Effect of water deprivation and hypertonic saline infusion on urinary AQP2 excretion in healthy humans

R. S. Pedersen, H. Bentzen, J. N. Bech, and E. B. Pedersen

Department of Medicine, Holstebro Hospital, DK-7500 Holstebro, Denmark


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Arginine vasopressin (AVP) mediates water transport in the renal collecting ducts by forming water channels of aquaporin-2 (AQP2) in the apical plasma membrane. AQP2 is excreted in human urine. We wanted to test the hypothesis that urinary excretion of AQP2 (u-AQP2) reflects the effect of AVP on the renal collecting ducts during water deprivation and hypertonic saline infusion in healthy subjects. Fifteen healthy subjects underwent a 24-h period of fluid restriction. Urine and blood samples were collected at timed intervals. Fifteen healthy subjects were given 7 ml/kg 3% hypertonic saline infusion for 30 min. Urine and blood samples were collected at timed intervals. During fluid restriction, the u-AQP2 rate increased from 3.9 (25th percentile: 3.1; 75th percentile: 5.2) to 7.6 (5.9-9.1; P < 0.001) ng/min, and the plasma AVP (p-AVP) level increased from 0.5 (0.4-0.6) to 3 (1.7-3.3) pmol/l. There was a positive correlation between the maximum change in u-AQP2 rate and the maximum change in p-AVP (r = 0.57, P < 0.03). During the infusion study, u-AQP2 rate was at maximum 90 min after the infusion [baseline: 4.5 ng/min (3.5-4.8); 90 min: 5 ng/min (4.5-6.0) P < 0.02]. p-AVP increased from 1.0 (0.9-1.1) to 1.5 (1.2-1.8; P < 0.002) pmol/l. There was a positive correlation between the maximum change in u-AQP2 rate and the maximum change in p-AVP (r = 0.83; P < 0.0001). It can be concluded that p-AVP and u-AQP2 are increased during thirst and hypertonic saline infusion and that u-AQP2 reflects the action of AVP on the collecting ducts.

arginine vasopressin; healthy subjects; urinary output; aquaporin-2


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

THE PEPTIDE HORMONE arginine vasopressin (AVP) controls body water balance by regulating renal water excretion. AVP induces a rapid and dramatic increase in the water permeability of the renal collecting ducts. The permeability depends on the formation of water channels by aquaporin-2 (AQP2). In animal studies, it has been shown that AVP binds to V2 receptors in the basolateral membrane of the collecting duct cells, thereby increasing water transport across the cells. These receptors are linked via a heterotrimeric G protein to adenylate cyclase and consequently cause an increase in cytosolic cAMP, which acts as the second messenger in the acute response. This cascade results in the transfer of AQP2 from intracellular vesicles to the apical plasma membrane, thereby increasing the water permeability of the collecting duct by forming channels for water passage. When AVP levels decline, AQP2 is retrieved endocytically, and the membrane returns to its resting state of lower water permeability (10, 12, 22). The function of AQP2 is also regulated through long-term effects that change the total abundance of AQP2 in collecting duct cells. In animals deprived of water for 24 h or more, a sustained increase in the collecting duct water permeability was observed (9). Moreover, immunoblotting studies and Northern blot assays showed that the levels of mRNA and protein for AQP2 were increased in response to water deprivation (4, 20). Recently, Rai et al. (16) demonstrated urinary excretion of AQP2 (u-AQP2) in rats. The fraction of AQP2 in the urine from rats was ~3% of the total amount of AQP2 in the kidney and did not change during water deprivation.

In patients with central diabetes insipidus, the AQP2 excretion was low or absent, and it could be restored after treatment with 1-desamino-8-D-AVP (dDAVP; synthetic AVP V2 receptor agonist; see Ref. 7). It was therefore considered that u-AQP2 might be used as a marker for the action of AVP on the collecting ducts. However, an analysis of the relationship between u-AQP2, plasma concentration of AVP (p-AVP), and urine osmolality on one hand and the renin-angiotensin system and the natriuretic peptides on the other has not been performed in healthy subjects during water deprivation or hypertonic saline infusion.

