Salt sensitivity in blacks. Salt intake and natriuretic substances

The Interaction of Kidneys and Gut in Development of Salt-Sensitive Hypertension

The incidence of salt-sensitive hypertension is quite common and varies between 30-60% in hypertensive patients. Regarding the causal role of high salt intake in the development of salt-sensitive hypertension, recent evidence has demonstrated that the gut through its microbiota plays a significant role in its genesis. Besides the gut, the kidneys also play important role in salt-sensitive hypertension and there is clinical and experimental evidence of an interrelationship between the gut and the kidneys in the development of salt-sensitive hypertension through the so-called "gastro-renal axis." The gut besides being an absorptive organ, it is also a hormonal secretory organ involving the secretion of gastrin, dopamine, norepinephrine, angiotensin, and aldosterone which through their action with the kidneys are involved in the development of salt-sensitive hypertension. In addition, the kidneys exert a protective role against the development of hypertension through the secretion of prostaglandins and their vasodilatory action. To assess the current evidence on the role of high salt intake and the interplay of the gut and kidneys in its development, a Medline search of the English literature was contacted between 2012 and 2022, and 46 pertinent papers were selected. These papers together with collateral literature will be discussed in this review.

Renal Dopamine Receptors and Oxidative Stress: Role in Hypertension

Significance: The kidney plays an important role in the long-term control of blood pressure. Oxidative stress is one of the fundamental mechanisms responsible for the development of hypertension. Dopamine, via five subtypes of receptors, plays an important role in the control of blood pressure by various mechanisms, including the inhibition of oxidative stress. Recent Advances: Dopamine receptors exert their regulatory function to decrease the oxidative stress in the kidney and ultimately maintain normal sodium balance and blood pressure homeostasis. An aberration of this regulation may be involved in the pathogenesis of hypertension. Critical Issues: Our present article reviews the important role of oxidative stress and intrarenal dopaminergic system in the regulation of blood pressure, summarizes the current knowledge on renal dopamine receptor-mediated antioxidation, including decreasing reactive oxygen species production, inhibiting pro-oxidant enzyme nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase, and stimulating antioxidative enzymes, and also discusses its underlying mechanisms, including the increased activity of G protein-coupled receptor kinase 4 (GRK4) and abnormal trafficking of renal dopamine receptors in hypertensive status. Future Directions: Identifying the mechanisms of renal dopamine receptors in the regulation of oxidative stress and their contribution to the pathogenesis of hypertension remains an important research focus. Increased understanding of the role of reciprocal regulation between renal dopamine receptors and oxidative stress in the regulation of blood pressure may give us novel insights into the pathogenesis of hypertension and provide a new treatment strategy for hypertension.

Distinct inflammatory mediator patterns in young black and white adults: The African-predict study

OBJECTIVE

Inflammatory mediators have been implicated in the early stages of cardiovascular disease development, including hypertension. Since global reports reflect a higher hypertension prevalence in black than white populations, we hypothesise the involvement of specific inflammatory mediators. We therefore compared a detailed range of 22 inflammatory mediators between young black and white adults, and determined the relationship with blood pressure.

APPROACH AND RESULTS

We included 1197 adults (20-30 years; 50% black; 52% female) with detailed ambulatory blood pressures. Blood samples were analysed for 22 inflammatory mediators. For pro-inflammatory mediators, the black adults had higher C-reactive protein, interferon-inducible T-cell alpha chemoattractant, macrophage inflammatory protein 3 alpha (all p ≤ 0.008), but lower interferon-gamma, interleukin (IL)-1β, IL-8, IL-12, IL-17A, and tumour necrosis factor alpha (all p ≤ 0.048). For anti-inflammatory mediators the black group consistently had lower levels (IL-5, IL-10 and IL-13 (all p ≤ 0.012)), resulting in generally higher pro-to-anti-inflammatory ratios in black than white adults (p ≤ 0.001). In mediators with pro- and anti-inflammatory functions, the black group had lower granulocyte-macrophage colony-stimulating factor and IL-6 (both p ≤ 0.010). These patterns were confirmed after adjustment for age, sex and waist circumference, or when stratifying by hypertensive status, sex and socio-economic status. Multi-variable adjusted regression analyses and factor analysis yielded no relationship between inflammatory mediators and blood pressure in this young healthy population.

CONCLUSIONS

Black and white ethnic groups each consistently presented with unique inflammatory mediator patterns regardless of blood pressure, sex or social class. No association with blood pressure was seen in either of the groups.

Ethnic differences in hypertension prevalence and contributing determinants - the HELIUS study

AIMS

There are important ethnic differences in the prevalence of hypertension and hypertension-mediated cardiovascular complications, but there is ongoing debate on the nature of these differences. We assessed the contribution of lifestyle, socio-economic and psychosocial variables to ethnic differences in hypertension prevalence.

METHODS

We used cross-sectional data from the Healthy Life In an Urban Setting (HELIUS) study, including 21,520 participants aged 18-70 years of South-Asian Surinamese ( n = 3032), African Surinamese ( n = 4124), Ghanaian ( n = 2331), Turkish ( n = 3594), Moroccan ( n = 3891) and Dutch ( n = 4548) ethnic origin. Ethnic differences in hypertension prevalence rates were examined using logistic regression models.

RESULTS

After adjustment for a broad range of variables, significant higher hypertension prevalence compared to the Dutch population remained in Ghanaian men (odds ratio 2.62 (95% confidence interval 2.14-3.22)) and women (4.16 (3.39-5.12)), African Surinamese men (1.62 (1.37-1.92)) and women (2.70 (2.29-3.17)) and South-Asian Surinamese men (1.22 (1.15-1.46)) and women (1.84 (1.53-2.22)). In contrast, Turkish men (0.72 (0.60-0.87)) and Moroccan men (0.50 (0.41-0.61)) and women (0.57 (0.46-0.71)) had a lower hypertension prevalence compared with the Dutch population. The differences in hypertension prevalence were present across different age groups and persisted after stratification for body mass index and waist-to-hip ratio.

CONCLUSION

Large ethnic differences in hypertension prevalence exist that are already present in young adulthood. Adjustment for common variables known to be associated with a higher risk of hypertension explained the higher adjusted prevalence rates among Turks and Moroccans, but not in African and South-Asian descent populations who remained to have a higher rate of hypertension compared to the Dutch host population.

