Intestinal Mineralocorticoid Receptor Contributes to Epithelial Sodium Channel-Mediated Intestinal Sodium Absorption and Blood Pressure Regulation

Incidence of hyperkalemia in anuric hemodialysis patients treated with sacubitril/valsartan

INTRODUCTION

Sacubitril/valsartan is increasingly used in hemodialysis patients due to its cardioprotective benefits. However, its impact on serum potassium levels in anuric patients undergoing hemodialysis remains controversial.

METHODS

We conducted a retrospective data from patients undergoing hemodialysis at two dialysis centers. A total of 71 out of 332 patients receiving hemodialysis treatment were enrolled. Mean serum potassium (mean value of 6-8 determinations), peak serum potassium (maximum K value observed during follow-up observations), and other biochemical parameters were recorded at baseline and during the follow-up period.

FINDINGS

After 6 months of follow-up, mean serum potassium increased from 4.84 ± 0.45 mmol/L at baseline to 5.07 ± 0.46 mmol/L at 3 months and 5.04 ± 0.46 mmol/L at 6 months (p < 0.001). Notably, no significant group differences were found in peak serum potassium concentrations between baseline and 6 months after sacubitril/valsartan therapy (5.69 ± 0.56 vs. 5.75 ± 0.41, p = 0.419). Prior to starting sacubitril/valsartan treatment, none of the patients had severe hyperkalemia; however, after 3 and 6 months of sacubitril/valsartan therapy, two (2.80%) and three (4.20%) patients experienced severe hyperkalemia, respectively; however, this difference was not statistically significant. Additionally, there was a significant reduction in blood pressure; however, serum sodium, bicarbonate, and Kt/V values did not change significantly during either period.

DISCUSSION

Sacubitril/valsartan therapy is associated with an increase in serum potassium levels in anuric hemodialysis patients. Nevertheless, the proportion of patients with severe hyperkalemia did not increase significantly. This suggests that the use of sacubitril/valsartan in anuric patients on hemodialysis is relatively safe.

Clinical and cardiac characteristics of primary bilateral macronodular adrenal hyperplasia

Background

Cardiovascular disease is the leading cause of death in Cushingžs syndrome (CS). Primary bilateral macro-nodular adrenal hyperplasia (PBMAH), is a rare cause of CS that is clinically distinct from the other common types of CS, but cardiac characteristics have been poorly studied.

Methods

The clinical data, steroid hormones and echocardiographic variables of 17 patients with PBMAH were collected. Twenty-one CS patients with cortisol-producing adenoma (CPA) were collected as controls. The similarities and differences of clinical and cardiac features between the two groups were compared.

Salt and Aldosterone - Reciprocal and Combined Effects in Preclinical Models and Humans

Primary aldosteronism is an endocrine disorder caused by excessive production of aldosterone by the adrenal glands, and is recognized as the most important cause of endocrine hypertension. With specific therapy, this type of hypertension is potentially curable. In the general population, high salt intake increases the risk for cardiovascular diseases like stroke. In populations with aldosterone excess, observational and experimental data suggest that aldosterone-induced organ damage requires a combination of high dietary salt intake and high plasma aldosterone, i.e., plasma aldosterone levels inappropriately high for salt status. Therefore, understanding the relationship between plasma aldosterone levels and dietary salt intake and the nature of their combined effects is crucial for developing effective prevention and treatment strategies. In this review, we present an update on findings about primary aldosteronism and salt intake and the underlying mechanisms governing their interaction.

Decaying kidney function during cirrhosis correlates with remodeling of distal colon aldosterone target gene expression

