title: Are the effects of T3 on resting metabolic rate in euthyroid rats entirely caused by T3 itself?
abstract: Because we previously reported that T3 and 3,5-diiodo-L-thyronine (3,5-T2) both increase resting metabolic rate (RMR), 3,5-T2 could be another thyroidal regulator of energy metabolism. This effect of 3,5-T2 is evident in rats made hypothyroid by propylthiouracil and iopanoic acid, not in normal euthyroid (N) rats. Possibly, under euthyroid conditions, active 3,5-T2 may need to be formed intracellularly from a precursor such as T3. We tested this hypothesis by giving a single injection of T3 to N rats and comparing the time course of the variations in RMR with those of the changes in the serum and hepatic levels of 3,5-T2. Acute injection had an evident effect on RMR, 25 h earlier, in N rats than in rats made hypothyroid by propylthiouracil and iopanoic acid, maximal values (+40%) being reached in the former at 24-26 h. In N rats, the simultaneous injection of actinomycin D with the T3 inhibited the late part of the effect (after 24 h) more strongly than the early part (14-24 h). In serum and liver, 3,5-T2 levels were increased significantly at 12-24 h after T3 injection into N rats, a time at which RMR was rising rapidly to peak. These results seem to indicate that when T3 is injected into N animals, not all the effects on RMR are attributable to T3 itself, the early effect presumably being largely because of its in vivo deiodination to 3,5-T2. Because the effects of T3 and 3,5-T2 are additive, in N rats, the two iodothyronines probably cooperate in vivo to determine the total metabolic rate.
authors.affiliation: Dipartimento di Scienze Biologiche ed Ambientali, Università degli Studi del Sannio, 82100 Benevento, Italy.
authors.forename: Maria
authors.initials: M
authors.lastname: Moreno
authors.forename: Assunta
authors.initials: A
authors.lastname: Lombardi
authors.forename: Luca
authors.initials: L
authors.lastname: Beneduce
authors.forename: Elena
authors.initials: E
authors.lastname: Silvestri
authors.forename: Graziano
authors.initials: G
authors.lastname: Pinna
authors.forename: Fernando
authors.initials: F
authors.lastname: Goglia
authors.forename: Antonia
authors.initials: A
authors.lastname: Lanni
mesh_terms: D000818:Animals; D013956:Antithyroid Agents; D001481:Basal Metabolism / Q000187:drug effects*; D003609:Dactinomycin / Q000494:pharmacology; D004104:Diiodothyronines / Q000097:blood / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007037:Hypothyroidism / Q000139:chemically induced / Q000378:metabolism; D007453:Iodide Peroxidase / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000378:metabolism; D007480:Iopanoic Acid; D008099:Liver / Q000378:metabolism; D008297:Male; D011441:Propylthiouracil; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D011863:Radioimmunoassay; D051381:Rats; D017208:Rats, Wistar; D013961:Thyroid Gland / Q000502:physiology*; D014284:Triiodothyronine / Q000097:blood / Q000494:pharmacology*
chemical_list: D013956:Antithyroid Agents; D004104:Diiodothyronines; D004791:Enzyme Inhibitors; D011500:Protein Synthesis Inhibitors; D014284:Triiodothyronine; D003609:Dactinomycin; C030103:3,5-diiodothyronine; D011441:Propylthiouracil; D007453:Iodide Peroxidase; D007480:Iopanoic Acid