Abstract
Actinomycin D was administered in moderate doses to rats. On the following day the rats were given 3H-cytidine, 3H-methionine or 3H-leucine and sacrificed 3 or 8 hr later. Radioautographs revealed significant decreases in nucleolar RNA and cytoplasmic RNA labeling in many types of cells, e.g. liver cells. Chromatin RNA labeling was undisturbed at 3 hr in liver parenchymal cells and slightly depressed at 8 hr. Nuclear and cytoplasmic protein labeling were unchanged. The liver and the pancreas also showed a decrease in nucleolar size, total nucleolar volume per nucleus, and the ratio of total nucleolar volume per nucleus to nuclear volume. There was no decrease in nuclear volume. The Purkinje cells of the cerebellum showed no change in nucleolar volume. Nerve cells of the central nervous system, unlike those of the peripheral nervous system, showed no decrease in RNA labeling. This might be explained by an inability of actinomycin D to penetrate the bloodbrain barrier.
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