Decrease in Blood and Ovarian 3α- and 3β-Androstanediol Levels in Rats Induced to Ovulate with Pregnant Mare Serum Gonadotropin*

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Abstract
Several hours before the first ovulation progesterone metabolism in the rat ovary, in vitro, is shifted from the production of 5α-androstane-3α,17β-diol (3α-diol) as the major metabolite toward the production of 4-ene-3-oxosteroids. In the present paper, changes in levels of 3α-diol and its 3β-epimer as well as testosterone in blood and ovaries around the time of the first ovulation have been studied in immature PMSG-treated rats. Forty-eight hours after PMSG, a considerable increase in blood and ovarian testosterone concentration was observed, whereas the concentrations of both androstanediols in blood decreased sharply. At 52 h, the level of ovarian 3β-diol was only one third of the control level and continued to fall. The decrease in ovarian 3α-diol was less pronounced, but reached about half, or less, of the control value. In PMSG-treated rats in which the LH surge was blocked by pentobarbitone, the decrease in blood diols was delayed but not prevented. It is concluded that the decrease in production of the androstanediols preceding the first ovulation observed previously in isolated ovaries, also occurs in the intact rat. The decrease in androstanediols occurs very shortly before an ovulation induced with an injection of PMSG and is dependent on the occurence of an LH surge. Since it is assumed that the androstanediols have a prepubertal role in inhibiting uterine and ovarian growth and in preventing cyclic LH release, it is essential that their concentration decrease several hours before the first ovulation.