206 - CELLULAR METABOLISM 



nounced regenerative power of the first-mentioned tissues responsible 

 for their greater radiosensitivity. It is more difficult to interpret on 

 such lines the finding of Jacobson that erythroblast vulnerability to ir- 

 radiation injury is not enhanced by increased mitotic activity (80). The 

 power of the tissue to wipe out changes produced by irradiation is as 

 important a fact in determining the radiosensitivity of the tissues as its 

 mitotic response to radiation. 



In all those cases in which the target theory is considered to be appli- 

 cable, as, for example, in the production of gene mutation, the changes 

 effected by irradiation are assumed to be irreversible. Work of Spencer 

 (81) and Bonnier (82) indicates, however, that besides the irreversible 

 direct hit action of ionizing radiation there is an additional reversible 

 effect. This additional effect becomes negligible as soon as the x-ray 

 dose amounts to a few hundred roentgens. Carlson (130) found a 

 gamma-ray dose of 128 r to be more effective in depressing mitosis of 

 the grasshopper neuroblast if given at 32 r per minute than at 2 r per 

 minute. 



In the adult organism the oxygen consumption due to mitotic proc- 

 esses is an almost neghgible fraction of the total oxygen uptake. In 

 such an organism a significant change of oxygen consumption due to 

 irradiation must be ascribed to interference of the irradiation with other 

 than mitotic processes. Measurements of both the oxygen consumption 

 and carbon dioxide output by irradiated animals indicate such an inter- 

 ference. 



Application of Radioactive Indicatoks 



Numerous and partly contradictory data are available on the effect 

 of irradiation on oxygen consumption. In a recent investigation on the 

 frog, after irradiation with 3000 r there was found a slight early decrease 

 in oxygen consumption with maximal depression of 34 per cent 15 days 

 after exposure. This was followed by return to control levels about 25 

 days after exposure (83). Rats, after total-body irradiation of 809- 

 972 r, showed, on the other side, an elevation of oxygen consumption of 

 approximately 35 per cent occurring within 24 hr (84). When C^^-labeled 

 glycine was injected into the rat 48 hr after exposure to roentgen rays, 

 a striking increase in the C^* activity of the expired CO2 was observed 

 by Altman (85). A depressing effect of irradiation on the metabolic 

 rate was observed by Barron in investigations in which the effect of 

 x-rays on tissue slices was studied (89a). 



The effect of irradiation on metabolic processes taking place in vivo 

 in a single organ can be studied by comparing the rate of incorporation 

 of labeled atoms into tissue fraction in irradiated animals and in controls. 



