Does the renin-angiotensin system determine the renal and systemic hemodynamic response to sodium in patients with essential hypertension?

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

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

The Sweet and Salty Dietary Face of Hypertension and Cardiovascular Disease in Lebanon

According to the World Health Organization (WHO), an estimated 1.28 billion adults aged 30–79 years worldwide have hypertension; and every year, hypertension takes 7.6 million lives. High intakes of salt and sugar (mainly fructose from added sugars) have been linked to the etiology of hypertension, and this may be particularly true for countries undergoing the nutrition transition, such as Lebanon. Salt-induced hypertension and fructose-induced hypertension are manifested in different mechanisms, including Inflammation, aldosterone-mineralocorticoid receptor pathway, aldosterone independent mineralocorticoid receptor pathway, renin-angiotensin system (RAS), sympathetic nervous system (SNS) activity, and genetic mechanisms. This review describes the evolution of hypertension and cardiovascular diseases (CVDs) in Lebanon and aims to elucidate potential mechanisms where salt and fructose work together to induce hypertension. These mechanisms increase salt absorption, decrease salt excretion, induce endogenous fructose production, activate fructose-insulin-salt interaction, and trigger oxidative stress, thus leading to hypertension. The review also provides an up-to-date appraisal of current intake levels of salt and fructose in Lebanon and their main food contributors. It identifies ongoing salt and sugar intake reduction strategies in Lebanon while acknowledging the country’s limited scope of regulation and legislation. Finally, the review concludes with proposed public health strategies and suggestions for future research, which can reduce the intake levels of salt and fructose levels and contribute to curbing the CVD epidemic in the country.

Sodium Intake Is Associated With Renal Resistive Index in an Adult Population-Based Study

Renal resistive index (RRI) has been associated with adverse renal and cardiovascular outcomes. Although traditionally considered a marker of intrinsic renal damage, RRI could also reflect systemic vascular dysfunction. As sodium intake was linked to alterations in vascular properties, we wished to characterize the association of salt consumption with RRI in the general adult population. Participants were recruited in a population-based study in Switzerland. RRI was measured by ultrasound in 3 segmental arteries. Sodium intake (UNa; mmol/24 h) was estimated on 24-hour urine samples. Carotido-femoral pulse wave velocity was obtained by applanation tonometry. Mixed multivariate regression models were used with RRI or pulse wave velocity as independent variables and UNa as dependent variable, adjusting for possible confounders. We included 1002 patients in the analyses with 528 (52.7%) women and mean age of 47.2±17.4. Mean values of UNa and RRI were 141.8±61.1 mmol/24 h and 63.8±5.5%, respectively. In multivariate analysis, UNa was positively associated with RRI (P=0.002) but not with pulse wave velocity (P=0.344). Plasma renin activity and aldosterone did not modify the relationship between UNa and RRI (P=0.087 for interaction). UNa/urinary potassium ratio was positively associated with pulse wave velocity ≥12 m/s (P=0.033). Our results suggest that dietary salt consumption has a direct impact on renal hemodynamic in the adult general population. Alterations in vascular properties likely explain those findings, but inadequate renal vaso-motor response is also possible. Sodium intake could thus potentially be linked to underlying structural systemic damages affecting this population.

Sodium Intake and Hypertension

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

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

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

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.

Renal Collectrin Protects against Salt-Sensitive Hypertension and Is Downregulated by Angiotensin II

Collectrin, encoded by the Tmem27 gene, is a transmembrane glycoprotein with approximately 50% homology with angiotensin converting enzyme 2, but without a catalytic domain. Collectrin is most abundantly expressed in the kidney proximal tubule and collecting duct epithelia, where it has an important role in amino acid transport. Collectrin is also expressed in endothelial cells throughout the vasculature, where it regulates L-arginine uptake. We previously reported that global deletion of collectrin leads to endothelial dysfunction, augmented salt sensitivity, and hypertension. Here, we performed kidney crosstransplants between wild-type (WT) and collectrin knockout (Tmem27^Y/- ) mice to delineate the specific contribution of renal versus extrarenal collectrin on BP regulation and salt sensitivity. On a high-salt diet, WT mice with Tmem27^Y/- kidneys had the highest systolic BP and were the only group to exhibit glomerular mesangial hypercellularity. Additional studies showed that, on a high-salt diet, Tmem27^Y/- mice had lower renal blood flow, higher abundance of renal sodium-hydrogen antiporter 3, and lower lithium clearance than WT mice. In WT mice, administration of angiotensin II for 2 weeks downregulated collectrin expression in a type 1 angiotensin II receptor-dependent manner. This downregulation coincided with the onset of hypertension, such that WT and Tmem27^Y/- mice had similar levels of hypertension after 2 weeks of angiotensin II administration. Altogether, these data suggest that salt sensitivity is determined by intrarenal collectrin, and increasing the abundance or activity of collectrin may have therapeutic benefits in the treatment of hypertension and salt sensitivity.

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.

Meta-Analysis of the Effect of Dietary Sodium Restriction with or without Concomitant Renin-Angiotensin-Aldosterone System-Inhibiting Treatment on Albuminuria

BACKGROUND AND OBJECTIVES

Urinary albumin excretion and/or albumin to creatinine ratio are associated with CKD and higher risk of cardiovascular events. Several studies investigated the effect of reduced dietary sodium intake on urinary albumin excretion and/or albumin to creatinine ratio in adult patient populations, but the majority was inconclusive because of insufficient statistical power. A meta-analysis of the randomized, controlled trials available could overcome this problem and lead to more definitive conclusions.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A systematic search of the online databases available (from 1996 to October of 2014) was conducted of randomized, controlled trials that expressed urinary albumin excretion or albumin to creatinine ratio as the difference between the effects of two different sodium intake regimens. For each study, the mean difference and 95% confidence intervals were pooled using a random effect model. Heterogeneity, publication bias, subgroup, and meta-regression analyses were performed.

RESULTS

Eleven studies met the predefined inclusion criteria and provided 23 cohorts with 516 participants and 1-6 weeks of follow-up time. In the pooled analysis, an average reduction in sodium intake of 92 mmol/d was associated with a 32.1% (95% confidence interval, -44.3 to -18.8) reduction in urinary albumin excretion. The effect of sodium restriction was higher in the cohorts including patients on concomitant renin-angiotensin-aldosterone system-blocking therapy, in the studies with intervention lasting at least 2 weeks, and among participants with evidence of kidney damage. A greater reduction of urinary albumin excretion was associated with a higher decrease in BP during the intervention. The analysis of changes in albumin to creatinine ratio provided similar results.