The purpose of the present study was to measure the effect of water deprivation during 24 h and hypertonic saline infusion on 1) u-AQP2, 2) p-AVP, 3) osmolality in plasma and urine, 4) the renin-angiotensin system, and 5) the natriuretic peptide system. We wanted to test the hypothesis that u-AQP2 reflects the effect of AVP on the collecting ducts in the kidney and measure the changes in the renin-angiotensin system and the natriuretic peptide during water deprivation and hypertonic saline infusion.


    MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Subjects

In the water deprivation study, the following inclusion criteria were used: 1) men and women and 2) age 20-50 yr. Exclusion criteria were 1) a history or clinical signs of disease of the heart, lungs, liver, kidneys, or endocrine organs, 2) neoplastic disease, 3) arterial hypertension, 4) alcohol or drug abuse, 5) medical treatment except for birth control pills, 6) unwillingness to participate, and 7) abnormal laboratory screening tests , i.e., Hb, white cell count, platelets, plasma sodium, plasma potassium, plasma creatinine, plasma albumin, blood-glucose, serum cholesterol, plasma bilirubin, and plasma alanine aminotransferase in blood samples and albumin and glucose in urine.

In the hypertonic saline infusion study, the same inclusion and exclusion criteria were used. The studies were approved by the local medical ethics committee, and written informed consent was obtained from all participants.

Experimental Procedures

Water deprivation. A 24-h urine collection was performed the day before the study and was used as baseline values. The subjects were allowed to drink water freely until the beginning of the study. During the following 24 h of fluid restriction, the subjects were allowed to eat foods low in fluid content but were instructed to avoid all liquid intake. Smoking was prohibited during the study. An indwelling catheter for blood sampling was placed in a forearm vein. Urine was collected every 4 h. Voiding took place in the standing or sitting position. Blood samples were drawn at 0800, 1400, 2200, and 0800 the following day, and plasma was analyzed for AVP, osmolality, renin concentration (PRC), ANG II, atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and hematocrit. The subjects were in the supine position a least 60 min before the blood samples were drawn. Blood pressure and pulse rate were measured before voiding, and afterward the subjects were weighed. If the subjects had lost 4% body wt or more they were excluded. There were no excluded subjects. Urinary collections were analyzed for u-AQP2, urine osmolality, and sodium.

Hypertonic saline infusion. The subjects were instructed to take 1-g sodium chloride tablets two times a day during 2 days before the study and to drink 45 ml/kg body wt of fluid 24 h before the study. A 24-h urine collection was performed the day before the study. On the study day, an oral water load of 200 ml tap water every 30 min was started at 0700, and after 1000 it was reduced to 100 ml every 60 min. Two indwelling catheters for blood sampling and administration of hypertonic sodium chloride were placed in forearm veins, one in each arm. Urine was collected at 0900, 0930 (baseline), 1000 (30 min), 1100 (90 min), 1200 (150 min), and 1300 (210 min). Voiding took place in the standing or sitting position. The subjects were otherwise in the supine position during the experiment. At 0930, 7 ml/kg of 3% NaCl were given over 30 min. Blood samples were drawn every 60 min. Analysis of the plasma levels of AVP, PRC, ANG II, ANP, BNP, and hematocrit was performed from samples drawn at 0900 (-30 min) and from samples drawn at 1100 (90 min after the beginning of the infusion) and 1300 (210 min after infusion start). In addition, plasma levels of osmolality and sodium were analyzed from each blood sample drawn. Urine was analyzed for u-AQP2, urine osmolality, and urine sodium. The subjects were weighed before and after the study. Thirst ratings were obtained each time the urine was collected. A scale from 0 to 10 was used; 0 indicated no thirst at all, and 10 indicated extreme thirst.

