Renal responses to sodium restriction in patients with early diabetes mellitus

Physiological confounders of renal blood flow measurement

OBJECTIVES

Renal blood flow (RBF) is controlled by a number of physiological factors that can contribute to the variability of its measurement. The purpose of this review is to assess the changes in RBF in response to a wide range of physiological confounders and derive practical recommendations on patient preparation and interpretation of RBF measurements with MRI.

METHODS

A comprehensive search was conducted to include articles reporting on physiological variations of renal perfusion, blood and/or plasma flow in healthy humans.

RESULTS

A total of 24 potential confounders were identified from the literature search and categorized into non-modifiable and modifiable factors. The non-modifiable factors include variables related to the demographics of a population (e.g. age, sex, and race) which cannot be manipulated but should be considered when interpreting RBF values between subjects. The modifiable factors include different activities (e.g. food/fluid intake, exercise training and medication use) that can be standardized in the study design. For each of the modifiable factors, evidence-based recommendations are provided to control for them in an RBF-measurement.

CONCLUSION

Future studies aiming to measure RBF are encouraged to follow a rigorous study design, that takes into account these recommendations for controlling the factors that can influence RBF results.

Altered dietary salt intake for preventing diabetic kidney disease and its progression

BACKGROUND

There is strong evidence that our current consumption of salt is a major factor in the development of increased blood pressure (BP) and that a reduction in our salt intake lowers BP, whether BP levels are normal or raised initially. Effective control of BP in people with diabetes lowers the risk of strokes, heart attacks and heart failure and slows the progression of chronic kidney disease (CKD) in people with diabetes. This is an update of a review first published in 2010.

OBJECTIVES

To evaluate the effect of altered salt intake on BP and markers of cardiovascular disease and of CKD in people with diabetes.

SEARCH METHODS

We searched the Cochrane Kidney and Transplant Register of Studies up to 31 March 2022 through contact with the Information Specialist using search terms relevant to this review. Studies in the Register were identified through searches of CENTRAL, MEDLINE, and EMBASE, conference proceedings, the International Clinical Trials Register (ICTRP) Search Portal and ClinicalTrials.gov.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) of altered salt intake in individuals with type 1 and type 2 diabetes. Studies were included when there was a difference between low and high sodium intakes of at least 34 mmol/day.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed studies and resolved differences by discussion. We calculated mean effect sizes as mean difference (MD) and 95% confidence intervals (CI) using the random-effects model. Confidence in the evidence was assessed using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach.

MAIN RESULTS

Thirteen RCTs (313 participants), including 21 comparisons (studies), met our inclusion criteria. One RCT (two studies) was added to this review update. Participants included 99 individuals with type 1 diabetes and 214 individuals with type 2 diabetes. Two RCTs (four studies) included some participants with reduced overall kidney function. The remaining studies either reported that participants with reduced glomerular filtration rate (GFR) were excluded from the study or only included participants with microalbuminuria and normal GFR. Five studies used a parallel study design, and 16 used a cross-over design. Studies were at high risk of bias for most criteria. Random sequence generation and allocation concealment were adequate in only three and two studies, respectively. One study was at low risk of bias for blinding of participants and outcome assessment, but no studies were at low risk for selective reporting. Twelve studies reported non-commercial funding sources, three reported conflicts of interest, and eight reported adequate washout between interventions in cross-over studies. The median net reduction in 24-hour urine sodium excretion (24-hour UNa) in seven long-term studies (treatment duration four to 12 weeks) was 76 mmol (range 51 to 124 mmol), and in 10 short-term studies (treatment duration five to seven days) was 187 mmol (range 86 to 337 mmol). Data were only available graphically in four studies. In long-term studies, reduced sodium intake may lower systolic BP (SBP) by 6.15 mm Hg (7 studies: 95% CI -9.27 to -3.03; I² = 12%), diastolic BP (DBP) by 3.41 mm Hg (7 studies: 95% CI -5.56 to -1.27; I² = 41%) and mean arterial pressure (MAP) by 4.60 mm Hg (4 studies: 95% CI -7.26 to -1.94; I² = 28%). In short-term studies, low sodium intake may reduce SBP by 8.43 mm Hg (5 studies: 95% CI -14.37 to -2.48; I² = 88%), DBP by 2.95 mm Hg (5 studies: 95% CI -4.96 to -0.94; I² = 70%) and MAP by 2.37 mm Hg (9 studies: 95% CI -4.75 to -0.01; I² = 65%). There was considerable heterogeneity in most analyses but particularly among short-term studies. All analyses were considered to be of low certainty evidence. SBP, DBP and MAP reductions may not differ between hypertensive and normotensive participants or between individuals with type 1 or type 2 diabetes. In hypertensive participants, SBP, DBP and MAP may be reduced by 6.45, 3.15 and 4.88 mm Hg, respectively, while in normotensive participants, they may be reduced by 8.43, 2.95 and 2.15 mm Hg, respectively (all low certainty evidence). SBP, DBP and MAP may be reduced by 7.35, 3.04 and 4.30 mm Hg, respectively, in participants with type 2 diabetes and by 7.35, 3.20, and 0.08 mm Hg, respectively, in participants with type 1 diabetes (all low certainty evidence). Eight studies provided measures of urinary protein excretion before and after salt restriction; four reported a reduction in urinary albumin excretion with salt restriction. Pooled analyses showed no changes in GFR (12 studies: MD -1.87 mL/min/1.73 m², 95% CI -5.05 to 1.31; I² = 32%) or HbA1c (6 studies: MD -0.62, 95% CI -1.49 to 0.26; I² = 95%) with salt restriction (low certainty evidence). Body weight was reduced in studies lasting one to two weeks but not in studies lasting for longer periods (low certainty evidence). Adverse effects were reported in only one study; 11% and 21% developed postural hypotension on the low-salt diet and the low-salt diet combined with hydrochlorothiazide, respectively.

AUTHORS' CONCLUSIONS

This systematic review shows an important reduction in SBP and DBP in people with diabetes with normal GFR during short periods of salt restriction, similar to that obtained with single drug therapy for hypertension. These data support the international recommendations that people with diabetes with or without hypertension or evidence of kidney disease should reduce salt intake to less than 5 g/day (2 g sodium).

Low plasma renin activity is independently associated with kidney disease progression in patients with type 2 diabetes and overt nephropathy, including those with impaired kidney function: a 2-year prospective study

Plasma renin activity (PRA) is lower in patients with diabetic nephropathy (DN) than in healthy individuals. However, the association, if any, between PRA and renal outcomes in patients with DN remains uncertain. In a 2-year prospective observational study, we aimed to investigate the association of PRA with the decline in kidney function in patients with DN. We studied 97 patients with DN who were categorized according to tertile (T1-T3) of PRA. The annual changes in estimated glomerular filtration rate (eGFR) (mL/min/1.73 m²/year) were determined from the slope of the linear regression curve for eGFR. The secondary endpoint was defined as a composite of the doubling of serum creatinine or end-stage renal disease. Results showed that kidney function rapidly declined with lower tertiles of PRA (median value [interquartile range] of the annual eGFR changes: -8.8 [-18.5 to -4.2] for T1, -8.0 [-14.3 to -3.2] for T2, and -3.1 [-6.3 to -2.0] for T3; p for trend <0.01). Multivariable linear regression analyses showed that, compared with T3, T1 was associated with a larger annual change in eGFR (coefficient, -4.410; 95% confidence interval [CI], -7.910 to -0.909 for T1). Composite renal events occurred in 46 participants. In multivariable Cox analysis, the lower tertiles of PRA (T1 and T2) were associated with higher incidences of the composite renal outcome (T2: hazard ratio [HR], 4.78; 95% CI, 1.64-13.89; T1: HR, 4.85; 95% CI 1.61-14.65) than T3. In conclusion, low PRA is independently associated with poor renal outcomes in patients with DN.

Effects of SGLT2 inhibitor and dietary NaCl on glomerular hemodynamics assessed by micropuncture in diabetic rats

Inhibitors of the main proximal tubular Na-glucose cotransporter (SGLT2) mitigate diabetic glomerular hyperfiltration and have been approved by the United States Food and Drug Administration for slowing the progression of diabetic kidney disease. It has been proposed that SGLT2 inhibitors improve hard renal outcomes by reducing glomerular capillary pressure (PGC) via a tubuloglomerular feedback (TGF) response to a decrease in proximal reabsorption (Jprox). However, the effect of SGLT2 inhibition on PGC has not been measured. Here, we studied the effects of acute SGLT2 blockade (ertugliflozin) on Jprox and glomerular hemodynamics in two-period micropuncture experiments using streptozotocin-induced diabetic rats fed high- or low-NaCl diets. PGC was measured by direct capillary puncture or computed from tubular stop-flow pressure (PSF). TGF is intact while measuring PGC directly but rendered inoperative when measuring PSF. Acute SGLT2 inhibitor reduced Jprox by ∼30%, reduced PGC by 5-8 mmHg, and reduced glomerular filtration rate (GFR) by ∼25% (all P < 0.0001) but had no effect on PSF. The decrease in PGC was larger with the low-NaCl diet (8 vs. 5 mmHg, P = 0.04) where PGC was higher to begin with (54 vs. 50 mmHg, P = 0.003). Greater decreases in PGC corresponded, unexpectedly, to lesser decreases in GFR (P = 0.04). In conclusion, these results confirm expectations that PGC would decline in response to acute SGLT2 inhibition and that a functioning TGF system is required for this. We infer a contribution of postglomerular vasorelaxation to the TGF responses where decreases in PGC were large and decreases in GFR were small.NEW & NOTEWORTHY It has been theorized that Na-glucose cotransporter (SGLT2) blockade slows progression of diabetic kidney disease by reducing physical strain on the glomerulus. This is the first direct measurement of intraglomerular pressure during SGLT2 blockade. Findings confirmed that SGLT2 blockade does reduce glomerular capillary pressure, that this is mediated through tubuloglomerular feedback, and that the tubuloglomerular feedback response to SGLT2 blockade involves preglomerular vasoconstriction and postglomerular vasorelaxation.