Liver cirrhosis is associated to circulatory abnormalities leading to hypovolemia and stimulation of the renin-angiotensin-aldosterone system (RAAS). Advanced stages of the disease cause renal failure, impairing K+ and Na+ homeostasis. It has been proposed that the distal colon undergoes functional remodeling during renal failure, in particular by aldosterone-driven increased K+ excretion. In this study, we compared the transcriptional response of aldosterone target genes in the rat distal colon under two models of increased circulating aldosterone (one with concomitant RAAS activation) and in a model of secondary hyperaldosteronism induced by cirrhosis. The expression of a subset of these genes was also tested in distal colon biopsies from control subjects or patients with cirrhosis with varying levels of disease progression and treated or not with mineralocorticoid receptor inhibitor spironolactone. We examined known aldosterone-regulated transcripts involved in corticosteroid signaling and transepithelial ion transport. In addition, we included aldosterone-regulated genes related to cell proliferation. Our comparison revealed multiple aldosterone target genes upregulated in the rat distal colon during decompensated cirrhosis. Epithelial Na+ channel β and γ subunit expression correlated positively with plasma aldosterone concentration and negatively with glomerular filtration rate. Patients with cirrhosis showed increased expression of 11-β-hydroxysteroid-dehydrogenase 2 (11βHSD2), which was reverted by spironolactone treatment, suggesting a sensitization of the distal colon to aldosterone action. In summary, our data show that decaying kidney function during cirrhosis progression toward a decompensated state with hypovolemia correlates with remodeling of distal colon ion transporter expression, supporting a role for aldosterone in the process.NEW & NOTEWORTHY Liver cirrhosis progression significantly alters ion transporter subunit expression in the rat distal colon, a change that correlated well with declining kidney function and the severity of the disease. Our data suggest that the steroid hormone aldosterone participates in this homeostatic response to maintain electrolyte balance.

Adverse effects of an aldosterone synthase (CYP11B2) inhibitor, fadrozole (FAD286), on inflamed rat colon

Recently, we described local aldosterone production in the murine large intestine. Upregulated local aldosterone synthesis in different tissues has been linked with inflammatory conditions, which have been attenuated by the aldosterone synthase (CYP11B2) inhibitor, fadrozole (FAD286). Therefore, we investigated the effect of inhibition of intestinal aldosterone synthesis on the development of intestinal inflammation. Sprague-Dawley rats were administered 5% (v/w) dextran sodium sulphate (DSS) for 7 days with or without daily FAD286 (30 mg/kg/d) subcutaneous injections on 3 days before, during and one day after DSS. Tissue aldosterone concentrations were evaluated by ELISA, CYP11B2 by Western blot and RT-qPCR. FAD286 halved adrenal aldosterone production but, intriguingly, increased the colonic aldosterone concentration. The lack of inhibitory effect of FAD286 in the colon might have been affected by the smaller size of colonic vs. adrenal CYP11B2, as seen in Western blot. When combined with DSS, FAD286 aggravated the macroscopic and histological signs of intestinal inflammation, lowered the animals' body weight gain and increased the incidence of gastrointestinal bleeding and the permeability to iohexol in comparison to DSS-animals. To conclude, FAD286 exerted harmful effects during intestinal inflammation. Local intestinal aldosterone did not seem to play any role in the inflammatory pathogenesis occurring in the intestine.

Intestinal Cckbr-specific knockout mouse as a novel model of salt-sensitive hypertension via sodium over-absorption

OBJECTIVES

To investigate the value of CCKBRfl/fl villin-Cre mice as a mouse model of salt-sensitive hypertension (SSH).

METHODS

In the first part, 2-month-old CCKBRfl/fl villin-Cre mice (CKO) and control CCKBRfl/fl mice (WT) were fed with normal diet (0.4% NaCl) or high salt diet (4% NaCl), separately for 6 weeks. In the rescue study, one week of hydrochlorothiazide or saline injection were treated with the CKO mice fed high salt diet. The blood pressure, biochemical indexes, and the expression of small intestinal sodium transporters (NHE3, NKCC1, eNaC) was detected. The organ injury markers (MMP2/MMP9) and the histopathological changes of kidneys were observed, whereas the changes of duodenal sodium absorption were detected by small intestinal perfusion in vivo.

RESULTS

The CCKBRfl/fl villin-Cre mice with high salt intake exhibited high blood pressure, increased duodenal sodium absorption and urinary sodium excretion, and with renal injury. The protein expression of NHE3, NKCC1 and eNaC were also significant increase in the intestine of CKO-HS mice. Treatment with hydrochlorothiazide remarkably attenuated the elevated blood pressure by high salt absorption in the CCKBRfl/fl villin-Cre mice, but no significant histopathological changes were observed.

CONCLUSIONS

These results support a crucial role of intestinal Cckbr deficiency on SSH development and the diuretic antihypertension effect in CCKBRfl/fl villin-Cre mice. The CCKBRfl/fl villin-Cre mice with the high salt intake may serve as a stable model of salt-sensitive hypertensive induced by sodium overloading.

