Is the relaxing effect of beta-adrenergic agonists on the human myometrium only transitory?

The beta-adrenergic receptor function in myometrial strips taken from pregnant women at cesarean section was studied. The material consisted of myometrial strips from 10 patients treated with terbutaline for threatening premature parturition and an untreated control group of 10 patients. The basal cAMP levels were lower and the cAMP production after receptor stimulation in vitro was less in myometrial strips taken from terbutaline-treated women than in the untreated control group. The phosphodiesterase activity was greater in myometrial strips taken from terbutaline-treated women, indicating a faster rate of degradation of cAMP in these women. Studies on the beta-adrenergic receptor function in leukocytes from venous blood samples during treatment with terbutaline for preterm labor demonstrated a gradual decline in receptor function, as shown by a successive reduction in cAMP production after stimulation with a beta-adrenergic agonist in vitro. The results presented may explain the merely temporary inhibitory effect on uterine contractions often obtained during treatment with beta-mimetics. It is suggested that intermittent usage or combined treatment with drugs inhibiting phosphodiesterase activity might improve the results in the treatment and prevention of premature birth.