Hepatic response to increased exogenous supply of plasma amino acids by infusion into the mesenteric vein of Holstein-Friesian cows in late gestation

Responses in splanchnic and mammary amino acid metabolism to short-term graded removal of methionine in lactating goats

Four multi-catheterized lactating goats were used in a 4 × 4 Latin square experiment to investigate the responses of amino acid metabolism in portal-drained viscera (PDV), liver, and mammary glands to short-term varying supplies of methionine (Met). During the last 45 h in each experimental period, goats were fasted for 12 h and then abomasally infused with an amino acid (AA) mixture plus glucose for 33 h. Treatments consisted of graded removal of Met from an infused AA mixture to achieve Met content in the infusate of 100% (complete), 60%, 30%, or 0% that in casein. Graded Met removal decreased the production of milk, milk protein, lactose, and fat linearly whilst also decreasing arterial Met concentration linearly (P < 0.05). Meanwhile, net PDV uptake and liver removal of Met decreased linearly (P < 0.05) due to decreased Met affinity of PDV and liver (P < 0.05). Net mammary uptake of Met (P > 0.1) was maintained as Met supply declined. This was achieved through increased mammary affinity (P < 0.05) and increased mammary blood flow (P < 0.05) totally offsetting the negative effect of decreased circulating Met concentration. Graded removal of Met from the infusate linearly decreased mammary uptake-to-milk output ratios of Met (P < 0.05) and tended to decrease essential amino acid (EAA) linearly (0.05 < P < 0.1). Treatments also linearly decreased circulating concentration of prolactin and linearly increased insulin concentration (P < 0.05). In conclusion, results of the present study indicated there were several mechanisms used to mitigate a Met deficiency, including reduced catabolism of Met in PDV, liver, and peripheral tissue (including mammary glands) and a linear increase in mammary blood flow. The observed decreases in milk protein production as Met supply decreased appear to be a result of regulatory events which may have been driven by decreased circulating prolactin, rather than as a result of decreased mammary Met uptake.

The effects of maternal supplementation of rumen-protected lysine during the close-up dry period on newborn metabolism and growth in Holstein calves

This study aimed to evaluate the effects of rumen-protected lysine (RPL) supplementation during the close-up period on blood metabolites and calf growth. Forty multiparous Holstein dams were selected based on parity, body condition score, and expected calving date, and randomly assigned to a group: with RPL (n = 22) or without (control [CON], n = 18). RPL dams were supplied daily with 80 g of RPL from Day 21 before the expected calving date to parturition. Blood samples were obtained from the dams before the start of supplementation, 1 week before calving, and immediately after calving, and from calves immediately after birth and weekly until 8 weeks of age. Body weight measurements were performed immediately after birth in all calves and at weekly intervals until 8 weeks of age in female calves. No significant difference was observed in serum metabolite levels and plasma amino acid concentrations between the RPL and CON dams before supplementation, whereas plasma lysine concentrations tended to be higher in RPL dams immediately after calving (p = 0.07). Serum total protein levels (p < 0.05) were higher, whereas plasma total amino acid, total essential amino acid, total non-essential amino acid, and other amino acid concentrations were lower in the calves of RPL dams than those of CON dams (p < 0.05). There were no significant differences in calf birth weight between the two groups, although female calves of RPL dams (n = 7) had higher serum total protein (p < 0.05) and tended to have greater body weight (p = 0.09) from 1 to 8 weeks of age than those of CON dams (n = 11). Overall, RPL supplementation during the close-up period may increase placenta-mediated amino acid transfer to the foetus and enhance protein synthesis in the calf, leading to improved weight gain during the suckling period.

The crucial role of lysine in the hepatic metabolism of growing Holstein dairy heifers as revealed by LC-MS-based untargeted metabolomics

The objective of this experiment was to evaluate the effect of supplementing rumen-protected Lys based on a Lys-deficient diet on liver metabolism in growing Holstein heifers. The experiment was conducted for 3 months with 36 Holstein heifers (initial body weight: 200 ± 9.0 kg; 7-month-old). Heifers were randomly assigned to 2 diets based on corn, soybean meal, alfalfa hay, and wheat bran: control, Lys-deficient diet (LD; 0.66% Lys in diet), and Lys-adequate diet (LA; 1.00% Lys in diet). The results showed no difference in growth performance between the 2 groups (P > 0.05). However, there was a clear trend of increasing feed conversion rate with Lys supplementation (0.05 < P < 0.01). The serum urea nitrogen concentration was significantly decreased, and the aspartate aminotransferase-to-alanine aminotransferase ratio was significantly decreased by Lys supplementation (P < 0.05). Moreover, growing heifers fed a Lys-adequate diet had lower levels of urine nitrogen excretion and higher levels of the biological value of nitrogen (P < 0.05). Metabolomic analysis revealed that 5 types of phosphatidylcholine and 3 types of ceramide were significantly increased and enriched in sphingolipid metabolism and glycerophospholipid metabolism (P < 0.05). His, Leu, and Asp levels were significantly decreased in the liver following Lys supplementation (P < 0.05). In conclusion, Lys supplementation may promote the synthesis of body tissue proteins, as evidenced by significantly decreased amino acids in the liver and urine N excretion, it also improves hepatic lipid metabolism by providing lipoprotein precursors.

