Quantitative aspects of cold-adaptation and its thyroxine model in cold- and warm-blooded animals

Abstract

1. In water frogs (Rana esculenta) and albino mice, as representatives of poikilothermic and homoiothermic animals, the metabolic effects of cold exposure are compared with those occurring after multiple injection of thyroxine. O2-uptake of whole animals and tissue respiration are measured in the temperature range of 4° (or 12.5°) to 37.5° C. The latter was also influenced by 4.6-dinitro-o-cresol (DNOC 10−5 M). The animals were exposed to cold (4° C) under constant light for 1 to 3 months. Thyroxine treatment (6.12 mg/kg) lasted 5 to 6 days; organ masses have been determined too. 2. Cold exposure exerts in winter frogs (WF's) an increase, in summer frogs (SF's) a decrease of whole body metabolism. In mice the whole body metabolism is like-wise enhanced by cold exposure, the thermoneutrality zone being displaced toward lower temperature, thus indicating a reduction in heat resistance. The increase of metabolism is to be interpreted as capacity adaptation, the decrease (as it occurs in SF's) is probably due to an overlapping of cold exposure with seasonal alterations of metabolism. The thyroxine action is in both instances similar to that of cold exposure. 3. As a further effect of cold adaptation in mice a highly significant weight increase of the (brown) interscapular fat can be recorded. 4. In WF's cold exposure causes a capacity adaptation of the respiration of brain, heart and kidneys (in vitro), whereas in SF's such an adaptation can only be found in the heart. 5. The earlier described differences in the tissue respiration of brain, liver and (partially) heart between unadapted WF's and mice, which could be demonstrated by means of the action of DNOC at different temperatures, can also be found between unadapted SF's and mice. However, cold exposure as well as thyroxine treatment brings about a complete disappearance of these differences. 6. On the basis of the present findings with whole body and tissue metabolism the chronic treatment with thyroxine can be considered as a model of cold adaptation.