TRANSFER OF AMINO-ACIDS ACROSS THE INVITRO PERFUSED HUMAN-PLACENTA

Universitats-Frauenklinik, Marburg, and Universitats-Frauenklinik, Munster, West GermanyAmino acid transfer across human placenta has been studied in an in yitro perfusion system. Transfer rates from maternal to fetal circuit of 12 amino acids were about the same when there was a downhill gradient permitting diffusion and active transport. This was termed “maximal transfer rate.” Cystine, aspartic acid, and glutamic acid were transferred at approximately half the rate of the others under these conditions. Measurement of exchange rates with radioactive tracers and with equimolar concentration of amino acids on both sides of the membrane provided a close estimate of maximal transfer rate. Fetal nutrition requires active transfer against a gradient. The rate of active transfer is a fraction of the exchange rate. The ability to establish a transplacental gradient by perfused placenta was investigated with the maternal circuit open and fetal circuit closed (recirculated). The fetal to maternal concentration ratio for D-leucine and antipyrine was 1.0, for L-leucine, 1.5, indicating a stereospecific transport mechanism for leucine. Gradients of 1.56 ± 0.11 and 1.62 ± 0.10 were demonstrated for l- alanine and L-Iysine, respectively. The establishment of transplacental gradients is a placental function, requiring no assistance from maternal or fetal factors. The ratios of the exchange rates for the L- and D-isomers were determined as a measure of the active, stereospecific transport system. The L:D ratio from the maternal to fetal circulation for leucine was 1.64 ± 0.19, for alanine 1.78 ± 0.49. In the reverse direction, the ratio for leucine was 1.03. The placental membrane exhibits a polarity in active transport directed towards the fetus. The transplacental gradient for leucine is the result of a dynamic equilibrium between active transport towards the fetus and diffusion back towards the maternal circulation. The availability of intracellular amino acids to serve as a reserve for fetal needs was studied by perfusing the placenta with buffer containing no amino acids. After 2 hr of perfusion, 53-84% of each of nine amino acids was still retained within the placenta. It is suggested that the high tissue concentrations of amino acids are related to the metabolic needs of the placenta rather than to its transport functions. © 1979 International Pediatric Research Foundation, Inc.