EFFECTS OF THYROID STATUS ON COLD-ADAPTIVE THERMOGENESIS IN BRANDT VOLE, MICROTUS-BRANDTI

Hyper- and hypothyroidism were induced by subcutaneous injection of th yroxine and by oral administration of methimazol in Brandt's voles. Th e effects of the two treatments on metabolic thermogenesis at 25 degre es C and 4 degrees C were investigated. The level of resting metabolic rate was closely related to thyroid status: high in the hyperthyroid case and low in the hypothyroid case. However, no increase in resting metabolic rate occurred in either case during further cold acclimation . Hyperthyroidism resulted in an increased nonshivering thermogenesis, which was much enhanced by lower temperature, but hypothyroidism led to a suppressed nonshivering thermogenesis in the cold. The state-4 an d state-3 respirations and the activities of cytochrome-c oxidase of l iver mitochondria were elevated in hyperthyroid animals but attenuated in hypothyroid ones. However, these levels were scarcely changed afte r further cold acclimation. Both hyperthyroidism and cold acclimation induced the recruitment of brown adipose tissue, but brown adipose tis sue was different biochemically in the two cases: in hyperthyroidism, the total protein was reduced, while fat content increased; in cold ac climation, the total and mitochondrial proteins were increased. Howeve r, in hypothyroid voles, the normal adaptive changes in brown adipose tissue were impaired in further told acclimation. The activity of cyto chrome-c oxidase in brown adipose tissue was increased by hyperthyroid ism and enhanced in further cold. In contrast, its activity was inhibi ted in hypothyroid animals, though activated to some extent in cold. T hese results demonstrate that normal thyroid function is essential for the cold-induced increase of resting metabolic rate and nonshivering thermogenesis and that there is a synergism between thyroid hormone an d cold acclimation in the regulation of nonshivering thermogenesis in Brandt's vole. In addition, the blunted response of brown adipocytes t o the cold may be the cytological mechanism for the suppressed nonshiv ering thermogenesis found with hypothyroidism.