Positive feedback between RyR phosphorylation and Ca leak promotes heterogeneous Ca 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 (Ca) 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 Ca 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 Ca cycling and RyR phosphorylation by Ca/calmodulin-dependent protein kinase II (CaMKII) create a positive feedback loop, which increases functional heterogeneity in the Ca spark size distribution. In large clusters, the Ca leak is substantial due to the large flux (number of channels recruited), leading to increased local Ca 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 Ca levels via the Na-Ca exchanger, further promoting the Ca leak and functional heterogeneity. We conclude that such positive feedback processes play a crucial role in arrhythmogenic Ca wave initiation and propagation, particularly in heart failure myocytes, where PTMs are often dysregulated.