JANSKY 



SUMMARY 



The measurement of maximal steady state metabolism is com- 

 plicated by the fact that both cold exposure and exercise may be re- 

 quired to elicit the maximal response and by the fact that the effect 

 of exercise and cold varies with environmental temperature, state 

 of acclimation, and other factors. In most small mammals studied, 

 the metabolic effect of exercise is added directly to the cold ther- 

 mogenesis, but in the bank vole and in warm acclimated rats, ex- 

 ercise substitutes for shivering and replaces cold thermogenesis. 

 In cold acclimated rats, the metabolic effect of exercise is added to 

 cold thermogenesis, except at the lowest test temperatures where 

 substitution is again observed. The varied responses of different 

 species and of cold- and warm- acclimated rats apparently depend 

 on the extent of participation of non- shivering thermogenesis, which 

 extends the range for activity and increases the maximal steady 

 state metabolism. Maximal steady state metabolism can be deter- 

 mined either by imposing exercise simultaneously with cold or by 

 exposing the subject to cold alone at very low temperatures. 



The maximal steady state metabolism of different species was 

 equal to about six times the basal metabolism, and the exponent re- 

 lating log metabolism to log body weight was not obviously different 

 from that for basal metabolism (W * ) for the species tested. Basal 

 and maximal metabolism, therefore, give two parallel curves. 



The total cytochrome oxidase activity was also measured in 

 homogenates of whole animals. It was found that the exponent of the 

 relationship between cytochrome oxidase activity and log of the 

 weight was very similar to that found for basal and maximal meta- 

 bolism. There was a close similarity between absolute values of 

 maximal metabolism, and the total cytochrome oxidase activity 

 provides a theoretical upper limit to the metabolic capability and is 

 useful for comparative purposes in various species and organs of 

 the same species. 



The study of body organ cytochrome oxidase activity in the gold- 

 en hamster illustrates the importance of the muscles, which com- 

 prise about three fourths of the total cytochrome oxidase activity. 



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