Placental transport of leucine, phenylalanine glycine, and proline in intrauterine growth-restricted pregnancies

L-[1-C-13]Leucine, [1-C-13]glycine, L-[1-C-13]phenylalanine, and L-[1-C-13] proline were infused as a bolus into the maternal circulation of seven appr opriate for gestational age at 30.3 +/- 3.0 wk and 7 intrauterine growth-re stricted pregnancies at 26.5 +/- 1.0 wk gestation to investigate placental transport in vivo. Umbilical venous samples were obtained at the time of in utero fetal blood sampling at 450 +/- 74 sec from the bolus injection. In normal pregnancies the fetal/maternal (F/M) enrichment ratios for leucine ( 0.76 +/- 0.06) and phenylalanine (0.77 +/- 0.06) were higher (P < 0.01) tha n the F/M ratios for glycine (0.18 +/- 0.04) and proline (0.22 +/- 0.02). T his suggests that these two essential amino acids rapidly cross the placent a in vivo. Compared with the essentials, both glycine and proline had signi ficantly lower F/M enrichment ratios, which were not different from each ot her. The results support the hypothesis that amino acids with high affinity for exchange transporters cross the placenta most rapidly. In intrauterine growth-restricted pregnancies, the F/M enrichment ratio was significantly lower (P < 0.01) for L-[1-C-13]leucine (0.76 +/- 0.06 vs. 0.48 +/- 0.07) an d for L-[1-C-13]phenylalanine (0.77 0.06 vs. 0.46 +/- 0.07) compared with a ppropriate for gestational age pregnancies reflecting impaired transplacent al flux. The F/M enrichment ratio did not differ for [1-C-13]glycine (0.18 0.04 vs. 0.17 +/- 0.03), and L-[1-C-13]proline (0.22 +/- 0.02 vs. 0.18 +/- 0.04).