Urinary excretion of aquaporin-2 in term and preterm infants.

Aquaporin-2 (AQP-2) is a vasopressin-regulated water channel of the renal collecting duct and is excreted in human urine. We measured the urinary excretion of AQP-2 by radioimmunoassay in 14 term and 12 preterm infants aged 1 month. Excretion of AQP-2 was low compared with adults, and correlated significantly with urine osmolality in preterm infants. Our results demonstrate that AQP-2 water channels are expressed in the renal collecting duct of both term and preterm infants.

Partial renal resistance to arginine vasopressin as an adaptation to high altitude living.

BACKGROUND: The normal physiological response to high altitude (HA) is a decrease in total body water (TBW) and plasma and extracellular volume. The present investigation was designed to determine the mechanisms of the decrease in TBW with HA adaptation. METHOD: There were 10 men from the Southern Colombian Andes (2600 m) in Pasto, Colombia, who were compared to age-matched men from sea level (SL), Tampa, FL, in the U.S., with respect to their TBW, ability to handle a water load and response to exogenous arginine vasopressin (AVP). Measurements included circulating AVP, atrial natriuretic peptides [atrial natriuretic factor (ANF) and vessel dilator], and urinary excretion of the AVP sensitive water channel, aquaporin-2 (AQP2). RESULTS: The HA subjects had significantly (p < 0.01) lower TBW (29.37 +/- 0.98 vs 39.71 +/- 1.66 Kg), AQP2 excretion and vessel dilator circulating levels at baseline compared to SL subjects. ANF levels were not significantly different between the two groups. With water loading (20 ml.kg-1 in 15 min) there was a rapid increase in urine volume at 30 min with a decline thereafter in HA subjects while SL subjects had a gradual increment peaking at 120 min. There was a significant (p < 0.05) decrease in plasma AVP in the SL subjects within 30 min after the water load while the HA subjects had no significant decrease in AVP levels. Excretion of AQP2 decreased significantly after the water load only in the SL subjects. Administration of exogenous AVP increased AQP2 3- to 4-fold in the HA in comparison to SL subjects. CONCLUSIONS: Present data demonstrate the following adaptations to HA: decrease in TBW, better ability to handle a water load despite high levels of AVP, a significant decrease in the circulation of vessel dilator, and diminished excretion of AQP2 water channel. These findings indicate an insensitivity of the collecting duct of HA subjects to the actions AVP. However, exogenous administration of AVP caused a marked excretion of AQP2.

Urinary excretion of aquaporin-2 water channel protein in human and rat.

Previous studies by the authors demonstrated that the response of urinary aquaporin-2 (AQP2) excretion to dDAVP (deamino-8-D-arginine vasopressin) infusion is an index of vasopressin action on the kidney (N Engl J Med 332: 1540-1545, 1995). In the study presented here, the characteristics of urinary excretion of AQP2 were examined further. An RIA suitable for AQP2 in the urine was established. Relatively high concentrations of detergent and bovine serum albumin in the RIA buffer allowed analysis of urine samples with a wide range of concentrations and increased the sensitivity of the assay. AQP2 in the urine existed as a high molecular weight form of approximately 190 kD by HPLC analysis. The mean urinary AQP2 concentration corrected for creatinine in spot urine samples of healthy subjects who voided in the morning was 1081 +/- 699 fmol/mg creatinine (mean +/- SD, n = 208). The amount of daily excretion of AQP2 in the urine was the same in men and women. Urinary AQP2 content was not affected by age of the subjects and showed a positive correlation with urine osmolality. Finally, the fraction of AQP2 excreted in the urine compared with whole kidney content was determined in the rat. Approximately 3% of AQP2 in the kidney was excreted daily, and this fraction did not change when rats were dehydrated for 3 d. These data demonstrate the necessity of establishing well-designed protocols to use urinary AQP2 as a marker of AVP action.

