Pathogenesis of salt sensitivity of blood pressure

Altered Neuronal Discharge in the Organum Vasculosum of the Lamina Terminalis Contributes to Dahl Salt-Sensitive Hypertension

BACKGROUND

Salt-sensitive hypertension in humans and experimental models is associated with higher plasma and cerebrospinal fluid sodium chloride (NaCl) concentrations. Changes in extracellular NaCl concentrations are sensed by specialized neurons in the organum vasculosum of the lamina terminalis (OVLT). Stimulation of OVLT neurons increases sympathetic nerve activity (SNA) and arterial blood pressure (ABP), whereas chronic activation produces hypertension. Therefore, the present study tested whether OVLT neuronal activity was elevated and contributed to SNA and ABP in salt-sensitive hypertension.

METHODS

Male Dahl salt-sensitive (Dahl S) and Dahl salt-resistant (Dahl R) rats were fed 0.1% or 4.0% NaCl diets for 3 to 4 weeks and used for single-unit recordings of OVLT neurons or simultaneous recording of multiple sympathetic nerves during pharmacological inhibition of the OVLT.

RESULTS

Plasma and cerebrospinal fluid Na+ and Cl- concentrations were higher in Dahl S rats fed 4% versus 0.1% or Dahl R rats fed either diet. In vivo single-unit recordings revealed a significantly higher discharge of NaCl-responsive OVLT neurons in Dahl S rats fed 4% versus 0.1% or Dahl R rats. Interestingly, intracarotid infusion of hypertonic NaCl evoked greater increases in OVLT neuronal discharge of Dahl S versus Dahl R rats regardless of NaCl diet. The activity of non-NaCl-responsive OVLT neurons was not different across strain or diets. Finally, inhibition of OVLT neurons by local injection of the gamma-aminobutyric acid agonist muscimol produced a greater decrease in renal SNA, splanchnic SNA, and ABP of Dahl S rats fed 4% versus 0.1% or Dahl R rats.

CONCLUSIONS

A high salt diet activates NaCl-responsive OVLT neurons to increase SNA and ABP in salt-sensitive hypertension.

Salt-dependent hypertension and inflammation: targeting the gut-brain axis and the immune system with Brazilian green propolis

Systemic arterial hypertension (SAH) is a major health problem around the world and its development has been associated with exceeding salt consumption by the modern society. The mechanisms by which salt consumption increase blood pressure (BP) involve several homeostatic systems but many details have not yet been fully elucidated. Evidences accumulated over the last 60 decades raised the involvement of the immune system in the hypertension development and opened a range of possibilities for new therapeutic targets. Green propolis is a promising natural product with potent anti-inflammatory properties acting on specific targets, most of them participating in the gut-brain axis of the sodium-dependent hypertension. New anti-hypertensive products reinforce the therapeutic arsenal improving the corollary of choices, especially in those cases where patients are resistant or refractory to conventional therapy. This review sought to bring the newest advances in the field articulating evidences that show a cross-talking between inflammation and the central mechanisms involved with the sodium-dependent hypertension as well as the stablished actions of green propolis and some of its biologically active compounds on the immune cells and cytokines that would be involved with its anti-hypertensive properties.

Excessive dietary salt promotes aortic stiffness in murine renovascular hypertension

Renovascular hypertension is characterized by activation of the renin-angiotensin-aldosterone system, blunted natriuretic responses, and elevated sympathetic nerve activity. Excess dietary salt intake exaggerates arterial blood pressure (ABP) in multiple models of experimental hypertension. The present study tested whether a high-salt diet exaggerated ABP and vascular dysfunction in a 2-kidney, 1-clip (2K1C) murine model. Male C57BL/6J mice (8-12 wk) were randomly assigned, and fed a 0.1% or 4.0% NaCl diet, and instrumented with telemetry units to measure ABP. Then, the 2K1C model was produced by placing a cuff around the right renal artery. Systolic, diastolic, and mean ABP were significantly higher in mice fed 4.0% vs. 0.1% NaCl at 1 wk but not after 3 wk. Interestingly, 2K1C hypertension progressively increased arterial pulse pressure in both groups; however, the magnitude was significantly greater in mice fed 4.0% vs. 0.1% NaCl at 3 wk. Moreover, pulse wave velocity was significantly greater in 2K1C mice fed 4.0% vs. 0.1% NaCl diet or sham-operated mice fed either diet. Histological assessment of aortas indicated no structural differences among groups. Finally, endothelium-dependent vasodilation was significantly and selectively attenuated in the aorta but not mesenteric arteries of 2K1C mice fed 4.0% NaCl vs. 0.1% NaCl or sham-operated control mice. The findings suggest that dietary salt loading transiently exaggerates 2K1C renovascular hypertension but promotes chronic aortic stiffness and selective aortic vascular dysfunction.NEW & NOTEWORTHY High dietary salt exaggerates hypertension in multiple experimental models. Here we demonstrate that a high-salt diet produces a greater increase in arterial blood pressure at 1 wk after induction of 2-kidney, 1-clip (2K1C) hypertension but not at 3 wk. Interestingly, 2K1C mice fed a high-salt diet displayed an exaggerated pulse pressure, elevated pulse wave velocity, and reduced endothelium-dependent vasodilation of the aorta but not mesenteric arteries. These findings suggest that dietary salt may interact with underlying cardiovascular disease to promote selective vascular dysfunction and aortic stiffness.

