Atrium-selective sodium channel block as a strategy for suppression of atrial fibrillation: differences in sodium channel inactivation between atria and ventricles and the role of ranolazine.
ABSTRACT: The development of selective atrial antiarrhythmic agents is a current strategy for suppression of atrial fibrillation (AF).Whole-cell patch clamp techniques were used to evaluate inactivation of peak sodium channel current (I(Na)) in myocytes isolated from canine atria and ventricles. The electrophysiological effects of therapeutic concentrations of ranolazine (1 to 10 micromol/L) and lidocaine (2.1 to 21 micromol/L) were evaluated in canine isolated coronary-perfused atrial and ventricular preparations. Half-inactivation voltage of I(Na) was approximately 15 mV more negative in atrial versus ventricular cells under control conditions; this difference increased after exposure to ranolazine. Ranolazine produced a marked use-dependent depression of sodium channel parameters, including the maximum rate of rise of the action potential upstroke, conduction velocity, and diastolic threshold of excitation, and induced postrepolarization refractoriness in atria but not in ventricles. Lidocaine also preferentially suppressed these parameters in atria versus ventricles, but to a much lesser extent than ranolazine. Ranolazine produced a prolongation of action potential duration (APD90) in atria, no effect on APD90 in ventricular myocardium, and an abbreviation of APD90 in Purkinje fibers. Lidocaine abbreviated both atrial and ventricular APD90. Ranolazine was more effective than lidocaine in terminating persistent AF and in preventing the induction of AF.Our study demonstrates important differences in the inactivation characteristics of atrial versus ventricular sodium channels and a striking atrial selectivity for the action of ranolazine to produce use-dependent block of sodium channels, leading to suppression of AF. Our results point to atrium-selective sodium channel block as a novel strategy for the management of AF.
Project description:Ranolazine is a Food and Drug Administration-approved antianginal agent. Experimental and clinical studies have shown that ranolazine has antiarrhythmic effects in both ventricles and atria. In the ventricles, ranolazine can suppress arrhythmias associated with acute coronary syndrome, long QT syndrome, heart failure, ischemia, and reperfusion. In atria, ranolazine effectively suppresses atrial tachyarrhythmias and atrial fibrillation (AF). Recent studies have shown that the drug may be effective and safe in suppressing AF when used as a pill-in-the pocket approach, even in patients with structurally compromised hearts, warranting further study. The principal mechanism underlying ranolazine's antiarrhythmic actions is thought to be primarily via inhibition of late I(Na) in the ventricles and via use-dependent inhibition of peak I(Na) and I(Kr) in the atria. Short- and long-term safety of ranolazine has been demonstrated in the clinic, even in patients with structural heart disease. This review summarizes the available data regarding the electrophysiologic actions and antiarrhythmic properties of ranolazine in preclinical and clinical studies.
Project description:Background:Adenosine leads to atrial action potential (AP) shortening through activation of adenosine 1 receptors (A1-R) and subsequent opening of G-protein-coupled inwardly rectifying K+ channels. Extracellular production of adenosine is drastically increased during stress and ischemia. Objective:The aim of this study was to address whether the pharmacological blockade of endogenous production of adenosine and of its signaling prevents atrial fibrillation (AF). Methods:The role of A1-R activation on atrial action potential duration, refractoriness, and AF vulnerability was investigated in rat isolated beating heart preparations (Langendorff) with an A1-R agonist [2-chloro-N 6-cyclopentyladenosine (CCPA), 50 nM] and antagonist [1-butyl-3-(3-hydroxypropyl)-8-(3-noradamantyl)xanthine (PSB36), 40 nM]. Furthermore, to interfere with the endogenous adenosine release, the ecto-5'-nucleotidase (CD73) inhibitor was applied [5'-(?,?-methylene) diphosphate sodium salt (AMPCP), 500 ?M]. Isolated trabeculae from human right atrial appendages (hRAAs) were used for comparison. Results:As expected, CCPA shortened AP duration at 90% of repolarization (APD90) and effective refractory period (ERP) in rat atria. PSB36 prolonged APD90 and ERP in rat atria, and CD73 inhibition with AMPCP prolonged ERP in rats, confirming that endogenously produced amount of adenosine is sufficiently high to alter atrial electrophysiology. In human atrial appendages, CCPA shortened APD90, while PSB36 prolonged it. Rat hearts treated with CCPA are prone to AF. In contrast, PSB36 and AMPCP prevented AF events and reduced AF duration (vehicle, 11.5 ± 2.6 s; CCPA, 40.6 ± 16.1 s; PSB36, 6.5 ± 3.7 s; AMPCP, 3.0 ± 1.4 s; P < 0.0001). Conclusion:A1-R activation by intrinsic adenosine release alters atrial electrophysiology and promotes AF. Inhibition of adenosine pathway protects atria from arrhythmic events.
