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.

Abstract

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 micrograms 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 (alpha 1-2-antagonist) and that after 2400 ng/kg/min adrenaline also by 2.5 mg/kg prazosin (alpha 1-antagonist) injected at the beginning of the infusion, but not by 6 mg/kg yohimbine (alpha 2-antagonist) injected every 60 min during 4-h infusion. In addition, increased rT3 excretion was found during the infusion of alpha 1-agonist methoxamine (1.5 mg/kg/4 h), while no such effect of the infusion of alpha 2-agonist azepexol (10 mg/kg/4 h) was observed. It may be suggested that the effect of adrenaline was mediated predominantly by alpha 1-adrenoceptors and that the observed changes in biliary excretion of T3 and rT3 were related to the inhibition of 5'-monodeiodination in the liver.