Net Urea Research Articles

Fermentable Fibers or Oligosaccharides Reduce Urinary Nitrogen Excretion by Increasing Urea Disposal in the Rat Cecum

The availability of fermentable carbohydrates could influence the digestive degradation and disposal of blood urea. The effects of a poorly fermented cellulosic oat fiber, a soluble fermentable fiber (gum arabic) or one of two oligosaccharides (fructooligosaccharide or xylooligosaccharide) on nitrogen excretion were compared with a wheat starch-based control diet in male Wistar rats. The fibers and oligosaccharides were added to the semipurified diets at 7.5 g/100 g in place of wheat starch. The diets contained 13 g casein/100 g. Oat fiber did not cause an enlargement of the cecum. In contrast, gum arabic and the oligosaccharides elicited a 35–60% enlargement of the cecal wall and a 2 to 2.6-fold mean increase in the cecal pool of short chain fatty acids. Compared with rats fed the oat fiber-based diet, urea flux from blood to cecum was nearly 50% greater and more than 120% greater in those fed the gum arabic and oligosaccharide diets, respectively. In those groups, net nitrogen retention in the cecum more than doubled (nitrogen retention was calculated as the difference between net urea nitrogen flux into the cecum and ammonia nitrogen reabsorption). As a percentage of total excreted nitrogen, fecal nitrogen was 20% in the oat fiber group and 27–29% in the gum arabic and oligosaccharide groups, compared with only 10% in fiber-free controls. Results indicate that under these dietary conditions, the addition of oligosaccharides to the diet induced a 20 to 30% decrease in blood urea and renal nitrogen excretion relative to the control, indicating a potential for oligosaccharide diet therapy in chronic renal disease.

Achieving dialysis adequacy: A global perspective.

Dialysis adequacy is conventionally quantified as net urea clearance. Single pool (sp) Kt/Vurea remains the best studied measure of dialysis adequacy globally. Other measures such as fluid status control, anemia correction, and mineral metabolism are monitored variably. Increasing use of hemodiafiltration across Europe and many parts of Japan and Australia is predicated on studies showing better patient survival with middle molecule clearance. Apart from local clinical practice guidelines, the income level and public health policy of a country determine quality of dialysis services. Among developed nations, small solute clearance adequacy targets are achieved with high frequency. In the United States, dialysis adequacy target is met by focussing on high blood flow rates and large dialyzer size, sometimes at the cost of session time. In Japan, Australia, and Germany, session length is given importance. Dialysis adequacy reporting is restricted and inconsistent in developing nations. The Gulf Cooperation Council countries, Russia and Malaysia, respectively, are close to achieving dialysis adequacy target (spKt/Vurea ≥1.2) universally in their dialysis populations. Patient-reported outcomes are typically measured only in developed countries. Patient survival on dialysis, partly linked to dialysis adequacy, varies greatly around the world, with Japan having the best survival rates. Until the development of better markers of dialysis adequacy, universal consistency in reporting of conventional parameters with a focus on patient-reported measures should be endeavored.

The effects of digesting a urea-rich meal on North Pacific spiny dogfish (Squalus acanthias suckleyi)

Marine elasmobranchs are nitrogen-limited owing to the requirement of nitrogen for both somatic growth and urea-based osmoregulation, and due to the loss of urea across the gills and kidney as nitrogenous waste. In this study we used in vitro stomach and intestinal gut sacs to investigate the effects of consuming a urea-rich meal (700 mM within a 2% body-mass ration of food-slurry) on nitrogen movement across the gastrointestinal (GI) tract of North Pacific spiny dogfish (Squalus acanthias suckleyi). Plasma urea concentrations did not differ between fasted (359 ± 19 mM), urea-poor fed (340 ± 16 mM), and urea-rich fed (332 ± 24 mM) dogfish. Interestingly, in vitro gut sacs of urea-rich fed dogfish showed no net urea absorption from the lumen over 3 h incubation, which contrasts previously published data on urea-poor fed dogfish that absorb urea from the lumen. In addition, ammonium (NH4+) concentration within the gut sac intestinal lumen significantly increased from 0.62 to 4.35 mM over 3 h. This is likely due to a combination of tissue production and microbial urease activity in the intestine. The overall results highlight the ability of S. a. suckleyi to regulate and maintain internal nitrogen concentrations despite the addition of excess dietary urea.

