Effect of blastocysts on rat ovarian steroidogenesis in early pregnancy

Ovarian progesterone (P) and 20α-hydroxypregn 4 en 3 one (20α-OHP) biosynthesis was studied in rats whose pituitaries had been autotransplanted beneath the kidney capsule (APTr rats) 1 to .5 days after ovulation. Pituitary autotransplantation suspends the initiation of implantation without decreasing the viability of the blastocysts. In some rats matings were made infertile by prior oviduct ligation. Ovaries were removed from all rats 9 days after ovulation, and the net accumulation of steroids by tissue slices incubated for 2 hr was measured. In mated APTr rats net accumulation of progesterone was 64 ± 11 μg/g of tissue with a P/20α-OHP ratio of 1.81 ± 0.44. In mated rats whose oviducts had been ligated 1 to 2 wk before mating, net progesterone accumulation was 25 ± 5 μg/g with a P/20α-OHP ratio of 0.32 ± 0.16. Tubal ligation itself may have a luteotropic effect. Yet, ovaries of APTr rats with viable blastocysts synthetized significantly more progesterone than mated APTr rats in which fertilization was prevented by tubal ligation. Increased progesterone formation was associated with decreased accumulation of 20α-OHP. Ovaries of unmated APTr rats, of similar rats cervically stimulated on proestrus, and of similar cervically stimulated rats whose uteri had been traumatized on day 5 accumulated 42 ± 5, 16 ± 4, and 18 ± 9 net μg progesterone g, respectively, during incubation. Thus, cervical stimulation with or without uterine traumatization significantly reduced progesterone accumulation from a relatively high level in APTr unmated rats to approximately that observed in mated rats whose oviducts had been ligated. The observed inhibitory effect of cervical stimulation, a presumed component of copulation was, therefore, overcome by stimuli arising from the blastocysts. Since progesterone secretion is prompted by either a pituitary autotransplant or prolactin in the absence but not in the presence of cervical stimulation, these results provide evidence for secretion by the unimplanted blastocyst of a substance with activity different from and more effective in promoting progesterone formation than pituitary prolactin.