In baboons, transplacental cortisol (F)/cortisone (E) metabolism changes from reduction (E to F) at midgestation to oxidation (F to E) near term when estrogen becomes elevated. Indeed, estrogen regulates the placental microsomal 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) enzyme catalyzing F oxidation. However, regulation of 11 beta-HSD-reductase (E to F) is unknown, because this enzyme is destroyed by microsomal isolation. Therefore, we used cell culture to determine the role of estrogen on placental reduction of E to F and to ascertain whether estrogen regulation of the oxidation of F to E was specific to trophoblast. Placentas were obtained on day 165 (n = 6; term, day 184) and on day 100 of gestation from baboons untreated (n = 8) or treated (n = 6) with 50-mg implants of androstenedione (delta 4A) inserted sc in the mother between days 70 and 100 of gestation to increase placental estrogen production. After removal of fetal membranes, the decidua basalis and trophoblast were separated, rinsed repeatedly in medium-199, minced, and then incubated in trypsin/DNase. Dispersed cells were layered onto a discontinuous Percoll gradient (5-70%), and purified cytotrophoblast (TC; 1.048-1.062 g/ml) and decidua (DC; 1.048-1.062 g/ml) were harvested. After incubation in media containing 10% fetal bovine serum to permit attachment, cells were incubated (24 h) in Dulbecco's modified Eagle's medium containing 10,100, or 500 ng [3H]F or [3H]E. F and E in medium were purified by HPLC and the interconversion of F/E calculated. Equilibrium was achieved by 12 h, and F/E metabolism was proportional to cell number and substrate (10-500 ng) concentration. At substrate concentrations of 500 ng/ml, the reduction of E to F (range, 81-195 ng F produced/24 h) in the DC (0.5 x 10(6) cells) was greater (P less than 0.05) than oxidation of F to E (19-28 ng E/24 h) in all groups. This pattern of metabolism by DC was not affected by time of gestation or treatment with delta 4A. In the TC (2.5 x 10(6) cells), oxidation of F to E always exceeded (P less than 0.05) reduction of E to F. Moreover, the conversion of F to E by TC of day 100 (86 +/- 26 ng E/24 h; mean +/- SE) was increased (P less than 0.05) by delta 4A (195 +/- 35) and greater (P less than 0.05) at day 165 (213 +/- 40). In contrast, TC metabolism of E (21-57 ng F/24 h) was similar in all groups.(ABSTRACT TRUNCATED AT 400 WORDS)