Renal expression of aquaporins in liver cirrhosis induced by chronic common bile duct ligation in rats

Aquaporins in Edema

One of the most prevalent indications of water-electrolyte imbalance is edema. Aquaporins (AQPs) are a protein family that can function as water channels. Osmoregulation and body water homeostasis are dependent on the regulation of AQPs. Human kidneys contain nine AQPs, five of which have been demonstrated to have a role in body water balance: AQP1, AQP2, AQP3, AQP4, and AQP7. Water imbalance is connected with AQP dysfunction. Hyponatremia with elevated AQP levels can accompany edema, which can be caused by disorders with low effective circulating blood volume and systemic vasodilation, such as congestive heart failure (CHF), hepatic cirrhosis, or the syndrome of incorrect antidiuretic hormone secretion (SIADH). In CHF, upregulation of AQP2 expression and targeting is critical for water retention. AQP2 is also involved in aberrant water retention and the formation of ascites in cirrhosis of the liver. Furthermore, water retention and hyponatremia in SIADH are caused by increased expression of AQP2 in the collecting duct. Fluid restriction, demeclocycline, and vasopressin type-2 receptor antagonists are widely utilized to treat edema. The relationship between AQPs and edema is discussed in this chapter.

Torsemide Pharmacometrics in Healthy Adult Populations Including CYP2C9 Genetic Polymorphisms and Various Patient Groups through Physiologically Based Pharmacokinetic-Pharmacodynamic Modeling

Torsemide is a widely used diuretic in clinical practice. In this study, pharmacokinetic (PK) and pharmacodynamic (PD) simulations of torsemide for various population groups and exposure scenarios were performed through human-scale physiologically-based PK-PD (PBPK-PD) modeling of torsemide. For PBPK-PD modeling of torsemide, invitro and clinical data of torsemide reported previously were used. After exposure to clinical doses of torsemide, observed plasma (or serum) concentration and urine torsemide excretion profiles were used as PK-data, and observed urinary sodium excretion rate was used as PD-data. The model was then extended to take into account physiological and biochemical factors according to different CYP2C9 phenotypes or patient populations. The established model captured various torsemide clinical results well. Differences in torsemide PKs and PDs between patient groups or CYP2C9 genetic polymorphisms were modelologically identified. It was confirmed that degrees of differences in torsemide PKs and PDs by disease groups were greater than those according to different CYP2C9 phenotypes. According to torsemide administration frequency or dose change, it was confirmed that although the difference in plasma PKs between groups (healthy adult and patient groups) could increase to 14.80 times, the difference in PDs was reduced to 1.01 times. Results of this study suggested that it is very important to consider disease groups in the setting of torsemide clinical therapy and that it is difficult to predict PD proportionally with only differences in PKs of torsemide between population groups. The PBPK-PD model established in this study is expected to be utilized for various clinical cases involving torsemide application in the future, enabling optimal drug therapy.

Aquaporins and diseases pathogenesis: From trivial to undeniable involvements, a disease-based point of view

Aquaporins (AQPs), as transmembrane proteins, were primarily identified as water channels with the ability of regulating the transmission of water, glycerol, urea, and other small-sized molecules. The classic view of AQPs involvement in therapeutic plan restricted them and their regulators into managing only a narrow spectrum of the diseases such as diabetes insipidus and the syndrome of inappropriate ADH secretion. However, further investigations performed, especially in the third millennium, has found that their cooperation in water transmission control can be manipulated to handle other burden-imposing diseases such as cirrhosis, heart failure, Meniere's disease, cancer, bullous pemphigoid, eczema, and Sjögren's syndrome.

Bile acid signaling in renal water regulation

Emerging evidence has shown that bile acids play important roles in renal physiology and diseases by activating two major receptors, the nuclear farnesoid X receptor (FXR) and the membrane G protein-coupled bile acid receptor-1 (Gpbar1; also known as TGR5). Both FXR and TGR5 have been identified in human and rodent kidneys, where they are deeply involved in renal water handling. In mice, FXR- or TGR5-related gene deficiency has been associated with reduced aquaporin-2 expression accompanied with impaired urinary concentration ability. In this mini-review, we briefly discuss the current understanding of FXR/TGR5 signaling in the kidneys, with a special focus on the regulation of aquaporin-2 expression by bile acids in the collecting ducts and its potential significance in disease conditions.

Aquaporin-2 excretion in hospitalized patients with cirrhosis: Relation to development of renal insufficiency and mortality

BACKGROUND AND AIM

Urinary aquaporin-2 (AQP2) is a parameter of water transport in the principal cells in the distal part of the nephron and involved in water retention in cirrhosis and may be a marker of renal function. The aim of the study was to evaluate AQP2 as a predictor of renal insufficiency and death in patients with cirrhosis.

METHODS

Urine samples from 199 patients (90 patients without organ failure [Group 1], 58 patients with organ failure excluding renal failure [Group 2], and 51 patients with organ failure including renal failure [Group 3]) from the CANONIC study were analyzed for urine AQP2 and urine osmolality.

