INFLUENCE OF OXYGEN ON RADIO-SENSlTI\ 1 IV OF CELLS AND TISSUES 



the aiuhcr at very low close rates, corresponding to exposure times of about 

 8 hours, more two-hit chromosome aberrations are produced if the irradia- 

 tion is given in the absence than in the presence of oxygen. The same result 

 was obtained if the radiation was delivered at high dose rate, but preceded 

 by 8 hours of anaerobiosis. The authors interpret their observations in terms 

 of the postulate of VVoUrand Luippold that oxidative phosphorylation speeds 

 chromosome rejoining and thereby reduces the yield of interchanges resulting 

 from exposure at very lo\v dose rates. 



A period of anaerobic culture greatly increases the radio-resistance of E. 

 coll Br and changes the dose response curve, but the cells still respond to 

 changes in oxygen tension at the time of irradiation in the same manner as 

 aerobically grown cells^^ Prolonged anaerobiosis, hkewise, does not appar- 

 ently have any influence on the relation between sensitivity and the environ- 

 mental oxygen concentration at the time of irradiation in the case of either 

 root meristem cells or Ehrlich ascites tumour cells. Reference will be made 

 in Paper 29 to the fact that for periods of up to half an hour after X irradia- 

 tion^--*^, and in some cases for much longer periods, oxygen can influence 

 the level of damage sustained by barley seeds, as reflected in subsequent 

 seedling growth and chromosome damage at the first root tip division. Much 

 light may be thrown on these observations by the experiments of Jackson 

 (Paper 21) with onion seed reported at this Conference. These effects have 

 chiefly been observed when seeds are very dry at the time of irradiation. It 

 seems that in biological materials such as seeds and spores**, which normally 

 have a low water content, the influence of oxygen is most marked when the 

 water content is reduced to the lowest possible level, and decreased or elim- 

 inated by small amounts of water. 



Dittrich (quoted by Kunkel and Schubert*^ and personal communication), 

 has recently observed that if Ehrlich ascites tumour cells are irradiated at 

 about 2 °C and allowed to warm up to room temperature in 2 atm pressure, 

 more chromosome structural damage is produced than if the cells warm up 

 in air. We have not observed a similar effect when Ehrlich ascites tumour 

 cells are allowed to warm up to room temperature in oxygen at 1 atm 

 pressure. 



In assessing all these results, account must be taken of the fact that oxygen 

 alone, without irradiation, produces chromosome structural damage in very 

 dry Tradescantia pollen**', and may, in combination with cyanide*'^ or Cup- 

 ferron*^'*", produce chromosome structural damage in Vicia meristem cells. 

 Moreover, irradiated medium is toxic to irradiated DNA and irradiated 

 phage {cf. Table 1), and to irradiated catalase-deficient bacteria^" '^^ 



As a general rule, somatic cells, growing in normal physiological condi- 

 tions, have been found to respond instantly to changes in oxygen tension 

 before irradiation and to be uninfluenced by changes in oxygen tension 

 immediately after irradiation. The time limits for Vicia root meristem cells 

 were set at less than 30 sec by Read^ and a similar figure was found by 

 Giles^2 for Tradescantia microspores. It is less than 4 sec for bone damage in 

 the mouse taiP^', less than 2 sec for Ehrlich ascites tumour cells (Deschner and 

 Gray, unpublished) and less than 1/50 sec for bacteria^^ j^ each case the 

 figure quoted is merely the shortest time in which the worker was able to 

 change the oxygen tension before and after irradiation. In experiments 



164 



