Aims: 

Propagating waves of elevated intracellular Ca²⁺ ([Ca²⁺]_i) are abnormal events that occur at the subcellular level in diverse models of heart dysfunction. Such Ca²⁺ waves activate the Ca²⁺ -dependent arrhythmogenic inward current that contributes to early and delayed afterdepolarizations. Although abnormally high levels of Ca²⁺ within the sarcoplasmic reticulum (SR) are suspected to underlie such Ca²⁺ waves, in some animal models of heart disease Ca²⁺ waves can develop when SR Ca²⁺ levels are relatively low. In this study we used sensitizers of RyR2 (the cardiac SR Ca²⁺ release channel) to activate Ca²⁺ waves in the presence and absence of SR Ca²⁺ overload.

Methods: 

Ca²⁺ waves initiated by sub-millisecond local UV photolysis of a caged caffeine analog or caged Ca²⁺ were viewed by confocal microscopy in cells loaded the Ca²⁺ indicator Fluo-4. Whole-cell patch-clamp technique was used to measure membrane current and control voltage.

Results: 

In isolated wild-type murine ventricular myocytes, localized UV photolysis initiated one of two responses: (1) a transitory but confined increase in [Ca²⁺]_i, or (2) a Ca²⁺ wave. Analysis suggests that, unlike Ca²⁺ waves seen in Ca²⁺ overload that can be initiated by a single Ca²⁺ spark, Ca²⁺ waves under normal SR Ca²⁺ load required the activation of a larger volume (40.2 ± 7.5 mm³) of Ca²⁺ release units.

Conclusions: 

Arrhythmogenic Ca²⁺ waves can be initiated without overloading the SR with Ca²⁺, but do require the activation of ~17 adjacent RYR2 clusters. How this may occur and the modeling implications of these findings will be discussed.