Urinary aquaporin 2 and calciuria correlate with the severity of enuresis in children

Early Biomarkers of Altered Renal Function and Orthostatic Intolerance During 10-day Bedrest

Exposure to actual or simulated microgravity results in alterations of renal function, fluid redistribution, and bone loss, which is coupled to a rise of urinary calcium excretion. We provided evidence that high calcium delivery to the collecting duct reduces local Aquaporin 2 (AQP2)-mediated water reabsorption under vasopressin action, thus limiting the maximal urinary concentration to reduce calcium saturation. To investigate early renal adaptation into simulated microgravity, we investigated the effects of 10 days of strict bedrest in 10 healthy volunteers. We report here that 10 days of inactivity are associated with a transient, significant decrease (day 5) in vasopressin (copeptin) paralleled by a decrease in AQP2 excretion, consistent with an increased central volume to the heart, resulting in reduced water reabsorption. Moreover, bedrest caused a significant increase in calciuria secondary to bone demineralization paralleled by a decrease in PTH. Urinary osteopontin, a glycoprotein exerting a protective effect on stone formation, was significantly reduced during bedrest. Moreover, a significant increase in adrenomedullin (day 5), a peptide with vasodepressor properties, was observed at day 5, which may contribute to the known reduced orthostatic capacity post-bedrest. We conclude that renal function is altered in simulated microgravity and is associated with an early increase in the risk of stone formation and reduced orthostatic capacity post-bedrest within a few days of inactivity.

Bibliometric and visual analysis of nocturnal enuresis from 1982 to 2022

Nocturnal enuresis is a common disorder among children that seriously affects physical and mental health and has become a social problem. Bibliometric analysis is a valid way to examine existing research results, current research hotspots and research frontiers. Current studies on nocturnal enuresis are numerous and complex, but a bibliometric analysis of the existing research on nocturnal enuresis has yet to be published. To better identify the research trends and frontiers in nocturnal enuresis, it is necessary to conduct a comprehensive review and analysis. We used bibliometric and visualization methods to analyze the 1,111 papers published between 1982 and 2022 from the Web of Science core collection. Basic information about the country, institution, and authors was analyzed, which led to a basic understanding of nocturnal enuresis. The United States is the most prolific country, Ghent University is the most influential institution, and Rittig Soren is the most prominent scholar. The frequency of keywords, clustering, and the cited literature were analyzed to understand the hotspots and frontiers of research, and a brief review of the highly cited literature was conducted. The current research hotspots are the treatment modalities for nocturnal enuresis, epidemiological investigations, and the exploration of pathogenesis. Clinical research, adenoidectomy, aquaporin 2, and response inhibition are potential research hotspots. The standardization of terminology in nocturnal enuresis and the pathologies of polyuria and sleep disorder are at the forefront of research. In summary, the results of our bibliometric analysis reveal views on the current situation and the trend of nocturnal enuresis research for the first time. This study may provide guidance for promoting research on nocturnal enuresis.

The long-term effects of rapamycin-based immunosuppressive protocols on the expression of renal aquaporins 1, 2, 3 and 4 water channels in rats

BACKGROUND

To this day, the effect of multi-drug immunosuppressive protocols on renal expression of AQPs is unknown. This study aimed to determine the influence of rapamycin-based multi-drug immunosuppressive regimens on the expression of aquaporins (AQPs) 1, 2, 3, and 4 in the rat kidney.

METHODS

For 6 months, 24 male Wistar rats were administered immunosuppressants, according to the three-drug protocols used in patients after organ transplantation. The rats were divided into four groups: the control group, the TRP group (tacrolimus, rapamycin, prednisone), the CRP group (cyclosporine A, rapamycin, prednisone), and the MRP group (mycophenolate mofetil, rapamycin, prednisone). Selected red cell indices and total calcium were measured in the blood of rats and quantitative analysis of AQP1, AQP2, AQP3 and AQP4 immunoexpression in the kidneys were performed.

RESULTS

In the TRP and CRP groups, a mild increase of mean corpuscular hemoglobin concentration, hematocrit and total calcium were observed. Moreover, decreased expression of AQP1-4 was found in all experimental groups, with the highest decrease in the CRP group.

CONCLUSIONS

The long-term immunosuppressive treatment using multi-drug protocols decreased AQP1-4 expressions in renal tubules, possibly leading to impaired urine-concentrating ability in rat.

Regulation of urinary calcium excretion by vasopressin

Background

The antidiuretic hormone (ADH) or arginine vasopressin (AVP) regulates the body's water balance. Recently, modifications in AVP levels have been related to osteoporosis during ageing and microgravity/bed rest. Therefore the present study was devised to assess whether the absence of AVP, as in patients with central diabetes insipidus (CDI), modulates renal calcium excretion.

Methods

We retrospectively analysed data from 12 patients with CDI with measured 24-h urinary excretion levels of calcium. Data were available at the moment of the diagnosis when patients were drug-free and after therapy with dDAVP, an analog of AVP. Hypercalciuria was defined as 24-h urinary Ca²⁺ >275 mg/day in males and >250 mg/day in females and a urinary calcium (Ca):creatinine (Cr) ratio >0.20 mg/mg.

Results

Untreated CDI patients had a daily urinary Ca²⁺ excretion of 383 ± 47 mg/day and a urinary Ca:Cr ratio of 0.26 ± 0.38 mg/mg. The urine osmolarity significantly increased after the administration of dDAVP by 210% and the urinary flow decreased by 72%. Furthermore, the estimated glomerular filtration rate (eGFR) increased by 7%, which did not reach statistical significance. dDAVP treatment did not significantly modify the urinary Ca²⁺ concentration; however, the daily calcium excretion and the urinary Ca:Cr ratio were significantly decreased (160 ± 27 mg/day and 0.11 ± 0.02 mg/mg, respectively).

