Renal function and blood pressure response to dietary salt restriction in normotensive men

Renal sympathetic activity: A key modulator of pressure natriuresis in hypertension

Hypertension is a complex disorder ensuing necessarily from alterations in the pressure-natriuresis relationship, the main determinant of long-term control of blood pressure. This mechanism sets natriuresis to the level of blood pressure, so that increasing pressure translates into higher osmotically driven diuresis to reduce volemia and control blood pressure. External factors affecting the renal handling of sodium regulate the pressure-natriuresis relationship so that more or less natriuresis is attained for each level of blood pressure. Hypertension can thus only develop following primary alterations in the pressure to natriuresis balance, or by abnormal activity of the regulation network. On the other hand, increased sympathetic tone is a very frequent finding in most forms of hypertension, long regarded as a key element in the pathophysiological scenario. In this article, we critically analyze the interplay of the renal component of the sympathetic nervous system and the pressure-natriuresis mechanism in the development of hypertension. A special focus is placed on discussing recent findings supporting a role of baroreceptors as a component, along with the afference of reno-renal reflex, of the input to the nucleus tractus solitarius, the central structure governing the long-term regulation of renal sympathetic efferent tone.

Phyllanthus amarus attenuated derangement in renal-cardiac function, redox status, lipid profile and reduced TNF-α, interleukins-2, 6 and 8 in high salt diet fed rats

High salt diet (HSD) has been implicated in the etiopathogenesis of immune derangement, cardiovascular disorders and, metabolic syndromes. This study investigated the protective effect of ethanol extract of Phyllanthus amarus (EEPA) against high salt diet (HSD) induced biochemical and metabolic derangement in male Wistar rats. The rats were divided into 5 groups of 6 animals each as follows; control group fed with normal rat chow, negative control group, fed HSD only, animals on HSD treated orally with 75 mg/kg, 100 mg/kg, and, 150 mg/kg EEPA once daily. At the end of 8 weeks treatment, lipid profile (TG, TC, LDL, and VLDL), oxidative stress (catalase, reduced glutathione, and malondialdehyde), inflammatory (TNF-a, interleukins 2, 6, and 8), cardiac (lactate dehydrogenase, creatine kinase) and kidney function markers (urea, uric acid, creatinine) were assessed. Serum TG, TC, LDL, and VLDL content were significantly (p < 0.05) elevated in HSD-only fed rats, while HDL was significantly elevated in a concentration-dependent manner in EEPA treated animals. The extract produced a significant (p < 0.05) and dose-dependent increase in the antioxidant enzymes activities and a significant reduction in the malondialdehyde level. A significant (p < 0.05) dose-dependent reduction in serum TNF-alpha, IL-2, 6, and 8 of EEPA treated rats compared with HSD-fed rats was observed. More so, reduction in serum LDH, creatine kinase, creatinine, urea, and uric acid activity of extract-treated animals were noted. EEPA attenuated high salt diet-induced oxidative stress, inflammation, and dyslipidemia in rats.

The Role of Thyroid in Renovascular Function: Independent Association of Serum TSH With Renal Plasma Flow

CONTEXT

There are well-established interactions between the thyroid and the kidney. Thyroid hypofunction is associated with reduced renal plasma flow (RPF), and hypothyroidism is highly prevalent in chronic kidney disease; however, less is known about the thyroid-kidney axis in the euthyroid state.

OBJECTIVE

This work aimed to study the association of thyroid function with renovascular parameters in a well-phenotyped cohort of euthyroid normotensive and hypertensive individuals.

METHODS

This cross-sectional, multicenter study of the HyperPATH Consortium took place in 5 US and European academic institutions. A total of 789 individuals, aged 18 to 65 years, with serum thyrotropin (TSH) 0.4 to 5.5 mIU/L, participated; individuals with uncontrolled or secondary hypertension or on medication affecting the hypothalamus-pituitary-thyroid axis were excluded. Hemodynamic parameters including RPF, thyroid function testing, and the Thr92Ala deiodinase 2 (D2) polymorphism were assessed in the setting of a liberal and restricted salt diet. We searched for associations between thyroid function and renovascular parameters and accounted for confounding factors, such as older age, hypertension, and diabetes.

RESULTS

Serum TSH was inversely associated with RPF assessed in the setting both of liberal and restricted salt diets. This association remained significant and independent when accounting for confounding factors, whereas free thyroxine index (fTI) and the Thr92Ala polymorphism, associated with lower D2 catalytic activity and disrupted thyroid hormone tissue availability, were not independently associated with RPF. Serum TSH remained an independent predictor of RPF on a liberal salt diet when the analysis was restricted to healthy young individuals.

CONCLUSION

Serum TSH levels, but not fTI nor the Thr92Ala D2 polymorphism, were independently inversely associated with RPF in individuals of the HyperPATH Consortium. These findings suggest a direct interconnection between TSH and renovascular dynamics even with TSH within reference range, warranting further investigation.

