Catecholaminergic polymorphic ventricular tachycardia is directly linked to mutations in proteins (eg, type 2 ryanodine receptor [RyR2]^R4496C) responsible for intracellular Ca²⁺ homeostasis in the heart. However, the mechanism of Ca²⁺ release dysfunction underlying catecholaminergic polymorphic ventricular tachycardia has only been investigated in isolated cells but not in the in situ undisrupted myocardium.

We investigated in situ myocyte Ca²⁺ dynamics in intact Langendorff-perfused hearts (ex vivo) from wild-type and RyR2^R4496C+/− mice using laser scanning confocal microscopy. We found that myocytes from both wild-type and RyR2^R4496C+/− hearts displayed uniform, synchronized Ca²⁺ transients. Ca²⁺ transients from beat to beat were comparable in amplitude with identical activation and decay kinetics in wild-type and RyR2^R4496C+/− hearts, suggesting that excitation-contraction coupling between the sarcolemmal Ca²⁺ channels and mutated RyR2^R4496C+/− channels remains intact under baseline resting conditions. On adrenergic stimulation, RyR2^R4496C+/− hearts exhibited a high degree of Ca²⁺ release variability. The varied pattern of Ca²⁺ release was absent in single isolated myocytes, independent of cell cycle length, synchronized among neighboring myocytes, and correlated with catecholaminergic polymorphic ventricular tachycardia. A similar pattern of action potential variability, which was synchronized among neighboring myocytes, was also revealed under adrenergic stress in intact hearts but not in isolated myocytes.

Our studies using an in situ confocal imaging approach suggest that mutated RyR2s are functionally normal at rest but display a high degree of Ca²⁺ release variability on intense adrenergic stimulation. Ca²⁺ release variability is a Ca²⁺ release abnormality, resulting from electric defects rather than the failure of the Ca²⁺ release response to action potentials in mutated ventricular myocytes. Our data provide important insights into Ca²⁺ release and electric dysfunction in an established model of catecholaminergic polymorphic ventricular tachycardia.