Renal Division, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In the terminal part of the kidney collecting duct, rapid urea reabsorption is essential to maintaining medullary hypertonicity, allowing maximal urinary concentration to occur. This process is mediated by facilitated urea transporters on both apical and basolateral membranes. Our previous studies have identified three rat urea transporters involved in the urinary concentrating mechanism, UT1, UT2 and UT3 , herein renamed UrT1-A, UrT1-B, and UrT2, which exhibit distinct spatial distribution in the kidney. Here we report the molecular characterization of an additional urea transporter isoform, UrT1-C, from rat kidney that encodes a 460-amino acid residue protein. UrT1-C has 70 and 62% amino acid identity to rat UrT1-B and UrT2 (UT3), respectively, and 99% identity to a recently reported rat isoform (UT-A3; Karakashian A, Timmer RT, Klein JD, Gunn RB, Sands JM, and Bagnasco SM. J Am Soc Nephrol 10: 230-237, 1999). We report the anatomic distribution of UrT1-C in the rat kidney tubule system as well as a detailed functional characterization. UrT1-C m RNA is primarily expressed in the deep part of the inner medulla. When expressed in Xenopus laevis oocytes, UrT1-C induced a 15-fold stimulation of urea uptake, which was inhibited almost completely by phloretin (0.7 mM) and 60-95% by thiourea analogs (150 mM). The characteristics are consistent with those described in perfusion studies with inner medullary collecting duct (IMCD) segments, but, contrary to UrT1-A, UrT1-C-mediated urea uptake was not stimulated by activation of protein kinase A. Our data show that UrT1-C is a phloretin-inhibitable urea transporter expressed in the terminal collecting duct that likely serves as an exit mechanism for urea at the basolateral membrane of IMCD cells.
urinary concentration; urea transporter; vasopressin; nomenclature
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
UREA IS A MAJOR END PRODUCT of protein catabolism, and urea transport in the kidney plays an important role in nitrogen balance and water conservation (1, 14). In the mammalian kidney, accumulation of urea in the inner medullary interstitium is an essential component of the urinary concentrating mechanism and is mostly achieved by passive urea reabsorption from inner medullary collecting ducts (IMCD). Urea permeability in the terminal IMCD is extremely high and is further stimulated by vasopressin via a cAMP-dependent pathway. The unique property of this segment allows the delivery of large amounts of urea into the deepest portions of the inner medulla. Physiological studies have demonstrated that this process is mediated by a specialized facilitated urea transporter (UrT), which is inhibited by urea analogs and phloretin and displays saturation kinetics with thiourea (13).
We initially cloned a phloretin-inhibitable urea transporter cDNA,
UrT1-B (formerly known as UT2 or UT-A2; see Table
1 for nomenclature), from rabbit kidney
medulla by expression cloning (30). Subsequently, we
identified three UrT isoforms from rat kidney, UrT1-A, UrT1-B, and UrT2
(formerly known as UT1, UT2, and UT3) from rat kidney (23, 24,
28). The two isoforms UrT1-A (4.0 kb) and UrT1-B (2.9 kb) are
derived from a single UrT gene, SLC14A1, by alternative splicing.
UrT1-A has a unique 532-amino acid residue stretch at the
NH2 terminus in addition to the common 397-amino acid
sequence at the COOH terminus (23). Interestingly, the two
isoforms are expressed in distinct parts of the kidney and are
regulated by different physiological stimuli. UrT1-A is expressed in
the inner medulla and upregulated by protein restriction
(24). Expression studies (23) showed that
urea uptake by UrT1-A is stimulated by cAMP analogs, and
immunocytochemical analysis (15) localized UrT1-A to the
apical membrane of the IMCD. These data indicate that UrT1-A serves as
the apical, vasopressin-regulated urea transporter, the major route
responsible for urea accumulation.
|
UrT1-B is expressed in the descending thin limbs of the loop of Henle (15, 22) and upregulated by chronic dehydration. It is likely involved in urea recycling between the interstitium and the loop of Henle.
