CALCITONIN-GENE-RELATED PEPTIDE RECEPTOR IN HUMAN PLACENTAL SYNCYTIOTROPHOBLAST BRUSH-BORDER AND BASAL PLASMA-MEMBRANES

Minerals, such as calcium and potassium, are essential for fetal devel opment, but their transplacental transport, and in particular, the eff ect of hormones on this process has not been extensively studied. Huma n a-calcitonin gene-related peptide (h alpha CGRP), a hormone constitu ted of 37 amino acids, is obtained by the alternative splicing of the mRNA from the calcitonin gene, and could be implicated in placental io n transport. In order to study the presence of this receptor, brush-bo rder and basal plasma membranes were purified, and membrane binding st udies were conducted using [I-1Z5]h alpha CGRP. The initiation of bind ing of [I-125]h alpha CGRP to both membranes was rapid and reached max imal value after 10 min of incubation at 37 degrees C. Scratchard anal ysis revealed single-affinity binding sites for h alpha CGRP with K-d equal to 4412.45 +/- 604.81 pM and 2673.24 +/- 552.51 pM for brush-bor der and basal plasma membranes, respectively, which were significantly different. Moreover, the maximal number of receptors was significantl y different (P<0.001) in both membranes, with B-max of 627.94 +/- 31.4 0 fmol/mg protein for brush-border membranes and 343.70 +/- 43.52 fmol /mg protein in basal-plasma membranes. Competitive displacement of [I- 125]h alpha CGRP with other ligands showed the following potencies; h alpha CGRP approximate to h beta CGRP approximate to Cys (acm)(2,7) CG RP>CGRP (8-37), but no competition was observed with human and salmon calcitonin. Half-maximal displacement for human alpha- and beta CGRP w as reached at approximately 10(-10) M for brush-border and basal-plasm a membranes. alpha- and beta CGRP, and their fragment and analogue, st imulated cyclic AMP production in placental homogenate ranging from 14 3-163 per cent. Thus, our results show the presence of CGRP-specific r eceptors in both the syncytiotrophoblast membranes of human placenta. The role(s) of this related peptide in placenta remains to be investig ated. (C) 1997 W. B. Saunders Company Ltd.