The Effects of Salt and Glucose Intake on Angiotensin II and Aldosterone in Obese and Nonobese Patients with Essential Hypertension

Background

The exact mechanisms for the development of essential hypertension are not known. Activation of the renin-angiotensin-aldosterone system (RAAS) in adipose tissue may represent an important link between obesity and hypertension. This study investigates the effects of oral intake of glucose with and without NaCl on angiotensin II (AngII) and aldosterone in obese and nonobese patients with essential hypertension.

Methods

Twenty newly diagnosed untreated essential hypertensive patients and 15 normotensive control subjects matched for age, gender, and BMI were studied. Participants fasted overnight (8–10 hrs), and then each subject took 75 gm glucose alone and with 3 gm NaCl, each dissolved in 250 ml. Subjects were monitored for 2 hours. Half hourly BP, plasma glucose (PG), serum Na⁺, K⁺, insulin, AngII, and aldosterone were measured. Subjects were classified into obese (BMI >30 Kg/m²) (11 patients and 8 control) and nonobese (BMI <30 Kg/m²) (9 patients and 7 control).

Results

After intake of glucose with NaCl serum, AngII was significantly higher in obese hypertensive patients compared with nonobese patients (P = 0.016). Intake of glucose with NaCl resulted in a significantly higher serum Na in obese hypertensive patients compared with nonobese patients Na (P = 0.009). Serum aldosterone was significantly higher in obese patients (P = 0.03, after glucose; P = 0.003, after glucose with NaCl) and in nonobese patients (P = 0.000 and P = 0.000, respectively) compared with their respective normotensive control subjects. In obese and nonobese patients, intake of glucose and glucose with NaCl showed no significant change in the levels of serum AngII and aldosterone which was associated a significant increase in serum Na in obese patients (P = 0.03) and a highly significant reduction in serum K in nonobese patients (P = 0.001).

Conclusion

Failure of suppression or inappropriate maintenance of secretion of AngII and aldosterone in both hypertensive groups by intake of glucose with NaCl may indicate a possible mechanism of essential hypertension.

Maintenance of long-term blood pressure control and vascular health by low-dose amiloride-based therapy in hyperaldosteronism

Whether aldosterone itself contributes directly to macro- or microcirculatory disease in man or to adverse cardiovascular outcomes is not fully known. We report our long-term single-practice experience in an unusual group of five patients with chronic hyperaldosteronism (HA, including three with glucocorticoid-remediable aldosteronism, GRA) treated with low-dose amiloride (a specific epithelial sodium channel [ENaC] blocker) 5-10 (mean 7) mg daily for 14-28 (mean 20) years. Except for one GRA diagnosed in infancy, all had severe resistant hypertension. In each case, BP was normalized within 1-4 weeks after starting amiloride and office BP's remained well controlled throughout the next two decades. 24-hour ambulatory BP monitoring with pulse wave analysis (cardiac output, vascular resistance, augmentation index, reflection magnitude), regional pulse wave velocities, pulse stiffening ratio, ankle-brachial index, serum creatinine, estimated glomerular filtration rate, and spot urinary albumin:creatinine ratio were measured after a mean of 18 years; all of these indicators were essentially normal. Over two additional years of observation (100 patient-years total), no cardiovascular or renal event occurred. We conclude that long-term ENaC blockade with amiloride can normalize BP and protect macro- and microvascular function in patients with HA. This suggests that either (a) putative vasculopathic effects of aldosterone are mediated via ENaC or (b) aldosterone may not play a direct role in age-dependent vasculopathic changes in humans independent of blood pressure. These findings, coupled with our literature review in both animal and human results, underscore the need for additional studies.

Long-term BP control and vascular health in patients with hyperaldosteronism treated with low-dose, amiloride-based therapy

Whether aldosterone itself contributes directly to macro- or microcirculatory disease in man or to adverse cardiovascular outcomes is not fully known. We report our long-term single-practice experience in 5 patients with chronic hyperaldosteronism (HA, including 3 with glucocorticoid remediable aldosteronism, GRA) treated with low-dose amiloride (a specific epithelial sodium channel [ENaC] blocker) 5-10 (mean 7) mg daily for 14-28 (mean 20) years. Except for 1 GRA diagnosed in infancy, all had severe resistant hypertension. In each case, BP was normal or near-normal within 1-4 weeks after starting amiloride and office BP's were well controlled for 20 years thereafter. Vascular studies and 24-hour ambulatory BP monitoring with pulse wave analysis (cardiac output, vascular resistance, augmentation index, and reflection magnitude) were assessed after a mean of 18 years as were regional pulse wave velocities, pulse stiffening ratio, ankle-brachial index, serum creatinine, estimated glomerular filtration rate, and spot urinary albumin:creatinine ratio. All indicators were completely normal in all patients after 18 years of amiloride, and none had a cardiovascular event during the 20-year mean follow-up. We conclude that long-term ENaC blockade can normalize BP and protect macro- and microvascular function in patients with HA. This suggests that (a) any vasculopathic effects of aldosterone are mediated via ENaC, not MR activation itself, and are fully preventable or reversible with ENaC blockade or (b) aldosterone may not play a major BP-independent role in human macro- and microcirculatory diseases. These and other widely divergent results in the literature underscore the need for additional studies regarding aldosterone, ENaC, and vascular disease.