RESULTS

There was no difference in AQP2 between the three groups. Urine osmolality was significantly lower in patients in Group 3 versus Group 1 and Group 2 (P = 0.0004). No relation was found between AQP2 and glomerular filtration rate or creatinine; however, AQP2 was a significant predictor of the development of renal insufficiency (P = 0.0485). In a univariate analysis, AQP2 was a significant predictor of 14 and 28-day survival, but this was not confirmed in multivariate analysis.

CONCLUSIONS

Aquaporin-2 was not associated with disease severity or markers of renal function but was a predictor for the development of renal insufficiency and death. Therefore, its future use as marker of renal insufficiency could be promising, but further research is needed before it can be considered a clinical useful tool.

Aquaporins in Urinary System

Several aquaporin (AQP )-type water channels are expressed in kidney: AQP1 in the proximal tubule, thin descending limb of Henle, and vasa recta; AQP2 -6 in the collecting duct; AQP7 in the proximal tubule; AQP8 in the proximal tubule and collecting duct; and AQP11 in the endoplasmic reticulum of proximal tubule cells. AQP2 is the vasopressin-regulated water channel that is important in hereditary and acquired diseases affecting urine-concentrating ability. The roles of AQPs in renal physiology and transepithelial water transport have been determined using AQP knockout mouse models. This chapter describes renal physiologic insights revealed by phenotypic analysis of AQP knockout mice and the prospects for further basic and clinical studies.

Renal aquaporins and water balance disorders

Aquaporins (AQPs) are a 13 member family (AQP0-12) of proteins that act as channels, through which water and, for some family members, glycerol, urea and other small solutes can be transported. Aquaporins are highly abundant in kidney epithelial cells where they play a critical role with respect to water balance. In this review we summarize the current knowledge with respect to the localization and function of AQPs within the kidney tubule, and their role in mammalian water homeostasis and the water balance disorders. Overviews of practical aspects with regard to differential diagnosis for some of these disorders, alongside treatment strategies are also discussed.

AVP-induced increase in AQP2 and p-AQP2 is blunted in heart failure during cardiac remodeling and is associated with decreased AT1R abundance in rat kidney

AIM

The objective was to examine the renal effects of long-term increased angiotensin II and vasopressin plasma levels in early-stage heart failure (HF). We investigated the regulations of the V2 vasopressin receptor, the type 1A angiotensin II receptor, the (pro)renin receptor, and the water channels AQP2, AQP1, AQP3, and AQP4 in the inner medulla of rat kidney.

METHODS

HF was induced by coronary artery ligation. Sixty-eight rats were allocated to six groups: Sham (N = 11), HF (N = 11), sodium restricted sham (N = 11), sodium restricted HF (N = 11), sodium restricted sham + DDAVP (N = 12), and sodium restricted HF + DDAVP (N = 12). 1-desamino-8-D-arginine vasopressin (0.5 ng h-1 for 7 days) or vehicle was administered. Pre- and post-treatment echocardiographic evaluation was performed. The rats were sacrificed at day 17 after surgery, before cardiac remodeling in rat is known to be completed.

RESULTS

HF rats on standard sodium diet and sodium restriction displayed biochemical markers of HF. These rats developed hyponatremia, hypo-osmolality, and decreased fractional excretion of sodium. Increase of AQP2 and p(Ser256)-AQP2 abundance in all HF groups was blunted compared with control groups even when infused with DDAVP and despite increased vasopressin V2 receptor and Gsα abundance. This was associated with decreased protein abundance of the AT1A receptor in HF groups vs. controls.

CONCLUSION

Early-stage HF is associated with blunted increase in AQP2 and p(Ser256)-AQP2 despite of hyponatremia, hypo-osmolality, and increased inner medullary vasopressin V2 receptor expression. Decreased type 1A angiotensin II receptor abundance likely plays a role in the transduction of these effects.

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.

Vasopressin and the regulation of aquaporin-2

Water excretion is regulated in large part through the regulation of osmotic water permeability of the renal collecting duct epithelium. Water permeability is controlled by vasopressin through regulation of the water channel, aquaporin-2 (AQP2). Two processes contribute: (1) regulation of AQP2 trafficking to the apical plasma membrane; and (2) regulation of the total amount of the AQP2 protein in the cells. Regulation of AQP2 abundance is defective in several water-balance disorders, including many polyuric disorders and the syndrome of inappropriate antidiuresis. Here we review vasopressin signaling in the renal collecting duct that is relevant to the two modes of water permeability regulation.

