Uterine uptake of amino acids and placental glutamine-glutamate balance in the pregnant ewe

The uterine uptake of amino acids was studied in 10 pregnant sheep with gestational ages of 114-146 days. After recovery from surgery, arterial and uterine venous samples were drawn simultaneously via indwelling catheters and analysed for amino acid and oxygen content. In seven ewes, amino acid concentrations were measured by a chromatographic technique. In four ewes, glutamate and glutamine arterio-venous differences across the uterine and umbilical circulations were measured by an enzymatic method. The uptake of neutral and basic amino acids was 66 μmol/mmol O₂ and 17.3 μmol/mmol O₂, respectively. Comparison of uterine and umbilical uptake shows that the bulk of the neutral and basic amino acids taken up by the pregnant uterus are transferred to the fetus. There was no significant uptake of acidic amino acids (i.e. glutamate, aspartate and taurine). Glutamate was delivered from the fetus to the placenta but excretion of glutamate into the uterine circulation was negligible. Glutamine and asparagine were delivered to the fetus in amounts which were two to three times larger than the placental uptake of glutamate and aspartate. Therefore placental conversion of exogenous glutamate and aspartate to glutamine and asparagine cannot account entirely for the fetal uptake of these amino acids.