The role of the kidney in the maintenance of water balance

The ubiquitin codes in cellular stress responses

Ubiquitination/ubiquitylation, one of the most fundamental post-translational modifications, regulates almost every critical cellular process in eukaryotes. Emerging evidence has shown that essential components of numerous biological processes undergo ubiquitination in mammalian cells upon exposure to diverse stresses, from exogenous factors to cellular reactions, causing a dazzling variety of functional consequences. Various forms of ubiquitin signals generated by ubiquitylation events in specific milieus, known as ubiquitin codes, constitute an intrinsic part of myriad cellular stress responses. These ubiquitination events, leading to proteolytic turnover of the substrates or just switch in functionality, initiate, regulate, or supervise multiple cellular stress-associated responses, supporting adaptation, homeostasis recovery, and survival of the stressed cells. In this review, we attempted to summarize the crucial roles of ubiquitination in response to different environmental and intracellular stresses, while discussing how stresses modulate the ubiquitin system. This review also updates the most recent advances in understanding ubiquitination machinery as well as different stress responses and discusses some important questions that may warrant future investigation.

Hsp27 over expression protect against cadmium induced nephrotoxicity in Drosophila melanogaster

Cadmium (Cd) exposure to the animals including humans is reported as nephrotoxic compounds i.e., disturbing redox status (increase oxidative stress), mitochondrial dysfunction, renal cell death and altered transporters in the renal system. Hsp27 (a small heat shock protein) has been shown as one of the modulators in the renal dysfunction and increased against the Cd induced toxicity. However, no studies are reported on the genetic modulation of stress protein against the Cd-induced nephrotoxicity. The current study aimed to examine the protective role of hsp27 overexpression against the Cd-induced nephrotoxicity using Drosophila melanogaster as an animal model. D. melanogaster renal system includes nephrocytes and Malpighian tubules (MTs) that show the functional similarity with mammalian kidney nephron. Overexpression of the hsp27 was found to reduce the Cd induced oxidative stress, rescue cell death in MTs of Cd exposed D. melanogaster larvae. The rescued GSH level, NADPH level and glucose 6 phosphate dehydrogenase (G6PD) activity were also observed in the MTs of the Cd exposed organism. Function (efflux activity and fluid secretion rate) of the MTs was restored in Cd exposed hsp27 overexpressed larvae. Further, results were confirmed by restored brush border microvilli density and reduced uric acid level. Tissue specific knockdown of hsp27 developed Cd like phenotypes in MTs and the phenotypes enhanced in Cd exposed condition. The present study clearly shows the role of hsp27 overexpression in restoration of the MTs function and protection against the Cd induced renal toxicity.

Ultrasound super-resolution imaging with a hierarchical Kalman tracker

Microbubble (MB) tracking plays an important role in ultrasound super-resolution imaging (SRI) by enabling velocity estimation and improving image quality. This work presents a new hierarchical Kalman (HK) tracker to achieve better performance at scenarios with high concentrations of MBs and high localization uncertainty. The method attempts to follow MBs with different velocity ranges using different Kalman filters. An extended simulation framework for evaluating trackers is also presented and used for comparison of the proposed HK tracker with the nearest-neighbor (NN) and Kalman (K) trackers. The HK tracks were most similar to the ground truth with the highest Jaccard similarity coefficient in 79% of the scenarios and the lowest root-mean-square error in 72% of the scenarios. The HK tracker reconstructed vessels with a more accurate diameter. In a scenario with an uncertainty of 51.2μm in MB localization, a vessel diameter of 250μm was estimated as 257μm by HK tracker, compared with 329μm and 389μm for the K and NN trackers. In the same scenario, the HK tracker estimated MB velocities with a relative bias down to 1.7% and a relative standard deviation down to 8.3%. Finally, the different tracking techniques were applied to in vivo data from rat kidneys, and trends similar to the simulations were observed. Conclusively, the results showed an improvement in tracking performance, when the HK tracker was employed in comparison with the NN and K trackers.