Effects of the Dietary Approaches to Stop Hypertension Diet and Sodium Reduction on Blood Pressure in Persons With Diabetes

Elevated blood pressure and blood pressure-related morbidity are extraordinarily common in persons with diabetes. The Dietary Approaches to Stop Hypertension dietary pattern and dietary sodium reduction are recommended as lifestyle interventions in individuals with diabetes. However, these recommendations have largely been based on studies conducted in persons without diabetes. In this review, we summarize available evidence from trials that tested the effects of these 2 dietary interventions on blood pressure in people with diabetes. Overall, of the 3 trials (total n=151) that tested the effects of the Dietary Approaches to Stop Hypertension dietary pattern in persons with diabetes, 2 trials documented that the Dietary Approaches to Stop Hypertension dietary pattern lowered blood pressure. While 16 trials (total n=445) tested the effects of sodium reduction in persons with diabetes, results were inconsistent, likely because of design limitations, for example, brief duration, small sample size, and low baseline blood pressure levels, as well as differences in the mode of intervention delivery (behavioral interventions, feeding studies, and sodium supplements). In conclusion, there is a substantial need for additional research on the blood pressure lowering effects of the Dietary Approaches to Stop Hypertension diet and sodium reduction in people with diabetes and hypertension, given the high prevalence of hypertension and the dearth of high-quality trials in this population.

Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterol, and triglyceride

BACKGROUND

Recent cohort studies show that salt intake below 6 g is associated with increased mortality. These findings have not changed public recommendations to lower salt intake below 6 g, which are based on assumed blood pressure (BP) effects and no side-effects.

OBJECTIVES

To assess the effects of sodium reduction on BP, and on potential side-effects (hormones and lipids) SEARCH METHODS: The Cochrane Hypertension Information Specialist searched the following databases for randomized controlled trials up to April 2018 and a top-up search in March 2020: the Cochrane Hypertension Specialised Register, the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (from 1946), Embase (from 1974), the World Health Organization International Clinical Trials Registry Platform, and ClinicalTrials.gov. We also contacted authors of relevant papers regarding further published and unpublished work. The searches had no language restrictions. The top-up search articles are recorded under "awaiting assessment."

SELECTION CRITERIA

Studies randomizing persons to low-sodium and high-sodium diets were included if they evaluated at least one of the outcome parameters (BP, renin, aldosterone, noradrenalin, adrenalin, cholesterol, high-density lipoprotein, low-density lipoprotein and triglyceride,.

DATA COLLECTION AND ANALYSIS

Two review authors independently collected data, which were analysed with Review Manager 5.3. Certainty of evidence was assessed using GRADE.

MAIN RESULTS

Since the first review in 2003 the number of included references has increased from 96 to 195 (174 were in white participants). As a previous study found different BP outcomes in black and white study populations, we stratified the BP outcomes by race. The effect of sodium reduction (from 203 to 65 mmol/day) on BP in white participants was as follows: Normal blood pressure: SBP: mean difference (MD) -1.14 mmHg (95% confidence interval (CI): -1.65 to -0.63), 5982 participants, 95 trials; DBP: MD + 0.01 mmHg (95% CI: -0.37 to 0.39), 6276 participants, 96 trials. Hypertension: SBP: MD -5.71 mmHg (95% CI: -6.67 to -4.74), 3998 participants,88 trials; DBP: MD -2.87 mmHg (95% CI: -3.41 to -2.32), 4032 participants, 89 trials (all high-quality evidence). The largest bias contrast across studies was recorded for the detection bias element. A comparison of detection bias low-risk studies versus high/unclear risk studies showed no differences. The effect of sodium reduction (from 195 to 66 mmol/day) on BP in black participants was as follows: Normal blood pressure: SBP: mean difference (MD) -4.02 mmHg (95% CI:-7.37 to -0.68); DBP: MD -2.01 mmHg (95% CI:-4.37, 0.35), 253 participants, 7 trials. Hypertension: SBP: MD -6.64 mmHg (95% CI:-9.00, -4.27); DBP: MD -2.91 mmHg (95% CI:-4.52, -1.30), 398 participants, 8 trials (low-quality evidence). The effect of sodium reduction (from 217 to 103 mmol/day) on BP in Asian participants was as follows: Normal blood pressure: SBP: mean difference (MD) -1.50 mmHg (95% CI: -3.09, 0.10); DBP: MD -1.06 mmHg (95% CI:-2.53 to 0.41), 950 participants, 5 trials. Hypertension: SBP: MD -7.75 mmHg (95% CI:-11.44, -4.07); DBP: MD -2.68 mmHg (95% CI: -4.21 to -1.15), 254 participants, 8 trials (moderate-low-quality evidence).   During sodium reduction renin increased 1.56 ng/mL/hour (95%CI:1.39, 1.73) in 2904 participants (82 trials); aldosterone increased 104 pg/mL (95%CI:88.4,119.7) in 2506 participants (66 trials); noradrenalin increased 62.3 pg/mL: (95%CI: 41.9, 82.8) in 878 participants (35 trials); adrenalin increased 7.55 pg/mL (95%CI: 0.85, 14.26) in 331 participants (15 trials); cholesterol increased 5.19 mg/dL (95%CI:2.1, 8.3) in 917 participants (27 trials); triglyceride increased 7.10 mg/dL (95%CI: 3.1,11.1) in 712 participants (20 trials); LDL tended to increase 2.46 mg/dl (95%CI: -1, 5.9) in 696 participants (18 trials); HDL was unchanged -0.3 mg/dl (95%CI: -1.66,1.05) in 738 participants (20 trials) (All high-quality evidence except the evidence for adrenalin).

AUTHORS' CONCLUSIONS

In white participants, sodium reduction in accordance with the public recommendations resulted in mean arterial pressure (MAP) decrease of about 0.4 mmHg in participants with normal blood pressure and a MAP decrease of about 4 mmHg in participants with hypertension. Weak evidence indicated that these effects may be a little greater in black and Asian participants. The effects of sodium reduction on potential side effects (hormones and lipids) were more consistent than the effect on BP, especially in people with normal BP.

Effect of high-salt diet on blood pressure and body fluid composition in patients with type 1 diabetes: randomized controlled intervention trial

INTRODUCTION

Patients with type 1 diabetes are susceptible to hypertension, possibly resulting from increased salt sensitivity and accompanied changes in body fluid composition. We examined the effect of a high-salt diet (HSD) in type 1 diabetes on hemodynamics, including blood pressure (BP) and body fluid composition.

RESEARCH DESIGN AND METHODS

We studied eight male patients with type 1 diabetes and 12 matched healthy controls with normal BP, body mass index, and renal function. All subjects adhered to a low-salt diet and HSD for eight days in randomized order. On day 8 of each diet, extracellular fluid volume (ECFV) and plasma volume were calculated with the use of iohexol and 125I-albumin distribution. Hemodynamic measurements included BP, cardiac output (CO), and systemic vascular resistance.

RESULTS

After HSD, patients with type 1 diabetes showed a BP increase (mean arterial pressure: 85 (5) mm Hg vs 80 (3) mm Hg; p<0.05), while BP in controls did not rise (78 (5) mm Hg vs 78 (5) mm Hg). Plasma volume increased after HSD in patients with type 1 diabetes (p<0.05) and not in controls (p=0.23). There was no significant difference in ECFV between diets, while HSD significantly increased CO, heart rate (HR) and N-terminal pro-B-type natriuretic peptide (NT-proBNP) in type 1 diabetes but not in controls. There were no significant differences in systemic vascular resistance, although there was a trend towards an HSD-induced decrease in controls (p=0.09).

CONCLUSIONS

In the present study, patients with type 1 diabetes show a salt-sensitive BP rise to HSD, which is accompanied by significant increases in plasma volume, CO, HR, and NT-proBNP. Underlying mechanisms for these responses need further research in order to unravel the increased susceptibility to hypertension and cardiovascular disease in diabetes.

TRIAL REGISTRATION NUMBERS

NTR4095 and NTR4788.

The tubular hypothesis of nephron filtration and diabetic kidney disease

Kidney size and glomerular filtration rate (GFR) often increase with the onset of diabetes, and elevated GFR is a risk factor for the development of diabetic kidney disease. Hyperfiltration mainly occurs in response to signals passed from the tubule to the glomerulus: high levels of glucose in the glomerular filtrate drive increased reabsorption of glucose and sodium by the sodium-glucose cotransporters SGLT2 and SGLT1 in the proximal tubule. Passive reabsorption of chloride and water also increases. The overall capacity for proximal reabsorption is augmented by growth of the proximal tubule, which (alongside sodium-glucose cotransport) further limits urinary glucose loss. Hyperreabsorption of sodium and chloride induces tubuloglomerular feedback from the macula densa to increase GFR. In addition, sodium-glucose cotransport by SGLT1 on macula densa cells triggers the production of nitric oxide, which also contributes to glomerular hyperfiltration. Although hyperfiltration restores sodium and chloride excretion it imposes added physical stress on the filtration barrier and increases the oxygen demand to drive reabsorption. Tubular growth is associated with the development of a senescence-like molecular signature that sets the stage for inflammation and fibrosis. SGLT2 inhibitors attenuate the proximal reabsorption of sodium and glucose, normalize tubuloglomerular feedback signals and mitigate hyperfiltration. This tubule-centred model of diabetic kidney physiology predicts the salutary effect of SGLT2 inhibitors on hard renal outcomes, as shown in large-scale clinical trials.

