Journal of the American Society of Nephrology
2007 JASN IMPACT FACTOR 7.111 HOME   AUTHOR INFO   EDITORIAL BOARD   SUBSCRIBE   FEEDBACK   ALERTS   HELP 
    advanced
CURRENT ISSUE ARCHIVES JASN Express ONLINE SUBMISSION


This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Sands, J. M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Sands, J. M.
J Am Soc Nephrol 13:2795-2806, 2002
© 2002 American Society of Nephrology

REVIEW

Molecular Approaches to Urea Transporters

Jeff M. Sands

Renal Division, Department of Medicine, and Department of Physiology, Emory University School of Medicine, Atlanta, Georgia.

Correspondence to Dr. Jeff M. Sands, Emory University School of Medicine, Renal Division, WMRB Room 338, 1639 Pierce Drive, NE, Atlanta, GA 30322. Phone: 404-727-2525; Fax: 404-727-3425; E-mail: jsands{at}emory.edu


   Abstract
Top
 Abstract
Introduction
 The Urinary-Concentrating...
 References
 
ABSTRACT. Urea plays a critical role in the urine-concentrating mechanism in the inner medulla. Physiologic data provided evidence that urea transport in red blood cells and kidney inner medulla was mediated by specific urea transporter proteins. Molecular approaches during the past decade resulted in the cloning of two gene families for facilitated urea transporters, UT-A and UT-B, encoding several urea transporter cDNA isoforms in humans, rodents, and several nonmammalian species. Polyclonal antibodies have been generated to the cloned urea transporter proteins, and the use of these antibodies in integrative animal studies has resulted in several novel findings, including: (1) the surprising finding that UT-A1 protein abundance and urea transport are increased in the inner medulla during conditions in which urine concentrating ability is reduced; (2) vasopressin increases UT-A1 phosphorylation in rat inner medullary collecting duct; (3) UT-A protein abundance is upregulated in uremia in both liver and heart; and (4) UT-B is expressed in many nonrenal tissues and endothelial cells. This review will summarize the knowledge gained from using molecular approaches to perform integrative studies into urea transporter protein regulation, both in normal animals and in animal models of human diseases, including studies of uremic rats in which urea transporter protein is upregulated in liver and heart.


   Introduction
Top
 Abstract
 Introduction
The Urinary-Concentrating...
 References
 
Urea is a small (molecular weight, 60 Da), highly polar molecule ([NH2] - [C = O] - [NH2]) that has low lipid solubility through artificial lipid bilayers (4 x 10-6 cm/s (1). Thus, urea should have a low permeability across cell membranes that lack a transport protein to facilitate its transfer. The high urea permeability across red blood cells and the kidney terminal inner medullary collecting duct IMCD) was the initial evidence suggesting the presence of specific urea transporter proteins (2,3). Transport studies of red blood cells and isolated perfused tubules, performed in the 1970s and 1980s, provided the physiologic evidence that established the concept of urea transporters (reviewed in references 4 and 5). In the last 10 yr, two genes and several cDNA isoforms for urea transporters were cloned (Table 1). Through the use of these cDNAs and polycolonal antibodies to the urea transporter proteins, urea transporters have now been found in liver, heart, testis, and brain, and in some of these tissues, their abundance is altered by uremia (6–8). This review will focus on the knowledge gained from molecular approaches to the study of urea transporters in kidney and other tissues.


View this table:
[in this window]
[in a new window]

  		Table 1. Facilitated urea transporter genes and isoformsa
 
Given that most textbooks state that urea is freely permeable across cell membranes, the reader may be puzzled at the existence of urea transporters in red blood cells, kidney, and other organs. Although urea’s permeability across artificial lipid bilayers is very low, it is not zero, and urea will diffuse across cell membranes slowly and achieve equilibrium in the steady state. However, the transit time for tubule fluid through the collecting duct or for red blood cells through the vasa recta is too fast to allow urea concentrations to reach equilibrium solely by passive diffusion. Furthermore, the finding that urea transporters are expressed in several nonrenal tissues suggests that urea was mistakenly considered freely permeable because of a lack of knowledge regarding the wide-spread distribution of urea transporters.


   The Urinary-Concentrating Mechanism: Role of Urea Transporters
Top
 Abstract
 Introduction
 The Urinary-Concentrating...
References
 
The original quest for a urea transporter resulted from urea’s key role in the urinary-concentrating mechanism. Urea’s importance to the generation of concentrated urine has been appreciated since Gamble et al. (9) described "an economy of water in renal function referable to urea" in 1934. Protein-deprived animals and humans have an impaired ability to concentrate their urine, but this is corrected by an infusion of urea (9–16). Thus, any hypothesis regarding the mechanism by which the kidney concentrates urine needs to take into account some effect derived from urea.

The passive mechanism hypothesis for urinary concentration was proposed in 1972 by Kokko and Rector (17) and Stephenson (18) (Figure 1). This hypothesis requires that the inner medullary interstitial urea concentration exceed the urea concentration in the lumen of the thin ascending limb. The inequality of urea concentration permits the interstitial NaCl concentration to be less than the NaCl concentration in the lumen of the thin ascending limb, thereby establishing a gradient for passive NaCl absorption in the absence of an osmotic gradient. If an inadequate amount of urea is delivered to the deep inner medullary interstitium, then the chemical gradients necessary for passive NaCl absorption from the thin ascending limb cannot be established, and urine-concentrating ability is reduced. Urea absorption from the terminal IMCD, mediated by a facilitated urea transporter, is the primary mechanism for delivering urea to the deep inner medullary interstitium (2,19). On the basis of functional measurements in isolated perfused IMCD and red blood cells that defined the properties of putative facilitated urea transporters, two genes encoding several cDNA isoforms have been cloned (Table 1).



View larger version (33K):
[in this window]
[in a new window]
[as a PowerPoint slide]
  		Figure 1. Diagram showing the location of the major medullary transport proteins involved in the urine concentrating mechanism. Shown are a loop of Henle (left) and collecting duct (right). UT, urea transporter; AQP, aquaporin; NKCC/BSC, Na-K-2Cl cotransporter; ROMK, renal outer medullary K channel.

 
The UT-A Urea Transporter Family
UT-A1 is the largest UT-A protein (Figure 2) and is expressed in the apical membrane of the IMCD in humans (20) and rats (21,22). Human and rodent UT-A1 are stimulated by cyclic AMP (cAMP) when expressed in Xenopus oocytes (20,23–26). UT-A2, which was the first urea transporter to be cloned (27), is expressed in thin descending limbs (21,22,28) and is not stimulated by cAMP analogs when expressed in either Xenopus oocytes or human embryonic kidney (HEK) 293 cells (23–25,27,29–31). UT-A1 and UT-A2 share identical C-terminal amino acid and 3' cDNA sequences, but they differ at their N-terminal (5') ends (20,23,32). Thus, UT-A2 is basically the C-terminal (3') half of UT-A1. UT-A3 has the same N-terminal amino acid and 5' cDNA sequence as UT-A1 but has a unique C-terminal (3') end and is basically the N-terminal (5') half of UT-A1 (31,33,34). UT-A3 is also expressed in the apical membrane of the IMCD (35) and is stimulated by cAMP analogs when expressed in HEK-293 cells or Xenopus oocytes in two studies (26,31) but not in a third study (34). Although UT-A4 has the same N- and C-terminal amino acid (5' and 3' cDNA) sequence as UT-A1, it is smaller than UT-A1 and basically consists of the N-terminal (5') quarter of UT-A1 spliced to the C-terminal (3') quarter of UT-A1 (31). Although its exact tubular location is unknown, UT-A4 mRNA is expressed in kidney medulla and is stimulated by cAMP analogs when expressed in HEK-293 cells (31). UT-A5 is expressed in testis but not in kidney (33), and the effect of vasopressin on UT-A5 has not been tested. UT-A5 is the shortest member of the UT-A family; its deduced amino acid sequence begins at methionine 139 of mouse UT-A3, after which it shares 100% homology and a common C-terminal amino acid (3' cDNA) end with UT-A3 (33).



View larger version (31K):
[in this window]
[in a new window]
[as a PowerPoint slide]
  		Figure 2. Diagram showing relationship between the four rat kidney UT-A proteins. There is a high degree of homology between these protein isoforms. There is no difference in the coding region of UT-A1, UT-A2, and UT-A3, and their respective b variants; therefore, the latter are not shown separately in this diagram. UT-A1, UT-A3, and UT-A4 share common amino-termini (N versus N'). UT-A1, UT-A2, and UT-A4 share common carboxy-termini (C versus C'). Consensus sites for phosphorylation and glycosylation are indicated. S, serine; T, threonine.

