Intracellular Na(+) Modulates Pacemaking Activity in Murine Sinoatrial Node Myocytes: An In Silico Analysis

Background: The mechanisms underlying dysfunction in the sinoatrial node (SAN), the heart’s primary pacemaker, are incompletely understood. Electrical and Ca(2+)-handling remodeling have been implicated in SAN dysfunction associated with heart failure, aging, and diabetes. Cardiomyocyte [Na(+)](i) is also elevated in these diseases, where it contributes to arrhythmogenesis. Here, we sought to investigate the largely unexplored role of Na(+) homeostasis in SAN pacemaking and test whether [Na(+)](i) dysregulation may contribute to SAN dysfunction. Methods: We developed a dataset-specific computational model of the murine SAN myocyte and simulated alterations in the major processes of Na(+) entry (Na(+)/Ca(2+) exchanger, NCX) and removal (Na(+)/K(+) ATPase, NKA). Results: We found that changes in intracellular Na(+) homeostatic processes dynamically regulate SAN electrophysiology. Mild reductions in NKA and NCX function increase myocyte firing rate, whereas a stronger reduction causes bursting activity and loss of automaticity. These pathologic phenotypes mimic those observed experimentally in NCX- and ankyrin-B-deficient mice due to altered feedback between the Ca(2+) and membrane potential clocks underlying SAN firing. Conclusions: Our study generates new testable predictions and insight linking Na(+) homeostasis to Ca(2+) handling and membrane potential dynamics in SAN myocytes that may advance our understanding of SAN (dys)function.