Pituitary Response of Prepuberal Lambs to Oestradiol-17ß

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Abstract
In 2 experiments, 48 prepuberal Merino ewe lambs were injected with estradiol-17.beta. (E2) or saline to study the effect of E2 on their plasma LH [luteinizing hormone] levels and on estrus and ovulation. In the 3 groups which received 30 (experiment I), 50 and 30 (experiment II) .mu.g E2, respectively, 27 of 28 lambs showed an LH response, the corresponding mean LH peaks being 64.3 .+-. 22.5, 153.6 .+-. 33.4 and 91.7 .+-. 16.9 ng/ml at mean intervals of 11.1, 11.2 and 10.5 h, respectively, after injection. None of the 20 lambs in the control groups had an LH level higher than 18 ng/ml 12 h after injection. In the 3 E2 groups, 41.7, 62.5 and 37.5% of animals showed estrus within 26 h of injection while in the control groups only 1 animals showed estrus. Of 13 animals showing estrus in the E2 groups, 11 failed to ovulate. The mean pre-injection plasma FSH [follicle-stimulating hormone] level in experiment I was 102.7 ng/ml, and in four 5-7 mo. old lambs over several weeks was 155.3 ng/ml. Despite these high pre-injection levels of FSH, the follicles were unable to respond to the LH peak which followed the E2 injection.

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