Plasma amino acid concentrations were determined in virgin female rats, in pregnant rats (12 and 21 days after impregnation) and in 21-day foetuses. The total amino acid concentration in plasma decreases significantly with pregnancy, being lower at 12 than at 21 days. Alanine, glutamine+glutamate and other ‘gluconeogenic’ amino acids decrease dramatically by mid-term, but regain their original concentrations at the end of the pregnancy. With most other amino acids, mainly the essential ones, the trend is towards lower concentrations which are maintained throughout pregnancy. These data agree with known nitrogen-conservation schemes in pregnancy and with the important demands on amino acids provoked by foetal growth. In the 21-day foetuses, concentrations of individual amino acids are considerably higher than in their mothers, with high plasma foetal/maternal concentration ratios, especially for lysine, phenylalanine and hydroxy-proline, suggesting active protein biosynthesis and turnover. All other amino acids also have high concentration ratios, presumably owing to their requirement by the foetuses for growth. Alanine, glutamine+glutamate, asparagine+aspartate, glycine, serine and threonine form a lower proportion of the total amino acids in foetuses than in the virgin controls or pregnant rats, probably owing to their role primarily in energy metabolism in the adults. The results indicate that at this phase of foetal growth, the placental amino acid uptake is considerable and seems to be higher than immediately before birth.
