Preliminary observations on the absorption of biliary iodothyronines from the intestine in vivo in rats.

From a drained bile duct of the rats injected i. v. with various doses of L-thyroxine (50 to 200 microgram per animal) bile was obtained which was then injected to various parts of intestine in recipient rats. The recipient animals were anesthetized with pentobarbiturate and their bile duct as well as femoral artery and vein were cannulated with polyethylene tubings. The level of thyroxine (T4), 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) in plasma and biliary excretion of these compounds was estimated by specific radioimmunoassay, the samples of plasma and bile being taken up to 4 h after the administration of donor bile. In one experiment also concentration of three different diiodothyronines (e.e. 3,5-T2, 3,3'T2 and 3',5'T2) in recipient bile was measured. A dose dependent increase of T4 excretion was fond in recipient bile. In some experiments also an increase of 3,3'-T2 and 3',5'-T2 excretion was observed and a higher ratio between the excreted and administered amount of rT3 was found than that of T3. An increase of T4 level in plasma was also directly related to the administered dose of this compound, while the increase of T3 and rT3 under the conditions used was observed only in some experiments where relatively higher doses of these substances were administered. It was also observed that the absorption of T4 from the administered bile was higher in fed than in fasted animals.