Triiodothyronine Action on RNA Synthesis in Rat‐Liver Mitochondria

Chronic treatment of thyroidectomized rats with physiological doses of triiodothyronine leads to a significant increase in the incorporation of [ 3 H]UTP into RNA by isolated rat liver mitochondria after the 4th day of treatment. The increased activity of mitochondrial RNA polymerase is followed by a net increase in the RNA content of organelles which can be observed after the 5th day of chronic treatment. Single injection of triiodothyronine at a dose of 250 μg/kg body wt also causes great stimulation of mitochondrial RNA synthesis which increases three‐fold after 40 h. At the same DNA/RNA ratio, RNA labelled in vitro with [ 3 H]UTP from liver mitochondria of thyroidectomized animals hybridizes with mitochondrial DNA less efficiently than the newly synthesized RNA from liver mitochondria of animals treated for 10 days with 5 μg/100 g body wt of the hormone. Hybridization‐competition experiments with unlabelled RNA extracted from mitochondria of treated and untreated animals seem to suggest that a change in the transcription of mitochondrial DNA under the influence of the hormone occurs also in vivo .