Mitochondrial instability during regional ischemia–reperfusion underlies arrhythmias in monolayers of cardiomyocytes

Abstract

Regional depolarization of the mitochondrial network can alter cellular electrical excitability and increase the propensity for reentry, in part, through the opening of sarcolemmal KATP channels. Mitochondrial inner membrane potential (ΔΨm) instability or oscillation can be induced in myocytes by exposure to reactive oxygen species (ROS), laser excitation, or glutathione depletion, and is thought to be a major factor in arrhythmogenesis during ischemia-reperfusion. Nevertheless, the correlation between ΔΨm recovery kinetics and reperfusion-induced arrhythmias has been difficult to demonstrate experimentally. Here, we investigate the relationship between subcellular changes in ΔΨm, cellular glutathione redox potential, electrical excitability, and wave propagation during coverslip-induced ischemia-reperfusion (IR) in neonatal rat ventricular myocyte (NRVM) monolayers. Ischemia led to decreased action potential amplitude and duration followed by electrical inexcitability after ~ 15 min of ischemia. ΔΨm depolarization occurred in two phases during ischemia: in phase 1 (< 30 min ischemia), mitochondrial clusters within individual NRVMs depolarized, while phase 2 ΔΨm depolarization (30–60 min) was characterized by global functional collapse of the mitochondrial network across the whole ischemic region of the monolayer, typically involving a propagating metabolic wave. Oxidation of the glutathione (GSSG:GSH) redox potential occurred during ischemia, followed by recovery upon reperfusion (i.e., lifting the coverslip). ΔΨm recovered in the mitochondria of individual myocytes quite rapidly upon reperfusion (< 5 min), but was highly unstable, characterized by subcellular oscillations or flickering of clusters of mitochondria in NRVMs across the reperfused region. Electrical excitability also recovered in a heterogeneous manner, providing an arrhythmogenic substrate which led to formation of sustained reentry. Treatment with 4′-chlorodiazepam, a peripheral benzodiazepine receptor ligand, prevented ΔΨm oscillation, improved GSH recovery rate, and prevented reentry during reperfusion, indicating that stabilization of mitochondrial network dynamics is an important component of preventing post-ischemic arrhythmias.

New and revisited approaches to preserving the reperfused myocardium

“…Numerous studies have shown that cardioprotection is associated with sustained, polarized Δψ m ; this energized Δψ m is essential during metabolic insult to replenish endogenous antioxidants via NADPH-linked reactions, which allows for the downstream detoxification of free radicals 99,100 . Cardioprotection inferred by sustained Δψ m has been observed after exercise preconditioning 91 , after administering serofendic acid (a cardioprotective substance found in fetal calf serum which inhibits the oxidant-induced mitochondrial death pathway 101 ), or by blocking energy-dissipating ion channels within the mitochondrial inner membrane 102,103 .…”

Functional protection against cardiac diseases depends on ATP‐sensitive potassium channels

“…ΔΨm instability or oscillation induced by ROS led to cardiac arrhythmia. This mechanism is necessary to increase the dispersion of refractoriness, slow the conduction velocity, and create a regional excitation block …”

“…This mechanism is necessary to increase the dispersion of refractoriness, slow the conduction velocity, and create a regional excitation block. 28 Recent data have shown that mutations of K ATP channels in myocardial cells can directly result in arrhythmias. The E23K variant of KCNJ11, identified in patients with type 2 diabetes, 29 results in the frequent opening of K ATP channels, which is associated with a greater left ventricular size with hypertension.…”

Solving mitochondrial mysteries

“…In addition to these state of the art review articles this special issue also contains several original research articles [27][28][29][30]. Taken together this special issue provides up to date reviews as well as original research articles on some of the hottest topics in mitochondrial biology.…”