Trial.

Importance: Dietary sodium recommendations are debated partly due to variable blood pressure (BP) response to sodium intake. Furthermore, the BP effect of dietary sodium among individuals taking antihypertensive medications is understudied. Objectives: To examine the distribution of within-individual BP response to dietary sodium, the difference in BP between individuals allocated to consume a high- or low-sodium diet first, and whether these varied according to baseline BP and antihypertensive medication use. Design, Setting, and Participants: Prospectively allocated diet order with crossover in community-based participants enrolled between April 2021 and February 2023 in 2 US cities. A total of 213 individuals aged 50 to 75 years, including those with normotension (25%), controlled hypertension (20%), uncontrolled hypertension (31%), and untreated hypertension (25%), attended a baseline visit while consuming their usual diet, then completed 1-week high- and low-sodium diets. Intervention: High-sodium (approximately 2200 mg sodium added daily to usual diet) and low-sodium (approximately 500 mg daily total) diets. Main Outcomes and Measures: Average 24-hour ambulatory systolic and diastolic BP, mean arterial pressure, and pulse pressure. Results: Among the 213 participants who completed both high- and low-sodium diet visits, the median age was 61 years, 65% were female and 64% were Black. While consuming usual, high-sodium, and low-sodium diets, participants' median systolic BP measures were 125, 126, and 119 mm Hg, respectively. The median within-individual change in mean arterial pressure between high- and low-sodium diets was 4 mm Hg (IQR, 0-8 mm Hg; P < .001), which did not significantly differ by hypertension status. Compared with the high-sodium diet, the low-sodium diet induced a decline in mean arterial pressure in 73.4% of individuals. The commonly used threshold of a 5 mm Hg or greater decline in mean arterial pressure between a high-sodium and a low-sodium diet classified 46% of individuals as "salt sensitive." At the end of the first dietary intervention week, the mean systolic BP difference between individuals allocated to a high-sodium vs a low-sodium diet was 8 mm Hg (95% CI, 4-11 mm Hg; P < .001), which was mostly similar across subgroups of age, sex, race, hypertension, baseline BP, diabetes, and body mass index. Adverse events were mild, reported by 9.9% and 8.0% of individuals while consuming the high- and low-sodium diets, respectively. Conclusions and Relevance: Dietary sodium reduction significantly lowered BP in the majority of middle-aged to elderly adults. The decline in BP from a high- to low-sodium diet was independent of hypertension status and antihypertensive medication use, was generally consistent across subgroups, and did not result in excess adverse events. Trial Registration: ClinicalTrials.gov Identifier: NCT04258332.

Colonic expression of calcium transporter TRPV6 is regulated by dietary sodium butyrate.

Dietary fibers have been shown to increase the intestinal absorption of calcium (Ca2+) and magnesium (Mg2+). However, the mechanisms that explain the enhanced electrolyte absorption remain unknown. Therefore, this study aims to investigate the short-term and long-term effects of 5% (w/w) sodium butyrate (Na-butyrate), an important end-metabolite of bacterial fermentation of dietary fibers, on Ca2+ and Mg2+ homeostasis in mice. Serum Ca2+ levels were only significantly increased in mice treated with Na-butyrate for 1 day. This was associated with a twofold increase in the mRNA expression levels of Trpv6 in the proximal and distal colon. Contrary, Na-butyrate did not affect serum Mg2+ concentrations at either of the intervention periods. However, we observed a reduction in urinary Mg2+ excretion, although not significantly, after 1 day of treatment. A significant reduction of 2.5-fold in urinary Mg2+ excretion was observed after 14 days of treatment. Indeed, 14-day Na-butyrate supplementation increased colonic Trpm7 expression by 1.2-fold compared to control mice. In conclusion, short-term Na-butyrate supplementation increases serum Ca2+ levels in mice. This was associated with increased mRNA expression levels of Trpv6 in the colon, suggesting that Na-butyrate regulates the expression of genes involved in active intestinal Ca2+ absorption.

New wrinkles in hypertension management 2022.

