Developmental expression of aquaporin 1 in the rat renal
vasculature
Jin
Kim1,
Wan-Young
Kim1,
Ki-Hwan
Han1,
Mark A.
Knepper2,
Søren
Nielsen3, and
Kirsten M.
Madsen4
1 Department of Anatomy,
Catholic University Medical College, Seoul, Korea;
2 Laboratory of Kidney and
Electrolyte Metabolism, National Heart, Lung, and Blood Institute,
National Institutes of Health, Bethesda, Maryland 20892-0951;
3 Department of Cell Biology, Institute of
Anatomy, University of Aarhus, DK-8000 Aarhus, Denmark; and
4 Division of Nephrology, Hypertension and
Transplantation, University of Florida College of Medicine,
Gainesville, Florida 32610-0224
 |
ABSTRACT |
Aquaporin 1 (AQP-1) is a water channel protein that is
constitutively expressed in renal proximal tubule and descending thin limb cells as well as in endothelial cells of the descending vasa recta. Studies in the developing rat kidney have demonstrated that
AQP-1 is expressed in renal tubules before birth. However, nothing is
known about the expression of AQP-1 in the renal vasculature during
kidney development. The purpose of this study was to establish the
distribution of AQP-1 in the renal vasculature of the developing rat
kidney and follow the differentiation of the vascular system during
kidney development. Kidneys from 16-, 17-, 18-, and 20-day-old fetuses
and 1-, 4-, 7-, 14-, 21-, and 28-day-old pups were preserved and
processed for immunohistochemical studies using a preembedding immunoperoxidase procedure. AQP-1 immunoreactivity was detected using
affinity-purified rabbit polyclonal antibodies to AQP-1. AQP-1 was
expressed throughout the arterial portion of the renal vasculature of
the fetal and neonatal kidney from gestational age 17 days to 1 wk
after birth. AQP-1 immunoreactivity gradually disappeared from the
renal vasculature between 1 and 2 wk of age and remained only in the
descending vasa recta. In contrast, AQP-1 immunoreactivity was not
observed in lymphatic vessels until 3 wk of age and persisted in the
adult kidney. AQP-1 was also expressed in a population of interstitial
cells in the terminal part of the renal papilla at 3 wk of
age as well as in the adult kidney. The transient expression
of AQP-1 in the arterial portion of the renal vasculature in the
developing rat kidney suggests that AQP-1 is important for fluid
equilibrium and/or drainage in the developing kidney or,
alternatively, plays a role in the regulation of growth and/or
branching of the vascular tree during kidney development.
kidney development; vasa recta; renal lymphatics; immunohistochemistry
 |
INTRODUCTION |
AQUAPORIN 1 (AQP-1) is a 28-kDa channel-forming
integral membrane protein that belongs to the AQP family of membrane
water channels (1, 8). AQP-1 was first discovered and purified from
human erythrocytes by Agre and colleagues (5, 16, 19), and it is the
major water channel in the erythrocyte membrane. AQP-1 is
widely expressed in the plasma membrane of secretory and absorptive
epithelia (1, 3, 13) as well as in nonfenestrated capillary endothelia
(13) and is believed to play a major role in fluid transport throughout
the body.
In the kidney, AQP-1 is heavily expressed in the apical and basolateral
plasma membranes of the proximal tubule and the descending thin limb
(14, 18), which are the constitutively water-permeable segments of the
nephron. It has also been immunolocalized to the nonfenestrated
endothelium of the descending vasa recta (12). In contrast, the
ascending vasa recta, which have a fenestrated endothelium, do not
express AQP-1 (12). In the human kidney, AQP-1 immunoreactivity has
also been reported in the fenestrated endothelium of the glomerular
capillaries and peritubular capillaries (10). However, in the
vasculature of the normal adult rat kidney, AQP-1 has been observed
only in the descending vasa recta.