The Epithelial Sodium Channel-An Underestimated Drug Target

Epithelial sodium channels (ENaC) are part of a complex network of interacting biochemical pathways and as such are involved in several disease states. Dependent on site and type of mutation, gain- or loss-of-function generated symptoms occur which span from asymptomatic to life-threatening disorders such as Liddle syndrome, cystic fibrosis or generalized pseudohypoaldosteronism type 1. Variants of ENaC which are implicated in disease assist further understanding of their molecular mechanisms in order to create models for specific pharmacological targeting. Identification and characterization of ENaC modifiers not only furthers our basic understanding of how these regulatory processes interact, but also enables discovery of new therapeutic targets for the disease conditions caused by ENaC dysfunction. Numerous test compounds have revealed encouraging results in vitro and in animal models but less in clinical settings. The EMA- and FDA-designated orphan drug solnatide is currently being tested in phase 2 clinical trials in the setting of acute respiratory distress syndrome, and the NOX1/ NOX4 inhibitor setanaxib is undergoing clinical phase 2 and 3 trials for therapy of primary biliary cholangitis, liver stiffness, and carcinoma. The established ENaC blocker amiloride is mainly used as an add-on drug in the therapy of resistant hypertension and is being studied in ongoing clinical phase 3 and 4 trials for special applications. This review focuses on discussing some recent developments in the search for novel therapeutic agents.

CASZ1: a promising factor modulating aldosterone biosynthesis and mineralocorticoid receptor activity

Hypertension is the definitive risk factor for cardiovascular disease. Primary aldosteronism (PA), a typical form of secondary hypertension, is responsible for treatment-resistant hypertension and carries an even higher risk of causing cardiovascular complications than essential hypertension. Several genes involved in the pathogenesis of hypertension have been identified recently using genome-wide association studies (GWASs). Among these, castor zinc finger 1(CASZ1) is considered to be involved in the pathophysiology of hypertension via modulation of aldosterone action. In 2021, using a biochemical approach with liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, we identified CASZ1b, an isoform of CASZ1, as a novel mineralocorticoid receptor (MR) coregulator. Our further analysis revealed that CASZ1b is coexpressed with MR in MR targets such as kidney tubule cells, and that a decrease in CASZ1 protein levels promotes aldosterone-dependent transcriptional activity of MR. Further, a recent study of GWAS on PA identified CASZ1 to be a PA-related gene and demonstrated that overexpression of CASZ1 suppresses aldosterone biosynthesis in adrenal cells. These results suggest CASZ1 plays a pivotal role in the pathophysiology of hypertension and PA via dual mechanisms: aldosterone biosynthesis and transcriptional activity of MR.

Biglycan Is a Novel Mineralocorticoid Receptor Target Involved in Aldosterone/Salt-Induced Glomerular Injury

The beneficial effects of mineralocorticoid receptor (MR) antagonists (MRAs) for various kidney diseases are established. However, the underlying mechanisms of kidney injury induced by MR activation remain to be elucidated. We recently reported aldosterone-induced enhancement of proteoglycan expression in mitral valve interstitial cells and its association with fibromyxomatous valvular disorder. As the expression of certain proteoglycans is elevated in several kidney diseases, we hypothesized that proteoglycans mediate kidney injury in the context of aldosterone/MR pathway activation. We evaluated the proteoglycan expression and tissue injury in the kidney and isolated glomeruli of uninephrectomy/aldosterone/salt (NAS) mice. The MRA eplerenone was administered to assess the role of the MR pathway. We investigated the direct effects of biglycan, one of the proteoglycans, on macrophages using isolated macrophages. The kidney samples from NAS-treated mice showed enhanced fibrosis and increased expression of biglycan accompanying glomerular macrophage infiltration and enhanced expression of TNF-α, iNOS, Nox2, CCL3 (C-C motif chemokine ligand 3), and phosphorylated NF-κB. Eplerenone blunted these changes. Purified biglycan stimulated macrophages to express TNF-α, iNOS, Nox2, and CCL3. This was prevented by a toll-like receptor 4 (TLR4) or NF-κB inhibitor, indicating that biglycan stimulation is dependent on the TLR4/NF-κB pathway. We identified the proteoglycan biglycan as a novel target of MR involved in MR-induced glomerular injury and macrophage infiltration via a biglycan/TLR4/NF-κB/CCL3 cascade.