CONCLUSIONS

This meta-analysis indicates that sodium intake reduction markedly reduces albumin excretion, more so during concomitant renin-angiotensin-aldosterone system-blocking therapy and among patients with kidney damage.

Higher filtration fraction in formerly early-onset preeclamptic women without comorbidity

Formerly preeclamptic women have an increased risk for developing end-stage renal disease, which has been attributed to altered renal hemodynamics and abnormalities in the renin-angiotensin-aldosterone system. Whether this is due to preeclampsia itself or to comorbid conditions is unknown. Renal hemodynamics and responsiveness to ANG II during low Na(+) intake (7 days, 50 mmol Na(+)/24 h) and high Na(+) (HS) intake (7 days, 200 mmol Na(+)/24 h) were studied in 18 healthy normotensive formerly early-onset preeclamptic women (fPE women) and 18 healthy control subjects (fHP women), all selected for absence of comorbidity. At the end of each diet, renal hemodynamics and blood pressure were measured before and during graded ANG II infusion. Both HS intake and former preeclampsia increased filtration fraction (FF) without an interaction between the two. FF was highest during HS intake in fPE women [0.31 ± 0.12 vs. 0.29 ± 0.11 in fHP women, generalized estimating equation analysis (body mass index corrected), P = 0.03]. The renal response to ANG II infusion was not different between groups. In conclusion, fPE women have a higher FF compared with fHP women. As this was observed in the absence of comorbidity, preeclampsia itself might exert long-term effects on renal hemodynamics. However, we cannot exclude the presence of prepregnancy alterations in renal function, which, in itself, lead to an increased risk for preeclampsia. In experimental studies, an elevated FF has been shown to play a pathogenic role in the development of hypertension and renal damage. Future studies, however, should evaluate whether the subtle differences in renal hemodynamics after preeclampsia contribute to the increased long-term renal risk after preeclampsia.

Effect of longer-term modest salt reduction on blood pressure

BACKGROUND

A reduction in salt intake lowers blood pressure (BP) and, thereby, reduces cardiovascular risk. A recent meta-analysis by Graudal implied that salt reduction had adverse effects on hormones and lipids which might mitigate any benefit that occurs with BP reduction. However, Graudal's meta-analysis included a large number of very short-term trials with a large change in salt intake, and such studies are irrelevant to the public health recommendations for a longer-term modest reduction in salt intake. We have updated our Cochrane meta-analysis.

OBJECTIVES

To assess (1) the effect of a longer-term modest reduction in salt intake (i.e. of public health relevance) on BP and whether there was a dose-response relationship; (2) the effect on BP by sex and ethnic group; (3) the effect on plasma renin activity, aldosterone, noradrenaline, adrenaline, cholesterol, low-density lipoprotein (LDL), high-density lipoprotein (HDL) and triglycerides.

SEARCH METHODS

We searched MEDLINE, EMBASE, Cochrane Hypertension Group Specialised Register, Cochrane Central Register of Controlled Trials, and reference list of relevant articles.

SELECTION CRITERIA

We included randomised trials with a modest reduction in salt intake and duration of at least 4 weeks.

DATA COLLECTION AND ANALYSIS

Data were extracted independently by two reviewers. Random effects meta-analyses, subgroup analyses and meta-regression were performed.

MAIN RESULTS

Thirty-four trials (3230 participants) were included. Meta-analysis showed that the mean change in urinary sodium (reduced salt vs usual salt) was -75 mmol/24-h (equivalent to a reduction of 4.4 g/d salt), the mean change in BP was -4.18 mmHg (95% CI: -5.18 to -3.18, I (2)=75%) for systolic and -2.06 mmHg (95% CI: -2.67 to -1.45, I (2)=68%) for diastolic BP. Meta-regression showed that age, ethnic group, BP status (hypertensive or normotensive) and the change in 24-h urinary sodium were all significantly associated with the fall in systolic BP, explaining 68% of the variance between studies. A 100 mmol reduction in 24 hour urinary sodium (6 g/day salt) was associated with a fall in systolic BP of 5.8 mmHg (95%CI: 2.5 to 9.2, P=0.001) after adjusting for age, ethnic group and BP status. For diastolic BP, age, ethnic group, BP status and the change in 24-h urinary sodium explained 41% of the variance between studies. Meta-analysis by subgroup showed that, in hypertensives, the mean effect was -5.39 mmHg (95% CI: -6.62 to -4.15, I (2)=61%) for systolic and -2.82 mmHg (95% CI: -3.54 to -2.11, I (2)=52%) for diastolic BP. In normotensives, the mean effect was -2.42 mmHg (95% CI: -3.56 to -1.29, I (2)=66%) for systolic and -1.00 mmHg (95% CI: -1.85 to -0.15, I (2)=66%) for diastolic BP. Further subgroup analysis showed that the decrease in systolic BP was significant in both whites and blacks, men and women. Meta-analysis of hormone and lipid data showed that the mean effect was 0.26 ng/ml/hr (95% CI: 0.17 to 0.36, I (2)=70%) for plasma renin activity, 73.20 pmol/l (95% CI: 44.92 to 101.48, I (2)=62%) for aldosterone, 31.67 pg/ml (95% CI: 6.57 to 56.77, I (2)=5%) for noradrenaline, 6.70 pg/ml (95% CI: -0.25 to 13.64, I (2)=12%) for adrenaline, 0.05 mmol/l (95% CI: -0.02 to 0.11, I (2)=0%) for cholesterol, 0.05 mmol/l (95% CI: -0.01 to 0.12, I (2)=0%) for LDL, -0.02 mmol/l (95% CI: -0.06 to 0.01, I (2)=16%) for HDL, and 0.04 mmol/l (95% CI: -0.02 to 0.09, I (2)=0%) for triglycerides.

AUTHORS' CONCLUSIONS

A modest reduction in salt intake for 4 or more weeks causes significant and, from a population viewpoint, important falls in BP in both hypertensive and normotensive individuals, irrespective of sex and ethnic group. With salt reduction, there is a small physiological increase in plasma renin activity, aldosterone and noradrenaline. There is no significant change in lipid levels. These results provide further strong support for a reduction in population salt intake. This will likely lower population BP and, thereby, reduce cardiovascular disease. Additionally, our analysis demonstrates a significant association between the reduction in 24-h urinary sodium and the fall in systolic BP, indicating the greater the reduction in salt intake, the greater the fall in systolic BP. The current recommendations to reduce salt intake from 9-12 to 5-6 g/d will have a major effect on BP, but are not ideal. A further reduction to 3 g/d will have a greater effect and should become the long term target for population salt intake.

