Increased insulin resistance in salt sensitive essential hypertension

Salt-Sensitivity of Blood Pressure and Insulin Resistance

Salt sensitivity of blood pressure (SSBP) is an independent risk factor for cardiovascular morbidity and mortality that is seen in both hypertensive and normotensive populations. Insulin resistance (IR) strongly correlates with SSBP and affects nearly 50% of salt sensitive people. While the precise mechanism by which IR and SSBP relate remains elusive, several common pathways are involved in the genesis of both processes, including vascular dysfunction and immune activation. Vascular dysfunction associated with insulin resistance is characterized by loss of nitric oxide (NO)-mediated vasodilation and heightened endothelin-1 induced vasoconstriction, as well as capillary rarefaction. It manifests with increased blood pressure (BP) in salt sensitive murine models. Another common denominator in the pathogenesis of insulin resistance, hypertension, and salt sensitivity (SS) is immune activation involving pro-inflammatory cytokines like tumor necrosis factor (TNF)-α, IL-1β, and IL-6. In the last decade, a new understanding of interstitial sodium storage in tissues such as skin and muscle has revolutionized traditional concepts of body sodium handling and pathogenesis of SS. We have shown that interstitial Na⁺ can trigger a T cell mediated inflammatory response through formation of isolevuglandin protein adducts in antigen presenting cells (APCs), and that this response is implicated in salt sensitive hypertension. The peroxisome proliferator-activated receptor γ (PPARγ) is a transcription factor that modulates both insulin sensitivity and BP. PPARγ agonists increase insulin sensitivity and ameliorate salt sensitivity, whereas deficiency of PPARγ results in severe insulin resistance and hypertension. These findings suggest that PPARγ plays a role in the common pathogenesis of insulin sensitivity and salt sensitivity, perhaps via effects on the immune system and vascular function. The goal of this review is to discuss those mechanisms that may play a role in both SSBP and in insulin resistance.

Association study of fasting blood glucose and salt sensitivity of blood pressure in community population: The EpiSS study

BACKGROUND AND AIMS

To evaluate the association between fasting blood glucose (FBG) and salt sensitivity of blood pressure (SSBP).

METHODS AND RESULTS

This study is based on the baseline survey of systemic epidemiology of salt sensitivity study. Subjects were classified into salt sensitive (SS) and salt resistant groups according to blood pressure (BP) changes during the modified Sullivan's acute oral saline load and diuresis shrinkage test. Multivariate logistic and linear regression were used to evaluate associations between FBG with SS or BP changes. A total of 2051 participants were included in the analyses with 581 (28.33%) for SS. Multiple analysis showed that for every interquartile range increase in FBG, the OR (95%CI) for SS was 1.140 (1.069, 1.215), β (95%CI) for mean arterial pressure change (ΔMAP₁), systolic and diastolic BP changes during saline load were 0.421 (0.221, 0.622), 0.589 (0.263, 0.914) and 0.340 (0.149, 0.531), respectively. Compared to the lowest FBG quartile (Q₁), the OR (95%CI) for SS in Q₃ and Q₄ were 1.342 (1.014, 1.776) and 1.577 (1.194, 2.084), respectively. Compared to subjects with normal FBG, the β (95%CI) for ΔMAP₁ was 0.973 (0.055, 1.891) in subjects with impaired FBG, and was 1.449 (0.602, 2.296) in patients with diabetes mellitus. Stratified analyses showed significant and stronger associations between FBG with SSBP in youngers, females, hypertensives, non-diabetics, non-current smokers and non-current drinkers.

CONCLUSION

Our findings suggest FBG is an independent, dose-dependent associated factor for SSBP, and prevention of SS focusing on controlling FBG elevation in the early stage is important.

24-h urinary sodium excretion and the risk of adverse outcomes

AIMS

The objective was to evaluate whether sodium intake, assessed with the gold standard 24-h urinary collections, was related to long-term incidence of death, cardiovascular disease (CVD) and diabetes mellitus (DM).

METHODS

A cohort of 4630 individuals aged 25-64 years collected 24-h urine samples in 1979-2002 and were followed up to 14 years for the incidence of any CVD, coronary heart disease (CHD), stroke, heart failure (HF) and DM event, and death. Cox proportional hazards models were used to estimate the association between the baseline salt intake and incident events and adjusted for baseline age, body mass index, serum cholesterol, prevalent DM, and stratified by sex and cohort baseline year.

RESULTS

During the follow-up, we observed 423 deaths, 424 CVD events (288 CHD events, 142 strokes, 139 HF events) and 161 DM events. Compared with the highest quartile of salt intake, persons in the lowest quartile had a lower incidence of CVD (hazard ratio [HR] 0.70; 95% confidence interval [CI], 0.51-0.95, p = .02), CHD (HR 0.63 [95% CI 0.42-0.94], p = .02) and DM (HR 0.52 [95% CI 0.31-0.87], p = .01). The results were non-significant for mortality, HF, and stroke.

