40. RADIATION' AND NUCLEIC ACID METABOLISM 539 



role of oxygen in potentiating radiation effect is well known from the classic 

 work of Gray and his associates. 54 To this, however, another factor should 

 be added: the general natural biochemical milieu, which is rarely, if ever, 

 reproduced in vitro. Most freshly explanted cells — if one is permitted to 

 use the term — are not "happy" in vitro. For the first few hours after ex- 

 plantation one is usually dealing with a predominantly "unhappy" popula- 

 tion of cells; later, by selection and adaptation, new and in many respects 

 different cell lines will develop. Some of these lines may become "happy" 

 even in completely synthetic media, but extrapolation from such cells to 

 in vivo cell behavior should be done only with caution. 



This is perhaps the reason why, in spite of being a system with simpler 

 parameters, relatively little has been done, until lately, with in vitro cell 

 systems in studying radiation effects on DNA synthesis. 



The importance of milieu was demonstrated by Ord and Stocken, who 

 have shown that thymus and spleen cells in vitro in Krebs Ringer phosphate 

 plus 0.3 M glucose show no constant significant depression of P 32 incorpora- 

 tion into DXA following 2000 r. (in vivo in the same experiment 1000 r. 

 produced a 60 % depression), 55 however, in a special medium originally made 

 for isolated nuclei 56 they found a 30% depression following 950 r. 57 It is 

 clear that even this medium is not yet ideal, but the demonstration of the 

 importance of choosing the right medium is obvious. 



This perhaps explains the lack of any significant depression of P 32 incor- 

 poration in vitro after 10,000 r. in vitro or in vivo in Ehrlich ascites cells as 

 reported by Harrington and Lavik. 44 They kept the cells in their own ascitic 

 fluid during a 3-hour incubation period, but although it was "fortified" 

 with 0.8 mg./ml. glucose, ascites fluid is a very poor medium for these cells 

 in vitro. In vivo its composition may be kept constant by the host animal, 

 but its reserves are quickly exhausted by the cells, even after 1-2 hours 

 in vitro. h% 



In bone marrow of rats, Lutwak-Mann has reported that in short term 

 experiments 10,000-25,000 r. in vitro did not affect the nucleic acid phos- 

 phorus content 59 ; but in view of Tro well's finding on the paradoxical resist- 

 ance of thymocytes to high doses of radiation, 60 and of a similar tendency 



54 L. H. Gray, in "Lectures on the Scientific Basis of Medicine." Athlone Press, 

 London, 1959. 



55 M. G. Ord and L. A. Stocken, Biochem. J. 63, 3 (1956). 



56 D. W. H. Barnes, M. P. Esnouf, and L. A. Stocken, in "Advances in Radiobiology" 

 (G. de Hevesy, A. G. Forssberg, and J. I). Abbott, eds.), p. 211. Oliver & Boyd, 

 Edinburgh, 1957. 



" M. G. Ord and L. A. Stocken, Biochem. J. 68, 410 (1958). 



58 E. Hell and L. G. Lajtha, To be published. 



59 C. Lutwak-Mann, Biochem. J. 49, 300 (1951). 



60 O. A. Trowell, M. J. Corp, and W. R. Lush, Radiation Research 7, 120 (1957). 