Methods

AQP2. AQP2 was measured by a newly developed RIA. Urine samples of 250-500 ml from a 24-h urine collection and from each urine sample were kept frozen at -20°C until assayed. The AQP2 standard (synthetic AQP2 peptide) and the AQP2 antibodies were gifts from professor Søren Nielsen, Department of Cell Biology, Institute of Anatomy, Aarhus University. Antibodies (LL358) were raised in rabbits to a synthetic peptide corresponding to the 15 COOH-terminal amino acids in human AQP2 (19) to which was added an NH2-terminal cystein for conjugation and affinity purification, as described previously for other anti-AQP2 antibodies (1, 12). The antibody was characterized by immunoblotting using membrane fractions from the human kidney inner medulla. This revealed 28- and 35- to 55-kDa bands corresponding to unglycosylated and glycosylated AQP2, consistent with previous reports using other antibody preparations (19). For the RIA, both the affinity-purified and the immune serum preparations were tested. Immune serum (LL358) was used for the measurements presented in this report. Iodination of AQP2 was performed by the chloramine T method using 40 mg of AQP2 and 37 MBq 125I. The reaction was stopped by addition of 22% human serum albumin. 125I-labeled AQP was separated from the iodination mixture by the use of a Sephadex G-10 column. The assay buffer was 40 mM sodium phosphate (pH = 7.4), 0.2% human albumin, 0.05% sodium azide, 0.1% Triton X-100, and 0.4% EDTA. A 1.5% solution of gamma globulins from pig (Sigma) and 25% polyethylene glycol 6000 (Merck) also containing 0.625% Tween 20 (Merck) was prepared using the 0.4 M phosphate buffer. Fresh urine samples [125-3,000 ml (depending on urinary osmolality)] were centrifuged for 5 min at 1.6 × 100 g (3,000 rpm). The supernatant was freeze-dried and kept frozen at -20°C until assayed. The mixture of 200 ml of standard or freeze-dried urine extracts redissolved in 200 ml assay buffer and 100 ml of antibody was incubated for 24 h at 4°C. Next, 100 ml of the tracer were added, and the mixture was incubated for a further 24 h at 4°C. Gamma globulin from pigs (100 ml) and 2 ml polyethylene glycol 6000 were added. The mixture was centrifuged at 3,500 rpm for 20 min at 4°C. The supernatant (free fraction) was poured off, and the precipitate (bound fraction) was counted in a gamma counter. The unknown content in urine extracts was read from a standard curve. For 21 consecutive standard curves, the zero standard was 32 ± 1.4%, the intercept for 50% binding inhibition was 987 ± 147 pg/tube, and the nonspecific binding was 4.0 ± 0.7%. The interassay variation was determined by quality controls from the same urine pool spiked with AQP2 standard. In 10 consecutive assays at a mean level of 429 pg/tube, the coefficient of variation was 11.7%, and after addition of 100, 200, and 400 pg AQP/tube the coefficients of variation were 11.9% (9 assays), 13.4% (9 assays), and 14.4% (8 assays), respectively. In 10 consecutive assays at a mean level of 240 pg/tube, the coefficient of variation was 7.7%, and after addition of 100, 200, and 400 pg AQP/tube the coefficients of variation were 11.5% (9 assays), 13.6% (9 assays), and 14.6% (8 assays), respectively. The intra-assay variation was determined on samples from the same urine pool in several assays at different concentration levels. At a mean level of 353 (n = 20), 500 (n = 20), 563 (n = 20), and 1,972 pg/tube (n = 20), the coefficients of variation were 5.3, 5.4, 5.3, and 7.7%, respectively. In addition, the coefficient of variation was calculated on the basis of 33 duplicate determinations in 20 different assays to 5.9% in the range of 200-600 pg/tube.

Recovery of unlabeled AQP2 added to urine with an AQP2 level of ~200 pg/tube was 95 ± 12% after addition of 200 pg AQP2 in nine consecutive assays and 98 ± 11% in eight consecutive assays after addition of 400 pg AQP2. Dilution of urine spiked with AQP2 with factor 2 from a mean level of ~600 and 400 pg/tube resulted in a measured level of the expected 105 ± 11% in eight consecutive assays and 104 ± 14% in seven consecutive assays. Recovery of the labeled tracer after freeze-drying was 98%. The sensitivity calculated as the smallest detectable difference at the 95% confidence limit was 32 pg/tube in 33 duplicate determinations in the range of 200-600 pg/tube using 20 different assays.

