The endocrine control of myometrial contractility in the uterus of the pregnant rabbit

Abstract

The present experiments clarify the following points:1. Provided that extreme care is taken to maintain light anesthesia, undisturbed breathing, constant body temperature, and constant temperature around the uterine segment from which recording is made, furthermore, if the tension is recorded isometrically, there is no significant difference between in vitro and in vivo findings. This suggests that the reactivity of the myometrium is determined by the endocrine state of the animal and that other mechanisms working in the body are of lesser importance.2. That the method here employed is satisfactory has been established in several different ways:a. Between the steps of an experiment in which electrical stimulation was employed, it was always possible to observe the same pattern of spontaneous activity as found at the beginning of the experiment.b. All animals in the same group exhibited the same pattern of uterine behavior, and even the quantitative differences from one animal to another in the same group were slight. Among all the tests employed in these studies, spontaneous activity showed the greatest and the staircase the smallest variation.c. When the patterns of behavior of the two horns of the uterus of one animal were compared, the similarity between the two records obtained was very close.d. When tension was recorded continuously for a period of 5 hours in an estrogen-treated nonpregnant rabbit, the uterus being stimulated electrically with optimum stimulus once every 4 minutes, tension remained unchanged. This, we believe, is one of the most convincing tests of the reliability of the technique and the unchanged condition of the animal during the experiment.3. These experiments demonstrate that until the twenty-ninth day of pregnancy progesterone dominates the myometrium. The effect of progesterone begins to decline only on the twenty-ninth day, its withdrawal being completed on the thirty-second day, the day on which labor occurs.4. A few hours before delivery, on the thirty-second day, the myometrium is clearly estrogen dominated. Its excitability is optimal. Its conductivity appears to be restored, its intrinsic stimulatory system is highly efficient, and development of spontaneous tension is at its best. Working capacity is maximal. It appears, therefore, that all conditions are optimal as required for maximum function, and the uterus is ready to perform its main physiological function, i.e., the delivery of the conceptuses.5. Shortly after delivery estrogen domination approaches an extreme condition so far as excitability of the myometrium is concerned, whereas working capacity begins to decline, and 48 hours after delivery it is similar to the value which can be obtained in the nonpregnant estrogen-dominated animal.It is to be hoped that the findings and methods reported in this paper will be useful in devising means by which studies on the human myometrium can be carried out in order to establish changes in endocrine domination in women during normal and abnormal pregnancies.