Figure 6. The glutamate gradient may drive uptake of OAT4 and OATP2B1 substrates from the fetus .

Once in the placenta these can be transported to the mother in the case of xenobiotics or, in the case of DHEAS, used for placental synthesis of oestrogen. Glutamate reuptake is mediated by high affinity EAAT glutamate transporters which maintain the glutamate gradient. Glutamate efflux in exchange for OAT substrates could only be observed in the perfused placenta if reuptake via EAAT was blocked with aspartate. This suggests recycling of glutamate back to the syncytiotrophoblast.