Intake and digestibility, rumen fermentation, and concentrations of metabolites in steers fed with peanut cake.

This study evaluated peanut cake as a replacement for soybean meal in the diet of steers. We used five castrated Holstein × Zebu steers, fistulated, with average live weight 550kg, in a 5 × 5 Latin square design. The diets were composed of Tifton hay and ground corn with peanut cake (0, 25, 50, 75, and 100%) as a replacement for soybean meal in the concentrate. Intake (kg/day) of dry matter, neutral detergent fiber, and crude protein were not affected by peanut cake, but ether extract intake increased linearly with the level of peanut cake in the diet. The digestibility of dry matter showed a quadratic effect, with a calculated maximum regression level of 56% at 37% replacement. There was no effect of the level of peanut cake on metabolizable energy intake and net energy, ammonia nitrogen, rumen pH, plasma urea, and glucose concentrations. We recommend replacing soybean meal in the concentrate with up to 40% peanut cake.

Hydration status affects sodium, potassium, and chloride transport across rat urothelia

Recent data suggest possible net transport of urinary constituents across mammalian urinary tract epithelia (urothelia). To evaluate the effect of animal hydration status on such transport, we instilled urine collected during 2-day water deprivation, water loading, or ad libitum water intake into isolated in situ bladder(s) of groups of rats undergoing one of the same three hydration states. After 1-h bladder dwell, we retrieved the urine and measured differences in volume and solute concentrations between instilled and retrieved urine. We previously reported results regarding changes in urine volume and net urea and creatinine transport and herein report the results of net urinary sodium, potassium, and chloride transport in the same animals. During water-loading conditions, urinary concentrations of Na, K, and Cl rose 4.9 (30.7%), 2.6 (16.5%), and 6.0 meq/l (26.8%), respectively, indicating urothelial secretion into urine. During ad libitum water intake, urinary K and Cl concentrations fell 33.6 (14.8%) and 28.4 meq/l (12%), respectively (Na did not change), and during water deprivation urine Na, K, and Cl concentrations fell dramatically by 53.2 (18.6%), 159.4 (34.6%) and 133.7 meq/l (33.8%), respectively, reflecting urothelial reabsorption of each ion. For each ionic species, two factors independently influenced transport: instilled urinary ion concentration and animal hydration state. These results demonstrate significant regulated ion transport across mammalian urothelia, support the notion that lower urinary tract modifies final urine, and suggest that the lower urinary tract may play a role in local and whole animal solute homeostasis.

An in vitro study of urea, water, ion and CO2/HCO3− transport in the gastrointestinal tract of the dogfish shark (Squalus acanthias): the influence of feeding

In vitro gut sac preparations made from the cardiac stomach (stomach 1), pyloric stomach (stomach 2), intestine (spiral valve) and colon were used to examine the impact of feeding on transport processes in the gastrointestinal tract of the dogfish shark. Preparations were made from animals that were euthanized after 1-2 weeks of fasting, or at 24-48 h after voluntary feeding on a 3% ration of teleost fish (hake). Sacs were incubated under initially symmetrical conditions with dogfish saline on both surfaces. In comparison to an earlier in vivo study, the results confirmed that feeding caused increases in H(+) secretion in both stomach sections, but an increase in Cl(-) secretion only in stomach 2. Na(+) absorption, rather than Na(+) secretion, occurred in both stomach sections after feeding. All sections of the tract absorbed water and the intestine strongly absorbed Na(+) and Cl(-), regardless of feeding condition. The results also confirmed that feeding increased water absorption in the intestine (but not in the colon), and had little influence on the handling of Ca(2+) and Mg(2+), which exhibited negligible absorption across the tract. However, K(+) was secreted in the intestine in both fasted and fed preparations. Increased intestinal water absorption occurred despite net osmolyte secretion into the mucosal saline. The largest changes occurred in urea and CO2/HCO3(-) fluxes. In fasted preparations, urea was absorbed at a low rate in all sections except the intestine, where it was secreted. Instead of an increase in intestinal urea secretion predicted from in vivo data, feeding caused a marked switch to net urea absorption. This intestinal urea transport occurred at a rate comparable to urea reabsorption rates reported at gills and kidney, and was apparently active, establishing a large serosal-to-mucosal concentration gradient. Feeding also greatly increased intestinal CO2/HCO3(-) secretion; if interpreted as HCO3(-) transport, the rates were in the upper range of those reported in marine teleosts. Phloretin (0.25 mmol l(-1), applied mucosally) completely blocked the increases in intestinal urea absorption and CO2/HCO3(-) secretion caused by feeding, but had no effect on Na(+), Cl(-) or water absorption.

