Progesterone effects on the evolution of myometrial activity in vivo in the non-pregnant rabbit

In order to study the influence of progesterone on the myometrial activity ovarian secretion of progesterone was stimulated by intravenous injection of human chorion gonadotrophin (HCG, 75 IU) in 9 nonpregnant rabbits. 4 rabbits, used as controls, were given only saline. Plasma progesterone concentration in blood samples taken at frequent intervals (2-4 h) during the recording was measured by radioimmunoassay. In the controls, the myometrial activity and the plasma levels of progesterone did not change. After 1 h of HCG administration the amplitude of contraction began to decrease and the frequency began to increase after 3 h. After about 5 h the changes in amplitude and frequency were generally fully developed. This pattern of myometrial activity persisted throughout the recording period (12 h) and was also present in recordings made after 24 and 48 h of HCG injection. A maximal increase in plasma progesterone concentration (5.0 ng/ml) had already occurred by 2 h after the HCG injection and gradually decreased thereafter. It is concluded that an elevation in the level of progesterone in non-pregnant rabbits resulting from increased ovarian output (through HCG) has a pronounced and immediate depressing effect on the character of myometrial activity which is indicative of a shift from nonpregnant to pregnant type.