Although the expression of AQP-1 and its segmental as well as cellular
distribution in the kidney have been studied extensively, little is
known about its expression and immunolocalization in the developing
kidney, and there is no information about AQP-1 protein expression in
the renal vasculature during development. In situ hybridization studies
of AQP-1 expression in fetal and neonatal rats showed very little
expression in the kidney before birth (2), and similar results were
obtained in studies using ribonuclease protection assay (24). However,
immunohistochemical studies demonstrated labeling for AQP-1 in both the
proximal tubule and the descending thin limb of the fetal rat 3 days
before birth (20). There is no information about AQP-1 immunoreactivity
in the renal vasculature in the fetal kidney.
The presence of AQP-1 in the descending thin limb as well as in
descending vasa recta is believed to play a major role in the urinary
concentrating mechanism. Recent studies in transgenic mice lacking
AQP-1 water channels have demonstrated a severe urinary concentrating
defect, indicating that expression of AQP-1 is necessary for the
development of a hypertonic medullary interstitium (9). Because rats
are not able to concentrate their urine at birth, the original focus of
this study was to determine whether AQP-1 was expressed in the renal
medulla and particularly in the vasa recta of the fetal and neonatal
kidney. The results of our initial studies revealed that, in contrast
to observations in the adult kidney, in the developing kidney, AQP-1 is
expressed in numerous blood vessels in the arterial part of the
vascular system. Therefore, another purpose of this study was to
establish the pattern of AQP-1 expression in the renal vasculature and
follow the differentiation of the arterial vascular system in the
developing rat kidney using AQP-1 as a marker.
 |
METHODS |
Animals and tissue preservation.
Sprague-Dawley rats were used in all experiments. Kidneys were obtained
from 16 (E16)-, 17 (E17)-, 18 (E18)-, and 20-day-old (E20) fetuses and 1 (P1)-, 4 (P4)-, 7 (P7)-, 14 (P14)-, 21 (P21)-, and 28-day-old
(P28) pups. The animals were
anesthetized with a 50-mg/kg body wt intraperitoneal injection of
pentobarbital sodium. The kidneys were preserved by in vivo perfusion
through the heart or abdominal aorta. The animals were initially
perfused briefly with PBS (osmolality 298 mosmol/kg
H2O, pH 7.4) to rinse away all
blood. This was followed by perfusion with a
periodate-lysine-paraformaldehyde (PLP) solution for 5 min. After
perfusion, the kidneys were removed and cut into 1- to 2-mm-thick
slices that were fixed additionally by immersion in the PLP solution
overnight at 4°C. Sections of tissue were cut transversely through
the entire kidney on a vibratome at a thickness of 50 µm and
processed for immunohistochemical studies using a horseradish
peroxidase preembedding technique.
Antibody.
Affinity-purified polyclonal antibody raised against a synthetic
peptide corresponding to the terminal 22 amino acids of rat AQP-1 was
used. This antibody recognizes AQP-1 in the rat kidney and has been
characterized in detail previously (22).
Immunohistochemistry.
Fifty-micrometer vibratome sections were processed for
immunohistochemistry using an indirect preembedding immunoperoxidase method. All sections were washed with 50 mM
NH4Cl in PBS three times for 15 min. Before incubation with the primary antibody, the sections were
pretreated with a graded series of ethanol followed by incubation for 3 h with PBS containing 1% BSA, 0.05% saponin, and 0.2% gelatin
(solution
A). The tissue sections were then
incubated overnight at 4°C with the antibody against AQP-1 diluted
1:300 in 1% BSA-PBS (solution
B). Control incubations were
performed in solution
B without primary antibody. After
several washes with solution
A, the tissue sections were incubated
for 2 h in peroxidase-conjugated goat anti-rabbit IgG Fab fragment
(Jackson ImmunoResearch Laboratories) diluted 1:50 in
solution
B. The tissues were then rinsed, first in solution
A and subsequently in 0.05 M Tris
buffer, pH 7.6. For the detection of horseradish peroxidase, sections
were incubated in 0.1% 3,3'-diaminobenzidine in 0.05 M Tris
buffer for 5 min, after which
H2O2
was added to a final concentration of 0.01% and the incubation was
continued for 10 min. After washing with 0.05 M Tris buffer, the
sections were dehydrated in a graded series of ethanol and embedded in
Epon-812. From all animals, 50-µm-thick vibratome sections through
the entire kidney were mounted in Epon-812 between polyethylene vinyl
sheets. Sections of each part of the kidney from flat-embedded
50-µm-thick vibratome sections were excised and glued onto empty
blocks of Epon-812. One-micrometer-thick sections were cut and treated
for 5 min with a mixture of saturated sodium hydroxide and absolute
ethanol (1:1) to remove the resin. After three brief rinses in absolute
ethanol, the sections were hydrated with graded ethanol and rinsed in
tap water. Some sections were counterstained with hematoxylin, whereas
others were examined unstained. The sections were washed with distilled
water, dehydrated with graded ethanol and xylene, mounted in balsam,
and examined by light microscopy.
