Amino Acid Utilization by Lactating Mammary Gland

Amino acids utilization comprises two phases, cellular uptake and intracellular metabolism. The paper reviews reports on amino acid uptake in different species and at different stages of lactation and presents new data on changes at lactogenesis. The relative importance of changes in factors which determine uptake, mammary blood flow, arterial concentration, and mammary extraction, are discussed. In addition to direct incorporation into milk protein, some absorbed amino acids are an energy source and carbon precursors of other amino acids synthesized in mammary cells. Branched-chain amino acids and ornithine act as nitrogen precursors in these syntheses. Ornithine, which is derived partly from absorbed arginine, is also a precursor of spermidine, a postulated intracellular mediator of hormone action. The question of whether milk protein yield is limited by supply to the mammary gland of certain amino acids is discussed. Much evidence indicates the importance of certain essential amino acids that are transferred stoichiometrically from blood to milk. Results of experiments involving administration of exogenous growth hormone to goats are presented. Increases in milk yield and protein yield are associated with increased rates of mammary blood flow, emphasizing the role of the latter in determining uptake.