ORGAN THERMOGENESIS 



their greater abilities to run at high and especially at low tempera- 

 tures. 



Recently ithas been shown (Hart, 1962, in press) that warm- and 

 cold- acclimated rats behave differently with respect to substitution 

 of exercise for cold thermogenesis. Warm- acclimated rats, having 

 shivering thermogenesis only, substituted heat production from shi- 

 vering by heat production from exercise. Oxygen consumption during 

 work did not change with decreasing temperature and was identical 

 to the maximal oxygen consumption in rest at the lowest tempera- 

 tures. This is apparently due to the fact that exercise in cold may 

 reduce or eliminate shivering. This was surmised long ago and re- 

 cently demonstrated in pigeons durirjg flight (Hart, 1960). On the 

 other hand, in cold- acclimated animals, which can produce heat with- 

 out shivering (Sellers et al., 1954; Heroux et al., 1956; Cottle and 

 Carlson, 1956), the addition of exercise heat production to cold ther- 

 mogenesis is made possible (Figiore 4). The result is that working 

 oxygen consumption increase with decreasing temperature parallel 

 to resting values and the maximum heat production is greatly in- 

 creased. 



However, at temperatures approaching the peak metabolic rate 

 for cold- acclimated rats, heat production during work did not in- 

 crease with loweringoftemperature. At these low temperatures, shi- 

 vering was clearly visible in the resting rats, and mechanical work 

 was substituted for shivering as in warm- acclimated rats (Hart, Jan- 

 sky, unpublished). The values followed closely the broken line shown 

 in Figure4. As shown for warm- acclimated animals, the resting me- 

 tabolism was, at very low temperatures, almost as great as the 

 values of working metabolism. 



It seems clear, therefore, that the substitution relationship be- 

 tween working and resting heat production exists in these animals 

 only when shivering is replacedby gross physical activity. The addi- 

 tional relationship occurs over a certain range of temperatures in 

 these animals, when non-shivering thermogenesis plays thedomin- 

 ant role in maintaining body temperature. 



All these data show that the values of maximal metabolism are 

 obtainable not only after simultaneous application of work and cold, 



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