Vascular sources of amino acids for milk protein synthesis in goats at two stages of lactation.

An arteriovenous technique, combined with a 30-h i.v. infusion of [5-(13)CH3]Met and [5,5,5-(2)H]Leu, was used to monitor mammary uptake of free amino acid (AA) and to estimate the proportion of casein synthesized from circulating peptides in goats in early and late lactation. At both stages, kinetics was performed on the last day of consecutive 5.5-d periods. The first period was an i.v. infusion of saline and the second an i.v. infusion of lysine (8.9 g/h) plus methionine (2 g/h). Net uptake of essential AA and protein yields were higher in early than in late lactation. Uptake of free Met, His, and Pro was less than, uptake of Tyr and Lys was equal to, and uptake of Arg, Leu, Val, and Ile was greater than milk protein synthesis. Peptide uptake, estimated from the difference in casein and plasma free AA enrichment, accounted for a larger fraction of casein-Met (17 vs. 8%) and casein-Leu (27 vs. 12%) in late than in early lactation. Small decreases in mammary blood flow, AA transport activity, and AA concentrations accounted for the lower uptake of AA in late compared with early lactation. Based on our studies of several AA, the utilization of circulating peptides for casein synthesis appears to be a general phenomenon.