Dietary salt modulates renal production of transforming growth factor-beta in rats

Self-Reported Frequency of Adding Salt to Food and Risk of Incident Chronic Kidney Disease

Importance

The self-reported frequency of adding salt to foods could reflect a person's long-term salt taste preference, and salt intake has been associated with increased risk of cardiovascular diseases (CVD). Whether self-reported adding of salt to foods is associated with increased risk of chronic kidney disease (CKD) remains unknown.

Objective

To prospectively examine the association of self-reported frequency of adding salt to foods with incident CKD risk in a general population of adults.

Design, Setting, and Participants

This population-based cohort study evaluated UK Biobank participants aged 37 to 73 years who were free of CKD at baseline. Participants were enrolled from 2006 to 2010 and prospectively followed up for disease diagnosis. Data were analyzed from October 2022 to April 2023.

Exposure

Self-reported frequency of adding salt to foods, categorized into never or rarely, sometimes, usually, and always.

Main Outcome and Measure

Incident CKD cases were defined by diagnostic codes. Hazard ratios (HRs) and 95% CIs were calculated by using Cox proportional hazards models. Models were adjusted for several potential confounders including age, sex, race and ethnicity, Townsend Deprivation Index, estimated glomerular filtration rate (eGFR), body mass index, (BMI), smoking status, alcohol drinking status, regular physical activity, high cholesterol, diabetes, CVD, hypertension, infectious disease, immune disease, and nephrotoxic drugs use at baseline.

Results

Within a cohort of 465 288 individuals (mean [SD] age 56.32 [8.08] years; 255 102 female participants [54.83%]; 210 186 male participants [45.17%]), participants with higher self-reported frequency of adding salt to foods were more likely to have a higher BMI, higher Townsend Deprivation Index score, and diminished baseline eGFR compared with those who reported a lower frequency of adding salt to foods. Participants who added salt to their foods were also more likely than those who did not add salt to their foods to be current smokers and have diabetes or CVD at baseline. During a median (IQR) follow-up of 11.8 (1.4) years, 22 031 incident events of CKD were documented. Higher self-reported frequency of adding salt to foods was significantly associated with a higher CKD risk after adjustment for covariates. Compared with those who reported never or rarely adding salt to foods, those who reported sometimes adding salt to food (adjusted HR [aHR], 1.04; 95% CI, 1.00-1.07), those who reported usually adding salt to food (aHR, 1.07; 95% CI, 1.02-1.11), and those who reported always adding salt to food (aHR, 1.11; 95% CI, 1.05-1.18) had an increased risk of CKD (P for trend < .001). In addition, eGFR, BMI, and physical activity significantly modified the associations, which were more pronounced among participants with a higher eGFR, lower BMI, or lower level of physical activity.

Conclusions and Relevance

In this cohort study of 465 288 individuals, a higher self-reported frequency of adding salt to foods was associated with a higher risk of CKD in the general population. These findings suggest that reducing the frequency of adding salt to foods at the table might be a valuable strategy to lower CKD risk in the general population.

Sodium Intake as a Cardiovascular Risk Factor: A Narrative Review

While sodium is essential for human homeostasis, current salt consumption far exceeds physiological needs. Strong evidence suggests a direct causal relationship between sodium intake and blood pressure (BP) and a modest reduction in salt consumption is associated with a meaningful reduction in BP in hypertensive as well as normotensive individuals. Moreover, while long-term randomized controlled trials are still lacking, it is reasonable to assume a direct relationship between sodium intake and cardiovascular outcomes. However, a consensus has yet to be reached on the effectiveness, safety and feasibility of sodium intake reduction on an individual level. Beyond indirect BP-mediated effects, detrimental consequences of high sodium intake are manifold and pathways involving vascular damage, oxidative stress, hormonal alterations, the immune system and the gut microbiome have been described. Globally, while individual response to salt intake is variable, sodium should be perceived as a cardiovascular risk factor when consumed in excess. Reduction of sodium intake on a population level thus presents a potential strategy to reduce the burden of cardiovascular disease worldwide. In this review, we provide an update on the consequences of salt intake on human health, focusing on BP and cardiovascular outcomes as well as underlying pathophysiological hypotheses.

Association between Daily Urinary Sodium Excretion, Ratio of Extracellular Water-to-Total Body Water Ratio, and Kidney Outcome in Patients with Chronic Kidney Disease

Whether dietary salt intake affects chronic kidney disease (CKD) progression remains unclear. We conducted a retrospective cohort study to analyze the effects of both daily salt intake (DSI) and volume status on renal outcomes in 197 CKD patients. DSI was estimated by 24-h urinary sodium excretion and volume status was assessed by the ratio of extracellular water (ECW) to total body water (TBW) measured by bioelectrical impedance analysis (BIA). We divided patients into two groups according to DSI (6 g/day) or median ECW/TBW (0.475) and compared renal outcomes of each group. Furthermore, we classified and analyzed four groups according to both DSI and ECW/TBW. The higher DSI group showed a 1.69-fold (95% confidence interval (CI) 1.12–2.57, p = 0.01) excess risk of outcome occurrence compared to the lower group. Among the four groups, compared with Group 1 (low DSI and low ECW/TBW), Group 3 (high DSI and low ECW/TBW) showed a 1.84-fold (95% CI 1.03–3.30, p = 0.04) excess risk of outcome occurrence; however, Group 2 (low DSI and high ECW/TBW) showed no significant difference. High salt intake appears to be associated with poor renal outcome independent of blood pressure (BP), proteinuria, and volume status.

Salt sensitivity and hypertension

Salt sensitivity refers to the physiological trait present in mammals, including humans, by which the blood pressure (BP) of some members of the population exhibits changes parallel to changes in salt intake. It is commoner in elderly, females, Afro-Americans, patients with chronic kidney disease (CKD) and insulin resistance. Increased salt intake promotes an expansion of extracellular fluid volume and increases cardiac output. Salt-sensitive individuals present an abnormal kidney reaction to salt intake; the kidneys retain most of the salt due to an abnormal over-reactivity of sympathetic nervous system and a blunted suppression of renin-angiotensin axis. Moreover, instead of peripheral vascular resistance falling, salt-sensitive subjects present increased vascular resistance due mainly to impaired nitric oxide synthesis in endothelium. Recent studies have shown that part of the dietary salt loading accumulates in skin. Hypertensive and patients with CKD seem to have more sodium in skin comparing to healthy ones. However, we still have not fully explained the link between skin sodium, BP and salt sensitivity. Finally, although salt sensitivity plays a meaningful role in BP pathophysiology, it cannot be used by the physician in everyday patient's care, mainly due to lack of a simple and practical diagnostic test.

Sodium Intake and Chronic Kidney Disease

In Chronic Kidney Disease (CKD) patients, elevated blood pressure (BP) is a frequent finding and is traditionally considered a direct consequence of their sodium sensitivity. Indeed, sodium and fluid retention, causing hypervolemia, leads to the development of hypertension in CKD. On the other hand, in non-dialysis CKD patients, salt restriction reduces BP levels and enhances anti-proteinuric effect of renin–angiotensin–aldosterone system inhibitors in non-dialysis CKD patients. However, studies on the long-term effect of low salt diet (LSD) on cardio-renal prognosis showed controversial findings. The negative results might be the consequence of measurement bias (spot urine and/or single measurement), reverse epidemiology, as well as poor adherence to diet. In end-stage kidney disease (ESKD), dialysis remains the only effective means to remove dietary sodium intake. The mismatch between intake and removal of sodium leads to fluid overload, hypertension and left ventricular hypertrophy, therefore worsening the prognosis of ESKD patients. This imposes the implementation of a LSD in these patients, irrespective of the lack of trials proving the efficacy of this measure in these patients. LSD is, therefore, a rational and basic tool to correct fluid overload and hypertension in all CKD stages. The implementation of LSD should be personalized, similarly to diuretic treatment, keeping into account the volume status and true burden of hypertension evaluated by ambulatory BP monitoring.

