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

Key points 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. Abstract In mammalian cardiac myocytes, the elementary calcium releases triggered by step voltage stimuli manifest either as solitary or as twin spikes that vary widely in kinetics and amplitude for unknown reasons. Here we examined the variability of calcium spikes measured using line‐scanning confocal microscopy in patch‐clamped rat ventricular myocytes. Amplitude distributions of the single and of the first of twin spikes were broader than those of the second spikes. All could be best approximated by a sum of a few elementary Gaussian probability distribution functions. The latency distributions of the single and the first spikes were identical, much shorter and less variable than those of the second spikes. The multimodal distribution of spike amplitudes and the probability of occurrence of twin spikes were stochastically congruent with activation of only a few of the many RyR2 channels present in the release site cluster. The occurrence of twin release events was rare due to refractoriness of release, induced with a probability proportional to the number of RyR2s activated in the primary release event. We conclude that the variability of the elementary calcium release events supports a calcium signalling mechanism that arises from stochastics of RyR2 gating and from inactivation of local origin.