Central body fat distribution associates with unfavorable renal hemodynamics independent of body mass index

Central distribution of body fat is associated with a higher risk of renal disease, but whether it is the distribution pattern or the overall excess weight that underlies this association is not well understood. Here, we studied the association between waist-to-hip ratio (WHR), which reflects central adiposity, and renal hemodynamics in 315 healthy persons with a mean body mass index (BMI) of 24.9 kg/m(2) and a mean (125)I-iothalamate GFR of 109 ml/min per 1.73 m(2). In multivariate analyses, WHR was associated with lower GFR, lower effective renal plasma flow, and higher filtration fraction, even after adjustment for sex, age, mean arterial pressure, and BMI. Multivariate models produced similar results regardless of whether the hemodynamic measures were indexed to body surface area. Thus, these results suggest that central body fat distribution, independent of BMI, is associated with an unfavorable pattern of renal hemodynamic measures that could underlie the increased renal risk reported in observational studies.

Association of sodium and potassium intake with left ventricular mass: coronary artery risk development in young adults

High salt intake may affect left ventricular mass (LVM). We hypothesized that urinary sodium (UNa) and sodium/potassium ratio (UNa/K) are associated with LVM in a predominantly normotensive cohort of young adults. The Coronary Artery Risk Development in Young Adults (CARDIA) Study is a multicenter cohort of black and white men and women aged 30±3.6 years at the time of baseline echocardiographic examination (1990-1991). 2D guided M-mode LVM indexed to body size (grams per meter(2.7)) was calculated, and UNa and potassium excretion assessed (average of three 24-hour urinary samples, n=1042). Linear and logistic regression analysis was used. Participants were 57% women and 55% black. Only 4% were hypertensive. UNa, urinary potassium, and UNa/K ratios were (mean±SD) 175.6±131.0, 56.4±46.3, and 3.4±1.4 mmol/24 h, respectively. Participants in the highest versus the lowest UNa excretion quartile had the greatest LVM (37.5 versus 34.0 g/m(2.7); P<0.001). Adjusted for age, sex, education, and race, LVM averaged 0.945 g/m(2.7) higher per SD of UNa/K (P=0.001). The relationship between UNa/K and LVM persisted among 399 participants with repeat echocardiographic measures 5 years later. In logistic regression analysis adjusted for age, sex, education, and race, each SD higher baseline UNa/K was associated with 23% and 38% greater chances of being in the highest quartile of LVM at baseline (odds ratio: 1.23; P=0.005) and 5 years later (odds ratio: 1.38; P=0.02). A higher sodium/potassium excretion ratio is significantly related to cardiac structure, even among healthy young adults.

Effect of sodium overload on renal function of offspring from diabetic mothers

The aim if this study was to evaluate the effect of sodium overload on blood pressure and renal function in the offspring of diabetic rat mothers. Diabetes was induced with a single dose of streptozotocin before mating. Experimental groups were control (C), offspring from diabetic mother (D), control with sodium chloride (NaCl) overload (CS), and offspring from diabetic mother submitted to NaCl overload (DS). After weaning, all groups received food ad libitum; groups C and D had water ad libitum, and CS and DS received NaCl 0.15 M as drinking water. Renal morphology and function were evaluated in 3-month-old rats. Glomerular area, macrophage infiltration, interlobular artery wall thickness, and renal vascular resistance were significantly increased in CS, D, and DS compared with C. Renal plasma flow (RPF) and glomerular filtration rate (GFR) were decreased in CS and D compared with C. In DS, GFR and fractional filtration were increased, suggesting a state of hyperfiltration. Hypertension was observed in groups D, CS, and DS from 2 months on and was more severe in DS. Our data suggest that diabetes during intrauterine development and salt overload beginning at an early age can cause hypertension and renal injury. When these conditions were associated, morphological and functional changes were much more intense, suggesting acceleration in the process of kidney injury.

Renal response to angiotensin II is blunted in sodium-sensitive normotensive men

BACKGROUND

In hypertension, sodium sensitivity (SS) of blood pressure is associated with renal hemodynamic abnormalities related to increased activity of the renal renin-angiotensin aldosterone system (RAAS). The renal mechanisms of SS in normotensives are unknown. Therefore, we studied whether SS is related to renal hemodynamics and renal responsiveness to angiotensin II (AngII) in young healthy adults.

METHODS

Blood pressure (mean arterial pressure (MAP)) and renal function were measured in 34 healthy men after 1-week low-sodium diet (LS; 50 mmol Na(+)/24 h), 1-week high-sodium diet (HS; 200 mmol Na(+)/24h), and 1-week HS-ACEi (enalapril 20 mg/day). The responses of effective renal plasma flow (ERPF; (131)I-Hippuran clearance) to graded infusion of AngII were assessed during each condition.

RESULTS

The sodium-induced change in MAP ranged from -7 to +14 mm Hg. SS (a sodium-induced increase in MAP >3 mm Hg) was present in 13 subjects. ERPF was lower in SS subjects during LS and during HS-ACEi. The AngII-induced decrease in ERPF was blunted in SS on LS (-25 +/- 6 vs. -29 +/- 7% in sodium-resistant (SR) subjects, P < 0.05) and on HS (-30 +/- 5 vs. -35 +/- 6%, P < 0.05). The blunting was corrected by angiotensin-converting enzyme inhibitors (ACEi) (-36 +/- 6 vs. -37 +/- 7%).

CONCLUSION

SS normotensive subjects have a blunted renal response to exogenous AngII. This is ameliorated by ACEi, supporting a role for inappropriately high intrarenal RAAS activity. As these findings cannot be attributed to subclinical renal hypertensive damage, high intrarenal RAAS activity and altered renal hemodynamics may be primary phenomena underlying SS.

