Studies on In-Vivo Capacity of Thyroxine Deiodinating System in Rat Liver: Biliary Excretion of Several Iodothyronines after Increasing Loading Doses of Thyroxine

Abstract

Eighteen male rats were inserted with polyethylene tubings into bile duct and femoral vein and two rats each were injected a series of doses from 5 to 1280 micrograms L-thyroxine into a venous cannula. The bile was collected for three subsequent 2-hr periods and the excretion of total thyroxine (T4), triiodothyronines (T3 and rT3) and all diiodothyronines (3,3'-T2, 3,5-T2 and 3',5'-T2) was estimated as well as that of conjugated T4 and T3. The excretion of all compounds was considerably increased as early as within the first 2-hr period. Almost linear dose-response relationship was found between the dose of T4 and its biliary excretion up to the dose of 640 micrograms, only smaller increase being observed after 1280 micrograms T4. Similar relationship was found also in the excretion of diiodothyronines, while that of triiodothyronines after 1280 micrograms T4 was slightly less than after 640 micrograms T4. The excretion of rT3 was consistently about twice as high as that of T3. The data on diiodothyronine excretion suggested a preferential conversion of rT3 to 3',5'-T2 and that of T3 to 3,5-T2 over that to 3,3'-T2. The ratio of 3,5-T2/T3 after different doses of T4 was about 2--4 times higher than that of 3',5'-T2/rT3 suggesting the higher deiodination rate of T3 than that of rT3.