Existence of Extrathyroidal Conversion Inhibitor (IEC) in Starved Animals and its Influence on Thyroid Hormones Deiodination in Liver and Kidney in Vitro

Summary

To find out whether an inhibitor of extrathyroidal conversion of iodothyronines is present in sera of starved animals, pig liver and kidney homogenates were incubated with T₄, T₃ or rT₃ and dithiotreitol in the presence of evaporated diethyl ether extracts of sera obtained from fed and starved (1-12 days) rabbits. Sera extracts of short-term (1-4 days) starved rabbits caused a significant inhibition of T₄ to T₃ conversion (54% on day 3) and T₄ to rT₃ deiodination (52% on day 2) in liver homogenates. Extracts of sera from long-term (8 and 12 days) starved animals diminished only liver T₄ to T₃ conversion on day 8 and had no influence on liver T₄ to rT₃ conversion. 5′-deiodination of rT₃ (to 3,3′-T₂) in liver was gradually decreased by extracts of sera from animals starved during 2-12 days. Liver rT₃-5-deiodination (to 3′,5′-T₂) was significantly impaired on day 4 and totally depressed by long-term starvation. In vitro T₃ to 3,3′-T₂ conversion in liver was markedly (59-103%) increased by ether extracts of sera from short-term fasted rabbits and considerably inhibited (62-72%) by long-term fasting. T₄ to T₃ conversion in kidney was significantly influenced by sera extracts obtained neither from short-term fasted rabbits and considerably inhibited (62-72%) by long-term fasting. T₄ to T₃ conversion in kidney was significantly influenced by sera extracts obtained neither from short-term nor from long-term fasted rabbits but T₄-5-deiodination (to rT₃) was reduced by sera extracts of short-term fasted animals. Further 5-deiodination of rT₃ was significantly changed only by the extracts obtained on the 3rd day of the experiment and rT₃-5′-deiodination was inhibited by extracts collected on days 3 and 4. T₃-5-monodeiodination (to 3,3′-T₂) in kidney homogenates was reduced (42-62%) by long-term starvation. Extracts of sera obtained from starved animals after intravenous glucose administration increased liver conversion of T₄ to T₃ and both deiodinations of rT₃ restoring them to the control level. These extracts augmented also kidney conversion of T₄ to T₃ and that of rT₃ to 3,3′-T₂, impaired kidney T₄ to rT₃ deiodination and had no significant influence on 5-monodeiodination of rT₃ and T₃.

Key-Words

Iodothyronines - Starvation - 5′- and 5-Monodeiodinations - Diethyl Ether Sera Extracts - Inhibitor of Extrathyroidal Conversion