A glance at the emerging diagnostic biomarkers in the most prevalent genitourinary cancers

Genitourinary cancers comprise of a heterogenous group of cancers of which renal cell carcinoma, urothelial bladder carcinoma, and prostate adenocarcinoma are the most commonly encountered subtypes. A lot of research is ongoing using various strategies for exploration of novel biomarkers for genitourinary cancers. These biomarkers would not reduce the need for invasive diagnostic techniques but also could be used for early and accurate diagnosis to improve the clinical management required for the disease. Moreover, selecting the appropriate treatment regimen for the responsive patients based on these biomarkers would reduce the treatment toxicity as well as cost. Biomarkers identified using various advanced techniques like next generation sequencing and proteomics, which have been classified as immunological biomarkers, tissue-specific biomarkers and liquid biomarkers. Immunological biomarkers include markers of immunological pathways such as CTLA4, PD-1/PDl-1, tissue biomarkers include tissue specific molecules such as PSA antigen and liquid biomarkers include biomarkers detectable in urine, circulating cells etc.

The purpose of this review is to provide a brief introduction to the most prevalent genitourinary malignancies, including bladder, kidney, and prostate cancers along with a major focus on the novel diagnostic biomarkers and the importance of targeting them prior to genitourinary cancers treatment. Understanding these biomarkers and their potential in diagnosis of genitourinary cancer would not help in early and accurate diagnosis as mentioned above but may also lead towards a personalized approach for better diagnosis, prognosis and specified treatment approach for an individual.

Evaluation of 2D super-resolution ultrasound imaging of the rat renal vasculature using ex vivo micro-computed tomography

Super-resolution ultrasound imaging (SRUS) enables in vivo microvascular imaging of deeper-lying tissues and organs, such as the kidneys or liver. The technique allows new insights into microvascular anatomy and physiology and the development of disease-related microvascular abnormalities. However, the microvascular anatomy is intricate and challenging to depict with the currently available imaging techniques, and validation of the microvascular structures of deeper-lying organs obtained with SRUS remains difficult. Our study aimed to directly compare the vascular anatomy in two in vivo 2D SRUS images of a Sprague-Dawley rat kidney with ex vivo μCT of the same kidney. Co-registering the SRUS images to the μCT volume revealed visually very similar vascular features of vessels ranging from ~ 100 to 1300 μm in diameter and illustrated a high level of vessel branching complexity captured in the 2D SRUS images. Additionally, it was shown that it is difficult to use μCT data of a whole rat kidney specimen to validate the super-resolution capability of our ultrasound scans, i.e., validating the actual microvasculature of the rat kidney. Lastly, by comparing the two imaging modalities, fundamental challenges for 2D SRUS were demonstrated, including the complexity of projecting a 3D vessel network into 2D. These challenges should be considered when interpreting clinical or preclinical SRUS data in future studies.

Association between the content of intracellular and extracellular fluid and the amount of water intake among Chinese college students

BACKGROUND

Normal distribution of body fluid is important for maintaining health through the balance of water metabolism. Studies have shown that disease states and diuretics perturb the balance and then induce abnormal intracellular/extracellular fluid ratio. However, there are relatively few researches on the associations between water intakes and body fluid. The objective of this study was to explore the association between body fluid and water intake.

METHODS

A total of 159 young adults in Baoding, China were recruited in this cross-sectional survey and completeness of follow-up was 98.1%. A 7-day fluid specific diary was used to record total fluid intake (TFI). Water intake from foods (FWI) for 3 days was measured using the methods of weighting, duplicate portion method and laboratory analysis by researchers. Body fluid was measured using bioelectrical impedance analysis.

RESULTS

Total body water (TBW), intracellular fluid (ICF) and extracellular fluid (ECF) of participants were 32.8[28.0,39.2], 20.5[17.3,24.5] and 12.4[10.7,14.7], (kg). This represented 55.2 ± 6.2, 34.4 ± 4.0 and 20.8 ± 2.3 (%) of body weight (BW), respectively. ICF, ECF and TBW among male participants who drank more than or equal to adequate TFI was higher than those who drank less (Z = -1.985, p = 0.047; Z = -2.134, p = 0.033; Z = -2.053, p = 0.040). Among both males and females, the values of TBW/BW in participants whose TWI met or exceeded the AI were higher than those with TWI less than AI (t = - 2.011, p = 0.046; t = - 2.716, p = 0.008). Among all participants, there was moderate correlation between water intakes (TFI/BW, FWI/BW and TWI/BW) and body fluid (ICF/BW,ECF/BW and TBW/BW) (p < 0.01 for all). Same correlations were found among both males and females.

