Downregulation of UT-A1/UT-A3 is associated with urinary concentrating defect in glucocorticoid-excess state.

Excessive glucocorticoid hormone, as occurs with Cushing syndrome, is known to be associated with altered body water homeostasis, but the molecular mechanisms are unknown. In this study, rats treated with daily dexamethasone (Dex) for 14 d provided a model of Cushing syndrome. Compared with control rats, Dex-treated rats demonstrated increased mean arterial pressure, urine flow rate, and urinary excretion of both sodium and urea. Dex-treated rats had increased abundance of aquaporin 1 (AQP1), AQP3, and Na-K-2Cl co-transporter proteins and a marked reduction of the urea transporters UT-A1 and UT-A3. In response to an acute water load, Dex-treated rats increased water excretion more than control rats, but both groups exhibited similar AQP2 expression. In response to fluid deprivation, Dex-treated rats demonstrated an impaired urinary concentrating capacity: Urine flow rate was higher and urine osmolality was lower than control rats despite an increase in AQP1, AQP3, and Na-K-2Cl co-transporter expression. AQP2 expression was similar between the two groups, but UT-A1 and UT-A3 were decreased and urinary urea excretion was increased in Dex-treated rats. Because Dex-treated rats ingested less food and water compared with controls, paired food and water studies were performed; these substantiated the previous results. In summary, the alterations in body water observed with glucocorticoid excess may be a result, in part, of impaired urinary concentrating capacity; downregulation of UT-A1 and UT-A3 and increased urea excretion may contribute to this impairment.

Molecular mechanisms of antidiuretic effect of oxytocin.

Oxytocin is known to have an antidiuretic effect, but the mechanisms underlying this effect are not completely understood. We infused oxytocin by osmotic minipump into vasopressin-deficient Brattleboro rats for five days and observed marked antidiuresis, increased urine osmolality, and increased solute-free water reabsorption. Administration of oxytocin also significantly increased the protein levels of aquaporin-2 (AQP2), phosphorylated AQP2 (p-AQP2), and AQP3 in the inner medulla and in the outer medulla plus cortex. Immunohistochemistry demonstrated increased AQP2 and p-AQP2 expression and trafficking to the apical plasma membrane of principal cells in the collecting duct, and increased AQP3 expression in the basolateral membrane. These oxytocin-induced effects were blocked by treatment with the vasopressin V2 receptor antagonist SR121463B, but not by treatment with the oxytocin receptor antagonist GW796679X. We conclude that vasopressin V2 receptors mediate the antidiuretic effects of oxytocin, including increased expression and apical trafficking of AQP2, p-AQP2, and increased AQP3 protein expression.

Effects of angiotensin receptor blockade on haemodynamics and gene expression after myocardial infarction.

INTRODUCTION: Despite the fact that congestive heart failure (CHF) remains the most common disease in the developed world and has been extensively studied, there is little known about the molecular and cellular mechanisms of cardiac dysfunction. Angiotensin has been implicated as a mediator of cardiac injury; however, the mechanisms of its action have not been delineated. The objective of this study was to examine the relationship between the haemodynamic and molecular events during cardiac dysfunction and the role of the angiotensin system. STUDY DESIGN: We examined the effects of the angiotensin receptor blocker, valsartan, on changes in the haemodynamic and gene expression patterns in a postmyocardial infarction model in the rat. METHODS: Myocardial infarction (MI) was induced in rats by coronary artery ligation. Cardiac haemodynamics were monitored using echocardiography. Gene expression profiles after myocardial infarction were identified using Affymetrix Genechip oligonucleotide arrays. RESULTS: Myocardial contractility, as assessed by cardiac output and left ventricle (LV) fraction of shortening, was reduced in untreated animals by week 3 after MI (p < 0.05 versus baseline), and preserved with valsartan treatment as observed by the nonsignificant changes versus baseline. LV dilatation, as demonstrated by increases in LV systolic and diastolic diameters, developed by week 3 in untreated animals (p < 0.05 versus baseline) while valsartan-treated animals were protected and showed no significant increases in diameter size compared with baseline. LV hypertrophy, as shown by LV posterior wall thickness, was more profound in untreated animals (p < 0.05 versus baseline) than in those treated with valsartan at weeks 3 and 4. Changes in gene expression at 4 weeks after MI included those encoding muscle-specific genes, fibrous tissue proliferation, immune response and various others. Treatment with valsartan reversed these changes in 67% of overexpressed genes and 83% of underexpressed genes. CONCLUSION: Angiotensin receptor blockade with valsartan was found to protect cardiac function, and this beneficial effect was accompanied by a reversal of changes in gene expression induced by MI.