Urinary Renin in Patients and Mice With Diabetic Kidney Disease

In patients with diabetic kidney disease (DKD), plasma renin activity is usually decreased, but there is limited information on urinary renin and its origin. Urinary renin was evaluated in samples from patients with longstanding type I diabetes mellitus and mice with streptozotocin-induced diabetes mellitus. Renin-reporter mouse model (Ren1d-Cre;mT/mG) was made diabetic with streptozotocin to examine whether the distribution of cells of the renin lineage was altered in a chronic diabetic environment. Active renin was increased in urine samples from patients with DKD (n=36), compared with those without DKD (n=38; 3.2 versus 1.3 pg/mg creatinine; P<0.001). In mice with streptozotocin-induced diabetes mellitus, urine renin was also increased compared with nondiabetic controls. By immunohistochemistry, in mice with streptozotocin-induced diabetes mellitus, juxtaglomerular apparatus and proximal tubular renin staining were reduced, whereas collecting tubule staining, by contrast, was increased. To examine the role of filtration and tubular reabsorption on urinary renin, mice were either infused with either mouse or human recombinant renin and lysine (a blocker of proximal tubular protein reabsorption). Infusion of either form of renin together with lysine markedly increased urinary renin such that it was no longer different between nondiabetic and diabetic mice. Megalin mRNA was reduced in the kidney cortex of streptozotocin-treated mice (0.70±0.09 versus 1.01±0.04 in controls, P=0.01) consistent with impaired tubular reabsorption. In Ren1d-Cre;mT/mG with streptozotocin-induced diabetes mellitus, the distribution of renin lineage cells within the kidney was similar to nondiabetic renin-reporter mice. No evidence for migration of cells of renin linage to the collecting duct in diabetic mice could be found. Renin mRNA in microdissected collecting ducts from streptozotocin-treated mice, moreover, was not significantly different than in controls, whereas in kidney cortex, largely reflecting juxtaglomerular apparatus renin, it was significantly reduced. In conclusion, in urine from patients with type 1 diabetes mellitus and DKD and from mice with streptozotocin-induced diabetes mellitus, renin is elevated. This cannot be attributed to production from cells of the renin lineage migrating to the collecting duct in a chronic hyperglycemic environment. Rather, the elevated levels of urinary renin found in DKD are best attributed to altered glomerular filteration and impaired proximal tubular reabsorption.

Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterol, and triglyceride

BACKGROUND

In spite of more than 100 years of investigations the question of whether a reduced sodium intake improves health is still unsolved.

OBJECTIVES

To estimate the effects of low sodium intake versus high sodium intake on systolic and diastolic blood pressure (SBP and DBP), plasma or serum levels of renin, aldosterone, catecholamines, cholesterol, high-density lipoprotein (HDL), low-density lipoprotein (LDL) and triglycerides.

SEARCH METHODS

The Cochrane Hypertension Information Specialist searched the following databases for randomized controlled trials up to March 2016: the Cochrane Hypertension Specialised Register, the Cochrane Central Register of Controlled Trials (CENTRAL) (2016, Issue 3), MEDLINE (from 1946), Embase (from 1974), the World Health Organization International Clinical Trials Registry Platform, and ClinicalTrials.gov. We also searched the reference lists of relevant articles.

SELECTION CRITERIA

Studies randomising persons to low-sodium and high-sodium diets were included if they evaluated at least one of the above outcome parameters.

DATA COLLECTION AND ANALYSIS

Two review authors independently collected data, which were analysed with Review Manager 5.3.

MAIN RESULTS

A total of 185 studies were included. The average sodium intake was reduced from 201 mmol/day (corresponding to high usual level) to 66 mmol/day (corresponding to the recommended level).The effect of sodium reduction on blood pressure (BP) was as follows: white people with normotension: SBP: mean difference (MD) -1.09 mmHg (95% confidence interval (CI): -1.63 to -0.56; P = 0.0001); 89 studies, 8569 participants; DBP: + 0.03 mmHg (MD 95% CI: -0.37 to 0.43; P = 0.89); 90 studies, 8833 participants. High-quality evidence. Black people with normotension: SBP: MD -4.02 mmHg (95% CI:-7.37 to -0.68; P = 0.002); seven studies, 506 participants; DBP: MD -2.01 mmHg (95% CI:-4.37 to 0.35; P = 0.09); seven studies, 506 participants. Moderate-quality evidence. Asian people with normotension: SBP: MD -0.72 mmHg (95% CI: -3.86 to 2.41; P = 0.65); DBP: MD -1.63 mmHg (95% CI:-3.35 to 0.08; P =0.06); three studies, 393 participants. Moderate-quality evidence.White people with hypertension: SBP: MD -5.51 mmHg (95% CI: -6.45 to -4.57; P < 0.00001); 84 studies, 5925 participants; DBP: MD -2.88 mmHg (95% CI: -3.44 to -2.32; P < 0.00001); 85 studies, 6001 participants. High-quality evidence. Black people with hypertension: SBP MD -6.64 mmHg (95% CI:-9.00 to -4.27; P = 0.00001); eight studies, 619 participants; DBP -2.91 mmHg (95% CI:-4.52, -1.30; P = 0.0004); eight studies, 619 participants. Moderate-quality evidence. Asian people with hypertension: SBP: MD -7.75 mmHg (95% CI:-11,44 to -4.07; P < 0.0001) nine studies, 501 participants; DBP: MD -2.68 mmHg (95% CI: -4.21 to -1.15; P = 0.0006). Moderate-quality evidence.In plasma or serum, there was a significant increase in renin (P < 0.00001), aldosterone (P < 0.00001), noradrenaline (P < 0.00001), adrenaline (P < 0.03), cholesterol (P < 0.0005) and triglyceride (P < 0.0006) with low sodium intake as compared with high sodium intake. All effects were stable in 125 study populations with a sodium intake below 250 mmol/day and a sodium reduction intervention of at least one week.

AUTHORS' CONCLUSIONS

Sodium reduction from an average high usual sodium intake level (201 mmol/day) to an average level of 66 mmol/day, which is below the recommended upper level of 100 mmol/day (5.8 g salt), resulted in a decrease in SBP/DBP of 1/0 mmHg in white participants with normotension and a decrease in SBP/DBP of 5.5/2.9 mmHg in white participants with hypertension. A few studies showed that these effects in black and Asian populations were greater. The effects on hormones and lipids were similar in people with normotension and hypertension. Renin increased 1.60 ng/mL/hour (55%); aldosterone increased 97.81 pg/mL (127%); adrenalin increased 7.55 pg/mL (14%); noradrenalin increased 63.56 pg/mL: (27%); cholesterol increased 5.59 mg/dL (2.9%); triglyceride increased 7.04 mg/dL (6.3%).

Sodium and Its Role in Cardiovascular Disease - The Debate Continues

Guidelines have recommended significant reductions in dietary sodium intake to improve cardiovascular health. However, these dietary sodium intake recommendations have been questioned as emerging evidence has shown that there is a higher risk of cardiovascular disease with a low sodium diet, including in individuals with type 2 diabetes. This may be related to the other pleotropic effects of dietary sodium intake. Therefore, despite recent review of dietary sodium intake guidelines by multiple organizations, including the dietary guidelines for Americans, American Diabetes Association, and American Heart Association, concerns about the impact of the degree of sodium restriction on cardiovascular health continue to be raised. This literature review examines the effects of dietary sodium intake on factors contributing to cardiovascular health, including left ventricular hypertrophy, heart rate, albuminuria, rennin-angiotensin-aldosterone system activation, serum lipids, insulin sensitivity, sympathetic nervous system activation, endothelial function, and immune function. In the last part of this review, the association between dietary sodium intake and cardiovascular outcomes, especially in individuals with diabetes, is explored. Given the increased risk of cardiovascular disease in individuals with diabetes and the increasing incidence of diabetes worldwide, this review is important in summarizing the recent evidence regarding the effects of dietary sodium intake on cardiovascular health, especially in this population.

Nutrition Interventions in Chronic Kidney Disease

Dietary modification is recommended in the management of chronic kidney disease (CKD). Individuals with CKD often have multiple comorbidities, such as high blood pressure, diabetes, obesity, and cardiovascular disease, for which dietary modification is also recommended. As CKD progresses, nutrition plays an important role in mitigating risk for cardiovascular disease and decline in kidney function. The objectives of nutrition interventions in CKD include management of risk factors, ensuring optimal nutritional status throughout all stages of CKD, preventing buildup of toxic metabolic products, and avoiding complications of CKD. Recommended dietary changes should be feasible, sustainable, and suited for patients' food preferences and clinical needs.

Sodium-glucose cotransporter-2 inhibition and the potential for renal protection in diabetic nephropathy

PURPOSE OF REVIEW

Renal hyperfiltration has been used as a surrogate marker for increased intraglomerular pressure in patients with diabetes mellitus. Previous human investigation examining the pathogenesis of hyperfiltration has focused on the role of neurohormones such as the renin-angiotensin-aldosterone system (RAAS). Unfortunately, RAAS blockade does not completely attenuate hyperfiltration or diabetic kidney injury. More recent work has therefore investigated the contribution of renal tubular factors, including the sodium-glucose cotransporter, to the hyperfiltration state, which is the topic of this review.

