The SLC6A18 Transporter Is Most Likely a Na-Dependent Glycine/Urea Antiporter Responsible for Urea Secretion in the Proximal Straight Tubule: Influence of This Urea Secretion on Glomerular Filtration Rate.

BACKGROUND: Urea is the major end-product of protein metabolism in mammals. In carnivores and omnivores, a large load of urea is excreted daily in urine, with a concentration that is 30-100 times above that in plasma. This is important for the sake of water economy. Too little attention has been given to the existence of energy-dependent urea transport that plays an important role in this concentrating activity. SUMMARY: This review first presents functional evidence for an energy-dependent urea secretion that occurs exclusively in the straight part of the proximal tubule (PST). Second, it proposes a candidate transmembrane transporter responsible for this urea secretion in the PST. SLC6A18 is expressed exclusively in the PST and has been identified as a glycine transporter, based on findings in SLC6A18 knockout mice. We propose that it is actually a glycine/urea antiport, secreting urea into the lumen in exchange for glycine and Na. Glycine is most likely recycled back into the cell via a transporter located in the brush border. Urea secretion in the PST modifies the composition of the tubular fluid in the thick ascending limb and, thus, contributes, indirectly, to influence the "signal" at the macula densa that plays a crucial role in the regulation of the glomerular filtration rate (GFR) by the tubulo-glomerular feedback. KEY MESSAGES: Taking into account this secondary active secretion of urea in the mammalian kidney provides a new understanding of the influence of protein intake on GFR, of the regulation of urea excretion, and of the urine-concentrating mechanism.

Fibroblast growth factor 23 but not copeptin is independently associated with kidney failure and mortality in patients with chronic kidney disease.

Background: Copeptin and intact fibroblast growth factor 23 (iFGF23) increase early during chronic kidney disease (CKD) and may be predictive of unfavourable outcomes. The aim of this study was to evaluate their respective associations with renal and vital outcomes in CKD patients. Methods: We included CKD patients from the NephroTest cohort with concomitant measurements of plasma copeptin and iFGF23 concentrations and isotopic glomerular filtration rate measurement (mGFR). The primary endpoint was a composite outcome including kidney failure (KF) (dialysis initiation, pre-emptive transplantation or a 57% decrease of mGFR, corresponding to doubling of serum creatinine) or death before KF. Hazard ratios (HRs) of the primary endpoint associated with log-transformed copeptin and iFGF23 concentrations were estimated by Cox models. The slope of mGFR over time was analysed using a linear mixed model. Results: A total of 329 CKD patients (243 men, mean age 60.3 ± 14.6 years) were included. Among them, 301 with an mGFR >15 ml/min/1.73 m2 were included in survival and mGFR slope analyses. During a median follow-up of 4.61 years (quartile 1-quartile 3: 3.72-6.07), 61 KFs and 32 deaths occurred. Baseline iFGF23 concentrations were associated with the composite outcome after multiple adjustments {HR 2.72 [95% confidence interval (CI) 1.85-3.99]}, whereas copeptin concentrations were not [HR 1.01 (95% CI 0.74-1.39)]. Neither copeptin nor iFGF23 were associated with mGFR slope over time. Conclusion: Our study shows for the first time in population of CKD patients an independent association between iFGF23 and unfavourable renal and vital outcomes and shows no such association regarding copeptin, encouraging the integration of iFGF23 measurement into the follow-up of CKD.

The urine-to-plasma urea concentration ratio is associated with eGFR and eGFR decline over time in a population cohort.