Inappropriate activity of local renin-angiotensin-aldosterone system during high salt intake: impact on the cardio-renal axis

Although there is a general agreement on the recommendation for reduced salt intake as a public health issue, the mechanism by which high salt intake triggers pathological effects on the cardio-renal axis is not completely understood. Emerging evidence indicates that the renin-angiotensin-aldosterone system (RAAS) is the main target of high Na⁺ intake. An inappropriate activation of tissue RAAS may lead to hypertension and organ damage. We reviewed the impact of high salt intake on the RAAS on the cardio-renal axis highlighting the molecular pathways that leads to injury effects. We also provide an assessment of recent observational studies related to the consequences of non-osmotically active Na⁺ accumulation, breaking the paradigm that high salt intake necessarily increases plasma Na⁺ concentration promoting water retention

Simulating a virtual population's sensitivity to salt and uninephrectomy

Salt sensitivity, with or without concomitant hypertension, is associated with increased mortality. Reduced functional renal mass plays an important role in causing salt-sensitive hypertension for many individuals. Factors that are important in the condition of decreased renal mass and how they affect blood pressure (BP) or salt sensitivity are unclear. We used HumMod, an integrative mathematical model of human physiology, to create a heterogeneous population of 1000 virtual patients by randomly varying physiological parameters. We examined potential physiological mechanisms responsible for the change in BP in response to high-salt diet (8× change in salt intake for three weeks) with full kidney mass and again after the removal of one kidney in the same group of virtual patients. We used topological data analysis (TDA), a clustering algorithm tool, to analyse the large dataset and separate patient subpopulations. TDA distinguished five unique clusters of salt-sensitive individuals (more than 15 mmHg change in BP with increased salt). While these clusters had similar BP responses to salt, different collections of variables were responsible for their salt sensitivity, e.g. greater reductions in glomerular filtration rate (GFR) or impairments in the renin-angiotensin system. After simulating uninephrectomy in these virtual patients, the three most salt-sensitive clusters were associated with a blunted increase in renal blood flow (RBF) and higher increase in loop and distal sodium reabsorption when compared with the salt-resistant population. These data suggest that the suppression of sodium reabsorption and renin-angiotensin system is key for salt resistance, and RBF in addition to GFR may be an important factor when considering criteria for kidney donors. Here, we show that in our model of human physiology, different derangements result in the same phenotype. While these concepts are known in the experimental community, they were derived here by considering only the data obtained from our virtual experiments. These methodologies could potentially be used to discover patterns in patient sensitivity to dietary change or interventions and could be a revolutionary tool in personalizing medicine.

Sodium Homeostasis in Chronic Kidney Disease

The pathologic consequences of sodium retention in the CKD population can lead to hypertension, edema, and progressive disease. Sodium excess is responsible for increases in oxidative stress, which alters kidney vasculature. As progression of CKD occurs, hyperfiltration by remaining nephrons compensates for an overall decrease in the filtered load of sodium. In the later stages of CKD, compensatory mechanisms are overcome and volume overload ensues. Nephrotic syndrome as it relates to sodium handling involves a different pathophysiology despite a common phenotype. Extrarenal sodium buffering is also examined as it has significant implications in the setting of advanced CKD.

A Quantitative Systems Physiology Model of Renal Function and Blood Pressure Regulation: Application in Salt-Sensitive Hypertension

Salt‐sensitivity (SS) refers to changes in blood pressure in response to changes in sodium intake. SS individuals are at greater risk for developing kidney disease, and also respond differently to antihypertensive therapies compared to salt‐resistant (SR) individuals. In this study we used a systems pharmacology model of renal function (presented in a companion article) to evaluate the ability of proposed mechanisms to produce salt‐sensitivity. The model reproduced previously published data on renal functional changes in response to salt‐intake, and also predicted that glomerular pressure, a variable that is not easily evaluated clinically but is a key factor in renal injury, increases with salt intake in SS hypertension. We then used the model to generate mechanistic insight into the differential blood pressure and glomerular pressure responses to angiotensin converting enzyme (ACE) inhibitors, thiazide diuretics, and calcium channel blockers observed in SS and SR hypertension.

Na/K-ATPase Signaling and Salt Sensitivity: The Role of Oxidative Stress

Other than genetic regulation of salt sensitivity of blood pressure, many factors have been shown to regulate renal sodium handling which contributes to long-term blood pressure regulation and have been extensively reviewed. Here we present our progress on the Na/K-ATPase signaling mediated sodium reabsorption in renal proximal tubules, from cardiotonic steroids-mediated to reactive oxygen species (ROS)-mediated Na/K-ATPase signaling that contributes to experimental salt sensitivity.

Altered renal sodium handling and risk of incident hypertension: Results of the Olivetti Heart Study

Renal tubular sodium (Na) handling plays a key role in blood pressure (BP) regulation. Several cross-sectional studies reported a positive association between higher proximal tubule fractional reabsorption of Na and BP, but no prospective investigation has been reported of this possible association. Hence, the purpose of this study was to estimate the predictive role of renal Na handling on the risk of incident hypertension and the changes in BP occurring in the 8-year follow-up observation of a sample of initially normotensive men (The Olivetti Heart Study). The study included 294 untreated normotensive non-diabetic men with normal renal function examined twice (1994–95 and 2002–04). Renal tubular Na handling was estimated by exogenous lithium clearance. Fractional reabsorption of Na in proximal and distal tubules was calculated and included in the analysis. At baseline, there was no association between BP and either proximal or distal fractional reabsorption of Na. At the end of the 8-year follow-up, direct associations were observed between baseline proximal (but not distal) Na fractional reabsorption and the changes occurred in systolic and diastolic BP over time (+2.79 and +1.53 mmHg, respectively, per 1SD difference in proximal Na-FR; p<0.01). Also multivariable analysis showed a direct association between baseline proximal Na fractional reabsorption and risk of incident hypertension, independently of potential confounders (OR: 1.34, 95%CI:1.06–1.70). The results of this prospective investigation strongly suggest a causal relationship between an enhanced rate of Na reabsorption in the proximal tubule and the risk of incident hypertension in initially normotensive men.

