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Concentrations of Seven Iodothyronine Metabolites in Brain Regions and the Liver of the Adult Rat

Departments of Radiology and Nuclear Medicine (Radiochemistry) (G.P., O.B., A.J., H.G., M.E., H.M., A.B.), Universitätsklinikum Benjamin-Franklin, Free University of Berlin, Berlin 12200 Germany; and Department of Internal Medicine III, Erasmus University Medical School (T.V.), 3062 PA Rotterdam, The Netherlands

Address all correspondence and requests for reprints to: Andreas Baumgartner, M.D., Department of Radiology and Nuclear Medicine (Radiochemistry), Hindenburgdamm 30, 12200 Berlin, Germany. E-mail: . Abaum{at}cipmail.ukbf.fu-berlin.de

The concentrations of the iodothyronine metabolites T₄, T₃, 3,5-diiodothyronine (3,5-T₂), 3,3'-diiodothyronine (3,3'-T₂), reverse T₃ (rT₃), 3,3'-T₂ sulfate (3,3'T₂S), and T₃ sulfate (T₃S) were measured in 12 regions of the brain, the pituitary gland, and liver in adult male rats. Quantification of iodothyronine was performed by RIA following a newly developed method of purification and separation by HPLC. 3,5-T₂, 3,3'-T₂, rT₃ and T₂S were detectable in the low femtomolar range (20–200 fmol/g) in most areas of the rat brain. T₃S was detectable only in the hypothalamus. The concentrations of T₃ and T₄ were approximately 20- to 60-fold higher, ranging between 1 and 6 pmol/g. There was a significant negative correlation between the activities of inner-ring deiodinase and T₃ concentrations across brain areas. In the liver, 3,5-T₂, rT₃, and T₃S were measurable in the low femtomolar range, whereas 3,3'-T₂ and 3,3'T₂S were not detectable. 3,5-T₂ and 3,3'-T₂ were not detectable in mitochondrial fractions of the brain regions. Tissue concentrations of 3,5-T₂ exhibited a circadian variation closely parallel to those of T₃ in the brain regions and liver. T₃ was not a substrate for outer-ring deiodination under different experimental conditions; thus, it remains unclear which substrate(s) and enzyme(s) are involved in the production of 3,5-T₂. These results indicate that five iodothyronine metabolites other than T₃ and T₄ are detectable in the low femtomolar range in the rat brain and/or liver. The physiological implications of this finding are discussed.

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