BACKGROUND: Evaluation of renal function and of factors associated with its decline are important public health issues. Besides markers of glomerular function [e.g. glomerular filtration rate (GFR)], those of tubular functions are rarely evaluated. Urea, the most abundant urinary solute, is markedly concentrated in urine when compared with plasma. We explored the urine-to-plasma ratio of urea concentrations (U/P urea ratio) as a marker of tubular functions. METHODS: We evaluated the relationship of the U/P urea ratio with eGFR at baseline in 1043 participants (48 ± 17 years) from the Swiss Kidney Project on Genes in Hypertension (SKIPOGH) population-based cohort, using mixed regression. In 898 participants, we assessed the relation between U/P urea ratio and renal function decline between two study waves 3 years apart. We studied U/P ratios for osmolarity, Na, K and uric acid for comparison. RESULTS: In a transversal study at baseline, estimated GFR (eGFR) was positively associated with U/P-urea ratio [ßscaled = 0.08, 95% CI (0.04; 0.13)] but not with the U/P ratio of osmolarity. Considering separately participants with renal function >90 or ≤90 mL/min × 1.73 m2, this association was observed only in those with reduced renal function. In the longitudinal study, eGFR declined at a mean rate of 1.2 mL/min per year. A significant association was observed between baseline U/P urea ratio and eGFR decline [ßscaled = 0.08, 95% CI (0.01; 0.15)]. A lower baseline U/P urea ratio was associated with a greater eGFR decline. CONCLUSION: This study provides evidence that the U/P urea ratio is an early marker of kidney function decline in the general adult population. Urea is easy to measure with well-standardized techniques and at low cost. Thus, the U/P urea ratio could become an easily available tubular marker for evaluating renal function decline.

Association of the Urine-to-Plasma Urea Ratio With CKD Progression.

RATIONALE & OBJECTIVES: The urine-to-plasma (U/P) ratio of urea is correlated with urine-concentrating capacity and associated with progression of autosomal dominant polycystic kidney disease. As a proposed biomarker of tubular function, we hypothesized that the U/P urea ratio would also be associated with progression of more common forms of chronic kidney disease (CKD). STUDY DESIGN: Observational cohort study. SETTING & PARTICIPANTS: 3,723 adults in the United States with estimated glomerular filtration rate (eGFR) of 20-70 mL/min/1.73 m2, enrolled in the Chronic Renal Insufficiency Cohort (CRIC) Study. EXPOSURE: U/P urea ratio, calculated from 24-hour urine collections and plasma samples at baseline. OUTCOME: Associations of U/P urea ratio with eGFR slope, initiation of kidney replacement therapy (KRT), and CKD progression, defined as 50% decline in eGFR or incident KRT. ANALYTICAL APPROACH: Multivariable linear mixed-effects models tested associations with eGFR slope. Cox proportional hazards models tested associations with dichotomous CKD outcomes. RESULTS: The median U/P urea ratio was 14.8 (IQR, 9.5-22.2). Compared with participants in the highest U/P urea ratio quintile, those in the lowest quintile had a greater eGFR decline by 1.06 mL/min/1.73 m2 per year (P < 0.001) over 7.0 (IQR, 3.0-11.0) years of follow-up observation. Each 1-SD lower natural log-transformed U/P urea ratio was independently associated with CKD progression (HR, 1.22 [95% CI, 1.12-1.33]) and incident KRT (HR, 1.22 [95% CI, 1.10-1.33]). Associations differed by baseline eGFR (P interaction = 0.009). Among those with an eGFR ≥30 mL/min/1.73 m2, each 1-SD lower in ln(U/P urea ratio) was independently associated with CKD progression (HR, 1.30 [95% CI, 1.18-1.45]), but this was not significant among those with eGFR <30 mL/min/1.73 m2 (HR, 1.00 [95% CI, 0.84-1.20]). LIMITATIONS: Possibility of residual confounding. Single baseline 24-hour urine collection for U/P urea ratio. CONCLUSIONS: In a large and diverse cohort of patients with common forms of CKD, U/P urea was independently associated with disease progression and incident kidney failure. Associations were not significant among those with advanced CKD at baseline.

Vaptans or voluntary increased hydration to protect the kidney: how do they compare?