Impact of a tannin extract on animal performance and nitrogen excretion of dairy cows grazing a tropical pasture

Context Tannin extracts have been investigated as natural feed additives with the potential to decrease the enteric emission of methane and urinary N and to improve the productive performance of ruminants. However, the impact of this additive in dairy cattle grazing tropical grass pastures has not been extensively evaluated. Aims To evaluate the impact of the Acacia mearnsii bark extract (TA) on productive and nutritional variables in dairy cows grazing a Cynodon dactylon pasture and receiving supplementary corn silage and concentrate. Methods Fourteen multiparous Holstein dairy cows were assigned in a randomised block design to either of the following two treatments: concentrate without TA (control) or with 10 g of TA/kg dry matter (DM). The concentrate and corn silage accounted for ~0.30 and 0.30 of total DM intake respectively. Key results The dietary concentration of TA was 2.9 g/kg DM and it did not affect nutrients intake, milk production and composition, rumen microbial protein synthesis, plasma urea concentration or faecal N excretion. Total N and urea N excreted in urine were higher, whereas total N excreted in manure tended to be higher in TA treatment. Conclusions No productive advantage was obtained by including TA in the diet of dairy cows grazing Cynodon dactylon pasture. Instead, it increased the excretion of labile urinary N. Implications The effect of the use of TA as a feed additive for cows fed tropical grass-based diets is negative from the environmental point of view.

Short-term lactation and mammary metabolism responses in lactating goats to graded removal of methionine from an intravenously infused complete amino acid mixture

To investigate the possible pathways of Met deficiency to depress milk protein synthesis, 4 lactating goats fitted with jugular vein, mammary vein, and carotid artery catheters and transonic blood flow detectors on the external pudic artery were used in a 4 × 4 Latin square experiment. Goats were fasted for 24 h followed by a 9-h intravenous infusion of an AA mixture plus glucose. Milk yield was recorded and samples were taken in h 2 to 8 of the infusion period, and mammary biopsy was performed in the last hour. Treatments were graded removal of Met from the infused AA mixture to achieve Met content in the infusate of 100 (complete), 60, 30, or 0% of that in casein. Graded Met removal decreased yield of milk, milk protein, and lactose linearly and tended to decrease yield of milk fat linearly. Milk protein yield decreased to 82, 78, and 69% that of complete mixture infusion, respectively, when the 60, 30, and 0% Met infusate was infused. Circulating Met decreased linearly with graded Met removal. Arterial and venous Met decreased to 36 and 23% that of complete mixture infusion, respectively, when all Met was removed out of the mixture. Concomitant with the decreased circulating concentration was a similar increase in mammary Met affinity as reflected by the linearly increased mammary Met clearance rate. The increased affinity plus the linearly increased mammary blood flow totally offset the negative effect of decreased circulating Met concentration on mammary Met uptake. The overall result was similar mammary Met uptakes across treatments ranging from 285.9 to 339.5 μmol/h. Mammary uptakes of the other AA measured were generally not affected by treatments except for a linearly decreased Thr uptake and a trend of linearly increased Glu uptake. Consistent with the behavior of an AA mainly catabolized in the liver and mainly used for protein synthesis in peripheral tissues, mammary uptake to milk output ratios of Met measured in the present study ranged from 1.25 to 1.49 and was not affected by treatments. For the other AA measured, the ratio of Thr was linearly decreased and that of Glu was linearly increased by graded Met removal. Graded Met removal linearly elevated circulating urea N and glucose concentrations, indicating enhanced whole-body catabolism of AA and hepatic gluconeogenesis. Treatments had no significant effects on circulating insulin, growth hormone, and the other hormones and metabolites measured. Phosphorylation status of eIF4E binding protein 1 tended to decrease linearly and that of p70S6k was linearly decreased by graded Met removal, indicating depressed signal in the intracellular mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway. In conclusion, results of the present study indicated that the mTORC1 pathway and whole-body AA catabolism rather than mammary uptake appeared the drivers for changes in milk protein synthesis in response to varying Met supply.

Changes in tissue abundance and activity of enzymes related to branched-chain amino acid catabolism in dairy cows during early lactation

Branched-chain α-keto acid dehydrogenase (BCKDH) complex catalyzes the irreversible oxidative decarboxylation of branched-chain α-keto acids. This reaction is considered as the rate-limiting step in the overall branched-chain amino acid (BCAA) catabolic pathway in mammals. For characterizing the potential enzymatic involvement of liver, skeletal muscle, adipose tissue (AT), and mammary gland (MG) in BCAA metabolism during early lactation, tissue and blood samples were examined on d 1, 42, and 105 after parturition from 25 primiparous Holstein cows. Serum BCAA profiles were analyzed and the mRNA and protein abundance as well as the activity in the different tissues were assessed for the BCAA catabolic enzymes, partly for the branched-chain aminotransferase and completely for BCKDH. Total BCAA concentration in serum was lowest on d 1 after parturition and increased thereafter to a steady level for the duration of the experiment. Pronounced differences between the tissues were observed at all molecular levels. The mRNA abundance of the mitochondrial isoform of branched-chain aminotransferase (BCATm) was greatest in AT as compared with the other tissues studied, indicating that AT might be an important contributor in the initiation of BCAA catabolism in dairy cows. From the different subunits of the BCKDH E1 component, only the mRNA for the β polypeptide (BCKDHB), not for the α polypeptide (BCKDHA), was elevated in liver. The BCKDHA mRNA abundance was similar across all tissues except muscle, which tended to lower values. Highest BCKDHA protein abundance was observed in both liver and MG, whereas BCKDHB protein was detectable in these tissues but could not be quantified. Adipose tissue and muscle only displayed abundance of the α subunit, with muscle having the lowest BCKDHA protein of all tissues. We found similarities in protein abundance for both BCKDH E1 subunits in liver and MG; however, the corresponding overall BCKDH enzyme activity was 7-fold greater in liver compared with MG, allowing for hepatic oxidation of BCAA transamination products. Reduced BCKDH activity in MG associated with no measurable activity in AT and muscle may favor sparing of BCAA for the synthesis of the different milk components, including nonessential AA. Deviating from previously published data on BCAA net fluxes and isotopic tracer studies in ruminants, our observed results might in part be due to complex counter-regulatory mechanisms during early lactation.

