Is respiratory activity in the brain mitochondria responsive to thyroid hormone action?: a critical re-evaluation

The effects of in vivo treatment with graded doses (0.5-1.5 micrograms/g body weight) of thyroid hormones, tri-iodothyronine (T3) and thyroxine (T4), for 4 consecutive days to euthyroid rats on the respiratory activity of isolated brain mitochondria were examined. T4 stimulated coupled State-3 respiration with glutamate, pyruvate + malate, ascorbate + tetramethyl-p-phenylenediamine and succinate, in a dose-dependent manner; T3 was effective only at the highest (1.5 micrograms) dose employed. T4 was more effective than T3 in stimulating respiratory activity. State-4 respiratory rates were in general not influenced except in the case of the ascorbate + tetramethyl-p-phenylenediamine system. Primary dehydrogenase activities, i.e. glutamate dehydrogenase, malate dehydrogenase and succinate dehydrogenase, were stimulated about 2-fold; interestingly mitochondrial but not cytosolic malate dehydrogenase activity was influenced under these conditions. The hormone treatments did not greatly influence the mitochondrial cytochrome content. The results therefore suggest that thyroid hormone treatment not only stimulates primary dehydrogenase activities but may also directly influence the process of mitochondrial electron transfer.