Effect of Salt Intake on Serum Glucagon-Like Peptide-1 Levels in Normotensive Salt-Sensitive Subjects

BACKGROUND/AIMS

Excess dietary salt is a critical risk factor of salt-sensitive hypertension. Glucagon-like peptide-1 (GLP-1) , a gut incretin hormone, conferring benefits for blood pressure by natriuresis and diuresis. We implemented a randomized trial to verify the effect of altered salt intake on serum GLP-1 level in human beings.

METHODS

The 38 subjects were recruited from a rural community of Northern China. All subjects were sequentially maintained a baseline diet period for 3 days, a low-salt diet period for 7 days (3.0g/day of NaCl) , and a high-salt diet period for additional 7 days (18.0g/day of NaCl).

RESULTS

Serum GLP-1 level increased significantly with the change from the baseline period to the low-salt diet period and decreased with the change from the low-salt to high-salt diet in normotensive salt-sensitive (SS) but not salt-resistant (SR) individuals. There was a significant inverse correlation between the serum GLP-1 level and the MAP in SS subjects. Inverse correlation between the serum GLP-1 level and 24-h urinary sodium excretion was also found among different dietary interventions in SS subjects.

CONCLUSIONS

Our study indicates that variations in dietary salt intake affect the serum GLP-1 level in normotensive salt-sensitive Chinese adults.

Salt sensitivity and its implication in clinical practice

Hypertension (HTN) is a complex multi-factorial disease and is considered one of the foremost modifiable risk factors for stroke, heart failure, ischemic heart disease and renal dysfunction. Over the past century, salt and its linkage to HTN and cardiovascular (CV) mortality has been the subject of intense scientific scrutiny. There is now consensus that different individuals have different susceptibilities to blood pressure (BP)-raising effects of salt and this susceptiveness is called as salt sensitivity. Several renal and extra-renal mechanisms are believed to play a role. Blunted activity of the renin–angiotensin–aldosterone system (RAAS), adrenal Rac1-MR-Sgk1-NCC/ENaC pathway, renal SNS-GR-WNK4-NCC pathway, defect of membrane ion transportation, inflammation and abnormalities of Na⁺/Ca²⁺ exchange have all been implicated as pathophysiological basis for salt sensitive HTN. While salt restriction is definitely beneficial recent observation suggests that treatment with Azilsartan may improve salt sensitivity by selectively reducing renal proximal tubule Na⁺/H⁺ exchange. This encourages the future potential benefits of recognizing and therapeutically addressing the salt sensitive phenotype in humans.

Human risk of diseases associated with red meat intake: Analysis of current theories and proposed role for metabolic incorporation of a non-human sialic acid

One of the most consistent epidemiological associations between diet and human disease risk is the impact of red meat consumption (beef, pork, and lamb, particularly in processed forms). While risk estimates vary, associations are reported with all-cause mortality, colorectal and other carcinomas, atherosclerotic cardiovascular disease, type II diabetes, and possibly other inflammatory processes. There are many proposed explanations for these associations, some long discussed in the literature. Attempts to explain the effects of red meat consumption have invoked various red meat-associated agents, including saturated fat, high salt intake, Trimethylamine-N-oxide (TMAO) generation by microbiota, and environmental pollutants contaminating red meat, none of which are specific for red meat. Even the frequently mentioned polycyclic aromatic carcinogens arising from high temperature cooking methods are not red meat specific, as these are also generated by grilling poultry or fish, as well as by other forms of cooking. The traditional explanations that appear to be more red meat specific invoke the impact of N-nitroso compounds, heme iron, and the potential of heme to catalyze endogenous nitrosation. However, heme can be denatured by cooking, high levels of plasma hemopexin will block its tissue delivery, and much higher amounts of heme likely originate from red blood cell breakdown in vivo. Therefore, red meat-derived heme could only contribute to colorectal carcinoma risk, via direct local effects. Also, none of these mechanisms explain the apparent human propensity i.e., other carnivores have not been reported at high risk for all these diseases. A more recently proposed hypothesis involves infectious agents in beef from specific dairy cattle as agents of colorectal cancer. We have also described another mechanistic explanation for the human propensity for risk of red-meat associated diseases that is consistent with most observations: metabolic incorporation of a non-human sialic acid N-glycolylneuraminic acid (Neu5Gc) into the tissues of red meat consumers and the subsequent interaction with inflammation-provoking antibodies against this "xenoautoantigen". Overall, we conclude that while multiple mechanisms are likely operative, many proposed theories to date are not specific for red meat, and that the viral and xenoautoantigen theories deserve further consideration. Importantly, there are potential non-toxic dietary antidotes, if the xenoautoantigen theory is indeed correct.

