40. RADIATION AND NUCLEIC ACID METABOLISM 537 



that even such mild shocks as tying down an animal may affect P 32 incor- 

 poration into DNA. The mechanism of this shock effect is not quite clear, 

 but it is not due to simple pool dilution. In this respect the experiments of 

 Smellie et a/. 34 are particularly interesting. They found no depression of re- 

 uptake into thymus of rabbits — whose abdominal viscera only were irradi- 

 ated with 1000 r. — during the first 2 hours after irradiation. However, 1 

 and 2 days after irradiation the uptake was depressed by about 50%. While 

 this late effect is almost certainly a depopulation effect, it is interesting 

 that it develops so late after irradiation. The interest in heightened by the 

 fact that apparently,- in their experiment the animals did not suffer acute 

 shock during irradiation as the 2-hour P 32 uptake was unaffected in the thy- 

 mus (excluded from the field of radiation) while the irradiated bone marrow 

 and appendix have shown a 50% depression. This late indirect effect merits 

 further investigation. 



Another aspect of indirect radiation effect on DNA synthesis was studied 

 by Harrington and Lavik. 44 They have irradiated mouse Ehrlich ascites 

 cells in vitro, and then injected the cells into unirradiated or previously 

 whole body irradiated recipient mice together with some P 32 . They have 

 measured the depression in P 32 uptake into DXA compared with that in 

 unirradiated controls and have reported that 1000 r. TBR to the recipient 

 animals, although itself without effect on DXA synthesis in the sub- 

 sequently injected tumor cells, enhances the effect of in vitro radiation in 

 these cells. The number of animals involved in their experiments, however, 

 was not large enough for an experiment of this nature and the work there- 

 fore needs confirmation. 



3. Effects in Regenerating Liver 



Regenerating liver is a very attractive system for the study of radiation 

 effects on DNA synthesis in vivo. The tissue behaves in a near synchronized 

 fashion for 30-40 hours both in respect of DNA synthesis and mitosis (Fig. 

 2). 



Consequently, radiation effects well within one cell cycle, and even within 

 parts of the cell cycle can be studied. A great deal of work has, in fact, been 

 done 45 " 53 ; the most detailed information coming from the work of Holmes 49 



44 H. Harrington and P. S. Lavik, Cancer Research 17, 38 (1957). 



45 J. F. Thomson, M. S. Carttar, and W. W. Tourtelotte, Radiation Research 1, 165 

 (1954). 



16 B. E. Holmes and L. K. Mee, Acta Radiol. Suppl. 116, p. 694 (1954). 



47 B. E. Holmes and L. K. Mee, in "Radiobiology Symposium" (Z. M. Bacq and P. 

 Alexander, eds.), p. 220. Butterworths, London, 1955. 



48 L. K. Mee, in "Progress in Radiobiology" (J. S. Mitchell, B. E. Holmes, and C. L. 

 Smith, eds.), p. 12. Oliver & Boyd, Edinburgh, 1955. 



49 D. B. Cater, B. E. Holmes, and L. K. Mee, Acta Radiol. 46, 655 (1956). 



50 L. S. Kelly, Proc. Am. Assoc. Cancer Research 1, 24 (1954). 



