Placental materno-fetal transfer of serine by amino acid exchangers

Objectives: The mechanisms mediating amino acid transport across the basal membrane of the placental syncytiotrophoblast into the fetal circulation are not well understood. Our previous data indicate that amino acid exchangers mediate serine transport into the fetoplacental circulation in exchange for alanine but not for glutamate. This study characterises amino acid stimulation of serine transfer into the fetoplacental circulation by exchange.Methods: Human placentas (n = 4) were collected within 30 minutes of delivery and an intact cotyledon was perfused with a modified Earl’s bicarbonate buffer. The maternal arterial circulation was perfused with 50 ?mol /1 L-serine & glycine, 0.6 ?mol /1 14Cserine and 20 ?mol /1 3H-glycine. Amino acid [12.5 ?mol] boluses were administered to the fetal side inflow perfusate. 14C-serine and 3H-glycine were measured in maternal and fetal venous samples by dual label liquid scintillation counting. Data (mean ± SEM) were expressed as area under the curve (AUC) and analysed by one way ANOVA.Results: Fetal side boluses of serine leucine threonine tryptophan and glutamine all resulted in increased serine release (AUC values 242 ± 34, 132 ± 33, 309 ± 69, 85 ± 21 and 216 ± 82 nmol respectively). Following a fetal side bolus of glutamate, BCH or lysine there was no significant increase in transfer of serine. Glycine release was not significantly altered following any of the boluses.Conclusion: This study demonstrates that serine is transported into the fetoplacental circulation by amino acid exchangers in response to a range of substrates, but not lysine or BCH, substrates of systems y+L and L. This is consistent with serine transport by Systems ASC and b0+ but not Systems y+L and L