Inhibition of thyrotropin- and dibutyryl cyclic AMP-induced secretion of thyroxine and triiodothyronine by catecholamines.

Thyrotropin (TSH), 1 MU/ml and N6, O2'-dibutyryl adenosine 3',5-cyclic monophosphoric acid (dbcAMP) greatly enhanced the release of thyroxine (T4) and triiodothyronine (T3) from mouse thyroids incubated in vitro. L-Epinephrine (E) and L-norepinephrine (NE) strongly inhibited the TSH and dbcAMP-stimulated release of thyroid hormones; L-isoproterenol (IPNE) exerted a relatively weak inhibition. The inhibition by catecholamines was prevented by the alpha-adrenergic blocker, phentolamine; L-propranolol, a beta-adrenergic blocker, had no effect on the inhibition. The TSH-induced release of thyroid hormones was not affected by adrenergic blockers. Epinephrine did not affect the increase in thyroidal cAMP content induced by TSH. These results indicate that catecholamines act by way of an alpha-adrenergic receptor to suppress TSH-stimulated release of thyroid hormones at a point beyond cAMP formation.