Abstract

The effects of various beta-adrenoceptor agents on radioiodine release from the thyroid were studied in mice pretreated with 125I and thyroxine. The non-selective beta-adrenoceptor agonist isopropylnoradrenaline and the selective beta 2-adrenoceptor agonist terbutaline both, and with the same efficacy, enhanced radioiodine release, whereas the selective beta 1-adrenoceptor agonist prenalterol had no such effect. The non-selective beta-adrenoceptor antagonist L-propranolol and the selective beta 2-adrenoceptor antagonist ICI 118,551 both abolished the radioiodine release induced by isopropylnoradrenaline or by terbutaline. The selective beta 1-adrenoceptor antagonist metoprolol inhibited the radioiodine response to isopropylnoradrenaline, but not that to terbutaline. Neither of the beta-adrenoceptor antagonists influenced the radioiodine release induced by TSH. It is concluded that the beta-adrenoceptors involved in the regulation of thyroid hormone secretion are mainly of the beta 2-subtype, and, further, that beta-adrenoceptor agonists and TSH exert their thyroid hormone secretory effects through different mechanisms.