TRANSFER OF LEUCIHE AND GLUTAMATE ACROSS HUMAN PLACEHTA

Abstract

Previous studies in this laboratory using tissue slices demonstrated significant differences in placental uptake of leucine and glutamate. The transfer of these two amino acids across human placenta has now been studied with an in vitro perfusion technique. With both circulations open (not recirculated), the transfer rate for L-leucine from maternal (M) to fetal (F) circulation was 1.7 times that of the D-isomer. The latter rate was attributed to diffusion; the L:D ratio provided a measure of active transport. The L:D ratio F→M was 1, indicating polarity of fhe placental membrane with respect to active transport. In similar studies with glutamate, the D-isorner was more efficiently transferred in both directions (L:D ratio, 0.6 and 0.4 respectively). This unexpected result was explained by the rapid metabolism of the natural L-amino acid, only 15% of the placental uptake appearing in the opposite circulation. With M circuit open and F recirculated, an F > H gradient was rapidly achieved with L-leucine. In contrast, F levels of L-asparate and L-glutamate progressively fell below M. A new peak appeared on the F aminogram consistent with glutamine-asparagine.

Rapid metabolism of the dicarboxylic acids by placenta serves as a barrier to transfer and may play a significant role in fetal H₂ metabolism.

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