Pre-ovulatory changes in steroidogenesis in ovaries from immature rats treated with pregnant mare serum gonadotrophin.

The metabolism of progesterone in the 1000 g supernatant fraction of homogenates of ovaries from PMSG-treated immature rats was determined. As early as 52 h after a single injection of 50 i.u. PMSG, still before the LH surge, 5alpha-pregnane-3alpha, 20alpha-diol was identified as the main metabolite, together with small quantities of 3alpha-hydroxy-kalpha-pregnan-20-one and 5alpha-androstane-3alpha, 17beta-diol. Similar incubations of untreated rat ovaries at the same age did not produce 5alpha-pregnane-3alpha, 20alpha-diol. The quantities of 3alpha-hydroxy-5alpha-pregnan-20-one and 5alpha-androstane-3alpha, 1mbeta-diol were reduced in PMSG-treated rat ovaries as compared with control ovaries. When progesterone metabolism was examined 64 h after PMSG administration, 5-7 h after the peak of LH surge but still before ovulation, 75% of the substrate was converted to 5alpha-pregnane-3alpha, 20alpha-diol, while 3alpha-hydroxy-5alpha-pregnan-20-one as well as 5alpha-androstane-3alpha, 17beta-diol could not be detected.