CONCLUSION

There is a certain degree of association between water intake and body fluid. However, whether TFI or TWI achieve AI or not do not disturb the balance on the distribution of body fluid. More studies should be conducted to find the diagnostic threshold on TFI and TWI which may disrupt the distribution of body fluid so as to prevent related diseases.

TRIAL REGISTRATION

Chinese clinical trial registry. Name of the registry: Relationship of drinking water and urination. Trial registration number: ChiCTR-ROC-17010320. Date of registration: 01/04/2017. URL of trial registry record: http://www.chictr.org.cn/edit.aspx?pid=17601&htm=4.

High-resolution MRI of kidney microstructures at 7.05 T with an endo-colonic Wireless Amplified NMR detector

To map the hemodynamic responses of kidney microstructures at 7.05 T with improved sensitivity, a Wireless Amplified NMR Detector (WAND) with cylindrical symmetry was fabricated as an endoluminal detector that can convert externally provided wireless signal at 600.71 MHz into amplified MR signals at 300.33 MHz. When this detector was inserted inside colonic lumens to sensitively observe adjacent kidneys, it could clearly identify kidney microstructures in the renal cortex and renal medullary. Owing to the higher achievable spatial resolution, differential hemodynamic responses of kidney microstructures under different breathing conditions could be individually quantified to estimate the underlying correlation between oxygen bearing capability and local levels of oxygen unsaturation. The WAND's ability to map Blood Oxygen Level Dependent (BOLD) signal responses in heterogeneous microstructures will pave way for early-stage diagnosis of kidney diseases, without the use of contrast agents for reduced tissue retention and toxicity.

Renal medullary tonicity regulates RNF183 expression in the collecting ducts via NFAT5

Nuclear factor of activated T-cells 5 (NFAT5) directly binds to the promoter of the RING finger protein 183 (RNF183) gene and induces its transcription under hypertonic conditions in mouse inner-medullary collecting duct (mIMCD-3) cells. However, there is no specific anti-RNF183 antibody for immunostaining; therefore, it is unclear whether NFAT5 regulates RNF183 expression in vivo and where RNF183 is localized in the kidney. This study investigated NFAT5-regulated in vivo RNF183 expression and localization using CRISPR/Cas9-mediated RNF183-green fluorescent protein (RNF183-GFP) knock-in mice. GFP with linker sequences was introduced upstream of an RNF183 open reading frame in exon 3 by homologous recombination through a donor plasmid. Immunofluorescence staining using GFP antibody revealed that GFP signals gradually increase from the outer medulla down to the inner medulla and colocalize with aquaporin-2. Furosemide treatment dramatically decreased RNF183 expression in the renal medulla, consistent with the decrease in NFAT5 protein and target gene mRNA expression. Furosemide treatment of mIMCD-3 cells did not affect mRNA expression and RNF183 promoter activities. These results indicated that RNF183 is predominantly expressed in the renal medullary collecting ducts, and that decreased renal medullary tonicity by furosemide treatment decreases RNF183 expression by NFAT5 downregulation.

Fasting Urinary Osmolality, CKD Progression, and Mortality: A Prospective Observational Study

RATIONALE & OBJECTIVE

Chronic kidney disease (CKD) characterized by decreased glomerular filtration rate (GFR) is often accompanied by various degrees of impaired tubular function in the cortex and medulla. Assessment of tubular function may therefore be useful in establishing the severity of kidney disease and identifying those at greater risk for CKD progression. We explored reductions in urinary concentrating ability, a well-known feature of CKD, as a risk factor for GFR decline and end-stage renal disease (ESRD).

STUDY DESIGN

Prospective longitudinal cohort study.

SETTING & PARTICIPANTS

2,084 adult patients with CKD stages 1 to 4 from the French NephroTest Cohort Study.

PREDICTOR

Fasting urinary osmolality measured using delta cryoscopy.

OUTCOMES

ESRD, mortality before ESRD, and measured GFR (mGFR) assessed using ⁵¹Cr-EDTA renal clearance.

ANALYTICAL APPROACH

Cause-specific hazards models were fit to estimate crude and adjusted associations of urinary osmolality with ESRD and death before ESRD. Linear mixed models with random intercepts were fit to evaluate the association of urinary osmolality with slope of decline in mGFR.