Cardiometabolic effects of DOCA-salt in male C57BL/6J mice are variably dependent on sodium and nonsodium components of diet

Hypertension characterized by low circulating renin activity accounts for roughly 25%-30% of primary hypertension in humans and can be modeled experimentally via deoxycorticosterone acetate (DOCA)-salt treatment. In this model, phenotypes develop in progressive phases, although the timelines and relative contributions of various mechanisms to phenotype development can be distinct between laboratories. To explore interactions among environmental influences such as diet formulation and dietary sodium (Na) content on phenotype development in the DOCA-salt paradigm, we examined an array of cardiometabolic endpoints in young adult male C57BL/6J mice during sham or DOCA-salt treatments when mice were maintained on several common, commercially available laboratory rodent "chow" diets including PicoLab 5L0D (0.39% Na), Envigo 7913 (0.31% Na), Envigo 2920x (0.15% Na), or a customized version of Envigo 2920x (0.4% Na). Energy balance (weight gain, food intake, digestive efficiency, and energy efficiency), fluid and electrolyte homeostasis (fluid intake, Na intake, fecal Na content, hydration, and fluid compartmentalization), renal functions (urine production rate, glomerular filtration rate, urine Na excretion, renal expression of renin, vasopressin receptors, aquaporin-2 and relationships among markers of vasopressin release, aquaporin-2 shedding, and urine osmolality), and blood pressure, all exhibited changes that were subject to interactions between diet and DOCA-salt. Interestingly, some of these phenotypes, including blood pressure and hydration, were dependent on nonsodium dietary components, as Na-matched diets resulted in distinct phenotype development. These findings provide a broad and robust illustration of an environment × treatment interaction that impacts the use and interpretation of a common rodent model of low-renin hypertension.

Bile acids and salt-sensitive hypertension: a role of the gut-liver axis

Salt-sensitivity of blood pressure (SSBP) affects 50% of the hypertensive and 25% of the normotensive populations. Importantly, SSBP is associated with increased risk for mortality in both populations independent of blood pressure. Despite its deleterious effects, the pathogenesis of SSBP is not fully understood. Emerging evidence suggests a novel role of bile acids in salt-sensitive hypertension and that they may play a crucial role in regulating inflammation and fluid volume homeostasis. Mechanistic evidence implicates alterations in the gut microbiome, the epithelial sodium channel (ENaC), the farnesoid X receptor, and the G protein-coupled bile acid receptor TGR5 in bile acid-mediated effects on cardiovascular function. The mechanistic interplay between excess dietary sodium-induced alterations in the gut microbiome and immune cell activation, bile acid signaling, and whether such interplay may contribute to the etiology of SSBP is still yet to be defined. The main goal of this review is to discuss the potential role of bile acids in the pathogenesis of cardiovascular disease with a focus on salt-sensitive hypertension.

Methods for the Comprehensive in vivo Analysis of Energy Flux, Fluid Homeostasis, Blood Pressure, and Ventilatory Function in Rodents

Cardiovascular disease represents the leading cause of death in the United States, and metabolic diseases such as obesity represent the primary impediment to improving cardiovascular health. Rodent (mouse and rat) models are widely used to model cardiometabolic disease, and as a result, there is increasing interest in the development of accurate and precise methodologies with sufficiently high resolution to dissect mechanisms controlling cardiometabolic physiology in these small organisms. Further, there is great utility in the development of centralized core facilities furnished with high-throughput equipment configurations and staffed with professional content experts to guide investigators and ensure the rigor and reproducibility of experimental endeavors. Here, we outline the array of specialized equipment and approaches that are employed within the Comprehensive Rodent Metabolic Phenotyping Core (CRMPC) and our collaborating laboratories within the Departments of Physiology, Pediatrics, Microbiology & Immunology, and Biomedical Engineering at the Medical College of Wisconsin (MCW), for the detailed mechanistic dissection of cardiometabolic function in mice and rats. We highlight selected methods for the analysis of body composition and fluid compartmentalization, electrolyte accumulation and flux, energy accumulation and flux, physical activity, ingestive behaviors, ventilatory function, blood pressure, heart rate, autonomic function, and assessment and manipulation of the gut microbiota. Further, we include discussion of the advantages and disadvantages of these approaches for their use with rodent models, and considerations for experimental designs using these methods.

