Metabolism of urea in late pregnancy and the possible contribution of amino acid carbon to glucose synthesis in sheep

Effects of exogenous urea infusion on glucose metabolism in acute heat stressed swamp buffaloes (Bubalus bubalis)

The effects of intravenous urea infusion on glucose turnover, glucose carbon recycling, glucose pool size and glucose clearance were studied in buffaloes kept in either normal ambient temperature or acute heat exposure. Heat stressed animals showed increases in glucose turnover rate, plasma glucose concentration and glucose clearance but decreased glucose carbon recycling. A marked reduction of glucose turnover and glucose clearance associated with increased plasma glucose concentration in heat stressed animals after urea infusion reflects under-utilization of this compound. Mechanisms involved in glucose metabolism during urea infusion in buffaloes are discussed.

Nitrogen and carbon flows between the caecum, blood and rumen in sheep given chopped lucerne (Medicago sativa) hay

1. Experiments involving 15N and 14C tracers were made in sheep consuming 800 g air-dry chopped lucerne (Medicago sativa) hay/d and providing 20.4 g N/d to study N and C flows within the caecal digesta and between the caecum, blood and rumen. 2. Continuous infusions of 15N tracers were made into the caecal ammonia, blood urea and rumen NH3 pools. The concentration and enrichment of caecal digesta NH3-N, caecal microbial N, caecal digesta non-urea, non-ammonia-N (NU-NAN), faecal NU-NAN, blood urea-N, rumen digesta NH3-N and rumen bacterial N were estimated at intervals during the infusions. A three-pool open-compartment model was solved to estimate N flows between the caecal digesta NH3-N, blood urea-N and rumen digesta NH3-N pools. 3. The rate of irreversible loss from the caecal digesta NH3-N pool was 2.17 (SE 0.623) g N/d. On average 0.9 (SE 0.56) g N/d of caecal digesta NH3-N was derived from blood urea and 0.1 (SE 0.08) g caecal digesta NH3-N/d was apparently derived from the fermentation of undigested rumen microbes in the caecum. The amount of NH3-N produced by proteolysis and deamination of dietary and endogenous N was 1.1 (SE 0.13) g/d. 4. There was net incorporation of 0.56 (SE 0.306) g caecal digesta NH3-N/d into caecal microbes. The microbial N synthesized de novo in the caecum was not determined, but 2.9 (SE 0.52) g microbial N/d of both rumen and caecal origin flowed out of the caecum and constituted 0.48 of the NU-NAN flow. The majority (mean 0.83 (SE 0.044] of this microbial N was excreted in faeces. 5. On average 1.8 (SE 0.80) g caecal digesta NH3-N/d were absorbed. Of this NH3-N, 0.92 (SE 0.054) was converted to blood urea, contributing 0.10 (SE 0.031) of blood urea-N. Only 0.012 (SE 0.0041) of rumen digesta NH3-N and 0.005 (SE 0.0009) of rumen bacterial N were derived from caecal digesta NH3-N. 6. Infusions of 14C tracers were made into the caecal digesta bicarbonate, blood bicarbonate, rumen digesta bicarbonate and blood urea pools, and samples were obtained at intervals to determine the specific radioactivity of each pool. A four-pool open-compartment model was solved to estimate C flows between these pools. 7. The rate of irreversible loss of blood urea estimated with [14C]urea (17.1 (SE 1.18) g N/d) was greater (P less than 0.01) than that estimated with [15N]urea (14.0 (SE 0.87) g N/d).(ABSTRACT TRUNCATED AT 400 WORDS)

Foetal and maternal rates of urea production and disposal in well-nourished and undernourished sheep

1. Five monotocous and two ditocous Scottish Blackface ewes with indwelling catheters in an umbilical artery and vein of one foetus only and in both maternal jugular veins were used. 2. Experiments were conducted over a period of 2 d when [14C]urea was infused intravenously over 9 h into either the mother or foetus, separate days being used for each infusion. Two series of experiments were completed, one in well-nourished and the other in undernourished sheep at 125-141 and 138-143 d of gestation respectively. 3. Plasma urea specific radioactivities of the mother and infused foetus at plateau were used to determine the urea flux-rates within and between mother and foetus. The mean rate of foetal urea production (Ffo) was 1.45 and 1.63 mg/min per kg foetus in well-nourished and undernourished ewes respectively. The corresponding rates of maternal urea production (Fmo) were 0.49 and 0.37 mg/min per kg live weight respectively and there was a close correlation between the rate of maternal urea disposal (Fom) and the dietary nitrogen intake. 4. The values of Ffo were used to calculate the maximum potential for foetal gluconeogenesis from deaminated amino acids. These calculations were compared with published information on the over-all rates of foetal gluconeogenesis in well-nourished ewes. 5. The foetal entry rate of urea expressed on a body-weight basis was high, approximately 8.5 times that of the mother, but it was a threefold overestimate of Ffo. The maternal entry rate was 1.3 times Fmo and the significance of this in relation to assessing differences in urea entry rates in pregnant and non-pregnant sheep is discussed.

