Molecular Mechanisms of Adenylyl Cyclase Desensitization in Pregnant Rat Myometrium following In Vivo Administration of the (beta)-Adrenergic Agonist, Isoproterenol

(beta)-Adrenergic agonists are widely used for preterm labor treatment, but their effectiveness may be limited by desensitization. We thus investigated the effects of a (beta)-agonist, isoproterenol, on the myometrial (beta)-adrenergic receptor ((beta)-AR)/adenylyl cyclase pathway after administration in vivo to late-pregnant rats (8 mg/kg, twice-daily injections). One hour after the first injection, isoproterenol-stimulated adenylyl cyclase activity was reduced by 37%. This was associated with a rapid and transient uncoupling of the (beta)-2-ARs (53% reduction of high-affinity receptors). After prolonged isoproterenol treatment (76 h), adenylyl cyclase activity was desensitized not only to isoproterenol but also to guanosine triphosphate and forskolin. Such treatment induced 1) a selective decrease of (beta)-2-ARs as assessed by 125I-cyanopindolol binding, which was reversed by 5ʹ-guanylylimidodiphosphate and thus probably did not involve irreversible loss of receptors, and 2) a rapid alteration of their transcript levels. Prolonged isoproterenol treatment also led to myometrial Gi2(alpha) and Gi3(alpha) increase (44% and 70%) as assessed by Western blotting. Furthermore, pertussis toxin pretreatment of membranes abolished the decrease in isoproterenol-stimulated adenylyl cyclase activity. Thus, we demonstrated that myometrial adenylyl cyclase desensitization after (beta)--agonist treatment results mainly from (beta)-2-AR uncoupling and increase in Gi activity.