ESTROGEN-INDUCED LH AND FSH RELEASE IN OVARIECTOMIZED EWES: EFFECT OF SEASON

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Abstract
Five ovariectomized ewes were each injected i.m. with 50μg estradiol-17β in oil at 1800 h on 9 June, 19 July, 2 Sept. and 20 Oct. Serum samples of jugular blood, collected at 1-h intervals from 12 or 13 h to 30 h after estrogen treatment were assayed for LH and FSH. With the exception of one ewe in July, estrogen consistently induced a rise in serum LH. A rise in serum FSH accompanied the LH rise in only two of the five animals. The interval between estrogen injection and peak gonadotropin levels decreased (P < 0.05) between July and October but no difference was observed in either the peak gonadotropin levels or amount of LH released. The data suggest that estrogen-treated ovariectomized ewes do not respond consistently with a release of both LH and FSH and that the transition from anestrous to the breeding season is associated with a slightly shorter interval between estrogen injection and peak gonadotropin levels.