Dietary sodium intake and its relationship to adiposity in young black and white adults: The African-PREDICT study

Obesity and salt intake are both established factors contributing to cardiovascular disease development. Recently, studies found a controversial positive relationship between dietary salt and obesity. Therefore, the authors investigated whether obesity-related measures are associated with 24-hour urinary sodium in a healthy biethnic population. The study included 761 adults (20-30 years) with complete 24-hour urinary sodium, anthropometry, and bioelectrical impedance measurements. In single regression analyses all obesity-related measures related positively with 24-hour urinary sodium (P ≤ .008). However, with multivariate adjustments for energy intake, accelerometery, age, sex, black and white ethnicity, and other covariates, only body surface area (BSA) remained independently associated with 24-hour urinary sodium (R2  = 0.72, β = .05, P = .039). To conclude, we found a consistent and robust positive relationship between BSA and estimated salt intake - but not with traditional obesity measures such as body mass index (BMI). Further studies are needed to investigate body surface area and potentially, skin area, in salt handling.

Predictive factors for successful renal denervation: should we use them in clinical trials?

Renal denervation (RDN) is facing various challenges to its initial claimed value in hypertension treatment. Major concerns are the choice of the patients and the technical efficacy of the RDN. Different factors have been described as predicting the capacity of RDN to decrease blood pressure. These factors are related to the patients, the procedure and the tools to confirm successful neural ablation. Their use in future trials should help to improve RDN trials understanding and outcomes. This review summarizes the different predictive factors available and their potential benefits in patient selection and in procedure guidance.

Biomarkers for Chronic Kidney Disease Associated with High Salt Intake

High salt intake has been related to the development to chronic kidney disease (CKD) as well as hypertension. In its early stages, symptoms of CKD are usually not apparent, especially those that are induced in a “silent” manner in normotensive individuals, thereby providing a need for some kind of urinary biomarker to detect injury at an early stage. Because traditional renal biomarkers such as serum creatinine are insensitive, it is difficult to detect kidney injury induced by a high-salt diet, especially in normotensive individuals. Recently, several new biomarkers for damage of renal tubular epithelia such as neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule-1 (Kim-1) have been identified. Previously, we found a novel renal biomarker, urinary vanin-1, in several animal models with renal tubular injury. However, there are few studies about early biomarkers of the progression to CKD associated with a high-salt diet. This review presents some new insights about these novel biomarkers for CKD in normotensives and hypertensives under a high salt intake. Interestingly, our recent reports using spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) fed a high-salt diet revealed that urinary vanin-1 and NGAL are earlier biomarkers of renal tubular damage in SHR and WKY, whereas urinary Kim-1 is only useful as a biomarker of salt-induced renal injury in SHR. Clinical studies will be needed to clarify these findings.

Race-ethnicity on blood pressure control after ischemic stroke: a prospective cohort study

Disparities in health care access and socioeconomic status (SES) have been associated with racial-ethnic differences in blood pressure (BP) control. We examined post-ischemic stroke BP in a multiethnic cohort with good health care access. We included all hypertensive patients (n = 2972) from a randomized quality improvement trial on secondary stroke prevention, conducted in 14 Kaiser Permanente hospitals in Northern California from 2004-2006 (QUISP). Average age 73.2 ± 12.2 years; 52% female, 66% non-Hispanic white, 14% African-American, 11% Asian, 8% Hispanic, and 1% other. Demographics, diagnoses, health care utilization, BP measurements, and medications were obtained as part of routine care. We used random effects logistic regression models to examine race as a predictor of blood pressure control (<140/90 mm Hg) at 6 months post-discharge, adjusted for SES, age, gender, dementia, antihypertensive therapy, and attendance at follow-up visits. At 6 months, BP was controlled in 52.7% of blacks compared to 61.4% of whites (OR = 0.63, 95% CI, 0.48-0.82, P = .001). Black race remained independently associated with poorer BP control in adjusted analysis, although blacks were as likely to attend post-discharge visits, and more likely to be on any antihypertensive therapy than whites. Greater difficulty in controlling BP and lifestyle differences may account for this difference.

Salt-sensitive hypertension: mechanisms and effects of dietary and other lifestyle factors

Salt sensitivity, which is an increase in blood pressure in response to high dietary salt intake, is an independent risk factor for cardiovascular disease and mortality. It is associated with physiological, environmental, demographic, and genetic factors. This review focuses on the physiological mechanisms of salt sensitivity in populations at particular risk, along with the associated dietary factors. The interplay of mechanisms such as the renin-angiotensin aldosterone system, endothelial dysfunction, ion transport, and estrogen decrease in women contributes to development of salt sensitivity. Because of their effects on these mechanisms, higher dietary intakes of potassium, calcium, vitamin D, antioxidant vitamins, and proteins rich in L-arginine, as well as adherence to dietary patterns similar to the DASH (Dietary Approaches to Stop Hypertension) diet, can be beneficial to salt-sensitive populations. In contrast, diets similar to the typical Western diet, which is rich in saturated fats, sucrose, and fructose, together with excessive alcohol consumption, may exacerbate salt-sensitive changes in blood pressure. Identifying potential mechanisms of salt sensitivity in susceptible populations and linking them to protective or harmful dietary and lifestyle factors can lead to more specific guidelines for the prevention of hypertension and cardiovascular disease.

Maternal ethnicity and its impact on the haemodynamic and blood pressure response to labetalol for the treatment of antenatal hypertension

Objective

Blood pressure (BP) control outside pregnancy is associated with a reduction in adverse cardiovascular events, and in pregnancy with improved outcomes. Outside pregnancy, there is evidence β-blockers are less effective in controlling BP in black populations. However, in pregnancy, labetalol is recommended as a universal first-line treatment, without evidence for the impact of ethnicity on its efficacy. We sought to compare haemodynamic responses to labetalol in black and white pregnant patients.

Methods

This was a prospective observational cohort study in a London teaching hospital. Maternal haemodynamics were assessed in 120 pregnant women treated with labetalol monotherapy. Measurements were taken at presentation, 1 and 24 h after treatment. Participants were monitored regularly until delivery. Statistical analysis was performed by multilevel modelling.

Results

Both groups exhibited similar temporal trends in haemodynamic changes over the first 24 h following labetalol. Both showed a reduction in BP and peripheral vascular resistance within 1 h and in heart rate after 24 h. There was no change in cardiac output and stroke volume in either group. BP control (<140/90) was achieved at 1 h in 79.7% of the white and 77% of the black cohort. At 24 h, control was achieved among 83.1% and 63.9%, and up to the immediate intrapartum period control was achieved in 89.8% and 70.4% of white and black patients, respectively.

Conclusions

There is no difference in the acute haemodynamic changes and hypertension can be controlled throughout pregnancy with labetalol monotherapy in excess of 70% pregnant black and white patients.