Role of AQP1 in endotoxemia-induced acute kidney injury.

The effect of endotoxemia (lipopolysaccharide, 2.5 mg/kg ip) was investigated in aquaporin (AQP) 1 knockout (KO) compared with wild-type (WT) mice. At baseline, KO mice exhibited higher water intake (WI) and urine output (UO). After endotoxemia, WI and UO remained higher in the KO than WT mice, and urine osmolality was lower. The higher serum osmolality in AQP1-KO mice during endotoxemia was associated with higher AQP2 (133 +/- 8 vs. 100 +/- 3%, P < 0.01), AQP3 (140 +/- 8 vs. 100 +/- 4%, P < 0.001) and Na(+)-K(+)-2Cl(-) cotransporter type 2 (NKCC2; 152 +/- 14 vs. 100 +/- 15%, P < 0.05) expression than that in WT mice. These responses during endotoxemia in the AQP1-KO mice compared with WT were associated with lower glomerular filtration rate (GFR) (69 +/- 8 vs. 96 +/- 8 ml/min, P < 0.05) and renal blood flow (0.77 +/- 0.1 vs. 1.01 +/- 0.1 ml/min, P < 0.01). Urinary sodium excretion and fractional sodium excretion were higher in KO compared with WT mice in endotoxemia and were accompanied by more severe tubular injury. With water repletion and comparable serum osmolalities, GFR was still lower in KO (57 +/- 13 vs. 120 +/- 6 ml/min, P < 0.01) compared with WT during endotoxemia. The abundance of AQP2 and AQP3 protein in KO mice was not different from WT mice; however, NKCC2, Na(+)/H(+) exchanger type 3, and fractional sodium excretion remained higher in KO compared with WT. Thus the polyuria in AQP1-KO mice does not protect against endotoxemia-induced acute kidney injury but rather absence of AQP1 predisposed to enhanced endotoximic renal injury.

Hyperosmolality in vivo upregulates aquaporin 2 water channel and Na-K-2Cl co-transporter in Brattleboro rats.

There are considerable experimental results that indicate that arginine vasopressin (AVP)-independent factors are involved in urinary concentration. This study examined the role of hyperosmolality in vivo to modulate aquaporin 2 (AQP2) and Na-K-2Cl co-transporter (NKCC2), pivotal factors in urinary concentration, in AVP-deficient Brattleboro (BB) rats. Hyperglycemia with associated hyperosmolality occurred in diabetic BB rats (BBDM). Protein abundance of AQP2 increased and was reversed by insulin in the inner medulla (IM; control 100+/-5%; BBDM 146+/-8%; BBDM+Ins 122+/-9%; P<0.001) and inner stripe of outer medulla (ISOM; control 100+/-4%; BBDM 123+/-8%; BBDM+Ins 93+/-6%; P<0.05). These results were confirmed by immunohistochemistry studies. NKCC2 rose in the ISOM but was not reversed with insulin treatment. For investigation of the role of hyperosmolality in the absence of hyperglycemia on the regulation of the expression of renal AQP and NKCC2, studies were performed with hyperosmolality that was induced by 0.5% NaCl in drinking water in BB rats. Hyperosmolality that was induced by NaCl increased significantly the protein abundance of IM AQP2 (121+/-2 versus 100+/-5%; P<0.01), ISOM AQP2 (135+/-6 versus 100+/-5%; P<0.001), cortex plus outer stripe of outer medulla AQP2 (121+/-4 versus 100+/-1%; P<0.001), ISOM NKCC2 (133+/-1 versus 100+/-4%; P<0.05), and cortex plus outer stripe of outer medulla NKCC2 (142+/-16 versus 100+/-9%; P<0.05). In conclusion, hyperosmolality, secondary to either glucose or NaCl, upregulated renal AQP2 and NKCC2 in vivo in BB rats.

