John C. MacRae

SOME FACTORS WHICH INFLUENCE THERMAL ENERGY LOSSES DURING THE METABOLISM OF RUMINANTS

Abstract Two aspects of thermal losses which arise as a result of the metabolism of ruminants are reviewed. The first considers the energy costs associated with basal metabolism, with emphasis on the contribution of protein turnover. The heat production associated with protein synthesis in the mature maintenance fed ruminant accounts for a minimum of 15–20% of total heat loss. From the few data available from growing ruminants, protein synthesis accounts for a similar proportion of thermal loss here also. Secondly the problem of poor utilisation of some roughages compared with concentrates is discussed and comparisons made of the possible differences in energy costs of ingestion, digestion, including fermentation and absorption. It is concluded that differences in these processes are insufficient between the two types of ration to account for the larger thermal losses with roughages and it is suggested that the major cause is a differential ability to handle excess acetate. In animals fed concentrate diets the larger production of propionate supplies adequate NADPH 2 to enable acetate to be converted to fat while in those fed roughages the larger amount of acetate and smaller proportion of propionate results in a metabolic excess of acetate which has to be eliminated as heat by some form of futile cycle. It is suggested that ruminants might be able to utilise efficiently larger quantities of acetate from roughage feeds provided an additional source of reduced coenzyme is available; such a source could well be amino acids.

Accretion of total protein and individual amino acids by organs and tissues of growing lambs and the ability of nitrogen balance techniques to quantitate protein retention

Twelve Suffolk-Finn Dorset lambs were reared from 25 to 40 or 25 to 55 kg body weight on either pelleted dried grass or a ration of pelleted grass plus barley (ratio 1:1) in a comparative slaughter experiment designed to determine the amounts of total nitrogen and individual amino acids accreted in different body components during growth. Nitrogen (N) balance measurements were determined frequently during this growth phase and accumulated N retentions were compared with the total N accretion determined by comparative slaughter. Total N and individual amino acids accumulated in carcass, wool, skin, offal and blood, head and feet, gastro-intestinal tract and liver were linearly related to body weight in all cases other than for cysteine in carcass. At 25 kg live weight, proportionately 0·52 of total body N was in carcass components, 0·115 in wool, 0·08 in skin, 0·10 in offal and blood, 0·095 in head and feet, 0·06 in the gastro-intestinal tract and 0·02 in liver. However as the animals grew from 25 to 55 kg, 0·256 of the total N accretion was in wool, which was rich in cysteine (98 g/kg total amino acid). Carcass accretion represented only 0·449 of total body N accretion. The N balance technique overestimated net protein accretion by 0·24 (s.e. 0·036).

Amino acid use by the gastrointestinal tract of sheep given lucerne forage

Essential amino acid (EAA) utilization by gastrointestinal tract (GIT) tissues has been investigated in sheep given 800 and 1,200 g/day lucerne pellets. Animals prepared with indwelling catheters into the aorta and the portal drained viscera plus cannulas into the small intestine were infused with mixed U-13C-labeled amino acid or (1-13C]leucine tracers into the jugular vein or directly into the small intestine. GIT sequestration of EAA from arterial and luminal AA pools was determined from tracer and tracee arterioportal concentration differences at both levels of intake. Proportional tracer 13C-labeled EAA extraction of the arterial supply, on first pass across the GIT during jugular infusion, ranged from 0.063 for histidine to 0.126 for leucine. Recovery of intestinally infused tracer 13C-EAA at the portal vein ranged from 0.61 for histidine to 0.83 for valine. These data were independent of intake. Calculated rates of tracee sequestration by GIT tissues represented 0.45-0.65 of whole body EAA flux, except for histidine, for which the values were much lower (0.25-0.32). With the exception of phenylalanine, more than 0.8 of the EAA used by the GIT was extracted from circulating blood, thus calling into question the theory that GIT tissues make preferential use of EAA during absorptive metabolism, restricting supply to peripheral tissues such as skeletal muscle (growth) or mammary glands (lactation). Instead the GIT seems to compete very successfully with these tissues for circulating blood EAA.Essential amino acid (EAA) utilization by gastrointestinal tract (GIT) tissues has been investigated in sheep given 800 and 1,200 g/day lucerne pellets. Animals prepared with indwelling catheters into the aorta and the portal drained viscera plus cannulas into the small intestine were infused with mixed U-13C-labeled amino acid or [1-13C]leucine tracers into the jugular vein or directly into the small intestine. GIT sequestration of EAA from arterial and luminal AA pools was determined from tracer and tracee arterioportal concentration differences at both levels of intake. Proportional tracer13C-labeled EAA extraction of the arterial supply, on first pass across the GIT during jugular infusion, ranged from 0.063 for histidine to 0.126 for leucine. Recovery of intestinally infused tracer13C-EAA at the portal vein ranged from 0.61 for histidine to 0.83 for valine. These data were independent of intake. Calculated rates of tracee sequestration by GIT tissues represented 0.45-0.65 of whole body EAA flux, except for histidine, for which the values were much lower (0.25-0.32). With the exception of phenylalanine, more than 0.8 of the EAA used by the GIT was extracted from circulating blood, thus calling into question the theory that GIT tissues make preferential use of EAA during absorptive metabolism, restricting supply to peripheral tissues such as skeletal muscle (growth) or mammary glands (lactation). Instead the GIT seems to compete very successfully with these tissues for circulating blood EAA.

