Western blotting data suggest that the Na⁺-H⁺ exchanger (NHE) isoforms, NHE1, NHE2 and NHE3, are expressed on the maternal-facing microvillous plasma membrane (MVM) of the human placental syncytiotrophoblast (Hughes et al. 2000; Pepe et al. 2001). NHE1 and NHE 3 are expressed on the fetal-facing basal plasma membrane (BM) (Pepe et al. 2001). Here, we used specific inhibitors of NHE1/NHE2 and NHE3 to investigate and compare the functional activity of NHE isoforms in MVM and BM vesicles from the term placental syncytiotrophoblast (TPS).

Initial rate of ²²Na uptake into vesicles was measured at room temperature in the presence of an outwardly directed proton gradient, without or with NHE inhibitors and expressed as pmol (mg protein)^-1 min^-1, mean ± S.E.M., n = number of placentas. Inhibitor experiments were performed over 30 s and results expressed as pmol (mg protein)^-1 (30 s)^-1 (mean ± S.E.M.). Initial rate in MVM was 289 ± 66 (n = 5), a significantly higher rate than BM, 71 ± 21 (n = 4, P < 0.05, Student’s unpaired t test). In MVM vesicles, uptake at 30 s was 336 ± 52 (n = 6); amiloride (500 mM), a non-specific inhibitor of NHE transport (Mahnensmith & Aronson, 1985), significantly reduced this to 116 ± 47 (n = 6, P < 0.05, ANOVA followed by Dunnett’s post-hoc test). HOE 694 (100 mM inhibits NHE1; Counillon et al. 1993), reduced uptake to 51 ± 23 (n = 5, P < 0.01) with an EC₅₀ of 0.12 mM. S3226 at 1 mM (inhibits NHE3; Schwark et al. 1998) had no effect (262 ± 54, n = 5; n.s.). At higher concentrations, S3226 (inhibits NHE1; Schwark et al. 1998) uptake was inhibited with an EC₅₀ of 4.04 mM. Control uptake of ²²Na into BM vesicles at 30 s was 84 ± 11 (n = 4), which was unaffected by inhibitors (amiloride, 63 ± 18, n = 4; HOE 694, 63 ± 13, n = 4; S3226, 95 ± 12, n = 4).

In conclusion, these data suggest that NHE3 is not a functional isoform in MVM vesicles isolated from the TPS. ²²Na uptake activity in the BM is 25 % of that seen in the MVM and was insensitive to all inhibitors, suggesting that NHE is not active in this membrane under these conditions. The polarization observed in this study may be of physiological significance with regard to maternofetal Na⁺ and proton exchange and possibly syncytiotrophoblast pH regulation.

This work was supported by The Wellcome Trust. HOE 694 and S3226 were kindly donated by Dr Jurgen Punter, Aventis Pharma.

All procedures accord with current local guidelines.