Angiotensin II mediates downregulation of aquaporin water channels and key renal sodium transporters in response to urinary tract obstruction

Obstructive nephropathy and molecular pathophysiology of renal interstitial fibrosis

The kidneys play a key role in maintaining total body homeostasis. The complexity of this task is reflected in the unique architecture of the organ. Ureteral obstruction greatly affects renal physiology by altering hemodynamics, changing glomerular filtration and renal metabolism, and inducing architectural malformations of the kidney parenchyma, most importantly renal fibrosis. Persisting pathological changes lead to chronic kidney disease, which currently affects ∼10% of the global population and is one of the major causes of death worldwide. Studies on the consequences of ureteral obstruction date back to the 1800s. Even today, experimental unilateral ureteral obstruction (UUO) remains the standard model for tubulointerstitial fibrosis. However, the model has certain limitations when it comes to studying tubular injury and repair, as well as a limited potential for human translation. Nevertheless, ureteral obstruction has provided the scientific community with a wealth of knowledge on renal (patho)physiology. With the introduction of advanced omics techniques, the classical UUO model has remained relevant to this day and has been instrumental in understanding renal fibrosis at the molecular, genomic, and cellular levels. This review details key concepts and recent advances in the understanding of obstructive nephropathy, highlighting the pathophysiological hallmarks responsible for the functional and architectural changes induced by ureteral obstruction, with a special emphasis on renal fibrosis.

[Post-obstructive diuresis, by the internal physician]

Post-Obstructive Diuresis (POD) is a polyuria that occurs following the release of an obstruction from the urinary tract that prevents the flow of urine. POD requires prompt diagnosis to avoid complications. Although its pathophysiology is better understood, there is little scientific evidence for its treatment. Restoration of renal homeostasis requires correction of blood volume and electrolyte disturbances to prevent complications, which can be serious. In this article, we propose a synthesis of knowledge on the subject, as well as a management strategy.

Renal expression and urinary excretion of aquaporin-2 in postobstructive uropathy in rats

This work assessed the time course of water renal management together with aquaporin-2 (AQP2) kidney expression and urinary AQP2 levels (AQP2u) in obstructive nephropathy. Adult male Wistar rats were monitored after 1, 2, and 7 days of bilateral ureteral release (bilateral ureteral obstruction (BUO); BUO-1, BUO-2 and BUO-7). Renal water handling was evaluated using conventional clearance techniques. AQP2 levels were assessed by immunoblotting and immunohistochemical techniques. AQP2 expression in apical membranes was downregulated in BUO-1 rats and upregulated both in BUO-2 and BUO-7 animals. AQP2 protein expression in whole cell lysate fraction from kidney cortex and medulla were significantly decreased in all the experimental groups. Concomitantly, mRNA levels of AQP2 decreased in renal medulla of all groups and in renal cortex from BUO-1; however, in renal cortex from BUO-2 and BUO-7 a recovery and an increase in the level of AQP2 mRNA were, respectively, observed. BUO-7 group showed a significant increase in AQP2u. The alterations observed in apical membranes AQP2 expression could explain, at least in part, the evolution time of water kidney management in the postobstructive phase of BUO. Additionally, the AQP2u increase after 7 days of ureteral release may be postulated as a biomarker of improvement in the kidney function.

Inhibition of IL-1β by Aliskiren Improved Renal AQP2 Expression and Urinary Concentration Defect in Ureteral Obstruction and Release

We previously demonstrated that ureteral obstruction is associated with a urinary concentrating defect and reduced expression of renal aquaporins (AQPs), in which the renin–angiotensin system (RAS) may play an important role. The aims of the present study were to examine whether the renin inhibitor aliskiren could prevent the reduction in AQP expression and improve the urinary concentrating capacity in mice with bilateral ureteral obstruction (BUO) and BUO release. BUO was performed for 24 h, and BUO release was performed for 1 (B-R1D) or 3 days (B-R3D) with or without aliskiren treatment. Aliskiren prevented polyuria and decreased urine osmolality induced by B-R3D. In mice with BUO and BUO release, aliskiren attenuated the reduction in AQP2 protein and mRNA expression in the obstructed kidneys. B-R3D increased the protein expression of NLRP3 inflammasome components ASC, caspase-1, and interleukin-1β in the obstructed kidneys, which was markedly prevented by aliskiren. Moreover, the NF-κB inhibitor Bay 11-7082 blocked NLRP3 inflammasome activation and attenuated the decrease in AQP2 protein expression in primary cultured rat inner medullary collecting duct cells treated with angiotensin II. These results indicate that the renin inhibitor aliskiren increases water channel AQP2 expression at least partially by suppressing NLRP3 inflammasome activation in the obstructed kidneys of mice with BUO and BUO release.

Role of sodium/glucose cotransporter inhibition on a rat model of angiotensin II-dependent kidney damage

Background

Renal proximal tubular sodium and glucose reabsorption are regulated by the sodium-glucose cotransporter (SGLT2). Changes in this transporter can play a role in hyperglycaemia and reactive oxygen species (ROS) production. We demonstrated increased glucose absorption in proximal tubule membrane vesicles and increased expression of SGLT2 in hypertensive rats. Here we investigated Angiotensin II (Ang II) -dependent SGLT2 expression induction and the role of SGLT2 induction in the development of Ang II-dependent kidney damage. The aim of this study was to determine whether SGLT2 induction by Ang II is associated with Ang II-dependent kidney damage. We propose the following objectives a) to demonstrate that Ang II induces SGLT2 expression and b) to demonstrate that prevention of SGLT2 expression and activity prevent Ang II-induced kidney damage.

Methods

We used chronic Ang II infusion as a model of kidney damage in male Wistar rats and evaluated systolic blood pressure by telemetric methods. SGLT2 mRNA and protein expression were evaluated by PCR and immunoblotting. SGLT2 activity was evaluated in brush border membrane vesicles by measuring glucose uptake. ROS production was measured by confocal microscopy. The glomerular filtration rate (GFR) was evaluated by the inulin excretion method, and urinary protein excretion was evaluated by the Bradford method. Biological parameter evaluations were performed, after two weeks of infusion of Ang II. We compared the effects of Angiotensin II (AT1) receptor blockade by Losartan and SGLT2 inhibition by Empagliflozin both as monotherapy treatments and in combination on the development of kidney damage.

Results

Chronic Ang II infusion led to a blood pressure elevation and increased SGLT2 mRNA expression and activity as well as kidney damage, as reflected by increased ROS production, decreased GFR and increased urinary protein excretion. AT1 receptor blockade prevented all these changes. By contrast, SGLT2 inhibition did not affect blood pressure and had a small effect on kidney damage. However, the combination of both drugs resulted in the potentiation of the effects observed by AT1 receptor blockade alone.

Conclusions

We suggest that Ang II-dependent increased SGLT2 induction is one mechanism by which Ang II induces kidney damage.

Electronic supplementary material

The online version of this article (10.1186/s12882-019-1490-z) contains supplementary material, which is available to authorized users.

The renal protect function of erythropoietin after release of bilateral ureteral obstruction in a rat model

Congenital urinary tract obstruction is one of the most frequent malformations in fetuses or neonates, which usually causes profound impairment of renal function including reductions in both glomerular filtration rate (GFR) and tubular handling of water and solutes. Although obstruction can be released by surgical operation, the child will suffer from diuresis for sometime. It has been reported that erythropoietin (EPO) could prevent the down-regulation of aquaporin-2 (AQP2) and urinary-concentrating defects induced by renal ischemia/reperfusion (I/R) injury. However, whether EPO could promote the recovery of renal function and AQP2 expression after releasing of ureteral obstruction has not been reported yet. The purposes of the present study were to investigate the effects of EPO on renal function and AQP2 expression after release of bilateral ureteral obstruction (BUO-R) in rats. The results showed that EPO could promote interleukin (IL) 10 (IL-10) expression; inhibit tumor necrosis factor-α (TNF-α), IL-6, and inducible nitric oxide synthase (iNOS) expressions; reduce the fractional excretion of sodium (FENa) and plasma creatinine (CREA) and urea; and promote the recovery of water and salt handling and AQP2 expression in BUO-R rats, especially in the high dose of EPO-treated group rats. In conclusion, EPO could promote the recovery of renal function and AQP2 expression in BUO-R rats, which may partially associate with its anti-inflammation effect.