Lysine-specific demethylase 1 as a corepressor of mineralocorticoid receptor

Mineralocorticoid receptor (MR) and its ligand aldosterone play a central role in controlling blood pressure by promoting sodium reabsorption in the kidney. Coregulators are recruited to regulate the activation of steroid hormone receptors. In our previous study, we identified several new candidates for MR coregulators through liquid chromatography-tandem mass spectrometry analysis using a biochemical approach. Lysine-specific demethylase 1 (LSD1) was identified as a candidate. The relationship between LSD1 and salt-sensitive hypertension has been reported; however, the role of MR in this condition is largely unknown. Here, we investigated the functions of LSD1 as a coregulator of MR. First, a coimmunoprecipitation assay using HEK293F cells showed specific interactions between MR and LSD1. A chromatin immunoprecipitation study demonstrated LSD1 recruitment to the gene promoter of epithelial Na⁺ channel (ENaC), a target gene of MR. Reduced LSD1 expression by treatment with shRNA potentiated the hormonal activation of ENaC and serum/glucocorticoid-regulated kinase 1, another target gene of MR, indicating that LSD1 is a corepressor of MR. In an animal study, mice with kidney-specific LSD1 knockout (LSD1^flox/floxKSP-Cre mice) developed hypertension after a high-salt diet without elevation of aldosterone levels, which was counteracted by cotreatment with spironolactone, an MR antagonist. In conclusion, our in vitro and in vivo studies demonstrated that LSD1 is a newly identified corepressor of MR.

“Greedy Organs Hypothesis” for sugar and salt in the pathophysiology of non-communicable diseases in relation to sodium-glucose co-transporters in the intestines and the kidney

Deposition of visceral fat and insulin resistance play central role in the development of non-communicable diseases (NCDs) including obesity, hypertension and type 2 diabetes. However, we shed more light upon the intestines and the kidney as a strong driver of NCDs. Based upon unexpected outcomes of clinical trials using sodium-glucose cotransporter (SGLT) 2 inhibitors to demonstrate their actions for not only body weight reduction and blood glucose fall but also remarkable cardiorenal protection, we speculate that hyperfunction of the intestines and the kidney is one of critical contributing factors for initiation of NCDs. By detecting high amount of glucose and sodium chloride around them by sweet/salt taste sensors, the intestines and the kidney are designed to (re)absorb these nutrients by up-regulating SGLT1 or SGLT2. We designate these hyperfunctioning organs for nutrient uptake as “greedy organs”. The greedy organs can induce NCDs (“greedy organ hypothesis”). SGLTs are regulated by glucose and sodium chloride, and SGLTs or other genes can be “greedy genes.” Regulating factors for greedy organs are renin-angiotensin system, renal sympathetic nervous activity, gut inflammation/microbiota or oxidative stress. Mitigation of organ greediness by SGLT2 inhibitors, ketone bodies, bariatric surgery, and regular lifestyle to keep rhythmicity of biological clock are promising.

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We propose the concept of “Greedy Organs” hypothesis as a possible cause of NCDs.

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Clinical implication of greedy kidney is supported by the effect of SGLT2 inhibitors.

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The significance of greedy intestines is suggested by the effect of bariatric surgery.

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The intestines and kidney become hyperactive through upregulation of SGLT1 or 2.

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To mitigate “greedy organs” should be a promising strategy against NCDs.

Salt Sensitivity of Blood Pressure in Blacks and Women: A Role of Inflammation, Oxidative Stress, and Epithelial Na+ Channel

Significance: Salt sensitivity of blood pressure (SSBP) is an independent risk factor for mortality and morbidity due to cardiovascular disease, and disproportionately affects blacks and women. Several mechanisms have been proposed, including exaggerated activation of sodium transporters in the kidney leading to salt retention and water. Recent Advances: Recent studies have found that in addition to the renal epithelium, myeloid immune cells can sense sodium via the epithelial Na+ channel (ENaC), which leads to activation of the nicotinamide adenine dinucleotide phosphate oxidase enzyme complex, increased fatty acid oxidation, and production of isolevuglandins (IsoLGs). IsoLGs are immunogenic and contribute to salt-induced hypertension. In addition, aldosterone-mediated activation of ENaC has been attributed to the increased SSBP in women. The goal of this review is to highlight mechanisms contributing to SSBP in blacks and women, including, but not limited to increased activation of ENaC, fatty acid oxidation, and inflammation. Critical Issues: A critical barrier to progress in management of SSBP is that its diagnosis is not feasible in the clinic and is limited to expensive and laborious research protocols, which makes it difficult to investigate. Yet without understanding the underlying mechanisms, this important risk factor remains without treatment. Future Directions: Further studies are needed to understand the mechanisms that contribute to differential blood pressure responses to dietary salt and find feasible diagnostic tools. This is extremely important and may go a long way in mitigating the racial and sex disparities in cardiovascular outcomes. Antioxid. Redox Signal. 35, 1477-1493.

