Role of Mitochondria in Cardiac Arrhythmias

Mitochondria are the major organelles responsible for providing energy to the cardiac myocytes. In ischemic conditions, the supply of nutrients and oxygen to a region of the heart, or the whole organ, is interrupted. Reperfusion, while critically important for tissue survival, can cause more damage and can lead to arrhythmias and sudden cardiac death. Dysfunctional mitochondria are known to contribute to ischemia/reperfusion injury, but more specifically, spatiotemporal heterogeneity of recovery of mitochondrial energetics has been suggested to cause metabolic sinks of current, which act to either shorten the wavelength of, or block, the electrical excitation wave. In this thesis, I examine how the failure of mitochondrial energetics can lead to electrical irregularity and arrhythmias. Monolayer cultures of neonatal rat ventricular myocytes were used for the experiments. Sarcolemmal voltage was recorded with optical mapping using the voltage-sensitive fluorescent dye, di-4-ANEPPS. Mitochondrial function was observed using the potentiometric fluorescent dye Tetramethyl Rhodamine Methyl Ester. Metabolic sinks were induced by chemically depolarizing mitochondria using a local perfusion system. Ischemia/reperfusion was modeled using a newly developed method of coverslip-induction of ischemia/reperfusion. Regional depolarization of the mitochondrial network, either through the use of a chemical uncoupler or after ischemia/reperfusion of the monolayers, resulted in an increased propensity for arrhythmias. Reperfusion of the monolayer after one hour of ischemia initiated dynamic instability of the mitochondrial network consisting of intracellular oscillations or global collapse of mitochondrial inner membrane potential (ΔΨm) along with reentrant arrhythmias. Compounds which stabilized the mitochondria prevented reentry and electrical instability. Uncoupler-mediated regional depolarization of mitochondria to induce metabolic sinks also caused inexcitability and reentry, which were significantly prevented by pharmacological inhibition of ATP-sensitive K+ channels (KATP). Mitochondrial function is a major factor in determining the fate of an ischemic heart. In our in vitro model system, mitochondrial instability was demonstrated to be present during reperfusion and was highly correlated with electrical instability. Spatiotemporal heterogeneity in ΔΨm contributes to dispersion of repolarization and, in some cases, can also contribute to ectopic electrical activity. The results support the hypothesis that mitochondria are important targets for therapeutic intervention to prevent post-ischemic arrhythmias and sudden cardiac death

Role of Mitochondria in Cardiac Arrhythmias

Mitochondria are the major organelles responsible for providing energy to the cardiac myocytes. In ischemic conditions, the supply of nutrients and oxygen to a region of the heart, or the whole organ, is interrupted. Reperfusion, while critically important for tissue survival, can cause more damage and can lead to arrhythmias and sudden cardiac death. Dysfunctional mitochondria are known to contribute to ischemia/reperfusion injury, but more specifically, spatiotemporal heterogeneity of recovery of mitochondrial energetics has been suggested to cause metabolic sinks of current, which act to either shorten the wavelength of, or block, the electrical excitation wave. In this thesis, I examine how the failure of mitochondrial energetics can lead to electrical irregularity and arrhythmias. Monolayer cultures of neonatal rat ventricular myocytes were used for the experiments. Sarcolemmal voltage was recorded with optical mapping using the voltage-sensitive fluorescent dye, di-4-ANEPPS. Mitochondrial function was observed using the potentiometric fluorescent dye Tetramethyl Rhodamine Methyl Ester. Metabolic sinks were induced by chemically depolarizing mitochondria using a local perfusion system. Ischemia/reperfusion was modeled using a newly developed method of coverslip-induction of ischemia/reperfusion. Regional depolarization of the mitochondrial network, either through the use of a chemical uncoupler or after ischemia/reperfusion of the monolayers, resulted in an increased propensity for arrhythmias. Reperfusion of the monolayer after one hour of ischemia initiated dynamic instability of the mitochondrial network consisting of intracellular oscillations or global collapse of mitochondrial inner membrane potential (ΔΨm) along with reentrant arrhythmias. Compounds which stabilized the mitochondria prevented reentry and electrical instability. Uncoupler-mediated regional depolarization of mitochondria to induce metabolic sinks also caused inexcitability and reentry, which were significantly prevented by pharmacological inhibition of ATP-sensitive K+ channels (KATP). Mitochondrial function is a major factor in determining the fate of an ischemic heart. In our in vitro model system, mitochondrial instability was demonstrated to be present during reperfusion and was highly correlated with electrical instability. Spatiotemporal heterogeneity in ΔΨm contributes to dispersion of repolarization and, in some cases, can also contribute to ectopic electrical activity. The results support the hypothesis that mitochondria are important targets for therapeutic intervention to prevent post-ischemic arrhythmias and sudden cardiac death