A third isoform, UrT2 (formerly named HUT11, UT3, or UTB1) has 60% amino acid sequence identity with UrT1-B and is derived from a distinct gene, SLC14A2 (16, 27). UrT2 shows a wider tissue distribution and is expressed in kidney descending vasa recta and pelvic epithelium, red blood cells, astrocytes in brain, and sertoli cells in testis (2, 4, 27). UrT2 is thought to participate in the countercurrent exchange process between the ascending and descending vasa recta (17, 27).
Taken together, the two splice products of the SLC14A1 gene, UrT1-A and UrT1-B, and the product of the SLC14A2 gene, UrT2, play distinct roles in the regulation of urea accumulation in the kidney inner medulla and the maintenance of water and nitrogen balance. Recently, two additional splice products of the SLC14A1 gene, UT-A3 and UT-A4 (see Table 1), were reported, but their functional characteristics and kidney localization have not been fully investigated (9).
Perfusion studies with IMCD segments demonstrated that transcellular transport of urea is mediated by a phloretin-sensitive urea transporter on both apical and basolateral membranes (13, 25). However, such a basolateral urea transporter has not yet been identified. High-stringency Northern analysis of rat inner medulla, using the probe corresponding to the NH2-terminal segment of UrT1-A, revealed a band of ~2.0 kb, in addition to the 4.0-kb band corresponding to UrT1-A mRNA (23). This observation prompted us to test the hypothesis that there is another splice variant in rat kidney inner medulla. In the present study, we isolated this cDNA from a rat kidney inner medullary cDNA library and characterized its function by using Xenopus laevis oocyte expression. The results show that UrT1-C mediates urea transport that is of high capacity and inhibited by phloretin and urea analogs in a manner similar to that of the basolateral urea transporter in the IMCD.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
cDNA library screening.
A rat kidney inner medullary gt-10 cDNA library was constructed from
the oligo(dT)-primed cDNA in the size range from 2.0 to 3.5 kb.
Approximately 500,000 clones were screened at high stringency by using
the first 652 nucleotides at the NH2 terminus of rat UrT1-A
cDNA as a probe. Inserts from
-phages were subcloned into the
plasmid pBluescript II SK (
), and the sequence from both ends was
determined by the dideoxy chain termination method. A full-length
clone containing a 2.0-kb insert was identified and thereafter
designated as
UrT1-C.1
In vitro transcription and oocyte expression. Capped cRNA was synthesized in vitro from the linearized UrT1-C cDNA clone by using T3 RNA polymerase and was microinjected into collagenase-treated and manually defolliculated X. laevis oocytes. Urea transport activity was measured by [14C]- urea uptake as previously described (23, 30). The effects of known urea transporter inhibitors were tested by preincubation for 30 min with various concentrations of phloretin or 150 mM of the urea analogs thiourea, 1,3-dimethylurea, and acetamide. The effects of protein kinase A activators were tested by preincubation with a mixture of the cAMP agonists (dibutyryl cAMP, IBMX, and forskolin) for 30 min before the uptake, as described previously (23). The cAMP concentration ranging from 0.1 to 0.5 mM was tested. All studies were performed in sodium-free solution containing 200 mM mannitol, except in studies of sodium-dependent urea uptake, in which 200 mM mannitol was replaced by 96 mM NaCl.
Northern analysis. By use of male Sprague-Dawley rats, poly (A)+ RNA was prepared in several tissues including different regions of the kidney, i.e., superficial cortex, deep cortex, outer and inner stripe of outer medulla, inner medulla, and papilla. About 3 µg of each sample were separated on a 1% agarose gel in the presence of 2.2 M formaldehyde and blotted onto a nitrocellulose filter. The filter was hybridized at 42°C with a [32P]-labeled probe synthesized from the full-length rat UrT1-C cDNA and washed in 0.1× standard sodium citrate (SSC)/0.1% SDS at 65°C.