The adverse effects of vasopressin (AVP) in diverse forms of chronic kidney disease have been well described. They depend on the antidiuretic action of AVP mediated by V2 receptors (V2R). Tolvaptan, a selective V2R antagonist, is now largely used for the treatment of patients with autosomal dominant polycystic kidney disease. Another way to reduce the adverse effects of AVP is to reduce endogenous AVP secretion by a voluntary increase in fluid intake. These two approaches differ in several ways, including the level of thirst and AVP. With voluntary increased drinking, plasma osmolality will decline and so will AVP secretion. Thus, not only will V2R-mediated effects be reduced, but also those mediated by V1a and V1b receptors (V1aR and V1bR). In contrast, selective V2R antagonism will induce a loss of fluid that will stimulate AVP secretion and thus increase AVP's influence on V1a and V1b receptors. V1aR is expressed in the luminal side of the collecting duct (CD) and in inner medullary interstitial cells, and their activation induces the production of prostaglandins, mostly prostaglandin E2 (PGE2). Intrarenal PGE2 has been shown to reduce sodium and water reabsorption in the CD and increase blood flow in the renal medulla, both effects contributing to increase sodium and water excretion and reduce urine-concentrating activity. Conversely, non-steroidal anti-inflammatory drugs have been shown to induce significant water and sodium retention and potentiate the antidiuretic effects of AVP. Thus, during V2R antagonism, V1aR-mediated actions may be responsible for part of the diuresis observed with this drug. These V1aR-dependent effects do not take place with a voluntary increase in fluid intake. In summary, while both strategies may have beneficial effects, the information reviewed here leads us to assume that pharmacological V2R antagonism, with resulting stimulation of V1aR and increased PGE2 production, may provide greater benefit than voluntary high water intake. The influence of tolvaptan on the PGE2 excretion rate and the possibility to use somewhat lower tolvaptan doses than presently prescribed remain to be evaluated.

Water intake and progression of chronic kidney disease: the CKD-REIN cohort study.

BACKGROUND: Optimal daily water intake to prevent chronic kidney disease (CKD) progression is unknown. Taking the kidney's urine-concentrating ability into account, we studied the relation of kidney outcomes in patients with CKD to total and plain water intake and urine volume. METHODS: Including 1265 CKD patients [median age 69 years; mean estimated glomerular filtration rate (eGFR) 32 mL/min/1.73 m2] from the Chronic Kidney Disease-Renal Epidemiology and Information Network cohort (2013-19), we assessed fluid intake at baseline interviews, collected 24-h urine volumes and estimated urine osmolarity (eUosm). Using Cox and then linear mixed models, we estimated hazard ratios (HRs) and 95% confidence intervals (CIs) for kidney failure and eGFR decline associated with hydration markers, adjusting for CKD progression risk factors and eUosm. RESULTS: Patients' median daily intake was 2.0 L [interquartile range (IQR) 1.6-2.6] for total water and 1.5 L (1-1.7) for plain water, median urine volume was 1.9 L/24 h (IQR 1.6-2.4) and mean eUosm was 374 ± 104 mosm/L. Neither total water intake nor urine volume was associated with either kidney outcome. Kidney failure risk increased significantly with decreasing eUosm ˂292 mosm/L. Adjusted HRs (95% CIs) for kidney failure associated with plain water intake were 1.88 (1.02-3.47), 1.59 (1.06-2.38), 1.76 (0.95-3.24) and 1.55 (1.03-2.32) in patients drinking <0.5, 0.5-1.0, 1.5-2.0 and >2.0 L/day compared with those drinking 1.0-1.5 L/day. High plain water intake was also significantly associated with faster eGFR decline. CONCLUSIONS: In patients with CKD, the relation between plain water intake and progression to kidney failure appears to be U-shaped. Both low and high intake may not be beneficial in CKD.

Low hydration status may be associated with insulin resistance and fat distribution: analysis of the Korea National Health and Nutrition Examination Survey (KNHANES) 2008-2010.

We aimed to identify the association of hydration status with insulin resistance (IR) and body fat distribution. A total of 14 344 adults participated in the Korea National Health and Nutrition Examination Survey 2008-2010. We used urine specific gravity (USG) to indicate hydration status, and HOMA-IR (homoeostasis model assessment of IR) and trunk:leg fat ratio (TLR) as primary outcomes. In multivariate logistic regression, the OR per 0·01 increase in USG for high IR was 1·303 (95 % CI 1·185, 1·433; P < 0·001). In multivariate generalised additive model plots, increased USG showed a J-shaped association with logarithmic HOMA-IR, with the lowest Akaike's information criterion score of USG 1·030. Moreover, increased USG was independently associated with increased trunk fat, decreased leg fat and increased TLR. In mediation analysis, the proportion of mediation effects of USG on TLR via IR was 0·193 (95 % CI 0·132, 0·285; P < 0·001), while the proportion of mediation effects of USG on IR via TLR was 0·130 (95 % CI 0·086, 0·188; P < 0·001). Increased USG, a sign of low hydration status and presumably high vasopressin, was associated with IR and poor fat distribution. Direct effect of low hydration status may be more dominant than indirect effect via IR or fat distribution. Further studies are necessary to confirm our findings.

