Role of Mitochondria in Cardiac Arrhythmias

Abstract

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