Urea is a dynamic pool of bioavailable nitrogen in coral reefs

Urea may be an important source of nitrogen in low nutrient coral reef environments because corals and other organisms can assimilate it easily and it is found throughout ocean waters. We measured the distribution and concentrations of urea in seagrass beds, areas of schooling fish, coral formations and bottom sediments in the Upper Florida Keys Reef Tract. The flux of urea from bottom sediments was also measured. Ambient concentrations of urea in the offshore reefs were similar to the concentrations of nitrate and ammonium. Seagrass beds, areas of schooling fish and coral formations had elevated concentrations of urea that were up to eight times higher than nitrate in the system. Numerous ephemeral hotspots of urea that were 8–20 times the ambient urea concentration existed in seagrass beds, areas of schooling fish, and above sediments. Coastal areas and inland canals had high urea concentrations where urban runoff and septic effluents were prevalent, but there was no anthropogenic influence in the offshore habitats. Urea concentrations above bottom sediments were not different from ambient concentrations and benthic flux chamber incubations showed biological activity in carbonaceous sediments but no net urea production. The decrease in urea concentrations from coasts and inland waterways to a consistent ambient concentration in the offshore reef system and ephemeral hotspots of high urea concentration suggest that urea is a dynamic pool of bioavailable nitrogen in the reefs of the Upper Florida Keys.

Role of UTB Urea Transporters in the Urine Concentrating Mechanism of the Rat Kidney

A mathematical model of the renal medulla of the rat kidney was used to investigate urine concentrating mechanism function in animals lacking the UTB urea transporter. The UTB transporter is believed to mediate countercurrent urea exchange between descending vasa recta (DVR) and ascending vasa recta (AVR) by facilitating urea transport across DVR endothelia. The model represents the outer medulla (OM) and inner medulla (IM), with the actions of the cortex incorporated via boundary conditions. Blood flow in the model vasculature is divided into plasma and red blood cell compartments. In the base-case model configuration tubular dimensions and transport parameters are based on, or estimated from, experimental measurements or immunohistochemical evidence in wild-type rats. The base-case model configuration generated an osmolality gradient along the cortico-medullary axis that is consistent with measurements from rats in a moderately antidiuretic state. When expression of UTB was eliminated in the model, model results indicated that, relative to wild-type, the OM cortico-medullary osmolality gradient and the net urea flow through the OM were little affected by absence of UTB transporter. However, because urea transfer from AVR to DVR was much reduced, urea trapping by countercurrent exchange was significantly compromised. Consequently, urine urea concentration and osmolality were decreased by 12% and 8.9% from base case, respectively, with most of the reduction attributable to the impaired IM concentrating mechanism. These results indicate that the in vivo urine concentrating defect in knockout mouse, reported by Yang et al. (J Biol Chem 277(12), 10633–10637, 2002), is not attributable to an OM concentrating mechanism defect, but that reduced urea trapping by long vasa recta plays a significant role in compromising the concentrating mechanism of the IM. Moreover, model results are in general agreement with the explanation of knockout renal function proposed by Yang et al.

Pharmacokinetics of Arginine and Related Amino Acids

Arginine (ARG) and its related amino acids (AAs) ornithine (ORN) and citrulline (CIT) find a range of applications as dietary supplements in subgroups of healthy subjects (e.g., bodybuilders) and patients with acute or chronic malnutrition. These AAs appear to be well utilized in humans with, in general, a rapid return of blood concentrations to basal values (i.e., within 5–8 h) and low absolute and relative excretion in urine (< 5% of administered dose). Based on published data for the maximum observed plasma concentrations (Cmax) after administration of doses in the range 5 to 10 g, CIT appeared to present relatively better absorption and systemic bioavailability than ARG and ORN. The few relevant dose-ranging studies available include 1 limited to a single subject receiving 5- to 20-g doses of ornithine α-ketoglutarate and another in which 8 subjects received from 5 to 15 g of CIT. Comparison of these 2 studies further indicates that CIT has higher bioavailability than ORN. The pharmacokinetics and metabolism of these AAs are modified by the coadministration of a salt such as α-ketoglutarate that modifies AA metabolism, as has clearly been demonstrated for ornithine α-ketoglutarate. Concomitant administration of a meal leads to a 15- to 30-min delay in Cmax. Finally, data from various pharmacokinetic studies together with basic physiology and biochemistry indicate that ARG is a net urea producer and ORN has a nitrogen-sparing effect, whereas CIT is neutral. However, most of the studies performed to date carry methodological weaknesses and are difficult to compare because of a number of confounding factors. To date, there have been no pharmacokinetic studies on the long-term administration of these AAs in healthy subjects despite the need to determine the safe upper limit of daily intake.

