The maternal to foetal transfers of S(+)- and R(-)-gamma-vinyl-GABA (VGB) across the human isolated perfused placenta were low and comparable with those of acidic alpha-amino acids. 2. The placental uptake of the active S(+)-isomer from the maternal circulation exceeded that of the R(-)-isomer and this was reflected by a corresponding difference in placental tissue concentrations. 3. During perfusion with recirculation of the foetal medium, the two enantiomers were present at a similar concentration and did not concentrate in foetal perfusate, indicating that the excess amount of S(+)-VGB cleared from the maternal circulation was not accessible to the foetal perfusate. Furthermore, stable concentrations of both isomers in the foetal perfusate suggested a lack of placental metabolism. 4. Possible explanations of these findings include the operation of a stereoselective sodium-dependent-GABA placental uptake system on the maternal side, similar to that observed in neuronal tissue, or stereoselective binding to a placental GABA transaminase.