Congestive heart failure in rats is associated with increased collecting duct vasopressin sensitivity and vasopressin type 2 receptor reexternalization

A number of studies have shown that rats with congestive heart failure (CHF) have increased protein levels of the vasopressin (AVP)-regulated water channel aquaporin-2 (AQP2) even during conditions with unchanged circulating levels of AVP, suggesting an increase in the sensitivity of the AVP type 2 (V2) receptor in experimental CHF. The present study was aimed at investigating AVP signaling in rats with moderate CHF (left ventricular end diastolic pressure >10 mmHg; normal plasma AVP levels) induced by ligation of the left anterior descending coronary artery. Sham-operated rats were used as controls. Western blotting analyses revealed an increased abundance of AQP2 in renal cortex (+33 ± 9% of sham; P < 0.05) and in inner medulla (IM) (+54 ± 15% of sham; P < 0.05) in CHF rats compared with sham-operated controls. Dose-response studies on isolated collecting ducts (CDs) showed an increased accumulation of cAMP in response to AVP in CHF rats compared with controls. V2 receptor surface-binding studies in isolated IMCDs showed a marked and comparable AVP-induced V2 receptor internalization in response to AVP in both CHF and control rats. As expected V2 receptor surface binding remained low after AVP challenge in control rats. In contrast to this, V2 receptor surface binding returned to pre-AVP levels within 30 min in the CHF rats, indicating an obtained recycling ability of the V2 receptor in CHF. Together the results indicate the presence of an increased AVP sensitivity in the CDs from CHF rats, associated with an acquired recycling ability of the V2 receptor.

Molecular biology of water and salt regulation in the kidney

The kidney plays a central role in the regulation of the salt and water balance, which depends upon an array of solute and water transporters in the renal tubules and upon vascular elements in the various regions of the kidney. Many recent studies have improved our understanding of this process. In this review, we summarize the current data on the molecules involved in sodium and water transport in the renal tubules, focusing in particular on aquaporins and renal sodium transporters and channels.

Age-related changes in expression in renal AQPs in response to congenital, partial, unilateral ureteral obstruction in rats

Previously we demonstrated that neonatally induced partial unilateral ureteral obstruction (PUUO) in rats is associated with changes in the abundance of renal acid-base transporters that were paralleled by reduction in renal functions dependent on the severity and duration of obstruction. The aim of the present study was to identify whether changes in renal aquaporin abundance are age-dependent. Semiquantitative immunoblotting and immunohistochemistry were used to examine the changes in abundance of AQP1, AQP2, p-S256AQP2 (AQP2 phosphorylated at consensus site Ser(256)) and AQP3 in the kidneys of rats with neonatally induced PUUO within the first 48 h of life, and then monitored for 7 or 14 weeks. Protein abundance of AQP2 and AQP3 increased in both obstructed and non-obstructed kidneys 7 weeks after induction of neonatal PUUO (PUUO-7W). In contrast, AQP1 and AQP2 protein abundance in the obstructed kidney were reduced after 14 weeks of PUUO (PUUO-14W). Importantly, pS256-AQP2 protein abundance was reduced in obstructed kidneys of both PUUO-7W and PUUO-14W. Immunohistochemistry confirmed the persistent pS256-AQP2 downregulation in both PUUO-7W and PUUO-14W rats. The study shows that the protein abundance of AQP1, AQP2, and AQP3 in the obstructed kidney is increased in PUUO-7W, which may be a compensatory phenomenon and reduced in PUUO-14W rats suggesting a time-/age-dependent dysregulation in response to PUUO. pS256-AQP2 protein abundance is reduced consistent with obstruction-induced direct effects in the apical part of the collecting duct principal cells in response to PUUO.

Influence of aquaporin-1 gene polymorphism on water retention in liver cirrhosis

Water retention is a major clinical problem in patients with liver cirrhosis. The factors that predispose to water retention are poorly understood but may involve genetic factors. Recent research suggests that renal aquaporins may be a pathophysiological factor involved in this condition. Aquaporin-1 (AQP1) is expressed in the proximal tubule and aquaporin-2 (AQP2) in the renal collecting duct cells. The aim of our study was to investigate the distribution of single nucleotide polymorphisms (SNPs) of AQP1: rs1049305 (C/G) and AQP2: rs3741559 (A/G) and rs467323 (C/T) in 100 cirrhotic patients with ascites and to analyze their relationship with dilutional hyponatremia.

METHODS

Genomic DNA was extracted from peripheral blood. Genotyping for the presence of different polymorphisms was performed using the Custom Taqman SNP Genotyping Assays. The possible influence of rs1049305 (C/G) in AQP1 gene expression was evaluated by luciferase assays in vitro.

RESULTS

The allelic frequencies of the AQP1 gene were the following: CC = 15%; CG = 49%; GG = 36%. Patients with CC genotype had significantly lower plasma sodium concentration than those with CG or GG genotype. Luciferase assays showed that the rs1049305 (C/G) in the AQP1 gene functionally affected the expression level in vitro. In addition, we did not find any relationship between AQP2 SNPs observed and plasma sodium concentration.

CONCLUSIONS

Our results suggest that the rs1049305 (C/G, UTR3) AQP1 polymorphism could be involved in the genetic susceptibility to develop water retention in patients with liver cirrhosis.