KINETICS OF BLOOD FREE AND MILK CASEIN-AMINO ACID LABELLING IN THE DAIRY GOAT AT TWO STAGES OF LACTATION

The kinetics of blood free amino acids (AA) transfer into milk casein were compared in goats (n 4) at 61 (SE 5) d (Expt 1; post-peak, 4.51 (SE 0.26) kg milk/d) and at 180 (SE 6) d (Expt 2; late, 2.36 (SE 0.16) kg milk/d) of lactation during non-primed, continuous (Expt 1, 12 h; Expt 2, 16 h) intravenous infusions of mixtures of L-[1-13C]leucine and L-[1-13C]phenylalanine with either L-[1-13C]valine (Expt 1) or L-[5-13C]methionine (Expt 2). The 13C enrichments of blood free and casein-bound AA were fitted to a single exponential model to estimate isotopic plateaux and the fractional rate constant for milk casein labelling. Milk protein output and its contribution to whole-body flux was higher in Expt 1 (post-peak) than in Expt 2 (late lactation), but the kinetics of 13C labelling of the casein-bound AA were similar for all AA tracers in both experiments. At both stages of lactation the delay (6-8 h) between the attainment of isotopic plateau for the blood free AA and the corresponding attainment of plateau for the casein-bound AA indicated that the blood free pool was not the immediate precursor pool for milk casein biosynthesis. Plateau enrichments of casein-bound AA were generally higher than those for the corresponding blood free AA in both experiments. These results indicate that the relative contributions of different AA sources to the immediate precursor pool for milk casein biosynthesis are similar at different stages of lactation despite major changes in the partitioning of whole-body flux towards milk protein output. Non-milk protein fluxes were also similar in post-peak and late lactation.

Physiological and metabolic implications of conventional and novel methods for the manipulation of growth and production

Abstract Growth-promoting strategies, both current and future, can only be most effectively introduced when the physiological consequences and nutritional requirements of the manipulation are understood. Beta-agonists increase protein gain, in part at least, by a reduction in protein breakdown, similar to the situation with anabolic steroids. Possibly as a consequence of this, the response is independent of feed quality. The specific myotrophic action of the beta-agonists is achieved at the expense of the growth of other tissues, including skin and fibre, and thus, while having potential for meat production, may have deleterious effects in other husbandry conditions. Alterations in the competitiveness of various organs for nutrients can produce marked anabolic and catabolic effects. The gastrointestinal tract represents a major contributor for both protein and energy dynamics in the animal, and manipulations which reduce the metabolic demands of the tissue (e.g. antibiotics, probiotics) increase nutrients available to other tissues, while stimulators of gastrointestinal secretions and hyperplasia (e.g. parasites, antinutritional factors) have an inhibitory effect on peripheral tissue anabolism. Growth promotion through stimulation of protein synthesis may be dependent on amino acid supply and there is considerable evidence that effective use of growth hormone is achieved only at medium to high levels of dietary protein. This will have implications for future manipulation of the somatotrophic axis via immunological and biotechnological means. Protein accretion may also be enhanced by procedures which divert substrates away from fat. This has been demonstrated in rodents by the use of antibodies against fat cell membranes, although whether the protein anabolic response occurs by ‘passive’ (increased substrate availability) or ‘active’ (alterations in endocrine status) mechanisms is not known.

Effect of the Methodology on Circulating Peptides Determination and Consequences on Net Flux Measurements Across the Gastrointestinal Tract of Sheep

Two methodologies for the measurement of peptide amino acids (PAA) in blood were compared to evaluate their effects on the measurement of the net flux of peptides across the gastrointestinal tract of sheep. These methods consisted of a chemical deproteinization of blood samples with sulfosalicylic acid (1.6M, 0.1 ml for 1 ml of sample) or perchloric acid (1M, 1 ml for 1 ml of sample) followed by ultrafiltration through a 3,000-Da cut-off filter (SSA+UF3kD) or gel filtration through a Sephadex G-15 column (1,500-Da cut-off filter; PCA+G-15), respectively, prior to PAA analysis. Peptide concentrations as determined by amino acid concentrations before and after hydrolysis of samples were slightly greater with the SSA+UF3kD (991 µM) than with the PCA+G-15 (605 µM) methodology. However, both methodologies gave similar net portal-drained viscera flux data in sheep fed on alfalfa pellets with histidine as the only significant uptake of peptide amino acid.