Molecular analysis of impaired urinary diluting capacity in glucocorticoid deficiency.

Urinary diluting ability and protein abundance of renal aquaporins (AQPs) and ion transporters in glucocorticoid-deficient (GD) rats were examined at baseline and in response to oral water loading. Rats underwent bilateral adrenalectomy followed by aldosterone (GD) or aldosterone + dexamethasone (CTL) replacement. Before oral water loading, urinary output was significantly decreased and urinary osmolality (U(osm)) was increased in GD compared with CTL rats. Protein abundance of inner medullary AQP2 (148 +/- 18%), phosphorylated AQP2 (pAQP2, 156 +/- 13%), and AQP3 (145 +/- 8%) was significantly upregulated in GD compared with CTL rats (all P < 0.05). GD rats also demonstrated a marked reduction in urinary Na(+) excretion compared with pair-fed CTL rats. Na(+)-K(+)-2Cl(-) cotransporter, Na(+)/H(+) exchanger type 3, and cortical beta- and gamma-subunits of the epithelial Na(+) channel were significantly upregulated in GD rats. At 1 h after an acute water load (40 ml/kg by oral gavage), GD rats demonstrated a decrease in percent water excretion (5 +/- 1 vs. 33 +/- 9%, P < 0.01) and urinary output (33 +/- 12 vs. 250 +/- 65 microl x kg(-1) x min(-1), P < 0.05) and an increase in U(osm) (1,894 +/- 292 vs. 316 +/- 92 mosmol/kgH(2)O, P < 0.001) compared with CTL rats. Plasma AVP was increased (1.6 +/- 0.2 vs. 0.9 +/- 0.2 pg/ml, P < 0.05), as was protein expression of inner medullary AQP2 (149 +/- 5%) and pAQP2 (177 +/- 9%, P < 0.01), in GD compared with CTL rats; apical expression of AQP2 was maintained in GD rats. The vasopressin V(2) receptor antagonist OPC-31260 increased percent water excretion and urinary output and reduced U(osm) compared with vehicle-treated GD rats. OPC-31260 also reversed the increased abundance and apical trafficking of inner medullary AQP2 and pAQP2 protein in GD rats. In conclusion, enhanced protein abundance of Na(+) transporters and Na(+) channels with Na(+) retention occurred with GD. OPC-31260 reversed upregulation and apical trafficking of AQP2 and pAQP2 in association with improved urinary diluting capacity and increased water excretion after oral water loading.

Molecular mechanisms of impaired urinary concentrating ability in glucocorticoid-deficient rats.

The purpose of this study was to examine urinary concentrating ability and protein expression of renal aquaporins and ion transporters in glucocorticoid-deficient (GD) rats in response to water deprivation as compared with control rats. Rats underwent bilateral adrenalectomies, followed only by aldosterone replacement (GD) or both aldosterone and dexamethasone replacement (control). As compared with control rats, the GD rats demonstrated a decrease in cardiac output and mean arterial pressure. In response to 36-h water deprivation, GD rats demonstrated significantly greater urine flow rate and decreased urine osmolality as compared with control rats at comparable serum osmolality and plasma vasopressin concentrations. The initiator of the countercurrent concentrating mechanism, the sodium-potassium-2 chloride co-transporter, was significantly decreased, as was the medullary osmolality in the GD rats versus control rats. There was also a decrease in inner medulla aquaporin-2 (AQP2) and urea transporter A1 (UT-A1) in GD rats as compared with control rats. There was a decrease in outer medulla Gsalpha protein, an important factor in vasopressin-mediated regulation of AQP2. Immunohistochemistry studies confirmed the decreased expression of AQP2 and UT-A1 in kidneys of GD rats as compared with control. In summary, impairment in the urinary concentrating mechanism was documented in GD rats in association with impaired countercurrent multiplication, diminished osmotic equilibration via AQP2, and diminished urea equilibration via UT-A1. These events occurred primarily in the relatively oxygen-deficient medulla and may have been initiated, at least in part, by the decrease in mean arterial pressure and thus renal perfusion pressure in this area of the kidney.