PURPOSE OF REVIEW: High blood pressure (BP) is the world's leading risk factor for cardiovascular disease (CVD) and death. This review highlights findings during the past 18 months that apply to the management of high BP in adults in the context of the 2017 American College of Cardiology/American Heart Association (AHA) BP guideline. RECENT FINDINGS: A comprehensive meta-analysis of clinical trials that employed a novel statistical method identified a substantially linear relationship between dietary sodium intake and BP, strongly supporting the AHA daily dietary sodium intake recommendation of less than 1500 mg/day but suggesting that any reduction in sodium intake is likely to be beneficial. Among adults with hypertension, use of a salt substitute (containing reduced sodium and enhanced potassium) led to striking reductions in CVD outcomes. Young adults with stage 1 hypertension and a low 10-year atherosclerotic CVD risk score should be started on a 6-month course of vigorous lifestyle modification; if their BP treatment goal is not achieved, a first-line antihypertensive agent should be added to the lifestyle modification intervention. In patients with stage 4 renal disease, the thiazide-like diuretic chlorthalidone (as add-on therapy) lowered BP markedly compared with placebo. Nonsteroidal mineralocorticoid receptor antagonists (MRAs) represent a new class of MRA that has been shown to lower BP and provide significant CVD protection. In Chinese adults aged 60-80 years at baseline, intensive BP control with a SBP target of 110-129 compared with 130-149 mmHg reduced CVD events with minimal side effects. SUMMARY: Recent findings have advanced our knowledge of hypertension management, clarifying, amplifying and supporting the 2017 ACC/AHA BP guideline recommendations.

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.

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.

Sodium-glucose co-transporter 2 inhibitor therapy: use in chronic kidney disease and adjunctive sodium restriction.

The global burden of chronic kidney disease (CKD) has increased significantly over the past few decades. This reflects the rising prevalence of type 2 diabetes mellitus (T2DM) and hypertension, two leading causes of CKD. Hypertension, which can also be a complication of CKD, accelerates renal disease progression and augments cardiovascular risk, especially in individuals with diabetic kidney disease. Hence, blood pressure (BP) reduction is a vital component of CKD management. Sodium-glucose co-transporter 2 (SGLT2) inhibitors are a relatively novel class of medications developed to treat T2DM by inducing glycosuria and hence, lowering glycaemia. Additionally, SGLT2 inhibitors are antihypertensive, renoprotective and cardioprotective, even in individuals without T2DM, making them effective therapeutic agents for CKD. Another therapy that has proven to be antihypertensive, renoprotective and cardioprotective is dietary sodium restriction. This review evaluates the potential combined benefits of SGLT2 inhibition and dietary sodium restriction on the BP and renal parameters of individuals with CKD.

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 ß ENaC and γ ENaC, as well as of Na,K-ATPase. HIF1 α silencing increased ß 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.

An in vivo protein landscape of the mouse DCT during high dietary K+ or low dietary Na+ intake.

The hormone aldosterone is essential for maintaining K+ and Na+ balance and controlling blood pressure. Aldosterone has different effects if it is secreted due to hypovolemia or hyperkalemia. The kidney distal convoluted tubule (DCT) is believed to play a central role in mediating the differential responses to aldosterone. To determine the alterations in the DCT that may be responsible for these effects, male mice with green fluorescent protein expression specifically in the DCT were maintained on diets containing low NaCl (hypovolemic state) or high potassium citrate (hyperkalemic state) for 4 days, and DCT cells were isolated using fluorescence-activated cell sorting. This pure population of DCT cells was subjected to analysis by liquid chromatography-coupled tandem mass spectrometry. Over 3,000 proteins were identified in the DCT, creating the first proteome of the mouse DCT. Of the identified proteins, 210 proteins were altered in abundance following a low-NaCl diet and 625 proteins following the high-K+ diet. Many of these changes were not detectable by analyzing whole kidney samples from the same animals. When comparing responses to high-K+ versus low-Na+ diets, protein translation, chaperone-mediated protein folding, and protein ubiquitylation were likely to be significantly altered in the DCT subsequent to a high-K+ diet. In conclusion, this study defines an in vivo protein landscape of the DCT in male mice following either a low-NaCl or a high-K+ diet and acts as an essential resource for the kidney research community.NEW & NOTEWORTHY The mineralocorticoid aldosterone, essential for maintaining body K+ and Na+ balance, has different effects if secreted due to hypovolemia or hyperkalemia. Here, we used proteomics to profile kidney distal convoluted tubule (DCT) cells isolated by a novel FACS approach from mice fed a low-Na+ diet (mimicking hypovolemia) or a high-K+ diet (mimicking hyperkalemia). The study provides the first in-depth proteome of the mouse DCT and insights into how it is physiologically regulated.

