The effect of chorion-uterine interaction upon free progesterone metabolism during advanced gestation in the guinea pig.

The in vitro fate of [3H]progesterone was studied after incubation with guinea pig tissues at 58/59 days (before pubic symphysis relaxation) and in the final week (post relaxation) of gestation. Buffered steroid was added to the fetal surface of chorion attached to the uterus or to the uterus alone. Neither the amount of recovered progesterone nor its metabolites (6.2% average conversion) differed between the two stages when only uterus was incubated. With chorion present, conversion averaged 28.3% at 58/59 days and 63.4% at the late stage. A 4.6-fold decrease in progesterone concentration, and 3.0-, 2.4- and 3.1-fold increases in the concentrations of 3alpha-hydroxy-5alpha-pregnan-20-one, 3beta-hydroxy-5alpha-pregnan-20-one and 5alpha-pregnane-3,20-dione, respectively, were found in the uterus in the late stage vs 58/59 days. Also, 2.8- and 6.4-fold decreases in progesterone concentration occurred in the myometrium and endometrium, respectively, from 58/59 days to the late stage. In endometrium, the concentrations of the 3alpha- and 3beta-isomers, and 5alpha-pregnane-3,20-dione, increased 2.6-, 2.6- and 5.0-fold, respectively. The above changes were all significant at P < 0.05 or better. Changes in the 3alpha-, 3beta- isomers and dione in the myometrium were not significant. The chorion-uterine interaction and gestation time thus affect the degree of progesterone conversion, and the amounts of metabolites reaching the uterus in the chorion-uterine in vitro system.