536 L. G. LAJTHA 



tion changes, as in the mouse spleen the DNA content also drops quickly 

 after irradiation. 3 



Perhaps the most crucial experiment on in vivo radiation effects was that 

 made by Ord and Stocken, 40 in which they have shown that within 3 min- 

 utes after 1000 r. TBR P 32 incorporation is depressed by about 50% in 

 rat thymus. The degree of depression was about the same during the first 

 2 hours following irradiation. The importance of this observation lies in the 

 fact that it is the first attempt to investigate immediate radiation effects 

 on DNA synthesis in vivo, and that a depression of DNA specific activity 

 was found. They also made a comprehensive study of P 32 and glycine-C 14 

 incorporation into histone, and of lysine-C 14 into residual DNA protein, 

 and found that they are all affected at 2 hours after 1000 r. TBR. 41 



d. Resting Liver 



Liver is a "radioresistent" organ, in so far as there is very little histologi- 

 cal damage detectable shortly after large doses of radiation. 42 Nevertheless 

 Hevesy found a 50% depression of incorporation of C 14 (from acetate) into 

 DNA 6 hours after exposure of rats to 950 r. 15 Less consistent depressions 

 were found 2 hours after 300-2500 r. using P 32 incorporation as the index 

 of DNA synthesis. 18 The situation, however, is complicated by the observa- 

 tion of Richmond et al. a who, while confirming a depression of P 32 incorpora- 

 tion 2 hours after 1000 r., found no effect of the same dose on the incorpora- 

 tion of glycine-C 14 into DNA of rat liver. As liver cells are particularly rich 

 in phosphorus compounds, a possible alteration of the local phosphorus 

 pool must be taken into consideration. 



To conclude: it is very difficult to collect good evidence for the inter- 

 ference of radiation with the process of DNA synthesis in small intestine, 

 bone marrow, spleen, thymus, and liver in vivo, using DNA specific radio- 

 activity as an index. All these organs (with the exception of liver) have a 

 fast cell turnover and show population changes within 1-2 hours after ir- 

 radiation. The best evidence for an effect on DNA synthesis is the demon- 

 stration of a depression of P 32 incorporation in the thymus 3 minutes after 

 irradiation. 40 



e. Indirect Effects 



There are some reports in the literature referring to an indirect effect of 

 radiation on DNA synthesis, i.e. depression of precursor incorporation in 

 shielded parts of the body. 12 ' 14, 16 ' 17 However, Holmes has demonstrated 43 



40 M. G. Ord and L. A. Stocken, in "Advances in Radiobiology" (G. de Hevesy, A. G. 

 Forssberg, and J. D. Abbott, eds.), p. 65. Oliver & Boyd, Edinburgh, 1957. 



41 J. E. Richmond, M. G. Ord, and L. A. Stocken, Biochem. J. 66, 123 (1957). 



42 S. Warren, M. W. Holt, and S. C. Sommers, Am. J. Clin. Pathol. 22, 411 (1952). 



43 B. E. Holmes and L. K. Mee, Brit. J. Radiol. 30, 305 (1957). 