Mechanisms of blood pressure salt sensitivity: new insights from mathematical modeling

Mathematical modeling is an important tool for understanding quantitative relationships among components of complex physiological systems and for testing competing hypotheses. We used HumMod, a large physiological model, to test hypotheses of blood pressure (BP) salt sensitivity. Systemic hemodynamics, renal, and neurohormonal responses to chronic changes in salt intake were examined during normal renal function, fixed low or high plasma angiotensin II (ANG II) levels, bilateral renal artery stenosis, increased renal sympathetic nerve activity (RSNA), and decreased nephron numbers. Simulations were run for 4 wk at salt intakes ranging from 30 to 1,000 mmol/day. Reducing functional kidney mass or fixing ANG II increased salt sensitivity. Salt sensitivity, associated with inability of ANG II to respond to changes in salt intake, occurred with smaller changes in renal blood flow but greater changes in glomerular filtration rate, renal sodium reabsorption, and total peripheral resistance (TPR). However, clamping TPR at normal or high levels had no major effect on salt sensitivity. There were no clear relationships between BP salt sensitivity and renal vascular resistance or extracellular fluid volume. Our robust mathematical model of cardiovascular, renal, endocrine, and sympathetic nervous system physiology supports the hypothesis that specific types of kidney dysfunction, associated with impaired regulation of ANG II or increased tubular sodium reabsorption, contribute to BP salt sensitivity. However, increased preglomerular resistance, increased RSNA, or inability to decrease TPR does not appear to influence salt sensitivity. This model provides a platform for testing competing concepts of long-term BP control during changes in salt intake.

Blood pressure, arterial stiffness and endogenous lithium clearance in relation to AGTR1 A1166C and AGTR2 G1675A gene polymorphisms

Introduction:

Although recently a matter of epidemiologic controversy, sodium overload and its interaction with genetic factors predispose to hypertension and related target organ complications.

Methods:

In 131 (66 male) treated hypertensives, we measured peripheral and central arterial pressures and pulse wave augmentation indexes (AIx_P, AIx_C1, AIx_C2), pulse wave velocity (PWV), daily urinary sodium excretion and did genetic studies of AGTR1 A1166C and AGTR2 G1675A polymorphisms. Proximal (FE_Li) and distal (FDR_Na) sodium reabsorption measurements were performed using endogenous lithium clearance.

Results:

In men, we found interaction between FDR_Na and AGTR2 G1675A polymorphism with respect to AIx_C1 (p_INT=0.01), AIx_C2 (p_INT=0.05) and AIx_P (p_INT=0.006). Arterial stiffness increased with higher sodium reabsorption in the distal tubule, in the presence of AGTR2 G allele with the opposite tendency in A allele carriers. In the subgroup with FDR_Na below median, as compared to those with FDR_Na above median, the AIx_C1 (139.6±3.8 vs 159.1±5.7%; p=0.009), AIx_C2 (26.3±1.8 vs 33.3±1.7%; p=0.016) and AIx_P (83.4±2.5 vs 96.5±2.6%; p<0.0001) were lower, in the G allele carrying men and GG homozygous women.

Conclusions:

The relation between sodium reabsorption in the distal tubule and the development of arterial stiffness depends on the AGTR2 G1675A polymorphism in blood pressure independent fashion.

Quantitative proteomics reveals novel therapeutic and diagnostic markers in hypertension

Hypertension is a prevalent disorder in the world representing one of the major risk factors for heart attack and stroke. These risks are increased in salt sensitive individuals. Hypertension and salt sensitivity are complex phenotypes whose pathophysiology remains poorly understood and, remarkably, salt sensitivity is still laborious to diagnose.

Here we present a urinary proteomic study specifically designed to identify urinary proteins relevant for the pathogenesis of hypertension and salt sensitivity. Despite previous studies that underlined the association of UMOD gene variants with hypertension, this work provides novel evidence showing different uromodulin protein level in the urine of hypertensive patients compared to healthy individuals. Notably, we also show that patients with higher level of uromodulin are homozygous for UMOD risk variant and display a decreased level of salt excretion, highlighting the essential role of UMOD in the regulation of salt reabsorption in hypertension. Additionally, we found that urinary nephrin 1, a marker of glomerular slit diaphragm, may predict a salt sensitive phenotype and positively correlate with increased albuminuria associated with this type of hypertension.

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We identified urinary proteins differently excreted in hypertensive patients.

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Nephrin 1 might predict salt sensitive phenotype and glomerular complications.

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Uromodulin impacts salt homeostasis in hypertension.

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We provide new insights into the pathogenesis of hypertension and salt sensitivity.

Cardiovascular and other effects of salt consumption

Salt is one of the most important determinants of high blood pressure and increased cardiovascular risk worldwide. However, a high salt intake has other adverse effects beyond those involving the cardiovascular system, so that there is renewed interest in the relationships between high salt intake and other diseases.