Project description:The aim of this study was to evaluate the effectiveness of a combination of dronedarone and ranolazine in suppression of atrial fibrillation (AF).Safe and effective pharmacological management of AF remains one of the greatest unmet medical needs.The electrophysiological effects of dronedarone (10 ?mol/l) and a relatively low concentration of ranolazine (5 ?mol/l) separately and in combination were evaluated in canine isolated coronary-perfused right and left atrial and left ventricular preparations as well as in pulmonary vein preparations.Ranolazine caused moderate atrial-selective prolongation of action potential duration and atrial-selective depression of sodium channel-mediated parameters, including maximal rate of rise of the action potential upstroke, leading to the development of atrial-specific post-repolarization refractoriness. Dronedarone caused little or no change in electrophysiological parameters in both atrial and ventricular preparations. The combination of dronedarone and ranolazine caused little change in action potential duration in either chamber but induced potent use-dependent atrial-selective depression of the sodium channel-mediated parameters (maximal rate of rise of the action potential upstroke, diastolic threshold of excitation, and the shortest cycle length permitting a 1:1 response) and considerable post-repolarization refractoriness. Separately, dronedarone or a low concentration of ranolazine prevented the induction of AF in 17% and 29% of preparations, respectively. In combination, the 2 drugs suppressed AF and triggered activity and prevented the induction of AF in 9 of 10 preparations (90%).Low concentrations of ranolazine and dronedarone produce relatively weak electrophysiological effects and weak suppression of AF when used separately but when combined exert potent synergistic effects, resulting in atrial-selective depression of sodium channel-dependent parameters and effective suppression of AF.
Project description:Wenxin Keli is a Chinese herb extract reported to be of benefit in the treatment of cardiac arrhythmias, cardiac inflammation, and heart failure.We evaluated the electrophysiologic effects of Wenxin Keli in isolated canine arterially perfused right atrial preparations with a rim of right ventricular tissue (n = 11). Transmembrane action potentials and a pseudoelectrocardiogram were simultaneously recorded. Acetylcholine (1 ?M) was used to induce atrial fibrillation (AF) and to test the anti-AF potential of Wenxin Keli (5 g/L). Wenxin Keli produced preferential abbreviation of action potential duration measured at 90% repolarization (APD(90)) in atria, but caused atrial-selective prolongation of the effective refractory period, due to the development of postrepolarization refractoriness. The maximum rate of rise of the action potential upstroke was preferentially reduced in atria. The diastolic threshold of excitation increased in both atria and ventricles, but much more in atria. The duration of the "P wave" (index of atrial conduction time) was prolonged to a much greater extent than the duration of the "QRS complex" (index of ventricular conduction time). Wenxin Keli significantly reduced I(Na) and shifted steady-state inactivation to more negative potentials in HEK293 cells stably expressing SCN5A. Wenxin Keli prevented the induction of persistent AF in 100% atria (6/6) and, in another experimental series, was found to terminate persistent acetylcholine-mediated AF in 100% of atria (3/3).Wenxin Keli produces atrial-selective depression of I(Na)-dependent parameters in canine isolated coronary-perfused preparations via a unique mechanism and is effective in suppressing AF and preventing its induction, with minimal effects on the ventricular electrophysiology.
