Adrenoceptors involved in the contractile activity of islated pregnant rat uterus.

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 beta 2-adrenoceptor agonist was parallel to the concentration of progesterone in plasma during pregnancy. Isoproterenol, norepinephrine and phenylephrine caused dual, alpha- and beta-mediated responses, their relative dominance varied with the concentration and the days of pregnancy, alpha-Adrenoceptor effects of the amines coincided with increased plasma concentrations of estrogens, whereas beta ones were in parallel with the increment of plasma progesterone. It is concluded that: (a) there exist in the pregnant rat uterus beta 2-receptor-mediated responses influenced by the length of gestation; and (b) the concentration-pregnancy-dependent biphasic actions of isoproterenol, norepinephrine or phenylephrine suggest that their variable hormone-modulated ability interacts with both alpha- and beta-adrenoceptive uterine sites.