An alternative hypothesis to the widely held view that renal excretion of sodium accounts for resistance to salt-induced hypertension

Research trends and hotpots on the relationship between high salt and hypertension: A bibliometric and visualized analysis

INTRODUCTION

A high salt diet is a significant risk factor for hypertension, and scholarly investigations into this relationship have garnered considerable attention worldwide. However, bibliometric analyses in this field remain underdeveloped. This study aimed to conduct a bibliometric and visual analysis of research progress on the link between high salt and hypertension from 2011 to 2022 with the goal of identifying future research trends and providing valuable insights for this field.

METHODS

High salt and hypertension data were obtained from the Web of Science Core Collection database. Microsoft Excel, Scimago Graphica, CiteSpace, and VOSviewer software were employed to analyze publication output trends, the most productive countries or regions, journals, authors, co-cited references, and keywords.

RESULTS

After screening, 1470 papers met the inclusion criteria. Relevant publications increased annually by 3.66% from 2011 to 2022. The United States led in research productivity, with The Journal of Hypertension publishing the most papers, and David L. Mattson as the most prolific author. Oxidative stress has emerged as a prominent research topic, and extensive investigations have been conducted on related mechanisms. "Oxidative stress," "gut microbiota," and "kidney injury" are recent hotspots that are expected to remain so, and this study carefully characterizes the mechanism of high salt-induced hypertension based on these hotspots.

CONCLUSION

This study utilized bibliometric and visualization analysis to identify the development trends and hotspots of publications related to high salt and hypertension. The findings of this study offer valuable insights into the forefront of emerging trends and future directions in this field.

Mechanisms of sodium-mediated injury in cardiovascular disease: old play, new scripts

There is a strong association between salt intake and cardiovascular diseases, particularly hypertension, on the population level. The mechanisms that explain this association remain incompletely understood and appear to extend beyond blood pressure. In this review, we describe some of the 'novel' roles of Na⁺ in cardiovascular health and disease: energetic implications of sodium handling in the kidneys; local accumulation in tissue; fluid dynamics; and the role of the microvasculature, with particular focus on the lymphatic system. We describe the interplay between these factors that involves body composition, metabolic signatures, inflammation and composition of the extracellular and intracellular milieus.

Mechanism-based strategies to prevent salt sensitivity and salt-induced hypertension

High-salt diets are a major cause of hypertension and cardiovascular (CV) disease. Many governments are interested in using food salt reduction programs to reduce the risk for salt-induced increases in blood pressure and CV events. It is assumed that reducing the salt concentration of processed foods will substantially reduce mean salt intake in the general population. However, contrary to expectations, reducing the sodium density of nearly all foods consumed in England by 21% had little or no effect on salt intake in the general population. This may be due to the fact that in England, as in other countries including the U.S.A., mean salt intake is already close to the lower normal physiologic limit for mean salt intake of free-living populations. Thus, mechanism-based strategies for preventing salt-induced increases in blood pressure that do not solely depend on reducing salt intake merit attention. It is now recognized that the initiation of salt-induced increases in blood pressure often involves a combination of normal increases in sodium balance, blood volume and cardiac output together with abnormal vascular resistance responses to increased salt intake. Therefore, preventing either the normal increases in sodium balance and cardiac output, or the abnormal vascular resistance responses to salt, can prevent salt-induced increases in blood pressure. Suboptimal nutrient intake is a common cause of the hemodynamic disturbances mediating salt-induced hypertension. Accordingly, efforts to identify and correct the nutrient deficiencies that promote salt sensitivity hold promise for decreasing population risk of salt-induced hypertension without requiring reductions in salt intake.

