Proarrhythmic electrical remodelling is associated with increased beat-to-beat variability of repolarisation

OBJECTIVE: Acquired long-QT syndrome in combination with increased beat-to-beat variability of repolarisation duration (BVR) is associated with lethal torsades de pointes arrhythmias (TdP) in dogs with remodelled heart after atrioventricular block (AVB). We evaluated the relative contributions of bradycardia and ventricular remodelling to proarrhythmic BVR with and without pharmacological I(Kr) block in order to identify the individual at risk. METHODS: Three groups of dogs were used: sinus rhythm dogs (n = 12), dogs with acute AVB (n = 8), and dogs with >3 weeks chronic AVB (n = 27). Under anaesthesia, ECG and monophasic action potential duration (MAPD) were measured. Local BVR was quantified as short-term variability from 30 consecutive left ventricular MAPD (STV = summation absolute value(D(n(i)-D(n+1))/[30 x square root of 2])). All dogs received dofetilide iv. RESULTS: The slower ventricular rate acutely after AVB affected neither QTc nor STV (288+/-18 to 293+/-38 ms and 0.7+/-0.1 to 0.7+/-0.1 ms, respectively; P = NS for both), whereas ventricular remodelling increased both (to 376+/-46 and 2.3+/-0.6 ms, respectively; P < 0.05 for both). Neither dogs in sinus rhythm nor acute AVB showed any TdP, whereas dofetilide induced TdP in 74% of the chronic-AVB dogs. Dofetilide increased the QTc interval in all groups (19-24%; P < 0.05 for all groups), whereas STV was elevated in chronic-AVB dogs only (to 4.2+/-1.5 ms; P < 0.05) and further confined to inducible chronic-AVB dogs (5.0+/-0.8 versus 1.9+/-0.4 ms for resistant dogs; P < 0.05). Variability of the idioventricular rate was increased directly after AVB and did not influence BVR. CONCLUSIONS: Under drug-free circumstances, a persistent high BVR in chronic-AVB dogs is remodelling dependent rather than a direct consequence of bradycardia acutely after AVB. Variability of this slower rate does not influence BVR. Dofetilide causes a transient increase in BVR only in proarrhythmic dogs. Thus, BVR may aid the identification of the TdP-susceptible patient.     

Heterogeneous distribution of the two components of delayed rectifier K+ current: a potential mechanism of the proarrhythmic effects of methanesulfonanilideclass III agents.

OBJECTIVE: To elucidate the regional difference of the K+ current blocking effects of methanesulfonanilide class III agents. METHODS: Regional differences in action potential duration (APD) and E-4031-sensitive component (IKr) as well as -insensitive component (IKs) of the delayed rectifier K+ current (IK) were investigated in enzymatically isolated myocytes from apical and basal regions of the rabbit left ventricle using the whole-cell clamp technique. RESULTS: At 1 Hz stimulation, APD was significantly longer in the apex than in the base (223.1 +/- 10.6 vs. 182.7 +/- 14.5 ms, p < 0.05); application of 1 microM E-4031 caused more significant APD prolongation in the apex than in the base (32.5 +/- 6.4% vs. 21.0 +/- 8.8%, p < 0.05), resulting in an augmentation of regional dispersion of APD. In response to a 3-s depolarization pulse to +40 mV from a holding potential of -50 mV, both IK tail and IKs tail densities were significantly smaller in apical than in basal myocytes (IK: 1.56 +/- 0.13 vs. 2.09 +/- 0.21 pA/pF, p < 0.05; IKs: 0.40 +/- 0.15 vs. 1.43 +/- 0.23, p < 0.01), whereas IKr tail density was significantly greater in the apex than in the base (1.15 +/- 0.13 vs. 0.66 +/- 0.11 pA/pF, p < 0.01). The ratio of IKs/IKr for the tail current in the apex was significantly smaller than that in the base (0.51 +/- 0.21 vs. 3.09 +/- 0.89; p < 0.05). No statistical difference was observed in the voltage dependence as well as activation and deactivation kinetics of IKr and IKs between the apex and base. Isoproterenol (1 microM) increased the time-dependent outward current of IKs by 111 +/- 8% during the 3-s depolarizing step at +40 mV and its tail current by 120 +/- 9% on repolarization to the holding potential of -50 mV, whereas it did not affect IKr. CONCLUSIONS: The regional differences in IK, in particular differences in its two components may underlie the regional disparity in APD, and that methanesulfonanilide class III antiarrhythmic agents such as E-4031 may cause a greater spatial inhomogeneity of ventricular repolarization, leading to re-entrant arrhythmias.     