Nonosmotic release of vasopressin and renal aquaporins in impaired urinary dilution in hypothyroidism.

The purpose of this study was to examine protein expression of renal aquaporins (AQP) and ion transporters in hypothyroid (HT) rats in response to an oral water load compared with controls (CTL) and HT rats replaced with l-thyroxine (HT+T). Hypothyroidism was induced by aminotriazole administration for 10 wk. Body weight, water intake, urine output, solute and urea excretion, and serum and urine osmolality were comparable among the three groups at the conclusion of the 10-wk treatment period. One hour after oral gavage of water (50 ml/kg body wt), HT rats demonstrated significantly less water excretion, higher minimal urinary osmolality, and decreased serum osmolality compared with CTL and HT+T rats. Despite the hyposmolality, plasma vasopressin concentration was elevated in HT rats. These findings in HT rats were associated with an increase in protein abundance of renal cortex AQP1 and inner medulla AQP2. AQP3, AQP4, and the Na-K-2Cl cotransporter were also increased. Moreover, 1 h following the oral water load, HT rats demonstrated a significant increase in the membrane-to-vesicle fraction of AQP2 by Western blot analysis. The defect in urinary dilution in HT rats was reversed by the V(2) vasopressin antagonist OPC-31260. In conclusion, impaired urinary dilution in HT rats is primarily compatible with the nonosmotic release of vasopressin and increased protein expression of renal AQP2. The impairment of maximal solute-free water excretion in HT rats, however, appears also to involve diminished distal fluid delivery.

Comparison of three methods to quantify urinary aquaporin-2 protein.

BACKGROUND: Urinary excretion of aquaporin-2 (AQP2) is measurable, and is regulated by renal vasopressin action in the principal cells of the collecting duct. To date, two methods [radioimmunoassay (RIA) and quantitative immunoblot analysis (IB)] have been used for quantitation of urinary AQP2 protein. However, the actual amount of urinary AQP2 measured has not been directly compared by the RIA and IB. Recently, we have established an enzyme-linked immunosorbent assay (ELISA) for quantitation of urinary AQP2. The purpose of our current study was to compare three different immunoassay methods for measurement of urinary AQP2. METHODS: After overnight dehydration, five normal subjects ingested an oral water load (20 mL/kg). Urine was collected at 0, 1, 2, 3, and 4 hours after oral water loading. Urinary AQP2 protein was quantitated in each sample by using the RIA, IB, and ELISA, and the correlation coefficients were compared among three different methods. RESULTS: Values of urinary AQP2 at 0, 1, 2, 3, and 4 hours after oral water loading for RIA, IB, and ELISA were, respectively (fmol/mg creatinine): 0 h: 266 +/- 28, 405 +/- 74, 294 +/- 41; 1 h: 159 +/- 59, 267 +/- 147, 195 +/- 95; 2 h: 48 +/- 17, 19 +/- 10, 38 +/- 18; 3 h: 79 +/- 18, 70 +/- 24, 80 +/- 15; 4 h: 147 +/- 21, 161 +/- 31, 136 +/- 15. All values were shown as means +/- SEM. There was a significant positive correlation between: the IB and ELISA (r = 0.91, P < 0.0001); the IB and RIA (r = 0.75, P < 0.0001); and the RIA and ELISA (r = 0.67, P < 0.0002). The correlation between the IB and ELISA was therefore the best. Also, urinary AQP2 was positively correlated with urine osmolality among all three methods. CONCLUSIONS: The results indicate that the newly developed IB and ELISA methods are useful for measurement of urinary AQP2 and have an excellent correlation.

Effect of mineralocorticoid deficiency on ion and urea transporters and aquaporin water channels in the rat.

