Daily delivery of dietary nitrogen to the periphery is stable in rats adapted to increased protein intake

Dietary nitrogen was traced in rats adapted to a 50% protein diet and given a meal containing 1.50 g (15)N-labeled protein (HP-50 group). This group was compared with rats usually consuming a 14% protein diet and fed a meal containing either 0.42 g (AP-14 group) or 1.50 g (AP-50 group) of (15)N-labeled protein. In the HP group, the muscle nonprotein nitrogen pool was doubled when compared with the AP group. The main adaptation was the enhancement of dietary nitrogen transferred to urea (2.2 +/- 0.5 vs. 1.3 +/- 0.1 mmol N/100 g body wt in the HP-50 and AP-50 groups, respectively). All amino acids reaching the periphery except arginine and the branched-chain amino acids were depressed. Consequently, dietary nitrogen incorporation into muscle protein was paradoxically reduced in the HP-50 group, whereas more dietary nitrogen was accumulated in the free nitrogen pool. These results underline the important role played by splanchnic catabolism in adaptation to a high-protein diet, in contrast to muscle tissue. Digestive kinetics and splanchnic anabolism participate to a lesser extent in the regulation processes.     

Postprandial exchange of urea and ammonia nitrogen between the digestive tract and portal blood in the conscious pig after ingestion of a diet with or without urea

The concentrations of urea and ammonia were measured in the portal and arterial blood simultaneously to the blood flow rate in the portal vein during the postprandial period (8 hrs.) after ingestion of a normal protein diet with 3% urea (10 meals) or without urea (12 meals) in conscious pigs (mean body weight: 55.5 +/- 2.3 kg). When no urea was present in the diet, there was a slight and permanent uptake of blood urea by the gut (570 mg/h, i.e. 9,5 mmoles/h) as well as a permanent appearance of ammonia in the portal vein (258 mg/h i.e. 15.2 mmoles/h), increasing with time (P less than 0.05). The absorbed ammonia nitrogen represented a maximum of 70% of urea nitrogen taken up. 2. Addition of urea to the diet brought about a large absorption of that substance (73% of the ingested amount) followed by a rather large excretion (960 mg/h, i.e. 16 mmoles/h), 5-6 hrs. after the meal and led to an increase (P less than 0.05) in the absorption of ammonia.     

Contribution of plasma proteins to splanchnic and total anabolic utilization of dietary nitrogen in humans

Splanchnic tissues are largely involved in the postprandial utilization of dietary amino acids, but little is yet known, particularly in humans, about the relative contributions of different splanchnic protein pools to splanchnic and total postprandial anabolism. Our aim was to develop a compartmental model that could distinguish dietary nitrogen (N) incorporation among splanchnic constitutive, plasma (splanchnic exported), and peripheral proteins after a mixed-protein meal in humans. Eight healthy subjects were fed a single mixed meal containing 15N-labeled soy protein, and dietary N postprandial kinetics were measured in plasma free amino acids, proteins, and urea and urinary urea and ammonia. These experimental data and others previously obtained for dietary N kinetics in ileal effluents under similar experimental conditions were used to develop the compartmental model. Six hours after the mixed-meal ingestion, 31.5, 7.5, and 21% of ingested N were predicted to be incorporated into splanchnic constitutive, splanchnic exported, and peripheral proteins, respectively. The contribution of splanchnic exported proteins to total splanchnic anabolism from dietary N was predicted to be approximately 19% and to remain steady throughout the simulation period. Model behavior and its predictions were strongly in line with current knowledge of the system and the scarce, specific data available in the literature. This model provides the first data concerning the anabolism of splanchnic constitutive proteins in the nonsteady postprandial state in humans. By use of only slightly invasive techniques, this model could help to assess how the splanchnic anabolism is modulated under different nutritional or pathophysiological conditions in humans.     