Repolarization abnormalities and their arrhythmogenic consequences in porcine tachycardia-induced cardiomyopathy

OBJECTIVES: Action potential prolongation related to the alteration of several membrane currents is constantly reported in heart failure (HF) but reports about its role in arrhythmogenesis are sparse. Our aim was to determine, by analogy with long QT syndromes, whether prolonged repolarization is associated with increased dispersion or linked to bradycardia-dependent ventricular arrhythmias in pacing-induced cardiomyopathy. METHODS: QT intervals, action potentials and transmural activation-to-recovery intervals (ARIs) along with whole-cell delayed rectifier (I(K)) and transient outward (I(to1)) K+ currents were recorded in left ventricle from pigs with HF and controls. HF was obtained after 14 days of rapid pacing at 250 ms. RESULTS: Repolarization was delayed as indexed by corrected QT intervals (13.7% increase, P<0.01) or ARIs (252+/-4 to 340+/-7 ms, P<0.01). ARIs were uniformly prolonged with disappearance of the transmural gradient, spatial dispersion of repolarization decreased by 50% (P<0.05). I(to1) density was reduced in HF from 1.35+/-0.1 to 0.57+/-0.04 pA/pF subepicardially, from 1.05+/-0.19 to 0.55+/-0.08 pA/pF midmyocardially and from 1.04+/-0.1 to 0.48+/-0.04 pA/pF subendocardially. I(K) density was significantly decreased in HF pigs vs. controls: subepicardially from 0.46+/-0.04 to 0.22+/-0.02 pA/pF; midmyocardially from 0.46+/-0.05 to 0.25+/-0.03 pA/pF; and subendocardially from 0.49+/-0.04 to 0.20+/-0.04 pA/pF following depolarization at +50 mV. Electrocardiogram (ECG) monitoring at the time of death did not disclose any polymorphic ventricular tachyarrhythmia. CONCLUSION: Despite a profound alteration in K+ currents, repolarization is uniformly prolonged in this model with no proclivity to develop bradycardia-dependent arrhythmias.     

The ionic mechanisms of long QT interval in diabetic rabbits

Objective Abnormal QT prolongation associated with arrhythmias is considered the major cardiac electrical disorder and a significant predictor of mortality in diabetic patients. The precise ionic mechanisms for diabetic QT prolongation remained unclear. The present study was designed to analyze the changes of ventricular repolarization and the underlying ionic mechanisms in diabetic rabbit hearts. Methods Diabetes was induced by a single injection ofalloxan （145mg/kg, Lv. ）. After the development of diabetes （10 weeks）, ECG was measured. Whole-cell patch-clamp technique was applied to record the action potential duration （APD50, APD90）, slowly activating outward rectifying potassium current （IKs）, L-type calcium current （ICa-L） and inward rectifying potassium current （IK1）. Results The action potential duration （APD50 and APD90） of ventricular myocytes was obviously prolonged from 271.5＋32.3 ms and 347.8＋36.3 ms to 556.6~72.5 ms and 647.9~72.2 ms respectively （P〈 0.05）. Meanwhile the normalized peak current densities of IKs in ventricular myocytes investigated by whole-cell patch clamp was smaller in diabetic rabbits than that in control group at test potential of＋50mV （1.27~0.20 pA/pF vs 3.08~0.67 pA/pF, P〈0.05）. And the density of the ICa-L was increased apparently at the test potential of 10 mV （-2.67~0.41 pA/pF vs -5.404-1.08 pA/pF, P〈0.05）. Conclusion Ventricular repolarization was prolonged in diabetic rabbits, it may be partly due to the increased L-type calcium current and reduced slow delayed rectifier K＋ current （IKs） （J Geriatr Cardio12010; 7：25-29）.     

A novel mutation in KCNQ1 associated with a potent dominant negative effect as the basis for the LQT1 form of the long QT syndrome

Introduction: Long QT Syndrome (LQTS) is an inherited disorder characterized by prolonged QT intervals and life-threatening polymorphic ventricular tachyarrhythmias. LQT1 caused by KCNQ1 mutations is the most common form of LQTS.
Methods and Results: Patients diagnosed with LQTS were screened for disease-associated mutations in KCNQ1, KCNH2, KCNE1, KCNE2, KCNJ2, and SCN5A . A novel mutation was identified in KCNQ1 caused by a three-base deletion at the position 824–826, predicting a deletion of phenylalanine at codon 275 in segment 5 of KCNQ1 (ΔF275). Wild-type (WT) and ΔF275- KCNQ1 constructs were generated and transiently transfected together with a KCNE1 construct in CHO-K1 cells to characterize the properties of the slowly activating delayed rectifier current (IKs) using conventional whole-cell patch–clamp techniques. Cells transfected with WT- KCNQ1 and KCNE1 (1:1.3 molar ratio) produced slowly activating outward current with the characteristics of IKs. Tail current density measured at −40 mV following a two-second step to +60 mV was 381.3 ± 62.6 pA/pF (n = 11). Cells transfected with ΔF275- KCNQ1 and KCNE1 exhibited essentially no current. (Tail current density: 0.8 ± 2.1 pA/pF, n = 11, P = 0.00001 vs WT). Cotransfection of WT- and ΔF275- KCNQ1 (50/50), along with KCNE1, produced little to no current (tail current density: 10.3 ± 3.5 pA/pF, n = 11, P = 0.00001 vs WT alone), suggesting a potent dominant negative effect. Immunohistochemistry showed normal membrane trafficking of ΔF275- KCNQ1 .
Conclusion: Our data suggest that a ΔF275 mutation in KCNQ1 is associated with a very potent dominant negative effect leading to an almost complete loss of function of IKs and that this defect underlies a LQT1 form of LQTS.     