Role of the microbiota in hypertension and antihypertensive drug metabolism

Recent evidence suggests that the gut microbiota plays an important role in the development and pathogenesis of hypertension. Dysbiosis, an imbalance in the composition and function of the gut microbiota, was shown to be associated with hypertension in both animal models and humans. In this review, we provide insights into host-microbiota interactions and summarize the evidence supporting the importance of the microbiota in blood pressure (BP) regulation. Metabolites produced by the gut microbiota, especially short-chain fatty acids (SCFAs), modulate BP and vascular responses. Harmful gut-derived metabolites, such as trimethylamine N-oxide and several uremic toxins, exert proatherosclerotic, prothrombotic, and proinflammatory effects. High-salt intake alters the composition of the microbiota, and this microbial alteration contributes to the pathogenesis of salt-sensitive hypertension. In addition, the microbiota may impact the metabolism of drugs and steroid hormones in the host. The drug-metabolizing activities of the microbiota affect the pharmacokinetic parameters of antihypertensive drugs and contribute to the pathogenesis of licorice-induced pseudohyperaldosteronism. Furthermore, the oral microbiota plays a role in BP regulation by producing nitric oxide, which lowers BP via its vasodilatory effects. Thus, antihypertensive intervention strategies targeting the microbiota, such as the use of prebiotics, probiotics, and postbiotics (e.g., SCFAs), are considered new therapeutic options for the treatment of hypertension.

Approaches towards tissue-selective pharmacology of the mineralocorticoid receptor

Mineralocorticoid receptor antagonists (MRAs) are highly effective therapies for cardiovascular and renal disease. However, the widespread clinical use of currently available MRAs in cardiorenal medicine is hampered by an increased risk of hyperkalemia. The mineralocorticoid receptor (MR) is a nuclear receptor responsible for fluid and electrolyte homeostasis in epithelial tissues, whereas pathophysiological MR activation in nonepithelial tissues leads to undesirable pro-inflammatory and pro-fibrotic effects. Therefore, new strategies that selectively target the deleterious effects of MR but spare its physiological function are needed. In this review, we discuss recent pharmacological developments starting from novel non-steroidal MRAs that are now entering clinical use, such as finerenone or esaxerenone, to concepts arising from the current knowledge of the MR signaling pathway, aiming at receptor-coregulator interaction, epigenetics, or downstream effectors of MR.

Hyperkalemia in patients undergoing hemodialysis: its pathophysiology and management

Potassium is a major intracellular cation in the body, regulating membrane potential of excitable cells, such as cardiomyocytes and skeletal muscle cells. Because the kidney plays a critical role in controlling potassium balance, the elevation in serum potassium levels is one of the most common complications in patients with maintenance hemodialysis (MHD). In addition to reduced renal potassium excretion, the alteration in body potassium distribution owing to comorbid conditions may also contribute to dyskalemia. Besides potassium elimination through hemodialysis in MHD patients, accumulating data indicate the potential importance of extra‐renal elimination involving the gastrointestinal system, which can be affected by the inhibitors of the renin‐angiotensin‐aldosterone system. In this article, the literature on potassium physiology in MHD patients is reviewed with an emphasis on the changes from individuals with normal kidney function. This article also summarizes the findings of recent studies on dietary control, dialysate prescription, and pharmacological therapy.

Immune Mechanisms of Dietary Salt-Induced Hypertension and Kidney Disease: Harry Goldblatt Award for Early Career Investigators 2020

