Characterization of myometrial desensitization to β-adrenergic agonists

Continuous exposure of ovine myometrial strips exposed to isoproterenol (10 μM) resulted in only transient inhibition with contractions returning within 60 min. Rechallenging these strips with isoproterenol failed to induce inhibition, confirming the occurrence of desensitization. In contrast, exposure of myometrial tissue to isoproterenol for only 5 min did not result in desensitization. Myometrial strips exposed to isoproterenol demonstrated a significant increase in cAMP content associated with inhibition of contractile activity and a subsequent fall in cAMP content upon desensitization. Elevation of endogenous cAMP levels by either inhibition of cAMP-dependent phosphodiesterase activity (0.5 mM isobutylmethylxanthine, in ovine strips) or direct activation of adenylyl cyclase (10 μM forskolin, in rat strips) induced a rapid and significant inhibition of myometrial contractile activity in desensitized tissue. Scatchard analysis of the binding of the β-adrenoceptor antagonist, [125I]iodocyanopindolol, revealed a significant reduction in the concentration of β-adrenergic receptors (but no change in binding affinity) in desensitized myometrial tissue. Incubation of desensitized tissue with fresh buffer for 3 h induced only a partial recovery in responsiveness to isoproterenol. These data suggest that prolonged, but not acute, exposure of the myometrium to β-adrenergic agonists induces a state of desensitization that is associated with a down-regulation of β-adrenoceptors but maintenance of postreceptor function.Key words: myometrium, contraction, β-adrenergic receptors, β-antagonists.