To determine whether the decrease in ovarian 5α-reduced androgen production before first ovulation might be caused by an increase in serum LH, prepuberal female rats were injected at 28–31 days of age with low doses of human chorionic gonadotrophin (hCG) (0·05–0·075 i.u., four times daily). This treatment resulted in ovulation of six to ten ova per rat on day 32 in all animals.
Treatment with hCG resulted in a gradual decrease in ovarian content and production (i.e. content in ovary and medium after 4 h of incubation) of 5α-dihydrotestosterone (DHT) and 5α-androstane-3α,17β-diol. The ovarian content of DHT and the production of 5α-androstane-3α,17β-diol decreased within 24 h after the first injection of hCG. Oestradiol content and production increased between 24 and 48 h after the start of treatment and was maximal on day 31 (day of pro-oestrus).
Activities of 5α-reductase and aromatase were measured in ovarian homogenates obtained on days 29–31. Activity of 5α-reductase in hCG-treated rats was lower than that in control rats on all days studied. Aromatase activity in hCG-treated rats increased between days 29 and 31.
It was concluded that multiple injections of low doses of hCG, which may induce ovulation, cause a decrease in 5α-reduced androgen production, which is probably due to a decrease in 5α-reductase activity. The subsequent increase in oestradiol production corresponds with an increase in aromatase activity. The results indicate that the decrease in 5α-reductase activity as observed in ovaries of spontaneously ovulating rats might be caused by the gradual increase in serum LH, which has been found to occur during the last week before first ovulation.