ORGAN THERMOGENESIS 



but also in the resting state after exposure to low temperatures ap- 

 proaching the lethal level. Under the latter condition it is necessary 

 to measure the oxygenconsumptionfor a very short period after ex- 

 posure to cold, because of the substantial drop in body temperature 

 (Figure 4) . This period is about 20 minutes long for the rat (Depocas 

 et al., 1957). 



Owing to the difficulties described above in measuring maximal 

 steady state metabolism, relatively few values have been published 

 for mammals (Ghevillard, 1935; Hart, 1950; Jansky, 1959a for the 

 white mouse; Hart and Heroux, 1955 for the lemming and rabbit; 

 Jansky, 1959b for the bank and common voles; Jansky 1959c for the 

 golden hamster; and most recently by Buskirk and Taylor, 1957, for 

 man). One can expect that rather larger species differences might 

 occur, owing to differences in body size, posture, hair insulation, 

 blood circulation, and air movement, which might be quite different 

 during maximal metabolism than during the resting state and could 

 have a different effect on the amount of heat dissipation. 



When all published values of maximal steady state metabolism 

 are plotted against the log body weight of the animals (Figure 5) , it 

 was found that maximal steady state metabolism is equal to about 

 six times and basal rate, with a body weight exponent very close to 

 that found for basal metabolism. All the species examined have, 

 therefore, nearly the same capacity to increase energy metabolism 

 from the basal to the maximal steady state level. Species differences 

 in posture, insulation, and other factors that affect heat dissipation 

 have no apparent effect on this capacity to increase heat production. 



Total Cytochrome- oxidase Activity 



Since the rapid cooling of small animals at low temperatures 

 makes the measurement of maximal steady state metabolism quite 

 difficult, an additional method was sought for measuring the highest 

 metabolic capability of animals and tissues. 



It can be assumed that the total oxidative activity of tissues can- 

 not be greater than that of the activity of the only terminal oxidative 

 enzyme, cytochrome oxidase. In other words, it is supposed that the 



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