RESULTS

At baseline, mean age was 58.7±15.2 (SD) years with a median mGFR of 40.2 (IQR, 29.1-54.5) mL/min/1.73m² and a median fasting urinary osmolality of 502.7±151.7mOsm/kg H₂O. Baseline fasting urinary osmolality was strongly associated with mGFR (R=0.54; P < 0.001). 380 ESRD events and 225 deaths before ESRD occurred during a median follow-up of 5.9 (IQR, 3.8-8.2) years. Patients with lower baseline fasting urinary osmolality had higher adjusted risk for ESRD but not for mortality (HRs of 1.97 [95% CI, 1.26-3.08] and 0.99 [95% CI, 0.68-1.44], respectively, for the lowest vs highest tertile). Based on a mixed linear model adjusted for baseline mGFR and clinical characteristics, patients in the lowest tertile of baseline urinary osmolality had a steeper decline in kidney function (-4.9% ± 0.9% per year; P < 0.001) compared with patients in the highest tertile.

LIMITATIONS

Fasting was self-reported.

CONCLUSIONS

Fasting urinary osmolality may be a useful tool, in addition to GFR and albuminuria, for assessing nonglomerular damage in patients with CKD who are at higher risk for CKD progression.

NFAT5 up-regulates expression of the kidney-specific ubiquitin ligase gene Rnf183 under hypertonic conditions in inner-medullary collecting duct cells

We previously reported that among the 37 RING finger protein (RNF) family members, RNF183 mRNA is specifically expressed in the kidney under normal conditions. However, the mechanism supporting its kidney-specific expression pattern remains unclear. In this study, we elucidated the mechanism of the transcriptional activation of murine Rnf183 in inner-medullary collecting duct cells. Experiments with anti-RNF183 antibody revealed that RNF183 is predominantly expressed in the renal medulla. Among the 37 RNF family members, Rnf183 mRNA expression was specifically increased in hypertonic conditions, a hallmark of the renal medulla. RNF183 up-regulation was consistent with the activation of nuclear factor of activated T cells 5 (NFAT5), a transcription factor essential for adaptation to hypertonic conditions. Accordingly, siRNA-mediated knockdown of NFAT5 down-regulated RNF183 expression. Furthermore, the -3,466 to -3,136-bp region upstream of the mouse Rnf183 promoter containing the NFAT5-binding motif is conserved among mammals. A luciferase-based reporter vector containing the NFAT5-binding site was activated in response to hypertonic stress, but was inhibited by a mutation at the NFAT5-binding site. ChIP assays revealed that the binding of NFAT5 to this DNA site is enhanced by hypertonic stress. Of note, siRNA-mediated RNF183 knockdown increased hypertonicity-induced caspase-3 activation and decreased viability of mIMCD-3 cells. These results indicate that (i) RNF183 is predominantly expressed in the normal renal medulla, (ii) NFAT5 stimulates transcriptional activation of Rnf183 by binding to its cognate binding motif in the Rnf183 promoter, and (iii) RNF183 protects renal medullary cells from hypertonicity-induced apoptosis.

De novo assembling and primary analysis of genome and transcriptome of gray whale Eschrichtius robustus

Background

Gray whale, Eschrichtius robustus (E. robustus), is a single member of the family Eschrichtiidae, which is considered to be the most primitive in the class Cetacea. Gray whale is often described as a “living fossil”. It is adapted to extreme marine conditions and has a high life expectancy (77 years). The assembly of a gray whale genome and transcriptome will allow to carry out further studies of whale evolution, longevity, and resistance to extreme environment.

Results

In this work, we report the first de novo assembly and primary analysis of the E. robustus genome and transcriptome based on kidney and liver samples. The presented draft genome assembly is complete by 55% in terms of a total genome length, but only by 24% in terms of the BUSCO complete gene groups, although 10,895 genes were identified. Transcriptome annotation and comparison with other whale species revealed robust expression of DNA repair and hypoxia-response genes, which is expected for whales.

Conclusions

This preliminary study of the gray whale genome and transcriptome provides new data to better understand the whale evolution and the mechanisms of their adaptation to the hypoxic conditions.

Electronic supplementary material

The online version of this article (doi: 10.1186/s12862-017-1103-z) contains supplementary material, which is available to authorized users.