Conclusions

Patients with CDI show hypercalciuria even though urine is more diluted than normal controls, and dDAVP reverses this effect. These data support the intriguing relationship between AVP and osteoporosis in ageing and microgravity/bed rest.

Calcium sensing receptor exerts a negative regulatory action toward vasopressin-induced aquaporin-2 expression and trafficking in renal collecting duct

Vasopressin (AVP) plays a major role in the regulation of water homeostasis by its antidiuretic action on the kidney, mediated by V2 receptors. An increase in plasma sodium concentration stimulates AVP release, which in turn promotes water reabsorption. Upon binding to the V2 receptors in the renal collecting duct, AVP induces the expression and apical membrane insertion of the aquaporin-2 (AQP2) water channels and subsequent water reabsorption. AVP regulates two independent mechanisms: the short-term regulation of AQP2 trafficking and long-term regulation of the total abundance of the AQP2 protein in the cells. On the other hand, several hormones, acting through specific receptors, have been reported to antagonize AVP-mediated water transport in kidney. In this respect, we previously described that high luminal Ca²⁺ in the renal collecting duct attenuates short-term AVP-induced AQP2 trafficking through activation of the Ca²⁺-sensing receptor (CaSR). This effect is due to reduction of AVP-dependent cAMP generation and possibly hydrolysis. Moreover, CaSR signaling reduces AQP2 abundance both via AQP2-targeting miRNA-137 and the proteasomal degradation pathway. This chapter summarizes recent data elucidating the molecular mechanisms underlying the physiological role of the CaSR-dependent regulation of AQP2 expression and trafficking.

Renal Ca2+ and Water Handling in Response to Calcium Sensing Receptor Signaling: Physiopathological Aspects and Role of CaSR-Regulated microRNAs

Calcium (Ca²⁺) is a universal and vital intracellular messenger involved in a diverse range of cellular and biological processes. Changes in the concentration of extracellular Ca²⁺ can disrupt the normal cellular activities and the physiological function of these systems. The calcium sensing receptor (CaSR) is a unique G protein-coupled receptor (GPCR) activated by extracellular Ca²⁺ and by other physiological cations, aminoacids, and polyamines. CaSR is the main controller of the extracellular Ca²⁺ homeostatic system by regulating parathyroid hormone (PTH) secretion and, in turn, Ca²⁺ absorption and resorption. Recent advances highlight novel signaling pathways activated by CaSR signaling involving the regulation of microRNAs (miRNAs). miRNAs are naturally-occurring small non-coding RNAs that regulate post-transcriptional gene expression and are involved in several diseases. We previously described that high luminal Ca²⁺ in the renal collecting duct attenuates short-term vasopressin-induced aquaporin-2 (AQP2) trafficking through CaSR activation. Moreover, we demonstrated that CaSR signaling reduces AQP2 abundance via AQP2-targeting miRNA-137. This review summarizes the recent data related to CaSR-regulated miRNAs signaling pathways in the kidney.

AQP1-Containing Exosomes in Peritoneal Dialysis Effluent As Biomarker of Dialysis Efficiency

The water channel Aquaporin 1 (AQP1) plays a fundamental role in water ultrafiltration during peritoneal dialysis (PD) and its reduced expression or function may be responsible for ultrafiltration failure (UFF). In humans, AQP1 is expressed in the endothelium of the peritoneal capillaries but its expression in mesothelial cells (MC) and its functional role in PD is still being debated. Here, we studied a cohort of 30 patients using PD in order to determine the presence of AQP1 in peritoneal biopsies, AQP1 release in the PD effluent through exosomes and the correlation of AQP1 abundance with the efficiency of peritoneal ultrafiltration. The experiments using immunofluorescence showed a strong expression of AQP1 in MCs. Immunoblotting analysis on vesicles isolated from PD effluents showed a consistent presence of AQP1, mesothelin and Alix and the absence of the CD31. Thus, this suggests that they have an exclusive mesothelial origin. The immunoTEM analysis showed a homogeneous population of nanovesicles and confirmed the immunoblotting results. Interestingly, the quantitative analysis by ELISA showed a positive correlation between AQP1 in the PD effluent and ultrafiltration (UF), free water transport (FWT) and Na-sieving. This evidence opens the discussion on the functional role of mesothelial AQP1 during PD and suggests that it may represent a potential non-invasive biomarker of peritoneal barrier integrity, with predictive potential of UFF in PD patients.

Circadian regulation of renal function

The kidneys regulate many vital functions that require precise control throughout the day. These functions, such as maintaining sodium balance or regulating arterial pressure, rely on an intrinsic clock mechanism that was commonly believed to be controlled by the central nervous system. Mounting evidence in recent years has unveiled previously underappreciated depth of influence by circadian rhythms and clock genes on renal function, at the molecular and physiological level, independent of other external factors. The impact of circadian rhythms in the kidney also affects individuals from a clinical standpoint, as the loss of rhythmic activity or clock gene expression have been documented in various cardiovascular diseases. Fortunately, the prognostic value of examining circadian rhythms may prove useful in determining the progression of a kidney-related disease, and chronotherapy is a clinical intervention that requires consideration of circadian and diurnal rhythms in the kidney. In this review, we discuss evidence of circadian regulation in the kidney from basic and clinical research in order to provide a foundation on which a great deal of future research is needed to expand our understanding of circadian relevant biology.

