Placental steroidogenesis in the rat: progesterone production by tissue of the basal zone.

Previous reports suggested that the rat placenta had a rather limited capacity for steroidogenesis, particularly with respect to progesterone production. We found that the basal zone in the rat placenta, which contains the steroidogenic giant cells, can be isolated by simple surgical separation, and using them for in vitro incubations, significant conversion of pregnenolone-7 alpha-3H to progesterone-1H could be demonstrated. The 5alpha-reduced metabolites of progesterone, as well as compounds of the delta4 pathway, including 17alpha-hydroxyprogesterone, androstenedione and testosterone were also isolated and characterized. When progesterone-7alpha-3H was added as precursor, most of it remained unmetabolized, but qualitatively identical products were isolated as were found when pregnenolong-7alpha-3H was used as precursor. Incubations of the whole placenta and of labyrinth tissue yielded relatively much lower progesterone-3H from pregnenolone-7alpha-3H. Our results have thus established that the rat placenta is a steroidogenic organ, just as in many other species. Past difficulties in isolating progesterone from in vitro studies may possibly be due to the intense activity of the enzyme 5alpha-reductase when the whole placenta was used.