Increased Urine Osmolality Research Articles

AT1a receptor knockout in mice impairs urine concentration by reducing basal vasopressin levels and its receptor signaling proteins in the inner medulla

Angiotensin II plays an important role in the regulation of blood pressure, body salt and fluid balance, and urine concentration. Mice with deletion of the AT(1a) receptor develop polyuria and urine concentration defects. We studied the mechanisms of these urine concentration defects by treating wild-type and AT(1a)-knockout mice with arginine vasopressin (AVP) for 2 weeks, controlling their water intake, or giving them an osmotic diuretic (sucrose) in order to determine whether central or nephrogenic mechanisms were involved. Under basal conditions, AT(1a)-knockout mice were hypotensive, had lower plasma AVP, and excreted more urine with a markedly reduced osmolality compared with wild-type mice. However, basal glomerular filtration rates were similar in both strains of mice. We isolated total lysate and membrane proteins from the inner medulla of wild-type and mutant mouse kidneys, and found that the amounts of aquaporin 2 (AQP2), adenylyl cyclases III and V/VI, and phosphorylated MAP kinases ERK 1/2 proteins were all reduced in the inner medulla of the knockout mice. Infusion of AVP raised plasma levels and blood pressure proportionally in both strains, but polyuria persisted and urine osmolality remained significantly lower in the knockout mice. Although AVP increased urine osmolality slightly in water-deprived knockout mice, this was well below the basal osmolality of wild-type mice. The diuretic response to the hyperosmotic sucrose was also impaired in the knockout mice. Neither AVP nor water rationing restored the levels of the inner medullary signaling proteins and membrane AQP2 proteins in the knockout mice. We suggest that AT(1a) receptor deletion causes polyuria and urine concentration defects by decreasing basal AVP release and impairing AVP-induced receptor signaling in the inner medulla.

Tonicity-dependent induction of Sgk1 expression has a potential role in dehydration-induced natriuresis in rodents

In various mammalian species, including humans, water restriction leads to an acute increase in urinary sodium excretion. This process, known as dehydration natriuresis, helps prevent further accentuation of hypernatremia and the accompanying rise in extracellular tonicity. Serum- and glucocorticoid-inducible kinase (Sgk1), which is expressed in the renal medulla, is regulated by extracellular tonicity. However, the mechanism of its regulation and the physiological role of hypertonicity-induced SGK1 gene expression remain unclear. Here, we identified a tonicity-responsive enhancer (TonE) upstream of the rat Sgk1 transcriptional start site. The transcription factor NFAT5 associated with TonE in a tonicity-dependent fashion in cultured rat renal medullary cells, and selective blockade of NFAT5 activity resulted in suppression of the osmotic induction of the Sgk1 promoter. In vivo, water restriction of rats or mice led to increased urine osmolality, increased Sgk1 expression, increased expression of the type A natriuretic peptide receptor (NPR-A), and dehydration natriuresis. In cultured rat renal medullary cells, siRNA-mediated Sgk1 knockdown blocked the osmotic induction of natriuretic peptide receptor 1 (Npr1) gene expression. Furthermore, Npr1-/- mice were resistant to dehydration natriuresis, which suggests that Sgk1-dependent activation of the NPR-A pathway may contribute to this response. Collectively, these findings define a specific mechanistic pathway for the osmotic regulation of Sgk1 gene expression and suggest that Sgk1 may play an important role in promoting the physiological response of the kidney to elevations in extracellular tonicity.

