Receptors mediating epinephrine effect on thyroid 131-i uptake and thyroxine synthesis in the rat.

The adrenergic receptors mediating the inhibitory effect of epinephrine on the rat thyroid in vivo have been studied by the use of more specific alpha- and beta-adrenergic stimulators, and blocking agents. The effect of methoxamine was similar to that of epinephrine, while isoproterenol stimulated 131-I uptake slightly, and did not alter the ratio of monoiodotyrosine (MIT) to diiodotyrosine (DIT). However, like methoxamine, isoproterenol resulted in a decrease in thyroxine (T4) formation. Phentolamine could partially prevent epinephrine-induced decrease in 131-I uptake and T4 synthesis. The changes in the MIT/DIT ratio could be completely prevented by high doses of phentolamine. Propranolol enhanced the effects of epinephrine. It is concluded that the effect of epinephrine on the rat thyroid in vivo is predominantly alpha-adrenergic, but T4 synthesis may be decreased by stimulation of either receptor.