Placental lactogen not growth hormone and prolactin regulates secretion ofprogesterone in vitro by the 40-45 day ovine corpus luteum of pregnancy

The study was designed to compare the direct effect of three prolactin-like hormones on steroidogenesis of ovine luteal cells collected at day 40-45 o f pregnancy. 100 ng/ml of ovine placental lactogen or 100 ng/ml of ovine gr owth hormone or 100 ng/ml of ovine prolactin were added to the media of lut eal cell cultures. After 48 h incubation, all cultures were terminated and the media were frozen until further steroid analysis. To determine to what extent growth hormone (GH), prolactin (PRL) and lactogen (PL) regulate the activity of 3 beta-HSD, an enzyme involved in progesterone synthesis, the c lassical steroidal competitive inhibitor of 3 beta-HSD trilostane, was inve stigated for its effects on basal and GH-, PRL-, and PL-stimulated progeste rone biosynthesis since there is a possibility that the luteotropic effect of these hormones are mediated via 3 beta-HSD. oPL resulted in an increase of progesterone secretion in a statistically significant manner, while GH o r PRL had no effect on progesterone secretion. A decrease in progesterone s ecretion as an effect of 100 mM trilostane was observed in all culture type s. An explanation for the luteotropic effect of PL and the lack of this eff ect for GH is that the GU receptor associates with a different molecule wit hin the ovarian tissue and forms a heterodimeric receptor for FL, and the p ossibility that physiological effects of native oPL may be mediated through its binding to specific PL receptors, which have low affinities for oGH an d oPRL.