Deletion of Kir5.1 abolishes the effect of high Na+ intake on Kir4.1 and Na+-Cl- cotransporter.

High sodium (HS) intake inhibited epithelial Na+ channel (ENaC) in the aldosterone-sensitive distal nephron and Na+-Cl- cotransporter (NCC) by suppressing basolateral Kir4.1/Kir5.1 in the distal convoluted tubule (DCT), thereby increasing renal Na+ excretion but not affecting K+ excretion. The aim of the present study was to explore whether deletion of Kir5.1 compromises the inhibitory effect of HS on NCC expression/activity and renal K+ excretion. Patch-clamp experiments demonstrated that HS failed to inhibit DCT basolateral K+ channels and did not depolarize K+ current reversal potential of the DCT in Kir5.1 knockout (KO) mice. Moreover, deletion of Kir5.1 not only increased the expression of Kir4.1, phospho-NCC, and total NCC but also abolished the inhibitory effect of HS on the expression of Kir4.1, phospho-NCC, and total NCC and thiazide-induced natriuresis. Also, low sodium-induced stimulation of NCC expression/activity and basolateral K+ channels in the DCT were absent in Kir5.1 KO mice. Deletion of Kir5.1 decreased ENaC currents in the late DCT, and HS further inhibited ENaC activity in Kir5.1 KO mice. Finally, measurement of the basal renal K+ excretion rate with the modified renal clearance method demonstrated that long-term HS inhibited the renal K+ excretion rate and steadily increased plasma K+ levels in Kir5.1 KO mice but not in wild-type mice. We conclude that Kir5.1 plays an important role in mediating the effect of HS intake on basolateral K+ channels in the DCT and NCC activity/expression. Kir5.1 is involved in maintaining renal ability of K+ excretion during HS intake. NEW & NOTEWORTHY Kir5.1 plays an important role in mediating the effect of high sodium intake on basolateral K+ channels in the distal convoluted tubule and Na+-Cl- cotransporter activity/expression.

Impact of Dietary Sodium Reduction on the Development of Obesity and Type 2 Diabetes in db/db Mice.

The impact of dietary sodium reduction on mouse models of type 2 diabetes is not well understood. Therefore, we analyzed the effect of a low-salt diet on obesity and parameters of type 2 diabetes in db/db mice. Five-week-old male db/db and lean db/m mice were fed a normal salt (0.19% Na+, NS) or a low-salt diet (<0.03% Na+, LS) for 5 weeks. Body and organ weight and parameters of glucose and insulin tolerance were analyzed. Plasma levels of steroids were determined by liquid chromatography tandem mass spectrometry. Body weight, glucose, and insulin tolerance were not affected by LS. The amount of gonadal adipose tissue showed a trend to be increased by LS whereas liver, pancreas, kidney, heart, and adrenal weight remained unaffected. LS reduced urinary sodium-to-creatinine ratio but did not affect plasma Na+ levels in both genotypes. Plasma and urinary potassium-to-creatinine ratio did not differ in all groups of mice. Aldosterone as a major determinant of changes in dietary sodium remained unaffected by LS in db/db mice as well as further investigated steroid hormones. The present study showed reduced sodium-to-creatinine ratio, but no additional effects of dietary sodium reduction on major metabolic parameters and steroid levels in obese and hyper-glycemic db/db mice.

Factors associated with heart failure hospitalization in patients with high sodium excretion: subanalysis of the ESPRIT, evaluation of sodium intake for the prediction of cardiovascular events in Japanese high-risk patients, cohort study.