Risk factors for postobstructive diuresis in pediatric patients with ureteropelvic junction obstruction, following open pyeloplasty in three high complexity institutions

INTRODUCTION

Postobstructive diuresis (POD) is a polyuric state in which large quantities of salt and water are eliminated after solving a urinary tract obstruction. These patients are at increased risk of severe dehydration, electrolytic disturbances, hypovolemic shock, and death. Ureteropelvic junction obstruction (UPJO) is the most common etiology of collecting system dilatation in the fetal kidney, and a significant number of patients require pyeloplasty. There are limited data regarding prognostic risk factors for POD in this scenario.

OBJECTIVE

To describe possible clinical risk factors for POD in the pediatric population after open pyeloplasty.

STUDY DESIGN

This was a retrospective case series study of consecutive patients diagnosed with UPJO at three high complexity centers, managed with open pyeloplasty from 2006 to 2016. Multiple qualitative and quantitative variables possibly associated with POD were included according to the literature review. They were statistically analyzed with STATA 14 software.

RESULTS

A total of 88 patients with UPJO following open pyeloplasty were analyzed. Twenty-seven patients (30%) had POD. A tendency to present POD in younger patients was found, with a mean age of 20.2 months vs. 72.3 months. There was also an increased risk of POD in patients with previous diagnosis of tubular acidosis.

CONCLUSIONS

There are no data about prognostic clinical risk factors for POD after open pyeloplasty in the pediatric population. Our study corresponds to one of the larger series reported so far. It suggests that younger patients and patients with a previous diagnosis of tubular acidosis could be at greater risk of POD. Consequently, prospective studies are required for validation of our results, and possible impact on patient follow-up.

Role of mitochondrial oxidative stress in modulating the expressions of aquaporins in obstructive kidney disease

Downregulation of aquaporins (AQPs) in obstructive kidney disease has been well demonstrated with elusive mechanisms. Our previous study indicated that mitochondrial dysfunction played a crucial role in this process. However, it is still uncertain how mitochondrial dysfunction affected the AQPs in obstructive kidney disease. This study investigated the role of mitochondria-derived oxidative stress in mediating obstruction-induced downregulation of AQPs. After unilateral ureteral obstruction for 7 days, renal superoxide dismutase 2 (SOD2; mitochondria-specific SOD) was reduced by 85%. Meanwhile, AQP1, AQP2, AQP3, and AQP4 were remarkably downregulated as determined by Western blotting and/or quantitative real-time PCR. Administration of the SOD2 mimic manganese (III) tetrakis(4-benzoic acid)porphyrin chloride (MnTBAP) significantly attenuated AQP2 downregulation in line with complete blockade of thiobarbituric acid-reactive substances elevation, whereas the reduction of AQP1, AQP3, and AQP4 was not affected. The cyclooxygenase (COX)-2/prostaglandin (PG) E2 pathway has been well documented as a contributor of AQP reduction in obstructed kidney; thus, we detected the levels of COX-1/2 and microsomal prostaglandin E synthase 1 (mPGES-1) in kidney and PGE2 secretion in urine. Significantly, MnTBAP partially suppressed the elevation of COX-2, mPGES-1, and PGE2. Moreover, a marked decrease of V2 receptor was significantly restored after MnTBAP treatment. However, the fibrotic response and renal tubular damage were unaffected by MnTBAP in obstructed kidneys. Collectively, these findings suggested an important role of mitochondrial oxidative stress in mediating AQP2 downregulation in obstructed kidney, possibly via modulating the COX-2/mPGES-1/PGE2/V2 receptor pathway.

The molecular biology of pelvi-ureteric junction obstruction

Over recent years routine ultrasound scanning has identified increasing numbers of neonates as having hydronephrosis and pelvi-ureteric junction obstruction (PUJO). This patient group presents a diagnostic and management challenge for paediatric nephrologists and urologists. In this review we consider the known molecular mechanisms underpinning PUJO and review the potential of utilising this information to develop novel therapeutics and diagnostic biomarkers to improve the care of children with this disorder.

MnTBAP therapy attenuates the downregulation of sodium transporters in obstructive kidney disease

Ureteral obstruction is associated with reduced expressions of renal sodium transporters, which contributes to impaired urinary concentrating capacity. In this study, we employed a synthetic mitochondrial superoxide dismutase 2 (SOD2) mimic MnTBAP to investigate the role of mitochondrial oxidative stress in modulating the sodium transporters in obstructive kidney disease. Following unilateral ureteral obstruction (UUO) for 7 days, a global reduction of sodium transporters including NHE3, NCC, NKCC2, and ENaCα was observed as determined by qRT-PCR, Western Blotting or immunohistochemistry. Among these sodium transporters, the downregulation of NHE3, NCC, and NKCC2 was partially reversed by MnTBAP treatment. In contrast, the reduction of ENaCα was not affected by MnTBAP. The β and γ subunits of ENaC were not significantly altered by ureteral obstruction or MnTBAP therapy. To further confirm the anti-oxidant effect of MnTBAP, we examined the levels of TBARs in the urine collected from the obstructed ureters of UUO mice and bladder of sham mice. As expected, the increment of urinary TBARs in UUO mice was entirely abolished by MnTBAP therapy, indicating an amelioration of oxidative stress. Meantime, we found that three types of SOD were all reduced in obstructed kidneys determined by qRT-PCR, which was unaffected by MnTBAP. Collectively, these results demonstrated an important role of mitochondrial oxidative stress in mediating the downregulation of sodium transporters in obstructive kidney disease.

Postobstructive diuresis in cats with naturally occurring lower urinary tract obstruction: incidence, severity and association with laboratory parameters on admission

OBJECTIVES

The objectives of this retrospective study were to investigate the actual incidence of postobstructive diuresis after relief of urethral obstruction in cats, as well as to identify changes in blood and urine parameters that might be associated with postobstructive diuresis (POD), and to assess the impact of fluid therapy.

METHODS

The medical records of 57 male cats with urethral obstruction that were treated with an indwelling urinary catheter were retrospectively analysed. Absolute urine output in ml/kg/h every 4 h and the incidence of cats with polyuria (urine volume >2 ml/kg/h) at any time point over a 48 h period after the re-establishment of urine flow were investigated. In addition, postobstructive diuresis in relation to fluid therapy (PODFR) was defined as urine output greater than the administered amount of intravenous fluids on at least two subsequent time points. Polyuria and PODFR were investigated for their association with blood and urine laboratory parameters.

RESULTS

After 4 h, 74.1% (40/54) of the cats had polyuria, with a urine output of >2 ml/kg/h. Metabolic acidosis was present in 46.2% of the cats. Venous blood pH and bicarbonate were inversely correlated with urine output in ml/kg/h after 4 h. The overall incidence of POD within 48 h of catheterisation was 87.7%. There was a significant correlation between intravenous fluid rate at time point x and urine output at time point x + 1 at all the time points except for the fluid rate at time point 0 and the urine output after 4 h. PODFR was seen in 21/57 cats (36.8%).

