Recent studies from our lab and others have shown that the hematopoietic cytokine erythropoietin (EPO) can protect the heart from ischemic damage in a red blood cell-independent manner. Here we examined any protective effects of the long-acting EPO analog darbepoetin alfa (DA) in a rat model of ischemia-reperfusion (I/R) injury. Rats were subjected to 30-min ischemia followed by 72-h reperfusion. In a dose-response study, DA (2, 7, 11, and 30 mug/kg) or vehicle was administered as a single bolus at the start of ischemia. To determine the time window of potential cardioprotection, a single high dose of DA (30 mug/kg) was given at either the initiation or the end of ischemia or at 1 or 24 h after reperfusion. After 3 days, cardiac function and infarct size were assessed. Acute myocyte apoptosis was quantified by TUNEL staining on myocardial sections and by caspase-3 activity assays. DA significantly reduced infarct size from 32.8 +/- 3.5% (vehicle) to 11.0 +/- 3.3% in a dose-dependent manner, while there was no difference in ischemic area between groups. Treatment with DA as late as 24 h after the beginning of reperfusion still demonstrated a significant reduction in infarct size (17.0 +/- 1.6%). Consistent with infarction data, DA improved in vivo cardiac reserve compared with vehicle. Finally, DA significantly decreased myocyte apoptosis and caspase-3 activity after I/R. These data indicate that DA protects the heart against I/R injury and improves cardiac function, apparently through a reduction of myocyte apoptosis. Of clinical importance pointing toward a relevant therapeutic utility, we report that even if given 24 h after I/R injury, DA can significantly protect the myocardium.