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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) Cbeta 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 PLCbeta1 and PLCbeta3 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 PLCbeta1 and PLCbeta3 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.

Up-regulation of rat myometrial phospholipases Cβ1 and Cβ3 correlates with increased term sensitivity to carbachol and oxytocin