In Silico Simulations Reveal That Ryr Distribution Affects the Dynamics of Calcium Release in Cardiac Myocytes

The dyads of cardiac myocytes contain ryanodine receptors (RYRs) that generate calcium sparks upon activation. To test how geometric factors of RYR distribution contribute to the formation of calcium sparks, which cannot be addressed experimentally, we performed in silico simulations on a large set of models of calcium release sites (CRSs). Our models covered the observed range of RYR number, density, and spatial arrangement. The calcium release function of CRSs was modeled by RYR openings, with an open probability dependent on concentrations of free Ca2+ and Mg2+ ions, in a rapidly buffered system, with a constant open RYR calcium current. We found that simulations of spontaneous sparks by repeatedly opening one of the RYRs in a CRS produced three different types of calcium release events (CREs) in any of the models. Transformation of simulated CREs into fluorescence signals yielded calcium sparks with characteristics close to the observed ones. CRE occurrence varied broadly with the spatial distribution of RYRs in the CRS but did not consistently correlate with RYR number, surface density, or calcium current. However, it correlated with RYR coupling strength, defined as the weighted product of RYR vicinity and calcium current, so that CRE characteristics of all models followed the same state-response function. This finding revealed the synergy between structure and function of CRSs in shaping dyad function. Lastly, rearrangements of RYRs simulating hypothetical experiments on splitting and compaction of a dyad revealed an increased propensity to generate spontaneous sparks and an overall increase in calcium release in smaller and more compact dyads, thus underlying the importance and physiological role of RYR arrangement in cardiac myocytes. © 2021 Iaparov et al.The research was supported by Russian Foundation for Basic Research according to research project no. 18-31-00153 (development of the computational model of CRS); by the Government of the Russian Federation (program 02.A03.21.0006); by the Ministry of Science and Higher Education of the Russian Federation (project no. FEUZ-2020-0054); by the Slovak Research and Development Agency (project APVV-15-0302); by SAV-TUBITAK (project JRP/2019/836/RyRinHeart); and by the Operational Program Integrated Infrastructure for the project “Long-term strategic research of prevention, intervention, and mechanisms of obesity and its comorbidities,” ITMS: 313011V344, co-financed by the European Regional Development Fund. The authors declare no competing financial interests