Abstract 12435: Mitochondrial Calcium Flux Modulates Pacemaker Activity in Embryonic Stem Cell-Derived Cardiomyocytes

Abstract

Introduction: Ca2+ release from the sarcoplasmic reticulum (SR) is known to contribute to spontaneous activity via the cytoplasmic Na+-Ca2+ exchanger (NCX). Mitochondria are known to participate in Ca2+ cycling. We studied the role of mitochondrial Ca2+ flux in spontaneously activity of embryonic stem cells (ESC)-derived CMs. Methods: CMs were derived from wild type (Wt) and ryanodine receptor type 2 knockout (RYR2-/-) mouse ESCs and differentiated for 19-21 days. Action potentials (APs) were recorded by perforated whole cell current-clamp. Cytoplasmic Ca2+ transients were determined by fluorescent imaging. Mitochondrial Ca2+ transients were monitored simultaneously with APs. Results: While If blocker, 10μM ivabradine, had no significant effect on the automaticity, SR Ca2+ handling inhibitors, 10 μM ryanodine and 2 μM 2-APB, reduced the spontaneous beating rate to 56% and 73% respectively in Wt CMs. Inhibition of mitochondrial Ca2+ flux by 10 μM Ru360 showed a similar inhibitory effect on the pacemaker activity.To isolate the contribution of mitochondrial Ca2+, we used RYR2-/- CMs. In RYR2-/- CMs, beating rate was dependent on SR Ca2+ uptake and release from IP3 and the Na+/Ca2+ exchange current but was independent of sarcolemmal Ca2+ influx through L-type Ca2+ channels. In these cells, MCU inhibition by pharmacological or molecular biological means reduced beating rate. Depolarizing mitochondria prevented spontaneous beating. The mitochondrial NCX blocker, 1 or 3μM CGP-37157, terminated AP firings. Conclusions: Mitochondrial Ca2+ cycling plays a role in spontaneous beating activity in embryonic stem (ES) cell-derived cardiomyocytes. Mitochondrial dysfunction may contribute to altered pacemaker activity in cardiomyopathy.