Hydrogen Sulfide Inhibits High-Salt Diet-Induced Renal Oxidative Stress and Kidney Injury in Dahl Rats

Background. The study was designed to investigate if H₂S could inhibit high-salt diet-induced renal excessive oxidative stress and kidney injury in Dahl rats. Methods. Male salt-sensitive Dahl and SD rats were used. Blood pressure (BP), serum creatinine, urea, creatinine clearance rate, and 24-hour urine protein were measured. Renal ultra- and microstructures were observed. Collagen-I and -III contents the oxidants and antioxidants levels in renal tissue were detected. Keap1/Nrf2 association and Keap1 s-sulfhydration were detected. Results. After 8 weeks of high-salt diet, BP was significantly increased, renal function and structure were impaired, and collagen deposition was abundant in renal tissues with increased renal MPO activity, H₂O₂, MDA, GSSG, and ^•OH contents, reduced renal T-AOC and GSH contents, CAT, GSH-PX and SOD activity, and SOD expressions in Dahl rats. Furthermore, endogenous H₂S in renal tissues was decreased in Dahl rats. H₂S donor, however, decreased BP, improved renal function and structure, and inhibited collagen excessive deposition in kidney, in association with increased antioxidative activity and reduced oxidative stress in renal tissues. H₂S activated Nrf2 by inducing Keap1 s-sulfhydration and subsequent Keap1/Nrf2 disassociation. Conclusions. H₂S protected against high-salt diet-induced renal injury associated with enhanced antioxidant capacity and inhibited renal oxidative stress.

Salt, blood pressure and cardiovascular risk: what is the most adequate preventive strategy? A Swiss perspective

Among the various strategies to reduce the incidence of non-communicable diseases reduction of sodium intake in the general population has been recognized as one of the most cost-effective means because of its potential impact on the development of hypertension and cardiovascular diseases. Yet, this strategic health recommendation of the WHO and many other international organizations is far from being universally accepted. Indeed, there are still several unresolved scientific and epidemiological questions that maintain an ongoing debate. Thus what is the adequate low level of sodium intake to recommend to the general population and whether national strategies should be oriented to the overall population or only to higher risk fractions of the population such as salt-sensitive patients are still discussed. In this paper, we shall review the recent results of the literature regarding salt, blood pressure and cardiovascular risk and we present the recommendations recently proposed by a group of experts of Switzerland. The propositions of the participating medical societies are to encourage national health authorities to continue their discussion with the food industry in order to reduce the sodium intake of food products with a target of mean salt intake of 5–6 grams per day in the population. Moreover, all initiatives to increase the information on the effect of salt on health and on the salt content of food are supported.

Vascular Type 1A Angiotensin II Receptors Control BP by Regulating Renal Blood Flow and Urinary Sodium Excretion

Inappropriate activation of the type 1A angiotensin (AT1A) receptor contributes to the pathogenesis of hypertension and its associated complications. To define the role for actions of vascular AT1A receptors in BP regulation and hypertension pathogenesis, we generated mice with cell-specific deletion of AT1A receptors in smooth muscle cells (SMKO mice) using Loxp technology and Cre transgenes with robust expression in both conductance and resistance arteries. We found that elimination of AT1A receptors from vascular smooth muscle cells (VSMCs) caused a modest (approximately 7 mmHg) yet significant reduction in baseline BP and exaggerated sodium sensitivity in mice. Additionally, the severity of angiotensin II (Ang II)-dependent hypertension was dramatically attenuated in SMKO mice, and this protection against hypertension was associated with enhanced urinary excretion of sodium. Despite the lower BP, acute vasoconstrictor responses to Ang II in the systemic vasculature were largely preserved (approximately 80% of control levels) in SMKO mice because of exaggerated activity of the sympathetic nervous system rather than residual actions of AT1B receptors. In contrast, Ang II-dependent responses in the renal circulation were almost completely eliminated in SMKO mice (approximately 5%-10% of control levels). These findings suggest that direct actions of AT1A receptors in VSMCs are essential for regulation of renal blood flow by Ang II and highlight the capacity of Ang II-dependent vascular responses in the kidney to effect natriuresis and BP control.

Neurogenic and sympathoexcitatory actions of NaCl in hypertension

Excess dietary salt intake is a major contributing factor to the pathogenesis of salt-sensitive hypertension. Strong evidence suggests that salt-sensitive hypertension is attributed to renal dysfunction, vascular abnormalities, and activation of the sympathetic nervous system. Indeed, sympathetic nerve transections or interruption of neurotransmission in various brain centers lowers arterial blood pressure (ABP) in many salt-sensitive models. The purpose of this article is to discuss recent evidence that supports a role of plasma or cerebrospinal fluid hypernatremia as a key mediator of sympathoexcitation and elevated ABP. Both experimental and clinical studies using time-controlled sampling have documented that a diet high in salt increases plasma and cerebrospinal fluid sodium concentration. To the extent it has been tested, acute and chronic elevations in sodium concentration activates the sympathetic nervous system in animals and humans. A further understanding of how the central nervous system detects changes in plasma or cerebrospinal fluid sodium concentration may lead to new therapeutic treatment strategies in salt-sensitive hypertension.

