[Regulation by thyroxine of the functional activity of liver and brain mitochondria in developing rats and participation of cytoplasmic protein factors in this process].

The physiological (10(-8)M) concentration of free thyroxine (T4) and specific thyroxine-binding protein (a thyroxine action modulator-MDT4) was studied for its effect on protein synthesis, respiration and oxidative phosphorylation of mitochondria in the liver and brain of developing rats. Free T4 was demonstrated to affect the studied parameters in the mitochondria of the liver and brain of the developing rats very rarely, if at all. In the liver MDT4 stimulated mitochondrial functional activity during all the studied developmental periods whereas in the brain mitochondria the modulator stimulated the synthesis of proteins and phosphorylating respiration only in early postnatal life. In the following periods of development there was a modulator-induced decrease in protein incorporation of labelled leucine, respiration, and conjugation strength of brain mitochondria. The findings suggest that thyroidal hormones ambiguously regulate the functional status of mitochondria in the liver and brain of the developing rats and the specific hormone-binding protein MDT4 takes an active part in this regulation.