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

Abstract 13376: Sleep-Disordered Breathing is Associated With Larger Ventricular Repolarization Instability

Byline: Soroosh Solhjoo, Johns Hopkins Univ Sch of Medicine, Baltimore, MD; Mark C Haigney, F. Edward HȨbert Sch of Medicine, Bethesda, MD; Trishul Siddharthan, Univ of Miami Miller Sch of Medicine, Miami, FL; Abigail L Koch, Univ of Miami Miller Sch of Medicine, Miami, FL; Ciprian M Crainiceanu, Johns Hopkins Bloomberg Sch of Public Health, Baltimore, MD; Naresh M Punjabi, Univ of Miami Miller Sch of Medicine, Miami, FL Introduction: Sleep-disordered breathing (SDB) is a common disorder in the general population that is associated with adverse cardiovascular events, such as sudden cardiac death, but the underlying mechanisms are unclear. Hypothesis: Ventricular repolarization instability is greater in those with severe SDB than those without SDB, independent of other risk factors. Methods: Among the participants of the Sleep Heart Health Study (SHHS), we identified those with SDB who had no diagnosed cardiovascular disease or other factors that could affect cardiac repolarization. Severe SDB was defined as having a respiratory disturbance index (RDI) > 33 events/hour (top 95%ile of the SHHS cohort). We also identified a group of participants matched to the severe SDB group on age, sex, BMI, and race and had RDI < 1.33 events/hour (bottom 25%ile; without SDB). Each group consisted of 61 (45 M and 16 F) participants. Oxygen saturation levels (SpO2) were used to measure hypoxemia burden as the percentage of sleep time with SpO2 < 90% (T90). Heart rate (HR), heart rate variability (SDNN), and QT variability index (QTVI, a measure of ventricular repolarization instability) were calculated from one-lead ECG recordings. Student's t-test was used to assess statistical associations. Results: The SDB group had a larger T90 than those without SDB (11.39 Ø 1.42% vs 1.32 Ø 0.61%, P < 0.001). Participants with SDB also had greater mean HR (P = 0.028), SDNN (P = 0.017), and QTVI (P = 0.027) than those without SDB. Based on a multivariable linear model that included hypoxemia burden, age, BMI, and smoking status, T90 was the only independent predictor of QTVI (P = 0.022). Conclusion: SDB is associated with higher HR, HRV, and QTVI, an indicator of increased instability in ventricular repolarization leading to increased risk for cardiac arrhythmias and sudden cardiac death. The severity of the destabilizing effect of SDB on ventricular repolarization appears to be modified by hypoxemia burden.Professiona

Abstract 12538: Intermittent Hypoxemia Progressively Destabilizes Ventricular Repolarization in Healthy Awake Subjects

Byline: Soroosh Solhjoo, Johns Hopkins Univ Sch of Medicine, Baltimore, MD; Naresh M Punjabi, Univ of Miami Miller Sch of Medicine, Miami, FL; Trishul Siddharthan, Univ of Miami Miller Sch of Medicine, Miami, FL; Abigail L Koch, Univ of Miami Miller Sch of Medicine, Miami, FL; Ciprian M Crainiceanu, Johns Hopkins Bloomberg Sch of Public Health, Baltimore, MD; Mark C Haigney, F. Edward HȨbert Sch of Medicine, Bethesda, MD Introduction : Obstructive sleep apnea (OSA) is highly prevalent in the general population. OSA is associated with adverse cardiovascular events, including sudden cardiac death, but the mechanism is unclear. Intermittent hypoxemia, a pathognomonic feature of OSA, could increase the propensity for abnormal ventricular repolarization. We previously found that short-term exposure of awake healthy subjects to recurrent hypoxic episodes increased their heart rate (HR) and HR variability. Hypothesis: Exposure to intermittent hypoxia increases ventricular repolarization instability in awake healthy subjects. Methods: Healthy adults (N = 19) were exposed to either intermittent hypoxia or air for 4 hours on two different days in a randomized order. Intermittent hypoxia was induced with exposure to hypoxic gas (95% N2 and 5% O2) or ambient air in an alternative fashion at a frequency of üô 25 times/hour, mimicking moderate to severe OSA. Repolarization stability was assessed using the QT variability index (QTVI), a validated predictor of cardiac arrhythmias and mortality. HR and QTVI were measured from one-lead ECG over consecutive 5-min epochs. Paired sample t-test was used to assess statistical associations. Results: Mean oxygen saturation levels were lower during exposure to intermittent hypoxia (90.86 Ø 0.19%) versus normoxia (97.37 Ø 0.20%, P < 10-13). Electrocardiographic measures at baseline were similar between the two conditions. No changes in these measures were noted during the conditions of normoxia. However, with intermittent hypoxia, mean HR and QTVI steadily increased (HR: 63.31 Ø 1.51 initial, 69.21 Ø 2.14 final, P < 0.001; QTVI: -1.85 Ø 0.04 initial, -1.64 Ø 0.07 final, P = 0.016). By the end of the 4-hour experiment, QTVI was greater in hypoxia vs. normoxia (final QTVI in hypoxia: -1.64 Ø 0.07, in normoxia: -1.86 Ø 0.06, P = 0.021). Conclusion: Four hours of intermittent hypoxia in awake normal healthy subjects progressively destabilized ventricular repolarization. Chronic exposure to recurrent periods of intermittent hypoxemia in OSA can increase ventricular repolarization and contribute to the OSA-associated adverse cardiovascular events, including sudden cardiac death.Professiona