158 J. SOSKA, L, BENES, V. DRASIL, Z. KARPFEL, E. PALECEK AND M. SKALKA 



diation (Berenbom and Peters, 1956). The injection of deoxycytidylic 

 acid after irradiation has modified the giianine/cytosine and adenine/ 

 thymine ratios towards the normal vah;e, while thymidylic acid modi- 

 fied only the ratio G/C and not A/T (Fig. 2). 



2-0 



1-5 



fc 



f 



m 



fi= 



K 400r I ; ; 400r + TMP 



4dOr-:-d-CMP 



Fig. 2. — The molar ratio of guanine/cj'tosine and adenine/thymine in the spleen DXA of 

 rats, on the tifth day after iri'adiation with a dose of 400 r on tlie fourth day after admin- 

 istration of 1 mg deoxycytidyhc or thyniidyhe acid. The height of the columns corres- 

 ponds to the molar ratio, standard deviations of the means are also indicated. K ^ 



control, non-irradiated rats. 



If the mechanism of the recovery effect of deoxynucleotides or of the 

 embryo extract lies in snpplying the missing DNA-precursors, it was 

 expected that after irradiation the content of free deoxyribonucleotides 

 would decrease in the tissue. We carried out the hrst experimental 

 series with spleens of rats irradiated with a dose of 600 r. However, in 

 the first hours after irradiation no decrease of deoxynucleotides was 

 observed in the tissue (Soska and Soskova, 1959). On the contrary, 

 their great increase was noticed. Only from about the 24th hr follow- 

 ing irradiation did the content of these substances fall below the initial 

 level. The level of free deoxyribosides rose somewhat more slowly and 

 the following decrease was also slower. The interpretation of results 

 obtained with the spleen is, however, complicated by the fact that the 

 cell population of this tissue is not homogeneous, that the contribution 

 of the particular cell types changes after irradiation, and the fact of 

 extensive cell death. 



For this reason, regenerating rat liver was used for oiu" next experi- 

 ments. This material has the advantage that its cell population is 