CONCLUSIONS AND RELEVANCE

POD is a frequent finding in cats treated for urethral obstruction, and can be very pronounced. Further studies are required to determine whether or not a change in venous blood pH actually interferes with renal concentrating ability. The discrepancy between the frequency of cats with polyuria and PODFR (87.7% vs 36.8%) in the present study indicates that administered intravenous fluid therapy might be the driving force for the high incidence of polyuria in some cats with naturally occurring obstructive feline lower urinary tract disease.

Inhibition of mitochondrial complex-1 restores the downregulation of aquaporins in obstructive nephropathy

Obstructive kidney disease is a common complication in the clinic. Downregulation of aquaporins (AQPs) in obstructed kidneys has been thought as a key factor leading to the polyuria and impairment of urine-concentrating capability after the release of kidney obstruction. The present study was to investigate the role of mitochondrial complex-1 in modulating AQPs in obstructive nephropathy. Following 7-day unilateral ureteral obstruction (UUO), AQP1, AQP2, AQP3, and vasopressin 2 (V₂) receptor were remarkably reduced as determined by qRT-PCR and/or Western blotting. Notably, inhibition of mitochondrial complex-1 by rotenone markedly reversed the downregulation of AQP1, AQP2, AQP3, and V₂ In contrast, AQP4 was not affected by kidney obstruction or rotenone treatment. In a separate study, rotenone also attenuated AQPs' downregulation after 48-h UUO. To study the potential mechanisms in mediating the rotenone effects on AQPs, we examined the regulation of the COX-2/microsomal prostaglandin E synthase (mPGES)-1/PGE₂/EP pathway and found that COX-2, mPGES-1, and renal PGE₂ content were all significantly elevated in obstructive kidneys, which was not affected by rotenone treatment. For EP receptors, EP₂ and EP₄ but not EP₁ and EP₃ were upregulated in obstructive kidneys. Importantly, rotenone strikingly suppressed EP₁ and EP₄ but not EP₂ and EP₃ receptors. However, treatment of EP₁ antagonist SC-51322 could not affect AQPs' reduction in obstructed kidneys. Collectively, these findings suggested an important role of mitochondrial dysfunction in modulating AQPs and V₂ receptor in obstructive nephropathy possibly via prostaglandin-independent mechanisms.

The Trafficking of the Water Channel Aquaporin-2 in Renal Principal Cells-a Potential Target for Pharmacological Intervention in Cardiovascular Diseases

Arginine-vasopressin (AVP) stimulates the redistribution of water channels, aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. By this AVP directs 10% of the water reabsorption from the 170 L of primary urine that the human kidneys produce each day. This review discusses molecular mechanisms underlying the AVP-induced redistribution of AQP2; in particular, it provides an overview over the proteins participating in the control of its localization. Defects preventing the insertion of AQP2 into the plasma membrane cause diabetes insipidus. The disease can be acquired or inherited, and is characterized by polyuria and polydipsia. Vice versa, up-regulation of the system causing a predominant localization of AQP2 in the plasma membrane leads to excessive water retention and hyponatremia as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure or liver cirrhosis. This article briefly summarizes the currently available pharmacotherapies for the treatment of such water balance disorders, and discusses the value of newly identified mechanisms controlling AQP2 for developing novel pharmacological strategies. Innovative concepts for the therapy of water balance disorders are required as there is a medical need due to the lack of causal treatments.

COX-2 inhibition attenuates lung injury induced by skeletal muscle ischemia reperfusion in rats

BACKGROUND

Skeletal muscle ischemia reperfusion accounts for high morbidity and mortality, and cyclooxygenase (COX)-2 is implicated in causing muscle damage. Downregulation of aquaporin-1 (AQP-1) transmembrane protein is implicated in skeletal muscle ischemia reperfusion induced remote lung injury. The expression of COX-2 in lung tissue and the effect of COX-2 inhibition on AQP-1 expression and lung injury during skeletal muscle ischemia reperfusion are not known. We investigated the role of COX-2 in lung injury induced by skeletal muscle ischemia reperfusion in rats and evaluated the effects of NS-398, a specific COX-2 inhibitor.

METHODS

Twenty-four Sprague Dawley rats were randomized into 4 groups: sham group (SM group), sham+NS-398 group (SN group), ischemia reperfusion group (IR group) and ischemia reperfusion+NS-398 group (IN group). Rats in the IR and IN groups were subjected to 3h of bilateral ischemia followed by 6h of reperfusion in hindlimbs, and intravenous NS-398 8 mg/kg was administered in the IN group. In the SM and SN groups, rubber bands were in place without inflation. At the end of reperfusion, myeloperoxidase (MPO) activity, COX-2 and AQP-1 protein expression in lung tissue, PGE2 metabolite (PGEM), tumor necrosis factor (TNF)-α and interleukin (IL)-1β levels in bronchoalveolar lavage (BAL) fluid were assessed. Histological changes in lung and muscle tissues and wet/dry (W/D) ratio were also evaluated.

RESULTS

MPO activity, COX-2 expression, W/D ratio in lung tissue, and PGEM, TNF-α and IL-1β levels in BAL fluid were significantly increased, while AQP-1 protein expression downregulated in the IR group as compared to that in the SM group (P<0.05). These changes were remarkably mitigated in the IN group (P<0.05). NS-398 treatment also alleviated histological signs of lung and skeletal muscle injury.

CONCLUSION

COX-2 protein expression was upregulated in lung tissue in response to skeletal muscle ischemia reperfusion. COX-2 inhibition may modulate pulmonary AQP-1 expression and attenuate lung injury.

Inhibition of Mitochondrial Complex-1 Prevents the Downregulation of NKCC2 and ENaCα in Obstructive Kidney Disease

Ureteral obstruction with subsequent hydronephrosis is a common clinical complication. Downregulation of renal sodium transporters in obstructed kidneys could contribute to impaired urinary concentrating capability and salt waste following the release of a ureteral obstruction. The current study was undertaken to investigate the role of mitochondrial complex-1 inhibition in modulating sodium transporters in obstructive kidney disease. Following unilateral ureteral obstruction (UUO) for 7 days, a global reduction of sodium transporters, including NHE3, α-Na-K-ATPase, NCC, NKCC2, p-NKCC2, ENaCα, and ENaCγ, was observed, as determined via qRT-PCR and/or Western blotting. Interestingly, inhibition of mitochondrial complex-1 by rotenone markedly reversed the downregulation of NKCC2, p-NKCC2, and ENaCα. In contrast, other sodium transporters were not affected by rotenone. To study the potential mechanisms involved in mediating the effects of rotenone on sodium transporters, we examined a number of known sodium modulators, including PGE2, ET1, Ang II, natriuretic peptides (ANP, BNP, and CNP), and nitric oxide synthases (iNOS, nNOS, and eNOS). Importantly, among these modulators, only BNP and iNOS were significantly reduced by rotenone treatment. Collectively, these findings demonstrated a substantial role of mitochondrial dysfunction in mediating the downregulation of NKCC2 and ENaCα in obstructive kidney disease, possibly via iNOS-derived nitric oxide and BNP.

