Induction of premature delivery in sheep following infusion of cortisol to the fetus: the effect of maternal progestagen treatment on the C21-steroid-17 alpha-hydroxylase, C-17,20 lyase and aromatase pathways.

Infusion of cortisol to the fetus of late pregnant sheep caused an increase in maternal and fetal plasma oestrone and oestrone sulphate concentration and subsequent delivery of the fetus. Administration of progesterone or medroxyprogesterone acetate to late pregnant sheep inhibited the induction of labour. The changes in maternal and fetal plasma concentrations of pregnenolone, 17 alpha-hydroxypregnenolone, dehydro-epiandrosterone and their respective sulphates were not significantly altered by the administration of progestagens during induction of delivery. Placental delta 5-3 beta-hydroxysteroid dehydrogenase was not, therefore, significantly inhibited by progestagen administration. Furthermore induction of 17 alpha-hydroxylase and C17-20 lyase by cortisol was not suppressed. Similarly, since the changes in maternal and fetal oestrone and oestrone sulphate concentrations were not affected by treatment of the ewe with either progesterone or medroxyprogesterone acetate, the increase in the activity of aromatase, observed during induced parturition, was not inhibited, not did the exogenous progesterone provide any further substrate for placental oestrogen production. Progestagen treatment is, however, able to inhibit delivery when administered in sufficient quantities to overcome the stimulatory effect of the oestrogens produced by the placenta from endogenous precursors.