Effect of salt intake and potassium supplementation on serum renalase levels in Chinese adults: a randomized trial

Renalase, a recently discovered enzyme released by the kidneys, breaks down blood-borne catecholamines and may thus regulate blood pressure (BP). Animal studies have suggested that high levels of dietary salt might reduce blood and kidney renalase levels. We conducted a randomized trial to assess the effects of altered salt and potassium intake on serum renalase levels and the relationship between serum renalase levels and BP in humans.Forty-two subjects (28-65 years of age) were selected from a rural community of northern China. All subjects were sequentially maintained on a low-salt diet for 7 days (3.0 g/day of NaCl), a high-salt diet for additional 7 days (18.0 g/day of NaCl), and a high-salt diet with potassium supplementation for final 7 days (18.0 g/day of NaCl + 4.5 g/day of KCl).Serum renalase levels were significantly higher than baseline levels during the low-salt diet intervention period. Renalase levels decreased with the change from the low-salt to high-salt diet, whereas dietary potassium prevented the decrease in serum renalase induced by the high-salt diet. There was a significant inverse correlation between the serum renalase level and 24-h urinary sodium excretion. No significant correlation was found between the renalase level and BP among the different dietary interventions.The present study indicates that variations in dietary salt intake and potassium supplementation affect the serum renalase concentration in Chinese subjects.

Postmenopausal hypertension and sodium sensitivity

It has been well established that women generally have lower incidence rates of hypertension than men at similar ages and these differences may vary with age. It also has been observed in many studies that after menopause, blood pressure (BP) increases in women to levels even higher than in men. The lack of estrogens may not be suggested as the only component involved in the development of postmenopausal hypertension. Thus, in this mini-review, the possible mechanisms by which sex hormones may influence the BP are discussed. This review also examines the renal regulatory mechanisms for gender differences in BP and explores the effects of salt intake on BP (salt-sensitivity) in pre and post-menopausal women.

Estrogen has been shown to stimulate nitric oxide (NO) production, thus female sex hormones have a beneficial effect on BP control. Evidences that angiotensin type 2 receptor (AT₂R) is up-regulated by estrogen support the favorable effects on BPs in women than men. The kidney plays an integral role in the regulation of arterial pressure through the mechanism of pressure-natriuresis, which has been shown to be modulated by the RAS. The prevalence of salt-sensitivity increases with age and low-salt diets has shown to help reduce systolic BP (SBP) and diastolic BP. While oral hormone replacement therapy has yielded only a neutral or minimal effect on the elevation of SBP, both the transdermal route replacement and a novel progestin with anti-aldosterone activity (drospirenone) has also shown to reduce SBP.

Orally active epoxyeicosatrienoic acid analog attenuates kidney injury in hypertensive Dahl salt-sensitive rat

Salt-sensitive hypertension leads to kidney injury. The Dahl salt-sensitive hypertensive rat (Dahl SS) is a model of salt-sensitive hypertension and progressive kidney injury. The current set of experimental studies evaluated the kidney protective potential of a novel epoxyeicosatrienoic acid analog (EET-B) in Dahl SS hypertension. Dahl SS rats receiving high-salt diet were treated with EET-B (10 mg/kg per day) or vehicle in drinking water for 14 days. Urine, plasma, and tissue samples were collected at the end of the treatment protocol to assess kidney injury, oxidative stress, inflammation, and endoplasmic reticulum stress. EET-B treatment in Dahl SS rats markedly reduced urinary albumin and nephrin excretion by 60% to 75% along with 30% to 60% reductions in glomerular injury, intratubular cast formation, and kidney fibrosis without affecting blood pressure. In Dahl SS rats, EET-B treatment further caused marked reduction in oxidative stress with 25% to 30% decrease in kidney malondialdehyde content along with 42% increase of nitrate/nitrite and a 40% reduction of 8-isoprostane. EET-B treatment reduced urinary monocyte chemoattractant protein-1 by 50% along with a 40% reduction in macrophage infiltration in the kidney. Treatment with EET-B markedly reduced renal endoplasmic reticulum stress in Dahl SS rats with reduction in the kidney mRNA expressions and immunoreactivity of glucose regulatory protein 78 and C/EBP homologous protein. In summary, these experimental findings reveal that EET-B provides kidney protection in Dahl SS rats by reducing oxidative stress, inflammation, and endoplasmic reticulum stress, and this protection was independent of reducing blood pressure.

Plasma Lp-PLA(2) mass and apoB-lipoproteins that carry Lp-PLA(2) decrease after sodium

BACKGROUND

Lipoprotein-associated phospholipase A(2) (Lp-PLA(2) ) is a novel cardiovascular risk marker, which is predominantly complexed to apolipoprotein (apo) B-containing lipoproteins in human plasma. As increasing dietary sodium intake may decrease plasma apoB-containing lipoproteins, we tested whether a sodium challenge lowers plasma Lp-PLA(2) mass, as well as the levels of apoB-containing lipoprotein particles carrying Lp-PLA(2) (apoB-Lp-PLA(2) ), employing a newly developed enzyme-linked immunosorbent assay.

MATERIALS AND METHODS

In 45 women and 31 men (mean age 44 ± 14 years), plasma Lp-PLA(2) mass (turbidimetric immunoassay), the level of apoB-Lp-PLA(2) , expressed in apoB concentration and lipoproteins were measured in response to a 3-day challenge with 9 g sodium chloride tablets daily.