Aliskiren restores renal AQP2 expression during unilateral ureteral obstruction by inhibiting the inflammasome

Ureteral obstruction is associated with reduced expression of renal aquaporins (AQPs), urinary concentrating defects, and an enhanced inflammatory response, in which the renin-angiotensin system (RAS) may play an important role. We evaluated whether RAS blockade by a direct renin inhibitor, aliskiren, would prevent the decreased renal protein expression of AQPs in a unilateral ureteral obstruction (UUO) model and what potential mechanisms may be involved. UUO was performed for 3 days (3UUO) and 7 days (7UUO) in C57BL/6 mice with or without aliskiren injection. In 3UUO and 7UUO mice, aliskiren abolished the reduction of AQP2 protein expression but not AQP1, AQP3, and AQP4. mRNA levels of renal AQP2 and vasopressin type 2 receptor were decreased in obstructed kidneys of 7UUO mice, which were prevented by aliskiren treatment. Aliskiren treatment was also associated with a reduced inflammatory response in obstructed kidneys of UUO mice. Aliskiren significantly decreased mRNA levels of several proinflammatory factors, such as transforming growth factor-β and tumor necrosis factor-α, seen in obstructed kidneys of UUO mice. Interestingly, mRNA and protein levels of the NOD-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome components apoptosis-associated speck-like protein containing a caspase recruitment domain, caspase-1, and IL-1β were dramatically increased in obstructed kidneys of 7UUO mice, which were significantly suppressed by aliskiren. In primary cultured inner medullary collecting duct cells, IL-1β significantly decreased AQP2 expression. In conclusions, RAS blockade with the direct renin inhibitor aliskiren increased water channel AQP2 expression in obstructed kidneys of UUO mice, at least partially by preventing NLRP3 inflammasome activation in association with ureteral obstruction.

A novel therapeutic effect of statins on nephrogenic diabetes insipidus

Statins competitively inhibit hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase, resulting in reduced plasma total and low-density lipoprotein cholesterol levels. Recently, it has been shown that statins exert additional 'pleiotropic' effects by increasing expression levels of the membrane water channels aquaporin 2 (AQP2). AQP2 is localized mainly in the kidney and plays a critical role in determining cellular water content. This additional effect is independent of cholesterol homoeostasis, and depends on depletion of mevalonate-derived intermediates of sterol synthetic pathways, i.e. farnesylpyrophosphate and geranylgeranylpyrophosphate. By up-regulating the expression levels of AQP2, statins increase water reabsorption by the kidney, thus opening up a new avenue in treating patients with nephrogenic diabetes insipidus (NDI), a hereditary disease that yet lacks high-powered and limited side effects therapy. Aspects related to water balance determined by AQP2 in the kidney, as well as standard and novel therapeutic strategies of NDI are discussed.

Signaling pathways involved in renal oxidative injury: role of the vasoactive peptides and the renal dopaminergic system

The physiological hydroelectrolytic balance and the redox steady state in the kidney are accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between antinatriuretic and natriuretic factors. Angiotensin II, atrial natriuretic peptide and intrarenal dopamine play a pivotal role in this interactive network. The balance between endogenous antioxidant agents like the renal dopaminergic system and atrial natriuretic peptide, by one side, and the prooxidant effect of the renin angiotensin system, by the other side, contributes to ensuring the normal function of the kidney. Different pathological scenarios, as nephrotic syndrome and hypertension, where renal sodium excretion is altered, are associated with an impaired interaction between two natriuretic systems as the renal dopaminergic system and atrial natriuretic peptide that may be involved in the pathogenesis of renal diseases. The aim of this review is to update and comment the most recent evidences about the intracellular pathways involved in the relationship between endogenous antioxidant agents like the renal dopaminergic system and atrial natriuretic peptide and the prooxidant effect of the renin angiotensin system in the pathogenesis of renal inflammation.

α-Melanocyte stimulating hormone treatment in pigs does not improve early graft function in kidney transplants from brain dead donors

Delayed graft function and primary non-function are serious complications following transplantation of kidneys derived from deceased brain dead (DBD) donors. α-melanocyte stimulating hormone (α-MSH) is a pleiotropic neuropeptide and its renoprotective effects have been demonstrated in models of acute kidney injury. We hypothesized that α-MSH treatment of the recipient improves early graft function and reduces inflammation following DBD kidney transplantation. Eight Danish landrace pigs served as DBD donors. After four hours of brain death both kidneys were removed and stored for 18 hours at 4°C in Custodiol preservation solution. Sixteen recipients were randomized in a paired design into two treatment groups, transplanted simultaneously. α-MSH or a vehicle was administered at start of surgery, during reperfusion and two hours post-reperfusion. The recipients were observed for ten hours following reperfusion. Blood, urine and kidney tissue samples were collected during and at the end of follow-up. α-MSH treatment reduced urine flow and impaired recovery of glomerular filtration rate (GFR) compared to controls. After each dose of α-MSH, a trend towards reduced mean arterial blood pressure and increased heart rate was observed. α-MSH did not affect expression of inflammatory markers. Surprisingly, α-MSH impaired recovery of renal function in the first ten hours following DBD kidney transplantation possibly due to hemodynamic changes. Thus, in a porcine experimental model α-MSH did not reduce renal inflammation and did not improve short-term graft function following DBD kidney transplantation.

Pre- or post-treatment with aminoguanidine attenuates a renal distal acidification defect induced by acute ureteral obstruction in rats

Acute unilateral ureteral obstruction (UUO) impairs distal nephron acid secretion and stimulates expression of inducible nitric oxide synthase (iNOS) in post-obstructed kidney (POK). This study investigated the influence of pre- or post-treatment with aminoguanidine as a selective iNOS inhibitor on UUO-induced renal functional disturbances. To induce acute UUO, the left ureter in rats was ligated and released after 24 h. Then, a 3 h clearance period followed by bicarbonate loading and thereafter a 30 min clearance period were allocated. Aminoguanidine was administered either prior to the UUO induction or after release of the obstruction in the different rat groups, while untreated and sham groups received normal saline. During the first clearance period, fractional bicarbonate excretion and urinary pH increased markedly in the POK of the untreated group compared with the left kidney of sham group, and a large drop in the difference between urine and blood pCO2 (U–B pCO2) was observed after bicarbonate loading; all of these parameters were ameliorated in the pre-treated and post-treated groups. However, the UUO-induced decreases in creatinine clearance, sodium reabsorption, urine osmolality, and free-water reabsorption in the POK were attenuated only in the post-treated group. Therefore, the in vivo application of a selective iNOS inhibitor partially improved the acute UUO-induced distal nephron acidification defect, while post-treatment but not pre-treatment with aminoguanidine ameliorated decrements of glomerular filtration, sodium reabsorption, and urine-concentrating ability.

Aquaporin-2 regulation in health and disease

Aquaporin-2 (AQP2), the vasopressin-regulated water channel of the renal collecting duct, is dysregulated in numerous disorders of water balance in people and animals, including those associated with polyuria (urinary tract obstruction, hypokalemia, inflammation, and lithium toxicity) and with dilutional hyponatremia (syndrome of inappropriate antidiuresis, congestive heart failure, cirrhosis). Normal regulation of AQP2 by vasopressin involves 2 independent regulatory mechanisms: (1) short-term regulation of AQP2 trafficking to and from the apical plasma membrane, and (2) long-term regulation of the total abundance of the AQP2 protein in the cells. Most disorders of water balance are the result of dysregulation of processes that regulate the total abundance of AQP2 in collecting duct cells. In general, the level of AQP2 in a collecting duct cell is determined by a balance between production via translation of AQP2 mRNA and removal via degradation or secretion into the urine in exosomes. AQP2 abundance increases in response to vasopressin chiefly due to increased translation subsequent to increases in AQP2 mRNA. Vasopressin-mediated regulation of AQP2 gene transcription is poorly understood, although several transcription factor-binding elements in the 5' flanking region of the AQP2 gene have been identified, and candidate transcription factors corresponding to these elements have been discovered in proteomics studies. Here, we review progress in this area and discuss elements of vasopressin signaling in the collecting duct that may impinge on regulation of AQP2 in health and in the context of examples of polyuric diseases.

Is there a role for PGE2 in urinary concentration?