 |
RESULTS |
Expression of AQP-1 in fetal kidneys.
At E16, AQP-1 immunoreactivity was
observed only in the renal cortex, where it was located in the
endothelium of a few scattered capillary plexuses (Fig.
1). The AQP-1-positive capillary plexuses were located at the border between the nephrogenic zone and the renal
medulla. In this area, AQP-1-negative capillary plexuses with wide
lumina were frequently observed along the medullary side of the
AQP-1-positive capillary plexuses. Small AQP-1-negative capillaries
containing red blood cells were distributed throughout the cortex and
medulla (Fig. 1).

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Fig. 1.
Light micrographs of a 50-µm-thick vibratome section
(A) and a 1-µm-thick section
(B) illustrating immunostaining for
aquaporin 1 (AQP-1) in developing blood vessels in 16-day-old fetal
kidneys. AQP-1 immunostaining is present in capillary plexus (arrows)
in nephrogenic zone, at site of future arcuate arteries. Note absence
of AQP-1 immunoreactivity in venous capillaries (arrowheads). MCD,
medullary collecting duct. Magnifications:
A, ×135;
B, ×250.
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At E17, the arcuate artery at the
border between the nephrogenic zone and the medulla exhibited strong
AQP-1 immunoreactivity (Fig. 2). AQP-1 was
also expressed in the endothelium of small blood vessels extending from
the arcuate artery into the renal cortex and medulla. From the blood
vessels that ascended into the cortex, several AQP-1-positive branches
extended to developing nephrons, including renal vesicles and S-shaped
bodies. At the corticomedullary junction, AQP-1-positive small vessels
extending from the arcuate artery formed the afferent arterioles of the juxtamedullary glomeruli. In addition, many AQP-1-positive vessels descended directly from the arcuate artery into the deep part of the
medulla and formed capillary plexuses surrounding the medullary collecting ducts (Fig. 3). However, AQP-1
immunoreactivity was not observed in the venous system, including the
arcuate vein (Fig. 4,
A and
B). In addition to being expressed
in blood vessels, AQP-1 immunoreactivity appeared in the proximal
anlage of juxtamedullary nephrons at this age (Figs. 2 and 3).

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Fig. 2.
Light micrographs of a 50-µm-thick vibratome section illustrating
immunostaining for AQP-1 in 17-day-old fetal kidney. AQP-1 is expressed
in arcuate artery (AA) and in small blood vessels extending from AA to
developing nephrons (arrowheads) and into medulla (open arrow). Area
marked by a rectangle in A is shown at
higher magnification in B. Note
immunoreactivity for AQP-1 in proximal tubule (PT; solid arrows)
anlage. Magnifications: A, ×80;
B, ×165.
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Fig. 3.
Light micrographs of 50-µm-thick vibratome section
(A) and 1-µm-thick sections
(B and
C) illustrating immunostaining for
AQP-1 in 17-day-old fetal kidneys
(A-C).