HSD11B2 CA-repeat and sodium balance

Type 2 11β-hydroxysteroid dehydrogenase encoded by the HSD11B2 gene converts cortisol to inactive cortisone and thus protects the mineralocorticoid receptor from cortisol exposure. Impaired activity of this enzyme leads to mineralocorticoid excess, suggesting HSD11B2 as a candidate locus for patients at risk of developing low renin or salt-sensitive essential hypertension. In the present study, we searched for frequent polymorphisms in 155 Japanese subjects but detected none in the proximal promoter or coding regions of HSD11B2. Following this result, we genotyped a highly polymorphic CA-repeat polymorphism within the first intron in 848 normotensive and 430 hypertensive Japanese patients, and we then analyzed its association with disease and clinical parameters. We confirmed 12 alleles (12, 15-25 CA repeats) in the population and found no significant difference in the distribution of the allele length between normotensive and hypertensive patients. In 174 normal subjects without medication, urinary cortisol excretion was higher in subjects with more CA repeats in the shorter allele, but the ratio of urinary cortisone to cortisol, a reliable marker of renal HSD11B2 activity, did not differ. However, longer CA-repeat length was positively correlated with 24-h urinary sodium excretion, fractional sodium excretion and potassium clearance, and this observation was confirmed when the longer CA-repeat length was dichotomized. Thus, HSD11B2 CA-repeat genotype is not associated with hypertension itself, but with renal sodium excretion, probably through salt intake/appetite.

A mechanism for salt-sensitive hypertension: abnormal dietary sodium-mediated vascular response to angiotensin-II

OBJECTIVES

Several mechanisms have been proposed for salt-sensitive hypertension, with most focusing on impaired renal sodium handling. We tested the hypothesis that abnormalities in peripheral vascular responsiveness to angiotensin-II (ANGII) might also exist in salt-sensitive hypertension because of the interplay of the renin-angiotensin system and dietary sodium.

METHODS

Blood pressure (BP) response to ANGII infusion was studied in 295 hypertensive and 165 normotensive individuals after 7 days of high (200 mEq/day) and low (10 mEq/day) dietary sodium.

RESULTS

Normotensive individuals demonstrated higher BP response to ANGII on high-salt than low-salt diet, whereas hypertensive individuals had similar responses on both diets; that is, the high-salt response was not enhanced as compared with low-salt response. Additionally, hypertensive individuals had a significantly greater high-salt BP response to norepinephrine than to ANGII. There was no correlation between the high-salt hormone levels and the difference in BP response to ANGII between the two diets. When stratified by BP response to dietary salt restriction, individuals with salt sensitivity of BP demonstrated abnormal high-salt BP responsiveness to ANGII. To assess if this represented increased tissue renin-angiotensin system activity in the vasculature, BP responses to angiotensin were compared before and after captopril in 20 hypertensive individuals on a high-salt diet. Individuals with the greatest BP-lowering effect to captopril had similar high and low-salt BP responses to ANGII at baseline and a significant increase in the high-salt response after captopril.

CONCLUSION

Hypertensive individuals have an abnormal vascular response to ANGII infusion on a high-salt diet. Dysregulated tissue renin-angiotensin system activity may play a role in this abnormal response. These findings raise an intriguing novel possibility for the pathophysiologic mechanism of salt-sensitive hypertension.

Effect of short-term high-protein compared with normal-protein diets on renal hemodynamics and associated variables in healthy young men

BACKGROUND

High-protein diets are effective for weight reduction; however, little is known about the potential adverse renal effects of such diets.

OBJECTIVE

The aim of our study was to compare the effect of a high-protein (HP) with a normal-protein (NP) diet on renal hemodynamics and selected clinical-chemical factors.

DESIGN

We prospectively studied the effect of an HP diet (2.4 g x kg(-1) x d(-1)) with that of an NP diet (1.2 g x kg(-1) x d(-1)) on the glomerular filtration rate (assessed on the basis of sinistrin-an inulin analog-clearance) and renal plasma flow (para-aminohippuric acid clearance) by using the constant infusion technique. Filtration fraction and renal vascular resistance were calculated. Twenty-four healthy young men followed the 2 diet protocols for 7 d each in a crossover design. They were individually advised by a dietitian to achieve the planned protein intake by selecting normal foods under isocaloric conditions. Serum and urinary variables and renal hemodynamics were measured on day 7 of both diets.

RESULTS

The glomerular filtration rate (NP: 125 +/- 5 mL/min; HP: 141 +/- 8 mL/min; P < 0.001) and filtration fraction (NP: 23 +/- 5%; HP: 28 +/- 5%; P < 0.05) increased significantly with the HP diet. Renal plasma flow was not significantly different between the HP (496 +/- 25 mL/min) and NP (507 +/- 18 mL/min) phases. Renal vascular resistance was not significantly different between the NP (94 +/- 6 mm Hg x mL(-1) x min(-1)) and HP (99 +/- 8 mm Hg x mL(-1) x min(-1)) phases. Blood urea nitrogen, serum uric acid, glucagon, natriuresis, urinary albumin, and urea excretion increased significantly with the HP diet.

CONCLUSIONS

A short-term HP diet alters renal hemodynamics and renal excretion of uric acid, sodium, and albumin. More attention should be paid to the potential adverse renal effects of HP diets.

Incidence of hypertension in individuals with different blood pressure salt-sensitivity: results of a 15-year follow-up study

OBJECTIVE

To evaluate the incidence of hypertension and the rate of decline in renal function in a sample of 47 Olivetti Heart Study (OHS) participants whose blood pressure (BP) salt-sensitivity and renal tubular sodium handling had been assessed in 1987-88.

