Uncoupling the mitochondria facilitates alternans formation in the isolated rabbit heart

Abstract

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

Mitochondrial depolarization and asystole in the globally ischemic rabbit heart: coordinated response to interventions affecting energy balance

“…Assuming that this method faithfully reproduces the actual ⌬⌿ m loss during ischemia, we have to conclude that a significant ⌬⌿ m depolarization occurs relatively late (at ϳ25 min of ischemia). This is much later than it was previously thought (14) and suggests that ⌬⌿ m depolarization has little or no role in early ischemic events, such as contraction failure, action potential shortening (2), and action potential alternans (25). It is of interest that the observed timing of ⌬⌿ m loss is close to the minimal duration of ischemia in the rabbit heart after which myocardial infarction is detectable (20 min) (15).…”

“…The effects of blebbistatin were fully abolished by the inhibitor of glycolysis iodoacetate, suggesting energy preservation as the underlying mechanism. Importantly, even in the absence of blebbistatin, a significant ⌬⌿ m depolarization occurred relatively late (at ϳ25 min of ischemia), which is much later than it was previously thought (14) and suggests that ⌬⌿ m has little or no role in early ischemic events, such as contraction failure, action potential shortening (2), and action potential alternans (25). However, under all tested conditions, there was a direct correlation between t asys and t mito_depol , which is compatible with a causal relationship between the two events, although it may also indicate a mutual dependence on a critical level of energy depletion.…”

“…It has been implicated in loss of excitability during ischemia and subsequent postreperfusion ventricular fibrillation (VF) (2), and in the development of proarrhythmic action potential alternans during ischemia (25). Despite the obvious importance of ⌬⌿ m loss for the outcomes of an ischemic insult, the timing and the determinants of this event are still poorly understood.…”

Effects of Late Sodium Current Blockade on Ventricular Refibrillation in a Rabbit Model

“…Ca 2+ Amplitude alternans ratio: Ca 2+ amplitude alternans ratio was calculated as x/y, where x is the amplitude of smaller and y is the amplitude of larger Ca 2+ transient. 27,28 Thus, a ratio of 1 is indicative of no alternans. The alternans ratio was measured during pacing at a rate of 5, 5.5, or 6 Hz after initial LDVF and compared between ranolazine-and GS-967-treated and nontreated control hearts.…”

“…Metabolic changes in the myocardial tissue during ischemia because of uncoupling of mitochondria is a proarrhythmic substrate for development of ventricular arrhythmias. 27 It has been demonstrated that, after ischemia, ranolazine improves mitochondrial function and that ranolazine is also protective of mitochondria in ischemia reperfusion injury. [37][38][39] In a previous study, we have demonstrated that improvement of mitochondrial function is cardioprotective.…”