In this study, we investigated the sex hormone regulation of 5’-iodothyronine deiodinase
activity, which is responsible for enzymatic conversion of thyroxine into the bioactive
form, triiodothyronine. Pituitary homogenates and liver microsomes from: 1) ovariectomized
rats injected with 17-β-estradiol benzoate and/or progesterone (0.7 and 250 µg/100
g body weight, respectively, subcutaneously, over 10 days); 2) male castrated rats
treated or not with 0.4 mg/100 g body weight testosterone propionate, intramuscular,
over 7 days, were assayed for type 1 and type 2 deiodinase activity in the pituitary.
Enzyme activities were measured by release of 125I from deiodination of 125I reverse triiodothyronine under varying assay conditions. Estrogen stimulated anterior
pituitary and liver type 1 deiodinase activity in ovariectomized rats (45 and 30 %,
p < 0.05). Progesterone inhibited the liver enzyme (40 %, p < 0.05), and had no effect
on the pituitary, but in both tissues, blocked estrogen stimulatory effect on type
1 deiodinase. In males, testosterone normalized the reduced liver type 1 deiodinase
of castrated rats. However, in the pituitary, castration increased (50 %) type 1 deiodinase
independent of testosterone treatment, suggesting the existence of a inhibitory testicular
regulator of pituitary type 1 enzyme. Treatments did not alter pituitary type 2 deiodinase
activity. In conclusion, gonads and sex steroids differentially modulate type 1 deiodinase
activity in rat pituitary and liver.
Key words:
5’-Deiodinase - Castration - Estradiol - Progesterone - Testosterone
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C. C. Pazos-Moura
Laboratório de Endocrinologia Molecular
Instituto de Biofisica Carlos Chagas Filho
Universidade Federal do Rio de Janeiro
CCS, Bloco G, Ilha do Fundão
Rio de Janeiro
Brazil
Fax: + 55 (21) 280-8193
Email: cpazosm@ibccf.biof.ufrj.br