AVP. AVP in plasma was measured by RIA, which was a modification of the method described previously (14). Before the assay procedure, C18 Sep-Pak extraction was performed. The antibody was a gift from professor Jacques Dürr (Miami, FL). The minimum detection level was 0.5 pmol/l. The coefficients of variation were 13% (interassay) and 9% (intra-assay).

Renin and ANG II. PRC was measured by a commercial immunoradiometric assay (Nichols Institute Diagnostics, Geneva, Switzerland). The coefficients of variation were 2.5% (intra-assay) and 9.9% (interassay). The minimal detection level was 1.4 mU/ml.

ANG II in plasma was determined by RIA using a modification (4) of the method originally described by Kappelgaard et al. (8). RIA was performed after previous extraction of plasma by Sep-Pak C18 cartridges (Waters, Milford, MA). The antibody was obtained from the Department of Clinical Physiology, Glostrup Hospital (Glostrup, Denmark). The minimal detection level was 2 pmol/l plasma. The coefficients of variation were 12% (interassay) and 8% (intra-assay).

Natriuretic peptides. ANP in plasma was determined by RIA, as previously described (15). ANP was extracted from plasma by means of Sep-Pak C18 cartridges, using ethanol, acetic acid, and water. For RIA, rabbit anti-ANP antibody was obtained from the Department of Clinical Chemistry, Bispebjerg Hospital (Copenhagen, Denmark). The minimum detection level was 0.5 pmol/l plasma. The coefficients of variation were 12% (interassay) and 10% (intra-assay).

BNP in plasma was measured by RIA, as previously described (6). Immunoreactive BNP was extracted from plasma by use of Sep-Pak C18 cartridges eluted by 80% ethanol in a 4% acetic acid solution. RIA was performed using a rabbit anti-BNP antibody without cross-reactivity with ANP and urodilatin. The minimum detection level was 0.5 pmol/l plasma. The coefficients of variation were 11% (interassay) and 6% (intra-assay).

Other measurements. Plasma and urinary concentrations of sodium and potassium were measured by routine methods at the Department of Clinical Chemistry, Holstebro Hospital. Plasma and urinary osmolality were measured by freezing-point depression (Advanced model 3900 multisample osmometer).

Blood pressure was determined by a UA-743 digital blood pressure meter (A&D).

Statistics

24 h of water deprivation. Data from 15 healthy subjects were included in the statistical analyses. The 24-h urine collection performed the day before the study was used as the baseline. All results are given as medians with 25th and 75th percentiles. Because a lack of normality or inhomogeneity of the variances was observed, we used Friedmann's ANOVA for paired comparisons within the groups followed by Wilcoxon's signed ranks test for comparisons between the groups. Correlations were calculated by Spearman's test. P < 0.05 was considered the limit of significance.

Hypertonic saline infusion study. Data from 15 healthy subjects were included in the statistical analyses. The start of the infusion (0930) was used as the baseline for all urine samples, and 30 min before (0900) infusion was used as the baseline for all blood samples. All results are given as medians with 25th and 75th percentiles. Because a lack of normality or inhomogeneity of the variances was observed, we used Friedmann's ANOVA for paired comparisons within the groups followed by Wilcoxon's signed ranks test for comparisons between the groups. Correlations were calculated by Spearman's test. P < 0.05 was considered the limit of significance.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Demographics

Fifteen healthy subjects with a mean age of 28 yr (range 23-33), eight men and seven women, were studied in the water deprivation study. Fifteen healthy subjects with a mean age of 31 yr (range 25-37), seven men and eight women, were studied in the infusion study. Clinical and laboratory data for both groups are given in Table 1.

