Urea Reabsorption Research Articles

Blood Urea Nitrogen and Serum Creatinine

Klein and colleagues1 have retrospectively analyzed results derived from the prospective, randomized Outcomes of a Prospective Trial of Intravenous Milrinone for Exacerbations of Chronic Heart Failure (OPTIME-CHF) study and contribute an interesting article to this inaugural issue of Circulation: Heart Failure . Their analysis provides further evidence that the level of renal function in patients with worsening heart failure and impaired systolic function is an important predictor of rehospitalization for cardiovascular events and death within 60 days of discharge. Renal function was assessed at admission. Change during hospitalization was recorded for blood urea nitrogen (BUN) and estimated glomerular filtration rate (GFR). Estimated GFR was calculated with the 4-variable equation of the Modification of Diet in Renal Disease study, which depends on serum creatinine, age, and sex.2 Of interest, the BUN on admission and change in BUN during the hospital stay (independent of the admission value) was a statistically better predictor of the 60-day death rate and days of rehospitalization than was estimated GFR. Because BUN is affected by protein intake, catabolism, and tubular reabsorption of urea, it is not as reliable an index of renal function as GFR. Thus, this observation by Klein et al1 is of particular interest and deserves explanation. Article p 25 Serum creatinine is freely filtered at the glomerulus, not reabsorbed, but undergoes tubular secretion. Thus, creatinine clearance exceeds inulin clearance, the gold standard for GFR. In contrast, urea is freely filtered, not secreted, but is reabsorbed by the renal tubules. This reabsorption of urea is flow dependent so that more urea is reabsorbed at lower urine flow rates (Figure 1).3 Most importantly, the reabsorption of urea in the collecting duct is mediated by the effect of arginine vasopressin (AVP) on the urea transporter in the collecting …

PKC stimulated by glucagon decreases UT-A1 urea transporter expression in rat IMCD

It is well-known that glucagon increases fractional excretion of urea in rats after a protein intravenous infusion. This effect was investigated by using: (a) in vitro microperfusion technique to measure [(14)C]-urea permeability (Pu x 10(-5)cm/s) in inner medullary collecting ducts (IMCD) from normal rats in the presence of 10(-7)M of glucagon and in the absence of vasopressin and (b) immunoblot techniques to determine urea transporter expression in tubule suspension incubated with the same glucagon concentration. Seven groups of IMCDs (n = 47) were studied. Our results revealed that: (a) glucagon decreased urea reabsorption dose-dependently; (b) the glucagon antagonist des-His(1)-[Glu(9)], blocked the glucagon action but not vasopressin action; (c) the phorbol myristate acetate, decreased urea reabsorption but (d) staurosporin, restored its effect; e) staurosporin decreased glucagon action, and finally, (f) glucagon decreased UT-A1 expression. We can conclude that glucagon reduces UT-A1 expression via a glucagon receptor by stimulating PKC.

No elevation of blood urea level in a dehydrated patient with central diabetes insipidus

Sir, Dehydrated patients usually present with elevated blood urea nitrogen (BUN) concentrations, reflecting a low urine flow rate and increased renal reabsorption of urea. This increased renal reabsorption of urea is thought to owe at least in part to the action of antidiuretic hormone (ADH).1 A 44-year-old Japanese man with mental retardation was admitted because of the recent onset of polyposia and polyuria. As he had taken any kinds of fluids (e.g. …

ACUTE INCREASE IN BLOOD UREA NITROGEN CAUSED BY ENTERIC NUTRITION

ACUTE INCREASE IN BLOOD UREA NITROGEN CAUSED BY ENTERIC NUTRITION

Cytokine-mediated regulation of urea transporters during experimental endotoxemia