Prostanoids are prominent, yet complex, components in the maintenance of body water homeostasis. Recent functional and molecular studies have revealed that the local lipid mediator PGE2 is involved both in water excretion and absorption. The biologic actions of PGE2 are exerted through four different G-protein-coupled receptors; designated EP1-4, which couple to separate intracellular signaling pathways. Here, we discuss new developments in our understanding of the actions of PGE2 that have been uncovered utilizing receptor specific agonists and antagonists, EP receptor and PG synthase knockout mice, polyuric animal models, and the new understanding of the molecular regulation of collecting duct water permeability. The role of PGE2 in urinary concentration comprises a variety of mechanisms, which are not fully understood and likely depend on which receptor is activated under a particular physiologic condition. EP3 and microsomal PG synthase type 1 play a role in decreasing collecting duct water permeability and increasing water excretion, whereas EP2 and EP4 can bypass vasopressin signaling and increase water reabsorption through two different intracellular signaling pathways. PGE2 has an intricate role in urinary concentration, and we now suggest how targeting specific prostanoid receptor signaling pathways could be exploited for the treatment of disorders in water balance.

Decrease of renal aquaporins 1-4 is associated with renal function impairment in pediatric congenital hydronephrosis

BACKGROUND

Renal aquaporins (AQP1-4) concentration is downregulated and is in proportion to the degree of hydronephrosis graded by ultrasound in pediatric congenital hydronephrosis (CH). However, the relationship between the expression of AQP1-4 with the changes of renal function impairment (RFI) evaluated by (99m)Tc-DTPA renal dynamic imaging is still unclear. This study aimed to investigate the relationship between AQP1-4 expression and degree of RFI in children with CH.

METHODS

The expression of AQP1-4 was evaluated in 45 children with unilateral ureteropelvic junction obstruction (28 boys and 17 girls, average age: 28±10 months) and 15 children undergoing nephrectomy for nephroblastoma (8 boys and 7 girls, average age: 26±8 months) by immunoblotting and immunohistochemistry. Renal function was graded into mild and severe RFI by (99m)Tc-DTPA renal dynamic imaging.

RESULTS

One-way analysis of variance with Bonferonni's correction showed a significantly reduced protein expression of AQP1-4 in the severe RFI group compared with those in both mild RFI group and controls (AQP1: 0.52±0.09 vs. 0.91±0.06 vs. 1.23±0.033; AQP2: 0.68±0.12 vs. 1.09±0.06 vs. 1.52±0.08; AQP3: 0.59±0.16 vs. 0.94±0.08 vs. 1.31±0.07; AQP4: 0.64±0.06 vs. 1.14±0.07 vs. 1.61±0.07; P<0.001, respectively). In kidneys with severe RFI, there was a reduction in the protein concentration of all four AQP isoforms which was more pronounced compared with those seen in kidneys with mild RFI and in the controls.

CONCLUSION

AQP1-4 expression is reduced in proportion with the impairment degree of renal function graded by (99m)Tc-DTPA renal dynamic imaging in human CH.

Early alteration of urinary exosomal aquaporin 1 and transforming growth factor β1 after release of unilateral pelviureteral junction obstruction

BACKGROUND/PURPOSE

Down-regulation of aquaporin 1 (AQP1) and up-regulation of transforming growth factor β(1) (TGF-β(1)) in the renal parenchyma have been demonstrated in children who underwent pyeloplasty for pelviureteral junction obstruction. However, no information about urinary exosomal AQP1 and TGF-β(1) during postobstructive polyuria in children with congenital unilateral hydronephrosis is available. The aim of the present study is to evaluate the urine concentration of exosomal AQP1 and TGF-β(1) on the first and the second day after surgery in children who underwent pyeloplasty.

METHODS

Twenty-two patients (age, 36.2 ± 17.1 months) with unilateral pelviureteral junction obstruction were examined in the study. For the first 2 days after the operation, the urine was collected separately from pyelostomy draining only from the postobstructed kidney and from the bladder catheter draining mostly from the contralateral kidney, which was used as an internal control. Urinary output, urinary osmolality, sodium, β(2)-microglobulin (β(2)-MG), and creatinine, as well as urinary exosomal AQP1 and TGF-β(1) excretion, were tested in each sample.

RESULTS

After pyeloplasty, a significantly decreased urinary excretion of exosomal AQP1 (≈ 64%) was found in the postobstructed kidney. The patients developed polyuria (807 ± 216 mL/24 h vs 484 ± 144 mL/24 h at day 1, 1021 ± 348 mL/24 h vs 603 ± 228 mL/24 h at day 2; P < .01) and reduced urine osmolality (115 ± 44 mOsm/kg vs 282 ± 61 mOsm/kg at day 1, 139 ± 39 vs 303 ± 46 mOsm/kg at day 2; P < .01) that persisted for 48 hours. In parallel, urinary TGF-β(1) and β(2)-MG (normalized for creatinine) from the postobstructed kidney were significantly higher compared with the contralateral kidney. The urine output and urinary sodium concentration from the postobstructed kidney elevated significantly on the second day after the release of obstruction compared with those on the first day. The contralateral kidney also showed same trends.

CONCLUSIONS

The down-regulation of urinary exosomal AQP1 in the postobstructed kidney may account for the polyuria, hypotonic urine, and elevated urinary β(2)-MG. The urinary TGF-β(1) level locally increased in the postobstructed kidney may be involved in renal AQP1 down-regulation.

Severe post-renal acute kidney injury, post-obstructive diuresis and renal recovery

Study Type--Therapy (case series) Level of Evidence 4. What's known on the subject? and What does the study add? The pathophysiology of post-renal acute kidney injury (PR-AKI), i.e. caused by urinary tract obstruction, has been extensively studied in animal models but clinical studies on this subject are outdated, and/or have focused on the mechanisms of 'post-obstructive diuresis' (POD), a potentially life-threatening polyuria that can develop after the release of obstruction. In severe PR-AKI, the risk of occurrence of POD is high. POD occurrence predicts renal recovery without the persistence of severe chronic kidney failure. In the present study, the occurrence of POD and the persistence of chronic renal sequelae could be predicted early from clinical variables at admission before the release of obstruction.

OBJECTIVE

• To identify predictors of post-obstructive diuresis (POD) occurrence or severe chronic renal failure (CRF) persistence after the release of urinary tract obstruction in the setting of post-renal acute kidney injury (PR-AKI).

PATIENTS AND METHODS

• Bi-centre retrospective observational study of all patients with PR-AKI treated in two intensive care units (ICUs) from 1998 to 2010. • Clinical, biological and imaging characteristics on admission and after the release of obstruction were analysed with univariate and, if possible, multivariate analysis to search for predictors of (i) occurrence of POD (diuresis >4 L/day) after the release of obstruction; (ii) persistence of severe CRF (estimated glomerular filtration rate <30 mL/min/1.73 m(2), including end-stage CRF) at 3 months.

RESULTS

• On admission, median (range) serum creatinine was 866 (247-3119) µmol/L. • POD occurred in 34 (63%) of the 54 analysable patients. On admission, higher serum creatinine (Odds ratio [OR] 1.002 per 1 µmol/L, 95% confidence interval [CI] 1.000-1.004, P = 0.004), higher serum bicarbonate (OR 1.36 per 1 mmol/L, 95% CI 1.13-1.65, P < 0.001), and urinary retention (OR 6.96, 95% CI 1.34-36.23, P = 0.01) independently predicted POD occurrence. • Severe CRF persisted in seven (21%) of the 34 analysable patients, including two (6%) cases of end-stage CRF. Predictors of severe CRF persistence after univariate analysis were: lower blood haemoglobin (P < 0.001) and lower serum bicarbonate (P = 0.03) on admission, longer time from admission to the release of obstruction (P = 0.01) and absence of POD (P = 0.04) after the release of obstruction.