Arterial Stiffness and Central Hemodynamics are Associated with Low Diurnal Urinary Sodium Excretion

BACKGROUND

Excessive salt intake is an important determinant of cardiovascular (CV) health, impacting arterial stiffness and central blood pressure. However, sodium exhibits several patterns of excretion in urine during day- and night-time, which could differently affect CV risk. Here, we sought to explore the relationship between the day:night urinary sodium excretion ratio and arterial stiffness and central hemodynamics in the general population.

METHODS

Cross-sectional analysis in 1062 subjects. Arterial stiffness (pulse-wave velocity, PWV), central blood pressure (central systolic blood pressure, cSBP; central diastolic blood pressure, cDBP), and other hemodynamic parameters were noninvasively assessed. Day- and night-time urinary sodium were separately detected. Analyses were performed according to the day:night urinary sodium excretion ratio tertiles (T1-T3).

RESULTS

Low day-time excretors (T1) showed significantly higher values of arterial stiffness when compared with high day-time excretors (T3) (cf-PWV 7.6 ± 1.9 vs 6.9 ± 1.5 m/sec; p ≤ 0.001), and higher central BP parameters (cSBP: 111.6 ± 12.1 vs 109.0 ± 11.1 mmHg, p ≤ 0.001; cDBP, 76.9 ± 9.2 vs 75.1 ± 9.3 mmHg, p ≤ 0.001). In multivariate linear-regression models (β, CI), the day:night ratio of sodium excretion was significantly associated with arterial stiffness (cf-PWV -0.386, -0.559, -0.213, p ≤ 0.001) and with central hemodynamic parameters (cSBP -1.655, -2.800, -0.510; p ≤ 0.001; cDBP -1.319, -2.218, -0.420, p ≤ 0.001). Associations persisted after controlling for multiple confounding factors. In logistic-regression models, the risk of increased arterial stiffness was significantly reduced as the day:night ratio of urinary sodium excretion increased (OR 0.40, 95% CI 0.25-0.65, p ≤ 0.001).

CONCLUSION

The individual, intra-daily pattern of urinary sodium excretion, characterised by low daytime excretion, is associated with increased arterial stiffness and central blood pressure. Further studies are advocated to clarify the clinical utility of assessing the daily pattern of sodium excretion.

Association of unhealthy dietary behaviors with renal function decline in patients with diabetes

OBJECTIVE

Balanced nutrition is important for patients with diabetes, and nutrition might well influence diabetes-related complications, although there is limited evidence for this supposition at present. Consequently, we investigate the association between dietary behaviors and renal function decline among patients with diabetes.

RESEARCH DESIGN AND METHODS

From 2011 to 2013, a total of 2797 patients with type 2 diabetes participated in the Diabetes Shared Care Program at Chang Gung Memorial Hospital. All received nutritional consulting by dieticians and an eight-item list of unhealthy dietary behaviors, which included the excessive intake of carbohydrates, fats, protein, fruit, pickled foods, dessert and alcohol, as well as inadequate dietary vegetable. Estimated glomerular filtration rate (eGFR) decline ≥40% was defined as a surrogate end point for kidney damage. Independent dietary risk factors predicting poor renal outcomes were assessed.

RESULTS

Stable mean glycated hemoglobin (A_1c) (7.78% to 7.75%, p=0.151), improved cholesterol (174.04 to 170.13 mg/dL, p<0.001) and low-density lipoprotein (104.19 to 98.07 mg/dL, p<0.001) were found in patients throughout 2 years of therapy. However, significant eGFR decline was noted (94.20 to 88.08 mL/min/1.73 m², p<0.001). A total of 125 subjects had eGFR decline ≥40% and 2672 had stable renal progression.In regression analysis, 625 stable renal patients (selected via propensity score matching) and 125 subjects with eGFR decline ≥40% demonstrated excessive pickled foods to be predictive of poor renal outcomes (OR 1.861, 95% CI 1.230 to 2.814, p=0.003).

CONCLUSIONS

Our study suggests that excessive pickled foods deteriorate renal function more than other unhealthy dietary behaviors in patients with diabetes.

Body Fluid-Independent Effects of Dietary Salt Consumption in Chronic Kidney Disease

The average dietary salt (i.e., sodium chloride) intake in Western society is about 10 g per day. This greatly exceeds the lifestyle recommendations by the WHO to limit dietary salt intake to 5 g. There is robust evidence that excess salt intake is associated with deleterious effects including hypertension, kidney damage and adverse cardiovascular health. In patients with chronic kidney disease, moderate reduction of dietary salt intake has important renoprotective effects and positively influences the efficacy of common pharmacological treatment regimens. During the past several years, it has become clear that besides influencing body fluid volume high salt also induces tissue remodelling and activates immune cell homeostasis. The exact pathophysiological pathway in which these salt-induced fluid-independent effects contribute to CKD is not fully elucidated, nonetheless it is clear that inflammation and the development of fibrosis play a major role in the pathogenic mechanisms of renal diseases. This review focuses on body fluid-independent effects of salt contributing to CKD pathogenesis and cardiovascular health. Additionally, the question whether better understanding of these pathophysiological pathways, related to high salt consumption, might identify new potential treatment options will be discussed.

Diet and Chronic Kidney Disease

Kidney disease affects almost 15% of the US population, and prevalence is anticipated to grow as the population ages and the obesity epidemic continues due to Western dietary practices. The densely caloric Western diet, characterized by high animal protein and low fruit and vegetable content, has fueled the growth of chronic diseases, including chronic kidney disease. The glomerulus or filtering unit of the kidney is very susceptible to barotrauma, and diets high in animal protein impede the glomerulus' ability to protect itself from hemodynamic injury. High animal protein intake combined with low intake of fruits and vegetables also leads to a high net endogenous acid production requiring augmentation of ammonium excretion in order to prevent acidosis. This higher workload of the kidney to maintain a normal serum bicarbonate level may further exacerbate kidney disease progression. This article reviews the potential mechanisms whereby several key characteristics of the typical Western diet may impact kidney disease incidence and progression. Reducing animal protein intake and egg yolk and increasing intake of fruits and vegetables and fiber may prevent or delay end-stage renal disease, but few clinical trials have examined vegetarian diets for management of chronic kidney disease. More research is needed to determine optimal dietary patterns for the prevention of kidney disease and its progression.

Contribution of volume overload to progression of cardiovascular disease in a rat model of chronic kidney disease

Volume overload is a common phenomenon in patients with chronic kidney disease that is associated with cardiovascular risk factors. However, its contribution to the development of adverse cardiovascular outcomes in those patients is not fully understood. Thus, the present work investigated the effect of salt-induced volume overload on cardiac functions and geometry in a rat model of chronic kidney disease. Thirty adult male Sprague-Dawley rats were randomly divided. One set of animals received a sham operation, while another set of animals underwent uninephrectomy. Rats were then fed either a normal-salt (0.4%) or high-salt (8.0%) diet for 6 weeks. The salt-loaded, uninephrectomized rats were treated with indapamide (3 mg·kg^-1·day^-1, orally) for 6 weeks. We found that uninephrectomized rats subjected to a high-salt diet (8.0%) for 6 weeks presented with hypertension, proteinuria, decreased renal Klotho expression, and deterioration in cardiac hemodynamics and histology. Echocardiography to assess cardiac function showed that ejection fraction and fractional shortening were positively correlated with relative renal Klotho expression. In conclusion, salt-induced volume overload in a rat model of chronic kidney disease has an adverse cardiovascular outcome and is associated with inflammatory activation and decrease in renal Klotho expression.

Sustained hyperosmolarity increses TGF-ß1 and Egr-1 expression in the rat renal medulla

BACKGROUND

Although TGF-ß and the transcription factor Egr-1 play an important role in both kidney fibrosis and in response to acute changes of renal medullary osmolarity, their role under sustained hypo- or hyperosmolar conditions has not been elucidated. We investigated the effects of chronic hypertonicity and hypotonicity on the renal medullary TGF-ß and Egr-1 expression.

