Oestrogen-induced sensitization of the uterine artery of the guinea-pig to acetylcholine.

1. It has been shown previously that a dilatation of the uterine artery of the guinea-pig in response to acetylcholine or to stimulation of cholinergic vasodilator nerves occurs only during pregnancy. Experiments have now been performed to determine the hormonal basis of this change in responsiveness.2. Chronic treatment of virgin guinea-pigs with oestradiol, either by intramuscular injection or by subcutaneous implantation, caused the development of a responsiveness of the uterine artery to acetylcholine which was similar in degree to that occurring during pregnancy.3. Simultaneous administration of progesterone did not significantly inhibit this oestrogen effect. In contrast the characteristic actions of oestrogen on uterine growth and fat catabolism were completely antagonized by progesterone.4. The time course of the development of responsiveness to acetylcholine of the uterine arteries during pregnancy was parallel to the rises in oestrogen production and placental blood flow as determined by other workers. This supports the concept that cholinergic dilator nerves participate in uterine hyperaemia during pregnancy and that their functioning is regulated by the maternal blood oestrogen level.