Mutations in human cardiac calsequestrin (CASQ2), a high-capacity 
calcium-binding protein located in the sarcoplasmic reticulum (SR), have 
recently been linked to effort-induced ventricular arrhythmia and sudden death 
(catecholaminergic polymorphic ventricular tachycardia). However, the precise 
mechanisms through which these mutations affect SR function and lead to 
arrhythmia are presently unknown. In this study, we explored the effect of 
adenoviral-directed expression of a canine CASQ2 protein carrying the 
catecholaminergic polymorphic ventricular tachycardia-linked mutation D307H 
(CASQ2(D307H)) on Ca2+ signaling in adult rat myocytes. Total CASQ2 protein 
levels were consistently elevated approximately 4-fold in cells infected with 
adenoviruses expressing either wild-type CASQ2 (CASQ2(WT)) or CASQ2(D307H). 
Expression of CASQ2(D307H) reduced the Ca2+ storing capacity of the SR. In 
addition, the amplitude, duration, and rise time of macroscopic I(Ca)-induced 
Ca2+ transients and of spontaneous Ca2+ sparks were reduced significantly in 
myocytes expressing CASQ2(D307H). Myocytes expressing CASQ2(D307H) also 
displayed drastic disturbances of rhythmic oscillations in [Ca2+]i and membrane 
potential, with signs of delayed afterdepolarizations when undergoing periodic 
pacing and exposed to isoproterenol. Importantly, normal rhythmic activity was 
restored by loading the SR with the low-affinity Ca2+ buffer, citrate. Our data 
suggest that the arrhythmogenic CASQ2(D307H) mutation impairs SR Ca2+ storing 
and release functions and destabilizes the Ca2+-induced Ca2+ release mechanism 
by reducing the effective Ca2+ buffering inside the SR and/or by altering the 
responsiveness of the Ca2+ release channel complex to luminal Ca2+. These 
results establish at the cellular level the pathological link between CASQ2 
mutations and the predisposition to adrenergically mediated arrhythmias observed 
in patients carrying CASQ2 defects.