Smith RM, Visweswaran R, Talkachova I, Wothe JK, Tolkacheva EG. Uncoupling the mitochondria facilitates alternans formation in the isolated rabbit heart. Am J Physiol Heart Circ Physiol 305: H9 -H18, 2013. First published May 3, 2013 doi:10.1152/ajpheart.00915.2012.-Alternans of action potential duration (APD) and intracellular calcium ([Ca 2ϩ ]i) transients in the whole heart are thought to be markers of increased propensity to ventricular fibrillation during ischemia-reperfusion injuries. During ischemia, ATP production is affected and the mitochondria become uncoupled, which may affect alternans formation in the heart. The aim of our study was to investigate the role of mitochondria on the formation of APD and [Ca 2ϩ ]i alternans in the isolated rabbit heart. We performed dual voltage and [Ca 2ϩ ]i optical mapping of isolated rabbit hearts under control conditions, global no-flow ischemia (n ϭ 6), and after treatment with 50 nM of the mitochondrial uncoupler FCCP (n ϭ 6). We investigated the formation of alternans of APD, [Ca 2ϩ ]i amplitude (CaA), and [Ca 2ϩ ]i duration (CaD) under different rates of pacing. We found that treatment with FCCP leads to the early occurrence of APD, CaD, and CaA alternans; an increase of intraventricular APD but not CaD heterogeneity; and significant reduction in conduction velocity compared with that of control. Furthermore, we demonstrated that FCCP and global ischemia have similar effects on the prolongation of [Ca 2ϩ ]i transients, whereas ischemia induces a significantly larger reduction of APD compared with that in FCCP treatment. In conclusion, our results demonstrate that uncoupling of mitochondria leads to an earlier occurrence of alternans in the heart. Thus, in conditions of mitochondrial stress, as seen during myocardial ischemia, uncoupled mitochondria may be responsible for the formation of both APD and [Ca 2ϩ ]i alternans in the heart, which in turn creates a substrate for ventricular arrhythmias.alternans; FCCP; calcium; action potential; mitochondria; ischemia T-WAVE ALTERNANS and alternans of action potential duration (APD) in the whole heart are thought to be markers of increased propensity to ventricular arrhythmias during ischemiareperfusion injuries (21,29,40). Repolarization T-wave alternans has been considered to be a strong marker of electrical instability and a precursor for ventricular arrhythmias (25,42,46,51,53), which is a major cause of sudden cardiac death (3, 4, 63). At the whole heart level, T-wave alternans, seen on the electrocardiogram, has been shown to correspond to a beat-tobeat variation in the APD at the single-cell level, a phenomenon known as APD alternans (46,63). It has been shown that APD and intracellular calcium ([Ca 2ϩ ] i ) transients are closely related. An action potential triggers a calcium influx leading to a release of calcium from the sarcoplasmic reticulum, whereas [Ca 2ϩ

Alternans of action potential duration (APD) and intracellular calcium ([Ca 2+ ] i ) transients in the whole heart are thought to be markers of increased propensity to ventricular fibrillation during ischemia reperfusion injuries. During ischemia, ATP production is affected and the mitochondria become uncoupled. This uncoupling may play an important role in alternans formation in the heart. The aim of our study was to investigate the role of mitochondrial depolarization on the formation of APD and [Ca 2+ ] i alternans in the isolated rabbit heart. We performed dual voltage and [Ca 2+ ] i optical mapping of isolated rabbit hearts under control conditions, global no-flow ischemia (n=6), and after treatment with 50 nM of the mitochondrial uncoupler FCCP (n=6). We investigated the formation of alternans of APD, [Ca 2+ ] i amplitude (CaA), and [Ca 2+ ] i duration (CaD) in the heart, under different rates of pacing. We found that treatment with FCCP leads to the early occurrence of APD (192 ± 18 ms vs 136 ± 6 ms, p<0.05), CaD (182 ± 20 ms vs 133 ± 6 ms, p<0.05), and CaA (152 ± 5 ms vs 141 ± 3 ms, p<0.05) alternans, and an increase of intraventricular APD (0.44 ± 0.02 vs 0.2 ± 0.05, p<0.05) but not CaD (0.20 ± 0.03 vs 0.16 ± 0.03, p=N/S) heterogeneity, when compared to control. On the other hand, FCCP does not affect the conduction velocity in the heart (0.95 ± 0.06 m/s vs 1.05 ± 0.11 m/s, p=N/S). Furthermore, we demonstrated that FCCP and global ischemia have similar effects on the prolongation of [Ca 2+ ] i transients, whereas ischemia induces a significantly larger reduction of APD compared to FCCP treatment. Our results demonstrate that uncoupling of mitochondria leads to earlier occurrence of alternans in the heart. Thus, in conditions of mitochondrial stress, as seen during myocardial ischemia, uncoupled mitochondria may be responsible for the formation of both APD and [Ca 2+ ] i alternans in the heart, which in turn creates a substrate conducive to formation of ventricular arrhythmias.