Sodium-selective salt sensitivity: its occurrence in blacks

We tested the hypothesis that the Na(+) component of dietary NaCl can have a pressor effect apart from its capacity to complement the extracellular osmotic activity of Cl(-) and, thus, expand plasma volume. We studied 35 mostly normotensive blacks who ingested a low-NaCl diet, 30 mmol/d, for 3 weeks, in the first and third of which Na(+) was loaded orally with either NaHCO(3) or NaCl, in random order (250 mmol/d). In subjects adjudged to be salt sensitive (n=18; Delta mean arterial pressure: >or=5 mm Hg with NaCl load), but not in salt-resistant subjects (n=17), loading with NaHCO(3) was also pressor. The pressor effect of NaHCO(3) was half that of NaCl: mean arterial pressure (millimeters of mercury) increased significantly from 90 on low NaCl to 95 with NaHCO(3) and to 101 with NaCl. The pressor effect of NaCl strongly predicted that of NaHCO(3.) As judged by hematocrit decrease, plasma volume expansion with NaCl was the same in salt-resistant and salt-sensitive subjects and twice that with NaHCO(3), irrespective of the pressor effect. In salt-sensitive subjects, mean arterial pressure varied directly with plasma Na(+) concentration attained with all Na(+) loading. In salt-sensitive but not salt-resistant subjects, NaHCO(3) and NaCl induced decreases in renal blood flow and increases in renal vascular resistance; changes in renal blood flow were not different with the 2 salts. Responses of renal blood flow and renal vascular resistance to NaHCO(3) were strongly predicted by those to NaCl. In establishing the fact of "sodium-selective" salt sensitivity, the current observations demonstrate that the Na(+) component of NaCl can have pressor and renal vasoconstrictive properties apart from its capacity to complement Cl(-) in plasma volume expansion.

Renal hemodynamics in overweight and obesity: pathogenetic factors and targets for intervention

Weight excess is a risk factor for progressive renal function loss, not only in subjects with renal disease or renal transplant recipients, but also in the general population. Considering the increasing prevalence of obesity worldwide, weight excess may become the main renal risk factor on a population basis, all the more so because the risk is not limited to morbid obesity, but is already apparent in the overweight range. The mechanism of the renal risk is multifactorial. In addition to the role of comorbid conditions such as hypertension and diabetes, current evidence supports a pathogenetic role for renal hemodynamics, specifically glomerular hyperfiltration, and also glomerular hypertension. Weight excess is associated with an elevated glomerular filtration rate and a less pronounced rise in renal plasma flow, resulting in an elevated filtration fraction. This suggests glomerular hypertension due to afferent-efferent dysbalance, which impairs glomerular protection from systemic hypertension. Data in renal transplant recipients support the pathogenetic role of elevated glomerular pressure for long-term renal prognosis. Blockade of the renin-angiotensin-aldosterone system can reverse the renal hemodynamic abnormalities. The obesity-associated renal risk is unfavourably affected by high sodium intake. This may be due to the effects of sodium on blood pressure, which is often sodium-sensitive in obesity, but direct renal effects are also present. Interestingly, sodium restriction ameliorates overweight-associated hyperfiltration in overweight subjects. More focus on weight excess as a renal risk factor is warranted. Preventive measures should focus on weight excess as well as on specific protection against renal damage, by renin-angiotensin-aldosterone system-blockade and moderate sodium restriction.

Dietary Salt, Blood Pressure, and Microalbuminuria

The relationship between dietary salt, blood pressure, and risk for cardiovascular disease has been debated for decades. Microalbuminuria is a biomarker for both cardiovascular and kidney disease. The presence of microalbuminuria correlates directly with the risk for myocardial infarction and stroke and indicates individuals at risk for the development of progressive kidney disease. Since patients with the metabolic syndrome, diabetes, or chronic kidney disease often are blood pressure salt sensitive, and it is well known that increasing dietary salt may offset both the antihypertensive and antiproteinuric effects of renin-angiotensin system blocking drugs, physicians must consider increased salt intake as a potential modifiable risk factor for progression of chronic kidney disease and possibly even cardiovascular disease.

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.

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.

Salt intake and progression of chronic kidney disease: An overlooked modifiable exposure? A commentary

The relationship between sodium chloride (salt) intake and blood pressure and cardiovascular disease has been debated for decades. Overlooked is whether there is a relationship between dietary electrolyte ingestion (both sodium and potassium) and risk for progression of kidney disease, particularly in patients who manifest early evidence of chronic kidney disease (CKD). Patients with CKD often are salt sensitive and respond to increased ingestion of sodium chloride with increased blood pressure. Of concern is the clinical evidence that salt-sensitive patients respond to increased salt intake, in the physiological range, with increased glomerular filtration fraction and proteinuria. Thus, these salt-induced changes in both systemic blood pressure and the renal microcirculation create a favorable theoretical scenario for progressive renal injury. Increased salt intake also attenuates the antihypertensive effects of most antihypertensive drugs. Consequently, salt intake must be considered a potential modifiable risk factor for the progression of kidney disease.

From secondary to primary prevention of progressive renal disease: The case for screening for albuminuria

Many subjects nowadays present with end-stage renal failure and its attendant cardiovascular complications without known prior renal damage. In this report we review the evidence available to strongly suggest that the present practice of secondary prevention in those with known prior renal disease should be extended to primary prevention for those subjects in the general population who are at risk for progressive renal failure, but who had never suffered from a primary renal disease. We show that such subjects can be detected by screening for albuminuria. Elevated urinary albumin loss is an indicator not only of poor renal, but also of poor cardiovascular prognosis. In addition to diabetic subjects who are at risk for albuminuria, we also show that hypertensive, obese, and smoking subjects are more susceptible. We suggest that therapies that have been shown to lower albumin excretion, such as ACE inhibitors, angiotensin II receptor antagonists, and statins be started early in such patients to prevent them from developing clinical renal disease and its attendant cardiovascular complications.

Body mass index is associated with altered renal hemodynamics in non-obese healthy subjects

BACKGROUND

Weight excess is associated with increased renal risk. Data in overt obesity suggest a role for altered renal hemodynamics. Whether body mass index (BMI) is also relevant to renal function in non-obese subjects is unknown.

METHODS

We studied the relation between BMI and renal hemodynamics in 102 healthy, non-obese (BMI <30 kg/m2) subjects [59 males, 43 females, mean age 39 (18-69) years] in a post-hoc analysis of subjects evaluated as prospective kidney donors or as healthy volunteers in renal hemodynamic studies.