In situ hybridization. Digoxigenin-labeled antisense and sense runoff transcripts were synthesized by using the Genius Kit (Boehringer Mannheim) from a PCR fragment that contained ~200 bp of UrT1-C cDNA-specific sequence (sense: 5'-TTCACCGCCAAGCCAAAATGTT-3', nucleotides 1760-1781; antisense: 5'-ATGTCGTACCAGCTTCCTTTAAT-3', nucleotides 1950-1974) and that was flanked by SP6 and T7 RNA polymerase initiation sites. For comparison of the detection of UrT1-C to UrT1-A and UrT1-B, subtype-specific cRNA probes were generated from UT cDNA (nucleotides 1-652, that recognizes both UrT1-A and UrT1-C) (23) and from UrT1-B cDNA (nucleotides 1-707) (24), respectively. Transcripts were alkali-hydrolyzed to an average length of 200-400 bp. Hybridization was performed on cryosections (10 µm) of freshly frozen tissues, on the basis of the protocol previously described (2). The hybridization buffer consisted of 50% formamide, 5× SSC, 2% blocking reagent (Boehringer Mannheim), 0.02% SDS, 0.1% N-laurylsarcosine, and probe (~200 ng/ml). Sections were immersed in slide mailers and hybridized at 68°C for 18 h. Sections were then washed three times in 2× SSC and twice for 30 min in 0.1× SSC at 68°C. The hybridized digoxigenin-labeled probes were visualized by using anti-digoxigenin Fab fragments (Boehringer Mannheim) and 5-bromo-4-chloro-indoylphosphate (BCIP)/nitro blue tetrazolium substrate. The sections were developed in substrate solution for 16 h, then rinsed in 10 mM Tris, 1 mM EDTA, pH 8.0, and coverslipped with Vectashield (Vecta Lab, Burlingame, CA).
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Cloning and sequencing of rat UrT1-C
cDNA.
To isolate a new splice variant and avoid contamination of 4.0-kb
transcripts, we constructed a size-selected cDNA library (ranging from
1.5 to 3.5 kb) by using rat kidney inner medulla mRNA and screened it
by a probe corresponding to the first 652 nucleotides of rat UrT1-A
(UT1) cDNA. Four positive clones of different sizes were isolated,
among which the clone UrT1-C carried the largest insert of 1,989 bp.
UrT1-C cDNA contains a polyadenylation sequence (ATTAAA) at position
1951, an open reading frame from nucleotide 377 to 1759, thus encoding
a protein of 460 amino acids with a relative molecular mass of 50.4 kDa
(Fig. 1A).
The amino acid sequence is identical to UT-A3 cDNA (GenBank AF041788) isolated from rat kidney on the basis of PCR amplification by other
investigators (9), except for two amino acids, Pro at position 17 (Leu in UT-A3) and Ala at position 95 (Gly in UT-A3). Compared with UrT1-A cDNA, UrT1-C cDNA is identical at the 5' ends from
position 1 to 1754, whereas it differs at the 3' end, 219 nucleotides
from position 1755 to 1973. The amino acid sequence of UrT1-C is
therefore identical to the NH2-terminal half of UrT1-A and
has 70 and 62% identity to rat UrT1-B and UrT2, respectively (Fig.
1A). The hydropathy plot of UrT1-C is in accordance with the
characteristic pattern of members of the urea transporter protein
family, which includes two extended hydrophobic membrane stretches
interspersed with hydrophilic regions (Fig. 1, B and C). UrT1-C polypeptide contains one potential
N-glycosylation site at Asn279, two potential
phosphorylation sites for protein kinase A (PKA; Ser84 and
Ser91), and three potential protein kinase C
phosphorylation sites (Ser23, Ser79,
and Thr447).
|
Tissue distribution and localization of URT1-C
mRNA in rat kidney.
Northern analysis using full-length UrT1-C as probe revealed a strong
hybridization signal of ~2.2 kb and a weak signal of 4.0 kb in rat
kidney inner medulla, in particular in the deeper portion (Fig.
2). No signal was detected in the regions
of the kidney or in other organs, indicating that the UrT1-C is
specific to the kidney inner medulla. Karakashian et al.
(9) also observed signals in testis; however, our Northern
analysis revealed no signal for testis when using high-stringency
washing conditions.
|
|
Functional properties of UrT1-C in X. laevis oocytes.
We examined the functional characteristics of UrT1-C in the X. laevis oocyte expression system. When expressed in the oocytes, UrT1-C mediated an ~15-fold increase in [14C]urea
uptake (1 mM) compared with water-injected control oocytes (Fig.
4A). Urea transport mediated
by UrT1-C reached a maximum after incubation for 30 min (Fig.
