Partial reversal of the myometrial progesterone 'block' in the non-pregnant ewe in vivo by oestradiol-17 beta.

In ovariectomized non-pregnant ewes myometrial activity, which was suppressed by continued injection of progesterone, showed a significant recovery of both spontaneous activity and oxytocin responsiveness by 12 and by 24 h after intrauterine administration of oestradiol-17 beta. No such recovery occurred in ewes given vehicle only into the uterus. The oestrogen-induced recovery was lost by 60 h after treatment. Infusion of PGF-2 alpha into the uterine lumen instead of oestrogen provided no support for the possibility that the oestrogen-induced recovery was mediated through PGF-2 alpha secretion. It is suggested that oestradiol may participate in the mechanism of parturition in sheep by accelerating myometrial recovery from the effects of progesterone.