Energy metabolism in the digestive tract and liver of cattle: influence of physiological state and nutrition

Major functions of portal-drained viscera (PDV) and liver of cattle include absorption of digestion products and modification of the body's supply of intermediary metabolites. The disproportionately high metabolic rate of PDV and liver (7-13% of body tissues) is exemplified by their oxygen uptake (40-50% of whole body). Extensive metabolism of glucose, volatile fatty acids and amino acids by PDV modulates nutrient supply from the diet such that most responses to diet or physiological state are a function of level of diet intake. Similarly, blood flow through PDV is highly correlated with energy intake across a range of body weight, physiological state or diet composition. Most common dietary responses in metabolite uptake by PDV are changes in uptake of ammonia and volatile fatty acids, which emphasize the strong energy: nitrogen interrelationship in the rumen and subsequently the rest of the body. The liver (tissue in series with PDV) removes glucose precursors and ammonia from its blood supply as part of its functions in gluconeogenesis, ammonia detoxification and urea synthesis. The liver also alters amounts and proportions of amino acids supplied by PDV. Accountable percentages of metabolizable energy from net PDV supply include: organic acids, 41-59%; amino acids, 5-13%; and heat energy (from oxygen uptake), 11-22%.     

Intestinal lysine metabolism is driven by the enteral availability of dietary lysine in piglets fed a bolus meal

Previous steady-state continuous-feeding studies have shown that the gut mucosa removes substantial amounts of both dietary and systemic amino acids. However, enteral nutrition is often given under non-steady-state conditions as a bolus meal, and this has been shown to influence systemic metabolism. Therefore, our aim was to quantify the relative metabolism of dietary and systemic lysine by the portal-drained viscera (PDV) under non-steady-state conditions after a single bolus meal. Five 28-day-old piglets implanted with arterial, venous, and portal catheters and with an ultrasonic portal flow probe were given an oral bolus feeding of a milk formula containing a trace quantity of intrinsically 15N-labeled soy protein and a continuous intravenous infusion of [U-13C]lysine for 8 h. Total lysine use by the PDV was maximal 1 h after the meal (891 micromol x kg(-1) x h(-1)) and was predominantly of dietary origin (89%), paralleling the enteral delivery of dietary lysine. Intestinal lysine use returned to a low level after 4 h postprandially and was derived exclusively from the arterial supply until 8 h. Cumulative systemic appearance of dietary lysine reached 44 and 80% of the ingested amount 4 and 8 h after the meal, respectively, whereas the PDV first-pass use of dietary lysine was 55 and 32% of the intake for these two periods, respectively. We conclude that the first-pass utilization rate of dietary lysine by the PDV is directly increased by the enteral lysine availability and that it is higher with a bolus than with continuous oral feeding.     

Metabolic and hormonal effects of test meals with various protein contents in pigs

The postprandial release of immunoreactive insulin, glucagon, gastrin, somatostatin, pancreatic polypeptide (PP), and gastric inhibitory polypeptide (GIP) was studied in parallel with the absorption of sugars and amino acids in conscious pigs. Six pigs fitted with permanent catheters in the portal vein and arterial blood system as well as within an electromagnetic flow probe around the portal vein received successively at 3-day intervals, three meals of 800 g each containing 0, 14, or 28% protein (semisynthetic diets based on fish protein). Blood samples were collected and portal blood flow was recorded during a postprandial period of 8 h. For the same level of feed intake, an increase in the dietary protein concentration led to a higher alpha-amino nitrogen absorption and to a lower appearance of reducing sugars in the portal vein; in addition, the carbohydrate absorption efficiency (amounts absorbed as a percentage of amounts ingested) was reduced, showing the competition between the absorption of amino acids and glucose. The largest absorption occurred during the first 4 h after the meal, but neither the digestion of proteins nor that of carbohydrates were finished 8 h after the meal since portoarterial differences could still be observed. All test meals induced a rise of portal and peripheral concentrations of insulin, gastrin, somatostatin, and PP, and of the systemic level of GIP. Glucagon increased after the 28% protein meal only. The rise of plasma insulin paralleled that of blood glucose, and bore a significant positive relationship to the systemic GIP level in the early postprandial period. In terms of absolute amounts, portoarterial concentration gradients increased postprandially. Insulin release was significantly the highest after intake of the 14% protein diet. The gastrin response was significantly correlated to the amount of protein. Similarly the release of glucagon and somatostatin tended to increase with increasing dietary amount, but differences failed to reach significance (P less than 0.05), except for glucagon 2 h after the meal. There were very close relationships between the hourly amounts of alpha-amino nitrogen absorbed and gastrin and glucagon production, as between insulin and PP secretions. From the present results, the induction of physiological increments of plasma peptide concentration in 60-kg pigs would require infusion rates of about 50-250 micrograms/h for insulin, 1-4 micrograms/h for gastrin 17, 5-10 micrograms/h for glucagon and somatostatin, and 5-50 micrograms/h for PP.     