RECENT FINDINGS

Novel sodium-glucose cotransporter-2 (SGLT2) inhibitors reduce proximal tubular sodium reabsorption, thereby increasing distal sodium delivery to the macula densa, causing tubuloglomerular feedback, afferent vasoconstriction and decreased hyperfiltration in animals. In humans, SGLT2 inhibition was recently shown to reduce hyperfiltration in normotensive, normoalbuminuric patients with type 1 diabetes. In clinical trials of type 2 diabetes, SGLT2 is associated with significant renal effects, including modest, acute declines in estimated glomerular filtration rate followed by the maintenance of stable renal function, and reduced albuminuria.

SUMMARY

Existing data are supportive of a potential renal-protective role for SGLT2 inhibition in patients with diabetes. Dedicated renal outcome trials are ongoing and have the potential to change the clinical practice.

Short-term dietary salt supplementation blunts telmisartan induced increases in plasma renin activity in hypertensive patients with type 2 diabetes mellitus

Current guidelines recommend low dietary salt intake (LDS) in patients with diabetes to reduce blood pressure (BP). However, low salt intake has been associated with higher mortality rates in people with diabetes. Our aim is to examine the effect of angiotensin II receptor blocker (ARB), telmisartan, with and without dietary sodium chloride (NaCl) supplementation, on BP [mean arterial pressure (MAP)], plasma renin activity (PRA), serum aldosterone level and estimated glomerular filtration rate (eGFR) in hypertensive patients with type 2 diabetes. In a randomized, double-blind, placebo-controlled study (RCT), 28 patients with type 2 diabetes, treated with telmisartan (40 mg daily), received 2 weeks of placebo or NaCl capsules (100 mmol/24 h). Following a 6-week washout, the protocol was repeated in reverse. Twenty-four-hour urinary sodium excretion (24hUNa), ambulatory BP (ABP) monitoring and blood tests were performed before and after each study phase. The telmisartan-associated increase in PRA was blunted by approximately 50% during salt supplementation compared with placebo; median PRA was 2.3 μg/l/h with placebo compared with 1.7 μg/l/h with salt (P<0.001). A trend towards blunting of ARB induced increases in serum aldosterone was also demonstrated. Salt supplementation significantly reduced the MAP lowering effects of telmisartan (P<0.05). The present study demonstrates that salt supplementation blunts the telmisartan induced increase in PRA in patients with type 2 diabetes.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Glomerular and tubular function in the diabetic kidney

Diabetes mellitus with its attendant complications is a significant cause of morbidity and mortality with diabetic nephropathy being the leading cause of end stage renal disease in the Western world. Characteristic structural and functional changes in the kidney early in the course of diabetes have been shown to have enduring effects on the progression of disease. A better understanding of the mechanisms underlying these changes is imperative to the development of new therapeutic strategies. Renal hypertrophy and hyperfiltration along with proximal tubular hyperreabsorption are among the distinctive features of early diabetic nephropathy. Additionally, there are particular alterations in the sensitivity of the glomerular and tubular function to dietary salt intake in early diabetes. Herein, we focus on these early physiologic changes and discuss some of the primary and secondary mechanisms discovered in recent years which lead to these alterations in kidney function.

Sodium/glucose cotransporter 2 inhibitors and prevention of diabetic nephropathy: targeting the renal tubule in diabetes

Optimal prevention and treatment of chronic kidney disease in diabetes requires implementing therapies that specifically interfere with the pathogenesis of diabetic nephropathy. In this regard, significant attention has been given to alterations of the proximal tubule and resulting changes in glomerular filtration rate. At the onset of diabetes mellitus, hyperglycemia causes increases in proximal tubular reabsorption secondary to induction of tubular growth with associated increases in sodium/glucose cotransport. The increase in proximal reabsorption leads to a decrease in solute load to the macula densa, deactivation of the tubuloglomerular feedback, and increases in glomerular filtration rate. Because glomerular hyperfiltration currently is recognized as a risk factor for progression of kidney disease in diabetic patients, limiting proximal tubular reabsorption constitutes a potential target to reduce hyperfiltration. The recent introduction of sodium/glucose cotransporter 2 (SGLT2) inhibitors opens new therapeutic perspectives for this high-risk patient population. Experimental studies have shown that these new agents attenuate the progressive nature of diabetic nephropathy by blood glucose-dependent and -independent mechanisms. SGLT2 inhibition may prevent glomerular hyperfiltration independent of the effect of lowering blood glucose levels while limiting kidney growth, inflammation, and albuminuria through reductions in blood glucose levels. Clinical data for the potential role of the proximal tubule in the pathophysiology of diabetic nephropathy and the nephroprotective effects of SGLT2 inhibitors currently are limited compared to the more extensive experimental literature. We review the evidence supporting this working hypothesis by integrating the experimental findings with the available clinical data.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

Salt sensitivity of tubuloglomerular feedback in the early remnant kidney

We previously reported internephron heterogeneity in the tubuloglomerular feedback (TGF) response 1 wk after subtotal nephrectomy (STN), with 50% of STN nephrons exhibiting anomalous TGF (Singh P, Deng A, Blantz RC, Thomson SC. Am J Physiol Renal Physiol 296: F1158-F1165, 2009). Presently, we tested the theory that anomalous TGF is an adaptation of the STN kidney to facilitate increased distal delivery when NaCl balance forces the per-nephron NaCl excretion to high levels. To this end, the effect of dietary NaCl on the TGF response was tested by micropuncture in STN and sham-operated Wistar rats. An NaCl-deficient (LS) or high-salt NaCl diet (HS; 1% NaCl in drinking water) was started on day 0 after STN or sham surgery. Micropuncture followed 8 days later with measurements of single-nephron GFR (SNGFR), proximal reabsorption, and tubular stop-flow pressure (PSF) obtained at both extremes of TGF activation, while TGF was manipulated by microperfusing Henle's loop (LOH) from the late proximal tubule. Activating TGF caused SNGFR to decline by similar amounts in Sham-LS, Sham-HS and STN-LS [ΔSNGFR (nl/min) = -16 ± 2, -11 ± 3, -11 ± 2; P = not significant by Tukey]. Activating TGF in STN-HS actually increased SNGFR by 5 ± 2 nl/min (P < 0.0005 vs. each other group by Tukey). HS had no effect on the PSF response to LOH perfusion in sham [ΔPSF (mmHg) = -9.6 ± 1.1 vs. -9.8 ± 1.0] but eliminated the PSF response in STN (+0.3 ± 0.9 vs. -5.7 ± 1.0, P = 0.0002). An HS diet leads to anomalous TGF in the early remnant kidney, which facilitates NaCl and fluid delivery to the distal nephron.

The effect of aliskiren on urinary cytokine/chemokine responses to clamped hyperglycaemia in type 1 diabetes

AIMS/HYPOTHESIS

Acute clamped hyperglycaemia activates the renin-angiotensin-aldosterone system (RAAS) and increases the urinary excretion of inflammatory cytokines/chemokines in patients with uncomplicated type 1 diabetes mellitus. Our objective was to determine whether blockade of the RAAS would blunt the effect of acute hyperglycaemia on urinary cytokine/chemokine excretion, thereby giving insights into potentially protective effects of these agents prior to the onset of clinical nephropathy.

METHODS

Blood pressure, renal haemodynamic function (inulin and para-aminohippurate clearances) and urinary cytokines/chemokines were measured after 6 h of clamped euglycaemia (4-6 mmol/l) and hyperglycaemia (9-11 mmol/l) on two consecutive days in patients with type 1 diabetes mellitus (n = 27) without overt nephropathy. Measurements were repeated after treatment with aliskiren (300 mg daily) for 30 days.

RESULTS

Before aliskiren, clamped hyperglycaemia increased filtration fraction (from 0.188 ± 0.007 to 0.206 ± 0.007, p = 0.003) and urinary fibroblast growth factor-2 (FGF2), IFN-α2 and macrophage-derived chemokine (MDC) (p < 0.005). After aliskiren, the filtration fraction response to hyperglycaemia was abolished, resulting in a lower filtration fraction after aliskiren under clamped hyperglycaemic conditions (p = 0.004), and none of the biomarkers increased in response to hyperglycaemia. Aliskiren therapy also reduced levels of urinary eotaxin, FGF2, IFN-α2, IL-2 and MDC during clamped hyperglycaemia (p < 0.005).

CONCLUSIONS/INTERPRETATION

The increased urinary excretion of inflammatory cytokines/chemokines in response to acute hyperglycaemia is blunted by RAAS blockade in humans with uncomplicated type 1 diabetes mellitus.

The effect of renal hyperfiltration on urinary inflammatory cytokines/chemokines in patients with uncomplicated type 1 diabetes mellitus

AIMS/HYPOTHESIS

High intraglomerular pressure causes renal inflammation in experimental models of diabetes. Our objective was to determine whether renal hyperfiltration, a surrogate for intraglomerular hypertension, is associated with increased excretion of urinary cytokines/chemokines in patients with type 1 diabetes mellitus.

METHODS

Blood pressure, renal haemodynamic function (inulin and para-aminohippurate clearances for glomerular filtration rate (GFR) and effective renal plasma flow (ERPF), respectively) and urine samples were obtained during clamped euglycaemia in individuals with type 1 diabetes with either hyperfiltration (GFR determined using inulin [GFRINULIN] ≥ 135 ml min⁻¹ 1.73 m⁻², n = 28) or normofiltration (n = 21) and healthy control individuals (n = 18).

