Abstract

• It has been well documented that animals exposed to cold show increased activity of thyroid gland. The calorigenic action of thyroid hormone has been demonstrated by a variety of in vivo and in vitro studies. According to Edelman et al, the thyroid thermogenesis is due to activation of energy consuming processes, especially the active sodium transport by the hormone in target tissues. If SO, the increase in thyroid activity during cold exposure should induce increased capacity of sodium transport in target tissue and the change in tissue metabolism should be precisely correlated with the change in Na+-K+-ATPase activity of the tissue. This possibility was tested in the present study: in one series, changes in oxygen consumption and Na+- K+-ATPase activity of liver preparations were measured in rats as a function of thyroid status, in order to establish the effect of thyroid hormone on the tissue respiration and enzyme system; in another series, the effect of cold stimulus on the serum thyroid hormone level, hepatic tissue oxygen consumption and Na+-K+-ATPase activity in rats. The results obtained are as follows:1. The Na+-dependent oxygen consumption of liver slices, the oxygen consumption of liver mitochondria and the Na+-K+-ATPase activity of liver preparations were significantly inhibited in hypothyroidism and activated in hyperthyroidisrn. Kinetic analysis indicated that the Vmax. of Na+-K+-ATPase was decreased in hypothyroidism and increased in hyperthyroidisrn. 2. In cold exposed rats, the serum triiodothyronine (T3) level increased rapidly during the initial one day of cold exposure, then declined slowly to the control level after two weeks. The serum thyroxine (T4) level decreased gradually throughout the cold exposure. Accordin'gly the T3/T4 ratio increased. The mitochondrial oxygen consumption and the Na+-dependent oxygen consumption of liver slices increased during the first two days and then remained unchanged thereafter. The activity of the Na+-K+-ATPase in liver preparations increased during cold exposure with a time course similar to that of oxygen consumption. Kinetic analysis indicated that the Vmax. of Na+-K+-ATPase increased. 3. Once the animal was adapted to cold, induction of hypothyroidism did not significantly alter the hepatic oxygen consumption and Na+-K+-ATPase activity. These results indicate that: 1) thyroid hormone increases capacities of mitochondrial respiration and active sodium transport in target tissues such as liver; 2) the increased T3 level during the initial period of cold exposure facilitates biosynthesis of Na+-K+-ATPase and mitochondrial enzymes for oxidative phosphorylation, leading to enhanced production and utilization of ATP, hence heat production.

Figure & Data

REFERENCES

Citations

Citations to this article as recorded by