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 Ca2+-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+/Ca2+ 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 Ca2+ and membrane potential clocks underlying SAN firing. Conclusions: Our study generates new testable predictions and insight linking Na+ homeostasis to Ca2+ handling and membrane potential dynamics in SAN myocytes that may advance our understanding of SAN (dys)function.

Muscle ion transporters and antioxidative proteins have different adaptive potential in arm than in leg skeletal muscle with exercise training

It was evaluated whether upper‐body compared to lower‐body musculature exhibits a different phenotype in relation to capacity for handling reactive oxygen species (ROS), H⁺, La⁻, Na⁺, K⁺ and also whether it differs in adaptive potential to exercise training. Eighty‐three sedentary premenopausal women aged 45 ± 6 years (mean ± SD) were randomized into a high‐intensity intermittent swimming group (HIS, n = 21), a moderate‐intensity swimming group (MOS, n = 21), a soccer group (SOC, n = 21), or a control group (CON, n = 20). Intervention groups completed three weekly training sessions for 15 weeks, and pre‐ and postintervention biopsies were obtained from deltoideus and vastus lateralis muscle. Before training, monocarboxylate transporter 4 (MCT4), Na⁺/K⁺ pump α₂, and superoxide dismutase 2 (SOD2) expressions were lower (P < 0.05) in m. deltoideus than in m. vastus lateralis, whereas deltoid had higher (P < 0.05) Na⁺/H⁺ exchanger 1 (NHE1) expression. As a result of training, Na⁺/K⁺ pump α₂ isoform expression was elevated only in deltoideus muscle, while upregulation (P < 0.05) of the α₁ and β₁ subunits, phospholemman (FXYD1), NHE1, and superoxide dismutase 1 expression occurred exclusively in vastus lateralis muscle. The increased (P < 0.05) expression of MCT4 and SOD2 in deltoid muscle after HIS and vastus lateralis muscle after SOC were similar. In conclusion, arm musculature displays lower basal ROS, La⁻, K⁺ handling capability but higher Na⁺‐dependent H⁺ extrusion capacity than leg musculature. Training‐induced changes in the ion‐transporting and antioxidant proteins clearly differed between muscle groups.

A structural rearrangement of the Na+/K+-ATPase traps ouabain within the external ion permeation pathway

With the use of the energy of ATP hydrolysis, the Na+/K+-ATPase is able to transport across the cell membrane Na+ and K+ against their electrochemical gradients. The enzyme is strongly inhibited by ouabain and its derivatives, some that are therapeutically used for patients with heart failure (cardiotonic steroids). Using lanthanide resonance energy transfer, we trace here the conformational changes occurring on the external side of functional Na+/K+-ATPases induced by the binding of ouabain. Changes in donor/acceptor pair distances are mainly observed within the α subunit of the enzyme. To derive a structural model matching the experimental lanthanide resonance energy transfer distances measured with bound ouabain, we carried out molecular dynamics simulations with energy restraints applied simultaneously using a novel methodology with multiple non-interacting fragments. The restrained simulation, initiated from the X-ray structure of the E2(2K+) state, became strikingly similar to the X-ray structure of the sodium-bound state. The final model shows that ouabain is trapped within the external ion permeation pathway of the pump.