Free amino acid levels during lactation in rats: effects of protein quality and protein quantity.

Free amino acid levels were determined in serum, mammary gland, liver and muscle during lactation in rats fed diets varying in protein. Using a 3 X 2 factorial design, after conception female rats were fed purified diets varying in protein quality (wheat, wheat + lysine + threonine or casein + methionine) and in protein level (11.6% vs. 22.3%). On d 15 of lactation dams were decapitated; blood was collected, and the serum was quickly frozen. Samples of mammary gland, liver and calf muscle were also removed and quickly frozen. After deproteinization, free amino acids were determined on an LKB 4400 Amino Acid Analyzer. As the protein nutritional value of the diet increased, milk yield increased threefold and the level of free lysine in the mammary gland increased six- to eightfold. With protein quality or quantity improvement, the level of free methionine increased two- to threefold, and no consistent changes were seen in the levels of free threonine, aromatic and branched chain amino acids in mammary glands. Free lysine concentrations in mammary gland, liver and muscle were highly correlated with free lysine in serum (r2 = 0.94, 0.96, 0.93, respectively). Under the conditions of this experiment, a direct relationship between dietary lysine, free lysine in the mammary gland and milk secretion was observed.