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Increase of uterine motility and simultaneous decrease of progesterone concentrations in the rat after bilateral ovariectomy at mid-pregnancy.

Comparison of uterine activities recorded during the control period to those obtained during the two recording periods after ovariectomy (0-30 min and 30-60 min) showed an increase of the amplitude of uterine contractions (P less than 0.005) and a decrease of the interval between two successive uterine contractions (P less than 0.005) and the delay of electrical activities (P less than 0.005). Progesterone treatment (50 mg/kg i.m.) of ovariectomized rats prevented the abrupt fall in plasma progesterone concentrations, measured by RIA, which in turn inhibited the increase of uterine mechanical and electrical activities. A close relation between the increase of myometrial activity and the decrease of progesterone concentrations after ovariectomy is suggested. The activation of the myometrium would be principally induced by the fall of progesterone or by the variation of the oestrogen/progesterone ratio; these changes in sexual steroid hormones would augment the uterine sensitivity to physiological stimuli or modify the activity of other factors involved in the regulation of the myometrium.