Knockout of Ku86 accelerates cellular senescence induced by high NaCl

NaCl induces DNA breaks, thus leading to cellular senescence. Here we showed that Ku86 deficiency accelerated the high NaCl-induced cellular senescence. We find that 1) high NaCl induces rapid cellular senescence in Ku86 deficient(xrs5) cells, 2) Ku86 deficiency shortens lifespan of C. elegans in high NaCl, and 3) cellular senescence is greatly accelerated in renal inner medullas of Ku86 (-/-) mice. Further, although water balance is known to be compromised in old mice, this occurs at much earlier age in Ku86(-/-) mice. When subjected to mild water restriction, 3 month old Ku86(-/-), but not Ku86(+/+),mice rapidly become dehydrated as evidenced by decrease in body weight, increased production of antidiuretic hormone,increased urine osmolality and decreased urine volume. The deficiency in water balance does not occur in Ku86(+/+)mice until they are much older (14 months). We conclude that Ku86 deficiency accelerates high NaCl(-) induced cellular senescence,particularly in the renal medulla where NaCl normally is high.

Thiol Antioxidants Regulate Angiotensin II AT1 and Arginine Vasopressin V1 Receptor Functions Differently in Vascular Smooth Muscle Cells

We compared the effects of the sulfhydryl-containing (thiol) antioxidant dithiothreitol (DTT), which disrupts disulfide bonds, on cell signaling through angiotensin II (AngII) Type 1 receptors (AT1Rs) and arginine vasopressin (AVP) V1 receptors (V1Rs). The AT1R contains two extracellular disulfides bonds but its ligand contains none, whereas the V1R contains no extracellular disufides bonds but its ligand contains 1. We measured radioligand binding, intracellular calcium responses, and extracellular signal-regulated kinase phosphorylation in cultured rat aortic vascular smooth muscle cells and alterations in urine osmolality in intact rats. Preincubation of cells with DTT, a maneuver designed to target receptor disulfides, resulted in concentration-dependent decreases in specific (125)I-AngII binding to AT1Rs and acute angiotensin-stimulated intracellular calcium mobilization but no decreases in specific (125)I-AVP binding to V1Rs or AVP-stimulated intracellular calcium mobilization. In contrast, preincubation of the ligands with DTT followed by acute exposure to the cells, a maneuver designed to target ligand disulfides, blunted calcium mobilization to AVP robustly but to AngII only minimally. In intact rats, the increase in urine osmolality caused by subcutaneous injection with the AVP analogue desmopressin was significantly diminished when the analogue was preincubated with an excess of DTT. DTT inhibits cell signaling to AngII AT1Rs and AVP V1Rs, at least in part through disruption of disulfide linkages, but the pattern of response depends upon whether disulfides of ligand or receptor are targeted.

Hsp90 inhibitor partially corrects nephrogenic diabetes insipidus in a conditional knock‐in mouse model of aquaporin‐2 mutation

Mutations in aquaporin-2 (AQP2) that interfere with its cellular processing can produce autosomal recessive nephrogenic diabetes insipidus (NDI). Prior gene knock-in of the human NDI-causing AQP2 mutation T126M produced mutant mice that died by age 7 days. Here, we used a novel "conditional gene knock-in" strategy to generate adult, AQP2-T126M mutant mice. Mice separately heterozygous for floxed wild-type AQP2 and AQP2-T126M were bred to produce hemizygous mice, which following excision of the wild-type AQP2 gene by tamoxifen-induced Cre-recombinase gave AQP2(T126M/-) mice. AQP2(T126M/-) mice were polyuric (9-14 ml urine/day) compared to AQP2(+/+) mice (1.6 ml/day) and had reduced urine osmolality (400 vs. 1800 mosmol). Kidneys of AQP2(T126M/-) mice expressed core-glycosylated AQP2-T126M protein in an endoplasmic reticulum pattern. Screening of candidate protein folding "correctors" in AQP2-T126M-transfected kidney cells showed increased AQP2-T126M plasma membrane expression with the Hsp90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG). 17-AAG increased urine osmolality in AQP2(T126M/-) mice by >300 mosmol but had no effect in AQP2(-/-) mice. Kidneys of 17-AAG-treated AQP2(T126M/-) mice showed partial rescue of defective AQP2-T126M cellular processing. Our results establish an adult mouse model of NDI and demonstrate partial restoration of urinary concentration function by a compound currently in clinical trials for other indications.

