UTERINE CONTRACTILITY AND ACTIONS OF CATECHOLAMINES IN LONGITUDINAL AND CIRCULAR UTERINE LAYERS FROM OVARIECTOMISED GUINEA‐PIGS: THE EFFECTS OF OVARIAN STEROIDS

Abstract

The influence of ovarian steroids upon responses to electrical stimulation and to activation of adrenoreceptors in field-stimulated preparations of longitudinal and circular myometrium from ovariectomised guinea-pigs has been investigated. Adult virgin guinea-pigs were bilaterally ovariectomised and were (i) treated two weeks later with thrice-weekly injections of oestradiol cypionate for two weeks, or (ii) treated as in (i) then given oestradiol cypionate and progesterone for a further four days. Control groups of bilaterally ovariectomised and sham ovariectomised animals remained untreated. Both myometrial layers from untreated ovariectomised guinea-pigs were atrophied. Responses to field stimulation in the circular myometrium were much smaller than those in the longitudinal layer. Steroid pretreatment, most notably treatment with oestradiol and progesterone, were associated with decreased and increased responsiveness to electrical stimulation in the circulation and longitudinal myometrial layers, respectively. Adrenaline and noradrenaline were consistently excitatory on preparations of circular myometrium from ovariectomised animals. Responses comprised either enhancement of electrically-evoked contractions, or, with the higher concentrations, the appearance of rapid contractions superimposed upon an increase in basal tone. The latter effects were also evident in preparations of circular myometrium from sham operated animals. In preparations of longitudinal myometrium from untreated ovariectomised animals noradrenaline consistently and adrenaline usually caused a simple enhancement of the magnitude of the evoked contractions. Phentolamine reduced the excitatory effects of both amines in both layers. In circular myometrium from the oestrogen-treated group both catecholamines produced phentolamine-sensitive enhancement of electrically-evoked contractions, but did not cause high frequency contractions or increased tonus. Noradrenaline and adrenaline produced qualitatively similar phentolamine-sensitive effects in preparations of longitudinal myometrium from this group. The co-administration of progesterone and oestradiol to oestradiol-treated animals led to a decrease in the excitatory potency of both amines on the circular layer compared to values obtained for the oestradiol-primed group: this effect achieved statistical significance in the case of noradrenaline. The effects of adrenaline and noradrenaline on the longitudinal layer were reversed to a propranolol-sensitive inhibition of the evoked contractions. These findings, when contrasted with the results of an earlier study, in which identical treatments were given to non-ovariectomized guinea-pigs, indicate that the precise nature of the influence of treatments with ovarian steroid hormones upon responses mediated by longitudinal and circular myometrial adrenoreceptors depends upon the presence or absence of functional ovaries.