Sodium Accumulation and Blood Capillary Rarefaction in the Skin Predispose Spontaneously Hypertensive Rats to Salt Sensitive Hypertension

Recent studies in humans and rats suggested that increased Na⁺ storage in the skin without parallel water retention may predispose to salt-sensitive hypertension. In the current studies, we compared tissue Na⁺ storage in salt sensitive spontaneously hypertensive rats (SHR) versus salt resistant normotensive Brown Norway (BN-Lx) rats. After salt loading (10 days drinking 1% NaCl solution), the SHR showed significant parallel increase in Na⁺-to-water as well as (Na⁺+K⁺)-to-water ratios suggesting increased storage of osmotically inactive Na⁺ in the skin while no significant changes in skin electrolyte concentrations were observed in BN-Lx rats. SHR rats after salt treatment exhibited a nonsignificant decrease in skin blood capillary number (rarefaction) while BN-Lx rats showed significantly increased skin blood capillary density. Analysis of dermal gene expression profiles in BN-Lx rats after salt treatment showed significant up-regulation of genes involved in angiogenesis and proliferation of endothelial cells contrary to the SHR. Since the skin harbors most of the body’s resistance vessels it is possible that blood capillary rarefaction may lead to increased peripheral resistance and salt sensitivity in the SHR.

QiShenYiQi ameliorates salt-induced hypertensive nephropathy by balancing ADRA1D and SIK1 expression in Dahl salt-sensitive rats

BACKGROUND

Hypertension is a leading risk factor for developing kidney disease. Current single-target antihypertensive drugs are not effective for hypertensive nephropathy, in part due to its less understood mechanism of pathogenesis. We recently showed that QiShenYiQi (QSYQ), a component-based cardiovascular Chinese medicine, is also effective for ischemic stroke. Given the important role of the brain-heart-kidney axis in blood pressure control, we hypothesized that QSYQ may contribute to blood pressure regulation and kidney protection in Dahl salt-sensitive hypertensive rats.

METHODS

The therapeutic effects of QSYQ on blood pressure and kidney injury in Dahl salt-sensitive rats fed with high salt for 9 weeks were evaluated by tail-cuff blood pressure monitoring, renal histopathological examination and biochemical indicators in urine and serum. RNA-seq was conducted to identify QSYQ regulated genes in hypertensive kidney, and RT-qPCR, immunohistochemistry, and Western blotting analysis were performed to verify the transcriptomics results and validate the purposed mechanisms.

RESULTS

QSYQ treatment significantly decreased blood pressure in Dahl salt-sensitive hypertensive rats, alleviated renal tissue damage, reduced renal interstitial fibrosis and collagen deposition, and improved renal physiological function. RNA-seq and subsequent bioinformatic analysis showed that the expression of ADRA1D and SIK1 genes were among the most prominently altered by QSYQ in salt-sensitive hypertensive rat kidney. RT-qPCR, immunohistochemistry and Western blotting results confirmed that the mRNA and protein expression levels of alpha-1D adrenergic receptor (ADRA1D) in the kidney tissue of the QSYQ-treated rats were markedly down-regulated, while the mRNA and protein levels of salt inducible kinase 1 (SIK1) were significantly increased.

CONCLUSION

QSYQ not only lowered blood pressure, but also alleviated renal damage via reducing the expression of ADRA1D and increasing the expression of SIK1 in the kidney of Dahl salt-sensitive hypertensive rats.

No evidence of racial disparities in blood pressure salt sensitivity when potassium intake exceeds levels recommended in the US dietary guidelines

On average, black individuals are widely believed to be more sensitive than white individuals to blood pressure (BP) effects of changes in salt intake. However, few studies have directly compared the BP effects of changing salt intake in black versus white individuals. In this narrative review, we analyze those studies and note that when potassium intake substantially exceeds the recently recommended US dietary goal of 87 mmol/day, black adults do not appear more sensitive than white adults to BP effects of short-term or long-term increases in salt intake (from an intake ≤50 mmol/day up to 150 mmol/day or more). However, with lower potassium intakes, racial differences in salt sensitivity are observed. Mechanistic studies suggest that racial differences in salt sensitivity are related to differences in vascular resistance responses to changes in salt intake mediated by vasodilator and vasoconstrictor pathways. With respect to cause and prevention of racial disparities in salt sensitivity, it is noteworthy that 1) on average, black individuals consume less potassium than white individuals and 2) consuming supplemental potassium bicarbonate, or potassium rich foods can prevent racial disparities in salt sensitivity. However, the new US dietary guidelines reduced the dietary potassium goal well below the amount associated with preventing racial disparities in salt sensitivity. These observations should motivate research on the impact of the new dietary potassium guidelines on racial disparities in salt sensitivity, the risks and benefits of potassium-containing salt substitutes or supplements, and methods for increasing consumption of foods rich in nutrients that protect against salt-induced hypertension.

