PGE2 inhibits water permeability at a post-cAMP site in rat terminal inner medullary collecting duct

A Vasopressin-Induced Change in Prostaglandin Receptor Subtype Expression Explains the Differential Effect of PGE2 on AQP2 Expression

Arginine vasopressin (AVP) stimulates the concentration of renal urine by increasing the principal cell expression of aquaporin-2 (AQP2) water channels. Prostaglandin E₂ (PGE₂) and prostaglandin_2α (PGF_2α) increase the water absorption of the principal cell without AVP, but PGE₂ decreases it in the presence of AVP. The underlying mechanism of this paradoxical response was investigated here. Mouse cortical collecting duct (mkpCCD_c14) cells mimic principal cells as they endogenously express AQP2 in response to AVP. PGE₂ increased AQP2 abundance without desmopressin (dDAVP), while in the presence of dDAVP, PGE₂, and PGF_2α reduced AQP2 abundance. dDAVP increased the cellular PGD₂ and PGE₂ release and decreased the PGF_2α release. MpkCCD cells expressed mRNAs for the receptors of PGE₂ (EP1/EP4), PGF₂ (FP), and TxB₂ (TP). Incubation with dDAVP increased the expression of EP1 and FP but decreased the expression of EP4. In the absence of dDAVP, incubation of mpkCCD cells with an EP4, but not EP1/3, agonist increased AQP2 abundance, and the PGE₂-induced increase in AQP2 was blocked with an EP4 antagonist. Moreover, in the presence of dDAVP, an EP1/3, but not EP4, agonist decreased the AQP2 abundance, and the addition of EP1 antagonists prevented the PGE₂-mediated downregulation of AQP2. Our study shows that in mpkCCD_c14 cells, reduced EP4 receptor and increased EP1/FP receptor expression by dDAVP explains the differential effects of PGE₂ and PGF_2α on AQP2 abundance with or without dDAVP. As the V2R and EP4 receptor, but not the EP1 and FP receptor, can couple to Gs and stimulate the cyclic adenosine monophosphate (cAMP) pathway, our data support a view that cells can desensitize themselves for receptors activating the same pathway and sensitize themselves for receptors of alternative pathways.

Mechanisms of Developmental Toxicity of Dioxins and Related Compounds

Dioxins and related compounds induce morphological abnormalities in developing animals in an aryl hydrocarbon receptor (AhR)-dependent manner. Here we review the studies in which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is used as a prototypical compound to elucidate the pathogenesis of morphological abnormalities. TCDD-induced cleft palate in fetal mice involves a delay in palatogenesis and dissociation of fused palate shelves. TCDD-induced hydronephrosis, once considered to be caused by the anatomical obstruction of the ureter, is now separated into TCDD-induced obstructive and non-obstructive hydronephrosis, which develops during fetal and neonatal periods, respectively. In the latter, a prostaglandin E₂ synthesis pathway and urine concentration system are involved. TCDD-induced abnormal development of prostate involves agenesis of the ventral lobe. A suggested mechanism is that AhR activation in the urogenital sinus mesenchyme by TCDD modulates the wingless-type MMTV integration site family (WNT)/β-catenin signaling cascade to interfere with budding from urogenital sinus epithelium. TCDD exposure to zebrafish embryos induces loss of epicardium progenitor cells and heart malformation. AHR2-dependent downregulation of Sox9b expression in cardiomyocytes is a suggested underlying mechanism. TCDD-induced craniofacial malformation in zebrafish is considered to result from the AHR2-dependent reduction in SRY-box 9b (SOX9b), probably partly via the noncoding RNA slincR, resulting in the underdevelopment of chondrocytes and cartilage.

PGE2 EP1 receptor inhibits vasopressin-dependent water reabsorption and sodium transport in mouse collecting duct

PGE2 regulates glomerular hemodynamics, renin secretion, and tubular transport. This study examined the contribution of PGE2 EP1 receptors to sodium and water homeostasis. Male EP1-/- mice were bred with hypertensive TTRhRen mice (Htn) to evaluate blood pressure and kidney function at 8 weeks of age in four groups: wildtype (WT), EP1-/-, Htn, HtnEP1-/-. Blood pressure and water balance were unaffected by EP1 deletion. COX1 and mPGE2 synthase were increased and COX2 was decreased in mice lacking EP1, with increases in EP3 and reductions in EP2 and EP4 mRNA throughout the nephron. Microdissected proximal tubule sglt1, NHE3, and AQP1 were increased in HtnEP1-/-, but sglt2 was increased in EP1-/- mice. Thick ascending limb NKCC2 was reduced in the cortex but increased in the medulla. Inner medullary collecting duct (IMCD) AQP1 and ENaC were increased, but AVP V2 receptors and urea transporter-1 were reduced in all mice compared to WT. In WT and Htn mice, PGE2 inhibited AVP-water transport and increased calcium in the IMCD, and inhibited sodium transport in cortical collecting ducts, but not in EP1-/- or HtnEP1-/- mice. Amiloride (ENaC) and hydrochlorothiazide (pendrin inhibitor) equally attenuated the effect of PGE2 on sodium transport. Taken together, the data suggest that EP1 regulates renal aquaporins and sodium transporters, attenuates AVP-water transport and inhibits sodium transport in the mouse collecting duct, which is mediated by both ENaC and pendrin-dependent pathways.

Polyuria-associated hydronephrosis induced by xenobiotic chemical exposure in mice

Hydronephrosis is a commonly found disease state characterized by the dilation of renal calices and pelvis, resulting in the loss of kidney function in the severest cases. A generally accepted etiology of hydronephrosis involves the obstruction of urine flow along the urinary tract. In the recent years, we have developed a mouse model of hydronephrosis induced by lactational exposure to dioxin and demonstrated a lack of anatomical obstruction in this model. We also showed that prostaglandin E2 synthesis system plays a critical role in the onset of hydronephrosis. In the present study, we found that neonatal hydronephrosis was not likely to be associated with functional obstruction (impaired peristalsis) but was found to be associated with polyuria and low urine osmolality with the downregulation of proteins involved in the urine concentrating process. The administration of an antidiuretic hormone analog to the dioxin-exposed pups not only suppressed the increased urine output but also decreased the incidence and severity of hydronephrosis. In contrast to the case in pups, administration of dioxin to adult mice failed to induce polyuria and upregulation of prostaglandin E2 synthesis system, and the adult mice were resistant to develop hydronephrosis. These findings suggest the possibility that polyuria could induce hydronephrosis in the absence of anatomical or functional obstruction of the ureter. It is concluded that the present animal model provides a unique example of polyuria-associated type of hydronephrosis, suggesting a need to redefine this disease state.

The vasopressin type 2 receptor and prostaglandin receptors EP2 and EP4 can increase aquaporin-2 plasma membrane targeting through a cAMP-independent pathway

Apical membrane targeting of the collecting duct water channel aquaporin-2 (AQP2) is essential for body water balance. As this event is regulated by Gs coupled 7-transmembrane receptors such as the vasopressin type 2 receptor (V2R) and the prostanoid receptors EP2 and EP4, it is believed to be cAMP dependent. However, on the basis of recent reports, it was hypothesized in the current study that increased cAMP levels are not necessary for AQP2 membrane targeting. The role and dynamics of cAMP signaling in AQP2 membrane targeting in Madin-Darby canine kidney and mouse cortical collecting duct (mpkCCD₁₄) cells was examined using selective agonists against the V2R (dDAVP), EP2 (butaprost), and EP4 (CAY10580). During EP2 stimulation, AQP2 membrane targeting continually increased during 80 min of stimulation; whereas cAMP levels reached a plateau after 10 min. EP4 stimulation caused a rapid and transient increase in AQP2 membrane targeting, but did not significantly increase cAMP levels. After washout of the EP2 agonist or dDAVP, AQP2 membrane abundance remained elevated for at least 80 min, whereas cAMP levels rapidly decreased. Similar effects of the EP2 agonist were also observed for AQP2 constitutively nonphosphorylated at ser-269. The adenylyl cyclase inhibitor SQ22536 did not prevent AQP2 targeting during stimulation of each receptor, nor after dDAVP washout. In conclusion, this study demonstrates that although direct stimulation with cAMP causes AQP2 membrane targeting, cAMP is not necessary for receptor-mediated AQP2 membrane targeting and Gs-coupled receptors can also signal through an alternative pathway that increases AQP2 membrane targeting.

