38 



L. H. GRAY 



1.000 



2,000 

 Dose (r) 



3.000 



4,000 



Fig. 7. — Tumour forming ability' of mouse leukaemia cells after exposure to '■"Co y-rays. 



(Hewitt and Wilson, 1959) 



approximately midway between the aerobic and anaerobic levels 

 is observed when cells are in eqnilibrium with a gas phase con- 

 taining 0-25 per cent oxygen (dissolved oxygen = 3-5 jLtmoles/1). 

 Nitric oxide enhances the sensitivity of the human liver cells 

 (Dewey, 1960a) as also that of Ehrlich mouse ascites tumour cells 

 (Gray et al, 1958) to about the same extent as oxygen. 

 The relation between sensitivity and the concentration of oxygen 

 in the immediate vicinity of the Ehrlich tumour cells at the time 

 of irradiation is shown in Fig. 8. Froese (personal communication), 

 while working in our laboratory, investigated the dependence of 

 the respiratory activity of these cells on oxygen tension. He ob- 

 served that respiration was maintained at nearly maximum 

 rate down to concentrations 1 /nmole/l of oxygen in the liquid phase 

 at which radiosensitivity is not much greater than that observed 

 in the complete absence of oxygen. After making full allowance for 

 experimental error in both sets of data it is evident that, as in the 

 case of bacteria, the variation in the sensitivity of the cells with 

 oxygen concentration cannot be ascribed to an influence of the 

 enzymes of the cytochrome system. 



The tumour cells change their sensitivity in less than 1 sec of 

 transfer from one medium to another of different oxygen tension 

 (Deschner and Gray, unpublished). 



In mammalian cells, therefore, as in bacteria, we may confi- 

 dently infer that one, two or, at most, a few ionizing particles 

 initiate loss of reproductive integrity, and that the influence of 

 oxygen concentration during an irradiation of short duration 



