Mind the midpoints: Rethinking variability in cardiac INa

The function of the heart depends critically on the precise timing and coordination of electrical signals generated by ion channels in cardiac cells. The voltage-gated sodium current (INa) plays a pivotal role in initiating the rapid depolarization that drives each heartbeat. Two important descriptive properties of cardiac INa are its activation and inactivation midpoints, which describe the membrane voltages at which there is a 50% probability of the channel being open or unavailable, respectively. These midpoints determine the voltage range over which sodium channels contribute to the action potential and influence how easily the heart can initiate and propagate electrical signals. Because even small shifts in these kinetic parameters can affect excitability, conduction, and arrhythmia risk, they are commonly used to characterize the effects of drugs, mutations, and disease states.