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).

Protein feeding and balancing for amino acids in lactating dairy cattle

This article summarizes the current literature as regards metabolizable protein (MP) and essential amino acid (EAA) nutrition of dairy cattle. Emphasis has been placed on research since the publication of the National Research Council Nutrient Requirements of Dairy Cattle, Seventh Revised Edition (2001). Postruminal metabolism of EAA is discussed in terms of the effect on requirements. This article suggests methods for practical application of MP and EAA balance in milking dairy cows.

Effect of plasma insulin and branched-chain amino acids on skeletal muscle protein synthesis in fasted lambs

The increase in fractional rate of protein synthesis (Ks) in the skeletal muscle of growing rats during the transition from fasted to fed state has been explained by the synergistic action of a rise in plasma insulin and branched-chain amino acids (BCAA). Since growing lambs also exhibit an increase inKswith level of feed intake, the objective of the present study was to determine if this synergistic relationship between insulin and BCAA also occurs in ruminant animals. Six 30 kg fasted (72 h) lambs (8 months of age) received each of four treatments, which were based on continuous infusion into the jugular vein for 6 h of: (1) saline (155 mmol NaCl/l); (2) a mixture of BCAA (0·778 μmol leucine, 0·640 μmol isoleucine and 0·693 μmol valine/min·kg); (3) 18·7 μmol glucose/min·kg (to induce endogenous insulin secretion); (4) co-infusion of BCAA and glucose. Within each period all animals received the same isotope of phenylalanine (Phe) as follows: (1) l-[1-13C]Phe; (2) l-phenyl-[ring2H5]-alanine; (3) l-[15N]Phe; (4) l-[ring 2,6-3H]Phe. Blood was sampled serially during infusions to measure plasma concentrations of insulin, glucose and amino acids, and plasma free Phe isotopic activity; biopsies were taken 6 h after the beginning of infusions to determineKsinm. longissimus dorsiandvastusmuscle. Compared with control (saline-infused) lambs,Kswas increased by an average of 40 % at the end of glucose infusion, but this effect was not statistically significant in either of the muscles sampled. BCAA infusion, alone or in combination with glucose, also had no significant effect onKscompared with control sheep.Kswas approximately 60 % greater forvastusmuscle than form. longissimus dorsi(P>0·01), regardless of treatment. It is concluded that there are signals other than insulin and BCAA that are responsible for the feed-induced increase inKsin muscle of growing ruminant animals.

The effect of intake on protein metabolism across splanchnic tissues in growing beef steers

The contribution of the total splanchnic tissue (TSP; portal-drained viscera (PDV) plus liver) to whole-body protein metabolism was estimated in relation to intake (0·6, 1·0 and 1·6 × maintenance requirements), in six multicatheterized growing beef steers used in a double 3 × 3 Latin square design. At the end of each 21 d experimental period, [1-13C]leucine was infused into a jugular vein (1·05 mmol/h for 5 h, preceded by a priming dose of 1·05 mmol). Arterial, portal and hepatic blood samples were collected hourly during the infusion. The increment in TSP leucine irreversible loss rate (ILR) observed with increasing intake reached significance (P< 0·10) only for PDV, while whole-body ILR increased markedly (P< 0·001) with intake. The relative contribution of TSP to whole-body leucine ILR averaged 44 % (25 % from PDV and 19 % from the liver). Although these proportions were not affected by intake, on an incremental basis more than 70 % of the increase of whole-body leucine ILR between the 0·6 and 1·0 × maintenance originated from the changes in TSP ILR, while the corresponding value was below 13 % between 1·0 and 1·6 × maintenance. Total whole-body leucine oxidation and fractional oxidation increased (P< 0·05) with intake. Protein retention increased with intake (P< 0·01), as a result of a greater increase in protein synthesis compared with protein degradation. Protein breakdown had a major impact on protein turnover as 65 % of the protein synthesized was degraded when intake varied from 1·0 to 1·6 × maintenance. Net leucine portal absorption increased (P< 0·001) with intake and represented 1, 16 and 23 % of whole body leucine ILR, for 0·6, 1·0 and 1·6 × maintenance, respectively. Although leucine oxidation was not a major component of whole body ILR (9·3–19·9 %), it represented 69 % of the net available leucine (portal absorption) even at 1·6 × maintenance. The lower relative contribution of the TSP to whole-body leucine ILR at higher intake indicates the proportional increase in the metabolic activity of peripheral tissues as the animals moved into positive protein balance.

Glutamine metabolism in ovine splanchnic tissues: effects of infusion of ammonium bicarbonate or amino acids into the abomasum

This study investigates the effects of increased NH3or 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 μmol/kg0·75per min), water (CONT), or a mixture of amino acids that excluded glutamine and glutamate (AA; 46·8 μmol amino acid-N/kg0·75per 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 [5-15N]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 NH3supply 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 NH3absorbed 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.

The route of absorbed nitrogen into milk protein

A database reviewing the metabolism of nitrogen (N) compounds from absorption to milk has been compiled from 14 published and unpublished studies (33 treatments) that measured the net flux of N compounds across the splanchnic tissues in dairy cows. Apparent N digestibility averaged 0·65, with this then partitioned between 0·34 excreted in urine and 0·31 secreted as milk.

