Revised T3 uptake and T4-to-T3 conversion in brain and cerebellum of 10-day-old rats: a compartment analysis.
The peripheral and cerebral metabolism of thyroid hormones in 10-day-old rats was evaluated by measuring the kinetics of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) fluxes. Labeled iodo-compounds were measured in the plasma, cerebellum and brain (without cerebellum) for 24 hours after the intravenous injection of [125I]T4 plus [131I]T3. Data were interpreted by compartment analysis. T4 was produced at 8.93 pmol x h(-1) and T3 at 2.26 pmol x h(-1) for 22.7 g body weight. The T4 and T3 distribution volumes were 4.26 and 22.7 ml, whereas the extra-cellular fluid volume was 9.42 ml. T4 was therefore considered to be mostly extra-cellular and T3 mostly intracellular. This was confirmed in the brain and cerebellum, where the extra-cellular fluid (ECF) fraction was 0.021 ml/g organ and the tissue-to-plasma ratio of labeled and endogenous hormones was 0.54-6.54 ml plasma/g tissue for T3 and 0.048-0.136 ml plasma/g tissue for T4. The T3 in the brain and cerebellum was distributed in several pools. The first, representing 11% of the cerebellum and 8% of the brain (without cerebellum) T3, was quickly exchanged with circulating T3. The second pool, derived from the local T4 5'-deiodination, represented 48% of the cerebellum and 94% of the brain (without cerebellum) hormone; a possible third pool (41% of the hormonal content) in the cerebellum appeared to be unlabeled by radioactive T3, and motionless. The in vivo T4 to T3 conversion, as a function of weight, accounted for 21% of cerebellum needs and 43% of brain needs. The rest was provided by T3 uptake.
Valentin
S
S
Laboratoire de Physiologie Comparée II, Université de Montpellier II, France.
Cheynel
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I
Vigouroux
E
E
D000818:Animals; D000833:Animals, Suckling; D001711:Biotransformation; D001825:Body Fluid Compartments; D002531:Cerebellum / Q000254:growth & development / Q000378:metabolism*; D002540:Cerebral Cortex / Q000254:growth & development / Q000378:metabolism*; D007275:Injections, Intravenous; D007453:Iodide Peroxidase / Q000378:metabolism; D007457:Iodine Radioisotopes / Q000032:analysis; D008954:Models, Biological; D009419:Nerve Tissue Proteins / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013974:Thyroxine / Q000008:administration & dosage / Q000493:pharmacokinetics*; D014284:Triiodothyronine / Q000008:administration & dosage / Q000493:pharmacokinetics*
D007457:Iodine Radioisotopes; D009419:Nerve Tissue Proteins; D014284:Triiodothyronine; D007453:Iodide Peroxidase; D013974:Thyroxine