Aims: 

Unlike healthy cells, failing cardiomyocytes typically exhibit a disorganised t-tubule network. This may cause disrupted excitation-contraction coupling, in which opening of ryanodine receptors (RyRs) may be triggered by Ca²⁺ diffusing from neighbouring RyRs rather than by depolarisation-induced Ca²⁺ influx. Ca²⁺ release from these RyRs is delayed compared to depolarisation-induced release and therefore reduces synchrony of Ca²⁺ release across the cell. In failing cardiomyocytes, reduced expression of the sarcoplasmic reticulum (SR) Ca²⁺ AT-Pase (SERCA) pump may also contribute to dyssynchrony by reducing the amount of Ca²⁺ available for release. We investigated Ca²⁺ release synchrony in healthy, failing and SERCA2 KO cells using experiments and mathematical modelling.

Methods: 

Experiments were performed on cardiomyocytes isolated from conditional SERCA2 KO mice (KO) 6 days following gene disruption. SERCA2^flox/flox (FF) mice served as controls. Ca²⁺ release synchrony was (i) analysed experimentally using confocal fluorescence imaging, and (ii) investigated theoretically using a mathematical model that describes Ca²⁺ influx and release, diffusion, and uptake from cytosol into SR.

Results: 

Theoretically, dyssynchrony increased with increasing t-tubular network disorganisation, and with decreasing SERCA pump rate. Experimentally, SERCA KO myocytes did not show more dyssynchrony of Ca²⁺ release than FF, contradicting model predictions. By assuming a simultaneous increased sensitisation of RyR in the KO, model and experiment were reconciled.

Conclusion: 

Both t-tubule disorganisation and reduction of SERCA function promote Ca²⁺ release dyssynchrony. However, increased RyR sensitivity can compensate for reduced SR content to maintain Ca²⁺ release uniformity.