Abstract Body (Do not enter title and authors here): Introduction: The voltage-gated sodium channel Na_V1.5 controls cardiac excitability. The mechanisms by which loss of function Na_V1.5 mutations cause diverse pathogenic outcomes (Brugada syndrome, conduction disease, heart failure) are not fully resolved. The physiological significance of non-Na_V1.5 Na⁺ channels in the heart is also uncertain.
Methods: We engineered knock-in mice in which Na_V1.5 was modified to contain a high-affinity binding site for acylsulfonamide (GX) drugs, enabling a pharmacological strategy to drive nonconducting channel conformations. We used optical mapping (di-4 ANEPPS) of Langendorff-perfused WT (n=6) and homozygous GX/GX hearts (n=14) to investigate how selective Na_V1.5 blockade influences action potential (AP) propagation and arrhythmogenesis.
Results: In the absence of the GX drug (GX-674) in sinus rhythm, action potential duration (APD; 71±7 vs. 68±10 ms), APD dispersion (13±5 vs. 12±3 ms), AP upstroke rise time (6±1 vs. 6±1 ms) and longitudinal (CVL, 0.71±0.16 vs. 0.85±0.16 m/s) and transverse conduction velocity (CVT, 0.43±0.11 vs. 0.49±0.1 m/s) were not statistically different between GX/GX and WT hearts. GX-674 application (0 ~ 300 nM) during optical mapping with S1S2 pacing resulted in loss of excitability (50% loss at 10 nM in GX/GX ; 0% loss up to 300 nM in WT) associated with an increase of AP upstroke rise time (10±3 vs. 5.4±0.5 ms) and conduction slowing mainly along the longitudinal direction (CVL=0.49±0.22 vs. 0.85±0.24 m/s); APD remained relatively stable (85±15 vs. 76±11 ms). The AP upstroke in GX/GX hearts often exhibited 2 steps (Fig-A), leading to frequent premature beats, reentry, and non-sustained ventricular tachycardia (Fig-B,C). The loss of excitability was heterogeneous, with sensitivity to GX-674 being higher in the right versus left ventricle (16% vs. 50% at 10 nM), Purkinje fibers and AV node (100% AV block at 100 nM), and atrial tissue (100% excitable at 100 nM but complete loss at 300 nM).
Conclusion: Regional heterogeneities in Na_V1.5 function, along with Na⁺ channels other than Na_V1.5, may contribute to cardiac conduction and arrhythmias in the mouse heart.