A new form of asymmetry in epithelia: kinetics of apical and basal sulphate transport in human placenta.

Sulphate transport into and out of membrane vesicles prepared from both the brush-border and basal surfaces of the human placental trophoblast has been studied. For both surfaces of this epithelium clear evidence of trans-stimulation of influx is seen but for efflux, acceleration of labelled sulphate movement by addition of external sulphate is seen only in basal and not in brush-border membrane vesicles. This kinetic asymmetry may underlie the previously observed, but unexplained, finding that the concentration of sulphate is greater in the fetal than in the maternal circulation.