Estradiol-Induced Blockade of Ovulation in the Cow: Effects on Luteinizing Hormone Release and Follicular Fluid Steroids1

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Abstract
Administration of 10 mg estradiol valerate (EV) to nonlactating Holstein cows on Days 16 of the estrous cycle prevented ovulation in 7 of 8 cows for 14 days post-injection. In these 7 cows, the timing of luteolysis and the leutinizing hormone (LH) surge was variable but within the normal range. At 14 days post-treatment, each of these cows had a large (>10 mm) follicle, with 558 .+-. 98 ng/ml estradiol-17.beta., 120 .+-. 31 ng/ml testosterone, and 31 .+-. 2 ng/ml progesterone in follicular fluid (means .+-. SE). A second group of animals was then either treated with EV as before (n = 22), or not injected (control, n = 17) and ovariectomized on either Day 17, Day 18.5, Day 20, or Day 21.5 (24, 60, 96, or 132 h post-EV). Treatment with EV did not influence the timing of luteolysis, but surges of LH occurred earlier (59 .+-. 8 h post-EV vs. 100 .+-. 11 h in controls). The interval from luteolysis to LH peak was reduced from 44 .+-. 6 h (controls) to 6.9 .+-. 1.5 h (treated). Histologically, the largest follicle in controls tended to be atretic before luteolysis, but nonatretic afterwards, whereas the largest follicle in treated animals always tended to be atretic. Nonatretic follicles contained high concentrations of estradiol (408 .+-. 59 ng/ml) and moderate amounts of testosterone (107 .+-. 33 ng/ml) and progesterone (101 .+-. 21 ng/ml), whereas atretic follicles contained low concentrations of estradiol (8 .+-. 4 ng/ml), and testosterone (12 .+-. 4 ng/ml), and either low (56 .+-. 24 ng/ml) or very high (602 .+-. 344 ng/ml) concentrations of progesterone. This study suggests that EV prevents ovulation by inducing atresia of the potential preovulatory follicle, which is replaced by a healthy large follicle by 14 days post-treatment.