Implantation in the Rabbit Following Administration of 20α-Hydroxypregnen-3-one and 20β-Hydroxy pregnen-3-one

Implantation occurred in rabbits ovariectomized 22-25 hr after mating with administration of either 20a-hydroxypregnen-3-one or 20j3-hydroxypregnen-3-one. The range of doses of progesterone required to achieve implantation was 0.2-0.4 mg/day. A comparable dose range for 20«-OH progesterone and 20/3-OH progesterone was 4-8 mg/day. On this basis the relative activity of 20«-OH progesterone and 20/3-OH progesterone was estimated to be one tenth that of progesterone. ((Endocrinology 76: 535, 1965) T PREDOMINANT steroid isolated from rabbit ovarian venous blood is 20ahydroxypregnen-3-one (20a-OH progesterone) (1, 2). I t is present during estrus and rises following coitus or an ovulating dose of human chorionic gonadotrophin, pregnant mares' serum, follicle-stimulating hormone or luteinizing hormone. Both it and 20/3hydroxypregnen-3-one (20/5-OH progesterone) possess progestational activity as demonstrated by the Hooker-Forbes and the Clauberg bio-assays (3). However, these compounds failed to maintain pregnancy in ovariectomized mice (4). In the present experiments these compounds have been given to rabbits to study their progestational activity. Mate r i a l s and Methods Mature young New Zealand white rabbits were kept in individual cages in a room controlled for light, humidity and temperature. They were fed an estrogen-free Purina Rat Chow and isolated for 21 days before mating. All matings were observed and that time was taken as day 0. The rabbits were ovariectomized 22-25 hr or 8 days after mating and were then divided into 3 groups. One group received 20a-OH progesterone; another 20/3-OH progesterone; the third group xeceived progesterone. These hormones were dissolved in sesame oil and were given subcutaneously at varying dose levels twice Received August 25, 1964. This investigation was supported by USPHS Research Grant HD00971 and by N.S.F. Grant GB 1639. daily from the time of operation. In order to make a clear solution of 20a-OH progesterone in oil, it was dissolved in ether, which was then mixed with sesame oil and the ether was evaporated. It was not necessary to treat 20/3-OH progesterone in this manner. Doses of 0.5 mg of either 20a-OH progesterone or 20/3-OH progesterone and all dose levels of progesterone were given in 0.5 ml sesame oil, while 2-5 mg dose levels of 20«-OH progesterone and 20/3-OH progesterone were given in 1.0 ml sesame oil. Those rabbits that were ovariectomized soon after mating were killed at the end of day 8, by which time implantation had begun in control rabbits. The rabbits ovariectomized on day 8 were killed on day 14. The gestation sacs were measured with a vernier caliper from their mesometrial to their antimesometrial side, and compared with those in control rabbits. If there was any doubt whether or not implantation had begun, the site was fixed in Bouin's fluid and serially sectioned at 10 p. Sham-operated ovariectomized control rabbits were injected with sesame oil twice daily for 7 days from day 1. Results and Discussion In the sham-operated control rabbits all the blastocysts had implanted. The average diameter of the gestation sacs was 1.04 cm, with a range of 0.74-1.35 cm. With increased dosage of progesterone the percentage of implanting blastocysts increased, as did their mean size. The size of the gestation sacs in the animals receiving 0.5 mg/day of progesterone was comparable with that seen in controls. In the 5 rabbits receiving 0.25 mg progesterone/day 4 blastocysts were observed. On sectioning, one of these showed antimesometrial attachment with minimal