CONCLUSION

High sodium intake is associated with an increased incidence of CVD and DM.

Cardiotonic Steroids Induce Vascular Fibrosis Via Pressure-Independent Mechanism in NaCl-Loaded Diabetic Rats

Endogenous cardiotonic steroid, marinobufagenin (MBG), induces Fli1-dependent tissue fibrosis. We hypothesized that an increase in MBG initiates the development of aortic fibrosis in salt-loaded rats with type 2 diabetes mellitus (DM2) via pressure-independent mechanism. DM2 was induced by a single intraperitoneal administration of 65 mg/kg streptozotocin to neonatal (4-5 days) male Wistar rats. Eight-week-old DM2 rats received water or 1.8% NaCl (DM-NaCl) solution for 4 weeks (n = 16); half of DM-NaCl rats were treated with anti-MBG monoclonal antibody (mAb) (DM-NaCl-AB) during week 4 of salt loading; control intact rats received water (n = 8/group). Blood pressure, MBG, erythrocyte Na/K-ATPase activity, aortic weights, levels of fibrosis markers (Fli1, protein kinase Cδ, transforming growth factor-β1, receptors of the transforming growth factor beta5, fibronectin, collagen-1), and sensitivity of the aortic explants to the vasorelaxant effect of sodium nitroprusside were assessed. No changes in systolic blood pressure were observed while erythrocyte Na/K-ATPase was inhibited by 30%, plasma MBG was doubled, and aortic markers of fibrosis became elevated in DM-NaCl rats versus control. Treatment of DM-NaCl rats with anti-MBG mAb activated Na/K-ATPase, prevented increases in aortic weights, and the levels of fibrosis markers returned to the control levels. The responsiveness of the aortic rings from DM-NaCl rats to the relaxant effect of sodium nitroprusside was reduced (half maximal effective concentration (EC50) = 29 nmol/L) versus control rings (EC50 = 7 nmol/L) and was restored by anti-MBG mAb (EC50 = 9 nmol/L). Our results suggest that in salt-loaded diabetic rats, MBG stimulates aortic collagen synthesis in a pressure-independent fashion and that 2 profibrotic mechanisms, Fli1 dependent and transforming growth factor-β dependent, underlie its effects.

Dietary sodium and potassium and risk of diabetes: A prospective study using data from the China Health and Nutrition Survey

AIMS

Dietary sodium and potassium intakes are well-known risk factors for cardiovascular outcomes. However, the associations between dietary sodium and potassium and diabetes are still controversial. Our study aimed to examine whether dietary sodium, potassium and the sodium-potassium ratio are associated with the risk of diabetes, based on a large sample of Chinese adults.

METHODS

The study data were from the 2004-2009 China Health and Nutrition Survey (CHNS), and 5867 participants were eligible for analysis. Sodium and potassium intakes were estimated based on three consecutive 24-h recalls at an individual level combined with a food inventory at a household level performed over the same 3-day period. Diabetes was defined as fasting glucose ≥7.0mmol/L (≥126mg/dL), HbA_1c ≥6.5% or use of antidiabetic drugs.

RESULTS

Over a mean follow-up of 4.7 years, there were 611 (10.4%) incident cases of diabetes. Participants in the higher quartiles (Q3 and Q4) of sodium intake had significantly higher risks of diabetes than those with the lowest sodium intake [Q3, RR: 1.41, 95% CI: 1.06-1.86 and Q4, RR: 1.35, 95% CI: 1.02-1.80; P<0.001 for trend]. In addition, high sodium intakes were significantly associated with levels of fasting glucose and HbA_1c (P<0.05 for trend), with similar associations also found with sodium-potassium ratios (P<0.05 for trend), but not for potassium intakes.

CONCLUSION

This study found that higher sodium intakes and sodium-potassium ratios were significantly associated with a higher risk of diabetes. Further clinical research is now necessary to confirm these results.

Various Effects of Omega 3 and Omega 3 Plus Vitamin E Supplementations on Serum Glucose Level and Insulin Resistance in Patients with Coronary Artery Disease

BACKGROUND

Omega 3 and vitamin E are two critical nutrients which include beneficial effects in coronary artery disease (CAD). The aim of this study was to assess the effects of omega 3 alone supplementation or in combination with vitamin E on serum glucose and lipid levels and insulin resistance in CAD patients.

METHODS

Participants of this clinical trial included 60 male patients with CAD who selected from Tehran Heart Center in Tehran, Iran in 2014. They received 4 g/day omega 3 plus 400 IU/day vitamin E (OE), 4 g/day omega 3 with vitamin E placebo (OP), or omega 3 and vitamin E placebo (PP) for two months. Serum glucose, lipids and insulin were assessed and HOMA-IR was calculated before and after the trial and effects of these nutrients on the highlighted parameters were compared within the study groups.