 
Three UT-A cDNA transcripts that have alternative 3'-untranslated regions have been cloned and named UT-A1b, UT-A2b, and UT-A3b, respectively (36). UT-A1b and UT-A2b transcripts are approximately 0.4 kb shorter than the original cDNAs, whereas the UT-A3b transcript is approximately 1.5 kb longer than the original cDNA (36). Northern analysis shows that UT-A1b, UT-A2b, and UT-A3b mRNAs are expressed in rat inner medulla (36).

Polyclonal antibodies have been made to three regions of UT-A1: the C-terminus (21,37); the N-terminus (28); and the intracellular loop region of UT-A1 (38). Due to the high degree of homology between the kidney UT-A cDNA isoforms, the C-terminus antibody should detect UT-A1 (97 and 117 kD), UT-A2 (55 kD), and UT-A4 (43 kD), the N-terminus antibody should detect UT-A1, UT-A3 (44 and 67 kD), and UT-A4, and the loop region antibody should detect only UT-A1 (5,31,35,38). Western analyses of inner medullary tip proteins show bands at both 117 and 97 kD using any of the anti-UT-A1 antibodies (21,37,38); both protein bands represent glycosylated versions of a non-glycosylated 88-kD UT-A1 protein (39). In addition, UT-A1 exists as a 206-kD protein complex in native inner medullary membranes (39). UT-A1 protein is most abundant in the inner medullary tip, present in the inner medullary base, but only as a 97-kD protein, and is not present in outer medulla or cortex (21,40,41).

Even though the C-terminus of UT-A3 differs from UT-A1 by only a single amino acid, Terris et al. (35) succeeded in making a UT-A3–specific antibody that detects bands at both 67 and 44 kD; both bands represent glycosylated versions of a non-glycosylated 40-kD UT-A3 protein. Both UT-A3 glycoproteins are most abundant in the inner medullary tip, weakly detected in the inner medullary base and outer medulla, and not detected in cortex (35).

Although the apical membrane is the rate-limiting barrier for vasopressin-stimulated urea transport, functional studies show that phloretin-inhibitable urea transport is present in both the apical and basolateral membranes of rat terminal IMCD (42). Both UT-A1 and UT-A3 immunostaining is detected in the apical plasma membrane and intracellular cytoplasmic vesicles of terminal IMCD, but not in the basolateral plasma membrane (21,35). Thus, the molecular identity of the IMCD basolateral membrane urea transporter remains undetermined.

The UT-A Gene
The UT-A gene Slc14a2 was initially cloned from rat (43), but it has now been cloned from both human and mouse (20,32). The rat UT-A gene contains 24 exons and extends for approximately 300 kb (43). UT-A1 is encoded by exons 1 to 12 spliced to exons 14 to 23. UT-A3 is encoded by exons 1 to 12. UT-A4 is encoded by exons 1 to 7 spliced to exons 18 to 23. These three UT-A isoforms share a common transcription start site in exon 1 and translation start site in exon 4. UT-A2 is unique because it is the only isoform that uses exon 13. It is encoded by exons 13 to 23 with a translation start site in exon 16. Exon 24 contains the alternative 3' untranslated region used by UT-A1b and UT-A2b.

The rat UT-A isoforms originate from a single gene (Slc14a2) that has two promoter elements: promoter I, which is upstream of exon 1 and drives transcription of UT-A1, UT-A1b, UT-A3, UT-A3b, and UT-A4; and promoter II, which is located within intron 12 and drives transcription of UT-A2 and UT-A2b (36,43). The initial 1.3 kb of UT-A promoter I contains 3 CCAAT elements but does not contain a TATA box (44). However, its expression in a luciferase reporter gene construct and transfection into MDCK, mIMCD3, or LLC-PK1 cells results in promoter activity (44,45). Hyperosmolality increases promoter I activity, consistent with the presence of a tonicity enhancer (TonE) element (44). Dexamethasone reduces promoter I activity and the mRNA abundance of UT-A1, UT-A3, and UT-A3b in the inner medulla of rats given a stress dose of dexamethasone (45). Although a consensus glucocorticoid response element (GRE) is present in promoter I, the repressive effect of dexamethasone does not occur via this GRE (45).

In contrast to the other UT-A isoforms, the transcription start site for UT-A2 is located in exon 13, almost 200 kb downstream from exon 1 (36,43). This distance suggested the hypothesis that there may be a second, internal promoter within intron 12. Cloning and sequencing of intron 12 shows that it does contain a TATA box 40 bp upstream of the UT-A2 transcription start site in exon 13 and a cAMP response element (CRE) 300 bp upstream of the UT-A2 transcription start site (36,43). Transfection of a luciferase reporter gene from this region into mIMCD3 cells shows evidence of promoter activity when the cells are stimulated with cAMP, but not under basal conditions (36,43).

The mouse UT-A gene has a similar structure (26,32). Like the rat (43), it contains 24 exons, is over 300 kb in length, has two promoter elements, and promoter I contains a TonE element, the activity of which is increased by hypertonicity (32). In contrast to rat, mouse UT-A promoter I activity is increased by cAMP, even though no consensus CRE element is present (32). Although all mouse UT-A isoforms, including UT-A5, originate from a single mouse UT-A gene, the transcription start site of UT-A5 has not been determined and could be located downstream from mouse UT-A1 and UT-A3 (32).

The human UT-A gene is located on chromosome 18, contains 20 exons, and extends for approximately 67.5 kb. The human gene is substantially shorter than the rodent gene: (1) the 5'-untranslated region is almost entirely located in exon 1, whereas in rat and mouse it spans the first three widely spaced exons; and (2) the 3'-untranslated region does not contain an exon analogous to rat exon 24 (20,32,43).

Although the mechanism is not known, single nucleotide polymorphisms in human UT-A2 are associated with variation in BP in men but not in women (46). Facilitated urea transporter cDNAs that are most homologous to UT-A2 have been cloned from frog, elasmobranch, eel, gulf toadfish, Lake Migadi tilapia, and pilot whale (47–53). A detailed discussion of these urea transporters is beyond the scope of this review, and the reader is referred to the original citations for more information about them.

Long-Term Regulation of UT-A
Vasopressin
Increasing plasma vasopressin (also called antidiuretic hormone [ADH]) by administering it exogenously, or by water restriction, decreases the abundance of both the 117- and 97-kD UT-A1 proteins in rat inner medulla (38) and decreases basal urea permeability in the perfused terminal IMCD (54). Thus, these studies led to the surprising finding that urea transport and UT-A1 protein abundance are decreased when vasopressin levels are increased. This decrease in UT-A1 protein abundance and basal urea permeability is not a result of a decrease in UT-A1 or UT-A1b mRNA, because Northern analysis shows no change in either mRNA abundance in response to either water loading or restriction in most studies (24,25,29,32,36,55), although one study does report that UT-A1 is decreased in water-restricted or vasopressin-treated Brattleboro rats (which have central diabetes insipidus) (56).

Feeding rats a low (10%) or high (40%) protein diet for 1 wk, compared with a control diet containing 20% protein, also has no effect on UT-A1 mRNA abundance in any portion of the kidney medulla (30,57). However, in Brattleboro rats, UT-A1 mRNA is decreased in the inner medullary tip of low-protein–fed rats (57), suggesting that there may be an interaction between dietary protein and vasopressin on UT-A1 mRNA abundance. Overall, transcriptional regulation does not appear to be the mechanism for changes in UT-A1 protein abundance in response to changes in hydration and/or vasopressin level.

