While the molecular mechanisms by which oxidants cause cytotoxicity are still poorly understood, disruption of Ca2+ homeostasis appears to be one of the critical alterations during the oxidant-induced cytotoxic process. Here, we examined the possibility that oxidative stress may alter the metabolism of cyclic ADP-ribose (cADPR), a potent Ca2+-mobilizing second messenger in the heart. Isolated heart perfused by Langendorff technique was subjected to ischemia/reperfusion injury and endogenous cADPR level was deter-mined using a specific radioimmunoassay. Following ischemia/reperfusion injury, a significant increase in intracellular cADPR level was observed. The elevation of cADPR content was closely correlated with the increase in ADP-ribosyl cyclase activity. Inclusion of oxygen free radical scavengers, 2,2,6,6-tetramethyl-1-piperidinyloxy and mannitol, in the reperfusate prevented the ischemia/reperfusion-induced increases in cADPR level and the ADP-ribosyl cyclase activity. Exposure of isolated cardiomyocytes to t-butyl hydroperoxide increased the ADP-ribosyl cyclase activity, cADPR level, and intracellular Ca2+ concentration ([Ca2+](i)) and consequently resulting in cell lethal damage. The oxidant-induced elevation of [Ca2+](i) as well as cell lethal damage was blocked by a cADPR antagonist, 8-bromo-cADPR. These results provide evidence for involvement of cADPR and its producing enzyme in alteration of Ca2+ homeostasis during the ischemia/reperfusion injury of the heart. (C) 2003 Elsevier Inc. All rights reserved.