0021-972X/99/$03.00/0
The Journal of Clinical Endocrinology & Metabolism
Copyright © 1999 by The Endocrine Society
Vol. 84, No. 6
Printed in U.S.A.
COMMENTS
Urinary Excretion of Aquaporin-2 Water Channel
Differentiates Psychogenic Polydipsia from Central
Diabetes Insipidus*
TAKAKO SAITO, SAN-E ISHIKAWA, TAKASHI ITO, HIDEO ODA, FUMIKO ANDO,
MINORI HIGASHIYAMA, SHOICHIRO NAGASAKA, MASASHI HIEDA, AND
TOSHIKAZU SAITO
Division of Endocrinology and Metabolism (Ta.S., S.I., F.A., Mi.H., S.N., To.S.), Department of
Medicine, Jichi Medical School, Tochigi 329-0498; and Departments of Medicine and Psychiatry (T.I.,
H.O., Ma.H.), Tokyo Metropolitan Matsuzawa Hospital, Tokyo 156, Japan
ABSTRACT
The present study was undertaken to determine whether urinary
excretion of aquaporin-2 (AQP-2) water channel under ad libitum
water intake is of value to differentiate polyuria caused by psychogenic polydipsia from central diabetes insipidus. A 30-min urine collection was made at 0900 h in 3 groups of: 11 patients with central
diabetes insipidus (22– 68 yr old), 10 patients with psychogenic polydipsia (28 – 60 yr old), and 15 normal subjects (21–38 yr old). In the
patients with central diabetes insipidus, the plasma arginine vasopressin level was low despite hyperosmolality, resulting in hypotonic
P
SYCHOGENIC polydipsia causes a marked polyuria
with hypotonic urine (1, 2). Arginine vasopressin
(AVP) secretion is suppressed by hypoosmolality caused by
excess intake of water. Suppression of AVP release obliges us
to differentiate psychogenic polydipsia from central diabetes
insipidus. Osmotic stimulation tests have been carried out to
determine the reserve function of the posterior pituitary
gland. Plasma AVP levels increase in response to an increase
in plasma osmolality (Posm) in patients with psychogenic
polydipsia but not in those with central diabetes insipidus.
In response to AVP, concentrated urine is produced by
water reabsorption across the renal collecting duct (3, 4).
Aquaporin-2 (AQP-2) is an AVP-regulated water channel of
the collecting duct; it is translocated from the cytoplasmic
vesicles to the apical plasma membranes by shuttle trafficking when the cells are stimulated by AVP (5–7), and it is again
redistributed into the cytoplasmic vesicles after removal of
AVP stimulation (8). Also, AQP-2 is, in part, excreted into the
urine (9, 10). We demonstrated that urinary excretion of
AQP-2 is of great value in diagnosing central diabetes inReceived November 10, 1998. Revision received February 8, 1999.
Accepted February 22, 1999.
Address all correspondence and requests for reprints to: San-e Ishikawa, M.D., Division of Endocrinology and Metabolism, Department of
Medicine, Jichi Medical School, 3311–1 Yakushiji Minamikawachi,
Tochigi 329-0498, Japan. E-mail: [email protected].
* This work was supported by grants from the Ministry of Welfare of
Japan.
urine. Urinary excretion of AQP-2 was 37 6 15 fmol/mg creatinine, a
value one-fifth less than that in the normal subjects. In the patients
with psychogenic polydipsia, plasma arginine vasopressin and urinary osmolality were as low as those in the patients with central
diabetes insipidus. However, urinary excretion of AQP-2 of 187 6 45
fmol/mg creatinine was not decreased, and its excretion was equal to
that in the normal subjects. These results indicate that urinary excretion of AQP-2, under ad libitum water drinking, participates in the
differentiation of psychogenic polydipsia from central diabetes insipidus. (J Clin Endocrinol Metab 84: 2235–2237, 1999)
sipidus in the hypertonic saline infusion test and impaired
water excretion in the acute oral water load test (11, 12).
The present study was undertaken to determine whether
urinary excretion of AQP-2, under ad libitum water intake, is
a useful tool for diagnosing psychogenic polydipsia.
Subjects and Methods
Subjects
Three groups of subjects were examined in the present study. The first
group had 11 patients who had been diagnosed as having idiopathic
central diabetes insipidus. They were 7 males and 4 females, whose ages
ranged from 22– 68 yr. They had taken 1-deamino-8-D-AVP (DDAVP)
intranasally, twice a day, and discontinued the DDAVP therapy 24 h
before the study. The second group of 10 patients were diagnosed as
having psychogenic polydipsia. They were 7 males and 3 females, with
ages ranging from 28 – 60 yr. Basically, they had been treated for psychiatric disorders, including schizophrenia, atypical psychiatric disorder, and chronic alcoholism. They were hospitalized in Tokyo Metropolitan Matsuzawa Hospital. Urine volume for the 24-h period before
the urine collection for AQP-2 analysis ranged from 2640 – 8490 mL
(4670 6 980 mL, mean 6 sem). The third group had 15 normal volunteers, with ages ranging from 21–38 yr. They were 10 males and 5
females. The present study was approved by the ethical committees of
Jichi Medical School Hospital and Tokyo Metropolitan Matsuzawa Hospital for human study. We obtained informed consent from all the
subjects to join the study.
