Title

Endothelin-1, a mediator of ovine luteal function

Date of Completion

January 2000

Keywords

Biology, Animal Physiology

Degree

Ph.D.

Abstract

Endothelin-1 (ET-1), a product of endothelial cells and the most potent vasoconstrictor known, was first discovered and characterized in 1988. Recent studies have suggested a role for ET-1 in bovine luteal function, but the role of ET-1 in ovine luteal function has not been examined. Endothelial cells comprise more than 50% of the cell population of the mature corpus luteum and their products are likely modulators of luteal function. A diversified series of experiments was conducted to determine the potential role of ET-1 in ovine luteal function. ET-1 inhibited basal and gonadotropin-stimulated progesterone production by dispersed ovine luteal cells during a 2 hour incubation. This inhibition was removed when cells were pre-incubated with cyclo-D-Trp 1-D-Asp2-Pro3-D-Val4-Leu 5 (BQ123), a highly specific endothelin receptor (ETA) antagonist, indicating that the anti-steroidogenic action of ET-1 is mediated through the ETA receptor subtype in these cells. Intra-luteal administration of a single dose of BQ123 to ewes on Day 8 or 9 of the estrous cycle mitigated the luteolytic effect of prostaglandin F2α (PGF2α) in vivo. Intra-muscular administration of 100 μg ET-1 to ewes at mid-cycle increased plasma progesterone concentrations 4 hours later and reduced plasma progesterone concentrations for the remainder of the estrous cycle. However, when ewes were pretreated with a subluteolytic dose of PGF 2α, intra-muscular administration of 100 μg ET-1 caused a rapid decline in plasma progesterone and shortened the length of the estrous cycle. Administration of a luteolytic dose of PGF2α stimulated gene expression for ET-1 in ovine CL collected at mid-cycle. Following PGF 2α administration, binding of ET-1 was immunolocalized in frozen sections to large and small luteal steroidogenic cells and to luteal endothelial cells. This is the first study to utilize ET-1 and its receptor antagonist in ewes in vivo. These data complement and extend previously published reports of the influence of ET-1 in bovine luteal function and are the strongest evidence presented to date in support of a novel role for ET-1 in PGF2α-mediated ruminant luteal function. ^