Tri-iodothyronine enhances the formation of light mitochondria during cold exposure

The effect of cold exposure and of PTU and PTU + T3 administration on the protein content and succinic dehydrogenase activity of three mitochondrial populations obtained from rat liver was examined. Our results indicated the following: Succinic dehydrogenase activity increases mainly in the light mitochondrial fraction of cold-exposed rats. PTU administration of cold-exposed animals does not affect the increment in enzyme activity of the heavy fraction but blocks the increment of the light fraction. PTU + T3 administration restores succinic dehydrogenase activity to the values prevalent in normal cold-exposed rats. These findings suggest that thyroid hormone may stimulate the formation of light mitochondria during cold exposure.