Characterization of Corticotropin-Releasing Hormone Binding Sites in the Human Placenta

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Abstract
The human placenta synthesizes and secretes large amounts of corticotropin-releasing hormone (CRH) which has been implicated in the triggering of parturition. The placental CRH was found to act in a paracrine manner to stimulate secretion of ACTH and beta-endorphin. In view of this we sought to characterize CRH binding sites in the human placenta. The specific binding of 125I-tyrosyl-ovine CRH (125I-oCRH) to placental membranes was dependent on time, temperature, pH, divalent cations and was reversible on addition of excess oCRH. Scatchard analysis revealed a high afinity binding site with a dissociation constant of approximately 0.7 nmol/L and maximum number of binding sites approximately 44 fmol/mg protein. Disuccinimidyl suberate, a chemical cross-linker, was used to covalently attach 125I-oCRH to placental membranes. The labelled placental membranes were analyzed by SDS-PAGE and autoradiography. A major radioactively labelled band with a molecular weight of 55,000 Da was identified. In this study we have identified placental binding sites for CRH with properties similar to CRH receptors described in a number of human and animal tissues and with a molecular weight similar to that of the brain CRH receptor. These binding sites may be involved in the regulation of the placental CRH/ACTH-beta-endorphin axis during pregnancy and parturition.