HUMAN PLACENTAL SYNCYTIOTROPHOBLAST EXPRESSES 2 PHARMACOLOGICALLY DISTINGUISHABLE TYPES OF NA+-H+ EXCHANGERS, NHE-1 IN THE MATERNAL-FACING (BRUSH-BORDER) MEMBRANE AND NHE-2 IN THE FETAL-FACING (BASAL) MEMBRANE

We investigated whether highly purified preparations of basal (fetal-facing) membrane isolated from normal term human placentas possess Na+-H+ exchanger activity. Uptake of Na+ into basal membrane vesicles was stimulated many-fold by an outwardly directed H+ gradient. This H+-gradient-dependent uptake was inhibitable by amiloride and its analogues. Na+ uptake in these vesicles did not occur via a Na+ channel, as it was not influenced by changes in membrane potential and, in addition, was inhibited by benzamil only at high micromolar concentrations. The results indicate that the human placental basal membrane possesses Na+-H+ exchanger activity. We then studied whether this exchanger is similar to or distinct from the Na+-H+ exchanger described in brush border (maternal-facing) membrane preparations. For this purpose, we compared the pharmacological characteristics of the basal membrane Na+-H+ exchanger with those of the brush border membrane Na+-H+ exchanger. The basal membrane exchanger was about 20-fold less sensitive to inhibition by amiloride and about 70-fold less sensitive to inhibition by dimethylamiloride than was the brush border membrane exchanger. The exchanger activity in both membrane preparations was inhibitable by clonidine and cimetidine, but the inhibition patterns with these compounds were markedly different between basal and brush border membrane preparations. These data demonstrate that the basal membrane Na+-H+ exchanger is distinct from the brush border membrane Na+-H+ exchanger. The pharmacological profiles of these exchangers indicate that the human placental brush border membrane possesses the housekeeping or non-epithelial type Na+-H+ exchanger (NHE-1), whereas the basal membrane possesses the epithelial or apical type Na+-H+ exchanger (NHE-2).