METHODS

During the 2002-04 OHS follow-up examination, medical history, physical examination and blood and urine sampling were performed in 36 of the 47 participants to the baseline study (age 60 +/- 6 years; average follow-up = 15.1 +/- 0.6 years). The renal length was measured in 23 participants by kidney ultrasonography. Based on the baseline salt-sensitivity evaluation, the subjects were classified into a lower salt-sensitivity (LSS, n = 20) and a higher salt-sensitivity group (HSS, n = 16).

RESULTS

In comparison with the LSS group, HSS participants had a significantly higher incidence of hypertension (87.5 versus 50.0%, P = 0.02), a higher glomerular filtration rate (median, first to fourth quartile: 81.9, 72.3-95.2 versus 72.3, 59.9-81.2 ml/min; P = 0.03) and greater kidney length (median, first to fourth quartile: 68.2, 63.3-72.1 versus 61.9, 58.7-62.7 mm/m of height; P = 0.003). The incidence of hypertension remained significantly higher in HSS individuals after adjustment for age, intercurrent changes in body mass index and baseline blood pressure on low sodium diet (P = 0.04).

CONCLUSION

Our findings indicate that individuals with higher BP salt-sensitivity have a higher rate of incident hypertension and suggest an altered renal tubular sodium handling involving a trend to increased glomerular filtration rate and blood pressure over time as a possible mechanism.

The effect of a shift in sodium intake on renal hemodynamics is determined by body mass index in healthy young men

A body mass index (BMI)>or=25 kg/m2 increases the risk for long-term renal damage, possibly by renal hemodynamic factors. As epidemiological studies suggest interaction of BMI and sodium intake, we studied the combined effects of sodium intake and BMI on renal hemodynamics. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured in 95 healthy men (median age 23 years (95% confidence interval: 22-24), BMI: 23.0+/-2.5 kg/m2) on low (50 mmol Na+, LS) and high (200 mmol Na+, HS) sodium intake. Mean GFR and ERPF significantly increased by the change to HS (both P<0.001). During HS but not LS, GFR and filtration fraction (FF) positively correlated with BMI (R=0.32 and R=0.28, respectively, both P<0.01). Consequently, BMI correlated with the sodium-induced changes in GFR (R=0.30; P<0.01) and FF (R=0,23; P<0.05). The effects of HS on GFR and FF were significantly different for BMI>or=25 versus <25 kg/m2, namely 7.8+/-12.3 versus 16.1+/-13.1 ml/min (P<0.05) and -0.1+/-2.2 and 1.1+/-2.3% (P<0.05). FF was significantly higher in BMI>or=25 versus <25 kg/m2, (22.6+/-2.9 versus 24.6+/-2.4%, P<0.05) only during HS. ERPF was not related to BMI. Urinary albumin excretion was increased by HS from 6.0 (5.4-6.7) to 7.6 (6.9-8.9). Results were essentially similar after excluding the only two subjects with BMI>30 kg/m2. BMI is a determinant of the renal hemodynamic response to HS in healthy men, and of GFR and FF during HS, but not during LS. Consequently, HS elicited a hyperfiltration pattern in subjects with a BMI>or=25 kg/m2 that was absent during LS. Future studies should elucidate whether LS or diuretics can ameliorate the long-term renal risks of weight excess.

Proximal tubular function and salt sensitivity

Blood pressure response to changes in dietary salt intake is highly variable among individuals. This heterogeneity results from the combined effects of genetic and environmental determinants. In recent years, considerable progress has been made in our understanding of the pathogenic mechanisms leading to the development of salt-sensitive hypertension. Much information has come from the investigation of rare monogenic forms of salt-sensitive hypertension, which has focused attention on alterations of renal sodium handling occurring essentially in the distal nephron. In this paper, we review the experimental, clinical, genetic, and epidemiologic evidence suggesting that proximal tubular function is also an important determinant of the blood pressure response to salt, which deserves greater attention.

Abnormalities of renal sodium handling in the metabolic syndrome. Results of the Olivetti Heart Study

OBJECTIVE

The mechanisms underlying high blood pressure in the framework of metabolic syndrome (MS) are not clarified: we thus analyzed the relationship of MS and its components to renal tubular sodium handling among participants of the Olivetti Heart Study, an epidemiological investigation of a representative sample of adult white male population in southern Italy.

METHODS

Proximal (FPRNa) and distal (FDRNa) fractional sodium reabsorption were estimated by the clearance of exogenous lithium in 702 participants aged 25-75 years examined in 1994-1995. Blood pressure and relevant anthropometric and biochemical variables were also measured. The diagnosis of MS was based on modified National Cholesterol Education Program (NCEP)-Adult Treatment Panel III (ATP III) criteria.

RESULTS

FPRNa, but not FDRNa, was directly associated with body mass index (BMI), waist circumference, diastolic pressure, serum triglyceride and uric acid, independently of age and of antihypertensive treatment. After adjustment for age, FPRNa, but not FDRNa, was significantly greater in individuals with MS, as compared to those without [77.6% (95% confidence interval = 76.7-80.1) versus 74.4% (73.7-75.1), P < 0.001]. A similar difference was observed after the exclusion of participants on current antihypertensive treatment (P = 0.018). In untreated individuals, a significant interaction was observed between obesity and insulin resistance as related to FPRNa (P = 0.002): the highest age-adjusted levels of FPRNa were detected in obese hypertensive and obese insulin-resistant participants.

