RATIONALE: Atrial fibrillation (AF) is the most common cardiac arrhythmia, 
however the mechanism(s) causing AF remain poorly understood and therapy is 
suboptimal. The ryanodine receptor (RyR2) is the major calcium (Ca2+) release 
channel on the sarcoplasmic reticulum (SR) required for excitation-contraction 
coupling in cardiac muscle.
OBJECTIVE: In the present study, we sought to determine whether intracellular 
diastolic SR Ca2+ leak via RyR2 plays a role in triggering AF and whether 
inhibiting this leak can prevent AF.
METHODS AND RESULTS: We generated 3 knock-in mice with mutations introduced into 
RyR2 that result in leaky channels and cause exercise induced polymorphic 
ventricular tachycardia in humans [catecholaminergic polymorphic ventricular 
tachycardia (CPVT)]. We examined AF susceptibility in these three CPVT mouse 
models harboring RyR2 mutations to explore the role of diastolic SR Ca2+ leak in 
AF. AF was stimulated with an intra-esophageal burst pacing protocol in the 3 
CPVT mouse models (RyR2-R2474S+/-, 70%; RyR2-N2386I+/-, 60%; RyR2-L433P+/-, 
35.71%) but not in wild-type (WT) mice (P<0.05). Consistent with these in vivo 
results, there was a significant diastolic SR Ca2+ leak in atrial myocytes 
isolated from the CPVT mouse models. Calstabin2 (FKBP12.6) is an RyR2 subunit 
that stabilizes the closed state of RyR2 and prevents a Ca2+ leak through the 
channel. Atrial RyR2 from RyR2-R2474S+/- mice were oxidized, and the RyR2 
macromolecular complex was depleted of calstabin2. The Rycal drug S107 
stabilizes the closed state of RyR2 by inhibiting the oxidation/phosphorylation 
induced dissociation of calstabin2 from the channel. S107 reduced the diastolic 
SR Ca2+ leak in atrial myocytes and decreased burst pacing-induced AF in vivo. 
S107 did not reduce the increased prevalence of burst pacing-induced AF in 
calstabin2-deficient mice, confirming that calstabin2 is required for the 
mechanism of action of the drug.
CONCLUSIONS: The present study demonstrates that RyR2-mediated diastolic SR Ca2+ 
leak in atrial myocytes is associated with AF in CPVT mice. Moreover, the Rycal 
S107 inhibited diastolic SR Ca2+ leak through RyR2 and pacing-induced AF 
associated with CPVT mutations.