The respiratory capacities of hepatocytes, derived from hypothyroid, euthyroid and
hyperthyroid rats, have been compared by measuring rates of
oxygen uptake and by titrating components of the respiratory chain with specific inhibitors.
Thyroid hormone increased the maximal rate of substrate-stimulated respiration and also increased the degree of
ionophore-stimulated
oxygen uptake. In titration experiments, similar concentrations of
oligomycin or
antimycin were required for maximal inhibition of respiration regardless of thyroid state, suggesting that the changes in respiratory capacity were not the result of variation in the amounts of
ATP synthase or
cytochrome b. However, less
rotenone was required for maximal inhibition of respiration in the hypothyroid state than in cells from euthyroid or
hyperthyroid rats, implying that hepatocytes from hypothyroid animals contain less
NADH dehydrogenase. The concentration of
carboxyatractyloside necessary for maximal inhibition of respiration was 100 microM in hepatocytes from hypothyroid rats, but 200 microM and 300 microM in hepatocytes from euthyroid and
hyperthyroid rats, respectively, indicating a possible correlation between levels of
thyroid hormone and the amount or activity of
adenine nucleotide translocase. The increased capacity for coupled respiration in response to
thyroid hormone is not associated with an increase in the components of the electron transport chain or
ATP synthase, but correlates with an increased activity of
adenine nucleotide translocase.