Carotid baroreflex responsiveness in normotensive African Americans is attenuated at rest and during dynamic leg exercise

Evidence suggests differences between African Americans (AAs) and Caucasian Americans (CAs) in cardiovascular responsiveness to physiological stressors. This study tested the hypothesis that carotid baroreflex (CBR) control of heart rate (HR) and blood pressure is reduced in AAs compared to CAs during exercise. Mean arterial pressure (MAP) and HR were continuously recorded at rest and during leg cycling in 23 non-hypertensive male subjects (12 AA; 11 CA; age 19–26 years). CBR control of HR and MAP was assessed with 5-s pulses of neck pressure (NP, simulated hypotension) and neck suction (NS, simulated hypertension) ranging from +45 to −80 Torr. Across all NS stimuli (−20, −40, −60, −80 Torr) at rest, the AA group demonstrated attenuated CBR-mediated reductions in HR (AA, −8.9 ± 1.9 vs. CA, −14.1 ± 2.3 bpm; P < 0.001) and MAP (AA, −6.4 ± 1 vs. CA, −7.8 ± 0.8 mmHg; P < 0.05). Despite similar gain and magnitude of resetting observed in the modeled stimulus response curves, an attenuation among AAs persisted in HR (AA, −8.2 ± 1.6 vs. CA, −11.8 ± 3 bpm; P < 0.05) and MAP (AA, −6.8 ± 0.9 vs. CA, −8.2 ± 1.1 mmHg; P < 0.05) responses to NS during exercise. No differences in CBR-mediated HR and MAP responses to NP were detected between groups at rest or during exercise. These data suggest impairment in the ability to defend against a hypertensive challenge among AAs during steady-state exercise compared to their CA counterparts.

Dietary sodium loading in normotensive healthy volunteers does not increase arterial vascular reactivity or blood pressure

BACKGROUND

Studies of dietary sodium on vascular function and blood pressure in normotensive volunteers have shown conflicting results. There are very limited data available on the effect of chronic sodium loading from a low-sodium diet to a high-sodium diet on vascular function and blood pressure in normotensive volunteers.

OBJECTIVE

To assess the effect of modifying dietary sodium intake on arterial function and surrogate markers of arterial remodelling in normal healthy volunteers.

DESIGN

Twenty-three normotensive volunteers met the inclusion criteria. After a 2 week run-in with a low-sodium diet (60 mmol/day), the participants maintained their low-sodium diets and were randomly assigned to receive sequentially one of three interventions for 4 weeks, with a 2 week washout between interventions: sodium-free tomato juice (A), tomato juice containing 90 mmol Na (B) and tomato juice containing 140 mmol Na (C). The outcomes measured were changes in pulse wave velocity (PWV), systolic blood pressure and diastolic blood pressure.

RESULTS

There was no difference in PWV between interventions (B-A 0.00 m/s, 95% CI: -0.30, 0.31 m/s; C-A 0.01 m/s, 95% CI: -0.38, 0.40 m/s). There was also no change in pulse wave analysis, systolic or diastolic blood pressure between interventions. There was an appropriate increase in urinary sodium excretion in the added sodium interventions.

CONCLUSION

Dietary salt loading did not produce significant increases in PWV and blood pressure in normotensive subjects with systolic blood pressure <130 mmHg. The lack of an observed effect supports Guyton's pressure-natriuresis hypothesis with appropriate renal excretion of the excess sodium load.

Carotid baroreflex responsiveness is impaired in normotensive African American men

There are important differences in autonomic function and cardiovascular responsiveness between African Americans (AA) and Caucasian Americans (CA). This study tested the hypothesis that carotid baroreflex (CBR) responsiveness is impaired in normotensive AA compared with normotensive CA at rest. CBR control of heart rate (HR) and mean arterial blood pressure (MAP) was assessed in 30 nonhypertensive male subjects (15 AA; 15 CA; age 18-33 yr) with 5-s periods of neck pressure (NP; simulated hypotension) and neck suction (NS; simulated hypertension) ranging from +45 to -80 Torr during rest. Carotid-cardiac stimulus-response curves revealed a significantly lower minimum HR response in the CA compared with AA (40.8 ± 2.4 vs. 49.8 ± 2.9 beats/min, respectively; P < 0.05). In addition, the magnitude of the mean HR response to all trials of NS (-20, -40, -60, and -80 Torr) was attenuated in the AA group (AA, -10.1 ± 1.7 vs. CA, -14.9 ± 2.2 beats/min; P < 0.05), while no significant differences were found in the magnitude of the mean HR response to NP (+15, +30, and +45 Torr) between racial groups. There were no significant differences in the carotid-vasomotor stimulus-response curves between racial groups. Also, while no racial differences were found in the magnitude of the mean MAP response to all trials of NS, the magnitude of the mean MAP response to all trials of NP was attenuated in the AA group (AA, 7.2 ± 1.3 vs. CA, 9.3 ± 1.1 mmHg; P < 0.05). Together, these findings support inherent differences in short-term blood pressure regulation between racial groups that exhibit different relative risk for the development of hypertension.

Higher aldosterone and lower N-terminal proatrial natriuretic peptide as biomarkers of salt sensitivity in the community

BACKGROUND

Salt sensitivity, a trait characterized by a pressor blood pressure response to increased dietary salt intake, has been associated with higher rates of cardiovascular target organ damage and cardiovascular disease events. Recent experimental studies have highlighted the potential role of the natriuretic peptides and aldosterone in mediating salt sensitivity.

DESIGN

Prospective cohort study.

METHODS

We evaluated 1575 non-hypertensive Framingham Offspring cohort participants (mean age 55 ± 9 years, 58% women) who underwent routine measurements of circulating aldosterone and N-terminal proatrial natriuretic peptide (NT-ANP) and assessment of dietary sodium intake. Participants were categorized as potentially 'salt sensitive' if their serum aldosterone was >sex-specific median but plasma NT-ANP was ≤sex-specific median value. Dietary sodium intake was categorized as lower versus higher (dichotomized at the sex-specific median). We used multivariable linear regression to relate presence of salt sensitivity (as defined above) to longitudinal changes (Δ) in systolic and diastolic blood pressure on follow-up (median four years).

RESULTS

Participants who were 'salt sensitive' (N = 437) experienced significantly greater increases in blood pressure (Δ systolic, +4.4 and +2.3 mmHg; Δ diastolic, +1.9 and -0.3 mmHg; on a higher versus lower sodium diet, respectively) as compared to the other participants (Δ systolic, +2.8 and +1.0 mmHg; Δ diastolic, +0.5 and -0.2 mmHg; on higher versus lower sodium diet, respectively; P = 0.033 and P = 0.0127 for differences between groups in Δ systolic and Δ diastolic blood pressure, respectively).

CONCLUSIONS

Our observational data suggest that higher circulating aldosterone and lower NT-ANP concentrations may be markers of salt sensitivity in the community. Additional studies are warranted to confirm these observations.