Calciuria in children with primary mono-symptomatic nocturnal enuresis

Introduction: The prevalence of idiopathic hypercalciuria (IH) in healthy pediatric population ranges from 3.0% to 7.0%. There is insufficient data about IH in children with mono-symptomatic enuresis. The aim of this study was to examine calcium excretion in urine (UCa) in patients with primary mono-symptomatic nocturnal enuresis (PMNE). Methods: In patients with PMNE, aged 5 to 17 years, IH was determined in 24-h urine and from second morning spot urine. The completeness of the 24-h urine collections was estimated via measuring 24h-urine creatinine excretion (UCr) of 0.1-0.2 mmol/kg/24h. Results: Sixty patients with PMNE, 32 males and 28 girls, median age of 9 years were enrolled in the study. Only 41.7% patients successfully completed 24 h urine collection. IH, defined as 24-h UCa > 0.1 mmol/kg body weight, was diagnosed in 12% of the patients, while when defined as UCa/UCr > 0.8 mmol/mmol in children 5-7 years and > 0.6 mmol/mmol in those > 7 years, IH was 8.3% and 6.7% from 24hurine and spot urine, respectively. Conclusion: Children and adolescents with PMNE are in risk of hypercalciuria. Therefore, it is useful to examine 24 hours of urine calcium excretion in these patients.

Urinary Excretion of Kidney Aquaporins as Possible Diagnostic Biomarker of Diabetic Nephropathy

Diabetic nephropathy (DN) is a microangiopathic complication of diabetes mellitus (DM) affecting one-third of diabetic patients. The large variability in the clinical presentation of renal involvement in patients with DM makes kidney biopsy a prerequisite for a correct diagnosis. However, renal biopsy is an invasive procedure associated with risk of major complications. Numerous studies aimed to identify a noninvasive biomarker of DN but, so far, none of these is considered to be sufficiently specific and sensitive. Water channel aquaporins (AQPs), expressed at the plasma membrane of epithelial tubular cells, are often dysregulated during DN. In this work, we analyzed the urine excretion of AQP5 and AQP2 (uAQP5 and uAQP2), via exosomes, in 35 diabetic patients: 12 normoalbuminuric with normal renal function (DM), 11 with proteinuric nondiabetic nephropathy (NDN), and 12 with histological diagnosis and classification of DN. ELISA and WB analysis independently showed that uAQP5 was significantly increased in DN patients. Interestingly, linear regression analysis showed a positive correlation between uAQP5 and the histological class of DN. The same analysis, focusing on uAQP2, showed comparable results. Taken together, these data suggest a possible use of AQP5 and AQP2 as novel noninvasive biomarkers to help in classifying the clinical stage of DN.

The pathophysiology of monosymptomatic nocturnal enuresis with special emphasis on the circadian rhythm of renal physiology

Nocturnal polyuria in monosymptomatic nocturnal enuresis (MNE) has so far mainly been attributed to a disturbed circadian rhythm of renal water handling. Low vasopressin levels overnight correlate with absent maximal concentrating activity, resulting in an increased nocturnal diuresis with low urinary osmolality. Therefore, treatment with desmopressin is a rational choice. Unfortunately, 20 to 60 % of children with monosymptomatic enuresis are desmopressin-resistant. There is increasing evidence that other disturbed circadian rhythms might play a role in nocturnal polyuria. This review focuses on renal aspects in the pathophysiology of nocturnal polyuria in MNE, with special emphasis on circadian rhythms. Articles related to renal circadian rhythms and enuresis were searched through the PubMed library with the goal of providing a concise review.

CONCLUSION

Nocturnal polyuria can only partially be explained by blunted circadian rhythm of vasopressin secretion. Other alterations in the intrinsic renal circadian clock system also seem to be involved, especially in desmopressin-resistant enuresis.

WHAT IS KNOWN

• Disturbance in the circadian rhythm of arginine vasopressin secretion is related to nocturnal polyuria in children with enuresis. • Desmopressin is recommended as a treatment for monosymptomatic nocturnal enuresis, working as a vasopressin analogue acting on V2 receptors in the collecting ducts of the kidney. What is New: • Other renal circadian rhythms might play a role in nocturnal polyuria, especially in desmopressin-resistant case.

Simvastatin increases AQP2 urinary excretion in hypercholesterolemic patients: A pleiotropic effect of interest for patients with impaired AQP2 trafficking

We previously reported that statins improve the symptoms of X-linked nephrogenic diabetes insipidus (X-NDI) in animal models. The aim of this study was to verify whether the pleiotropic effect of statins on AQP2 trafficking and kidney-concentrating ability, observed in rodents, was attainable in humans at therapeutic doses. We enrolled 24 naïve hypercholesterolemic patients and measured urine excretion of AQP2 (uAQP2) at baseline and during 12 weeks of treatment with simvastatin 20 mg/day. Simvastatin induced a rapid and significant increase of uAQP2, reduced the 24-hour diuresis, and increased urine osmolality. These effects were also maintained in patients chronically treated with statins for at least 1 year. This study strongly suggests that statins may effectively enhance the efficacy of current pharmacological treatment of patients with urine-concentrating defects caused by defective AQP2 plasma membrane trafficking, like X-NDI.

Adenotonsillectomy Normalizes Hormones and Urinary Electrolytes in Children With Nocturnal Enuresis and Sleep-Disordered Breathing

OBJECTIVE

To assess (1) plasma levels of antidiuretic hormone (ADH) and brain natriuretic peptide (BNP) and urinary levels of electrolytes in children with sleep disordered breathing (SDB), with or without nocturnal enuresis (NE), and (2) the effect of adenotonsillectomy (T&A) on urinary electrolytes and the secretion of ADH and BNP in children with NE and SDB. We previously reported post-T&A improvements in plasma levels of BNP and ADH in children with SDB and NE. However, the differences in plasma concentration of these hormones in SDB children with and without NE, and their relationships with urinary electrolytes, have not yet been addressed.

METHODS

This prospective study compared concentrations of urinary electrolytes and plasma ADH and BNP in (1) children with SDB and NE (study group) and an age- and sex-matched control group of children with SDB without NE, and (2) the study group before and 1-month after T&A.

