POIKILOTHERMIC ADAPTATIONS 



PROSSER: What about the Krebs cycle enzymes? 



HART: No alteration. 



PROSSER: How about the glycolytic ones? 



HANNON: In general, we have found that the overall metabolic 

 capacity of the Krebs cycle is increased by pro longed cold exposure. 

 In the one month cold-exposed animal this is evidenced by an in- 

 crease in the activity of all the Krebs cycle oxidases we have stu- 

 died. At the enzyme level, cold exposure may or may not lead to 

 an increased activity. Thus, in the electron transport system of the 

 liver, it was found that the activities succinic and malic dehydro- 

 genase and cytochrome oxidase were elevated following one month 

 in the cold. DPNH-cytochrome reductase, on the other hand was 

 unaffected. Similar increases in Krebs cycle activity have also been 

 seen in muscle. On the basis of these data, we might tentatively con- 

 clude that the primary effect of cold exposure (at least after one 

 month) is an increase in the metabolic capacity of those reactions 

 that are rate- limiting, e.g. succinic and malic dehydrogenase and 

 cytochrome oxidase. This conclusion, however, may have tobe modi- 

 fied for animals that have been exposed for intervals longer than 

 one month. Also, we have only limited data on how cold exposure 

 affects muscle tissue, and we have no idea how changes in relative 

 mass might affect the results that are obtained. 



PROSSER: Were these Krebs cycle activity measurements cal- 

 culated on a unit weight basis? 



HANNON: That is correct. The increase in oxidation will pro- 

 bably disappear with exposures that would lead to an increase in the 

 relative amount of tissue. In response to an earlier comment, I 

 should mention that fatty acid oxidation, at least the oxidation of 

 palmitic acid, does proceed at a greater rate in the liver tissue cf 

 the cold-acclimatized rat. 



PROSSER: The factor of exposure time, I am sure, is very 

 important. We have used periods of one to three weeks because 

 this agrees with acclimation time for lethal temperatures. How- 

 ever, Dr. Murphy showed me some data taken from experiments 



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