Estimating fermentative amino acid catabolism in the small intestine of growing pigs

Fermentative catabolism (FAAC) of dietary and endogenous amino acids (AA) in the small intestine contributes to loss of AA available for protein synthesis and body maintenance functions in pigs. A continuous isotope infusion study was performed to determine whole body urea flux, urea recycling and FAAC in the small intestine of ileal-cannulated growing pigs fed a control diet (CON, 18.6% CP; n=6), a high fibre diet with 12% added pectin (HF, 17.7% CP; n=4) or a low-protein diet (LP, 13.4% CP; n=6). ¹⁵N-ammonium chloride and ¹³C-urea were infused intragastrically and intravenously, respectively, for 4 days. Recovery of ammonia at the distal ileum was increased by feeding additional fibre when compared with the CON (P<0.05) but was not affected by dietary protein (0.24, 0.39 and 0.14 mmol nitrogen/kg BW/day for CON, HF and LP, respectively; P<0.05). Lowering protein intake reduced urea flux (25.3, 25.7 and 10.3 mmol nitrogen/kg BW/day; P<0.01), urinary urea excretion (14.4, 15.0 and 6.2 mmol N/kg BW/day; P<0.001) and urea recycling (12.1, 11.3 and 3.23 mmol nitrogen/kg BW/day; P<0.01) compared with CON. There was a rapid reduction in ¹⁵N-ammonia enrichment in digesta along the small intestine suggesting rapid absorption of ammonia before the distal ileum and lack of uniformity of enrichment in the digesta ammonia pool. A two-pool model was developed to determine possible value ranges for nitrogen flux in the small intestine assuming rapid absorption of ammonia. Maximum estimated FAAC based on this model was significantly lower when dietary protein content was decreased (32.9, 33.4 and 17.4 mmol nitrogen/kg BW/day; P<0.001). There was no impact of dietary fibre on estimates of small intestine nitrogen flux (P>0.05) compared with CON. The two-pool model developed in the present study allows for estimation of FAAC but still has limitations. Quantifying FAAC in the small intestine of pigs, as well as other non-ruminants and humans, offers a number of challenges but warrants further investigation.     

Interorgan ammonia, glutamate, and glutamine trafficking in pigs with acute liver failure

Ammonia reduction is the target for therapy of hepatic encephalopathy, but lack of quantitative data about how the individual organs handle ammonia limits our ability to develop novel therapeutic strategies. The study aims were to evaluate interorgan ammonia metabolism quantitatively in a devascularized pig model of acute liver failure (ALF). Ammonia and amino acid fluxes were measured across the portal drained viscera (PDV), kidneys, hind leg, and lungs in ALF pigs. ALF pigs developed hyperammonemia and increased glutamine levels, whereas glutamate levels were decreased. PDV contributed to the hyperammonemic state mainly through increased shunting and not as a result of increased glutamine breakdown. The kidneys were quantitatively as important as PDV in systemic ammonia release, whereas muscle took up ammonia. Data suggest that the lungs are able to remove ammonia from the circulation during the initial stage of ALF. Our study provides new data supporting the concept of glutamate deficiency in a pig model of ALF. Furthermore, the kidneys are quantitatively as important as PDV in ammonia production, and the muscles play an important role in ammonia removal.     

Selective partitioning of dietary fatty acids into the VLDL TG pool in the early postprandial period

Circulating triacylglycerol (TG) arises mainly from dietary fat. However, little is known about the entry of dietary fat into the major TG pool, very low-density lipoprotein (VLDL) TG. We used a novel method to study the specific incorporation of dietary fatty acids into postprandial VLDL TG in humans. Eight healthy volunteers (age 25.4 +/- 2.2 years, body mass index 22.1 +/- 2.3 kg/m2) were fed a mixed meal containing 30 g fish oil and 600 mg [1-13C]palmitic acid. Chylomicrons and VLDL were separated using immunoaffinity against apolipoprotein B-100. The fatty acid composition of lipoproteins was analyzed by gas chromatography/mass spectrometry. [1-13C]palmitic acid started to appear in VLDL TG 3 h after meal intake, and a similar delay was observed for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Approximately 20% of dietary fatty acids entered the VLDL TG pool 6 h after meal intake. DHA was clearly overincorporated into this pool compared with [1-13C]palmitic acid and EPA. This seemed to depend on a marked elevation of this fatty acid in the nonesterified fatty acid pool. In summary, the contribution of dietary fatty acids to early postprandial VLDL TG is substantial. The role of DHA in VLDL TG production will require further investigation.     

