Effects of different supplements on splanchnic oxygen consumption and net fluxes of nutrients in sheep consuming bromegrass (Bromus inermis) hay ad libitum

Nutritional regulation of the anabolic fate of amino acids within the liver in mammals: concepts arising from in vivo studies

At the crossroad between nutrient supply and requirements, the liver plays a central role in partitioning nitrogenous nutrients among tissues. The present review examines the utilisation of amino acids (AA) within the liver in various physiopathological states in mammals and how the fates of AA are regulated. AA uptake by the liver is generally driven by the net portal appearance of AA. This coordination is lost when demands by peripheral tissues is important (rapid growth or lactation), or when certain metabolic pathways within the liver become a priority (synthesis of acute-phase proteins). Data obtained in various species have shown that oxidation of AA and export protein synthesis usually responds to nutrient supply. Gluconeogenesis from AA is less dependent on hepatic delivery and the nature of nutrients supplied, and hormones like insulin are involved in the regulatory processes. Gluconeogenesis is regulated by nutritional factors very differently between mammals (glucose absorbed from the diet is important in single-stomached animals, while in carnivores, glucose from endogenous origin is key). The underlying mechanisms explaining how the liver adapts its AA utilisation to the body requirements are complex. The highly adaptable hepatic metabolism must be capable to deal with the various nutritional/physiological challenges that mammals have to face to maintain homeostasis. Whereas the liver responds generally to nutritional parameters in various physiological states occurring throughout life, other complex signalling pathways at systemic and tissue level (hormones, cytokines, nutrients, etc.) are involved additionally in specific physiological/nutritional states to prioritise certain metabolic pathways (pathological states or when nutritional requirements are uncovered).

Relation of net portal flux of nitrogen compounds with dietary characteristics in ruminants: a meta-analysis approach

Decrease of N intake (NI) with the aim of increasing efficiency of N utilization and decreasing the negative environmental effects of animal production requires assessment of the forms in which N is absorbed. A meta-analysis was conducted on 68 publications (90 experiments and 215 treatments) to study the effect of NI on net portal appearance (NPA) of nitrogenous nutrients [amino acids (AA), ammonia, and urea] in ruminants. In addition, the effect of several dietary energy and protein factors on this relationship was investigated. These factors were: dry matter intake; proportion of concentrate; diet concentrations and intakes of nonfiber carbohydrates and neutral detergent fiber (NDF); diet concentrations of total digestible nutrients (TDN) and crude protein; rumen-degradable protein and rumen-undegradable protein, as percent dry matter or percent crude protein. The effect of species and physiological stage was also investigated. Within-experiment analyses revealed that the NPA of AA-N and ammonia-N increased linearly, whereas the NPA of urea-N decreased (or recycling of urea-N increased) linearly with NI. Besides NI, many significant covariates could be introduced in each NPA model. However, only TDN and neutral detergent fiber intake (NDFi) were common significant covariates of NI in each NPA model. In this database, ruminants converted 60% of incremental NI into NPA of AA-N with no species effect on that slope. However, at similar NI, TDN, and NDFi, sheep absorbed more AA-N than did cattle and dairy cows. On the other hand, species tended to affect the slope of the relationship between NPA of ammonia-N and NI, which varied from 0.19 for the sheep to 0.38 for dairy cows. On average, the equivalent of 11% of incremental NI was recycled as urea-N to the gut through the portal-drained viscera, which excludes salivary contribution, and no species difference was detected. Overall, at similar TDN and NDFi, sheep and cattle increased their NPA of AA-N relative to NI increment by a similar magnitude. The higher absorption of AA-N observed in sheep compared with cattle, at similar NI, TDN, and NDFi, might result from lower losses of AA through portal-drained viscera metabolism.

Effects of ruminal protein degradability and frequency of supplementation on nitrogen retention, apparent digestibility, and nutrient flux across visceral tissues in lambs fed low-quality forage