RESULTS

Serum glucose level increased significantly in OP group (P=0.004), but not in OE group. OE and OP groups showed a significant decrease in fasting serum TG (P=0.020 and P=0.001, respectively). Serum insulin and HOMA-IR decreased significantly in OE group (P=0.044 and P=0.039, respectively) but did not change significantly in OP group.

CONCLUSION

Although, omega 3 supplementation may include adverse effects on serum glucose level, co-administration of omega 3 and vitamin E can beneficially decrease serum insulin and insulin resistance in CAD patients.

Evaluating the impact of type 2 diabetes mellitus on cardiovascular risk in persons with metabolic syndrome using the UKPDS risk engine

BACKGROUND

The aim of this study is to evaluate the impact of coexistence of metabolic syndrome (MS) and type 2 diabetes mellitus (T2DM) on the estimated cardiovascular risk as calculated using the United Kingdom Prospective Diabetic Study risk engine (UKPDS-RE) and also to determine the impact of the coexistence of MS and T2DM on the 10-year risk of developing coronary heart disease and stroke.

METHODOLOGY

This is a cross-sectional study in which convenience sampling technique was used to recruit 124 consecutive persons with T2DM and 96 controls using a questionnaire administered technique. The World Health Organization (WHO) criterion was used to define MS and the UKPDS-RE was used to identify persons with increased risk for stroke and those with increased risk for coronary heart disease. The data obtained were analyzed using SPSS version 16. Statistical comparisons were made with chi-square for comparison of proportions. A P-value of less than 0.05 was taken as statistically significant.

RESULTS

Fifteen subjects were identified as having an increased 10-year risk for stroke and ten as having an increased risk for a coronary event. The odds of a T2DM subject with MS having an increased risk for stroke compared with a T2DM subject without MS was 0.9579≈1 while the odds of a T2DM subject with MS developing an increased risk for a coronary event compared with a T2DM subject without MS was =3.451≈3.

CONCLUSION

MS was more common in subjects with T2DM compared with controls (irrespective of the diagnostic criteria used) and MS appears to increase the risk of a coronary event in subjects with T2DM by threefold. Also from this study, MS did not appear to cause an additional increase in the risk of stroke in subjects with T2DM.

Metabolic syndrome: Performance of five different diagnostic criterias

BACKGROUND

The aim of this study is to describe the metabolic syndrome (MS) and to evaluate five diagnostic criteria of the MS with respect to their sensitivity and specificity in patients with type 2 diabetes mellitus (T2DM).

MATERIALS AND METHODS

It is a cross-sectional case control study of T2DM patients and their first degree relatives (FDRs) recruited using convenience sampling and data collected through questionnaire administered technique. Variables of interest included anthropometric indices, blood pressure, serum lipid profile, fasting blood sugar (FBS), proteinuria, and microalbuminuria. The Chi-square test was used for comparison of proportions. A P value of less than 0.05 was taken as statistically significant. Kappa statistic was used to test the degree of agreement between the diagnostic criteria.

RESULTS

The World Health Organization (WHO), International Diabetes Federation (IDF), revised National Cholesterol Education Program (NCEP-R), NCEP Adult Treatment Panel (ATP)-III, and American Association of Clinical Endocrinologists (AACE) criteria reported a prevalence of 87.1, 64.5, 61.3, 55.6, and 22.6%, respectively in persons with T2DM. Using the WHO criteria as a reference or gold standard, the sensitivity of the IDF, NCEP-R, NCEP ATP-III, and AACE criteria among persons with T2DM were 71.3, 67.6, 61.1, and 25.9% respectively. Using the WHO criteria as a reference or gold standard, the specificity of the IDF, NCEP-R, NCEP ATP-III, and AACE criteria among persons with T2DM were 81.3, 81.3, 81.3, and 100%, respectively. Using the WHO criteria as a reference or gold standard, the level of agreement of the IDF, NCEP-R, NCEP ATP-III, and AACE criteria with the WHO criteria among persons with T2DM (as estimated by the kappa statistics) were 0.30, 0.26, 0.21, and 0.08 respectively.

CONCLUSION

THE LEVEL OF AGREEMENT APPEARS TO BE GENERALLY POOR, THOUGH THE IDF CRITERIA SHOWED A FAIR LEVEL OF AGREEMENT WITH THE WHO CRITERIA: Therefore the IDF criteria is recommended for screening of the MS in persons with T2DM because of its ease of application and its level of agreement with the WHO criteria being the best compared to the other three criteria.

Nonmodulation as the mechanism for salt sensitivity of blood pressure in individuals with hypertension and type 2 diabetes mellitus

CONTEXT

It is assumed that in individuals with type 2 diabetes mellitus (T2DM), blood pressure sensitivity to salt intake and the frequency of a low renin state are both increased compared with the nondiabetic population. However, studies supporting these assumptions may have been confounded by participant inclusion criteria, and study results may reflect target organ damage.

OBJECTIVE

The objective of this study was to examine in a cohort of T2DM 1) the frequency of salt sensitivity of blood pressure and 2) whether alterations of the renin-angiotensin-aldosterone system (RAAS) contribute to salt sensitivity in this population.