RESULTS

Baseline clinical characteristics, dietary sodium and protein intake and blood pressure levels were similar in the diabetic and healthy control groups. In addition, HbA1c levels were similar in the two diabetic groups. As expected baseline GFR was higher in hyperfilterers than either normofiltering diabetic patients or healthy control patients (165 ± 9 vs 113 ± 2 and 116 ± 4 ml min⁻¹ 1.73 m⁻², respectively, p < 0.01). ERPF and renal blood flow were also comparatively higher and renal vascular resistance was lower in hyperfiltering patients (p < 0.01). Hyperfiltering diabetic patients had higher excretion rates for eotaxin, IFNα2, macrophage-derived chemokine, platelet-derived growth factor (PDGF)-AA, PDGF-AB/BB and granulocyte-macrophage colony-stimulating factor (p ≤ 0.01). Urinary monocyte chemoattractant protein (MCP)-1 and RANTES (regulated on activation, normal T expressed and secreted) excretion was also higher in hyperfiltering vs normofiltering diabetic individuals (p < 0.01) and fibroblast growth factor-2, MCP-3 and CD40K excretion was elevated in hyperfiltering diabetic individuals vs healthy controls (p < 0.01).

CONCLUSIONS/INTERPRETATION

Renal hyperfiltration is associated with increased urinary excretion of inflammatory cytokines/chemokines in patients with uncomplicated type 1 diabetes.

Effect of longer-term modest salt reduction on blood pressure

BACKGROUND

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

OBJECTIVES

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

SEARCH METHODS

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

SELECTION CRITERIA

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

DATA COLLECTION AND ANALYSIS

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

MAIN RESULTS

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

AUTHORS' CONCLUSIONS

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

Increased susceptibility to hypertensive renal disease in streptozotocin-treated diabetic rats is not modulated by salt intake

AIMS/HYPOTHESIS

In early type 1 diabetes mellitus, renal salt handling is dysregulated, so that the glomerular filtration rate becomes inversely proportional to salt intake. The salt paradox occurs in both humans and rats and, with low salt intake, results in diabetic hyperfiltration. We tested whether increased salt intake could reduce the susceptibility to injury of non-clipped kidneys in diabetic rats with pre-existing Goldblatt hypertension.

METHODS

Male Long-Evans rats were made hypertensive and half were then made diabetic. Blood glucose was maintained at ~20-25 mmol/l by insulin implants. One half of each received only the salt in normal chow (1% by weight) and the other half received added salt in drinking water to equal 2.7% by weight of food intake. Weekly 24 h blood pressure records were acquired by telemetry during the 4-month experiment.

RESULTS

Systolic blood pressure was not affected by diabetes or increased salt intake, alone or together. Autoregulation was highly efficient in the non-clipped kidney of both intact and diabetic rats. Histological examination showed minor injury in the clipped kidney, which did not differ among groups. The non-clipped kidney showed extensive pressure-dependent glomerular and vascular injury in both intact and diabetic rats.

CONCLUSIONS/INTERPRETATION

The relationship between pressure and injury was shifted toward lower blood pressure in diabetic rats, indicating that diabetes increased the susceptibility of the kidney to injury despite preservation of autoregulation. The increased susceptibility was not affected by high salt intake in the diabetic rats, thus disproving the hypothesis.

Renal hemodynamic response to L-arginine in uncomplicated, type 1 diabetes mellitus: the role of buffering anions and tubuloglomerular feedback

According to the "tubulocentric" hypothesis of the glomerular hyperfiltration of diabetes mellitus (DM), tubuloglomerular feedback (TGF) is the critical determinant of the related renal hemodynamic dysfunction. To examine the role of TGF in human type 1 DM, 12 salt-replete healthy (C) and 11 uncomplicated DM individuals underwent measurements of glomerular filtration rate (GFR), renal blood flow (RBF), and lithium-derived absolute "distal" sodium delivery (DDNa). Measurements were made during two 3-h infusions of 0.012 mmol·kg(-1)·min(-1) l-arginine (ARG) buffered with either equimolar HCl (ARG.HCl) or citric acid (ARG.CITR). Our hypothesis was that changes in TGF signaling would be directionally opposite ARG.HCl vs. ARG.CITR according to the effects of the ARG-buffering anion on DDNa. Similar changes in C and DM followed ARG.CITR, with declines in DDNa (-0.26 ± 0.07 mmol/min C vs. -0.31 ± 0.07 mmol/min DM) and increases in RBF (+299 ± 25 vs. +319 ± 29 ml·min(-1)·1.73 m(-2)) and GFR (+6.6 ± 0.8 vs. +11.6 ± 1.2 ml·min(-1)·1.73 m(-2)). In contrast, with ARG.HCl, DDNa rose in both groups (P = 0.001), but the response was 73% greater in DM (+1.50 ± 0.15 mmol/min C vs. +2.59 ± 0.22 mmol/min DM, P = 0.001). RBF also increased (P = 0.001, +219 ± 20 ml·min(-1)·1.73 m(-2) C, +105 ± 14 DM), but ΔRBF after ARG.HCl was lower vs. ARG.CITR in both groups (P = 0.001). After ARG.HCl, ΔRBF also was 50% lower in DM vs. C (P = 0.001) and GFR, unchanged in C, declined in DM (-7.4 ± 0.9 ml·min(-1)·1.73 m(-2), P = 0.02 vs. C). After ARG.HCl, unlike ARG.CITR, DDNa increased in C and DM, associated with less ΔRBF and ΔGFR vs. ARG.CITR. This suggests that the renal hemodynamic response to ARG is influenced substantially by the opposite actions of HCl vs. CITR on DDNa and TGF. In DM, the association of ARG.HCl-induced exaggerated ΔDDNa, blunted ΔRBF, and the decline in GFR vs. C shows an enhanced TGF dependence of renal vasodilatation to ARG, in agreement with a critical role of TGF in DM-related renal hemodynamic dysfunction.

Elucidating mechanisms underlying altered renal autoregulation in diabetes

Previous studies have reported that high-salt intake paradoxically activates tubuloglomerular feedback (TGF) in type 1 diabetes. Using Zucker lean (ZL) and diabetic fatty (ZDF) rats on normal and high-salt diets, renal hemodynamics and the renin-angiotensin system (RAS) were characterized. On normal salt diet, glomerular filtration rate (GFR) was higher in ZDF than ZL rats. Autoregulation of GFR was less efficient and lithium clearance was lower in ZDF rats than ZL rats. Salt load reduced GFR in ZDF rats with restoration of lithium clearance and partial improvement in autoregulatory index (AI). The administration of 8-cyclopentyl-1,3-dipropylxanthine, a selective adenosine-1 receptor antagonist to ZDF rats on a high-salt diet abolished the improvement of AI in GFR. However, this effect was seen by neither (Cx40)GAP27 nor (Cx37,43)GAP27, which inhibits connexin (Cx) 40 or Cx37. Renal ANG II was higher in ZDF than ZL rats on normal salt diet, but the difference was eliminated by a salt load. The present data provide the first demonstration for a salt paradox in type 2 diabetes and implicate that in addition to Cx alterations, an enhanced proximal reabsorption attenuates TGF, underlying glomerular hyperfiltration and RAS activation. These data suggest that a high-salt diet standardizes distal delivery in diabetes, suppressing the RAS, and improving GFR autoregulation and hyperfiltration through adenosine.

Renal function in diabetic disease models: the tubular system in the pathophysiology of the diabetic kidney

Diabetes mellitus affects the kidney in stages. At the onset of diabetes mellitus, in a subset of diabetic patients the kidneys grow large, and glomerular filtration rate (GFR) becomes supranormal, which are risk factors for developing diabetic nephropathy later in life. This review outlines a pathophysiological concept that focuses on the tubular system to explain these changes. The concept includes the tubular hypothesis of glomerular filtration, which states that early tubular growth and sodium-glucose cotransport enhance proximal tubule reabsorption and make the GFR supranormal through the physiology of tubuloglomerular feedback. The diabetic milieu triggers early tubular cell proliferation, but the induction of TGF-β and cyclin-dependent kinase inhibitors causes a cell cycle arrest and a switch to tubular hypertrophy and a senescence-like phenotype. Although this growth phenotype explains unusual responses like the salt paradox of the early diabetic kidney, the activated molecular pathways may set the stage for tubulointerstitial injury and diabetic nephropathy.

Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterol, and triglyceride

BACKGROUND

In spite of more than 100 years of investigations the question of reduced sodium intake as a health prophylaxis initiative is still unsolved.

OBJECTIVES

To estimate the effects of low sodium versus high sodium intake on systolic and diastolic blood pressure (SBP and DBP), plasma or serum levels of renin, aldosterone, catecholamines, cholesterol, high-density lipoprotein (HDL), low-density lipoprotein (LDL) and triglycerides.

SEARCH METHODS

PUBMED, EMBASE and Cochrane Central and reference lists of relevant articles were searched from 1950 to July 2011.

SELECTION CRITERIA

Studies randomizing persons to low sodium and high sodium diets were included if they evaluated at least one of the above outcome parameters.

DATA COLLECTION AND ANALYSIS

Two authors independently collected data, which were analysed with Review Manager 5.1.

