Recovery of cardiac calcium release is controlled by sarcoplasmic reticulum refilling and ryanodine receptor sensitivity

Abstract

Aims

In heart cells, the mechanisms underlying refractoriness of the elementary units of sarcoplasmic reticulum (SR) Ca²⁺ release, Ca²⁺ sparks, remain unclear. We investigated local recovery of SR Ca²⁺ release using experimental measurements and mathematical modelling.

Methods and results

Repeated Ca²⁺ sparks were induced from individual clusters of ryanodine receptors (RyRs) in quiescent rat ventricular myocytes, and we examined how changes in RyR gating influenced the time-dependent recovery of Ca²⁺ spark amplitude and triggering probability. Repeated Ca²⁺ sparks from individual sites were analysed in the presence of 50 nM ryanodine with: (i) no additional agents (control); (ii) 50 µM caffeine to sensitize RyRs; (iii) 50 µM tetracaine to inhibit RyRs; or (iv) 100 nM isoproterenol to activate β-adrenergic receptors. Sensitization and inhibition of RyR clusters shortened and lengthened, respectively, the median interval between consecutive Ca²⁺ sparks (caffeine 239 ms; control 280 ms; tetracaine 453 ms). Recovery of Ca²⁺ spark amplitude, however, was exponential with a time constant of ∼100 ms in all cases. Isoproterenol both accelerated the recovery of Ca²⁺ spark amplitude (τ = 58 ms) and shortened the median interval between Ca²⁺ sparks (192 ms). The results were recapitulated by a mathematical model in which SR [Ca²⁺] depletion terminates Ca²⁺ sparks, but not by an alternative model based on limited depletion and Ca²⁺-dependent inactivation of RyRs.

Conclusion

Together, the results strongly suggest that: (i) local SR refilling controls Ca²⁺ spark amplitude recovery; (ii) Ca²⁺ spark triggering depends on both refilling and RyR sensitivity; and (iii) β-adrenergic stimulation influences both processes.

Ca²⁺ spark, Triggered activity, Calcium transient, Ventricular myocyte, Computer modelling