Role of UTB Urea Transporters in the Urine Concentrating Mechanism of the Rat Kidney

A mathematical model of the renal medulla of the rat kidney was used to investigate urine concentrating mechanism function in animals lacking the UTB urea transporter. The UTB transporter is believed to mediate countercurrent urea exchange between descending vasa recta (DVR) and ascending vasa recta (AVR) by facilitating urea transport across DVR endothelia. The model represents the outer medulla (OM) and inner medulla (IM), with the actions of the cortex incorporated via boundary conditions. Blood flow in the model vasculature is divided into plasma and red blood cell compartments. In the base-case model configuration tubular dimensions and transport parameters are based on, or estimated from, experimental measurements or immunohistochemical evidence in wild-type rats. The base-case model configuration generated an osmolality gradient along the cortico-medullary axis that is consistent with measurements from rats in a moderately antidiuretic state. When expression of UTB was eliminated in the model, model results indicated that, relative to wild-type, the OM cortico-medullary osmolality gradient and the net urea flow through the OM were little affected by absence of UTB transporter. However, because urea transfer from AVR to DVR was much reduced, urea trapping by countercurrent exchange was significantly compromised. Consequently, urine urea concentration and osmolality were decreased by 12% and 8.9% from base case, respectively, with most of the reduction attributable to the impaired IM concentrating mechanism. These results indicate that the in vivo urine concentrating defect in knockout mouse, reported by Yang et al. (J Biol Chem 277(12), 10633-10637, 2002), is not attributable to an OM concentrating mechanism defect, but that reduced urea trapping by long vasa recta plays a significant role in compromising the concentrating mechanism of the IM. Moreover, model results are in general agreement with the explanation of knockout renal function proposed by Yang et al.

Do different haemodialysis regimen interfere with genomic damage in patients with end-stage-renal disease?

Continuous renal replacement therapies have practical and theoretical advantages compared with conventional intermittent hemodialysis in hemodynamically unstable or severely catabolic patients with acute renal failure (ARF). Sustained low-efficiency dialysis (SLED) is a hybrid modality introduced July 1998 at the University of Arkansas for Medical Sciences that involves the application of a conventional hemodialysis machine with reduced dialysate and blood flow rates for 12-hour nocturnal treatments. Nine critically ill patients with ARF were studied during a single SLED treatment to determine delivered dialysis dose and the most appropriate model for the description of urea kinetics during treatment. Five patients were men, mean Acute Physiology and Chronic Health Evaluation (APACHE) II score was 28.9 and mean weight was 92.5 kg. Kt/V was determined by the reference method of direct dialysate quantification (DDQ) combined with an equilibrated postdialysis plasma water urea nitrogen (PUN) concentration and four other methods that were either blood or dialysate based, single or double pool, or model independent (whole-body kinetic method). Solute removal indices (SRIs) were determined from net urea removal and urea distribution volume supplied from DDQ (reference method) and by mass balance using variables supplied from blood-based formal variable-volume single-pool (VVSP) urea kinetic modeling. Equivalent renal urea clearances (EKRs) were calculated from urea generation rates and time-averaged concentrations for PUN based on weekly mass balance with kinetic variables supplied by either DDQ (reference method) or formal blood-based VVSP modeling. Mean Kt/V determined by the reference method was 1.40 and not significantly different when determined by formal VVSP modeling, DDQ using an immediate postdialysis PUN, or the whole-body kinetic method. Correction of single-pool Kt/V by a Daugirdas rate equation did not yield plausible results. Mean SRI and EKR by the reference methods were 0.61 and 24.8 mL/min, respectively, and not significantly different when determined by blood-based methods. A single-pool urea kinetic model adequately described intradialytic PUN profiles, indicating that SLED was associated with minimal urea disequilibrium. This was supported by the parity between hemodialyzer and whole-body urea clearances, and the mean postdialytic urea rebound of 4.1% (P = 0.13 versus zero). Additional prospective studies are needed in this setting to define the optimal method for dialysis quantification, targets for azotemic control, and optimal modality of renal replacement therapy. In conclusion, SLED delivers a high dose of dialysis with minimal associated urea disequilibrium and can be quantified by Kt/V, SRI, and EKR from blood-based methods using single-pool urea kinetic models. © 2002 by the National Kidney Foundation, Inc.