Decreased ATP-sensitive K(+) current density during chronic human atrial fibrillation

Chronic atrial fibrillation (AF) is associated with shortening of action potential duration (APD), which involves modified activity of atrial ion currents. However, little is known about the activity of ATP-sensitive K(+) channels (I(K,ATP)) during chronic AF. An AF-related increase in the activity of I(K,ATP) would reduce APD and could contribute to initiation and/or perpetuation of AF. Here, we studied the activity of I(K,ATP) in atrial myocytes from patients with sinus rhythm (SR) and chronic AF. Human atrial myocytes were isolated from atrial tissue obtained from patients undergoing open-heart surgery. Inward rectifier currents were measured with the whole-cell patch-clamp technique by applying a depolarizing ramp pulse (1245 ms) from -100 to +40 mV (0.5 Hz). I(K,ATP) was activated with the I(K,ATP) channel opener rilmakalim. The inward rectifier I(K1) and I(K,ATP) were identified by their sensitivity to 1 mM Ba(2+). Density of I(K1) did not differ between cells from patients with AF (at -100 mV: -14.8 +/- 1.3 pA/pF, n = 38/10 (cells/patients)) and SR (-13.8 +/- 1.5 pA/pF, n = 33/16). In both types of cells, rilmakalim stimulated I(K,ATP) (defined as rilmakalim-inducible current) in a concentration-dependent manner (0.3-10 microM). However, maximum activation of I(K,ATP) with 10 microM rilmakalim was smaller in AF than in SR cells (at -100 mV: -5.3 +/- 0.8 pA/pF, n = 22/7 vs. -11.2 +/- 2.9 pA/pF, n = 19/9; at +40 mV: +9.6 +/- 2.1 pA/pF, n = 22/7 vs. +23.7 +/- 3.4 pA/pF, n = 19/9 for AF and SR, respectively; P < 0.05). Only aortic valve disease and pulmonary hypertension were found to be independent contributors to I(K,ATP) current density. We provide evidence that chronic AF is associated with a downregulation of ATP-sensitive K(+) currents. These changes may provide an additional molecular mechanism for electrical remodeling in chronic AF.     

Mechanisms of L-type Ca(2+) current downregulation in rat atrial myocytes during heart failure

Downregulation of the L-type Ca(2+) current (I(Ca)) is an important determinant of the electrical remodeling of diseased atria. Using a rat model of heart failure (HF) due to ischemic cardiopathy, we studied I(Ca) in isolated left atrial myocytes with the whole-cell patch-clamp technique and biochemical assays. I(Ca) density was markedly reduced (1.7+/-0.1 pA/pF) compared with sham-operated rats (S) (4.1+/-0.2 pA/pF), but its gating properties were unchanged. Calcium channel alpha(1C)-subunit quantities were not significantly different between S and HF. The beta-adrenergic agonist isoproterenol (1 micromol/L) had far greater stimulatory effects on I(Ca) in HF than in S (2.5- versus 1-fold), thereby suppressing the difference in current density. Dialyzing cells with 100 micromol/L cAMP or pretreating them with the phosphatase inhibitor okadaic acid also increased I(Ca) and suppressed the difference in density between S and HF. Intracellular cAMP content was reduced more in HF than in S. The phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine had a greater effect on I(Ca) in HF than in S (76.0+/-11.2% versus 15.8+/-21.2%), whereas the inhibitory effect of atrial natriuretic peptide on I(Ca) was more important in S than in HF (54.1+/-4.8% versus 24.3+/-8.8%). Cyclic GMP extruded from HF myocytes was enhanced compared with S (55.8+/-8.0 versus 6.2+/-4.0 pmol. mL(-1)). Thus, I(Ca) downregulation in atrial myocytes from rats with heart failure is caused by changes in basal cAMP-dependent regulation of the current and is associated with increased response to catecholamines.     

Glucocorticoid regulation of cardiac K+ currents and L-type Ca2+ current in neonatal mice.