An Optimal Mathematical Model for the Urine Concentrating Process of Neonatal Rats

Immature rats have limited abilities to concentrate the urine. A mathematical model for the concentrating mechanism in the neonatal rat medulla was formulated. A nonlinear optimization method was used to identify model parameter sets that maximize a model efficiency measure, which was taken as the ratio of free‐water absorption rate to total active solute transport rate. The optimization method systematically changes the parameter values within their prescribed ranges, seeking maximal efficiency. The variable parameters were: NaCl and urea concentration in the cortico‐medullary junction of descending limbs of Henle (DLH) and collecting ducts (CD); urea permeability in DLH, ascending limbs (ALH) and inner medullary CD (IMCD); NaCl maximum active transport rates in ALH and IMCD; CD water permeability; and the exponential decrease rates of loops of Henle and CD population. Parameter baseline values were obtained from reported means in physiological experiments or estimated using physiological assumptions. By allowing most parameters to fluctuate by ± 25% from baseline values the maximal model efficiency was 15% above baseline, resulting in only 5% increase in urine osmolality. This is consistent with experimental data from animals and isolated CD from immature rats.

Tonicity‐responsive activation of NFATc by calcineurin Aβ in the inner medullae

Previously, we found that nuclear factor of activated T cell (NFATc) is regulated by the β isoform of calcineurin (CaN). In vivo, NFATc‐driven luciferase expression is highest in the renal inner medullae (IM) and loss of the β isoform significantly decreases NFATc‐transcriptional activity. Expression and activity of NFATc in the IM suggests a role for CaNβ/NFATc in response to tonicity. To explore this, activation of CaN in response to calcium ionophore, glucose, mannitol, urea, and NaCl was examined. CaN activity in wildtype renal fibroblasts was increased by changes in tonicity in a dose‐dependent manner. CaN activity in β−/− cells, in contrast, was no longer regulated by tonicity. To examine the tonicity response in vivo, wildtype and β−/− mice were thirsted for 32 hours. Urine output and osmolarity were measured and kidneys were harvested for analyses. Wildtype mice had decreased urine output and increased urine concentration, as expected. This was accompanied by an increase in NFATc‐luciferase activity in the IM. β−/− mice, however, did not reduce urine output and did not concentrate their urine despite the period of thirsting. In addition, despite normal basal urine concentration, β−/− mice failed to increase urine osmolality in response to the vasopressin analog DDAVP. In conclusion, CaNβ is activated in vitro in response to hypertonicity and loss of β in vivo impairs urine concentration.

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.

Water Balance and Arginine Vasotocin in the Cocooning FrogCyclorana platycephala(Hylidae)

It is well established that forming a cocoon, for frog species capable of doing so, markedly reduces evaporative water loss; however, the capacity of cocooned frogs to maintain hydration during extended estivation is not well understood. The combined effects of long-term estivation and water loss were examined in the cocoon-forming species Cyclorana platycephala by assessing the hydration state of the frogs throughout a 15-mo estivation period. Frogs lost mass throughout the 15-mo period to a maximum of 36%+/-6.5% of their initial standard mass. Plasma osmolality reached maximal levels by the ninth month of estivation at 487 mOsm kg(-1) and then remained stable to the fifteenth month of estivation. Urine osmolality continued to increase to the fifteenth month of estivation, at which point plasma and urine concentrations were isosmotic. The use of bladder water to counter losses from circulation was indicated by the relatively slow rate of increase in plasma osmolality with mass loss and the progressive increase in urine osmolality. For estivating frogs, evidence was found for a possible threshold relationship between plasma osmolality and plasma arginine vasotocin (AVT) concentration. After estivation, plasma AVT concentrations decreased markedly after 15-mo estivators were placed in water for 2 h, suggesting that high levels of AVT may not be integral to rapid rehydration in this species.

