Evaluation of whole blood and plasma in the interorgan supply of free amino acids for the mammary gland of lactating dairy cows

We investigated the contribution of plasma and red blood cells to amino aci d (AA) supply for milk protein synthesis during a combination of treatments that included abomasal infusion of casein and AA and utilization of a hype rinsulinemic-euglycemic clamp. Treatments resulted in substantial differenc es in circulating concentrations of AA, mammary uptake of AA, and rates of milk protein synthesis. Arterial concentrations of all AA in plasma were hi ghly correlated with that of whole blood. Concentrations of AA in red blood cells were either higher (Asn+Asp, Gly, His, Leu, Met, Om, Ser, Tau, Thr, and Tyr), lower (Ala, Arg, Cit, Cys, lie, and Val), or similar (Gln+Glu, Ph e, and Pro) to that of plasma. Arteriovenous difference measurements demons trated that interorgan transfer of AA to the mammary gland was primarily by plasma. There was little involvement of red blood cells except for small q uantities of Leu, Met, and Thr to the mammary gland; this contribution was greatest for Met and accounted for 14% of the total mammary uptake. Counter current transport of Gln+Glu, Asn+Asp, and Pro was also evident where these AA were extracted from plasma, but were released into red blood cells as b lood passed through the mammary gland. This net influx of Gln+Glu, Asn+Asp, and Pro into red blood cells was equivalent bo 26, 17, and 30% of their ma mmary uptake from plasma. Overall, the interorgan transport of free AA for the mammary gland was predominantly by plasma, and red blood cells were lim ited to minor contributions in mammary uptake for a few BA. Furthermore, ar teriovenous differences of essential AB across the mammary gland were highl y correlated between plasma and whole blood.