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Triggered arrhythmias due to spontaneous cytoplasmic calcium oscillations occur in a variety of disease 28 conditions; however, their cellular mechanisms in tissue are not clear. We hypothesize that spontaneous 29 calcium oscillations in the whole heart are due to calcium release from the sarcoplasmic reticulum (SR) 30 and are facilitated by calcium diffusion through gap junctions. Optical mapping of cytoplasmic calcium 31 from Langendorff perfused guinea pig hearts (n= 10) was performed using oxygenated Tyrode’s solution 32 (mM): 140 NaCl, 0.7 MgCl, 4.5 KCl, 5.5 dextrose, 5 HEPES and 5.5 CaCl2 (pH 7.45, 34 C). Rapid pacing 33 was used to induce diastolic calcium oscillations. In all preparations, pacing-induced multi-cellular 34 diastolic calcium oscillations (m-SCR) occurred across most of the mapping field, at all pacing rates 35 tested. Ryanodine (1 µM) eliminated all m-SCR activity. Low dose caffeine …