Acute renal failure (ARF) is a frequent complication of sepsis and has a high mortality. Sepsis-induced ARF is known to be associated with significant impairment of tubular capacity. However, the pathogenesis of endotoxemic tubular dysfunction with failure of urine concentration is poorly understood. Urea plays an important role in the urinary concentrating mechanism and expression of the urea transporters UT-A1, UT-A2, UT-A3, UT-A4, and UT-B is essential for tubular urea reabsorption. The present study attempts to assess the regulation of renal urea transporters during severe inflammation in vivo. Lipopolysaccharide-(LPS)-injected mice presented with reduced glomerular filtration rate, fractional urea excretion, and inner medulla osmolality associated with a marked decrease in expression of all renal urea transporters. Similar alterations were observed after application of tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, interferon (IFN)-gamma, or IL-6. LPS-induced downregulation of urea transporters was not affected in knockout mice with deficient TNF-alpha, IL-receptor-1, IFN-gamma, or IL-6. Glucocorticoid treatment inhibited LPS-induced increases of tissue TNF-alpha, IL-1beta, IFN-gamma, or IL-6 concentration, diminished LPS-induced renal dysfunction, and attenuated the downregulation of renal urea transporters. Renal ischemia induced by renal artery clipping did not influence the expression of urea transporters. Our data demonstrate that renal urea transporters are downregulated by severe inflammation, which likely accounts for tubular dysfunction. Furthermore, they suggest that the downregulation of renal urea transporters during LPS-induced ARF is mediated by proinflammatory cytokines and is independent from renal ischemia because of sepsis-induced hypotension.

High urea and creatinine concentrations and urea transporter B in mammalian urinary tract tissues

Although the mammalian urinary tract is generally held to be solely a transit and storage vehicle for urine made by the kidney, in vivo data suggest reabsorption of urea and other urine constituents across urinary tract epithelia. To determine whether urinary tract tissue concentrations are increased as a result of such reabsorption, we measured urea nitrogen and creatinine concentrations and determined whether urea transporter B (UT-B) was present in bladder, ureter, and other tissues from dogs and rats. Mean urea nitrogen and creatinine concentrations in dogs and rats were three- to sevenfold higher in urinary tract tissues than in serum and were comparable to those in renal cortex. In water-restricted or water-loaded rats, urea nitrogen concentrations in bladder tissues fell inversely with the state of hydration, were proportional to urine urea nitrogen concentrations, and were greater than the corresponding serum urea nitrogen concentration in every animal. Immunoblots of rat and dog urinary tract tissues demonstrated the presence of UT-B in homogenates of bladder and ureter, and immunocytochemical analysis localized UT-B to epithelial cell membranes. These findings are consistent with the notion that urea and creatinine are continuously reabsorbed from the urine across the urothelium, urea in part via UT-B, and that urine is thus altered in its passage through the urinary tract. Urea reabsorption across urinary tract epithelia may be important during conditions requiring nitrogen conservation and may contribute to pathophysiological states characterized by high blood urea nitrogen, such as prerenal azotemia and obstructive uropathy.

Cloning and functional characterization of a second urea transporter from the kidney of the Atlantic stingray,Dasyatis sabina

The cloning of cDNAs encoding facilitated urea transporters (UTs) from the kidneys of the elasmobranchs indicates that in these fish renal urea reabsorption occurs, at least in part, by passive processes. The previously described elasmobranch urea transporter clones from shark (shUT) and stingray (strUT-1) differ from each other primarily because of the COOH-terminus of the predicted strUT-1 translation product being extended by 51-amino acid residues compared with shUT. Previously, we noted multiple UT transcripts were present in stingray kidney. We hypothesized that a COOH terminally abbreviated UT isoform, homologous to shUT, would also be present in stingray kidney. Therefore, we used 5'/3' rapid amplification of cDNA ends to identify a 3'UTR-variant (strUT-1a) of the cDNA that encodes (strUT-1), as well as three, 3'UTR-variant cDNAs (strUT-2a,b,c) that encode a second phloretin-sensitive, urea transporter (strUT-2). The 5'UTR and the first 1,132 nucleotides of the predicted coding region of the strUT-2 cDNAs are identical to the strUT-1 cDNAs. The remainder of the coding region contains only five novel nucleotides. The strUT-2 cDNAs putatively encode a 379-amino acid protein, the first 377 amino acids identical to strUT-1 plus 2 additional amino acids. We conclude that 1) a second UT isoform is expressed in the Atlantic stingray and that this isoform is similar in size to the UT previously cloned from the kidney of the dogfish shark, and 2) at least five transcripts encoding the 2 stingray UTs are derived from a single gene product through alternative splicing and polyadenylation.