We have reported that high sodium excretion ≥ 4.0 g/day, assessed by repeated measurements of spot urine, is associated with composite cardiovascular (CV) events of heart failure (HF) hospitalization, acute coronary syndrome, cerebrovascular events, and documented CV deaths in Japanese high-risk patients with either stable and compensated congestive HF, high brain natriuretic peptide, coronary artery disease, cerebrovascular disease, chronic kidney disease, or atrial fibrillation. A total of 520 patients were enrolled. During the median follow-up period of 5.2 years, 105 (20%) experienced composite CV events, which were predominantly driven by 60 (12%) HF hospitalizations. The aim of the present study was to elucidate which subgroups of patients with high sodium excretion were associated with HF hospitalization. We divided the enrolled patients into three groups according to the amount of sodium excretion (< 3.0 g/day, 3.0-3.99 g/day (reference), and ≥ 4.0 g/day) based on a median of 14 measurements during follow-up. We assessed the hazard ratio for HF hospitalization according to age, bodyweight, and gender, using the Cox hazard model. In the total population, high sodium excretion ≥ 4.0 g/day was associated with HF hospitalization [hazard ratio (HR) 1.75, confidence interval (CI) 1.05-2.83] after adjustment for gender, age, and bodyweight, but was not associated with other CV events. In older patients (≥ 75 years old), high sodium excretion ≥ 4.0 g/day was associated with HF hospitalization after adjustment for gender and bodyweight (HR 3.25, CI 1.55-6.55), which was not observed in younger (< 75 years old) patients. In patients with lower bodyweight (< 60 kg), high sodium excretion ≥ 4.0 g/day was associated with HF hospitalization after adjustment for age and gender (HR 3.05, CI 1.34-6.61), which was not observed in heavier (≥ 60 kg) patients. High sodium excretion is associated with HF hospitalization in patients with older age and lower bodyweight in Japanese high-risk patients.

Kir4.1/Kir5.1 Activity Is Essential for Dietary Sodium Intake-Induced Modulation of Na-Cl Cotransporter.

BACKGROUND: Dietary sodium intake regulates the thiazide-sensitive Na-Cl cotransporter (NCC) in the distal convoluted tubule (DCT). Whether the basolateral, inwardly rectifying potassium channel Kir4.1/Kir5.1 (a heterotetramer of Kir4.1/Kir5.1) in the DCT is essential for mediating the effect of dietary sodium intake on NCC activity is unknown. METHODS: We used electrophysiology, renal clearance techniques, and immunoblotting to examine effects of Kir4.1/Kir5.1 in the DCT and NCC in wild-type and kidney-specific Kir4.1 knockout mice. RESULTS: Low sodium intake stimulated basolateral Kir4.1/Kir5.1 activity, increased basolateral K+ conductance, and hyperpolarized the membrane. Conversely, high sodium intake inhibited the potassium channel, decreased basolateral K+ currents, and depolarized the membrane. Low sodium intake increased total and phosphorylated NCC expression and augmented hydrochlorothiazide-induced natriuresis; high sodium intake had opposite effects. Thus, elevated NCC activity induced by low sodium intake was associated with upregulation of Kir4.1/Kir5.1 activity in the DCT, whereas inhibition of NCC activity by high sodium intake was associated with diminished Kir4.1/Kir5.1 activity. In contrast, dietary sodium intake did not affect NCC activity in knockout mice. Further, Kir4.1 deletion not only abolished basolateral K+ conductance and depolarized the DCT membrane, but also abrogated the stimulating effects induced by low sodium intake on basolateral K+ conductance and hyperpolarization. Finally, dietary sodium intake did not alter urinary potassium excretion rate in hypokalemic knockout and wild-type mice. CONCLUSIONS: Stimulation of Kir4.1/Kir5.1 by low intake of dietary sodium is essential for NCC upregulation, and inhibition of Kir4.1/Kir5.1 induced by high sodium intake is a key step for downregulation of NCC.

Dietary sodium butyrate ameliorated the blood stress biomarkers, heat shock proteins, and immune response of Nile tilapia (Oreochromis niloticus) exposed to heat stress.

The present study investigated the effects of sodium butyrate (SB) on the growth performance, histomorphology, immune response, and stress related markers of Nile tilapia subjected to heat stress. SB was incorporated at 0, 0.5, 1, 1.5, and 2 g per kg diet and fed to fish for 8 weeks. The obtained results revealed significantly improved growth performance with a decreased feed conversion ratio in the fish fed SB (P < 0.05). In the anterior, middle, and distal parts of the intestine, villus length and width and internal villi distance as well as the number of goblet cells were increased in the fish fed SB (P < 0.05). The blood total protein, hemoglobin, and white and red blood cell counts showed a significant quadratic influence (P < 0.05). The survival rate for Nile tilapia exposed to heat stress for 48 h revealed that the SB fed groups had noticeably higher survival rates. Dietary SB significantly increased the phagocytic index and lysozyme and phagocytic activities both before and after heat stress (P < 0.05). After heat stress, blood glucose decreased significantly with SB feeding at 0.5, 1, or 1.5 g per kg diet, while cortisol was reduced in fish fed 1.5 or 2 g per kg diet (P < 0.05). Additionally, in fish fed SB, superoxide dismutase (SOD), catalase, and glutathione peroxidase activities were significantly increased both before and after heat stress, while malondialdehyde was decreased by SB feeding (P < 0.05). Liver heat shock protein 70 and SOD gene expression were significantly upregulated in fish fed on SB at 1 g per kg diet (P < 0.05). Thus, supplementation with SB at 1-2 g per kg diet can be used effectively in tilapia diets for improving growth, feed efficiency, and immune response as well as for tolerance to heat stress.