METHODS

Male adult Sprague Dawley rats (n = 6/group) were treated with 15 mg/day furosemide, or the rats were water restricted to 15 ml/200 g body weight per day. Control rats had free access to water and rodent chow. Kidneys were harvested after 5 days of treament. In cultured inner medullary collecting duct (IMCD) cells, osmolarity was increased from 330 mOsm to 900 mOsm over 6 days. Analyses were performed at 330, 600 and 900 mOsm.

RESULTS

Urine osmolarity has not changed due to furosemide treatment but increased 2-fold after water restriction (p < 0.05). Gene expression of TGF-ß and Egr-1 increased by 1.9-fold and 7-fold in the hypertonic medulla, respectively (p < 0.05), accompanied by 6-fold and 2-fold increased c-Fos and TIMP-1 expression, respectively (p < 0.05) and positive immunostaining for TGF-ß and Egr-1 (p < 0.05). Similarly, hyperosmolarity led to overexpression of TGF-ß and Egr-1 mRNA in IMCD cells (2.5-fold and 3.5-fold increase from 330 to 900 mOsm, respectively (p < 0.05)) accompanied by significant c-Fos and c-Jun overexpressions (p < 0.01), and increased Col3a1 and Col4a1 mRNA expression.

CONCLUSION

We conclude that both TGF-ß and Egr-1 are upregulated by sustained hyperosmolarity in the rat renal medulla, and it favors the expression of extracellular matrix components.

Kidney dysfunction in patients with pulmonary arterial hypertension

Pulmonary arterial hypertension (PH) and chronic kidney disease (CKD) both profoundly impact patient outcomes, whether as primary disease states or as co-morbid conditions. PH is a common co-morbidity in CKD and vice versa. A growing body of literature describes the epidemiology of PH secondary to chronic kidney disease and end-stage renal disease (ESRD) (WHO group 5 PH). But, there are only limited data on the epidemiology of kidney disease in group 1 PH (pulmonary arterial hypertension [PAH]). The purpose of this review is to summarize the current data on epidemiology and discuss potential disease mechanisms and management implications of kidney dysfunction in PAH. Kidney dysfunction, determined by serum creatinine or estimated glomerular filtration rate, is a frequent co-morbidity in PAH and impaired kidney function is a strong and independent predictor of mortality. Potential mechanisms of PAH affecting the kidneys are increased venous congestion, decreased cardiac output, and neurohormonal activation. On a molecular level, increased TGF-β signaling and increased levels of circulating cytokines could have the potential to worsen kidney function. Nephrotoxicity does not seem to be a common side effect of PAH-targeted therapy. Treatment implications for kidney disease in PAH include glycemic control, lifestyle modification, and potentially Renin-Angiotensin-Aldosterone System (RAAS) blockade.

Novel Paradigms of Salt and Hypertension

Salt resistance/sensitivity refers specifically to the effect of dietary sodium chloride (salt) intake on BP. Increased dietary salt intake promotes an early and uniform expansion of extracellular fluid volume and increased cardiac output. To compensate for these hemodynamic changes and maintain constant BP in salt resistance, renal and peripheral vascular resistance falls and is associated with an increase in production of nitric oxide. In contrast, the decline in peripheral vascular resistance and the increase in nitric oxide are impaired or absent in salt sensitivity, promoting an increase in BP in these individuals. Endothelial dysfunction may pose a particularly significant risk factor in the development of salt sensitivity and subsequent hypertension. Vulnerable salt-sensitive populations may have in common underlying endothelial dysfunction due to genetic or environmental influences. These individuals may be very sensitive to the hemodynamic stress of increased effective blood volume, setting in motion untoward molecular and biochemical events that lead to overproduction of TGF-β, oxidative stress, and limited bioavailable nitric oxide. Finally, chronic high-salt ingestion produces endothelial dysfunction, even in salt-resistant subjects. Thus, the complex syndrome of salt sensitivity may be a function of the endothelium, which is integrally involved in the vascular responses to high salt intake.

Effects of Vitamin D Receptor Activation and Dietary Sodium Restriction on Residual Albuminuria in CKD: The ViRTUE-CKD Trial

Reduction of residual albuminuria during single-agent renin-angiotensin-aldosterone blockade is accompanied by improved cardiorenal outcomes in CKD. We studied the individual and combined effects of the vitamin D receptor activator paricalcitol (PARI) and dietary sodium restriction on residual albuminuria in CKD. In a multicenter, randomized, placebo (PLAC)-controlled, crossover trial, 45 patients with nondiabetic CKD stages 1-3 and albuminuria >300 mg/24 h despite ramipril at 10 mg/d and BP<140/90 mmHg were treated for four 8-week periods with PARI (2 μg/d) or PLAC, each combined with a low-sodium (LS) or regular sodium (RS) diet. We analyzed the treatment effect by linear mixed effect models for repeated measurements. In the intention-to-treat analysis, albuminuria (geometric mean) was 1060 (95% confidence interval, 778 to 1443) mg/24 h during RS + PLAC and 990 (95% confidence interval, 755 to 1299) mg/24 h during RS + PARI (P=0.20 versus RS + PLAC). LS + PLAC reduced albuminuria to 717 (95% confidence interval, 512 to 1005) mg/24 h (P<0.001 versus RS + PLAC), and LS + PARI reduced albuminuria to 683 (95% confidence interval, 502 to 929) mg/24 h (P<0.001 versus RS + PLAC). The reduction by PARI beyond the effect of LS was nonsignificant (P=0.60). In the per-protocol analysis restricted to participants with ≥95% compliance with study medication, PARI did provide further albuminuria reduction (P=0.04 LS + PARI versus LS + PLAC). Dietary adherence was good as reflected by urinary excretion of 174±64 mmol Na⁺ per day in the combined RS groups and 108±61 mmol Na⁺ per day in the LS groups (P<0.001). In conclusion, moderate dietary sodium restriction substantially reduced residual albuminuria during fixed dose angiotensin-converting enzyme inhibition. The additional effect of PARI was small and nonsignificant.

Is low-protein diet a possible risk factor of malnutrition in chronic kidney disease patients?

Chronic kidney disease (CKD) is becoming increasingly widespread in the world. Slowing its progression means to prevent uremic complications and improve quality of life of patients. Currently, a low-protein diet (LPD) is one of the tools most used in renal conservative therapy but a possible risk connected to LPD is protein-energy wasting. The aim of this study is evaluate the possible correlation between LPD and malnutrition onset. We enrolled 41 CKD patients, stages IIIb/IV according to K-DIGO guidelines, who followed for 6 weeks a diet with controlled protein intake (recommended dietary allowance 0.7 g per kilogram Ideal Body Weight per day of protein). Our patients showed a significant decrease of serum albumin values after 6 weeks of LDP (T2) compared with baseline values (T0) (P=0.039), whereas C-reactive protein increased significantly (T0 versus T2; P=0.131). From body composition analysis, a significant impairment of fat-free mass percentage at the end of the study was demonstrated (T0 versus T2; P=0.0489), probably related to total body water increase. The muscular mass, body cell mass and body cell mass index are significantly decreased after 6 weeks of LDP (T2). The phase angle is significantly reduced at the end of the study compared with basal values (T0 versus T2; P=0.0001, and T1 versus T2; P=0.0015). This study indicated that LPD slows down the progression of kidney disease but worsens patients' nutritional state.

Pancreatic functions in high salt fed female rats

Salt consumption has been increased worldwide and the association of high salt diets with enhanced inflammation and target organ damage was reported. Little data were available about the effect of high salt diet on exocrine function of pancreas, while the relation between high salt intake and insulin sensitivity was controversial. This study was designed to investigate the effect of high salt diet on exocrine and endocrine pancreatic functions, and to elucidate the possible underlying mechanism(s). Twenty adult female Wistar rats were randomly divided into two groups; control group; fed standard rodent diet containing 0.3% NaCl, and high salt fed group; fed 8% NaCl for 8 weeks. On the day of sacrifice, rats were anesthized by i.p. pentobarbitone (40 μg/kg B.W.). Nasoanal length was measured and fasting blood glucose was determined from rat tail. Blood samples were obtained from abdominal aorta for determination of plasma sodium, potassium, amylase, lipase, aldosterone, insulin, transforming growth factor-β (TGF-β1), and interleukin 6 (IL6). Pancreata of both groups were histologically studied. Compared to control group, 8-week high salt fed group showed: significant elevation in body weight, body mass index, Lee index, plasma sodium, TGF-β1 and IL6, however, plasma aldosterone, amylase, lipase, and insulin levels were significantly decreased. A nonsignificant increase in plasma potassium and nonsignificant changes in fasting blood glucose and HOMA-IR were detected between groups. Pancreatic fibrosis was observed in test group. High salt diet for 8 weeks caused pancreatic fibrosis evidenced by decline of both exocrine and endocrine functions of pancreas in Wistar rats.

