Latent injury and repair in rat liver induced to regenerate at intervals after x-radiation.

When young, adult, male rats were exposed to a single dose of x-radiation of 365 or 600 r to the whole body and were partially hepatectomized immediately afterward, the regenerating livers exhibited a lag in initiating both D N A synthesis and mitosis. The lag was most pronounced in livers tha t had been regenerating for ~7 hours, a time when controls exhibited near maximal levels of activity. When days elapsed between irradiation and the induction of hepatic regeneration the lag gradually diminished so that after 8 weeks the response of the irradiated animals was similar to tha t of the controls. When a second partial hepatectomy was performed 4 or 5 days after the first, recovery from the above effects of irradiation was expedited. The number of chromosome bridges and other signs of structural damage resulting from irradiation did not diminish with time under any of the above conditions, however. Thus it appears tha t certain functions, although presumably inactive at the time, are impaired by x-radiation administered when cells are in the nondividing state, but the recovery is tmcelerated by induction of cell proliferation. Other functions, involving the distribution of genetic material, appear to be more permanently damaged.