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Department of Physiology, Queen Elizabeth College, University of London, Campden Hill Road, London, W8 7 AH

1. The unidirectional influx of amino acids into the guinea-pig syncytiotrophoblast was measured using a single circulation paired-tracer dilution technique which allows separate characterization of both fetal and maternal interfaces. An in situ preparation perfused through the fetal circulation was used to examine the fetal side, while an isolated preparation perfused through both the fetal and maternal circulations was used to study both interfaces simultaneously.

2. On the fetal side the maximal uptake (U_max) determined at tracer concentrations was high for the short-chain neutral amino acid alanine (76%) and the long-chain neutrals, leucine (75%), phenylalanine (90%) and tyrosine (82%) and for the basic amino acid lysine (65%). In contrast, U_max was negligible for -methylaminoisobutyric acid and taurine, a -amino acid.

3. The uptake of alanine and phenylalanine on the fetal side was inhibited by both short-chain (alanine, serine, cysteine) and long-chain (phenylalanine, methionine, leucine) neutral amino acids. D-alanine had no effect on L-alanine uptake whereas D-phenylalanine significantly inhibited that of L-phenylalanine. Diaminobutyric acid, lysine and arginine were effective inhibitors of alanine uptake but had no effect on phenylalanine uptake.

4. On the maternal side uptake of alanine, phenylalanine and lysine was measured. Over a wide range of concentrations self-inhibition of alanine influx was similar to the cross-inhibition observed with phenylalanine. In contrast, the influx of phenylalanine, which was strongly self-inhibited, was only partially cross-inhibited by alanine.

5. Influx of alanine and phenylalanine was measured at various perfusate concentrations and was found to be saturable on both maternal and fetal sides. The data were fitted to a single hyperbola and, on the maternal side, the K_m for alanine (10.3±2.7 mM, mean±S.E., n = 3) was three-fold higher than the value measured for phenylalanine (3·1±0·8 mM). On the fetal side the K_m values for alanine (8·4±1·4 mM, n = 4) and phenylalanine (11·9±1·9 mM, n = 3) were similar.

6. The uptake of alanine, phenylalanine and lysine appeared to be highly sodium-dependent accounting for 40-70% of the total influx. However, the inhibited fractions were found to be different on the two sides of the placenta.

7. The results of uptake, cross-inhibition and Na⁺-dependency experiments suggest the presence of an alanine-serine-cysteine (ASC) type system and a leucine (L) type system with markedly overlapping specificities at both the fetal and maternal interfaces. Separate kinetic characterization of a two carrier system was not possible under the conditions of these experiments. However, kinetic parameters for the over-all transport of alanine and phenylalanine were measured.