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

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) (Indol-AM), L-type Ca2+-current (I-CaL) 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 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. (Circ Arrhythmia Electrophysiol. 2009;2:295-304.)