Nitrogen metabolites are absorbed from the lumen of the gut into the portal vein, mainly as free amino acids (AA) and ammonia; these represented 0·58 and 0·57 of digested N, respectively. All of the ammonia absorbed was removed by the liver with, as a result, a net splanchnic flux of zero. Detoxification of ammonia by the liver and catabolism of AA results in production of urea as an end-product. Hepatic ureagenesis is a major cross-road in terms of whole body N exchange, being the equivalent of 0·81 of digested N. Therefore, salvage of a considerable part of this ureagenesis is needed to support milk protein synthesis. This salvage occurs via transfer of urea from the blood circulation into the lumen of the gut. On average, 0·47 of hepatic ureagenesis was returned to the gut via the portal-drained viscera (equivalent to 0·34 of digested N) with 0·56 of this then used for anabolic purposes e.g. as precursor N for microbial protein synthesis. On average, 0·65 of estimated digestible AA was recovered in the portal vein. This loss (0·35) is due to oxidation of certain AA across the gut wall and non-absorption of endogenous secretions. The magnitude of this loss is not uniform among AA and varies between less than 0·05 for histidine to more than 0·90 for some non-essential AA, such as glutamine.

A second database (six studies, 14 treatments) was constructed to further examine the subsequent fate of absorbed essential AA. When all AA are aggregated, the liver removed, on average, 0·45 of portal absorption but this value hides the considerable variation between individual AA. Simplistically, the AA behave as two major groups: one group undergoes very little hepatic removal and includes the branched-chain AA and lysine. For the second group, removal varies between 0·35 and 0·50 of portal absorption, and includes histidine, methionine and phenylalanine. For both groups, however, the efficiency of transfer of absorbed AA into milk protein decreases with increasing supply of protein. This loss of efficiency is linked directly with increased hepatic removal of AA from the second group and, probably, increased catabolism by peripheral tissues, including the mammary gland, of AA from the first group. Therefore, we must stop using fixed factors of conversion of digestible AA to milk in our predictive schemes and acknowledge that metabolism of AA between delivery from the duodenum and conversion to milk protein will vary with nutrient supply. New information evolving from re-analysis of the literature and recent studies will allow better models to be devised for the prediction of nutrient-based responses by the lactating cow. Consideration of biological efficiency, rather than maximal milk yield, will lead to systems that are economically more sensible for the farmer and that have better environmental impacts.

Effect of Level of Metabolizable Protein on Splanchnic Flux of Amino Acids in Lactating Dairy Cows

The response of splanchnic tissue metabolism to different levels of metabolizable protein (MP) was measured in 6 catheterized multiparous lactating Holstein cows. Three diets, balanced to provide similar energy intakes and increasing amounts of MP (g/d)-1922 (low), 2264 (medium), and 2517 (high)-were fed during 21-d experimental periods according to a replicated Latin square. On d 18, 19, or 20, six hourly blood samples were collected simultaneously from the portal and hepatic veins plus an artery to determine net fluxes of nutrients across the portal-drained viscera and the liver. Yields of milk and protein increased, as did urinary N excretion with increasing MP. Portal absorption of essential amino acids (EAA) increased linearly with increasing MP supply, as did liver removal of His, Met, and Phe. In contrast, liver removal of the branched-chain AA (BCAA) and lysine was unaffected by diets. With increasing MP, the ratio of milk output to postliver supply of BCAA, Thr, and Lys decreased linearly, indicating oxidation of these AA in the peripheral tissues. Concomitant to a decreased catabolism of EAA in the liver (His, Met, Phe, and Thr) and/or in peripheral tissues (BCAA, Lys, and Thr), the efficiency of transfer of absorbed EAA into milk protein decreases markedly as protein supply increases. The efficiency of transfer of absorbed AA into milk also varies greatly between AA. These 2 important factors should be taken into account when building predictive schemes for milk protein output.

An integrative model of amino acid metabolism in the liver of the lactating dairy cow

The objective of this work was to construct a dynamic model of hepatic amino acid metabolism in the lactating dairy cow that could be parameterized using net flow data from in vivo experiments. The model considers 22 amino acids, ammonia, urea, and 13 energetic metabolites, and was parameterized using a steady-state balance model and two in vivo, net flow experiments conducted with mid-lactation dairy cows. Extracellular flows were derived directly from the observed data. An optimization routine was used to derive nine intracellular flows. The resulting dynamic model was found to be stable across a range of inputs suggesting that it can be perturbed and applied to other physiological states. Although nitrogen was generally in balance, leucine was in slight deficit compared to predicted needs for export protein synthesis, suggesting that an alternative source of leucine (e.g. peptides) was utilized. Simulations of varying glucagon concentrations indicated that an additional 5 mol/d of glucose could be synthesized at the reference substrate concentrations and blood flows. The increased glucose production was supported by increased removal from blood of lactate, glutamate, aspartate, alanine, asparagine, and glutamine. As glucose output increased, ketone body and acetate release increased while CO(2) release declined. The pattern of amino acids appearing in hepatic vein blood was affected by changes in amino acid concentration in portal vein blood, portal blood flow rate and glucagon concentration, with methionine and phenylalanine being the most affected of essential amino acids. Experimental evidence is insufficient to determine whether essential amino acids are affected by varying gluconeogenic demands.