CONCLUSIONS

• In severe PR-AKI treated in ICU, POD occurrence was a frequent event that predicted renal recovery without severe CRF. • POD occurrence or severe CRF persistence could be predicted early from clinical and biological variables at admission before the release of obstruction.

Salt-induced downregulation of renal aquaporins is prevented by losartan

AIMS

The purpose of this study was to investigate the expression of aquaporin-1 (AQP-1) and aquaporin-2 (AQP-2) in the renal tubule of rats fed with a high-salt diet and its modulation by the AT1 receptor blocker losartan.

MAIN METHODS

The experiments were performed in four groups of rats fed for 3 weeks with the following diets: regular rat chow (NS); high-salt (8% NaCl) chow (HS), NS plus losartan (NS-L) and HS plus losartan (HS-L). Losartan (40 mg x kg(-1)) was administered in the drinking water. Systolic blood pressure (SBP) and renal function were evaluated. The intrarenal levels of angiotensin II (Ang II), TGF-β(1), α-smooth muscle actin (α-SMA), endothelial nitric oxide synthase (eNOS), AQP-1 and AQP-2 were determined by immunohistochemistry. AQP-1 and AQP-2 protein levels were measured by western blot analysis.

KEY FINDINGS

A high-sodium diet downregulated AQP-1 and AQP-2 expression levels in the proximal tubule and collecting duct, respectively. The high-sodium diet also induced Ang II, TGF-β(1) and α-SMA overexpression and decreased eNOS expression in the renal cortex and medulla. Losartan increased the diuresis and natriuresis, favoring urinary sodium concentration. Additionally, losartan prevented the profibrogenic response, decreasing Ang II, TGF-β(1) and α-SMA levels and normalizing AQP-2 expression in the HS-L group. AQP-1 expression was upregulated by losartan in both the NS-L and HS-L groups.

SIGNIFICANCE

These results show that increased intrarenal Ang II in rats fed with a high-salt diet downregulates renal AQP-1 and AQP-2 expressions. In addition, although losartan increased diuresis and natriuresis, it prevented the downregulation of aquaporins, favoring urinary sodium concentration.

Disruption of COX-2 and eNOS does not confer protection from cardiovascular failure in lipopolysaccharide-treated conscious mice and isolated vascular rings

It was hypothesized that a serial stimulation of vascular cyclooxygenase-2 (COX-2) with subsequent activation of endothelial nitric oxide synthase (eNOS) is responsible for decrease in blood pressure, cardiac performance, and vascular reactivity in endotoxemia caused by LPS. The hypothesis was tested in catheterized, conscious, freely moving, wild-type mice and mice (C57BL/6J background) with targeted deletion of COX-2 and eNOS that were given an intravenous LPS bolus (2 mg/kg, 055:B5). In vitro studies were performed on murine aorta rings. LPS caused a concomitant decrease in mean arterial blood pressure (MAP) and heart rate (HR) that was significant after 3 h and was sustained throughout the experiment (8 h). The LPS-induced changes in MAP and HR were not different from control in COX-2(-/-) and eNOS(-/-) mice. A prostacyclin receptor antagonist (BR5064) blocked the hypotensive effect of a prostacyclin agonist (beraprost), but did not attenuate the LPS-induced decrease in MAP and HR. LPS decreased eNOS and neuronal NOS mRNA abundances in several organs, while inducible NOS mRNA was enhanced. In aortic rings, LPS suppressed α(1)-adrenoceptor-mediated vascular tone. Inhibition of COX-2 activity (NS 398), disruption of COX-2, endothelium removal, or eNOS deletion (eNOS(-/-)) did not improve vascular reactivity after LPS, while the NO synthase blockers 1400W and N(G)-nitro-l-arginine methyl ester prevented loss of tone. COX-2 and eNOS activities are not necessary for LPS-induced decreases in blood pressure, heart rate, and vascular reactivity. Inducible NOS activity appears crucial. COX-2 and eNOS are not obvious therapeutic targets for cardiovascular rescue during gram-negative endotoxemic shock.

Angiotensin II induces phosphorylation of the thiazide-sensitive sodium chloride cotransporter independent of aldosterone

We studied here the independent roles of angiotensin II and aldosterone in regulating the sodium chloride cotransporter (NCC) of the distal convoluted tubule. We adrenalectomized three experimental and one control group of rats. Following surgery, the experimental groups were treated with either a high physiological dose of aldosterone, a non-pressor, or a pressor dose of angiotensin II for 8 days. Aldosterone and both doses of angiotensin II lowered sodium excretion and significantly increased the abundance of NCC in the plasma membrane compared with the control. Only the pressor dose of angiotensin II caused hypertension. Thiazides inhibited the sodium retention induced by the angiotensin II non-pressor dose. Both aldosterone and the non-pressor dose of angiotensin II significantly increased phosphorylation of NCC at threonine-53 and also increased the intracellular abundance of STE20/SPS1-related, proline alanine-rich kinase (SPAK). No differences were found in other modulators of NCC activity such as oxidative stress responsive protein type 1 or with-no-lysine kinase 4. Thus, our in vivo study shows that aldosterone and angiotensin II independently increase the abundance and phosphorylation of NCC in the setting of adrenalectomy; effects are likely mediated by SPAK. These results may explain, in part, the hormonal control of renal sodium excretion and the pathophysiology of several forms of hypertension.

A reduction of unilateral ureteral obstruction-induced renal fibrosis by a therapy combining valsartan with aliskiren

The protective effect of combination therapy with valsartan and aliskiren against renal fibrosis remains to be defined. This study was undertaken to examine the protective effects of the combination of valsartan and aliskiren against renal fibrosis induced by unilateral ureteral obstruction (UUO). Combination therapy with valsartan (15 mg·kg(-1)·day(-1)) and aliskiren (10 mg·kg(-1)·day(-1)), valsartan monotherapy (30 mg·kg(-1)·day(-1)), and aliskiren monotherapy (20 mg·kg(-1)·day(-1)) all significantly ameliorated the increase in blood urea nitrogen and the degree of hydronephrosis determined by the increase in weight and length of the obstructed kidney. The dose titration study and blood pressure measurement confirmed that the combination therapy provided a greater benefit independent of the vasodilatory effect. There were no significant changes in serum levels of creatinine, sodium, and potassium in UUO rats and any treatment groups. Combination therapy also attenuated UUO-related increases in the scores of tubular dilatation, interstitial volume, interstitial collagen deposition, α-smooth muscle actin, the activation of ERK 1/2, the infiltration of monocytes/macrophages, the mRNA expression of snail-1, and transforming growth factor-β1 to a greater extent compared with aliskiren or valsartan used alone. The mRNA expression of renin and the (pro)renin receptor significantly increased after UUO. Combination therapy and monotherapy of valsartan and aliskiren had a comparable enhancing effect on the mRNA expression of renin, whereas all these treatments did not affect the expression of the (pro)renin receptor. In conclusion, a direct renin inhibitor in conjunction with an angiotensin II receptor blocker exerts increased renal protection against renal fibrosis and inflammation during obstruction over either agent alone.

Nonsteroidal Anti-Inflammatory Drugs and the Kidney

Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit the isoenzymes COX-1 and COX-2 of cyclooxygenase (COX). Renal side effects (e.g., kidney function, fluid and urinary electrolyte excretion) vary with the extent of COX-2-COX-1 selectivity and the administered dose of these compounds. While young healthy subjects will rarely experience adverse renal effects with the use of NSAIDs, elderly patients and those with co-morbibity (e.g., congestive heart failure, liver cirrhosis or chronic kidney disease) and drug combinations (e.g., renin-angiotensin blockers, diuretics plus NSAIDs) may develop acute renal failure. This review summarizes our present knowledge how traditional NSAIDs and selective COX-2 inhibitors may affect the kidney under various experimental and clinical conditions, and how these drugs may influence renal inflammation, water transport, sodium and potassium balance and how renal dysfunction or hypertension may result.