Urea Synthesis by Ruminal Epithelial and Duodenal Mucosal Cells from Growing Sheep*,†

The objective was to determine whether ruminant gut tissues have the capability to synthesize urea in a short-term incubation. Mixed primary cell cultures containing ruminal epithelial (REC) or duodenal mucosal cells (DMC) were isolated from growing sheep (n=4) fed a mixed forage-concentrate diet. Cells were incubated (90min) in a Krebs salts-based buffer with either acetate (5mM) or propionate (5mM) plus a combination of substrate intermediates (5mM) for urea synthesis: arginine, aspartate + citrulline (AspC), aspartate + ornithine + ammonia (AspON), or AspON + N-carbamoylglutamate (AspONG) in a 2×4 factorial arrangement of treatments. Volatile fatty acid, propionate vs. acetate, did not influence net urea synthesis. For REC, net urea synthesis (nmoles•(106 cells)−1•90min−1) was greatest with Arg (54.5±6.3) followed by AspC (4.6±1.1) and AspONG (3.6±1.4). For DMC, net urea synthesis for Arg (2.1±0.7) and AspONG (1.9±0.7) treatments was greater than for AspC (0.3±0.7) and AspON (−0.6±0.7) treatments. Thus, for both REC and DMC, arginase activity appeared to be sufficient for catabolism of arginine to urea. Furthermore, greater urea synthesis from ammonia, ornithine and aspartate in the presence of the N-acetylglutamate analogue suggests that carbamoyl phosphate synthetase is probably rate-limiting for urea synthesis and ammonia detoxification by ruminant gut tissues.

Expression, localization, and regulation of urea transporter B in rat urothelia.

Although mammalian urothelia are generally considered impermeable to urinary constituents, in vivo studies in several species suggest urothelial transport of water, urea, and solutes under certain conditions. This study investigates the expression, localization, and regulation of urea transporter-B (UT-B) in rat renal pelvis, ureter, and bladder tissues. Immunoblots of homogenates of tissues identified characteristic approximately 40- to 55- and approximately 32-kDa bands in the ureter, bladder, and renal inner medulla, but not renal cortex. UT-B was localized by immunocytochemistry and was strongly expressed in all cell membranes (and to a limited extent in intracellular vesicles in the cytoplasm) of epithelial cells lining the rat bladder, ureter, and renal pelvis lumens except the apical membrane of the umbrella cells. It was also present in single-layer papillary surface epithelial cells. There was no difference in immunoblot expression of UT-B in the bladder or ureteral homogenates between groups of rats fed high- or low-protein or high- or low-sodium diets. Water restriction resulted in an increase in UT-B expression in ureters (49%, P = 0.001) but not in bladders (14%, P = not significant). The functional role of UT-B in the genitourinary tract epithelia is unknown. UT-B may participate in the regulation of epithelial cell volume and osmolality, in the dissipation of urea gradients, and in possible net urea transport across uroepithelia.

The precision of estimating protein intake of patients with chronic renal failure

