APPLICATION OF A U-C-13-LABELED AMINO-ACID TRACER IN LACTATING DAIRY GOATS FOR SIMULTANEOUS MEASUREMENTS OF THE FLUX OF AMINO-ACIDS IN PLASMA AND THE PARTITION OF AMINO-ACIDS TO THE MAMMARY-GLAND

A preliminary study was conducted using lactating British Saanen goats (n = 5) at 109 to 213 d in milk that yielded 1.67 to 3.68 kg of milk/ d to examine the application of a U-C-13-labeled amino acid (AA) mixtu re obtained from hydrolyzed algal proteins as a tracer for measuring p lasma flux (n = 5) and partition to the mammary gland (n = 3; arteriov enous difference) of 13 AA simultaneously. Except for lie and Ser, the re was incomplete (6 to 54%) equilibration of the tracer with AA from packed blood cells (>90% erythrocytes) during the 6-h infusions. This result agreed with the large ratio of packed cells to gradients for pl asma AA concentration that was also observed. However, net mass and is otope removals by the mammary gland were predominately from plasma, in dicating that the erythrocytes did not participate in kinetic exchange s. Plasma AA fluxes (millimoles per kilogram of metabolizable protein intake per kilogram of body weight(0.75)) differed among goats that co nsumed different protein sources; however, overall rates were lowest f or Met (5 to 14) and His (8 to 17) and highest for Leu (48 to 70) and Ala (53 to 88). On average, 25% of plasma flux was partitioned to the mammary gland. Less than 20% of His, Ser, Phe, and Ala were directed t o the mammary gland; 20 to 30% of Arg, Thr, Tyr, and Leu were directed to the mammary gland; and 30 to 40% of Pro, Ile, Lys, and Val were di rected to the mammary gland. The unidirectional AA flux in the mammary gland (AA apparently available for protein syntheses, oxidation, and metabolite formation) did not match the pattern that is required for c asein synthesis, suggesting differences in the metabolic requirements of AA for nonmilk protein synthesis.