MAIN RESULTS

A total of 167 studies were included in this 2011 update.The effect of sodium reduction in normotensive Caucasians was SBP -1.27 mmHg (95% CI: -1.88, -0.66; p=0.0001), DBP -0.05 mmHg (95% CI: -0.51, 0.42; p=0.85). The effect of sodium reduction in normotensive Blacks was SBP -4.02 mmHg (95% CI:-7.37, -0.68; p=0.002), DBP -2.01 mmHg (95% CI:-4.37, 0.35; p=0.09). The effect of sodium reduction in normotensive Asians was SBP -1.27 mmHg (95% CI: -3.07, 0.54; p=0.17), DBP -1.68 mmHg (95% CI:-3.29, -0.06; p=0.04). The effect of sodium reduction in hypertensive Caucasians was SBP -5.48 mmHg (95% CI: -6.53, -4.43; p<0.00001), DBP -2.75 mmHg (95% CI: -3.34, -2.17; p<0.00001). The effect of sodium reduction in hypertensive Blacks was SBP -6.44 mmHg (95% CI:-8.85, -4.03; p=0.00001), DBP -2.40 mmHg (95% CI:-4.68, -0.12; p=0.04). The effect of sodium reduction in hypertensive Asians was SBP -10.21 mmHg (95% CI:-16.98, -3.44; p=0.003), DBP -2.60 mmHg (95% CI: -4.03, -1.16; p=0.0004).In plasma or serum there was a significant increase in renin (p<0.00001), aldosterone (p<0.00001), noradrenaline (p<0.00001), adrenaline (p<0.0002), cholesterol (p<0.001) and triglyceride (p<0.0008) with low sodium intake as compared with high sodium intake. In general the results were similar in studies with a duration of at least 2 weeks.

AUTHORS' CONCLUSIONS

Sodium reduction resulted in a 1% decrease in blood pressure in normotensives, a 3.5% decrease in hypertensives, a significant increase in plasma renin, plasma aldosterone, plasma adrenaline and plasma noradrenaline, a 2.5% increase in cholesterol, and a 7% increase in triglyceride. In general, these effects were stable in studies lasting for 2 weeks or more.

The proximal tubule in the pathophysiology of the diabetic kidney

Diabetic nephropathy is a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved in the early changes of the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. This review focuses on the proximal tubule in the early diabetic kidney, particularly on its exposure and response to high glucose levels, albuminuria, and other factors in the diabetic glomerular filtrate, the hyperreabsorption of glucose, the unique molecular signature of the tubular growth phenotype, including aspects of senescence, and the resulting cellular and functional consequences. The latter includes the local release of proinflammatory chemokines and changes in proximal tubular salt and fluid reabsorption, which form the basis for the strong tubular control of glomerular filtration in the early diabetic kidney, including glomerular hyperfiltration and odd responses like the salt paradox. Importantly, these early proximal tubular changes can set the stage for oxidative stress, inflammation, hypoxia, and tubulointerstitial fibrosis, and thereby for the progression of diabetic renal disease.

Altered dietary salt intake for preventing and treating diabetic kidney disease

BACKGROUND

There is strong evidence that our current consumption of salt is a major factor for increased blood pressure (BP) and a modest reduction in salt intake lowers BP whether BP levels are normal or raised. Tight control of BP in diabetics lowers the risk of strokes, heart attacks and heart failure and slows the progression of diabetic kidney disease (DKD). Currently there is no consensus in restricting salt intake in diabetic patients.

OBJECTIVES

To evaluate the effect of altered salt intake on BP and markers of cardiovascular disease and DKD.

SEARCH STRATEGY

In January 2010, we searched the Cochrane Renal Group's Specialised Register, CENTRAL (in The Cochrane Library), MEDLINE (from 1966) and EMBASE (from 1980) to identify appropriate articles.

SELECTION CRITERIA

We included all randomised controlled trials of salt reduction in individuals with type 1 and type 2 diabetes.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed studies and resolved differences by discussion with a third independent author. We calculated mean effect sizes using both the fixed-effect and random-effects models.

MAIN RESULTS

Thirteen studies (254 individuals) met our inclusion criteria. These included 75 individuals with type 1 diabetes and 158 individuals with type 2 diabetes. The median reduction in urinary sodium was 203 mmol/24 h (11.9 g/day) in type 1 diabetes and 125 mmol/24 h (7.3 g/day) in type 2 diabetes. The median duration of salt restriction was one week in both type 1 and type 2 diabetes. BP was reduced in both type 1 and type 2 diabetes. In type 1 diabetes (56 individuals), salt restriction reduced BP by -7.11/-3.13 mm Hg (systolic/diastolic); 95% CI: systolic BP (SBP) -9.13 to -5.10; diastolic BP (DBP) -4.28 to -1.98). In type 2 diabetes (56 individuals), salt restriction reduced BP by -6.90/-2.87 mm Hg (95% CI: SBP -9.84 to -3.95; DBP -4.39 to -1.35). There was a greater reduction in BP in normotensive patients, possibly due to a larger decrease in salt intake in this group.

AUTHORS' CONCLUSIONS

Although the studies are not extensive, this meta-analysis shows a large fall in BP with salt restriction, similar to that of single drug therapy. All diabetics should consider reducing salt intake at least to less than 5-6 g/day in keeping with current recommendations for the general population and may consider lowering salt intake to lower levels, although further studies are needed.

Effect of direct renin inhibition on renal hemodynamic function, arterial stiffness, and endothelial function in humans with uncomplicated type 1 diabetes: a pilot study

OBJECTIVE

Blockade of the renin-angiotensin system (RAS) plays an important role in preventing end-organ injury associated with diabetes. The recent development of direct renin inhibitors (DRIs) provides a new approach to block the RAS, but the effects of DRIs on renal and systemic vascular function in uncomplicated type 1 diabetes have not been elucidated.

RESEARCH DESIGN AND METHODS

Renal hemodynamic function (inulin and paraaminohippurate clearance), augmentation index and pulse wave velocity, endothelial dependent vasodilatation (flow-mediated dilation [FMD]), and endothelial independent vasodilatation (response to sublingual nitroglycerin) were evaluated before and after administration of aliskiren (300 mg daily for 30 days) in 10 adult subjects with uncomplicated type 1 diabetes during clamped euglycemia (4-6 mmol/l) and hyperglycemia (9-11 mmol/l).

RESULTS

In response to the DRI, plasma renin activity decreased (from 0.40 to 0.13 ng . ml(-1) . h(-1), P < 0.05) and plasma renin increased (from 5.2 to 75.0 ng/l, P < 0.05). Peripheral and central blood pressures decreased, and effective renal plasma flow and glomerular filtration rate increased during clamped euglycemia and hyperglycemia (P < 0.05). The carotid augmentation index during clamped euglycemia decreased (from 26 +/- 6 to 20 +/- 5%, P < 0.05) as did pulse wave velocity during clamped hyperglycemia (from 7.8 +/- 0.6 to 6.8 +/- 0.5 m/s, P < 0.05). In response to the DRI, FMD increased during both clamped euglycemia (from 1.92 +/- 1.13 to 5.55 +/- 0.81%) and hyperglycemia (from 1.86 +/- 0.98 to 5.63 +/- 0.62) as did the vasodilatory response to sublingual nitroglycerin.

CONCLUSIONS

DRIs exert a renal vasodilatory effect and improve parameters of systemic vascular function, suggesting that blockade of the RAS with this new class of agents has important functional effects in subjects with uncomplicated type 1 diabetes.

Effects of salt supplementation on the albuminuric response to telmisartan with or without hydrochlorothiazide therapy in hypertensive patients with type 2 diabetes are modulated by habitual dietary salt intake

OBJECTIVE This prospective randomized double-blind placebo-controlled crossover study examined the effects of sodium chloride (NaCl) supplementation on the antialbuminuric action of telmisartan with or without hydrochlorothiazide (HCT) in hypertensive patients with type 2 diabetes, increased albumin excretion rate (AER), and habitual low dietary salt intake (LDS; <100 mmol sodium/24 h on two of three consecutive occasions) or high dietary salt intake (HDS; >200 mmol sodium/24 h on two of three consecutive occasions). RESEARCH DESIGN AND METHODS Following a washout period, subjects (n = 32) received 40 mg/day telmisartan for 4 weeks followed by 40 mg telmisartan plus 12.5 mg/day HCT for 4 weeks. For the last 2 weeks of each treatment period, patients received either 100 mmol/day NaCl or placebo capsules. After a second washout, the regimen was repeated with supplements in reverse order. AER and ambulatory blood pressure were measured at weeks 0, 4, 8, 14, 18, and 22. RESULTS In LDS, NaCl supplementation reduced the anti-albuminuric effect of telmisartan with or without HCT from 42.3% (placebo) to 9.5% (P = 0.004). By contrast, in HDS, NaCl supplementation did not reduce the AER response to telmisartan with or without HCT (placebo 30.9%, NaCl 28.1%, P = 0.7). Changes in AER were independent of changes in blood pressure. CONCLUSIONS The AER response to telmisartan with or without HCT under habitual low salt intake can be blunted by NaCl supplementation. By contrast, when there is already a suppressed renin angiotensin aldosterone system under habitual high dietary salt intake, the additional NaCl does not alter the AER response.