A and
B: MCD is surrounded by AQP-1-positive
capillary plexus (arrows), which is connected with small blood vessels
(arrowheads) extending from AA. C:
AQP-1 immunostaining in afferent arteriole (aa) of a juxtamedullary
glomerulus. There is no immunoreactivity in arcuate vein (AV) or in
developing glomerular capillaries. Magnifications:
A, ×170;
B and
C, ×420.
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Fig. 4.
Light micrographs of 50-µm-thick vibratome sections
(A and
D) and 1-µm-thick sections
(B and
C) illustrating immunostaining for
AQP-1 in renal cortex from 17-day-old fetus
(A and
B) and 14 (C)- and 21-day-old
(D) pups. AQP-1 is expressed in
apical and basolateral plasma membrane of endothelial cells in AA in
fetal kidneys (A and
B) but gradually disappears after
birth (C and
D). Note absence of AQP-1
immunoreactivity in AV at all ages. D:
AQP-1-positive lymphatic vessels (arrows) are located in connective
tissue sheath surrounding AA. Arrowheads indicate AQP-1-negative
capillaries in developing glomeruli. Magnifications:
A, ×300;
B, ×460;
C, ×370;
D, ×460.
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At E18 and
E20, AQP-1 was expressed in the
endothelium of arcuate arteries, interlobular arteries, and afferent
arterioles in the renal cortex (Fig.
5A). In
the medulla, AQP-1-positive descending vasa recta extended into the
papillary tip and were loosely connected to each other by small
branches (not shown). At these ages, the AQP-1 immunoreactivity in the
blood vessels was decreased in intensity compared with the labeling at
E16 and E17. There was no AQP-1
immunoreactivity in the efferent arterioles of cortical glomeruli.
However, a transient expression of AQP-1 was observed in efferent
arterioles of juxtamedullary glomeruli at
E20 (Fig.
6A).

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Fig. 5.
Light micrographs of 50-µm-thick vibratome sections illustrating
immunostaining for AQP-1 in kidneys from 20-day-old fetus
(A) and 7 (B)-, 14 (C)-, and 21-day-old
(D) pups.
A: in fetal kidney, AQP-1 is expressed
in entire arterial tree (arrowheads), including AA. AQP-1 is expressed
in interlobular arteries (IA) and aa until 7 days of age
(B) and then gradually disappears.
Note AQP-1-negative IA and aa in 14 (C)- and 21-day-old
(D) kidneys. Magnifications in
A-D,
×460.
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Fig. 6.
Light micrographs of 50-µm-thick vibratome sections illustrating
AQP-1 immunostaining in kidneys from 20-day-old fetus
(A) and 4-day-old pup
(B).
A: AQP-1 is expressed in efferent
arteriole (ea) of a juxtamedullary glomerulus as well as in aa and AA
in the 20-day-old fetus. B:
AQP-1-negative ea is connected with AQP-1-positive descending vasa
recta (arrowheads). Magnifications in
A and
B, ×460.
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Expression of AQP-1 in neonatal kidneys.
In 1-, 4-, and 7-day-old pups, there was a gradual decrease in AQP-1
immunoreactivity in the arcuate artery but AQP-1 was still expressed in
interlobular arteries and afferent arterioles (Figs.
5B and
7A). Weak AQP-1 immunostaining was
also observed in efferent arterioles of juxtamedullary glomeruli from
P1, but there was no labeling of
efferent arterioles from P4 (Fig.
6B) or
P7. In the renal medulla, there was an
increase in the number of AQP-1-positive descending vasa recta after
birth (Fig. 7,
A and
B). However, they did not form
vascular bundles at this age (Fig.
7B). On 50-µm-thick vibratome
sections from P4 and
P7, it was apparent that most of the
AQP-1-positive descending vasa recta were connected with the
AQP-1-negative efferent arterioles of juxtamedullary glomeruli (Fig.
6B).

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Fig. 7.
