Reduced Ca Flux via Ryanodine Receptor Cluster Size or Unitary Current can both Promote Long-Lasting Ca Sparks

Abstract

Calcium (Ca) sparks are fundamental E-C coupling events with typical duration of 10-50 ms. Long-lasting Ca release events, lasting >10 times longer than typical Ca sparks, have been observed experimentally when ryanodine receptors (RyRs) are partially blocked by tetracaine or ruthenium red (Zima et al., 2008 BJ & Circ Res). However, the mechanism of these events is not fully understood. Here, we use a physiologically detailed mathematical model of subcellular Ca cycling, and show how RyR cluster size (or ‘effective’ cluster size when RyRs are partially blocked) can affect long-lasting Ca release events. The single cluster contains a few to several hundred RyRs, and we use a 4-state Markov model of the RyR. Each RyR opens stochastically and is regulated by cytosolic and luminal Ca. The number of RyR channels in the cluster, diffusion within the SR network, diffusion between network and junctional SR, SERCA uptake rate and RyR open probability were varied. In order for long-lasting release events, opening events within the cluster must occur continuously since the typical open time of the RyR is only a few milliseconds. We found (1) if the number of RyRs is too small, it is difficult to keep consecutive openings and stochastic attrition terminates the release, (2) if the number of RyRs is too large, the depletion of Ca from the junctional SR terminates the release, and (3) the involvement of moderate sized RyR clusters (∼8; or reduced flux/RyR) allows stable release flux lasting >500 ms, wherein local [Ca]SR can be maintained by intra-SR diffusion.