Eight-week-old rats were irradiated with x- and γ-rays, and their brains were studied histochemically. They showed a transient increase in their glycogen content. The lowest exposure capable of producing a detectable increase in glycogen was 500 R. Visible accumulation of glycogen started approximately 6-8 hours postirradiation and reached a maximum at 18-48 hours. A simultaneous increase in phosphorylase and phosphorylase plus branching enzyme activity was recorded. This was mild initially but gradually became more prominent. The branching enzyme and some oxidoreductive enzymes were apparently unaffected except 6-phosphogluconate dehydrogenase, which was slightly increased. The activities of the enzymes glycogen synthetase and glyceraldehyde 3-phosphate dehydrogenase were unaltered in the irradiated brains. Glycogen synthetase activity even seemed to be diminished below normal levels 24 hours post-irradiation. Hexokinase reaction was slightly increased about 2 hours post-irradiation. The changes in phosphorylase activity and glycogen content are probably initiated by a transient hypoxia at the molecular level causing diminished aerobic as well as anaerobic glycolysis. A simultaneous increase in the glucose level of blood-and brain-probably plays a major role in raising the glycogen content. The phosphorylase reaction emerged from this study as the most sensitive histochemical parameter of radiation injury.

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