Published Online
on April 17, 2009

A more recent version of this article appeared on June 1, 2009

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Original Article

The Calcium/Calmodulin/Kinase System and Arrhythmogenic Afterdepolarizations in Bradycardia-related Acquired Long QT Syndrome

XiaoYan Qi¹; Yung-Hsin Yeh²; Denis Chartier¹; Ling Xiao³; Yukiomi Tsuji⁴; Bianca J.J.M. Brundel⁵; Itsuo Kodama⁴ and Stanley Nattel^3,6

¹ Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada;
² Montreal Heart Inst and Uni of Montreal, Canada; Chang Gung Memorial Hosp and Uni, Tao-Yuan, Taiwan;
³ Montreal Heart Inst and Uni of Montreal and McGill Uni, Montreal, Quebec, Canada;
⁴ Research Institute of Environmental Medicine, Nagoya University, Japan;
⁵ University of Groningen, The Netherlands

⁶ E-mail: stanley.nattel{at}icm-mhi.org

Background—Sustained bradycardia is associated with long QT syndrome in man and causes spontaneous Torsades de Pointes (TdP) in rabbits with chronic atrioventricular block (CAVB), at least partly by downregulating delayed-rectifier K⁺-current to cause action potential (AP) prolongation. Here, we addressed the importance of altered Ca²⁺-handling, studying underlying mechanisms and consequences.

Methods and Results—We measured ventricular-cardiomyocyte [Ca²⁺]_i (Indo1-AM), L-type Ca²⁺-current (ICaL) and APs (whole-cell perforated-patch), and Ca²⁺-handling protein expression (immunoblot). CAVB increased AP-duration, cell-shortening, systolic [Ca²⁺]_i-transients and caffeine-induced [Ca²⁺]_i-release, and CAVB-cells showed spontaneous early afterdepolarizations (EADs). I_CaL-density was unaffected by CAVB, but inactivation was shifted to more positive voltages, increasing the activation-inactivation overlap-zone for I_CaL window-current. Ca²⁺-calmodulin-dependent protein kinase-II (CaMKII) autophosphorylation was enhanced in CAVB, indicating CaMKII-activation. CAVB also enhanced CaMKII-dependent phospholamban-phosphorylation and accelerated [Ca²⁺]_i-transient decay, consistent with phosphorylation-induced reductions in phospholamban-inhibition of sarcoplasmic-reticulum (SR) Ca²⁺-ATPase as a contributor to enhanced SR Ca²⁺-loading. The CaMKII-inhibitor KN93 reversed CAVB-induced changes in caffeine-releasable [Ca²⁺]_i and I_CaL inactivation-voltage, and suppressed CAVB-induced EADs. Similarly, the calmodulin-inhibitor W7 suppressed CAVB-induced I_CaL inactivation-voltage shifts and EADs, and a specific CaMKII inhibitory peptide prevented I_CaL inactivation-voltage shifts. The SR Ca²⁺-uptake inhibitor thapsigargin and the SR Ca²⁺-release inhibitor ryanodine also suppressed CAVB-induced EADs, consistent with an important role for SR Ca²⁺-loading and release in arrhythmogenesis. AP-duration changes reached a maximum after 1-week bradypacing, but peak alterations in CaMKII and [Ca²⁺]_i required 2 weeks, paralleling the EAD time-course.

Conclusions—CAVB-induced remodeling enhances [Ca²⁺]_i-load and activates the Ca²⁺-calmodulin-CaMKII system, producing [Ca²⁺]_i-handling abnormalities that contribute importantly to CAVB-induced arrhythmogenic afterdepolarizations.

Key Words: calcium • electrophysiology • ion channels • long-QT syndrome • remodeling

Author contributions: Drs XiaoYan Qi and Yung-Hsin Yeh contributed equally to this work.