EFFECT OF INTRAVENOUS AMINO-ACID INFUSION ON LEUCINE OXIDATION ACROSSTHE MAMMARY-GLAND OF THE LACTATING GOAT

Changes in the kinetics of leucine in the mammary gland were examined in four lactating goats (25, 38, 45, and 135 DIM) that were given an i .v. infusion of a mixture of 18 AA, not including leucine, to alter th e availability of leucine to the gland relative to other AA. Arteriove nous monitoring of [1-C-13]leucine kinetics across one-half of the mam mary gland was conducted on the last day (d 6 or 7) of the saline (con trol) and the AA infusion periods. Although blood flow to the mammary gland and the arterial concentration of most AA other than leucine wer e increased by the AA infusion, milk and protein yields did not change . For goats in early lactation (n = 3), arterial leucine concentration s fell considerably during AA infusion; however, the arteriovenous dif ference of leucine was maintained, resulting in uncommonly low leucine concentrations in venous plasma (8 mu M). Whole body leucine flux (pr otein synthesis plus oxidation) was unaffected by AA infusion, but, be cause whole body leucine oxidation was reduced, whole body utilization of leucine for protein synthesis increased. The AA infusion reduced m ammary oxidation of leucine to approximately one-third of control valu es. These results suggest that leucine oxidation can be reduced consid erably without affecting milk protein output; thus, leucine oxidation may not be an irrevocable consequence of mammary metabolism. If catabo lism of other AA either by the gland or in the whole body can be reduc ed, then the efficiency of milk yield can be improved.