IDENTIFICATION, CHARACTERIZATION, AND BIOLOGICAL-ACTIVITY OF ENDOTHELIN RECEPTORS IN HUMAN OVARY

We previously reported the expression of endothelin-1 (ET-1) in granul osa cells (GCs) of the human ovary and the presence of ET-1-like immun oreactivity in human follicular fluid obtained from women in an in vit ro fertilization program. In follicular fluid, but not in plasma, the levels of ET-1-like immunoreactivity were higher in gonadotropin-stimu lated vs. spontaneous cycles, suggesting hormonal regulation of follic ular ET-1. To identify and characterize ET receptors in human ovary, w e performed autoradiography, radioligand binding, and functional studi es. Mathematical analysis of families of self- and cross-competition c urves among [I-125]ET-1, [I-125]ET-3, and selective analogs indicates that human ovary expresses both subtypes of ET receptors, i.e. ETA and ETB receptors. However, the concentration of the ETB site was 100-fol d lower than that of the ETA one. By using [I-125]ET-1, we demonstrate d that the density of binding sites in human ovary is not affected by the hormonal milieu (similar concentrations in normal cycling, postmen opausal, and combined oral contraceptive-treated women). In situ bindi ng studies indicate that the majority of ETA and ETB receptors are exp ressed in the blood vessels of the ovary. In particular, ETA receptors are abundant in the ovulatory follicles and localized in the theca in terna, in close proximity to the granulosa layer. Few GCs of the ovula tory follicle were specifically labeled. Conversely, in the rat ovary, used as a control, ETB receptors were predominantly expressed and loc alized in GCs. Accordingly, ETB receptors negatively regulated estroge n accumulation in rat GCs. In human granulosa-luteal cells, neither ET -1 (unselective ligand) nor ET-3 or sarafotoxin 6c (ETB ligands) affec ted estrogen or progesterone secretion. ET-1 was 2.5-fold more potent than noradrenaline in eliciting contraction of ovarian artery, acting through the ETA receptor. Our results indicate that in human ovary, at variance with rat ovary, the endothelin system is primarily involved in the regulation of ovarian blood flow and not steroidogenesis.