RESULTS

Compared with the control group (n = 31), the study group (n = 37) exhibited significantly lower ADH (P = .04) and higher BNP (P = .009) plasma levels. The differences in urinary electrolytes were not significant. Post-T&A, the study group showed significantly decreased BNP (P = .018), urinary sodium-to-creatinine ratio (P = .02), and urinary calcium-to-creatinine ratio (P = .007) compared with the pre-T&A values. Post-T&A changes in urinary calcium were significantly correlated with changes in sodium excretion (P = .002) and in plasma levels of BNP (P <.001).

CONCLUSION

The presence of NE is associated with altered ADH and BNP levels in children with SDB. T&A led to normalization of ADH and BNP, probably through a calcium- and sodium-dependent mechanism.

Negative feedback from CaSR signaling to aquaporin-2 sensitizes vasopressin to extracellular Ca2

We previously described that high luminal Ca(2+) in the renal collecting duct attenuates short-term vasopressin-induced aquaporin-2 (AQP2) trafficking through activation of the Ca(2+)-sensing receptor (CaSR). Here, we evaluated AQP2 phosphorylation and permeability, in both renal HEK-293 cells and in the dissected inner medullary collecting duct, in response to specific activation of CaSR with NPS-R568. In CaSR-transfected cells, CaSR activation drastically reduced the basal levels of AQP2 phosphorylation at S256 (AQP2-pS256), thus having an opposite effect to vasopressin action. When forskolin stimulation was performed in the presence of NPS-R568, the increase in AQP2-pS256 and in the osmotic water permeability were prevented. In the freshly isolated inner mouse medullar collecting duct, stimulation with forskolin in the presence of NPS-R568 prevented the increase in AQP2-pS256 and osmotic water permeability. Our data demonstrate that the activation of CaSR in the collecting duct prevents the cAMP-dependent increase in AQP2-pS256 and water permeability, counteracting the short-term vasopressin response. By extension, our results suggest the attractive concept that CaSR expressed in distinct nephron segments exerts a negative feedback on hormones acting through cAMP, conferring high sensitivity of hormone to extracellular Ca(2+).

Hypercalciuria in children with monosymptomatic nocturnal enuresis

OBJECTIVE

The aim of this study was to measure the 24 h urinary calcium content in children with monosymptomatic nocturnal enuresis (MNE) and compare with those in healthy children to investigate whether there is any relation with enuresis and hypercalciuria.

MATERIAL AND METHODS

This study included 120 children and adolescents with MNE aged between 7 and 14 years. Eighty age- and sex-matched healthy children served as a control group. To determine urinary calcium excretion, 24 h urine samples were collected. The children with enuresis were divided into two sub-groups as hypercalciuric and normocalciuric groups according to the amount of urinary calcium excretion.

RESULTS

Hypercalciuria was found in 27 (23%) of the MNE patients compared with two (4%) of continent children (p<0.001). In addition, the mean 24 h urine calcium/body weight ratio was higher in the enuresis group than in the control group, 2.94±2.42 versus 1.59±1.72, respectively (p=0.002). When the children with enuresis were divided into two groups as normokalsiuric and hypercalciuric, the hypercalciuric children were younger and the majority of this group were boys..

CONCLUSIONS

Our study showed that hypercalciuria is common in children with MNE, so we suggested measuring urine calcium levels in NE patients. However, further studies are needed to clarify the relationship between hypercalciuria and NE in larger series..

Aquaporin 2: Identification and analysis of expression in calves’ urine during their first month of life

Michałek, K., Dratwa-Chałupnik, A., Ciechanowicz, A. K. and Malinowski, E. 2014. Aquaporin 2: Identification and analysis of expression in calves’ urine during their first month of life. Can. J. Anim. Sci. 94: 653–659. Aquaporin 2 (AQP2) is a vasopressin-regulated water channel of the renal collecting duct and is excreted into urine. Renal excretion of AQP2 has been studied in humans, dogs, rats and mice. The aim of this study was to identify and analyze expression of AQP2 in calves’ urine. Short and controlled diarrhea was induced in the calves in order to determine any change in renal excretion of AQP2 and to evaluate a possible role of this channel in water balance during the neonatal period. For this purpose, an additional portion of lactose was administered to the calves with a milk replacer. The present work shows that after transient diarrhea associated with excessive supply of lactose in the diet, a significant increase in renal AQP2 excretion was reported in the first 3 wk of the calves’ lives. Renal excretion of this protein in the urine increased significantly despite the absence of changes in the concentration of vasopressin or the osmotic pressure of the blood plasma. The increase observed in the expression of the protein might be associated with the reduction of the total Ca2+ concentration in the blood plasma of calves. The results clearly indicate a different mechanism regulating urinary excretion of AQP2 in newborn calves, hitherto unknown. Although elevated renal excretion of AQP2 was observed, there was no increase in the osmolality of the urine in the calves tested.

Urinary calcium excretion in children with monosymptomatic enuresis

Background

Among many factors predisposing to monosymptomatic enuresis (MNE) disturbances in urinary electrolites excretion play an important role. Because of many controversies in this field there is a need to debate the role of hypercalciuria in MNE. The aim of our study was to determine the urinary calcium in children with MNE.

Methods

The investigation was conducted on 204 children (83 MNE children and 121 reference group). Urinary calcium excretion (in 24-h collection and per kg of body mass), Ca/creatinine ratio, Ca²⁺ in urine sample and in 24-h collection of urine were estimated.

Results

Hypercalciuria in MNE group was diagnosed in 18/83 (21.69 %) patients. We found statistically significant differences between children with MNE in Ca²⁺ in urine sample and 24-h collection and Ca/creat. ratio. Median urinary calcium excretion (mg/kg/24-h and mmol/24-h) was significantly higher in hypercalciuric enuretic patients. The urinary total calcium (mmol/24-h), urinary bound calcium and urinary calcium concentration (mmol/L) demonstrated a significant positive correlation with height, weight and age in reference group but not in MNE group.