RESULTS

Urinary sodium excretion increased from 165 ± 60 to 321 ± 70 mmol/24 h (P<0.001) after salt loading. Plasma Lp-PLA(2) mass decreased from 618 (493-719) to 588 (465-698) μg/L (P<0.001), and apoB-Lp-PLA(2) decreased from 0.276 (0.200-0.351) to 0.256 (0.189-0.328) g LDL protein/L (P=0.004) in response to the sodium challenge together with decreases in plasma total cholesterol, nonhigh-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, apolipoprotein B and the total cholesterol/HDL cholesterol ratio (P<0.01 for all). Changes in plasma Lp-PLA(2) mass were correlated positively with changes in total cholesterol, LDL cholesterol and non-HDL cholesterol (r=0.260-0.276, P<0.05 to P<0.02), whereas changes in apoB-Lp-PLA(2) were correlated positively with changes in non-HDL cholesterol and in the total cholesterol/HDL cholesterol ratio (r=0.232-0.385, P<0.05-0.01).

CONCLUSION

Both plasma Lp-PLA(2) mass levels and apoB-Lp-PLA(2) decrease in response to a short-term oral sodium challenge.

Salt sensitivity of blood pressure is associated with polymorphisms in the sodium-bicarbonate cotransporter

Previous studies have demonstrated that single nucleotide polymorphisms (SNPs) of the sodium-bicarbonate co-transporter gene (SLC4A5) are associated with hypertension. We tested the hypothesis that SNPs in SLC4A5 are associated with salt sensitivity of blood pressure in 185 whites consuming an isocaloric constant diet with a randomized order of 7 days of low Na(+) (10 mmol/d) and 7 days of high Na(+) (300 mmol/d) intake. Salt sensitivity was defined as a ≥ 7-mm Hg increase in mean arterial pressure during a randomized transition between high and low Na(+) diet. A total of 35 polymorphisms in 17 candidate genes were assayed, 25 of which were tested for association. Association analyses with salt sensitivity revealed 3 variants that associated with salt sensitivity, 2 in SLC4A5 (P<0.001) and 1 in GRK4 (P=0.020). Of these, 2 SNPs in SLC4A5 (rs7571842 and rs10177833) demonstrated highly significant results and large effects sizes, using logistic regression. These 2 SNPs had P values of 1.0 × 10(-4) and 3.1 × 10(-4) with odds ratios of 0.221 and 0.221 in unadjusted regression models, respectively, with the G allele at both sites conferring protection. These SNPs remained significant after adjusting for body mass index and age (P=8.9 × 10(-5) and 2.6 × 10(-4) and odds ratios 0.210 and 0.286, respectively). Furthermore, the association of these SNPs with salt sensitivity was replicated in a second hypertensive population. Meta-analysis demonstrated significant associations of both SNPs with salt sensitivity (rs7571842 [P=1.2 × 10(-5)]; rs1017783 [P=1.1 × 10(-4)]). In conclusion, SLC4A5 variants are strongly associated with salt sensitivity of blood pressure in 2 separate white populations.

Endogenous ouabain and the renin-angiotensin-aldosterone system: distinct effects on Na handling and blood pressure in human hypertension

OBJECTIVE

To evaluate whether the renin-angiotensin-aldosterone system (RAAS) and endogenous ouabain system differently affect renal Na handling and blood pressure.

METHODS

Three hundred and one patients in whom we compared blood pressure, and renal Na tubular reabsorption in the basal condition and 2 h (T120) after saline infusion.

RESULTS

Following multivariate-adjusted linear and quartiles analysis, baseline mean blood pressure (MBP) was significantly higher (113.7 ± 1.33 mmHg) in the fourth versus the first endogenous ouabain quartile (103.8 ± 1.04 mmHg) and the trend across the quartiles was highly significant (β = 0.23, P = 3.53e-04). In contrast, an inverse relationship was present in the renin activity (PRA) quartiles with MBP highest in the first (112.5 ± 1.26) and lowest in the fourth PRA quartile (107.6 ± 1.48, P = 0.039). Following an acute saline load, changes in MBP and the slope of the pressure-natriuresis relationship were inversely related across the PRA quartiles. The fractional excretion of sodium (FENa) showed a negative linear trend going from the first to the third endogenous ouabain quartiles (2.35 ± 0.17 and 1.90 ± 0.14%, P = 0.05). Patients in the fourth endogenous ouabain quartile (>323 pmol/l) showed increased FENa T120 (2.78 ± 0.18%, P < 0.01) and increased Na tubular rejection fraction (P = 0.007) after Na load. After the saline load, there was a biphasic relationship between plasma endogenous ouabain and FENa favoring Na retention at low endogenous ouabain and Na excretion at high endogenous ouabain levels.

CONCLUSION

The RAAS and endogenous ouabain system are two independent and complementary systems having an inverse (RAAS) or a direct (endogenous ouabain system) relationship with hemodynamic parameters.

Vasopressin V2 receptors, ENaC, and sodium reabsorption: a risk factor for hypertension?

