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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.

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Myometrial responses to different agents acting on adrenoceptors were examined in vivo in the pro‐oestrous rat. Changes in spontaneous uterine mechanical activity were recorded isometrically and evaluated in terms of amplitude and duration of uterine contractions. Phenylephrine (10 μg kg−1) markedly increased the amplitude and duration of contractions and 40 μg kg−1 gave rise to tetanic contractions. Administration of either nicergoline (400 μg kg−1) or phentolamine (1000 μg kg−1) to phenylephrine‐primed rat uterus reduced the strength of contractions and phentolamine abolished the phenylephrine‐induced uterine contracture. Following blockade of α2‐adrenoceptors by yohimbine (1000 μg kg−1) and β‐adrenoceptors by propranolol (2400 μg kg−1), a single injection of phenylephrine (100 μg kg−1) increased the amplitude of uterine contractions by 30%. Noradrenaline reduced the amplitude of contractions and caused elevation of the baseline level. The response of myometrium to the combination of both propranolol and noradrenaline was the establishment of uterine contracture with subsequent increase of the duration of contractions. These results clearly demonstrate the involvement of α‐adrenoceptors in the myometrial activity of the rat in vivo during pro‐oestrus.

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