Radiation induced alterations in the synthesis of ribosomal and informosomal particles in rat liver

Rats were sacrificed from 3 to 24 hrs after 2000 R of X-ray total-body irradiation. In one series of experiments, ¹⁴C-orotate was injected 40 to 50 min. before sacrifice, and the specific radioactivity was determined in the liver polyribosome fraction and in the cytoplasmic 40S particles. In these conditions, the polysome specific radioactivity had increased above control level 3 to 9 hrs after irradiation, and decreased below control level later on. Analyses of the 40S particles on CsCl gradients revealed that the specific radioactivity of both m RN proteins r RN proteins had increased 6 hrs after irradiation and fell progressively back to control level 12 to 24 hrs after irradiation. In another series, ¹⁴(-orotate was injected immediately before irradiation and the specific radioactivity determined on liver polysome fraction and on r-RNA preparations analyzed on sucrose gradients. In these conditions the specific radioactivity of both polysomes and r RNA had progressively increased above control levels from 6 to 24 hrs after irradiation. These results indicate that irradiation at first stimulates the synthesis of m-RNA and r-RNA in normal rat liver, and that this phase of enhanced synthesis is followed by a depression of synthesis during which the r-RNA molecules labelled in greater amount; the preceding stimulation phase retain a higher specific radioactivity, because they have a relatively longer life span than m-RNA and because they are less 'diluted' by newly synthesized r-RNA, due to the radiation-induced depression of synthesis.