Salt-resistant blood pressure and salt-sensitive renal autoregulation in chronic streptozotocin diabetes

Hyperfiltration occurs in early type 1 diabetes mellitus in both rats and humans. It results from afferent vasodilation and thus may impair stabilization of glomerular capillary pressure by autoregulation. It is inversely related to dietary salt intake, the "salt paradox." Restoration of normal glomerular filtration rate (GFR) involves increased preglomerular resistance, probably mediated by tubuloglomerular feedback (TGF). To begin to test whether the salt paradox has pathogenic significance, we compared intact vs. diabetic (streptozotocin) Long-Evans rats with normal and increased salt intake, 1 and approximately 3% by weight of food eaten, respectively. Weekly 24-h blood pressure records were acquired by telemetry before and during diabetes. Blood glucose was maintained at approximately 20 mmol/l by insulin implants. GFR was significantly elevated only in diabetic rats on normal salt intake, confirming diabetic hyperfiltration and the salt paradox. Renal blood flow dynamics show strong contributions to autoregulation by both TGF and the myogenic mechanism and were not impaired by diabetes or by increased salt intake. Separately, systolic pressure was not elevated in diabetic rats at any time during 12 wk with normal or high salt intake. Autoregulation was effective in all groups, and the diabetic-normal salt group showed significantly improved autoregulation at low perfusion pressures. Histological examination revealed very minor glomerulosclerosis and modest mesangial expansion, although neither was diagnostic of diabetes. Periodic acid-Schiff-positive droplets found in distal tubules and collecting duct segments were diagnostic of diabetic kidneys. Biologically significant effects attributable to increased salt intake were abrogation of hyperfiltration and of the left shift in autoregulation in diabetic rats.

Adenosine A(1) receptors determine glomerular hyperfiltration and the salt paradox in early streptozotocin diabetes mellitus

BACKGROUND

In early type 1 diabetes mellitus, changes in proximal reabsorption influence glomerular filtration rate (GFR) through tubuloglomerular feedback (TGF). Due to TGF, a primary increase in proximal reabsorption causes early diabetic hyperfiltration, while a heightened sensitivity of the proximal tubule to dietary salt leads to the so-called salt paradox, where a change in dietary salt causes a reciprocal change in GFR ('tubulocentric principle'). Here, experiments were performed in adenosine A(1) receptor knockout mice (A(1)R-/-), which lack an immediate TGF response, to determine whether A(1)Rs are essential for early diabetic hyperfiltration and the salt paradox.

METHODS

GFR was measured by inulin disappearance in conscious A(1)R-/- and wild-type (WT) mice after 4 weeks of streptozotocin diabetes on a control NaCl diet (1%), and measurements were repeated after 6 days of equilibration on a low-NaCl (0.1%) or a high-NaCl (4%) diet.

RESULTS

A(1)R-/- and WT were similar with respect to blood glucose, dietary intakes and body weight changes on a given diet. Diabetic hyperfiltration occurred in WT, but was blunted in A(1)R-/-. A reciprocal relationship between GFR and dietary salt was found in WT diabetics, but not A(1)R-/- diabetics or nondiabetics of either strain.

CONCLUSION

A(1)Rs determine glomerular hyperfiltration and the salt paradox in early diabetes, which is consistent with the tubulocentric principle.

Proteinuria in mice expressing PKB/SGK-resistant GSK3

SGK1 is critically important for mineralocorticoid/salt-induced glomerular injury. SGK1 inactivates GSK3, which downregulates Snail, a DNA-binding molecule repressing the transcription of nephrin, a protein critically important for the integrity of the glomerular slit membrane. PKB/SGK-dependent GSK regulation is disrupted in mice carrying a mutation, in which the serine in the SGK/PKB-phosphorylation consensus sequence is replaced by alanine. The present study explored whether PKB/SGK-dependent GSK3 regulation influences glomerular proteinuria. Gene-targeted knockin mice with mutated and thus PKB/SGK-resistant GSK3alpha,beta (gsk3(KI)) were compared with their wild-type littermates (gsk3(WT)). gsk3(KI) and gsk3(WT) mice were implanted with DOCA release pellets and offered 1% saline as drinking water for 21 days. Under standard diet, tap water intake and absence of DOCA, urinary flow rate, glomerular filtration rate, and urinary albumin excretion were significantly larger and blood pressure was significantly higher in gsk3(KI) than in gsk3(WT) mice. Within 18 days, DOCA/salt treatment significantly increased fluid intake and urinary flow rate, urinary protein and albumin excretion, and blood pressure in both genotypes but the respective values were significantly higher in gsk3(KI) than in gsk3(WT) mice. Plasma albumin concentration was significantly lower in gsk3(KI) than in gsk3(WT) mice. Proteinuria was abrogated by lowering of blood pressure with alpha(1)-blocker prazosin (1 microg/g body wt) in 8-mo-old mice. According to immunofluorescence, nephrin at 3 and 8 mo and podocin expression at 3 mo were significantly lower in gsk3(KI) than in gsk3(WT) mice. After 18 days, DOCA/salt treatment renal glomerular sclerosis and tubulointerstitial damage were significantly more pronounced in gsk3(KI) than in gsk3(WT) mice. The observations reveal that disruption of PKB/SGK-dependent regulation of GSK3 leads to glomerular injury with proteinuria, which may at least partially be secondary to enhanced blood pressure.

Ornithine decarboxylase inhibitor eliminates hyperresponsiveness of the early diabetic proximal tubule to dietary salt

Heightened sensitivity of the diabetic proximal tubule to dietary salt leads to a paradoxical effect of salt on glomerular filtration rate (GFR) via tubuloglomerular feedback. Diabetic hyperfiltration is a feedback response to growth and hyperreabsorption by the proximal tubule. The present studies were performed to determine whether growth and hyperfunction of the proximal tubule are essential for its hyperresponsiveness to dietary salt and, hence, to the paradoxical effect of dietary salt on GFR. Micropuncture was performed in four groups of inactin-anesthetized Wistar rats after 10 days of streptozotocin diabetes drinking tap water or 1% NaCl. Kidney growth was suppressed with ornithine decarboxylase (ODC) inhibitor, DFMO (200 mg.kg(-1).day(-1)), or placebo. Single nephron GFR (SNGFR) was manipulated by perfusing Henle's loop so that proximal reabsorption (Jprox) could be expressed as a function of SNGFR in each nephron, dissociating primary effects on the tubule from the effects of glomerulotubular balance. Alone, DFMO or high salt reduced SNGFR and suppressed Jprox independent of SNGFR. Suppression of Jprox was eliminated and SNGFR increased when high salt was given to rats receiving DFMO. ODC is necessary for hyperresponsiveness of the proximal tubule to dietary salt and for the paradoxical effect of dietary salt on GFR in early diabetes. This coupling of effects adds to the body of evidence that feedback from the proximal tubule is the principal governor of glomerular filtration in early diabetes.

Can rodent models of diabetic kidney disease clarify the significance of early hyperfiltration?: recognizing clinical and experimental uncertainties

In the past, hyperfiltration and increased glomerular capillary pressure have been identified as important determinants of the development of DN (diabetic nephropathy). Recently, some basic research and clinical reviews on DN have omitted identifying hyperfiltration as an important risk factor. At the same time, different rodent models of DN have been described without and with documented hyperfiltration. In the present review, the importance of hyperfiltration is reassessed, reviewing key clinical and research studies, including the first single nephron studies in a mouse model of DN. From clinical studies of Type 1 and Type 2 diabetes mellitus, it is clear that many patients do not have early hyperfiltration and, even when present, its contribution to subsequent DN remains uncertain. Key mechanisms underlying hyperfiltration in rodent models are reviewed. Findings on intrarenal NO metabolism and the control of single-nephron GFR (glomerular filtration rate) in rodent models of DN are also presented. Characterization of valid experimental models of DN should include a careful delineation of the absence or presence of early hyperfiltration, with special efforts made to establish the specific role hyperfiltration may play in the emergence of DN.

CHRONIC SALT LOADING AND CARDIOVASCULAR-ASSOCIATED CHANGES IN EXPERIMENTAL DIABETES IN RATS

1. High-sodium intake may increase blood pressure and diabetes is a salt-sensitive condition. In the present study, we evaluated cardiovascular changes and their neurohumoral mechanisms in streptozotocin (STZ)-diabetic rats that underwent chronic salt loading. 2. We studied male Wistar rats (150-280 g) 14 days after the injection of either STZ (50 mg/kg, i.v.; D; n = 18) or citrate buffer (C; n = 16). After the induction of diabetes, animals were maintained for 14 days with free access to standard rat chow and tap water (C and D groups) or 1% NaCl solution (C-S and D-S groups). We conducted two experiments. Experiment 1 consisted of basal arterial pressure (AP) measurement (30 min) followed by the evaluation of AP responsiveness to phenylephrine and sodium nitroprusside. One day later, with the rats anaesthetized, a blood sample was collected to test for glycaemia, plasma angiotensin-converting enzyme (ACE) activity and renin. Kidneys were removed for the determination of tissue ACE activity. Experiment 2 comprised 24 h urine collection followed by 3 days of cardiovascular records, which consisted of a 30 min basal AP measurement, followed by injection of blockers of the vasopressin system, the renin-angiotensin system (RAS) and the sympathetic system. Basal haemodynamic data, baroreflex evaluation and AP responses to blockade of the vasopressin system with vasopressin V(1) receptor antagonist (aAVP; 10 mg/kg, i.v.), the RAS by losartan (10 mg/kg, i.v.) and the sympathetic system by hexamethonium (20 mg/kg, i.v.) were determined. 3. Glycaemia was similar between C and C-S (P = 0.612) and between D and D-S (P = 0.552), but higher in diabetic compared with non-diabetic rats (P < 0.0001). The D-S rats had an increment of 24% in mean AP compared with D (120 +/- 4 vs 97 +/- 2 mmHg, respectively; P = 0.0001), which was not seen in C-S compared with C rats. A positive association was noted between urinary sodium and mean AP (r = 0.37; P = 0.04). Plasma renin was undetectable in D-S rats. The response to acute drug blockade of vasopressin and the RAS was similar among groups, but hexamethonium elicited a more pronounced decrease in AP in D-S compared with D rats (P = 0.001). 4. The main neurohumoral mechanisms of salt-induced cardiovascular changes in STZ-diabetes are increased sodium and vascular sensitivity to adrenergic stimuli, which act in combination to produce a final result of higher AP levels, a finding not observed in control rats. Baroreflex derangements induced by diabetes were not affected by salt overload.