The EP3 receptor regulates water excretion in response to high salt intake

The mechanisms by which prostanoids contribute to the maintenance of whole body water homeostasis are complex and not fully understood. The present study demonstrates that an EP3-dependent feedback mechanism contributes to the regulation of water homeostasis under high-salt conditions. Rats on a normal diet and tap water were placed in metabolic cages and given either sulprostone (20 μg·kg^-1·day^-1) or vehicle for 3 days to activate EP3 receptors in the thick ascending limb (TAL). Treatment was continued for another 3 days in rats given either 1% NaCl in the drinking water or tap water. Sulprostone decreased expression of cyclooxygenase 2 (COX-2) expression by ∼75% in TAL tubules from rats given 1% NaCl concomitant with a ∼60% inhibition of COX-2-dependent PGE₂ levels in the kidney. Urine volume increased after ingestion of 1% NaCl but was reduced ∼40% by sulprostone. In contrast, the highly selective EP3 receptor antagonist L-798106 (100 μg·kg^-1·day^-1), which increased COX-2 expression and renal PGE₂ production, increased urine volume in rats given 1% NaCl. Sulprostone increased expression of aquaporin-2 (AQP2) in the inner medullary collecting duct plasma membrane in association with an increase in phosphorylation at Ser269 and decrease in Ser261 phosphorylation; antagonism of EP3 with L-798106 reduced AQP2 expression. Thus, although acute activation of EP3 by PGE₂ in the TAL and collecting duct inhibits the Na-K-2Cl cotransporter and AQP2 activity, respectively, chronic activation of EP3 in vivo limits the extent of COX-2-derived PGE₂ synthesis, thereby mitigating the inhibitory effects of PGE₂ on these transporters and decreasing urine volume.

PGE2 receptor EP3 inhibits water reabsorption and contributes to polyuria and kidney injury in a streptozotocin-induced mouse model of diabetes

AIMS/HYPOTHESIS

The first clinical manifestation of diabetes is polyuria. The prostaglandin E2 (PGE2) receptor EP3 antagonises arginine vasopressin (AVP)-mediated water reabsorption and its expression is increased in the diabetic kidney. The purpose of this work was to study the contribution of EP3 to diabetic polyuria and renal injury.

METHODS

Male Ep 3 (-/-) (also known as Ptger3 (-/-)) mice were treated with streptozotocin (STZ) to generate a mouse model of diabetes and renal function was evaluated after 12 weeks. Isolated collecting ducts (CDs) were microperfused to study the contribution of EP3 to AVP-mediated fluid reabsorption.

RESULTS

Ep 3 (-/-)-STZ mice exhibited attenuated polyuria and increased urine osmolality compared with wild-type STZ (WT-STZ) mice, suggesting enhanced water reabsorption. Compared with WT-STZ mice, Ep 3 (-/-)-STZ mice also had increased protein expression of aquaporin-1, aquaporin-2, and urea transporter A1, and reduced urinary AVP excretion, but increased medullary V2 receptors. In vitro microperfusion studies indicated that Ep 3 (-/-) and WT-STZ CDs responded to AVP stimulation similarly to those of wild-type mice, with a 60% increase in fluid reabsorption. In WT non-injected and WT-STZ mice, EP3 activation with sulprostone (PGE2 analogue) abrogated AVP-mediated water reabsorption; this effect was absent in mice lacking EP3. A major finding of this work is that Ep 3 (-/-)-STZ mice showed blunted renal cyclooxygenase-2 protein expression, reduced renal hypertrophy, reduced hyperfiltration and reduced albuminuria, as well as diminished tubular dilation and nuclear cysts.

CONCLUSIONS/INTERPRETATION

Taken together, the data suggest that EP3 contributes to diabetic polyuria by inhibiting expression of aquaporins and that it promotes renal injury during diabetes. EP3 may prove to be a promising target for more selective management of diabetic kidney disease.

Physiology and pathophysiology of cyclooxygenase-2 and prostaglandin E2 in the kidney

The cyclooxygenase (COX) enzyme system is the major pathway catalyzing the conversion of arachidonic acid into prostaglandins (PGs). PGs are lipid mediators implicated in a variety of physiological and pathophysiological processes in the kidney, including renal hemodynamics, body water and sodium balance, and the inflammatory injury characteristic in multiple renal diseases. Since the beginning of 1990s, it has been confirmed that COX exists in 2 isoforms, referred to as COX-1 and COX-2. Even though the 2 enzymes are similar in size and structure, COX-1 and COX-2 are regulated by different systems and have different functional roles. This review summarizes the current data on renal expression of the 2 COX isoforms and highlights mainly the role of COX-2 and PGE2 in several physiological and pathophysiological processes in the kidney.

Chronic elevation of IL-1β induces diuresis via a cyclooxygenase 2-mediated mechanism

Chronic renal inflammation is an increasingly recognized phenomenon in multiple disease states, but the impact of specific cytokines on renal function is unclear. Previously, we found that 14-day interleukin-1β (IL-1β) infusion increased urine flow in mice. To determine the mechanism by which this occurs, the current study tested the possible involvement of three classical prodiuretic pathways. Chronic IL-1β infusion significantly increased urine flow (6.5 ± 1 ml/day at day 14 vs. 2.3 ± 0.3 ml/day in vehicle group; P < 0.05) and expression of cyclooxygenase (COX)-2, all three nitric oxide synthase (NOS) isoforms, and endothelin (ET)-1 in the kidney (P < 0.05 in all cases). Urinary prostaglandin E metabolite (PGEM) excretion was also significantly increased at day 14 of IL-1β infusion (1.21 ± 0.26 vs. 0.29 ± 0.06 ng/day in vehicle-infused mice; P = 0.001). The selective COX-2 inhibitor celecoxib markedly attenuated urinary PGEM excretion and abolished the diuretic response to chronic IL-1β infusion. In contrast, deletion of NOS3, or inhibition of NOS1 with L-VNIO, did not blunt the diuretic effect of IL-1β, nor did pharmacological blockade of endothelin ETA and ETB receptors with A-182086. Consistent with a primary effect on water transport, IL-1β infusion markedly reduced inner medullary aquaporin-2 expression (P < 0.05) and did not alter urinary Na⁺ or K⁺ excretion. These data indicate a critical role for COX-2 in mediating the effects of chronic IL-1β elevation on the kidney.

Lithium reduces aquaporin-2 transcription independent of prostaglandins

Vasopressin (AVP)-stimulated translocation and transcription of aquaporin-2 (AQP2) water channels in renal principal cells is essential for urine concentration. Twenty percent of patients treated with lithium develop nephrogenic diabetes insipidus (NDI), a disorder in which the kidney is unable to concentrate urine. In vivo and in mouse collecting duct (mpkCCD) cells, lithium treatment coincides with decreased AQP2 abundance and inactivation of glycogen synthase kinase (Gsk) 3β. This is paralleled in vivo by an increased renal cyclooxygenase 2 (COX-2) expression and urinary prostaglandin PGE(2) excretion. PGE(2) reduces AVP-stimulated water reabsorption, but its precise role in lithium-induced downregulation of AQP2 is unclear. Using mpkCCD cells, we here investigated whether prostaglandins contribute to lithium-induced downregulation of AQP2. In these cells, lithium application reduced AQP2 abundance, which coincided with Gsk3β inactivation and increased COX-2 expression. Inhibition of COX by indomethacin, leading to reduced PGE(2) and PGF(2α) levels, or dexamethasone-induced downregulation of COX-2 both increased AQP2 abundance, while PGE(2) addition reduced AQP2 abundance. However, lithium did not change the prostaglandin levels, and indomethacin and dexamethasone did not prevent lithium-induced AQP2 downregulation. Further analysis revealed that lithium decreased AQP2 protein abundance, mRNA levels and transcription, while PGE(2) reduced AQP2 abundance by increasing its lysosomal degradation, but not by reducing AQP2 gene transcription. In conclusion, our data reveal that in mpkCCD cells, prostaglandins decrease AQP2 protein stability by increasing its lysosomal degradation, indicating that in vivo paracrine-produced prostaglandins might have a role in lithium-induced NDI via this mechanism. However, lithium affects also AQP2 gene transcription, which is prostaglandin independent.

