TRANSFER OF GLUTAMIC ACID ACROSS THE HUMAN PLACENTA PERFUSED IN VITRO

Abstract

SummaryTransfer of amino acids was studied with an in vitro perfusion technique of human placenta in which independant maternal and fetal circulations were established. When the fetal perfusate was recirculated, a concentration gradient was established with fetal concentrations of alanine, glycine and lysine about 1.5 times higher than maternal. In this system, the placenta could actively transfer 4.1 mM per day of each of these amino acids. Glutamic acid and aspartic acid were progressively removed from the fetal circulation so that fetal concentrations fell below maternal. The exchange rates of L‐ and D‐glutamic acid were compared, the latter providing a measure of simple diffusion. The ratio of L:D exchange rates was 0.64 from the maternal to the fetal circulation and 0.41 in the reverse direction, indicating a more rapid bidirectional exchange of the D‐isomer. Simultaneously measured exchange rates of leucine gave a L:D ratio of 1.73 and 1 00 respectively, indicating active transfer of L‐leucine towards the fetus. While for L‐glutamic acid, transfer across the placenta was even lower than expected on the basis of simple diffusion, uptake into the tissue was high. Of the amount presented to the placenta, 49 percent was taken up, of which only 10 per cent passed to the other side. The rest was retained and presumably metabolized. For L‐leucine, the uptake was only 16 percent of which 75 per cent was transferred. The placenta serves as an effective barrier to the transfer of glutamic acid by rapidly metabolizing this amino acid. It is postulated that maternal glutamine, which is efficiently transferred across the placenta, contributes its amid‐N2 to the fetal synthesis of purines and pyrimidines and that the resulting surplus of glutamic acid is returned to the placenta which removes it from the circulation and metabolizes it.