Responses Mediated by Adrenoceptors in the Separated Layers of the Myometrium and in the Costo-Uterine Muscle of the Guinea Pig During the Estrous Cycle

The mechanical responses of field-stimulated preparations of the longitudinal and circular myometrium, and of the costo-uterine muscle of the guinea pig to adrenoceptor agonists have been examined on Days 1, 4, 10 and 15-16 of the estrous cycle. The alpha-adrenoceptor agonist, phenylephrine, was exclusively excitatory in all three preparations taken from animals at each of the cycle days studied. Concentration-response curves for phenylephrine in all three tissues were unaffected by the fluctuations in ovarian hormones occurring during the estrous cycle. Isoproterenol (1 nmol/1-5 mumol/1) usually inhibited field-stimulation-induced contractions of the longitudinal uterine layer and the costo-uterine muscle; higher doses usually caused enhancement of contractions. In neither tissue did the potency or maximum effect of this inhibitory agonist differ at differing cycle stages. In the circular myometrium, the inhibitory effects of isoproterenol were less consistently observed, particularly in preparations taken at proestrus (Days 15-16). It is concluded that, except perhaps in the circular myometrium at proestrus, the fluctuations in ovarian hormones occurring during the estrous cycle in this species are insufficient to modify uterine responses mediated via alpha- and beta-adrenoceptors. It remains to be established whether the steroid hormones are capable of modulating responses mediated by adrenoceptors in the costo-uterine muscle.