α-Adrenergic receptor blockade attenuates pressor response during mental stress in young black adults

Black individuals exhibit increased blood pressure (BP) responses to sympathetic stimulation that are associated with an increased risk of hypertension (HTN). We tested the hypothesis that α1 -adrenergic blockade inhibits the increased BP response during and after 45-min stress in young normotensive Black adults, which may be mediated, in part, by dampened vasoconstriction and decreased renal sodium retention. Utilizing a double-masked randomized, crossover study design, 51 normotensive Black adults (31 ± 8 yr) were treated with either a placebo or 1 mg/day of prazosin for 1 week. On the final day of each treatment, hemodynamic measures and urinary sodium excretion (UNaV) were collected before (Rest), during (Stress) and after (Recovery) 45 min of mental stress induced via a competitive video game task. During the Stress period, diastolic BP and total peripheral resistance (TPR) were significantly lower with prazosin compared to placebo (p < .05 for both). Similarly, we observed lower systolic BP, diastolic BP, and TPR during the Recovery period with prazosin versus placebo (p < .05 for both). There was no effect of prazosin on stress-associated UNaV. The change in systolic BP from Rest to Recovery was positively associated with the change in TPR with both treatments (p < .05 for both). In summary, prazosin treatment dampened BP reactivity to 45-min mental stress and lowered post-stress BP over the recovery period, which was linked to reduce TPR in young normotensive Black adults. These results suggest that α1 -adrenergic receptor activity may contribute to BP responses and delayed BP recovery to prolonged mental stress through increased vasoconstriction in Black adults.

Effect of high-salt diet on blood pressure and body fluid composition in patients with type 1 diabetes: randomized controlled intervention trial

INTRODUCTION

Patients with type 1 diabetes are susceptible to hypertension, possibly resulting from increased salt sensitivity and accompanied changes in body fluid composition. We examined the effect of a high-salt diet (HSD) in type 1 diabetes on hemodynamics, including blood pressure (BP) and body fluid composition.

RESEARCH DESIGN AND METHODS

We studied eight male patients with type 1 diabetes and 12 matched healthy controls with normal BP, body mass index, and renal function. All subjects adhered to a low-salt diet and HSD for eight days in randomized order. On day 8 of each diet, extracellular fluid volume (ECFV) and plasma volume were calculated with the use of iohexol and 125I-albumin distribution. Hemodynamic measurements included BP, cardiac output (CO), and systemic vascular resistance.

RESULTS

After HSD, patients with type 1 diabetes showed a BP increase (mean arterial pressure: 85 (5) mm Hg vs 80 (3) mm Hg; p<0.05), while BP in controls did not rise (78 (5) mm Hg vs 78 (5) mm Hg). Plasma volume increased after HSD in patients with type 1 diabetes (p<0.05) and not in controls (p=0.23). There was no significant difference in ECFV between diets, while HSD significantly increased CO, heart rate (HR) and N-terminal pro-B-type natriuretic peptide (NT-proBNP) in type 1 diabetes but not in controls. There were no significant differences in systemic vascular resistance, although there was a trend towards an HSD-induced decrease in controls (p=0.09).

CONCLUSIONS

In the present study, patients with type 1 diabetes show a salt-sensitive BP rise to HSD, which is accompanied by significant increases in plasma volume, CO, HR, and NT-proBNP. Underlying mechanisms for these responses need further research in order to unravel the increased susceptibility to hypertension and cardiovascular disease in diabetes.

TRIAL REGISTRATION NUMBERS

NTR4095 and NTR4788.

