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Calcium spike variability in cardiac myocytes results from activation of small cohorts of ryanodine receptor 2 channels

•  Cardiac myocytes contract due to the upsurge in intracellular calcium concentration resulting from the activation of ryanodine receptor (RyR) channels residing at calcium releasing sites. •  Calcium release can be observed as local calcium spikes that vary in amplitude, time course and frequency of occurrence due to reasons that are not well understood. •  In this study we quantify the characteristics of calcium spikes in isolated myocytes, typify them by stochastic analysis and interpret them by mathematical modelling. •  We show that the observable properties of calcium spikes are compatible with gating properties, calcium/magnesium sensitivity and distribution of RyR channels, if only a few out of the many RyRs clustered at the release sites activate at a single release event. •  The proposed mechanism of stimulated calcium release clarifies the concepts of the structure and function of calcium releasing sites and of plasticity and dynamics of calcium signalling in cardiac development and disease.