Two experiments were conducted to determine the effect of ruminal protein degradability and supplementation frequency on intake, apparent digestibility, N retention, and nutrient flux across visceral tissues of lambs fed a low-quality forage diet. In both experiments, wethers were fed a basal diet of mature crested wheatgrass hay (4.2% CP) for ad libitum consumption plus 1 of 4 supplements: 1) a high RDP supplement provided daily (RDP-D), 2) the high RDP supplement provided on alternate days (RDP-A), 3) a high RUP provided on alternate days (RUP-A), or 4) a 50:50 mixture of the RDP and RUP supplements provided on alternate days. In Exp. 1, 12 lambs (29.9 +/- 2.7 kg initial BW) were used. Forage OM, NDF, and ADF intake were not affected by treatment. Total tract digestibilities (OM, NDF, ADF, and N) were unaffected (P >or= 0.15) by treatment. Neither protein degradability nor supplementation frequency had an effect (P >or= 0.52) on N retention. In Exp. 2, 15 lambs (34 +/- 4 kg initial BW) fitted with indwelling catheters in a hepatic vein, the hepatic portal vein, a mesenteric vein, and a mesenteric artery were used. Release of ammonia N by the portal-drained viscera (PDV) was reduced (P = 0.004) in alternate-day-supplemented lambs compared with RDP-D. Consequently, hepatic uptake of ammonia N was least (P = 0.003) in all alternate-day lambs. Alpha-amino nitrogen (AAN) release by the PDV and hepatic uptake of AAN were not affected by treatment or supplementation frequency. Additionally, hepatic output and PDV uptake of urea N were not affected by treatment. Hepatic N uptake (ammonia N + AAN) accounted for urea synthesized by the liver in all treatments; however, hepatic urea synthesis was approximately 4.5-fold less for RUP-A lambs. This suggests that the provision of AA as RUP may provide a delay in ureagenesis, thus altering the timing of N recycling.

Effects of supplemental ruminally degradable protein versus increasing amounts of supplemental ruminally undegradable protein on nitrogen retention, apparent digestibility, and nutrient flux across visceral tissues in lambs fed low-quality forage

Two experiments were conducted to determine effects of supplemental ruminally degradable protein (RDP) vs. increasing amounts of supplemental ruminally undegradable protein (RUP) on intake, apparent digestibility, N retention, and nutrient flux across visceral tissues in lambs fed low-quality forage. Lambs were fed a basal diet of crested wheatgrass hay (4.2% CP) for ad libitum consumption, plus 1 of 4 protein supplements: isolated soy protein (RDP source) fed to meet estimated RDP requirements (CON), or corn gluten meal (RUP source) fed at 50, 100, or 150% of the supplemental N provided by CON (C50, C100, and C150, respectively). In Exp. 1, 12 lambs (29.9 +/- 2.7 kg) were used. Forage OM intake was not affected (P = 0.46) by protein degradability or by increasing RUP (P >/= 0.31). Apparent total tract OM digestibility was not affected (P = 0.10) by protein degradability, but increased (P </= 0.004) with increasing RUP. Urinary N excretion was not affected (P = 0.20) by protein degradability, but increased (P </= 0.006) with increasing RUP. Similarly, N retention (g/d) was not affected (P = 0.69) by protein degradability, but increased (P = 0.001) as RUP increased. However, N retention (% of digested N) was not affected (P >/= 0.40) by protein degradability or level of RUP. In Exp. 2, 16 catheterized lambs (32 +/- 5 kg) were used. Net release of ammonia-N from the portal-drained viscera (PDV) was greater (P = 0.02) for CON than for C100 and increased linearly (P = 0.002) as RUP increased. Net uptake of ammonia-N by liver was not affected (P = 0.23) by protein degradability, but increased linearly (P = 0.04) as RUP increased. Net urea-N release from liver was not affected (P >/= 0.49) by protein degradability or level of RUP. Net uptake of urea-N by PDV was greater (P = 0.02) for C100 compared with CON and increased (P = 0.04) with increasing RUP. Neither net release from PDV nor hepatic uptake of alpha-amino N were affected (P >/= 0.12) by protein degradability or level of RUP. Hepatic ammonia-N uptake accounted for 82, 38, 98, and 79% of net urea-N release from the liver for CON, C50, C100, and C150, respectively. Hepatic alpha-amino N uptake for all treatments greatly exceeded that required for the remaining urea-N release by the liver, suggesting that alpha-amino N may serve as a temporary means of storing excess N by liver between supplementation events. The pattern of net release or uptake of N metabolites between supplementation events requires further investigation.

