Revised T3 Uptake And T4-To-T3 Conversion In Brain And Cerebellum of 10-Day-Old Rats: A Compartment Analysis

Abstract

The peripheral and cerebral metabolism of thyroid hormones in 10-day-old rats was evaluated by measuring the kinetics of thyroxine (T 4) and 3,5,3'-triiodothyronine (T 3) fluxes. Labeled iodo-compounds were measured in the plasma, cerebellum and brain (without cerebellum) for 24 hours after the intravenous injection of [125 I]T 4 plus [13 I]T 3. Data were interpreted by compartment analysis. T 4 was produced at 8.93 pmol.h –1 and T 3 at 2.26 pmol.h –1 for 22.7 g body weight. The T 4 and T 3 distribution volumes were 4.26 and 22.7 ml, whereas the extra-cellular fluid volume was 9.42 ml. T 4 was therefore considered to be mostly extra-cellular and T 3 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 T 3 and 0.048–0.136 ml plasma/g tissue for T 4. The T 3 in the brain and cerebellum was distributed in several pools. The first, representing 11% of the cerebellum and 8% of the brain (without cerebellum) T 3, was quickly exchanged with circulating T 3. The second pool, derived from the local T 4 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 T 3, and motionless. The in vivo T 4 to T 3 conversion, as a function of weight, accounted for 21% of cerebellum needs and 43% of brain needs. The rest was provided by T 3 uptake.