Characteristics of sodium sensitivity in Korean populations

Sodium sensitivity (SS) is a variable response of blood pressure (BP) to changes in sodium intake. The present study evaluated the existence and the characteristics of subjects with SS in Koreans. One hundred one subjects with (n = 31, 57.7 ± 9.8 yr) or without hypertension (n = 70, 40.8 ± 16.5 yr) were given a low-sodium dietary approache to stop hypertension (DASH) diet (LSD) for 7 days and a high-sodium DASH diet (HSD) for the following 7 days. The prevalence of SS in the present study population was 27.7% (17.6% in the non-hypertensive subjects and 51.6% in the hypertensive subjects). Analysis of the non-hypertensive subjects showed that systolic BP, diastolic BP, and mean arterial pressure at baseline and after HSD were higher in the subjects with SS than the subjects without SS, and there were no differences after LSD. In the hypertensive subjects, there was no difference in the BP at baseline and after HSD whether or not the subjects had SS. However, the systolic BP of hypertensive subjects with SS was lower than hypertensive subjects without SS after LSD. In the present study population, subjects with SS have distinctive BP features unlike to subjects without SS.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Genetics of salt-sensitive hypertension

The assessment of salt sensitivity of blood pressure is difficult because of the lack of universal consensus on definition. Regardless of the variability in the definition of salt sensitivity, increased salt intake, independent of the actual level of blood pressure, is also a risk factor for cardiovascular morbidity and mortality and kidney disease. A modest reduction in salt intake results in an immediate decrease in blood pressure, with long-term beneficial consequences. However, some have suggested that dietary sodium restriction may not be beneficial to everyone. Thus, there is a need to distinguish salt-sensitive from salt-resistant individuals, but it has been difficult to do so with phenotypic studies. Therefore, there is a need to determine the genes that are involved in salt sensitivity. This review focuses on genes associated with salt sensitivity, with emphasis on the variants associated with salt sensitivity in humans that are not due to monogenic causes. Special emphasis is given to gene variants associated with salt sensitivity whose protein products interfere with cell function and increase blood pressure in transgenic mice.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

Salt sensitivity is associated with insulin resistance, sympathetic overactivity, and decreased suppression of circulating renin activity in lean patients with essential hypertension

BACKGROUND

The mechanisms by which a derangement of glucose metabolism causes high blood pressure are not fully understood.

OBJECTIVES

This study aimed to clarify the relation between salt sensitivity of blood pressure and insulin resistance, which are important subcharacteristics of hypertension and impaired glucose metabolism, respectively. Effects on the renin-angiotensin and sympathetic nervous systems were also studied.

DESIGN

The state of glucose metabolism was assessed by a hyperinsulinemic euglycemic glucose clamp technique and a 75-g oral-glucose-tolerance test in 24 essential hypertensive patients who were lean and without diabetes or chronic kidney disease. The subjects were classified as salt-sensitive or salt-resistant on the basis of the difference (Delta mean blood pressure > or =5%) between 24-h ambulatory blood pressure monitoring results on the seventh day of low-salt (34 mmol/d) and high-salt (252 mmol/d) diets. Urine and blood samples were collected for analyses.

RESULTS

There was a robust inverse relation between the glucose infusion rate (GIR) and the salt sensitivity index. The GIR correlated directly with the change in urinary sodium excretion and was inversely related to the change in hematocrit when the salt diet was changed from low to high, which is indicative of salt and fluid retention in salt-sensitive subjects. The GIR also showed an inverse correlation compared with the changes in urinary norepinephrine excretion, plasma renin activity, and plasma aldosterone concentration.

CONCLUSIONS

Salt sensitivity of blood pressure is strongly associated with insulin resistance in lean, essential hypertensive patients. Hyperinsulinemia, sympathetic overactivation, and reduced suppression of the renin-angiotensin system may play a role in this relation.

Urinary dopamine excretion and renal responses to fenoldopam infusion in blacks and whites

Dopamine is an endogenous natriuretic amine that contributes to the maintenance of sodium homeostasis. Deficiencies in the renal production of dopamine and the action of dopamine on renal tubular receptors have been observed in human hypertension and may contribute to salt sensitivity of blood pressure. Ethnic differences in the sodium-to-dopamine relationship may contribute to the higher prevalence of salt sensitivity in blacks. The authors assessed dopaminergic activity in two studies. In the first, daytime and nighttime excretion of sodium and dopamine were compared in 11 black and 17 white normotensive patients. No racial difference in the rate of sodium or dopamine excretion during either period was observed. In the second study, a graded infusion of the dopamine-1 receptor agonist, fenoldopam, was performed in 14 black and 17 white normotensive patients. There was no racial difference in the natriuretic responses. Previously described lower rates of renal free water clearance and potassium excretion in blacks compared with whites were maintained during fenoldopam infusion, suggesting that dopamine is not a mediator of those differences. The authors conclude that there are no race-related differences in dopamine excretion or activity in normotensive patients.

Sodium, blood pressure, and ethnicity: what have we learned?

An enormous amount of research has yielded significant knowledge about ethnic differences in sodium homeostasis and blood pressure regulation. Consistent findings such as greater sodium-sensitivity, lower potassium excretion and high higher serum sodium levels in African Americans need further exploration to define more precise physiological mechanisms. The genetic alleles associated with sodium homeostasis in relation to blood pressure have accounted for only a small proportion of the variance in blood pressure. Several allelic variants differ in frequency among ethnic groups and heat-adapted genetic variants have a high prevalence in low latitudes and hot, wet climates which lends support to the "sodium retention" hypothesis. The blood pressure disparities between African Americans and whites may, in part, be due to different allelic frequencies of genes associated with sodium homeostasis. However, with advances in genomics, environmental factors tend to be neglected in research. Better measures of environmental stress have recently been developed by anthropologists and should be included in research designs by investigators in other disciplines. Public health efforts should encourage food producers to reduce sodium content of its products, and physicians should encourage patients to reduce consumption of high sodium packaged and fast foods.

