214 CELLULAR METABOLISM 



lowered in the postradiation state, the appearance of radiation damage is de- 

 layed until the animal has been warmed up again. 



Dowdy: 



I feel that we should distinguish between oxygen presence and oxygen utili- 

 zation or metabolic rate, particularly in the warm-blooded mammals. Bennett, 

 in our laboratory, has demonstrated very well the protection obtained from 

 anoxic anoxia in rabbits, rats, and mice. The degree of anoxia permissible is 

 dependent upon a very fine balance. For example, the rat tolerates an atmos- 

 phere of 95 per cent nitrogen and 5 per cent oxygen. The adult mouse, how- 

 ever, will succumb to such a low atmosphere of oxygen. With 7 per cent oxygen 

 in the inspired air the mouse will live and if irradiated at this time will derive a 

 relative degree of protection. There is little or no protection when the mouse is 

 irradiated in an atmosphere of 10 per cent oxygen. When rats are irradiated in 

 an atmosphere of 5 per cent oxygen, there is complete protection from 600 r 

 total-body radiation for an indefinite period of time. The 30-day LDso for rats 

 in our laboratory is approximately 600 r. By these acute anoxic experiments we 

 can raise the 30-day LDbo in rats from 600 r to between 1200 and 1400 r total- 

 body radiation. However, at 1000 r and above, the rats die between 60 and 250 

 days, depending upon the total dose of radiation. This late death is entirely 

 different from what we are accustomed to see. 



Bennett also tied up the respiratory enzymes with cyanide without protection 

 to acute total-body radiation. Patt and his associates have demonstrated that, 

 if frogs are irradiated at a low temperature, while this low temperature is main- 

 tained the animals appear normal; but once they are brought back to normal 

 temperatures destructive changes take place and the animals die similarly to the 

 controls. Haley, in association with our group in our laboratory, has failed to 

 obtain protection in rats who were thyroidectomized. Large doses of thyroxin, 

 on the other hand, accelerated mortality in irradiated rats. 



It therefore seems that the presence of the oxygen molecule in the tissues at 

 the time of irradiation permits certain chemical reactions to take place. These 

 reactions may be slowed down or accelerated, depending upon the metabolic 

 rate. If the oxygen molecules are greatly reduced in the tissues, certain chemical 

 reactions either do not take place or occur at a greatly reduced frequency. It 

 seems obvious, however, that certain chemical reactions occur which do not re- 

 quire the presence of molecular oxygen and are manifested by late deaths 60-250 

 days later. 



Tahmisian : 



I wish to bring up the question of metabolism. We have irradiated grass- 

 hopper eggs with high doses such as 25,000-200,000 r. Grasshoppers normally 

 have a stage known as diapause. When they enter this stage there is no further 

 development unless this block is broken in nature by the onset of winter or by 

 chilling to 0° C in the laboratory for a period of 3 months. When eggs at this 

 stage are irradiated, immediately after irradiation, even after 200,000 r, no 

 morphological or cytological differences are observed. One cannot tell a control 