Hyponatremia in Critically Ill Patients

Disorders of sodium and water metabolism are frequently encountered in hospitalized patients. Hyponatremia in critically ill patients can cause significant morbidity and mortality. Inappropriate treatment of hyponatremia can add to the problem. The diagnosis and management of salt and water abnormalities in critically ill patients is often challenging. The increasing knowledge about aquaporins and the role of vasopressin in water metabolism has enhanced our understanding of these disorders. The authors have outlined the general approach to the diagnosis and management of hyponatremia. A systematic approach by clinicians, using a detailed history, physical examination, and relevant diagnostic tests, will assist in efficient management of salt and water problems.

New insights into urea and glucose handling by the kidney, and the urine concentrating mechanism

The mechanism by which urine is concentrated in the mammalian kidney remains incompletely understood. Urea is the dominant urinary osmole in most mammals and may be concentrated a 100-fold above its plasma level in humans and even more in rodents. Several facilitated urea transporters have been cloned. The phenotypes of mice with deletion of the transporters expressed in the kidney have challenged two previously well-accepted paradigms regarding urea and sodium handling in the renal medulla but have provided no alternative explanation for the accumulation of solutes that occurs in the inner medulla. In this review, we present evidence supporting the existence of an active urea secretion in the pars recta of the proximal tubule and explain how it changes our views regarding intrarenal urea handling and UT-A2 function. The transporter responsible for this secretion could be SGLT1, a sodium-glucose cotransporter that also transports urea. Glucagon may have a role in the regulation of this secretion. Further, we describe a possible transfer of osmotic energy from the outer to the inner medulla via an intrarenal Cori cycle converting glucose to lactate and back. Finally, we propose that an active urea transporter, expressed in the urothelium, may continuously reclaim urea that diffuses out of the ureter and bladder. These hypotheses are all based on published findings. They may not all be confirmed later on, but we hope they will stimulate further research in new directions.

Decreased expression of AQP2 and AQP4 water channels and Na, K-ATPase in kidney collecting duct in AQP3 null mice

BACKGROUND INFORMATION

Phenotype analysis has demonstrated that AQP3 (aquaporin 3) null mice are polyuric and manifest a urinary concentration defect. In the present study, we report that deletion of AQP3 is also associated with an increased urinary sodium excretion. To investigate further the mechanism of the decreased urinary concentration and significant natriuresis, we examined the segmental and subcellular localization of collecting duct AQPs [AQP2, p-AQP2 (phosphorylated AQP2), AQP3 and AQP4], ENaC (epithelial sodium channel) subunits and Na,K-ATPase by immunoperoxidase and immunofluorescence microscopy in AQP3 null (-/-), heterozygous (+/-) mice, wild-type and unrelated strain of normal mice.

RESULTS

The present study confirms that AQP3 null mice exhibit severe polyuria and polydipsia and demonstrated that they exhibit increased urinary sodium excretion. In AQP3 null mice, there is a marked down-regulation of AQP2 and p-AQP2 both in CNT (connecting tubule) and CCD (cortical collecting duct). Moreover, AQP4 is virtually absent from CNT and CCD in AQP3 null mice. Basolateral AQP2 was virtually absent from AQP3 null mice and normal mice in contrast with rat. Thus the above results demonstrate that no basolateral AQPs are expressed in CNT and CCD of AQP3 null mice. However, in the medullary-collecting ducts, there is no difference in the expression levels and subcellular localization of AQP2, p-AQP2 and AQP4 between AQP3 +/- and AQP3 null mice. Moreover, a striking decrease in the immunolabelling of the alpha1 subunit of Na,K-ATPase was observed in CCD in AQP3 null mice, whereas a medullary-collecting duct exhibited normal labelling. Immunolabelling of all the ENaC subunits in the collecting duct was comparable between the two groups.

CONCLUSIONS

The results improve the possibility that the severe urinary concentrating defect in AQP3 null mice may in part be caused by the decreased expression of AQP2, p-AQP2 and AQP4 in CNT and CCD, whereas the increased urinary sodium excretion may in part be accounted for by Na,K-ATPase in CCD in AQP3 null mice.