DESIGN, PATIENTS, AND METHODS

Within participants of the HyperPATH cohort, four groups were analyzed: 1) T2DM with hypertension (HTN), n=51; 2) T2DM without HTN, n=30; 3) HTN only, n=451; and 4) normotensive, n=209. Phenotype studies were conducted after participants completed two dietary phases: liberal sodium (200 mmol/d) and low sodium (10 mmol/d) for 7 d each. Participants were admitted overnight to a clinical research center after each diet, and supine measurements of the RAAS before and after a 60-min angiotensin II infusion (3 ng/kg·min) were obtained.

RESULTS

Multivariate regression analysis demonstrated that T2DM status (all individuals with T2DM vs. individuals without T2DM) was not associated with the change in mean arterial pressure between the low and liberal sodium diets after accounting for age, gender, body mass index, race, and baseline blood pressure (T2DM status, P=0.5). Furthermore, two intermediate phenotypes of altered RAAS, low renin, and nonmodulation (NMOD), were associated with salt-sensitive blood pressure but occurred at different frequencies in the T2DM-HTN and HTN groups (low renin, 12% T2DM-HTN vs. 29% HTN; NMOD, 41% T2DM-HTN vs. 27% HTN; P=0.01).

CONCLUSION

The frequency of NMOD in participants with T2DM was significantly higher compared with HTN, suggesting that the salt sensitivity often seen in T2DM is driven by NMOD.

The antihypertensive effect of arginine

Hypertension is a leading cause of morbidity and mortality worldwide. Individuals with hypertension are at increased risk of stroke, heart disease and kidney failure. Although the etiology of essential hypertension has a genetic component, lifestyle factors such as diet play an important role. Reducing dietary salt is effective in lowering blood pressure in salt-sensitive individuals. Insulin resistance and altered glucose metabolism are common features of hypertension in humans and animal models, with or without salt sensitivity. Altered glucose metabolism leads to increased formation of advanced glycation end products. Insulin resistance is also linked to oxidative stress, and alterations in the nitric oxide pathway and renin angiotensin system. A diet rich in protein containing the semiessential amino acid, arginine, and arginine treatment, lowers blood pressure in humans and in animal models. This may be due to the ability of arginine to improve insulin resistance, decrease advanced glycation end products formation, increase nitric oxide, and decrease levels of angiotensin II and oxidative stress, with improved endothelial cell function and decreased peripheral vascular resistance. The Dietary Approaches to Stop Hypertension (DASH) study demonstrated that the DASH diet, rich in vegetables, fruits and low-fat dairy products; low in fat; and including whole grains, poultry, fish and nuts, lowered blood pressures even more than a typical North American diet with similar reduced sodium content. The DASH diet is rich in protein; the blood pressure-lowering effect of the DASH diet may be due to its higher arginine-containing protein, higher antioxidants and low salt content.

The antihypertensive effect of cysteine

Hypertension is a leading cause of morbidity and mortality worldwide. Individuals with hypertension are at an increased risk for stroke, heart disease and kidney failure. Essential hypertension results from a combination of genetic and lifestyle factors. One such lifestyle factor is diet, and its role in the control of blood pressure has come under much scrutiny. Just as increased salt and sugar are known to elevate blood pressure, other dietary factors may have antihypertensive effects. Studies including the Optimal Macronutrient Intake to Prevent Heart Disease (OmniHeart) study, Multiple Risk Factor Intervention Trial (MRFIT), International Study of Salt and Blood Pressure (INTERSALT) and Dietary Approaches to Stop Hypertension (DASH) study have demonstrated an inverse relationship between dietary protein and blood pressure. One component of dietary protein that may partially account for its antihypertensive effect is the nonessential amino acid cysteine. Studies in hypertensive humans and animal models of hypertension have shown that N-acetylcysteine, a stable cysteine analogue, lowers blood pressure, which substantiates this idea. Cysteine may exert its antihypertensive effects directly or through its storage form, glutathione, by decreasing oxidative stress, improving insulin resistance and glucose metabolism, lowering advanced glycation end products, and modulating levels of nitric oxide and other vasoactive molecules. Therefore, adopting a balanced diet containing cysteine-rich proteins may be a beneficial lifestyle choice for individuals with hypertension. An example of such a diet is the DASH diet, which is low in salt and saturated fat; includes whole grains, poultry, fish and nuts; and is rich in vegetables, fruits and low-fat dairy products.