Pathogenetic background for treatment of ascites and hepatorenal syndrome

Ascites and hepatorenal syndrome (HRS) are the major and challenging complications of cirrhosis and portal hypertension that significantly affect the course of the disease. Liver insufficiency, portal hypertension, arterial vasodilatation, and systemic cardiovascular dysfunction are major pathophysiological hallmarks. Modern treatment of ascites is based on this recognition and includes modest salt restriction and stepwise diuretic therapy with spironolactone and loop diuretics. Tense and refractory ascites should be treated with a large volume paracentesis, followed by volume expansion or transjugular intrahepatic portosystemic shunt. New treatment strategies include the use of vasopressin V(2)-receptor antagonists and vasoconstrictors. The HRS denotes a functional and reversible impairment of renal function in patients with severe cirrhosis with a poor prognosis. Attempts of treatment should seek to improve liver function, ameliorate arterial hypotension and central hypovolemia, and reduce renal vasoconstriction. Ample treatment of ascites and HRS is important to improve the quality of life and prevent further complications, but since treatment of fluid retention does not significantly improve survival, these patients should always be considered for liver transplantation.

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.

Expression of aquaporin 3 (AQP3) in normal and neoplastic lung tissues

Aquaporin 3 (AQP3) acts as the membrane channel of water and other small solutes and plays a major role in fluid homeostasis. To investigate the expression of AQP3 in normal and neoplastic lung tissues, we studied a series of 149 lung carcinoma tissues and 2 cell lines by immunohistochemistry, Western blotting, and reverse transcriptase-polymerase chain reaction. In normal lung tissues, immunohistochemical expression of AQP3 was demonstrated in bronchial basal cells, alveolar type II cells, bronchiolar epithelial cells, and secretory cells of submucosal glands. In lung carcinomas, AQP3 expression was observed in 59 (70.2%) of 84 adenocarcinomas. Squamous cell carcinoma and large cell carcinoma had rather low positive ratios (35.8% and 13.4%, respectively). No AQP3 expression was demonstrated in small cell carcinoma, pleomorphic carcinoma, or metastatic colon adenocarcinoma. In adenocarcinomas, AQP3 was detected in all tumors of bronchioloalveolar subtype. Papillary subtype also showed a higher positive ratio of AQP3 compared with that in acinar and solid with mucin subtypes. In addition, AQP3 expression was related to tumor differentiation and clinical stage in adenocarcinomas. Western blotting and reverse transcriptase-polymerase chain reaction analyses confirmed the expression of AQP3 protein and messenger RNA in cell lines and tissues of lung adenocarcinoma. We conclude that AQP3 is widely expressed in the normal respiratory tract and can play an important role in the maintenance of water homeostasis. In addition, lung carcinomas, especially adenocarcinomas, can produce AQP3, possibly in connection with their functional and/or biological nature, although the detailed mechanism of AQP3 expression in lung carcinomas remains to be clarified.

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.

Aquaporin-1 and aquaporin-2 urinary excretion in cirrhosis: Relationship with ascites and hepatorenal syndrome

Several experimental models of cirrhosis have shown dysregulation of renal aquaporins in different phases of liver disease. We investigated the urinary excretion of both aquaporin-1 and aquaporin-2 in patients with cirrhosis at different stages of the disease. Twenty-four-hour urine was collected from 11 healthy volunteers, 13 patients with compensated cirrhosis (without ascites), and 20 patients with decompensated cirrhosis (11 with ascites without renal failure and 9 with hepatorenal syndrome). Aquaporin-1 and aquaporin-2 excretion was analyzed by immunoblotting. Urinary aquaporin-2 excretion was reduced in patients with cirrhosis compared to healthy subjects. A progressive decrease in urinary aquaporin-2 excretion was observed as the severity of cirrhosis increased, from compensated cirrhosis to cirrhosis with ascites and hepatorenal syndrome. Patients with hyponatremia had lower urinary aquaporin-2 excretion than patients without hyponatremia. Vasopressin plasma level did not correlate with aquaporin-2 excretion. There were no differences between healthy subjects and patients with cirrhosis with or without ascites in urinary excretion of aquaporin-1, but urinary aquaporin-1 excretion of those with hepatorenal syndrome was extremely low. In conclusion, patients with cirrhosis appear to exhibit a decreased abundance of renal aquaporin-2 and therefore lower water permeability in the collecting tubules. This may represent an adaptive renal response to sodium retention, with expansion of extracellular fluid volume and dilutional hyponatremia observed in those who have cirrhosis with ascites. Finally, aquaporin-1 does not appear to play a role in the progressive dysregulation of extracellular fluid volume in cirrhosis.

Role of the aldosterone-sensitive distal nephron in the sodium retention associated with liver cirrhosis

The renal mechanisms underlying sodium retention during liver cirrhosis have been difficult to elucidate. Kim and associates describe a biphasic pattern of regulation of the renal epithelial sodium channel in the common bile duct ligation model, shedding some light on this issue.

Clinical update on renal aquaporins

Following the discovery of the aquaporin-1 water channel over a decade ago, molecular techniques have been developed to examine the role of renal aquaporin water channels under numerous physiological and pathological conditions. The present article reviews current knowledge regarding the function and dysfunction of renal aquaporins in disorders of water metabolism.