Previous studies have reported that dexamethasone (Dex) prolongs cardiac action potential repolarization in mice and rats. However, the cellular mechanisms of this effect have not been addressed. Because action potential duration is influenced by a complex interplay of both inward and outward currents, this study evaluated the role of K+ currents and the L-type Ca2+ current in response to chronic in vivo Dex treatment. Accordingly, neonatal mice were randomly allocated to treatment with Dex (1 mg/kg per day) or placebo (saline) given subcutaneously for 5 days. At 14 to 15 days of age, the L-type Ca2+ current and K+ currents were recorded in ventricular myocytes using whole-cell patch-clamp techniques. The density of peak outward K+ currents was significantly decreased in the chronic Dex-treated group, but the current measured at the end of a 1-second depolarization pulse was similar in both groups. We further measured the magnitudes of the fast-inactivating (I(to)) and the slowly inactivating (I(slow)) currents that contribute to the peak outward K+ currents. I(to) was reduced from 17.5+/-3.0 pA/pF (control) to 10.6+/-2.5 pA/pF (Dex) at +50 mV (P<0.05), but I(slow) was not significantly different. These data suggest that downregulation of I(to) is responsible for the reduced peak outward current. Time courses of the onset and offset of in vivo Dex effects were also assessed. A period of 3 days of treatment was required to observe the Dex effect on peak outward K(+) currents, whereas a 7-day period after discontinuation of Dex was required to recover the baseline current density. Acute in vitro treatment with Dex (1 micromol/L) had no effect on K+ current densities. In addition, chronic Dex treatment significantly increased the density of the L-type Ca2+ current (I(Ca-L)) from -7.2+/-0.5 pA/pF of control to -8.9+/-0.6 pA/pF of Dex at +10 mV, P<0.05. In conclusion, chronic in vivo Dex treatment decreases I(to) and increases I(Ca-L) in neonatal mouse ventricular myocytes, both of which contribute to the prolongation of cardiac action potential repolarization induced by glucocorticoids.     

The Jervell and Lange-Nielsen syndrome. Natural history, molecular basis and clinical outcome

Data on the Jervell and Lange-Nielsen syndrome (JLN), the long QT syndrome (LQTS) variant associated with deafness and caused by homozygous or compound heterozygous mutations on the KCNQ1 or on the KCNE1 genes encoding the IKs current, are still largely based on case reports. We analyzed data from 186 JLN patients obtained from the literature (31%) and from individual physicians (69%). Most patients (86%) had cardiac events and 50% were symptomatic already by age 3. Their QTc was markedly prolonged (557 +/- 65 ms). Most of the arrhythmic events (95%) were triggered by emotions or exercise. Females are at lower risk for cardiac arrest and sudden death (CA/SD). A QTc>550 ms and history of syncope during the first year of life are independent predictors of subsequent CA/SD. Most mutations (90.5%) are on the KCNQ1 gene; mutations on the KCNE1 gene are associated with a more benign course. beta-blockers have only partial efficacy as 51% of the patients had events despite therapy and 29% had CA/SD. Conclusions: JLN syndrome is a most severe variant of LQTS, with a very early onset, major QTc prolongation, and is not well responsive to beta-blockers. Subgroups at relatively lower risk for CA/SD are identifiable and include females, patients with a QTc pound550 ms, without events in the first year of life, and with mutations on KCNE1. Early therapy with ICDs has to be considered.     

Contribution of IKr to rate-dependent action potential dynamics in canine endocardium

Previous modeling studies have suggested that the rapid component of the delayed rectifier (I(Kr)) may contribute importantly to action potential dynamics during tachycardia. To test this idea experimentally, I(Kr) was measured as the E-4031-sensitive current in isolated canine endocardial myocytes at 37 degrees C using the perforated patch-clamp technique. Command potentials were trains of action potential waveforms recorded at cycle lengths (CLs) of 1000, 500, 320, 170, and 120 ms. Action potential duration (APD) alternans occurred at CLs of 170 and 120 ms. During an action potential, I(Kr) increased gradually to a maximum at -55 to -60 mV. Peak I(Kr) increased initially as CL was shortened from 1000 to 500 ms (from 0.55+/-0.03 to 0.57+/-0.03 pA/pF), but decreased progressively as CL was shortened further (to 0.45+/-0.03 pA/pF at CL=120 ms). Baseline I(Kr) was negligible at CLs of 1000 to 320 ms, but increased to 0.12+/-0.01 pA/pF at a CL of 120 ms. During APD alternans, peak I(Kr) was larger for the short than for the long action potential (0.48+/-0.03 versus 0.46+/-0.03 pA/pF). A computer model of I(Kr) based on these data indicated that increasing I(Kr) suppressed alternans and decreasing I(Kr) increased alternans. In support of the latter result, inhibition of I(Kr) by E-4031 increased the maximal amplitude of alternans. These results indicate that I(Kr) contributes importantly to rate-related alterations of repolarization, including APD alternans. Modifying I(Kr) may be a promising approach to suppressing alternans and thereby preventing ventricular tachyarrhythmias.     