Hypotonicity-induced reduction of aquaporin-2 transcription in mpkCCD cells is independent of the tonicity responsive element, vasopressin, and cAMP

The syndrome of inappropriate antidiuretic hormone secretion is characterized by excessive water uptake and hyponatremia. The extent of hyponatremia, however, is less than anticipated, which is ascribed to a defense mechanism, the vasopressin-escape, and is suggested to involve a tonicity-determined down-regulation of the water channel aquaporin-2 (AQP2). The underlying mechanism, however, is poorly understood. To study this, we used the mouse cortical collecting duct (mpkCCD) cell line. MpkCCD cells, transfected with an AQP2-promoter luciferase construct showed a reduced and increased AQP2 abundance and transcription following culture in hypotonic and hypertonic medium, respectively. This depended on tonicity rather than osmolality and occurred independently of the vasopressin analog dDAVP, cAMP levels, or protein kinase A activity. Although prostaglandins and nitric oxide reduced AQP2 abundance, inhibition of their synthesis did not influence tonicity-induced AQP2 transcription. Also, cells in which the cAMP or tonicity-responsive element (CRE/TonE) in the AQP2-promoter were mutated showed a similar response to hypotonicity. Instead, the tonicity-responsive elements were pin-pointed to nucleotides -283 to -252 and -157 to -126 bp. In conclusion, our data indicate that hypotonicity reduces AQP2 abundance and transcription, which occurs independently of vasopressin, cAMP, and the known TonE and CRE in the AQP2-promoter. Increased prostaglandin and nitric oxide, as found in vivo, may contribute to reduced AQP2 in vasopressin-escape, but do not mediate the effect of hypotonicity on AQP2 transcription. Our data suggest that two novel segments (-283 to -252 and -157 to -126 bp) in the AQP2-promoter mediate the hypotonicity-induced AQP2 down-regulation during vasopressin-escape.

alphaENaC-mediated lithium absorption promotes nephrogenic diabetes insipidus

Lithium-induced nephrogenic diabetes insipidus (NDI) is accompanied by polyuria, downregulation of aquaporin 2 (AQP2), and cellular remodeling of the collecting duct (CD). The amiloride-sensitive epithelial sodium channel (ENaC) is a likely candidate for lithium entry. Here, we subjected transgenic mice lacking αENaC specifically in the CD (knockout [KO] mice) and littermate controls to chronic lithium treatment. In contrast to control mice, KO mice did not markedly increase their water intake. Furthermore, KO mice did not demonstrate the polyuria and reduction in urine osmolality induced by lithium treatment in the control mice. Lithium treatment reduced AQP2 protein levels in the cortex/outer medulla and inner medulla (IM) of control mice but only partially reduced AQP2 levels in the IM of KO mice. Furthermore, lithium induced expression of H(+)-ATPase in the IM of control mice but not KO mice. In conclusion, the absence of functional ENaC in the CD protects mice from lithium-induced NDI. These data support the hypothesis that ENaC-mediated lithium entry into the CD principal cells contributes to the pathogenesis of lithium-induced NDI.

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.

Potential involvement of P2Y2 receptor in diuresis of postobstructive uropathy in rats

AVP resistance of the medullary collecting duct (mCD) in postobstructive uropathy (POU) has been attributed to increased production of PGE2. P2Y2 receptor activation causes production of PGE2 by the mCD. We hypothesize that increased P2Y2 receptor expression and/or activity may contribute to the diuresis of POU. Sprague-Dawley rats were subjected to bilateral ureteral obstruction for 24 h followed by release (BUO/R, n = 17) or sham operation (SHM/O, n = 15) and euthanized after 1 wk or 12 days. BUO/R rats developed significant polydipsia, polyuria, urinary concentration defect, and increased urinary PGE2 and decreased aquaporin-2 protein abundance in the inner medulla compared with SHM/O rats. After BUO/R, the relative mRNA expression of P2Y2 and P2Y6 receptors was increased by 2.7- and 4.9-fold, respectively, without significant changes in mRNA expression of P2Y1 or P2Y4 receptor. This was associated with a significant 3.5-fold higher protein abundance of the P2Y2 receptor in BUO/R than SHM/O rats. When freshly isolated mCD fractions were challenged with different types of nucleotides (ATPgammaS, ADP, UTP, or UDP), BUO/R and SHM/O rats responded to only ATPgammaS and UTP and released PGE2, consistent with involvement of the P2Y2, but not P2Y6, receptor. ATPgammaS- or UTP-stimulated increases in PGE2 were much higher in BUO/R (3.20- and 2.28-fold, respectively, vs. vehicle controls) than SHM/O (1.68- and 1.30-fold, respectively, vs. vehicle controls) rats. In addition, there were significant 2.4- and 2.1-fold increases in relative mRNA expression of prostanoid EP1 and EP3 receptors, respectively, in the inner medulla of BUO/R vs. SHM/O rats. Taken together, these data suggest that increased production of PGE2 by the mCD in POU may be due to increased expression and activity of the P2Y2 receptor. Increased mRNA expression of EP1 and EP3 receptors in POU may also help accentuate PGE2-induced signaling in the mCD.

Nucleotides downregulate aquaporin 2 via activation of apical P2 receptors

Vasopressin regulates water reabsorption in the collecting duct, but extracellular nucleotides modulate this regulation through incompletely understood mechanisms. We investigated these mechanisms using immortalized mouse collecting duct (mpkCCD) cells. Basolateral exposure to dDAVP induced AQP2 localization to the apical membrane, but co-treatment with ATP internalized AQP2. Because plasma membrane-bound P2 receptors (P2R) mediate the effects of extracellular nucleotides, we examined the abundance and localization of P2R in mpkCCD cells. In the absence of dDAVP, P2Y(1) and P2Y(4) receptors localized to the apical membrane, whereas P2X(2), P2X(4), P2X(5), P2X(7), P2Y(2), P2Y(11), and P2Y(12) receptors localized to the cytoplasm. dDAVP induced gene expression of P2X(1), which localized to the apical domain, and led to translocation of P2X(2) and P2Y(2) to the apical and basolateral membranes, respectively. In co-expression experiments, P2R activation decreased membrane AQP2 and AQP2-mediated water permeability in Xenopus oocytes expressing P2X(2), P2Y(2,) or P2Y(4) receptors, but not in oocytes expressing other P2R subtypes. In summary, these data suggest that AQP2-mediated water transport is downregulated not only by basolateral nucleotides, mediated by P2Y(2) receptors, but also by luminal nucleotides, mediated by P2X(2) and/or P2Y(4) receptors.

P2Y(2) receptors and water transport in the kidney

The kidneys play a critical role in the maintenance of water homeostasis. This is achieved by the inherent architecture of the nephron along with the expression of various membrane transporters and channels that are responsible for the vectorial transport of salt and water. The collecting duct has become a focus of attention by virtue of its ability to transport water independent of solutes (free-water transport), and its apparent involvement in various water balance disorders. It was originally believed that the water transport capability of the collecting duct was solely under the influence of the circulating hormone, arginine vasopressin (AVP). However, during the past decade, locally produced autocrine and/or paracrine factors have emerged as potent modulators of transport of water by the collecting duct. Recently, much attention has been focused on the purinergic regulation of renal water transport. This review focuses on the role of the P2Y(2) receptor, the predominant purinergic receptor expressed in the collecting duct, in the modulation of water transport in physiological and pathophysiological conditions, and its therapeutic potential as a drug target to treat water balance disorders in the clinic. Studies carried out by us and other investigators are unravelling potent interactions among AVP, prostanoid and purinergic systems in the medullary collecting duct, and the perturbations of these interactions in water balance disorders such as acquired nephrogenic diabetes insipidus. Future studies should address the potential therapeutic benefits of modulators of P2Y(2) receptor signalling in water balance disorders, which are extremely prevalent in hospitalised patients irrespective of the underlying pathology.