Blunted hypertensive effect of combined fructose and high-salt diet in gene-targeted mice lacking functional serum- and glucocorticoid-inducible kinase SGK1

Serum- and glucocorticoid-inducible kinase (SGK1) is transcriptionally upregulated by mineralocorticoids and activated by insulin. The kinase stimulates the renal epithelial Na(+) channel and may thus participate in blood pressure regulation. Hyperinsulinemia is triggered by dietary fructose, which sensitizes blood pressure for salt intake. The role of SGK1 in hypertensive effects of combined fructose and high-salt intake was thus explored in SGK1 knockout mice (sgk1(-/-)) and their wild-type littermates (sgk1(+/+)). Renal SGK1 transcript levels of sgk1(+/+) mice were significantly elevated after fructose diet. Under control diet, fluid intake, urinary flow rate, urinary Na(+), K(+), and Cl(-) excretion, and blood pressure were similar in sgk1(-/-) and sgk1(+/+) mice. Addition of 10% fructose to drinking water increased fluid intake and urinary flow rate in both genotypes, and did not significantly alter urinary Na(+), K(+), and Cl(-) output in either genotype. Additional high NaCl diet (4% NaCl) did not significantly alter fluid intake and urine volume but markedly increased urinary output of Na(+) and Cl(-), approaching values significantly (P < 0.05) larger in sgk1(-/-) than in sgk1(+/+) mice (Na(+): 2,572 +/- 462 vs. 1,428 +/- 236; Cl(-): 2,364 +/- 388 vs. 1,379 +/- 225 micromol/24 h). Blood pressure was similar in sgk1(+/+) and sgk1(-/-) mice at control diet or fructose alone but increased only in sgk1(+/+) mice (115 +/- 1 vs. 103 +/- 0.7 mmHg, P < 0.05) after combined fructose and high-salt intake. Acute intravenous insulin infusion (during glucose clamp) caused antinatriuresis in sgk1(+/+) mice, an effect significantly blunted in sgk1(-/-) mice. The observations reveal a pivotal role of SGK1 in insulin-mediated sodium retention and the salt-sensitizing hypertensive effect of high fructose intake.

The salt paradox and its possible implications in managing hypertensive diabetic patients

Diabetes mellitus is one of the leading causes of end-stage renal disease. The pathogenesis of diabetic nephropathy is still poorly understood, but glomerular injury has been ascribed, at least in part, to glomerular hyperfiltration, which occurs early in the course of diabetes mellitus. Therefore, a better understanding of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. In this review, we discuss the pathophysiology for the paradoxical relationship between dietary salt and glomerular filtration rate observed in early diabetes mellitus and possible implications in managing diabetic patients.

High Sodium Intake Increases Blood Pressure and Alters Renal Function in Intrauterine Growth–Retarded Rats

A suboptimal fetal environment increases the risk to develop cardiovascular disease in the adult. We reported previously that intrauterine stress in response to reduced uteroplacental blood flow in the pregnant rat limits fetal growth and compromises renal development, leading to an altered renal function in the adult offspring. Here we tested the hypothesis that high dietary sodium intake in rats with impaired renal development attributable to intrauterine stress, results in increased blood pressure, altered renal function, and organ damage. In rats, intrauterine stress was induced by bilateral ligation of the uterine arteries at day 17 of pregnancy. At the age of 12 weeks, the offspring was given high-sodium drinking water (2% sodium chloride). At the age of 16 weeks, rats were instrumented for monitoring of blood pressure and renal function. After intrauterine stress, litter size and birth weight were reduced, whereas hematocrit at birth was increased. Renal blood flow, glomerular filtration rate, and the glomerular filtration fraction were increased significantly after intrauterine stress. High sodium intake did not change renal function and blood pressure in control animals. However, during high sodium intake in intrauterine stress offspring, renal blood flow, glomerular filtration rate, and the filtration fraction were decreased, and blood pressure was increased. In addition, these animals developed severe albuminuria, an important sign of renal dysfunction. Thus, a suboptimal fetal microenvironment, which impairs renal development, results in sodium-dependent hypertension and albuminuria.

Glomerular hyperfiltration in type 1 diabetes mellitus results from primary changes in proximal tubular sodium handling without changes in volume expansion

BACKGROUND

Glomerular hyperfiltration plays a role in the pathophysiology of diabetic nephropathy. An increase in the glomerular filtration rate (GFR) could result from primary actions at the glomerular/vascular level or could be the consequence of a primary increase in proximal tubular sodium reabsorption resulting in systemic volume expansion. Recently it was hypothesized that an increase in sodium reabsorption may lead to glomerular hyperfiltration through the tubulo-glomerular feedback mechanism (tubular-hypothesis) without volume expansion.

DESIGN

We have studied 54 normoalbuminuric patients with type 1 diabetes. The GFR was measured by inulin clearance. Proximal and distal sodium reabsorption were calculated according to standard formulas using the free water clearance technique. Plasma volume, measured by the (125)I-albumin method, atrial natriuretic peptide (ANP) and the second messenger cyclic guanosine-3,5-monophosphate (c-GMP) were used as markers of extracellular volume expansion.

RESULTS

Glomerular hyperfiltration (GFR >or= 130 mL min(-1) 1.73 m(-2)) was present in 14 out of 55 patients with diabetes (25%). There were no differences in plasma volume between normo-(NF) and hyper-filtrating (HF) patients (2933 +/- 423 in NF vs. 3026 +/- 562 mL in HF, NS). Also plasma ANP and c-GMP levels were not significantly different between the groups. The fractional proximal reabsorption of sodium was significantly increased in HF [fPRNa(+) (%) 90.1 +/- 2.0 vs. 91.5 +/- 1.6, P = 0.02]. There were no differences in distal sodium reabsorption or distal sodium load (approximately macula densa concentration of NaCl) in both groups.

CONCLUSIONS

Our data suggest that the primary event in diabetic glomerular hyperfiltration is an increase in proximal tubular sodium reabsorption. They do not support the hypothesis that systemic volume expansion or ANP mediate glomerular hyperfiltration in patients with normoalbuminuric type 1 diabetes. As such, changes in tubular sodium handling most probably influence tubulo-glomerular feedback.

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

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

Kidney function in early diabetes: the tubular hypothesis of glomerular filtration

At the onset of diabetes mellitus, the glomerular filtration rate becomes supranormal. Discovery science has identified many abnormalities in the early diabetic kidney that apparently contribute to this phenotype. A serviceable understanding of the early diabetic kidney requires this information to fit together. It is the purpose of this article to present an archetype that explains multiple nuances of kidney function in early diabetes. We refer to this archetype as the "tubular hypothesis of glomerular filtration." Its basic tenet is that strange effects of diabetes on glomerular filtration stem from primary effects on the proximal tubule or loop of Henle that impact glomerular filtration by feedback through the macula densa. This theory can explain diabetic hyperfiltration, a paradoxical effect of dietary salt on glomerular filtration rate in diabetes, and the renal response to dietary protein and amino acid infusion in diabetes. The discussion centers on the kidney as an integrated system of parts rather than on the specific cellular mechanisms that comprise those parts.

Effect of longer-term modest salt reduction on blood pressure

BACKGROUND

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

OBJECTIVES

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

SEARCH STRATEGY

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

SELECTION CRITERIA

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

DATA COLLECTION AND ANALYSIS

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

MAIN RESULTS

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

REVIEWERS' CONCLUSIONS

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

Tubuloglomerular feedback and the control of glomerular filtration rate

In every nephron the glomerular filtration rate is adapted to changes in the salt concentration of early distal tubular fluid through the mechanism of tubuloglomerular feedback. Recent studies indicate that adenosine and possibly ATP mediate this mechanism and demonstrate its role in glomerular hemodynamic alterations in the early diabetic kidney.

Paracrine factors in tubuloglomerular feedback: adenosine, ATP, and nitric oxide

The tubuloglomerular feedback response, the change in afferent arteriolar tone caused by a change in NaCl concentration at the macula densa, is likely initiated by the generation of a vasoactive mediator within the confines of the juxtaglomerular apparatus. Substantial progress has been made in identifying the nature of this mediator and the factors that modulate its effect on vascular tone. In support of earlier studies using P1 purinergic antagonists, the application of the knockout technique has shown that adenosine 1 receptors are absolutely required for eliciting TGF responses. The background level of angiotensin II appears to be an important cofactor determining the efficiency of A1AR-induced vasoconstriction, probably through a synergistic interaction at the level of the G protein-dependent transduction mechanism. The source of the adenosine is still unclear, but it is conceivable that adenosine is generated extracellularly from released ATP through a cascade of ecto-nucleotidases. There is also evidence that ATP may activate P2 receptors in preglomerular vessels, which may contribute to autoregulation of renal vascular resistance. Nitric oxide (NO), generated by the neuronal isoform of nitric oxide synthase in macula densa cells, reduces the constrictor effect of adenosine, but the regulation of NO release and its exact role in states of TGF-induced hyperfiltration are still unclear.