Different actions of RyR2 open and closed channel block explained by a multiscale Ca²⁺ release model

RyR2 channels regulate Ca²⁺ release from the sarcoplasmic reticulum (SR), the Ca storage organelle in cardiac muscle. Altered RyR2 activity is a key factor promoting atrial and ventricular Ca-arrhythmias, but it is unclear how changes in RyR2 open probability underlie a sustained pro- or anti-arrhythmic affect against a background of "autoregulation" whereby SR load adjusts until the RyR2 Ca²⁺ leak once again balances Ca²⁺ uptake by SERCA2a. Accordingly, tetracaine, a closed channel blocker that increases RyR2 closed durations, increases SR Ca²⁺ load and promotes Ca²⁺ waves in CSQ knockout cells. However, flecainide, an open channel blocker that decreases RyR2 open durations, inhibits RyR2 activity with no change in SR Ca²⁺ load and has a sustained anti-arrhythmic action. In quiescent cells, flecainide increases Ca²⁺ spark frequency and decreases spark mass, whereas tetracaine decreases spark frequency but not spark mass. We present a multiscale, dynamic model for SR Ca²⁺ release and uptake in permeabilized cardiomyocytes using experimentally determined RyR2 gating kinetics and SERCA2a uptake that explains the initiation and termination of Ca²⁺ sparks and these different actions of open and closed RyR2 block on SR Ca²⁺ load and Ca²⁺ spark properties. Spark initiation results from opening of a single RyR2 followed by a regenerative phase of Ca²⁺-induced activation between RyR2s within the dyad cleft. Termination of spark fluorescence occurs as [Ca²⁺] in the terminal SR declines, reducing the RyR2 Ca²⁺ flux and dyad [Ca²⁺] to a point where RyR2s deactivate. Both RyR2 open and closed block produced similar acute inhibition of Ca²⁺ release during a spark with a consequent increase in SR load. Acute closed block also decreased spark frequency due to reduced frequency of RyR2 opening. This resulted in a much stronger inhibition of overall spark Ca²⁺ leak and a larger increase in SR load to achieve a new steady state.