RESULTS

Mean (+/-SD) BMI was 24.0 +/- 2.8 kg/m2, mean arterial pressure (MAP) 93 +/- 11 mm Hg, glomerular filtration rate (GFR, iothalamate clearance) 111 +/- 19 mL/min/1.73 m2, effective renal plasma flow (ERPF, hippuran clearance) 458 +/- 108 mL/min/1.73 m2, FF (GFR/ERPF) 0.25 +/- 0.04. On univariate analysis, BMI correlated negatively with ERPF/1.73 m2 body surface area (BSA) (r=-0.46; P < 0.001), GFR/1.73 m2 BSA (r=-0.24, P= 0.013) and positively with FF (r= 0.45, P < 0.001), and age (r= 0.47, P < 0.001). On multivariate analysis both BMI and age were independent predictors of ERPF/1.73 m2 BSA (negative) and FF (positive, all P < 0.05). Age was the only predictor of GFR/1.73 m2 BSA (negative). Analyzed for renal function indexed for height (h), BMI correlated negatively with ERPF/h (r=-0.274, P= 0.005), but not with GFR/h (r= 0.13, P= 0.899). On multivariate analysis both BMI (positive) and age (negative) were independent predictors for GFR/h (both P < 0.001). Age was the only predictor for ERPF/h (negative). Predictors for FF (BMI and age, both positive) were by definition unaltered.

CONCLUSION

The impact of BMI on renal function is not limited to overt obesity, as in subjects with BMI <30 kg/m2 a higher BMI is associated with higher FF, that is, a higher GFR relative to ERPF. This suggests an altered afferent/efferent balance and higher glomerular pressure (i.e., a potentially unfavorable renal hemodynamic profile) that may confer enhanced renal susceptibility when other factors, such as hypertension or diabetes are superimposed.

Hypertension but Not Sodium Intake Determines Progression of Renal Failure in Experimentally Uremic Rats

BACKGROUND/AIMS

High sodium intake is implicated in contributing to progression of chronic renal failure. We studied the effect of high sodium consumption on progression of rat experimental renal failure while sodium-induced hypertension was pharmacologically controlled.

METHODS

64 Sprague-Dawley rats underwent 5/6 nephrectomy. Subsequently, they were divided in three groups which were fed either low, normal, or high sodium diet. Only the high sodium-consuming group developed hypertension. This group was further divided in two subgroups in which hypertension was either untreated or titrated to normotension by hydralazine alone or with propranolol.

RESULTS

Sequential GFR values did not differ between the respective normotensive groups. Survival downslopes of all three normotensive groups (including the pharmacologically treated, high sodium-consuming subgroup) were also similar, extending over 10 weeks. By contrast, pharmacologically untreated animals exhibited severe hypertension and 100% mortality within 3 weeks. In all experimental groups, 24-hour urinary sodium excretion paralleled sodium intake. Proteinuria rose similarly and significantly in all animals on high sodium. A significant correlation between 24-hour sodium and proteinuria was evident throughout the experimental period.

CONCLUSIONS

(1) In 5/6 nephrectomized rats, renal function deterioration was not affected by dietary sodium, provided hypertension was pharmacologically controlled. (2) Enhanced proteinuria secondary to high sodium consumption had no adverse effect on progression of renal failure in this model.

Effect of longer-term modest salt reduction on blood pressure

BACKGROUND

Many randomised trials assessing the effect of salt reduction on blood pressure show reduction in blood pressure in individuals with high blood pressure. However, there is controversy about the magnitude and the clinical significance of the fall in blood pressure in individuals with normal blood pressure. Several meta-analyses of randomised salt reduction trials have been published in the last few years. However, most of these included trials of very short duration (e.g. 5 days) and included trials with salt loading followed by salt deprivation (e.g. from 20 to 1 g/day) over only a few days. These short-term experiments are not appropriate to inform public health policy which is for a modest reduction in salt intake over a prolonged period of time. A meta-analysis by Hooper et al is an important attempt to look at whether advice to achieve a long-term salt reduction (i.e. more than 6 months) in randomised trials causes a fall in blood pressure. However, most trials included in this meta-analysis achieved a small reduction in salt intake; on average, salt intake was reduced by 2 g/day. It is, therefore, not surprising that this analysis showed a small fall in blood pressure, and that a dose-response to salt reduction was not demonstrable.

OBJECTIVES

To assess the effect of the currently recommended modest reduction in salt intake (WHO 2003; SACN 2003; Whelton 2002), on blood pressure in individuals with normal and elevated blood pressure. To assess whether the magnitude of the reduction in blood pressure is dependent on the magnitude of the reduction in salt intake.

SEARCH STRATEGY

We searched MEDLINE, EMBASE, Cochrane library, CINAHL, and reference list of original and review articles.

SELECTION CRITERIA

We included randomised trials with a modest reduction in salt intake and a duration of 4 or more weeks.

DATA COLLECTION AND ANALYSIS

Data were extracted independently by two persons. Mean effect sizes were calculated using both fixed and random effect models using Review Manager 4.2.1 software. Weighted linear regression was used to examine the relationship between the change in urinary sodium and the change in blood pressure. We used funnel plots to detect publication and other biases in the meta-analysis.

MAIN RESULTS

Seventeen trials in individuals with elevated blood pressure (n=734) and 11 trials in individuals with normal blood pressure (n=2220) were included. In individuals with elevated blood pressure the median reduction in 24-h urinary sodium excretion was 78 mmol (4.6 g/day of salt), the mean reduction in systolic blood pressure was -4.97 mmHg (95%CI:-5.76 to -4.18), and the mean reduction in diastolic blood pressure was -2.74 mmHg (95% CI:-3.22 to -2.26). In individuals with normal blood pressure the median reduction in 24-h urinary sodium excretion was 74 mmol (4.4 g/day of salt), the mean reduction in systolic blood pressure was -2.03 mmHg (95% CI: -2.56 to -1.50) mmHg, and the mean reduction in diastolic blood pressure was -0.99 mmHg (-1.40 to -0.57). Weighted linear regression analyses showed a correlation between the reduction in urinary sodium and the reduction in blood pressure.

REVIEWERS' CONCLUSIONS

Our meta-analysis demonstrates that a modest reduction in salt intake for a duration of 4 or more weeks has a significant and, from a population viewpoint, important effect on blood pressure in both individuals with normal and elevated blood pressure. These results support other evidence suggesting that a modest and long-term reduction in population salt intake could reduce strokes, heart attacks, and heart failure. Furthermore, our meta-analysis demonstrates a correlation between the magnitude of salt reduction and the magnitude of blood pressure reduction. Within the daily intake range of 3 to 12 g/day, the lower the salt intake achieved, the lower the blood pressure.

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.

