scholarly journals Regulation of Renal Urea Transporters

1999 ◽  
Vol 10 (3) ◽  
pp. 635-646
Author(s):  
JEFF M. SANDS
Keyword(s):  
Apical Membrane ◽  
Collecting Duct ◽  
Basolateral Membrane ◽  
Molecular Evidence ◽  
Urea Transport ◽  
Urea Transporter ◽  
Transporter Family ◽  
Vasa Recta ◽  
Urea Reabsorption ◽  
Urine Concentrating Ability

Abstract. Urea is important for the conservation of body water due to its role in the production of concentrated urine in the renal inner medulla. Physiologic data demonstrate that urea is transported by facilitated and by active urea transporter proteins. The facilitated urea transporter (UT-A) in the terminal inner medullary collecting duct (IMCD) permits very high rates of transepithelial urea transport and results in the delivery of large amounts of urea into the deepest portions of the inner medulla where it is needed to maintain a high interstitial osmolality for concentrating the urine maximally. Four isoforms of the UT-A urea transporter family have been cloned to date. The facilitated urea transporter (UT-B) in erythrocytes permits these cells to lose urea rapidly as they traverse the ascending vasa recta, thereby preventing loss of urea from the medulla and decreasing urine-concentrating ability by decreasing the efficiency of countercurrent exchange, as occurs in Jk null individuals (who lack Kidd antigen). In addition to these facilitated urea transporters, three sodium-dependent, secondary active urea transport mechanisms have been characterized functionally in IMCD subsegments: (1) active urea reabsorption in the apical membrane of initial IMCD from low-protein fed or hypercalcemic rats; (2) active urea reabsorption in the basolateral membrane of initial IMCD from furosemide-treated rats; and (3) active urea secretion in the apical membrane of terminal IMCD from untreated rats. This review focuses on the physiologic, biophysical, and molecular evidence for facilitated and active urea transporters, and integrative studies of their acute and long-term regulation in rats with reduced urine-concentrating ability.

scholarly journals Urea transport processes are induced in rat IMCD subsegments when urine concentrating ability is reduced

1999 ◽  
Vol 276 (1) ◽  
pp. F62-F71 ◽  
Author(s):  
Akihiko Kato ◽  
Jeff M. Sands
Keyword(s):  
Collecting Duct ◽  
Urine Concentration ◽  
Transport Processes ◽  
Urea Transport ◽  
Urea Transporter ◽  
Water Diuresis ◽  
Low Protein ◽  
Urea Reabsorption ◽  
Medullary Collecting Duct ◽  
Urine Concentrating Ability

Infusing urea into low-protein-fed mammals increases urine concentration within 5–10 min. To determine which urea transporter may be responsible, we measured urea transport in perfused IMCD3 segments [inner medullary collecting duct (IMCD) segments from the deepest third of the IMCD] from low-protein-fed rats. Basal facilitated urea permeability increased 78%, whereas active urea secretion was completely inhibited. This suggests that upregulation of facilitated urea transport may mediate the rapid increase in urine concentration. Next, expression of active urea transporter(s) in perfused IMCDs was determined in rats with other causes of reduced urine concentrating ability. In untreated and water diuretic rats, IMCD1 segments showed no active urea transport, nor did IMCD2segments from untreated or hypercalcemic rats. In IMCD1 segments from hypercalcemic rats, active urea reabsorption was induced. The induced active urea reabsorption was completely inhibited by replacing perfusate Na+ with N-methyl-d-glucamine (NMDG+). Active urea secretion was completely inhibited in IMCD3 segments from hypercalcemic rats. In contrast, water diuresis stimulated active urea secretion in IMCD2 segments. The induced active urea secretion was inhibited by phloretin, ouabain, triamterene, or replacing perfusate Na+ with NMDG+. In conclusion, the response of active urea transporters to reductions in urine concentrating ability follows two paradigms: one occurs with hypercalcemia or a low-protein diet, and the second occurs only in water diuresis.