Tissue Sodium Content and Arterial Hypertension in Obese Adolescents

Early-onset obesity is known to culminate in type 2 diabetes, arterial hypertension and subsequent cardiovascular disease. The role of sodium (Na⁺) homeostasis in this process is incompletely understood, yet correlations between Na⁺ accumulation and hypertension have been observed in adults. We aimed to investigate these associations in adolescents. A cohort of 32 adolescents (13–17 years), comprising 20 obese patients, of whom 11 were hypertensive, as well as 12 age-matched controls, underwent ²³Na-MRI of the left lower leg with a standard clinical 3T scanner. Median triceps surae muscle Na⁺ content in hypertensive obese (11.95 mmol/L [interquartile range 11.62–13.66]) was significantly lower than in normotensive obese (13.63 mmol/L [12.97–17.64]; p = 0.043) or controls (15.37 mmol/L [14.12–16.08]; p = 0.012). No significant differences were found between normotensive obese and controls. Skin Na⁺ content in hypertensive obese (13.33 mmol/L [11.53–14.22] did not differ to normotensive obese (14.12 mmol/L [13.15–15.83]) or controls (11.48 mmol/L [10.48–12.80]), whereas normotensive obese had higher values compared to controls (p = 0.004). Arterial hypertension in obese adolescents is associated with low muscle Na⁺ content. These findings suggest an early dysregulation of Na⁺ homeostasis in cardiometabolic disease. Further research is needed to determine whether this association is causal and how it evolves in the transition to adulthood.

Sodium Intake and Hypertension

The close relationship between hypertension and dietary sodium intake is widely recognized and supported by several studies. A reduction in dietary sodium not only decreases the blood pressure and the incidence of hypertension, but is also associated with a reduction in morbidity and mortality from cardiovascular diseases. Prolonged modest reduction in salt intake induces a relevant fall in blood pressure in both hypertensive and normotensive individuals, irrespective of sex and ethnic group, with larger falls in systolic blood pressure for larger reductions in dietary salt. The high sodium intake and the increase in blood pressure levels are related to water retention, increase in systemic peripheral resistance, alterations in the endothelial function, changes in the structure and function of large elastic arteries, modification in sympathetic activity, and in the autonomic neuronal modulation of the cardiovascular system. In this review, we have focused on the effects of sodium intake on vascular hemodynamics and their implication in the pathogenesis of hypertension.

Testing Computer Models Predicting Human Responses to a High-Salt Diet

Recently, mathematical models of human integrative physiology, derived from Guyton's classic 1972 model of the circulation, have been used to investigate potential mechanistic abnormalities mediating salt sensitivity and salt-induced hypertension. We performed validation testing of 2 of the most evolved derivatives of Guyton's 1972 model, Quantitative Cardiovascular Physiology-2005 and HumMod-3.0.4, to determine whether the models accurately predict sodium balance and hemodynamic responses of normal subjects to increases in salt intake within the real-life range of salt intake in humans. Neither model, nor the 1972 Guyton model, accurately predicts the usual changes in sodium balance, cardiac output, and systemic vascular resistance that normally occur in response to clinically realistic increases in salt intake. Furthermore, although both contemporary models are extensions of the 1972 Guyton model, testing revealed major inconsistencies between model predictions with respect to sodium balance and hemodynamic responses of normal subjects to short-term and long-term salt loading. These results demonstrate significant limitations with the hypotheses inherent in the Guyton models regarding the usual regulation of sodium balance, cardiac output, and vascular resistance in response to increased salt intake in normal salt-resistant humans. Accurate understanding of the normal responses to salt loading is a prerequisite for accurately establishing abnormal responses to salt loading. Accordingly, the present results raise concerns about the interpretation of studies of salt sensitivity with the various Guyton models. These findings indicate a need for continuing development of alternative models that incorporate mechanistic concepts of blood pressure regulation fundamentally different from those in the 1972 Guyton model and its contemporary derivatives.

