Uterine Activity in Late Pregnancy and During Parturition in the Rat

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Abstract
Uterine activity was recorded in 17 unanesthetized, unrestrained pregnant rats by means of intrauterine balloons inserted at a previous operation. Recordings of intrauterine pressure changes were obtained from Day 18 of gestation onwards and throughout spontaneous delivery, which occurred on Days 22 or 23 of gestation. Oxytocin was injected intravenously through an indwelling polyethylene catheter at intervals. The uterus of the pregnant rat was never quiescent except for short periods of time and it responded to moderate doses of oxytocin (10 mU) throughout the period studied. However, a changing pattern and character of contractions was consistently observed in late pregnancy and toward term the intensity of the uterine response to oxytocin increased markedly. Parturition occurred at a time when the uterine oxytocin sensitivity was maximal. The expulsion of the litter was preceded by a period of continuous uterine activity lasting 2.5 hr on the average and by a period of 5-30 min of bearing down efforts before the delivery of the first fetus. The delivery of successive littermates was associated with continuous uterine activity and only a few abdominal straining efforts, and lasted 1.5 hr on the average. After the last fetus and placenta were born the uterine activity disappeared quickly and in 10-30 mm the uterus was completely quiescent except for infrequently occurring cycles consisting of 3-4 contractions. Rhythmic regular activity started 8-12 hr postpartum.

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