Biosynthesis of Progesterone and 20α-Hydroxypregn-4-en-3-one by the Rat Ovary During the Estrous Cycle and Early Pregnancy

Abstract

Rat ovaries were removedfor incubation during the middle of the light cycle each day: prior to the hypophyseal release of LH [luteinizing hormone] on the day of proestrus. Significant in vitro synthesis ofprogesterone and 20a-hydroxypregn-4-en-3-one (20[alpha]-OH-P)was found to occur from endogenous precursors in ovarian slices obtained from rats at each of 4 stages of the estrous cycle. The synthesis of 20[alpha]-OH-P was twice the synthesis of progesterone with no significant differences in synthesis of either compound among days of the estrous cycle. Luteinizing hormone caused a significant increase in both steroids when added to the incubation medium and was similarly effective at all stages of the cycle. In early diestrus, the corpora lutea were shown to be responsible for most of the progestin synthesis as determined by incubation of the isolated structures. The incorporation of radioactivity from acetate-l-14C was unrelated to total biosynthesis; the specific activities of both progesterone and 20a-OH-P were lowest at estrus and 6-9 times higher at metestrus. LH decreased the specific activity of both progestins throughout the cycle by approximately half by increasing synthesis without changing the rate of 14C-acetate incorporation. Under the conditions of incubation, in vitro progesterone biosynthesis was not greater in early pregnancy than during the estrous cycle. However, the ratio of 20[alpha]-OH-P to progesterone decreased progressively from 1-5 days of pregnancy. Serum levels confirm the lack of change in progestin biosynthesis during the cycle, whereas the serum progesterone concentration rose with ensuing pregnancy.