Positive feedback between RyR phosphorylation and Ca²⁺ leak promotes heterogeneous Ca²⁺ release

Structural heterogeneity in the distribution of ryanodine receptor (RyR) clusters in cardiac myocytes has been shown to have pro-arrhythmic effects. The presence of a mixture of large and small RyR clusters can potentiate arrhythmogenic calcium (Ca²⁺) waves. RyRs are subject to posttranslational modifications (PTMs), such as phosphorylation, that are linked to heart failure and other pathological conditions. This study aims to investigate how PTMs interact with the structural heterogeneity of RyR clusters and further increase heterogeneous Ca²⁺ release activities in cardiac myocytes. Using a physiologically detailed three-dimensional ventricular myocyte model containing approximately 2 million stochastic RyR channels, we simulated heterogeneous distributions of RyR clusters with and without PTMs. The results demonstrate that Ca²⁺ cycling and RyR phosphorylation by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) create a positive feedback loop, which increases functional heterogeneity in the Ca²⁺ spark size distribution. In large clusters, the Ca²⁺ leak is substantial due to the large flux (number of channels recruited), leading to increased local Ca²⁺ concentrations, CaMKII activation, and further RyR sensitization, amplifying the leak. Conversely, in small clusters, the leak is limited, and sensitization is restricted. Furthermore, CaMKII activation can enhance late sodium (Na⁺) currents, increasing Na⁺ influx and subsequently raising Ca²⁺ levels via the Na⁺-Ca²⁺ exchanger, further promoting the Ca²⁺ leak and functional heterogeneity. We conclude that such positive feedback processes play a crucial role in arrhythmogenic Ca²⁺ wave initiation and propagation, particularly in heart failure myocytes, where PTMs are often dysregulated.