The study investigates the relationship between reactive oxygen species (ROS) and the mitochondrial permeability transition (MPT) in cardiac myocytes. Using a novel method involving photoactivation of tetramethylrhodamine derivatives, the researchers observed incremental ROS accumulation in individual mitochondria, which triggered abrupt and sometimes reversible mitochondrial depolarization. This phenomenon was attributed to MPT induction, as evidenced by the prevention of depolarization by bongkrekic acid and the concurrent permeability of mitochondria to calcein. The ROS scavenger Trolox prevented MPT induction, suggesting that ROS play a crucial role. The time required for ROS to induce MPT was dependent on cellular ROS-scavenging mechanisms, particularly glutathione. The MPT induction coincided with a burst of mitochondrial ROS generation, termed "ROS-induced ROS release" (RIRR). This RIRR phenomenon often occurred synchronously and reversibly among adjacent mitochondria, demonstrating apparent cooperativity. The study highlights a fundamental link between MPT and RIRR in cardiac myocytes, which may have implications for mitochondrial and cell biology.The study investigates the relationship between reactive oxygen species (ROS) and the mitochondrial permeability transition (MPT) in cardiac myocytes. Using a novel method involving photoactivation of tetramethylrhodamine derivatives, the researchers observed incremental ROS accumulation in individual mitochondria, which triggered abrupt and sometimes reversible mitochondrial depolarization. This phenomenon was attributed to MPT induction, as evidenced by the prevention of depolarization by bongkrekic acid and the concurrent permeability of mitochondria to calcein. The ROS scavenger Trolox prevented MPT induction, suggesting that ROS play a crucial role. The time required for ROS to induce MPT was dependent on cellular ROS-scavenging mechanisms, particularly glutathione. The MPT induction coincided with a burst of mitochondrial ROS generation, termed "ROS-induced ROS release" (RIRR). This RIRR phenomenon often occurred synchronously and reversibly among adjacent mitochondria, demonstrating apparent cooperativity. The study highlights a fundamental link between MPT and RIRR in cardiac myocytes, which may have implications for mitochondrial and cell biology.