Involvement of alpha-adrenoceptors in myometrial responses in the pro-oestral rat.

1. 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. 2. Phenylephrine (10 micrograms kg-1) markedly increased the amplitude and duration of contractions and 40 micrograms kg-1 gave rise to tetanic contractions. 3. Administration of either nicergoline (400 micrograms kg-1) or phentolamine (1000 micrograms kg-1) to phenylephrine-primed rat uterus reduced the strength of contractions and phentolamine abolished the phenylephrine-induced uterine contracture. 4. Following blockade of alpha 2-adrenoceptors by yohimbine (1000 micrograms kg-1) and beta-adrenoceptors by propranolol (2400 micrograms kg-1), a single injection of phenylephrine (100 micrograms kg-1) increased the amplitude of uterine contractions by 30%. 5. 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. 6. These results clearly demonstrate the involvement of alpha-adrenoceptors in the myometrial activity of the rat in vivo during pro-oestrus.