Renal dopaminergic defect in C57Bl/6J mice

The C57Bl/6J mouse strain, the genetic background of many transgenic and gene knockout models, is salt sensitive and resistant to renal injury. We tested the hypothesis that renal dopaminergic function is defective in C57Bl/6J mice. On normal NaCl (0.8%, 1 wk) diet, anesthetized and conscious (telemetry) blood pressures were similar in C57Bl/6J and SJL/J mice. High NaCl (6%, 1 wk) increased blood pressure (approximately 30%) in C57Bl/6J but not in SJL/J mice and urinary dopamine to greater extent in SJL/J than in C57Bl/6J mice. Absolute and fractional sodium excretions were lower in SJL/J than in C57Bl/6J mice. The blood pressure-natriuresis plot was shifted to the right in C57Bl/6J mice. Renal expressions of D(1)-like (D(1)R and D(5)R) and angiotensin II AT(1) receptors were similar on normal salt, but high salt increased D(5)R only in C57Bl/6J. GRK4 expression was lower on normal but higher on high salt in C57Bl/6J than in SJL/J mice. Salt increased the excretion of microalbumin and 8-isoprostane (oxidative stress marker) and the degree of renal injury to a greater extent in SJL/J than in C57Bl/6J mice. A D(1)-like receptor agonist increased sodium excretion whereas a D(1)-like receptor antagonist decreased sodium excretion in SJL/J but not in C57Bl/6J mice. In contrast, parathyroid hormone had a similar natriuretic effect in both strains. These results show that defective D(1)-like receptor function is a major cause of salt sensitivity in C57Bl/6J mice, decreased renal dopamine production might also contribute. The relative resistance to renal injury of C57Bl/6J may be a consequence of decreased production of reactive oxygen species.

Functional genomics of the dopaminergic system in hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. Under normal conditions, D(1)-like receptors (D(1) and D(5)) inhibit sodium transport in the kidney and intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats (SHRs) and in humans with essential hypertension, the D(1)-like receptor-mediated inhibition of epithelial sodium transport is impaired because of an uncoupling of the D(1)-like receptor from its G protein/effector complex. The uncoupling is receptor specific, organ selective, nephron-segment specific, precedes the onset of hypertension, and cosegregates with the hypertensive phenotype. The defective transduction of the renal dopaminergic signal is caused by activating variants of G protein-coupled receptor kinase type 4 (GRK4: R65L, A142V, A486V). The GRK4 locus is linked to and GRK4 gene variants are associated with human essential hypertension, especially in salt-sensitive hypertensive subjects. Indeed, the presence of three or more GRK4 variants impairs the natriuretic response to dopaminergic stimulation in humans. In genetically hypertensive rats, renal inhibition of GRK4 expression ameliorates the hypertension. In mice, overexpression of GRK4 variants causes hypertension either with or without salt sensitivity according to the variant. GRK4 gene variants, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic factors (e.g., angiotensin II type 1 receptor) to predominate, may be responsible for salt sensitivity. Subclasses of hypertension may occur because of additional perturbations caused by variants of other genes, the quantitative interaction of which may vary depending upon the genetic background.

Update on disparities in the pathophysiology and management of hypertension: focus on African Americans

Hypertension treatment and control is of paramount importance in the prevention of premature cardiovascular disease. African Americans present a special challenge to the clinician due, in part, to their earlier age of onset,greater prevalence, and increased rates of untoward events. A review of the recent studies of genetic epidemiology has not revealed unique genotypes that explain human hypertension or the disparate impact suffered by African Americans. Moreover, a clear message has emerged that environmental factors predominate in their effect on cardiovascular risk and are mutable. These findings suggest that to have an immediate and substantial impact on the ethnic disparity of hypertension, resources and research should be directed toward social and behavioral factors. Prompt and aggressive control of blood pressure is an effective global strategy for cardiovascular risk reduction. In most cases, this approach requires multiple interventions including lifestyle modification and an antihypertensive regimen that is tailored to the individual under the current guidelines and not stipulated by race.

Influence of two doses of irbesartan on non-dipper circadian blood pressure rhythm in salt-sensitive black hypertensives under high salt diet

The authors examined whether the blockage of angiotensin II receptors by irbesartan (IRB) can reverse the "non-dipper" circadian rhythm of blood pressure (BP) to a "dipper" pattern in black salt-sensitive hypertensive patients submitted to a high-sodium loading. Twelve black salt-sensitive hypertensive patients (seven men; age, 35-58 years) on a high-sodium diet (300 mmol Na+ per day) were followed for 8 weeks. A placebo was given during the first 2 weeks, followed by 2 weeks on IRB 150 mg/d, 2 weeks on placebo, and 2 weeks on IRB 300 mg/d. On the last day of placebo, IRB 150 mg/d, and IRB 300 mg/d treatments, 24-hour BP and urinary 24-hour excretion of Na+ and potassium were measured. On placebo, ambulatory mean arterial pressure (MAP) was 112 mm Hg+/-2 (24 h), 112 mm Hg+/-2 (daytime), and 111 mm Hg+/-2 (nighttime), showing a clear circadian non-dipper profile. Versus placebo, IRB 150 mg/d reduced MAP by 4.2 mm Hg+/-1.1 (24 h), 2.6 mm Hg+/-0.8 (daytime) and 6.0 mm Hg+/-1.3 (nighttime; P<0.05 vs. placebo) and IRB 300 mg/d reduced MAP by 7.8 mm Hg+/-1.4 (24 h), 3.9 mm Hg+/-1.1 (daytime), and 11.8 mm Hg+/-2.1 mm Hg (all P<0.02 vs. placebo); nighttime/daytime MAP decrease was 0.7+/-0.8% on placebo, 3.5+/-2.1% on IRB 150 mg/d, and 7.0+/-1.2% on IRB 300 mg/d (P<0.02 for trend). Compared with placebo, IRB significantly increased serum potassium and plasma renin activity and reduced fractional excretion of potassium and plasma aldosterone levels in a dose-dependent manner. Body weight and urinary sodium excretion did not change throughout the study. It was concluded that the angiotensin receptor blocker IRB can reverse the BP non-dipper profile in salt-sensitive hypertensive patients on a high-salt diet, restoring nocturnal BP decline by a predominantly dose-dependent reduction of nighttime BP. Although the increment of potassium balance and reduction of aldosterone may account for this effect, it occurs independently of increased natriuresis. It is speculated that blunting of nighttime BP decrease in black salt-sensitive hypertensive patients may be related to a deficient suppression of the renin-angiotensin system during the night.

The renin-angiotensin system in blacks: active, passive, or what?

Plasma renin activity in blacks has been consistently reported to be lower than in whites. Many mechanisms for the low plasma renin activity have been proposed, including volume status, renal sodium handling, and the reduction of renin release. The status of the RAS is paramount in the regulation of salt and water balance and its implications in disease processes such as hypertension and renal failure. In this review, we present data to suggest that low systemic plasma renin activity in blacks may not be the primary abnormality, but rather the reflection of an overactive RAS at the tissue level in the kidney.

Dopamine and the kidney: a role in hypertension?

PURPOSE OF REVIEW

Defective transduction of the dopamine receptor signal in the kidney has been shown to be important in the pathogenesis of hypertension This review will discuss the genetic mechanism for the defective renal dopaminergic function and the interaction with other gene variant products in the pathogenesis of salt sensitivity and essential hypertension.