Cyclooxygenase 2 inhibition exacerbates AQP2 and pAQP2 downregulation independently of V2 receptor abundance in the postobstructed kidney

Previously we demonstrated that ANG II receptor (AT1R) blockade attenuates V2 receptor (V2R), AQP2, and pS256-AQP2 downregulation in the postobstructed kidney and partially reverses obstruction-induced inhibition of cAMP generation and cyclooxygenase 2 (COX-2) induction. Therefore, we speculated whether the effects of AT1R blockade on V2R and the vasopressin-regulated pathway are attributable to attenuated COX-2 induction. To examine this, rats were subjected to 24-h bilateral ureteral obstruction (BUO) followed by 48-h release and treated with the COX-2 inhibitor parecoxib or saline. Control rats were sham-operated. Parecoxib treatment significantly reduced urine output 24 h after release of BUO whereas urine osmolality and solute-free water reabsorption was comparable between saline- and parecoxib-treated BUO rats. Immunoblotting revealed a significant decrease in AQP2 and pS256-AQP2 abundance to 20 and 23% of sham levels in parecoxib-treated BUO rats compared with 40 and 55% of sham levels in saline-treated BUO rats. Immunohistochemistry confirmed the exacerbated AQP2 and pS256-AQP2 downregulation in parecoxib-treated BUO rats. Finally, parecoxib treatment had no effect on V2R downregulation and the inhibited, vasopressin-stimulated cAMP generation in inner medullary membrane fractions from the postobstructed kidney. In conclusion, COX-2 inhibition exacerbates AQP2 and pS256-AQP2 downregulation 48 h after release of 24-h BUO independently of V2R abundance and vasopressin-stimulated cAMP generation. The results indicate that COX-2 inhibition does not mimic AT1R blockade-mediated effects and that AT1R-mediated AQP2 regulation in the postobstructed kidney collecting duct is independent of COX-2 induction.

Rapid and segmental specific dysregulation of AQP2, S256-pAQP2 and renal sodium transporters in rats with LPS-induced endotoxaemia

BACKGROUND

Acute renal failure (ARF) is a frequent complication of sepsis. Characteristics of ARF in sepsis are impaired urinary concentration, increased natriuresis and decreased glomerular filtration rate (GFR), in which inducible nitric oxide synthase (iNOS) has been revealed to play a role. Aims. We aimed to investigate renal water and sodium excretion and in parallel the segmental regulation of renal AQP2 and major sodium transporters in rats with acute LPS-induced endotoxaemia. Next, we aimed to examine the changes of iNOS expression and activated macrophage infiltration in the kidney and the effects of iNOS inhibition on AQP2 and NKCC2 expression in LPS rats.

METHODS

Rats were treated with LPS (i.p.) or with LPS + iNOS inhibitor L-NIL, and 6 h later kidneys were subjected to semiquantitative immunoblotting and immunohistochemistry.

RESULTS

Polyuria and increased natriuresis were seen 6 h after LPS injection alongside downregulation of both AQP2 and S256-phosphorylated AQP2 in CTX/OSOM and ISOM but not in inner medulla (IM). Thick ascending limb sodium transporters NHE3 and NKCC2 were downregulated in ISOM and NaPi2 was decreased in CTX/OSOM, whereas NCC and ENaC were not consistently downregulated. Immunolabelling intensity of iNOS was increased in vascular structures and transitional epithelium, and an infiltration of activated macrophages was seen in CTX and ISOM. L-NIL co-treatment prevented the downregulation of NKCC2 but not AQP2 in LPS rats.

CONCLUSIONS

Early downregulation of AQP2 and sodium transporters takes place segmentally in the kidney after LPS administration. In addition, an infiltration of activated macrophages and increased iNOS expression may play a role in the urinary concentrating defect in acute LPS-induced entotoxaemia.

Angiotensin II regulates V2 receptor and pAQP2 during ureteral obstruction

Release of bilateral ureteral obstruction (BUO) is associated with nephrogenic diabetes insipidus (NDI) and a reduced abundance of the vasopressin-regulated aquaporins. To evaluate the role of the vasopressin type 2 receptor (V2R), we determined V2R abundance in kidneys from rats subjected to 24-h BUO or 24-h unilateral ureteral obstruction (UUO) followed by 48-h release. Because angiotensin II type 1 (AT1) receptor blockade attenuates postobstructive polyuria and aquaporin-2 (AQP2) downregulation, we examined the effect of AT1 receptor blockade on AQP2 phosphorylated at serine 256 (pS256-AQP2) and V2 receptor complex abundance in kidney inner medulla (IM). Furthermore, cAMP generation in sodium fluoride- and forskolin-stimulated inner medullary membrane fractions was studied after release of BUO. V2R was significantly reduced to 12% of sham levels in IM and to 52% of sham levels in cortex and outer stripe of outer medulla (OSOM) from BUO rats. In UUO rats, V2R abundance in the obstructed kidney IM decreased to 35% of sham levels, whereas it was comparable to sham levels in the nonobstructed kidney IM. No significant change was observed in cortex and OSOM. AT1 receptor blockade attenuated V2R, pS256-AQP2, and G(s)alpha protein downregulation in IM and partially reversed the obstruction-induced inhibition of sodium fluoride- and forskolin-stimulated cAMP generation in inner medullary membrane fractions from BUO rats. In conclusion, V2R downregulation plays a pivotal role in development of NDI after release of BUO. In addition, we have shown that angiotensin II regulates the V2 receptor complex and pS256-AQP2 in postobstructive kidney IM, probably by stimulating cAMP generation.

N-acetylcysteine protects against renal injury following bilateral ureteral obstruction

Background. Obstructive nephropathy decreases renal blood flow (RBF) and glomerular filtration rate (GFR), causing tubular abnormalities, such as urinary concentrating defect, as well as increasing oxidative stress. This study aimed to evaluate the effects of N-acetylcysteine (NAC) on renal function, as well as on the protein expression of aquaporin 2 (AQP2) and endothelial nitric oxide synthase (eNOS), after the relief of bilateral ureteral obstruction (BUO).

Methods. Adult male Wistar rats were divided into four groups: sham (sham operated); sham operated + 440 mg/kg body weight (BW) of NAC daily in drinking water, started 2 days before and maintained until 48 h after the surgery; BUO (24-h BUO only); BUO + NAC-pre (24-h BUO plus 440 mg/kg BW of NAC daily in drinking water started 2 days before BUO); and BUO + NAC-post (24-h BUO plus 440 mg/kg BW of NAC daily in drinking water started on the day of BUO relief). Experiments were conducted 48 h after BUO relief.

Results. Serum levels of thiobarbituric reactive substances, which are markers of lipid peroxidation, were significantly lower in NAC-treated rats than in the BUO group rats. The administration of NAC provided significant protection against post-BUO GFR drops and reductions in RBF. Renal cortices and BUO rats presented decreased eNOS protein expression of eNOS in the renal cortex of BUO group rats, whereas it was partially recovered in BUO + NAC-pre group rats. Urine osmolality was significantly lower in BUO rats than in sham group rats or NAC-treated rats, the last also presenting less interstitial fibrosis. Post-BUO downregulation of AQP2 protein expression was averted in the BUO + NAC-pre group rats.

Conclusions. This study demonstrates that NAC administration ameliorates the renal function impairment observed 48 h after the relief of 24-h BUO. Oxidative stress is important for the suppression of GFR, RBF, tissue AQP2 and eNOS in the polyuric phase after the release of BUO.