Ryegrass-based diet and barley supplementation: partition of energy-yielding nutrients among splanchnic tissues and hind limbs in finishing lambs

Splanchnic metabolism of energy-yielding nutrients and their uptake by the hind limb were studied in finishing lambs receiving ryegrass harvested at grazing stage (ear at 10 cm) with or without barley supplementation. Six ruminally cannulated and multicatherized lambs (40.2 +/- 1.5 kg) were fed with frozen ryegrass (RG) at 690 kJ of metabolizable energy intake (MEI) x d(-1) x BW(-0.75) successively with and without barley supplementation (RG + B), according to a triplicated Latin square design. Barley supplementation represented 21% of DM intake and increased the MEI by 32% (P < 0.002). In ruminal fluid, barley supplementation increased the acetate and butyrate concentrations by 21.2 and 49.6%, respectively (P < 0.04), without modifying those of propionate. Thus, molar proportions of acetate and butyrate were not modified, and those of propionate tended (P < 0.06) to decrease from 26 to 23%. As a result, the net portal appearance of propionate was not modified. Net portal appearance of butyrate and beta-hydroxybutyrate increased (P < 0.03), and that of acetate was not modified. Consequently, hepatic uptake of butyrate increased and probably spared acetate from hepatic metabolism. The hepatic fractional extraction of propionate decreased (P < 0.03), whereas the net flux of lactate switched from a net release to a net uptake, suggesting an alteration in the contribution of gluconeogenic substrates to glucose synthesis without modification in net hepatic glucose release. As a consequence, barley supplementation increased net splanchnic release of acetate (P < 0.02), propionate (P < 0.001), and beta-hydroxybutyrate (P < 0.01) by 60, 157, and 78%, respectively. In addition, the net splanchnic release of insulin increased (P < 0.03) because of a decrease (P < 0.02) in its hepatic extraction. Despite those changes, the net uptake of nutrients by the hind limb was not modified and even decreased in the case of glucose (P < 0.02), suggesting a stimulation of lipogenesis in adipose tissues. Results from the present study suggested that supplementation of a ryegrass-based diet would likely have little effect on the orientation of muscle energy metabolism and on meat quality because the net uptake of nutrients by the hind limb was unchanged.

Oxygen consumption by splanchnic tissues in wethers consuming ad libitum different proportions of bermudagrass and ryegrass-wheat

Crossbred wethers (n = 18, 7.5 month of age and 31 +/- 0.8 kg) were used in a 23-day experiment to determine effects of ad libitum consumption of diets differing in proportions of coarsely chopped bermudagrass and ryegrass-wheat hay (0, 33, 67 and 100%) on oxygen consumption by splanchnic tissues. Bermudagrass and ryegrass-wheat were 9 and 13% CP and 78 and 71% NDF, respectively. Intake of dry matter (1.03, 0.92, 0.92 and 0.76 kg/d) and digestible energy (13.5, 10.7, 10.6 and 8.2 MJ/d for 0, 33, 67 and 100% bermudagrass, respectively) changed linearly and cubically (P < 0.05) as bermudagrass level increased. Consumption of oxygen by the portal-drained viscera tended to decrease linearly (P = 0.14) with increasing bermudagrass (182, 154, 156 and 137 mM/h), and hepatic oxygen consumption decreased linearly (P < 0.05) and changed cubically (P = 0.07; 150, 113, 116 and 103 mM/h for 0, 33, 67 and 100% bermudagrass, respectively). Splanchnic tissue energy consumption expressed as a percentage of digestible energy intake increased linearly (P = 0.08) with increasing bermudagrass (24.0, 27.6, 28.6 and 33.2% for 0, 33, 67 and 100% bermudagrass, respectively). In conclusion, the level rather than presence alone of different grass sources consumed ad libitum affected energy use by the splanchnic bed, and as a percentage of digestible energy intake splanchnic bed energy consumption increased with increasing dietary bermudagrass level and decreasing digestible energy intake.

Influence of hepatic ammonia removal on ureagenesis, amino acid utilization and energy metabolism in the ovine liver