In contrast, UT-A2, UT-A2b, UT-A3, and UT-A3b mRNA abundances fall in the inner medulla of water-loaded rats and rise in rodents with increased vasopressin levels, due either to vasopressin administration or water restriction (24,29,32,36,55,56,58). UT-A2 mRNA abundance is increased in the base of the inner medulla from rats fed an 8% (low) protein diet for 1 wk (30), but not in Sprague-Dawley or Brattleboro rats fed a 10% protein diet (57). UT-A2 protein abundance is increased by administering dDAVP (Desmopressin, a V2-selective vasopressin receptor agonist) to Brattleboro rats (28) and decreased by treating rats with furosemide (59). Thus, vasopressin may regulate UT-A2 by a transcriptional mechanism, consistent with promoter II containing a CRE element and promoter activity being increased by cAMP (44). UT-A2 expression can also be induced in mIMCD3 cells grown in hypertonic culture media in which osmolality is increased by adding equiosmolar NaCl and urea; mIMCD3 cells grown in isotonic culture media do not express UT-A2 (or any other UT-A isoform) (43,59). UT-A3 protein also increases in water-deprived rats (35), which could be transcriptionally mediated by the TonE element in promoter I (44). The long-term regulation of UT-A4 and UT-A5 have not been studied because UT-A4’s mRNA abundance in the renal medulla is too low to detect by Northern analysis (31) and UT-A5 is not expressed in kidney (33). Thus, there may be multiple mechanisms by which vasopressin regulates the different UT-A protein and mRNA isoforms.

UT-A Proteins during Development
UT-A immunostaining is not detected in the fetal kidney but appears in 1-d-old rats, both in the IMCD (UT-A1) and the thin descending limb (UT-A2), and increases progressively in both segments until adult levels are achieved at 21 d of age (22). Thus, the time course for the development of urine-concentrating ability in rats coincides with the increase in UT-A1 and UT-A2 immunostaining.

Impaired Urine-Concentrating Ability and UT-A1
The long-term regulation of UT-A1 protein abundance has been studied in several conditions associated with reduced urine-concentrating ability: water diuresis; low-protein diet; hypercalcemia; furosemide diuresis; adrenalectomy; and lithium administration (30,37,38,41,54,60–64). Surprisingly, in every condition except lithium administration (which is discussed in more detail below), both UT-A1 protein abundance and basal facilitated urea permeability are increased in the deepest portion of the IMCD during conditions with reduced urine-concentrating ability. The increase in UT-A1 protein abundance and urea absorption could be a mechanism for the rapid increase in urine-concentrating ability that occurs within 5 to 10 min after urea is infused into malnourished or low-protein-fed people or rats (9,10,12,65): UT-A1 protein abundance is increased when urine-concentrating ability is impaired, and this response "prepares" the IMCD to restore inner medullary urea rapidly once urea (or protein) intake rises.

Glucocorticoids
Adrenalectomy causes a urinary-concentrating defect in humans and rats (66–68). Administering dexamethasone to adrenalectomized rats decreases UT-A1 protein abundance and facilitated urea permeability in the rat terminal IMCD (37). Administering dexamethasone to normal rats decreases UT-A1, UT-A3, and UT-A3b mRNA abundances, but not UT-A2 mRNA abundance, in the tip of the inner medulla (45). This repressive effect is likely to be transcriptionally regulated because dexamethasone decreases the activity of promoter I (which controls transcription of UT-A1 and UT-A3) but has no effect on promoter II (which controls transcription of UT-A2) (45).

Volume Expansion
Rats (and people) become volume-expanded when given aldosterone and a high-NaCl diet, but do not become volume expanded when given aldosterone and a NaCl-free diet (69). Volume-expanded rats have decreased levels of UT-A1 and UT-A3 protein abundances in the inner medulla, whereas UT-A2 protein is unchanged (69). After volume expansion, the decrease in UT-A1 protein parallels the decrease in serum urea concentration while the decrease in UT-A3 is delayed (69). Inhibition of AT1-receptors (for 2 d) also decreases UT-A1 and UT-A3 protein abundances, suggesting that the suppression of the renin-angiotensin system that accompanies aldosterone-induced volume expansion may mediate the reduction in these two urea transporters (69).

UT-A1 Protein in Animal Models of Human Diseases
Diabetes Mellitus
In rats, uncontrolled diabetes mellitus (induced by streptozotocin) increases corticosterone production and urea excretion (70). UT-A1 protein abundance is downregulated in the inner medullary tip of rats at 3 d after streptozotocin-injection (62). However, UT-A1 protein abundance does not decrease in adrenalectomized rats injected with streptozotocin, suggesting that the diabetes-induced increase in glucocorticoids is the mechanism for reducing UT-A1 protein in rats with uncontrolled diabetes for 3 d (62). In contrast, at 21 d post-streptozotocin, UT-A1 mRNA and protein are increased in the inner medulla (71). However, UT-A1 protein is decreased in 6-mo-old, obese Zucker rats, a model of type II diabetes (72). Thus, UT-A1 abundance may vary with time from streptozotocin treatment, and hence with the duration of diabetes, and/or the type of diabetes.

Renal Failure
Administering adriamycin to rats for 3 wk results in proteinuria and decreased UT-A1 protein abundance in the inner medulla (73). Administering cisplatin to rats for 5 d results in acute renal failure, accompanied by an increase in urine volume and a decrease in urine osmolality, but no change in UT-A1, UT-A2, or UT-A4 protein abundances in the outer or inner medulla (74). Inducing uremia in rats by 5/6 nephrectomy results in an increase in urine output and a decrease in urine osmolality at 5 wk post-nephrectomy accompanied by undetectable levels of UT-A1 mRNA and protein and reduced levels of UT-A2 mRNA and protein (8).

Lithium
Lithium is widely used to treat patients with manic-depressive (bipolar) disorders but can cause nephrogenic diabetes insipidus and an inability to concentrate urine (reviewed in reference 75). Although the mechanisms by which lithium causes nephrogenic diabetes insipidus are not entirely understood, lithium-treated rats do have a marked reduction in AQP2 protein (64,76,77) and inner medullary interstitial osmolality (78).

Lithium-fed rats have a marked reduction in UT-A1 protein abundance in both the inner medullary tip and base (64). In addition, vasopressin does not increase UT-A1 phosphorylation in IMCD suspensions from lithium-fed rats, in contrast to vasopressin’s effect on IMCD suspensions from normal rats (64). Thus, lithium differs from the other conditions associated with reduced urine-concentrating ability (discussed above) because it reduces UT-A1 protein abundance. The reason for this difference will require future studies.

Rapid Regulation of UT-A in the IMCD
Studies of perfused IMCD have been the primary method for investigating the rapid regulation of urea transport. This method provides physiologically relevant, functional data, but it cannot determine which urea transporter isoform is responsible for a functional effect because the terminal IMCD expresses both UT-A1 and UT-A3. Thus, the functional studies reviewed below may be due to urea transport mediated by UT-A1, UT-A3, or both. In the past few years, new findings have been published for two regulators of urea transport: vasopressin and angiotensin II, and these studies will be reviewed below. The reader is referred to older reviews (4,5,79) that discuss other agents that rapidly regulate urea transport.

Vasopressin
Adding vasopressin to the bath of a perfused rat terminal IMCD results in binding to V2-receptors, stimulating adenylyl cyclase, generating cAMP, and ultimately increasing facilitated urea transport (2,80–82). One possible mechanism for rapid regulation is that vasopressin alters the phosphorylation of UT-A1 and/or UT-A3. The deduced amino acid sequences for UT-A1 and UT-A3 contain several consensus sites for phosphorylation by protein kinase A (PKA), as well as PKC and tyrosine kinase (Figure 2) (31). Vasopressin increases the phosphorylation of both the 117- and 97-kD UT-A1 proteins within 2 min in rat IMCD suspensions (83); it is not known whether vasopressin also alters UT-A3 phosphorylation. The time course and dose response for vasopressin-stimulated phosphorylation of UT-A1 is consistent with the time course and dose response for vasopressin-stimulated urea transport in the perfused rat terminal IMCD (80,82–84). Both dDAVP and cAMP also increase UT-A1 phosphorylation, and PKA inhibitors block the phosphorylation of UT-A1 by vasopressin (85). These findings strongly support the hypothesis that vasopressin rapidly increases urea transport in the rat terminal IMCD by increasing UT-A1 phosphorylation.

Another possible mechanism by which vasopressin could rapidly increase urea transport is regulated trafficking of UT-A1 and/or UT-A3. However, regulated trafficking of UT-A1 does not occur in the rat IMCD (86). Whether UT-A3 undergoes regulated trafficking has not been studied.