Evidence against a crucial role of renal medullary perfusion in blood pressure control of hypertensive rats.

KEY POINTS: The development of new effective methods of treating arterial hypertension is hindered by uncertainty regarding its causes. According to one widespread concept hypertension is caused by abnormal blood circulation in the kidney, specifically by reduction of blood flow through the kidney medulla; however, this causal relationship has never been rigorously verified. We investigated whether in rats with three different forms of experimental hypertension prolonged selective elevation of renal medullary blood flow using local infusion of the vasodilator bradykinin would lower arterial pressure. We found that increasing medullary blood flow by almost 50% did not result in alleviation of hypertension, which argues against a causal role of such changes in the control of arterial pressure and suggests that attempts at improving renal medullary circulation are not likely to be a promising approach to combating hypertension. ABSTRACT: The crucial role of renal medullary blood flow (MBF) in the control of arterial pressure (MAP) has been widely accepted but not rigorously verified. We examined the effects of experimental selective MBF elevation on MAP, medullary tissue hypertonicity and renal excretion in hypertensive rats. We used three hypertensive rat models: (1) rats with hypertension induced by chronic angiotensin II infusions (AngII model), (2) rats with hypertension induced by unilateral nephrectomy followed by high salt diet (HS/UNX), and (3) spontaneously hypertensive rats (SHR). In acute experiments, MBF (laser-Doppler measurement) was selectively increased with an intramedullary infusion of bradykinin (Bk) at 0.27 mg h-1  kg-1 BW over 4 h. MAP, renal artery blood flow (Transonic probe) and renal excretion parameters were measured simultaneously. In chronic studies with AngII and HS/UNX rats, Bk was infused over 2 weeks and MAP (telemetry probe) and renal excretion were repeatedly determined. In acute studies, with AngII, SHR and HS/UNX groups, Bk infusion caused a 47% increase in MBF (P < 0.01-0.001), whereas solvent infusion was without effect. During the experiments MAP decreased slightly and to the same extent with Bk and solvent infusion. Medullary tissue osmolality and [Na+ ] were lower in Bk- than in solvent-infused AngII rats and in SHR. Two weeks of intramedullary Bk infusion tested in AngII and HS/UNX rats did not alter MAP or renal excretion; though in the latter group a significant MBF increase and medullary hypertonicity decrease was observed. Since no decrease in MAP in hypertensive rats was seen with Bk-induced major renal medullary hyperperfusion or with a wash-out of medullary solutes, our data argue against a crucial role of MBF in the pathogenesis of arterial hypertension.

Dietary Na+ intake in healthy humans changes the urine extracellular vesicle prostasin abundance while the vesicle excretion rate, NCC, and ENaC are not altered.

Low Na+ intake activates aldosterone signaling, which increases renal Na+ reabsorption through increased apical activity of the NaCl cotransporter (NCC) and the epithelial Na+ channel (ENaC). Na+ transporter proteins are excreted in urine as an integral part of cell-derived extracellular vesicles (uEVs). It was hypothesized that Na+ transport protein levels in uEVs from healthy humans reflect their physiological regulation by aldosterone. Urine and plasma samples from 10 healthy men (median age: 22.8 yr) were collected after 5 days on a low-Na+ (70 mmol/day) diet and 5 days on a high-Na+ (250 mmol/day) diet. uEVs were isolated by ultracentrifugation and analyzed by Western blot analysis for EV markers (CD9, CD63, and ALIX), transport proteins (Na+-K+-ATPase α1-subunit, NCC, ENaC α- and γ-subunits, and aquaporin 2), and the ENaC-cleaving protease prostasin. Plasma renin and aldosterone concentrations increased during the low-Na+ diet. uEV size and concentration were not different between diets by tunable resistive pulse sensing. EV markers ALIX and CD9 increased with the low-Na+ diet, whereas CD63 and aquaporin 2 excretion were unchanged. Full-length ENaC γ-subunits were generally not detectable in uEVs, whereas ENaC α-subunits, NCC, and phosphorylated NCC were consistently detected but not changed by Na+ intake. Prostasin increased with low Na+ in uEVs. uEV excretion of transporters was not correlated with blood pressure, urinary Na+ and K+ excretion, plasma renin, or aldosterone. In conclusion, apical Na+ transporter proteins and proteases were excreted in uEVs, and while the excretion rate and size of uEVs were not affected, EV markers and prostasin increased in response to the low-Na+ diet.