Role of prostaglandins in collecting duct-derived endothelin-1 regulation of blood pressure and water excretion

Collecting duct (CD)-derived endothelin-1 (ET-1) exerts natriuretic, diuretic, and hypotensive effects. In vitro studies have implicated cyclooxygenase (COX) metabolites, and particularly PGE(2), as important mediators of CD ET-1 effects. However, it is unknown whether PGE(2) mediates CD-derived ET-1 actions in vivo. To test this, CD ET-1 knockout (KO) and control mice were studied. During normal salt and water intake, urinary PGE(2) excretion was unexpectedly increased in CD ET-1 KO mice compared with controls. Salt loading markedly increased urinary PGE(2) excretion in both groups of mice; however, the levels remained relatively higher in KO animals. Acutely isolated inner medullary collecting duct (IMCD) from KO mice also had increased PGE(2) production. The increased IMCD PGE(2) was COX-2 dependent, since NS-398 blocked all PGE(2) production. However, increased CD ET-1 KO COX-2 protein or mRNA could not be detected in inner medulla or IMCD, respectively. Inner medullary COX-1 mRNA and protein levels and IMCD COX-1 mRNA levels were unaffected by Na intake or CD ET-1 KO. KO mice on a normal or high-Na diet had elevated blood pressure compared with controls; this difference was not altered by indomethacin or NS-398 treatment. However, indomethacin or NS-398 did increase urine osmolality and reduce urine volume in KO, but not control, animals. In summary, IMCD COX-2-dependent PGE(2) production is increased in CD ET-1 KO mice, indicating that CD-derived ET-1 is not a primary regulator of IMCD PGE(2). Furthermore, the increased PGE(2) in CD ET-1 KO mice partly compensates for loss of ET-1 with respect to maintaining urinary water excretion, but not in blood pressure control.

Short-Term Effects of Desmopressin on Water and Electrolyte Excretion in Adults With Nocturnal Polyuria

Increased calcium excretion due to desmopressin has been reported in children with nocturnal enuresis. Desmopressin is often used to treat adult patients with nocturnal polyuria. However, data on the effect of desmopressin on water/electrolyte excretion in adults are scarce. We present the short-term effects of desmopressin on water and electrolyte excretion in adult patients with nocturnal polyuria. A total of 16 male patients with nocturnal polyuria, mean age 76.3 years, received 0.1 or 0.2 mg desmopressin before sleep. Frequency volume chart was recorded, and daytime and nighttime urine samples were collected separately before and after desmopressin administration. Urinary excretions of sodium, potassium and calcium were determined, and compared before and after treatment with desmopressin. Desmopressin significantly increased urine osmolality, decreased nocturnal total urine volume, reduced the ratio of nocturnal urine volume-to-whole day urine volume and decreased nocturnal voiding frequency. Nocturnal urinary excretion of calcium (mean 0.137 vs 0.169 mg/kg body weight per hour, p = 0.004) and whole day excretion of calcium (mean 165.9 vs 200.0 mg per day, p = 0.012) were increased after desmopressin treatment. Nocturnal urinary potassium excretion (mean 0.030 vs 0.025 mEq/kg body weight per hour, p = 0.030) and whole day potassium excretion (mean 40.7 vs 36.1 mEq per day, p = 0.017) were decreased by desmopressin treatment. However, desmopressin treatment did not significantly change urinary secretion of sodium and chloride at nighttime or for the whole day. Desmopressin reduces nocturnal urine volume and nocturnal voiding frequency in male patients with nocturnal polyuria. However, increased calcium and decreased potassium excretion following desmopressin treatment deserve attention particularly when it is used on a long-term basis.