Light micrographs of 50-µm-thick vibratome sections illustrating
immunostaining for AQP-1 in kidneys from 1 (A)-, 7 (B)-, 14 (C)-, and 21-day-old
(D) pups.
A: AQP-1 immunoreactivity is present
in entire arterial tree (solid arrow) and in descending vasa recta
(open arrow) in 1-day-old pup. There was a gradual increase in
AQP-1-positive vasa recta during development, but vascular bundles (VB)
were not formed until 14 days after birth
(C) and were fully developed at 21 days of age (D). Arrowheads indicate
AQP-1-positive descending thin limbs of Henle's loop. Magnifications:
A, ×120;
B-D,
×180.
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In 14- and 21-day-old pups, there was no immunoreactivity for AQP-1 in
the arcuate or interlobular arteries or afferent arterioles (Figs.
4C and 5,
C and
D). In the renal vasculature, AQP-1
was expressed only in descending vasa recta, which gradually increased in number (Fig. 7, C and
D). Vascular bundles were not
observed until 14 days after birth and were fully developed at 21 days of age (Fig. 7D). In 21-day-old
pups, AQP-1 immunoreactivity was also observed in a small number of
interstitial cells in the terminal part of the renal papilla (Fig.
8B) as
well as in lymphatic vessels in the renal cortex (Fig.
9A).

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Fig. 8.
Light micrographs of 50-µm-thick vibratome sections
(A-C)
and a 1-µm-thick section (D)
illustrating immunostaining for AQP-1 in terminal part of renal papilla
from 14 (A)- and 21-day-old
(B) pups and adult rats
(C and
D). AQP-1-positive interstitial
cells (arrowheads) were observed at terminal part of renal papilla from
21 days of age (B). Note AQP-1
immunoreactivity on plasma membrane of cell body and processes of some
interstitial cells (D). Arrows
indicate AQP-1-negative interstitial cells. Magnifications:
A and
B, ×180;
C, ×230;
D, ×920.
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Fig. 9.
Light micrographs of 50-µm-thick vibratome sections
(A,
B, and
C) and a 1-µm-thick section
(D) illustrating immunostaining
AQP-1 in renal cortex from 21 (A)-
and 28-day-old (B) pups and adult
rat (C and
D).
A: AQP-1 is expressed in lymphatic
vessels (open arrow) from 21 days of age.
B: AQP-1 appeared in lymphatic vessels
(arrows) along aa and IA from 28 days of age. Note AQP-1-positive
lymphatic vessels (arrows) along IA in adult kidneys
(C and
D). Magnifications in
A-D,
×460.
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In the renal cortex of 28-day-old pups as well as that of adult rats,
besides the strong labeling of proximal tubules, AQP-1 was expressed
only in lymphatic vessels in the periarterial connective tissue sheath
along the interlobular and arcuate arteries (Fig. 9). The intensity of
labeling in the lymphatic vessels was very weak compared with that
observed in the proximal tubule (Fig. 9,
B-D). There was no
immunoreactivity for AQP-1 in arcuate arteries, interlobular arteries,
or afferent arterioles. In the medulla, AQP-1 immunostaining was
observed in the descending vasa recta and in a subpopulation of
interstitial cells in the terminal part of the renal papilla in
addition to the labeling of thin descending limbs of Henle's loop
(Fig. 8, C and
D). The AQP-1-positive interstitial cells were few in number and possessed many slender cytoplasmic processes with globular terminal endings that appeared to be connected with neighboring tubules.
AQP-1 immunoreactivity was not observed in glomerular and peritubular
capillaries or venous blood vessels in the developing kidney at any of
the ages examined (Figs. 1B,
3C, and 4).
 |
DISCUSSION |
The present study provides the first description of AQP-1 expression in
the vascular system of the developing kidney. Although previous studies
have demonstrated low levels of AQP-1 mRNA (2, 24) and protein (20) in
both the proximal tubule and descending thin limb of the developing rat
kidney before birth, those studies provided no information about AQP-1
expression in the renal vasculature.
