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.
Proceedings of the New Zealand Society of Animal Production, Volume 59, , 62-65, 1999
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