The Radioprotective Action of Parathyroid Extract

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Abstract
The protective action of a single post-irradiation injection of parathyroid extract in x-irradiated rats has been confirmed. The effectiveness of the extract (as a difference in survival percentage) was greatest in rats exposed to 800 r of x-rays. With this dose, the level of protection was maximum and independent of the amount of extract (administered 5 min after irradiation) between 50 and 200 USP units. The increase in survival obtained with 25 units of extract was about one-half that resulting from the higher doses. The protective action of 200 units was maximum whether injected 5 min or 1 hour after irradiation, but was about one-half the maximum with a delay of 3 hours. The extract was totally ineffective when injected 5 hours after irradiation. A number of lines of evidence suggest the calcium-mobilizing activity of the extract as the most likely basis of its radioprotective action.