Effects of thyroid hormone on the arrhythmogenic activity of pulmonary vein cardiomyocytes.

OBJECTIVES: This study was conducted to investigate the effects of thyroid hormone on the electrophysiological characteristics of pulmonary vein (PV) cardiomyocytes. BACKGROUND: Hyperthyroidism is an important etiology of paroxysmal atrial fibrillation (AF). Pulmonary veins are known to initiate paroxysmal AF. METHODS: The action potential and ionic currents were investigated in single rabbit PV and atrial cardiomyocytes with (hyperthyroid) and without (control) incubation of L-triiodothyronine using the whole-cell clamp technique. RESULTS: Compared with the control cardiomyocytes, hyperthyroid PV and atrial cardiomyocytes had shorter action potential duration. Hyperthyroid PV cardiomyocytes had faster beating rates (1.82 +/- 0.13 Hz vs. 1.03 +/- 0.15 Hz, p < 0.005) and a higher incidence of delayed after depolarization (beating: 92% vs. 6%, p < 0.0001; non-beating: 45% vs. 3%, p < 0.005). However, only hyperthyroid PV beating cardiomyocytes had a higher incidence of early after depolarization (46% vs. 0%, p < 0.0001). The ionic current experiments showed that hyperthyroid PV beating cardiomyocytes had larger densities of overall slow inward (2.72 +/- 0.21 pA/pF vs. 2.07 +/- 0.19 pA/pF, p < 0.05), overall transient outward (1.39 +/- 0.21 pA/pF vs. 0.48 +/- 0.08 pA/pF, p < 0.001) and steady state outward currents (0.78 +/- 0.06 pA/pF vs. 0.58 +/- 0.04 pA/pF, p < 0.05) on depolarization and larger transient inward (0.021 +/- 0.004 pA/pF vs. 0.005 +/- 0.001 pA/pF, p < 0.001) on repolarization. By contrast, the hyperthyroid PV non-beating cardiomyocytes had larger densities of overall transient outward (1.01 +/- 0.14 pA/pF vs. 0.37 +/- 0.07 pA/pF, p < 0.001), steady state outward (0.61 +/- 0.06 pA/pF vs. 0.44 +/- 0.04 pA/pF, p < 0.05) and transient inward currents (0.011 +/- 0.002 pA/pF vs. 0.003 +/- 0.001 pA/pF, p < 0.05). CONCLUSIONS: Thyroid hormone changes the electrophysiological activity of the PV cardiomyocytes. Increased automaticity and enhanced triggered activity may increase the arrhythmogenic activity of PVs in hyperthyroidism.     

Gender differences in autonomic modulation of ventricular repolarization in humans

BACKGROUND: Gender differences in the incidence of ventricular arrhythmias have been reported and torsades de pointes associated with long QT syndrome are more common in women than men. Although increased sympathetic tone has an important role in vulnerability to arrhythmia, little is currently known regarding gender differences in the dynamic electrophysiological response to sympathetic stimulation. Therefore, we investigated whether there is a gender difference in humans with respect to the dynamic response of ventricular repolarization to beta-adrenergic stimulation and to autonomic blockade. METHODS: Twelve-lead ECGs were continuously recorded during isoproterenol infusion (protocol 1) and autonomic blockade with propranolol and atropine infusion (protocol 2) in 24 healthy volunteers (12 men, 23 +/- 2 years; 12 women, 23 +/- 5 years). QT (QTc) intervals were measured at the baseline and at a heart rate of 75, 100, and 120 beats/min. RESULTS: (1) The morphology of the T wave dynamically and transiently changed to bifid or biphasic during the acute phase of isoproterenol infusion. The incidence of these morphologic changes was higher in women than men (P < 0.05). (2) The QTc interval was initially prolonged and then shortened in both men and women during isoproterenol administration. However, QTc prolongation was significantly greater in women (0.44 +/- 0.02 to 0.55 +/- 0.03 sec) than men (0.42 +/- 0.03 to 0.51 +/- 0.04 sec; P < 0.05). (3) The QTc interval was significantly prolonged under autonomic blockade and the intrinsic QTc interval was longer in women than men (P < 0.05). CONCLUSION: While sympathetic stimulation and autonomic blockade modulated the dynamics of ventricular repolarization in both sexes, it was more pronounced in women. This gender difference may partially account for the susceptibility of women to arrhythmogenesis.     

Magnitude and time course of beta-adrenergic antagonism during oral amiodarone therapy.