Angiotensin II
Both RT-PCR and in situ hybridization studies show that mRNA for the type 1 angiotensin II (AT 1) receptor is present in rat IMCD (87–89), and radioligand binding studies show that AT1 receptors are present (90). Angiotensin II has no effect on basal urea permeability, but it increases vasopressin-stimulated facilitated urea permeability in rat terminal IMCD and 32P incorporation into both the 117- and 97-kD UT-A1 proteins via a PKC-mediated effect (91). Mice that lack tissue angiotensin converting enzyme (ACE.2 mice) have a histologically normal medulla and a urine-concentrating defect (92). In the inner medulla of these mice, UT-A1 protein is decreased to 25% of the level in wild-type mice (93). Neither the urine-concentrating defect nor the reduction in UT-A1 protein is corrected by administering angiotensin II to ACE.2 mice (93). Thus, angiotensin II may play a physiologic role in the urinary-concentrating mechanism by augmenting the maximal urea permeability response to vasopressin.

UT-B Urea Transporter
The red blood cell facilitated urea transporter, UT-B, was originally cloned from a human erythropoietic cell line (94), but it has also been cloned from rodents (4,95–97). The human Slc14a1 (UT-B) gene arises from a single locus located on chromosome 18q12.1-q21.1, which is close to, but distinct from, the human Slc14a2 (UT-A) gene (98–100). The mouse Scl14a1 (UT-B) and Scl14a2 (UT-A) genes also occur in tandem on chromosome 18 (101). A minor blood group antigen, the Kidd (or Jk) antigen, is also located in the same region of human chromosome 18 as are the two urea transporter genes (100). In humans, the Kidd antigen is the UT-B protein (98–100). Several mutations of the Kidd antigen/UT-B (Scl14a1) gene have been reported (100,102). Red blood cells from individuals that lack the Kidd antigen (Jk(a-b-) or Jk null) do not have phloretin-sensitive facilitated urea transport (103).

The human UT-B gene includes 11 exons, with the coding region beginning in exon 4 and extending through exon 11, and is approximately 30 kb in length (100). Reticulocytes express two mRNA transcripts, 4.4 and 2.0 kb, due to alternative polyadenylation signals; both mRNA transcripts encode a single 45-kD protein (100). Although two rat cDNA sequences have been reported (UT-B1, UT-B2), they differ by only a few nucleotides at their 3' end (95,96), and it is uncertain whether UT-B1 and UT-B2 truly represent different rat UT-B isoforms, a polymorphism, or a sequencing artifact. At present, most investigators favor the hypothesis that rat UT-B1 and UT-B2 are not distinct isoforms because humans have only a single isoform, but this hypothesis has not been tested. UT-B1/UT-B2 mRNA is widely expressed and has been detected in kidney and several other organs, including brain, testis, bone marrow, spleen, prostate, bladder, thymus, heart, skeletal muscle, lung, liver, colon, small intestine, and pancreas (8,24,94–97,99,104–106). However, Northern analysis has not detected UT-B1/UT-B2 mRNA in placenta, salivary glands, ovary, leukocytes, monocytes, or B lymphocytes (24,94,96,104).

Three studies have addressed the question of whether UT-B transports urea only, or water and urea, by injecting UT-B1/UT-B2 cRNA into Xenopus oocytes. Two studies report that UT-B can function as a water channel when expressed in oocytes (97,107). However, a third study reports that UT-B is specific for urea transport if a physiologic expression level is achieved in oocytes, but that higher levels of UT-B expression result in an increase in water permeability (108).

Antibodies have been made to the N- or C-terminus of UT-B (8,106,109,110) that should detect both UT-B1 and UT-B2 proteins, if indeed there are two rat isoforms. Thus in this review, I will refer to the rat protein(s) detected by the anti-UT-B antibodies as UT-B protein. UT-B protein appears on Western analysis as a broad band between 45 to 65 kD in human red blood cells and 37 to 51 kD in rat or mouse red blood cells (97,106). In kidney outer or inner medulla, a broad band between 41 to 54 kD is detected; deglycosylation converts the broad band seen by Western analysis of either red blood cells or kidney medulla to a sharp 32 kD band (106,110). In addition, a 98-kD band is detected in kidney (106). However, the molecular explanation for this 98 kD band is uncertain (106).

Human and rodent kidney show UT-B immunostaining in nonfenestrated endothelial cells that are characteristic of descending vasa recta (8,97,106,109,110). UT-B protein is also present in rodent testis, brain, colon, heart, liver, lung, aorta, bladder, spinotrapezius muscle, and mesenteric artery (8,106,110,111) and in several cultured endothelial cell lines (106,111). UT-B promotes urea entry into cultured endothelial cells, thereby increasing intracellular urea and inhibiting L-arginine transport (111). If a similar mechanism is present in patients with chronic kidney disease, then the inhibition of arginine transport, a precursor of nitric oxide, could be another mechanism contributing to hypertension in these patients (111).

UT-B immunostaining is only weakly detected in rat kidney at fetal day 20, but it increases progressively after birth in the descending vasa recta, both in terms of the intensity of staining and the number of endothelial cells that stain for UT-B, until adult levels are achieved at 21 d of age (22). Thus, the time course for the development of urine concentrating ability in rats coincides with the increase in UT-B staining in the descending vasa recta.

Role of UT-B in Urine-Concentrating Ability
Kidd antigen null individuals are unable to concentrate their urine above 800 mOsm/kg H2O, even following overnight water deprivation and exogenous vasopressin administration (112). A UT-B knockout mouse has a similar impairment in urine-concentrating ability, achieving a maximal urine osmolality of 2400 mOsm/kg H2O compared with 3400 in a wild-type mouse (97). These findings support the hypothesis that facilitated urea transport in red blood cells or descending vasa recta is necessary to preserve the efficiency of countercurrent exchange (113). UT-B protein and phloretin-inhibitable urea transport are present in both red blood cells and perfused rat descending vasa recta (8,106,109,114–116), suggesting that urea transport in red blood cells and descending vasa recta occurs via UT-B protein.

Mathematical models of microcirculatory exchange between the ascending and descending vasa recta predict that urea transporters (UT-B) are necessary to counterbalance the effect of aquaporin-1 water channels in the descending vasa recta, i.e., the efficiency of small solute trapping within the renal medulla will be decreased in the absence of UT-B, thereby decreasing the efficiency of countercurrent exchange and urine-concentrating ability (117,118). Consistent with this hypothesis, urea recycling is impaired in the UT-B knockout mouse (97). Thus, the production of maximally concentrated urine appears to require UT-B protein expression in red blood cells and/or descending vasa recta (119).

Long-Term Regulation of UT-B in Kidney
The long-term regulation of UT-B has not been studied as extensively as UT-A. In Brattleboro rats, administering vasopressin or dDAVP for 6 h reduces UT-B mRNA abundance in both the inner and outer medulla (55). However, administering vasopressin or dDAVP for 5 d increases UT-B mRNA abundance in the inner stripe of the outer medulla and the inner medullary base, but it decreases it in the inner medullary tip (55). In normal rats, dDAVP administration for 7 d decreases UT-B protein abundance in the inner medulla, but furosemide administration also results in a more modest decrease in UT-B protein (110). Varying dietary protein between 10 and 40% had no effect on UT-B mRNA abundance in any portion of the medulla in either Brattleboro or normal rats (57). Inducing uremia in rats by 5/6 nephrectomy results in a reduction of UT-B mRNA and protein after 5 wk (8). Lastly, lithium-fed rats have a marked reduction in UT-B protein abundance in the inner medullary base (64).

Effect of Uremia on Urea Transporters in Nonrenal Tissues
Liver
The liver performs ureagenesis and has phloretin-inhibitable urea transport, suggesting that liver expresses a urea transporter, possibly to accelerate urea efflux after ureagenesis (6,120–122). HepG2 cells are a cultured human hepatoblastoma cell line that has a high rate of urea influx that is inhibited by two urea transport inhibitors: phloretin and thionicotinamide (123). Western blot analysis of HepG2 cells and rat liver reveals two protein bands: a 49-kD UT-A protein in the plasma membrane and a 36-kD UT-A protein in the cytoplasm (123). Rat liver expresses a 2.6-kb UT-A mRNA (124). This size is consistent with either UT-A2b or UT-A4 (Table 1), and sequencing of the liver UT-A mRNA will be required to identify the isoform.