Biochemical methods for estimating protein intake are based on the concept that nitrogen-containing products of protein in diet plus the products arising from endogenous protein are excreted as either urea or non-urea nitrogen (NUN). This formulation is based on the fact that the urea is the principal end product of amino acid degradation and, hence, the urea appearance rate (or net urea production) is parallel to protein intake. The urea nitrogen appearance (UNA) rate is measured as the amount of urea excreted in urine plus the net amount accumulated in body water. A more difficult problem is how to estimate NUN, the sum of fecal nitrogen, and all forms of non-urea urinary nitrogen. Maroni, Steinman, and Mitch (Kidney Int 27:58-65, 1985) proposed estimating nitrogen intake (IN MARONI) from UNA plus NUN excretion rate of 0.031 g nitrogen/kg body weight/day, as they found NUN correlated with body weight but not with dietary nitrogen. Kopple, Gao, and Qing (Kidney Int 27:486-494, 1997) proposed a different equation for estimating nitrogen intake (IN KOPPLE) = 1.20 UNA + 1.74, concluding that dietary nitrogen directly correlates with fecal nitrogen and that NUN is constant for all patients. Their report prompted us to review all nitrogen balance measurements we had conducted in order to address the following questions. Does dietary protein increase fecal nitrogen excretion? Does NUN vary with weight or is it constant? How do the two methods (IN MARONI and IN KOPPLE) compare in estimating dietary protein from UNA? We examined nitrogen balance and its components measured in 33 patients with chronic renal failure (CRF) who were eating diets varying from 4.1 to 10.1 g nitrogen/day. We evaluated relationships between dietary nitrogen [intake nitrogen (IN)], NUN, fecal nitrogen, body weight, and the predictability of the two methods. Neither fecal nitrogen nor NUN were significantly correlated with IN (r = 0.04 and r = -0.07, respectively). NUN significantly correlated with body weight (P = 0.008). Measured IN averaged 5.75 +/- 0.41 g nitrogen/day; the estimated IN MARONI value was 5.61 +/- 0.27 g nitrogen/day; the estimated IN KOPPLE was 6.04 +/- 0.44 g nitrogen/day. The prediction errors associated with the IN KOPPLE equation were slightly but not statistically higher than that associated with IN MARONI. Fecal nitrogen is not correlated with IN. NUN is not constant but varies with weight, and the traditional method of estimating IN in stable chronic renal insufficiency (CRI) patients from UNA and weight as proposed by Maroni, Steinman, and Mitch is valid.

Urea kinetics during sustained low-efficiency dialysis in critically ill patients requiring renal replacement therapy

Continuous renal replacement therapies have practical and theoretical advantages compared with conventional intermittent hemodialysis in hemodynamically unstable or severely catabolic patients with acute renal failure (ARF). Sustained low-efficiency dialysis (SLED) is a hybrid modality introduced July 1998 at the University of Arkansas for Medical Sciences that involves the application of a conventional hemodialysis machine with reduced dialysate and blood flow rates for 12-hour nocturnal treatments. Nine critically ill patients with ARF were studied during a single SLED treatment to determine delivered dialysis dose and the most appropriate model for the description of urea kinetics during treatment. Five patients were men, mean Acute Physiology and Chronic Health Evaluation (APACHE) II score was 28.9 and mean weight was 92.5 kg. Kt/V was determined by the reference method of direct dialysate quantification (DDQ) combined with an equilibrated postdialysis plasma water urea nitrogen (PUN) concentration and four other methods that were either blood or dialysate based, single or double pool, or model independent (whole-body kinetic method). Solute removal indices (SRIs) were determined from net urea removal and urea distribution volume supplied from DDQ (reference method) and by mass balance using variables supplied from blood-based formal variable-volume single-pool (VVSP) urea kinetic modeling. Equivalent renal urea clearances (EKRs) were calculated from urea generation rates and time-averaged concentrations for PUN based on weekly mass balance with kinetic variables supplied by either DDQ (reference method) or formal blood-based VVSP modeling. Mean Kt/V determined by the reference method was 1.40 and not significantly different when determined by formal VVSP modeling, DDQ using an immediate postdialysis PUN, or the whole-body kinetic method. Correction of single-pool Kt/V by a Daugirdas rate equation did not yield plausible results. Mean SRI and EKR by the reference methods were 0.61 and 24.8 mL/min, respectively, and not significantly different when determined by blood-based methods. A single-pool urea kinetic model adequately described intradialytic PUN profiles, indicating that SLED was associated with minimal urea disequilibrium. This was supported by the parity between hemodialyzer and whole-body urea clearances, and the mean postdialytic urea rebound of 4.1% (P = 0.13 versus zero). Additional prospective studies are needed in this setting to define the optimal method for dialysis quantification, targets for azotemic control, and optimal modality of renal replacement therapy. In conclusion, SLED delivers a high dose of dialysis with minimal associated urea disequilibrium and can be quantified by Kt/V, SRI, and EKR from blood-based methods using single-pool urea kinetic models.

Cycles and separations in a model of the renal medulla.