The 4.5Å Structure of Human AQP2

Located in the principal cells of the collecting duct, aquaporin-2 (AQP2) is responsible for the regulated water reabsorption in the kidney and is indispensable for the maintenance of body water balance. Disregulation or malfunctioning of AQP2 can lead to severe diseases such as nephrogenic diabetes insipidus, congestive heart failure, liver cirrhosis and pre-eclampsia. Here we present the crystallization of recombinantly expressed human AQP2 into two-dimensional protein-lipid arrays and their structural characterization by atomic force microscopy and electron crystallography. These crystals are double-layered sheets that have a diameter of up to 30 microm, diffract to 3 A(-1) and are stacked by contacts between their cytosolic surfaces. The structure determined to 4.5 A resolution in the plane of the membrane reveals the typical aquaporin fold but also a particular structure between the stacked layers that is likely to be related to the cytosolic N and C termini.

Pathophysiological basis of pharmacotherapy in the hepatorenal syndrome

Hepatorenal syndrome (HRS) is a functional and reversible impairment of renal function in patients with severe cirrhosis. Major pathophysiological elements include liver dysfunction, a circulatory derangement with central hypovolaemia and neurohumoral activation of potent vasoactive systems leading to a pronounced renal vasoconstriction. The prognosis of patients with HRS is poor but recent research has spread new enthusiasm for treatment. Efforts at treatment should seek to improve liver function, to ameliorate arterial hypotension and central hypovolaemia, and to reduce renal vasoconstriction. Therefore a combined approach should be applied with reduction of portal pressure with e.g. ss-adrenergic blockers and transjugular intrahepatic portosystemic shunt (TIPS), with amelioration of arterial hypotension and central hypovolaemia with vasoconstrictors such as terlipressin and plasma expanders. New experimental treatments with endothelin- and adenosine antagonists and long-acting vasoconstrictors may have a future role in the management of HRS.

Review article: pathogenesis and pathophysiology of hepatorenal syndrome--is there scope for prevention?

The hepatorenal syndrome (HRS) is a functional impairment of the kidneys in chronic liver disease caused by a circulatory failure. The prognosis is poor, particularly with type 1 HRS, but also type 2, and only liver transplantation is of lasting benefit. However, recent research into the pathophysiology of ascites and HRS has stimulated new enthusiasm in their prevention and treatment. Patients with HRS have hyperdynamic circulatory dysfunction with reduced arterial blood pressure and reduced central blood volume, owing to preferential splanchnic arterial vasodilatation. Activation of potent vasoconstricting systems, including the sympathetic nervous and renin-angiotensin-aldosterone systems, counteracts the arterial vasodilatation and leads to a pronounced renal vasoconstriction with renal hypoperfusion, a reduced glomerular filtration rate, and intense sodium-water retention. Thus prevention of HRS should seek to improve liver function, limit arterial hypotension and central hypovolaemia, and reduce renal vasoconstriction and the renal and interstitial pressures. Portal pressure can be reduced with beta-adrenergic blockers and transjugular intrahepatic portosystemic shunt (TIPS). Precipitating events, like infections, bleeding, and postparacentesis circulatory syndrome, should be treated to avoid further circulatory failure. Improvement in arterial blood pressure and central hypovolaemia can be achieved with vasoconstrictors, such as terlipressin (Glypressin), and plasma expanders such as human albumin. In the future endothelins, adenosine antagonists, long-acting vasoconstrictors, and antileukotriene drugs may play a role in preventing and treating HRS.

Opioid receptor-like 1 stimulation in the collecting duct induces aquaresis through vasopressin-independent aquaporin-2 downregulation

Nociceptin, the endogenous ligand of the inhibitory G protein-coupled opioid receptor-like 1 receptor, produces aquaresis (i.e., increases the excretion of solute-free urine) in rats. However, the mechanism underlying this effect has not yet been explained. Using immunohistochemistry, we found the opioid receptor-like 1 receptor in the rat kidney colocalized with the vasopressin-regulated water channel aquaporin-2 in inner medullary collecting ducts. We investigated the aquaretic effect of opioid receptor-like 1 receptor stimulation by infusing the selective nociceptin analog ZP120C; volume depletion was prevented by computer-driven, servo-controlled intravenous volume replacement with 50 mM glucose. ZP120C induced a marked and sustained aquaresis in normal and congestive heart failure rats in the absence of changes in vasopressin plasma concentrations. The ZP120C-induced aquaresis was associated with downregulation of the aquaporin-2 protein level in both rat groups, suggesting that opioid receptor-like 1 receptor stimulation produces aquaresis by inhibiting the vasopressin type-2 receptor-mediated stimulation on collecting duct water reabsorption. However, substantial amounts of PKA-mediated serine 256 phosphorylated aquaporin-2 were still present after 4 h of ZP120C treatment. Furthermore, neither preincubation with nociceptin nor ZP120C inhibited vasopressin-mediated cAMP accumulation in isolated collecting ducts. We conclude that renal opioid receptor-like 1 receptor stimulation in normal and congestive heart failure rats produces aquaresis by a direct renal effect, via aquaporin-2 downregulation, through a mechanism not involving inhibition of vasopressin type-2 receptor-mediated cAMP production.