The vasopressin-regulated urea transporter in renal inner medullary collecting duct

1990 ◽  
Vol 259 (3) ◽  
pp. F393-F401 ◽  
Author(s):  
M. A. Knepper ◽  
R. A. Star
Keyword(s):  
Apical Membrane ◽  
Collecting Duct ◽  
Water Channel ◽  
Toad Urinary Bladder ◽  
Urea Transport ◽  
Urea Transporter ◽  
Transport Pathway ◽  
Inner Medullary Collecting Duct ◽  
Urea Permeability ◽  
Medullary Collecting Duct

The terminal part of the inner medullary collecting duct (terminal IMCD) is unique among collecting duct segments in part because its permeability to urea is regulated by vasopressin. The urea permeability can rise to extremely high levels (greater than 100 x 10(-5) cm/s) in response to vasopressin. Recent studies in isolated perfused IMCD segments have established that the rapid movement of urea across the tubule epithelium occurs via a specialized urea transporter, presumably an intrinsic membrane protein, present in both the apical and basolateral membranes. This urea transporter has properties similar to those of the urea transporters in mammalian erythrocytes and in toad urinary bladder, namely, inhibition by phloretin, inhibition by urea analogues, saturation kinetics in equilibrium-exchange experiments, and regulation by vasopressin. The urea transport pathway is distinct from and independent of the vasopressin-regulated water channel. The increase in transepithelial urea transport in response to vasopressin is mediated by adenosine 3',5'-cyclic monophosphate and is associated with an increase in the urea permeability of the apical membrane. However, little is known about the physical events associated with the activation or insertion of urea transporters in the apical membrane. Because of the importance of this transporter to the urinary concentrating mechanism, efforts toward understanding its molecular structure and the molecular basis of its regulation appear to be justified.

scholarly journals Vasopressin regulation of multisite phosphorylation of UT-A1 in the inner medullary collecting duct

2015 ◽  
Vol 308 (1) ◽  
pp. F49-F55 ◽  
Author(s):  
Carol A. Hoban ◽  
Lauren N. Black ◽  
Ronald J. Ordas ◽  
Diane L. Gumina ◽  
Fadi E. Pulous ◽  
...  
Keyword(s):  
Apical Membrane ◽  
Collecting Duct ◽  
Phosphorylation Site ◽  
Urea Transport ◽  
Urea Transporter ◽  
Inner Medullary Collecting Duct ◽  
Camp Pathway ◽  
Multisite Phosphorylation ◽  
Medullary Collecting Duct ◽  
Stimulation Of

Vasopressin signaling is critical for the regulation of urea transport in the inner medullary collecting duct (IMCD). Increased urea permeability is driven by a vasopressin-mediated elevation of cAMP that results in the direct phosphorylation of urea transporter (UT)-A1. The identification of cAMP-sensitive phosphorylation sites, Ser486 and Ser499, in the rat UT-A1 sequence was the first step in understanding the mechanism of vasopressin action on the phosphorylation-dependent modulation of urea transport. To investigate the significance of multisite phosphorylation of UT-A1 in response to elevated cAMP, we used highly specific and sensitive phosphosite antibodies to Ser486 and Ser499 to determine cAMP action at each phosphorylation site. We found that phosphorylation at both sites was rapid and sustained. Furthermore, the rate of phosphorylation of the two sites was similar in both mIMCD3 cells and rat inner medullary tissue. UT-A1 localized to the apical membrane in response to vasopressin was phosphorylated at Ser486 and Ser499. We confirmed that elevated cAMP resulted in increased phosphorylation of both sites by PKA but not through the vasopressin-sensitive exchange protein activated by cAMP pathway. These results elucidate the multisite phosphorylation of UT-A1 in response to cAMP, thus providing the beginning of understanding the intracellular factors underlying vasopressin stimulation of urea transport in the IMCD.

Urea transporters in kidney and erythrocytes

1997 ◽  
Vol 273 (3) ◽  
pp. F321-F339 ◽  
Author(s):  
J. M. Sands ◽  
R. T. Timmer ◽  
R. B. Gunn
Keyword(s):  
Structural Model ◽  
Collecting Duct ◽  
Molecular Data ◽  
Urea Transport ◽  
Urea Transporter ◽  
Vasa Recta ◽  
Countercurrent Exchange ◽  
Systematic Nomenclature ◽  
Medullary Collecting Duct

Physiological and molecular data demonstrate that urea transport in kidney and erythrocytes is regulated by specific urea transporter proteins. The urea transporter in the terminal inner medullary collecting duct permits very high rates of regulated transepithelial urea transport and results in the delivery of large amounts of urea into the deepest portions of the inner medulla, where it is needed to maintain a high interstitial osmolality for concentrating the urine maximally. The urea transporter in erythrocytes permits these cells to lose urea rapidly as they ascend through the ascending vasa recta, thereby preventing loss of urea from the medulla. Urea lost from the medulla would decrease concentrating ability by decreasing the efficiency of countercurrent exchange, as occurs in individuals who lack the Kidd antigen. The recent cloning of cDNAs for these two urea transporters has begun to yield new insights into the mechanisms underlying acute and long-term regulation of urea transport and should permit exciting new insights in the future. This review focuses on the physiological and biophysical evidence that established the concept of urea transporters, the subsequent cloning of cDNAs for urea transporters, and the recent integrative studies into the regulation of urea transport. We also propose a new systematic nomenclature and a new structural model for urea transporters.