Sympathetic regulation of NCC in norepinephrine-evoked salt-sensitive hypertension in Sprague-Dawley rats.

Salt sensitivity of blood pressure is characterized by inappropriate sympathoexcitation and renal Na+ reabsorption during high salt intake. In salt-resistant animal models, exogenous norepinephrine (NE) infusion promotes salt-sensitive hypertension and prevents dietary Na+-evoked suppression of the Na+-Cl- cotransporter (NCC). Studies of the adrenergic signaling pathways that modulate NCC activity during NE infusion have yielded conflicting results implicating α1- and/or ß-adrenoceptors and a downstream kinase network that phosphorylates and activates NCC, including with no lysine kinases (WNKs), STE20/SPS1-related proline-alanine-rich kinase (SPAK), and oxidative stress response 1 (OxSR1). In the present study, we used selective adrenoceptor antagonism in NE-infused male Sprague-Dawley rats to investigate the differential roles of α1- and ß-adrenoceptors in sympathetically mediated NCC regulation. NE infusion evoked salt-sensitive hypertension and prevented dietary Na+-evoked suppression of NCC mRNA, protein expression, phosphorylation, and in vivo activity. Impaired NCC suppression during high salt intake in NE-infused rats was paralleled by impaired suppression of WNK1 and OxSR1 expression and SPAK/OxSR1 phosphorylation and a failure to increase WNK4 expression. Antagonism of α1-adrenoceptors before high salt intake or after the establishment of salt-sensitive hypertension restored dietary Na+-evoked suppression of NCC, resulted in downregulation of WNK4, SPAK, and OxSR1, and abolished the salt-sensitive component of hypertension. In contrast, ß-adrenoceptor antagonism attenuated NE-evoked hypertension independently of dietary Na+ intake and did not restore high salt-evoked suppression of NCC. These findings suggest that a selective, reversible, α1-adenoceptor-gated WNK/SPAK/OxSR1 NE-activated signaling pathway prevents dietary Na+-evoked NCC suppression, promoting the development and maintenance of salt-sensitive hypertension.

An exaggerated blood pressure response to exercise is associated with the dietary sodium, potassium, and antioxidant vitamin intake in normotensive subjects.

AIM: This study was designed to examine the associations between an exaggerated systolic blood pressure (SBP) response to exercise and the nutrient intake in normotensive subjects. METHODS: The subjects consisted of 302 normotensive subjects (64 males and 238 females; age, 48.4 ± 11.3 years) without a history of cardiovascular disease or stroke who were not taking any medications. Each subject performed a multistage graded submaximal exercise stress test using an electric bicycle ergometer, and their blood pressure was measured at rest and during the last minute of each stage. The nutrient intake was assessed using a self-administered food frequency questionnaire. An exaggerated SBP response to exercise was defined according to the criteria of the Framingham Study (peak SBP ≥210 mmHg in males, or ≥190 mmHg in females). RESULTS: An exaggerated SBP response to exercise was observed in 85 subjects. A multiple logistic regression analysis revealed that the dietary sodium-to-potassium (Na/K) ratio (odds ratio [OR]: 5.75, 95% confidence interval [CI]: 2.37-13.75, p = 0.001) and vitamin E intake (OR: 0.67, 95% CI: 0.51-0.93, p = 0.012) were significantly associated with an exaggerated SBP response to exercise. Furthermore, the percent change in SBP during exercise was found to be significantly associated with an increase in the dietary Na/K ratio (p for trend = 0.0005) and a decrease in the vitamin E intake (p for trend = 0.018). CONCLUSIONS: These results suggest that an exaggerated SBP response to exercise was associated with the dietary sodium, potassium, and antioxidant vitamin intake in normotensive subjects.

High salt intake shifts the mechanisms of flow-induced dilation in the middle cerebral arteries of Sprague-Dawley rats.

