Amino acid utilisation by the mammary gland: Whole blood versus
plasma free amino acid pools.
D. Pacheco-Rios, B.P. Treloar, J. Lee, T.N. Barry and W.C.
McNabb
AgResearch, Grasslands Research Centre, PB 11008, Palmerston
North, New Zealand.
E-Mail:
Proceedings of the New Zealand Society of Animal Production. 1999, 59: 62-65
Utilisation of amino acids (AA) by the mammary gland of dairy
cows has been studied in order to understand the responses in milk
protein production to alterations in nutrient supply. One approach has
been the use of balance studies, in which the amount of AA taken up from
the blood by the mammary gland is compared with their respective output
in milk protein(s). Although it is generally accepted that the free AA
pool in blood is the major precursor for milk protein synthesis, there
are differences when plasma or whole blood arterio-venous differences
(A-V) are used to measure the uptake of AA by the mammary gland.
Arterial and venous blood samples were obtained at 2 h intervals from
cows fed fresh pasture over two 12-h sampling periods. The samples were
subdivided into whole blood (WB) and plasma (P) and analysed for free AA
concentrations, which were used to estimate amino acid concentrations in
the erythrocyte. There were no significant differences in the A-V
calculated from WB or P for the majority of AA. Plasma A-V was
significantly higher than WB for threonine (13.9 vs 8.9 nmol/g blood)
whilst WB A-V was higher than P for isoleucine (26.9 vs 23.3 nmol/g
blood) and tyrosine (11.8 vs 10.3 nmol/g blood). Assuming the
differences in A-V between WB and P can be attributed to AA exchange
with the red blood cell (RBC), significant (t<0.01) exchange
from RBC to the mammary gland occurred for only isoleucine (15% of WB
A-V) and tyrosine (14% of WB A-V). These results indicate that plasma AA
concentrations are valid measures of the immediate free amino acid pool
available for uptake by the mammary gland.
Keywords: NZSAPAB;
amino acids; arterio-venous differences; milk protein; dairy
cows.
Last Updated 06-08-1999
