POIKILOTHERMIC ADAPTATIONS 



tiiat this increase is reflected in a similar increase in the meta- 

 bolic activity of several tissues such as liver and muscle. We find 

 essentially the same results with small mammals. Then to carry 

 such studies a step further you have also given a number of instances 

 where cold acclimatization in the poikilotherm produces changes 

 in enzyme activity that are the same as we find in rats. I would 

 include among these latter effects succinic dehydrogenase, cyto- 

 chrome oxidase, glucose- 6- phosphate dehydrogenase, and lactic 

 dehydrogenase as well as DPNH and TPNH cytochrome c reductase 

 which are not affected by prolonged cold exposure in either type 

 of animal. It is true that there are several instances where poikilo- 

 therms and homeotherms are at first glance quite different in 

 their responses to prolonged cold exposure. This, however, may 

 or may not be significant since even in one species cold exposure 

 can lead to quite a variety of effects. 



There are a number of factors that can influence the nature 

 of the results obtained from in vitro tissue metabolism studies. 

 In intact cell preparations, for example, we have the problem of 

 quite limited exogenous substrate permeability or utilization. This 

 is particularly true for those substrates that exist in an ionic form. 

 But it is also true for such a common metabolite as glucose. In 

 vitro metabolic rate of whole cell preparations, therefore, is 

 largely dependent upon the availability of endogenous substrate, 

 and we must be quite cautious in interpreting them. 



PROSSER: That is why we used homogenation. 



HANNON; Homogenates are also notoriously bad for oxidizing 

 free glucose. They will not do it like the intact animal will. 



PROSSER: We have used succinate, too. 



HANNON: Practically everybody, I think, has reported an 

 increase in succinate oxidation in the Liver and muscle of cold- 

 acclimated mammals. Skin has also shown this increase. 



PROSSER: Do you find an increase in the monophosphate 

 shunt enzymes ? 



39 