Urinary excretion of aquaporin-2 in the diagnosis of central diabetes insipidus.

We determined whether alteration in urinary excretion of aquaporin-2 (UAQP-2) is of value to diagnose central diabetes insipidus (CDI). First, UAQP-2 was determined in 16 normal subjects under ad libitum water drinking (n = 6) and after an overnight dehydration (n = 10). UAQP-2 has a positive correlation with plasma arginine vasopressin (AVP) levels (r = 0.61, P < 0.05) but not with urinary osmolality (Uosm). Second, a hypertonic saline (5% NaCl)-infusion test was studied in 5 normal subjects (21 to 25 yr old) and 10 patients with CDI (22-68 yr). After drinking water ad libitum, they were given 20 mL/kg water orally and then given 5% NaCl (0.05 mL/kg x min) i.v. for 120 min. Finally, 0.1 U of AVP was administered i.v. During the period, 30-min urine collections were made. In the normal subjects, after the infusion of 5% NaCl, plasma AVP levels and Uosm markedly increased in parallel with an increase in plasma osmolality (Posm, 294-320 mOsm/kg H2O; Uosm, 102-737 mOsm/kg H2O; AVP, 0.4-2.6 pg/mL, P < 0.001). In the CDI patients, plasma AVP and Uosm failed to increase, despite an increase in Posm (Posm, 306-332; Uosm, 102-164; AVP, 0.9-1.2). UAQP-2 was markedly greater in the normal subjects than the CDI patients (7.2 vs. 0.9 pmol/L/mg creatinine, P < 0.05) under water intake ad libitum. UAQP-2 was changeable in the wide range in physiological condition. After the 5%-NaCl infusion, UAQP-2 elevated to 12.5 from 0.9 pmol/L x mg creatinine in the normal subjects. In contrast, UAQP-2 remained low during the 5%-NaCl infusion in the CDI patients. Exogenous AVP promptly increased UAQP-2 to a similar extent in two groups of the normal subjects and the CDI patients. These results indicate that measurement of UAQP-2 is of value to diagnose CDI in the 5%-NaCl infusion test.

Urinary content of aquaporin 1 and 2 in nephrogenic diabetes insipidus.

Hereditary nephrogenic diabetes insipidus (NDI) is caused by mutations in either the X-chromosomal gene encoding the vasopressin V2-receptor or in the autosomal gene encoding aquaporin-2. Expressed in Xenopus oocytes, the AQP2 gene mutations found in NDl have been shown to reduce the stability of the encoded protein. This study investigated the in vivo stability of mutant and wild-type aquaporin-2 proteins by measuring their excretion in urine of NDl patients and healthy individuals. On immunoblots, the urine samples from healthy volunteers revealed clear aquaporin-1 and aquaporin-2 signals in antidiuretic but not diuretic states. In the urine of a female patient, whose NDl is explained by low expression of the wild-type V2-receptor gene, aquaporin-2 excretion was high and comparable with that in a healthy individual during antidiuresis. In the urine of a male patient with a non-sense mutation in the V2-receptor gene, a weak aquaporin-2 signal was detected. In NDl patients with mutations in the aquaporin-2 gene, aquaporin-2 could not be detected in urine, suggesting a low stability of mutant aquaporin-2 proteins. In four out of seven NDl patients, aquaporin-1 excretion was relatively high, which suggests a compensatory increase in proximal reabsorption in NDl.

Urinary excretion of aquaporin-2 in humans: a potential marker of collecting duct responsiveness to vasopressin.

