Recruiting RyRs to open in a Ca2+ release unit: Single-RyR gating properties make RyR group dynamics

Abstract

ABSTRACTIn cardiac myocytes, clusters of type-2 ryanodine receptors (RyR2s) release Ca2+ from the sarcoplasmic reticulum (SR) via a positive feedback mechanism where fluxed Ca2+ activates nearby RyRs. While the general principles of this are understood, less is known about how single-RyR gating properties define the RyR group dynamics in an array of many channels. Here, we examine this using simulations with three models of RyR gating that have identical open probabilities. The commonly-used two-state Markov gating model produces frequent, large, and long Ca2+ release events because the single exponential that defines its open time (OT) and closed time (CT) distributions reproduces the experimental data poorly. In contrast, simulations that utilize complete single-channel OT and CT distributions fit with multiple exponentials produce infrequent Ca2+ release events with far fewer open RyRs. Moreover, when experimentally-measured correlations between single-channel OTs and CTs are included, Ca2+ release events become even smaller. This occurs because the correlations produce a small but consistent bias against recruiting more RyRs to open during the middle of a Ca2+ release event, between the initiation and termination phases (which are unaltered compared to the uncorrelated simulations). Beyond the effects of full OT and CT distributions and OT/CT correlations on SR Ca2+ release, we also show that Ca2+ release events can terminate spontaneously without any reduction in SR [Ca2+] or physical coupling between RyRs when Ca2+ flux is below a threshold value. This both supports and extends the pernicious attrition/induction decay hypothesis that SR Ca2+ release events terminate below a threshold Ca2+ flux.STATEMENT OF SIGNIFICANCEThis work provides insights into RyR2-mediated Ca2+ release by a cluster of RyRs interacting only via their fluxed Ca2+. It is shown that: 1) common proxies like the single-RyR open probability versus cytosolic [Ca2+] curve and mean open or closed times are poor predictors of SR Ca2+ release dynamics; 2) Ca2+ release events can self-terminate below a flux threshold without any physical coupling between channels; 3) commonly-used two-state Markov gating models can produce qualitatively different Ca2+ release events (larger and longer) compared to simulations where complete single-channel open and closed time distributions are used; 4) correlations between a RyR’s open times and previous closed duration (and vice versa) significantly limit Ca2+ release by tamping down the number of open RyRs.