Changes of renal AQP2, ENaC, and NHE3 in experimentally induced heart failure: response to angiotensin II AT1 receptor blockade

Heart failure (HF) was induced by ligation of the left anterior descending artery (LAD). Left ventricular end-diastolic pressure (LVEDP) >25 mmHg (at day 23 after LAD ligation) was the inclusion criterion. The rats were divided into three groups: sham-operated (Sham, n = 23, LVEDP: 5.6 +/- 0.6 mmHg), HF (n = 14, LVEDP: 29.4 +/- 1.4 mmHg), and candesartan (1 mg.kg(-1).day(-1) sc)-treated HF (HF + Can, n = 9, LVEDP: 29.2 +/- 1.2 mmHg). After 7 days (i.e., 29 days after LAD ligation) semiquantitative immunoblotting revealed increased abundance of inner medulla aquaporin-2 (AQP2) and AQP2 phosphorylated at Ser(256) (p-AQP2) in HF. There was also markedly enhanced apical targeting of AQP2 and p-AQP2 in inner medullary collecting duct (IMCD) in HF compared with Sham rats, shown by immunocytochemistry. Candesartan treatment significantly reversed the increases in both AQP2 and p-AQP2 expression and targeting. In contrast, there were only modest changes in other collecting duct segments. Semiquantitative immunoblots revealed increased expression of type 3 Na(+)/H(+) exchanger (NHE3) and Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) in kidneys from HF compared with Sham rats: both effects were reversed or prevented by candesartan treatment. The protein abundance of alpha-epithelial sodium channel (alpha-ENaC) was increased while beta-ENaC and gamma-ENaC expression was decreased in the cortex and outer stripe of the outer medulla in HF compared with Sham rats, which was partially reversed by candesartan treatment. These findings strongly support an important role of angiotensin II in the pathophysiology of renal water and sodium retention associated with HF.

A comparative study of renal function in the desert-adapted spiny mouse and the laboratory-adapted C57BL/6 mouse: response to dietary salt load

The desert-adapted spiny mouse has a significantly lower glomerular number, increased glomerular size, and a more densely packed renal papillae compared with the similar-sized laboratory-adapted C57BL/6 mouse. In the present study we examined the functional consequences of these structural differences in young adult male spiny and C57BL/6 mice and detailed the impact of 1 wk of a high-salt (10% wt/wt NaCl) diet. Basal food and water intake, urine and feces production, and urinary electrolyte concentrations were not different between species, although urinary urea concentrations were higher in spiny mice (P < 0.05). On normal salt, MAP of the anesthetized spiny mouse was approximately 18 mmHg lower, effective renal plasma flow (ERPF) was 40% lower (P < 0.001), and glomerular filtration rate (GFR) tended to be lower than in the C57BL/6 mouse. On the high-salt diet, both species had similar 24-h NaCl excretions; but C57BL/6 mice required a significantly increased amount of water (lower urine NaCl concentration) than the spiny mice. Filtration fraction was greater in both species on the high-salt diet. Spiny mice had greater GFR and ERPF after the high-salt diet, whereas the C57BL/6 mouse showed little change in GFR. The ability of the spiny mouse to tolerate a significantly higher plasma osmolality after salt, measured by a decreased drinking response, and the ability to increase ERPF at a lower MAP are features that allow this species to conserve water more efficiently than can be done in the C57BL/6 mouse. These features are important, particularly since the desert mouse has a smaller kidney, with fewer nephrons.

Urea and urine concentrating ability: new insights from studies in mice

Urea is the most abundant solute in the urine in humans (on a Western-type diet) and laboratory rodents. It is far more concentrated in the urine than in plasma and extracellular fluids. This concentration depends on the accumulation of urea in the renal medulla, permitted by an intrarenal recycling of urea among collecting ducts, vasa recta and thin descending limbs, all equipped with specialized, facilitated urea transporters (UTs) (UT-A1 and 3, UT-B, and UT-A2, respectively). UT-B null mice have been recently generated by targeted gene deletion. This review describes 1) the renal handling of urea by the mammalian kidney; 2) the consequences of UT-B deletion on urinary concentrating ability; and 3) species differences among mice, rats, and humans related to their very different body size and metabolic rate, leading to considerably larger needs to excrete and to concentrate urea in smaller species (urea excretion per unit body weight in mice is 5 times that in rats and 23 times that in humans). UT-B null mice have a normal glomerular filtration rate but moderately reduced urea clearance. They exhibit a 30% reduction in urine concentrating ability with a more severe defect in the capacity to concentrate urea (50%) than other solutes, despite a twofold enhanced expression of UT-A2. The urea content of the medulla is reduced by half, whereas that of chloride is almost normal. When given an acute urea load, UT-B null mice are unable to raise their urinary osmolality, urine urea concentration (Uurea), and the concentration of non-urea solutes, as do wild-type mice. When fed diets with progressively increasing protein content (10, 20, and 40%), they cannot prevent a much larger increase in plasma urea than wild-type mice because they cannot raise Uurea. In both wild-type and UT-B null mice, urea clearance was higher than creatinine clearance, suggesting the possibility that urea could be secreted in the mouse kidney, thus allowing more efficient excretion of the disproportionately high urea load. On the whole, studies in UT-B null mice suggest that recycling of urea by countercurrent exchange in medullary vessels plays a more crucial role in the overall capacity to concentrate urine than its recycling in the loops of Henle.

