1. In hypothyroid rats, we determined the effects of administration of different doses of 3,3',5-triiodo-L-thyronine (T3), 3,3'-diiodo-L-thyronine (3,3'-T2) and 3,5-diiodo-L-thyronine (3,5-T2) ("T2 isomers' refers specifically to these latter two isomers throughout this paper) on resting metabolism (RM) and on the oxidative capacity (measured as cytochrome oxidase activity) of tissues that are metabolically very active. 2. The T2 isomers induced a dose-dependent calorigenic effect when injected I.P. into hypothyroid rats. The increase in RM was already evident at a dose of 2.5 micrograms (100 g body wt)(-1), and the greatest effect was observed at the highest dose, 10 micrograms (100 g body wt)(-1), when RM reached a value not significantly different from that of the euthyroid controls (1.92 +/- 0.08 and 1.93 +/- 0.13 (1 O2) kg(-1) h(-1) for 3,5'-T2, respectively, vs. 2.1 +/- 0.12 (1 O2) kg(-1) h(-1) for euthyroid controls). T3 administration restored RM to normal euthyroid values, even at a dose of 2.5 micrograms (100 g body wt)(-1). 3. The effect of T2 isomers on RM was paralleled by an increase in the oxidative capacity of tissues that are metabolically very active (liver, skeletal muscle, brown adipose tissue (BAT) and heart). The increases were between 33% (liver + 3,3'-T2) and 63% (muscle + 3,3'-T2). By contrast, T3 induced its greatest effect on the liver, with a smaller effect on skeletal muscle, but no significant stimulation in heart and BAT, whatever the dose. 4. These results suggest that T8 isomers might be mediators of the direct thyroid hormone regulation of energy metabolism.
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