Histidine utilization by growing steers is not negatively affected by increased supply of either ammonia or amino acids

Two experiments were conducted with ruminally cannulated Holstein steers to determine effects of N supply on histidine (His) utilization. All steers received 2.5 kg DM/d of a diet based on soybean hulls; abomasal infusion of 250 g/d amino acids, which supplied adequate amounts of all essential amino acids except His; abomasal infusion of 300 g/d glucose; and ruminal infusion of 180 g/d acetate, 180 g/d propionate, and 45 g/d butyrate. Both experiments were 6 x 6 Latin squares with treatments arranged as 3 x 2 factorials. No significant (P < 0.05) interactions between main effects were noted for N balance criteria in either Exp. 1 or 2. For Exp. 1, steers (146 +/- 7 kg) received 0, 1.5, or 3 g/d of L-His infused abomasally in combination with 0 or 80 g/d urea infused ruminally to supply a metabolic ammonia load. Urea infusions increased (P < 0.05) ruminal ammonia concentration from 8.6 to 19.7 mM and plasma urea from 2.7 to 5.1 mM. No change in N retention occurred in response to urea (35.1 and 37.1 g/d for 0 and 80 g/d urea, respectively, P = 0.16). Retained N increased linearly (P < 0.01) with His (31.5, 37.8, and 39.0 g/d for 0, 1.5, and 3 g/d L-His, respectively). Efficiency of deposition of supplemental His between 0 and 1.5 g/d averaged 65%. In Exp. 2, steers (150 +/- 6 kg) were infused abomasally with 0 or 1 g/d of L-His in combination with no additional amino acids (Control), 100 g/d of essential + 100 g/d of nonessential amino acids (NEAA+EAA), or 200 g/d of essential amino acids (EAA). Retained N increased (P = 0.02) from 34.2 to 38.3 g/d in response to His supplementation. Supplementation with NEAA+EAA increased (P < 0.05) N retention (33.9, 39.3, and 35.6 g/d for Control, NEAA+EAA, and EAA, respectively), likely in response to increased energy supply. Plasma urea concentrations of steers receiving NEAA+EAA (3.8 mM) and EAA (3.8 mM) were greater (P < 0.05) than those of Control steers (2.7 mM). The average efficiency of His utilization was 63%, a value similar to the value of 65% observed in Exp. 1, as well as the 71% value predicted by the Cornell net carbohydrate and protein system model. Under our experimental conditions, increases in N supply above requirements, as either ammonia or amino acids, did not demonstrate a metabolic cost in terms of His utilization for whole-body protein deposition by growing steers.

Effect of feed intake on amino acid transfers across the ovine hindquarters

Responses in variables of amino acid (AA) metabolism across peripheral tissues to feed intake were studied in six sheep (mean live weight 32 kg) prepared with arterio-venous catheters across the hindquarters. Four intakes (0.5, 1.0, 1.5 and 2.5 x maintenance energy) were offered over 2-week periods to each sheep in a Latin square design with two animals replicated. Animals were infused intravenously with a mixture of U-13C-labelled AA for 10 h and integrated blood samples withdrawn from the aorta and vena cava hourly between 5 and 9 h of infusion. Biopsy samples were also taken from skin and m. vastus lateralis. Data from both essential (histidine, isoleucine, leucine, lysine, phenylalanine, threonine) and nonessential (glycine, proline, serine, tyrosine) AA were modelled to give rates of inward and outward transport, protein synthesis and degradation, plus the fraction of total vascular inflow that exchanged with the hindquarter tissues. Rates of inward transport varied more than 10-fold between AA. For all essential AA (plus serine), inward transport increased with food intake (P<0.04). There were corresponding increases in AA efflux (P<0.05) from the tissues for threonine and the branched-chain AA. Protein synthesis rates estimated from the kinetics of these AA also increased with intake (P<0.02). Rates of inward transport greatly exceeded the amount of AA necessary to support protein retention, but were more similar to rates of protein synthesis. Nutritional or other strategies to enhance AA transport into peripheral tissues are unlikely to increase anabolic responses.

Effect of supply of metabolizable protein on whole body and splanchnic leucine metabolism in lactating dairy cows

The effect of the supply of metabolizable protein (MP) on protein metabolism across the splanchnic tissues was determined in six catheterized lactating Holstein cows. In a crossover design, two isonitrogenous (16.3% CP) diets balanced to provide a low (Lo-MP) or high (Hi-MP) supply of MP were fed over 35-d periods. After 24 d of feeding, N balance was determined over a 6-d period. On d 33, [13C] sodium bicarbonate was infused into one jugular vein for 6 h, and hourly breath samples were collected. On d 34 or 35, L[1-(13)C] leucine was infused into one jugular vein, and between 2 to 6 h of infusion, breath and blood samples were taken hourly from the portal and hepatic veins and an artery. Isotopic enrichments of plasma leucine, 4-methyl-2-oxopentanoate, and expired CO2 were determined for calculation of leucine kinetics. Net leucine absorption was greater, either on a direct basis (leucine transfer only) or corrected for portal-drained viscera metabolism to 4-methyl-2-oxopentanoate and CO2 for the Hi-MP diet. There were no effects of diet on hepatic net flux of leucine across the liver, and, thus, more leucine was available to peripheral tissues with the Hi-MP diet. Combined with an increment in portal absorption of most of essential AA, this led to increased milk protein output, although it only represented 16% of the additional available leucine. Whole body leucine oxidation was also greater for the Hi-MP diet, as was leucine used for protein synthesis. Despite these changes, MP supply did not affect irreversible loss rate of leucine by portal-drained viscera and the liver; these averaged 35 and 20% of whole body irreversible loss rate, respectively. These ratios confirm the high metabolic activity of splanchnic tissues in lactating dairy cows, which are even greater than previously reported in growing ruminants.