Conclusion

Urinary calcium excretion was significantly disturbed and further studies are needed to assess the role of hypercalciuria in the pathogenesis of MNE.

Absence of PKC-alpha attenuates lithium-induced nephrogenic diabetes insipidus

Lithium, an effective antipsychotic, induces nephrogenic diabetes insipidus (NDI) in ∼40% of patients. The decreased capacity to concentrate urine is likely due to lithium acutely disrupting the cAMP pathway and chronically reducing urea transporter (UT-A1) and water channel (AQP2) expression in the inner medulla. Targeting an alternative signaling pathway, such as PKC-mediated signaling, may be an effective method of treating lithium-induced polyuria. PKC-alpha null mice (PKCα KO) and strain-matched wild type (WT) controls were treated with lithium for 0, 3 or 5 days. WT mice had increased urine output and lowered urine osmolality after 3 and 5 days of treatment whereas PKCα KO mice had no change in urine output or concentration. Western blot analysis revealed that AQP2 expression in medullary tissues was lowered after 3 and 5 days in WT mice; however, AQP2 was unchanged in PKCα KO. Similar results were observed with UT-A1 expression. Animals were also treated with lithium for 6 weeks. Lithium-treated WT mice had 19-fold increased urine output whereas treated PKCα KO animals had a 4-fold increase in output. AQP2 and UT-A1 expression was lowered in 6 week lithium-treated WT animals whereas in treated PKCα KO mice, AQP2 was only reduced by 2-fold and UT-A1 expression was unaffected. Urinary sodium, potassium and calcium were elevated in lithium-fed WT but not in lithium-fed PKCα KO mice. Our data show that ablation of PKCα preserves AQP2 and UT-A1 protein expression and localization in lithium-induced NDI, and prevents the development of the severe polyuria associated with lithium therapy.

A decrease in aquaporin 2 excretion is associated with bed rest induced high calciuria

Background

Exposure to microgravity or immobilization results in alterations of renal function, fluid redistribution and bone loss, which couples to a rise of urinary calcium excretion. We recently demonstrated that high calcium delivery to the collecting duct reduces local Aquaporin-2 (AQP2) mediated water reabsorption under vasopressin action, thus limiting the maximal urinary concentration and reducing calcium saturation. To investigate renal water balance adaptation during bed rest, a model to mimic the effects of microgravity on earth, the effect of changes in urinary calcium on urinary AQP2 excretion were assessed.

Methods

Ten healthy men (aged 21-28 years) participated in the experiment. Study design included 7 days of adaptation and 35 days of continuous bed rest (days -6 to 0 and 1 to 35, respectively) under controlled diet. Food records and 24-hour urine samples were collected daily from day -3 to 35. Changes in blood hematocrit were used as an indirect index of plasma volume changes. AQP2 excretion was measured by ELISA.

Results

Bed rest induced bone demineralization and a transient increase in urinary calcium followed by transient decrease in AQP2 excretion, which can reduce the urine concentrating ability causing plasma volume reduction. The return of calciuria to baseline was followed by a recovery of AQP2 excretion, which allows for a partial restoration of plasma volume.

Conclusions

These results further support the view that urinary calcium can modulate the vasopressin-dependent urine concentration through a down-regulation of AQP2 expression/trafficking. This mechanism could have a key role in the prevention of urine super-saturation due to hypercalciuria.

Renal aquaporins and water balance disorders

BACKGROUND

Aquaporins (AQPs) are a family of proteins that can act as water channels. Regulation of AQPs is critical to osmoregulation and the maintenance of body water homeostasis. Eight AQPs are expressed in the kidney of which five have been shown to play a role in body water balance; AQP1, AQP2, AQP3, AQP4 and AQP7. AQP2 in particular is regulated by vasopressin.

SCOPE OF REVIEW

This review summarizes our current knowledge of the underlying mechanisms of various water balance disorders and their treatment strategies.

MAJOR CONCLUSIONS

Dysfunctions of AQPs are involved in disorders associated with disturbed water homeostasis. Hyponatremia with increased AQP levels can be caused by diseases with low effective circulating blood volume, such as congestive heart failure, or osmoregulation disorders such as the syndrome of inappropriate secretion of antidiuretic hormone. Treatment consists of fluid restriction, demeclocycline and vasopressin type-2 receptor antagonists. Decreased AQP levels can lead to diabetes insipidus (DI), characterized by polyuria and polydipsia. In central DI, vasopressin production is impaired, while in gestational DI, levels of the vasopressin-degrading enzyme vasopressinase are abnormally increased. Treatment consists of the vasopressin analogue dDAVP. Nephrogenic DI is caused by the inability of the kidney to respond to vasopressin and can be congenital, but is most commonly acquired, usually due to lithium therapy. Treatment consists of sufficient fluid supply, low-solute diet and diuretics.

GENERAL SIGNIFICANCE

In recent years, our understanding of the underlying mechanisms of water balance disorders has increased enormously, which has opened up several possible new treatment strategies. This article is part of a Special Issue entitled Aquaporins.

