In the present study, we compared rat uterine contractility and myometrial inositol phosphate (InsP) production in response to activation of muscarinic and oxytocin receptors during pregnancy and at term. The level of myometrial phospholipase (PL) Cβ was also determined by Western blotting at different stages of pregnancy and following administration of oestradiol, progesterone or vehicle. The results showed an increased potency of carbachol (CCh), a cholinergic muscarinic agonist, and oxytocin (OT) to enhance myometrial InsP production at term. This correlated with an increased potency of both agonists to induce contraction of the circular but not the longitudinal muscle. For both InsP production and contractile activity, the maximal response of CCh was unaltered, while that of OT was significantly increased. Interestingly, the increased responsiveness to CCh and OT was associated with an up-regulation of PLCβ1 and PLCβ3 enzymes. Such regulation is under the control of oestradiol since administration of this steroid to pregnant rats increased the amount of both enzymes by 200–260%. In contrast, progesterone administration was without effect. The present study presents the first evidence that the expression of rat myometrial PLCβ1 and PLCβ3 is under the positive control of oestradiol. This could participate in the enhancement of myometrial InsP accumulation and uterine contraction at term in response to CCh and OT. Based on contraction studies, we also propose that the longitudinal and circular uterine muscles differ in the regulation of the PLC pathway during pregnancy.

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.