Effect of oestrogen and progesterone on pituitary goandotropic content of the cyclic hamster.

Abstract

ABSTRACT Oestradiol cyclopentylpropionate (ECP) and progesterone alter the normal pattern of LH synthesis and release in the mature female hamster. Both compounds also inhibit ovulation, but through different mechanisms. A dose of 50 μg ECP, injected s. c. on day 1 of the oestrous cycle, causes atresia of all vesicular follicles within 48 hours. This is related temporally to a decrease in pituitary LH content. On the other hand, 5 mg progesterone, administered at day 1, does not change the normal pattern of follicular development, but inhibits ovulation by preventing the ovulatory surge of LH on day 4. The number of healthy antral follicles is halved between days 3 and 4 of the oestrous cycle in untreated controls and animals given 5 mg progesterone on day 1. This coincides with a decrease in pituitary FSH content of both groups. Animals administered 50 μg ECP on day 1 show no decrease in pituitary FSH between days 3 and 4. Concurrent administration of 50 μg ECP and 5 mg progesterone on day 1 inhibits both synthesis and release of LH, as evidenced by a decreased pituitary content and atrophic ovaries. The present study indicates that ECP causes atresia indirectly via a decrease in LH synthesis and release during the first two days of the oestrous cycle. Progesterone, however, does not alter the normal pattern of follicular development or pituitary LH content until day 4, at which time the release of LH is blocked.