Possible role of AQP3 in the concentration of non‐urea solutes in urine

AQP3 is located in the cortical and medullary collecting duct (CD). Mice with AQP3 deletion (KO) exhibit a severe urinary concentrating defect, but are able, after 36 h dehydration, to concentrate urine to about 1/3 of the osmolality seen in wild-type (WT). In order to explore their concentrating defect further, we studied the influence of an acute urea load (UL = 300 μl of 1M urea, i.p.) in conscious KO and WT mice. Urine was collected every 2 h from 2 h before (Basal = B) to 8 h after the UL. During B, urine flow rate (V) was 3-fold higher and urine osmolality (Uosm) 3-fold lower in KO than in WT. Both genotypes excreted the UL in about 4 h with the same time-course. In the last 4 h, in WT mice, V fell below B values and Uosm, urea concentration (Uurea) and non-urea solute (NUS) concentration (U-NUS) rose above B values, in agreement with the well-known ability of urea to improve the capacity of the kidney to concentrate urine. Interestingly, after the UL, KO mice raised their Uosm and Uurea progressively to a dramatic extent so that these variables became almost equal to those in WT mice at 8 h. However, U-NUS did not change at all. V fell in the last 4 h to reach only 1/4 of B values. Thus, the excretion of NUS was significantly reduced, but not that of urea. These observations show that the lack of AQP3 improves the ability of the kidney to concentrate urea but impairs its ability to concentrate NUS. Because AQP3 is not only permeable to water but also to urea, it is conceivable that, in normal mice, some AQP3-mediated urea reabsorption in the CD may osmotically drive additional water and thus contribute to concentrate other solutes in the urine.

Long-Term Regulation of Renal Urea Transporters during Antidiuresis.

To produce a concentrated urine, the renal medulla needs hypertonicity for the reabsorption of free water from collecting duct. The single effect that increases interstitial tonicity in the outer medulla is the active NaCl reabsorption in the thick ascending limb, while the single effect in the inner medulla is the passive efflux of NaCl through the thin ascending limb. The passive mechanism in the inner medulla requires high interstitial urea concentration. Two main groups of urea transporters (UT-A, UT-B) are present in the kidney, which maintains the high concentration of urea in the deepest portion of the inner medulla by intra-renal urea recycling. Recent studies suggest that UT-A1 in the terminal inner medullary collecting duct is up-regulated when urine or inner medullary interstitial urea is depleted in order to enhance the reabsorption of urea, while UT-A2 in the descending thin limb of loops of Henle and UT-B in the descending vasa recta are increased when outer medullary interstitial urea concentration is high, in order to prevent the loss of urea from the medulla to the systemic circulation, thereby increasing intra-renal urea recycling. This review will summarize the functions of the renal urea transporters in urine concentration mechanism and the recent knowledge about their long-term regulation.

Hypercreatininemia and hyperglycemia: diabetic nephropathy or “inverted peritoneal auto-dialysis”?

We describe a case of 51-year-old male with fever, abdominal pain and inguino-scrotal hernia. Laboratory examination revealed hypercreatininemia and hyperglycemia, firstly interpreted as diabetic nephropathy. US and CT scan showed a hernia of the bladder into the scrotum. Surgery revealed multiple bladder perforations with peritoneal diffusion of urine. So, hypercreatininemia was caused by peritoneal reabsorption of urea and creatinine, a condition that may be described as "inverted peritoneal auto-dialysis". Surgical reposition and repairment of the bladder led to rapid normalization of serum urea and creatinine. Discharged diagnosis was intraperitoneal rupture of inguino-scrotal hernia of the bladder in patient with recent onset of diabetes mellitus.