Volume overload and adverse outcomes in chronic kidney disease: clinical observational and animal studies

BACKGROUND

Volume overload is frequently encountered and is associated with cardiovascular risk factors in patients with chronic kidney disease (CKD). However, the relationship between volume overload and adverse outcomes in CKD is not fully understood.

METHODS AND RESULTS

A prospective cohort of 338 patients with stage 3 to 5 CKD was followed for a median of 2.1 years. The study participants were stratified by the presence or absence of volume overload, defined as an overhydration index assessed by bioimpedance spectroscopy exceeding 7%, the 90th percentile for the healthy population. The primary outcome was the composite of estimated glomerular filtration rate decline ≥50% or end-stage renal disease. The secondary outcome included a composite of morbidity and mortality from cardiovascular causes. Animal models were used to simulate fluid retention observed in human CKD. We found that patients with volume overload were at a higher risk of the primary and secondary end points in the adjusted Cox models. Furthermore, overhydration appears to be more important than hypertension in predicting an elevated risk. In rats subjected to unilateral nephrectomy and a high-salt diet, the extracellular water significantly increased. This fluid retention was associated with an increase in blood pressure, proteinuria, renal inflammation with macrophage infiltration and tumor necrosis factor-α overexpression, glomerular sclerosis, and cardiac fibrosis. Diuretic treatment with indapamide attenuated these changes, suggesting that fluid retention might play a role in the development of adverse outcomes.

CONCLUSIONS

Volume overload contributes to CKD progression and cardiovascular diseases. Further research is warranted to clarify whether the correction of volume overload would improve outcomes for CKD patients.

Management of hypertension in CKD: beyond the guidelines

Hypertension (HTN) and CKD are closely associated with an intermingled cause and effect relationship. Blood pressure (BP) typically rises with declines in kidney function, and sustained elevations in BP hasten progression of kidney disease. This review addresses current management issues in HTN in patients with CKD including altered circadian rhythm of BP, timing of antihypertensive medication dosing, BP targets, diagnostic challenges in evaluating secondary forms of HTN, and the role of salt restriction in CKD. HTN in patients with CKD is often accompanied by a decrease in the kidney's ability to remove salt. Addressing this salt sensitivity is critical for the management of HTN in CKD. In addition to the well-established use of an ACEI or angiotensin receptor blocker, dietary salt restriction and appropriate diuretic therapy make up the mainstay of HTN treatment in patients with CKD. Bedtime dosing of antihypertensive medications can restore nocturnal dips in BP, and future clinical practice guidelines may recommend bedtime dosing of 1 or more antihypertensive medications in patients with CKD.

Hyperosmolarity drives hypertension and CKD--water and salt revisited

An epidemic of chronic kidney disease (CKD) in Mesoamerica is providing new insights into the mechanisms by which salt and water might drive hypertension and CKD. Increasingly, evidence suggests that recurrent dehydration and salt loss might be a mechanism that causes CKD, and experimental studies suggest a key role for increased plasma osmolarity in activating both intrarenal (polyol-fructokinase) and extrarenal (vasopressin) pathways that drive renal injury. Thus, we propose that water and salt might influence blood pressure and kidney disease through the timing and combination of their intake, which affect plasma osmolarity as well as intrarenal and extrarenal mechanisms of renal injury. The type of fluid intake might also be important, as fluids containing fructose can trigger activation of these pathways. Future studies should investigate the effects of salt, sugar and fluid intake on plasma osmolarity as a potential pathogenetic mechanism in renal injury and high blood pressure.

Sodium and potassium regulate endothelial phospholipase C-γ and Bmx

The amount of Na(+) and K(+) in the diet promotes significant changes in endothelial cell function. In the present study, a series of in vitro and in vivo experiments determined the role of Na(+) and K(+) in the regulation of two pleckstrin homology domain-containing intracellular signaling molecules, phospholipase C (PLC)-γ1 and epithelial and endothelial tyrosine kinase/bone marrow tyrosine kinase on chromosome X (Bmx), and agonist-generated Ca(2+) signaling in the endothelium. Extracellular K(+) concentration regulated the levels of activated PLC-γ1, Bmx, and carbachol-stimulated intracellular Ca(2+) mobilization in human endothelial cells. Additional experiments confirmed that high-conductance Ca(2+)-activated K(+) channels and phosphatidylinositol 3-kinase mediated these effects. The content of Na(+) and K(+) in the diet also regulated Bmx levels in endothelial cells and activated PLC-γ1 levels in rats in vivo. The effects of dietary K(+) on Bmx were more pronounced in rats fed a high-salt diet compared with rats fed a low-salt diet. These experiments elucidated an endothelial cell signaling mechanism regulated by electrolytes, further demonstrating an integral relationship between endothelial cell function and dietary Na(+) and K(+) content.

The role of transforming growth factor β1 in the regulation of blood pressure

Although human association studies suggest a link between polymorphisms in the gene encoding transforming growth factor (TGF) β1 and differing blood pressure levels, a causative mechanism for this correlation remains elusive. Recently we have generated a series of mice with graded expression of TGFβ1, ranging from approximately 10% to 300% compared to normal. We have found that blood pressure and plasma volume are negatively regulated by TGFβ1. Of note, the 10% hypomorph exhibits primary aldosteronism and markedly impaired urinary excretion of water and electrolytes. We here review previous literature highlighting the importance of TGFβ signaling as a natriuretic system, which we postulate is a causative mechanism explaining how polymorphisms in TGFβ1 could influence blood pressure levels.

Transforming growth factor-β regulates endothelial function during high salt intake in rats

Previous studies have demonstrated that an increase in dietary NaCl (salt) intake stimulated endothelial cells to produce transforming growth factor-β (TGF-β). The intent of the present study was to determine the functional significance of increased TGF-β on endothelial cell function. Young Sprague-Dawley rats were fed diets containing 0.3 or 8.0% NaCl for 2 days before treatment with a specific inhibitor of the TGF-β receptor I/activin receptor-like kinase 5 kinase, or vehicle for another 2 days. At day 4 of study, endothelial phosphorylated Smad2 (S465/467) increased and phosphatase and tensin homologue deleted on chromosome 10 (PTEN) levels decreased in the high-salt-treated rats. In addition, phosphorylated Akt (S473) and phosphorylation of the endothelial isoform of NO synthase (NOS3) at S1177 increased. Treatment with the TGF-β receptor I/activin receptor-like kinase 5 inhibitor reduced Smad2 phosphorylation to levels observed in rats on the low-salt diet and prevented the downstream signaling events induced by the high-salt diet. In human umbilical vein endothelial cells, reduction in PTEN levels increased phosphorylated Akt and NOS3. Treatment of macrovascular endothelial cells with TGF-β1 increased phosphorylated NOS3 and the concentration of NO metabolites in the medium but had no effect on either of these variables in cells pretreated with small interfering RNA directed against PTEN. Thus, during high salt intake, an increase in TGF-β directly promoted a reduction in endothelial PTEN levels, which in turn regulated Akt activation and NOS3 phosphorylation. This effect closes a feedback loop that potentially mitigates the effect of TGF-β on the vasculature.