Effect of supply of metabolizable protein on splanchnic fluxes of nutrients and hormones in lactating dairy cows

The effect of the supply of metabolizable protein on splanchnic fluxes of nutrients and hormones was measured in six catheterized late-lactation Holstein cows in a crossover design. Two isonitrogenous diets (16.3% CP), but differing in rumen protein degradability and estimated metabolizable protein (MP) supply (1654 g/ d, Lo-MP; 1930 g/d, Hi-MP) were fed, each over a 35-d experimental period. On d 34 or 35, net fluxes of nutrients and hormones across the portal-drained viscera, the liver, and total splanchnic tissues were determined. Portal absorption of total, essential, nonessential, and branched-chain amino acids (AA) increased with the Hi-MP diet. Approximately 76% of the additional metabolizable protein supply was recovered as extra AA-N absorption in the portal vein. Liver removal of AA was not different between diets, and this resulted in a greater net release across the splanchnic tissues for the Hi-MP diet. This extra AA supply provided substrates for the observed increased milk protein yield for the Hi-MP diet. Fractional efficiencies of conversion of absorbed individual essential AA into milk protein ranged from 0.42 to 0.68. The corresponding efficiencies for utilization of postsplanchnic AA supply were 0.42 to 1.80. Provision of methionine, phenylalanine, and histidine beyond the liver were similar to outputs in milk protein but the other essential AA were supplied to peripheral tissues in excess of milk output, indicative of oxidative mechanisms in nonhepatic tissues. Net fluxes of glucose, NH3-N, and urea were not affected by the diets. Neither arterial concentrations of insulin, somatotropin, or IGF-1, nor net transfers across the portal-drained viscera or liver of insulin, were affected by the diets. Although portal release of glucagon was not different between the diets, a smaller proportion was removed by the liver on the Hi-MP diet. Metabolism of AA across the splanchnic tissue bed is a major determinant of the quantity and the profile of AA delivered to peripheral tissues.

The hepatic extraction of plasma free amino acids and response to hepatic portal venous infusion of methionine sources in anesthetized SCWL males (Gallus domesticus)

This study was conducted to investigate the hepatic extraction of plasma free amino acids in anesthetized Single Comb White Leghorn (SCWL) males (Gallus domesticus). SCWL males were anesthetized and implanted with cannulae in the carotid artery, hepatic vein, hepatic portal vein and the left hepatic duct. Free amino acids in plasma and bile were determined before, during and after 30-min infusions of Saline (control), DL-Methionine (DL-Met) or DL-2-hydroxy-4-methylthio-butanoic acid (DL-HMB) into the hepatic portal vein. Hepatic extraction rates (HER) of amino acids were calculated based on the concentration of amino acids in plasma multiplied by estimations of blood flow in the hepatic portal vein, hepatic artery and hepatic vein. For the non-essential amino acids, alanine had the highest HER (46%). The liver also removed more than 20% of hepatic inflow of tyrosine and asparagine with substantial extraction (14-18%) of serine, glycine and glutamine, also. In contrast, less than 5% of hepatic inflow of glutamate and cystine were removed by liver. For the essential amino acids, HER for methionine, histidine and phenylalanine were 30, 14 and 17%, respectively, with less than 5% for branched-chain amino acids, lysine, arginine and threonine. Biliary secretion of amino acids represented a small percentage (<0.2%) of total hepatic extraction turnover of the amino acids. Infusion of methionine sources, DL-Met and DL-HMB, had no effect on hepatic metabolism of amino acids other than methionine. The results demonstrated for the first time, the hepatic extraction of circulating free amino acids in avian species in vivo.

Response in hepatic removal of amino acids by the sheep to short-term infusions of varied amounts of an amino acid mixture into the mesenteric vein

Under conditions of chronic supply the liver removes most amino acids (AA) in excess of net anabolic needs. Little information is available, however, on how acute alterations in AA supply (as might occur with once-daily feeding regimens) are controlled by the liver. Are these also extracted completely in a 'first-pass' manner or are there limitations to hepatic uptake? Furthermore, is the rate of removal 'saturable' (by Michaelis-Menten kinetics) over the range of supply experienced under normal feeding conditions? These questions have been addressed in a study that involved acute (4.5 h) increases in AA supply. Four sheep were prepared with trans-hepatic vascular catheters and were offered a basal diet (equivalent to 1.6xenergy maintenance) throughout. On four occasions, at 7 d intervals, they were infused with various amounts of an AA mixture into the mesenteric vein over a 4.5 h period. The mixture contained fourteen AA in the proportions present in rumen microbial protein. The amounts infused were calculated to provide an additional one, two, three and four times that absorbed from the basal diet. Continuous blood collections were removed over 2 h intervals before (basal diet only) and at 0.5-2.5 and 2.5-4.5 h of AA infusion. Transfers of AA, from the digestive tract and to the liver, were calculated for both plasma and total blood. The recovery of the infused AA across the portal-drained viscera (PDV) was quantitative (100%) only for histidine and proline, the remaining AA were recovered at 56-83 %. These losses correlated with the arterial concentrations and were probably due to removal of AA from the systemic circulation by the tissues of the digestive tract. Despite the wide range of net PDV appearances (i.e. absorbed plus infused), the percentage of most AA removed by the liver remained constant, but the percentage varied with AA (from 34 for proline to 78 for tryptophan for blood transfers). Thus, even when supply was increased 5-fold over baseline there was no indication that the transport into the liver declined, indeed the absolute removals continued to increase. In contrast, the branched-chain AA (isoleucine, leucine and valine) did not exhibit constant percentage extractions. Their percentage extractions were always the lowest (16, 10 and 25 respectively) and tended to decline at the highest infusion rates, indicative of saturation in hepatic transport and/or metabolism. The arterial concentrations of all infused AA increased with rate of infusion, again indicative that the liver did not extract all the net AA available across the PDV. Absolute amounts removed were similar between plasma and blood, indicating that most of the hepatic transfers occurred from plasma. The fractional rates of transfer from total inflow to the liver (i.e. with re-circulated AA included) were 3- to 4-fold lower than rates based on the amounts absorbed plus infused. The highest percentage extraction for total blood inflows was for serine (27), but most were between 6 and 16, except for the branched-chain AA, which were all <1. Use of percentage extractions based on total inflows are probably more appropriate for development of mathematical models of liver metabolism, and the current data suggest that constant values may be applied. The needs of the liver for specific mechanisms involving phenylalanine and histidine (plasma protein synthesis), glycine (detoxification of xenobiotics) and alanine (gluconeogenesis) probably also require to be included in such models.