Small Amounts of Inorganic Nitrate or Beetroot Provide Substantial Protection From Salt-Induced Increases in Blood Pressure

To reduce the risk of salt-induced hypertension, medical authorities have emphasized dietary guidelines promoting high intakes of potassium and low intakes of salt that provide molar ratios of potassium to salt of ≥1:1. However, during the past several decades, relatively few people have changed their eating habits sufficiently to reach the recommended dietary goals for salt and potassium. Thus, new strategies that reduce the risk of salt-induced hypertension without requiring major changes in dietary habits would be of considerable medical interest. In the current studies in a widely used model of salt-induced hypertension, the Dahl salt-sensitive rat, we found that supplemental dietary sodium nitrate confers substantial protection from initiation of salt-induced hypertension when the molar ratio of added nitrate to added salt is only ≈1:170. Provision of a low molar ratio of added nitrate to added salt of ≈1:110 by supplementing the diet with beetroot also conferred substantial protection against salt-induced increases in blood pressure. The results suggest that on a molar basis and a weight basis, dietary nitrate may be ≈100× more potent than dietary potassium with respect to providing substantial resistance to the pressor effects of increased salt intake. Given that leafy green and root vegetables contain large amounts of inorganic nitrate, these findings raise the possibility that fortification of salty food products with small amounts of a nitrate-rich vegetable concentrate may provide a simple method for reducing risk for salt-induced hypertension.

6-Gingerol Normalizes the Expression of Biomarkers Related to Hypertension via PPARδ in HUVECs, HEK293, and Differentiated 3T3-L1 Cells

Hypertension is a disease with a high prevalence and high mortality rates worldwide. In addition, various factors, such as genetic predisposition, lifestyle factors, and the abnormality of organs related to blood pressure, are involved in the development of hypertension. However, at present, there are few available drugs for hypertension that do not induce side effects. Although the therapeutic effects of ginger on hypertension are well established, the precise mechanism has not been elucidated. Therefore, this study was designed to evaluate the antihypertensive mechanism of 6-gingerol, one of the main ingredients of ginger, and to assist in the development of new drugs for hypertension without side effects. The antihypertensive effects and mechanism of 6-gingerol were identified through reverse transcription polymerase chain reaction (RT-PCR), western blotting, and immunocytochemical staining for biomarkers involved in hypertension in human umbilical vein endothelial cells (HUVECs), human embryonal kidney cells (HEK293 cells), and mouse preadipocytes (3T3-L1 cells). The lipid accumulation in differentiated 3T3-L1 cells was evaluated by using Oil Red O staining. 6- Gingerol increased the level of phosphorylated endothelial nitric oxide synthase (eNOS) protein but decreased that of vascular cell adhesion protein 1 (VCAM1) and tumor necrosis factor alpha (TNFα) in HUVECs. In HEK293 cells, the expression of the epithelial sodium channel (ENaC) protein was reduced by 6-gingerol. Lipid accumulation was attenuated by 6-gingerol treatment in differentiated 3T3-L1 cells. These effects were regulated via peroxisome proliferator-activated receptor delta (PPARδ). 6-Gingerol ameliorated the expression of biomarkers involved in the development of hypertension through PPARδ in HUVECs, HEK293, and differentiated 3T3-L1 cells.

Reduced Renal Mass, Salt-Sensitive Hypertension Is Resistant to Renal Denervation

Aim: Activation of the sympathetic nervous system is common in resistant hypertension (RHT) and also in chronic kidney disease (CKD), a prevalent condition among resistant hypertensives. However, renal nerve ablation lowers blood pressure (BP) only in some patients with RHT. The influence of loss of nephrons per se on the antihypertensive response to renal denervation (RDNx) is unclear and was the focus of this study.

Methods: Systemic hemodynamics and sympathetically mediated low frequency oscillations of systolic BP were determined continuously from telemetrically acquired BP recordings in rats before and after surgical excision of ∼80% of renal mass and subsequent RDNx.

Results: After reduction of renal mass, rats fed a high salt (HS) diet showed sustained increases in mean arterial pressure (108 ± 3 mmHg to 128 ± 2 mmHg) and suppression of estimated sympathetic activity (∼15%), responses that did not occur with HS before renal ablation. After denervation of the remnant kidney, arterial pressure fell (to 104 ± 4 mmHg), estimated sympathetic activity and heart rate (HR) increased concomitantly, but these changes gradually returned to pre-denervation levels over 2 weeks of follow up. Subsequently, sympathoinhibition with clonidine did not alter arterial pressure while significantly suppressing estimated sympathetic activity and HR.

