Effect of progesterone on the electric activity and intrauterine pressure of pregnant and parturient rabbits

Using a new method, the simultaneous recording of the electric and mechanic activity of the pregnant and parturient rabbit uterus, we obtained detailed information about the character of progesterone effect. When the uterus is effectively blocked by progesterone (endogenous or exogenous) electric activity is greatly suppressed and is restricted to the point of origin. As a result, the mechanic activity of the uterus is insignificant. If the progesterone block is moderately weakened, asynchronic electric activity is generated at different uterine portions but the spread of activity is limited to restricted areas. Mechanic activity improves some-what but carries asynchronic symptoms. When the progesterone block is largely or completely withdrawn, the action potentials generated at the pacemaker area in the form of regular trains readily propagate to distant uterine portions. Electric activity is synchronic resulting in pressure cycles of large amplitude and of uncomplicated shape. The progesterone block is enforced upon the uterus by the endocrine functions of the corpora lutea and placentae. The removal of these endocrine glands results in the withdrawal of the block and the evolution of uterine activity. This evolution can be delayed or reversed by progesterone therapy. Cyclic administration of progesterone results in the cyclic evolution and suppression of uterine activity. The two pregnant horns of the same animal can be made to develop activities of different character and extent. This is achieved by the earlier removal of one set of placentas and their local effect on the emptied horn. The systemic (intravenous) administration of phospholipids also weakens the progesterone block and results in a premature evolution of uterine activity.