Uncoupling of vasopressin signaling in collecting ducts from rats with CBL-induced liver cirrhosis

Vasopressin (AVP) stimulates collecting duct water reabsorption through cAMP-mediated membrane targeting and increased expression of the aquaporin-2 (AQP2) water channel. Rats with liver cirrhosis induced by common bile duct ligation (CBL) show decreased protein expression of AQP2 despite increased plasma concentrations of AVP. The present study was conducted to investigate possible mechanisms behind this uncoupling of AVP signaling. The rats were examined 4 wk after CBL or sham operation. The CBL rats had increased plasma AVP concentrations (CBL: 3.2 +/- 0.2 vs. sham: 1.4 +/- 0.4 pg/ml, P < 0.05) and reduced AQP2 (0.62 +/- 0.11) and phosphorylated AQP2 (0.50 +/- 0.06) protein expression compared with sham-operated rats. However, examination of subcellular AQP2 localization by immunohistochemistry showed unchanged plasma membrane targeting in CBL rats, indicating a sustained ability of AQP2 short-term regulation. In a separate series of animals, thirsting was found to normalize AQP2 expression, indicating that AVP uncoupling in CBL rats is a physiological compensatory mechanism aimed at avoiding dilutional hyponatremia. Studies on microdissected collecting ducts from CBL rats showed decreased cAMP accumulation in response to AVP stimulation. The presence of the nonspecific phosphodiesterase inhibitor IBMX normalized the cAMP accumulation, indicating that cAMP-phosphodiesterase activity is increased in CBL rats. However, in contrast to this, Western blotting showed a decreased expression of several phosphodiesterase splice variants. We conclude that CBL rats develop an escape from AVP to prevent the formation of dilutional hyponatremia in response to increased plasma AVP concentrations. The mechanism behind AVP escape seems to involve decreased collecting duct sensitivity to AVP as a result of increased cAMP-phosphodiesterase activity.

Urinary aquaporin-2 in healthy humans and patients with liver cirrhosis and chronic heart failure during baseline conditions and after acute water load

BACKGROUND

Patients with liver cirrhosis and chronic heart failure (CHF) have a reduced capacity to excrete water. Studies in healthy humans have shown that an acute water load reduces the excretion of aquaporin-2 in urine (u-AQP-2). We wanted to test the hypothesis that an acute water load reduces u-AQP-2 less in patients with liver cirrhosis or CHF than in healthy humans.

METHODS

Fourteen healthy subjects, 14 patients with liver cirrhosis, and 14 patients with CHF were given an oral water load of 20 mL/kg. Urine was collected every 30 minutes for 4 hours for analysis of u-AQP-2. Blood samples were drawn at the beginning and at the end of the study for analysis of arginine vasopressin (AVP). u-AQP-2 was determined by radioimmunoassay.

RESULTS

During the study period, urinary output was 22.8% higher than water intake in the healthy controls and increased 14-fold from baseline, but in patients with liver cirrhosis and CHF urinary output was 14% and 24% less than the intake, while urinary output increased 7- and 19-fold from baseline, respectively. u-AQP2 decreased significantly more in patients with CHF (39%) than in healthy controls (17%) but it was unchanged in those with liver cirrhosis. AVP decreased 46% in patients with CHF, but was unchanged in healthy controls and those with liver cirrhosis. A 24-hour urinary excretion of AQP-2 was significantly elevated in patients with CHF (median, 25.7 nmol/mol creatinine) compared to healthy controls (15.7 nmol/mol creatinine) and those with liver cirrhosis (17 nmol/mol creatinine).

CONCLUSION

The excretion of AQP-2 in urine is abnormal both in liver cirrhosis in which we find less suppression of u-AQP2 by an acute water load and in CHF in which we find a high baseline level and an exaggerated suppression of u-AQP2 by an acute water load.

Renal aquaporin water channels: from molecules to human disease

Following the discovery of the aquaporin-1 water channel in 1991, molecular techniques have been developed to examine the roles of renal aquaporins-1, -2, -3, and -4 in disorders of water balance. This article reviews current knowledge regarding aquaporin function and dysfunction in water-losing and water-retaining states.

Regulation of collecting duct AQP3 expression: response to mineralocorticoid

Adrenocortical steroid hormones are importantly involved in the regulation of extracellular fluid volume. The present study was aimed at examining whether aldosterone and/or glucocorticoid regulates the abundance of aquaporin-3 (AQP3), -2, and -1 in rat kidney. In protocol 1, rats were adrenalectomized, followed by aldosterone replacement, dexamethasone replacement, or combined aldosterone and dexamethasone replacement (rats had free access to water but received a fixed amount of food). Protocol 2 was identical to protocol 1, except that all groups received fixed daily food and water intake. In both protocols 1 and 2, aldosterone deficiency was associated with increased fractional Na excretion and severe hyperkalemia. Semiquantitative immunoblotting revealed that aldosterone deficiency was associated with a dramatic downregulation of AQP3 abundance. Consistent with this, immunocytochemistry and immunoelectron microscopy revealed a marked decrease in AQP3 labeling in the basolateral plasma membranes of collecting duct principal cells. In contrast, AQP1 and AQP2 abundance and distribution were unchanged. Glucocorticoid deficiency revealed no changes in AQP3, -2, or -1 abundance. In protocol 3, Na restriction (to increase endogenous aldosterone levels) or exogenous aldosterone infusion in either normal rats or vasopressin-deficient Brattleboro rats was associated with a major increase in AQP3 abundance. In protocol 4, aldosterone levels were clamped by infusion of aldosterone, while Na intake was altered from a low to a high level. Under these circumstances, there were no changes in AQP3 or AQP2 abundance, although the level of the thiazide-sensitive Na-Cl cotransporter was decreased. In conclusion, the results uniformly demonstrate that aldosterone regulates AQP3 abundance independently of Na intake. In contrast, changes in glucocorticoid levels in these models do not influence AQP3 or AQP2 abundance. Therefore, in the collecting duct aldosterone may regulate, at least in part, AQP3 expression in addition to regulating Na and K transport.

