Effect of age of rat and dose of a single injection of estradiol benzoate (EB) on ovulation and the facilitation of ovulation by progesterone (P).

Abstract

Ovulation occurred in 50 or 80% of 31-or 33-day-old rats receiving 0.25 and 0.5 μg doses of EB at 28 or 30 days of age, respectively. Higher or lower doses of EB were less effective and on day 22 doses ranging from 0.1 to 8.0 μg produced no ovulation. Injection of P sc (0.5 mg) at 8:00 AM on the second day after EB injection in varying doses on days 25, 28, or 30 increased the percentage of rats ovulating; most strikingly, 1.0 μg EB on day 22 and 0.5 mg P on day 24 produced ovulation in 9 of 16 rats. Evidence obtained from the injection of FSH iv alone and subsequent to EB suggests that follicular sensitivity to exogenous gonadotropin is higher in animals primed with EB. Younger animals treated with EB are less responsive to FSH than the older rats primed with EB. Phenobarbital prevented both EB-and EB-P-induced ovulation if given at 1:30 PM on the second day after EB, but not when given at 5:30 PM. These data suggest that a) EB plays an important role in the maturation of follicles to be ovulated and als...