Effect of meal and propranolol on whole body and splanchnic oxygen consumption in patients with cirrhosis

Our aim was to measure whole body energy expenditure after a mixed liquid meal, with and without simultaneous propranolol infusion, in patients with cirrhosis. We also wanted to investigate the effect of propranolol on substrate fluxes and oxygen uptake in the tissues drained by the hepatic vein and azygos vein in the postprandial period in these patients. Whole-body oxygen uptake, hepatic blood flow, hepatic venous pressure gradient and net-hepatic fluxes of oxygen, lactate, glucose, glycerol, and free fatty acids (FFA) were measured in 12 patients with alcoholic cirrhosis before and for 2 h after ingestion of a mixed liquid meal (700 kcal). Half of the patients (n = 6) were randomized to a treatment group receiving intravenous infusion of propranolol in combination with the meal. The meal-induced energy expenditure was significantly lower in patients given propranolol [15.0 +/- 18.9 vs. 67.0 +/- 26.1 kJ/120 min (means +/- SD), P < 0.01]. Meal-induced whole body oxygen uptake was lower in patients receiving propranolol (19.2 +/- 38 vs. 135.7 +/- 61 mmol/120 min, P < 0.01), and the meal-induced increase in splanchnic oxygen uptake was nonexistent when propranolol was administered in combination (-13.2 +/- 34.8 vs. 110.4 +/- 34.8 mmol/120 min, P = 0.04). Postprandially, the propranolol group had a tendency toward a reduced splanchnic glucose output, and the FFA uptake was significantly reduced. Propranolol reduces meal-induced whole body oxygen uptake and energy expenditure as well as splanchnic oxygen uptake. The splanchnic reduction in oxygen consumption can explain almost the entire reduction in whole body oxygen consumption.     

Dietary protein source affects lipid metabolism in the European seabass (Dicentrarchus labrax)

The study was undertaken to evaluate the effects of dietary protein sources on lipogenesis and fat deposition in a marine teleost, the European seabass (Dicentrarchus labrax). Four isonitrogenous (crude protein (CP, Nx6.25), 44% DM) and isoenergetic (22-23 kJ/g DM) diets were formulated to contain one of the following as the major protein source: fish meal (FM), one of two soy protein concentrates (SPC) and corn gluten meal (CGM). Apparent digestibility coefficients of the diets and raw ingredients, as well as soluble nitrogen (ammonia and urea) and phosphorus excretion were measured. Growth rates of seabass fed plant protein-based diets were significantly lower than those fed fish meal based diet. The protein utilisation was strongly correlated to the dietary essential amino acids index. Measurements of N excretion (ammonia and urea nitrogen) confirmed these data. Daily fat gain at the whole body level ranged between 1.1 to 1.7 g/kg BW, with the highest values being recorded in fish fed the fish meal based diet. Levels of plasma triglycerides and cholesterol were lower in fish fed soy protein diets than in those fed the diet solely based on fish meal. Soy protein rich diets decreased the activities of selected hepatic lipogenic enzymes (glucose 6-phosphate dehydrogenase, malic enzyme, ATP-citrate lysase, acetylcoenzyme A carboxylase and fatty acid synthetase). Highest lipogenic enzyme activities where found in fish fed the fish meal diet, except for fatty acid synthetase which was increased in seabass fed the corn-gluten meal based diets. Overall data suggest that dietary protein sources affects fat deposition and the lipogenic potential in European seabass.     

