PARATHYROID-HORMONE RECEPTOR IN HUMAN PLACENTAL SYNCYTIOTROPHOBLAST BRUSH-BORDER AND BASAL PLASMA-MEMBRANES

The syncytiotrophoblast of the placenta is the site of exchange of nutrients and minerals between the mother and fetus. We have recently demonstrated that PTH influences, in vitro, phosphate transport through the placenta brush border membranes (BBM) and increases cAMP accumulation in placental tissue (1). To demonstrate the site of binding of PTH in the cytoplasmic membrane, we have purified two polar membranes: the first located on the apical side, the BBM, and the second, on the fetal side, the basal plasma membrane (BPM). BBM were enriched 24-fold in alkaline phosphatase (marker for BBM), and the BPM was enriched 37-fold in binding of [3H] dihydroalprenolol (marker for BPM) compared to homogenate. Both placental membranes contain binding sites (maximum binding = 0.550 .+-. 0.032 and 0.298 .+-. 0.065 pmol/mg protein for BBM and BPM, respectively) with similar affinities (Kd = 2.05 .+-. 0.23 and 1.78 .+-. 0.19 nM, respectively) for 125I-[Nle8,Nle18, Tyr34] bovine (b) PTH-(1-34) amide. The three bovine preparations [bPTH-(1-34), its analog [Nle8,Nle18, Try34]bPTH-(1-34) amide, and the antagonist bPTH-(3.34)] were equipotent in binding to both placental membranes. In contrast, human PTH-(1-84) was more effective in displacing the bovine radioligand in BBM. Thyrocalcitonin and insulin, two non-PTH peptides, did not significantly displace the radioligand in BBM and BPM. Adenylate cyclase activity, located exclusively in BPM, was stimulated by PTH. Since the enyzme is absent from BBM, it is probable that the binding of the hormone to this membrane activates another system of messengers.