The vasopressin-sensitive water channel (aquaporin 2; AQP-2) mediates water transport across the apical plasma membrane of the renal collecting ducts and is excreted in human urine. This study presents the hypothesis that measurements of the AQP-2 excretion rate might be used as a marker of collecting-duct responsiveness to vasopressin, and therefore could be useful in the clinical evaluation of various water-balance disorders. This study presents information about the development of an antibody to human AQP-2, and measures the urinary excretion of AQP-2 by quantitative Western analysis. A standard curve of band densities was generated by using known quantities of the modified immunizing peptide to derive the amount of AQP-2 contained in aliquots of urine. AQP-2 urinary excretion changed with short-term alterations in hydration status produced either by water loading (76% decrease, P < 0.01) or by 3% sodium chloride (760% increase, P < 0.01). Steady-state 24-h urinary excretion of AQP-2 was 43 +/- 10 nmol/24 h (or 28.5 +/- 6.9 pmol/mg creatinine), and 20 +/- 6 nmol/24 h (or 18.3 +/- 7.9 pmol/mg creatinine) in men and women, respectively. Therefore, urinary AQP-2 excretion can be quantified by using Western analysis, and may serve as a marker of collecting-duct responsiveness to vasopressin in different physiologic settings.

Urinary excretion of aquaporin-2 in patients with diabetes insipidus.

BACKGROUND: Urine-concentrating ability is regulated by vasopressin. Recently, the specific water-channel protein of the renal collecting duct, known as aquaporin-2, was cloned. However, it is not certain whether this molecule is responsive to vasopressin. METHODS: We measured the urinary excretion of aquaporin-2 and its response to vasopressin in 11 normal subjects and 9 patients with central or nephrogenic diabetes insipidus. The urine samples were collected during periods of dehydration and hydration and after the administration of vasopressin. Urine samples were analyzed for aquaporin-2 by the Western blot assay and immunogold labeling, and the amount of aquaporin-2 was determined by radioimmunoassay. RESULTS: Aquaporin-2 was detectable in the urine in both soluble and membrane-bound forms. In the five normal subjects tested, the mean (+/- SE) urinary excretion of aquaporin-2 was 11.2 +/- 2.2 pmol per milligram of creatinine after a period of dehydration, and it decreased to 3.9 +/- 1.9 pmol per milligram of creatinine (P = 0.03) during the second hour after a period of hydration. In the six other normal subjects, an infusion of desmopressin (1-desamino-8-D-arginine vasopressin) increased the urinary excretion of aquaporin-2 from 0.8 +/- 0.3 to 11.2 +/- 1.6 pmol per milligram of creatinine (P < 0.001). The five patients with central diabetes insipidus also had increases in urinary excretion of aquaporin-2 in response to the administration of vasopressin, but the four patients with X-linked or non-X-linked nephrogenic diabetes insipidus did not. CONCLUSIONS: Aquaporin-2 is detectable in the urine, and changes in the urinary excretion of this protein can be used as an index of the action of vasopressin on the kidney.

Mutations in aquaporin-1 in phenotypically normal humans without functional CHIP water channels.

The gene aquaporin-1 encodes channel-forming integral protein (CHIP), a member of a large family of water transporters found throughout nature. Three rare individuals were identified who do not express CHIP-associated Colton blood group antigens and whose red cells exhibit low osmotic water permeabilities. Genomic DNA analyses demonstrated that two individuals were homozygous for different nonsense mutations (exon deletion or frameshift), and the third had a missense mutation encoding a nonfunctioning CHIP molecule. Surprisingly, none of the three suffers any apparent clinical consequence, which raises questions about the physiological importance of CHIP and implies that other mechanisms may compensate for its absence.

A cationic protein from a urate-calcium oxalate stone: isolation and purification of a shared protein.

A protein extracted from a urate-calcium oxalate stone by electrodialysis is also excreted in the urine which served as the source material for its purification by FPLC after separation on an ACA44 column. It has an amino acid composition appropriate for a cationic protein. One peptide obtained by cyanogen bromide cleavage has significant (approximately 60%) homology with CD59 protein (protectin). Both proteins have wide distribution, the unknown having been found in bile, cholesterol gallstones, and the wall of the aorta. However, the two proteins appear to be immunologically different.