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Abstract-Although Ca 2ϩ waves in cardiac myocytes are regarded as arrhythmogenic substrates, their properties in the heart in situ are poorly understood. On the hypothesis that Ca 2ϩ waves in the heart behave diversely and some of them influence the cardiac function, we analyzed their incidence, propagation velocity, and intercellular propagation at the subepicardial myocardium of fluo 3-loaded rat whole hearts using real-time laser scanning confocal microscopy. We classified Ca 2ϩ waves into 3 types. ), with a velocity of 84 m/s, a decline half-time (t 1/2 ) of 0.16 seconds, and rare intercellular propagation (propagation ratio Ͻ0.06) (sporadic wave). In contrast, in presumably Ca 2ϩ -overloaded regions showing higher fluorescent intensity (113% versus the intact regions), Ca 2ϩ waves occurred at 28 waves ⅐ min Ϫ1 ⅐ cell Ϫ1 under quiescence with a higher velocity (116 m/s), longer decline time (t 1/2 ϭ0.41 second), and occasional intercellular propagation (propagation ratioϭ0.23) (Ca 2ϩ -overloaded wave). In regions with much higher fluorescent intensity (124% versus the intact region), Ca 2ϩ waves occurred with a high incidence (133 waves ⅐ min Ϫ1 ⅐ cell Ϫ1 ) and little intercellular propagation (agonal wave). We conclude that the spatiotemporal properties of Ca 2ϩ waves in the heart are diverse and modulated by the Ca 2ϩ -loading state. The sporadic waves would not affect cardiac function, but prevalent Ca 2ϩ -overloaded and agonal waves may induce contractile failure and arrhythmias.

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Here, we present an optical technique that can induce waves of calcium ion concentration in live biological cells. Ca2+ waves were induced by femtosecond pulsed-laser illumination. Living HeLa cells were exposed to focused 140 fs pulses of 780 nm wavelength at 30 mW average power. Ca2+ waves were imaged by fluorescence and were observed to propagate from the laser focal point inside the cell. Photoinduced generation of Ca2+ waves can be performed at any point inside the cell, an improvement over previous mechanical or biochemical stimulation techniques.