Balancing SR Ca²⁺ uptake and release in the cycle of heart rhythm

Heart contraction is normally initiated by spontaneous action potentials in the sinoatrial node (SAN, the pacemaker region), which propagate through the heart via electrical connections between cells. Proper timing of initiation and propagation of action potentials is crucial for the coordinated contraction of atria and ventricles necessary for blood circulation.

The cardiac action potential triggers a rise in cytoplasmic [Ca²⁺] that stimulates muscle contraction (Bers, 2002). This occurs by opening voltage‐dependent L‐type Ca²⁺ channels (LCCs) in the sarcolemma and transverse tubules. The resulting influx of Ca²⁺ into the cytoplasm activates Ca²⁺ release in the sarcoplasmic reticulum (SR) at specialised intracellular synapses (dyad junctions) between transverse tubules and SR. Elevated [Ca²⁺] in the cytoplasm of the dyad cleft activates cardiac Ca²⁺ release channels (Type 2 ryanodine receptors, RyR2) in the SR membrane via their cytoplasmic facing activation sites. The subsequent release of Ca²⁺ from the SR into the dyad cleft feeds back on RyR2 to cause further activation (Fabiato, 1983). This Ca²⁺‐induced Ca²⁺ release (CICR) provides a strong positive feedback on Ca²⁺ release that is ultimately broken by Ca²⁺ depletion within subcompartments of the SR associated with the dyad junction (Guo et al. 2012; Laver et al. 2013). Once RyR2s deactivate, [Ca²⁺] in the cytoplasm declines as Ca²⁺ is either pumped back into the SR by ATP driven Ca²⁺ pumps in the SR membrane (SERCA2a) or extruded from the cell by the Na⁺/Ca²⁺ exchanger (NCX) in the sarcolemma, allowing muscle relaxation.

The cardiac action potential normally drives the cyclic release and uptake of Ca²⁺ by the SR (Ca²⁺ cycling) in ventricular cells, as described above. However, in cells of the SAN, SERCA2a can increase [Ca²⁺] in the SR lumen to the point where it can activate RyR2 via luminal facing sites on RyR2. The NCX, that extrudes 1 Ca²⁺ in exchange for 3 Na⁺, contributes to depolarisation of the sarcolemma and the triggering of action potentials, thus providing a link by which Ca²⁺ cycling can advance or retard spontaneous action potentials. There is now substantial evidence that this process plays a key role in heart pacemaking and that coordinated increases in RyR2 and SERCA2a activity during stress and exercise contribute to increased heart rate (Rigg and Terrar, 1996; Ju and Allen, 2007; Lakatta and DiFrancesco, 2009). However, pathological over‐activation of RyR2 due to inherited mutations in RyR2, certain drugs or chemical modifications of RyR2 that occur during or following ischaemia (periods of reduced blood flow) or during heart failure causes heart arrhythmias and sudden cardiac death (Blayney and Lai, 2009; Belevych et al. 2012). Thus, excess RyR2 activity tends to promote SAN‐like Ca²⁺ cycling in ventricular myocytes, leading to alternative foci for pacemaking and arrhythmia.

Our understanding of the role of SR Ca²⁺ uptake in maintaining heart rhythm is hampered by conflicting experimental findings. Many studies show that increasing Ca²⁺ uptake by increased SERCA2a expression protects against Ca²⁺‐driven action potentials and arrhythmias and that inhibition of SERCA2a is proarrhythmic, while other studies show that decreasing Ca²⁺ uptake is protective. The paper by Mazzocchi et al. (2016) in this issue of The Journal of Physiology has addressed the role of SR Ca²⁺ uptake in maintaining heart rhythm using both experimental and computational modelling approaches. They used a knockout mouse, deficient in the SERCA2a inhibiting protein, phospholamban (PLN), to assess the effects of SERCA2a up‐regulation on the incidence of adrenergically induced arrhythmic events in intact hearts and on Ca²⁺ cycling and action potentials in ventricular cardiac cells. By crossing PLN knockout mice with RyR2 knockin mice containing a pseudo‐phosphorylation S2814D mutation, they were able to test if increased Ca²⁺ uptake could prevent arrhythmic events due to excess Ca²⁺ release. Their resultant data confirmed this hypothesis showing that it is possible to compensate the pathological effects of excess Ca²⁺ release by increasing Ca²⁺ uptake.

From the literature, it appears that the roles of the main Ca²⁺ transport proteins (RyR2, SERCA2a, NCX and LCC) involve complex interactions between their transport functions as well as Ca²⁺ diffusion and buffering in the cytoplasm and SR lumen. To understand these interactions and their role in heart function, Mazzocchi et al. rely on a computer model of human ventricular myocytes that incorporates the function of these transporters into a simplified dyad and cell structure. Their model simulates their experimentally observed properties of cardiac action potentials, Ca²⁺ cycling and the incidence of arrhythmia events in isolated myocytes from wild‐type and genetically modified mice. Importantly, this model successfully predicts their experimental observation that increased SERCA2a activity in the PLN‐deficient mouse protects against arrhythmias generated by excess Ca²⁺ release in the S2814D mutation in RyR2. A particularly interesting output of their model was a heat map of arrhythmia incidence in relation to imposed changes in SR Ca²⁺ uptake and release. The map shows an island of increased arrhythmia incidence between 50% and 600% increase in RyR2 activity and between ±20% changes in SERCA2a activity. In other words, either increasing or decreasing SERCA2a activity by more than 20% can rescue the S2814D mouse from arrhythmia events. Interrogation of their model revealed that increasing SERCA2a activity increases the rate of Ca²⁺ removal from the cytoplasm, both decreasing cytoplasmic [Ca²⁺] and stimulating NCX interactions with action potentials, whereas decreasing SERCA2a activity reduces Ca²⁺ loading of the SR and hence luminal [Ca²⁺] and resultant RyR2 activity, these changes also resulting in a decreased cytoplasmic [Ca²⁺]. The predictions go some way to explaining the contradictory experimental findings on the effect of SERCA2a activity on heart rhythm and provide an outstanding example of how experimental studies in combination with computational modelling can illuminate complex cellular processes.

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Competing interests

None declared.