Abstract-The Brugada syndrome is a major cause of sudden death, particularly among young men of Southeast Asian and Japanese origin. The syndrome is characterized electrocardiographically by an ST-segment elevation in V1 through V3 and a rapid polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation. Our group recently linked the disease to mutations in SCN5A, the gene encoding for the ␣ subunit of the cardiac sodium channel. When heterologously expressed in frog oocytes, electrophysiological data recorded from the Thr1620Met missense mutant failed to adequately explain the electrocardiographic phenotype. Therefore, we sought to further characterize the electrophysiology of this mutant. We hypothesized that at more physiological temperatures, the missense mutation may change the gating of the sodium channel such that the net outward current is dramatically augmented during the early phases of the right ventricular action potential. In the present study, we test this hypothesis by expressing Thr1620Met in a mammalian cell line, using the patch-clamp technique to study the currents at 32°C. Our results indicate that Thr1620Met current decay kinetics are faster when compared with the wild type at 32°C. Recovery from inactivation was slower for Thr1620Met at 32°C, and steady-state activation was significantly shifted. Our findings explain the features of the ECG of Brugada patients, illustrate for the first time a cardiac sodium channel mutation of which the arrhythmogenicity is revealed only at temperatures approaching the physiological range, and suggest that some patients may be more at risk during febrile states. (Circ Res. 1999;85:803-809.)Key Words: Brugada syndrome Ⅲ Na ϩ channel Ⅲ temperature P olymorphic ventricular tachycardia (VT) and ventricular fibrillation (VF) developing in patients with structurally normal hearts accounts for 5% to 12% of the Ͼ300 000 sudden deaths of Americans each year. 1,2 Approximately half of these are attributed to the Brugada syndrome, a familial disease electrocardiographically characterized by a downsloping ST-segment elevation terminating in a negative T wave in the right precordial leads, an apparent right bundle branch block, 3,4 and rapid polymorphic VT capable of degenerating to VF. 5,6 Slightly prolonged H-V intervals are observed in 60% of patients 7 with Brugada syndrome.Chen et al 8 recently uncovered the first gene defects linked to the Brugada syndrome, identifying different mutations in SCN5A, the cardiac sodium channel gene, in each of the 3 families studied. A frameshift mutation resulted in an inframe stop codon in the pore region of domain III in one family. Because the syndrome has an autosomal dominant pattern of inheritance, the frameshift mutation is likely to result in a decrease in the number of functional channels, which can explain the clinical manifestation of the syndrome. 3,4 Missense mutations involving a double substitution of arginine at position 1232 by a tryptophan (Arg1232Trp) and the threonine at position 1620 by...

Background-Epicardial pacing of the left ventricle (LV) has been shown to prolong the QT interval and predispose to the development of torsade de pointes arrhythmias. The present study examines the cellular basis for QT prolongation and arrhythmogenesis after reversal of the direction of activation of the LV wall. Methods and Results-A transmural ECG and transmembrane action potentials were simultaneously recorded from epicardial, M, and endocardial cells of arterially perfused canine LV wedge preparations. QT interval increased from 297.6Ϯ3.9 to 314.0Ϯ5.7 ms (nϭ12; PϽ0.001) and transmural dispersion of repolarization (TDR) increased from 35.5Ϯ5.2 to 70.3Ϯ6.2 ms (nϭ12; PϽ0.001) as pacing was shifted from endocardium to epicardium. Conduction time between M and epicardial cells increased from 12.1Ϯ1.2 to 24.2Ϯ1.5 ms (nϭ12; PϽ0.001).Amplification of TDR was further accentuated in the presence of rapidly activating delayed rectifier potassium current blockers (E-4031 and cisapride), increasing from 50.5Ϯ7.6 to 86.1Ϯ6.2 ms (nϭ8; PϽ0.01). Torsade de pointes arrhythmias could be induced during epicardial, but not endocardial, pacing of LV in the presence of rapidly activating delayed rectifier potassium current blockade. Key Words: electrocardiography Ⅲ torsade de pointes Ⅲ heart failure Ⅲ pacemakers Ⅲ electrophysiology R ecent studies have highlighted the benefits of resynchronization therapy involving biventricular pacing for patients with congestive heart failure, demonstrating enhanced cardiac output and New York Heart Association class improvement. [1][2][3][4] Despite improvements in hemodynamics and patient quality of life, the incidence of sudden death in patients treated with biventricular pacing remains high. 5,6 Recent reports document the development of R-on-T ventricular extrasystoles and ventricular tachyarrhythmias after the initiation of biventricular pacing. 7,8 Resynchronization therapy most commonly involves the placement of one stimulating catheter in the right ventricular (RV) apex and another in contact with the left ventricular (LV) epicardium via the coronary sinus. Although the mechanical benefits of resynchronization therapy have been studied extensively, little attention has been directed toward the consequences of reversing the electric activation of the LV free wall. Using a rabbit wedge preparation, Medina-Ravell et al 8 demonstrated the development of early afterdepolarizations and increased dispersion of epicardial and endocardial repolarization after reversal of the transmural sequence of activation, suggesting that these mechanisms may underlie the development of torsade de pointes in patients undergoing resynchronization therapy. Conclusions-ReversalOur laboratory first described the contribution of M cells to transmural dispersion of repolarization (TDR) in 1991 9 -11 and in more recent years has shown these cells to be the chief culprits in the development of torsade de pointes under a wide variety of conditions. 12 The present study tests the hypothesis that delayed activation and...