Medullary and cortical thick ascending limb: similarities and differences.

The thick ascending limb of the loop of Henle (TAL) is the first segment of the distal nephron, extending through the whole outer medulla and cortex, two regions with different composition of the peritubular environment. The TAL plays a critical role in the control of NaCl, water, acid, and divalent cation homeostasis, as illustrated by the consequences of the various monogenic diseases that affect the TAL. It delivers tubular fluid to the distal convoluted tubule and thereby affects the function of the downstream tubular segments. The TAL is commonly considered as a whole. However, many structural and functional differences exist between its medullary and cortical parts. The present review summarizes the available data regarding the similarities and differences between the medullary and cortical parts of the TAL. Both subsegments reabsorb NaCl and have high Na+-K+-ATPase activity and negligible water permeability; however, they express distinct isoforms of the Na+-K+-2Cl- cotransporter at the apical membrane. Ammonia and bicarbonate are mostly reabsorbed in the medullary TAL, whereas Ca2+ and Mg2+ are mostly reabsorbed in the cortical TAL. The peptidic hormone receptors controlling transport in the TAL are not homogeneously expressed along the cortical and medullary TAL. Besides this axial heterogeneity, structural and functional differences are also apparent between species, which underscores the link between properties and role of the TAL under various environments.

What can copeptin tell us in patients with autosomal dominant polycystic disease?

Vasopressin is known to contribute to disease progression in autosomal dominant polycystic disease (ADPKD) by its influence on cyclic adenosine monophosphate that directly promotes cyst growth. In addition, vasopressin probably contributes to progression by inducing glomerular hyperfiltration as shown in other forms of chronic kidney diseases. The measurement of plasma copeptin, a marker of vasopressin secretion, could help identify patients at higher risk of fast progression and those expected to benefit the most from vasopressin V2 receptor blockade. Further studies should evaluate the optimal level of suppression of vasopressin effects in autosomal dominant polycystic disease.

Effects of hydration on plasma copeptin, glycemia and gluco-regulatory hormones: a water intervention in humans.

PURPOSE: High plasma copeptin, a marker of vasopressin, predicts diabetes mellitus. We tested if copeptin could be suppressed by increased water intake in healthy individuals, and if a water-induced change in copeptin was accompanied by altered concentrations of glucose, insulin or glucagon. METHODS: Thirty-nine healthy individuals underwent, in random order, 1 week of high water intake (3 L/day on top of habitual intake) and 1 week of normal (habitual) fluid intake (control). Fasting plasma concentrations of copeptin, glucose, insulin and glucagon were compared between the ends of both periods. Furthermore, acute copeptin kinetics were mapped for 4 h after ingestion of 1 L of water. RESULTS: After acute intake of 1 L water, copeptin was significantly reduced within 30 min, and reached maximum reduction within 90 min with on average 39% reduction (95% confidence interval (95 CI) 34-45) (p < 0.001) and remained low the entire test period (4 h). One week of increased water intake led to a 15% reduction (95 CI 5-25) (p = 0.003) of copeptin compared to control week. The greatest reduction occurred among subjects with habitually high copeptin and concentrated urine ("water-responders"). Water-responders had significant water-induced reduction of glucagon, but glucose and insulin were unaffected. CONCLUSIONS: Both acute and 1 week extra water intake potently reduced copeptin concentration. In those with the greatest decline (water-responders), who are typically low drinkers with high baseline copeptin, water induced a reduction in fasting glucagon. Long-term trials assessing the effect of water on glucometabolic traits should focus on low-water drinkers with high copeptin concentration.

Glucagon revisited: Coordinated actions on the liver and kidney.