[Postoperative hyponatremia in children: pathophysiology, diagnosis and treatment]

OBJECTIVES

To review the current data on pathophysiology, causes and management of postoperative hyponatremia in children.

DATA SOURCES AND EXTRACTION

The Pubmed database was searched for articles, combined with references analysis of major articles on the field.

DATA SYNTHESIS

The incidence of postoperative hyponatremia has been evaluated at 0.34% and its mortality significant. Postoperative hyponatremia is triggered by the diminished renal ability to excrete free water, due to antidiuretic hormone release. Inappropriate secretion of antidiuretic hormone is frequently seen after spine, cardiac and neurosurgery but can occur even after minor surgery. In this context, the infusion of hypotonic fluids represents a strong risk factor for developing hyponatremia. Other causes of hyponatremia are represented by extrarenal fluid losses, cerebral salt wasting syndrome, desalination phenomenon, adrenal insufficiency or some medications. Preventive treatment is essential and based on prohibition of hypotonic fluids infusion and the use of isotonic fluids infusions, maintenance of a normal total blood volume, the observance of the good practice recommendations for fluid infusion in children, and frequent blood and urine sodium concentration determinations in patients at risk for developing hyponatremia. Hyponatremic encephalopathy requires an emergent management, consisting in respiratory care and hypertonic sodium chloride infusion. Chronic hyponatremia is most often asymptomatic and the main neurological risk factor is represented by a too rapid correction of plasma sodium, which may lead to centropontine myelinolysis.

Lack of UT-B in vasa recta and red blood cells prevents urea-induced improvement of urinary concentrating ability

Recycling of urea within the renal medulla is known to play an important role in the capacity of the kidney to concentrate urine. This recycling occurs simultaneously through a tubular and a vascular route (i.e., through the loops of Henle and vasa recta, respectively). In the present study, transgenic mice with a selective deficiency in UT-B (the urea transporter protein expressed in descending vasa recta and red blood cells), were used to evaluate the specific contribution of vascular urea recycling to overall urine-concentrating ability (UCA). The renal handling of urea was studied in normal conditions and after acute or chronic alterations in urea excretion (acute urea loading or variations in protein intake, respectively). In normal conditions, UT-B null mice exhibited a 44% elevation in plasma urea (Purea), a normal creatinine clearance, but a 25% decrease in urea clearance, with no change in that of sodium and potassium. Acute urea loading induced a progressive increase in urinary urea concentration (Uurea) in wild-type mice and a subsequent improvement in their UCA in contrast to UT-B null mice, in which urinary osmolality and Uurea did not rise, due to the failure to accumulate urea in the medulla. With increasing protein intake (from 10 to 40% protein in diet, leading to a 5-fold increase in urea excretion), Purea was further increased in null mice while little change was observed in wild-type mice, and null mice were not able to increase Uurea as did wild-type mice. In conclusion, this study in UT-B-deficient mice reveals that countercurrent exchange of urea in renal medullary vessels and red blood cells accounts for a major part of the kidney's concentrating ability and for the adaptation of renal urea handling during a high-protein intake.