                              
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Table 1.   Clinical and laboratory data in the water deprivation study and the hypertonic saline infusion study

Water Deprivation

AQP2, AVP, and osmolality. Figure 1 shows that u-AQP2 given as nanograms per millimole creatinine gradually increased during fluid restriction and reached a maximum after 20 h (732 ng/mmol creatinine; 25th percentile: 617; 75th percentile: 962) that was significantly higher than the baseline level (437 ng/mmol creatinine; 382-464). After 12 h of thirst, u-AQP2 was significantly elevated over baseline (P < 0.01). The u-AQP2 excretion rate given in nanograms per minute changed in a similar way (Table 2) but was already significantly increased after 8 h (baseline: 3.9 ng/min, 3.1-5.2; 8 h: 6 ng/min, 4.9-6.4; P < 0.02). The maximal increase was seen after 20 h and was 89% (P < 0.001) when u-AQP2 was expressed as nanograms per millimole creatinine and 79% (P < 0.001) when expressed as nanograms per minute.


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Fig. 1.   Urinary (u) aquaporin-2 (AQP2) excretion during 24 h of water deprivation in 15 healthy subjects. Baseline, 24-h urine collection before the experiment; 4 h, 1200; 8 h, 1600; 12 h, 2000; 16 h, 2400; 20 h, 0400; 24 h, 0800 the next morning. Results are medians with 25th, 75th, 10th, and 90th percentiles. *P < 0.01 and **P < 0.001, significant deviation from baseline.


                              
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Table 2.   u-AQP2, u-osm, u-Na, and V during 24 h of water deprivation in 15 healthy subjects

Figure 2 shows that AVP changed almost linearly during the study from 0.5 (0.4-0.6) to 3 (1.7-3.3) pmol/l. The increase was significant compared with baseline (P < 0.001) after 6 h.


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Fig. 2.   Plasma arginine vasopressin (AVP) during 24 h of water deprivation in 15 healthy subjects. Baseline, beginning of the test at 0800; 6 h, 1400; 14 h, 2200; 24 h, 0800 the next morning. Results are medians with 25th, 75th, 10th, and 90th percentiles. *P < 0.001, significant deviation from baseline.

Table 2 shows that urine osmolality increased 109% (P < 0.001) during the 24 h of water deprivation. There was a initial fall in urine osmolality between baseline and 4 h. The greatest increase was seen from 4 h (246 mosmol/kgH2O) and 8 h (765 mosmol/kgH2O) after baseline. During the rest, a plateau of ~1,000 mosmol/kgH2O was reached. The increase was already significant after 8 h. As expected, the urine volume was gradually reduced during the study (Table 2).

Relationship between AQP2, AVP, and osmolality. Figure 3 shows a positive correlation between the maximum changes in u-AQP2 rate (ng/min) and the maximum changes in AVP (r = 0.67, P < 0.006) and between the maximum changes in u-AQP2 excretion (ng/mmol creatinine) and the maximum changes in AVP (r = 0.57, P < 0.03) during water deprivation. There was no correlation between urinary osmolality on one hand and AQP2-creatinine and AQP2 rate on the other.


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Fig. 3.   Correlation between the maximum change (triangle ) in AVP and u-AQP2-creatinine (A) and between AVP and AQP rate (B) in 15 healthy subjects during 24 h of water deprivation.

Renin, ANG II, ANP, and BNP. Table 3 shows that PRC and ANG II increased significantly but PRC only to a modest extent during water deprivation. ANP was unchanged, whereas BNP was reduced significantly.

                              
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Table 3.   PRC, ANG II, ANP, and BNP in 15 healthy subjects during 24 h of water deprivation

Blood pressure, body weight, serum osmolality, serum sodium, and hematocrit. The systolic blood pressure decreased during the study from 120 mmHg (114) to 110 mmHg (102) at 0400 in the morning. The diastolic blood pressure changed in a similar way from 70 mmHg (63-77) to 61 mmHg (55-66). The pulse rate did not change significantly during the study. Serum osmolality increased from 284 mosmol/kgH2O (281) to 290 mosmol/kgH2O (288-295; P < 0.001), and the body weight decreased with a mean of 3.2%. During the study, the hematocrit increased from 0.38 (0.37-0.43) to 0.39 (0.38-0.46; P < 0.002) liters. Serum sodium increased at the beginning from 141 mmol/l (139) to a maximum 143 mmol/l (142) after 14 h (P < 0.001) and remained at that level during the rest of the experiment.

