Objective:To investigate the effect of brain tissue extract of hypoxia-preconditioned mice （BHP） on tolerance of NGF-induced differentiated PC12 cells to hypoxia and the optimal concentration for protective effect.Methods:The mice model of acute repetitive hypoxia was reproduced and BHP was prepared.BHP was added into the cultures of 4～5 day differentiated PC12 cells and the final protein concentrations of BHP were 1.0、10.0、100.0μg/ml（BHP groups） respectively.Brain tissue ex- tract of normal mice（BN） at the same three protein concentrations were used as controls（BN groups） and PBS solvent control groups were also prepared in which only PBS was added（PBS groups）.Then differentiated PC12 cells were cultured in hypoxia （2%O

2

）.After 24

48

72 hours’hypoxia

colorimetric method(OD

570nm

) of tetrazolium salt MTT staining was adopted to determine cell viability and lactate dehydrogenase（LDH） leakage assays were also conducted at the same time.Besides

apoptotic per- centages at late stage（72 hours’ hypoxia） were detected.Results:After 24 hours’ hypoxia

OD

570nm

values of BHP groups with protein concentrations being 10.0、100.0μg/ml were significantly higher

but their LDH leakage percentages were significantly lower than those of BN groups with the same protein concentrations and of hypoxic PBS group.With hypoxia prolonging

BHP gradually lost the protective effect.At the time point of 72 hours’ hypoxia

OD

570nm

values of only BHP group at 100.0μg/ml protein concentration was significantly higher than that of the corresponding BN groups

its LDH leakage percentage and apop- totic percentage was significantly lower than those of the hypoxic PBS group.Conclusion:The effects of BHP on tolerance of differentiated PC12 cells to hypoxia depend on its concentrations and on its treating time.Besides

BHP at final concentration of 100.01μg/ml protein showed steady

prolonged protective effect.