Date of Graduation

2007

Document Type

Thesis

Degree Type

MS

Committee Chair

Jorge Flores

Abstract

Recent evidence supports a role for luteal endothelial cells in mediating PGF2αinduced luteolysis via production of endothelin-1 (ET-1). Three experiments were designed to examine how the endothelin system mediates luteolytic actions of PGF2α. The first objective was to examine in vitro the anti-steroidogenic actions of PGF2α and ET-1 in the presence of a type-A or B receptor antagonist in day-8 luteal minces. The second experiment was designed to measure in vivo, the effectiveness of PGF2α in inducing luteolysis when actions of ET-1 had been blocked by sustained administration of either a type-A or type-B endothelin receptor antagonist. In the third experiment, immunohistochemistry (IHC) was used to determine luteal cellular expression of ET-A and ET-B receptors. Finally, relative gene expression of the ET-A and ET-B receptors was examined by semi-quantitative RT-PCR in day-8 sheep and day-10 cow CL. ET-A, but not ET-B, participated in the anti-steroidogenic actions of ET-1, and at least some luteolytic actions of PGF2α were not mediated by ET-1. During the first 12 h after injection of PGF2α, ET receptor antagonists did not prevent the PGF2α-induced decline in serum progesterone. Thus, early luteolytic actions of PGF2α are either direct on steroidogenic cells, or mediated by something other than ET-1. However, beyond 12 h post injection of PGF2α, serum concentrations of progesterone increased in ET-A antagonist-treated animals until they were not significantly different from saline-treated controls. No changes in the luteolytic actions of PGF2α were observed in vivo or in vitro when a selective ET-B antagonist was used alone. Immunohistochemically, ET-A receptor was located in small luteal steroidogenic, endothelial, and smooth muscle cells. ET-B immunoreactivity was not observed in CL, however, RT-PCR data demonstrated ET-B expression, albeit at one tenth the expression of ET-A. In summary, these studies in ovine CL provided strong evidence that: 1) ET-A but not ET-B receptors mediate antisteroidogenic actions of ET-1; 2) actions of PGF2α are both dependent upon, and independent of mediation by ET-1; 3) during PGF2α-induced luteolysis, ET-1 may be more important in later stages of luteal regression; and 4) small steroidogenic cells are the target for luteal anti-steroidogenic effects of ET-1.

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