Prandial lactate infusion inhibits spontaneous feeding in rats

To investigate the acute effects of lactate on spontaneous feeding, we infused lactate in the hepatic portal vein (0.5, 1.0, and 1.5 mmol lactate/meal) or in the vena cava (1.0 and 1.5 mmol lactate/meal) of ad libitum-fed rats during their first spontaneous nocturnal meal. Infusions (5 min, 0.1 ml/min) were remotely controlled, and a computerized feeding system recorded meal patterns. In separate crossover tests, meal size decreased independent of the infusion route after 1.0 and 1.5 mmol but not after 0.5 mmol lactate. The subsequent intermeal interval (IMI) tended to decrease only after vena cava infusion of 1.0 mmol lactate. The size of the second nocturnal meal increased after the 1.0 mmol lactate infusion. Hepatic portal infusion of 1.5 mmol lactate increased the satiety ratio [subsequent IMI (min)/meal size (g)] by 175%, which was higher than the insignificant 43% increase after vena cava infusion. Hepatic portal infusion of 1.5 mmol lactate also increased systemic plasma lactate but not glucose concentration at 1 min after the end of infusion. The results are consistent with the idea that meal-induced increases in circulating lactate play a role in the control of meal size (satiation). Moreover, the results suggest that lactate also contributes to postprandial satiety and that the liver is involved in this effect. The exact mechanisms of lactate's inhibitory effects on feeding and the site(s) where lactate acts to terminate meals remain to be identified.     

Cysteine fluxes across the portal-drained viscera of enterally fed minipigs: effect of an acute intestinal inflammation

Cysteine is considered as a conditionally indispensable amino acid. Its dietary supply should thus be increased when endogenous synthesis cannot meet metabolic need, such as during inflammatory diseases. However, studies in animal models suggest a high first-pass extraction of dietary cysteine by the intestine, limiting the interest for an oral supplementation. We investigated here unidirectional fluxes of cysteine across the portal-drained viscera (PDV) of multi-catheterized minipigs, using simultaneous intragastric l-[¹⁵N] cysteine and intravenous l-[3,3D2] cysteine continuous infusions. We showed that in minipigs fed with an elemental enteral solution, cysteine first-pass extraction by the intestine is about 60% of the dietary supply, and that the PDV does not capture arterial cysteine. Beside dietary cysteine, the PDV release non-dietary cysteine (20% of the total cysteine release), which originates either from tissue metabolism or from reabsorption of endogenous secretion, such as glutathione (GSH) biliary excretion. Experimental ileitis induced by local administration of trinitrobenzene sulfonic acid, increased liver and ileal GSH fractional synthesis rate during the acute phase of inflammation, and increased whole body flux of cysteine. However, cysteine uptake and release by the PDV were not affected by ileitis, suggesting an adaptation of the intestinal sulfur amino acid metabolism in order to cover the additional requirement of cysteine linked to the increased GSH synthesis. We conclude that the small intestine sequesters large amounts of dietary cysteine during absorption, limiting its release into the bloodstream, and that the other tissues of the PDV (colon, stomach, pancreas, spleen) preferentially use circulating methionine or cysteine-containing peptides to cover their cysteine requirement.     

Postprandial increases in nitrogenous excretion and urea synthesis in the Chinese soft-shelled turtle, <Emphasis Type="Italic">Pelodiscus sinensis</Emphasis>

The objective of this study was to determine the effects of feeding on the excretory nitrogen (N) metabolism of the aquatic Chinese soft-shelled turtle, Pelodiscus sinensis, with a special emphasis on the role of urea synthesis in ammonia detoxification. P. sinensis is ureogenic and possesses a full complement of ornithine-urea cycle enzymes in its liver. It is primarily ureotelic in water, and the estimated rate of urea synthesis in unfed animals was equivalent to only 1.5% of the maximal capacity of carbamoyl phosphate synthetase I (CPS I) in its liver. Approximately 72 h was required for P. sinensis to completely digest a meal of prawn meat. During this period, there were significant increases in ammonia contents in the stomach at hour 24 and in the intestine between hours 12 and 36, which could be a result of bacterial activities in the intestinal tract. However, ammonia contents in the liver, muscle, brain and plasma remained unchanged throughout the 72-h post-feeding. In contrast, at hour 24, urea contents in the stomach, intestine, liver, muscle, brain and plasma increased significantly by 2.9−, 3.5−, 2.6−, 2.9−, 3.4 and 3.0-fold, respectively. In addition, there was a 3.3- to 8.0−fold increase in the urea excretion rate between hours 0 and 36 post-feeding, which preceded the increase in ammonia excretion between hours 12 and 48. By hour 48, 68% of the assimilated N from the feed was excreted, 54% of which was excreted as urea-N. The rate of urea synthesis apparently increased sevenfold during the initial 24 h after feeding, which demanded only 10% of the maximal CPS I capacity in P. sinensis. The postprandial detoxification of ammonia to urea in P. sinensis effectively prevented postprandial surges in ammonia contents in the plasma and other tissues, as observed in other animals, during the 72-h period post-feeding. In addition, postprandial ammonia toxicity was ameliorated by increased transamination and synthesis of certain amino acids in the liver and muscle of P. sinensis. After feeding, a slight but significant increase in the glutamine content occurred in the brain at hour 24, indicating that the brain might experience a transient increase in ammonia and ammonia was detoxified to glutamine.     

