Effect of thyroid hormone on respiratory control of liver mitochondria from adult and senescent rats

Abstract

Changes in the State-3 (ADP-stimulated) and State-4 (controlled) rates of succinate oxidation in liver mitochondria induced by thyroid hormone were examined in adult (12–13 months old) and senescent (27–29 months old) rats. Adult animals, made hypothyroid by feeding thiouracil, had significantly decreased State-3 rates of respiration and respiratory control ratios. The State-4 rate of oxidation was not lowered significantly. A single injection of triiodo- l-thyronine (T 3) to these hypothyroid rats resulted in increases in States-3 and -4 rates of respiration as well as in the respiratory control ratios. The enhancement of the State-3 rate was evident at 6 hr after hormonal administration, reaching a maximum (2.3-fold the hypothyroid value) in about 2 days, with this high level being maintained for at least an additional 3 days. The increase in State-4 rate of succinate oxidation was 1.6-fold; this was attained in 1 day after injection of T 3 and was maintained for at least an additional 4 days. The rates of induction by T 3 of the increases in States-3 and -4 rates of succinate oxidation and respiratory control ratio as well as the maximal levels achieved with hormonal administration were the same in adult and senescent rats. These findings suggest that the biochemical machinery necessary for the stimulation in respiratory rates and respiratory control ratio in the livers of aged animals is in no way impaired and that senescent rats are fully capable of responding, at least in this system, to the direct adaptive stress of thyroid hormone.