Thyroid hormone and the mitochondrial population in the rat heart.

The effects of the thyroid hormone on the number and protein content of mitochondria were investigated in rat heart. The specific mitochondrial population, determined by direct counting, was estimated to be about 3.7 X 10(11) mitochondria per g wet weight in young hypothyroid male rats (T) and about 2.4 X 10(11) (65%) in euthyroid animals, sex- and age-matched. Triiodothyronine (T3) treatment of T animals restored the levels to normal values. The protein content per mitochondrion, on the contrary, was higher in euthyroid animals or hypothyroid animals, following T3 treatment, compared to T. Finally, thyroid hormone enhanced the heart mass and, therefore, the cardiocytal and mitochondrial populations of the whole organ. These results differ from previous data from our laboratory indicating that liver weight is increased by thyroid treatment, albeit at a slower rate, while the number of mitochondria per g tissue increases and the protein content per mitochondrion decreases. In conclusion, the effects of the thyroid hormone on mitochondria are different in the hepatocyte and in the cardiocyte. It appears that the hormone differently modulates the machinery of mitochondrial protein synthesis in the two target cells according to the physiological role of mitochondria in liver and heart cells, i.e. heat production or mechanical energy output, respectively.