Background— Many long-QT syndrome (LQTS) mutations in the cardiac Na⁺ channel result in a gain of function due to a fraction of channels that fail to inactivate (burst), leading to sustained current (I_sus) during depolarization. However, some Na⁺ channel mutations that are causally linked to cardiac arrhythmia do not result in an obvious gain of function as measured using standard patch-clamp techniques. An example presented here, the SCN5A LQTS mutant I1768V, does not act to increase I_sus (<0.1% of peak) compared with wild-type (WT) channels. In fact, it is difficult to reconcile the seemingly innocuous kinetic alterations in I1768V as measured during standard protocols under steady-state conditions with the disease phenotype.

Methods and Results— We developed new experimental approaches based on theoretical analyses to investigate Na⁺ channel gating under non-equilibrium conditions, which more closely approximate physiological changes in membrane potential that occur during the course of a cardiac action potential. We used this new approach to investigate channel-gating transitions that occur subsequent to channel activation.

Conclusions— Our data suggest an original mechanism for development of LQT-3 arrhythmias. This work demonstrates that a combination of computational and experimental analysis of mutations provides a framework to understand complex mechanisms underlying a range of disorders, from molecular defect to cellular and systems function.