Characteristics of L-lactic acid transport in basal membrane vesicles of human placental syncytiotrophoblast

The characteristics of L-lactic acid transport across the trophoblast basal membrane were investigated and compared with those across the brush-border membrane by using membrane vesicles isolated from human placenta. The uptake of L-[¹⁴C]lactic acid into basal membrane vesicles was Na⁺ independent, and an uphill transport was observed in the presence of a pH gradient ([H⁺]_out > [H⁺]in). L-[¹⁴C]lactic acid uptake exhibited saturation kinetics with a K_m value of 5.89 ± 0.68 mM in the presence of a pH gradient. p-Chloromercuribenzenesulfonate and α-cyano-4-hydroxycinnamate inhibited the initial uptake, whereas phloretin or 4,4′-diisothiocyanostilbene-2,2′-disulfonate did not. Mono- and dicarboxylic acids suppressed the initial uptake. In conclusion, L-lactic acid transport in the basal membrane is H⁺ dependent and Na⁺ independent, as is also the case for the brush-border membrane transport, and its characteristics resemble those of monocarboxylic acid transporters. However, there were several differences in the effects of inhibitors between basal and brush-border membrane vesicles, suggesting that the transporter(s) involved in L-lactic acid transport in the basal membrane of placental trophoblast may differ from those in the brush-border membrane.