4B). The level of transport activity of UrT1-A and UrT1-C
was compared in the same batch of oocytes (Fig. 4D). Urea
transport mediated by UrT1-C was about three- to fourfold higher than
that mediated by UrT1-A, 20-30 vs. 5-10 × 106 cm/s. Reducing the amount of UrT1-C cRNA for
microinjection to as low as 10 ng/oocyte did not significantly affect
UrT1-C transport activity and gave consistently higher uptake levels
than UrT1-A. Furthermore, this uptake was about fivefold higher than
that mediated by AQP3 (~5 × 10
6 cm/s; Ref.
29). UrT1-C-mediated urea uptake was not changed by
replacing 200 mM mannitol with 96 mM NaCl, indicating that transport is
not sodium dependent (Fig. 4A). Uptake of urea (1 mM) was
inhibited by preincubation for 30 min with phloretin (0.2-0.7 mM)
in a dose-dependent fashion (Fig. 4A). The level of
phloretin sensitivity was similar to those observed for UrT1-A and
UrT1-B (23, 24) and consistent with the properties of urea
transport characterized in perfusion studies of the IMCD (3,
25).
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In the present study, we show that another splice variant of the SLC14A1 gene, UrT1-C, is primarily expressed in the terminal IMCD. Sequence analysis indicates that UrT1-C has a hydrophobic transmembrane protein structure similar to the other isoforms UrT1-B (UT2) and UrT2 (UT3). X. laaevis oocytes expressing UrT1-C facilitate transport of urea with high capacity, and transport is inhibited by urea analogs and phloretin. The transport characteristics of UrT1-C are consistent with those previously reported for perfused isolated IMCD. In contrast to the apical urea transporter UrT1-A (UT1), the transport activity mediated by UrT1-C is constitutively high, whereas UrT1-A requires activation by protein kinase A to reach similar activity.
We have summarized the properties of urea transporter family members in Table 1. Most of the nucleotide sequences of UrT1-C cDNA correspond to the 5' part of UrT1-A cDNA. On the basis of genomic analysis (23), we have previously showed that UrT1-A arises by splicing of exon group I and group III, which are located about 7 kb apart (Fig. 1D). UrT1-C mRNA presumably arises as a splice product of exon group I and group Ia. Other investigators recently reported the cloning of two splicing variants of the SLC14A1 gene, UT-A3 and UT-A4, by PCR from rat kidney cDNA (9). Of these isoforms, the physiological role of UT-A4 (UrT1D) remains unclear because its transcript was too low to be detected by Northern analysis. The sequence of UT-A3 cDNA is 99% identical to UrT1-C cDNA and induced phloretin-inhibitable urea transport when expressed in human kidney HEK-293 cells, similar to our expression data in the oocytes.
Our localization and functional characterization data suggest that
UrT1-C may be responsible for urea transport on the basolateral membrane of the kidney IMCD cells. Urea reabsorption in the terminal part of the IMCD is an important aspect of urinary concentration (Fig.
5). To ensure maximal urinary
concentrating capacity, urea absorption is delayed to the most terminal
part of the IMCD, where effective blood flow is low. This process
prevents the escape of urea, through the vasa recta, in the upper part
of the inner medulla, where blood flow is much higher (1,
13). Urea permeability measurements in isolated perfused rat and
rabbit IMCD segments showed that the urea permeability of the IMCD in
the initial part is at least one order of magnitude lower than that in
the terminal portions (19). Because transepithelial urea
permeability is extremely high in the terminal IMCD, urea movement
across the IMCD cells is considered to occur via urea transporters
present on both apical and basolateral membranes. Previous perfusion
studies indicated that both apical and basolateral membranes of the
IMCD contain phloretin-sensitive, urea analog-inhibitable urea
transporters (25). Immunocytochemical studies showed that
UrT1-A is expressed on the apical membranes throughout the entire
length of the IMCD (15). Subsequent quantitative
measurement of UrT1-A protein in the IMCD segments demonstrated that
UrT1-A expression levels are almost comparable between initial and
terminal parts (11). These observations suggest that
transepithelial urea permeability in the IMCD is mediated by another
basolateral transporter, in addition to the apical transporter UrT1-A.
