A New Simplified 3D Model of Cardiac Pacemaker Cell Based on Superresolution Structured Illumination Microscopy (SIM)

Abstract

The contribution of diastolic Local Calcium-Releases (LCRs) to sinoatrial-node-cell pacemaker function is presently approximated by three numerical models: 1)“common-pool” (Maltsev-Lakatta,2009); 2)“2D” (Anna Maltsev et al.,2011-2013); 3)“3D” (Stern et al.,2014). While the most advanced 3D-model describes stochastic states of each ryanodine receptor (RyR) and L-type calcium-channel, its high computational demand prevents parametric sensitivity analyses. Here we developed a new 3D-model having a lower computational demand, but reproducing all essential features of calcium dynamics measured in isolated rabbit SA node cells by SIM (achieving approximately double the resolution of conventional microscopy). Our cell-cross-section SIM measurements revealed that LCRs occur mainly within ∼1 µm under the plasma membrane, in line with immunofluorescence data on RyR cluster localization. Therefore, the model cell interior is approximated by only three layers of diffusively linked intracellular voxels: submembrane (20 nm), ring (1 µm), and core cylinder. Each submembrane RyR cluster is approximated as a Calcium-Release Unit (CRU) residing within the respective junctional SR linked to free SR which pumps/collects cytosolic calcium. Instead of assuming a fixed restitution period like prior 2D-models, the release activation and termination are controlled in each CRU by local calcium-dependent mechanisms (based on RyR interactions via Calium-Induced-Calcium-Release, reported recently). The new model simulations are substantially faster (vs. the original 3D-model), but predict all essential features of LCRs crucial for pacemaker rate autonomic modulation. Thus, super-resolution SIM allowed fine localization of calcium-dynamics and validated the new model of integrated cardiac pacemaker cell function at the level of individual CRUs, filling an important niche between individual-molecule-level detail and common pool models (lacking LCRs). This new faster 3D-model allows parametric sensitivity analyses and provides a new mechanistic formulation of CRU function that is important for multi-scale modeling of heart function.