Conclusion: These results indicate that RDNx does not chronically lower arterial pressure in this model of salt-sensitive hypertension associated with substantial nephron loss, but without ischemia and increased sympathetic activity, thus providing further insight into conditions likely to impact the antihypertensive response to renal-specific sympathoinhibition in subjects with CKD.

The American Heart Association Scientific Statement on salt sensitivity of blood pressure: Prompting consideration of alternative conceptual frameworks for the pathogenesis of salt sensitivity?

: Recently, the American Heart Association (AHA) published a scientific statement on salt sensitivity of blood pressure which emphasized a decades old conceptual framework for the pathogenesis of this common disorder. Here we examine the extent to which the conceptual framework for salt sensitivity emphasized in the AHA Statement accommodates contemporary findings and views of the broader scientific community on the pathogenesis of salt sensitivity. In addition, we highlight alternative conceptual frameworks and important contemporary theories of salt sensitivity that are little discussed in the AHA Statement. We suggest that greater consideration of conceptual frameworks and theories for salt sensitivity beyond those emphasized in the AHA Statement may help to advance understanding of the pathogenesis of salt-induced increases in blood pressure and, in consequence, may lead to improved approaches to preventing and treating this common disorder.

Speculations on salt and the genesis of arterial hypertension

Blood pressure salt sensitivity and salt resistance are mechanistically imperfectly explained. A prescient systems medicine approach by Guyton and colleagues-more than 50 years ago-suggested how salt intake might influence blood pressure. They proposed that a high-salt diet engenders sodium accumulation, volume expansion, cardiac output adjustments, and then autoregulation for flow maintenance. The autoregulation in all vascular beds increases systemic vascular resistance, causing the kidneys to excrete more salt and water, thus reducing systems to normal and minimizing any changes in blood pressure. This schema, which is remarkably all encompassing, included all regulatory mechanisms Guyton could identify at the time. Guyton introduced the idea that the kidney is central, particularly concerning the regulation of renal pressure natriuresis. Numerous criticisms have been subsequently raised, particularly recently. Kurtz and colleagues argue that the ability of individuals to respond with an appropriate vasodilatory response to increased salt intake is pivotal. Data exist to address that issue. Salt-resistant hypertensive models provide additional information. We identified a mendelian form of hypertension not related to sodium reabsorption in the distal nephron. The hypertension develops because of increased systemic vascular resistance. In addition, we rediscovered a third salt-storage glycose-aminoglycan-related compartment, largely in the skin. This compartment operates independently of renal function, and when perturbed, is associated with salt sensitivity. More recently, we found novel molecular mechanisms demonstrating how large salt quantities are excreted by the kidneys with minimal water losses. We introduce novel interpretations as to how the kidneys excrete salt when the intake is high. The findings could have relevance as to how blood pressure may be regulated at varying salt intakes. Our purposes are to provide the readership with a banquet of thoughts to digest, to pursue Guyton's ideas, and to adjust them accordingly.

Functional foods for augmenting nitric oxide activity and reducing the risk for salt-induced hypertension and cardiovascular disease in Japan

High salt intake is one of the major dietary determinants of hypertension and cardiovascular disease in Japan and throughout the world. Although dietary salt restriction may be of clinical benefit in salt-sensitive individuals, many individuals may not wish, or be able to, reduce their intake of salt. Thus, identification of functional foods that can help protect against mechanistic abnormalities mediating salt-induced hypertension is an issue of considerable medical and scientific interest. According to the "vasodysfunction" theory of salt-induced hypertension, the hemodynamic abnormality initiating salt-induced increases in blood pressure usually involves subnormal vasodilation and abnormally increased vascular resistance in response to increased salt intake. Because disturbances in nitric oxide activity can contribute to subnormal vasodilator responses to increased salt intake that often mediate blood pressure salt sensitivity, increased intake of functional foods that support nitric oxide activity may help to reduce the risk for salt-induced hypertension. Mounting evidence indicates that increased consumption of traditional Japanese vegetables and other vegetables with high nitrate content such as table beets and kale can promote the formation of nitric oxide through an endothelial independent pathway that involves reduction of dietary nitrate to nitrite and nitric oxide. In addition, recent studies in animal models have demonstrated that modest increases in nitrate intake can protect against the initiation of salt-induced hypertension. These observations are: (1) consistent with the view that increased intake of many traditional Japanese vegetables and other nitrate rich vegetables, and of functional foods derived from such vegetables, may help maintain healthy blood pressure despite a high salt diet; (2) support government recommendations to increase vegetable intake in the Japanese population.