RECENT FINDINGS

Single nucleotide polymorphisms of G protein-coupled receptor kinase type 4 (GRK4) phosphorylate, desensitize, and diminish the inhibitory action of D receptors on sodium transport in the kidney. Inhibition of GRK4 expression normalizes renal proximal tubule D receptor function in humans and rodents and ameliorates the hypertension in genetically hypertensive rats. Expression of the GRK4 variant, GRK4gammaA142V, produces hypertension and impairs the natriuretic effect of D receptor stimulation in mice. In humans, GRK4 single nucleotide polymorphisms are associated with essential hypertension, particularly salt sensitive hypertension. The prediction of the hypertensive phenotype is most accurate when elements of the renin-angiotensin system and GRK4 are included in the analysis.

SUMMARY

GRK4 single nucleotide polymorphisms, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic function of angiotensin II type 1 receptors to predominate, may be responsible for salt sensitivity. Hypertension develops with additional perturbations caused by the variants of other genes (e.g., alpha-adducin, angiotensin converting enzyme, angiotensinogen, angiotensin II type 1 receptor, aldosterone synthase, 11beta-hydroxysteroid dehydrogenase type 2), the quantitative interaction of which may vary depending upon the genetic background.

Dietary electrolytes and hypertension in the elderly

The effect of age on older hypertensive patient's blood pressure response to increased sodium intake is well known. Salt sensitivity which does increase with age and the decrease in renal function limiting the ability of aged kidney to excrete sodium load are major factors, responsible for rise in blood pressure during Na consumption in the elderly. Clinical studies encourage salt reduction with and without weight loss. Although potassium consumption is highly recommended, one should be aware of potassium overload in the elderly.

Diet and arterial hypertension: is the sodium ion alone important?

Hypertension is a widespread phenomenon whose ultimate cause is still unknown. Many factors contribute to this disease, and partially for this reason, hypertension responds to different treatments in different individuals. It is difficult to generalize about therapies for general populations. In particular, the role of electrolytes in hypertension varies widely across individuals. This review focuses its attention on sodium, potassium, calcium, and magnesium ions in order to investigate whether these electrolytes play a role in the pathogenesis of arterial hypertension and its treatment. Some individuals are especially sensitive to sodium, and changing their intake of dietary sodium may lead to variations in the levels of the other electrolytes. These changes in electrolyte levels can complicate treatments for arterial hypertension in some patients.

Pathophysiology of chronic progressive renal disease in the African American patient with hypertension

Chronic renal failure and ESRD are major causes of morbidity, mortality, and chronic disability in patients in the United States. Hypertension is a major underlying cause of chronic progressive renal disease and continues to be a leading reason for the heavy burden of ESRD observed in African Americans. Hypertension is actually a syndrome of vascular pathology manifesting itself in patients by a constellation of common findings and attributes. These pathophysiologic alterations include dysregulation of arterial compliance, endothelial dysfunction, obesity and insulin resistance, abnormal sympathetic nervous system activation, accelerated atherosclerosis, left ventricular hypertrophy, and a propensity for increased vascular thrombogenesis among others. This review will focus on some of the important mechanisms possibly involved in the progression of renal disease in the setting of chronic hypertension.

Natural bioactive substances in milk and colostrum: effects on the arterial blood pressure system

High blood pressure is a significant public health problem worldwide which is associated with increased risk of cardiovascular disease, stroke, and renal disease. The development of this disease is influenced by genetic and environmental factors. The results of many studies have linked increased consumption of milk and milk products with lower blood pressure and reduced risk of hypertension. The intake of several minerals found in milk has been demonstrated to have an inverse relationship with blood pressure. Peptides formed during the digestion of milk proteins have also been demonstrated to have a blood pressure lowering effect. Other components in milk that have been examined for their effects on blood pressure have been less promising. More recent data indicate that a dietary pattern that is low in fat, with fruits, vegetables, and low fat dairy products can significantly reduce blood pressure and lower risk of developing high blood pressure.

Intrarenal dopamine: a key signal in the interactive regulation of sodium metabolism

The kidney regulates sodium metabolism with extraordinary precision and sensitivity. This is accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between anti-natriuretic and natriuretic factors. Dopamine, produced in renal proximal tubule cells, plays a central role in this interactive network. Natriuretic hormones that are released from extrarenal sources, such as atrial natriuretic peptide, mediate some of their effects via renal dopamine receptors. On the level of the tubules, dopamine acts by opposing the effects of anti-natriuretic factors, such as angiotensin II and alpha-adrenergic receptors. Sodium retention leads to an increase in renal dopamine tonus, and the natriuretic effects of dopamine are more prominent under this condition. Inhibition or down-regulation of dopamine receptors significantly attenuates the natriuretic response to salt loading. Renal dopamine is modulated by the supply of filtered L-DOPA and the metabolism of dopamine via catechol-O-methyldopamine. The importance of dopamine as a natriuretic hormone is reflected by its capacity to inhibit the majority of renal tubule sodium transporters. Notably, the activity of Na+, K+ ATPase is inhibited in most tubule segments by dopamine. Recent studies have elucidated many of the signaling pathways for renal dopamine receptors. Novel principles for homologous and heterologous sensitization of dopamine receptors have been detected that may explain some of the interaction between dopamine and other first messengers that modulate renal tubule sodium transport. A broad understanding of the renal dopamine system has become increasingly important, since there is now strong evidence from both clinical and experimental studies that dysregulation of the renal dopamine system plays a role in many forms of multigenetic hypertension.

Hypertension in black patients: an emerging role of the endothelin system in salt-sensitive hypertension

The prevalence of essential hypertension in blacks is much higher than that in whites. In addition, the pathogenesis of hypertension appears to be different in black patients. For example, black patients present with a salt-sensitive hypertension characterized by low renin levels. Racial differences in renal physiology and socioeconomic factors have been suggested as possible causes of this difference, but reasons for this difference remain unclear. Endothelial cells are important in the regulation of vascular tonus and homeostasis, in part through the secretion of vasoactive substances. One of these factors, endothelin-1 (ET-1), is a 21 amino acid residue peptide with potent vasopressor actions. In addition to its contractile effects, it has been shown to stimulate mitogenesis in a number of cell types. Moreover, ET-1 displays modulatory effects on the endocrine system, including stimulation of angiotensin II and aldosterone production and inhibition of antidiuretic hormone in the kidney. Recent data from several laboratories indicate that ET-1 is overexpressed in the vasculature in several salt-sensitive models of experimental hypertension. Moreover, circulating plasma ET-1 levels are significantly increased in black hypertensives compared with white hypertensives. Thus, the ET system might be particularly important in the development or maintenance of hypertension in this population.

