Effects of ovarian steroids upon responses mediated by adrenoceptors in separated layers of the myometrium and in the costo‐uterine muscle of the guinea‐pig

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Abstract
This study describes the effects of ovarian steroid hormones upon the responses to adrenoceptor agonists of isolated myometrium, separated into its longitudinal and circular layers, and of costo‐uterine muscle from guinea‐pigs. The preparations were field‐stimulated at 100 s intervals, and the adrenoceptor agonists phenylephrine and isoprenaline produced enhancement or inhibition of the evoked contractions. Isoprenaline produced propranolol‐sensitive inhibitory effects in longitudinal and circular myometrium and costo‐uterine muscle preparations from animals from all experimental groups: i.e. from nonsteroid‐treated animals (ovariectomized and intact); intact animals treated with either oestrogen or progesterone alone; ovariectomized animals treated with oestrogen; ovariectomized and intact animals treated with progesterone following oestrogen priming; and from animals 1–4 days post‐partum. Longitudinal myometrial preparations from progesterone‐treated oestrogen‐primed and from post‐partum animals were most sensitive to this agonist. Phenylephrine produced phentolamine‐sensitive excitatory effects in circular myometrial and costo‐uterine muscle preparations from animals from all the experimental groups. In contrast, propranolol‐sensitive inhibitory responses to phenylephrine occurred in longitudinal myometrial preparations taken from animals treated with progesterone following oestrogen priming, and from post‐partum animals. Longitudinal myometrium from animals from the remaining experimental groups exhibited phentolamine‐sensitive excitatory responses to phenylephrine. The basis for the selective effect upon the longitudinal myometrium of exposure to progesterone following a period of oestrogen priming, is discussed. The results described are consistent with the possibility that in the longitudinal layer of guinea‐pig uterus exposed to progesterone following oestrogen priming there is an increase in the proportion of β‐adrenoceptors in this layer. This increase may reduce the likelihood of contractions arising via direct stimulation of α‐adrenoceptors in this layer in response to sympathetic activation during pregnancy.

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