Background Brugada syndrome (BrS) is a sudden death predisposing genetic condition characterized electrocardiographically by ST-segment elevation in the leads V1-V3. Given the prominent role of the transient outward current (Ito) in BrS pathogenesis, we hypothesized that rare gain-of-function mutations in KCND3 may serve as a pathogenic substrate for BrS. Methods Comprehensive mutational analysis of KCND3-encoded Kv4.3 (Ito) was conducted using PCR, DHPLC, and direct sequencing of DNA derived from 86 unrelated BrS1-8 genotype negative BrS patients. DNA from 780 healthy individuals was examined to assess allelic frequency for non-synonymous variants. Putative BrS-associated Kv4.3 mutations were engineered and co-expressed with wild-type KChIP2 in HEK293 cells. Wild-type and mutant Ito ion currents were recorded using whole cell patch clamp. Results Two BrS1-8 genotype-negative cases possessed novel Kv4.3 missense mutations. Both Kv4.3-L450F and Kv4.3-G600R were absent in 1560 reference alleles and involved residues highly conserved across species. Both Kv4.3-L450F and Kv4.3-G600R demonstrated a gain-of-function phenotype, increasing peak Ito current density by 146.2% (n=15, p<0.05) and 50.4% (n=15, p<0.05) respectively. Simulations employing a Luo-Rudy II AP model demonstrated the stable loss of the AP dome as a result of the increased Ito maximal conductance associated with the heterozygous expression of either L450F or G600R. Conclusions These findings provide the first molecular and functional evidence implicating novel KCND3 gain-of-function mutations in the pathogenesis and phenotypic expression of BrS, with the potential for a lethal arrhythmia being precipitated by a genetically enhanced Ito current gradient within the right ventricle were KCND3 expression is the highest.

Action potentials and whole cell sodium current were recorded in canine epicardial, midmyocardial, and endocardial myocytes in normal sodium at 37 degrees C. Tetrodotoxin (TTX) reduced the action potential duration of midmyocardial cells to a greater degree than either epicardial or endocardial cells. Whole cell recordings in potassium-free and very-low-chloride solutions revealed a slowly decaying current that was completely inhibited by 5 microM TTX or replacement of external and internal sodium with the impermeant cation N-methyl-D-glucamine. Late sodium current density at 0 mV was 47% greater in midmyocardial cells and averaged -0.532 +/- 0.058 pA/pF in endocardial, -0.463 +/- 0.068 pA/pF in epicardial, and -0.785 +/- 0.070 pA/pF in midmyocardial cells. Neither the frequency dependence of late sodium current nor its recovery from inactivation exhibited transmural differences. After a 4.5-s pulse to -30 mV, late sodium current recovered with a single time constant of 140 ms. We conclude that a larger late sodium conductance in midmyocardial cells will favor longer action potentials in these cells. More importantly, drugs that slow inactivation of sodium channels will produce a nonuniform response across the ventricular wall that is proarrhythmic.