Amino acids in the rat liver and plasma and some metabolites in the liver after sodium benzoate treatment

The effect of sodium benzoate administration on amino acids in the liver and plasma and various metabolites in the liver was studied. Changes in glutamine and ornithine were noted only at a higher dose (10 mmol/kg body wt) of benzoate, whereas even a lower dose caused a significant decrease in glycine, serine, and alanine levels of plasma and liver. A dose- and time-dependent decrease in glycine levels was studied. A decrease of up to 50% in the glycine concentration may limit its own transport into mitochondria and availability for the formation of hippurate. A decrease in alanine may have resulted from stimulation of gluconeogenesis from alanine, by increased ammonia. Among the metabolites studied, ATP and acetyl-CoA decreased and ammonia increased significantly even at a lower dose (5 mmol/kg body wt) of benzoate. The compounds that require ATP for their synthesis such as N-acetylglutamate and glutamine decreased significantly only at the higher dose of benzoate, whereas urea and glutathione levels were unaffected under our experimental conditions.     

Urea-triazone N characteristics and uses

Urea-triazone nitrogen (N) is a stable solution resulting from a controlled reaction in aqueous medium of urea, formaldehyde, and ammonia which contains at least 25% total N. This N source contains no more than 40%, nor less than 5%, of total N from unreacted urea and not less that 40% from triazone. All other N shall be derived from water-soluble dissolved reaction products of the above reactants. It is a source of slowly available N. The rate of mineralization of urea-triazone is about 66% that of urea after 8 days when incorporated in a Munjor sandy loam. Ammonia volatilization losses of N applied as urea-triazone were about 41% of those from urea on a Cecil sandy loam in the first week after application. N leaching losses through saturated Yolo loam columns of urea-triazone were about two thirds that of urea or nitrate N. This N source has proven to be a safer and more effective material for direct application on plant foliage. Tomato growth was enhanced with foliar application of urea-triazone relative to that obtained from ammonium nitrate or urea. The stability of this N source from potential losses via ammonia volatilization and nitrate leaching when soil applied is also documented by results from university trials.     

Uptake and transport of nonprotein nitrogen by the ruminant gut

Ruminants can use dietary or endogenous nonprotein nitrogen (N) to meet protein requirements largely because of the symbiotic relationship between the ruminant and its gut microbes. Because of gut fermentation, a substantial portion (16-80%) of N is absorbed as ammonia N (NH3N). Net uptake of NH3N by portal-drained viscera ranges from 0.4 to 6.5 times net uptake of alpha-amino N, with proportionally greater net uptake of NH3N with forage diets than with high-energy diets. Uptake of NH3N appears to be by diffusion; therefore, rates of absorption are controlled by factors regulating NH3N concentrations in chyme. Urea N is transferred directly to the lumen of the gut from blood and indirectly from blood as a constituent of saliva. Therefore, rate of urea transfer is controlled in part by blood concentrations of urea. However, other less clearly defined mechanisms relating to type of diet, ruminal fermentation patterns, and intraruminal concentrations of metabolites affect urea transfer to the rumen. Urea N transfer to the lumen of the gut ranges from 10 to 42% of N intake. Nucleic acid N is absorbed from the small intestine as part of purines and pyrimidines, some of which ruminants may incorporate directly in nucleotides. Estimated nucleic acid N absorption is 7-8% of N intake.     