Glucagon secretion is stimulated by a low plasma glucose concentration. By activating glycogenolysis and gluconeogenesis in the liver, glucagon contributes to maintain a normal glycemia. Glucagon secretion is also stimulated by the intake of proteins, and glucagon contributes to amino acid metabolism and nitrogen excretion. Amino acids are used for gluconeogenesis and ureagenesis, two metabolic pathways that are closely associated. Intriguingly, cyclic AMP, the second messenger of glucagon action in the liver, is released into the bloodstream becoming an extracellular messenger. These effects depend not only on glucagon itself but on the actual glucagon/insulin ratio because insulin counteracts glucagon action on the liver. This review revisits the role of glucagon in nitrogen metabolism and in disposal of nitrogen wastes. This role involves coordinated actions of glucagon on the liver and kidney. Glucagon influences the transport of fluid and solutes in the distal tubule and collecting duct, and extracellular cAMP influences proximal tubule reabsorption. These combined effects increase the fractional excretion of urea, sodium, potassium and phosphates. Moreover, the simultaneous actions of glucagon and extracellular cAMP are responsible, at least in part, for the protein-induced rise in glomerular filtration rate that contributes to a more efficient excretion of protein-derived end products.

Plasma copeptin and chronic kidney disease risk in 3 European cohorts from the general population.

BACKGROUND: The prevalence of chronic kidney disease (CKD) is increasing worldwide. The identification of factors contributing to its progression is important for designing preventive measures. Previous studies have suggested that chronically high vasopressin is deleterious to renal function. Here, we evaluated the association of plasma copeptin, a surrogate of vasopressin, with the incidence of CKD in the general population. METHODS: We studied 3 European cohorts: DESIR (n = 5,047; France), MDCS-CC (n = 3,643; Sweden), and PREVEND (n = 7,684; the Netherlands). Median follow-up was 8.5, 16.5, and 11.3 years, respectively. Pooled data were analyzed at an individual level for 4 endpoints during follow-up: incidence of stage 3 CKD (estimated glomerular filtration rate [eGFR] < 60 ml/min/1.73 m2); the KDIGO criterion "certain drop in eGFR"; rapid kidney function decline (eGFR slope steeper than -3 ml/min/1.73 m2/yr); and incidence of microalbuminuria. RESULTS: The upper tertile of plasma copeptin was significantly and independently associated with a 49% higher risk for stage 3 CKD (P < 0.0001); a 64% higher risk for kidney function decline, as defined by the KDIGO criterion (P < 0.0001); a 79% higher risk for rapid kidney function decline (P < 0.0001); and a 24% higher risk for microalbuminuria (P = 0.008). CONCLUSIONS: High copeptin levels are associated with the development and the progression of CKD in the general population. Intervention studies are needed to assess the potential beneficial effect on kidney health in the general population of reducing vasopressin secretion or action. FUNDING: INSERM and Danone Research Centre for Specialized Nutrition.

Renal potassium handling in carriers of the Gly40Ser mutation of the glucagon receptor suggests a role for glucagon in potassium homeostasis.

Plasma potassium concentration (PK ) is tightly regulated. Insulin is known to favor potassium entry into cells. But how potassium leaves the cells later on is not often considered. Previous studies in rats showed that glucagon infusion increased urinary potassium excretion dose-dependently and reversibly. This prompted us to investigate the possible influence of glucagon on potassium handling in humans. We took advantage of the Gly40Ser mutation of the glucagon receptor (GR) that results in a partial loss of function of the GR. In the Olivetti cohort (male workers), 25 subjects who carried this mutation were matched 1:4 to 100 noncarriers for age and weight. Estimated osmolarity of serum and 24-h urine (Sosm and Uosm, respectively) was calculated from the concentrations of the main solutes: [(Na+K)*2 + urea (+glucose for serum)]. Transtubular potassium gradient (TTKG), reflecting the intensity of K secretion in the distal nephron, was calculated as [(urine K/serum K)(Uosm /Sosm )]. There was no significant difference in serum K, or 24-h urine urea, Na and K excretion rates. But urine K concentration was significantly lower in carriers than in noncarriers. Means (interquartile range): 38 (34-43) versus 47 (43-51) mmol/L, P = 0.030. TTKG was also significantly lower in carriers: 4.2 (3.9-4.6) versus 5.0 (4.7-5.2), P = 0.015. This difference remained statistically significant after adjustments for serum insulin and 24-h Na and urea excretions. These results in humans suggest that glucagon stimulates K secretion in the distal nephron. Thus, in conjunction with insulin, glucagon may also participate in K homeostasis by promoting renal K excretion.