Renal dopaminergic mechanisms in renal parenchymal diseases, hypertension, and heart failure

The recovery of renal function in renal transplant recipients is accompanied by an enhanced ability to synthesize dopamine (DA), which may contribute to maintain sodium homeostasis. Patients suffering from chronic renal parenchymal disease, a well-recognized form of salt sensitive (SS) hypertension, have a reduced ability to produce DA that correlates well with deterioration of renal function. In patients afflicted with IgA nephropathy, but normal renal function, urinary excretion of DA correlated positively with BP responses to changes from 200 to 20 mmol/day salt intake. In black salt resistant (SR) normotensives (NT) and SR hypertensives, under low salt intake (40 mmol/day), but not SS-NT and SS-HT, the saline infusion induced increments of DA and DOPAC urinary excretion correlated significantly with increments of sodium urinary excretion and sodium fractional excretion. Patients afflicted with heart failure (HF) have a reduced delivery of L-DOPA to the kidney, accompanied by an increase in DA/L-DOPA urinary ratios. This suggests that HF patients have an increased ability to take up or decarboxylate L-DOPA. Sodium restriction resulted in a significant decrease in urinary L-DOPA, DA and DOPAC in HF patients, suggesting that the system responds to sodium. It is concluded that activity of renal dopaminergic system may be altered in SS subjects, despite the level of their BP, and an enhanced delivery of L-DOPA to the kidney may be beneficial in edema formation states.

Deficiency of renal dopaminergic-dependent natriuretic response to acute sodium load in black salt-sensitive subjects in contrast to salt-resistant subjects

OBJECTIVE

To evaluate the involvement of the renal dopaminergic system in the natriuretic responses to acute saline load in salt-resistant (SR) and salt-sensitive (SS) black normotensive (NT) and hypertensive (HT) subjects.

DESIGN AND METHODS

We studied the relationship between the urinary excretion of dopa, dopamine (DA) and its metabolite DOPAC and the natriuretic responses to acute volume expansion (2 l NaCl 0.9% over 2 h) in 20 black NT subjects (12 SR and 8 SS) and 19 black HT subjects (10 SS and 9 SR). Subjects received a low salt (LS) diet (40 mmol sodium/day) for 1 week and a high salt (HS) diet (300 mmol sodium/day) for 1 week; the sequence of the dietary regimens was randomized. Comparisons were made between the results before the saline infusion (baseline) and the results 2 h after the infusion.

RESULTS

In all the groups saline infusion induced significant increases in urinary volume (ml/4 h) of two- to three-fold and in urinary sodium excretion (mmol/4 h) of three- to ten-fold; these increases were significantly greater during the HS diet than during the LS diet. Saline infusion significantly increased the mean arterial pressure (MAP) by 5 mmHg in HT-SS subjects and by 4-5 mmHg in NT-SS subjects, but the MAP did not changed in the NT-SR and HT-SR groups. Under the LS diet, saline infusion changed the DA excretion (in nmol/4 h) by -49+/-89 in HT-SS subjects, by 17+/-52 in NT-SS subjects, by 235+/-72 in HT-SR subjects and by 220+/-86 in NT-SR subjects (P < 0.05 between SR and SS subjects). The saline infusion-induced changes in DA excretion correlated significantly with the increases in urinary sodium excretion (r = 0.71, P < 0.01) in the NT-SR and HT-SR subjects under the LS diet, but not in the SR groups on the HS diet nor in the SS groups (HT and NT) on either diet. Saline infusion significantly reduced the DA/dopa ratio in SS (NT and HT) but not SR (NT and HT) subjects, whereas the DA/DOPAC (dihydroxyphenylacetic acid) ratios were similar in all the groups.

CONCLUSIONS

The urinary dopaminergic system may participate in the natriuretic responses to acute sodium load only in SR subjects (NT and HT) and only under LS diets, but not in SS subjects (NT and HT). This strongly suggests that black NT- and HT-SS subjects have an underlying impairment in the activity of the renal dopaminergic system which may be associated with a reduced decarboxylation of dopa into DA.

Acute hypotensive, natriuretic, and hormonal effects of nifedipine in salt-sensitive and salt-resistant black normotensive and hypertensive subjects

In a randomized double-blind study, we compared the short-term effects of nifedipine (10 mg 3x daily for 1 day) versus placebo on 24-h blood pressure, diuresis, natriuresis, urinary excretion of dopamine and metabolites, and on plasma renin activity (PRA) and plasma aldosterone levels in 18 black hypertensive (HT) patients [eight salt-resistant (HT-SR) and 10 salt-sensitive (HT-SS)], and in 20 black normotensive (NT) subjects (12 NT-SR and eight NT-SS) who were studied randomly with both a high- (HS) and a low-salt (LS) diet. In comparison to placebo, nifedipine significantly decreased 24-h mean BP in all groups either with HS or LS diets (all p<0.05). With HS, greater hypotensive effects were achieved in NT-SS (-10+/-2 mm Hg) versus NT-SR (-3+/-1 mm Hg; p<0.05) and in HT-SS (-18+/-2 mm Hg) versus HT-SR (-12+/-2 mm Hg; p<0.05). In NT-SS and HT-SS, nifedipine induced greater (p<0.05) BP decrease with HS (-10+/-2 and -18+/-2 mm Hg) than with LS (-4+/-1 and -9+/-1 mm Hg, respectively), whereas in NT-SR and HT-SR, the hypotensive effect did not differ between HS and LS. Nifedipine versus placebo significantly increased natriuresis and fractional excretion of sodium in all groups only with HS (p<0.05) but not with LS diets. Only in HT-SS were the hypotensive and natriuretic effects of nifedipine significantly correlated (r = -0.77; p<0.01). Nifedipine produced a similar increase of the urinary excretion of dopamine, L-DOPA, and of DOPAC in all subjects, which did not correlate with hypotensive and natriuretic effects. Nifedipine did not modify plasma levels of renin and of aldosterone except in NT-SS with HS, in whom nifedipine increased PRA levels (p <0.05). We conclude that although nifedipine reduces BP in all groups of NT and HT with LS and HS diets, the effect is greater in salt-sensitive subjects with HS. Although in HT-SS with HS, the short-term natriuretic response to nifedipine may contribute to its hypotensive effects, the diuretic-natriuretic effect of nifedipine is not necessary for the expression of its hypotensive effect. Moreover, it is unlikely that any short-term effects of nifedipine either on the renal dopaminergic system or on the secretion of aldosterone explain nifedipine short-term hypotensive and diuretic-natriuretic effects.

Dietary salt reduction in hypertension--what is the evidence and why is it still controversial?

