Relevance of changes in adrenoceptor characteristics after terbutaline treatment

Rats were treated with terbutaline in order to specify the in vivo effects of beta 2-sympathicomimetic treatment in relation with desensitization phenomena on the level of pulmonary adrenoceptors. Maximal binding of 3H-dihydroalprenolol (3H-DHA) to lung membrane preparations, binding affinity and the relative amount of beta 2-adrenoceptors were used as parameters. These were established 1, 3, and 6 days after a 10 days terbutaline treatment and compared with control values. Ten days treatment did not cause significant changes in the parameters measured. However, maximal binding was enhanced 3 and 6 days after the last treatment, binding affinity was enhanced 3 days after the last treatment, while the relative amount of beta 2-adrenoceptors was diminished 1 and 6 days after termination of the treatment. When the amount of beta 2-adrenoceptors was calculated in terms of 3H-DHA binding, an increase could be shown 3 days after the last terbutaline treatment. Therefore the direct influence of the treatment on the beta-adrenoceptor population appeared to be small, dependent on the parameter used and probably not eliciting a functional desensitization.