Excessive sodium reabsorption by the kidney has long been known to participate in the pathogenesis of some forms of hypertension. In the kidney, the final control of NaCl reabsorption takes place in the distal nephron through the amiloride-sensitive epithelial sodium channel (ENaC). Liddle's syndrome, an inherited form of hypertension due to gain-of-function mutations in the genes coding for ENaC subunits, has demonstrated the key role of this channel in the sodium balance. Although aldosterone is classically thought to be the main hormone regulating ENaC activity, several studies in animal models and in humans highlight the important effect of vasopressin on ENaC regulation and sodium transport. This review summarizes the effect of vasopressin V2 receptor stimulation on ENaC activity and sodium excretion in vivo. Moreover, we report the experimental and clinical data demonstrating the role of renal ENaC in water conservation at the expense of a reduced ability to excrete sodium. Acute administration of the selective V2 receptor agonist dDAVP not only increases urine osmolality and reduces urine flow rate but also reduces sodium excretion in rats and humans. Chronic V2 receptor stimulation increases blood pressure in rats, and a significant correlation was found between blood pressure and urine concentration in healthy humans. This led us to discuss how excessive vasopressin-dependent ENaC stimulation could be a risk factor for sodium retention and resulting increase in blood pressure.

Excess dietary salt intake alters the excitability of central sympathetic networks

The ingestion of excess dietary salt (defined as NaCl) is strongly correlated with cardiovascular disease, morbidity, mortality, and is regarded as a major contributing factor to the pathogenesis of hypertension. Although several mechanisms contribute to the adverse consequences of dietary salt intake, accumulating evidence suggests that dietary salt loading produces neurogenically-mediated increases in total peripheral resistance to raise arterial blood pressure (ABP). Evidence from clinical studies and experimental models clearly establishes a hypertensive effect of dietary salt loading in a subset of individuals who are deemed "salt-sensitive". However, we will discuss and present evidence to develop a novel hypothesis to suggest that while chronic increases in dietary salt intake do not elevate mean ABP in "non-salt-sensitive" animals, dietary salt intake does enhance several sympathetic reflexes thereby predisposing these animals and/or individuals to the development of salt-sensitive hypertension. Additional evidence raises an intriguing hypothesis that these enhanced sympathetic reflexes are largely attributed to the ability of excess dietary salt intake to selectively enhance the excitability of sympathetic-regulatory neurons in the rostral ventrolateral medulla. Insight into the cellular mechanisms by which dietary salt intake alters the responsiveness of RVLM circuits will likely provide a foundation for developing new therapeutic approaches to treat salt-sensitive hypertension. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Therapeutic potential of endothelin receptor antagonists for chronic proteinuric renal disease in humans

Diabetes and arterial hypertension continue to be the main causes of chronic renal failure in 2010, with a rising prevalence in part due to the worldwide obesity epidemic. Proteinuria is a main feature of chronic renal disease and mediated by defects in the glomerular filtration barrier and is as a good predictor of cardiovascular events. Indeed, chronic renal disease due to glomerulosclerosis is one of the important risk factors for the development of coronary artery disease and stroke. Glomerulosclerosis develops in response to inflammatory activation and increased growth factor production. Preclinical and first preliminary clinical studies provide strong evidence that endogenous endothelin-1 (ET-1), a 21-amino-acid peptide with strong growth-promoting and vasoconstricting properties, plays a central role in the pathogenesis of proteinuria and glomerulosclerosis via activation of its ET(A) subtype receptor involving podocyte injury. These studies have not only shown that endothelin participates in the disease processes of hypertension and glomerulosclerosis but also that features of chronic renal disease such as proteinuria and glomerulosclerosis are reversible processes. Remarkably, the protective effects of endothelin receptors antagonists (ERAs) are present even on top of concomitant treatments with inhibitors of the renin-angiotensin system. This review discusses current evidence for a role of endothelin for proteinuric renal disease and podocyte injury in diabetes and arterial hypertension and reviews the current status of endothelin receptor antagonists as a potential new treatment option in renal medicine.

Stress-induced sodium retention and hypertension: a review and hypothesis

Hypertension--an important health problem in industrialized nations--is particularly significant in blacks and obese individuals, in whom it is hypothesized to result from impaired renal sodium regulation. We reviewed studies that identified individuals with impaired sodium regulation by examining the natriuretic response to mental stress. A significant percentage of black and obese individuals retain or have a diminished natriuretic response to mental stress despite increased blood pressure (BP). This contributes a volume component to the normal resistance-mediated BP increase, and BP remains elevated after the stressor ceases until the volume expansion diminishes. The stress exposes these individuals to greater cardiovascular load. This response pattern has been linked to renin-angiotensin-aldosterone system activity, and is associated with premature target-organ damage. Assessing stress-induced sodium retention provides a method to identify patients with impaired sodium regulation without using a dietary protocol that poses adherence difficulties, or complicated laboratory assessments. Furthermore, research using this technique indicates the effectiveness of renin-angiotensin-aldosterone system blockers in correcting impaired sodium regulation and consequent hypertension in these individuals.