This study gives the first quantitative analysis of net steady-state transmural fluxes of water, urea, and NaCl in a numerical model of the rat renal medulla in antidiuresis, revealing the model's predictions of water, urea, and NaCl cycling patterns. These predictions are compared both to in vivo micropuncture data from the literature and to earlier qualitative proposals (e.g., K. V. Lemley and W. Kriz. Kidney Int. 31: 538-548, 1987) of cycling and exchange patterns based on medullary anatomy and available permeability and transport parameter measurements. The analysis is based on our most recent three-dimensional model [X. Wang, S. R. Thomas, and A. S. Wexler. Am. J. Physiol. 274 (Renal Physiol. 43): F413-F424, 1998]. In general agreement with earlier proposed patterns, this analysis predicts the following: 1) important water short-circuiting from descending structures to ascending vasa recta in most medullary regions, 2) massive urea recycling in the upper inner medulla, 3) a progressive increase of the ratio of urea to total osmoles along the corticopapillary axis, 4) urea dumped from the collecting ducts (CD) into the deep papilla is returned to the cortex essentially via outer medullary short vasa recta, bearing witness to a shift from the long descending limbs and vasa recta of the inner medulla (IM) to short structures in the outer medulla (OM). The analysis also shows that the known radial heterogeneity of the inner stripe (IS) implies unequal osmolalities in long descending limbs, vasa recta, and CDs entering the IM across the OM/IM border and explains the model's unorthodox osmolality profile along the CD. In conflict with micropuncture evidence of a doubling of urea flow in superficial Henle's loops (SHL) between the end proximal and early distal tubule (T. Armsen and H. W. Reinhardt. Pflügers Arch. 326: 270-280, 1971), the model predicts net urea loss from SHL due to the model's inclusion of nonneglible measured urea permeability of medullary thick ascending limbs [M. A. Knepper, Am. J. Physiol. 245 (Renal Fluid Electrolyte Physiol. 14): F634-F639, 1983]. We present a suite of adjusted model permeabilities that improves agreement with the micropuncture data on this point. In conclusion, this modeling analysis of solute and water recycling serves as a quantitative check on qualitative propositions in the literature and allows closer critical comparison of model behavior with published experimental results than was heretofore possible.

Evidence for sodium-dependent active urea secretion in the deepest subsegment of the rat inner medullary collecting duct.

Active reabsorption of urea appears in the initial IMCD (IMCD1) of rats fed a low-protein diet. To determine whether active urea transport also occurs in the deepest IMCD subsegment, the IMCD3, we isolated IMCDs from the base (IMCD1), middle (IMCD2), and tip (IMCD3) regions of the inner medulla from rats fed a normal protein diet and water ad libitum. IMCDs were perfused with identical perfusate and bath solutions. A significant rate of net urea secretion was present only in IMCD3s. Replacing perfusate Na+ with NMDG+ reversibly inhibited net urea secretion but replacing bath Na+ with NMDG+ or perfusate Cl- with gluconate- had no effect. Net urea secretion was significantly inhibited by: (a) 250 microM phloretin (perfusate); (b) 100 nM triamterene (perfusate); (c) 1 mM ouabain (bath); and (d) cooling the tubule to 23 degrees C. Net urea secretion was significantly stimulated by 10 nM vasopressin (bath). Next, we perfused IMCD3s from water diuretic rats (given food ad libitum) and found a significant, fivefold increase in net urea secretion. In summary, we identified a secondary active, secretory urea transport process in IMCD3s of normal rats which is upregulated in water diuretic rats. This new urea transporter may be a sodium- urea antiporter.

On-line urea kinetics in haemodiafiltration

Calculation of Kt/V and assessment of nutrition have so far been dependent upon off-line urea measurements of blood or dialysate samples. Here we describe a biosensor for on-line urea measurement during haemodiafiltration. Methods. The biosensor consisted of a cartridge containing covalently linked urease placed between two conductivity cells. The biosensor was placed on the outlet line of a haemofilter in series with a dialyser in order to obtain an aliquot of plasma ultrafiltrate for on-line measurement of urea. Urea nitrogen concentrations were highly correlated to the difference (Delta) in conductivity measured by the two conductivity cells both in aqueous solutions (in-vitro studies, y=-6. 676+32.12x, R2=0.998, P<0.0001) and in ultrafiltrates (ex-vivo studies, y=-637+32.01x, R2=0.98, P<0.00001). Delta conductivity was highly reproducible (% variation: ).8-5.3%) and stable (maximal % variation at 150 mg/dl after 100 min. 0.9+/-0.3 vs initial values). The intradialytic plasma water urea profile was obtained in 10 haemodialysis patients. To study recirculation, the plasma water urea profile was analysed before and 3 min after stopping the dialysate flow. The pre- and post-stopped flow ratio (1.21+/-0.1, mean+/-1 SD) was superimposable to conventional blood sampling data (opposite arm venous arterial: 1.22+/-0.11) and allowed correction for recirculation. A novel approach to urea kinetic modelling was described and used to reliably project end-dialysis and post-dialysis rebound urea concentration as early as 90 min. Projected (29.2+/-10.4 g) or measured (29.8+/-10.5 g) net urea removal was highly correlated with the amount of urea collected in the total spent dialysate (29.7+/-10.6 g) (R2=0.99, R2=0.97 respectively). These results indicate that on-line, real-time analysis of urea kinetics may provide information on delivery of adequate dialysis in high-efficiency techniques.