Salt sensitivity of blood pressure is an independent risk factor for cardiovascular mortality not only in hypertensive but also in normotensive adults. The diagnosis of salt sensitivity of blood pressure is not feasible in the clinic due to lack of a simple diagnostic test, making it difficult to investigate therapeutic strategies. Most research efforts to understand the mechanisms of salt sensitivity of blood pressure have focused on renal regulation of sodium. However, salt retention or plasma volume expansion is not different between salt-sensitive and salt-resistant individuals. In addition, over 70% of extracellular fluid is interstitial and, therefore, not directly controlled by renal salt and water excretion. We discuss in this review how the seminal work by Harry Goldblatt paved the way for our attempts at understanding the mechanisms that underlie immune activation by salt in hypertension. We describe our findings that sodium, entering antigen-presenting cells via an epithelial sodium channel, triggers a PKC (protein kinase C)- and SGK1 (serum/glucocorticoid kinase 1)-stimulated activation of nicotinamide adenine dinucleotide phosphate oxidase, which, in turn, enhances lipid oxidation with generation of highly reactive isolevuglandins. Isolevuglandins adduct to proteins, with the potential to generate degraded peptide neoantigens. Activated antigen-presenting cells increase production of the TH17 polarizing cytokines, IL (interleukin)-6, IL-1β, and IL-23, which leads to differentiation and proliferation of IL-17A producing T cells. Our laboratory and others have shown that this cytokine contributes to hypertension. We also discuss where this sodium activation of antigen-presenting cells may occur in vivo and describe the multiple experiments, with pharmacological antagonists and knockout mice that we used to unravel this sequence of events in rodents. Finally, we describe experiments in mononuclear cells obtained from normotensive or hypertensive volunteers, which confirm that analogous processes of salt-induced immunity take place in humans.

The role of NHE3 (Slc9a3) in oxalate and sodium transport by mouse intestine and regulation by cAMP

Intestinal oxalate transport involves Cl⁻/HCO₃⁻ exchangers but how this transport is regulated is not currently known. NHE3 (Slc9a3), an apical Na⁺/H⁺ exchanger, is an established target for regulation of electroneutral NaCl absorption working in concert with Cl⁻/HCO₃⁻ exchangers. To test whether NHE3 could be involved in regulation of intestinal oxalate transport and renal oxalate handling we compared urinary oxalate excretion rates and intestinal transepithelial fluxes of ¹⁴C‐oxalate and ²²Na⁺ between NHE3 KO and wild‐type (WT) mice. NHE3 KO kidneys had lower creatinine clearance suggesting reduced GFR, but urinary oxalate excretion rates (µmol/24 h) were similar compared to the WT but doubled when expressed as a ratio of creatinine. Intestinal transepithelial fluxes of ¹⁴C‐oxalate and ²²Na⁺ were measured in the distal ileum, cecum, and distal colon. The absence of NHE3 did not affect basal net transport rates of oxalate or sodium across any intestinal section examined. Stimulation of intracellular cAMP with forskolin (FSK) and 3‐isobutyl‐1‐methylxanthine (IBMX) led to an increase in net oxalate secretion in the WT distal ileum and cecum and inhibition of sodium absorption in the cecum and distal colon. In NHE3 KO cecum, cAMP stimulation of oxalate secretion was impaired suggesting the possibility of a role for NHE3 in this process. Although, there is little evidence for a role of NHE3 in basal intestinal oxalate fluxes, NHE3 may be important for cAMP stimulation of oxalate in the cecum and for renal handling of oxalate.

Intestinal Flora Modulates Blood Pressure by Regulating the Synthesis of Intestinal-Derived Corticosterone in High Salt-Induced Hypertension

Rationale:

High-salt diet is one of the most important risk factors for hypertension. Intestinal flora has been reported to be associated with high salt–induced hypertension (hSIH). However, the detailed roles of intestinal flora in hSIH pathogenesis have not yet been fully elucidated.

Objective:

To reveal the roles and mechanisms of intestinal flora in hSIH development.

Methods and Results:

The abovementioned issues were investigated using various techniques including 16S rRNA gene sequencing, untargeted metabolomics, selective bacterial culture, and fecal microbiota transplantation. We found that high-salt diet induced hypertension in Wistar rats. The fecal microbiota of healthy rats could dramatically lower blood pressure (BP) of hypertensive rats, whereas the fecal microbiota of hSIH rats had opposite effects. The composition, metabolism, and interrelationship of intestinal flora in hSIH rats were considerably reshaped, including the increased corticosterone level and reduced Bacteroides and arachidonic acid levels, which tightly correlated with BP. The serum corticosterone level was also significantly increased in rats with hSIH. Furthermore, the above abnormalities were confirmed in patients with hypertension. The intestinal Bacteroides fragilis could inhibit the production of intestinal-derived corticosterone induced by high-salt diet through its metabolite arachidonic acid.

Conclusions:

hSIH could be transferred by fecal microbiota transplantation, indicating the pivotal roles of intestinal flora in hSIH development. High-salt diet reduced the levels of B fragilis and arachidonic acid in the intestine, which increased intestinal-derived corticosterone production and corticosterone levels in serum and intestine, thereby promoting BP elevation. This study revealed a novel mechanism different from inflammation/immunity by which intestinal flora regulated BP, namely intestinal flora could modulate BP by affecting steroid hormone levels. These findings enriched the understanding of the function of intestinal flora and its effects on hypertension.