Mineralocorticoid deficiency is associated with impaired urinary concentration and dilution. The present investigation was undertaken to determine the effects of selective mineralocorticoid deficiency on renal sodium and urea transporters and aquaporin water channels and whether these perturbations can be reversed by maintenance of extracellular fluid volume. Mineralocorticoid deficiency was induced by bilateral adrenalectomies with glucocorticoid replacement. Mineralocorticoid deficient rats receiving plain drinking water (MDW) were compared with mineralocorticoid deficient rats receiving saline-drinking water (MDS) in order to maintain extracellular fluid volume, and with controls (CTL). In MDW rats, there was a significant decrease in renal outer medulla Na-K-2Cl co-transporter and outer medulla Na-K-ATPase as well as an increase in inner medulla aquaporins 2 and 3. There were no significant changes in aquaporin-1, aquaporin-4, or urea transporters. These alterations were reversed with maintenance of extracellular fluid volume in MDS rats. Our findings indicate that mineralocorticoid deficiency in the rat is associated with alterations in factors involved in the countercurrent concentrating mechanism (Na-K-2Cl, Na-K-ATPase) and osmotic water equilibration in the collecting duct (AQP2, AQP3). Maintenance of sodium balance and extracellular fluid volume is associated with normalization of these perturbations.

Increased vascular heme oxygenase-1 expression contributes to arterial vasodilation in experimental cirrhosis in rats.

Vascular heme oxygenase (HO) regulates vascular tone in normal conditions and in some pathologic circumstances (e.g., sepsis). However, its possible role in the pathogenesis of arterial vasodilation in cirrhosis is unknown. To address this question, the expression and activity of HO in arterial vessels was studied in rats at 1, 2, and 4 weeks after bile duct ligation (BDL) or sham operation. A progressively increased expression of HO-1 was found in aorta and mesenteric arteries of BDL rats in a close chronologic relationship with the progression from acute cholestatic liver injury (1 week) to the fully developed cirrhosis with intense systemic arterial vasodilation (4 weeks). No changes were found in the expression of the constitutive isoform HO-2. HO-1 was mainly located in vascular smooth muscle cells of the arterial wall. Aortic HO activity increased in parallel with the expression of HO-1 (up to 600% in rats with cirrhosis compared with sham rats) and correlated with hemodynamic parameters. Increased expression of HO-1 and HO activity were also found in other organs, such as liver and spleen, though to a lesser extent compared with vascular tissue. The acute administration of an inhibitor of HO to cirrhotic rats, at a dose that normalized aortic HO activity, was associated with significantly greater effects on arterial pressure, total peripheral vascular resistance, and cardiac index, compared with effects in sham rats. In conclusion, these findings are consistent with a role for HO in the pathogenesis of arterial vasodilation in cirrhosis.

Effect of primary polydipsia on aquaporin and sodium transporter abundance.

Chronic primary polydipsia (POLY) in humans is associated with impaired urinary concentrating ability. However, the molecular mechanisms responsible for this finding have not been elucidated. The purpose of this study was to examine the effect of chronic primary POLY on water metabolism and renal aquaporin (AQP) water channels and sodium and urea transporter abundance in rats. Primary POLY was induced in male Sprague-Dawley rats by daily administration of 15 g powdered rat chow mixed in 100 ml water for 10 days. Control rats (CTL) received 15 g powdered rat chow per day and ad libitum drinking water. Rats were studied following this period before further intervention and with a 36-h period of water deprivation to examine maximal urinary concentrating ability. At baseline, POLY rats demonstrated significantly greater water intake (100 +/- 1 vs. 22 +/- 2 ml/day, P < 0.0001) and urinary output (80 +/- 1 vs. 11 +/- 1 ml/day, P < 0.0001) and decreased urinary osmolality (159 +/- 13 vs. 1,365 +/- 188 mosmol/kgH2O, P < 0.001) compared with CTL rats. These findings were accompanied by decreased inner medulla AQP-2 protein abundance in POLY rats compared with CTL rats before water deprivation (76 +/- 2 vs. 100 +/- 7% CTL mean, P < 0.007). With water deprivation, maximal urinary osmolality was impaired in POLY vs. CTL rats (2,404 +/- 148 vs. 3,286 +/- 175 mosmol/kgH2O, P < 0.0005). This defect occurred despite higher plasma vasopressin concentrations and similar medullary osmolalities in POLY rats. In response to 36-h water deprivation, inner medulla AQP-2 protein abundance was decreased in POLY rats compared with CTL rats (65 +/- 5 vs. 100 +/- 5% CTL mean, P < 0.0006). No significant differences were noted in renal protein abundance of either AQP-3 or AQP-4 or sodium and urea transporters. We conclude that the impaired urinary concentrating ability associated with primary POLY in rats is due to impaired osmotic equilibration in the collecting duct that is mediated primarily by decreased AQP-2 protein abundance.

