Calcium Mediated Mechanism of Early Afterdepolarizations in LQT2 Ventricular Myocytes

Abstract

Background: Loss of function mutations in hERG potassium channels underlie the long QT syndrome type 2 (LQT2). LQT2 is associated with fatal ventricular arrhythmias promoted by triggered activity in the form of early afterdepolarizations (EADs). However, the cellular mechanism of EAD formation remains unexplored.Methods: We have investigated the mechanism of EAD formation in LQT2 ventricular myocytes using a physiologically detailed computational model of calcium (Ca2+) cycling and membrane voltage dynamics. This model bridges the submicron scale of individual couplons of plasmalemmal L-type Ca2+ channels clusters and sarcoplasmic reticulum (SR) Ca2+ release units (CRUs) and the whole cell. We incorporate the novel experimental finding that ryanodine receptors (RyRs) Ca2+ release channels are remodeled in ventricular mycoytes isolated from LQT2 transgenic rabbits; RyRs are hyperphosphorylated leading to enhanced channel activity and hence increased Ca2+ leak.Results: Computer simulations with RyR hyperactivity modeled as an increased rate of channel opening show that hyperactivity is causally linked to EAD formation. Under stimulation with the β-adrenergic agonist isoproterenol (ISO), LQT2 myocytes with hyperactive RyRs exhibit EADs together with decreased SR load and Ca2+ transient (CaT) amplitude, while myocytes with normal RyR activity exhibit a prolonged action potential without reductions of SR load and CaT amplitude. Simulations show that RyR hyperactivity shortens RyR refractoriness at the CRU level, resulting in late aberrant Ca2+ releases during repolarization. Those releases promote onset of EADs by driving the forward mode Na+-Ca2+ exchanger NCX1 current, which slows repolarization and allows reactivation of L-type Ca2+ current. Modeling predictions are in good agreement with experimental observations, which show that EADs in ISO-stimulated LQT2 myocytes are accompanied by late Ca2+ releases together with decreased SR load and CaT amplitude, and that both late releases and EADs are abolished by inhibition of CaMKII.