Changes in ovarian steroidogenesis in prepuberal rats induced to ovulate by low doses of human chorionic gonadotrophin.

To determine whether the decrease in ovarian 5 alpha-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 4h of incubation) of 5 alpha-dihydrotestosterone (DHT) and 5 alpha-androstane-3 alpha, 17 beta-diol. The ovarian content of DHT and the production of 5 alpha-androstane-3 alpha, 17 beta-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 5 alpha-reductase and aromatase were measured in ovarian homogenates obtained on days 29-31. Activity of 5 alpha-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 5 alpha-reduced androgen production, which is probably due to a decrease in 5 alpha-reductase activity. The subsequent increase in oestradiol production corresponds with an increase in aromatase activity.(ABSTRACT TRUNCATED AT 250 WORDS)