Altered expression of renal NHE3, TSC, BSC-1, and ENaC subunits in potassium-depleted rats

The purpose of this study was to examine whether hypokalemia is associated with altered abundance of major renal Na+ transporters that may contribute to the development of urinary concentrating defects. We examined the changes in the abundance of the type 3 Na+/H+ exchanger (NHE3), Na+ - K+-ATPase, the bumetanide-sensitive Na+ - K+ - 2Cl- cotransporter (BSC-1), the thiazide-sensitive Na+ - Cl- cotransporter (TSC), and epithelial sodium channel (ENaC) subunits in kidneys of hypokalemic rats. Semiquantitative immunoblotting revealed that the abundance of BSC-1 (57%) and TSC (46%) were profoundly decreased in the inner stripe of the outer medulla (ISOM) and cortex/outer stripe of the outer medulla (OSOM), respectively. These findings were confirmed by immunohistochemistry. Moreover, total kidney abundance of all ENaC subunits was significantly reduced in response to the hypokalemia: alpha-subunit (61%), beta-subunit (41%), and gamma-subunit (60%), and this was confirmed by immunohistochemistry. In contrast, the renal abundance of NHE3 in hypokalemic rats was dramatically increased in cortex/OSOM (736%) and ISOM (210%). Downregulation of BSC-1, TSC, and ENaC may contribute to the urinary concentrating defect, whereas upregulation of NHE3 may be compensatory to prevent urinary Na+ loss and/or to maintain intracellular pH levels.

Pharmacological characterization of YM471, a novel potent vasopressin V(1A) and V(2) receptor antagonist

The pharmacologic profile of YM471 ((Z)-4'-[4,4-difluoro-5-[2-(4-dimethylaminopiperidino)-2-oxoethylidene]-2,3,4,5-tetrahydro-1H-1-benzoazepine-1-carbonyl]-2-phenylbenzanilide monohydrochloride), a novel potent vasopressin V(1A) and V(2) receptor antagonist, was investigated using several in vitro and in vivo techniques. YM471 showed high affinity for rat vasopressin V(1A) and V(2) receptors, exhibiting K(i) values of 0.16 and 0.77 nM, respectively. In contrast, YM471 exhibited much lower affinity for rat vasopressin V(1B) and oxytocin receptors, with K(i) values of 10.5 microM and 31.0 nM, respectively. In conscious rats, oral administration of YM471 (0.1-3.0 mg/kg) produced dose-dependent inhibition of the pressor response caused by exogenous vasopressin and increased urine excretion and decreased urine osmolality; this effect lasted more than 8 h. In all biological assays used, YM471 exhibited no agonistic activity. These results demonstrate that YM471 exerts potent and long-lasting antagonistic activity on both vasopressin V(1A) and V(2) receptors, and that this compound may be a useful tool for clarifying the physiologic and pathophysiologic roles of vasopressin and the therapeutic usefulness of the vasopressin receptor antagonist.

Aquaporins in the kidney: from molecules to medicine

The discovery of aquaporin-1 (AQP1) answered the long-standing biophysical question of how water specifically crosses biological membranes. In the kidney, at least seven aquaporins are expressed at distinct sites. AQP1 is extremely abundant in the proximal tubule and descending thin limb and is essential for urinary concentration. AQP2 is exclusively expressed in the principal cells of the connecting tubule and collecting duct and is the predominant vasopressin-regulated water channel. AQP3 and AQP4 are both present in the basolateral plasma membrane of collecting duct principal cells and represent exit pathways for water reabsorbed apically via AQP2. Studies in patients and transgenic mice have demonstrated that both AQP2 and AQP3 are essential for urinary concentration. Three additional aquaporins are present in the kidney. AQP6 is present in intracellular vesicles in collecting duct intercalated cells, and AQP8 is present intracellularly at low abundance in proximal tubules and collecting duct principal cells, but the physiological function of these two channels remains undefined. AQP7 is abundant in the brush border of proximal tubule cells and is likely to be involved in proximal tubule water reabsorption. Body water balance is tightly regulated by vasopressin, and multiple studies now have underscored the essential roles of AQP2 in this. Vasopressin regulates acutely the water permeability of the kidney collecting duct by trafficking of AQP2 from intracellular vesicles to the apical plasma membrane. The long-term adaptational changes in body water balance are controlled in part by regulated changes in AQP2 and AQP3 expression levels. Lack of functional AQP2 is seen in primary forms of diabetes insipidus, and reduced expression and targeting are seen in several diseases associated with urinary concentrating defects such as acquired nephrogenic diabetes insipidus, postobstructive polyuria, as well as acute and chronic renal failure. In contrast, in conditions with water retention such as severe congestive heart failure, pregnancy, and syndrome of inappropriate antidiuretic hormone secretion, both AQP2 expression levels and apical plasma membrane targetting are increased, suggesting a role for AQP2 in the development of water retention. Continued analysis of the aquaporins is providing detailed molecular insight into the fundamental physiology and pathophysiology of water balance and water balance disorders.

