Action of several progestagens on the electrical activity of the pregnant rat myometrium after bilateral ovariectomy

8-day-pregnant rats were ovariectomized then injected im with progestagens and estrogens while their uterine motility and electrical activity were being recorded simultaneously with a strain gauge and 2 bipolar electrodes. The injections known to maintain pregnancy after ovariectomy were progesterone 50 mg/kg progesterone with estradiol 5 mcg/kg or medroxyprogesterone acetate 25 mg/kg. Base line recordings for 20 minutes showed the normal pregnant state of brief periods of synchromous electrical and mechanical activity. After ovariectomy untreated controls exhibited increased duration of synchronized mechanical and electrical activity and height of contractions. When any of the 3 progestagen treatments coincided with ovariectomy synchrony was decreased and the intervening interval lasted longer. These procedures demonstrated how quickly the hormonal deficit due to ovariectomy is compensated for when ablation is performed before placental progesterone synthesis has begun.