The source of the 5-alpha-pregnanes that occur during gestation in mares.

[1,2,6,7-3H]Progesterone was injected into the uterine artery of umbilical vein of 4 pregnant Ponies to determine whether 5 alpha-pregnane-3,20-dione (DHP), 20 alpha-hydroxy-5 alpha-pregnan-3-one (20 alpha-ol), and 3 beta-hydroxy-5 alpha-pregnan-20-one (3 beta-ol) are produced by the placenta, fetus, or mare during late gestation. Plasma samples were collected from indwelling catheters in the uterine artery and vein and the umbilical artery and vein at frequent intervals until 6 h after isotope injection. The plasma samples were extracted with organic solvents and the respective pregnanes were isolated by thin-layer chromatography. Mean plasma levels of radioactive DHP in the umbilical vein were greater (P less than 0.05) than those in the umbilical artery but were similar to those in the uterine artery and vein. Plasma levels of radioactive 20 alpha-ol in the umbilical artery and vein were less (P less than 0.05) than those in the uterine artery but similar to those in the uterine vein. Levels of radioactive 3 beta-ol in the umbilical artery were greater (P less than 0.05) than those in uterine artery and vein and similar to those in the umbilical vein. We suggest that DHP was produced by the placenta and a maternal source, that 20 alpha-ol was primarily of maternal origin, and that 3 beta-ol was formed primarily by the fetus.