Amino acid transport systems in bovine mammary tissue.

Nutrient provision to the lactating mammary gland involves three factors: blood nutrient concentration, blood flow, and cellular uptake. This paper reviews uptake of amino acids by bovine mammary tissue relative to interorgan blood flows, red blood cell contribution, arteriovenous differences, specific mammary amino acid transport systems, and glutathione and gamma-glutamyl transpeptidase. Recent studies with ruminant amino acid blood fluxes and the role of the red blood cell in providing nitrogenous substrates to tissues have brought to light new considerations of nutrient availability to mammary tissue. Previous studies measured arteriovenous differences to quantitate net amino acid uptake. These studies are considered relative to seven specific and separate amino acid transport systems, some of which have been identified in bovine mammary tissue. Uptake of sulfur amino acids by mammary tissue has been of interest because it appeared that insufficient quantities were provided during lactation. Glutathione, a tripeptide, may be a principle source of cysteine to mammary tissue via mechanisms involving gamma-glutamyl transpeptidase, glutathione, and red blood cells. The paper considers these mechanisms in relationship to amino acid transport systems.