The effect of epinephrine on thyroid hormone synthesis in the rat

ABSTRACT The effect of epinephrine (100 μg/100 g body weight) on the distribution of 131I among the iodo-amino acids in the rat thyroid was studied. Chromatography of thyroid homogenates was carried out, using as solvent n-butanol:ethanol:2 n ammonia (5:1:2). Epinephrine resulted in elevation of the ratio of monoiodotyrosine (MIT) to diiodotyrosine (DIT), and decreased the incorporation of 131I into iodothyronines. The ratio of triiodothyronine (T3) to thyroxine (T4) was unchanged. Epinephrine did not influence the rate or extent of protein-binding of iodine, determined both by precipitation with trichloroacetic acid and by chromatography. The nature of the iodoprotein formed was studied by its solubility characteristics in phosphate buffer. No change was observed with epinephrine. The changes caused in the MIT/DIT ratio by epinephrine could be completely prevented by the administration of excess iodide, while the incorporation of iodide into T4 improved, but remained slightly below normal. It is postulated that the changes caused by epinephrine in 131I distribution occurred as a result of decreased iodination of thyroglobulin, but an additional effect on iodothyronine synthesis cannot be excluded.