Potential role of purinergic signaling in lithium-induced nephrogenic diabetes insipidus

Lithium (Li)-induced nephrogenic diabetes insipidus (NDI) has been attributed to the increased production of renal prostaglandin (PG)E(2). Previously we reported that extracellular nucleotides (ATP/UTP), acting through P(2y2) receptor in rat medullary collecting duct (mCD), produce and release PGE(2). Hence we hypothesized that increased production of PGE(2) in Li-induced NDI may be mediated by enhanced purinergic signaling in the mCD. Sprague-Dawley rats were fed either control or Li-added diet for 14 or 21 days. Li feeding resulted in marked polyuria and polydipsia associated with a decrease in aquaporin (AQP)2 protein abundance in inner medulla ( approximately 20% of controls) and a twofold increase in urinary PGE(2). When acutely challenged ex vivo with adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), UTP, or ADP, mCD of Li-fed rats showed significantly higher increases (50-130% over control diet-fed rats) in PGE(2) production, indicating that more than one subtype of P(2y) receptor is involved. This was associated with a 3.4-fold increase in P(2y4), but not P(2y2), receptor mRNA expression in the inner medulla of Li-fed rats compared with control diet-fed rats. Confocal laser immunofluorescence microscopy revealed predominant localization of both P(2y2) and P(2y4) receptors in the mCD of control or Li diet-fed rats. Together, these data indicate that in Li-induced NDI 1) purinergic signaling in the mCD is sensitized with increased production of PGE(2) and 2) P(2y2) and/or P(2y4) receptors may be involved in the enhanced purinergic signaling. Our study also reveals the potential beneficial effects of P(2y) receptor antagonists in the treatment and/or prevention of Li-induced NDI.

Potential role of purinergic signaling in urinary concentration in inner medulla: insights from P2Y2 receptor gene knockout mice

Osmotic reabsorption of water through aquaporin-2 (AQP2) in the inner medulla is largely dependent on the urea concentration gradients generated by urea transporter (UT) isoforms. Vasopressin (AVP) increases expression of both AQP2 and UT-A isoforms. Activation of the P2Y2 receptor (P2Y2-R) in the medullary collecting duct inhibits AVP-induced water flow. To gain further insights into the overarching effect of purinergic signaling on urinary concentration, we compared the protein abundances of AQP2 and UT-A isoforms between P2Y2-R knockout (KO) and wild-type (WT) mice under basal conditions and following AVP administration. Under basal conditions (a gel diet for 10 days), KO mice concentrated urine to a significantly higher degree, with 1.8-, 1.66-, and 1.29-fold higher protein abundances of AQP2, UT-A1, and UT-A2, respectively, compared with WT, despite comparable circulating AVP levels in both groups. Infusion of 1-desamino-8-d-arginine vasopressin (dDAVP; desmopressin; 1 ng/h sc) for 5 days resulted in 2.14-, 2.6-, and 2.22-fold higher protein abundances of AQP2, AQP3, and UT-A1, respectively, in the inner medullas of KO mice compared with WT mice. In response to acute (45 min) stimulation by AVP (0.2 unit/mouse sc), UT-A1 protein increased by 1.39- and 1.54-fold in WT and KO mice, respectively. These data suggest that genetic deletion of P2Y2-R results in increased abundances of key proteins involved in urinary concentration in the inner medulla, both under basal conditions and following AVP administration. Thus purinergic regulation may play a potential overarching role in balancing the effect of AVP on the urinary concentration mechanism.

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.

Treating lithium-induced nephrogenic diabetes insipidus with a COX-2 inhibitor improves polyuria via upregulation of AQP2 and NKCC2

Prostaglandin E(2) may antagonize vasopressin-stimulated salt absorption in the thick ascending limb and water absorption in the collecting duct. Blockade of prostaglandin E(2) synthesis by nonsteroidal anti-inflammatory drugs (NSAIDs) enhances urinary concentration, and these agents have antidiuretic effects in patients with nephrogenic diabetes insipidus (NDI) of different etiologies. Because renal prostaglandins are derived largely from cyclooxygenase-2 (COX-2), we hypothesized that treatment of NDI with a COX-2 inhibitor may relieve polyuria through increased expression of Na-K-2Cl cotransporter type 2 (NKCC2) in the thick ascending limb and aquaporin-2 (AQP2) in the collecting duct. To test this hypothesis, semiquantitative immunoblotting and immunohistochemistry were carried out from the kidneys of lithium-induced NDI rats with and without COX-2 inhibition. After male Sprague-Dawley rats were fed an LiCl-containing rat diet for 3 wk, the rats were randomly divided into control and experimental groups. The COX-2 inhibitor DFU (40 mg.kg(-1).day(-1)) was orally administered to the experimental rats for an additional week. Treatment with the COX-2 inhibitor significantly relieved polyuria and raised urine osmolality. Semiquantitative immunoblotting using whole-kidney homogenates revealed that COX-2 inhibition caused significant increases in the abundance of AQP2 and NKCC2. Immunohistochemistry for AQP2 and NKCC2 confirmed the effects of COX-2 inhibition in lithium-induced NDI rats. The upregulation of AQP2 and NKCC2 in response to the COX-2 inhibitor may underlie the therapeutic mechanisms by which NSAIDs enhance antidiuresis in patients with NDI.

Dysregulation of Renal Cyclooxygenase-2 in Rats with Lithium-induced Nephrogenic Diabetes Insipidus

This study aimed to examine whether the expression of major prostaglandin E₂ (PGE₂) synthesis enzyme, cyclooxygenase-2 (COX-2), is changed in the kidneys of the rats with lithium-induced nephrogenic diabetes insipidus (Li-NDI). Sprague-Dawley rats treated with lithium for 4 weeks were used as the NDI model and expression of renal COX-2 was determined by immunoblotting and immunohistochemistry. In Li-NDI where urine output was markedly increased and urine osmolality was significantly decreased, COX-2 expression in the inner medulla was decreased (28% of control), while it increased 18-fold in the cortex and outer medulla. Consistent with this, labeling intensity of COX-2 in macula densa region was increased, whereas it was decreased in the interstitial cells in the inner medulla, indicating a differential regulation of COX-2 between the cortex and inner medulla in Li-NDI. Accordingly, urinary PGE₂ excretion was significantly increased in Li-NDI. In conclusion, there is a differential regulation of COX-2 between cortex and inner medulla in Li-NDI and urinary PGE₂ excretion is increased in Li-NDI, possibly due to an increased renal production. This may suggest that increased renal production of PGE₂ could play a role in modulating water reabsorption in the renal collecting duct in Li-NDI.

P2 receptors in the regulation of renal transport mechanisms

Extracellular nucleotides (e.g., ATP) regulate physiological and pathophysiological processes through activation of nucleotide P2 receptors in the plasma membrane. Examples include such diverse processes as communication from taste buds to gustatory nerves, platelet aggregation, nociception, or neutrophil chemotaxis. Over approximately the last 15 years, evidence has also accumulated that cells in renal epithelia release nucleotides in response to physiological stimuli and that these nucleotides act in a paracrine and autocrine way to activate P2 receptors and play a significant role in the regulation of transport mechanisms and cell volume regulation. This review discusses potential stimuli and mechanisms involved in nucleotide release in renal epithelia and summarizes the available data on the expression and function of nucleotide P2 receptors along the native mammalian tubular and collecting duct system. Using established agonist profiles for P2 receptor subtypes, significant insights have been gained particularly into a potential role for P2Y(2)-like receptors in the regulation of transport mechanisms in the collecting duct. Due to the lack of receptor subtype-specific antagonists, however, the in vivo relevance of P2 receptor subtypes is unclear. Studies in gene knockout mice provided first insights including an antihypertensive activity of P2Y(2) receptors that is linked to an inhibitory influence on renal Na(+) and water reabsorption. We are only beginning to unravel the important roles of extracellular nucleotides and P2 receptors in the regulation of the diverse transport mechanisms of the kidney.

Prostaglandin E2 inhibits vasotocin-induced osmotic water permeability in the frog urinary bladder by EP1-receptor-mediated activation of NO/cGMP pathway

PGE(2) is a well-known inhibitor of the antidiuretic hormone-induced increase of osmotic water permeability (OWP) in different osmoregulatory epithelia; however, the mechanisms underlying this effect of PGE(2) are not completely understood. Here, we report that, in the frog Rana temporaria urinary bladder, EP(1)-receptor-mediated inhibition of arginine-vasotocin (AVT)-induced OWP by PGE(2) is attributed to increased generation of nitric oxide (NO) in epithelial cells. It was shown that the inhibitory effect of 17-phenyl-trinor-PGE(2) (17-ph-PGE(2)), an EP(1) agonist, on AVT-induced OWP was significantly reduced in the presence of 7-nitroindazole (7-NI), a neuronal NO synthase (nNOS) inhibitor. NO synthase (NOS) activity in both lysed and intact epithelial cells measured as a rate of conversion of l-[(3)H]arginine to l-[(3)H]citrulline was Ca(2+) dependent and inhibited by 7-NI. PGE(2) and 17-ph-PGE(2), but not M&B-28767 (EP(3) agonist) or butaprost (EP(2) agonist), stimulated NOS activity in epithelial cells. The above effect of PGE(2) was abolished in the presence of SC-19220, an EP(1) antagonist. 7-NI reduced the stimulatory effect of 17-ph-PGE(2) on NOS activity. 17-ph-PGE(2) increased intracellular Ca(2+) concentration and cGMP in epithelial cells. Western blot analysis revealed an nNOS expression in epithelial cells. These results show that the inhibitory effect of PGE(2) on AVT-induced OWP in the frog urinary bladder is based at least partly on EP(1)-receptor-mediated activation of the NO/cGMP pathway, suggesting a novel cross talk between AVT, PGE(2), and nNOS that may be important in the regulation of water transport.