Furosemide as a possible treatment of X‐linked nephrogenic diabetes insipidus (NDI)

In NDI due to mutations of the V2 vasopressin receptor, indomethacin (Indo) + thiazide (TZ) induce a modest reduction in diuresis (V). Here, we tested whether furosemide (FURO), which inhibits urinary dilution in the thick ascending limb of Henle's loop, could also reduce V and increase urine osmolality (Uosm, mosm/kg H2O) in NDI patients. Seven children with NDI (6–17 y) were included in a double-blind study involving 2 weeks on placebo (P) and 2 weeks on FURO (0.5 mg/kg 3 times/d) in random order with 1 week washout between the two periods. Their usual Indo + TZ treatment was not interrupted. Data during FURO and P were compared by paired Student's t test. Results. Uosm was higher during FURO than during P: 155±48 vs 128±34, p=0.03, and V was significantly lower: 4.85 vs 7.12 l/d, p=0.015. Kalemia decreased by 0.3 mmol/l (NS). In Brattleboro rats with central DI (6 rats per group), FURO (50 mg/kg/d) and TZ (10 mg/kg/d) decreased V (ml/d) with additive effects: control=202±14, FU=139±19, TZ=80±6, FU+TZ=57±6. Conclusion. FURO could be a possible treatment for NDI. The benefit/risk of FURO and the withdrawal from NSAID need to be evaluated in a larger group of subjects.

Diabetes insipidus in a patient with germinoma

Central diabetes insipidus (cDI) is characterized by a decreased release of antidiuretic hormone (ADH), resulting in a variable degree of polyuria. Besides lesions affecting the posterior pituitary, lack of appropriate ADH-secretion can be caused by disorders that act at one or more of the sites involved in the regulation of ADH secretion, e.g. the hypothalmic osmoreceptors, the supraoptic paraventricular nuclei or the superior portion of the supraoptical hypophyseal tract. In contrast to adult patients, where the vast majority of causes of cDI is post traumatic or following neurosurgical interventions, in children and adolescents tumors or infiltrative diseases are more common causes of central DI. Here, we report the case of a 17-year-old adolescent who developed polyuria due to a central nervous germinoma associated with diabetes insipidus. Water deprivation led to a persistent urine production with low osmolality as well as ADH plasma concentrations. Administration of vasopressin resulted in increased urine osmolality, thereby excluding nephrogenic DI. Magnetic resonance imaging (MRI) of the brain and spinal cord revealed a bifocal mass in the pineal region and suprasellar area surrounded by marginal oedema. Both, signalling sequences and localisation appeared typical for a germinoma. Furthermore, lumbar puncture revealed increased liquor concentrations of placental alkaline phosphatase and human chorionic gonadotropin, thereby providing further indirect evidence for a central germinoma. A combined test of the anterior pituitary demonstrated partial defects of the thyreo- gonado- and adrenocorticotropic function.

Plasma concentration of antidiuretic hormone in response to intraruminal administration of butyrate in suckling calves

Our previous study has revealed that providing dry feeds increased the plasma concentration of antidiuretic hormone (ADH) in suckling calves, leading to altered water balance. To examine whether ketone bodies formed from ruminal fermentation-derived butyrate induced ADH secretion in suckling calves, the effects of intraruminal administration of butyrate on plasma concentration of ADH and ketone bodies, plasma and urine osmolality, and urine volume were examined. Six male Holstein calves aged 4 wk were used. Three levels of butyrate (0 g, 22 g and 44 g) were intraruminally administrated in a 3 × 3 Latin square design, and blood plasma and urine were analyzed. Plasma concentration of ketone bodies was increased by intraruminal administration of butyrate within 15 min in a dose-dependent manner, and the elevation of plasma levels continued until 4 h. Plasma concentration of ADH was also increased by the butyrate treatment, and it was higher in the 44 g butyrate group than in the 22 g butyrate group from 15 min to 2 h. The duration of the elevated plasma concentration of ADH was shorter than that of plasma concentration of ketone bodies. The relationship between plasma concentrations of ADH and those of ketone bodies was statistically significant, although the relationship was weaker. In accordance with the elevation of plasma ADH levels, the butyrate treatment resulted in the decreases in urine volume and increases in urine osmolality. Plasma osmolality was not different among the groups. The present results suggest that ruminal butyrate-derived ketone bodies are at least partly responsible for ADH secretion in suckling calves fed with dry feeds, and that the secreted ADH decreases urine volume through the increase in urine osmolality.

