Abstract 15491: Mitochondria Calcium Flux Plays an Important Role in Triggered Activity in Cardiomyocytes With Reduced Sarcoplasmic Reticulum Calcium Release
Introduction: Decreased ryanodine receptor 2 (RYR2) levels and reduced RYR2 activity has been detected in several heart failure (HF) models. Up to 50% of HF associated sudden cardiac deaths are caused by arrhythmia. The mechanisms linking HF, altered Ca2+ handling, and arrhythmic risk remain to be elucidated, however. Mitochondria play a role in Ca2+ handling, and we investigated whether altered mitochondrial Ca2+ handling contributed to arrhythmic risk in ventricular myocytes with reduced sarcoplasmic reticulum (SR) Ca2+ release.
Methods: Ventricular-like cardiomyocytes (CMs) were derived by differentiation for 19 days, from control (Wt) and RYR2 knockout (RYR2-/-) mouse embryonic stem cells. Action potentials (APs) were recorded by perforated current-clamp. Potassium, sodium calcium exchanger (NCX) and L-type Ca2+ currents were measured by whole cell voltage-clamp. Ca2+ transients were determined by fluorescent imaging using Fluo-4.
Results: RyR2-/- CMs showed less L-type Ca2+ current density and shorter AP duration at 90% repolarization (173.1 ± 22.7 ms vs. 274.3 ± 36.3 ms) than those of Wt CMs. Spontaneous Ca2+ transient amplitudes were considerably smaller in RyR2-/- CMs (0.74 ± 0.09 vs. 0.38 ± 0.02) and had prolonged transient duration at 50% amplitude (211 ± 19 ms vs. 613 ± 43 ms). RyR2-/- ventricular-like CMs displayed more delayed after-depolarizations (DADs, 83% vs. 0% in Wt, p<0.05). In spontaneously contracting RyR2-/- cells, Ca2+ transients were mediated by IP3R or mitochondrial Ca2+ release. In the presence of the IP3R blocker 2-APB, AP frequency decreased (from 1.3±0.1 to 0.5±0.2 Hz) supporting its role in pacemaker activity, but DADs remained. Caffeine depletion experiments showed other ryanodine receptor subtypes didn’t contribute Ca2+ release in RyR2-/- CMs. Block of mitochondrial Ca2+ uptake by Ru360 prevented DADs in RyR2-/- CMs, however. Conversely application of PPT, a MCU activator, enhanced DADs in RyR2-/- CMs.
Conclusions: DADs may occur in the presence of reduced SR Ca2+ release and seem to be dependent on mitochondrial Ca2+. Mitochondrial Ca2+ handling plays an important role in triggered activity in states of reduced SR Ca2+ release.
- © 2013 by American Heart Association, Inc.