ORGAN THERMOGENESIS 



the increased thermogenesis of visceral organs (Weiss, 1954). How- 

 ever, direct evidence on the magnitude of the contribution is lacking. 



On the other hand, evidence against the visceral organs as the 

 important site of heat production in non- shivering was provided by 

 Depocas (1958) who found, that the metabolic response to cold in 

 curarized cold acclimated rats was not reduced by functional evis- 

 ceration. Supporting evidence that the liver did not greatly contri- 

 bute to increased heat production in cold was provided by Kawahata 

 and Carlson (1959) in cold acclimated rats. Similar observations 

 have recently been obtained for the kidney ( Jansky and Hart, unpub- 

 lished). On the other hand, direct evidence for the participation of 

 muscle in cold thermogenesis was obtained by Jansky and Hart (un- 

 published) in the leg muscles of cold acclimated curarized rats 

 where elevation in oxygen consumption equal to that in the whole ani- 

 mal were found during exposure to cold. 



While thermogenesis from visceral organs still cannot be ex- 

 cluded, it can be concluded that both shivering and non- shivering 

 thermogenesis are dependent to an important extent on the muscles. 

 The total increase in heat production of cold-apclimated rats from 

 the basal level (125 CaL/(hour x body weight ' )) is about 295 Gal 

 /(hour X body weight * ), which is 75% of the maximal rate (420 

 Gal/hour). If muscleaccountsfor the entire cold thermogenesis, this 

 would be equivalent to 77%of maximal metabolism, when the contri- 

 bution of muscle to the basal metabolism is also considered. 



The calculations again agree closely with the large proportion 

 of muscle to the total cytochrome oxidase activity in hamsters and 

 rats and show that as an upper limit, muscle could contribute about 

 the same proportion to metabolism in both warm and cold acclimated 

 rats. However, it is clear that the absolute increase in maximal me- 

 tabolism of cold acclimated rats would require an increase in ab- 

 solute valuesof cytochrome oxidase activity in the organs concerned. 

 These observations, which are incomplete and permit only tentative 

 conclusions, will be extended by work now in progress on the cyto- 

 chrome oxidase activity of the muscles and other organs in warm 

 and cold acclimated rats. 



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