Dietary potassium: a key mediator of the cardiovascular response to dietary sodium chloride

Potassium and sodium share a yin/yang relationship in the regulation of blood pressure (BP). BP is directly associated with the total body sodium and negatively correlated with the total body potassium. Epidemiologic, experimental, and clinical studies have shown that potassium is a significant regulator of BP and further improves cardiovascular outcomes. Hypertensive cardiovascular damage, stroke, and stroke-related death are accelerated by salt intake but might be curbed by increasing dietary potassium intake. The antihypertensive effect of potassium supplementation appears to occur through several mechanisms that include regulation of vascular sensitivity to catecholamines, promotion of natriuresis, limiting plasma renin activity, and improving endothelial function. In the absence of chronic kidney disease, the combined evidence suggests that a diet rich in potassium content serves a vasculoprotective function, particularly in the setting of salt-sensitive hypertension and prehypertension.

Antihypertensive and renoprotective effect of the kinin pathway activated by potassium in a model of salt sensitivity following overload proteinuria

The albumin overload model induces proteinuria and tubulointersitial damage, followed by hypertension when rats are exposed to a hypersodic diet. To understand the effect of kinin system stimulation on salt-sensitive hypertension and to explore its potential renoprotective effects, the model was induced in Sprague-Dawley rats that had previously received a high-potassium diet to enhance activity of the kinin pathway, followed with/without administration of icatibant to block the kinin B₂ receptor (B₂R). A disease control group received albumin but not potassium or icatibant, and all groups were exposed to a hypersodic diet to induce salt-sensitive hypertension. Potassium treatment increased the synthesis and excretion of tissue kallikrein (Klk1/rKLK1) accompanied by a significant reduction in blood pressure and renal fibrosis and with downregulation of renal transforming growth factor-β (TGF-β) mRNA and protein compared with rats that did not receive potassium. Participation of the B₂R was evidenced by the fact that all beneficial effects were lost in the presence of the B₂R antagonist. In vitro experiments using the HK-2 proximal tubule cell line showed that treatment of tubular cells with 10 nM bradykinin reduced the epithelial-mesenchymal transdifferentiation and albumin-induced production of TGF-β, and the effects produced by bradykinin were prevented by pretreatment with the B₂R antagonist. These experiments support not only the pathogenic role of the kinin pathway in salt sensitivity but also sustain its role as a renoprotective, antifibrotic paracrine system that modulates renal levels of TGF-β.

Heterozygous knockout of transforming growth factor-β1 protects Dahl S rats against high salt-induced renal injury

The present study employed a zinc-finger nuclease strategy to create heterozygous knockout (KO) rats for the transforming growth factor-β1 (Tgfb1) gene on the Dahl SS/Jr genetic background (TGF-β1(+/-) Dahl S). Intercrossing TGF-β1(+/-) rats did not produce any homozygous KO rats (66.4% +/-, 33.6% +/+), indicating that the mutation is embryonic lethal. Six-week-old wild-type (WT) littermates and TGF-β1(+/-) Dahl S rats were fed a 0.4% (low salt, LS) or 8% NaCl (high salt, HS) diet for 5 wk. Renal cortical expression of TGF-β1, urinary TGF-β1 excretion, proteinuria, glomerular injury and tubulointerstitial fibrosis, and systolic blood pressure were similar in WT and TGF-β1(+/-) Dahl S rats maintained on the LS diet. The expression and urinary excretion of TGF-β1 increased to a greater extent in WT than in TGF-β1(+/-)Dahl S rats fed an HS diet for 1 wk. Systolic blood pressure rose by the same extent to 235 ± 2 mmHg in WT and 239 ± 4 mmHg in TGF-β1(+/-) Dahl S rats fed a HS diet for 5 wk. However, urinary protein excretion was significantly lower in TGF-β1(+/-) Dahl S than in the WT animals. The degree of glomerular injury and renal cortical and outer medullary fibrosis was markedly less in TGF-β1(+/-) than in WT rats. These findings suggest that the loss of one copy of the TGF-β1 gene blunts the increase in renal TGF-β1 protein expression and slows the progression of proteinuria, glomerulosclerosis, and renal interstitial fibrosis in Dahl S rats fed an HS diet independently of changes in blood pressure.

How does TGF-β mediate tubulointerstitial fibrosis?

Tubulointerstitial fibrosis mediates the development of end-stage renal disease from renal injuries of all etiologies and is considered an important predictor of renal survival. Transforming growth factor-β (TGF-β) is one of the most important growth factors that promotes tubulointerstitial fibrosis, but the mechanisms whereby this occurs are not well defined. This is because TGF-β has pleiotropic effects that depend on the target cell type. This review discusses how TGF-β signaling in each of the relevant cell types (eg, tubular epithelium, fibroblasts) may contribute to tubulointerstitial fibrosis progression and suggests ways in which future research can improve our understanding of TGF-β-mediated tubulointerstitial fibrosis.

Effect of aging and dietary salt and potassium intake on endothelial PTEN (Phosphatase and tensin homolog on chromosome 10) function

Aging promotes endothelial dysfunction, defined as a reduction in bioavailable nitric oxide (NO) produced by the endothelial isoform of nitric oxide synthase (NOS3). This enzyme is critically regulated by phosphorylation by protein kinase B (Akt), which in turn is regulated by the lipid phosphatase, PTEN. The present series of studies demonstrated a reduction in bioavailable NO as the age of rats increased from 1 to 12 months. At 12 months of age, rats no longer demonstrated increases in phosphorylated NOS3 in response to high dietary salt intake. Endothelial cell levels of PTEN increased with age and became refractory to change with increased salt intake. In contrast to the reduction in NO production, endothelial cell production of transforming growth factor-ß (TGF-ß) relative to NO increased progressively with age. In macrovascular endothelial cells, PTEN was regulated in a dose-dependent fashion by TGF-ß, which was further regulated by extracellular [KCl]. When combined with prior studies, the present series of experiments suggested an integral role for PTEN in endothelial cell pathobiology of aging and an important mitigating function of TGF-ß in endothelial PTEN regulation. The findings further supported a role for diet in affecting vascular function through the production of TGF-ß and NO.

Effect of dietary salt on regulation of TGF-β in the kidney

Dietary sodium chloride (salt) has long been considered injurious to the kidney by promoting the development of glomerular and tubulointerstitial fibrosis. Endothelial cells throughout the vasculature and glomeruli respond to increased dietary salt intake with increased production of transforming growth factor-β (TGF-β) and nitric oxide. High-salt intake activates large-conductance, voltage- and calcium-activated potassium (BK(Ca)) channels in endothelial cells. Activation of BK(Ca) channels promotes signaling through proline-rich tyrosine kinase-2, cellular-sarcoma (c-Src), Akt (also known as protein kinase B), and mitogen-activated protein kinase pathways that lead to endothelial production of TGF-β and nitric oxide. TGF-β signaling is broadly accepted as a strong stimulator of renal fibrosis. The classic description of TGF-β signaling pathology in renal disease involves signaling through Smad proteins resulting in extracellular matrix deposition and fibrosis. Active TGF-β1 also causes fibrosis by inducing epithelial-mesenchymal transition and apoptosis. By enhancing TGF-β signaling, increased dietary salt intake leads to progressive renal failure from nephron loss and glomerular and tubulointerstitial fibrosis.

Renoprotective effects of anti-TGF-β antibody and antihypertensive therapies in Dahl S rats

This study examined the effects of anti-TGF-β antibody (1D11) therapy in Dahl S (S) rats fed a 4% NaCl diet. Baseline renal expression of TGF-β1 and the degree of injury were lower in female than male S rats maintained on a 0.4% NaCl diet. 4% NaCl diet increased mean arterial pressure (MAP), proteinuria, and renal injury to the same extent in both male and female S rats. Chronic treatment with 1D11 had renoprotective effects in both sexes. The ability of 1D11 to oppose the development of proteinuria when given alone or in combination with antihypertensive agents was further studied in 6-wk-old female S rats, since baseline renal injury was less than that seen in male rats. 1D11, diltiazem, and hydrochlorothiazide (HCT) attenuated the development of hypertension, proteinuria, and glomerular injury. 1D11 had no additional effect when given in combination with these antihypertensive agents. We also explored whether 1D11 could reverse renal injury in 9-wk-old male S rats with preexisting renal injury. MAP increased to 197 ± 4 mmHg and proteinuria rose to >300 mg/day after 3 wk on a 4% NaCl diet. Proteinuria was reduced by 30-40% in rats treated with 1D11, HCT, or captopril + 1D11, but the protective effect was lost in rats fed the 4% NaCl diet for 6 wk. Nevertheless, 1D11, HCT, and captopril + 1D11 still reduced renomedullary and cardiac fibrosis. These results indicate that anti-TGF-β antibody therapy reduces renal and cardiac fibrosis and affords additional renoprotection when given in combination with various antihypertensive agents in Dahl S rats.