The abundance of the 49-kD UT-A protein in liver varies with uremia and/or acidosis in rats (123,125); it increases in livers from rats made uremic by 5/6 nephrectomy but not in uremic rats given bicarbonate to correct their acidosis. The abundance of this 49-kD UT-A protein also increases in liver from nonuremic rats made acidotic by HCl-feeding (125). HCl-feeding also increases the 117 kD, but not the 97 kD, UT-A1 protein abundance in kidney inner medulla (125). Thus acidosis, either directly or through a change in ammonium concentration, increases the abundance of the 49-kD UT-A protein in liver and the 117-kD UT-A1 protein in kidney inner medulla (125).

Heart
Somewhat surprisingly, heart also expresses a urea transporter (7). Although the rationale is not as obvious as for kidney, red blood cells, or liver, cardiac hypertrophy is associated with an increase in polyamine synthesis (7). Urea is a by-product of the production of ornithine from arginine; therefore, it is possible that the heart needs a urea transporter to dispose of any urea produced within its cells (7). Western analysis shows that rat heart expresses 3 UT-A proteins: 56, 51, and 39 kD (7). Uremia increases the abundance of the 56-kD UT-A glycoprotein (7). The abundance of the 56-kD UT-A protein also increases in hypertrophic hearts from non-uremic DOCA/salt hypertensive rats, and in short-term hypertension induced by a 3 d infusion of angiotensin II (7). Rat heart expresses only a single 2.7-kb UT-A mRNA transcript (7,31), and cDNA sequencing shows that this mRNA is UT-A2b (7).

Human heart expresses four UT-A proteins: 97, 56, 51, and 39 kD (7). The abundance of the 56- and 51-kD UT-A proteins increases in terminally failing (NYHA class IV) human hearts (7). Thus, UT-A proteins are expressed in human and rat heart, and the abundance the 56-kD UT-A protein increases in conditions such as uremia and hypertension that predispose to left ventricular hypertrophy.

Testis
Seminiferous tubules have phloretin-inhibitable urea transport and express four UT-A mRNA transcripts (approximately 4.0, 3.3, 2.8, and 1.7 kb), some of which have not been detected in other rodent tissues (31,33,124,126); the 1.7-kb transcript is UT-A5 (33). UT-B protein and mRNA are also expressed in seminiferous tubules (8,95,96,106,126). Uremic does not change UT-B mRNA abundance in rat testis (8).

Brain
Both UT-A and UT-B mRNA and protein are expressed in brain (8,95,96,105,106,123,124). UT-B mRNA is unchanged at 1 wk post-5/6 nephrectomy, but it is reduced to about 30% of control levels after 5 wk (8).


   Acknowledgments
 
This work was supported by National Institutes of Health grants R01-DK41707, R01-DK63657, and P01-DK50268.


