Evidence that progesterone may influence subsequent luteal function in the ewe by modulating preovulatory follicle development

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Abstract
Ovulation was induced in seasonally anoestrous ewes by repeated 2-h injections of 250 ng GnRH [gonadotorpin releasing hormone; LHRH] after 12 days (group 1, n = 7; group 2, n = 8), 2 days (group 3, n = 8) or no (group 4, n = 7) progesterone pretreatment. A preovulatory LH [luteinizing hormone] peak occurred spontaneously at a mean (.+-. SEM [standard error of the mean]) time of 43.1 .+-. 2.0 h, 38.5 .+-. 3.1 h and 26.8 .+-. 1.7 h after the start of GnRH treatment in groups 1, 3 and 4, respectively, and was artificially induced in ewes in group 2, after 24 h of treatment, by a single i.v. injection of 150 .mu.g GnRH. Normal luteal function occurred in all progesterone-pretreated ewes, but in only 1/7 animals not treated with progesterone. Although normal luteal function in progesterone-primed ewes induced to ovulate with repeated injections of low doses of GnRH was associated with a delayed preovulatory LH peak, it was this extended period of follicle development which was responsible for functional competence of the resultant corpus luteum. Since as little as 2 days of exposure to elevated plasma progesterone concentrations is effective, progesterone may act directly on the preovulatory follicle.