To examine the presence and time course of beta-adrenergic antagonism produced by amiodarone, the heart rate, QT interval and arrhythmia frequency in response to graded doses of isoproterenol were evaluated in eight patients treated with oral amiodarone for sustained ventricular tachycardia. Measurements were made before and every 2 days after beginning oral amiodarone therapy (600 mg twice daily). Isoproterenol was given in doses of 12.5, 25 and 50 ng/kg body weight per min. The mean heart rate at rest decreased from 73.1 +/- 17.8 beats/min on day 0 to 57.8 +/- 15.0 beats/min after 12 days of amiodarone therapy. A significant linear decline in heart rate at rest was observed until day 6 (p less than 0.05 for all comparisons). On all days isoproterenol produced a progressive increase in heart rate that reached 115.5 +/- 20.2 beats/min on day 0 and 94.2 +/- 18.5 beats/min on day 12. Amiodarone blunted the heart rate increase produced by isoproterenol on days 2 to 12 (p less than 0.05 versus day 0). This effect was present by day 2 and did not change significantly thereafter. The mean corrected QT (QTc) interval increased from 430 +/- 30 ms on day 0 to 449 +/- 63 ms on day 12. A significant linear increase in QTc interval was observed until day 6 (p less than 0.05 for all comparisons). There was no systematic effect of isoproterenol on the QTc interval. Five of eight patients had a significant number of isoproterenol-induced premature ventricular complexes. Ventricular ectopic activity in response to isoproterenol was abolished after 4 days of amiodarone therapy.(ABSTRACT TRUNCATED AT 250 WORDS)     

Long QT syndrome in neonates: conduction disorders associated with HERG mutations and sinus bradycardia with KCNQ1 mutations

OBJECTIVES: We hypothesized that neonatal long QT syndrome (LQTS) with 2:1 atrioventricular block (AVB) could be related to HERG mutations. BACKGROUND: Early onset of LQTS is rare but carries a high risk of life-threatening events such as ventricular arrhythmias and conduction disorders. There are no data on possible gene specificity. METHODS: We analyzed the characteristics and outcomes of 23 neonate probands from our LQTS population. Samples of DNA were available in 18 cases. RESULTS: Long QT syndrome was diagnosed because of corrected QT interval (QTc) prolongation (mean QTc of 558 +/- 62 ms) and neonatal bradycardia attributable to sinus bradycardia (n = 8) or 2:1 AVB (n = 15). Symptoms included syncope (n = 2), torsades de pointes (n = 7), and hemodynamic failure (n = 6). Three infants with 2:1 AVB died during the first month of life. During the neonatal period, all living patients received beta-blockers (BB) and 13 had a combination of BB and permanent cardiac pacing. Under treatment, patients remained asymptomatic, with a mean follow-up of seven years. Mutations were identified in HERG (n = 8) and KCNQ1 (n = 8), and one child had three mutations (HERG, KCNQ1, and SCN5A). Conduction disorders were associated with LQT2, whereas sinus bradycardia was associated with LQT1. CONCLUSIONS: Two-to-one AVB seems preferentially associated with HERG mutations, either isolated or combined. Long QT syndrome with relative bradycardia attributable to 2:1 AVB has a poor prognosis during the first month of life. In contrast, sinus bradycardia seems to be associated with KCNQ1 mutations, with a good short-term prognosis under BB therapy.     

Properties of the transient outward current in rabbit sino-atrial node cells

The electrophysiological properties of the transient outward current were investigated in voltage-clamped single cells from the rabbit sino-atrial node. To make a regional comparison, some experiments were repeated in atrial myocytes. The current-voltage relationship showed a characteristic outward rectification with an activation threshold of -30 mV. External 4-aminopyridine (0.01-5 mM) inhibited this current in a dose-dependent manner (IC50 = 0.28 mM, Hill coefficient = 1.38). The steady-state inactivation exhibited a half-maximum voltage of -35 mV and a slope factor of -.4 mV. The current density of the transient outward current was 6.3 +/- 0.5 pA/pF in sino-atrial node cells and 12.3 +/- 1.2 pA/pF in atrial cells. The inactivation time constant was faster in sino-atrial node cells (time constants 4.2 +/- 0.5 and 26.0 +/- 0.6 ms, respectively, for the fast and slow components) than in atrial cells (9.7 +/- 1.2 and 44.8 +/- 3.2 ms, respectively). Recovery from inactivation was much faster in sino-atrial node cells (time constants 44.7 +/- 9.0 ms) than in atrial cells (time constants 1.39 +/- 0.32 and 6.70 +/- 0.1 s, respectively, for the fast and slow components). These results suggest that the kinetic properties, as well as the current density, of the transient outward current differs between sino-atrial node and atrial cells. Taking the current density of Ito at +10 mV as 2.5 +/- 0.3 pA/pF gives a total Ito of approximately 100 pA at the peak of the action potential in rabbit sino-atrial node cells. The action potential duration was increased by 24.8 +/- 1.3% by 0.5 mM 4-AP. Thus, Ito may contribute significantly to the repolarization phase in mammalian sino-atrial node cells.     

