Excitation-Dependent Intracellular Ca2+ Waves at the Border Zone of the Cryo-injured Rat Heart Revealed by Real-time Confocal Microscopy

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Intracellular Ca2+ waves, which develop under Ca2+-overloaded conditions of the injured myocardium, are regarded as an important substrate for triggered arrhythmias. However, little is known about whether Ca2+ waves arise or become proarrhythmic in the injured heart in situ. On the hypothesis that injured myocardium manifests frequent Ca2+ waves and produce an oscillatory [Ca2+]i rise leading to triggered activity, we applied cryo-injury to the epicardial surface of fluo 3-AM-loaded perfused rat hearts and analyzed spatiotemporal [Ca2+]i changes at border zones of the injured myocardium using real-time confocal microscopy. In intact regions Ca2+ waves barely emerged, whereas the border zone myocardium exhibited frequent Ca2+ waves, propagating randomly within the individual cells. Two different types of Ca2+ waves were identified: highly frequent waves (159.6+/-86.5 waves/min/cell, n=266) adjacent to the cryo-ablated regions, and less frequent waves (79.0+/-50.1 waves/min/cell, n=160) slightly farther (2 cells) away from the ablated regions (vicinities). The former Ca2+ waves emerged asynchronously to Ca2+ transients. Contrariwise, the latter depended on ventricular excitation: they vanished instantaneously on Ca2+ transients, but emerged more frequently and propagated more swiftly after cessation of higher-frequency pacing. Immediately after 3-Hz pacing, some cryo-injured hearts exhibited oscillatory [Ca2+]i rises; an instantaneous and synchronous elevation of [Ca2+]i followed by burst occurrence of Ca2+ waves with a gradual decrease in incidence and propagation velocity in a considerable number of cells. These observations indicate that myocardial injury induces Ca2+ waves in the heart, and that their synchronous occurrence could become a substrate for triggered arrhythmias.

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