Inhibitory effect of ovine and human placental lactogens on progesterone catabolism in luteinized rat ovaries in vitro.

: Pseudopregnant rat ovaries, obtained by PMSG and hCG treatment, were incubated or perfused. Progesterone and 20 alpha-dihydroprogesterone (20 alpha-DHP) secretion and synthesis as well as 20 alpha-hydroxysteroid dehydrogenase (20 alpha-SDH) activities were determined by radioimmunoassays. Ovine and human chorionic somatomammotrophin (oCS; hCS) and ovine prolactin (10(-8) M) significantly (P less than 0.001) depressed 20 alpha-DHP ovarian contents (49, 64 and 69 per cent, respectively) and 20 alpha-DHP release (49, 55 and 55 per cent, respectively) in comparison with those of controls without hormones. The activity of 20 alpha-SDH was also inhibited by these three hormones (59, 68 and 56 per cent, respectively). The kinetics of 20 alpha-DHP release, studied by perfusion or incubation, showed a maximum inhibition from the first hour onwards with all lactogenic hormones. The fall in 20 alpha-DHP content of luteinized ovarian tissue coincided with a sharp rise in the progesterone/20 alpha-DHP ratio, from 0.5 in the controls to 1.5 after lactogenic hormone treatment. This study in vitro showed that two placental lactogens of sheep or human origin exhibit an inhibitory effect on 20 alpha-SDH activity analogous to that of prolactin. It is suggested that these placental hormones may be involved in the regulation of luteal and placental progesterone metabolism during pregnancy.