Blunted renal dopaminergic system in a mouse model of diet-induced obesity

Obesity has reached epidemic proportions in the Western world and is implicated in the pathophysiology of essential hypertension. The aim of the present study was to evaluate sodium handling, blood pressure and renal dopaminergic system activity in a mouse model of obesity induced by exposure to a hypercaloric diet. From six to 18 weeks of age, animals were fed with a control diet or a high-fat high-simple-carbohydrate (HFHSC) diet. Renal function, blood pressure and urinary and plasmatic catecholamines and biochemical parameters were evaluated in both groups. In parallel, the effects of high sodium intake (HS, 1.0% NaCl, 3 days) on natriuresis, urinary catecholamine excretion and aromatic l-amino acid decarboxylase (AADC) activity were evaluated in control and obese mice. Mice exposed to the HFHSC diet presented obesity, hyperglycemia, glucose intolerance, insulin resistance, hyperinsulinemia and increased blood pressure. This was accompanied, in obese mice, by decreases in urinary excretion of dopamine and metabolites as well as reduced AADC activity in renal tissues. During HS intake, absolute urinary dopamine excretion increased in control, but not in obese mice. This was accompanied in obese mice by a natriuretic resistance on day 1 of the HS diet. In addition, obese mice presented increased urinary and plasmatic noradrenaline levels, as well as an increased heart rate when compared with control mice. In conclusion, in this model of diet-induced obesity hyperinsulinemia, insulin resistance and increased sympathetic tone are associated with blunted renal dopaminergic activity. It is suggested that this may contribute to compromised sodium excretion and increased blood pressure in obesity.

Metabolic syndrome and oxidative stress

Metabolic syndrome is an obesity-associated collection of disorders, each of which contributes to cardiovascular risk. Metabolic syndrome is also associated with overproduction of reactive oxygen species (ROS). ROS contribute to the interrelationship between metabolic syndrome and salt-sensitive hypertension, which are both caused by obesity and excess salt consumption and are major threats to health in developed countries. ROS can induce insulin resistance, which is indispensable for the progression of metabolic syndrome, and salt-sensitive hypertension stimulates ROS production, thereby promoting the development of metabolic syndrome. Moreover, ROS activate mineralocorticoid receptors (MRs) and the sympathetic nervous system, which can contribute to the development of metabolic syndrome and salt-sensitive hypertension. Salt-induced progression of cardiovascular disease (CVD) is also accelerated in animal models with metabolic syndrome, probably owing to further stimulation of ROS overproduction and subsequent ROS-induced MR activation and sympathetic excitation. Therefore, ROS contribute to the progression of the metabolic syndrome itself and to the CVD accompanying it, particularly in conjunction with excessive salt consumption.

Potassium, calcium, magnesium, and sodium levels in biological samples of hypertensive and nonhypertensive diabetes mellitus patients

There is accumulating evidence that the metabolism of several essential elements is altered in diabetes mellitus and that these nutrients might have specific roles in the pathogenesis and progress of this disease. The aim of the present study was to compare the level of essential elements, potassium (K), calcium (Ca), magnesium (Mg), and sodium (Na), in biological samples (whole blood, urine, and scalp hair) of patients who have hypertensive diabetes mellitus type 2 (n = 254) and nonhypertensive diabetes mellitus type 2 (n = 228) with those of nondiabetic as control subjects (n = 182; age range of both genders 45-75). The element concentrations were measured by means of an atomic absorption spectrophotometer after microwave-induced acid digestion. The validity and accuracy was checked by conventional wet acid digestion method and using certified reference materials. The overall recoveries of all elements were found in the range of 99.1-99.9% of certified values. The results of this study showed that the mean values of K, Mg, and Ca were significantly reduced, while Na level were higher in blood and scalp hair samples of hypertensive diabetic (HD) patients and nonhypertensive diabetic (NHD) patients as compared to control subjects of both genders (p < 0.05), but level of K in the biological samples of nonhypertensive diabetic patient was found to be higher, but it was not significant (p = 0.05).The urinary levels of these elements were found to be higher in both HD and NHD patients than in the age-matched healthy controls. These results are consistent with those obtained in other studies, confirming that deficiency and efficiency of some essential trace metals may play a role in the development of diabetes mellitus.

Pathophysiological Mechanisms of Salt-Dependent Hypertension

Changes in salt intake are associated in general with corresponding changes in arterial blood pressure. An exaggerated increment in blood pressure driven by a salt load is characteristic of salt-sensitive hypertension, a condition affecting more than two thirds of individuals with essential hypertension who are older than 60 years. In the last decade, significant insight was gained about the role of the kidney in the increment in blood pressure induced by sodium retention. The present review focuses on the pathophysiological characteristics of the blood pressure increase driven by expansion of extracellular fluid and the increment in plasma sodium concentration. In addition, we discuss systemic and renal conditions that result in decreased urinary sodium excretion and were implicated in the development of salt-sensitive hypertension.

Microvascular function: a potential link between salt sensitivity, insulin resistance and hypertension

OBJECTIVE

Generalized microvascular dysfunction may contribute to the development of salt sensitivity, insulin resistance and hypertension, and may thus link these cardiovascular risk factors. To test this hypothesis, we examined skin microvascular function, salt sensitivity, insulin sensitivity and blood pressure in 27 normotensive and 26 hypertensive individuals.