All the subjects drank water ad libitum, and 30-min urine collection
was made and blood drawn at 0900 h. Urine samples were subjected to
measurements of urinary osmolality (Uosm) and urinary excretion of
creatinine and AQP-2. Blood samples were used to measure Posm and
plasma AVP levels. Uosm and Posm were measured by freezing-point
depression (Model 3W2, Advanced Instruments, Needham Height,
2235
2236
JCE & M • 1999
Vol 84 • No 6
COMMENTS
MA). Urinary creatinine was measured with an automatic clinical analyzer (Model 736, Hitachi Co., Tokyo, Japan). Plasma AVP levels were
determined by RIA using AVP RIA kits (Mitsubishi Chemistry, Tokyo,
Japan) (13). Urinary excretion of AQP-2 was measured as described
below.
RIA of AQP-2
The RIA of urinary AQP-2 was performed by the method described
in our previous reports (11, 12). Urinary AQP-2-like immunoreactivity
was measured by a specific RIA that used the polyclonal antibody
against a synthetic portion (Tyr0-AQP-2 [V257-A271]) of the C-terminal
of human AQP-2 raised in rabbits. A synthetic peptide [Tyr0-AQP-2
(V257-A271)] was radioiodinated with iodine-125 (New England Nuclear, Boston, MA) by the chloramine-T method. For the assay, 0.1 mL
of the urine samples (diluted 1– 8 times) or a standard, 0.1 mL assay
buffer [ 0.05 mol/L sodium phosphate (pH 7.4), 0.08 mol/L sodium
chloride, 0.01 mol/L EDTA, 0.5% BSA, 0.5% Noridet P-40, and 0.01%
sodium azide], and 0.1 mL of the antibody (final dilution, 1:12,000) was
incubated at 4 C for 48 h, followed by the addition of 0.1 mL of the
radiolabeled synthetic peptide (;10,000 cpm) and further incubation at
4 C for 48 h. Bound and free quantities of radiolabeled ligand were
separated by the double-antibody method. All samples were analyzed
in duplicate. The intra- and interassay coefficients of variation were less
than 10%. The minimal detectable quantity of AQP-2 was 0.86 pmol/
tube, and an amount equivalent to 6.9 pmol/tube caused 50% inhibition
of binding of the radiolabeled ligand.
Statistical analysis
Uosm, Posm, plasma AVP, and urinary excretion of AQP-2 were
expressed as the mean 6 sem. All values were compared with Fisher’s
t test. A P value less than 0.05 was considered significant.
Results
In the patients with central diabetes insipidus, the plasma
AVP level was low despite hyperosmolality of 297.8 6 3.4
mosmol/kg H2O, resulting in hypotonic urine (Fig. 1). Urinary excretion of AQP-2 was one-fifth less in the patients
with central diabetes insipidus than in the normal subjects.
AQP-2 is the AVP-dependent water channel of collecting
duct cells and is recycling between the cytoplasmic vesicles
and the apical plasma membranes in the cells (5– 8). AQP-2
is partly excreted into the urine, which is approximately 3%
of AQP-2 in the collecting duct cells (14). In normal subjects,
urinary excretion of AQP-2 is changeable in a wide range in
physiological conditions (11). Because urinary excretion of
AQP-2 has a positive correlation with plasma AVP levels in
normal subjects (11), the reduced urinary excretion of AQP-2
was in concert with the impaired secretion of AVP in central
diabetes insipidus.
In the patients with psychogenic polydipsia, Uosm was as
low as that in the patients with central diabetes insipidus
(Fig. 1). The plasma AVP level was low because of the reduced Posm, which was derived from an exaggerated intake
of water. Urinary excretion of AQP-2, however, was not
decreased; and rather, its excretion kept the normal range.
The relationship between plasma AVP levels and urinary
excretion of AQP-2 is shown in Fig. 2. The urinary excretion
of AQP-2 in the patients with psychogenic polydipsia was
dissociated from the positive correlation between plasma
AVP and urinary excretion of AQP-2 in the normal subjects.
Discussion
The present study demonstrated the clinical tool, of urinary excretion of AQP-2, in differentiating psychogenic polydipsia from central diabetes insipidus. What is involved in
the marked difference in urinary excretion of AQP-2 in these
FIG. 1. Posm, plasma AVP (PAVP), Uosm, and urinary excretion of AQP-2 (UAQP-2), under ad libitum water drinking, in 15 normal subjects
(NL, F), 11 patients with central diabetes insipidus (CDI, E) and 10 patients with psychogenic polydipsia (PP, M). *, P , 0.01; **, P , 0.05
vs. the normal subjects. Value are means 6 SEM.
