The mechanisms for protection through parabiosis against both the acute intestinal and hematopoietic radiation death were investigated in two separate experiments. The length of time after irradiation that the parabiosis must remain intact for protection to occur was investigated in the first experiment. The data show that if surgical separation is delayed to 48 hours postexposure, protection against 3- to 5-day death occurs, and if delayed to 96 hours, protection against 30-day lethality also occurs. The second experiment was designed to test the quantity of whole-body radiation that one member of a parabiotic pair could receive (range 250-1000 R) and still provide protection to the simultaneously irradiated partner (1500 R). The results of this experiment show that over the range tested, none of the doses alters the animal's capacity to protect an irradiated partner (1500 R) against the 3- to 5-day death. Substantial 30-day survival also occurred with all the test doses employed except at the highest exposure level (1000 R). The findings are consistent with the hypothesis that protection through parabiosis against 3- to 5-day death is one of electrolyte and fluid maintenance, and protection against 30-day mortality is through transfer in peripheral blood of stem cells capable of repopulation of bone marrow.

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