Nocturnal increases in the triiodothyronine/thyroxine ratio in the rat thymus and pineal gland follow increases of type II 5'-deiodinase activity.
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.
Soutto
M
M
Department of Medical Biochemistry and Molecular Biology, School of Medicine, University of Sevilla, Spain.
Guerrero
J M
JM
Osuna
C
C
Molinero
P
P
D000818:Animals; D002940:Circadian Rhythm; D003624:Darkness; D005260:Female; D007453:Iodide Peroxidase / Q000378:metabolism*; D007457:Iodine Radioisotopes; D008297:Male; D010870:Pineal Gland / Q000201:enzymology / Q000378:metabolism*; D010903:Pituitary Gland, Anterior / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013950:Thymus Gland / Q000201:enzymology / Q000378:metabolism*; D013974:Thyroxine / Q000378:metabolism*; D014284:Triiodothyronine / Q000378:metabolism*; D000097564:Iodothyronine Deiodinase Type II
D007453:Iodide Peroxidase; D007457:Iodine Radioisotopes; D013974:Thyroxine; D014284:Triiodothyronine; D000097564:Iodothyronine Deiodinase Type II