Abstract:

Cumulative log dose-response curves of soterenol, isoproterenol, phenylephrine and norepinephrine on isolated pregnant rat uterus at different days of gestation, were investigated. Soterenol produced a sustained inhibition of spontaneous motility during the whole pregnancy and this effect was blocked by butoxamine. The affinity of myometrium for the β2-adrenoceptor agonist was parallel to the concentration of progesterone in plasma during pregnancy. Isoproterenol, norepinephrine and phenylephrine caused dual, α- and β-mediated responses; their relative dominance varied with the concentration and the days of pregnancy. α-Adrenoceptor effects of the amines coincided with increased plasma concentrations of estrogens, whereas β ones were in parallel with the increment of plasma progesterone. It is concluded that: there exist in the pregnant rat uterus β2-receptor-mediated responses influenced by the length of gestation; and the concentration-pregnancy-dependent biphasic actions of isoproterenol, norepinephrine or phenylephrine suggest that their variable hormone-modulated ability interacts with both α- and β-adrenoceptive uterine sites.