Four equally spaced injections of serotonin (5 mg/kg) during the first hour following exposure to 800 R of X-rays has been found to increase the survival of rats. This effect was inhibited by a single injection of lysergic acid diethylamide (0.16 mg/kg), a serotonin antagonist, given immediately prior to irradiation. No therapeutic effect of serotonin was observed with one (10 mg/kg) or two (6 or 8 mg/kg) injections, or when the four injections were spaced at hourly intervals. These data suggest that a prolonged and intense vasoconstriction in the vital radiosensitive tissues of animals is basic to the radiotherapeutic role of serotonin. The recovery from lethal radiation damage may be due to an hypoxia-induced reduction in cellular respiration.

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