Steroid and prostaglandin concentrations in the plasma of pregnant ewes during infusion of adrenocorticotrophin or dexamethasone to intact or hypophysectomized foetuses

Abstract

Catheters were implanted into 16 ewes and their foetuses between days 110 and 124 of gestation. Hypophysectomy was attempted in eight of these foetuses. Continuous infusion of synthetic ACTH (10 microgram/h) or dexamethasone (1mg/24 h) into the foetus, starting between days 124 and 129, induced premature parturition. The concentration of progesterone in the maternal peripheral plasma decreased before parturition in all animals while the level of oestradiol increased in ewes with intact foetuses or in those in which hypophysectomy was incomplete. When hypophysectomy was complete, no increase in the maternal level of oestradiol occurred before delivery. The concentration of 13,14-dihydro-15-oxo-prostaglandin F2alpha increased in the peripheral plasma of ewes with intact or hypophysectomized foetuses infused with ACTH. It is suggested that an intact foetal pituitary gland is required for the rise in the level of oestrogen prepartum, but that this rise is not essential for increased prostaglandin production of parturition.

PIP: The effectiveness of dexamethasone or adrenocorticotrophin (ACTH) in inducing parturition in the hypophysectomized fetus of the ewe was investigated. 16 cross-bred ewes were used. In 8 hypophysectomy was attempted between Days 110-120 of pregnancy. Dexamethasone or ACTH were infused into a fetal vein from Days 124 to 129 of pregnancy until delivery occurred. Progesterone, estrone, and estradiol were measured by radioimmunoassay. Cortisol was measured by competitive protein binding or radioimmunoassay. Prostaglandin (PGFM) was also determined by radioimmunoassay as was ACTH growth hormone and prolactin. The interval from the start of the infusion to delivery in fetuses adminstered dexamethasone was the same whether they were intact or hypophysectomized. The induction-delivery interval was significantly longer in the hypophysectomized fetuses administered ACTH. Hypophysectomized fetuses were less viable than intact ones. Premature delivery following infusion in the intact fetus resulted in a decrease in progesterone in maternal plasma and an increase in estradiol. In maternal plasma of intact fetuses PGFM increased in the last 20-30 hours before delivery when ACTH was infused. Differences between intact and hypophysectomized fetuses in concentrations of progesterone in the maternal plasma and in the cortisol levels were insignificant. When hypophysectomy was complete the estradiol level was without the increase seen in intact fetuses. The results suggest that an intact fetal pituitary gland is required for the rise in the level of estrogen prepartum, but that this rise is unessential for increased PGFM production and parturition.