Salt loading affects cortisol metabolism in normotensive subjects: relationships with salt sensitivity

We studied cortisol metabolism together with insulin sensitivity [homeostatic model assessment (HOMA)] and renal hemodynamics in 19 salt-resistant (sr) and nine salt-sensitive (ss) normotensive subjects after a low- and high-salt diet. Results are described as high- vs. low-salt diet. Sum of urinary cortisol metabolite excretion (sum(metabolites)) increased in sr subjects (3.8 +/- 1.6 vs. 3.1 +/- 1.1 microg/min per square meter, P < 0.05) and decreased in ss subjects (2.3 +/- 1.0 vs. 2.9 +/- 1.1 microg/min per square meter, P < 0.05). Plasma 0830 h cortisol decreased in sr subjects but did not change significantly in ss subjects. In all subjects, the absolute blood pressure change correlated negatively with the percentage change in sum(metabolites) (P < 0.05) and positively with the percentage change in renal vascular resistance (P < 0.05). Sum(metabolites) during high-salt diet correlated negatively with the percentage changes in plasma 0830 h cortisol (P < 0.05) and renal vascular resistance (P = 0.05). HOMA did not change in either group, but the percentage change in HOMA correlated positively with the percentage change in plasma cortisol (P = 0.001) and negatively with the percentage change in sum(metabolites) (P < 0.01). Parameters of 11 beta-hydroxysteroid dehydrogenase activity were not different between groups and did not change. In conclusion, these data suggest that cortisol elimination is affected differently after salt loading in sr and ss subjects. Changes in circulating cortisol might contribute to individual sodium-induced alterations in insulin sensitivity.

Salt, blood pressure and the renin-angiotensin system

Much evidence from epidemiological, migration, intervention, animal and genetic studies suggests that salt intake plays an important role in regulating blood pressure (BP). At the same time, many clinical trials have shown that reducing salt intake lowers BP. However, the magnitude of the fall in BP for a given reduction in salt intake varies with age, ethnic group and BP levels. This difference has been suggested to be related to the responsiveness of the renin-angiotensin system (RAS). However, the sympathetic nervous system, the kallikrein-kinin system, the nitric oxide system, and many eicosanoids may also play a role. In this article, we address the important role of the RAS in determining the fall in BP with salt reduction.

Between-patient differences in the renal response to renin-angiotensin system intervention: clue to optimising renoprotective therapy?

Renin-angiotensin-aldosterone system (RAAS) blockade with angiotensin-converting enzyme inhibitors (ACE-I) or angiotensin II (Ang II), AT(1)-receptor blockers (ARB) is the cornerstone of renoprotective therapy. Still, the number of patients with end-stage renal disease is increasing worldwide, prompting the search for improved renoprotective strategies. In spite of proven efficacy at group level, the long-term renoprotective effect of RAAS blockade displays a marked between-patient heterogeneity, which is closely linked to between-patient differences in the intermediate parameters of blood pressure, proteinuria and renal haemodynamics. Of note, the between-patient differences by far exceed the between-regimen differences, and thus may provide a novel target for exploration and intervention. The responsiveness to RAAS blockade appears to be an individual characteristic as demonstrated by studies applying a rotation-schedule design. The type and severity of renal disease, obesity, insulin-resistance, glycaemic control, and genetic factors may all be involved in individual differences in responsiveness, as well as dietary factors, such as dietary sodium and protein intake. Several strategies, such as dietary sodium restriction and diuretic therapy, dose-titration for proteinuria, and dual RAAS blockade with ACE-I and ARB, can improve the response to therapy at a group level. However, when analysed for their effect in individuals, it appears that these measures do not allow poor responders to catch up with the good responders, i.e. in spite of their efficacy at group level, the available measures are usually not sufficient to overcome individual resistance to RAAS blockade. We conclude that between-patient differences in responsiveness to renoprotective intervention should get specific attention as a target for intervention. Unravelling of the underlying mechanisms may allow development of specific intervention. Based on the currently available data, we propose that response-based treatment schedules, with a multidrug approach titrated and adapted at individual responses rather than fixed treatment schedules, may provide a fruitful strategy for more effective renoprotection.

A low-sodium diet potentiates the effects of losartan in type 2 diabetes

OBJECTIVE

Diabetic subjects have a high prevalence of hypertension, increased total body exchangeable sodium levels, and an impaired ability to excrete a sodium load. This study assessed the effect of dietary sodium restriction on the efficacy of losartan in hypertensive subjects with type 2 diabetes and albumin excretion rates of 10-200 microg/min.

RESEARCH DESIGN AND METHODS

In this study, 20 subjects were randomized to losartan 50 mg/day (n = 10) or placebo (n = 10). Drug therapy was given in two 4-week phases separated by a washout period. In the last 2 weeks of each phase, patients were assigned to low- or regular-sodium diets, in random order. In each phase, 24-h ambulatory blood pressure, urinary albumin-to-creatinine ratio (ACR), and renal hemodynamics were measured.

RESULTS

Achieved urinary sodium on a low-sodium diet was 85 +/- 14 and 80 +/- 22 mmol/day in the losartan and placebo groups, respectively. In the losartan group, the additional blood pressure-lowering effects of a low-sodium diet compared with a regular-sodium diet for 24-h systolic, diastolic, and mean arterial blood pressures were 9.7 mmHg (95% confidence interval [CI], 2.2-17.2; P = 0.002), 5.5 mmHg (2.6-8.4; P = 0.002), and 7.3 mmHg (3.3- 11.3; P = 0.003), respectively. In the losartan group, the ACR decreased significantly on a low-sodium diet versus on a regular-sodium diet (-29% [CI -50.0 to -8.5%] vs. + 14% [-19.4 to 47.9%], respectively; P = 0.02). There was a strong correlation between fall in blood pressure and percent reduction in the ACR (r = 0.7, P = 0.02). In the placebo group, there were no significant changes in blood pressure or ACR between regular- and low-sodium diets. There were no significant changes in renal hemodynamics in either group.

CONCLUSIONS

These data demonstrated that a low-sodium diet potentiates the antihypertensive and antiproteinuric effects of losartan in type 2 diabetes. The blood pressure reduction resulting from the addition of a low-sodium diet to losartan was of similar magnitude to that predicted from the addition of a second antihypertensive agent.

Cardiovascular indices of peripheral and central sympathetic activation

OBJECTIVE

A number of sympathetic nervous system (SNS) parameters have been used in cardiovascular psychophysiology. This study aimed to describe the pattern and redundancy of a set of SNS parameters during peripherally induced changes of cardiac sympathetic activation and reflex modulation of central SNS control. Preejection period (PEP) was assessed as a marker of peripheral sympathetic activation. Low-frequency blood pressure variability (BPV) was assessed as an estimate of central SNS control.

