Regulation of Iodothyronine Deiodinase Activity as Studied in Thyroidectomized Rats Infused with Thyroxine or Triiodothyronine

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Abstract
10 pages, 7 figures, 2 tables.-- Presented in part at the 76th Annual Meeting of The Endocrine\ud Society, Anaheim, CA, June 1994, and the 22nd Annual Meeting of the\ud European Thyroid Association, Vienna, Austria, September 1994.To provide new insights into the in vivo regulation of iodothyronine deiodinases in the different tissues of the rat, we have evaluated the effects on these enzymatic activities of T4 or T3 infusions into thyroidectomized rats. Thyroidectomized rats were infused with placebo, T4, or T3. Placebo-infused intact rats served as euthyroid controls. Plasma and samples of cerebral cortex, brown adipose tissue, pituitary, liver, and lung were obtained after 12-13 days of infusion. Plasma TSH, plasma and tissue T4 and T3, and iodothyronine deiodinase activities were determined. Type II 5'-deiodinase (DII) was increased in cortex, brown adipose tissue, and pituitary from animals infused with placebo. DII activity returned to normal only with T4 infusion, remaining elevated in the animals infused with T3 alone despite normal tissue T3 concentrations. Cortex type III 5-deiodinase was only increased when hyperthyroidism was induced by infusion of T3. Liver type I 5'-deiodinase (DI) paralleled the changes in plasma and tissue T3 regardless of whether T4 or T3 was infused. On the contrary, the increase in lung DI, proportional to the increases in plasma and tissue T3, was higher when T4 was infused. As a rule, the tissues with DII presented a tighter homeostasis in their T3 concentrations than the tissues with DI. In conclusion, the regulation of iodothyronine deiodinases depends on the hormone infused into the thyroidectomized animals and on the tissue in which the deiodinase is studied, demonstrating the existence of tissue-specific regulation of its thyroid hormone concentrations.This\ud work was supported by the Fondo de Investigaciones de la Seguridad\ud Social (Proyecto 92/0888, BAE 93/5168 and 94/5082) and Henning\ud Berlin (Berlin, Germany).Peer reviewe

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