The link between sodium intake and hypertension remains controversial because of inconsistency between early epidemiologic studies, which showed a strong positive relationship between salt intake and blood pressure/incidence of hypertension, and more recent studies, which showed only modest decreases in blood pressure with sodium reduction, particularly in the normotensive population. In addition, there is clinical evidence that sodium is related to target organ damage such as left ventricular hypertrophy and renal disease. Although the evidence available linking sodium intake and blood pressure in the general population is weak, sodium reduction has been shown to be useful in hypertensive patients, particularly salt-sensitive patients. Whether dietary sodium reduction should be recommended for the general population remains questionable because of marginal benefit and the suggestion of possible deleterious effects on cardiovascular outcomes independent of blood pressure. This paper will review the definition and methods used in determining salt sensitivity, the evidence linking sodium intake and target organ damage, and modern studies of salt and blood pressure.

Normotensive salt sensitivity: effects of race and dietary potassium

-Normotensive salt sensitivity, a putative precursor of hypertension, might be quite frequent in African Americans (blacks) and less frequent in Caucasian Americans (whites), but only when dietary potassium is deficient and not when maintained well within the normal range. We tested this hypothesis in 41 metabolically controlled studies of 38 healthy normotensive men (24 blacks, 14 whites) who ate a basal diet low in sodium (15 mmol/d) and marginally deficient in potassium (30 mmol/d) for 6 weeks. Throughout the last 4 weeks, NaCl was loaded (250 mmol/d); throughout the last 3, potassium was supplemented (as potassium bicarbonate) to either mid- or high-normal levels, 70 and 120 mmol/d. Salt sensitivity, defined as an increase in mean arterial blood pressure >/=3 mm Hg with salt loading, was deemed "moderate" if increasing Collapse

Key Words Collapse

MESH Headings

• Adult
• Aged
• Bicarbonates/administration & dosage
• Black People
• Blood Pressure
• Data Interpretation, Statistical
• Humans
• Hypertension/etiology
• Hypertension/prevention & control
• Linear Models
• Male
• Middle Aged
• Potassium Compounds/administration & dosage
• Potassium Deficiency/complications
• Potassium, Dietary/administration & dosage
• Racial Groups
• Sodium Chloride, Dietary/administration & dosage
• Sodium Chloride, Dietary/adverse effects
• White People
• Black or African American

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Grants

• HL-47943 NHLBI NIH HHS
• M01-00079 PHS HHS

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Affiliation(s)

R C Morris Department of Medicine, General Clinical Research Center, University of California, San Francisco 94143-0126, USA.
A Sebastian
A Forman
M Tanaka
O Schmidlin

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Collaborators Collapse

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Influence of race and dietary salt on the antihypertensive efficacy of an angiotensin-converting enzyme inhibitor or a calcium channel antagonist in salt-sensitive hypertensives

Dietary salt restriction is a recommended adjunct with antihypertensive therapy. There may be racial differences in blood pressure response to salt restriction while on antihypertensive therapy. We performed a multicenter, randomized, double-blind, placebo-controlled, parallel-group clinical trial (black, n=96; Hispanic, n=63; white, n=232). Participants were initially preselected for stage I to III hypertension and then further selected for salt sensitivity (> or = 5 mm Hg increase in diastolic blood pressure after 3 weeks of low salt [< or = 88 mmol/d Na+] and high salt [>190 mmol/d Na+] diet). We compared the antihypertensive effect of an angiotensin-converting enzyme inhibitor (enalapril 5 or 20 mg BID) or a calcium channel antagonist (isradipine 5 or 10 mg BID) during alternating periods of high and low salt intake. The main outcome measure was blood pressure change and absolute blood pressure level achieved with therapy. During the high salt diet (314.7+/-107.5 mmol/d urinary Na+) there was greater downward change in blood pressure with both enalapril and isradipine compared with the low salt diet (90.1+/-50.8 mmol/d Na+); however, the absolute blood pressure achieved in all races was consistently lower on a low salt diet for both agents. Black, white, and Hispanic isradipine-treated salt-sensitive hypertensives demonstrated a smaller difference between high and low salt diets (black, -3.6/-1.6 mmHg; white, -6.2/-3.9 mmHg; Hispanic, -8.1/-5.3 mm Hg) than did enalapril-treated patients (black, -9.0/-5.3 mm Hg; white, -11.8/-7.0 mm Hg; Hispanic, -11.1/-5.6 mm Hg). On the low salt diet, blacks, whites, and Hispanics had similar blood pressure control with enalapril and isradipine. On the high salt diet, blacks had better blood pressure control with isradipine than with enalapril, whereas there was no difference in the blood pressure control in whites and Hispanics treated with either drug. Dietary salt reduction helps reduce blood pressure in salt-sensitive hypertensive blacks, whites, and Hispanics treated with enalapril or isradipine. These data demonstrate that controlling for salt sensitivity diminishes race-related differences in antihypertensive activity.

Hypertension hemodynamics

The hemodynamic factors in hypertension should be evaluated in terms of early versus late stages, autoregulation versus amplifying mechanisms, and arterial compliance versus arteriolar vasoconstrictive responses. In addition, evaluation of hemodynamic changes in hypertension should include the role of vascular endothelium, genetic factors, volume factors, salt intake, vascular reactivity, presence or absence of left ventricular hypertrophy or left ventricular diastolic dysfunction, and finally the presence of left ventricular systolic failure. Whether therapy directed by knowledge of hemodynamic profiling will be more efficacious or more cost-effective than standard therapy in reducing morbidity and mortality of hypertension needs to be proven.

Reduced dietary potassium reversibly enhances vasopressor response to stress in African Americans

Acute vasopressor responses to stress are adrenergically mediated and hence potentially subject to differential modulation by dietary potassium and sodium. The greater vasopressor responsiveness in blacks compared with whites might then be consequent not only to a high dietary salt intake but also to a marginally reduced dietary potassium intake. Under controlled metabolic conditions, we compared acute vasopressor responses to cold and mental stress in black and white normotensive men during three successive dietary periods: (1) while dietary potassium was reduced (30 mmol K+/70 kg per day) and salt was restricted (10 to 14 days); (2) while salt was loaded (15 to 250 mmol Na+/70 kg per day) (7 days); and (3) while salt loading was continued and potassium was either supplemented (70 mmol K+/70 kg per day) (7 to 21 days) in 9 blacks and 6 whites or continued reduced (30 mmol K+/70 kg per day) (28 days) in 4 blacks (time controls). At the lower potassium intake, cold-induced increase in forearm vascular resistance in blacks was twice that in whites during both salt restriction and salt loading. Normalization of dietary potassium attenuated cold-induced increases in both forearm vascular resistance and systolic and diastolic blood pressures in blacks but only in systolic pressure in whites. In blacks but not in whites, normalization of dietary potassium attenuated mental stress-induced increases in systolic and diastolic pressures. In normotensive blacks but not whites, a marginally reduced dietary intake of potassium reversibly enhances adrenergically mediated vasopressor responsiveness to stress. That responsiveness so enhanced over time might contribute to the pathogenesis of hypertension in blacks.