Leukotrienes (LTs), known primarily for their pathological roles, may also be capable of exerting "cytoprotection" against toxic agents in a manner similar to that of prostaglandins. In this report, it is shown that treatment of mice with leukotrienes${\rm C}_{4},{\rm D}_{4},{\rm E}_{4}$, or B4 prior to sublethal irradiation increased the number of endogenous hematopoietic stem cells (E-CFU), with LTC4 producing the greatest response$({\rm LTC}_{4}\gg {\rm B}_{4}>{\rm E}_{4}>{\rm D}_{4}).\ {\rm LTC}_{4}\text{-induced}$ hematopoietic radioprotection was examined in greater detail using the exogenous spleen colony (CFU-S) and granulocyte/macrophage progenitor cell (GM-CFC) assays. The dose reduction factors for these cells in${\rm LTC}_{4}\text{-treated}$ mice at radiation doses resulting in 37% cell survival were 1.65 and 2.01, respectively.

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