[Action of thyroxine on the cytochrome content in the brain and liver mitochondria during the postnatal development of rats].

The content of cytochromes c + c1, b and a in brain and liver mitochondria in 7-day-old rats reaches the level seen in adult animals. Administration of L-T4 in a dose of 0.7 micrograms/g rat bw for 4 days before sacrifice results in activation of cytochrome synthesis in both test organs within the first week of the suckling rats' life. On the 20th day of the postnatal period the effect of T4 is seen only in the liver while the brain tissue turns out indifferent to the thyroid hormone. Thus, T4 activates cytochrome biosynthesis in brain mitochondria during the first week of the rats' life, that leads to the acceleration of the functional activity and higher differentiation of the developing brain mitochondria.