METHODS

Capillary density was examined by videomicroscopy during venous congestion and postocclusive reactive hyperaemia. Endothelium-(in)dependent vasodilation was assessed by iontophoresis of acetylcholine and sodium nitroprusside and by laser Doppler flowmetry. Salt sensitivity was determined as the difference in mean arterial pressure (MAP) between a 1-week high-salt diet (approximately 235 mmol NaCl/day) versus low-salt diet (approximately 55 mmol NaCl/day). Insulin sensitivity was measured with the hyperinsulinaemic, euglycaemic clamp, and blood pressure was assessed by 24-h ambulatory blood pressure monitoring.

RESULTS

Salt sensitivity of blood pressure was inversely associated with postocclusive capillary recruitment and endothelium-dependent vasodilation (r=-0.67, P<0.001 and r=-0.60, P<0.01, respectively), but not with capillary density during venous congestion or endothelium-independent vasodilation. Salt sensitivity was negatively associated with insulin sensitivity (r=-0.55, P<0.001) and positively with MAP (r=0.58, P<0.001). Multiple regression analyses suggested that associations between salt sensitivity and both insulin sensitivity and MAP were dependent on microvascular function.

CONCLUSION

Our results suggest a close inverse association between skin microvascular function and salt sensitivity and a role for generalized microvascular defects as a link between salt sensitivity, insulin resistance and hypertension.

Mechanical forces in diabetic kidney disease: a trigger for impaired glucose metabolism

Nephropathy is one of the major microvascular complications of diabetes, and both hemodynamic and metabolic stimuli participate in its development and progression toward ESRD. There is now a greater understanding of the molecular pathways that are activated by high glomerular capillary pressure and hyperglycemia and how they interplay to produce kidney pathology. The observation that overexpression of glucose transporter 1 (GLUT-1) in mesangial cells could induce a "diabetic cellular phenotype" has led to the postulation that the expression of GLUT-1 could be upregulated in glomeruli that are exposed to high pressure. This review suggests a mechanism by which mechanical forces may aggravate a metabolic insult by stimulating excessive cellular glucose uptake. Proposed is the existence of a self-maintaining cycle whereby a hemodynamic stimulus on glomerular cells induces GLUT-1 overexpression followed by greater glucose uptake and activation of intracellular glucose metabolic pathways, resulting in excess TGF-beta1 production. TGF-beta1 in turn, maintains overexpression of GLUT-1, perpetuating a signaling sequence that has, as its ultimate effect, increased extracellular matrix synthesis. This mechanical and metabolic coupling suggests a novel pathophysiologic mechanism of injury in the kidney in diabetes and possibly other glomerular diseases.

The antinatriuretic effect of insulin: an unappreciated mechanism for hypertension associated with insulin resistance?

Insulin resistance is proposed to be causally related to the metabolic syndrome disorders, but a direct cause-and-effect relationship between insulin resistance and hypertension was not originally obvious. Previous data suggested that insulin promotes sodium retention from the kidney, and thus research efforts focused on this action among several other possible pathways connecting insulin resistance and hyperinsulinemia with hypertension. A review of numerous studies provides evidence that this antinatriuretic effect of insulin is preserved in states of metabolic insulin resistance, representing a major mechanism for blood pressure elevation. More recent experimental and clinical studies have added data about the exact tubular sites of this insulin action, its relation with the respective insulin action on potassium handling, its possible role in the development of salt sensitivity in essential hypertension, as well as the involvement of oxidant stress in these associations. This review summarizes the current state of knowledge in this area and attempts to highlight an important but rather overlooked pathway for hypertension development in the metabolic syndrome, the influence of high insulin levels leading to volume expansion.

Serum- and glucocorticoid-inducible kinase 1 mediates salt sensitivity of glucose tolerance

Excess salt intake decreases peripheral glucose uptake, thus impairing glucose tolerance. Stimulation of cellular glucose uptake involves phosphatidylinositide-3-kinase (PI-3K)-dependent activation of protein kinase B/Akt. A further kinase downstream of PI-3K is serum- and glucocorticoid-inducible kinase (SGK)1, which is upregulated by mineralocorticoids and, thus, downregulated by salt intake. To explore the role of SGK1 in salt-dependent glucose uptake, SGK1 knockout mice (sgk1(-/-)) and their wild-type littermates (sgk1(+/+)) were allowed free access to either tap water (control) or 1% saline (high salt). According to Western blotting, high salt decreased and deoxycorticosterone acetate (DOCA; 35 mg/kg body wt) increased SGK1 protein abundance in skeletal muscle and fat tissue of sgk1(+/+) mice. Intraperitoneal injection of glucose (3 g/kg body wt) into sgk1(+/+) mice transiently increased plasma glucose concentration approaching significantly higher values ([glucose]p,max) in high salt (281 +/- 39 mg/dl) than in control (164 +/- 23 mg/dl) animals. DOCA did not significantly modify [glucose]p,max in control sgk1(+/+) mice but significantly decreased [glucose]p,max in high-salt sgk1(+/+) mice, an effect reversed by spironolactone (50 mg/kg body wt). [Glucose]p,max was in sgk1(-/-) mice insensitive to high salt and significantly higher than in control sgk1(+/+) mice. Uptake of 2-deoxy-d-[1,2-(3)H]glucose into skeletal muscle and fat tissue was significantly smaller in sgk1(-/-) mice than in sgk1(+/+) mice and decreased by high salt in sgk1(+/+) mice. Transfection of HEK-293 cells with active (S422D)SGK1, but not inactive (K127N)SGK, stimulated phloretin-sensitive glucose uptake. In conclusion, high salt decreases SGK1-dependent cellular glucose uptake. SGK1 thus participates in the link between salt intake and glucose tolerance.