   References
Top
 Abstract
 Introduction
 The Urinary-Concentrating...
 References
 

  1. Galluci E, Micelli S, Lippe C: Non-electrolyte permeability across thin lipid membranes. Arch Int Physiol Biochim 79: 881–887, 1971[Medline]
  2. Sands JM, Nonoguchi H, Knepper MA: Vasopressin effects on urea and H20 transport in inner medullary collecting duct subsegments. Am J Physiol Renal Physiol 253: F823–F832, 1987[Abstract/Free Full Text]
  3. Wieth JO, Funder J, Gunn RB, Brahm J: Passive transport pathways for chloride and urea through the red cell membrane. In: Comparative Biochemistry and Physiology of Transport, edited by Bolis K, Bloch K, Luria SE, Lynen F, Amsterdam, Elsevier/North-Holland, 1974, pp 317–337
  4. Sands JM, Timmer RT, Gunn RB: Urea transporters in kidney and erythrocytes. Am J Physiol Renal Physiol 273: F321–F339, 1997[Abstract/Free Full Text]
  5. Sands JM: Regulation of renal urea transporters. J Am Soc Nephrol 10: 635–646, 1999[Abstract/Free Full Text]
  6. Effros RM, Jacobs E, Hacker A, Ozker K, Murphy C: Reversible inhibition of urea exchange in rat hepatocytes. J Clin Invest 91: 2822–2828, 1993
  7. Duchesne R, Klein JD, Velotta JB, Doran JJ, Rouillard P, Roberts BR, McDonough AA, Sands JM: UT-A urea transporter protein in heart: Increased abundance during uremia, hypertension, and heart failure. Circ Res 89: 139–145, 2001[Abstract/Free Full Text]
  8. Hu MC, Bankir L, Michelet S, Rousselet G, Trinh-Trang-Tan M-M: Massive reduction of urea transporters in remnant kidney and brain of uremic rats. Kidney Int 58: 1202–1210, 2000[CrossRef][Medline]
  9. Gamble JL, McKhann CF, Butler AM, Tuthill E: An economy of water in renal function referable to urea. Am J Physiol 109: 139–154, 1934[Free Full Text]
  10. Pennell JP, Sanjana V, Frey NR, Jamison RL: The effect of urea infusion on the urinary concentrating mechanism in protein-depleted rats. J Clin Invest 55: 399–409, 1975
  11. Crawford JD, Doyle AP, Probst H: Service of urea in renal water conservation. Am J Physiol 196: 545–548, 1959[Abstract/Free Full Text]
  12. Levinsky NG, Berliner RW: The role of urea in the urine concentrating mechanism. J Clin Invest 38: 741–748, 1959
  13. Peil AE, Stolte H, Schmidt-Nielsen B: Uncoupling of glomerular and tubular regulations of urea excretion in rat. Am J Physiol Renal Physiol 258: F1666–F1674, 1990[Abstract/Free Full Text]
  14. Epstein FH, Kleeman CR, Pursel S, Hendrikx A: The effect of feeding protein and urea on the renal concentrating process. J Clin Invest 36: 635–641, 1957
  15. Klahr S, Alleyne GAO: Effects of chronic protein-calorie malnutrition on the kidney. Kidney Int 3: 129–141, 1973[Medline]
  16. Hendrikx A, Epstein FH: Effect of feeding protein and urea on renal concentrating ability in the rat. Am J Physiol 195: 539–542, 1958[Abstract/Free Full Text]
  17. Kokko JP, Rector FC: Countercurrent multiplication system without active transport in inner medulla. Kidney Int 2: 214–223, 1972[Medline]
  18. Stephenson JL: Concentration of urine in a central core model of the renal counterflow system. Kidney Int 2: 85–94, 1972[Medline]
  19. Sands JM, Knepper MA: Urea permeability of mammalian inner medullary collecting duct system and papillary surface epithelium. J Clin Invest 79: 138–147, 1987
  20. Bagnasco SM, Peng T, Janech MG, Karakashian A, Sands JM: Cloning and characterization of the human urea transporter UT-A1 and mapping of the human Slc14a2 gene. Am J Physiol Renal Physiol 281: F400–F406, 2001[Abstract/Free Full Text]
  21. Nielsen S, Terris J, Smith CP, Hediger MA, Ecelbarger CA, Knepper MA: Cellular and subcellular localization of the vasopressin-regulated urea transporter in rat kidney. Proc Natl Acad Sci USA 93: 5495–5500, 1996[Abstract/Free Full Text]
  22. Kim Y-H, Kim D-U, Han K-H, Jung J-Y, Sands JM, Knepper MA, Madsen KM, Kim J: Expression of urea transporters in the developing rat kidney. Am J Physiol Renal Physiol 282: F530–F540, 2002[Abstract/Free Full Text]
  23. Shayakul C, Steel A, Hediger MA: Molecular cloning and characterization of the vasopressin-regulated urea transporter of rat kidney collecting ducts. J Clin Invest 98: 2580–2587, 1996[Medline]
  24. Promeneur D, Rousselet G, Bankir L, Bailly P, Cartron JP, Ripoche P, 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]
  25. Shayakul C, Knepper MA, Smith CP, DiGiovanni SR, Hediger MA: Segmental localization of urea transporter mRNAs in rat kidney. Am J Physiol Renal Physiol 272: F654–F660, 1997[Abstract/Free Full Text]
  26. Fenton RA, Stewart GS, Carpenter B, Howorth A, Potter EA, Cooper GJ, Smith CP: Characterization of the mouse urea transporters UT-A1 and UT-A2. Am J Physiol Renal Physiol 283: F817–F825, 2002[Abstract/Free Full Text]
  27. You G, Smith CP, Kanai Y, Lee W-S, Stelzner M, Hediger MA: Cloning and characterization of the vasopressin-regulated urea transporter. Nature 365: 844–847, 1993[CrossRef][Medline]
  28. Wade JB, Lee AJ, Liu J, Ecelbarger CA, Mitchell C, Bradford AD, Terris J, Kim G-H, Knepper MA: UT-A2: A 55 kDa urea transporter protein in thin descending limb of Henle’s loop whose abundance is regulated by vasopressin. Am J Physiol Renal Physiol 278: F52–F62, 2000[Abstract/Free Full Text]
  29. Smith CP, Lee W-S, Martial S, Knepper MA, You G, Sands JM, Hediger MA: Cloning and regulation of expression of the rat kidney urea transporter (rUT2). J Clin Invest 96: 1556–1563, 1995
  30. Ashkar ZM, Martial S, Isozaki T, Price SR, Sands JM: Urea transport in initial IMCD of rats fed a low-protein diet: Functional properties and mRNA abundance. Am J Physiol Renal Physiol 268: F1218–F1223, 1995[Abstract/Free Full Text]
  31. Karakashian A, Timmer RT, Klein JD, Gunn RB, Sands JM, Bagnasco SM: Cloning and characterization of two new mRNA isoforms of the rat renal urea transporter: UT-A3 and UT-A4. J Am Soc Nephrol 10: 230–237, 1999[Abstract/Free Full Text]
  32. Fenton RA, Cottingham CA, Stewart GS, Howorth A, Hewitt JA, Smith CP: Structure and characterization of the mouse UT-A gene (Slc14a2). Am J Physiol Renal Physiol 282: F630–F638, 2002[Abstract/Free Full Text]
  33. Fenton RA, Howorth A, Cooper GJ, Meccariello R, Morris ID, Smith CP: Molecular characterization of a novel UT-A urea transporter isoform (UT-A5) in testis. Am J Physiol Cell Physiol 279: C1425–C1431, 2000[Abstract/Free Full Text]
  34. Shayakul C, Tsukaguchi H, Berger UV, Hediger MA: Molecular characterization of a novel urea transporter from kidney inner medullary collecting ducts. Am J Physiol Renal Physiol 280: F487–F494, 2001[Abstract/Free Full Text]
  35. Terris JM, Knepper MA, Wade JB: UT-A3: Localization and characterization of an additional urea transporter isoform in the IMCD. Am J Physiol Renal Physiol 280: F325–F332, 2001[Abstract/Free Full Text]
  36. Bagnasco SM, Peng T, Nakayama Y, Sands JM: Differential expression of individual UT-A urea transporter isoforms in rat kidney. J Am Soc Nephrol 11: 1980–1986, 2000[Abstract/Free Full Text]
  37. Naruse M, Klein JD, Ashkar ZM, Jacobs JD, Sands JM: Glucocorticoids downregulate the rat vasopressin-regulated urea transporter in rat terminal inner medullary collecting ducts. J Am Soc Nephrol 8: 517–523, 1997[Abstract]
  38. Terris J, Ecelbarger CA, Sands JM, Knepper MA: Long-term regulation of collecting duct urea transporter proteins in rat. J Am Soc Nephrol 9: 729–736, 1998[Abstract]
  39. Bradford AD, Terris J, Ecelbarger CA, Klein JD, Sands JM, Chou C-L, Knepper MA: 97 and 117 kDa forms of the collecting duct urea transporter UT-A1 are due to different states of glycosylation. Am J Physiol Renal Physiol 281: F133–F143, 2001[Abstract/Free Full Text]
  40. Sands JM: Urea transport: It’s not just "freely diffusible" anymore. NIPS 14: 46–47, 1999[Free Full Text]
  41. Sands JM, Flores FX, Kato A, Baum MA, Brown EM, Ward DT, Hebert SC, Harris HW: Vasopressin-elicited water and urea permeabilities are altered in the inner medullary collecting duct in hypercalcemic rats. Am J Physiol Renal Physiol 274: F978–F985, 1998[Abstract/Free Full Text]
  42. Star RA: Apical membrane limits urea permeation across the rat inner medullary collecting duct. J Clin Invest 86: 1172–1178, 1990
  43. Nakayama Y, Naruse M, Karakashian A, Peng T, Sands JM, Bagnasco SM: Cloning of the rat Slc14a2 gene and genomic organization of the UT-A urea transporter. Biochim Biophys Acta 1518: 19–26, 2001[Medline]
  44. Nakayama Y, Peng T, Sands JM, Bagnasco SM: The TonE/TonEBP pathway mediates tonicity-responsive regulation of UT-A urea transporter expression. J Biol Chem 275: 38275–38280, 2000[Abstract/Free Full Text]
