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 Ca(2+) handling, studying underlying mechanisms and consequences.


RESULTS
We measured ventricular cardiomyocyte [Ca(2+)](i) (Indo1-AM), L-type Ca(2+)-current (I(CaL)) and APs (whole-cell perforated-patch), and Ca(2+)-handling protein expression (immunoblot). CAVB increased AP duration, cell shortening, systolic [Ca(2+)](i) transients, and caffeine-induced [Ca(2+)](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. Ca(2+)-calmodulin-dependent protein kinase-II (CaMKII) autophosphorylation was enhanced in CAVB, indicating CaMKII activation. CAVB also enhanced CaMKII-dependent phospholamban-phosphorylation and accelerated [Ca(2+)](i)-transient decay, consistent with phosphorylation-induced reductions in phospholamban inhibition of sarcoplasmic reticulum (SR) Ca(2+)-ATPase as a contributor to enhanced SR Ca(2+) loading. The CaMKII-inhibitor KN93 reversed CAVB-induced changes in caffeine-releasable [Ca(2+)](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 Ca(2+)-uptake inhibitor thapsigargin and the SR Ca(2+) release inhibitor ryanodine also suppressed CAVB-induced EADs, consistent with an important role for SR Ca(2+) loading and release in arrhythmogenesis. AP-duration changes reached a maximum after 1 week of bradypacing, but peak alterations in CaMKII and [Ca(2+)](i) required 2 weeks, paralleling the EAD time course.


CONCLUSIONS
CAVB-induced remodeling enhances [Ca(2+)](i) load and activates the Ca(2+)-calmodulin-CaMKII system, producing [Ca(2+)](i)-handling abnormalities that contribute importantly to CAVB-induced arrhythmogenic afterdepolarizations.
Qi
XiaoYan
X
Department of Medicine and Research Center, Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada.
Yeh
Yung-Hsin
YH
Chartier
Denis
D
Xiao
Ling
L
Tsuji
Yukiomi
Y
Brundel
Bianca J J M
BJ
Kodama
Itsuo
I
Nattel
Stanley
S
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D054537:Atrioventricular Block / Q000378:metabolism / Q000503:physiopathology; D001919:Bradycardia / Q000378:metabolism / Q000503:physiopathology*; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000502:physiology; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2 / Q000378:metabolism*; D002304:Cardiac Pacing, Artificial; D004195:Disease Models, Animal; D008133:Long QT Syndrome / Q000378:metabolism / Q000503:physiopathology*; D009200:Myocardial Contraction / Q000502:physiology; D032383:Myocytes, Cardiac / Q000502:physiology*; D018408:Patch-Clamp Techniques; D011817:Rabbits; D016171:Torsades de Pointes / Q000378:metabolism / Q000503:physiopathology*
D020746:Calcium Channels, L-Type; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D002118:Calcium