Urinary sodium and potassium excretion and the risk of type 2 diabetes: a prospective study in Finland

AIMS/HYPOTHESIS

No previous studies on the association between salt intake and the risk of type 2 diabetes have been reported. The aim of this study was to assess whether high salt intake, measured by 24-h urinary sodium excretion, is an independent risk factor for type 2 diabetes.

METHODS

We followed prospectively 932 Finnish men and 1,003 women aged 35-64 years with complete data on 24-h urinary sodium and potassium excretion and other study parameters. Hazard ratios for the incidence of type 2 diabetes were estimated for different levels of 24-h urinary sodium and potassium excretion.

RESULTS

During a mean follow-up of 18.1 years, there were 129 incident cases of type 2 diabetes. The multivariate-adjusted (age, sex, study year, body mass index, physical activity, systolic blood pressure, antihypertensive drug treatment, education, smoking and coffee, alcohol, fruit, vegetable, sausage, bread and saturated fat consumption) hazard ratio for diabetes for the highest vs combined lower quartiles of 24-h urinary sodium excretion was 2.05 (95% CI, 1.43-2.96). This positive association persisted in non-obese and obese subjects, in normotensive and hypertensive subjects, as well as in men and women. Potassium excretion was not associated with the risk of type 2 diabetes.

CONCLUSIONS/INTERPRETATION

High sodium intake predicted the risk of type 2 diabetes, independently of other risk factors including physical inactivity, obesity and hypertension. These results provide direct evidence of the harmful effects of high salt intake in the adult population, although the confounding effect of other dietary factors cannot be fully excluded.

Dietary lipoic acid supplementation attenuates hypertension in Dahl salt sensitive rats

There is strong evidence that excess dietary salt (NaCl) is a major factor contributing to the development of hypertension. Salt sensitive humans and rats develop hypertension even on a normal salt diet. Salt sensitivity is associated with glucose intolerance and insulin resistance in both humans and animal models, including Dahl salt sensitive (DSS) rats. In insulin resistance, impaired glucose metabolism leads to elevated endogenous aldehydes. These aldehydes bind sulfhydryl groups of membrane proteins, altering calcium channels, increasing cytosolic free calcium ([Ca2+]i) and blood pressure. Treatment with lipoic acid, an endogenous sulfur-containing fatty acid, normalizes insulin resistance and lowers tissue aldehyde conjugates, cytosolic [Ca2+]i, and blood pressure in spontaneously hypertensive rats (SHR). The objective of this study was to investigate the effects of a normal salt diet on tissue aldehyde conjugates, cytosolic [Ca2+]i and blood pressure in DSS rats and to determine whether lipoic acid supplementation prevents the increase in blood pressure and biochemical changes. Starting at 7 weeks of age, DSS rats were divided into three groups of six animals each and treated for 6 weeks with diets as follows: DSS-low salt, 0.4% NaCl; DSS-normal salt, 0.7% NaCl, and; DSS-normal salt + lipoic acid, 0.7% NaCl + lipoic acid 500 mg/kg feed. At completion, animals in the normal salt group had elevated systolic blood pressure, cytosolic [Ca2+]i and tissue aldehyde conjugates as compared to the low salt group. They also showed smooth muscle cell hyperplasia in small arteries and arterioles of the kidney. Dietary lipoic acid supplementation attenuated the increase in systolic blood pressure and associated biochemical and histopathological changes.

Should we restrict chloride rather than sodium?