The effect of CP concentration in the diet on urea kinetics and microbial usage of recycled urea in cattle: a meta-analysis

In ruminants, urea recycling is considered an evolutionary advantage. The amount of urea recycled mainly depends of the nitrogen (N) intake and the amount of organic matter (OM) digested in the rumen. Because recycled N contributes to meeting microbial N requirements, accurate estimates of urea recycling can improve the understanding of efficiency of N utilization and N losses to the environment. The objective of this study was to evaluate urea kinetics and microbial usage of recycled urea N in ruminants using a meta-analytical approach. Treatment mean values were compiled from 25 studies with ruminants (beef cattle, dairy cows and sheep) which were published from 2001 to 2016, totalling 107 treatment means. The data set was analyzed according to meta-analysis techniques using linear or non-linear mixed models, taking into account the random variations among experiments. Urea N synthesized in the liver (UER) and urea N recycled to the gut (GER) linearly increased (P<0.001) as N intake (g/BW^0.75) increased, with increases corresponding to 71.5% and 35.2% of N intake, respectively. The UER was positively associated (P<0.05) with dietary CP concentration and the ratio of CP to digestible OM (CP:DOM). Maximum curvature analyses identified 17% dietary CP as the point where there was a prominent increase in hepatic synthesis of urea N, likely due to an excess of dietary N leading to greater ammonia absorption. The GER:UER decreased with increasing dietary CP concentration (P<0.05). At dietary CP⩾19%, GER:UER reached near minimal values. The fraction of UER eliminated as urinary urea N and the contribution of urea N to total urinary N were positively associated with dietary CP (P<0.05), both reaching values near the plateau when dietary CP was 17%. The fractions of GER excreted in the feces and utilized for anabolism decreased, whereas the fraction of GER returned to the ornithine cycle increased with dietary CP concentration (P<0.05). Recycled urea N assimilated by ruminal microbes (as a fraction of GER) decreased as dietary CP and CP:DOM increased (P<0.05). The efficiency of microbial assimilation of recycled urea N was near plateau values at 194 g CP/kg DOM. The models obtained in this study contribute to the knowledge on N utilization, and they could be used in feeding models to predict urea recycling and thus to improve formulation of diets to reduce N losses that contribute to air and water pollution.     

Metabolic flux measurements across portal drained viscera, liver, kidney and hindquarter in mice

A method was developed to measure metabolic fluxes across either portally-drained viscera (PDV) and liver or kidney and hindquarter (HQ) in anesthetized mice. The method includes a primed-constant infusion of ketamine-medetomidine anaesthesia to stabilize the mice for the surgical procedures. For measurement of metabolic fluxes across PDV and liver, blood sampling catheters were inserted in the carotid artery, portal vein and hepatic vein and infusion catheters in the jugular vein and mesenteric vein. For measurement of metabolic flux across kidney and HQ, blood sampling catheters were inserted in the carotid artery, renal vein and caval vein and infusion catheters in the jugular vein and abdominal aorta. 14C-PAH was infused to enable plasma flow measurement using an indicator dilution method. In addition, we developed a blood sampling procedure without waste of blood. We measured plasma flow and metabolic fluxes across PDV, liver, kidney and HQ. Mean plasma flow in post-absorptive mice was: PDV: 0.9+/-0.2, liver: 1.2+/-0.3, kidney: 1.0+/-0.1, HQ: 1.1+/-0.3 ml/10 g body weight (b.w.)/min. Significant glutamine release by the HQ and uptake of glutamine by the kidney and PDV was observed. In PDV, citrulline is produced from glutamine and is in turn used by the kidney for the production of arginine. In conclusion, the described model enables measurement of metabolic fluxes across PDV, liver, kidney and HQ in mice. The availability of such a small animal model allows the potential for measuring metabolic parameters in transgenic and knockout mice, and therefore may lead to an important refinement in metabolic research.     

Effect of dietary protein on the capacity of urea synthesis in rats

The in vivo capacity of urea nitrogen synthesis (CUNS) during alanine stimulation was measured within the blood amino acid concentration interval 7.3-11.6 mmol/l, where urea synthesis is at maximum and independent of substrate concentration. Three groups of rats were fed for 14 days, either a low protein diet (8%), a normal diet (17%), or a high protein diet (53%). Diet protein modified both CUNS and plasma glucagon concentration. CUNS was 5.86 +/- 2.93, 7.43 +/- 2.16, and 19.31 +/- 4.32 mumol/(min.100 g BW) (mean +/- SD, N = 6), respectively. The corresponding plasma glucagon concentrations after alanine stimulation were 222 +/- 400, 633 +/- 229, and 1700 +/- 627 ng/l, respectively. The in vivo kinetics of urea production is regulated by dietary protein, possibly via glucagon. This implies that the liver plays an active part in adaptation of whole body nitrogen homeostasis to dietary changes.