Objective To evaluate the effects of age factors on hypothermia-induced reduction of ischemia-reperfusion(I/R)injury in isolated rat hearts.Methods Pathogen-free healthy aged male Sprague-Dawley rats,aged 18-20 months,weighing 400-600 g,and young rats,aged 4-6 months,weighing 280-350 g,were used in the study.After the animals were anesthetized with intraperitoneal chloral hydrate and heparinized,their hearts were excised and perfused with K-H solution in a Langendorff apparatus.Twenty-four isolated hearts of aged rats were assigned into 2 groups(n=12 each)using a random number table:I/R group(group AI/R)and hypothermia group(group AH).Twenty-four isolated hearts of young rats were assigned into 2 groups(n=12 each)using a random number table:I/R group(group YI/R)and hypothermia group(group YH).Perfusion was suspended for 30 min followed by 120 min reperfusion to establish the model of I/R.The temperature was maintained at 37 ℃ during the whole process in AI/R and YI/R groups.The hearts were perfused with 34 ℃ K-H solution until 120 min of reperfusion starting from onset of reperfusion in AH and YH groups.At 30 min of equilibration(T0)and 15,30,60 and 120 min of reperfusion(T1-4),heart rate(HR),left ventricular developed pressure(LVDP),the maximum rate of increase in left ventricular pressure(+dp/dtmax),and the minimum rate of increase in left ventricular pressure(+dp/dtmin)were recorded.Six hearts from each group were randomly selected at T4,and myocardial specimens were obtained for determination of ATP,superoxide dismutase(SOD)and malondialdehyde(MDA)levels and myocardial infarct size(IS).Results Compared with group YI/R,HR was significantly decreased at T1-4,ATP and SOD levels were increased,and the MDA content and myocardial IS were decreased in group YH,and the HR,LVDP,+dp/dtmax and +dp/dtmin at T0 and ATP and SOD levels at T4 were significantly decreased,and the MDA content and myocardial IS were increased in group AI/R(P<0.05).Compared with group YH,HR,LVDP,+dp/dtmax and +dp/dtmin at T0 and ATP and SOD levels at T4 were significantly decreased,and the MDA content and myocardial IS were increased in group AH(P<0.05).Compared with group AH,the levels of ATP and SOD were significantly decreased,and the MDA content and myocardial IS were increased in group AI/R(P<0.05).Conclusion Age factors affect the efficacy of hypothermia in reducing I/R injury in isolated rat hearts,and hypothermia provides better cardioprotection for young rats than for aged rats.