Urinary concentrating defect in hypothyroid rats: role of sodium, potassium, 2-chloride co-transporter, and aquaporins.

Hypothyroidism is associated with impaired urinary concentrating ability in humans and animals. The purpose of this study was to examine protein expression of renal sodium chloride and urea transporters and aquaporins in hypothyroid rats (HT) with diminished urinary concentration as compared with euthyroid controls (CTL) and hypothyroid rats replaced with L-thyroxine (HT+T). Hypothyroidism was induced by aminotriazole administration. Body weight, water intake, urine output, solute and urea excretion, serum and urine osmolality, serum creatinine, 24-h creatinine clearance, and fractional excretion of sodium were comparable among the three groups. However, with 36 h of water deprivation, HT rats demonstrated significantly greater urine flow rates and decreased urine and medullary osmolality as compared with CTL and HT+T rats at comparable plasma vasopressin concentrations. Western blot analyses revealed decreased renal protein abundance of transporters, including Na-K-2Cl, Na-K-ATPase, and NHE3, in HT rats as compared with CTL and HT+T rats. Protein abundance of renal AQP1 and urea transporters UTA(1) and UTA(2) did not differ significantly among study groups. There was however a significant decrease in protein abundance of AQP2, AQP3, and AQP4 in HT rats as compared with CTL and HT+T rats. These findings demonstrate a decrease in the medullary osmotic gradient secondary to impaired countercurrent multiplication and downregulation of aquaporins 2, 3, and 4 as contributors to the urinary concentrating defect in the hypothyroid rat.

Evidence for bradykinin as a stimulator of thirst.

Angiotensin-converting enzyme inhibition (ACEI) with captopril has been shown to increase water intake and urine output in rats, but the mechanism is unknown. ACEI impairs the conversion of ANG I to ANG II, a dipsogenic hormone, and impairs the degradation of bradykinin. The goal of this study was to examine the role of bradykinin in the polydipsia and polyuria associated with ACEI. Male Sprague-Dawley rats received captopril (CPT; 20 mg.kg(-1).day(-1)) in ground chow for 48 h. Water intake, food intake, and urine output were monitored and compared with control rats (CTL), rats receiving captopril treatment with limited water intake (CPT-LIM), and rats receiving captopril treatment with ad libitum water intake plus 24-h treatment with the bradykinin antagonist B-9430 (CPT-BK1). CPT rats consumed significantly more water and produced more urine vs. CTL. Urine osmolality was significantly decreased in CPT rats vs. CTL. Inner medullary aquaporin-2 (AQP2) protein abundance was also markedly decreased in CPT rats vs. CTL. These findings were reversed in CPT-LIM rats, suggesting captopril-induced primary polydipsia. CPT-BKI rats demonstrated parameters no different from CTL despite ad libitum water intake. Mean arterial pressure and 24-h creatinine clearance did not differ among groups. We conclude that ACEI with captopril induces primary polydipsia despite impaired production of the dipsogen ANG II and that this primary increase in water intake is likely the cause of the decreased protein abundance of inner medullary AQP2. Furthermore, this dipsogenic effect was reversed by antagonism of bradykinin, thus implicating this hormone in thirst regulation in the rat.