Cycles and separations in a model of the renal medulla

This study gives the first quantitative analysis of net steady-state transmural fluxes of water, urea, and NaCl in a numerical model of the rat renal medulla in antidiuresis, revealing the model's predictions of water, urea, and NaCl cycling patterns. These predictions are compared both to in vivo micropuncture data from the literature and to earlier qualitative proposals (e.g., K. V. Lemley and W. Kriz. Kidney Int. 31: 538-548, 1987) of cycling and exchange patterns based on medullary anatomy and available permeability and transport parameter measurements. The analysis is based on our most recent three-dimensional model [X. Wang, S. R. Thomas, and A. S. Wexler. Am. J. Physiol. 274 (Renal Physiol. 43): F413-F424, 1998]. In general agreement with earlier proposed patterns, this analysis predicts the following: 1) important water short-circuiting from descending structures to ascending vasa recta in most medullary regions, 2) massive urea recycling in the upper inner medulla, 3) a progressive increase of the ratio of urea to total osmoles along the corticopapillary axis, 4) urea dumped from the collecting ducts (CD) into the deep papilla is returned to the cortex essentially via outer medullary short vasa recta, bearing witness to a shift from the long descending limbs and vasa recta of the inner medulla (IM) to short structures in the outer medulla (OM). The analysis also shows that the known radial heterogeneity of the inner stripe (IS) implies unequal osmolalities in long descending limbs, vasa recta, and CDs entering the IM across the OM/IM border and explains the model's unorthodox osmolality profile along the CD. In conflict with micropuncture evidence of a doubling of urea flow in superficial Henle's loops (SHL) between the end proximal and early distal tubule (T. Armsen and H. W. Reinhardt. Pflügers Arch. 326: 270-280, 1971), the model predicts net urea loss from SHL due to the model's inclusion of nonneglible measured urea permeability of medullary thick ascending limbs [M. A. Knepper, Am. J. Physiol. 245 (Renal Fluid Electrolyte Physiol. 14): F634-F639, 1983]. We present a suite of adjusted model permeabilities that improves agreement with the micropuncture data on this point. In conclusion, this modeling analysis of solute and water recycling serves as a quantitative check on qualitative propositions in the literature and allows closer critical comparison of model behavior with published experimental results than was heretofore possible.

Outer medullary anatomy and the urine concentrating mechanism

In earlier work, mathematical models of the urine concentration mechanism were developed incorporating the features of renal anatomy. However, several anatomic observations showed inconsistencies in the modeling representation of the outer stripe (OS) anatomy. In this study, based on observations from comparative anatomy and morphometric studies, we propose a new structural model of outer medullary anatomy, different from that previously presented [A. S. Wexler, R. E. Kalaba, and D. J. Marsh. Am. J. Physiol. 260 (Renal Fluid Electrolyte Physiol. 29): F368-F383, 1991]. The modifications include the following features of rat outer medullary anatomy, for example, 1) in the OS, the limbs of long loops of Henle surround the descending and ascending vasa recta that develop into the vascular bundles in the inner stripe (IS), whereas the limbs of short loops are close to the collecting ducts; and 2) the descending limbs of short loops shift from the tubular region in the OS to near the vascular bundle in the IS, whereas the limbs of long loops are situated away from the vascular bundles in the tubular region. The sensitivity of the concentrating process to the relative position of loops and vessels was investigated in the different medullary regions. With these modifications, the model predicts a more physiological, axial osmolarity gradient in both outer and inner medulla with membrane parameters that are all in the range of measured physiological values, including the urea permeabilities of descending vasa recta reported by Pallone and co-workers (T. L. Pallone, J. Work, R. L. Myers, and R. L. Jamison. J. Clin.Invest. 93: 212-222, 1994).

Direct and indirect cost of urea excretion

Urea, the major end product of protein metabolism in mammals, is the most abundant solute in the urine. Urea excretion is thought to result from filtration curtailed by some passive reabsorbtion along the nephron. This reabsorption is markedly enhanced by vasopressin and slow urinary flow rate (V), the fraction of filtered urea excreted in the urine (FEurea) falling from approximately 60% at high V to only approximately 20% at low V. In concentrated urine, normal urea excretion can be maintained only if urea filtration is elevated. This can be achieved by increasing plasma urea concentration (Purea) and/or GFR. We have shown that both parameters do increase when normal rats are submitted to chronic alterations in the water intake/vasopressin axis within the normal range of physiologic regulation. This situation is very similar to that observed after alterations in protein intake. In both cases more urea needs to be filtered, either because more of it has to be excreted, or because the efficiency of its excretion is reduced. A common mechanism is proposed to explain the rise in GFR observed in the two situations. In summary, our studies demonstrate that the antidiuretic effects of vasopressin are responsible for a significant elevation of GFR. This GFR adaptation limits the rise in Purea, a favorable effect because urea is not as harmless as usually thought. However, this hyperfiltration might have deleterious consequences in diseased kidneys.