The relationship between urea and pyrimidine de novo synthesis in ruminant liver (original) (raw)
Related papers
Animal Feed Science and Technology, 2006
Three experiments were conducted with Angus or Holstein steers to evaluate effects of dietary urea-calcium (a slow rumen-release urea source) on absorption of ammonia N from the gut and urea N production in the liver. Steers were fed a high-grain diet (Experiment 1) or an all-forage diet (Experiments 2 and 3). Urea or urea-calcium (0.25 g/kg body weight) was dosed into the esophagus (Experiments 1 and 2) or rumen (Experiment 3), and blood samples were serially collected for 180 min. Blood concentrations of ammonia N and urea N were measured in all experiments, and net flux of metabolites across splanchnic tissues was measured in Experiment 3. Compared to urea, urea-calcium reduced (P<0.05) plasma concentrations of ammonia N in steers fed all-forage diets, and tended (P<0.06) to reduce arterial glucose concentrations in Experiment 3. Plasma concentrations of urea N were not affected by treatment in any experiment. Treatment and time post-dosing interactions (P<0.05) in Experiment 3 were due to increased ruminal fluid concentrations of ammonia N, net release of ammonia N by portal-drained viscera and total Abbreviations: ADF, acid detergent fibre; AUC, area under the curve; BW, bodyweight; CP, crude protein; DM, dry matter; NDF, neutral detergent fibre; PDV, portal-drained viscera; TSP, total splanchnic splanchnic tissues with urea versus urea-calcium treatment shortly after dosing. Similar interactions (P<0.05) indicated that urea caused higher hepatic glucose release and increased l-lactate release by total splanchnic tissues after dosing than urea-calcium. Urea-calcium was effective in mitigating rapid ammonia release in the rumen and subsequent effects on glucose and lactate metabolism.
Dynamic aspects of ammonia and urea metabolism in sheep
British Journal of Nutrition, 1972
1. To obtain a quantitative model for nitrogen pathways in sheep, a study of ammonia and urea metabolism was made by using isotope dilution techniques with [15N]ammonium sulphate and [15N]urea and [14C]urea.2. Single injection and continuous infusion techniques of isotope dilution were used for measuring ammonia and urea entry rates.3. Sheep were given 33 g of chaffed lucerne hay every hour; the mean dietary N intake was 23.4 g/d.4. It was estimated that 59% of the dietary N was digested in the reticulo-rumen; 29% of the digested N was utilized as amino acids by the micro-organisms, and 71% was degraded to ammonia.5. Of the 14.2 g N/d entering the ruminal ammonia pool, 9.9 g N/d left and did not return to the pool, the difference of 4.3 g N/d represented recycling, largely within the rumen itself (through the pathways: ruminal ammonia → microbial protein → amino acids → ammonia).6. Urea was synthesized in the body at a rate of 18.4 g N/d from 2.0 g N/d of ammonia absorbed through th...
Practical aspects of urea and ammonia metabolism in ruminants
1999
Nitrogen was recognized over 200 yr ago as an element essential for normal function of farm animals. During the first half of the 19th century, the roles of proteins and urea in N metabolism were discovered. By the middle of the 20th century, the substrates, products, and enzymes of the urea cycle were elucidated. Work since then has quantified dietary crude protein requirements for specific production goals, protein synthesis and breakdown, ruminal ammonia production, endogenous urea synthesis, and urea recycling. In ruminants fed conventional diets, N absorbed as ammonia can be several times the amount of N absorbed in the form of amino acids or peptides. Nitrogen recycled to the digestive tract as urea in saliva or urea transported from blood ranges from 10 to 40% of N consumed in feed. Under production conditions, from 0 to 20% of N consumed by ruminants is retained as tissue N or excreted as milk protein. This review describes the quantitative aspects of urea and ammonia metabolism in ruminants and it relates the metabolic or economic costs of that metabolism to practical feeding situations. The review concludes with a discussion of conflicts and considerations among three main priorities in ruminant N metabolism: 1) maximizing microbial function in the rumen; 2) optimizing amino acid supply to the host ruminant; and 3) minimizing negative environmental effects of cycling N through ruminant production systems.