Because our in situ hybridization studies demonstrate that the
expression of UrT1-C is strongest in the terminal part of the IMCD,
where massive urea transport is expected, it is reasonable to speculate
that UrT1-C serves as a basolateral transporter to allow basolateral
exit of urea in this segment. The greater transport activities of
UrT1-C might be crucial for the IMCD cells to respond promptly to
changes in interstitial urea concentration without many changes in cell volume, especially when vasopressin is stimulated during water restriction. Immunolocalization of UrT1-C will be the ultimate approach
to confirm the basolateral localization of UrT1-C. However, these
studies have been hampered due to the amino acid sequence identity of
UrT1-C to UrT1-A at the NH2 terminus and difficulties in
raising specific antibodies against its NH2 terminus. This study requires comparison of immunocytochemical staining using 1) antibodies made from the NH2-terminal domain
of UrT1-A that also recognize UrT1-C and 2) antibodies that
recognize only UrT1-A at the COOH-terminal domain, in parallel with in
situ hybridization studies using specific probes for UrT1-A and UrT1-C.
|
It is possible that basolateral urea exit is mediated by a pathway
other than UrT1-C. Several water channels have been reported to be
permeable to water as well as urea (6, 7, 29). Among those, aquaporin-3 (AQP3) was shown to be expressed on the basolateral membranes of cells throughout the IMCD (5, 8). However, AQP3 does not appear to play an important role in basolateral exit of
urea, because it is expressed most abundantly in the outer half of the
inner medulla and it is absent in the terminal part of the IMCD
(5). Furthermore, on the basis of studies in X. laevis oocytes, the calculated urea permeability mediated by AQP3 was relatively small (~5 × 106 cm/s). Finally, on
the basis of the perfusion studies with isolated IMCD, separate
pathways were shown for urea and water transport, because the urea
reflection coefficient was
1 and because distinct phloretin
sensitivities were observed for water and urea permeability (13,
25). Recently, Kato and Sands (10) showed evidence for a secondary active, sodium-dependent urea transport in the deepest
subsegment of the rat IMCD. The actual role of this sodium/urea antiporter in urea secretion and maintenance of water homeostasis remains to be determined.
UrT1-C has two potential PKA sites at the NH2 terminus. However, despite the findings of Karakashian et al. (9), we did not find PKA activation of UrT1-C transport in the oocytes, making it less likely that UrT1-C is regulated by vasopressin. Because we observed cAMP activation of UrT1-A, which has PKA sites at the same position (Ser84 and Ser91) as UrT1-C, activation of UrT1-A must be mediated by other UrT1-A-specific PKA sites located in intracellular loops of the COOH-terminal portion of the transporter (21, 23). Previous studies using video-enhanced contrast microscopy (25) showed that urea permeability of the basolateral membrane of IMCD cells is about twofold greater than that of the apical membrane, when corrected by their surface area. The apical membrane is therefore thought to be rate limiting for overall transcellular urea transport and the site of vasopressin regulation. However, vasopressin regulation on the basolateral side has not been well examined. Given that the time course of urea permeability stimulated by vasopressin is biphasic (26), this complex response might involve the multiple transport steps, including apical urea uptake by UrT1-A and basolateral urea exit by UrT1-C. Further studies are necessary to clarify whether and how UrT1-C is regulated.
In summary, we have isolated and characterized an additional urea transporter isoform, UrT1-C, also known as UTA-3, from the rat kidney inner medulla. UrT1-C mRNA is exclusively expressed in the terminal IMCD, and its transport properties are consistent with those observed in perfusion studies of isolated IMCDs, i.e., in terms of phloretin sensitivities and inhibition by urea analogs. We propose that UrT1-C is the urea transporter on the basolateral side of the IMCD cells allowing basolateral exit of urea and maintenance of medullary hypertonicity that are essential to ensure maximal urinary concentration.
![]() |
ACKNOWLEDGEMENTS |
---|
This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant RO1-DK-46289 (to M. A. Hediger), a research fellowship grant from the Siriraj-China Medical Board, Mahidol University, Thailand (to C. Shayakul), and a National Kidney Foundation Fellowship (to H. Tsukaguchi).