Association of a Low-Protein Diet With Slower Progression of CKD.

INTRODUCTION: Reducing protein intake is recommended for slowing chronic kidney disease (CKD) progression, but assessment of its true effectiveness is sparse. METHODS: Using the Maroni formula, we assessed dietary protein intake (DPI) from 24-hour urinary urea excretion in 1594 patients (67% men and 33% women) with CKD, 784 of whom also had 7-day food records. Cause-specific hazard ratios (HRs) and 95% confidence intervals for the competing risks of DPI-associated end-stage renal disease (ESRD) or death were estimated in 1412 patients with baseline glomerular filtration rate ≥15 ml/min per 1.73 m2, measured by 51Cr-EDTA renal clearance (mGFR). RESULTS: Overall, mean DPI estimated from urea excretion was 1.09 ± 0.30 g/kg of body weight per day (range = 0.34-2.76); 20% of patients had values > 1.3 g/kg per day, and 1.9% had values < 0.6 g/kg per day. Urea excretion and food records produced similar estimates of mean DPI. The lower the mGFR, the lower the mean DPI. Over a median follow-up of 5.6 years, there were 319 ESRD events and 189 pre-ESRD deaths. After adjusting for relevant covariates, each 0.1 g/kg daily higher baseline urea excretion-based DPI or food record-based DPI was associated with an HR for ESRD of 1.05 (95% confidence interval 1.01-1.10) or 1.09 (95% confidence interval 1.04-1.14), respectively. HRs were stronger in patients with baseline mGFR < 30 ml/min per 1.73 m2. There was no association with mortality. The mean age of the patients was 59 ± 15 years, and mean body mass index was 26.6 ± 5.2 kg/m2. CONCLUSION: In this prospective observational study, the lower the baseline DPI, the slower the progression toward ESRD. Most importantly, the absence of threshold for the relation between DPI and ESRD risk indicates that there is no optimal DPI in the range observed in this cohort.

Improved protocols for the study of urinary electrolyte excretion and blood pressure in rodents: use of gel food and stepwise changes in diet composition.

Many experimental protocols in rodents require the comparison of groups that are fed different diets. Changes in dietary electrolyte and/or fat content can influence food intake, which can potentially introduce bias or confound the results. Unpalatable diets slow growth or cause weight loss, which is exacerbated by housing the animals in individual metabolic cages or by surgery. For balance studies in mice, small changes in body weight and food intake and low urinary flow can amplify these challenges. Powder food can be administered as gel with the addition of a desired amount of water, electrolytes, drugs (if any), and a small amount of agar. We describe here how the use of gel food to vary water, Na, K, and fat content can reduce weight loss and improve reproducibility of intake, urinary excretion, and blood pressure in rodents. In addition, mild food restriction reduces the interindividual variability and intergroup differences in food intake and associated variables, thus improving the statistical power of an experiment. Finally, we also demonstrate the advantages of using gel food for weight-based drug dosing. These protocols can improve the accuracy and reproducibility of experimental data where dietary manipulations are needed and are especially advisable in rodent studies related to water balance, obesity, and blood pressure.

Validation of Surrogates of Urine Osmolality in Population Studies.

BACKGROUND: The importance of vasopressin and/or urine concentration in various kidney, cardiovascular, and metabolic diseases has been emphasized recently. Due to technical constraints, urine osmolality (Uosm), a direct reflect of urinary concentrating activity, is rarely measured in epidemiologic studies. METHODS: We analyzed 2 possible surrogates of Uosm in 4 large population-based cohorts (total n = 4,247) and in patients with chronic kidney disease (CKD, n = 146). An estimated Uosm (eUosm) based on the concentrations of sodium, potassium, and urea, and a urine concentrating index (UCI) based on the ratio of creatinine concentrations in urine and plasma were compared to the measured Uosm (mUosm). RESULTS: eUosm is an excellent surrogate of mUosm, with a highly significant linear relationship and values within 5% of mUosm (r = 0.99 or 0.98 in each population cohort). Bland-Altman plots show a good agreement between eUosm and mUosm with mean differences between the 2 variables within ±24 mmol/L. This was verified in men and women, in day and night urine samples, and in CKD patients. The relationship of UCI with mUosm is also significant but is not linear and exhibits more dispersed values. Moreover, the latter index is no longer representative of mUosm in patients with CKD as it declines much more quickly with declining glomerular filtration rate than mUosm. CONCLUSION: The eUosm is a valid marker of urine concentration in population-based and CKD cohorts. The UCI can provide an estimate of urine concentration when no other measurement is available, but should be used only in subjects with normal renal function.