Acute regulation of mUT-A3 urea transporter expressed in a MDCK cell line

2007 ◽  
Vol 292 (4) ◽  
pp. F1157-F1163 ◽  
Author(s):  
Gavin S. Stewart ◽  
Sarah L. King ◽  
Elizabeth A. Potter ◽  
Craig P. Smith
Keyword(s):  
Cell Line ◽  
Collecting Duct ◽  
Basolateral Membrane ◽  
Vital Role ◽  
Urea Transport ◽  
Urea Transporter ◽  
Epithelial Monolayers ◽  
Concentrating Mechanism ◽  
Dimethyl Urea ◽  
And Control

Renal facilitative urea transporters play a vital role in the urinary concentrating mechanism. UT-A3 is a phloretin-sensitive urea transporter that in the mouse is expressed on the basolateral membrane of renal inner medullary collecting duct (IMCD) cells. In this study, we engineered a Madin-Darby canine kidney (MDCK) I cell line that stably expresses mouse UT-A3 (MDCK-mUT-A3). Immunoblotting using the UT-A-targeted antibody ML446 detected a ∼40-kDa signal in MDCK-mUT-A3 protein that corresponds to mUT-A3. Using cultured epithelial monolayers, radioactive 14C-urea flux experiments determined that basolateral urea transport was no different between MDCK-mUT-A3 and control MDCK-FLZ cells under basal conditions [not significant (NS), ANOVA]. However, exposure to arginine vasopressin (AVP) significantly stimulated basolateral urea flux in MDCK-mUT-A3 monolayers ( P < 0.05, ANOVA), while it had no effect in control MDCK-FLZ monolayers (NS, ANOVA). The AVP-stimulated basolateral urea transport in MDCK-mUT-A3 was inhibited by 1,3 dimethyl urea ( P < 0.05, ANOVA) or phloretin ( P < 0.05, ANOVA), both known inhibitors of facilitative urea transporters. MDCK-mUT-A3 basolateral urea flux was also stimulated by increasing intracellular levels of cAMP, via forskolin ( P < 0.05, ANOVA), or intracellular calcium, via ATP ( P < 0.05, ANOVA). Finally, 1-h preincubation with a specific PKA inhibitor, H89, significantly inhibited the increase in urea transport produced by AVP ( P < 0.05, ANOVA). In conclusion, we have produced the first renal cell line to stably express the mUT-A3 urea transporter. Our results indicate that mUT-A3 is acutely regulated by AVP, via a PKA-dependent pathway. These findings have important implications for the regulation of urea transport in the renal IMCD and the urinary concentrating mechanism.

scholarly journals Vasopressin regulation of the renal UT-A3 urea transporter

2009 ◽  
Vol 296 (3) ◽  
pp. F642-F648 ◽  
Author(s):  
G. S. Stewart ◽  
A. Thistlethwaite ◽  
H. Lees ◽  
G. J. Cooper ◽  
Craig Smith
Keyword(s):  
Plasma Membrane ◽  
Collecting Duct ◽  
Basolateral Membrane ◽  
Vital Role ◽  
Urea Transport ◽  
Urea Transporter ◽  
Inner Medullary Collecting Duct ◽  
Kinase C ◽  
Medullary Collecting Duct ◽  
Dependent Pathway

Facilitative urea transporters in the mammalian kidney play a vital role in the urinary concentrating mechanism. The urea transporters located in the renal inner medullary collecting duct, namely UT-A1 and UT-A3, are acutely regulated by the antidiuretic hormone vasopressin. In this study, we investigated the vasopressin regulation of the basolateral urea transporter UT-A3 using an MDCK-mUT-A3 cell line. Within 10 min, vasopressin stimulates urea flux through UT-A3 transporters already present at the plasma membrane, via a PKA-dependent process. Within 1 h, vasopressin significantly increases UT-A3 localization at the basolateral membrane, causing a further increase in urea transport. While the basic trafficking of UT-A3 to basolateral membranes involves both protein kinase C and calmodulin, its regulation by vasopressin specifically occurs through a casein kinase II-dependent pathway. In conclusion, this study details the effects of vasopressin on UT-A3 urea transporter function and hence its role in regulating urea permeability within the renal inner medullary collecting duct.