Glucose and urea kinetics in cows in early lactation

1. Four mature Friesian cows were used. For 2 weeks after calving they were fed 3 kg hay (147 g crude protein (nitrogen X 6.25; CP)/kg dry matter (DM) plus 9 kg concentrates (177 g CP/kg DM) per d and thereafter 4 kg hay plus 12 kg concentrates/d. 2. At approximately the 2nd, 4th and 9th weeks post-partum each cow was given a single intravenous dose of [14C]urea (1 mCi) and [6-3H]glucose (1.5 mCi). Following this, jugular venous blood samples were withdrawn up to 26 h post-injection. 3. The log (specific activity) v. time curves were not linear for either metabolite. Glucose irrversible loss rates (IRL) were calculated by a stochastic procedure. The linear part of the urea log (specific activity) v. time curve gave the same estimate for urea IRL rate as a stochastic method of calculation, but urea pool sizes were overestimated so that stochastic analysis of results was preferred. 4. Mean milk yields at the time of the three measurements were 27.1, 30.8 and 27.9 kg milk/d. 5. Urea IRL was significantly lower (P < 0.05) in the first 4 weeks of lactation than in the 9th, and blood urea concentration was lower (P < 0.05) in the 4th than in the 2nd and 9th week, but there was no close correlation between urea IRL and concentrations. 6. Glucose IRL rose (P < 0.05) between the 2nd and 9th weeks post-partum, perhaps due to the increase in feed intake. The results were generally consistent with the relationship between milk yield and glucose IRL established by Paterson & Linzell (1974). 7. Urea IRL (y) and milk protein-N (x) production values were used to calculate the efficiency of protein utilization (EPU) as x divided by (x+0.35 y). This assumes that 0.35 of urea IRL was due to protein catabolism. EPU was found to be 0.59-0.80 (mean 0.69). 8. The possible contribution of catabolized amino acids to glucose IRL was calculated by assuming that 0.35 of urea IRL was due to protein catabolism and 0.2 of catabolized protein-C passed through the glucose pool. On this basis the contribution of protein-C top glucose-C was < 25 g/kg glucose IRL.

Gluconeogenesis in cattle: significance and methodology

Gluconeogenesis is a continual process that is of great importance in ruminants because almost all dietary carbohydrates are fermented to volatile fatty acids in the rumen. In turn, propionate is the only major volatile fatty acid that contributes to gluconeogenesis. Many different techniques and analytical procedures are involved in studying ruminant gluconeogenesis. Glucose kinetics can be examined by single-injection or continuous-infusion isotope dilution techniques with a variety of glucose labels. Correcting for recycling of label is an important consideration. Absorption of glucose from the gut can be measured by combining arterial-venous differences and flow rates of portal blood or can be estimated by determining the amount of glucose plus alpha-glucose polymers passing into the small intestine. Production of propionate in the ruminoreticulum can be measured by isotope dilution techniques. Quantitating the conversion of propionate to glucose requires the use of [carbon-14]propionate with careful corrections for propionate carbon entering the citric acid cycle before incorporation into glucose; The same fundamental techniques used with propionate are required to quantitate the contributions of amino acids and other precursors to glucose. In vitro studies of gluconeogenic enzymes, and cellular, tissue, or organ preparations provide valuable insights into the gluconeogenic processes and controls but must be validated by in vivo experiments. Progress has been considerable in understanding some aspects of ruminant gluconeogenesis, but many more studies will be required to obtain a complete understanding.

Further studies of the dynamics of nitrogen metabolism in sheep

1. A study of ammonia and urea metabolism in sheep was made using isotope dilution techniques with (15NH4)2SO4,[15N]urea and [14C]urea in order to determine quantitatively the movements of urea-N and NH3-N throughout the body of normal, feeding sheep. 2. Single injections of 15N-labelled compounds were made into the rumen fluid NH3, caecal fluid NH3 and the blood urea pools, in order to estimate the rates of flux through, and the transfer of N between, these and other nitrogenous pools in the body. 51CrEDTA was injected into the rumen and caecum with (15NH4)2SO4 to allow estimation of fluid volumes and to provide an indication of mixing, and of times of transit of isotopes between different sampling sites in the digestive tract. 3. The sheep ate approximately 22 g lucerne chaff/h and the mean dietary N intake was 16-3 g/d. 4. The rate of flux of NH3 through the rumen NH3 pool was 15-0 g/d (i.e. 90% of the dietary N ingested; however, this amount also included N from plasma urea (1-1 g/d) and other endogenous sources including NH3 derived from caecal NH3 (0-4 g/d). 5. Only 40% of the N in isolated rumen bacteria was derived from NH3, indicating that a considerable proportion of their N requirements were obtained from compounds other than NH3 (e.g. peptides and amino acids). 6. There was evidence of recycling of N between nitrogenous pools in the rumen, probably through rumen NH3 leads to microbial N leads to NH3. 7. It was estimated that 5-3 g blood urea-N/d entered the digestive tract; 20% of this urea was degraded in the rumen, 25% in the caecum and the remainder was apparently degraded elsewhere; there was evidence of urea degradation in the large intestine posterior to the caecum and it is suggested that urea degradation and absorption of the resultant NH3 may occur in the ileum. 8. Of the 4-8 g N/d entering the caecal NH3 pool, 4-2 g N/d left and did not return and the difference (0-6 g N/d) was recycled, possibly through caecal NH3 leads to microbial N leads to NH3. 9. A large proportion of the NH3 entering the caecal NH3 pool (70% or 3-2 g N/d) was apparently derived from degradation of nitrogenous products, other than urea, including rumen microbial N (1-0 g N/d) passing undigested from the small intestine. 10. Less than half the NH3-N of caecal origin entering the rumen passed through the blood urea pool; the remainder was apparently transported as other nitrogenous compounds in the blood or body fluids. 11. The results of the three experiments were combined in a general three-pool, open-compartment model which formally recognizes an unlimited number of other unspecified, interconnected pools together comprising the whole-animal system. Rates of flux through, and transfer of N between these and other nitrogenous pools in the body were calculated by solving this model and the information derived has been applied to whole-animal models with a view to subsequently using these models in computer simulation studies.