High salt intake causes adverse fetal programming--vascular effects beyond blood pressure

BACKGROUND

High salt intake causes hypertension, adverse cardiovascular outcomes and potentially also blood pressure (BP)-independent target organ damage. Excess salt intake in pregnancy is known to affect BP in the offspring. The present study was designed to assess whether high salt intake in pregnancy affects BP and vascular morphology in the offspring.

METHODS

Sprague-Dawley rats were fed a standard rodent diet with low-normal (0.15%) or high (8.0%) salt content during pregnancy and lactation. After weaning at 4 weeks of age, offspring were maintained on the same diet or switched to a high- or low-salt diet, respectively. Vascular geometry was assessed in male offspring at 7 and 12 weeks postnatally.

RESULTS

Up to 12 weeks of age, there was no significant difference in telemetrically measured BP between the groups of offspring. At 12 weeks of age, wall thickness of central (aorta, carotid), muscular (mesenteric) and intrapulmonary arteries was significantly higher in offspring of mothers on a high-salt diet irrespective of the post-weaning diet. This correlated with increased fibrosis of the aortic wall, more intense nitrotyrosine staining as well as elevated levels of marinobufagenin (MBG) and asymmetric dimethyl arginine (ADMA).

CONCLUSIONS

High salt intake in pregnant rats has long-lasting effects on the modeling of central and muscular arteries in the offspring independent of postnatal salt intake and BP. Circulating MBG and ADMA and local oxidative stress correlate with the adverse vascular modeling.

Pyrrole-imidazole polyamide targeting transforming growth factor β1 ameliorates encapsulating peritoneal sclerosis

OBJECTIVE

Encapsulating peritoneal sclerosis (EPS) is a devastating fibrotic complication in patients treated with peritoneal dialysis (PD). Transforming growth factor β1 (TGF-β1) is a pivotal factor in the induction of EPS.

METHODS

To develop pyrrole-imidazole (PI) polyamide, a novel gene silencer, targeted to the TGF-β1 promoter (Polyamide) for EPS, we examined the effects of Polyamide on messenger RNA (mRNA) expression of TGF-β1, vascular endothelial growth factor (VEGF), and extracellular matrix (ECM) in mesothelial cells in vitro, and on the thickness of injured peritoneum evaluated by histology and high-resolution regional elasticity mapping in rats in vivo.

RESULTS

Polyamide significantly lowered mRNA expression of TGF-β1 and ECM in vitro. Polyamide labeled with fluorescein isothiocyanate was taken up into the injured peritoneum and was strongly localized in the nuclei of most cells. Polyamide 1 mg was injected intraperitoneally 1 or 3 times in rats receiving a daily intraperitoneal injection of chlorhexidine gluconate and ethanol (CHX) for 14 days. Polyamide significantly suppressed peritoneal thickening and the abundance of TGF-β1 and fibronectin mRNA, but did not affect expression of VEGF mRNA in the injured peritoneum. Elasticity distribution mapping showed that average elasticity was significantly lower in Polyamide-treated rats than in rats treated solely with CHX.

CONCLUSIONS

Polyamide suppressed the stiffness, ECM formation, and thickening of the injured peritoneum that occurs during EPS pathogenesis. These data suggest that PI polyamide targeted to the TGF-β1 promoter will be a specific and feasible therapeutic strategy for patients with EPS.

Effects of dietary salt restriction on puromycin aminonucleoside nephrosis: preliminary data

Proteinuria is a major promoter that induces tubulointerstitial injury in glomerulopathy. Dietary salt restriction may reduce proteinuria, although the mechanism is not clear. We investigated the effects of dietary salt restriction on rat kidneys in an animal model of glomerular proteinuria. Male Sprague-Dawley rats were used and divided into 3 groups: vehicle-treated normal-salt controls, puromycin aminonucleoside (PA)-treated normal-salt rats, and PA-treated low-salt rats. PA was given at a dose of 150 mg/kg BW at time 0, followed by 50 mg/kg BW on days 28, 35, and 42. Sodium-deficient rodent diet with and without additional NaCl (0.5%) were provided for normal-salt rats and low-salt rats, respectively. On day 63, kidneys were harvested for histopathologic examination and immunohistochemistry. PA treatment produced overt proteinuria and renal damage. Dietary salt restriction insignificantly reduced proteinuria in PA-treated rats, and PA-treated low-salt rats had lower urine output and lower creatinine clearance than vehicle-treated normal-salt controls. When tubulointerstitial injury was semiquantitatively evaluated, it had a positive correlation with proteinuria. The tubulointerstitial injury score was significantly increased by PA treatment and relieved by low-salt diet. ED1-positive infiltrating cells and immunostaining for interstitial collagen III were significantly increased by PA treatment. These changes appeared to be less common in PA-treated low-salt rats, although the differences in PA-treated normal-salt versus low-salt rats did not reach statistical significance. Our results suggest that renal histopathology in PA nephrosis may potentially be improved by dietary salt restriction. Non-hemodynamic mechanisms induced by low-sodium diet might contribute to renoprotection.

Effects of antiproteinuric intervention on elevated connective tissue growth factor (CTGF/CCN-2) plasma and urine levels in nondiabetic nephropathy

BACKGROUND AND OBJECTIVES

Connective tissue growth factor (CTGF/CCN-2) is a key player in fibrosis. Plasma CTGF levels predict end-stage renal disease and mortality in diabetic chronic kidney disease (CKD), supporting roles in intra- and extrarenal fibrosis. Few data are available on CTGF in nondiabetic CKD. We investigated CTGF levels and effects of antiproteinuric interventions in nondiabetic proteinuric CKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In a crossover randomized controlled trial, 33 nondiabetic CKD patients (3.2 [2.5 to 4.0] g/24 h proteinuria) were treated during 6-week periods with placebo, ARB (100 mg/d losartan), and ARB plus diuretics (100 mg/d losartan plus 25 mg/d hydrochlorothiazide) combined with consecutively regular and low sodium diets (193 ± 62 versus 93 ± 52 mmol Na(+)/d).

RESULTS

CTGF was elevated in plasma (464 [387 to 556] pmol/L) and urine (205 [135 to 311] pmol/24 h) of patients compared with healthy controls (n = 21; 96 [86 to 108] pmol/L and 73 [55 to 98] pmol/24 h). Urinary CTGF was lowered by antiproteinuric intervention, in proportion to the reduction of proteinuria, with normalization during triple therapy (CTGF 99 [67 to 146] in CKD versus 73 [55 to 98] pmol/24 h in controls). In contrast, plasma CTGF was not affected.

CONCLUSIONS

Urinary and plasma CTGF are elevated in nondiabetic CKD. Only urinary CTGF is normalized by antiproteinuric intervention, consistent with amelioration of tubular dysfunction. The lack of effect on plasma CTGF suggests that its driving force might be independent of proteinuria and that short-term antiproteinuric interventions are not sufficient to correct the systemic profibrotic state in CKD.

Mechanisms and consequences of salt sensitivity and dietary salt intake

PURPOSE OF REVIEW

Investigation into the underlying mechanisms of salt sensitivity has made important advances in recent years. This review examines in particular the effects of sodium and potassium on vascular function.

RECENT FINDINGS

Sodium chloride (salt) intake promotes cutaneous lymphangiogenesis mediated through tissue macrophages and directly alters endothelial cell function, promoting increased production of transforming growth factor-β (TGF-β) and nitric oxide. In the setting of endothelial dysfunction, such as occurs with aging, diminished nitric oxide production exacerbates the vascular effects of TGF-β, promoting decreased arterial compliance and hypertension. Dietary potassium intake may serve as an important countervailing influence on the effects of salt in the vasculature.