scholarly journals Expression of urea transporters in potassium-depleted mouse kidney

2003 ◽  
Vol 285 (6) ◽  
pp. F1210-F1224 ◽  
Author(s):  
Ju-Young Jung ◽  
Kirsten M. Madsen ◽  
Ki-Hwan Han ◽  
Chul-Woo Yang ◽  
Mark A. Knepper ◽  
...  
Keyword(s):  
Collecting Duct ◽  
Basolateral Membrane ◽  
Mouse Kidney ◽  
Potassium Depletion ◽  
Urea Transporter ◽  
Vasa Recta ◽  
Low Potassium ◽  
Normal Chow ◽  
Medullary Collecting Duct ◽  
And Control

Urea transport in the kidney is mediated by a family of transporter proteins that include the renal urea transporter (UT-A) and the erythrocyte urea transporter (UT-B). The purpose of this study was to determine the location of the urea transporter isoforms in the mouse kidney and to examine the effects of prolonged potassium depletion on the expression and distribution of these transporters by ultrastructural immunocytochemistry. C57BL6 mice were fed a low-potassium diet for 2 wk, and control animals received normal chow. After 2 wk on a low-potassium diet, urinary volume increased and urinary osmolality decreased (833 ± 30 vs. 1,919 ± 174 mosmol/kgH2O), as previously demonstrated. Kidneys were processed for immunocytochemistry with antibodies against UT-A1 (L446), UT-A1 and UT-A2 (L194), UT-A3 (Q2), and UT-B. In normal mice, UT-A1 and UT-A3 were expressed mainly in the cytoplasm of the terminal inner medullary collecting duct (IMCD). UT-A2 immunoreactivity was observed mainly on the basolateral membrane of the type 1 epithelium of the descending thin limb (DTL) of short-looped nephrons. The intensity of UT-A1 and UT-A3 immunoreactivity in the IMCD was markedly reduced in potassium-depleted mice. In contrast, there was a significant increase in UT-A2 immunoreactivity in the DTL. The intensity of UT-B immunoreactivity in the descending vasa recta (DVR) was reduced in potassium-depleted animals compared with controls. In control animals, UT-B immunoreactivity was predominantly observed in the plasma membrane, whereas in potassium-depleted mice, it was mainly observed in cytoplasmic granules in endothelial cells of the DVR. In summary, potassium depletion is associated with reduced expression of UT-A1, UT-A3, and UT-B but increased expression of UT-A2. We conclude that reduced expression of urea transporters may play a role in the impaired urine-concentrating ability associated with potassium deprivation.

Water permeability of apical and basolateral cell membranes of rat inner medullary collecting duct

1990 ◽  
Vol 259 (6) ◽  
pp. F986-F999 ◽  
Author(s):  
B. Flamion ◽  
K. R. Spring
Keyword(s):  
Water Flow ◽  
Water Permeability ◽  
Cell Membranes ◽  
Apical Membrane ◽  
Collecting Duct ◽  
Basolateral Membrane ◽  
Volume Regulatory Decrease ◽  
Water Permeation ◽  
Inner Medullary Collecting Duct ◽  
Medullary Collecting Duct

To quantify the pathways for water permeation through the kidney medulla, knowledge of the water permeability (Posmol) of individual cell membranes in inner medullary collecting duct (IMCD) is required. Therefore IMCD segments from the inner two thirds of inner medulla of Sprague-Dawley rats were perfused in vitro using a setup devised for rapid bath and luminal fluid exchanges (half time, t1/2, of 55 and 41 ms). Differential interference contrast microscopy, coupled to video recording, was used to measure volume and approximate surface areas of single cells. Volume and volume-to-surface area ratio of IMCD cells were strongly correlated with their position along the inner medullary axis. Transmembrane water flow (Jv) was measured in response to a variety of osmotic gradients (delta II) presented on either basolateral or luminal side of the cells. The linear relation between Jv and delta II yielded the cell membrane Posmol, which was then corrected for membrane infoldings. Basolateral membrane Posmol was 126 +/- 3 microns/s. Apical membrane Posmol rose from a basal value of 26 +/- 3 microns/s to 99 +/- 5 microns/s in presence of antidiuretic hormone (ADH). Because of amplification of basolateral membrane, the ADH-stimulated apical membrane remained rate-limiting for transcellular osmotic water flow, and the IMCD cell did not swell significantly. Calculated transcellular Posmol, expressed in terms of smooth luminal surface, was 64 microns/s without ADH and 207 microns/s with ADH. IMCD cells in anisosmotic media displayed almost complete volume regulatory decrease but only partial volume regulatory increase.