Changes in the renal handling of urea in sheep on a low protein diet exposed to saline drinking water

Previous trials have demonstrated that sheep on a low protein diet and free access to water, and sheep dosed with boluses of NaCl intraruminally also with free access to water, showed decreases in urea loss via the urine compared to control animals. We monitored urea excretion in sheep on a relatively poor protein diet when they were exposed to saline drinking water, i.e. they were unable to vary their intake of NaCl:water. Sheep on isotonic saline drinking water (phase 3) excreted significantly more urea via the urine (284 mM/day) compared to phase 1 when they were on non-saline drinking water (urea excretion = 230 mM/day) and phase 2 when they were on half isotonic saline drinking water (urea excretion = 244 mM/day). This finding was explained by the high glomerular filtration rate (GFR) 91.9 l/day, compared to 82.4 l/day (phase 1) and 77.9 l/day (phase 2), together with a significantly raised fractional excretion of urea (FEurea) (51.1 %) during this phase, and was in spite of the significantly lower plasma concentrations of urea in phase 3 compared to phase 1. The FEurea probably results from the osmotic diuresis caused by the salt. There were indications of a raised plasma antidiuretic hormone (ADH) concentration and this would have opposed urea loss, as ADH promotes urea reabsorption. However, this ADH effect was probably counteracted to some extent by a low plasma angiotensin II concentration, for which again there were indications, inhibiting urea reabsorption during the phases of salt loading. As atrial natriuretic peptide both increases GFR and decrease sodium reabsorption from the tubule, it was probably instrumental in causing the increase in GFR and the increase in the fractional excretion of sodium (FE(Na)).

Upregulation of urea transporter UT-A2 and water channels AQP2 and AQP3 in mice lacking urea transporter UT-B.

The UT-B urea transporter is the major urea transporter in red blood cells and kidney descending vasa recta. Humans and mice that lack UT-B have a mild urine-concentrating defect. Whether deletion of UT-B altered the expression of other transporter proteins involved in urinary concentration was tested. Fluorescence-based real-time reverse transcription-PCR and Northern blot analysis showed upregulation of the UT-A2 urea transporter and the aquaporin 2 (AQP2) and AQP3 water channel transcripts but no change in other urea transporters or AQP. Western blot analysis showed that UT-A2 protein abundance in the outer medulla of UT-B null mice increased to 122 +/- 6% of wild-type control. AQP2 protein abundance increased to 177 +/- 32% and 127 +/- 7% in the outer and inner medulla, respectively, of UT-B null versus wild-type mice. The abundance of UT-A1, AQP1, renal outer medullary potassium channel, and NKCC2/BSC1 proteins were not significantly different between UT-B null and wild-type mice. The increases in AQP2 and AQP3 would reduce water loss and improve concentrating ability. The lack of UT-B does not result in a change in expression of urea transporters involved in urea reabsorption from the inner medullary collecting duct (UT-A1 and UT-A3). However, UT-B null mice have a selective increase in UT-A2 protein abundance. This may be an adaptive response to the loss of UT-B, because UT-B and UT-A2 are involved in different intrarenal urea recycling pathways.

Does urea reabsorption occur via the glucose pathway in the kidney of the freshwater rainbow trout?

This study tested the hypothesis that the renal reabsorption of urea occurs via the glucose transport pathway in the freshwater rainbow trout (Oncorhynchus mykiss). The relationship between glucose transport and urea transport was examined by experimentally elevating the rate of renal glucose reabsorption via infusion of the fish with exogenous glucose, and by inactivating the glucose transporters via the administration of phlorizin. Under all treatments, urea was reabsorbed against a concentration gradient, with plasma levels of urea being higher than urine levels. Glucose was almost completely reabsorbed (88%) whereas urea was reabsorbed less efficiently (47%) but to a greater extent than water (22%). Glucose and urea reabsorption were both found to be correlated with Na+ and Cl− reabsorption, though the latter were 20 fold and 200–300 fold higher than glucose and urea transport rates, respectively. Glucose infusions greatly increased glucose reabsorption but urea reabsorption was unaffected. Phlorizin treatment completely blocked glucose reabsorption, but urea reabsorption was again unaffected. We conclude that there is no relationship between glucose and urea handling in the trout kidney, thus disproving the hypothesis.

