Changes in the concentrations of plasma steroid hormone and plasma 13, 14-dihydro-15-oxo-prostaglandin F2 alpha in late pregnancy rabbits treated with an inhibitor of 3 beta-hydroxysteroid dehydrogenase.

Changes in the concentrations of progesterone, 17 beta-estradiol and 13, 14-dihydro-15-oxo-prostaglandin F2 alpha (PGFM) were evaluated in the peripheral plasma of rabbits during late pregnancy by treating trilostane, an inhibitor of 3 beta-hydroxysteroid dehydrogenase, in an attempt to obtain further insight into the involvement of progesterone and prostaglandin (PG) in the initiation of parturition. The concentrations of progesterone were 18.8 +/- 2.2 ng/ml (mean +/- SE, n = 6) before administration of the inhibitor, significantly (p less than 0.05) fell to 7.6 +/- 1.0 ng/ml (n = 6) at 30 min, and remained low until 10 h after the drug administration. The concentrations of progesterone were still low (5.4 +/- 0.5 ng/ml, n = 6) at 20-24 h after administration of the inhibitor, and were also low (4.9 +/- 2.2 ng/ml, n = 6) at delivery. Premature deliveries occurred at 28.8 +/- 2.0 h after injection of trilostane (on days 29 of gestation). Increased concentrations of PGFM were observed at delivery. However, administration of trilostane induced no discernible changes in the concentration of estradiol. These findings suggest that delivery is induced by progesterone withdrawal and that synthesis prostaglandin F2 alpha is remarkably increased at delivery in the rabbit.