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

The renin-angiotensin system is well known to be involved in the pathophysiological changes in renal function after obstruction of the ureter. Previously, we demonstrated that bilateral ureteral obstruction (BUO) is associated with dramatic changes in the expression of both renal sodium transporters and aquaporin water channels (AQPs). We now examined the effects of the AT1-receptor antagonist candesartan on the dysregulation of AQPs and key renal sodium transporters in rats subjected to 24-h BUO and followed 2 days after release of BUO (BUO-2R). Consistent with previous observations, BUO-2R resulted in a significantly decreased expression of AQP1, -2, and -3 compared with control rats. Concomitantly, the rats developed polyuria and reduced urine osmolality. Moreover, expression of the type 2 Na-phosphate cotransporter (NaPi-2) and type 1 bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) was markedly reduced, consistent with postobstructive natriuresis. Candesartan treatment from the onset of obstruction attenuated the reduction in GFR (3.1 ± 0.4 vs. 1.7 ± 0.3 ml·min−1·kg−1) and partially prevented the reduction in the expression of AQP2 (66 ± 21 vs. 13 ± 2%, n = 7; P < 0.05), NaPi-2 (84 ± 6 vs. 57 ± 10%, n = 7; P < 0.05), and NKCC2 (89 ± 12 vs. 46% ± 11, n = 7; P < 0.05). Consistent with this, candesartan treatment attenuated the increase in urine output (58 ± 4 vs. 97 ± 5 μl·min−1·kg−1, n = 7; P < 0.01) and the reduction in sodium reabsorption (433 ± 62 vs. 233 ± 45 μmol·min−1·kg−1, n = 7; P < 0.05) normally found in rats subjected to BUO. Moreover, candesartan treatment attenuated induction of cyclooxygenase 2 (COX-2) expression in the inner medulla, suggesting that COX-2 induction in response to obstruction is regulated by ANG II. In conclusion, candesartan prevents dysregulation of AQP2, sodium transporters, and development of polyuria seen in BUO. This strongly supports the view that candesartan protects kidney function in response to urinary tract obstruction.

Decreased expression of aquaporin water channels in denervated rat kidney

AIMS

A neural mechanism regulating aquaporin (AQP) water channels in the kidney was investigated.

METHODS

Male Sprague-Dawley rats were used. Renal denervation was induced by painting the renal vessels with 10% phenol. The expression of AQP1-4 proteins was determined in the denervated and contralateral kidneys. The expression was also examined in rats which were renally denervated and subjected to water restriction or deoxycorticosterone acetate (DOCA)-salt treatment.

RESULTS

Following the unilateral denervation, tissue contents of norepinephrine were significantly decreased in the denervated kidney, while increased in the contralateral kidney. Accordingly, the expression of AQP1-4 proteins was decreased by 15-40% in the denervated kidney, and increased by 30-50% in the contralateral kidney. Immunohistochemistry of AQP2 confirmed its decreases in the denervated kidney and increases in the contralateral kidney. In bilaterally denervated rats, the urine flow increased along with decreased osmolarity. The water restriction increased the expression of AQP channels, however, the magnitude of which was lower in the denervated than in the contralateral kidney. Renal denervation decreased the degree of DOCA-salt hypertension, along with lower expression of AQP channels.

CONCLUSION

It is suggested that the sympathetic nerve should play a specific excitatory role in the regulation of AQP channels in the kidney.

Aldosterone increases urine production and decreases apical AQP2 expression in rats with diabetes insipidus

Vasopressin and aldosterone are essential hormones in the regulation of water and sodium balance. Aldosterone regulates sodium reabsorption, although synergistic effects on collecting duct water permeability have been shown. We investigated the effects of 7-day aldosterone infusion or oral spironolactone treatment on water balance and aquaporin (AQP) 2 expression in rats with 21 days of lithium-induced nephrogenic diabetes insipidus (Li-NDI). In rats with Li-NDI, aldosterone markedly increased (271 +/- 14 ml/24 h), whereas spironolactone decreased (74 +/- 11 ml/24 h) urine production compared with rats treated with lithium only (120 +/- 11 ml/24 h). Aldosterone increased free-water clearance and creatinine clearance, whereas spironolactone caused a decreased creatinine clearance but unchanged free-water clearance. Immunoblotting showed unchanged AQP2 expression in cortex/outer stripe of the outer medulla and inner medulla. In the inner stripe of the outer medulla aldosterone caused a decreased AQP2 expression, whereas spironolactone caused an increase compared with rats treated with lithium only. Semiquantitative confocal immunofluorescence microscopy of AQP2 immunolabeling showed reduced AQP2 expression in the apical plasma membrane domain in connecting tubule (CNT) and initial cortical collecting ducts (iCCD) in response to aldosterone-treated rats compared with rats treated with lithium only. Spironolactone significantly increased apical AQP2 expression in the iCCD compared with rats treated with lithium only. We also tested whether similar changes could be observed in vasopressin-deficient BB rats and found similar changes in urine production and subcellular AQP2 expression in the CNT and iCCD in response to aldosterone and spironolactone. This study shows that aldosterone treatment perturbs diabetes insipidus and is associated with AQP2 redistribution in CNT and iCCD likely mediated by the spironolactone-sensitive mineralocorticoid receptor.

COX-2 inhibition prevents downregulation of key renal water and sodium transport proteins in response to bilateral ureteral obstruction

Bilateral ureteral obstruction (BUO) is associated with marked changes in the expression of renal aquaporins (AQPs) and sodium transport proteins. To examine the role of prostaglandin in this response, we investigated whether 24-h BUO changed the expression of cyclooxygenases (COX-1 and -2) in the kidney and tested the effect of the selective COX-2 inhibitor parecoxib (5 mg·kg−1·day−1via osmotic minipumps) on AQPs and sodium transport. Sham and BUO kidneys were analyzed by semiquantitative immunoblotting, and a subset of kidneys was perfusion fixed for immunocytochemistry. BUO caused a significant 14-fold induction of inner medullary COX-2 (14.40 ± 1.8 vs. 1.0 ± 0.4, n = 6; P < 0.0001) and a reduction in medullary tissue osmolality, whereas COX-1 did not change. Immunohistochemistry confirmed increased COX-2 labeling associated with medullary interstitial cells. COX isoforms did not change in cortex/outer medulla after 24-h BUO. In BUO kidneys, inner medullary AQP2 expression was reduced, and this decrease was prevented by parecoxib. In the inner stripe of outer medulla, the type 3 Na+/H+exchanger (NHE3) and apical Na+-K+-2Cl−cotransporter (BSC-1) were significantly reduced by BUO, and this decrease was significantly attenuated by parecoxib. Immunohistochemistry for AQP2, NHE3, and BSC-1 confirmed the effect of parecoxib. Parecoxib had no significant effect on the Na-K-ATPase α1-subunit, type II Na-Picotransporter, or AQP3. In conclusion, acute BUO leads to marked upregulation of COX-2 in inner medulla and selective COX-2 inhibition prevents dysregulation of AQP2, BSC-1, and NHE3 in response to BUO. These data indicate that COX-2 may be an important factor contributing to the impaired renal water and sodium handling in response to BUO.

