The mechanism(s) of action of ionizing radiation on RNA synthesis, processing, and transport was studied in liver of intact and adrenalectomized male rats exposed to 1800 rad. The animals (1) were whole-body irradiated; (2) were irradiated to the liver only, with the rest of the body shielded: (3) had the liver shielded and the rest of the body irradiated; or (4) served as unirradiated controls. Newly synthesized RNA as measured by 1/2-hr uptake of <tex-math>$(6\text{-}{}^{14}{\rm C})\text{orotic}$</tex-math> acid was determined in various cell fractions at times ranging from 1/2 to 72 hr after either laparotomy or exposure to irradiation. The major response was an increase in polyribosomal specific radioactivity 3 and 6 hr after exposure in adrenal intact animals. Adrenalectomy 3 days prior to treatment prevented the increase. Two phases were observed in the liver exteriorization studies. In the first phase (0-48 hr), when RNA synthesis was enhanced, it appeared to be mainly adrenal-mediated. In the second phase (72 hr), however, when increases in both RNA synthetic rate and transport were observed, the action on the liver was direct.

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