![]() |
FOOTNOTES |
---|
* C. Shayakul and H. Tsukaguchi contributed equally to this study.
Present address of C. Shayakul: Renal Unit, Dept. of Medicine, Faculty of Medicine, Siriraj Hospital, Mahidol Univ., Bangkok, 10700 Thailand.
Address for reprint requests and other correspondence: M. A. Hediger, Harvard Institutes of Medicine, 77 Louis Pasteur Ave., Boston, MA 02115 (E-mail: mhediger{at}rics.bwh.harvard.edu).
1 The sequence of UrT1-C cDNA has been deposited in the GenBank database under the accession no, AF031642.
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 29 November 1999; accepted in final form 7 November 2000.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Bankir, L.
Urea and the kidney.
In: The Kidney (6th ed.), edited by Brenner BM,
and Trinh-Trang-Tan MM.. Philadelphia, PA: Saunders, 1999, p. 637-679.
2.
Berger, UV,
Tsukaguchi H,
and
Hediger MA.
Distribution of mRNA for the facilitated urea transporter UrT2 in the rat nervous system.
Anat Embryol
197:
405-414,
1998[ISI][Medline].
3.
Chou, CL,
and
Knepper MA.
Inhibition of urea transport in inner medullary collecting duct by phloretin and urea analogues.
Am J Physiol Renal Fluid Electrolyte Physiol
257:
F359-F365,
1989
4.
Couriaud, C,
Ripoche P,
and
Rousselet G.
Cloning and functional characterization of a rat urea transporter; expression in the brain.
Biochem Biophys Acta
1309:
197-199,
1996[ISI][Medline].
5.
Ecelbarger, CA,
Terris J,
Frindt G,
Echevarria M,
Marples D,
Nielsen S,
and
Knepper MA.
Aquaporin-3 water channel localization and regulation in rat kidney.
Am J Physiol Renal Fluid Electrolyte Physiol
269:
F663-F672,
1995
6.
Echevarria, M,
Windhager EE,
Tate SS,
and
Frindt G.
Cloning and expression of AQP3, a water channel from the medullary collecting duct of rat kidney.
Proc Natl Acad Sci U SA
91:
10997-11001,
1994
7.
Ishibashi, K,
Sasaki S S,
Fushimi K,
Uchida S,
Kuwahara M,
Saito H,
Furukawa T,
Nakajima K,
Yamaguchi Y,
Gojobori T,
and
Marumo F.
Molecular cloning and expression of a member of the aquaporin family with permeability to glycerol and urea in addition to water expressed at the basolateral membrane of kidney collecting.
Proc Natl Acad Sci USA
91:
6269-6273,
1994[Abstract].
8.
Ishibashi, K,
Sasaki S,
Fushimi K,
Yamamoto T,
Kuwahara M,
and
Marumo F.
Immunolocalization and effect of dehydration on AQP3, a basolateral water channel of kidney collecting ducts.
Am J Physiol Renal Physiol
272:
F235-F241,
1997
9.
Karakashian, A,
Timmer RT,
Klein JD,
Gunn RB,
Sands JM,
and
Bagnasco SM.
Cloning and characterization of two new isoforms of the rat kidney urea transporter: UT-A3 and UT-A4.
J Am Soc Nephrol
10:
230-237,
1999
10.
Kato, A,
and
Sands JM.
Evidence for sodium-dependent active urea secretion in the deepest subsegment of the rat inner medullary collecting duct.
J Clin Invest
101:
423-428,
1998
11.
Kishore, BK,
Terris J,
Fernandez-Llama P,
and
Knepper MA.
Ultramicrodetermination of vasopressin-regulated urea transporter protein in microdissected renal tubules.
Am J Physiol Renal Physiol
272:
F531-F537,
1997
12.
Knepper, MA,
and
Roch-Ramel F.
Pathways of urea transport in the mammalian kidney.
Kidney Int
3:
629-633,
1987.
13.
Knepper, MA,
and
Star RA.
The vasopressin-regulated urea transporter in renal inner medullary collecting duct.
Am J Physiol Renal Fluid Electrolyte Physiol
259:
F393-F401,
1990
14.
March, DJ,
and
Knepper MA.