  45. Peng T, Sands JM, Bagnasco SM: Glucocorticoids inhibit transcription and expression of the rat UT-A urea transporter gene. Am J Physiol Renal Physiol 282: F853–F858, 2002[Abstract/Free Full Text]
  46. Ranade K, Wu KD, Hwu CM, Ting CT, Pei D, Pesich R, Hebert J, Chen YDI, Pratt R, Olshen R, Masaki K, Risch N, Cox DR, Botstein D: Genetic variation in the human urea transporter-2 is associated with variation in blood pressure. Hum Mol Genet 10: 2157–2164, 2001[Abstract/Free Full Text]
  47. Couriaud C, Leroy C, Simon M, Silberstein C, Bailly P, Ripoche P, Rousselet G: Molecular and functional characterization of an amphibian urea transporter. Biochim Biophys Acta Bio-Membr 1421: 347–352, 1999[Medline]
  48. Smith CP, Wright PA: Molecular characterization of an elasmobranch urea transporter. Am J Physiol Regul Integr Comp Physiol 276: R622–R626, 1999[Abstract/Free Full Text]
  49. Mistry AC, Honda S, Hirata T, Kato A, Hirose S: Eel urea transporter is localized to chloride cells and is salinity dependent. Am J Physiol Regul Integr Comp Physiol 281: R1594–R1604, 2001[Abstract/Free Full Text]
  50. Walsh PJ, Heitz MJ, Campbell CE, Cooper GJ, Medina M, Wang YXS, Goss GG, Vincek V, Wood CM, Smith CP: Molecular characterization of a urea transporter in the gill of the gulf toadfish (Opsanus beta). J Exp Biol 203: 2357–2364, 2000[Abstract]
  51. Walsh PJ, Grosell M, Goss GG, Bergman HL, Bergman AN, Wilson P, Laurent P, Alper SL, Smith CP, Kamunde C, Wood CM: Physiological and molecular characterization of urea transport by the gills of the Lake Magadi tilapia (Alcolapia grahami). J Exp Biol 204: 509–520, 2001[Abstract]
  52. Walsh PJ, Wang Y, Campbell CE, De Boeck G, Wood CM: Patterns of nitrogenous waste excretion and gill urea transporter mRNA expression in several species of marine fish. Marine Biol 139: 839–844, 2001[CrossRef]
  53. Janech MG, Chen R, Klein J, Nowak MW, McFee W, Paul RV, Fitzgibbon WR, Ploth DW: Molecular and functional characterization of a urea transporter from the kidney of a short-finned pilot whale. Am J Physiol Regul Integr Comp Physiol 282: R1490–R1500, 2002[Abstract/Free Full Text]
  54. Kato A, Naruse M, Knepper MA, Sands JM: Long-term regulation of inner medullary collecting duct urea transport in rat. J Am Soc Nephrol 9: 737–745, 1998[Abstract]
  55. Promeneur D, Bankir L, Hu MC, Trinh-Trang-Tan M-M: Renal tubular and vascular urea transporters: Influence of antidiuretic hormone on messenger RNA expression in Brattleboro rats. J Am Soc Nephrol 9: 1359–1366, 1998[Abstract]
  56. Shayakul C, Smith CP, Mackenzie HS, Lee W-S, Brown D, Hediger MA: Long-term regulation of urea transporter expression by vasopressin in Brattleboro rats. Am J Physiol Renal Physiol 278: F620–F627, 2000[Abstract/Free Full Text]
  57. Hu MC, Bankir L, Trinh-Trang-Tan MM: mRNA expression of renal urea transporters in normal and Brattleboro rats: Effect of dietary protein intake. Exp Nephrol 7: 44–51, 1999
  58. Knox FG, Burnett JC Jr, Kohan DE, Spielman WS, Strand JC: Escape from the sodium-retaining effects of mineralocorticoids. Kidney Int 17: 263–276, 1980[Medline]
  59. Leroy C, Basset G, Gruel G, Ripoche P, Trinh-Trang-Tan M-M, Rousselet G: Hyperosmotic NaCl and urea synergistically regulate the expression of the UT-A2 urea transporter in vitro and in vivo. Biochem Biophys Res Commun 271: 368–373, 2000[CrossRef][Medline]
  60. Isozaki T, Verlander JW, Sands JM: Low protein diet alters urea transport and cell structure in rat initial inner medullary collecting duct. J Clin Invest 92: 2448–2457, 1993
  61. Isozaki T, Gillin AG, Swanson CE, Sands JM: Protein restriction sequentially induces new urea transport processes in rat initial IMCDs. Am J Physiol Renal Physiol 266: F756–F761, 1994[Abstract/Free Full Text]
  62. Klein JD, Price SR, Bailey JL, Jacobs JD, Sands JM: Glucocorticoids mediate a decrease in the AVP-regulated urea transporter in diabetic rat inner medulla. Am J Physiol Renal Physiol 273: F949–F953, 1997
  63. Kato A, Sands JM: Urea transport processes are induced in rat IMCD subsegments when urine concentrating ability is reduced. Am J Physiol Renal Physiol 276: F62–F71, 1999[Abstract/Free Full Text]
  64. Klein JD, Gunn RB, Roberts BR, Sands JM: Down-regulation of urea transporters in the renal inner medulla of lithium-fed rats. Kidney Int 61: 995–1002, 2002[CrossRef][Medline]
  65. Wilson DR, Sonnenberg H: Urea reabsorption in the medullary collecting duct of protein-depleted young rats before and after urea infusion. Pfluegers Arch 393: 302–309, 1982[CrossRef][Medline]
  66. Kamoi K, Tamura T, Tanaka K, Ishikashi M, Yamagi T: Hyponatremia and osmoregulation of thirst and vasopressin secretion in patients with adrenal insufficiency. J Clin Endocrin Metabol 77: 1584–1588, 1993[Abstract]
  67. Schwartz MJ, Kokko JP: Urinary concentrating defect of adrenal insufficiency. Permissive role of adrenal steroids on the hydroosmotic response across the rabbit cortical collecting duct. J Clin Invest 66: 234–242, 1980
  68. Jackson BA, Braun-Werness JL, Kusano E, Dousa TP: Concentrating defect in the adrenalectomized rat. Abnormal vasopressin-sensitive cyclic adenosine monophosphate metabolism in the papillary collecting duct. J Clin Invest 72: 997–1004, 1983
  69. Wang X-Y, Beutler K, Nielsen J, Nielsen S, Knepper MA, Masilamani S: Decreased abundance of collecting duct urea transporters UT-A1 and UT-A3 with ECF volume expansion. Am J Physiol Renal Physiol 282: F577–F584, 2002[Abstract/Free Full Text]
  70. Mitch WE, Bailey JL, Wang X, Jurkovitz C, Newby DN, Price SR: Evaluation of signals activating ubiquitin-proteasome proteolysis in a model of muscle wasting. Am J Physiol Cell Physiol 276: C1132–C1138, 1999[Abstract/Free Full Text]
  71. Bardoux P, Ahloulay M, Le Maout S, Bankir L, Trinh-Trang-Tan MM: Aquaporin-2 and urea transporter-A1 are up-regulated in rats with Type I diabetes mellitus. Diabetologia 44: 637–645, 2001[CrossRef][Medline]
  72. Bickel CA, Knepper MA, Verbalis JG, Ecelbarger CA: Dysregulation of renal salt and water transport proteins in diabetic Zucker rats. Kidney Int 61: 2099–2110, 2002[CrossRef][Medline]
  73. Fernández-Llama P, Andrews P, Nielsen S, Ecelbarger CA, Knepper MA: Impaired aquaporin and urea transporter expression in rats with adriamycin-induced nephrotic syndrome. Kidney Int 53: 1244–1253, 1998[CrossRef][Medline]
  74. Ecelbarger CA, Sands JM, Doran JJ, Cacini W, Kishore BK: Expression of salt and urea transporters in rat kidney during cisplatin-induced polyuria. Kidney Int 60: 2274–2282, 2001[CrossRef][Medline]
  75. Timmer RT, Sands JM: Lithium intoxication. J Am Soc Nephrol 10: 666–674, 1999[Free Full Text]
  76. Okusa MD, Crystal LJT: Clinical manifestations and management of acute lithium intoxication. Am J Med 97: 383–389, 1994[CrossRef][Medline]
  77. Marples D, Christensen S, Christensen EI, Ottosen PD, Nielsen S: Lithium-induced downregulation of Aquaporin-2 water channel expression in rat kidney medulla. J Clin Invest 95: 1838–1845, 1995
  78. Christensen S, Kusano E, Yusufi ANK, Murayama N, Dousa TP: Pathogenesis of nephrogenic diabetes insipidus due to chronic administration of lithium in rats. J Clin Invest 75: 1869–1879, 1985
  79. Sands JM: Regulation of urea transport proteins in kidney and liver. Mt Sinai J Med 67: 112–119, 2000[Medline]
  80. Star RA, Nonoguchi H, Balaban R, Knepper MA: Calcium and cyclic adenosine monophosphate as second messengers for vasopressin in the rat inner medullary collecting duct. J Clin Invest 81: 1879–1888, 1988
  81. Sands JM, Schrader DC: An independent effect of osmolality on urea transport in rat terminal IMCDs. J Clin Invest 88: 137–142, 1991
  82. Nielsen S, Knepper MA: Vasopressin activates collecting duct urea transporters and water channels by distinct physical processes. Am J Physiol Renal Physiol 265: F204–F213, 1993[Abstract/Free Full Text]
  83. Zhang C, Sands JM, Klein JD: Vasopressin rapidly increases the phosphorylation of the UT-A1 urea transporter activity in rat IMCDs through PKA. Am J Physiol Renal Physiol 282: F85–F90, 2002[Abstract/Free Full Text]
  84. Wall SM, Suk Han J, Chou C-L, Knepper MA: Kinetics of urea and water permeability activation by vasopressin in rat terminal IMCD. Am J Physiol Renal Physiol 262: F989–F998, 1992[Abstract/Free Full Text]
  85. Wu MS, Yang CW, Bens M, Peng KC, Yu HM, Vandewalle A: Cyclosporine stimulates Na+-K+-Cl- cotransport activity in cultured mouse medullary thick ascending limb cells. Kidney Int 58: 1652–1663, 2000[CrossRef][Medline]
