ATRIAL FIBRILLATION INDUCES A MITOCHONDRIAL STRESS RESPONSE WHICH FACILITATES SUSTAINED RAPID CONDUCTION

Abstract

Atrial Fibrillation (AF) is the most common sustained cardiac arrhythmia. A progressive disorder, AF begins with brief paroxysms, becoming increasingly persistent and resistant to therapies, ultimately becoming permanent. At the cellular level, fibrillation quickly results in functional changes in human atrium which permit the ultra-rapid conduction of multiple re-entrant waves and/or rotors which previously could not have been maintained. Ultra-rapid conduction results in ultra-rapid calcium flux and increased oxidative stress which demands a sustained increase in mitochondrial respiration for cell survival. We hypothesized that sustained high frequency stimulation results in a mitochondrial stress response which subsequently enables sustained fibrillatory conduction. We performed rapid (5 Hz) electrical field stimulation (EFS) experiments for 24 hours at 37oC, “fibrillatory stress”, or 1 Hz “control”, in cultured HL-1 cells. These atrial cells maintain an atrial phenotype, beating spontaneously in vitro at 0.5- 1 Hz. HL-1 cells have previously been shown by us, and others, to demonstrate electrophysiological remodeling and altered gene transcription in response to fibrillatory stress similar to those observed in human atrial myocytes isolated from patients suffering from persistent AF. Electron and confocal microscopy were used to perform morphometric analysis and to record calcium release events, oxidative stress and mitochondrial potential (Ψm). Fibrillatory stress resulted in enlarged hyper-fused mitochondria, increased expression of mitofusin-2 and co-localization of mitochondria with the sarcoplasmic reticulum (n= 50, p< 0.001 for all comparisons). Following fibrillatory stress, Ψm became synchronized with calcium release events and prevented detection of both mitochondrial and cytoplasmic oxidative stress upon subsequent rapid activation (n= 50, p< 0.001 for all comparisons) [Figure 1]. These observations were reproducible when fibrillatory stress was performed in the presence of reducing agents or the L-type calcium antagonist Verapamil. Sustained fibrillatory stress in our cell model resulted in a mitochondrial stress response which was associated with enhanced structural and functional co-ordination with intracellular calcium handling. These changes appeared to facilitate subsequent rapid activation without inducing oxidative stress. This “mitochondrial remodeling” process did not appear to be mediated by oxidative stress or sarcolemmal L-type calcium channels. These data suggest that the mitochondrial stress response may represent a novel mechanism to prevent AF progression. Much further work is required to determine whether the mitochondrial stress response is truly arrhythmogenic.