Endogenous cardiotonic steroids: physiology, pharmacology, and novel therapeutic targets

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na(+)/K(+)-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

Blood volume, blood pressure and total body sodium: internal signalling and output control

Total body sodium and arterial blood pressure (ABP) are mutually dependent variables regulated by complex control systems. This review addresses the role of ABP in the normal control of sodium excretion (NaEx), and the physiological control of renin secretion. NaEx is a pivotal determinant of ABP, and under experimental conditions, ABP is a powerful, independent controller of NaEx. Blood volume is a function of dietary salt intake; however, ABP is not, at least not in steady states. A transient increase in ABP after a step-up in sodium intake could provide a causal relationship between ABP and the regulation of NaEx via a hypothetical integrative control system. However, recent data show that subtle sodium loading (simulating salty meals) causes robust natriuresis without changes in ABP. Changes in ABP are not necessary for natriuresis. Normal sodium excretion is not regulated by pressure. Plasma renin is log-linearly related to salt intake, and normally, decreases in renin secretion are a precondition of natriuresis after increases in total body sodium. Renin secretion is controlled by renal ABP, renal nerve activity and the tubular chloride concentrations at the macula densa (MD). Renal nerve activity is related to blood volume, also at constant ABP, and elevates renin secretion by means of beta(1)-adrenoceptors. Recent results indicate that renal denervation reduces ABP and renin activity, and that sodium loading may decrease renin without changes in ABP, glomerular filtration rate or beta(1)-mediated nerve activity. The latter indicates an essential role of the MD mechanism and/or a fourth mediator of the physiological control of renin secretion.

Endogenous digitalis: pathophysiologic roles and therapeutic applications

Endogenous digitalis-like factors, also called cardiotonic steroids, have been thought for nearly half a century to have important roles in health and disease. The endogenous cardiotonic steroids ouabain and marinobufagenin have been identified in humans, and an effector mechanism has been delineated by which these hormones signal through the sodium/potassium-transporting ATPase. These findings have increased interest in this field substantially. Although cardiotonic steroids were first considered important in the regulation of renal sodium transport and arterial pressure, subsequent work has implicated these hormones in the control of cell growth, apoptosis and fibrosis, among other processes. This Review focuses on the role of endogenous cardiotonic steroids in the pathophysiology of essential hypertension, congestive heart failure, end-stage renal disease and pre-eclampsia. We also discuss potential therapeutic strategies that have emerged as a result of the increased understanding of the regulation and actions of cardiotonic steroids.

Salt-sensitive men show reduced heart rate variability, lower norepinephrine and enhanced cortisol during mental stress

Salt sensitivity (SS) represents a risk factor for essential hypertension, which has been related to enhanced cardiovascular stress reactivity possibly mediated by increased noradrenergic susceptibility. We investigated biophysiological responses to mental stress in salt-sensitive (ss) and salt-resistant (sr) subjects, hypothesizing lower heart rate variability (HRV) and higher cortisol in the ss. A total of 48 healthy normotensive Caucasian men (age 25.6+/-2.6, body mass index 22.9+/-2.3) were phenotyped for SS (defined as significant drop in mean arterial pressure>3 mm Hg under the low-salt diet) by a 2-week high- versus low-salt diet. Subjects underwent a standardized mental stress task with continuous cardiovascular monitoring before, during and after the test (Finapres; Ohmeda, Louisville, CO, USA). Blood samples were drawn to examine cortisol and catecholamines before, after and 20 min after stress. The task elicited significant increases of systolic blood pressure (SBP), diastolic BP (DBP) and heart rate (HR) and a significant decrease of HRV (all time effects P<0.0001). The ss subjects showed lower norepinephrine (NE) and higher cortisol, indicated by significant group effects (P=0.009 and 0.025, respectively). HR increased and HRV decreased more in the ss under the stress, shown by significant time by group interactions (P=0.045 and 0.003, respectively). The observation of a more pronounced HR rise coupled with a greater decrease of HRV in healthy ss men under the influence of brief mental stress confirms their enhanced physiological stress reactivity. The lower peripheral NE may represent an effort to compensate for increased noradrenergic receptor sensitivity. The enhanced cortisol levels are backed by recent genetic findings on HSD11B2 polymorphisms and may promote hypertension.

Rat chromosome 19 transfer from SHR ameliorates hypertension, salt-sensitivity, cardiovascular and renal organ damage in salt-sensitive Dahl rats

OBJECTIVES

Unlike Dahl salt-sensitive (SS) rats, some strains of spontaneously hypertensive (SHR) rats develop only minor organ damage even when exposed to high-salt diet. In previous linkage studies, we identified quantitative trait loci on rat chromosome 19 (RNO19) linked to the SHR allele suggesting a protective effect against salt-induced hypertensive organ damage in SS.

METHODS

To test the relevance of this finding, we generated and characterized a consomic strain SS-19SHR in which RNO19 from SHR was introgressed into the susceptible background of SS. We compared the effects of low-salt (0.2% NaCl) and high-salt (4% NaCl) diet exposure for 8 weeks on the development of hypertension and target organ damage in male consomic and SS animals (n=14-20, each).

RESULTS

Systolic blood pressure, relative left ventricular weight and urinary protein excretion were significantly lower in SS-19SHR compared to SS under both low-salt and high-salt diet (P < 0.05, respectively). Left ventricular atrial natriuretic peptide mRNA expression showed a more pronounced 4.5-fold increase in SS compared to SS-19 (two-fold) after high-salt (P < 0.05). In comparison to low diet, high-salt exposure induced a significant increase in vascular aortic hypertrophy index, left ventricular interstitial fibrosis (+210%) and perivascular fibrosis (+195%) in SS but not in consomic SS-19SHR (P < 0.05, respectively).

CONCLUSIONS

These results demonstrate a strong protective effect of RNO19 from SHR on the development of hypertension, salt-sensitivity, cardiovascular and renal organ damage in SS. In particular, we demonstrate a genetic effect protecting against the development of cardiac fibrosis in salt-sensitive hypertension.