An arteriovenous kinetics technique was used to monitor mammary gland lysine and protein metabolism in goats (n = 4) at two stages of lactation (80 ± 17 vs. 233 ± 14 DIM) in response to an i.v. infusion of lysine (Lys) plus methionine (Met). At each stage of lactation [2-15N] and [1-13C; 6,6-2H2] Lys kinetics were performed on the last day of 5-d i.v. infusion of saline followed by Lys (370 mg/h) plus Met (84 mg/h, LM). Milk and protein yields and dry matter intake were higher in early than in late lactation, but LM infusion did not affect these variables. Regardless of stage of lactation, the absolute and fractional oxidation rates of Lys by the mammary gland increased in response to LM infusion. When corrected for Lys oxidation, net uptake of Lys by the gland was less than milk protein Lys secretion. However, correction for the contribution of peptides (15.8%) to Lys uptake brought net Lys uptake close into balance with milk Lys secretion. The present data suggests that when Lys is in excess of requirements, the mammary gland appears to dispose of the extra supply via the oxidative mechanism.
Bibliographical noteFunding Information:
The work is part of a joint project at the Rowett Research Institute and Reading University cosponsored by a consortium of the Ministry of Agriculture, Fisheries and Food; the Biotechnology and Biological Sciences Research Council; the Scottish Office Executive Rural Affairs Department; Purina Mills, Inc.; Hendrix Voeders B.V.; and the Milk Development Council of England and Wales. Special thanks are extended to Vivien Buchan and David Brown for their expertise in AA determinations. The expertise offered by S. Anderson and A. G. Calder for mass spectrometry analysis is gratefully acknowledged. S. J. Mabjeesh was the recipient of a British Council post-doctoral Fellowship.
- Amino acid metabolism
- Mammary gland