Intestinal disappearance and mesenteric and portal appearance of amino acids in dairy cows fed ruminally protected methionine

An experiment was conducted to compare the rates of disappearance of amino acids (AA) from the small intestine and their net appearance in the blood draining only the small intestine (mesenteric-drained viscera) and the whole gastrointestinal tract (portal-drained viscera) of cows fed a silage-based diet supplemented or not with ruminally protected Met. Five lactating dairy cows (118+/-4 DIM) equipped with duodenal and ileal cannulae (n = 2) or a duodenal cannula only (n = 3), two of which were multicatheterized, were fed a TMR top dressed with 0 or 72 g of ruminally protected Met per day. The addition of ruminally protected Met to the diet increased the duodenal flux of Met leading to a higher apparent digestibility of Met in the small intestine. Sixty-six percent of Met from ruminally protected Met bypassed the rumen and 82% of that Met disappeared from the small intestine. Arterial plasma Met concentrations numerically increased with ruminally protected Met (45 vs. 18 microM), while total AA concentration decreased. Feeding ruminally protected Met resulted in higher concentrations of urea-N and glucose in arterial plasma. Milk production and milk composition were unaffected. The disappearance of essential AA across the small intestine was equivalent (101%) to their flux through the mesenteric-drained viscera while the portal:mesenteric-drained viscera flux ratio for each essential AA varied from 38% for Thr to 76% for Phe. The portal:mesenteric-drained viscera flux ratio for Met was 66%. These results confirm observations made with pigs and sheep.

Nitrogen metabolism of early lactation cows fed diets with two different levels of protein and different amino acid profiles

Four multiparous Holstein cows (569+/-122 kg) surgically prepared with indwelling catheters in the mesenteric, portal, and hepatic veins and carotid artery were allocated in a 4 x 4 Latin square to determine the effects of dietary crude protein (CP) level and amino acid (AA) profile on N metabolism during early lactation (from 25 to 65 d in milk). Cows received their diets in two equal meals and were milked twice daily. The dietary treatments were: 18% CP with a high (18H) or a low (18L) quality AA profile, and 15% CP with a high (15H) or a low (15L) quality AA profile. The four diets were similar in net energy for lactation (1.75 NEL Mcal/kg) and contained the same amount of RUP (34% of CP). The quality of the AA profile pertained only to the essential AA (EAA), and was assessed by comparison with the EAA profile of casein and considered the potential contribution of EAA from ruminal bacteria. The 18H and 15H diets were supplemented with 50 and 25 g/d of ruminally protected Met, respectively. After 10 d on treatment, a blood flow marker (p-amino-hippurate) was infused into a mesenteric vein, and arterial, portal, hepatic, and mammary blood samples were obtained at 3, 6, and 12 h after feeding. Dry matter intake was similar across treatments (23.4+/-0.5 kg/d). Amino acid oxidation, and consequent urea production, in the liver were numerically greater with the 18% CP rations, and, as a result, arterial urea concentrations were greatest (P < 0.01) with these rations. The amount of total AA extracted by the mammary gland tended to be greater with the H than with the L diets (21.4 vs. 18.2 mmol/ h, respectively). Milk yield tended to be greater (P = 0.16) with the 18H and 15H diets (47.7 and 46.3 kg/d, respectively) compared with the 18L and 15L diets (45.9 and 44.6 kg/d, respectively). Also, milk CP and casein contents were greatest (P = 0.09) with the H diets compared with the L diets. Milk and plasma urea N were greatest (P < 0.01) with the 18% CP diets. The efficiency of N utilization for milk protein synthesis was greatest (P < 0.09) with the 15% CP diets. It is concluded that milk protein production during early lactation is less susceptible to variations in dietary CP contents than variations in the AA profile of the dietary protein.

Changes in mRNA expression for gluconeogenic enzymes in liver of dairy cattle during the transition to lactation

The objective of this study was to profile phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC) mRNA expression in the liver of dairy cattle during the peripartum transition and determine changes in abundance of these mRNA in response to protein fed during the prepartum period. Thirty-eight multiparous Holstein cows were fed diets containing either 12% crude protein (CP) and 26% rumen undegradable protein (RUP), 16% CP and 26% RUP, 16% CP and 33% RUP, or 16% CP and 40% RUP on a dry-matter basis beginning 28 d before expected calving. After calving, all cows were fed a common diet through 56 d in milk (DIM). Northern analysis of RNA from liver biopsy samples obtained on days -28, -14, +1, +28, and +56 relative to calving indicated that PC and PEPCK mRNA expression were responsive to onset of lactation but not to prepartum protein or RUP concentration. Abundance of PEPCK mRNA was similar at -28, -14, and +1 DIM but was elevated by +28 and +56 DIM relative to precalving levels. Liver PC mRNA abundance was elevated on +1 DIM, remained elevated through 28 DIM, and declined to precalving levels by 56 DIM. The activity of PC enzyme was correlated (r2 = 0.89) with PC mRNA abundance. The data demonstrate increased abundance of PC mRNA during the early transition period followed by increased abundance of PEPCK mRNA during the postpartum period and suggest increased potential metabolism of lactate, pyruvate, and amino acids that contribute to the liver pyruvate pool.