Hypertonic Saline Infusion

AQP2, AVP, and osmolality. Figure 4 shows that already after 30 min of saline infusion there was a significant increase in u-AQP2 from 433 ng/mmol creatinine (341) to 494 ng/mmol creatinine (371-708; P < 0.03). U-AQP2 still increased to a maximum 595 ng/mmol creatinine (535-758; P < 0.006) 90 min after the infusion was started, i.e., a 37% increase. Hereafter, it decreased to the preinfusion level after 210 min. AQP2 rate was only significantly increased 90 min after the start of infusion when it had increased from 4.5 ng/min (3.5-4.8) to 5.0 ng/min (4.5-6.0; Table 4).


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Fig. 4.   u-AQP2 during the hypertonic saline infusion in 15 healthy subjects. Times are before (0 min) and after (30, 90, 150, and 210 min) hypertonic saline infusion start. Results are medians with 25th, 75th, 10th, and 90th percentiles. *P < 0.05 and **P < 0.01, significant deviation from preinfusion level (0 min) within group.


                              
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Table 4.   u-AQP2, u-osm, and V in 15 healthy subjects given hypertonic saline infusion

Figure 5 shows that AVP increased from 1.0 pmol/l (0.9-1.1) to 1.5 pmol/l (1.2-1.8; P < 0.002) 90 min after the infusion started. The level of AVP was normalized after 210 min.


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Fig. 5.   Plasma AVP in 15 healthy subjects during hypertonic saline infusion. Times are before (-30 min) and after (90 and 210 min) start of hypertonic saline infusion. Results are medians with 25th, 75th, 10th, and 90th percentiles. *P < 0.01, significant deviation from preinfusion level (-30 min) within group.

The urinary osmolality changed from 110 mosmol/kgH2O (88) to a maximal 588 mosmol/kgH2O (536-660; P < 0.001) 150 min after the infusion start (Table 4). The urine volume followed the same pattern as urinary osmolality by decreasing from 9.9 ml/min (7.2-12.9) to 2.3 ml/min (range 1.9-2.8) 150 min after the infusion start.

Relationship among AQP2, AVP, and osmolality. Figure 6 shows the maximum change in AVP plotted against the maximal change in AQP2. A highly significant correlation was found between u-AQP2 rate and AVP (r = 0.83, P < 0.0001). No correlation between u-AQP2-creatinine and AVP was found. There was no correlation between AQP2-creatinine and AQP2 rate on one hand and urinary osmolality on the other.


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Fig. 6.   Correlation between the maximum change in AQP2 rate and maximum change in AVP during hypertonic saline infusion in 15 healthy subjects.

Renin, ANG II, ANP, and BNP. Table 5 shows that both PRC and ANG II were suppressed by saline infusion and ANP and BNP increased.

                              
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Table 5.   PRC, ANG II, ANP, and BNP in 15 healthy subjects given hypertonic saline infusion

Blood pressure, serum osmolality, serum sodium, and hematocrit. The blood pressure was compared with the pressure taken at -30 min, and there was a significant increase in both the systolic and diastolic blood pressure immediately after the infusion (30 min) was stopped (P < 0.05). During the rest of the experiment, there was no change compared with baseline. The pulse rate did not change significantly during the experiment.

The mean amount of saline infusion was 496 ml. During the study, serum sodium and serum osmolality increased from 141 mmol/l (139) to 145 mmol/l (144-147; P < 0.001) and from 285 mosmol/kgH2O (283) to 292 mosmol/kgH2O (289-295; P < 0.001), respectively, after the infusion. Thereafter, serum sodium and serum osmolality slowly decreased during the experiment, reaching 142 mmol/l (140) and 284 mosmol/kgH2O (282) at the end of the study. The hematocrit decreased from 0.40 liters (0.38-0.43) at baseline to 0.38 liters (0.36-0.41; P < 0.001) after 210 min. The feeling of thirst was registered on a scale from 0 to 10 and changed from 1.9 before the infusion to 4.4 right after the infusion (P < 0.003). The thirst sensation reached preinfusion levels 150 min after baseline.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

