Effect of experimentally induced thyrotoxicosis on oxidative energy metabolism in rat heart mitochondria.

ABSTRACT

Treatment of rats with T3 resulted in a significant decrease in body weight, while the heart weight increased. T4 treatment had less marked effect on body weights but resulted in decreased heart weights. Serum T4 levels decreased significantly with simultaneous increase of T3 level following T3 treatment, whereas with T4 treatment, levels of both T4 and T3 increased in the serum. Low doses of T3 (0·5 μg) caused decrease in mitochondrial protein content while high dose of T4 (1 μg), caused significant increase in mitochondrial mass. The state 3 respiration rates were significantly depressed following T3 and T4 treatments, in a substrate specific manner with the effects being more pronounced with T3; these responses with T4 were dose-dependent for succinate and ascorbate + N,N,N',N'-tetramethyl-p-phenylenedίamme. State 4 respiration rates also exhibited similar corresponding changes. ADP/O ratios were not changed but ADP-phosphorylation rates were decreased significantly particularly so with the T3-treated animals. Treatment with T3 also resulted in lowering of intramitochondrial cytochrome contents. Similar effects were seen also with higher doses of T4. The results thus indicate that T3- and T4- thyrotoxicosis results in impaired energy metabolism in heart mitochondria.