The influence of thyroid hormones on normal rat-liver mitochondria II. Thyroxine effects on pretreated mitochondria

Abstract

1. 1. Dilution of a sucrose suspension of mitochondria with an equal volume of deionized H 2O lowered the respiratory control and permitted a number of thyroxine- induced effects to be observed. 2. 2. The most immediate effect of thyroxine was an increase in the rate of succinate oxidation. This was followed at about 1 min by a further drop in respiratory control and at 2 min by an increase in adenosine-5′-triphosphatase activity. These effects all contributed to a reduction in phosphorylation efficiency in the presence of thyroxine. 3. 3. Although thyroxine increased the rate of mitochondrial swelling, beginning at approx. 2 min after the start of the incubation, this effect was clearly not associated with any of the other thyroxine effects except possibly the stimulation of adenosine-5′-triphosphatase activity. 4. 4. The results support other work in which thyroxine stimulates succinate oxidation under conditions in which it is not limited by the phosphorylative capacity of the mitochondria.