Understanding the Two Faces of Low-Salt Intake

Fierce debate has developed whether low-sodium intake, like high-sodium intake, could be associated with adverse outcome. The debate originates in earlier epidemiological studies associating high-sodium intake with high blood pressure and more recent studies demonstrating a higher cardiovascular event rate with both low- and high-sodium intake. This brings into question whether we entirely understand the consequences of high- and (very) low-sodium intake for the systemic hemodynamics, the kidney function, the vascular wall, the immune system, and the brain. Evolutionarily, sodium retention mechanisms in the context of low dietary sodium provided a survival advantage and are highly conserved, exemplified by the renin-angiotensin system. What is the potential for this sodium-retaining mechanism to cause harm? In this paper, we will consider current views on how a sodium load is handled, visiting aspects including the effect of sodium on the vessel wall, the sympathetic nervous system, the brain renin-angiotensin system, the skin as "third compartment" coupling to vascular endothelial growth factor C, and the kidneys. From these perspectives, several mechanisms can be envisioned whereby a low-sodium diet could potentially cause harm, including the renin-angiotensin system and the sympathetic nervous system. Altogether, the uncertainties preclude a unifying model or practical clinical guidance regarding the effects of a low-sodium diet for an individual. There is a very strong need for fundamental and translational studies to enhance the understanding of the potential adverse consequences of low-salt intake as an initial step to facilitate better clinical guidance.

[How does salt intake influence blood pressure? Associated aetiopathogenic mechanisms]

Abundant evidence from epidemiological and experimental studies has established a link between salt and blood pressure. However, there is heterogeneity in the blood pressure responses of humans to changes in sodium intake. Those individuals in whom a severe, abrupt change in salt intake causes the least change in arterial pressure and are termed salt-resistant, whereas in those in whom this leads to large changes in blood pressure, are called salt sensitive. Classically, Guyton's theory of the pressure-natriuresis phenomenon has been accepted to explain the pressor effect of salt, as well as the fundamental role played by the different protein sodium transporters of the renal tubules. In recent years, new theories have emerged pointing to the possible role of the immune system and the existence of a third sodium store in the body as aetiopathogenic factors.

Eplerenone-Resistant Salt-Sensitive Hypertension in Nedd4-2 C2 KO Mice

The epithelial sodium channel (ENaC) plays critical roles in maintaining fluid and electrolyte homeostasis and is located in the aldosterone-sensitive distal nephron (ASDN). We previously found that Nedd4-2 C2 knockout (KO) mice showed salt-sensitive hypertension with paradoxically enhanced ENaC gene expression in ASDN under high oral salt intake. Eplerenone (EPL), a selective aldosterone blocker, is a promising therapeutic option for resistant or/and salt-sensitive hypertension. We examined the effect of EPL on Nedd4-2 C2 KO mice with respect to blood pressure, metabolic parameters, and molecular level changes in ASDN under high oral salt intake. We found that EPL failed to reduce blood pressure in KO mice with high oral salt intake and upregulated ENaC expression in ASDN. Thus, salt-sensitive hypertension in Nedd4-2 C2 KO was EPL-resistant. Gene expression analyses of laser-captured specimens in ASDN suggested the presence of non-aldosterone-dependent activation of ENaC transcription in ASDN of Nedd4-2 C2 KO mice, which was abolished by amiloride treatment. Our results from Nedd4-2 C2 KO mice suggest that enhanced ENaC gene expression is critically involved in salt-sensitive hypertension under certain conditions of specific enzyme isoforms for their ubiquitination.

Involvement of ENaC in the development of salt-sensitive hypertension

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