534 L. G. LAJTHA 



ever, the net DNA synthesis was measured in a 50-hour post irradiation 

 period, the first point being at about 2 hours, while the glycine incorpora- 

 tion was measured only for the first 2-hour period, and the reported 25% 

 depression is only the mean figure whereas the actual depression increases 

 from 5.7% at 60 minutes to 44.9% at 135 minutes; thus the discrepancy is 

 not much beyond the experimental error, especially considering that the 

 two experiments were not performed simultaneously. 



The general conclusion which can be drawn from the tumor experiments 

 is twofold: First, in vivo irradiation can depress P 32 or C 14 incorporation 

 into DNA within 2 hours — a period well within one interphase for these 

 cells. The degree of depression varies somewhat with the dose of radiation, 

 but rarely exceeds 50% in the 1000-5000 r. range. Second, it appears that 

 there are differences in radit (sensitivity in respect of P 32 or C 14 incorporation 

 into DNA between solid and ascites tumors and even between different 

 types of solid or ascites tumors, some ascites tumors not showing significant 

 depression of isotope incorporation into DNA even after 5000 r. 



2. Effects in Normal Tissue in Vivo 



All "radiosensitive" organs — intestinal epithelium, bone marrow, and 

 lymphatic tissues (spleen, thymus, appendix) — have been investigated ex- 

 tensively. 



a. Small Intestine 



A decrease of ultraviolet absorbing materials (2654 A.) was reported in 

 the small intestine at 4 hours following 600 r. whole body radiation in rats. 25 

 Kelly et a/. 18 found that 300 r. TBR produced a slight, and 800 r. a 50 % 

 depression of P 32 incorporation into DNA of mouse small intestine. Similar 

 observations were made with adenine-C 14 incorporation 26 with glycine-N 15 

 uptake into rat small intestine following 2000 r., 27 and with thymidine-C 14 

 uptake 1 hour after 400-800 r. 28 However, it has been pointed out that 

 within the first 1-2 hours following such doses of radiation there are mor- 

 phological signs of cell death in the intestinal epithelium. 29 ' 30 This obser- 

 vation becomes particularly significant if one considers the structure of the 

 tissue sampled 31 • 32 in which the cells capable of division and DNA synthesis 



25 J. O. Ely and M. H. Ross, Cancer Research 8, 285 (1948). 



26 E. L. Bennett, L. S. Kelly, and B. Krueckel, Federation Proc. 13, 181 (1954). 



27 J. N. Toal, J. C. Reid, R. B. Williams, and J. White, J. Natl. Cancer Inst. 21, 63 

 (1958) . 



28 (). F. Nygaard and R. L. Potter, Radiation Research 10, 462 (1959). 



29 W. Montagna and J. W. Wilson, J. Natl. Cancer Inst. 15, 1703 (1955). 



30 R. B. Williams, J. N. Toal, J. White, and H. M. Carpenter, J. Natl. Cancer Inst. 21, 

 17 (1958). 



31 H. Quastler, Radiation Research 4, 303 (1956). 



32 H. Quastler and F. Sherman, unpublished results (1958). 