CONCLUSION

In this sample of an adult male population, MS was associated with an increased rate of FPRNa. This finding is relevant to the pathophysiology of MS and possibly to the prevention of its cardiovascular and renal consequences.

Salt sensitivity and hypertension after menopause: role of nitric oxide and angiotensin II

Hypertension is a major risk factor for cardiovascular disease and renal disease. After menopause, the incidence of hypertension increases in women to levels that equal or exceed that in men, suggesting a protective role of female sex hormones. Salt sensitivity of blood pressure is associated with an increased risk for development of hypertension and cardiovascular disease. We and others have demonstrated that after menopause, the prevalence of salt sensitivity increases, suggesting that female sex hormones influence renal sodium handling and blood pressure regulation. A homeostatic balance between the counteracting effects of nitric oxide (NO) and angiotensin (Ang) II on pressure natriuresis, renal hemodynamics, tubular sodium reabsorption, and oxidative stress plays an important role in modulating salt sensitivity as well as hypertensive end-organ injury. Estrogens modulate the activity and expression of NO and Ang II. We infer that after menopause, estrogen deficiency promotes an unbalance between NO and Ang II, resulting in disturbed renal sodium handling, oxidative stress, and hypertension, particularly in genetically prone women. A better understanding of the mechanisms underlying the development of postmenopausal hypertension and associated cardiovascular and renal diseases should provide insights into preventive and therapeutic strategies.

Long-term mathematical model involving renal sympathetic nerve activity, arterial pressure, and sodium excretion

This paper presents a physiological long-term model of the cardiovascular system. It integrates the previous models developed by Guyton, Uttamsingh and Coleman. Additionally it introduces mechanisms of direct effects of the renal sympathetic nerve activity (rsna) on tubular sodium reabsorption and renin secretion in accordance with experimental data from literature. The resulting mathematical model constitutes the first long-term model of the cardiovascular system accounting for the effects of rsna on kidney functions in such detail. The objective of developing such a model is to observe the consequences of long-term rsna increase and impairment of rsna inhibition under volume loading. This model provides an understanding of the rsna-related mechanisms, which cause mean arterial pressure increase in hypertension and total sodium amount increase (sodium retention) in congestive heart failure, nephrotic syndrome and cirrhosis.

Links between dietary salt intake, renal salt handling, blood pressure, and cardiovascular diseases

Epidemiological, migration, intervention, and genetic studies in humans and animals provide very strong evidence of a causal link between high salt intake and high blood pressure. The mechanisms by which dietary salt increases arterial pressure are not fully understood, but they seem related to the inability of the kidneys to excrete large amounts of salt. From an evolutionary viewpoint, the human species is adapted to ingest and excrete <1 g of salt per day, at least 10 times less than the average values currently observed in industrialized and urbanized countries. Independent of the rise in blood pressure, dietary salt also increases cardiac left ventricular mass, arterial thickness and stiffness, the incidence of strokes, and the severity of cardiac failure. Thus chronic exposure to a high-salt diet appears to be a major factor involved in the frequent occurrence of hypertension and cardiovascular diseases in human populations.

Serum electrolyte, serum protein, serum fat and renal responses to a dietary sodium challenge: allostasis and allostatic load

OBJECTIVE

The purpose of this study was to assess, in borderline hypertensive subjects, the homeostatic and allostatic responses of serum electrolytes, proteins, lipids, hematocrit and renal function to an extreme dietary sodium challenge, and to evaluate whether the responses in these clinical parameters were associated with a concomitant response in blood pressure.

SUBJECTS AND METHODS

Data from middle-aged adults with a diagnosis of mild, uncomplicated borderline hypertension were collected at the end of 1-month randomized trials of low (24 +/- 13 mmol/day) and high (309 +/- 88 mmol/day) dietary sodium intake. A total of 48 subjects (38 men and 10 women) were examined.

RESULTS

Serum sodium increased (p < 0.001), while all other serum electrolytes, except chloride, decreased (p < 0.01) from the low to high sodium diets. Serum proteins (p < 0.05) and hematocit (p < 0.001) also declined among subjects on a high sodium diet. However, creatinine clearance (an indicator of glomerular filtration) increased with sodium intake (p = 0.004). None of these biochemical or renal functional responses was associated with a change in blood pressure.

CONCLUSION

There are modest yet significant changes in serum electrolytes associated with changes in dietary sodium intake, suggesting that these ions are under an allostatic control mechanism. Serum proteins also appear to function as allostatic compensatory mechanisms, offsetting the net effect of increased serum salinity. It is speculated that the adaptive allostatic renal response to a high sodium diet (an increase in GFR) may result in loss of the ability to appropriately vary renal filtration if that diet is chronically maintained.

