Ca2+ sparks were first detected as spontaneous, highly localized elevations of Ca2+ indicator fluorescence in confocal microscope images of rat cardiac myocytes studied under resting conditions (Cheng et al., 1993). It was immediately suggested that these events are likely to reflect the localized release of Ca2+ from a small cluster of sarcoplasmic reticulum (SR) Ca2+ release channels or perhaps even from a single SR channel (Cheng et al, 1993). Ca2+ sparks were found to occur at increased rates during small depolarizations of rat cardiac myocytes, leading to the concept that the macroscopic [Ca2+] transient during larger depolarizations of cardiac myocytes might be due to the spatio-temporal summation of such events occurring at high frequencies throughout the myocyte (Cannell et al., 1993, 1995; López-López et al., 1994).

Ca2+ sparks also occur in frog skeletal muscle fibers,...

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