Abstract 16909: Sarcoplasmic Reticulum Calcium Leak and Enhanced NCX Increase Occurrence of Delayed Afterdepolarisations in Atrial Myocytes from Patients with Chronic Atrial Fibrillation
Introduction: Sarcoplasmic reticulum (SR) Ca2+ leak activates the inward Na+-Ca2+ exchange current (INCX) causing delayed afterdepolarsations (DADs) that promote arrhythmias. Whether this mechanism contributes to promotion of atrial fibrillation (AF) is unknown and was the object of this study.
Methods: Membrane currents and potentials (patch clamp) and [Ca2+]i (Fluo-3) were measured in right-atrial myocytes from sinus rhythm (Ctl) or chronic AF (cAF) patients. Tetracaine (1 mM) or single-channel recordings were used to quantify SR Ca2+ leak through ryanodine-receptor channels (RyR2). Proteins were measured by immunoblotting.
Results: Diastolic [Ca2+]i and SR Ca2+ content (integrated INCX during caffeine-induced [Ca2+]i transient [cCaT]) were unchanged, whereas diastolic SR Ca2+ leak was ∼50% higher in cAF than in Ctl (Figure). Calmodulin expression (+60%), CaMKII autophosphorylation (activation) at Thr287 (+40%) and Ser2808-phosphorylation (PKA/CaMKII-site) of RyR2 (+250%) were higher in cAF than in Ctl. Single-channel recordings from RyR2 reconstituted into lipid bilayers revealed enhanced open probability of RyR2 in cAF, providing a molecular correlate for the increased SR Ca2+ leak. The selective CaMKII blocker KN-93 (1 μM) decreased SR Ca2+ leak and reduced single-RyR2 channel open probability in cAF, suggesting a mechanistic link between enhanced CaMKII and increased SR Ca2+ leak in cAF. The decay of cCaT that is attributable to Ca2+ transport by NCX was faster and a defined SR Ca2+ release produced larger INCX in cAF vs Ctl, pointing to NCX up-regulation. Spontaneous (non-stimulated) Ca2+-release events accompanied by inward INCX currents together with DADs/triggered APs occurred more often in cAF than Ctl.
Conclusions: Elevated SR Ca2+ leak due to increased CaMKII-mediated RyR2 phosphorylation and generation of larger depolarising INCX for a given SR Ca2+ release cause cellular DADs/triggered activity that may promote AF.
- © 2010 by American Heart Association, Inc.