COMMENTS
2237
the positive relationship between urinary excretion of AQP-2
and plasma AVP levels. At the present time, however, other
factors involved in urinary excretion of AQP-2 remain undetermined. Further study will be necessary to elucidate the
exact mechanism.
In conclusion, urinary excretion of AQP-2, under ad libitum
water drinking, participates in the differentiation of polyuria
caused by psychogenic polydipsia from central diabetes
insipidus.
References
FIG. 2. Relationship between plasma AVP levels and UAQP-2. F,
Normal subjects (n 5 15); E, patients with central diabetes insipidus
(n 5 11); M, patients with psychogenic polydipsia (n 5 10). Values are
means 6 SEM.
two disorders? There is a possibility that, as patients with
psychogenic polydipsia reduce water intake during sleep,
antidiuresis may occur periodically at night and the production of AQP-2 be somewhat restored. Because approximately
3% of AQP-2 in collecting duct cells is excreted into the urine,
urinary excretion of AQP-2 may keep relatively high, despite
hypotonic urine. It seemed that the present results are distinct from down-regulation of kidney AQP-2 content in rats
under chronic water load (15). The difference may come from
the periodicity of water intake in a day, in the patients with
psychogenic polydipsia. Collecting-duct flow rate is increased and renal medullary tissue tonicity may be reduced
in patients with psychogenic polydipsia. These environmental alterations, according to a peculiar drinking behavior,
could cause accompanying changes in the action of AVP or
AQP-2 in collecting duct cells and the pattern of AQP-2
excretion from them. As a whole, these changes may disrupt
1. Jose CI, Perez-Cruet J. 1979 Incidence and morbidity of self-induced water
intoxication in stare mental hospital patients. Am J Psychiatry. 136:221–222.
2. Goldman MB, Luchins DJ, Robertson GL. 1988 Mechanisms of altered water
metabolism in psychiatric patients with polydipsia and hyponatremia. N Engl
J Med. 318:397– 403.
3. Ishikawa S. 1993 Cellular action of arginine vasopressin in the kidney. Endocr
J. 40:373–386.
4. Knepper MA, Rector Jr FC. 1995 Urinary concentration, and dilution. In:
Brenner BM, Rector Jr FC, eds. The kidney. Philadelphia: Saunders; 532–570.
5. Fushimi K, Uchida S, Hara Y, Hirata Y, Marumo F, Sasaki S. 1993 Cloning
and expression of apical membrane water channel of rat kidney collecting
tubule. Nature. 361:549 –552.
6. Sasaki S, Fushimi K, Saito H, et al. 1994 Cloning, characterization and chromosomal mapping of human aquaporin of collecting duct. J Clin Invest.
93:1250 –1256.
7. Nielsen S, DiGiovanni SR, Christensen EI, Knepper MA, Harris HW. 1993
Cellular and subcellular immunolocalization of vasopressin-regulated water
channel in rat kidney. Proc Natl Acad Sci USA. 90:11663–11667.
8. Saito T, Ishikawa S, Sasaki S, et al. 1997 Alteration in water channel AQP-2
by removal of AVP stimulation in collecting duct cells of dehydrated rats. Am J
Physiol. 272:F183–F191.
9. Kanno K, Sasaki S, Ishikawa S, et al. 1995 Urinary excretion of aquaporin-2
in patients with diabetes insipidus. N Engl J Med. 332:1540 –1545.
10. Elliot S, Goldsmith P, Knepper MA, Haughey M, Olson B. 1996 Urinary
excretion of aquaporin-2 in humans: a potential marker of collecting duct
responsiveness to vasopressin. J Am Soc Nephrol. 7:403– 409.
11. Saito T, Ishikawa S, Sasaki S, et al. 1997 Urinary excretion of aquaporin-2 in
the diagnosis of central diabetes insipidus. J Clin Endocrinol Metab.
82:1823–1827.
12. Saito T, Ishikawa S, Ando F, et al. 1998 Exaggerated urinary excretion of
aquaporin-2 in the pathological state of impaired water excretion dependent
upon arginine vasopressin. J Clin Endocrinol Metab. 83:4034 – 4040.
13. Ishikawa S, Saito T, Okada K, Tsutsui K, Kuzuya T. 1986 Effect of vasopressin
antagonist on water excretion in inferior vena cava constriction. Kidney Int.
30:49 –55.
14. Rai T, Sekine K, Kanno K, et al. 1997 Urinary excretion of aquaporin-2 water
channel protein in human and rat. J Am Soc Nephrol. 8:1357–1362.
15. Terris J, Ecelbarger CA, Nielsen S, Knepper MA. 1996 Long-term regulation
of four renal aquaporins in rats. Am J Physiol. 271:F414 –F422.