Calmodulin (CaM), one of the accessory proteins of the cardiac ryanodine receptor (RyR2), is known to play a significant role in the channel regulation of the RyR2 However, the possible involvement of calmodulin in the pathogenic process of catecholaminergic polymorphic ventricular tachycardia (CPVT) has not been investigated In this study, we investigated the state of RyR2-bound CaM and channel dysfunctions using a knock-in (KI) mouse model with CPVT-linked RyR2 mutation (R24745) Without added effectors, the affinity of CaM binding to the RyR2 was indistinguishable between KI and WT hearts In response to cAMP (1 mu mol/L), the RyR2 phosphorylation at Ser2808 increased in both WT and KI hearts to the same extent However, cAMP caused a significant decrease of the CaM-binding affinity in KI hearts, but the affinity was unchanged in WT Dantrolene restored a normal level of CaM-binding affinity in the cAMP-treated KI hearts, suggesting that defective inter-domain interaction between the N-terminal domain and the central domain of the RyR2 (the target of therapeutic effect of dantrolene) is involved in the cAMP-induced reduction of the CaM-binding affinity In saponin-permeabilized cardiomyocytes, the addition of cAMP increased the frequency of spontaneous Ca2+ sparks to a significantly larger extent in KI cardiomyocytes than in WT cardiomyocytes, whereas the addition of a high concentration of CaM attenuated the aberrant increase of Ca2+ sparks In conclusion, CPVT mutation causes defective interdomain interaction, significant reduction in the ability of CaM binding to the RyR2, spontaneous Ca2+ leak, and then lethal arrhythmia (C) 2010 Elsevier Inc All rights reserved