Corticotropin-releasing hormone binding to the syncytiotrophoblast membranes

Background The human placenta secretes large amounts of corticotropin-relea sing hormone (CRH) which was thought to exert a paracrine action in the pla centa. We have recently characterized high-affinity binding sites for CRH i n the human placenta. However, our studies utilized whole placental membran es, which did not identify the site of binding of CRH in the plasma membran e. Materials and methods In this study we investigated the characteristics of CRH binding to purified mother-facing, brush border membranes (BBM) and fet us-facing, basal plasma membranes (BPM) of the syncytiotrophoblast. The two membranes were separated by a series of differential and density-gradient centrifugations. The purity of the membranes was determined by measuring al kaline phosphatase, as a marker of BBM and Na+/K(+)ATPase as a marker of BP M. Results Each membrane showed specific and high-affinity binding. Scatchard analysis revealed a high-affinity binding site for CRH with K-d of 1.0 +/- 0.15 and 1.3 +/- 0.176 for BBM and BPM, respectively. The maximal number of binding sites was significantly different (P < 0.01) in the two plasma mem branes: B-max of 79 +/- 6.4 fmol/mg protein for BBM and 23 +/- 3.9 fmol/mg protein for BPM. Conclusions Both the mother-facing and fetus-facing membranes of the syncyt iotrophoblast contain binding proteins for CRH, with significantly more bin ding sites on the mother-facing membranes. The functional consequences of C RH binding could be different for the two polar membranes due to differenti al localization of second messenger systems between the two membrane types. It is proposed that partial purification of BBM and BPM provides a better system to study CRH action in the placenta, than whole placental membrane p reparations.