Aim:To investigate whether hypoxia reoxygenation induces premature senes-cence in neonatal Sprague-Dawley(SD)rat cardiomyocytes.Methods:Cardio-myocytes were isolated from neonatal SD rat heart and identified by immunohisto-chemistry.The control cultures were incubated at 37℃ in a humidified atmo-sphere of 5% CO_2 and 95% air.The hypoxic cultures were incubated in a modularincubator chamber filled with 1% O_2,5% CO_2,and balance N_2 for 6 h.The reoxygen-ated cultures were subjected to 1% O_2 and 5% CO2 for 6 h,then 21% oxygen for 4,8,12,24,and 48 h,respectively.Cell proliferation was determined using bromo-deoxyuridine labeling.The ultrastructure of cardiomyocytes was observed byusing an electron microscope.β-Galactosidase activity was determined by usinga senescence β-galactosidase Staining Kit.p16~(INK4a)and telomerase reverse tran-scriptase(TERT)mRNA levels were measured by real time quantitative PCR.TERTprotein expression was determined by immunohistochemistry.Telomerase activi-ties were assayed by using the Telo TAGGG Telomerase PCR ELISA(plus)kit.Results:The initial cultures consisted of pure cardiomyocytes identified by immunohisto-chemistry.The proportion of BrdU positive cells was reduced significantly in thehypoxia reoxygenation-treated group(P<0.01).Under the condition of hypoxiareoxygenation,mitochondrial dehydration appeared;p16~(INK4a)and TERT mRNAlevels,β-galactosidase activity,TERT protein expression and telomerase activi-ties were all significantly increased(P<0.01 or P<0.05).Conclusion:These dataindicate that premature senescence could be induced in neonatal SD rat cardiomyo-cytes exposed to hypoxia reoxygenation.Although TERT significantly increased,it could not block senescence. Aim: To investigate whether hypoxia reoxygenation induces premature senes-cence in neonatal Sprague-Dawley (SD) rat cardiomyocytes. Methods: Cardio-myocytes were isolated from neonatal SD rat heart and identified by immunohisto-chemistry. in a humidified atmo-sphere of 5% CO 2 and 95% air. The hypoxic cultures were incubated in a modular incubator chamber filled with 1% O 2, 5% CO 2, and balance N 2 for 6 h. The reoxygen-ated cultures were subjected to 1 % O2 and 5% CO2 for 6 h, then 21% oxygen for 4, 8, 12, 24 and 48 h, respectively. Cell proliferation was determined using bromo-deoxyuridine labeling. The ultrastructure of cardiomyocytes was observed by using an electron microscope. β-Galactosidase activity was determined by using senescence β-galactosidase Staining Kit.p16 ~ (INK4a) and telomerase reverse tran-scriptase (TERT) mRNA levels were measured by real time quantitative PCR. TTERprotein expression was determined by immunohistochemistry. ties were assayed by using the Telo TAGGG Telomerase PCR ELISA (plus) kit. Results: The initial cultures consisted of pure cardiomyocytes identified by immunohisto-chemistry. The proportion of BrdU positive cells was reduced significantly in the hypoxia reoxygenation-treated group (P <0.01 ) .Under the condition of hypoxiareoxygenation, mitochondrial dehydration appeared; p16 ~ (INK4a) and TERT mRNA levels, β-galactosidase activity, TERT protein expression and telomerase activi-ties were all significantly increased (P <0.01 or P < These dataindicate that premature senescence could be induced in neonatal SD rat cardiomyo-cytes exposed to hypoxia reoxygenation. Although TERT significantly increased, it could not block senescence.