The goal of the present study was to examine the effect of 1 wk of high salt (HS) intake and the role of oxidative stress in changing the mechanisms of flow-induced dilation (FID) in isolated pressurized middle cerebral arteries of male Sprague-Dawley rats ( n = 15-16 rats/group). Reduced FID in the HS group was restored by intake of the superoxide scavenger tempol (HS + tempol in vivo group). The nitric oxide (NO) synthase inhibitor Nω-nitro-l-arginine methyl ester, cyclooxygenase inhibitor indomethacin, and selective inhibitor of microsomal cytochrome P-450 epoxidase activity N-(methylsulfonyl)-2-(2-propynyloxy)-benzenehexanamide significantly reduced FID in the low salt diet-fed group, whereas FID in the HS group was mediated by NO only. Cyclooxygenase-2 mRNA (but not protein) expression was decreased in the HS and HS + tempol in vivo groups. Hypoxia-inducible factor-1α and VEGF protein levels were increased in the HS group but decreased in the HS + tempol in vivo group. Assessment by direct fluorescence of middle cerebral arteries under flow revealed significantly reduced vascular NO levels and increased superoxide/reactive oxygen species levels in the HS group. These results suggest that HS intake impairs FID and changes FID mechanisms to entirely NO dependent, in contrast to the low-salt diet-fed group, where FID is NO, prostanoid, and epoxyeicosatrienoic acid dependent. These changes were accompanied by increased lipid peroxidation products in the plasma of HS diet-fed rats, increased vascular superoxide/reactive oxygen species levels, and decreased NO levels, together with increased expression of hypoxia-inducible factor-1α and VEGF. NEW & NOTEWORTHY High-salt (HS) diet changes the mechanisms of flow-induced dilation in rat middle cerebral arteries from a combination of nitric oxide-, prostanoid-, and epoxyeicosatrienoic acid-dependent mechanisms to, albeit reduced, a solely nitric oxide-dependent dilation. In vivo reactive oxygen species scavenging restores flow-induced dilation in HS diet-fed rats and ameliorates HS-induced increases in the transcription factor hypoxia-inducible factor-1α and expression of its downstream target genes.

Salt-sensitive (Rapp) rats from Envigo spontaneously develop accelerated hypertension independent of ovariectomy on a low-sodium diet.

Inbred salt-sensitive (SS) rats developed by John Rapp and distributed by Harlan (SS/JrHsd) were shown to model ovariectomy-induced hypertension because on a low-sodium (LS) diet, ovariectomized SS (SS-OVX) animals became hypertensive in contrast to their sham-operated (SS-SHAM) normotensive littermates. After Harlan merged with Envigo in 2015, inconsistencies in the LS normotensive phenotype were reported. To further investigate these inconsistencies, we studied the effects of ovariectomy on SS and salt-resistant (SR) rats purchased from Envigo (SS/JrHsd/Env) between 2015 and 2017. The mean arterial pressure (MAP) in SS rats on a LS diet exceeded 160 mmHg at 7 mo old. Ovariectomy at 3 mo had no detectable effect on MAP from 4 to 7 mo, nor did ovariectomy at 1.5 mo significantly affect MAP at 10 mo in either strain; only strain differences in MAP were observed [MAP: SR-SHAM ( n = 7 rats), 102 ± 3 mmHg; SR-OVX ( n = 6 rats), 114 ± 1 mmHg; SS-SHAM ( n = 7 rats), 177 ± 6 mmHg; SS-OVX ( n = 5 rats), 190 ± 12 mmHg; where P < 0.0001 vs. SR, same ovarian-status for SS-SHAM and SS-OVX, respectively]. Whole genome sequencing revealed more genomic variants of SS/JrHsd/Env, including single nucleotide and insertion deletion polymorphisms and higher heterozygous/homozygous ratios compared with the reference genome, than for SS/JrHsd/Mcwi and SS/Jr rats maintained in Milwaukee, WI and Toledo, OH, respectively, and which still exhibit normal blood pressure on a LS diet. These findings demonstrate that the female SS/JrHsd/Env rat has genetically diverged from the original phenotype, which was normotensive on a LS diet when the ovaries were intact but rapidly developed hypertension when the ovaries were removed. Nonetheless, the SS/JrHsd/Env rat could be a valuable model that complements other animal models of spontaneous hypertension used to investigate mechanisms of essential hypertension.