The results of our study revealed that in the fetal kidney, AQP-1 is
expressed not only in proximal tubules and descending thin limbs of
Henle's loop but throughout the arterial portion of the renal
vasculature, including arcuate arteries, interlobular arteries, and
afferent arterioles, as well as in the descending vasa recta (see Fig.
10). This distribution is in contrast to
observations in the adult rat kidney, in which AQP-1, in addition to
its expression in renal tubules (14, 18), has been described only in
the descending vasa recta (12). Studies by Nielsen and co-workers (12)
demonstrated that in the adult rat kidney, AQP-1 is expressed in the
nonfenestrated endothelium of the descending vasa recta, which is
consistent with the high water permeability of these vessels. However,
there is no evidence that AQP-1 is present in other parts of the
vascular system of the adult rat kidney. Surprisingly, AQP-1 expression
in the arcuate arteries, interlobular arteries, and afferent arterioles
was gradually downregulated after birth and had disappeared at 2 wk of
age (Fig. 10). In contrast, AQP-1 immunoreactivity in the descending
vasa recta, which appeared at E18,
persisted after birth and increased in intensity in a fashion similar
to that observed in the proximal tubule and descending thin limb and
reported previously by other investigators (20).

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Fig. 10.
Diagram illustrating changes in AQP-1 immunostaining along renal
vasculature of developing rat kidney. AQP-1 immunoreactivity is
indicated by black color in blood vessels and lymphatic vessels. AVR,
ascending vasa recta CP, capillary plexus; DVR, descending vasa recta;
G, glomerulus; LV, lymphatic vessel; d, day.
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Previous studies have provided evidence that AQP-1 in the descending
vasa recta is involved in transendothelial water transport (15).
However, the exact functional significance of this observation is not
known with certainty. The demonstration that AQP-1 remains in the
descending vasa recta, whereas it disappears in the other parts of the
renal vasculature after birth, suggests that it may be important for
the function of the descending vasa recta. It is noteworthy that a
striking increase in the level of AQP-1 expression in the descending
vasa recta as well as in the proximal tubule and descending thin limb
of Henle's loop occurs during the first 3 wk after birth and coincides
with the development of the renal concentrating mechanism (7, 17, 23).
Moreover, a recent study (9) in transgenic mice lacking AQP-1
demonstrated a severe impairment of the urinary concentrating ability,
suggesting that AQP-1 is necessary for the development of a hypertonic
medullary interstitium. Interestingly, a transient weak expression of
AQP-1 was also observed in efferent arterioles of juxtamedullary
glomeruli that give rise to the AQP-1-positive descending vasa recta.
However, AQP-1 immunoreactivity was not observed in efferent arterioles after 4 days of age, although it remained in the descending vasa recta.
The functional significance of the transient expression of AQP-1 in the
arterial part of the renal vascular system during kidney development is
not known. However, it should be pointed out that the vascular system
is not fully developed at the time when AQP-1 is expressed in the
arterial system. Therefore, a possible function of AQP-1 in the
developing kidney may be to prevent localized fluid build up by
providing a means of drainage at a time when there appear to be no
functional lymphatic vessels. Another putative function of AQP-1 would
be to allow fluid to equilibrate across the wall of sprouting vessels
that may in part consist of distally closed structures at this time. It
is also possible that AQP-1 is involved in the regulation of growth
and/or branching of the arterial vascular tree during kidney
development. In a previous study by Bondy and co-workers (2), a
transient expression of AQP-1 mRNA was observed in the periosteum
surrounding developing bone, in developing endocardium, and in the
fetal cornea. However, the physiological importance of the transient
expression of AQP-1 in these tissues during development remains to be
established. Finally, it should be mentioned that a protein does not
necessarily play an important role at all sites where it is expressed.
There are several reports of superfluous expression of proteins in
cells or tissues where they have no apparent function (6).
