Structure, function, and pharmacology of the cardiac sodium channel: Implications for the Long QT syndrome

The Long QT syndrome (LQTS) is a rare inherited disorder that is associated with an increased propensity to arrhythmogenic syncope, polymorphous ventricular tachycardia, and sudden cardiac death. In particular, LQT-3 syndrome, which is the result of mutation in the cardiac isoform of the sodium channel (Na V1.5), leads to prolongation of the action potential through an increase in sodium conductance throughout the duration of depolarization. Examination of mutations in the cardiac sodium channel has contributed to the understanding of basic channel function, mechanisms of disease, and mutation specific pharmacology that is needed to develop effective treatments.;A careful study of a second disease causing mutation that was referred by clinical collaborator revealed a de novo SCN5A mutation (F1473C) discovered in a newborn who had presented with extreme QT prolongation (approximately 800 ms) and 2:1 heart block. Patch clamp experiments revealed multiple biophysical changes in Na+ channel gating present in channels harboring the F1473C mutation that all likely contribute to delayed repolarization of cellular action potentials of the patient. In addition, the patient showed a significantly variable response to treatment to the Na + channel blockers lidocaine, mexiletine, and flecainide, which prompted careful examination of the pharmacology of these channels. The pronounced QT prolongation in the proband and the distinct therapeutic effectiveness of different Na+ channel blockers used to treat the patient were in large part due to the Na+ channel dysfunction caused by the mutation. However, the analysis suggests that the patient's genetic background and age impact both the severity of the disease phenotype and the differences in the effectiveness of two sodium channel blockers, flecainide and mexiletine as therapeutic agents.;Finally, the ability of three beta-blockers to block current in Na V1.5 channels is examined and two are shown to block sodium current in a use-dependent manner similar to block by local anesthetic drugs. A series of patch clamp studies are presented which show that beta-blockers block Na + channels in a use-dependent manner; that beta-blocker efficacy is dependent on the inactivated state of the channel; that beta-blockers block late non-inactivating current more effectively than peak sodium current; and that mutation of the local anesthetic binding site greatly reduces the ability of beta-blockers to block both peak and late Na+ channel current. Furthermore the results indicate that this activity, like that of local anesthetic drugs, differs both with drug structure and the biophysical changes in Na + channel function caused by specific LQT-3 mutations.;A structural model of the NaV1.5 carboxyl terminus (C-T) based on homology to the amino-terminal lobe of Calmodulin predicts six alpha helices (H1--H6), the first four forming two EF hand pairs. Studies presented here using whole cell patch clamp and tryptophan fluorescence suggest a role of the putative interface between H1 and H4 in control of channel inactivation and stability of the C-T structure. The results indicate that mutation of hydrophobic residues integral to the H1--H4 interface disrupts protein stability and markedly alters channel inactivation, providing evidence that stabilization of the C-T structure via the H1--H4 hydrophobic interface is necessary to preserve physiologically essential inactivation of the Nav1.5 channel. In addition, experiments shown here demonstrate that a naturally occurring LQT-3 mutation, which occurs within the region of the Nav1.5 carboxyl terminal (CT) domain that is critical in the coordination of an inactivation gate complex consisting of the CT and the DIII-DIV linker, disrupts this inactivation gate complex and causes distinct biophysical consequences. The biophysical phenotype of S1904L, channels are not dominated by channel bursting, but instead, by altered inactivation kinetics that are predicted to cause a novel disease phenotype wherein arrhythmogenesis is not predicted to be exacerbated by bradycardia. The results reinforce the importance of the inactivation gate complex in coordinating physiologically correct inactivation. The consistency of the patient phenotype with the unique rate dependence of the mutant channel indicate that caution must be used in extrapolating risk factors from disease genotype without a more complete understanding of the physiological consequences of specific gene mutations.