Molecular and functional characterization of a urea transporter from the kidney of the Atlantic stingray.

In general, marine elasmobranch fishes (sharks, skates, and rays) maintain body fluid osmolality above seawater, principally by retaining large amounts of urea. Maintenance of the high urea concentration is due in large part to efficient renal urea reabsorption. Regulation of renal urea reabsorption also appears to play a role in maintenance of fluid homeostasis of elasmobranchs that move between habitats of different salinities. We identified and cloned a novel 2.7-kb cDNA from the kidney of the euryhaline Atlantic stingray Dasyatis sabina (GenBank accession no. AF443781). This cDNA putatively encoded a 431-amino acid protein (strUT-1) that had a high degree of sequence identity (71%) to the shark kidney facilitated urea transporter (UT). However, the predicted COOH-terminal region of strUT-1 appears to contain an additional sequence that is unique among cloned renal UTs. Injection of strUT-1 cRNA into Xenopus oocytes induced a 33-fold increase in [(14)C]urea uptake that was inhibited by phloretin. Four mRNA bands were detected in kidney by Northern blot: a transcript at 2.8 kb corresponding to the expected size of strUT-1 mRNA and bands at 3.8, 4.5, and 5.5 kb. Identification of a facilitated UT in the kidney of the Atlantic stingray provides further support for the proposal that passive mechanisms contribute to urea reabsorption by elasmobranch kidney.

Correction of age-related polyuria by dDAVP: molecular analysis of aquaporins and urea transporters.

Senescent female WAG/Rij rats exhibit polyuria without obvious renal disease or defects in vasopressin plasma level or V(2) receptor mRNA expression. Normalization of urine flow rate by 1-desamino-8-d-arginine vasopressin (dDAVP) was investigated in these animals. Long-term dDAVP infusion into 30-mo-old rats reduced urine flow rate and increased urine osmolality to levels comparable to those in control 10-mo-old rats. The maximal urine osmolality in aging rat kidney was, however, lower than that in adult kidney, despite supramaximal administration of dDAVP. This improvement involved increased inner medullary osmolality and urea sequestration. This may result from upregulation of UT-A1, the vasopressin-regulated urea transporter, in initial inner medullary collecting duct (IMCD), but not in terminal IMCD, where UT-A1 remained low. Expression of UT-A2, which contributes to medullary urea recycling, was greatly increased. Regulation of IMCD aquaporin (AQP)-2 (AQP2) expression by dDAVP differed between adult and senescent rats: the low AQP2 abundance in senescent rats was normalized by dDAVP infusion, which also improved targeting of the channel; in adult rats, AQP2 expression was unaltered, suggesting that IMCD AQP2 expression is not regulated by dDAVP directly. Increased AQP3 expression in senescent rats may also be involved in improved urine-concentrating capacity owing to higher basolateral water and urea reabsorption capacity.

Transport physiology of the urinary bladder in teleosts: A suitable model for renal urea handling?

The transport physiology of the urinary bladder of both the freshwater rainbow trout (Oncorhychus mykiss) and the marine gulf toadfish (Opsanus beta) was characterized with respect to urea, and the suitability of the urinary bladder as a model for renal urea handling was investigated. Through the use of the in vitro urinary bladder sac preparation urea handling was characterized under control conditions and in the presence of pharmacological agents traditionally used to characterize urea transport such as urea analogues (thiourea, acetamide), urea transport blockers (phloretin, amiloride), and hormonal stimulation (arginine vasotocin; AVT). Na(+)-dependence and temperature sensitivity were also investigated. Under control conditions, the in vitro trout bladder behaved as in vivo, demonstrating significant net reabsorption of Na(+), Cl(-), water, glucose, and urea. Bladder urea reabsorption was not affected by pharmacological agents and, in contrast to renal urea reabsorption, was not correlated to Na(+). However, the trout bladder showed a threefold greater urea permeability compared to artificial lipid bilayers, a prolonged phase transition with a lowered E(a) between 5 degrees C and 14 degrees C, and differential handling of urea and analogues, all suggesting the presence of a urea transport mechanism. The in vitro toadfish bladder did not behave as in vivo, showing significant net reabsorption of Na(+) but not of Cl(-), urea, or water. As in the trout bladder, pharmacological agents were ineffective. The toadfish bladder showed no differential transport of urea and analogues, consistent with a low permeability storage organ and intermittent urination. Our results, therefore, suggest the possibility of a urea transport mechanism in the urinary bladder of the rainbow trout but not the gulf toadfish. While the bladders may not be suitable models for renal urea handling, the habit of intermittent urination by ureotelic tetrapods and toadfish seems to have selected for a low permeability storage function in the urinary bladder.

