Nocturnal increases in the triiodothyronine/thyroxine ratio in the rat thymus and pineal gland follow increases of type II 5′-deiodinase activity

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• 作者：Soutto ; Mohammed ; Guerrero ; Juan M. ; Osuna ; Carmen ; Molinero ; Patrocinio
• 关键词：Thymus ; Pineal gland ; 5&prime ; Deiodinase ; Thyroid hormones ; Circadian rhythms
• 刊名：The International Journal of Biochemistry and Cell Biology
• 出版年：1998
• 出版时间：March 27, 1998
• 年：1998
• 卷：30
• 期：2
• 页码：235-241
• 全文大小：456 K

文摘

The type II 5′-deiodinase (5′D-II) is regulated by the light–dark cycle in some tissues in which the enzyme is present. This prompted us to investigate putative influences of light–dark cycle on thyroid hormone concentrations in these tissues. The results revealed the following facts: (a) Deiodinase activity in the rat thymus exhibits a nyctohemeral profile with peak values late at night and basal values during the day. The thyroid hormone concentrations in the thymus also show a 24 h rhythm with an increase in the triiodothyronine/thyroxine (TT₃/TT₄) ratio at night. (b) The content of thyroid hormones in the pineal gland exhibits, like in the thymus, nyctohemeral variations with increase values in the TT₃/TT₄ ratio during the dark period coinciding with the maximal enzyme activity. (c) Other tissue, like the anterior pituitary, in which 5′D-II activity does not exhibit a diurnal variation, the concentration of thyroid hormones does not show modifications. In conclusion, the nocturnal increase of 5′D-II activity produces an increase of T₃ concentration and a decrease of T₄ concentration in both thymus and pineal gland. Therefore, these diurnal changes in 5′D-II activity is a mean by which the cell can regulate the intracellular availability of the most active thyroid hormone T₃.