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Regulation of Myometrial Phospholipase C System and Uterine Contraction by ß-Adrenergic Receptors in Midpregnant Rat

Laboratoire de Physiologie et Physiopathologie,² UMR-CNRS 7079, 75252 Paris CEDEX 05, France Unité de Neuro-Gastroentérologie et Nutrition,³ INRA, 31931 Toulouse CEDEX 09, France

We investigated whether ß-adrenergic receptors (ß-AR) regulate the phospholipase C (PLC) system in midpregnant rat myometrium. PLCß isoforms were characterized, and the effect of isoproterenol (ß-adrenergic agonist) was tested on myometrial inositol phosphate (InsP) production and uterine contraction. Using specific antibodies, we showed that rat myometrium expresses PLCß1, PLCß3, and PLCß4, and to a lesser degree PLCß2. Quantitative analysis revealed that PLCß isoforms are differentially expressed during pregnancy. Indeed, the amount of PLCß4 is increased at midpregnancy, whereas PLCß1, PLCß2, and PLCß3 are up-regulated at term. At midpregnancy, pretreatment of myometrial strips with isoproterenol significantly reduced basal and agonist-stimulated InsP production. Forskolin, a diterpene that increases cAMP accumulation by directly activating adenylyl cyclases, had no effect on InsP production. In contrast, two global potassium (K⁺) channel inhibitors, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), prevented attenuation of InsP production by isoproterenol. Isoproterenol also significantly decreased spontaneous and agonist-induced contraction of the longitudinal layer of midpregnant rat myometrium. Preincubation of uterine strips with TEA plus 4-AP prior to ß-AR activation blocked only partial uterine relaxation, whereas Forskolin was as potent as isoproterenol. This indicates that ß-AR operate through both K⁺ channels and cAMP to induce uterine relaxation. In conclusion, we show for the first time that three myometrial PLCß isoforms (PLCß1, PLCß2, and PLCß3) are down-regulated at midpregnancy. At this period, ß-AR reduce basal and agonist-stimulated InsP production through activation of K⁺ channels. Altogether, these mechanisms could act to decrease responsiveness of the longitudinal layer of myometrium to contractant factors.

¹ Correspondence and current address: Sakina Mhaouty-Kodja, Institut Pasteur, Bâtiment Jacques Monod. 25, rue du Docteur Roux, 75724 Paris CEDEX 15, France. FAX: 331 40 61 31 09; smhaouty{at}pasteur.fr

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