Chronic candesartan alters expression and activity of NKCC2, NCC, and ENaC in the obese Zucker rat

The obese Zucker rat reportedly has increased activity of the intrarenal renin-angiotensin-aldosterone system, which conceptually could contribute to elevated salt sensitivity and blood pressure (BP). Our aim was to determine whether there was increased angiotensin II type 1 receptor (AT(1)R)-mediated upregulation of expression or activity of the bumetanide-sensitive Na-K-2Cl cotransporter, the thiazide-sensitive Na-Cl cotransporter (NCC), and/or the epithelial sodium channel (ENaC) in obese vs. lean Zucker rats. Male obese and lean Zucker rats (10-wk old) were fed either 1) control chow (1% NaCl) or 2) chow with candesartan (CAN), an AT(1)R antagonist (25 mg/kg.diet) for 14 wk (n = 8/treatment/body type). BP measured by radiotelemetry, was markedly reduced by CAN ( approximately 20-25 mmHg) in both lean and obese rats with no body-type differences. Obese rats had significantly greater net natriuretic response to single injections of hydrochlorothiazide and benzamil, suggesting increased activity of NCC and ENaC, respectively; however, only the response to benzamil was reduced by CAN. CAN led to a significant reduction in whole kidney levels of NCC and gamma-ENaC (70-kDa band) in both lean and obese rats. However, it significantly increased alpha-ENaC and Na-K-2Cl cotransporter levels, and these increases were greater in obese rats. These studies suggest that relatively increased ENaC, but not NCC activity, in obese rats is due to enhanced AT(1)R activity. CAN attenuated the reduction of several renal transporters in the obese rat kidney. Finally, differences in intrarenal AT(1)R activity do not seem directly responsible for BP differences between lean and obese rats.

ANG II provokes acute trafficking of distal tubule Na+-Cl(-) cotransporter to apical membrane

The distal convoluted tubule (DCT) Na+-Cl(-) cotransporter (NCC), the target of thiazide diuretics, is responsible for the reabsorption of 5-10% of filtered NaCl. The aim of this study was to test the hypothesis that acute infusion of the angiotensin-converting enzyme (ACE) inhibitor captopril (at 12 microg/min) for 20 min provokes trafficking of NCC from apical plasma membranes (APM) to subapical cytoplasmic vesicles (SCV), which is reversed by acute ANG II infusion (ANG II at 20 ng.kg(-1).min(-1) along with 12 microg/min captopril) for 20 min in male Sprague-Dawley rats (250-350 g). By immuno-electron microscopy using an anti-NCC (D. Ellison) 71.5 +/- SD 4.9% of the NCC gold labeling was associated with the APM in control, sham operated, and infused rats, while captopril infusion reduced NCC in APM to 54.9 +/- 6.9% (P < 0.001) and markedly increased immunogold labeling of SCV. Subsequent infusion of ANG II with captopril restored NCC immunogold labeling of APM to 72.4 +/- 4.2%, that is, 20% of the total NCC trafficked between APM and SCV. Likewise, on density gradients of cortex, captopril provoked redistribution of 27.3% of total NCC from low-density APM-enriched membranes to higher-density membranes and ANG II+captopril restored 20.3% of the NCC to APM-enriched fractions. Redistribution occurred independent of a change in NCC total abundance. In conclusion, this study demonstrates that ACE inhibition provokes acute trafficking of NCC out of the plasma membrane, which likely decreases DCT Na+ reabsorption, while ANG II provokes rapid trafficking of NCC from stores in subapical vesicles to the plasma membrane, which likely increases DCT Na+ reabsorption.

Angiotensin AT1-receptor inhibition exacerbates renal injury resulting from partial unilateral ureteral obstruction in the neonatal rat

The renin-angiotensin system is activated in the developing kidney and is necessary for normal renal development, but is further activated by unilateral ureteral obstruction (UUO). During nephrogenesis, there is a switch from a preponderance of angiotensin AT(2) to AT(1) receptors in the rat. We examined the renal cellular response to angiotensin II receptor inhibition in the neonatal rat subjected to partial UUO under anesthesia within 48 h of birth. Group I ("early") received saline vehicle, losartan (AT(1) inhibitor), or PD-123319 (AT(2) inhibitor) during the completion of nephrogenesis in the first 10 days of life. Group II ("late") received each of the three treatments throughout the subsequent 10 days of life. Kidneys were harvested at 21 days, and the distribution of renin, apoptosis, macrophages, alpha-smooth muscle actin, and collagen was determined. Losartan and PD-123319 each increased vascular renin distribution in both kidneys. Partial UUO reduced growth and increased apoptosis, macrophages, alpha-smooth muscle actin, and collagen in the obstructed kidney. Early losartan treatment further increased alpha-smooth muscle actin and collagen in the obstructed kidney and induced apoptosis, macrophages, and collagen in the contralateral kidney. Late losartan treatment had no effect on any of the parameters in either kidney, and PD-123319 had no effect on either kidney. We conclude that selective inhibition of AT(1) receptors during nephrogenesis (but not during subsequent renal maturation) exacerbates injury to the obstructed kidney and also injures the contralateral kidney. These results suggest that angiotensin II receptor blockers should be avoided in the developing hydronephrotic kidney.

Candesartan prevents long-term impairment of renal function in response to neonatal partial unilateral ureteral obstruction

Angiotensin II (ANG II) plays an important role in the development of obstructive nephropathy. Here, we examined the effects of the ANG II receptor type 1 (AT1R) blockade using candesartan on long-term renal molecular and functional changes in response to partial unilateral ureteral obstruction (PUUO). Newborn rats were subjected to severe PUUO or sham operation (Sham) within the first 48 h of life. Candesartan was provided in the drinking water (10 mg·kg−1·day−1) from day 21 of life until 10 wk of age. Renal blood flow (RBF) was evaluated by MRI, glomerular filtration rate (GFR) was measured using the renal clearance of51Cr-EDTA, and the renal expression of Na-K-ATPase and the collecting duct water channel aquaporin-2 (AQP2) was examined by immunoblotting and immunocytochemistry. At 10 wk of age, PUUO significantly reduced RBF (0.8 ± 0.1 vs. 1.6 ± 0.1 ml·min−1·100 g body wt−1; P < 0.05) and GFR (37 ± 16 vs. 448 ± 111 μl·min−1·100 g body wt−1; P < 0.05) compared with Sham. Candesartan prevented the RBF reduction (PUUO+CAN: 1.6 ± 0.2 vs. PUUO: 0.8 ± 0.1 ml·min−1·100 g body wt−1; P < 0.05) and attenuated the GFR reduction (PUUO+CAN: 265 ± 68 vs. PUUO: 37 ± 16 μl·min−1·100 g body wt−1; P < 0.05). PUUO was also associated with a significant downregulation in the expression of Na-K-ATPase (75 ± 12 vs. 100 ± 5%, P < 0.05) and AQP2 (52 ± 15 vs. 100 ± 4%, P < 0.05), which were also prevented by candesartan (Na-K-ATPase: 103 ± 8 vs. 100 ± 5% and AQP2: 74 ± 13 vs. 100 ± 4%). These findings were confirmed by immunocytochemistry. Consistent with this, candesartan treatment partly prevented the reduction in solute free water reabsorption and attenuated fractional sodium excretion in rats with PUUO. In conclusion, candesartan prevents or attenuates the reduction in RBF, GFR and dysregulation of AQP2 and Na-K-ATPase in response to congenital PUUO in rats, suggesting that AT1R blockade may protect the neonatally obstructed kidney against development of obstructive nephropathy.