Kinetics of blood free and milk casein-amino acid labelling in the dairy goat at two stages of lactation

Abstract

The kinetics of blood free amino acids (AA) transfer into milk casein were compared in goats (n 4) at 61 (SE . The I3C 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 caseinbound 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.Milk protein: Amino acid kinetics: Casein: Lactating goats Lactation requires a major increase in the partitioning of amino acids (AA) towards the mammary gland for milk protein synthesis (3 1 4 6 % of whole-body protein flux compared with I-6% in dry, non-pregnant animals; Champredon et al. 1990;Baracos et al. 1991). However, milk protein output represents only a proportion (W609'0) of total protein synthesis in the lactating mammary gland (Oddy et al. Champredon et al. 1990;Baracos et al. 1991) and therefore approximately half of the synthesized protein must be degraded, assuming that there is no appreciable net gain by the mammary tissue protein pool. This may represent either general turnover of both constitutive and milk proteins, or a specific channelling of milk protein precursor AA through protein(s) which constitutes an intermediate mixing pool(s).The results of Oddy et al. (1988) would seem to indicate that the latter phenomenon exists, but their study involved only short-term infusions of a single AA tracer (leucine) which is known to be extensively metabolized by the gland. The present study involved the * For reprints.

Effects of stage of lactation on protein metabolism in dairy cows

“…This is the first study, however, to consider how stage of lactation might impact on whole body protein kinetics in dairy cows and where very little effect on ILR was observed. This contrasts with reports in dairy goats, where animals in mid to late lactation had a lower whole body ILR based on either leucine (by 35%: Riis, 1988;22%: Bequette et al, 1994), phenylalanine (by 16%: Bequette et al, 1994) or lysine (by 38%: Mabjeesh et al, 2000) compared with goats in early lactation. Such differences highlight the caution needed when extrapolations are made between ruminant species but raise the intriguing question -why are there differences between lactating caprines and bovines?…”

“…In contrast, milk protein output in the current dairy cows decreased by only 14%. Thus, for the dairy cows the proportion of ILR used for milk protein secretion showed a modest decrease during lactation from 0.39 to 0.34±0.007, while in the dairy goats studies, the proportion declined from 0.30 to 0.19 (Bequette et al, 1994) or remained unaltered at a much lower proportion (approximately 0.15; Riis, 1988;Mabjeesh et al, 2000).…”

“…This suggests an interaction between folic acid supplementation and stage of lactation related to a possible decreased demand for methionine as lactation progresses. First, in dairy cows, the demand for methionine to support protein synthesis likely decreases as lactation progresses: whole body irreversible loss rates (ILR) of amino acids (which include usage for both protein synthesis and oxidation) decreased with advancing lactation in goats (Riis, 1988;Bequette et al, 1994;Mabjeesh at al., 2000). In addition, the use of methionine as a methyl group precursor may also decline as lactation progresses.…”

Quantitative aspects of blood and amino acid flows in cattle

“…Usually, the analysis of amino acids in the plasma is used to monitor the mammary gland amino acid uptake even though it does not seem sufficiently accurate (Guinard and Rulquin, 1994). Red blood cells (Hanigan et al, 1991) and peptides (Backwell, 1994;Bequette et al, 1994) also deliver amino acids to the mammary gland for milk protein synthesis and mammary gland tissue maintenance and growth.…”

Nitrogen-15 labelling of the nitrogenous fractions of milk using oral administration of labelled ammonium sulphate in the diet

“…Currently, the best way to obtain homogeneously marked products has been via the synthesis of milk proteins by ruminants (Gruhn and Thelemann, 1973;Bequette et al, 1994;Mahé et al, 1994a). Though stable isotopes could be administered intravenously (Oddy et al, 1988;Bequette et al, 1994;Boirie et al, 1995), or directly in the rumen via a fistula (Mahé et al, 1994a), it was easier and more harmless for the animais to incorporate them into the di et. However, the production of labelled milk proteins required the research of food easily enriched in stable isotope.…”