Nephrogenic diabetes insipidus in mice caused by deleting COOH-terminal tail of aquaporin-2

In mammals, the hormonal regulation of water homeostasis is mediated by the aquaporin-2 water channel (Aqp2) of the collecting duct (CD). Vasopressin induces redistribution of Aqp2 from intracellular vesicles to the apical membrane of CD principal cells, accompanied by increased water permeability. Mutations of AQP2 gene in humans cause both recessive and dominant nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin. In this study, we generated a line of mice with the distal COOH-terminal tail of the Aqp2 deleted (Aqp2(Delta230)), including the protein kinase A phosphorylation site (S256), but still retaining the putative apical localization signal (221-229) at the COOH-terminal. Mice heterozygous for the truncation appear normal. Homozygotes are viable to adulthood, with reduced urine concentrating capacity, increased urine output, decreased urine osmolality, and increased daily water consumption. Desmopressin increased urine osmolality in wild-type mice but had no effect on Aqp2(Delta230/Delta230) mice. Kidneys from affected mice showed CD and pelvis dilatation and papillary atrophy. By immunohistochemical and immunoblot analyses using antibody against the NH(2)-terminal region of the protein Aqp2(Delta230/Delta230) mice had a markedly reduced protein abundance. Expression of the truncated protein in MDCK cells was consistent with a small amount of functional expression but no stimulation. Thus we have generated a mouse model of NDI that may be useful in studying the physiology and potential therapy of this disease.

Expression of SPAM1 (PH-20) in the Murine Kidney Is Not Accompanied by Hyaluronidase Activity: Evidence for Potential Roles in Fluid and Water Reabsorption

Background:A role for <X_Underline>S</X_Underline>perm <X_Underline>A</X_Underline>dhesion <X_Underline>M</X_Underline>olecule <X_Underline>1</X_Underline> (SPAM1) hyaluronidase in murine kidney, where Spam1 transcript levels have been reported to be higher in males, has not been clarified. Methods:Spam1 RNA and protein were studied using RT-PCR, in situhybridization, Western blotting, immunohistochemistry and hyaluronic acid substrate gel electrophoresis. Urine volume and osmolality were studied in wild-type and Spam1 null mice. Results: While RT-PCR supported a tendency of higher RNA expression in males, no sex difference for the protein was detectable in the cortex, medulla, and urine. Transcripts were predominantly localized in the proximal tubules and glomeruli, with lower levels in the medulla. Similarly, Western blotting and immunohistochemistry revealed that SPAM1 is more abundant in the cortex. Hyaluronidase activity was absent at neutral and acidic pH: suggesting non-enzymatic role(s) for SPAM1. Wild-type and Spam1 null mice given free access to water showed significantly reduced urine volumes (p < 0.01; n = 12) in the latter.Baseline urine osmolality was similar in both, leading to a significantly (p < 0.05) lower osmolar output in the nulls. After water deprivation (24 h), a significant (p < 0.01) increase in urine osmolality was seen only for wild-type mice. Conclusion: SPAM1 is implicated in fluid reabsorption and urine concentration.