METHODS

Peripheral beta-sympathetic stimulation and blockade were achieved with epinephrine and esmolol hydrochloride (beta1-blockade), respectively. Changes in central SNS output were induced by loading and unloading arterial baroreceptors with norepinephrine and nitroprusside sodium, respectively. This single-blinded, crossover study in 24 healthy men also included two placebo control periods. PEP was derived from impedance cardiography and adjusted individually for heart rate. BPV was calculated by power spectral analyses of beat-to-beat heart rate and systolic blood pressure (Finapres system) data.

RESULTS

PEP decreased during epinephrine infusion (-40.1 +/- 3.8 ms, p <.0001) and increased during esmolol infusion (+6.6 +/- 3.5 ms, p =.05). PEP was shortened after central SNS activation by nitroprusside (-16.8 +/- 2.9 ms, p < 0.0001). Systolic BPV in the low-frequency range (0.07-0.14 Hz, Mayer waves) increased during nitroprusside infusion (+0.44 +/- 0.19 ln mm Hg(2), p =.03) and decreased during norepinephrine infusion (-0.67 +/- 0.13 ln mm Hg(2), p < 0.0001). Low-frequency BPV did not change significantly during epinephrine or esmolol infusion.

CONCLUSIONS

Our data provide empirical evidence of separable peripheral and central sympathetic response components. The combined report of low-frequency BPV and PEP gives distinct information on both central SNS control and the level of sympathetic cardiac activation achieved.

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 in acute and chronic salt loading/depletion protocols: the confounding influence of acute water loading

OBJECTIVES

Renal tubular sodium handling was measured in healthy subjects submitted to acute and chronic salt-repletion/salt-depletion protocols. The goal was to compare the changes in proximal and distal sodium handling induced by the two procedures using the lithium clearance technique.

METHODS

In nine subjects, acute salt loading was obtained with a 2 h infusion of isotonic saline, and salt depletion was induced with a low-salt diet and furosemide. In the chronic protocol, 15 subjects randomly received a low-, a regular- and a high-sodium diet for 1 week. In both protocols, renal and systemic haemodynamics and urinary electrolyte excretion were measured after an acute water load. In the chronic study, sodium handling was also determined, based on 12 h day- and night-time urine collections.

RESULTS

The acute and chronic protocols induced comparable changes in sodium excretion, renal haemodynamics and hormonal responses. Yet, the relative contribution of the proximal and distal nephrons to sodium excretion in response to salt loading and depletion differed in the two protocols. Acutely, subjects appeared to regulate sodium balance mainly by the distal nephron, with little contribution of the proximal tubule. In contrast, in the chronic protocol, changes in sodium reabsorption could be measured both in the proximal and distal nephrons. Acute water loading was an important confounding factor which increased sodium excretion by reducing proximal sodium reabsorption. This interference of water was particularly marked in salt-depleted subjects.

CONCLUSION

Acute and chronic salt loading/salt depletion protocols investigate different renal mechanisms of control of sodium balance. The endogenous lithium clearance technique is a reliable method to assess proximal sodium reabsorption in humans. However, to investigate sodium handling in diseases such as hypertension, lithium should be measured preferably on 24 h or overnight urine collections to avoid the confounding influence of water.

Renin inhibition improves pressure natriuresis in essential hypertension

Pressure natriuresis (PN), i.e., a rise in renal sodium excretion in response to a higher BP, is involved in long-term BP regulation. PN is blunted in essential hypertension, but the mechanism is unknown. This study assessed the role of the renin-angiotensin-aldosterone system (RAAS) in PN in eight essential hypertensive men from the individual correlations between spontaneous fluctuations in BP and time corresponding changes in sodium excretion (collected at 2- and 4-h intervals for 48 h), during strict sodium balance, without treatment, and during renin inhibition (remikiren, 600 mg oral compound). Without treatment, daily values for mean arterial pressure were 109.5 +/- 1.9 and 107 +/- 1.9 mmHg, for urinary sodium excretion were 37.2 +/- 2.8 and 42.0 +/- 2.8 mmol/24 h, and for plasma renin activity were 2.34 +/- 0.48 and 2.23 +/- 0.44 nmol/L per h, respectively, for two consecutive days. During remikiren treatment, mean arterial pressure was 101.9 +/- 1.7 and 100.8 +/- 1. 7 mmHg (P: < 0.05, versus baseline). Urinary sodium excretion was 39. 3 +/- 3.7 and 45.2 +/- 5.3 mmol/24 h (not significant versus baseline), and plasma renin activity was 0.79 +/- 0.11 and 0.82 +/- 0.13 nmol/L per h (P: < 0.05 versus baseline). During remikiren treatment, BP correlated positively with sodium excretion in all patients but in only three of eight patients without treatment. The slope of the regression equation was steeper during remikiren treatment in seven of eight patients. Thus, the relationship between BP and natriuresis was more readily apparent during RAAS blockade, suggesting that RAAS activity blunts PN in hypertensive patients. Improved PN may contribute to the hypotensive effect of RAAS blockade and to maintenance of sodium balance at a lower BP level without volume expansion.

Enhanced antinatriuresis in response to angiotensin II in essential hypertension

Angiotensin II regulates sodium homeostasis by modulating aldosterone secretion, renal vascular response, and tubular sodium reabsorption. We hypothesized that the antinatriuretic response to angiotensin II is enhanced in human essential hypertension. We therefore studied 48 white men with essential hypertension (defined by ambulatory blood pressure measurement) and 72 normotensive white control persons, and measured mean arterial pressure, sodium excretion, renal plasma flow, glomerular filtration rate, and aldosterone secretion in response to angiotensin II infusion (0.5 and 3.0 ng/kg/min). Hypertensive subjects exhibited a greater increase of mean arterial pressure (16.7+/-8.2 mm Hg v 13.4+/-7.1 mm Hg in normotensives, P < .05) and a greater decrease of renal plasma flow (-151.5+/-73.9 mL/ min v -112.6+/-68.0 mL/min in controls, P < .01) when 3.0 ng/kg/min angiotensin II was infused. The increase of glomerular filtration rate and serum aldosterone concentration was similar in both groups. Sodium excretion in response to 3.0 ng/kg/min angiotensin II was diminished in both groups (P < .01). However, the decrease in sodium excretion was more pronounced in hypertensives than in normotensives (-0.18+/-0.2 mmol/min v -0.09+/-0.2 mmol/min, P < .05), even if baseline mean arterial pressure and body mass index were taken into account (P < .05). We conclude that increased sodium retention in response to angiotensin II exists in subjects with essential hypertension, which is unrelated to changes in glomerular filtration rate and aldosterone concentration. Our data suggest a hyperresponsiveness to angiotensin II in essential hypertension that could lead to increased sodium retention.