Relationship between Sodium Intake and Water Intake: The False and the True.

BACKGROUND: Generally, eating salty food items increases thirst. Thirst is also stimulated by the experimental infusion of hypertonic saline. But, in steady state, does the kidney need a higher amount of water to excrete sodium on a high than on a low sodium intake? This issue is still controversial. The purpose of this review is to provide examples of how the kidney handles water in relation to salt intake/output. It is based on re-analysis of previously published studies in which salt intake was adjusted to several different levels in the same subjects, and in databases of epidemiologic studies in populations on an ad libitum diet. Summary and Key Messages: These re-analyses allow us to draw the following conclusions: (1) In a steady state situation, the urine volume (and thus the fluid intake) remains unchanged over a large range of sodium intakes. The adaptation to a higher sodium excretion rests only on changes in urinary sodium concentration. However, above a certain limit, this concentration cannot increase further and the urine volume may then increase. (2) In population studies, it is not legitimate to assume that sodium is responsible for changes in urine volume, since people who eat more sodium also eat more of other nutrients leading to an increase in the excretion of potassium, urea and other solutes, besides sodium. (3) After an abrupt increase in sodium intake, fluid intake is increased in the first few days, but urine volume does not change. The extra fluid drunk is responsible for an increase in body weight.

Antagonism of vasopressin V2 receptor improves albuminuria at the early stage of diabetic nephropathy in a mouse model of type 2 diabetes.

AIMS: Vasopressin is increased in diabetes and was shown to contribute to development of diabetic nephropathy through V2 receptor (V2R) activation in an experimental model of type 1 diabetes. The role of V2R in type 2 diabetes remains undocumented. This study addresses the issue in a mouse model of type 2 diabetes. METHODS: Male obese diabetic db/db mice were treated for 12weeks with a selective V2R antagonist (SR121463) and compared to non-treated db/db and non-diabetic db/m mice. All animals were previously uninephrectomized. RESULTS: The V2R antagonist did not alter glycemia or glycosuria in db/db mice. It induced a two-fold increase in urine output and a 52% decrease in urine osmolality compared to non-treated db/db mice. After four weeks of treatment urinary albumin to creatinine ratio was 50% lower in treated mice compared to non-treated mice, and remained significantly lower until end of experiment. Glomerular filtration rate increased significantly over time in non-treated db/db mice but remained stable in treated mice. CONCLUSIONS: This study shows that vasopressin contributes to albuminuria and glomerular hyperfiltration via V2R in a mouse model of type 2 diabetes. It documents causality behind the association of vasopressin with renal disease observed in diabetic patients.

Vasopressin and diabetic nephropathy.

PURPOSE OF REVIEW: The prevalence of diabetic kidney disease (DKD) is increasing worldwide. Despite major therapeutic advances in the last decades in DKD, the current standard of care let many people progress to severe stages. Vasopressin secretion is increased in diabetes, and its potential role in the onset and progression of DKD is being re-investigated. RECENT FINDINGS: Recently, observational studies evidenced an association between surrogates of vasopressin secretion (daily fluid intake or urine volume, and plasma copeptin concentration) and chronic kidney disease in the community, but also specifically in type 1 and in type 2 diabetes. Causality is strongly supported by a series of studies in rats conducted more than a decade ago, and by additional recent experimental data. The mechanism underlying these adverse effects likely involves the hyperfiltration induced indirectly as a consequence of the tubular effects of the hormone mediated by the V2 receptor. SUMMARY: If chronic vasopressin action on the kidney is detrimental in diabetes as suggested so far, intervention studies should be designed. Available tools include V2 receptor blockade, and changes in daily water intake in vulnerable patients. Safety and effectiveness should be tested, as it is currently done in patients with CKD (NCT01766687).