Intrafollicular Concentrations of Steroids and Steroidogenic Enzymes in Relation to Follicular Development in the Mare1

Abstract

To determine whether prostaglandin (PG) F_2α had a dose-dependent effect upon secretion of progesterone, oligonucleosome formation, or loss of luteal weight, ewes on Day 9 or 10 of the estrous cycle were administered 0, 3, 10, or 30 mg PGF_2α per 60 kg BW (i.v.), and luteal tissue was collected 9 and 24 h after injection. All doses of PGF_2α decreased (P < 0.05) concentrations of progesterone in sera by 9 h; however, in ewes treated with 3 mg PGF_2α, concentrations of progesterone were similar to control values at 24 h and higher (P < 0.05) than those in the 10- or 30-mg groups. Concentrations of progesterone in sera over all dose levels were highly correlated to luteal concentrations of mRNA encoding steroidogenic acute regulatory protein (P < 0.001), cytochrome P450 side-chain cleavage (P < 0.02), and 3β-hydroxysteroid dehydrogenase (P < 0.01). Corpora lutea collected at 24 h from ewes treated with the 10- and 30-mg doses of PGF_2α weighed less (P < 0.05) than those from controls. Oligonucleosomes were not present in luteal tissues from control ewes. Surprisingly, all doses of PGF_2α-induced oligonucleosomes in a majority of animals at 9 h and in a majority of ewes treated with 10 and 30 mg of PGF_2α at 24 h. In conclusion, 3 mg of PGF_2α per 60 kg BW transiently decreased serum concentrations of progesterone and induced oligonucleosome formation, but did not result in reduced luteal weight. The 10- and 30-mg doses of PGF_2α decreased secretion of progesterone and induced oligonucleosome formation and luteolysis.