Cross-regulations between Gs and Gi mediated pathways controlling the adenylyl cyclase activity have been clearly demonstrated in vitro. To elucidate whether activation of the beta-adrenergic pathway in the pregnant myometrium might affect Gi proteins and alpha(2)-adrenergic receptors (ARs), we treated late pregnant rats from day 18 to day 21 with twice-daily administration of isoproterenol (8 mg/kg). This treatment increased myometrial cAMP levels and led after 76 h to a significant and maximal rise in the immunoreactive amount of myometrial Gi alpha 2 and Gi alpha 3 proteins (1.4- and 1.7-fold respectively) associated with a parallel increase of the steady-state levels of both Gi alpha 2 and Gi alpha 3 mRNA (1.6- and 1.9-fold respectively). Propranolol antagonized this response indicating the implication of the beta-adrenergic pathway. Nuclear run-on assays demonstrated that isoproterenol enhanced respectively by 1.3- and 1.2-fold the transcription rate of the Gi alpha 2 and Gi alpha 3 genes. Quantification of myometrial alpha(2)-ARs by [3H]rauwolscine binding revealed that the total number of receptors was also increased at 76 h by 1.7-fold when compared with controls, with no change in the affinity of the alpha(2)-ARs for the ligand. This effect was antagonized by propranolol. Quantification of both alpha(2A)- and alpha(2B)-subtypes by Northern blotting analysis demonstrated that this elevation was due to a selective increase of the alpha(2A)-subtype mRNAs. The present results indicate that in vivo stimulation of the beta-adrenergic pathway by isoproterenol increases both Gi alpha 2/Gi alpha 3 and alpha(2A)-AR expression in the pregnant rat myometrium. The possible contribution of such a mechanism in pregnancy-related changes of both entities is discussed.

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