In the present study, we showed that u-AQP2 was increased during antidiuresis in normal subjects both when it was induced by water deprivation and by hypertonic saline infusion. During water deprivation, the increase in u-AQP2 occurred within hours, whereas the increase in u-AQP2 appeared within minutes during hypertonic saline infusion. During both tests, p-AVP increased, and there was a strong correlation between the maximum excretion rate of AQP2 and the maximum increase in p-AVP. The AQP2 results were only expressed in values that were independent of the diuresis because in our experiments we affected the diuresis either by infusion of hypertonic saline or by water deprivation.

Previous studies have shown that acute changes in plasma osmolality resulted in changes in u-AQP2. Elliot et al. (3) found an increase in u-AQP2 during a period of fluid restriction followed by infusion of hypertonic saline. The effect of water deprivation and hypertonic saline infusion has been addressed previously by Saito et al. (18). In that small study, only five healthy subjects were pretreated with an acute water load (20 ml/kg body wt) followed by a 5% hypertonic saline infusion during 120 min. During the acute water load, u-AQP2 decreased to one-third of that during ad libitum water drinking, whereafter it increased 12-fold during the 5% hypertonic saline infusion. Saito et al. used a combined stimulus of volume depletion and hypertonic saline infusion. In contrast, we demonstrated the effect of water deprivation alone and hypertonic saline infusion alone. In addition, our study comprises larger groups of control subjects, with 15 subjects in each group. These differences in the design and number of subjects can explain the differences in the increase in u-AQP2 between the present study and the previous one (18). In the study of Saito et al. (18), the effect of water deprivation was used to establish a relationship between u-AQP2 and AVP or urine osmolality, but only after an overnight dehydration. In the present study, a 24-h water deprivation was used, and we did repeated measurements in a larger group of subjects during the whole water deprivation period. Those normal subjects with an unrestricted water intake and subjects who had undergone an overnight dehydration were included in the correlation analysis between u-AQP2 and AVP or urine osmolality in the study of Saito et al. (18). In the present study, the relationship between u-AQP2 and AVP was only analyzed after one type of stimulation, and a positive correlation was demonstrated between the two parameters both during water deprivation and during hypertonic saline infusion.

These are new observations, and for the first time positive correlations between the maximum changes in AVP and the maximum changes in u-AQP2 rate are described. Our results show that u-AQP2 reflects the events taking place in the collecting ducts.

The release of AVP is controlled by both osmotic and nonosmotic stimuli. The major stimulus for AVP secretion is hypertonicity. An increase in tonicity is sensed by hyperthalamic osmoreceptors, leading to an enhancement of the AVP secretion. In addition, AVP secretion is regulated by volume and baroreceptors in the large vessels close to the heart. The sensitivity of these receptors with regard to volume depletion and blood pressure reduction is significantly lower than the osmoreceptors; i.e., a small increase in osmolality but a larger decrease in blood volume or blood pressure is needed to stimulate AVP secretion. During the hypertonic saline infusion, the increase in serum osmolality resulted in an increase in AVP, even if blood pressure increased. During water deprivation, there might have been an osmotic and a nonosmotic stimuli for release of AVP because the plasma sodium increased simultaneously with a decrease in blood pressure.

In addition to endogenous AVP, the renin-angiotensin-aldosterone system is a major factor in water and sodium homeostasis. In the present study, increasing serum osmolality either by water deprivation or hypertonic saline infusion resulted in a similar change in AVP but a different response in the renin-angiotensin system. During water deprivation, there was a significant increase in PRC and ANG II, consistent with an extracellular volume depletion and a decrease in blood pressure. In addition, thirst is also known to increase ANG II secretion. Presumably, as a consequence of the increase in ANG II, there was a decrease in urinary sodium excretion because ANG II increases the reabsorption of sodium in the proximal tubules. Hypertonic sodium infusion leads to an increase in blood pressure and an inhibition in the renin-angiotensin-aldosterone system, most likely due to extracellular volume expansion. During the hypertonic saline infusion, there was a significant increase in both ANP and BNP. An increase in BNP during hypertonic saline infusion is not reported earlier. The increase in plasma concentration of the natriuretic peptides can be seen as a compensatory phenomenon to restore and maintain sodium and water balance. An increased release of ANP is known to inhibit the renin-angiotensin axis, consistent with the findings in the hypertonic saline infusion study.

