The placental syncytiotrophoblast is the site for mineral and nutrient exchange across the maternal-fetal interface. It has been proposed that parathyroid hormone-related protein (PTHrP) is a key factor in the maintenance of a maternal-fetal calcium gradient. Using simultaneously prepared microvillous (maternal facing) and basal (fetal facing) syncytiotrophoblast membranes from term human placentae (n=8), we determined the relative contribution of PTH(1-34), PTHrP(1-34) and PTHrP(67-94) to the regulation of syncytiotrophoblast calcium efflux. The vesicles had correct right-side-out membrane orientation and specific markers validated the fractionation of microvillous and basal membrane vesicles. Calcium efflux was studied by preloading vesicles with calcium-45 in the presence of calcium and magnesium and then incubating the vesicles at 37 degrees C for 15 min with the peptides. In basal membranes, PTHrP(1-! 34) significantly stimulated calcium efflux at a dose of 12.5 nmol/l, whereas PTH(1-34)-stimulated efflux was significant at 50 nmol/l (P<0.05, ANOVA). This efflux was significantly reduced in the presence of the PTH/PTHrP receptor antagonist (PTHrP(7-34)). Midmolecule PTHrP(67-94) had no significant effect on basal membrane calcium efflux. PTH(1-34), PTHrP(1-34) or PTHrP(67-94) had no significant effects on MVM calcium efflux. This study, using the human syncytiotrophoblast in vitro membrane system, demonstrated that PTHrP(1-34) and PTH(1-34) stimulate calcium transport across the basal, but not microvillous, syncytiotrophoblast membrane vesicles, mediated via the PTH/PTHrP receptor.

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