Active urea transport in the rat inner medullary collecting duct: Functional characterization and initial expression cloning

Active transport of urea has been proposed to exist in the inner medullary collecting duct (IMCD) of low-protein fed mammals for over 30 years. We perfused IMCD subsegments from rats fed a standard (18%) or a low (8%) protein diet and tested for the presence of active urea transport. We found no active urea transport in terminal IMCDs, regardless of diet. In initial IMCDs from rats fed 18% protein or fed 8% protein for one to two weeks, we again found no active urea transport. However, in rats fed 8% protein for three to four weeks, we found significant net urea reabsorption. This active urea reabsorption was inhibited when Na+, K(+)-ATPase activity was inhibited by adding 1 mM ouabain or removing bath potassium, suggesting a secondary active transport process. Removing sodium from the perfusate completely inhibited net urea reabsorption, demonstrating that this active urea transport is dependent upon the presence of sodium in the tubule lumen. Unlike the facilitated urea transporter, the active urea transporter was not inhibited by phloretin nor stimulated by vasopressin, suggesting that it is a distinct transport protein. To test this hypothesis, we size-separated poly(A)(+)-RNA prepared from inner medullae of rats fed 8% protein for three weeks and injected it into Xenopus laevis oocytes. RNA from a 4.4 to 8.4 kb size fraction increased urea permeability fourfold compared to water-injected oocytes or injecting RNA from other size-fractions. We conclude that feeding rats a low-protein diet for three weeks induces the expression of an unique, secondary active, sodium-dependent urea transporter whose cDNA is between 4.4 and 8.4 kb in size. In addition, our results suggest that it will be possible to clone the cDNA for this sodium-urea cotransporter by expression in Xenopus laevis oocytes.

Effects of recombinant human growth hormone on plasma and dialysate amino acid profiles in CAPD patients

Protein and calorie malnutrition is common in chronic dialysis patients. Several interventions have been proposed to prevent and/or to treat malnutrition including the use of recombinant human growth hormone (rhGH) as an anabolic agent. We have previously reported a significant decrease in net urea nitrogen appearance along with modest but statistically significant decrements in several blood chemistries including serum potassium, phosphorus, albumin and a small increase in creatinine concentration during rhGH administration in CAPD patients. In order to evaluate the underlying mechanism of these changes, we systematically evaluated the plasma and dialysate amino acid profiles in blood and dialysate samples of the same patients during their participation in the study. The design of the study was prospective, cross-over with the patients serving as their own controls. There were three study periods: baseline (preGH), treatment (Tx), and follow-up (PostGH). During the seven days Tx period, patients self-administered 5 mg/day s.c of rhGH. Compared to PreGH period, administration of rhGH resulted in a significant decrease in essential amino acids (EAA), in both plasma (935 +/- 243 mumol/liter vs. 801 +/- 186 mumol/liter; P < 0.05) and dialysate (623 +/- 244 mumol/liter vs. 415 +/- 122 mumol/liter; P < 0.05). This decrease was evident in 8 out of 10 individual EAA, and the extent of decrease ranged from 15% to 28% for plasma EAA and from 30% to 45% for dialysate EAA. On the other hand, plasma non-essential AA levels increased significantly during treatment (2537 +/- 776 mumol/liter vs. 3177 +/- 1259 mumol/liter; P < 0.05). All changes returned to baseline values after discontinuation of rhGH. Our findings suggest that the net anabolic processes induced by rhGH reflect a shift in AA metabolism towards peripheral muscle tissues.