Chronic dDAVP infusion in rats decreases the expression of P2Y2 receptor in inner medulla and P2Y2 receptor-mediated PGE2 release by IMCD

Activation of P2Y2 receptor (P2Y2-R) in inner medullary collecting duct (IMCD) of rat decreases AVP-induced water flow and releases PGE(2). We observed that dehydration of rats decreases the expression of P2Y2 receptor in inner medulla (IM) and P2Y2-R-mediated PGE(2) release by IMCD. Because circulating vasopressin (AVP) levels are increased in dehydrated condition, we examined whether chronic infusion of desmopressin (dDAVP) has a similar effect on the expression and activity of P2Y2-R. Groups of rats were infused with saline or dDAVP (5 or 20 ng/h sc, 5 or 6 days) via osmotic minipumps and euthanized. Urine volume, osmolality, and PGE(2) metabolite content were determined. AQP2- and P2Y2- and V2-R mRNA and/or protein in IM were quantified by real-time RT-PCR and immunoblotting, respectively. P2Y2-R-mediated PGE(2) release by freshly prepared IMCD was assayed using ATPgammaS as a ligand. Chronic dDAVP infusion resulted in low-output of concentrated urine and significantly increased the AQP2 protein abundance in IM. On the contrary, dDAVP infusion at 5 or 20 ng/h significantly decreased P2Y2-R protein abundance (approximately 40% of saline-treated group). In parallel, the relative expression of P2Y2-R vs. AQP2- or V2-R mRNA was significantly decreased. Furthermore, the P2Y2-R-mediated PGE(2) release by IMCD was significantly decreased in rats infused 20 ng/h but not 5 ng/h of dDAVP. Urinary PGE(2) metabolite excretion, however, did not change with dDAVP infusion. In conclusion, chronic dDAVP infusion decreases the expression and activity of P2Y2-R in IM. This may be due to a direct effect of dDAVP or dDAVP-induced increase in medullary tonicity.

Altered expression of COX-1, COX-2, and mPGES in rats with nephrogenic and central diabetes insipidus

Prostaglandins have an important role in renal salt and water reabsorption. PGE2is the main kidney prostaglandin and is thought to be mainly produced in the kidney inner medulla (IM). There are indications that PGE2synthesis in nephrogenic (NDI) and central (CDI) diabetes insipidus is altered. We hypothesize that the expression of the major PGE2synthesis enzymes cyclooxygenases 1 and 2 (COX-1, COX-2) and membrane-associated PGE2synthase (mPGES) is altered in the kidneys of rats with NDI and CDI. Wistar rats treated with lithium for 4 wk were used as the NDI model. One-half of the NDI model rats were additionally dehydrated for 48 h. Brattleboro (BB) rats that lack endogenous antidiuretic hormone were used as the CDI model. Expression and localization of COX-1, COX-2, and mPGES in IM, inner stripe of outer medulla (ISOM), and cortex were determined by immunoblotting and immunohistochemistry. In lithium-induced NDI, expression of COX-1, COX-2, and mPGES was markedly decreased in IM. In ISOM and cortex, COX-1 expression was marginally reduced and mPGES expression was unaltered. COX-2 expression was undetected in ISOM and marginally increased in cortex. Consistent with this, the density of COX-2-expressing cells in macula densa was significantly increased, indicating differential regulation of COX-2 in IM and cortex. Dehydration of NDI rats resulted in a marked increase in COX-2 immunolabeling in IM interstitial cells, and there was no significant change in COX-1 and mPGES expression in any kidney zone. Treatment of DDAVP in BB rats for 6 days resulted in a markedly increased expression of COX-1, COX-2, and mPGES in IM. In the cortex, there were no changes in the expression of COX-1 and mPGES, whereas COX-2 expression was decreased. These results identify markedly reduced expression of COX-1, COX-2, and mPGES in IM in lithium-induced NDI. Furthermore, there were major changes in the expression of COX-1, COX-2, and mPGES in rats with CDI.

P2Y2 receptor-mediated release of prostaglandin E2 by IMCD is altered in hydrated and dehydrated rats: relevance to AVP-independent regulation of IMCD function

Circulating vasopressin levels change in hydrated and dehydrated conditions and thus control osmotic water permeability (P(f)) of the inner medullary collecting duct (IMCD). Prostaglandin E2 (PGE2) antagonizes vasopressin-induced P(f) of IMCD. Previously, we showed that activation of P2Y2 receptor (P2Y2-R) in IMCD results in production and release of PGE2, and P2Y2-R mRNA and protein are significantly elevated in inner medullas of hydrated rats compared with dehydrated rats. Therefore, we examined whether the altered expression of P2Y2-R in hydrated and dehydrated states is associated with corresponding changes in P2Y2-R-mediated PGE2 release by the IMCD. Rats were hydrated by providing sucrose water as the sole drinking fluid or dehydrated by water deprivation for 2 days. This resulted in high output-low osmolality and low output-high osmolality urines in hydrated and dehydrated rats, respectively. In hydrated rats, there was a significant increase in tubular fluid PGE2, measured indirectly by assessing the urinary PGE2 metabolite. Stimulation of freshly isolated IMCD preparations in vitro with P2Y2-R agonist (ATPgammaS) showed a marked increase in the release of PGE2 in hydrated rats compared with normal rats. These responses were blunted in the IMCD prepared from dehydrated rats. The P2Y2-R-mediated PGE2 release in the IMCD of hydrated rats was mediated largely by cyclooxygenase (COX)-1 as COX-1-specific inhibitor valeroyl salicylate completely blocked the release. The COX-2-specific inhibitor N5398 had only a modest and insignificant inhibitory effect. In conclusion, the increased sensitivity of purinergic-prostanoid interaction seen in the IMCD of hydrated rats may represent a novel vasopressin-independent regulatory mechanism of IMCD function.

P2Y2 receptor mRNA and protein expression is altered in inner medullas of hydrated and dehydrated rats: relevance to AVP-independent regulation of IMCD function

Arginine vasopressin (AVP), acting through a cAMP second messenger system, regulates osmotic water permeability (Pf) of the collecting duct. In the collecting duct, the activities of cAMP and phosphonositides (PI) are mutually inhibitory. The P2Y2 receptor (P2Y2-R) is a G protein-coupled extracellular nucleotide receptor associated with PI signaling pathway. Previously, we showed that P2Y2-R is expressed in inner medullary collecting duct (IMCD) of rat, and its agonist (ATP/UTP) activation decreased AVP-induced Pf and resulted in enhanced production of prostaglandin E2. Hydrated and dehydrated states are associated with alterations in the circulating levels of AVP, expression and/or subcellular distribution of AVP-regulated aquaporin-2 water channel in IMCD and thus Pf of IMCD. We hypothesized that altered expression and/or signaling via P2Y2-R may also modulate IMCD function in these conditions. Sprague-Dawley rats were subjected to dehydration by water deprivation (48 h) or hydration (48 or 96 h) by providing sucrose water. Hydration or dehydration resulted in marked alterations in mRNA expression (Northern blot analysis and real-time RT-PCR) and protein abundance (Western blot analysis) of P2Y2-R, with hydrated rats showing significantly higher levels compared with dehydrated rats. Sequential hydration and dehydration experiments also revealed that the regulated expression profiles of P2Y2-R mRNA and protein are discordant. Conversely, the expression of V2-R mRNA remained unaltered during hydration and dehydration. Because virtually all renal cells release ATP in a regulated fashion, the observed alterations in P2Y2-R expression in the inner medulla in hydrated and dehydrated states may constitute a novel mechanism of purinergic modulation of IMCD function.

Expression of NTPDase1 and NTPDase2 in murine kidney: relevance to regulation of P2 receptor signaling

The regulation of renal function by extracellular nucleotides encompasses alterations in glomerular hemodynamics, microvascular function, tubuloglomerular feedback, tubular transport, cell growth or apoptosis, and transport of water and solutes in the medullary collecting duct. Nearly all cells can release ATP or other nucleotides that are then rapidly hydrolyzed in the extracellular milieu. However, little information is available on the cellular expression of ectoenzymes that hydrolyze extracellular nucleotides within the kidney. Nucleoside triphosphate diphosphohydrolases (NTPDases) are plasma membrane-bound ectonucleotidases. NTPDase1 has identity with CD39, a B lymphocyte activation marker, and hydrolyzes extracellular ATP and ADP to AMP within the vasculature, whereas NTPDase2/CD39L(ike)1 preferentially converts ATP to ADP outside of blood vessels. Using immunohistochemical and in situ hybridization approaches, we localized the protein and mRNA of NTPDase1 and 2 in murine renal tissues. In the renal cortex, NTPDase1 is expressed by vascular smooth muscle cells and endothelium in interlobular arteries, afferent glomerular arterioles, and peritubular capillaries. In the inner medulla, NTPDase1 is expressed in ascending thin limbs of Henle's loop, ducts of Bellini, and in the pelvic wall. In contrast, NTPDase2 is expressed in Bowman's capsule, glomerular arterioles, adventitia of blood vessels, and pelvic wall. Thus the distribution patterns of NTPDases have parallels to the known distribution of P2 receptors within the kidney. NTPDases may modulate regulatory effects of ATP and degradation products within the vasculature and other sites and thereby potentially influence physiological as well as multiple pathological events in the kidney.

