RUTH MOORE AND H. A. S. VAN DEN BRENK 



but was not increased Ijy an increase in pure oxygen tension from 1 to 2 

 atmospheres. His results show a twelvefold increase in radio-sensitivity from 

 to 100 per cent oxygen. However, this increase is largely dependent on the 

 physical gradients which prevailed from the outside to the inside of the nodes. 

 The figures he gives for oxygen tensions are those at the surface. For lymph 

 nodes in vitro the tension decreases in proportion to the distance from the 

 surface of the node^ and at oxygen tensions of less than 1 atmosphere, the 

 centres of the larger nodes are anoxic and often necrotic. Owing to variations 

 in the size of the explants it is impossible to assess the 'oxygen eflfect ' factor. 



In the experiments described, for intact rats breathing air, the oxygen 

 tension in the tissue fluid of a well-vascularized organ such as a lymph node 

 would be much more uniform. Also, the actual available oxygen in the 

 presence of haemoglobin would be much larger than for tissue fluid, and this 

 factor must be taken into consideration in evaluating the extent of a given 

 rise in gaseous oxygen pressure, in ecjuilibrium with the particular fluid. A 

 significant change in radio-sensitivity in vivo would only be expected, for 

 lymph nodes, under conditions of severe anoxia. 



Both cystamine and 5-OHT exert a protective effect in vivo. No information 

 is available concerning their eflfect in vitro. In the case of 5-OHT, increasing 

 the respired oxygen tension to 4 atmospheres significantly reduces radio- 

 protection. This result has not been found for the oxidized protective 

 substance cystamine (S-S). 



These results closely agree with the findings described for lethality of 

 protected rats respiring oxygen at various partial pressures. It will be noted 

 that the protective effect of both amines is small for this radio-biological 

 indicator in comparison with whole-body effects. For mouse spleen and 

 thymus, irradiated in vivo'' the protective action of cysteamine is also slight. 

 It is difficult to escape the conclusion that the radio-protection afforded by 

 these substances is less in well-vascularized organs and may be largely due 

 to an upset in intracellular oxygen tension — partly the result of cellular 

 anoxia due to anoxaemia, and partly due to a histotoxic anoxia. 



REFERENCES 



iTrowell, O. a. Exp. Cell Res. 3 (1951) 79 

 2TROWELL, O. A. /. Path. Bad. 64 (1952) 687 

 3TROWELL, O. A. Brit. J. Radiol. N.S. 26 (1953) 302 



^Trowell, O. a., Corp, M.J. and Lush, W. R. Radiation Res. 7 (1957) 120 

 ^Patt, H. M., Blackford, Margaret E. and Staube, R. L. Proc. Sac. exp. Biol., 

 N.Y. 80 (1952) 92 



eScHREK, R. Radiology 65 (1955) 912 

 ' Gerebtzoff, M. a. and Bacq, Z. M. Experientia 10 (1954) 341 



189 