Project description:Slowly inactivating Na+ channels conducting "late" Na+ current (INa,late) contribute to ventricular arrhythmogenesis under pathological conditions. INa,late was also reported to play a role in chronic atrial fibrillation (AF). The objective of this study was to investigate INa,late in human right atrial cardiomyocytes as a putative drug target for treatment of AF. To activate Na+ channels, cardiomyocytes from transgenic mice which exhibit INa,late (?KPQ), and right atrial cardiomyocytes from patients in sinus rhythm (SR) and AF were voltage clamped at room temperature by 250-ms long test pulses to -30 mV from a holding potential of -80 mV with a 100-ms pre-pulse to -110 mV (protocol I). INa,late at -30 mV was not discernible as deviation from the extrapolated straight line IV-curve between -110 mV and -80 mV in human atrial cells. Therefore, tetrodotoxin (TTX, 10 ?M) was used to define persistent inward current after 250 ms at -30 mV as INa,late. TTX-sensitive current was 0.27±0.06 pA/pF in ventricular cardiomyocytes from ?KPQ mice, and amounted to 0.04±0.01 pA/pF and 0.09±0.02 pA/pF in SR and AF human atrial cardiomyocytes, respectively. With protocol II (holding potential -120 mV, pre-pulse to -80 mV) TTX-sensitive INa,late was always larger than with protocol I. Ranolazine (30 ?M) reduced INa,late by 0.02±0.02 pA/pF in SR and 0.09±0.02 pA/pF in AF cells. At physiological temperature (37°C), however, INa,late became insignificant. Plateau phase and upstroke velocity of action potentials (APs) recorded with sharp microelectrodes in intact human trabeculae were more sensitive to ranolazine in AF than in SR preparations. Sodium channel subunits expression measured with qPCR was high for SCN5A with no difference between SR and AF. Expression of SCN8A and SCN10A was low in general, and lower in AF than in SR. In conclusion, We confirm for the first time a TTX-sensitive current (INa,late) in right atrial cardiomyocytes from SR and AF patients at room temperature, but not at physiological temperature. While our study provides evidence for the presence of INa,late in human atria, the potential of such current as a target for the treatment of AF remains to be demonstrated.
Project description:Animal studies show that transforming growth factor-?1 (TGF-?1) is an important mediator of atrial fibrosis and atrial fibrillation (AF). This study investigated the role of TGF-?1 in human AF and the mechanism of atrial-selective fibrosis.Atrial specimens from 17 open heart surgery patients and left atrial and ventricular specimens from 17 explanted hearts were collected to assess the relationship between TGF-?1, AF, and differential atrial vs. ventricular TGF-?1 levels. A transgenic mouse model overexpressing active TGF-?1 was used to study the mechanisms underlying the resultant atrial-selective fibrosis. Higher right atrial total TGF-?1 levels (2.58 ± 0.16-fold, P < 0.0001) and active TGF-?1 (3.7 ± 0.7-fold, P = 0.013) were observed in those that developed post-operative AF. Although no ventricular differences were observed, 11 explanted heart failure hearts exhibited higher atrial TGF-?1 levels than 6 non-failing hearts (2.30 ± 0.87 fold higher, P < 0.001). In the transgenic mouse, TGF-?1 receptor-1 kinase blockade resulted in decreased atrial expression of fibrosis-related genes. By RNA microarray analyses in that model, 80 genes in the atria and only 2 genes in the ventricle were differentially expressed. Although these mice atria, but not the ventricles, exhibited increased expression of fibrosis-related genes and phosphorylation of Smad2, there were no differences in TGF-?1 receptor levels or Smads in the atria compared with the ventricles.TGF-?1 mediates selective atrial fibrosis in AF that occurs via TGF-? Receptor 1/2 and the classical Smad pathway. The differential atrial vs. ventricular fibrotic response occurs at the level of TGF-?1 receptor binding or phosphorylation.
Project description:Acute effects of heptanol (0.1 to 2 mM) on atrial electrophysiology were explored in Langendorff-perfused mouse hearts. Left atrial bipolar electrogram or monophasic action potential recordings were obtained during right atrial stimulation. Regular pacing at 8 Hz elicited atrial activity in 11 out of 11 hearts without inducing atrial arrhythmias. Programmed electrical stimulation using a S1S2 protocol provoked atrial tachy-arrhythmias in 9 of 17 hearts. In the initially arrhythmic group, 2 mM heptanol exerted anti-arrhythmic effects (Fisher's exact test, P < 0.05) and increased atrial effective refractory period (ERP) from 26.0 ± 1.9 to 57.1 ± 2.5 ms (ANOVA, P < 0.001) despite increasing activation latency from 18.7 ± 1.1 to 28.9 ± 2.1 ms (P < 0.001) and leaving action potential duration at 90% repolarization (APD90) unaltered (25.6 ± 1.2 vs. 27.2 ± 1.2 ms; P > 0.05), which led to increases in ERP/latency ratio from 1.4 ± 0.1 to 2.1 ± 0.2 and ERP/APD90 ratio from 1.0 ± 0.1 to 2.1 ± 0.2 (P < 0.001). In contrast, in the initially non-arrhythmic group, heptanol did not alter arrhythmogenicity, increased AERP from 47.3 ± 5.3 to 54.5 ± 3.1 ms (P < 0.05) and activation latency from 23.7 ± 2.2 to 31.3 ± 2.5 ms and did not alter APD90 (24.1 ± 1.2 vs. 25.0 ± 2.3 ms; P > 0.05), leaving both AERP/latency ratio (2.1 ± 0.3 vs. 1.9 ± 0.2; P > 0.05) and ERP/APD90 ratio (2.0 ± 0.2 vs. 2.1 ± 0.1; P > 0.05) unaltered. Lower heptanol concentrations (0.1, 0.5 and 1 mM) did not alter arrhythmogenicity or the above parameters. The present findings contrast with known ventricular pro-arrhythmic effects of heptanol associated with decreased ERP/latency ratio, despite increased ERP/APD ratio observed in both the atria and ventricles.