Development of water transport in the collecting duct

The ability of the immature kidney to concentrate urine is lower than in adults. This can lead to severe water and electrolyte disorders, especially in premature babies. Resistance to AVP and lower tonicity of the medullary interstitium seem to be the major factors limiting urine concentration in newborns. AVP-stimulated cAMP generation is impaired. This is the result of inhibition of the production by PGE(2) acting through EP3 receptors and increased degradation by phosphodiesterase IV. The expression of aquaporin-2 (AQP2) in the immature kidney is low; however, under conditions of water deprivation and after stimulation with DDAVP, it rises to adult levels. The expression of AQP3 and AQP4 is intact at birth and does not seem to contribute to the hyporesponsiveness to AVP. Low sodium transport by thick ascending loops of Henle, immaturity of the medullary architecture, and adaptations in the transport of urea contribute to the lower tonicity of the medullary interstitium. This paper reviews the alterations in the AVP signal transduction pathway in the immature kidney.

Bidirectional regulation of AQP2 trafficking and recycling: involvement of AQP2-S256 phosphorylation

The present study examined the role of PKA and serine256 (S256) phosphorylation for AQP2 trafficking and recycling using cells transfected with wild-type AQP2 (AQP2-WT) or mutant AQP2 and high-resolution confocal microscopic techniques. In transiently transfected MDCK-C7 cells, stimulation with forskolin induced translocation of AQP2-WT to the plasma membrane. Treatment of AQP2-WT cells with the PKA inhibitor H-89 following forskolin stimulation resulted in internalization of AQP2-WT. Moreover, H-89 treatment of AQP2-S256D (mimicking constitutively phosphorylated AQP2 and hence localized to the plasma membrane) resulted in redistribution of AQP2-S256D to intracellular vesicles, even in the presence of forskolin. Both PGE2 and dopamine stimulation induced endocytosis of AQP2-WT and AQP2-S256D, respectively, in forskolin-stimulated cells. Consistent with this, dopamine in the presence of vasopressin stimulated endocytosis of AQP2 in slices of rat kidney inner medulla without substantial dephosphorylation. In conclusion, these results strongly suggest that 1) S256 phosphorylation is necessary but not sufficient for AQP2 plasma membrane expression, 2) active PKA is required for AQP2 plasma membrane expression, 3) PGE2 and dopamine induce internalization of AQP2 independently of AQP2 dephosphorylation, and 4) preceding activation of cAMP production is necessary for PGE2 and dopamine to cause AQP2 internalization.

Acute endotoxemia in rats induces down-regulation of V2 vasopressin receptors and aquaporin-2 content in the kidney medulla

BACKGROUND

Endotoxemia can lead to fluid metabolism alterations despite unchanged or elevated plasma vasopressin (VP) levels, suggesting a refractoriness of the kidney to the effect of the peptide. To test this hypothesis, we examined the effect of lipopolysaccharide (LPS) injection on the expression of V2 receptors and aquaporin-2 in the kidney.

METHODS

Plasma VP and urine osmolality, and binding of [3H]VP to kidney membranes, Western blot, and immunohistochemical analysis of aquaporin-2, in situ hybridization for V2 VP receptors and cytokines mRNAs were measured in the kidney 3 to 24 hours after LPS injection, 250 microg/100 g, intraperitoneally.

RESULTS

LPS injection caused prolonged decreases in urine osmolality (up to 24 hours) without significant changes in plasma levels of sodium or VP. This was associated with marked decreases in V2 VP receptor mRNA and VP receptor number in the kidney, which were evident for up to 12 hours after LPS injection. Aquaporin-2 in kidney inner medulla was also reduced by about 50%. LPS induced interleukin (IL)-1beta in the kidney medulla by 3 hours, reached maximum at 6 hours, and started to decline by 12 hours, while it increased IL-6 mRNA significantly only at 3 hours. Interleukin mRNA expression was absent in kidneys of control rats. In vitro incubation of kidney medulla slices with IL-1beta reduced VP binding.

CONCLUSION

The inflammatory response to acute endotoxemia down regulates V2 VP receptors and aquaporin-2 of the kidney inner medulla resulting in prolonged impairment of the renal capacity to concentrate urine.

P2Y2 receptor-stimulated release of prostaglandin E2 by rat inner medullary collecting duct preparations

Extracellular nucleotides, acting through the P2Y2 receptor and the associated phosphoinositide-Ca2+ signaling pathway, inhibit AVP-stimulated osmotic water permeability in rat inner medullary collecting duct (IMCD). Because a rise in intracellular Ca2+ is frequently associated with enhanced arachidonic acid metabolism, we examined the effect of activation of the P2Y2 receptor on release of PGE2 in freshly prepared rat IMCD suspensions. Unstimulated IMCD released moderate, but significant, amounts of PGE2, which were more sensitive to cyclooxygenase (COX)-2 than COX-1 inhibition. Agonist activation of P2Y2 receptor by adenosine 5'-O-(3-thiotriphosphate) enhanced release of PGE2 from IMCD in a time- and concentration-dependent fashion. Purinergic-stimulated release of PGE2 was completely blocked by nonspecific COX inhibitors (flurbiprofen and 2-acetoxyphenylhept-2-ynyl sulfide). Differential COX inhibition studies revealed that purinergic-stimulated release of PGE2 was more sensitive to a COX-1-specific inhibitor (valeroyl salicylate) than a COX-2-specific inhibitor (NS-398). Thus purinergic stimulation resulted in significantly more release of PGE2 in the presence of COX-2 inhibitor than COX-1 inhibitor. If it is assumed that increased release of PGE2 is related to its increased production, our results suggest that purinergic stimulation of IMCD results in enhanced production and release of PGE2 in a COX-1-dependent fashion. Because PGE2 is known to affect transport of water, salt, and urea in IMCD, interaction of the purinergic system with the prostanoid system in IMCD can modulate handling of water, salt, and urea by IMCD and, thus, may constitute an AVP-independent regulatory mechanism.

The prostaglandin E2 analogue sulprostone antagonizes vasopressin-induced antidiuresis through activation of Rho

Arginine-vasopressin (AVP) facilitates water reabsorption in renal collecting duct principal cells by activation of vasopressin V2 receptors and the subsequent translocation of water channels (aquaporin-2, AQP2) from intracellular vesicles into the plasma membrane. Prostaglandin E2 (PGE2) antagonizes AVP-induced water reabsorption; the signaling pathway underlying the diuretic response is not known. Using primary rat inner medullary collecting duct (IMCD) cells, we show that stimulation of prostaglandin EP3 receptors induced Rho activation and actin polymerization in resting IMCD cells, but did not modify the intracellular localization of AQP2. However, AVP-, dibutyryl cAMP- and forskolin-induced AQP2 translocation was strongly inhibited. This inhibitory effect was independent of increases in cAMP and cytosolic Ca2+. In addition, stimulation of EP3 receptors inhibited the AVP-induced Rho inactivation and the AVP-induced F-actin depolymerization. The data suggest that the signaling pathway underlying the diuretic effects of PGE2 and probably those of other diuretic agents include cAMP- and Ca2+-independent Rho activation and F-actin formation.

Synergistic effects of nitric oxide and prostaglandins on renal escape from vasopressin-induced antidiuresis

Recent results from our laboratories indicate that renal escape from AVP-induced antidiuresis is accompanied by marked downregulation of kidney aquaporin-2 (AQP2) and AVP V2 receptors. The present studies evaluated the effect of nitric oxide (NO) and PG synthesis blockade on escape from antidiuresis. dDAVP-infused rats were water loaded (WL) for 5 days. l-NAME, an NO synthesis inhibitor, or diclofenac, a cyclooxygenase inhibitor, was infused subcutaneously beginning 1 day before WL. As early as 2 days after WL, urine volume increased and urine osmolality decreased, indicating the onset of escape. Endogenous NO synthesis, measured as urinary NO2 + NO3 excretion, was significantly increased in the WL group compared with the non-WL controls during all 5 days of WL. l-NAME (20 mg. kg(-1). day(-1)) markedly decreased urine volume on days 4 and 5 of WL, indicating inhibition of the escape phenomenon. Kidney AQP2 protein was significantly increased by this dose of l-NAME as well. A lower dose of l-NAME (10 mg. kg(-1). day(-1)) or diclofenac (2.5 mg. kg(-1). day(-1)) did not significantly affect the escape phenomenon by itself, but the combination of l-NAME and diclofenac showed a marked inhibitory effect on the escape phenomenon, which was also accompanied by a significant increase in kidney AQP2 expression. These results therefore suggest that renal NO and PG both play important roles in escape from AVP-induced antidiuresis by acting synergistically to downregulate kidney AQP2 expression.