Large-scale purification of functional recombinant human aquaporin-2

The homotetrameric aquaporin-2 (AQP2) water channel is essential for the concentration of urine and of critical importance in diseases with water dysregulation, such as nephrogenic diabetes insipidus, congestive heart failure, liver cirrhosis and pre-eclampsia. The structure of human AQP2 is a prerequisite for understanding its function and for designing specific blockers. To obtain sufficient amounts of AQP2 for structural analyses, we have expressed recombinant his-tagged human AQP2 (HT-AQP2) in the baculovirus/insect cell system. Using the protocols outlined in this study, 0.5 mg of pure HT-AQP2 could be obtained per liter of bioreactor culture. HT-AQP2 had retained its homotetrameric structure and exhibited a single channel water permeability of 0.93+/-0.03x10(-13) cm3/s, similar to that of other AQPs. Thus, the baculovirus/insect cell system allows large-scale expression of functional recombinant human AQP2 that is suitable for structural studies.

Renal water handling in rats with decompensated liver cirrhosis

The present study was performed to investigate the renal handling of water in rats with decompensated liver cirrhosis. Liver cirrhosis was induced by intraperitoneal administration of carbon tetrachloride twice weekly for 16 wk. Control rats were treated with vehicle. The cirrhotic rats developed severe disturbances in water homeostasis: urine production was decreased and hyperosmotic, the rats had significantly decreased plasma sodium concentration and ascites, and the ability to excrete an intravenous water load was significantly impaired. Plasma concentrations of vasopressin and aldosterone were increased. Mean arterial pressure, glomerular filtration rate (GFR), and fractional lithium excretion were decreased. Acute vasopressin type 2-receptor blockade with the selective nonpeptide antagonist OPC-31260 (800 microg. kg(-1). h(-1)) was performed during conditions whereby volume depletion was prevented by computer-driven, servo-controlled intravenous volume replacement with 150 mM glucose. The aquaretic response to OPC-31260 was similar in cirrhotic and control rats. However, the OPC 31260-induced rises in fractional water excretion (delta V/GFR; +24%) and fractional distal water excretion (delta V/C(Li); +46%) were significantly increased in the cirrhotic rats, where V is flow rate and delta is change. This suggests that vasopressin-mediated renal water reabsorption capacity was increased in the cirrhotic rats. Semiquantitative immunoblotting revealed that the expression of the vasopressin-regulated water channel aquaporin-2 was unchanged in membrane fractions of both whole kidney and inner medulla from cirrhotic rats. Together, these results suggest a relative escape from vasopressin on collecting duct water reabsorption in rats with decompensated liver cirrhosis.

Dysregulation of renal aquaporins and Na-Cl cotransporter in CCl4-induced cirrhosis

BACKGROUND

Severe hepatic cirrhosis is associated with abnormal renal water retention.

METHODS

Semiquantitative immunoblotting was employed to investigate the abundance of the major renal aquaporins (water channels) and sodium-dependent cotransporters in kidneys from control rats and rats with cirrhosis secondary to chronic CCl4 inhalation.

RESULTS

The cirrhotic rats had ascites and manifested a water excretion defect detected by a standard water-loading test. The abundance of aquaporin-1 (the major aquaporin in the proximal tubule) was increased, an effect markedly accentuated in high-density membrane fractions prepared by differential centrifugation. Differential centrifugation studies demonstrated a redistribution of aquaporin-2 from high-density to low-density membranes, compatible with increased trafficking of aquaporin-2 to the plasma membrane. The abundance of aquaporin-3, but not aquaporin-2, was increased in collecting ducts of rats with CCl4-induced cirrhosis. The Na-K-2Cl cotransporter of the thick ascending limb showed no change in abundance. However, the abundance of the thiazide-sensitive Na-Cl cotransporter of the distal convoluted tubule was markedly suppressed in cirrhotic rats, possibly contributing to a defect in urinary dilution.

CONCLUSIONS

In this model of cirrhosis, the development of a defect in urinary dilution may be multifactorial, with contributions from at least four abnormalities in transporter regulation: (1) an increase in the renal abundance of aquaporin-1, (2) a cellular redistribution of aquaporin-2 in the collecting duct compatible with trafficking to the plasma membrane without an increase in total cellular aquaporin-2, (3) an increase in the renal abundance of aquaporin-3, and (4) a decrease in the abundance of the thiazide-sensitive cotransporter of the distal convoluted tubule.