Evidence for Early Ovarian Recognition of Blastocysts in Rabbits

Abstract

Plasma progesterone and 20±-hydroxy-pregn-4-ene-3-one concentrations were determined in peripheral blood from rabbits mated with intact or vasectomized males. Both progestins were measured by the protein binding assay method after separation of the ether-extracted steroids with paper chromatography. Samples were collected 5, 30 and 300 min after mating and daily thereafter until the tenth postcoital day. Plasma progesterone, as well as 20±-hydroxy-pregn-4-ene-3-one levels rose immediately after mating, and were maintained at high levels for at least 30 min. The concentration of both steroids had decreased at 5 hr, but was still significantly higher than in nonpregnant rabbits. Twenty-four hr later, the values had returned to precoital levels and remained low for several days. In the pre-gnant rabbits, plasma progesterone started to rise on day 4 and was significantly higher than in pseudopregnant rabbits on day 5 (8.7 ‡ 1.8 vs 3.4 ‡ 0.65 ng/ml) and on the following days until day 10. In the pseudopregnant rabbits, plasma progesterone rose significantly over nonpregnant levels on day 6 or 7, and increased thereafter more gradually than in the pregnant rabbits. On day 10, the dif-ference between pregnant and pseudopregnant rabbits (12.8 ‡ 1.8 vs 9.4 ‡ 1.5 ng⁄ml) was not statistically significant. The 20±-hydroxy-pregn-4-ene-3-one concentration increased parallel to progesterone in the pregnant rabbits and the ratio progesterone:20±-hydroxy-pregn-4-ene-3-one was near unity in all postcoital samples, whereas in the pseudopregnant rabbits, 20±-hydroxy-pregn-4-ene-3-one increased more than progesterone and remained the dominant proges-terone until day 10. These results suggest that in the rabbit, the maternal ovary recognizes the presence of blasto-cysts prior to implantation, and would imply that the fertilized ova exert a luteotropic influence already at the blastocyst stage. (Endocrinology95: 1054, 1974)