ORGAN THEPMOGENESIS 



the theoretical total metabolic capacity (Figure 6). The role of other 

 organs is relatively small. The most important are skin (9%), liver 

 (5%), brain (3%), and gonads (3%). Similar observations were also 

 made on the rat. 



Organ Thermogenesis in vivo 



Owing to the large contribution of the muscles and the minor 

 contribution of visceral organs in the total cytochrome oxidase ac- 

 tivity of hamsters and rats, it is important to consider the relative 

 contribution of these organs in vivo to the maximal steady state me- 

 tabolism. At present only indirect estimation can be made on the 

 role of muscle in intact animals. In warm acclimated rats (Figure 

 4) the maximal steady state metabolism is approximately 270 Cal/ 

 (hour x body weight ' ). The increase of metabolism from the basal 

 level (100 CaL/(hour x body weight * )) is about 170 Gal/(hour x 

 body weight * ), which is 63% of maximal metabolism (270 Cal/ 

 hour) . Since shivering is the principal source of heat in these rats 

 exposed to cold, it can be assumed that the cold thermogenesis is 

 due to muscles. In addition to the cold thermogenesis, it has to be 

 estimated that the participation of muscles in the basal state a- 

 mounts to about 30% of the total metabolic rate (Field et al., 1939). 

 This would be approximately 10%of the maximal rate in these tests. 

 The total contribution of the muscles to the maximal metabolism 

 would therefore be 73% (63% + 10%). 



In cold acclimated rats the quantitative estimation of the role of 

 the muscles is more complicated. In addition to shivering, the non- 

 shivering thermogenesis is developed (Sellers et al., 1954; Heroux 

 et al., 19 56; Cottle and Carlson, 19 56), which increases the maximal 

 metabolic rate to about 420 Cal/(hour x body weight) (Figure 4) . In 

 order to estimatethecontributionof muscles under these conditions, 

 the site of non-shivering tnermogenesis must first be ascertained. 



The visceral organs have been considered as important sites of 

 non-shivering heat production for many years. Much of the evidence 

 has come from measurements of temperatures near the liver (Gray- 

 son andMendell956;Donhofferetal.,1957).ln cold acclimated rats, 

 the elevation of BMR and the elevation QO in vitro give support to 



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