  86. Inoue T, Terris J, Ecelbarger CA, Chou C-L, Nielsen S, Knepper MA: Vasopressin regulates apical targeting of aquaporin-2 but not of UT1 urea transporter in renal collecting duct. Am J Physiol Renal Physiol 276: F559–F566, 1999[Abstract/Free Full Text]
  87. Kakinuma Y, Fogo A, Inagami T, Ichikawa I: Intrarenal localization of angiotensin II type 1 receptor mRNA in the rat. Kidney Int 43: 1229–1235, 1993[Medline]
  88. Terada Y, Tomita K, Nonoguchi H, Marumo F: PCR localization of angiotensin II receptor and angiotensin mRNAs in rat kidney. Kidney Int 43: 1251–1259, 1993[Medline]
  89. Miyata N, Park F, Li XF, Cowley AW, Jr: Distribution of angiotensin AT1 and AT2 receptor subtypes in the rat kidney. Am J Physiol Renal Physiol 277: F437–F446, 1999[Abstract/Free Full Text]
  90. Mujais SK, Kauffman S, Katz AI: Angiotensin II binding sites in individual segments of the rat nephron. J Clin Invest 77: 315–318, 1986
  91. Kato A, Klein JD, Zhang C, Sands JM: Angiotensin II increases vasopressin-stimulated facilitated urea permeability in rat terminal IMCDs. Am J Physiol Renal Physiol 279: F835–F840, 2000[Abstract/Free Full Text]
  92. Esther CR, Jr., Marrero MB, Howard TE, Machaud A, Corvol P, Capecchi MR, Bernstein KE: The critical role of tissue angiotensin-converting enzyme as revealed by gene targeting in mice. J Clin Invset 99: 2375–2385, 1997[Medline]
  93. Klein JD, Quach DL, Cole JM, Disher K, Mongiu AK, Wang X, Bernstein KE, Sands JM: Impaired urine concentration and the absence of tissue ACE: The involvement of medullary transport proteins. Am J Physiol Renal Physiol 283: F517–F524, 2002[Abstract/Free Full Text]
  94. Olives B, Neau P, Bailly P, Hediger MA, Rousselet G, Cartron JP, Ripoche P: Cloning and functional expression of a urea transporter from human bone marrow cells. J Biol Chem 269: 31649–31652, 1994[Abstract/Free Full Text]
  95. Couriaud C, Ripoche P, Rousselet G: Cloning and functional characterization of a rat urea transporter: Expression in the brain. Biochim Biophys Acta Gene Struct Expression 1309: 197–199, 1996[Medline]
  96. Tsukaguchi H, Shayakul C, Berger UV, Tokui T, Brown D, Hediger MA: Cloning and characterization of the urea transporter UT3. Localization in rat kidney and testis. J Clin Invest 99: 1506–1515, 1997[Medline]
  97. Yang B, Bankir L, Gillespie A, Epstein CJ, Verkman AS: Urea-selective concentrating defect in transgenic mice lacking urea transporter UT-B. J Biol Chem 277: 10633–10637, 2002[Abstract/Free Full Text]
  98. Olivès B, Mattei M-G, Huet M, Neau P, Martial S, Cartron J-P, Bailly P: Kidd blood group and urea transport function of human erythrocytes are carried by the same protein. J Biol Chem 270: 15607–15610, 1995[Abstract/Free Full Text]
  99. Olivès B, Martial S, Mattei MG, Matassi G, Rousselet G, Ripoche P, Cartron JP, Bailly P: Molecular characterization of a new urea transporter in the human kidney. FEBS Lett 386: 156–160, 1996[CrossRef][Medline]
  100. Lucien N, Sidoux-Walter F, Olivès B, Moulds J, Le Pennec PY, Cartron JP, Bailly P: Characterization of the gene encoding the human Kidd blood group urea transporter protein — Evidence for splice site mutations in Jknull individuals. J Biol Chem 273: 12973–12980, 1998[Abstract/Free Full Text]
  101. Fenton RA, Hewitt JE, Howorth A, Cottingham CA, Smith CP: The murine urea transporter genes Slc14a1 and Slc14a2 occur in tandem on chromosome 18. Cytogenet Cell Genet 87: 95–96, 1999[CrossRef][Medline]
  102. Sidoux-Walter F, Lucien N, Nissinen R, Sistonen P, Henry S, Moulds J, Cartron J-P, Bailly P: Molecular heterogeneity of the Jknull phenotype: Expression analysis of the Jk(S291P) mutation found in Finns. Blood 96: 1566–1573, 2000[Abstract/Free Full Text]
  103. Kimoto Y, Constantinou CE: Effects of [1-desamino-8-D-arginine] vasopressin and papaverine on rabbit renal pelvis. Eur J Pharmacol 175: 359–362, 1990[CrossRef][Medline]
  104. Prichett WP, Patton AJ, Field JA, Brun KA, Emery JG, Tan KB, Rieman DJ, McClung HA, Nadeau DP, Mooney JL, Suva LJ, Gowen M, Nuttall ME: Identification and cloning of a human urea transporter HUT11, which is downregulated during adipogenesis of explant cultures of human bone. J Cell Biochem 76: 639–650, 2000[CrossRef][Medline]
  105. Berger UV, Tsukaguchi H, Hediger MA: Distribution of mRNA for the facilitated urea transporter UT3 in the rat nervous system. Anat Embryol 197: 405–414, 1998[CrossRef][Medline]
  106. Timmer RT, Klein JD, Bagnasco SM, Doran JJ, Verlander JW, Gunn RB, Sands JM: Localization of the urea transporter UT-B protein in human and rat erythrocytes and tissues. Am J Physiol Cell Physiol 281: C1318–C1325, 2001[Abstract/Free Full Text]
  107. Yang BX, Verkman AS: Urea transporter UT3 functions as an efficient water channel — Direct evidence for a common water/urea pathway. J Biol Chem 273: 9369–9372, 1998[Abstract/Free Full Text]
  108. Sidoux-Walter F, Lucien N, Olivès B, Gobin R, Rousselet G, Kamsteeg EJ, Ripoche P, Deen PMT, Cartron JP, Bailly P: At physiological expression levels the Kidd blood group/urea transporter protein is not a water channel. J Biol Chem 274: 30228–30235, 1999[Abstract/Free Full Text]
  109. Xu Y, Olives B, Bailly P, Fischer E, Ripoche P, Ronco P, Cartron J-P, Rondeau E: Endothelial cells of the kidney vasa recta express the urea transporter HUT11. Kidney Int 51: 138–146, 1997[Medline]
  110. Trinh-Trang-Tan M-M, Lasbennes F, Gane P, Roudier N, Ripoche P, Cartron J-P, Bailly P: UT-B1 proteins in rat: Tissue distribution and regulation by antidiuretic hormone in kidney. Am J Physiol Renal Physiol 283: 2002, inpress
  111. Wagner L, Klein JD, Sands JM, Baylis C: Urea transporters are widely distributed in endothelial cells and mediate inhibition of L-arginine transport. Am J Physiol Renal Physiol 283: F578–F582, 2002[Abstract/Free Full Text]
  112. Sands JM, Gargus JJ, Fröhlich O, Gunn RB, Kokko JP: Urinary concentrating ability in patients with Jk(a-b-) blood type who lack carrier-mediated urea transport. J Am Soc Nephrol 2: 1689–1696, 1992[Abstract]
  113. Macey RI: Transport of water and urea in red blood cells. Am J Physiol 246: C195–C203, 1984
  114. Pallone TL, Work J, Myers RL, Jamison RL: Transport of sodium and urea in outer medullary descending vasa recta. J Clin Invest 93: 212–222, 1994
  115. Pallone TL: Characterization of the urea transporter in outer medullary descending vasa recta. Am J Physiol Regul Integr Comp Physiol 267: R260–R267, 1994[Abstract/Free Full Text]
  116. Pallone TL, Nielsen S, Silldorff EP, Yang S: Diffusive transport of solute in the rat medullary microcirculation. Am J Physiol Renal Physiol 269: F55–F63, 1995[Abstract/Free Full Text]
  117. Edwards A, Pallone TL: Facilitated transport in vasa recta: Theoretical effects on solute exchange in the medullary microcirculation. Am J Physiol Renal Physiol 272: F505–F514, 1997[Abstract/Free Full Text]
  118. Edwards A, Pallone TL: A multiunit model of solute and water removal by inner medullary vasa recta. Am J Physiol Heart Circ Physiol 274: H1202–H1210, 1998[Abstract/Free Full Text]
  119. Macey RI, Yousef LW: Osmotic stability of red cells in renal circulation requires rapid urea transport. Am J Physiol Cell Physiol 254: C669–C674, 1988[Abstract/Free Full Text]
  120. Von Dahl S, Haussinger D: Evidence for a phloretin-sensitive glycerol transport mechanism in the perfused rat liver. Am J Physiol Gastrointest Liver Physiol 272: G563–G574, 1997[Abstract/Free Full Text]
  121. Hasegawa H, Verkman AS: Functional expression of cAMP-dependent and independent urea transporters in Xenopus oocytes. Am J Physiol Cell Physiol 265: C514–C520, 1993[Abstract/Free Full Text]
  122. Walsh PJ, Wood CM, Perry SF, Thomas S: Urea transport by hepatocytes and red blood cells of selected elasmobrach and teleost fishes. J Exp Med 193: 321–335, 1994
  123. Klein JD, Timmer RT, Rouillard P, Bailey JL, Sands JM: UT-A urea transporter protein expressed in liver: Upregulation by uremia. J Am Soc Nephrol 10: 2076–2083, 1999[Abstract/Free Full Text]
  124. Doran JJ, Timmer RT, Sands JM: Accurate mRNA size determination in northern analysis using individual lane size markers. BioTechniques 27: 280–282, 1999[Medline]
  125. Klein JD, Rouillard P, Roberts BR, Sands JM: Acidosis mediates the up-regulation of UT-A protein in livers from uremic rats. J Am Soc Nephrol 13: 581–587, 2002[Abstract/Free Full Text]
  126. Fenton RA, Cooper GJ, Morris ID, Smith CP: Coordinated expression of UT-A and UT-B urea transporters in rat testis. Am J Physiol Cell Physiol 282: C1492–C1501, 2002[Abstract/Free Full Text]



This article has been cited by other articles:


Home page
 Am. J. Physiol. Renal Physiol.Home page