Cationic amino acid transporter activity in the syncytiotrophoblast microvillous plasma membrane and oxygenation of the uteroplacental unit

The purpose of this study was to determine whether there is any relationship between the activity of cationic amino acid transporters in the microvillous plasma membrane (MVM) of the Syncytiotrophoblast and the oxygenation of the uteroplacental unit. Oxygenation data were obtained at the time of caesarean section from the uterine veins, the maternal radial artery and the umbilical vessels of 7 normal (AGA) and 13 intrauterine growth restricted (IUGR) pregnancies. Microvillous plasma membranes were isolated from the same placentas and the activity of the system y⁺ and y⁺L cationic amino acid transporters determined by measuring ³H-L- arginine uptake in the presence and absence of L-glutamine. In IUGR pregnancies uterine venous PO₂ was significantly higher (AGA=44.7 ± 8.0 mmHg; IUGR = 57.2 ± 2.3 mmHg, P2 was significantly lower (AGA=33.4 ± 3.0 mmHg; IUGR = 25.1 ± 2.0 mmHg, P+L activity, but not system y⁺ activity, was inversely correlated with uterine venous PO₂ (P2=0.4) in AGA and IUGR pregnancies. In IUGR pregnancies without associated maternal pre-eclampsia, y⁺L activity, but not y⁺ activity, was also directly related to the umbilical O₂ content difference (P2=0.9). A significant negative correlation was found between system y⁺ and the umbilical O₂ content difference in AGA pregnancies (P2=0.9). Our data are consistent with the hypothesis that in IUGR fetuses uterine oxygenation is not reduced and can be increased. The inverse correlation between system y⁺L activity and uterine venous PO₂ and the correlations with umbilical venous-arterial O₂ content difference suggest a relationship between cationic amino acid transporter activity and oxygen tension in the uteroplacental unit.