Detection of renal tissue and urinary tract proteins in the human urine after space flight

The urine protein composition samples of ten Russian cosmonauts (male, aged of 35 up to 51) performed long flight missions and varied from 169 up to 199 days on the International Space Station (ISS) were analyzed. As a control group, urine samples of six back-up cosmonauts were analyzed. We used proteomic techniques to obtain data and contemporary bioinformatics approaches to perform the analysis. From the total number of identified proteins (238) in our data set, 129 were associated with a known tissue origin. Preflight samples contained 92 tissue-specific proteins, samples obtained on Day 1 after landing had 90 such proteins, while Day 7 samples offered 95 tissue-specific proteins. Analysis showed that consistently present proteins in urine (under physiological conditions and after space flight) are cubilin, epidermal growth factor, kallikrein-1, kininogen-1, megalin, osteopontin, vitamin K-dependent protein Z, uromodulin. Variably present proteins consists of: Na(+)/K(+) ATPase subunit gamma, β-defensin-1, dipeptidyl peptidase 4, maltasa-glucoamilasa, cadherin-like protein, neutral endopeptidase and vascular cell adhesion protein 1. And only three renal proteins were related to the space flight factors. They were not found in the pre-flight samples and in the back-up cosmonaut urine, but were found in the urine samples after space flight: AFAM (afamin), AMPE (aminopeptidase A) and AQP2 (aquaporin-2). This data related with physiological readaptation of water-salt balance. The proteomic analysis of urine samples in different phases of space missions with bioinformation approach to protein identification provides new data relative to biomechemical mechanism of kidney functioning after space flight.

Is there any correlation between hypercalciuria and nocturnal enuresis?

Nocturnal enuresis is a common problem among children. Hypercalciuria has been proposed as an important etiology of bedwetting. We investigated the incidence of hypercalciuria among children with nocturnal enuresis and age- and sex-matched healthy controls. In this case–control study 118 children with nocturnal enuresis and 100 age-, sex-, and educational district-matched healthy controls were recruited. Urine samples were obtained from each subject twice: immediately after awakening and 2 hours later at school. Urinary calcium and creatinine levels were measured and the subjects with a urinary calcium/creatinine ratio more than 0.2 were considered as hypercalciuric. Serum Ca, P, Na, K, and PTH levels were measured for all hypercalciuric subjects. The mean urine calcium to creatinine ratio in the second urine sample was 0.070±0.06 mg/mg and 0.050±0.046, respectively in the case and control groups (P=0.008). There were 12/118 and 3/100 hypercalciuric subjects in the case and control groups respectively (P=0.032). The serum Ca, P, Na, K, and PTH levels were in normal range in all hypercalciuric subjects. In our study there was a significant difference in the frequency of hypercalciuria among children with nocturnal enuresis and healthy controls, so we can suggest adding the measurement of urine electrolytes especially the calcium level in patients with nocturnal enuresis.

Bone disease in pediatric idiopathic hypercalciuria

Idiopathic hypercalciuria (IH) is the leading metabolic risk factor for urolithiasis and affects all age groups without gender or race predominance. IH has a high morbidity with or without lithiasis and reduced bone mineral density (BMD), as described previously in pediatric patients as well as in adults. The pathogenesis of IH is complex and not completely understood, given that urinary excretion of calcium is the end result of an interplay between three organs (gut, bone and kidney), which is further orchestrated by hormones, such as 1,25 dihydroxyvitamin D, parathyroid hormone, calcitonin and fosfatonins (i.e., fibroblast growth-factor-23). Usually, a primary defect in one organ induces compensatory mechanisms in the remaining two organs, such as increased absorption of calcium in the gut secondary to a primary renal loss. Thus, IH is a systemic abnormality of calcium homeostasis with changes in cellular transport of this ion in intestines, kidneys and bones. Reduced BMD has been demonstrated in pediatric patients diagnosed with IH. However, the precise mechanisms of bone loss or failure of adequate bone mass gain are still unknown. The largest accumulation of bone mass occurs during childhood and adolescence, peaking at the end of the second decade of life. This accumulation should occur without interference to achieve the peak of optimal bone mass. Any interference may be a risk factor for the reduction of bone mass with increased risk of fractures in adulthood. This review will address the pathogenesis of IH and its consequence in bone mass.

Calcium-sensing receptor and aquaporin 2 interplay in hypercalciuria-associated renal concentrating defect in humans. An in vivo and in vitro study

One mechanism proposed for reducing the risk of calcium renal stones is activation of the calcium-sensing receptor (CaR) on the apical membranes of collecting duct principal cells by high luminal calcium. This would reduce the abundance of aquaporin-2 (AQP2) and in turn the rate of water reabsorption. While evidence in cells and in hypercalciuric animal models supports this hypothesis, the relevance of the interplay between the CaR and AQP2 in humans is not clear. This paper reports for the first time a detailed correlation between urinary AQP2 excretion under acute vasopressin action (DDAVP treatment) in hypercalciuric subjects and in parallel analyzes AQP2-CaR crosstalk in a mouse collecting duct cell line (MCD4) expressing endogenous and functional CaR. In normocalciurics, DDAVP administration resulted in a significant increase in AQP2 excretion paralleled by an increase in urinary osmolality indicating a physiological response to DDAVP. In contrast, in hypercalciurics, baseline AQP2 excretion was high and did not significantly increase after DDAVP. Moreover DDAVP treatment was accompanied by a less pronounced increase in urinary osmolality. These data indicate reduced urinary concentrating ability in response to vasopressin in hypercalciurics. Consistent with these results, biotinylation experiments in MCD4 cells revealed that membrane AQP2 expression in unstimulated cells exposed to CaR agonists was higher than in control cells and did not increase significantly in response to short term exposure to forskolin (FK). Interestingly, we found that CaR activation by specific agonists reduced the increase in cAMP and prevented any reduction in Rho activity in response to FK, two crucial pathways for AQP2 translocation. These data support the hypothesis that CaR–AQP2 interplay represents an internal renal defense to mitigate the effects of hypercalciuria on the risk of calcium precipitation during antidiuresis. This mechanism and possibly reduced medulla tonicity may explain the lower concentrating ability observed in hypercalciuric patients.

Increased excretion of glycosaminoglycans in children with urinary incontinence compared to those with monosymptomatic nocturnal enuresis

OBJECTIVE

To analyse the urinary excretion of glycosaminoglycans (GAGs) in patients with either urinary incontinence or nocturnal enuresis.

