Is increased metabolism in rats in the cold mediated by the thyroid?

Abstract

1. In the rat variation of metabolic heat production is the principal effector of thermoregulation. There is a continuous relationship between ambient temperature and metabolic rat over the whole range of tolerable environmental temperature. The mechanism that controls metabolic rate is unknown; this paper reports an attempt to test whether thyroid hormones provide the controlling pathway. 2. First, the changes in metabolic rate and in the plasma concentrations of thyroid stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4) were measured in rats living in a controlled environment, first at 23 degrees C and then at 6 degrees C. Metabolic rate increased from approximately 290 to 470 kJ day-1 when the temperature was lowered, a factor of ca 1.6, and the diurnal rhythm disappeared. The concentration of TSH increased from approximately 320 to 450 ng ml-1 (with loss of diurnal rhythm) and of T3 from ca 0.7 to 1.0 nmol l-1, a factor of ca 1.4 in each case. T4 concentration did not change. 3. Next, a dose schedule of T3 was found that, when injected I.V. via indwelling jugular cannulae in the same rats in an environment at 23 degrees C, maintained an increase in T3 concentration rather greater than had been found at 6 degrees C. 4. This dose of T3, given to the same rats at 23 degrees C, did not affect metabolic rate (or its diurnal pattern). 5. It is therefore unlikely that the increase in T3 concentration evoked the increase in metabolic rate when ambient temperature was changed from 23 to 6 degrees C; and therefore that the thyroid controls variation of metabolic rate in 'everyday' thermoregulation in the rat.

Calorigenic effect of diiodothyronines in the rat.

“…In the literature, reported values for the resting metabolism of euthyroid rats vary over a considerable range, namely between 1 2 and 2 1 (1 02) kg-' h-' (Nagasaka, Hirata, Sugano & Shibata, 1979;Liverini, Iossa & Barletta, 1994). The values obtained in our experiments are the same as the means reported by the latter group and close to those in other reports (Buser, Kopp, Gehr, Weibel & Hoppeler, 1982;Whitaker, Hussain, Hervey, Tobin & Rayfield, 1990;Himms-Hagen et al 1994).…”

“…The values obtained in our experiments are the same as the means reported by the latter group and close to those in other reports (Buser, Kopp, Gehr, Weibel & Hoppeler, 1982; Whitaker, Hussain, Hervey, Tobin & Rayfield, 1990;Himms-Hagen et al 1994). …”

Seasonal Changes in Serum Total Thyroxine, Free Thyroxine, and Canine Thyroid-Stimulating Hormone in Clinically Healthy Beagles in Hokkaido.

“…Many studies have reported that thyroid function in humans adapts to environmental conditions such as ambient temperature, photoperiod, and season, but the results have not been consistent because more than one parameter may mediate thyroid function [2][3][4]10]. Although similar findings were reported in rat [11,13], sheep [15], and dog [12], the only hormones measured were serum total thyroxine (tT 4 ), triiodothyronine (tT 3 ), and TSH. There has been only one report about seasonal changes in thyroid function in the dog [12], but the details of such changes have remained unclear because only tT 4 and tT 3 levels were measured.…”

“…tT 4 post TSH level in November is also significantly higher than in Dec. 17.0 17.1 Exposure to cold in January might create a condition like Cushing's syndrome because of stress, although the cortisol level was not measured in the present study. Many reports about the influence of environmental temperature--specifically, exposure to cold--on the thyroid hormones of the human being [2,7] and rat [11,13] suggest that cold exposure activates pituitary and thyroid functions and increases the basal metabolic rate. In our present study, the response to the TSH-stimulating test was increased in August and November.…”

Cold-induced changes in thyroid function in a poikilothermic mammal, the naked mole-rat

“…Most small mammals respond to cold exposure by augmenting thyroid-mediated processes (9,17). These increase basal metabolism and heat production (45,48). Poikilotherms on the other hand, generally show a decrease in both thyroid activity and metabolism in the cold (5,12).…”