The mass transfers of O2, glucose, NH3, urea and amino acids across the portal-drained viscera (PDV) and the liver were quantified, by arterio-venous techniques, during the last 4 h of a 100 h infusion of 0 (basal), 150 or 400 mumol NH4HCO3/min into the mesenteric vein of three sheep given 800 g grass pellets/d and arranged in a 3 x 3 Latin-square design. Urea irreversible loss rate (ILR) was also determined by continuous infusion of [14C]urea over the last 52 h of each experimental period. PDV and liver movements of glucose, O2 and amino acids were unaltered by NH4HCO3 administration, although there was an increase in PDV absorption of non-essential amino acids (P = 0.037) and a trend for higher liver O2 consumption and portal appearance of total amino acid-N, glucogenic and non-essential amino acids at the highest level of infusion. PDV extraction of urea-N (P = 0.015) and liver removal of NH3 (P < 0.001), release of urea-N (P = 0.002) and urea ILR (P = 0.001) were all increased by NH4HCO3 infusion. Hepatic urea-N release (y) and NH3 extraction (x) were linearly related (R2 0.89), with the slope of the regression not different from unity, both for estimations based on liver mass transfers (1.16; SE 0.144; P(b) not equal to 1 = 0.31) and [14C]urea (0.97; SE 0.123; P(b) not equal to 1 = 0.84). The study indicates that a sustained 1.5 or 2.4-fold increase in the basal NH3 supply to the liver did not impair glucose or amino acid supply to non-splanchnic tissues; nor were additional N inputs to the ornithine cycle necessary to convert excess NH3 to urea. Half of the extra NH3 removed by the liver was, apparently, utilized by periportal glutamate dehydrogenase and aspartate aminotransferase for sequential glutamate and aspartate synthesis and converted to urea as the 2-amino moiety of aspartate.

Absorption and metabolism of short-chain fatty acids in ruminants

Short-chain fatty acids (SCFA), viz. acetate, propionate and butyrate are quantitatively important substrates in ruminant energy metabolism. In the reviewed literature, 16 44% of ME intake was recovered as portal appearance of SCFA. This is considerably lower than expected when related to the estimated intragastric flux of SCFA. The discrepancy is caused by portal drained viscera metabolism of arterially abundant metabolites e.g., acetate and the metabolism of acetate and butyrate to acetoacetate and D-3-hydroxybutyrate in the absorptive epithelia. Even though considerable variations between experiments on acetate and propionate appearance are found, there seems to be a great deal of evidence that the proportion of gastrointestinally produced acetate and propionate absorbed to the portal blood is 50-75%. The portal recovery of butyrate has been found to be between 10 and 36% dependent on intraruminal infusion rate. It is concluded that major parts of acetate and propionate are directly absorbed to the portal vein. The true absorption rate of acetate can only be estimated by taking the portal drained viscera metabolism of arterial acetate into account. Butyrate is generally found to have a low recovery in the portal vein, but the production of D-3-hydroxybutyrate seems to be underestimated in major parts of the literature. It is therefore necessary to measure portal appearance as well as portal drained viscera metabolism to assess the quantitative as well as the qualitative contribution of SCFA and SCFA metabolites to whole animal metabolism.

Influence of grass source on net flux of nutrients across splanchnic tissues in sheep with restricted intake

Crossbred wethers (22 months old; 46 +/- 1.3 kg body weight), with catheters in a hepatic vein, the portal vein and a mesenteric vein and artery, consumed warm (W; bermudagrass hay) or cool season grass hay (C; ryegrass-wheat) at 1.6% body weight (dry matter basis) in a crossover design experiment. Warm and cool season grasses were 13.6 and 9.9% crude protein, 77 and 66% neutral detergent fibre and 4.6 and 4.0% acid detergent lignin, respectively. Neutral detergent fibre digestibility (70.3 and 77.4%) and digestible energy intake (8.5 and 9.3 mJ/d) were greater (P < 0.02) for C than for W, and digestible nitrogen intake (11.5 and 8.0 g/d for W and C, respectively) was greater (P < 0.01) for W. Ruminal fluid concentrations of ammonia nitrogen and total volatile fatty acids were not altered by grass source, and acetate: propionate was greater (P < 0.02) for W (3.80) than for the C (3.54). Portal-drained viscera blood flow (118 and 119 l/h; SE 8.0), oxygen consumption (141 and 142 mM/h; SE 3.7), alpha-amino nitrogen release (13.4 and 13.1 mM/h; SE 3.42), urea nitrogen uptake (22.8 and 22.5 mM/h; SE 4.97), ammonia nitrogen release (14.9 and 15.7 mM/h; SE 3.36), glucose uptake (10.0 and 6.5 mM/h; SE 1.30), propionate release (14.5 and 16.4 mM/h; SE 1.88), lactate release (4.64 and 5.03 mM/h; SE 1.908) and acetate release (54.8 and 55.4 mM/h for W and C, respectively; SE 8.82) were similar between grasses. Energy consumption by the portal-drained viscera accounted for a slightly greater (P < 0.01) percentage of digestible energy intake with W vs C (18.8 vs 17.0%; SE 0.10). In conclusion, with restricted consumption of W or C by mature sheep, grass source had little impact on net flux of oxygen and nutrients across the portal-drained viscera and splanchnic bed.