Changes in Serum Levels of Lh and Fsh in Progesterone-Treated Ovariectomized Ewes following a Single Injection of Estrogen

Howland, B.E.; Sanford, L.M.; Palmer, W.M.

Canadian Journal of Animal Science 58(1): 15-22


ISSN/ISBN: 0008-3984
DOI: 10.4141/cjas78-002
Accession: 068493054

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The effect of dose and duration of progesterone treatment on estrogen-induced Lh and Fsh release was studied in ovariectomized ewes. In each of two years, groups of ewes were injected with 5 or 10 mg progesterone daily for 4 or 10 days before an injection of 1 mg estradiol-17β. Experiments were conducted in early July (year 1) or mid-August (year 2). Jugular blood was collected immediately before and hourly from 14 to 30-h after estrogen injection. Serum hormone levels were determined by radioimmunoassay. Combined data from the two years showed that an Lh release occurred in four of five ewes treated with 5 mg progesterone for either 4 or 10 days, five of five ewes treated with 10 mg progesterone for 4 days but only one of five ewes treated with 10 mg progesterone for 10 days. The onset of estrus and the rise in Lh both tended to occur earlier after estrogen treatment in year 2 than in year 1, suggesting an effect of season. Only one ewe showed a simultaneous release of Lh and FSH; in the remainder, the levels of Fsh declined after estrogen injection and remained depressed throughout the sampling period. Mean Fsh levels tended to be higher in ewes receiving 10 mg progesterone than in ewes receiving 5 mg progesterone in year 2, but the reverse tended to occur in year 1 (progesterone dose × year, P <.05). The results suggest that a 10-day treatment with a moderate dose of progesterone can reduce the incidence of Lh release induced by a high dose of estrogen and that the estrogen-induced Lh release in progesterone-treated ovariectomized ewes is usually not accompanied by a release of FSH.