Pretreatment renal vascular tone predicts the effect of specific renin inhibition on natriuresis in essential hypertension

BACKGROUND

In essential hypertension an elevated renal vascular resistance (RVR) may be a marker of renin-angiotensin-aldosterone system-mediated impairment of renal sodium excretion. This hypothesis was tested by investigating whether, in subjects with essential hypertension, the natriuretic response to specific renin-angiotensin-aldosterone system (RAAS) blockade by renin-inhibitor remikiren could be predicted from pretreatment renal vascular tone.

MATERIALS AND METHODS

Renal hemodynamics, and the effects of single (n = 17) and multiple doses (n = 8, 8 days) of remikiren (600 mg day-1) on sodium excretion were studied under conditions of carefully controlled sodium balance.

RESULTS

Pretreatment renal vascular tone showed considerable individual differences: filtration fraction (FF) ranged from 21.2 to 30.3% and RVR from 18.8 to 33.5 10-2 mmHg min mL-1 in the single dose study, and FF from 20.8 to 24.9% and RVR from 14.8 to 28.8 10-2 mmHg min mL-1 in the multiple dose study. Remikiren induced a fall in blood pressure, FF and RVR, with considerable interindividual variability in natriuretic response. During single dose, cumulative sodium loss was 5.1 mmol per 5 h (-8.8 to +24.6), whereas after 8 days treatment cumulative sodium loss was 72 +/- 30 mmol (-46 to +187). The natriuretic response to remikiren during single as well as multiple dose significantly correlated with pretreatment renal vascular tone (estimated from FF and RVR) but not with remikiren-induced changes in renal hemodynamics or in hormonal parameters. Cumulative sodium loss was largest in patients with a higher pretreatment FF and RVR (r = 0.74, P < 0.001 and r = 0.52, P < 0.05, respectively, single dose; and r = 0.75, P < 0.05 and r = 0.73, P < 0.05, respectively, multiple dose).

CONCLUSION

These data support the hypothesis that in essential hypertension an elevated renal vascular tone is a marker of RAAS-mediated impairment of sodium excretion.

Impaired renal vascular response to a D1-like receptor agonist but not to an ACE inhibitor in conscious spontaneously hypertensive rats

The natriuretic response to a dopamine 1-like receptor agonist is blunted in spontaneously hypertensive rats (SHRs). Whether the renal vasodilator response to D1-like receptor stimulation in SHRs is defective also is unclear. To determine whether the renal hemodynamic response to a D1-like receptor is impaired in SHR, we examined the effect of a continuous infusion of the D1-like receptor agonist fenoldopam (2 microg/kg/min) on systemic and renal hemodynamics in conscious SHRs and Wistar-Kyoto (WKY) rats. As an active control, we used an equivalent antihypertensive dosage of captopril (10 mg/kg). Fenoldopam significantly increased effective renal plasma flow (ERPF) in WKY rats (+22 +/- 5%; p < 0.01), whereas this response was absent in SHRs (+7 +/- 3%; NS). Mean arterial pressure (MAP) was significantly reduced in SHRs (-11 +/- 2%; p < 0.001), demonstrating a systemic vasodilator response to fenoldopam in SHRs. The reduction in renal vascular resistance (RVR) was more pronounced in WKY rats (-24 +/- 2%) than in SHRs (-13 +/- 4%; p < 0.05). Captopril significantly increased ERPF in SHRs (+16 +/- 3%; p < 0.001), demonstrating a preserved renal vasodilatory capacity in SHRs. The blunting of the renal vasodilatory response to fenoldopam in SHRs is present during a high as well as a low sodium intake. In conscious SHRs, the renal vasodilatory response to a D1-like receptor agonist is impaired, whereas the blood pressure response is more pronounced. The preserved renal vasodilatory response to captopril indicates that the defective vasodilatory response in SHRs is functional rather than due to altered structural properties of the renal vascular bed.

The cellular ionic basis of hypertension and allied clinical conditions

Two central concepts of human hypertensive disease remain poorly understood: (1) elevated blood pressure as merely one component of an underlying systemic condition, characterized by multiple defects in diverse tissues (eg, "Syndrome X"), and (2) the heterogeneity of hypertension, in which different and even opposite clinical responses to different dietary and drug therapies are routinely observed among equally hypertensive subjects. To help explain these clinical phenomena, a unifying "ionic hypothesis" is proposed, in which steady-state elevations of cytosolic free calcium and suppressed intracellular free magnesium levels, characteristic features of all hypertension, concomitantly alter the function of many tissues. In blood vessels this causes vasoconstriction, arterial stiffness, and/or hypertension; in the heart, cardiac hypertrophy; in platelets, increased aggregation and thrombosis; in fat and skeletal muscle, insulin resistance; in pancreatic beta cells, other endocrine tissues, and sympathetic neurons, potentiated stimulus-secretion coupling resulting in hyperinsulinemia, increased sympathetic nerve activity, and so on. Furthermore, an analysis of cellular biochemical, dietary-nutrient, and hormonal factors that normally regulate steady-state levels of these intracellular ions suggests an ionic equivalent to Laragh's volume-vasoconstriction analysis of hypertension. This provides a cellular-based explanation for the heterogeneity of hypertension and a rational basis for individualizing dietary and drug recommendations among different hypertensive subjects.

The kidney in blood pressure regulation and development of hypertension

Systemic arterial pressure is a dynamic and responsive physiologic parameter that can be influenced by many different factors. In particular, short-term changes in arterial pressure are caused by a myriad of mechanisms that affect cardiac output, total peripheral resistance, and cardiovascular capacitance. In the long run, however, most of these actions can be buffered or compensated by appropriate renal adjustments of sodium balance, ECFV, and blood volume. As long as the mechanisms regulating sodium excretion can maintain sodium balance by appropriately modulating the sensitivity of the pressure-natriuresis relationship, normal arterial pressure can be sustained. Derangements that compromise the ability of the kidneys to maintain sodium balance, however, can result in the kidney's need for an elevated arterial pressure to reestablish net salt and water balance.