Corticotropin-releasing hormone binding to the syncytiotrophoblast membranes

The human placenta secretes large amounts of corticotropin-releasing hormone (CRH) which was thought to exert a paracrine action in the placenta. We have recently characterized high-affinity binding sites for CRH in the human placenta. However, our studies utilized whole placental membranes, which did not identify the site of binding of CRH in the plasma membrane. In this study we investigated the characteristics of CRH binding to purified mother-facing, brush border membranes (BBM) and fetus-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 alkaline phosphatase, as a marker of BBM and Na+/K+ATPase as a marker of BPM. Each membrane showed specific and high-affinity binding. Scatchard analysis revealed a high-affinity binding site for CRH with Kd 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 membranes: Bmax of 79 +/- 6.4 fmol/mg protein for BBM and 23 +/- 3.9 fmol/mg protein for BPM. Both the mother-facing and fetus-facing membranes of the syncytiotrophoblast contain binding proteins for CRH, with significantly more binding sites on the mother-facing membranes. The functional consequences of CRH binding could be different for the two polar membranes due to differential 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 preparations.