Renal handling of urea.
In: Renal Physiology, edited by Windhager E. E.. Oxford: Oxford University Press, 1992, p. 1317-1347.
15.
Nielsen, S,
Terris J,
Smith CP,
Hediger MA,
Ecelbarger CA,
and
Knepper MA.
Cellular and subcellular localization of the vasopressin-regulated urea transporter in rat kidney.
Proc Natl Acad Sci USA
93:
5495-5500,
1996
16.
Olives, B,
Martial S,
Mattei MG,
Matassi G,
Rousselet G,
Ripoche P,
Cartron JP,
and
Bailly P.
Molecular characterization of a new urea transporter in the human kidney.
FEBS Lett
386:
156-160,
1996[ISI][Medline].
17.
Olives, B,
Neau P,
Bailly P,
Hediger MA,
Rousselet G,
Cartron JP,
and
Rippoche P.
Cloning and functional expression of a urea transporter from human bone marrow cells.
J Biol Chem
269:
31649-31652,
1994
18.
Promeneur, D,
Rousselet G,
Bankir L,
Bailly P,
Carton JP,
Rippoche P,
and
Trinh-Trang-Tan MM.
Evidence for distinct vascular and tubular urea transporters in the rat kidney.
J Am Soc Nephrol
7:
852-860,
1996[Abstract].
19.
Sands, JM,
and
Knepper MA.
Urea permeability of mammalian inner medullary collecting duct system and papillary surface epithelium.
J Clin Invest
79:
138-147,
1987[ISI][Medline].
20.
Sands, JM,
Timmer RT,
and
Gunn RB.
Urea transporters in kidney and erythrocytes.
Am J Physiol Renal Physiol
273:
F321-F339,
1997
21.
Shayakul, C,
and
Hediger MA
Regulation of the inner medullary collecting duct (IMCD) urea transporter by protein kinases (Abstract).
FASEB J
11:
A23,
1997.
22.
Shayakul, C,
Knepper MA,
Smith CP,
DiGiovanni SSR,
and
Hediger MA.
Segmental localization of urea transporter mRNA in rat kidney.
Am J Physiol Renal Physiol
272:
F654-F660,
1997
23.
Shayakul, C,
Steel A,
and
Hediger MA.
Molecular cloning and characterization of the vasopressin-regulated urea transporter of rat kidney collecting ducts.
J Clin Invest
98:
2580-2587,
1996
24.
Smith, CP,
Lee WS,
Martial S,
You G,
Sands JM,
and
Hediger MA.
Cloning and regulation of expression of the rat kidney urea transporter (rUrT1-B).
J Clin Invest
92:
2448-2457,
1995.
25.
Star, RA.
Apical membrane limits urea permeation across the rat inner medulla collecting ducts.
J Clin Invest
86:
1172-1178,
1990[ISI][Medline].
26.
Star, RA,
Nonoguchi H,
Balaban R,
and
Knepper MA.
Calcium and cyclic adenosine monophosphate as second messenger for vasopressin in the rat inner medulla collecting ducts.
J Clin Invest
81:
1879-1888,
1988[ISI][Medline].
27.
Tsukaguchi, H,
Shayakul C,
Berger UV,
Brown D,
Tokui T,
and
Hediger MA.
Cloning and characterization of the urea transporter UrT2 localization in rat kidney and testis.
J Clin Invest
99:
1506-1515,
1997
28.
Tsukaguchi, H,
Shayakul C,
Berger UV,
and
Hediger MA.
Urea transporters in kidney: molecular analysis and contribution to the urinary concentrating process.
Am J Physiol Renal Physiol
275:
F319-F324,
1998
29.
Tsukaguchi, H,
Shayakul C,
Berger UV,
Mackenzie B,
Devidas S,
Guggino WB,
van Hoek AN,
and
Hediger MA.
Molecular characterization of a broad selectivity neutral solute channel.
J Biol Chem
273:
24737-24743,
1998
30.
You, G,
Smith CP,
Kanai Y,
Lee WS,
Stelzner M,
and
Hediger MA.
Cloning and characterization of the vasopressin- regulated urea transporter.
Nature
365:
844-847,
1993[ISI][Medline].