Project description:Comparison of total RNA of atrial and ventricular chambers from adult zebrafish heart. The goal was to identify chamber specific transcripts and atrial enriched transcripts that can be linked to atrial or ventricular septal defects in mammals. Overall design: The hearts of 12 adult zebrafish (6mpf) were extracted and the atria were dissected from the ventricles. The total RNA of the atria was compared to the total RNA from the ventricles.
Project description:Background: Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and one of the major causes of cardiovascular morbidity and mortality. Despite good progress within the past years, safe and effective treatment of AF remains an unmet clinical need. The anti-anginal agent ranolazine has been shown to exhibit antiarrhythmic properties via mainly late INa and IKr blockade. This results in prolongation of the atrial action potential duration (APD) and effective refractory period (ERP) with lower effect on ventricular electrophysiology. Furthermore, ranolazine has been shown to be effective in the treatment of AF. TASK-1 is a two-pore domain potassium (K2P) channel that shows nearly atrial specific expression within the human heart and has been found to be upregulated in AF, resulting in shortening the atrial APD in patients suffering from AF. We hypothesized that inhibition TASK-1 contributes to the observed electrophysiological and clinical effects of ranolazine. Methods: We used Xenopus laevis oocytes and CHO-cells as heterologous expression systems for the study of TASK-1 inhibition by ranolazine and molecular drug docking simulations to investigate the ranolazine binding site and binding characteristics. Results: Ranolazine acts as an inhibitor of TASK-1 potassium channels that inhibits TASK-1 currents with an IC50 of 30.6 ± 3.7 µM in mammalian cells and 198.4 ± 1.1 µM in X. laevis oocytes. TASK-1 inhibition by ranolazine is not frequency dependent but shows voltage dependency with a higher inhibitory potency at more depolarized membrane potentials. Ranolazine binds within the central cavity of the TASK-1 inner pore, at the bottom of the selectivity filter. Conclusions: In this study, we show that ranolazine inhibits TASK-1 channels. We suggest that inhibition of TASK-1 may contribute to the observed antiarrhythmic effects of Ranolazine. This puts forward ranolazine as a prototype drug for the treatment of atrial arrhythmia because of its combined efficacy on atrial electrophysiology and lower risk for ventricular side effects.
Project description:BACKGROUND:Evidence has emerged that small-conductance Ca2+-activated K+ (SK) channels constitute a new target for treatment of atrial fibrillation (AF). SK channels are predominantly expressed in the atria as compared with the ventricles. Various marketed antiarrhythmic drugs are limited by ventricular adverse effects and efficacy loss as AF progresses. METHODS AND RESULTS:A total of 43 pigs were used for the studies. AF reversion in conscious long-term tachypaced pigs: Pigs were subjected to atrial tachypacing (7 Hz) until they developed sustained AF that could not be reverted by vernakalant 4 mg/kg (18.8±3.3 days of atrial tachypacing). When the SK channel inhibitor AP14145 was tested in these animals, vernakalant-resistant AF was reverted to sinus rhythm, and reinduction of AF by burst pacing (50 Hz) was prevented in 8 of 8 pigs. Effects on refractory period and AF duration in open chest pigs: The effects of AP14145 and vernakalant on the effective refractory periods and acute burst pacing-induced AF were examined in anaesthetized open chest pigs. Both vernakalant and AP14145 significantly prolonged atrial refractoriness and reduced AF duration without affecting the ventricular refractoriness or blood pressure in pigs subjected to 7 days atrial tachypacing, as well as in sham-operated control pigs. CONCLUSIONS:SK currents play a role in porcine atrial repolarization, and pharmacological inhibition of these with AP14145 demonstrates antiarrhythmic effects in a vernakalant-resistant porcine model of AF. These results suggest SK channel blockers as potentially interesting anti-AF drugs.