Our short-term studies suggest that u-AQP2 reflects the action of AVP on the collecting duct. Previous animal studies have shown that two distinct mechanisms are involved in the function of AQP2 (12). During short-term regulation, AVP induces an increase in collecting duct water permeability within minutes by insertion of AQP2-bearing vesicles in the apical plasma membrane (12, 13). This mechanism is readily reversible because withdrawal of AVP causes AQP2 retrieval from the apical plasma membrane into vesicles for potential reuse by recycling. The total amount of AQP2 is not being reused because AQP2 is also excreted in the urine. The mechanism by which AQP2 is excreted in urine remains unknown. Rai et al. (16) estimated the urine fraction of AQP2 to be 3% of the whole kidney content of AQP2 in rats, indicating that the excretion of AQP2 is constant but not a major route of AQP2 elimination. Recent experiments in rats done by Wen et al. (21) have confirmed that the daily excretion of AQP2 corresponds to ~3-4% of the total kidney content of AQP2. In addition, when rats undergo water deprivation or receive dDAVP treatment, u-AQP2 increases in parallel with the increase in kidney expression of AQP2 in the apical plasma membrane, and it exceeds the increase in total kidney AQP2 levels. This strongly supports that u-AQP2 closely parallels changes in AVP action.

Long-term regulation of AQP2 is only partly influenced by AVP. During water restriction for 24-48 h, there was a marked increase in AQP2 protein level in rat renal collecting ducts, thereby increasing the total amount of AQP2 available per cell (11, 12). Several studies have recognized a cAMP responsive element in the 5'-untranslated region of the AQP2 gene, thereby raising the possibility that cAMP may increase the expression of AQP2 (21). However, Ecelbarger et al. (2) raised the possibility of AVP-independent mechanisms for the regulation of AQP2 expression. They showed that, despite a continuous subcutaneous infusion of dDAVP, AQP2 was downregulated after water loading. This has been called the AVP escape phenomenon (2). The mechanism behind this phenomenon is at present unknown.

In the present study, u-AQP2 excretion and urinary osmolality were not increased further after 20 h of fluid restriction, despite a continued increase in AVP. The AVP response may have saturated the capacity of the concentrating mechanism to respond. Thus u-AQP2 parallels the physiological response to AVP, even if it does not increase with AVP concentration. A number of possible signals other than AVP could therefore be involved in the control of AQP2, i.e., prostaglandin production or other unknown local factors.

In summary, we have shown that during antidiuresis p-AVP and u-AQP2 were increased, and there was a strong correlation between the maximum change in u-AQP2 rate and AVP. It is concluded that u-AQP2 reflects the action of p-AVP on the collecting ducts.


    ACKNOWLEDGEMENTS

We thank laboratory technicians Lisbeth Mikkelsen, Anne Jaritz-Nielsen, Susan Milton Rasmussen, Henriette Hedelund Vorup Simonsen, and Anne Mette Ravn Torstensen for skillful technical assistance and commitment. We also thank staff in medical department M3, Holstebro Hospital, for the excellent help with the study subjects during 24 h of water deprivation.


    FOOTNOTES

This study was supported by grants from Ringkjoebing County, the Foundation for Medical Research in the County of Ringkjoebing, Ribe and South Jutland, and the Danish Kidney Foundation.

Address for reprint requests and other correspondence: R. S. Pedersen, Dept. of Medicine, Holstebro Hospital, DK-7500 Holstebro, Denmark (E-mail: arsp{at}ringamt.dk).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 20 September 2000; accepted in final form 8 January 2001.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

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Am J Physiol Renal Fluid Electrolyte Physiol 280(5):F860-F867
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