Male rats of the Sherman strain were fed for 2 weeks a diet of ground purina rat chow containing 0.04 per cent thioacetamide. Animals were injected intraperitoneally with tritiated cytidine, 200µc/100 gm body weight, and sacrificed in pairs, a control and a thioacetamide-treated rat, at prescribed intervals. Liver tissues were preserved with the freeze-substitution method and postfixed in anhydrous OsO4. Other samples were fixed directly with an acetic acid-ethanol mixture (1:3). AR-10 stripping film was applied to 2- and 4-µ sections and exposed for appropriate lengths of time. Nuclear and nucleolar volumes were obtained by direct measurement. Cytoplasmic volumes were obtained with the aid of Chalkley ratios. Nucleolar and cytoplasmic RNA concentrations were calculated from cytophotometric extinction (E540 mµ) measurements. Data were expressed as grains/unit area, grains/unit area/concentration (or specific activity) and grains/total structure. In the liver parenchymal cells of thioacetamide-treated rats, the nucleolus shows vast increases in volume, RNA content, and grain count/total structure, 14-fold, 25-fold, and over 30-fold, respectively. The nucleus increases 2-fold in volume and about 3-fold in total grain count. Cytoplasmic volume increases only 20 per cent and displays a total grain count about equal to that in the control. The time course of incorporation curves for nucleolus and non-nucleolar nucleus (NNN) contain 2 distinct turnover fractions, rapid and slow. Both fractions were increased after thioacetamide treatment but remained proportional to those of controls. The unique stimulated RNA turnover in the nucleus and nucleolus, coupled to a "normal" turnover in the cytoplasm, suggests that this nuclear-nucleolar loss of label does not represent an exclusive passage of formed nuclear RNA to the cytoplasm.

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