Sleep-disordered Breathing Destabilizes Ventricular Repolarization: Cross-sectional, Longitudinal, and Experimental Evidence – The Role of Intermittent Hypoxemia

Sleep-disordered breathing (SDB) increases the risk of cardiac arrhythmias and sudden cardiac death. To characterize the associations between SDB, intermittent hypoxemia, and the beat-to-beat QT variability index (QTVI), a measure of ventricular repolarization lability associated with cardiac arrhythmias and sudden cardiac death. Three distinct cohorts were used: (1) a matched sample of 122 participants with and without severe SDB for cross-sectional analysis, (2) a matched sample of 52 participants with and without incident SDB for longitudinal analysis, and (3) a sample of 19 healthy adults exposed to acute intermittent hypoxia and ambient air on two separate days. The cross-sectional and longitudinal cohorts were the Sleep Heart Health Study participants with no known comorbidities who were not on any drugs known to affect cardiac repolarization and satisfied the inclusion criteria. Electrocardiographic measures were calculated from one-lead electrocardiograms. Participants with severe SDB had greater QTVI than those without SDB (P = 0.027). Total sleep time with less than 90% oxygen saturation, but not the arousal frequency, was a predictor of QTVI. QTVI during sleep was predictive of all-cause mortality. With incident SDB, mean QTVI increased from -1.23 to -0.86 over 5 years (P = 0.017). Finally, experimental exposure of healthy adults to acute intermittent hypoxia for four hours progressively increased QTVI (P = 0.016). The results show that both prevalent and incident SDB are associated with ventricular repolarization instability and suggest intermittent hypoxemia as the underlying mechanism that may contribute to increased mortality in SDB

Sleep-Disordered Breathing Destabilizes Ventricular Repolarization

Sleep-disordered breathing (SDB) increases the risk of cardiac arrhythmias, sudden death, and all-cause mortality. To characterize the associations between SDB, intermittent hypoxemia, and the QT variability index (QTVI), a measure of ventricular repolarization lability associated with a higher risk for cardiac arrhythmias, sudden death, and cardiovascular mortality. Polysomnograms from three cohorts were used: a matched sample of 122 participants with and without severe SDB, a matched sample of 52 participants with and without incident SDB, and a cohort of 19 healthy adults exposed to intermittent hypoxia and ambient air on two separate days. Electrocardiographic measures were calculated from one-lead electrocardiograms. Compared to those without SDB, participants with severe SDB had larger QTVI (-1.19 vs. -1.43, =0.027), heart rate (68.34 vs. 64.92 beats/minute; =0.028), and hypoxemia burden during sleep as assessed by the total sleep time with oxygen saturation less than 90% (TST ; 11.39% vs. 1.32%, <0.001). TST , but not the frequency of arousals, was a predictor of QTVI. Heart rate and QTVI during sleep were predictive of all-cause mortality. In the cohort with incident SDB, the mean QTVI increased from -1.23 to -0.86 over 5 years ( =0.017). Finally, in the cohort of healthy adults, exposure to intermittent hypoxia for four hours increased QTVI (-1.85 vs. -1.64; =0.016). Prevalent and incident SDB are associated with ventricular repolarization instability, which predisposes to ventricular arrhythmias and sudden cardiac death. Intermittent hypoxemia can destabilize ventricular repolarization and may mediate the increased mortality in SDB