Calcium current in single cells isolated from normal and hypertrophied rat heart. Effects of beta-adrenergic stimulation

The L-type calcium current was investigated in normal and hypertrophied rat ventricular myocytes as a possible cause of the action potential lengthening that has been reported during hypertrophy. Regulation of the calcium current (ICa) by a beta-adrenergic agonist (isoproterenol) was also analyzed since beta-agonist-induced positive inotropy is less marked in hypertrophied heart. Left ventricular hypertrophy was induced by stenosis of the abdominal aorta. For recording ICa, the whole-cell patch-clamp technique was used. Potassium currents were suppressed by replacing K+ ions with Cs+ ions in both the extracellular and intracellular media, and sodium current was blocked by 50 microM tetrodotoxin. The Ca2+ current was larger in hypertrophied cells (2.2 +/- 0.6 nA [n= 31]) than in normal cells (1.2 +/- 0.5 nA [n = 33]). However, if one relates ICa amplitude to the cell membrane area, as estimated by membrane capacitance measurement, no significant difference was observed in current density (8.5 +/- 2.5 pA/pF [n = 31] and 8.3 +/- 2.1 pA/pF [n = 33] in hypertrophied and in normal cells, respectively). In both cell types, ICa displayed the same voltage and time dependence. When expressed as a percentage, the maximal increase in ICa amplitude that was obtained with 100 nM isoproterenol was less in hypertrophied cells (+78%) than in normal cells (+120%). The sensitivity of ICa to beta-adrenergic stimulation was not modified: EC50 was 3.8 nM for hypertrophied cells and 4.8 nM for normal cells. Forskolin and cyclic AMP were as effective in both cell types. Stimulation of ICa by beta-adrenergic agonist was decreased in agreement with a reduced number of binding sites of beta-agonists and/or an altered coupling of the G-proteins.     

Difference between electrical remodelling after pulmonary veins and right atrium appendage pacing.

OBJECTIVE: Pulmonary veins (PVs) are important sources of paroxysmal atrial fibrillation (AF). Rapid atrial pacing changes atrial electrophysiology, and facilitates the induction and maintenance of AF. The purpose of our study was to evaluate the changes in atrial effective refractory period (AERP) proprieties and in ionic currents in PVs myocytes from dogs subjected to rapid atrial pacing in PVs and right atrial appendage (RAA) and to relate these changes to the ability to induce AF. METHODS: Twelve mongrel dogs in normal sinus rhythm were paced from the superior left PVs or RAA at 500 bpm for 4 h. Electrophysiological studies were conducted to determine the changes in AERP, dispersion, and rhythm. Ionic currents were evaluated using patch clamp technique in single PVs myocytes in sham-operated dogs, and the results were compared with those from PVs and RAA pacing groups. RESULTS: The presence of rapid atrial pacing was associated with a marked shortening in AERP in both PVs and RAA pacing group with a marked increase in AERP dispersion in PVs pacing. Both L-type calcium current (I(Ca,L)) and the transient outward current (I(to)) were reduced in both groups with an increased significance in PVs pacing group. The density of I(Ca,L) was decreased significantly from (-6.03 +/- 0.63) pA/pF in the control group to (-3.21 +/- 0.34) pA/pF in the PVs pacing group and (-4.75 +/- 0.41) pA/pF in the RAA pacing group (n = 6, P < 0.05), whereas the density of I(to) was decreased significantly from (8.45 +/- 0.71) pA/pF in the control group to (5.21 +/- 0.763) pA/pF in the PVs pacing group and (6.84 +/- 0.69) pA/pF in the RAA pacing group (n = 6, P < 0.05). CONCLUSION: Our findings provide likely ionic mechanisms of shortened repolarization in induced atrial tachycardia with a decrease in I(Ca,L) and I(to) densities, which is the likely mechanism for a decrease in action potential duration rate adaptation in the canine rapid pacing model more pronounced in the PVs pacing group underlying the crucial role of PVs in initiating AF.     

The ultrarapid and the transient outward K(+) current in human atrial fibrillation. Their possible role in postoperative atrial fibrillation

