Effects of multiple small doses of progesterone on estrogen and gonadotropin secretion in the cyclic hamster.

Abstract

AbstractThe sc injection of 10 μg of progesterone at 0900, 1200, and 1500 h of Day 3 of the hamster estrous cycle (Day 1 = morning of ovulation) delayed ovulation by at least one day. Following progesterone treatment, the temporal changes in hormone levels on Day 3 were determined. Serum estradiol declined significantly at 1300 hr (1 hr after the second injection of progesterone) and was depressed until at least 2200 hr of Day 3. Serum estradiol returned to normal levels by 0900 hr, Day 4. Serum levels of progesterone ranging from 1.2 to 2.8 ng/ml were associated with the decline in estradiol; these levels of progesterone are within the physiological range of 5 ng/ml produced by the corpora lutea on Days 1 and 2 of the cycle. The rapid initial fall in estradiol produced by exogeneous progesterone was not paralleled at 1300 hr by similar declines in FSH, LH, or PRL. However, at subsequent time intervals on Day 3, serum LH was 3-4 ng/ml less in the progesterone-treated hamsters. Therefore, the use of multip...