Effect of adrenaline on biliary excretion of triiodothyronines in rats mediated by alpha 1 -adrenoceptors and related to the inhibition of 5’-monodeiodination in liver

Biliary excretion of thyroxine (T4), 3,5,3’-triiodothyronine (T3), 3,3,5’-triiodothyronine (rT3) and diiodothyronines (3, 3’-T2, 3, 5-T2 and 3’, 5’-T2) was estimated with the aid of radioimmunoassay in 3–4 subsequent 2-h samples of bile obtained from pentobarbital anesthetized rats through the tubing inserted in bile duct. The excretion of T3 was significantly decreased during 4-h infusion of 2400 ng/kg/min adrenaline in normal rats or during 6-h infusion of the latter dose in the animals preinjected with 2 µg T4. Moreover, the excretion of rT3 was significantly increased after the infusion of 1200 and 2400 ng/kg/min adrenaline. Such increase after 1200 and 2400 ng/kg/min adrenaline was prevented by a single dose of 10 mg/kg phentolamine (alpha1–2-antagonist) and that after 2400 ng/kg/min adrenaline also by 2.5 mg/kg prazosin (alpha1-antagonist) injected at the beginning of the infusion, but not by 6 mg/kg yohimbine (alpha2-antagonist) injected every 60 min during 4-h infusion. In addition, increased rT3 excretion was found during the infusion of alpha1-agonist methoxamine (1.5 mg/kg/4 h), while no such effect of the infusion of alpha2-agonist azepexol (10 mg/kg/4 h) was observed. It may be suggested that the effect of adrenaline was mediated predominantly by alpha1-adrenoceptors and that the observed changes in biliary excretion of T3 and rT3 were related to the inhibition of 5’-monodeiodination in the liver.