Stimulatory and Inhibitory Effects of Progesterone on Foilicuiar Development in the Hypophysectom ized Foil icie-Stimu iating Hormone/Luteinizing Hormone-Treated Hamster1

Cyclic hamsters hypophysectomized at estrus (Day 1 of the cycle) and injected with 5 μg follicle-stimulating hormone (FSH) on Day 1 and 20 μg luteinizing hormone (LH) in polyvinylpyrrolidone (PVP) from Days 1–4 ovulated 15.3 ova, in response to 30 IU human chorionic gonadotropin (hCG) administered at 1500 h on Day 4 (Kim and Greenwald, 1984). When 1 mg progesterone (P4) was administered daily from Days 1–4 concurrent with the above regimen, ovulation increased to 38 ova, a clearcut superovulatory response. However, daily injection of 1, 10, or 100 μg P4 plus FSH and LH reduced the number of antral follicles present on the afternoon of Day 4 to 3–4 per ovary, compared to 9 per ovary after FSH-LH alone, and the ovulation rate was drastically reduced with most animals being anovulatory. Substituting 1 mg 17α-hydroxyprogesterone or estradiol cyclopentylpropionate for P4 on Days 1–4 did not alter the number of antral follicles on Day 4 from FSH-LH alone, whereas 1 mg androstenedione or 1 mg testosterone cyclopentylpropionate reduced the number of antral follicles to 3 or less. Hence, the stimulatory effects of 1 mg P4 are not attributable to its conversion to other P4 derivatives. After the concurrent injection of 1 mg P4 and FSH-LH, on the afternoon of Day 3, an average of only 1.8 large preantral follicles was present per ovary. By the morning of Day 4, however, the ovary contained 14 large preantral and early antral follicles in addition to 8 large antral follicles. Injection of hCG at this time resulted in the ovulation of 14.5 ova. By the afternoon of Day 4, 22 antral follicles per ovary were present, with the majority recruited from the smaller categories. The major effect of 1 mg P4 was to enable smaller preantral follicles to grow and ultimately to be recruited into the ovulatory group. It is therefore suggested that in the hamster high doses of P4 act as a mitogenic stimulus to preantral follicles and that it has the same effects as daily doses of FSH.