Objective To investigate the effects of 5-(N,N-dimethyl) amiloride(DMA),a Na +/H + exchange inhibitor(NHEI),on myocardial ischemia reperfusion injury(MIRI) Methods In the langendorff's perfused rat hearts,the myocardial ischemia reperfusion injury was induced by the 30 min reperfusion following the 30 min ischemia The hearts were randomly allocated to receiving no drug administration(control group,n=8), 5μmol/L(n=8),10μmol/L(n=8) or 20μmol/LMA(n=8) during reperfusion, respectively During the experiment,the cardiac contractile ability and electrocardiogram(ECG) were recorded At the end of reperfusion,the left ventricular myocardial samples were collected for measuring Ca 2+ content and observing myocardial ultrastructure with transmission electron microscopy Results Compared with those in control group,the recovery degrees of cardiac contractile ability,heart rate and coronary flow rate increased significantly in DMA 10μmol/L and DMA 20μmol/L groups(P<0 05 or 0 01), but the incidence of arrhythmias and myocardial Ca 2+ content decreased obviously(P<0 05 or 0 01) and the ultrastructure was preserved better in DMA 10μmol/L or DMA 20μmol/L groups; there were no significant differences in the parameters mentioned above in DMA 5μmol/L group (P>0 05),except for the obvious increases of the recovery degrees of cardiac contractile ability 10 min and 15 min following the reperfusion Conclusions DMA can protect myocardium from ischemia reperfusion injury to some extent in dose related way