An evaluation of postabsorptive protein and amino acid metabolism in the lactating dairy cow

The current protein system utilized in the US was formulated in 1985 with minor modifications in 1989 and has gained widespread acceptance. However, some of the assumptions that were adopted by the National Research Council (NRC) appear to be inconsistent with observational data. The marginal efficiency of conversion of absorbed protein to milk protein was assumed by NRC to be 70% until the requirement for absorbed protein was met and was 0% thereafter. The mean marginal efficiency observed for abomasal casein infusions reported in the literature and collected at the Purina Mills Research Center was 21%. Sorting the data into protein-sufficient and protein-deficient classes did not support the assumptions of 70% marginal efficiency in a deficient state and 0% marginal efficiency in the sufficient state. Analyses of nitrogen balance data and abomasal flow data and the work of Van Straalen et al. (77) indicated that energy status of the animal plays a role in determining the response to absorbed protein. Such a consideration was not included in the NRC model. The adoption of equations that describe metabolism at the organ level as opposed to the animal level would allow direct use of organ level data for parameterization and may provide better predictions. Simple representations of digestion and absorption, splanchnic metabolism, and mammary metabolism of amino acids or protein in aggregate are described. These representations could be used to improve the current system and could serve as a bridge to adoption of more complex models.

Transfers of N metabolites across the ovine liver in response to short-term infusions of an amino acid mixture into the mesenteric vein

The effect of acute (4.5 h) infusions into the mesenteric vein of an amino acid (AA) mixture, which simulated the composition of rumen microbial protein, on net transfers of NH3, urea and total AA across the portal-drained viscera (PDV) and liver in the ovine has been examined. Four wether sheep were surgically prepared with vascular catheters across the PDV and liver (Lobley et al. 1995) and were offered a basal diet of 1000 g grass pellets/d (approximately 1.4 × energy maintenance). Each animal was infused at weekly intervals with one of four dilutions of the AA mixture. These dilutions provided 0.44, 0.88, 1.32 and 1.84mmol AA-N/min infused, the lowest of which approximately doubled the net absorption of AA-N from the basal diet. Animals were treated with heparin to allow continuous collection of blood by peristaltic pump for 2 h preceding, and between 0.5–2.5 and 2.5–4.5 h after, the start of the AA infusions. Blood flow in the hepatic artery increased (100 v. 208 g/min; P = 0.002) in response to AA infusion, while hepatic portal venous flow decreased (2090 v. 1854 g/min; P = 0.006). The AA infusion also stimulated O2 uptake by the PDV (P < 0.001) and liver (P = 0.016). Absorption across the PDV and hepatic removal of NH3 were unchanged between basal and amino acid infusion conditions. Urea-N removal across the PDV was unaltered, but hepatic production increased (P < 0.001) with level of AA infusion. During infusions, net appearance of AA across the PDV was below the theoretical level. This may have been due to inhibition of AA uptake from the small intestine, and/or increased removal by the digestive tract of AA from the systemic circulation associated with greater arterial concentrations. Hepatic extraction of AA increased with level of infusion, both for total AA and those included in the infusate. Total hepatic urea-N production tended towards a maximum (estimated as 2 μmol N/g liver wet weight per min). The AA removed by the liver and not used for ureagenesis remained similar (170 μmol AA-N/min) between basal and AA infusions. This was presumed available for anabolic purposes (mainly synthesis of export proteins). The proportion of net AA-N appearance (absorption plus infused) across the PDV removed by the liver declined from 0.71 to 0.53 between basal and AA infusions. In contrast to findings from cattle (Wray-Cahen et al. 1997), increased AA infusion did not alter the net removal of glutamine across the liver. This may reflect differences between the studies in NH3: AA-N absorbed. Further differences between the cattle study and the current findings may relate to the different physiological state (pregnancy v. growth), which may alter the partition of AA between anabolic and catabolic fates.

Alternative models for analyses of liver and mammary transorgan metabolite extraction data

Alternative models for analyses of liver and mammary transorgan data were formulated and fitted to liver and mammary data sets respectively. The models considered metabolite inputs to and effluxes from an extracellular pool. In general, fits were greatly improved over previous efforts using other models (Miller et al. 1991a; Hanigan et al. 1992; Wray-Cahen et al. 1997). Errors of prediction were generally less than 15% for liver and less than 20% for mammary glands. With the possible exception of glutamine for the udder, all metabolites exhibited linear responses to extracellular concentrations within the observed ranges of inputs. However, prediction biases were evident for beta-hydroxybutyrate, acetate, and propionate by liver and for arginine, histidine, citrulline and glycerol by mammary tissue. These biases were hypothesized to be caused by the existence of additional regulatory complexity. With the exception of histidine, parameter estimates for essential amino acid removal by liver were 2-3-fold lower than for mammary gland. Infusion of an amino acid mixture into the mesenteric vein did not alter parameter estimates for removal of amino acids by the liver. Treatment of cows with bovine somatotropin resulted in changes in mammary parameter estimates for aspartate, glutamate, leucine, phenylalanine, glucose, and glycerol.