Evaluation of Whole Blood and Plasma in the Interorgan Supply of Free Amino Acids for the Mammary Gland of Lactating Dairy Cows

¹ Department of Animal Science, Cornell University, Ithaca, NY 14853-4801

We investigated the contribution of plasma and red blood cells to amino acid (AA) supply for milk protein synthesis during a combination of treatments that included abomasal infusion of casein and AA and utilization of a hyperinsulinemic-euglycemic clamp. Treatments resulted in substantial differences in circulating concentrations of AA, mammary uptake of AA, and rates of milk protein synthesis. Arterial concentrations of all AA in plasma were highly correlated with that of whole blood. Concentrations of AA in red blood cells were either higher (Asn+Asp, Gly, His, Leu, Met, Orn, Ser, Tau, Thr, and Tyr), lower (Ala, Arg, Cit, Cys, Ile, and Val), or similar (Gln+Glu, Phe, and Pro) to that of plasma. Arteriovenous difference measurements demonstrated that interorgan transfer of AA to the mammary gland was primarily by plasma. There was little involvement of red blood cells except for small quantities of Leu, Met, and Thr to the mammary gland; this contribution was greatest for Met and accounted for 14% of the total mammary uptake. Countercurrent 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 blood passed through the mammary gland. This net influx of Gln+Glu, Asn+Asp, and Pro into red blood cells was equivalent to 26, 17, and 30% of their mammary uptake from plasma. Overall, the interorgan transport of free AA for the mammary gland was predominantly by plasma, and red blood cells were limited to minor contri butions in mammary uptake for a few AA. Furthermore, arteriovenous differences of essential AA across the mammary gland were highly correlated between plasma and whole blood.

Key Words: amino acids • insulin • red blood cells • plasma

Submitted on August 17, 1999
Accepted on January 4, 2000

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