Molecular characterization of a novel urea transporter from kidney inner medullary collecting ducts.

In the terminal part of the kidney collecting duct, rapid urea reabsorption is essential to maintaining medullary hypertonicity, allowing maximal urinary concentration to occur. This process is mediated by facilitated urea transporters on both apical and basolateral membranes. Our previous studies have identified three rat urea transporters involved in the urinary concentrating mechanism, UT1, UT2 and UT3, herein renamed UrT1-A, UrT1-B, and UrT2, which exhibit distinct spatial distribution in the kidney. Here we report the molecular characterization of an additional urea transporter isoform, UrT1-C, from rat kidney that encodes a 460-amino acid residue protein. UrT1-C has 70 and 62% amino acid identity to rat UrT1-B and UrT2 (UT3), respectively, and 99% identity to a recently reported rat isoform (UT-A3; Karakashian A, Timmer RT, Klein JD, Gunn RB, Sands JM, and Bagnasco SM. J Am Soc Nephrol 10: 230-237, 1999). We report the anatomic distribution of UrT1-C in the rat kidney tubule system as well as a detailed functional characterization. UrT1-C m RNA is primarily expressed in the deep part of the inner medulla. When expressed in Xenopus laevis oocytes, UrT1-C induced a 15-fold stimulation of urea uptake, which was inhibited almost completely by phloretin (0.7 mM) and 60-95% by thiourea analogs (150 mM). The characteristics are consistent with those described in perfusion studies with inner medullary collecting duct (IMCD) segments, but, contrary to UrT1-A, UrT1-C-mediated urea uptake was not stimulated by activation of protein kinase A. Our data show that UrT1-C is a phloretin-inhibitable urea transporter expressed in the terminal collecting duct that likely serves as an exit mechanism for urea at the basolateral membrane of IMCD cells.

Nitrogen metabolism and renal function in the dik-dik antelope ( Rhynchotragus kirkii)

Nitrogen metabolism and kidney function of the dik-dik antelope ( Rhynchotragus kirkii) were studied under a range of controlled, experimental conditions and diets. Dik-dik antelope remained in nitrogen balance even when fed a diet low in protein and high in fibre. When fed a diet high in protein (20%) and water ad-libitum, 55.3% of the urea filtered by the kidney was reabsorbed. Limiting water intake increased urea reabsorption to 77.2%. The U/P urea concentrations were maintained at similar ratios on all diets, as well as during dehydration and solute loading. Minimum endogenous nitrogen excreted was 74 mg/kg 0.75/day. Dehydration (water deprivation) and solute loading (intraruminal infusion of 0.25 M NaCl) had varying effects on nitrogen metabolism. It is concluded that the metabolic nitrogen economy of the dik-dik antelope is qualitatively similar to that of other domestic and wild ruminants.

The medullary collecting duct urea transporters.

The terminal inner medullary collecting duct facilitated urea transporter, UT-A1, belongs to a family of four urea transporters. The inner medullary collecting duct also expresses three active urea transporters. Reducing urine concentrating ability in vivo upregulates UT-A1 and alters active urea secretion in the terminal inner medullary collecting duct, and induces active urea reabsorption in the initial inner medullary collecting duct.

Regulation of renal urea transporters.

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.