Role of the Endothelin-1 System in the Luteolytic Process of Pseudopregnant Rabbits

Abstract

The aim of this study was to better understand the role of the endothelin-1 (ET-1) system in the process of controlling the corpora lutea (CL) life span in rabbits. ET-1 (10 μg iv) administration at d 9 and 12 of pseudopregnancy induced a functional luteolysis within 24 h of injection, but it was ineffective at both d 4 and 6. Pretreatments with Bosentan, a dual ETA/ETB receptor antagonist, or cyclooxygenase (COX) inhibitor blocked the luteolytic action of ET-1 but not that induced by prostaglandin F2α (PGF2α). In CL cultured in vitro, ET-1 increased (P ≤ 0.01) both PGF2α production and luteal nitric oxide synthase activity but decreased (P ≤ 0.01) progesterone release. Addition of ETA receptor antagonist BQ123 or COX inhibitor blocked the ET-1 luteolytic effects. Positive staining for ET-1 receptors was localized in ovarian blood vessels, granulosa cells of large follicles, and luteal cells. Immunoblot analysis of ET-1 receptor protein revealed a strong band of approximately 48 kDa in d-9 CL. Up to d 6 of pseudopregnancy, ET-1 mRNA abundance in CL was poorly expressed but then increased (P ≤ 0.01) at d 9 and 13. ETA-receptor transcript increased (P ≤ 0.01) at d 6, remained at the same level up to d 13, and then declined to the lowest (P ≤ 0.01) levels at d 22. ETB-receptor mRNA increased (P ≤ 0.01) throughout the late-luteal stage from d 13 up to d 18. Our data suggest that the luteolytic action of ET-1 may be a result of PGF2α synthesis from both luteal and accessory cells, via the COX pathways.