MATERIAL AND METHODS

The study population comprised 65 patients with either nocturnal enuresis (n=34) or urinary incontinence (n=31) and 67 controls. Excretion of urinary GAGs was assessed using the sodium tetraborate-carbazole method.

RESULTS

GAG excretion in patients with urinary incontinence was significantly higher than that in controls (p<0.000129) and in children with nocturnal enuresis (p<0.016). There were no age or sex differences in GAG excretion in the three groups studied.

CONCLUSION

Increased GAG excretion in patients with urinary incontinence suggests an association with urothelial/detrusor pathology.

Evidence of partial anti-enuretic response related to poor pharmacodynamic effects of desmopressin nasal spray

PURPOSE

Desmopressin is an evidence-based medicine level I, category A therapy for monosymptomatic nocturnal enuresis. However, in up to 40% of patients only partial desmopressin response is obtained. While the poor pharmacokinetic characteristics of the different available formulations may have a role in apparent therapy resistance, there are limited data available to support this theory. We sought to identify pharmacodynamic factors involved in partial desmopressin response or desmopressin resistance in children with monosymptomatic nocturnal enuresis, with special emphasis on concentrating performance, and time to reach and duration of maximal urine concentration.

MATERIALS AND METHODS

We evaluated 64 children with monosymptomatic nocturnal enuresis and proved nocturnal polyuria lacking full response to desmopressin treatment. The study involved 2 separate home based test days (A and B), each consisting of 9 timed urine collections starting in the evening 1 hour before desmopressin administration and continuing for 16 hours following desmopressin administration. Test A was done during fluid restriction, and test B was done during an oral fluid load.

RESULTS

Under fluid restriction 16 patients failed to achieve urine concentration greater than 850 mOsmol/l at the midnight collection following desmopressin administration. After an oral fluid load given at the start of the test the majority of patients failed to reach maximal concentration of urine as voided during hydropenia, and 45 patients failed to regain appropriate dilution of urine even when an oral water load of 15 ml/kg (urine osmolality less than 750 mOsmol/l) was given in the morning at the end of the test. This finding is suggestive of a prolonged duration of action of the drug.

CONCLUSIONS

Pharmacodynamic tests reveal a suboptimal effect of desmopressin on urine concentration in a significant percentage of patients, which worsens when fluid is not restricted before desmopressin administration. Also the time to reach maximal antidiuretic effect and the duration of pharmacodynamic action show a wide range, requiring individualization of mode and time of administration. Our data demonstrate that a simple pharmacodynamic test as described may give important information on time of dosing, duration of action and influence of oral fluid intake, allowing individualization of therapy. Data also reveal that desmopressin should be administered at least 1 hour before bedtime, and that in case of therapy resistance a longer interval, up to 2 hours, might further reduce diuresis rate in the early night. Because of the documented prolonged action of desmopressin in some patients, increasing the dose without performing pharmacodynamic testing is no longer acceptable.

Increased morning brain natriuretic peptide levels in children with nocturnal enuresis and sleep-disordered breathing: a community-based study

INTRODUCTION

Habitual snoring and obstructive sleep apnea have been associated with bed-wetting in children, and effective obstructive sleep apnea treatment may improve enuresis.

OBJECTIVES

The purpose of this work was to assess whether habitual snoring is associated with increased incidence of enuresis and whether severity of obstructive sleep apnea correlates with enuretic frequency and to evaluate brain natriuretic peptide levels.

METHODS

Parental surveys of 5- to 7-year-old children were reviewed for habitual snoring and enuresis. Enuresis was also assessed in a cohort of 378 children with habitual snoring undergoing overnight polysomnographic evaluation, and brain natriuretic peptide plasma levels were determined in 20 children with obstructive sleep apnea, 20 with habitual snoring without obstructive sleep apnea, and 20 nonsnoring children, matched for enuresis.

RESULTS

There were 17,646 surveys completed (50.6% boys; 18.3% black). A total of 1976 (11.2%) of these children were habitual snoring (53% boys; 25.2% black). A total of 531 habitual snoring children also had enuresis (26.9%), with a predominant representation of boys (472 boys [87.5%]). Among the 15670 nonsnoring children, enuresis was reported in 1821 children (11.6%), of whom 88.8% were boys. However, enuresis among 378 children with habitual snoring did not correlate with the magnitude of sleep respiratory disturbances. Indeed, enuresis was reported in 33 of 149 children with obstructive sleep apnea (obstructive apnea hypopnea index: >2 per hour of total sleep time; 53% boys) as compared with 36 habitual snoring children with enuresis (62% boys) and obstructive apnea hypopnea index <2 per hour of total sleep time. Brain natriuretic peptide levels were elevated among children with enuresis and were marginally increased among children with obstructive sleep apnea.

CONCLUSIONS

Habitual snoring is associated with increased prevalence of enuresis, and brain natriuretic peptide levels are increased in enuretic children with further increases with obstructive sleep apnea. However, the prevalence of enuresis is not modified by severity of sleep disturbance. Even mild increases in sleep pressure because of habitual snoring may raise the arousal threshold and promote enuresis, particularly among prone children, that is, those with elevated brain natriuretic peptide levels.