Sodium Intake and Cardiac Sympatho-Vagal Balance in Young Men with High Blood Pressure

We have previously reported that a high sodium intake increases sleep-time blood pressure (BP) in young men. However, there are cases in which this relation does not apply. To account for them, we investigated the relation between sodium intake and cardiac sympatho-vagal balance (SVB) in young men with high BP. Sodium intake was estimated from the amount of urinary sodium excretion over 1 week. Twenty-four-hour (24-h) urinary sodium excretion (Salt24), 24-h ambulatory BP and ECG were obtained on the last day of the observation period. As an index of sodium intake, the expression In(Salt24/Cr24) (Cr24, 24-h urinary creatinine excretion) was used. From power-spectral analysis of ECG-RR intervals during sleep, we obtained the LF/HF ratio between the low-frequency component (LF) and the high frequency component (HF) and used it as an index of SVB. The subjects were male medical students divided into a normal BP group (N-group; n=103) and a high BP group (H-group; n=26, 24-h BP>125/75 mmHg). Mean In(Salt24/Cr24) and LF/HF in the H-group were significantly higher than those in the N-group (LF/HF: 1.86+/-0.44 [SD] vs. 1.37+/-0.30, p<0.001). The calculated discriminant function (D) for the H-group and N-group was D=1.6x + 5y - 11, where x is In(Salt24/Cr24) and y is LF/HF. This formula (D) resulted in high discriminant predictive accuracy (82%) between the groups. If D=0 (the value of the cut-off line determining separation of the groups), the relation y=-0.32x + 2.2 (negative relation between y and x) was obtained. These results suggest that excessive sodium intake in combination with accentuated SVB (LF/HF) increases BP in young men.

Nonhypertensive cardiac effects of a high salt diet

Dietary salt intake plays a major role in variation of blood pressure levels and cardiovascular conditions. High salt intake is associated with the occurrence of hypertension. The evidence that high salt intake increases risk of cardiovascular disease is inconsistent. Some studies indicate a significant and positive association between salt intake and risk of cardiovascular disease, whereas several other studies report that such an association may not exist and that low sodium may even be harmful. It is clear that left ventricular hypertrophy is closely related to salt intake. There is evidence indicating that high salt intake increases renal glomerular filtration rate and glomerular filtration fraction in salt-sensitive patients. The association of high salt intake, insulin resistance, and salt sensitivity, and the interaction between high salt intake, heart rate, and blood pressure are debated.

Does a low-salt diet exert a protective effect on endothelial function in normal rats?

Sodium restriction is often used as an adjunct in the treatment of conditions characterized by endothelial dysfunction, such as hypertension and heart or kidney disease. However, the effect of sodium restriction on endothelial function is not known. Therefore, male Wistar rats were studied after a fixed salt diet had been maintained (low-salt group: 0.05% NaCl, n = 10; normal-salt group: 0.3% NaCl, n = 10) for 6 weeks. Blood pressure and sodium excretion values were measured once a week. Subsequently the rats were killed, the aorta was removed, and rings were cut. Endothelium-independent (sodium nitrite [SN]) and endothelium-dependent (acetylcholine [ACh]) vasodilator responses were assessed in the presence of indomethacin (a cyclo-oxygenase inhibitor) and in the presence or absence of NG-monomethyl-L-arginine (L-NMMA; a competitive inhibitor of nitric oxide [NO] synthase). Endothelium-independent vasodilatation was not different for the two salt groups. Endothelium-dependent vasodilatation, on the other hand, was different. The response to ACh was almost completely abolished by L-NMMA in the normal-salt group, whereas vasodilatation was partially preserved during L-NMMA in the low-salt group. Accordingly, the L-NMMA-sensitive contribution to ACh-dependent vasodilatation was smaller in the low-salt group. Thus, salt restriction induced a non-NO and non-prostaglandin-dependent vasodilating pathway. By exclusion this could be endothelium-derived hyperpolarizing factor, a pathway of vasculoprotective potential. Accordingly, the relative contributions of the different vasoactive endothelial pathways were affected by salt intake. Further research will be needed to clarify the nature and importance of this non-NO, non-prostaglandin-dependent pathway in the clinical setting as well.

Renal sodium handling and haemodynamics are equally affected by hyperinsulinaemia in salt-sensitive and salt-resistant hypertensives

OBJECTIVE

It is well-known that insulin induces renal sodium retention. It is not yet known whether insulin's renal effects are involved in the development of salt-sensitive hypertension. We assessed the effects of insulin on renal sodium handling and haemodynamics in 10 salt-sensitive (SS) and 10 salt-resistant (SR) essential hypertensives.

DESIGN

After a baseline period of 90 min, all subjects underwent a euglycaemic clamp with sequential infusion of a physiological and supraphysiological dose of insulin (50 and 150 mU/kg per h) during 90 min periods each. Time-control studies were performed in the same subjects. Clearances of 131I-hippuran, 125I-iothalamate, sodium and lithium were used to evaluate renal plasma flow (RPF), CNa/glomerular filtration rate (GFR) and fractional proximal and distal sodium reabsorption.

RESULTS

Plasma insulin levels and insulin-mediated glucose uptake did not differ between both groups. RPF and GFR showed similar increases during both insulin infusions in both groups. During physiological hyperinsulinaemia, fractional sodium excretion decreased 38% (P = 0.009) in the SS group and 36% (P = 0.002) in the SR group. During supraphysiological hyperinsulinaemia, fractional sodium excretion decreased 49% (P = 0.01) in the SS group and 19% (P = 0.2) in the SR group, not statistically different between both groups. Fractional proximal sodium reabsorption was unaffected and fractional distal sodium reabsorption increased to a similar magnitude in both groups.

CONCLUSION

The comparable renal effects of acute exogenous hyperinsulinaemia in SS and SR hypertensives do not support a role for insulin in the development of salt-sensitive hypertension. However, the results do not yet exclude a role for chronic hyperinsulinaemia.