Low-salt diets have potential for prevention and treatment of hypertension, and may also reduce risk for stroke, left ventricular hypertrophy, osteoporosis, renal stones, asthma, cataract, gastric pathology, and possibly even senile dementia. Nonetheless, the fact that salt restriction evokes certain counter-regulatory metabolic responses-- increased production of renin and angiotensin II, as well as increased sympathetic activity--that are potentially inimical to vascular health, has suggested to some observers that salt restriction might not be of unalloyed benefit, and might in fact be contraindicated in some "salt-resistant" subjects. Current epidemiology indicates that lower-salt diets tend to reduce coronary risk quite markedly in obese subjects, whereas the impact of such diets on leaner subjects (who are less likely to be salt sensitive) is equivocal--seemingly consistent with the possibility that salt restriction can exert countervailing effects on vascular health. There is considerable evidence that sodium chloride, rather than sodium per se, is responsible for the known adverse effects of dietary salt. Other non-halide sodium salts, such as sodium citrate or bicarbonate, do not raise plasma volume, increase blood pressure, boost urinary calcium loss, or promote stroke in stroke-prone rats. Nonetheless, these compounds have been shown to blunt the impact of salt restriction on renin, angiotensin II, and sympathetic activity in humans. This may rationalize limited clinical evidence that organic sodium salts can decrease blood pressure in salt-restricted hypertensives. Furthermore, organic sodium salts have an alkalinizing metabolic impact favorable to bone health. These considerations suggest that restricting dietary salt to the extent feasible, while encouraging consumption of organic sodium salts in mineral waters, soft drinks, or other nutraceuticals--preferably in conjunction with organic potassium salts and taurine--may represent a superior strategy for controlling blood pressure, promoting vascular health, and preserving bone density. Further clinical studies should determine whether a moderately salt-restricted diet supplemented with organic sodium salts has a better and more uniform impact on hypertension than salt restriction alone, while rodent studies should examine the comparative impact of these regimens on rodents prone to vascular disease.

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.

Influence of plasma insulin levels on antinatriuretic and vasoconstrictor actions of angiotensin-II

The objective of the present study is to investigate whether plasma insulin levels play a role in the antinatriuretic and vasoconstrictor actions of angiotensin-II (Ang-II). We evaluated antinatriuretic function of endogenous Ang-II using an AT1 receptor antagonist, candesartan in anesthetized Sprague-Dawley rats. In control rats, candesartan produced significant increases in natriuresis and diuresis and these effects were abolished in streptozocin (STZ, 55 mg/kg i.p.) treated rats. Replacement of insulin restored these renal effects of candesartan. In a separate group of rats pretreated with an autonomic ganglionic blocker, pressor responses to Ang-II and norepinephrine (NE) before or after L-NNA, a nitric oxide synthase inhibitor were not affected by STZ treatment. However, insulin replacement greatly augmented these responses. These data provide evidence in vivo showing that insulin can enhance both antinatriuretic and vasoconstrictor actions of Ang-II. Hence exaggerated renal and vascular effects of Ang-II in the obese Zucker rats observed in our previous studies may be related to hyperinsulimemia and this phenomena could contribute to salt-sensitivity and development of sustained hypertension.

Rosiglitazone improves insulin sensitivity and lowers blood pressure in hypertensive patients

OBJECTIVE

To examine the effect of rosiglitazone on insulin resistance and blood pressure in patients with essential hypertension, classified based on abnormalities of their renin-angiotensin system.

RESEARCH DESIGN AND METHODS

A total of 24 hypertensive nondiabetic patients (age 58 +/- 6 years, BMI 30 +/- 5 kg/m2) were studied before and after rosiglitazone treatment. After 2 weeks off antihypertensive medication, subjects received a euglycemic-hyperinsulinemic clamp (40 mU. m(-2). min(-1)) with 6,6-[2H2]glucose infusion, ambulatory blood pressure monitoring, and blood tests for cardiovascular risk factors. Subjects were then placed on rosiglitazone (4 mg orally b.i.d.) and their usual antihypertensive medications (but not ACE inhibitors) for 16 weeks, and baseline tests were repeated.

RESULTS

There was no change in fasting plasma glucose (83 +/- 2 vs. 82 +/- 2 mg/dl, P = 0.60), but fasting insulin decreased (16.1 +/- 1.4 vs. 12.5 +/- 0.9 micro U/ml, P < 0.01). Total glucose disposal during the clamp increased (5.0 +/- 0.4 vs. 5.9 +/- 0.5 mg. kg(-1). min(-1), P < 0.001), with no change in suppression of hepatic glucose output. There were significant decreases in mean 24-h systolic (138 +/- 2 vs. 134 +/- 2 mmHg, P < 0.02) and diastolic (85 +/- 2 vs. 80 +/- 2 mmHg, P < 0.0001) blood pressure, and the decline in systolic blood pressure was correlated with the improvement in insulin sensitivity (r = 0.59, P < 0.005). Triglycerides (135 +/- 16 vs. 89 +/- 8 mg/dl, P < 0.01), LDL cholesterol (129 +/- 6 vs. 122 +/- 8 mg/dl, P = 0.18), and HDL cholesterol (51 +/- 3 vs. 46 +/- 3 mg/dl, P < 0.02) all decreased, with no change in the LDL-to-HDL ratio. Plasminogen activator inhibitor-1 and C-reactive protein also declined significantly.

CONCLUSIONS

Rosiglitazone treatment of nondiabetic hypertensive patients improves insulin sensitivity, reduces systolic and diastolic blood pressure, and induces favorable changes in markers of cardiovascular risk. Insulin sensitizers may provide cardiovascular benefits when used in the treatment of patients with hypertension.

Nonhypertensive cardiac effects of a high salt diet

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