Published online before print April 17, 2009, doi: 10.1161/CIRCEP.108.815654
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Original Articles |
The Calcium/Calmodulin/Kinase System and Arrhythmogenic Afterdepolarizations in Bradycardia-Related Acquired Long-QT Syndrome
From the Department of Medicine and Research Center, Montreal Heart Institute and Université de Montréal (X.Y.Q., Y.-H.Y., D.C., L.X., S.N.), Montreal, Quebec, Canada; the Department of Pharmacology and Therapeutics, McGill University (L.X., S.N.), Montreal, Quebec, Canada; First Cardiovascular Division, Chang Gung Memorial Hospital and Chang Gung University (Y.-H.Y.), Tao-Yuan, Taiwan; the Department of Cardiovascular Research, Research Institute of Environmental Medicine, Nagoya University, Japan (Y.T., I.K.); and the Department of Clinical Pharmacology (B.J.J.M.B.), University of Groningen, The Netherlands.
Correspondence to Stanley Nattel, MD, 5000 Belanger St East, Montréal, Québec, H1T 1C8, Canada. E-mail stanley.nattel{at}icm-mhi.org
Received August 19, 2008; accepted April 8, 2009.
Background— Sustained bradycardia is associated with long-QT syndrome in human beings and causes spontaneous torsades de pointes in rabbits with chronic atrioventricular block (CAVB), at least partly by downregulating delayed-rectifier K+-current to cause action potential (AP) prolongation. 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). ICaL density was unaffected by CAVB, but inactivation was shifted to more positive voltages, increasing the activation-inactivation overlap zone for ICaL 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 ICaL inactivation voltage and suppressed CAVB-induced EADs. Similarly, the calmodulin inhibitor W7 suppressed CAVB-induced ICaL inactivation voltage shifts and EADs, and a specific CaMKII inhibitory peptide prevented ICaL 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 of 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: long-QT syndrome • remodeling • calcium • ion channels • electrophysiology
CLINICAL PERSPECTIVE
Drs Qi and Yeh contributed equally to this work.The online-only Data Supplement is available at http://circep.ahajournals.org/cgi/content/full/CIRCEP.108.815654/DC1.