Refractory hyponatremia

Refractory hyponatremia

Maximum Urine Concentrating Capability for Transport Parameters and Urine Flow within Prescribed Ranges

In a mathematical model of the urine concentrating mechanism of the rat inner medulla, a nonlinear optimization technique was used to estimate parameter sets that maximize the urine-to-plasma osmolality ratio (U/P) and that maintain a urine flow rate within a plausible physiological range. The model assumed low solute permeabilities in the terminal part of the descending thin limb (DTL) and in the ascending thin limb of Henle’s loop. The parameters varied were: water flow and urea concentration in tubular fluid entering the DTL and collecting duct (CD) at the outer-inner medullary boundary; scaling factors for loops of Henle and CD population distributions; location and exponential increase rate of the urea permeability profile along the CD; and a scaling factor for the CD NaCl maximum active transport rate. The algorithm sought an optimum by simultaneously changing parameter values in the direction in which U/P increased. Parameters were allowed to vary within ranges suggested by physiologic experiments. The algorithm obtained a maximum U/P that increased urine osmolality by 42% above its base-case value. In the optimum case, only one parameter, CD water inflow, took on a value interior to its range; the other parameters assumed values at extrema of their ranges. This research was supported by NIH grants DK-042091 and S06GM08102, and NSF grant DMS-0340654.

Aquaporin-2 Excretion and Renal Function during the 1st Week of Life in Preterm Newborn Infants

In many preterm infants, a characteristic pattern of fluid and electrolyte homeostasis occurs during the 1st week of life, consisting of three phases: prediuretic, diuretic, and postdiuretic. In this study, we evaluated the possible role of aquaporin-2 (AQP2) in renal concentrating ability and correlated it with other markers of the renal function in healthy preterm infants. Daily urine and spot blood samples were collected from 9 healthy preterm (32 ± 1 weeks) infants at postnatal ages 1, 3, and 7 days. Urine and serum osmolality, creatinine, electrolytes, and AQP2 excretion were measured. All infants showed a significant (about 7%) weight loss on day 3 associated with a more than threefold increase in urine output without a significant change in fluid intake (diuretic phase). The creatinine clearance increased on day 3, indicating an increase in glomerular filtration rate. Interestingly, on day 3, the level of total excreted AQP2 (pmol/h) was significantly higher when compared to day 1 and day 7, and the same tendency was observed for urine osmolality. To conclude, the observed increase in urine osmolality and creatinine clearance during the diuretic phase, paralleled by an increase in total AQP2 excretion, suggests that AQP2 can contribute to the urinary concentrating ability early in postnatal life.

Disturbed Homeostasis in Sodium-Restricted Mice Heterozygous and Homozygous for Aldosterone Synthase Gene Disruption

We have determined that differences in expression of aldosterone synthase (AS) affect responses to a low-salt diet. In AS-null mice (AS(-/-)), but not in wild-type, low salt significantly decreased plasma sodium and increased potassium. The increased urine volume (1.5xwild-type) and decreased urine osmolality (0.7xwild-type), present in AS(-/-) mice on normal salt, became more severe (2.3xwild-type and 0.5xwild-type) on low salt, but neither changed in wild-type. In both genotypes, plasma vasopressin was similar on normal and low salt, and desmopressin injection significantly increased urine osmolality. Renal mRNA levels for aquaporin 1 and 3 were unchanged by genotype or diet and epithelial sodium channel and Na(+)-K(+)-2Cl(-)-cotransporter by genotype. In AS(-/-) mice, aquaporin 2 mRNA increased on normal salt, whereas Na(+)Cl(-)-cotransporter and cortex K(+) channel mRNAs decreased on both diets. The low blood pressure of AS(-/-) mice was decreased further by low salt, despite additional increases in renin, intrarenal arterial wall thickness, and macula densa cyclogenase-2 mRNA. In AS(+/-) mice on normal salt, adrenal AS mRNA was slightly decreased (0.7xwild-type), but blood pressure was normal. On low salt, their blood pressure was less than wild-type (101+/-2 mm Hg versus 106+/-2 mm Hg), even though renin mRNA increased to 2xwild-type. We conclude that aldosterone is critical for urine concentration and maintenance of blood pressure and even a mild reduction of AS expression makes blood pressure sensitive to low salt, suggesting that genetic differences of AS levels in humans may influence how blood pressure responds to dietary salt.