Effects of sodium intake on the pressor and renal responses to nitric oxide synthesis inhibition in normotensive individuals with different sodium sensitivity

OBJECTIVE

The present study evaluated the role of nitric oxide (NO) in the systemic vascular and renal adaptation to changes in dietary sodium intake.

DESIGN AND METHODS

Seven healthy normotensive male subjects were randomized to high or low sodium diets in a double blind crossover design (7 days on each diet). The NO synthesis inhibitor, NGmonomethyl-L-arginine (L-NMMA) was infused systemically (1.8 mg/kg over 30 min) at the end of each dietary period and its effects on blood pressure, renal plasma flow, glomerular filtration rate, urinary flow rate and sodium excretion were measured.

RESULTS

Blood pressure increased in response to L-NMMA on a high sodium diet only (area under time curve percentage change in mean blood pressure, low sodium = -94.5 +/- 164.3; high sodium = 391.1 +/- 228.6; P < 0.05 low versus high). The increase in blood pressure was directly and significantly associated with the individual salt sensitivity, defined by the difference in systemic mean blood pressure between high and low sodium diets (r = 0.756; P < 0.05). L-NMMA also reduced renal plasma flow and urinary flow rate in subjects on high sodium diet.

CONCLUSIONS

The data support a significant influence of endogenous NO in the systemic and renal vascular adaptation to a high sodium diet in normotensive men. In addition, the direct association between the individual sodium-sensitivity and the pressor response to L-NMMA suggests that there is increased dependence of vascular tone on NO in normotensive subjects whose blood pressure is more sodium sensitive.

Sodium sensitivity in young adults with high resting end-tidal CO2

BACKGROUND

Previous research with normotensive adults aged over 40 years ('older') found that sensitivity of blood pressure of subjects with high resting end-tidal partial pressures of CO2 to high sodium intake was greater than normal.

OBJECTIVE

To test the hypothesis that the lesser sensitivity of blood pressure of young normotensive adults to high sodium intake is also a function of resting end-tidal partial pressure of CO2.

DESIGN

Forty-eight Caucasian men and women (age 28.5 +/- 1.4 years) had a lower than normal dietary intake of sodium chloride for 4 days, and then ingested sodium chloride capsules for 7 days (an additional 190 mmol/day sodium chloride). Resting end-tidal partial pressure of CO2 and blood pressure, and 24 h ambulatory blood pressure, were measured before and after the high-sodium diet. Overnight urine samples were collected before and after the high-sodium diet to determine dietary compliance, and to assess changes in urinary excretion of endogenous digitalis-like factors (a ouabain-like factor, and a marinobufagenin-like factor) that covary with plasma volume.

RESULTS

Subjects with high end-tidal partial pressures of CO2 had lower resting heart rates and lower urinary excretion of ouabain-like factor before sodium loading. Sodium loading decreased mean partial pressure of CO2 (by 0.8 +/- 0.2 mmHg) and increased only ambulatory systolic blood pressure (by 2.1 +/- 0.8 mmHg) for the whole group. However, the changes in resting systolic (r = 0.32, P < 0.025) and diastolic (r = 0.36, P < 0.01) blood pressures and in 24 h systolic (r = 0.28, P < 0.05) blood pressure after sodium loading were all positive functions of individual resting end-tidal partial pressures of CO2. Sodium loading increased urinary excretion of marinobufagenin-like factor (by 1.78 +/- 0.88 nmol) and the magnitude of the individual increase was a function of end-tidal partial pressure of CO2.

CONCLUSIONS

The results indicate that a high resting partial pressure of CO2 augments the effects of high sodium intake on plasma volume, levels of endogenous digitalis-like factors, and blood pressure in young normotensive humans.

Hypertension and nephropathy in diabetes mellitus: what is inherited and what is acquired?

Prolonged duration of diabetes mellitus, poor long term glycemic control and raised blood pressure have all been clearly related to the development of diabetic nephropathy. Evidence exists to suggest that a subset of individuals with diabetes have a genetic predisposition to diabetic nephropathy. Cases of diabetic nephropathy cluster in families and a parental history of hypertension is more common in patients with diabetic nephropathy. Current evidence suggests an important role for hypertension in the genetic susceptibility to diabetic nephropathy but the extent of this is unknown. While cellular and animal studies have generated a plethora of data regarding mechanisms involved in the role of hypertension and diabetic nephropathy, these are not helpful for drawing conclusions in humans. In the following review, we examine the available clinical, epidemiologic and family studies to assess the relationship between the development of hypertension and diabetic nephropathy in IDDM and NIDDM. We will demonstrate the differences in the epidemiology of hypertension in diabetes depending on the type of diabetes and thus, move the emphasis of nephropathy susceptibility away from hypertension per se. We hope to emphasize instead the homogeneity of nephropathy risk in both IDDM and NIDDM and also the idea that a common genetic susceptibility exists for all types of diabetes and is conditional on cumulative exposure to hyperglycemia. Regarding the interaction of hypertension and nephropathy in diabetes mellitus, any conclusions at this time about what is inherited and what is acquired must be regarded as speculative. However we will discuss some potential mechanisms of hypertension in the evolution of nephropathy and we will allude to the role for novel genetic studies in the search for nephropathy susceptibility gene(s).