Because AQP-1 was expressed throughout the arterial system of the
developing rat kidney, immunostaining for AQP-1 using a preembedding
method and plastic-embedded tissue made it possible to follow the
differentiation of the renal vasculature during kidney development. The
results indicate that the arterial vascular system in the kidney cortex
is derived from primitive capillary networks that could be
distinguished already at E16 at the
sites of the future arcuate arteries and by
E17 had differentiated into arcuate
arteries, interlobular arteries, and sprouting afferent arterioles.
This study also revealed the presence of AQP-1-positive vessels
descending directly from the arcuate artery, forming so-called true
vasa recta vera. These vessels descended into the deep
part of the medulla, where they continued into a capillary plexus
surrounding the medullary collecting duct. Vasa recta vera have been
described previously in the rat kidney but were observed only in old
animals (4, 21). It was suggested in those studies that vasa recta vera
may develop secondary to glomerular degeneration. In the present study,
AQP-1-positive vasa recta vera were observed only in the fetal kidney.
It is not known whether these vessels disappeared after birth or we
failed to recognize them because of the downregulation of AQP-1.
However, true vasa recta vera are not believed to exist in normal young
adult rats.
From 21 days of age, AQP-1 was expressed in lymphatic vessels located
in the periarterial connective tissue along arcuate and interlobular
arteries. However, AQP-1-positive lymphatics were not observed in the
fetal or neonatal kidney. Thus the initial expression of AQP-1 in
lymphatic vessels in the renal cortex appears to coincide with or
follow the downregulation and disappearance of AQP-1 from the arterial
portion of the renal vasculature. The demonstration of AQP-1
in lymphatic vessels in the renal cortex confirms observations in the
adult kidney reported previously (12). Expression of AQP-1 in lymphatic
vessels has also been reported in other tissues, including
the intestine (13).
The role of AQP-1 in interstitial cells in the terminal part of the
renal papilla is not known. However, it is possible that AQP-1 is
involved in osmotic regulation in these cells. Under the experimental
conditions of the present study, AQP-1 appeared to be expressed in only
a few of the interstitial cells in the renal papilla. Whether the
differential expression of AQP-1 indicates the presence of different
types of interstitial cells or simply reflects different functional
states of the same cell type remains to be explored. The presence of
AQP-1 in interstitial cells is consistent with the results of previous
studies in which AQP-1 immunoreactivity was observed in fibroblasts
along the respiratory tract and nasopharynx (11).
In summary, the results of this study demonstrate that AQP-1 is
transiently expressed in the arterial portion of the renal vascular
system during development and remains only in the descending vasa recta
after 2 wk of age. In addition to the well-established expression in
proximal tubules, thin descending limbs of Henle's loop, and
descending vasa recta, AQP-1 is present in lymphatic vessels of the
renal cortex and in a population of interstitial cells in the terminal
renal papilla at 3 wk of age as well as in adult kidney. These
observations suggest that AQP-1 plays an important role during the
development and differentiation of the renal vasculature.
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ACKNOWLEDGEMENTS |
We gratefully acknowledge the technical assistance of Byung-Ouk
Hong, Hee-Duk Roh, Wendy Wilber, and Li Zhang.
 |
FOOTNOTES |
This work was supported by Korean Ministry of Health and Welfare Grant
HMP-96-M-2-1037 and National Institute of Diabetes and Digestive and
Kidney Diseases Grant DK-28330.
Address for other correspondence: K. M. Madsen, Division of Nephrology,
Hypertension and Transplantation, PO Box 100224, Univ. of Florida,
Gainesville, FL 32610-0224 (E-mail:
madsekm{at}medicine.ufl.edu).
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. §1734 solely to indicate this fact.
Address for reprint requests: J. Kim, Dept. of Anatomy, Catholic Univ.
Medical College, 505 Banpo-Dong, Socho-Ku, Seoul 137-701, Korea
(E-mail: jinkim{at}cmc.cuk.ac.kr).
Received 26 August 1998; accepted in final form 17 November 1998.
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G. M. Preston,
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