Journal of Animal Physiology and Animal Nutrition, 2014
Changes in N balance, urinary excretion of purine derivative (PD), urea, creatinine and ammonia and plasma ammonia, glucose, urea, insulin and IGF-1 were examined in four wethers (37 AE 2.6 kg BW). The animals were fitted with permanent ruminal catheters, fed lucerne hay (9.4 MJ/day; 23 g N/day; 7 g soluble N/day, 6 equal meals/day) and treated with contrasting rates of urea infusion into the rumen: first, a continuous infusion (CT), at 3.2 mg urea-N/min for 10 days and then a discontinuous infusion (DT) at 156 mg urea-N/min for 4 min; in 6 daily doses with the meals for 7 days. N balance was calculated from pooled samples of faeces and urine. Jugular blood samples were collected before and 1.5 h after the morning meal (M1) on days CT10, DT2, DT4 and DT6. N retention decreased during DT (p = 0.01) due to a significant increase of N excretion in urine (4 g/day; p = 0.009) and faeces (1 g/day; p = 0.02). Dry matter (p < 0.001) and N digestibility in vivo (p = 0.01) decreased significantly during DT. Urinary urea and PD excretion were not altered by treatment. Significant linear (p = 0.004) and quadratic (p = 0.001) effects were observed for plasma ammonia in M1 (from 170 CT10 to 235 lM DT2 and returned to 120 lM DT6). No changes were observed in plasma glucose, urea, insulin and IGF-1. Results indicate that changes from CT to DT reduced N retention in sheep due to enhanced urinary N excretion, but it was not associated with changes in urinary urea or PD excretion; or plasma concentrations of insulin and IGF-1. As the dry matter (DM) an N digestibility could account a 0.23 of the decrease in N retention; the largest fraction of the reduction in N retention remained unexplained by the results.
Detoxication of Ammonia in Sheep Fed Soy Protein or Urea
The Journal …, 1970
Urea-fed sheep were able to detoxify additional ammonia absorbed from the digestive tract by a mechanism involving increased concentrations of liver ornithine. Feeding urea as the sole nitrogen source caused decreases in activities of carbamyl phosphate synthetase, ornithine transcarbamylase and argüÃ-ase while no differences were noted in activities of arginine synthetase and argininosuccinase. Decreases in these enzyme systems were concluded to be the result of ammonia caus ing derangements in cellular energy metabolism or a suboptimum amino acid nutritive in urea-fed animals, both of which could cause decreases in enzyme synthesis. Although activity of the rate-limiting enzyme system, arginine synthetase, was sufficient over a 24-hour period to account for the quantity of urea excreted, reserve capacity was minimal. It was suggested that for several hours after feeding a urea diet, liver ammonia detoxication mechanisms may be exceeded. In view of the effects of ammonia on cellular energy metabolism, part of the lowered productive capacities obtained when urea diets are fed may be due to ammonia-producing biochemical derangements in liver and other tissues.
Effect of ammonia on Na transport across isolated rumen epithelium of sheep is diet dependent
British Journal of Nutrition, 2003
The cellular uptake of ammonia affects the intracellular pH (pH i ) of polar and non-polar cells. A predominant uptake of NH 3 and its intracellular protonation tend to alkalinise the cytoplasm, whereas a predominant uptake of NH 4 þ acidifies the cytoplasm by reversing this reaction. Hence, the well-known absorption of ammonia across the rumen epithelium probably causes a change in the pH i . The magnitude and direction of this change in pH i (acid or alkaline) depends on the relative transport rates of NH 3 and NH 4 þ . Consequently, the intracellular availability of protons will influence the activity of the Na þ -H þ exchanger, which could affect transepithelial Na þ transport. The aim of the present study has been to test this possible interaction between ruminal ammonia concentrations and Na þ transport. The term ammonia is used to designate the sum of the protonated (NH 4 þ ) and unprotonated (NH 3 ) forms. Isolated ruminal epithelium of sheep was investigated by using the Ussing-chamber technique in vitro. The present results indicate that ammonia inhibits Na þ transport across the rumen epithelium of hay-fed sheep, probably by binding intracellular protons and thus inhibiting Na þ -H þ exchange. By contrast, ammonia stimulates Na þ transport in concentrate-fed and urea-fed sheep, which develop an adaptation mechanism in the form of an increased metabolism of ammonia in the rumen mucosa and/or an increased permeability of rumen epithelium to the charged ammonium ion (NH 4 þ ). Intracellular dissociation of NH 4 þ increases the availability of protons, which stimulate Na þ -H þ exchange. This positive effect of ruminal ammonia on Na þ absorption may significantly contribute to the regulation of osmotic pressure of the ruminal fluid, because intraruminal ammonia concentrations up to 40 mmol/l have been reported.
