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Key points It has been proposed that ephaptic conduction, relying on interactions between the sodium (Na+) current and the extracellular potential in intercalated discs, might contribute to cardiac conduction when gap junctional coupling is reduced, but this mechanism is still controversial. In intercalated discs, Na+ channels form clusters near gap junction plaques, but the functional significance of these clusters has never been evaluated. In HEK cells expressing cardiac Na+ channels, we show that restricting the extracellular space modulates the Na+ current, as predicted by corresponding simulations accounting for ephaptic effects. In a high-resolution model of the intercalated disc, clusters of Na+ channels that face each other across the intercellular cleft facilitate ephaptic impulse transmission when gap junctional coupling is reduced. Thus, our simulations reveal a functional role for the clustering of Na+ channels in intercalated discs, and suggest that rearrangement of these clusters in disease may influence cardiac conduction. Abstract It has been proposed that ephaptic interactions in intercalated discs, mediated by extracellular potentials, contribute to cardiac impulse propagation when gap junctional coupling is reduced. However, experiments demonstrating ephaptic effects on the cardiac Na+ current (INa) are scarce. Furthermore, Na+ channels form clusters around gap junction plaques, but the electrophysiological significance of these clusters has never been investigated. In patch clamp experiments with HEK cells stably expressing human Nav1.5 channels, we examined how restricting the extracellular space modulates INa elicited by an activation protocol. In parallel, we developed a high-resolution computer model of the intercalated disc to investigate how the distribution of Na+ channels influences ephaptic interactions. Approaching the HEK cells to a non-conducting obstacle always increased peak INa at step potentials near the threshold of INa activation and decreased peak INa at step potentials far above threshold (7 cells, P = 0.0156, Wilcoxon signed rank test). These effects were consistent with corresponding control simulations with a uniform Na+ channel distribution. In the intercalated disc computer model, redistributing the Na+ channels into a central cluster of the disc potentiated ephaptic effects. Moreover, ephaptic impulse transmission from one cell to another was facilitated by clusters of Na+ channels facing each other across the intercellular cleft when gap junctional coupling was reduced. In conclusion, our proof-of-principle experiments demonstrate that confining the extracellular space modulates cardiac INa, and our simulations reveal the functional role of the aggregation of Na+ channels in the perinexus. These findings highlight novel concepts in the physiology of cardiac excitation.

Cardiac arrhythmias frequently result from disorders of impulse propagation. Recent studies proposed that electrical potentials occurring in intercalated discs may contribute to cardiac conduction in addition to gap junctional coupling (ephaptic conduction). Furthermore, Na+ channels are found in clusters near gap junctions, suggesting that Na+ channel distribution in intercalated discs may modulate ephaptic intercellular interactions. Our aim was to investigate computationally how cardiac Na+ current (INa) dynamics are influenced in a narrow extracellular cleft by the extracellular potential (Ve) and by the distribution of Na+ channels. The model consisted of a disc-shaped cleft discretized at a high resolution separating either one cell membrane containing Na+ channels and a non-conductive obstacle, or two excitable membranes representing an intercalated disc. INa was modeled using the Hodgkin-Huxley formalism. The intracellular domain of the first cell was subjected to a voltage-clamp activation protocol while the intracellular potential of the second cell was clamped to the resting potential. In the model with one membrane facing a non-conducting obstacle, decreasing cleft width from 1000 to 20 nm resulted in decreasing INa peak intensity at voltage steps ≥-30 mV. This decrease was caused by the negative Ve in the cleft, which decreased the driving force for INa. However, at voltage steps just above threshold, INa was activated in a ring-shaped peripheral region. This resulted in larger INa in simulations with clefts 10-200 nm wide. When total INa was considered, the negative Ve resulted in a cleft-dependent shift of the steady state activation curve to more negative potentials and a steepening of the curve. Narrowing the cleft thus lowered and accentuated the threshold of INa. These effects were more prominent in larger discs and when Na+ channels were redistributed in the center of the disc. In the two-cell model, the negative Ve caused by INa in the first cell resulted in activation of INa in the second cell after a ∼1 ms delay for clefts