Effects of non-selective and selective beta-adrenergic agonists on spontaneous contractions and cyclic AMP levels in myometrial strips from pregnant women.

Isoprenaline and terbutaline relaxed myometrial strips from pregnant women and increased their content of cAMP. The highest measured cAMP concentration, about twice the control level, was determined after a 3-min incubation. Subsequently, the cAMP level declined and approached the basal level after about 60 min. The spontaneous contractions of the myometrial strips were abolished for about 20 min, but reappeared gradually after that time, although the beta-agonists were present in the incubation medium all the time. The decrease of the beta-adrenoceptor responses was not due to an inactivation of the drug tested, or dependent on any diffusible inhibitor substance, since the incubation medium containing the beta-agonist from the 60-min incubation elicited a normal response when added to a fresh myometrial preparation. We suggest that in vitro incubation with high concentrations of beta-agonists induces a desensitization of the beta-receptor of human myometrium.