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Background: The right ventricular outflow tract (RVOT) is known to be the origin of potentially dangerous ventricular tachycardias (VT). Arrhythmias can occur in what are otherwise assume to be structurally normal hearts as well as in arrhythmia syndromes like Brugada syndrome, catecholaminergic polymorphic VT and arrhythmogenic right ventricular dysplasia. In adults with surgically corrected tetralogy of Fallot (ToF), VT often originates in the RVOT. Methods: To investigate potential arrhythmia substrates in the structurally normal heart, the RVOT of adult rats was investigated and compared to the right ventricle (RV). In the human, the right atrium (RA), RV and the RVOT were examined in patients free of electrical and structural disease at the time of clinically indicated surgery on the left heart. The same tissue samples were collected from adults with surgically corrected ToF at the time of pulmonary valve replacement. Real-time reverse transcription quantitative polymerase chain reaction (RT-qPCR) was used to explore changes on the mRNA level. Antibody based techniques (immunohistochemistry and Western blot) and mass spectrometry were used to identify changes on the protein level. In the rat RVOT, fine microelectrodes were employed to investigate electrophysiological characteristics. Results: A reduction in the Na+ channel Nav1.5 was found in the structurally normal RVOT in the rat and human. In the rat, RVOT-nodal like myocytes were identified. Unlike ventricular myocytes, the myocytes did not express the gap junction channel Cx43 and the K+ channel Kir2.1, but they did express the pacemaker Ca2+ channel Cav3.2 and the transcription factor Tbx3. Ectopic pacemaker and nodal-like action potentials with phase 4 depolarization were recorded from the region of the pulmonary valve. In the RA of ToF patients, SERCA2A was found to be downregulated (mRNA and protein) as seen in heart failure and atrial fibrillation. Further widespread remodelling was found in the RA, the RV and RVOT identifying multiple changes as seen in heart failure with downregulation of K+ channels and β-receptors and an increase in inflammatory markers and extracellular matrix molecules. Multiple molecules involved in the mitochondrial respiration chain were found to be downregulated in the RVOT of ToF patients. Conclusion: This is the first study that demonstrates reduction in Nav1.5 in the RVOT of structurally normal human hearts. The changes found in the rat RVOT could explain why arrhythmias originate in the RVOT. The widespread remodelling in ToF, especially in the ECM, could form a substrate for reentry arrhythmias. Mitochondrial dysfunction could point towards potential treatment targets for heart failure in this patient group. |
