Calcitonin receptor in human placental syncytiotrophoblast brush border and basal plasma membranes

Abstract

The physiology of calcium transport through the placenta has not been studied thoroughly. In particular, the effect of calcaemic hormones on this process has never been reported. In this paper we questioned if calcitonin, a hypocalcaemic hormone, is also implicated in the regulation of calcium transport by one of the placental syncytiotrophoblast bipolar membranes. In order to investigate the implication of calcitonin on calcium transport, we first studied whether this hormone binds to any of these bipolar membranes i.e. purified syncytiotrophoblast brush border (facing the mother) and basal plasma membranes (facing the fetus). The initiation of binding of human [ 125I]calcitonin to the two types of membranes was rapid and reached a steady state after 10 min of incubation at 37°C. The number of binding sites and the affinity of these receptors for the hormone were studied for each type of membrane, with concentrations of [ 125I]calcitonin varying from 0.01 to 1.8 nM. Scatchard analysis revealed a single affinity binding site for human calcitonin with K d s of 0.83 ± 0.09 nM and 0.67 ± 0.26 nM for brush border and basal plasma membranes respectively. The maximal number of receptors was significantly different ( p < 0.001) in the two membranes: B max of 66.64 ± 9.15 fmol/mg protein for brush border membranes and 19.66 ± 2.80 fmol/mg protein for basal plasma membranes. Competitive displacement of [ 125I]calcitonin with other ligands showed the following potencies between human calcitonin > salmon calcitonin > calcitonin gene-related peptides and segments analogues but no competition with some human calcitonin gene-related peptides fragments. Half-maximal displacement concentration for human calcitonin was reached at approximatively 1 nM for BBM and 0.1 nM for BPM. Calcitonin stimulated inositol phosphate production in both membranes by 175% and 330% in BBM and BPM, respectively. We conclude that calcitonin receptors are present in the two polar syncytiotrophoblast membranes, but are more abundant in brush border membranes, facing the mother. These results also suggest that in the placenta, both maternal and fetal calcitonin might intervene in ion transport and particularly calcium transport or on the intracellular metabolism of the placental syncytiotrophoblast cells.