Molecular physiology of urinary concentration defect in elderly population

It is estimated that by the year 2050 one in five Americans will be 65 years or older. This mandates the growing need for clinical and basic research in the field of geriatric medicine to understand age-related maladies. The most prominent abnormality in renal function in the aging population is the inability to handle water, frequently resulting in hypo- or hyperosmolar states, and the associated electrolyte imbalances. During the past decade, thanks to the advent of powerful molecular techniques, rapid strides have been made in the approaches employed to understand and dissect the physiology of renal function in general and the urinary concentration mechanism in particular. Using an integrated approach of clinical observations, experimental model systems, molecular analysis, and functional genomics, a more comprehensive picture of the interplay of physiological systems in the genesis of urinary concentration defect in the elderly is beginning to emerge. Much remains to be deciphered regarding the complex interactions between the role of environment, genetics, diet, pharmacological agents and the general effects of aging on kidney function. The emerging importance of socio-economic and quality of life issues surrounding geriatric medicine encourage public and private support and funding for research in the area of age-related diseases, especially as they are related to the kidney.

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.

Molecular and biochemical characterization of prostacyclin receptors in rat kidney

The prostacyclin (IP) message was detected by RT-PCR in the renal cortex, outer (OM) and inner medulla (IM), and in freshly isolated (IMCD-f) and cultured inner medullary collecting duct (IMCD-c), and also the E-prostanoid (EP)1,3,4 receptor subtypes, but not EP2. Digoxigenin in situ hybridization localized IP mRNA in the tubules of the OM and IM, and the vasculature, and also in the glomeruli, arteries, and tubules of the cortex. IP splice variants or subtypes could not be detected by RT-PCR followed by TA cloning, though several nonfunctional point mutations or single base pair deletions were observed. Iloprost (ILP), cicaprost (CCP), PGE2, and arginine vasopressin (AVP) stimulated cAMP in both IMCD preparations. In addition, AVP-stimulated cAMP in IMCD-f was inhibited by all three prostanoids, but not in IMCD-c. Calcium experiments were performed on IMCD-c or microdissected IMCD (IMCD-m). CCP, ILP, and PGE2 did not alter intracellular calcium concentration ([Ca2+]i) in IMCD-c. However, on IMCD-m, both PGE2 and ILP increased [Ca2+]i levels equipotently and CCP had no effect. Pretreatment with the EP1 antagonist AH-6809 indicates that the response to ILP and PGE2 is mediated via EP1. These results suggest that IP receptors in the rat IMCD mediate the cAMP but not calcium signaling linked to PGI2; to date no subtypes or splice variants have been identified.

Role of PGE(2) in alpha(2)-induced inhibition of AVP- and cAMP-stimulated H(2)O, Na(+), and urea transport in rat IMCD

PGE(2) inhibits osmotic water permeability (P(f)) in the rat inner medullary collecting duct (IMCD) via cellular events occurring after the stimulation of cAMP, i.e., post-cAMP-dependent events. The alpha(2)-agonists also inhibit P(f) in the rat IMCD via post-cAMP-dependent events. The purpose of this study was to determine whether PGE(2) plays a role in alpha(2)-mediated inhibition of P(f), Na(+), and urea transport in the rat IMCD. Isolated terminal IMCDs from Wistar rats were perfused to measure, in separate experiments, P(f), lumen-to-bath (22)Na(+) transport (J(lb)), and urea permeability (P(u)). Transport was stimulated with 220 pM arginine vasopressin (AVP) or 0.1 mM 8-(4-chlorophenylthio)-cAMP (CPT-cAMP). Indomethacin was used to inhibit endogenous prostaglandin synthesis, and the alpha(2)-agonists clonidine, oxymetazoline, and dexmedetomidine were used to test the role of PGE(2) in the alpha(2)-mediated mechanism that inhibits transport. All agents were added to the bath. Indomethacin at 5 microM significantly elevated CPT-cAMP-stimulated P(f), J(lb), and P(u), and subsequent addition of 100 nM PGE(2) reduced these transport parameters. Indomethacin reversed alpha(2) inhibition of CPT-cAMP-stimulated P(f), J(lb), and P(u), and subsequent addition of PGE(2) reduced transport in each case. Indomethacin partially reversed alpha(2) inhibition of AVP-stimulated P(f), J(lb), and P(u), and PGE(2) reduced transport back to the alpha(2)-inhibited level. These results indicate that PGE(2) is a second messenger involved in the mechanism of transport inhibition mediated by alpha(2)-adrenoceptors via post-cAMP-dependent events in the rat IMCD.

Prostaglandin E(2) interaction with AVP: effects on AQP2 phosphorylation and distribution

Prostaglandin E(2) (PGE(2)) antagonizes the action of arginine vasopressin (AVP) on collecting duct water permeability. To investigate the mechanism of this antagonism, rat renal inner medulla (IM) was incubated with the two hormones, and the phosphorylation and subcellular distribution of the water channel, aquaporin-2 (AQP2) were studied. Using a phosphorylation state-specific AQP2 antibody, we demonstrated that AVP stimulates AQP2 phosphorylation at the Ser(256) protein kinase A consensus site in a time- and dose-dependent manner. In parallel studies using a differential centrifugation technique, we demonstrated that AVP induced translocation of AQP2 from an intracellular vesicle-enriched fraction to a plasma membrane-enriched fraction. PGE(2) (10(-7) M) added after AVP (10(-8) M) did not decrease AQP2 phosphorylation but reversed AVP-induced translocation of AQP2 to the plasma membrane. Preincubation of IM with PGE(2) did not prevent the effects of AVP on AQP2 phosphorylation and trafficking. PGE(2) alone did not influence AQP2 phosphorylation and subcellular distribution. Our data indicate that 1) recruitment of AQP2 to the plasma membrane and its retrieval to a pool of intracellular vesicles may be regulated independently, 2) PGE(2) may counteract AVP action by activation of AQP2 retrieval, 3) dephosphorylation of AQP2 is not a prerequisite for its internalization.

Endothelin inhibits vasopressin-stimulated water permeability in rat terminal inner medullary collecting duct

Renal tubule solute and water transport is subject to regulation by numerous factors. To characterize direct effects of the recently discovered peptide endothelin (ET) on renal tubule transport, we determined signaling mechanisms for ET effects on vasopressin (AVP)-stimulated water permeability (PF) in rat terminal inner medullary collecting duct (IMCD) perfused in vitro. ET caused a rapid, dose-dependent, and reversible fall in AVP- but not cyclic AMP-stimulated PF, suggesting that its effect on PF is by inhibition of cyclic AMP accumulation. Indomethacin did not block ET actions, ruling out a role for prostaglandins in its effect. The protein kinase C (PKC) inhibitor calphostin, or pretreatment of perfused tubules with pertussis toxin, blocked ET-mediated inhibition of AVP-stimulated PF. ET caused a transient increase in intracellular calcium ([Ca2+]i) in perfused tubules, an effect unchanged in zero calcium bath or by PT pretreatment. ET effects on PF and [Ca2+]i desensitized rapidly. Inhibition of PF was transient and largely abolished by 20 min ET preexposure, and repeat exposure to ET did not alter [Ca2+]i. In contrast, PGE2-mediated inhibition of AVP-stimulated PF and increase of [Ca2+]i were sustained and unaltered by prior exposure of IMCD to ET. Thus desensitization to ET is homologous. We conclude that ET is a potent inhibitor of AVP-stimulated water permeability in rat terminal IMCD. Signaling pathways for its effects involve both an inhibitory guanine nucleotide-binding protein and phospholipase-mediated activation of PKC. Since ET is synthesized by IMCD cells, this peptide may be an important autocrine modulator of renal epithelial transport.