Physiology and pathophysiology of the vasopressin-regulated renal water reabsorption

To prevent dehydration, terrestrial animals and humans have developed a sensitive and versatile system to maintain their water homeostasis. In states of hypernatremia or hypovolemia, the antidiuretic hormone vasopressin (AVP) is released from the pituitary and binds its type-2 receptor in renal principal cells. This triggers an intracellular cAMP signaling cascade, which phosphorylates aquaporin-2 (AQP2) and targets the channel to the apical plasma membrane. Driven by an osmotic gradient, pro-urinary water then passes the membrane through AQP2 and leaves the cell on the basolateral side via AQP3 and AQP4 water channels. When water homeostasis is restored, AVP levels decline, and AQP2 is internalized from the plasma membrane, leaving the plasma membrane watertight again. The action of AVP is counterbalanced by several hormones like prostaglandin E2, bradykinin, dopamine, endothelin-1, acetylcholine, epidermal growth factor, and purines. Moreover, AQP2 is strongly involved in the pathophysiology of disorders characterized by renal concentrating defects, as well as conditions associated with severe water retention. This review focuses on our recent increase in understanding of the molecular mechanisms underlying AVP-regulated renal water transport in both health and disease.

Primary monosymptomatic nocturnal enuresis in children and adolescents

Nocturnal enuresis (NE) is one of the most frequent paediatric pathologies. The prevalence of primary nocturnal enuresis (PNE) is around 9% in children between 5 and 10 years of age and about 40% of them have one or more episodes per week. Still for too long, PNE has not been recognised as a pathological condition, particularly by the medical community; as a consequence, there was no specific education at medical school, and a poor involvement by the practitioners. Enuretic children have a sense of social difference and isolation; some of them do express a low self-esteem. Also, self-esteem is improved by the management NE even if this management fails to cure the condition. Primary monosymptomatic nocturnal enuresis (PMNE) is an heterogeneous condition for which various causative factors have been identified such as: nocturnal polyuria, sleep disturbances, reduced bladder capacity or bladder dysfunction, upper airway obstruction. The positive diagnosis of PMNE is based on a complete questionnaire and a careful physical examination. A drinking and voiding chart is an essential non-invasive tool: first, to collect information about the initial drinking and voiding habits of the child, then to reassess the accuracy of the diagnosis. Only motivated patients should receive a specific treatment for their NE and the treatment should be proposed based on the type of PMNE. PMNE associated with nocturnal polyuria should be treated with desmopressin, which reduces nighttime urine production. For PMNE with a reduced bladder capacity alarms should be the first-line treatment. Oxybutinin, a drug with anticholinergic properties, is not theoretically indicated for the treatment of PMNE except for a very small subgroup of patients who have an overactive bladder only during sleep. In cases refractory to monotherapy, NE is probably the result of an association of different physiopathological factors (e.g. both a nocturnal polyuria together with a small bladder capacity) some of them are still unknown. In these patients, a combination of treatments may be more effective than monotherapy. Various combination therapies can be proposed to improve the cure rates.

Diagnostic Potential of Urine Proteome: A Broken Mirror of Renal Diseases

This brief overview of studies into the urine proteome illustrates its potential value for diagnostic, prognostic, and pathophysiologic discovery. Hypothesis-targeted investigations of individual proteins as well as proteome-wide searches for urinary biomarkers of various diseases and their progression are reviewed. The majority of urine proteins appear as cleavage products that are found not only as free solutes but also in secreted membrane vesicles called exosomes. Described are several recent examples of important diagnostic findings using urine proteomics along with the idea that signature profiles of injury to individual nephron segments can be measured by this technology. Shared are some thoughts on the most challenging step: Integration of seemingly unrelated findings of various protein fragments into a rational pathogenetic pathway(s). The future chance that the centuries-old technique of uroscopy will reveal its secrets using modern proteomic approaches makes gradual improvement.

Urine proteomics--prospects for future diagnostics

This brief overview of the potential diagnostic, prognostic and pathophysiological value of studies into the urine proteome describes hypothesis-driven investigations of individual proteins and proteome-wide search for urinary biomarkers of various diseases and their progression. It is intended to illustrate the recent progress in the area of urine proteomics and proselytize for the promise of this centuries-old technique of uroscopy, yet to reveal its secrets, using modem approaches.

Relief of Nocturnal Enuresis by Desmopressin Is Kidney and Vasopressin Type 2 Receptor Independent

Primary nocturnal enuresis (PNE) is a common problem in childhood and adolescence. Although various treatments are highly effective, a common underlying hypothesis on the pathogenesis is lacking. The success of desmopressin, a synthetic analogue of the antidiuretic hormone vasopressin, has been attributed to increased renal water reabsorption that is mediated by activation of the renal vasopressin V2 receptor (V2R). However, this effect does not explain other symptoms of PNE, such as the failure to arouse upon bladder distension. This study identified a family in which one child displayed PNE and coexisting nephrogenic diabetes insipidus, as a result of a novel nonsense mutation in the V2R gene (C358X). Cell-biologic investigations revealed that V2R-C358X is retained in the endoplasmic reticulum and is unstable, which explains his nephrogenic diabetes insipidus. Consistently, extrarenal V2R-mediated responses were absent in the patient who was treated with desmopressin. Administration of desmopressin, however, changed his PNE into nocturia, because he now still voided unchanged high urinary volumes at night but woke up and went to the bathroom. Withdrawal of desmopressin was accompanied by bedwetting, whereas reintroduction again relieved the symptoms. Therefore, these data indicate that neither a functioning renal concentration system nor a functional V2R is needed for the therapeutic benefit of desmopressin in PNE. Rather, it suggests that another vasopressin receptor and other organ(s) is the target for desmopressin to relieve PNE.

Regulation and dysregulation of aquaporins in water balance disorders

The discovery of aquaporin-1 (AQP1) explained the long-standing biophysical question of how water specifically crosses biological membranes. These studies led to the identification of a whole new family of membrane proteins, the aquaporin water channels. At present, at least eight aquaporins are expressed at distinct sites in the kidney and four members of this family (AQP1-4) have been demonstrated to play pivotal roles in the physiology and pathophysiology for renal regulation of body water balance. In the present review, a number of inherited and acquired conditions characterized by urinary concentration defects as well as common diseases associated with severe water retention are discussed with relation to the role of aquaporins in regulation and dysregulation of renal water transport.