SUMMARY

There is growing appreciation that, independently of alterations in blood pressure, dietary intake of sodium and potassium promotes functional changes in the vasculature and lymphatic system. These changes may protect against development of salt-sensitive hypertension. While salt sensitivity cannot be ascribed exclusively to these factors, perturbation of these processes promotes hypertension during high-salt intake. These studies add to the list of genetic and environmental factors that are associated with salt sensitivity, but in particular provide insight into adaptive mechanisms during high salt intake.

Regulation of renal sodium handling through the interaction between serine proteases and serine protease inhibitors

Sodium balance, extracellular fluid volume, and ultimately blood pressure are maintained by precise regulation of the activity of epithelial sodium channels (ENaC). Multiple mechanisms such as hormones, intracellular factors, and other regulatory factors contribute to regulation of ENaC activity. Prostasin, a glycosylphosphatidylinositol-anchored serine protease, has been identified as an activator of ENaC that increases its open probability. Furin cleaves αENaC at two sites and γENaC at one site at the Golgi. Prostasin cleaves γENaC at one site that is distinct from the furin site at the plasma membrane. Dual cleavage of α- and γ-subunit releases inhibitory segments from ENaC, leading to channel activation. Protease nexin-1 (PN-1), an endogenous prostasin inhibitor, inhibits ENaC activity through suppression of prostasin activity. Aldosterone and transforming growth factor-β1 reciprocally regulate expression of prostasin, PN-1, and ENaC in renal epithelial cell, resulting in sodium retention or natriuresis, respectively. These findings strongly suggest the possibility that coordinated regulation of serine protease, serpin, and ENaC expression plays a key role in sodium handling in the kidney.

Intratubular hydrodynamic forces influence tubulointerstitial fibrosis in the kidney

PURPOSE OF REVIEW

Renal epithelial cells respond to mechanical stimuli with immediate transduction events (e.g. activation of ion channels), intermediate biological responses (e.g. changes in gene expression), and long-term cellular adaptation (e.g. protein expression). Progressive renal disease is characterized by disturbed glomerular hydrodynamics that contributes to glomerulosclerosis, but how intratubular biomechanical forces contribute to tubulointerstital inflammation and fibrosis is poorly understood.

RECENT FINDINGS

In-vivo and in-vitro models of obstructive uropathy demonstrate that tubular stretch induces robust expression of transforming growth factor beta-1, activation of tubular apoptosis, and induction of nuclear factor-kappaB signaling, which contribute to the inflammatory and fibrotic milieu. Nonobstructive structural kidney diseases associated with nephron loss follow a course characterized by compensatory increases of single nephron glomerular filtration rate and tubular flow rate. Resulting increases in tubular fluid shear stress reduce tissue-plasminogen activator and urokinase enzymatic activity, which diminishes breakdown of extracellular matrix. In models of high dietary Na intake, which increases tubular flow, urinary transforming growth factor beta-1 concentrations and renal mitogen-activated protein kinase activity are increased.

SUMMARY

In conclusion, intratubular biomechanical forces, stretch, and fluid shear stress generate changes in intracellular signaling and gene expression that contribute to the pathobiology of obstructive and nonobstructive kidney disease.

Potassium inhibits dietary salt-induced transforming growth factor-beta production

Human and animal studies demonstrate an untoward effect of excess dietary NaCl (salt) intake on cardiovascular function and life span. The endothelium in particular augments the production of transforming growth factor (TGF)-beta, a fibrogenic growth factor, in response to excess dietary salt intake. This study explored the initiating mechanism that regulates salt-induced endothelial cell production of TGF-beta. Male Sprague-Dawley rats were given diets containing different amounts of NaCl and potassium for 4 days. A bioassay for TGF-beta demonstrated increased (35.2%) amounts of active TGF-beta in the medium of aortic ring segments from rats on the high-salt diet compared with rats maintained on a 0.3% NaCl diet. Inhibition of the large-conductance, calcium-activated potassium channel inhibited dietary salt-induced vascular production of TGF-beta but did not affect production of TGF-beta by ring segments from rats on the low-salt diet. Immunohistochemical and Western analyses demonstrated the alpha subunit of the calcium-activated potassium channel in endothelial cells. Increasing medium [K+] inhibited production of dietary salt-induced vascular production levels of total and active TGF-beta but did not alter TGF-beta production by aortic rings from rats on the 0.3% NaCl diet. Increasing dietary potassium content decreased urinary active TGF-beta in animals receiving the high-salt diet but did not change urinary active TGF-beta in animals receiving the low-salt diet. The findings demonstrated an interesting interaction between the dietary intake of potassium and excess NaCl and further showed the fundamental role of the endothelial calcium-activated potassium channel in the vascular response to excess salt intake.

Vascular consequences of dietary salt intake

Animal and human studies support an untoward effect of excess dietary NaCl (salt) intake on cardiovascular and renal function and life span. Recent work has promoted the concept that the endothelium, in particular, reacts to changes in dietary salt intake through a complex series of events that are independent of blood pressure and the renin-angiotensin-aldosterone axis. The cellular signaling events culminate in the intravascular production of transforming growth factor-beta (TGF-beta) and nitric oxide in response to increased salt intake. Plasticity of the endothelium is integral in the vascular remodeling consequences associated with excess salt intake, because nitric oxide serves as a negative regulator of TGF-beta production. Impairment of nitric oxide production, such as occurs with endothelial dysfunction in a variety of disease states, results in unopposed excess vascular TGF-beta production, which promotes reduced vascular compliance and augmented peripheral arterial constriction and hypertension. Persistent alterations in vascular function promote the increase in cardiovascular events and reductions in renal function that reduce life span during increased salt intake.

Acquisition of brain Na sensitivity contributes to salt-induced sympathoexcitation and cardiac dysfunction in mice with pressure overload

In animal models of salt-sensitive hypertension, high salt augments sympathetic outflow via central mechanisms. It is not known, however, whether pressure overload affects salt sensitivity, thereby modifying central sympathetic outflow and cardiac function. We induced left ventricular hypertrophy with aortic banding in mice. Four weeks after aortic banding (AB-4), the left ventricle wall thickness was increased without changing the percentage fractional shortening. AB-4 mice were then fed either a high-salt (8%) diet or regular-salt diet for additional 4 weeks. Cardiac dysfunction, wall thickness, and 24-hour urinary catecholamine excretion were increased with high-salt diet compared with regular-salt diet. We then examined brain Na sensitivity. Intracerebroventricular infusion of high-Na (0.2 mol/L) artificial cerebrospinal fluid into AB-4 mice and mice Sham-4 increased urinary catecholamine excretion, arterial pressure, and heart rate more in AB-4 mice than in Sham-4 mice. Intracerebroventricular infusion of an epithelial Na channel blocker (benzamil) into mice with high-salt diet significantly decreased urinary catecholamine excretion and improved cardiac function. Infusion of either an angiotensin II type 1 receptor blocker or a Rho-kinase inhibitor also attenuated the salt-induced sympathetic hyperactivation and cardiac dysfunction in mice with high-salt diet. The levels of angiotensin II type 1 receptor and phosphorylated moesin, a substrate of Rho-kinase, were significantly greater in AB-4 mice than in Sham-4 mice. These results suggest that mice with pressure overload acquire brain Na sensitivity because of the activation of epithelial Na channel via Rho-kinase and angiotensin II, and this mechanism contributes to salt-induced sympathetic hyperactivation, further pressure overload, and cardiac dysfunction.

Salt restriction in kidney disease--a missed therapeutic opportunity?

The importance of salt restriction in the treatment of patients with renal disease has remained highly controversial. In the following we marshal the current evidence that salt plays a definite role in the genesis of hypertension and target organ damage, point to practical problems of salt restriction, and report on novel pathomechanisms of how salt affects blood pressure and causes target organ damage.

