BIOCHEMICAL-CHARACTERIZATION AND AUTORADIOGRAPHIC LOCALIZATION OF [I-125] ENDOTHELIN-1 BINDING-SITES ON TROPHOBLAST AND BLOOD-VESSELS OF HUMAN PLACENTA

The presence of endothelin binding sites in the human placenta raises the question of the precise localization of these receptors on well de fined placental constituents. In order to find an answer to this probl em various approaches were used. Specific binding sites for [I-125] en dothelin-1 (ET-1) were identified on human term placenta, not only on membranes of smooth muscle stem villi vessels, but also on trophoblast ic plasma membranes prepared from trophoblast in culture. Scatchard an alysis of binding data revealed a single class of high affinity bindin g sites with K(d) values of 26 +/- 4 pmol/L for stem villi vessels and 126 +/- 4 pmol/L for trophoblast in culture, with maximum binding cap acities of 681 +/- 61 and 224 +/- 53 fmol/mg protein, respectively. Th e anatomical localization of these binding sites was determined by in vitro autoradiography. Autoradiograms obtained from placental sections incubated with [I-125]ET-1 indicate that [I-125]ET-1 high affinity bi nding sites exist on placental stem villi vessels and on the trophobla stic layer of the villi. The latter localization was also found on aut oradiograms of trophoblast in culture. The human placental syncytiotro phoblast is a polaraized epithelium with the microvillous membrane, fa cing maternal blood space and the basal plasma membrane, facing fetal circulation. [I-125]ET-1 high affinity binding sites are present on bo th membranes but the number of binding sites is higher on the basal pl asma membrane. These findings lead to the suggestion that ET-1 may be involved in the regulation of the feto-placental circulation and may s ubserve specific trophoblastic functions.