British Journal of Nutrition, 1996
Changes in splanchnic energy and N metabolism were studied in sheep, prepared with vascular catheters across the portal-drained viscera (PDV) and the Liver, and maintained on supramaintenance intakes of either grass or grass + barley pellets. The animals were challenged, on both diets, with 4 d intra- mesenteric vein infusions of NH4CI (25 µmol/min) plus NH4HCO3 (at either 0 or 125 µmol/min). On the final day of each treatment the natural abundance NH4Cl was replaced with 15NH4Cl over a 10 h infusion while over the same period [l-13C]leucine was infused via a jugular vein. Measurements were made of blood flow plus mass transfers of NH3, urea, free amino acids and O2, across the PDV and liver. Enrichments of [14N15N]urea and [15N15N]urea plus [15N]glutamine, aspartate and glutamate were also monitored. Whole-body urea flux was determined by infusion of [14C]urea. At the end of the study the animals were infused for 3 h with 15NH4CI, killed and liver samples assayed for intracellular ...
Modulation of sheep ruminal urea transport by ammonia and pH
American journal of physiology. Regulatory, integrative and comparative physiology, 2014
Ruminal fermentation products such as short-chain fatty acids (SCFA) and CO2 acutely stimulate urea transport across the ruminal epithelium in vivo, whereas ammonia has inhibitory effects. Uptake and signaling pathways remain obscure. The ruminal expression of SLC14a1 (UT-B) was studied using polymerase chain reaction (PCR). The functional short-term effects of ammonia on cytosolic pH (pHi) and ruminal urea transport across native epithelia were investigated using pH-sensitive microelectrodes and via flux measurements in Ussing chambers. Two variants (UT-B1 and UT-B2) could be fully sequenced from ovine ruminal cDNA. Functionally, transport was passive and modulated by luminal pH in the presence of SCFA and CO2, rising in response to luminal acidification to a peak value at pH 5.8 and dropping with further acidification, resulting in a bell-shaped curve. Presence of ammonia reduced the amplitude, but not the shape of the relationship between urea flux and pH, so that urea flux remai...
Journal of Dairy Science, 1980
Catalyzed phenol-hypochlorite and ninhydrin colorimetric procedures were adapted to the Technicon AutoAnalyzer for simultaneous determination of ammonia and total amino acids in ruminal fluid or ruminal in vitro media. The manifold developed was compatible with a sampling rate of 40/h without significant sample-to-sample carryover. With proper storage, reagents for both the phenol-hypochlorite and the air-stable ninhydrin systems were stable for 8 mo or more. Response of individual amino acids in the phenol-hypochlorite system were generally 1% or less than equimolar amounts of ammonia. Certain amino acids inhibited ammonia color yield 10 to 15% when with equimolar amounts of ammonia; however, the inhibitory effect of casein amino acids was only 2 to 3%. Although ninhydrin response, relative to leucine, of individual alpha-amino acids ranged from 62 (valine) to 151% (histidine), recoveries of casein amino acids from ruminal fluid had coefficients of variation of 1% or less. Coefficients of variation for ammonia recoveries from ruminal fluid by the phenol-hypochlorite procedure were about half of those for the Conway microdiffusion technique. Intraclass correlations for the adapted procedures indicated high degrees of accuracy and precision for both ammonia and amino acid analyses.
British Journal of Nutrition, 2002
This study investigates the effects of increased NH 3 or amino acid supply on glutamine utilisation and production by the splanchnic tissues of fed sheep. Six sheep, prepared with vascular catheters in the aorta, mesenteric, portal and hepatic veins, were fed grass pellets to 1·1 £ energy maintenance requirements. Each treatment involved a 4 d abomasal infusion, of either ammonium bicarbonate (AMM; 23·4 mmol/kg 0·75 per min), water (CONT), or a mixture of amino acids that excluded glutamine and glutamate (AA; 46·8 mmol amino acid-N/kg 0·75 per min). The treatments simulated nutritional extremes in terms of the balance of absorbed N. Kinetics across the whole gut and the liver were monitored during an intra-jugular infusion of [5-15 N]glutamine. Blood flow across the whole gut or liver were unaffected by treatment. Both AMM and AA infusions doubled the hepatic release of urea-N compared with CONT (P,0·02). AA infusion decreased arterial glutamine concentration by 26 % (P,0·01) and 23 % (P,0·05) compared with AMM and CONT respectively. Despite this, whole-body glutamine flux was not affected by treatment. In contrast, AMM infusion increased hepatic glutamine production by 40 % compared with CONT (P,0·02). This provided a mechanism to ensure NH 3 supply to the periphery was maintained within the normal low physiological levels. Hepatic glutamine utilisation tended to increase during AA infusion, probably to ensure equal inflows of N to the ornithine cycle. Between 6 and 10 % of NH 3 absorbed across the digestive tract was derived from the amido-N of glutamine. Overall, splanchnic glutamine utilisation accounted for 45 -70 % of whole-body glutamine flux.