Atrial fibrillation (AF) causes distinct changes in atrial conduction, characterized as electrical remodeling. Experimental data on the possible significance of alterations of specific K(+)outward currents in this process are still limited in human AF. The ultra-rapid delayed rectifier current (I(Kur)) has not been studied in AF with respect to its sensitivity to 4-Aminopyridine (4-AP). To clarify the role of (1) the 4-AP sensitive I(Kur)current, compared to recordings without using 4-AP (I(Kur*)), and (2) the transient outward current (I(to)) in changes of atrial repolarization associated with AF, whole cell voltage-clamp recordings were obtained from atrial myocytes of patients undergoing elective cardiac surgery, with and without a history of atrial fibrillation (AF/non-AF). Further, a possible relation between experimental data and postoperative AF was studied. In AF patients, I(Kur*)was reduced by 40% [5.00+/-0.32 pA/pF (non-AF) and 2.91+/-0. 45 pA/pF (AF) at +50 mV, P<0.0001, n=22/11], I(Kur)by 55% [3.81+/-0. 30 pA/pF (non-AF) and 1.71+/-0.20 pA/pF (AF) at +50 mV, P<0.0001, n=22/11]. The mean amplitude of I(Kur)was significantly smaller than I(Kur*). Consistently, I(to)was reduced by 44% [11.57+/-0.77 pA/pF (non-AF) and 6.51+/-1.31 pA/pF (AF), P<0.01, n=25/11]. In 48% of non-AF patients, postoperative AF was detected. The corresponding voltage-clamp recordings showed a trend to reduced I(Kur*)and I(Kur)currents, although it did not reach statistical significance. The consistent reduction of all three K(+)currents investigated due to the presence of AF indicates an important association of abnormalities in cellular repolarization with the onset and the self-sustaining nature of human AF.     

L型钙通道在LQTl发病机制中作用的实验研究

目的 分析L型钙电流(IcaL)在犬三层心室肌细胞中的特点,探讨其在LQTl发病机制中的作用.方法 成年杂种犬14只,体重13～15 kg,雌雄不拘.分离犬三层心室肌细胞,采用全细胞膜片钳技术记录动作电位(AP)和ICaL,依次用Chromanol 293B(50ìμmoL/L)阻断慢激活延迟整流性钾电流(IKs)模拟LQTl,用异丙肾上腺素(100 nmo/L)激活13肾上腺素受体(β-AR),观察AP和,ICaL的变化.分三层取少量心室肌组织,采用实时定量逆转录聚合酶链反应(RT-PCR)技术,检测各层L型钙通道a1C亚单位的mRNA含量.结果 正常情况下,犬三层心室肌细胞ICaL电流密度差异无统计学意义[外层(4.253±0.782)pA/pF,中层(4.392±0.714)pA/pF,内层(4.182±0.665)pA/pF,P＞0.05],而中层心室肌细胞动作电位时限(APD)较内层和外层的长[外层(721.48±26.59)ms,中层(911.80±31.24)ms,内层(783.52±25.27)ms,P＜0.05];阻断IKs后ICaL电流密度没有变化,而APD均明显延长[(外层(835.21±27.34)ms,中层(1089.21±30.55)ms,内层(830.64±27.12)ms,与阻断IKs前相比,P＜0.05)];β-AR兴奋使三层心室肌细胞ICaL显著增加,且三者变化差异无统计学意义[(外层(5.654±0.756)pA/pF,中层(5.458±0.702)pA/pF,内层(5.600±0.819)pAZpF,P＞0.05].但β-AR兴奋使外层和内层心室肌细胞APD缩短,中层心室肌细胞APD延长,三者变化差异有统计学意义[外层(792.63±26.71)ms,中层(1127.85±32.10)ms,内层(811.32±27.52)ms,P＜0.05].实时定量RT-PCR结果显示,三层心室肌细胞中alC亚单位的mRNA含量差异无统计学意义(外层0.112±0.019,中层0.077±0.018,内层0.109±0.012,P＞0.05).结论 L型钙通道在犬三层心室肌中的分布没有差异,在LQTl模型中,Iso使三层心室肌细胞,ICaL均匀增加,推测ICsL本身没有引起LQTl复极不稳定.     

Pharmacological activation of normal and arrhythmia-associated mutant KCNQ1 potassium channels

KCNQ1 alpha-subunits coassemble with KCNE1 beta-subunits to form channels that conduct the slow delayed rectifier K+ current (IKs) important for repolarization of the cardiac action potential. Mutations in KCNQ1 reduce IKs and cause long-QT syndrome, a disorder of ventricular repolarization that predisposes affected individuals to arrhythmia and sudden death. Current therapy for long-QT syndrome is inadequate. R-L3 is a benzodiazepine that activates IKs and has the potential to provide gene-specific therapy. In the present study, we characterize the molecular determinants of R-L3 interaction with KCNQ1 channels, use computer modeling to propose a mechanism for drug-induced changes in channel gating, and determine its effect on several long-QT syndrome-associated mutant KCNQ1 channels heterologously expressed in Xenopus oocytes. Scanning mutagenesis combined with voltage-clamp analysis indicated that R-L3 interacts with specific residues located in the 5th and 6th transmembrane domains of KCNQ1 subunits. Most KCNQ1 mutant channels responded to R-L3 similarly to wild-type channels, but one mutant channel (G306R) was insensitive to R-L3 possibly because it disrupted a key component of the drug-binding site.