Single-Channel Analysis of the Molecular Pharmacology of the Long QT Syndrome Variant 3

Abstract

The Long QT Syndrome (LQTS) is characterized by a prolongation of the QT interval on an ECG and occurrences of ventricular fibrillation, polymorphic ventricular tachycardia, and sudden cardiac death. In patients with LQTS variant 3 (LQT3), mutations in the cardiac sodium channel alpha subunit, Nav1.5, disrupt channel inactivation by multiple mechanisms and can cause a sustained depolarizing current (INaL) sufficient to prolong the ventricular action potential. LQT3 mutant sodium channels are therefore a reliable experimental model for the study of the function and pharmacology of dysfunctional sodium currents in a wide range of more common acquired disorders that are linked by disease-altered INaL, including ischemic heart disease and heart failure. While preferential pharmacological block of INaL by several Na+-channel blocking drugs has proven to be an important and useful clinical approach to manage LQT-3 arrhythmias, we still lack a fundamental understanding of the pharmacological determinants of this selectivity. We have previously identified a novel lethal LQT3 variant, F1473C, which displays slowed inactivation, altered channel availability, and a significant INaL. Here we report for the first time single channel recordings from F1473C channels and find that late reopening events during prolonged depolarizations underlie pathological INaL for this mutation. To further unravel the mechanism of action of of Ranolazine, a drug known to preferentially block INaL, we studied its effects on single F1473C channels. Ranolazine reduced the number of late reopening events in a manner consistent with block of whole cell currents. These experiments represent the first exploration of the block of INaL caused by late reopening, and present a framework for further studies of the mechanisms of drug block in these channels.