Uterine uptake of amino acids throughout gestation in the unstressed ewe

The uterine uptake of 21 amino acids was determined in chronically catheterized ewes from 35 to 145 days of gestation. Total uptake of amino acids increased as gestation progressed as described by the equation: Q (mmoles/kg·day)=0.19 gestational age (days)+0.30. We were unable to detect a statistically significant correlation of uterine A-V concentration difference and gestational age for most amino acids. The uptakes of only eight amino acids contributed to the total increase (arginine, glutamine, valine, leucine, ornithine, isoleucine, tyrosine, phenylalanine). The amino acids which made the largest contribution to uterine uptake are similar to those which furnish the largest contributions to umbilical uptake previously reported, except for glycine, which exhibits significant umbilical uptake, but no uterine uptake. Glutamate, which is removed by the placenta from the umbilical circulation, exhibited no significant uterine uptake nor production. The total amino acid nitrogen uptake by the uterus extrapolated to 130 days’ gestation, 1.20 gm/kg fetal weight·day, closely approximates the previously reported value for umbilical amino acid nitrogen uptake at 130 days. Comparison of the individual amino acids removed from the uterine circulation with those which enter the umbilical circulation, however, suggests that metabolic conversions which involve glutamate and glycine occur in the placenta.