Upregulation of Claudin-7 Expression by Angiotensin II in Colonic Epithelial Cells of Mice Fed with NaCl-Depleted Diets

Dietary NaCl depletion increases Na⁺ and Cl⁻ absorption in the colon, but the mechanisms are not fully understood. So far, we reported that the expression of claudin-7 (CLDN7), a tight junction (TJ) protein, was upregulated in the mice fed with NaCl-depleted diets, but the regulatory mechanism has not been clarified. Here, we found that angiotensin II (ANGII) increases the mRNA level of CLDN7, which was inhibited by losartan, a type 1 ANGII (AT1) receptor antagonist. Immunofluorescence measurement showed that CLDN7 is colocalized with zonula occludens-1 at the TJ in untreated and ANGII-treated cells. ANGII decreased transepithelial electrical resistance (TER) and increased permeability to C1⁻ without affecting permeability to lucifer yellow, a paracellular flux marker. In contrast, TER was increased by CLDN7 knockdown in the absence and presence of ANGII. ANGII increased the nuclear distribution of phosphorylated p65 subunit of NF-κB, which was inhibited by losartan. The ANGII-induced elevation of CLDN7 expression was blocked by BAY 11-7082 (BAY), an NF-κB inhibitor. Luciferase reporter assay showed that ANGII increases promoter activity of CLDN7, which was inhibited by the treatment with losartan or BAY, and introduction of mutations in κB-binding motifs in the promoter. The binding of p65 on the promoter region of CLDN7 was increased by ANGII, which was inhibited by losartan and BAY in chromatin immunoprecipitation assay. Our data suggest that ANGII acts on AT1 receptor and increases paracellular permeability to Cl⁻ mediated by the elevation of CLDN7 expression in the colon.

Hypertension as a Metabolic Disorder and the Novel Role of the Gut

Purpose of Review

Hypertension is related to impaired metabolic homeostasis and can be regarded as a metabolic disorder. This review presents possible mechanisms by which metabolic disorders increase blood pressure (BP) and discusses the importance of the gut as a novel modulator of BP.

Recent Findings

Obesity and high salt intake are major risk factors for hypertension. There is a hypothesis of “salt-induced obesity”; i.e., high salt intake may tie to obesity. Heightened sympathetic nervous system (SNS) activity, especially in the kidney and brain, increases BP in obese patients. Adipokines, including adiponectin and leptin, and renin-angiotensin-aldosterone system (RAAS) contribute to hypertension. Adiponectin induced by a high-salt diet may decrease sodium/glucose cotransporter (SGLT) 2 expression in the kidney, which results in reducing BP. High salt can change secretions of adipokines and RAAS-related components. Evidence has been accumulating linking the gastrointestinal tract to BP. Glucagon-like peptide-1 (GLP-1) and ghrelin decrease BP in both rodents and humans. The sweet taste receptor in enteroendocrine cells increases SGLT1 expression and stimulates sodium/glucose absorption. Roux-en-Y gastric bypass improves glycemic and BP control due to reducing the activity of SGLT1. Na/H exchanger isoform 3 (NHE3) increases BP by stimulating the intestinal absorption of sodium. Gastrin functions as an intestinal sodium taste sensor and inhibits NHE3 activity. Intestinal mineralocorticoid receptors also regulate sodium absorption and BP due to changing ENaC activity. Gastric sensing of sodium induces natriuresis, and gastric distension increases BP. Changes in the composition and function of gut microbiota contribute to hypertension. A high-salt/fat diet may disrupt the gut barrier, which results in systemic inflammation, insulin resistance, and increased BP. Gut microbiota regulates BP by secreting vasoactive hormones and short-chain fatty acids. BP-lowering effects of probiotics and antibiotics have been reported. Bariatric surgery improves metabolic disorders and hypertension due to increasing GLP-1 secretion, decreasing leptin secretion and SNS activity, and changing gut microbiome composition. Strategies targeting the gastrointestinal system may be therapeutic options for improving metabolic abnormalities and reducing BP in humans.

Summary

SNS, brain, adipocytes, RAAS, the kidney, the gastrointestinal tract, and microbiota play important roles in regulating BP. Most notably, the gut could be a novel target for treatment of hypertension as a metabolic disorder.