IXCORPORATIOX OF "C INTO TISSUE 797 



of the increase in the activity level, due to the strongly inhibiting effect 

 of urethane on nucleic acid formation, the '^HJ content of the nucleic 

 acid content of the urethane injected mouse is markedly lowered (the 

 last mentioned effect overcompensates the first mentioned one). 



As seen in Table 1, irradiation acts in a similar way not only on incor- 

 poration of i*C into nucleic acid as does urethane but an increased 

 incorporation of ^^C into tissue proteins is observed as well. The last 

 mentioned effect is, however, less pronounced after irradiation and more 

 difficult to reproduce than the effect observed after administration of 

 urethane. In fully grown mice we found a significant increase in ^'^C incor- 

 poration due to irradiation in the brain and possibly into the liver pro- 

 teins (cf. Table 8). 



C:iANGE IN THE SENSITIVITY OF THE RADIOACTIVE INDICATOR 



WITH TIME 



The change in the sensitivity of the radioactive indicator with time 

 plays an important role in many applications of radioactive tracers, 

 this being especially the case in the study of the metabolism of acetate 

 and other rapidly metabolizing carbon compounds. 



Some years ago, MarinelliW studied the incorporation of ^sp into the 

 Ijones a few days after irradiating one leg of the rabbit with a 3000 r 

 dose, while the other legs were protected. He found that if ^^p was 

 injected into the animal several days after irradiation of one of the legs, 

 the irradiated leg contained only about 70 per cent of the ^^P content of 

 the non-irradiated leg. The inhibition of radiophosphate uptalce by the 

 irradiated leg leads to a higher activity level of the plasma inorganic 

 P and correspondingly to a higher ^sp uptake by the phosphorus com- 

 pounds of the soft tissues. We meet thus indirect radiation effects. Radio- 

 lesion of the bone may reflect itself in the rate of incorporation of ^^p 

 into liver phosphatides and many other compounds. It is not the turn- 

 over rate itself of the liver phosphatides which is influenced, but the 

 sensitivity of the registration of the turnover. 



The application of i*C as an indicator, especially in the form of rapidly 

 metabolizing compounds such as acetate, may reveal a great variety of 

 indirect radiation effects. 



If we inhibit by irradiation i*C labelled protein or phosphatide for- 

 mation in the liver, the protein resp. phosphatide content of the plasma 

 will decrease and all steps in which these compounds participate will be 

 influenced as well. 



^i^L. D. Marinelli, Radiology 37, 169 (1941). 