Ultramicrodetermination of vasopressin-regulated urea transporter protein in microdissected renal tubules

1997 ◽  
Vol 272 (4) ◽  
pp. F531-F537 ◽  
Author(s):  
B. K. Kishore ◽  
J. Terris ◽  
P. Fernandez-Llama ◽  
M. A. Knepper
Keyword(s):  
Collecting Duct ◽  
Enzyme Linked Immunosorbent Assay ◽  
Apical Plasma Membrane ◽  
Renal Tubules ◽  
Urea Transport ◽  
Urea Transporter ◽  
Transporter Protein ◽  
Low Protein ◽  
Collecting Duct Cells ◽  
Medullary Collecting Duct

The vasopressin-regulated urea transporter (VRUT) is a 97-kDa protein (also called “UT-1”) responsible for facilitated urea transport across the apical plasma membrane of inner medullary collecting duct (IMCD) cells. To determine the abundance of VRUT protein in collecting duct cells of the rat, we designed a sensitive fluorescence-based enzyme-linked immunosorbent assay capable of detecting <5 fmol of VRUT protein. In collecting duct segments, measurable VRUT was found in microdissected IMCD segments but not in other portions of the collecting duct. In the mid-IMCD, the measured level averaged 5.3 fmol/mm tubule length, corresponding to approximately 5 million copies of VRUT per cell. Thus VRUT is extremely abundant in the IMCD, accounting, in part, for the extremely high urea permeability of this segment. Feeding a low-protein diet (8% protein) markedly decreased urea clearance but did not alter the quantity of VRUT protein in the IMCD. Thus increased urea transport across the collecting duct with dietary protein restriction is not a consequence of increased expression of VRUT. Based on urea fluxes measured in the IMCD and our measurements of the number of copies of VRUT, we estimate a turnover number of > or = 0.3-1 x 10(5) s. In view of the large magnitude of this value and previously reported biophysical properties of urea transport in collecting ducts, we hypothesize that the VRUT may function as a channel rather than a carrier.

EGF and PGE2 inhibit rabbit CCD Na+ transport by different mechanisms: PGE2 inhibits Na(+)-K+ pump

1993 ◽  
Vol 264 (4) ◽  
pp. F670-F677 ◽  
Author(s):  
D. H. Warden ◽  
J. B. Stokes
Keyword(s):  
Apical Membrane ◽  
Collecting Duct ◽  
Basolateral Membrane ◽  
Membrane Conductance ◽  
Membrane Voltage ◽  
Na Channel ◽  
Cortical Collecting Duct ◽  
Na Transport ◽  
Flux Measurements ◽  
Epidermal Growth

The rabbit cortical collecting duct absorbs Na+ by a transport system comprised of an apical membrane Na+ channel and a basolateral membrane Na(+)-K(+)-adenosinetriphosphatase. The rate of Na+ absorption across this epithelium is acutely inhibited by several hormones and autacoids including epidermal growth factor (EGF) and prostaglandin E2 (PGE2). We used electrophysiological analysis to determine which Na+ transport mechanism is primarily regulated in response to EGF and PGE2. We used concentrations of EGF and PGE2 that inhibited Na+ absorption to a comparable degree. We assessed the effects of these agents on Na+ transport primarily by the calculated equivalent current; the validity of this indicator was verified using simultaneous tracer flux measurements. EGF and PGE2 had different effects on the intracellular electrophysiological parameters. EGF (in the presence of a cyclooxygenase inhibitor) hyperpolarized the apical membrane voltage in a manner analogous to the Na(+)-channel blocker amiloride, reduced the transepithelial conductance, and increased the fractional resistance of the apical membrane. In comparison, PGE2 depolarized the apical membrane voltage in a manner analogous to the Na(+)-K+ pump inhibitor ouabain, and caused no significant changes in transepithelial conductance or apical membrane conductance. The finding that EGF hyperpolarized the apical membrane indicates that this agent attenuates Na+ absorption by reducing apical Na+ entry due to a decrease in the magnitude of the apical membrane Na+ conductance. In contrast, the electrophysiological changes produced by PGE2 indicate primary inhibition of the basolateral Na(+)-K+ pump following PGE2 treatment.

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