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.

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.

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.

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 Glucocorticoids downregulate the vasopressin-regulated urea transporter in rat terminal inner medullary collecting ducts.

1997 ◽  
Vol 8 (4) ◽  
pp. 517-523 ◽  
Author(s):  
M Naruse ◽  
J D Klein ◽  
Z M Ashkar ◽  
J D Jacobs ◽  
J M Sands
Keyword(s):  
Collecting Duct ◽  
Northern Analysis ◽  
Sham Operation ◽  
Urea Transport ◽  
Urea Transporter ◽  
Transporter Protein ◽  
Urea Permeability ◽  
Collecting Ducts ◽  
Medullary Collecting Duct ◽  
Adrenalectomized Rats

This study tested whether glucocorticoids regulate tubular urea transport. Urea permeability was measured in perfused inner medullary collecting duct (IMCD) subsegments from rats that underwent adrenalectomy, adrenalectomy plus replacement with a physiologic dose of glucocorticoid (dexamethasone), or sham operation. Compared with sham rats, basal urea permeability in terminal IMCD was significantly increased in adrenalectomized rats and reduced in dexamethasone-treated rats. Vasopressin significantly increased urea permeability in all three groups. In contrast, there was no difference in basal or vasopressin-stimulated urea permeability in initial IMCD between the three groups. Next, membrane and vesicle fraction proteins were isolated from inner medullary tip or base and Western analysis was performed by use of an antibody to the rat vasopressin-regulated urea transporter. Vasopressin-regulated urea transporter protein was significantly increased in both membrane and vesicle fractions from the inner medullary tip of adrenalectomized rats. There was no change in vasopressin-regulated urea transporter protein in the inner medullary base, and Northern analysis showed no change in urea transporter mRNA abundance in either inner medullary region. It was concluded that glucocorticoids can downregulate function and expression of the vasopressin-regulated urea transporter in rat terminal IMCD.

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.

scholarly journals Epac Regulation of Urea Transport and the UT‐A1 Urea Transporter in Rat Inner Medullary Collecting Duct.

2009 ◽  
Vol 23 (S1) ◽  
Author(s):  
Yanhua Wang ◽  
Janet D. Klein ◽  
Christopher F. Martin ◽  
Kimilia J. Kent ◽  
Susan M. Wall ◽  
...  
Keyword(s):  
Collecting Duct ◽  
Urea Transport ◽  
Urea Transporter ◽  
Inner Medullary Collecting Duct ◽  
Medullary Collecting Duct

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.

Characteristics of osmolarity-stimulated urea transport in rat IMCD

1992 ◽  
Vol 262 (6) ◽  
pp. F1061-F1067 ◽  
Author(s):  
A. G. Gillin ◽  
J. M. Sands
Keyword(s):  
Adenylyl Cyclase ◽  
Inhibitory Concentration ◽  
Collecting Duct ◽  
Urea Transport ◽  
Urea Transporter ◽  
Inner Medullary Collecting Duct ◽  
Independent Mechanism ◽  
Additive Increase ◽  
Medullary Collecting Duct ◽  
Different Levels

Urea transport across the terminal inner medullary collecting duct (IMCD) is mediated by a urea transporter that is stimulated by vasopressin (AVP) or hyperosmolarity. To determine whether hyperosmolarity stimulates urea transport by an adenylyl cyclase-dependent or -independent mechanism, terminal IMCDs were perfused with 10 microM forskolin followed by an increase in osmolality or with increasing osmolality followed by 10 nM AVP. In both protocols, stimulating adenylyl cyclase caused an additive increase in urea permeability (Purea) to that stimulated by hyperosmolarity. Next, we investigated whether hyperosmolarity stimulates the same urea transporter as AVP by studying the inhibitor profile and IMCD subsegment response of hyperosmolarity-stimulated urea transport and comparing it to properties already demonstrated for AVP-stimulated urea transport. In terminal IMCDs, luminal phloretin (250 microM) reversibly inhibited Purea by 63%. Thiourea (100 mM) inhibited Purea by 73% at two different levels of osmolality, 690 and 290 mosmol/kgH2O. The half-maximal inhibitory concentration (K1/2) for thiourea at 690 mosmol/kgH2O was not significantly different from the K1/2 value at 290 mosmol/kgH2O, suggesting that stimulation by hyperosmolarity is related to an increase in the Vmax for the urea transporter. Finally, we found that hyperosmolarity did not stimulate Purea in the initial IMCD. In summary, the data suggests that hyperosmolarity stimulates urea transport by an adenylyl cyclase-independent mechanism. However, the inhibitor profile and the IMCD subsegment response for hyperosmolarity-stimulated and AVP-stimulated Purea are similar, suggesting that both hyperosmolarity and AVP stimulate the same urea transporter.

scholarly journals Long-term regulation of inner medullary collecting duct urea transport in rat.

1998 ◽  
Vol 9 (5) ◽  
pp. 737-745
Author(s):  
A Kato ◽  
M Naruse ◽  
M A Knepper ◽  
J M Sands
Keyword(s):  
Arginine Vasopressin ◽  
Water Deprivation ◽  
Collecting Duct ◽  
Urea Transport ◽  
Water Diuresis ◽  
Sugar Water ◽  
Inner Medullary Collecting Duct ◽  
Medullary Collecting Duct ◽  
Ad Libitum

Facilitated urea transport is regulated acutely by arginine vasopressin (AVP) and hyperosmolality in rat terminal inner medullary collecting duct (IMCD). This study tested whether chronic diuresis or antidiuresis regulates facilitated urea transport. Basal and AVP-stimulated urea permeabilities (Purea) were measured in perfused IMCD subsegments. Rats were made: (1) diuretic by giving them sugar water (with or without food) or furosemide; or (2) antidiuretic by water deprivation. They were then compared with untreated rats given food and water ad libitum. Terminal IMCD from untreated rats had a high basal Purea that was significantly increased by AVP. Diuresis significantly increased basal Purea in terminal IMCD in all five diuresis protocols. Water deprivation for 1 or 3 d had no effect on basal or AVP-stimulated Purea in the IMCD2 subsegment of the terminal IMCD. In contrast, 3 d of water deprivation significantly increased both basal and AVP-stimulated Purea in the IMCD3 subsegment; 1 d of water deprivation had no effect on basal or AVP-stimulated Purea. Next, initial IMCD (IMCD1) were studied. Initial IMCD from untreated rats had a low basal Purea that was not increased by AVP (10 nM). Water diuresis (with or without food) for 3 to 5 d had no effect on basal Purea but significantly increased AVP-stimulated Purea. Furosemide diuresis and water diuresis for 1 or 7 d had no effect on either basal or AVP-stimulated Purea in initial IMCD. Water deprivation for 2 to 3 d, but not for 1 d, significantly increased basal Purea in initial IMCD, whereas water deprivation for 1 d increased AVP-stimulated Purea. It is concluded that chronic changes in hydration cause heterogeneous changes in facilitated urea transport in rat IMCD subsegments.

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