Dietary salt activates an endothelial proline-rich tyrosine kinase 2/c-Src/phosphatidylinositol 3-kinase complex to promote endothelial nitric oxide synthase phosphorylation

Although many laboratories have shown that dietary NaCl (salt) intake increases NO production in rodents and humans, the mechanism has not been uncovered. In the present study, pharmacological and dominant-negative strategies were used to show that feeding a formulated diet containing increased amounts of salt to young male Sprague-Dawley rats induced the formation of an endothelial cell-signaling complex that contained proline-rich tyrosine kinase 2, c-Src (also known as pp60(c-src)), and phosphatidylinositol 3-kinase. In the setting of a high-salt diet, proline-rich tyrosine kinase 2 served as the scaffold for c-Src-mediated phosphatidylinositol 3-kinase activation. Phosphatidylinositol 3-kinase was the upstream activator of protein kinase B (Akt), which was responsible for phosphorylation of the rat endothelial isoform of NO synthase at S1176 and thereby promoted the increase in NO production. The combined findings illustrated the crucial role for a proline-rich tyrosine kinase 2-signaling complex in the endothelial response to salt intake.

Mechanism of dietary salt-mediated increase in intravascular production of TGF-beta1

Clinical and preclinical studies have demonstrated an important effect of arterial pathobiology on the progressive loss of renal function that occurs in chronic kidney disease. Chronic kidney disease, in turn, promotes alterations in vascular function. A modulating role for dietary salt has been suggested, with the amount of salt intake regulating endothelial cell production of transforming growth factor-beta1 (TGF-beta1), a fibrogenic growth factor that promotes arteriosclerosis and glomerulosclerosis. The purpose of the present studies was to determine how the interaction between dietary salt intake and vasculature promoted the production of TGF-beta1 in rats. Two different vascular tissues, aortic rings and glomeruli, were chosen for study. Dietary salt induced, in a dose-dependent fashion, activation of proline-rich tyrosine kinase-2 (Pyk2) and further identified c-Src as an important binding partner of Pyk2 in these tissues. Use of pharmacological inhibitors and dominant negative strategies confirmed that dietary salt induced complex formation of Pyk2 and c-Src with downstream activation of p38 and p42/44 mitogen-activated protein kinases and generation of TGF-beta1. The experiments defined the molecular signaling events that promoted the production of TGF-beta1, a key growth factor involved in the vascular response to increased salt intake.

Pathophysiological Mechanisms of Salt-Dependent Hypertension

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

Recovery from acute renal failure predisposes hypertension and secondary renal disease in response to elevated sodium

Recovery of renal function is a well-characterized feature of models of acute renal failure; however, more recent studies have reported a predisposition to chronic renal disease. This study sought to determine the susceptibility to sodium-dependent hypertension following recovery from ischemic acute renal failure. Following ischemia-reperfusion (I/R) injury, rats were allowed to recover for 35 days on a 0.4% salt diet, then were switched to 4.0% salt diet for an additional 28 days. Blood pressure was significantly increased in postischemic rats switched to high-sodium diet at day 35 (19 ± 9 mmHg) compared with postischemic rats maintained on low-sodium diet. Plasma renin activity and creatinine clearance were not affected by I/R injury. The ischemic injury combined with transfer to 4.0% salt diet resulted in marked renal hypertrophy characterized by interstitial cellular deposition, tubular dilation, and enhanced rates of albumin excretion. Glomerular structure was altered in post-I/R rats switched to high-sodium diet but not in those maintained on low-sodium diets. When rats were acclimated to high-sodium diet before I/R injury, the early injury was similar to that observed in animals acclimated to low-sodium diet, and these animals progressed rapidly toward chronic kidney disease, as evidenced by advancement of albuminuria. These data suggest that the recovery from acute I/R injury is not complete, compromises Na homeostasis, and predisposes hypertension and secondary renal disease.

Effect of salt intake on progression of chronic kidney disease

PURPOSE OF REVIEW

The attempt of this review is to bring into focus the potential role of dietary salt intake in progression of chronic kidney disease.

RECENT FINDINGS

Ongoing work has elucidated a role for dietary salt intake in modulating intrarenal production of transforming growth factor-beta1. The mechanism is independent of angiotensin II and systemic blood pressure and involves activation of vascular endothelium by dietary salt intake with release of this growth factor. In this model, transforming growth factor-beta1 serves an autacoid function by stimulating nitric oxide production by the endothelium. In turn, endothelium-derived nitric oxide modulates production of this growth factor. The model further predicts that individuals who have lost the requisite endothelial cell flexibility to adapt to this environmental stress (a high salt diet) are potentially at increased risk of developing end-organ damage from excess salt intake. Animal and human studies are presented to support this working hypothesis.

SUMMARY

Overproduction of transforming growth factor-beta1 permits excess biological activity of this important fibrogenic growth factor with subsequent development or acceleration of vascular and kidney damage. In patients with diseases whose pathogenesis is related to excess production of transforming growth factor-beta1, such as chronic allograft nephropathy and diabetic nephropathy, increased salt intake may hasten loss of function, particularly if nitric oxide production does not increase. The role that endothelial cell plasticity plays in altering vascular tone and renal function, especially in response to changes in dietary salt intake, should be examined further in chronic kidney disease.

Enhanced expression of EGF receptor in a model of salt-sensitive hypertension

Chronic kidney disease in the Dahl/Rapp salt-sensitive (S) rat is related to an arteriolopathic process that occurs following the onset of hypertension and involves vascular smooth muscle cell (VSMC) hyperplasia and luminal constriction. Because previous studies have shown that activation of the epidermal growth factor receptor (EGFR) produces a mitogenic stimulus in VSMC and the EGFR participates integrally in the vasoconstrictor responses of renal arterioles, the present study analyzed the expression of EGFR in these animals. Compared with Sprague-Dawley (SD) rats, renal cortical expression of EGFR was increased in both prehypertensive and hypertensive S rats. Immunohistochemistry using a polyclonal antibody to EGFR demonstrated that EGFR expression was prominent in the renal vasculature, particularly in the media of afferent and efferent arterioles and the aorta of S rats. When examined, primary cultures of VSMC from S rats showed increased expression of EGFR, compared with VSMC from SD and Dahl/Rapp salt-resistant rats. Following addition of EGF, autophosphorylation of the EGFR was enhanced in cells from S rats, as was the downstream signaling events that included activation of p42/44 MAPK and Akt pathways. Thus in vivo and in vitro studies demonstrated augmented expression and functional activity of the EGFR in S rats.

Mechanism of hypertensive nephropathy in the Dahl/Rapp rat: a primary disorder of vascular smooth muscle

The Dahl/Rapp salt-sensitive (S) rat is a model of salt-sensitive hypertension and hypertensive renal disease. This study explored the role of vascular remodeling in the development of renal failure in S rats. Groups of S and Sprague-Dawley rats were given 0.3 and 8.0% NaCl diets for up to 21 days and evidence of smooth muscle proliferation identified using immunohistochemistry that showed nuclear accumulation of proliferating cell nuclear antigen and 5-bromo-2′-deoxy-uridine. Compared with the other three groups, S rats on 8.0% NaCl diet showed increased nuclear labeling of cells of the aorta and arteries and arterioles of the kidney by the end of the first week of study. Progressive luminal narrowing of the interlobular arteries and preglomerular arterioles occurred in S rats over the 3 wk on the 8.0% NaCl diet. Accumulation of pimonidazole adducts and nuclear accumulation of hypoxia-inducible factor-1α (HIF-1α) were used as markers of tissue hypoxia. By the end of the second week of study, pimonidazole levels increased in S rats on 8.0% NaCl diet and deposition was apparent in tubular cells in the cortex and medulla. At the completion of the experiment, HIF-1α levels were increased in nuclear extracts from the cortex and medulla of S rats on this diet, compared with the other three groups of rats. The data demonstrated a disorder of the vascular remodeling process with proliferation of vascular smooth muscle cells temporally followed by development of tissue hypoxia in the hypertensive nephropathy of S rats on 8.0% NaCl diet.