Functional Roles of Cl-/HCO3- Exchanger in the Sinoatrial Node

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Introduction: The sinoatrial node (SAN) initiates the electrical signals to generate normal heart rhythm. Hyperpolarization and cyclic nucleotide (HCN)-gated channels as well as localized subsarcolemmal Ca2+ releases (LCR) play central roles in the pacemaking activities of SAN. The collaborative activities of the membrane voltage clock and Ca2+ clock were considered to be the mechanisms of pacemaking in the heart. However, other ion channels and transporters also regulate the SAN function. We recently identified a solute carrier member, Slc26a6, in the heart which is an electrogenic Cl-/HCO3- exchanger. Importantly, we found that knockout of Slc26a6 results in bradycardia and fragmented QRS. We hypothesize that Slc26a6 may regulate cardiac pacemaking function. Methods: Wild type (WT) and Slc26a6 knockout (Slc26a6-/-) mice were examined. Cardiac-specific knockout of Slc26a6 was generated using CRISPR/Cas9 gene editing. Electrocardiography (ECG), ambulatory ECG, and echocardiographic recordings were performed. Single-cell RT-PCR and immunostaining were used to test the expression of Slc26a6 in SAN cells, and LCR were measured by confocal imaging. Whole-cell and perforated patch clamp was applied to record the spontaneous action potentials, funny currents and Cl-/HCO3- exchanger currents. Intracellular Cl- concentration and pH were quantified in the SAN cells. Results: We identified significant expression of Slc26a6 as well as robust Cl-/HCO3- exchange currents in the SAN cells. Slc26a6-/- mice show evidence of sinus bradycardia and fragmented QRS. At the cellular level, the Slc26a6-/- SAN cells demonstrate irregular spontaneous action potentials and reduced firing frequency. Moreover, intracellular Cl- and pH in SAN cells were significantly regulated during cardiac cycles. Conclusion: Slc26a6 plays critical roles in the regulation of SAN function. Cl-/HCO3- exchangers may serve as a pH clock in addition to the membrane voltage clock and Ca2+ clock.