Spontaneous atrial fibrillation initiated by tyramine in canine atria with increased sympathetic nerve sprouting

Aging-associated atrial fibrillation: A comprehensive review focusing on the potential mechanisms

Atrial fibrillation (AF) has been receiving a lot of attention from scientists and clinicians because it is an extremely common clinical condition. Due to its special hemodynamic changes, AF has a high rate of disability and mortality. So far, although AF has some therapeutic means, it is still an incurable disease because of its complex risk factors and pathophysiologic mechanisms, which is a difficult problem for global public health. Age is an important independent risk factor for AF, and the incidence of AF increases with age. To date, there is no comprehensive review on aging-associated AF. In this review, we systematically discuss the pathophysiologic evidence for aging-associated AF, and in particular explore the pathophysiologic mechanisms of mitochondrial dysfunction, telomere attrition, cellular senescence, disabled macroautophagy, and gut dysbiosis involved in recent studies with aging-associated AF. We hope that by exploring the various dimensions of aging-associated AF, we can better understand the specific relationship between age and AF, which may be crucial for innovative treatments of aging-associated AF.

Linking Electrical Drivers With Atrial Cardiomyopathy for the Targeted Treatment of Atrial Fibrillation

The relationship between atrial fibrillation (AF) and underlying functional and structural abnormalities has received substantial attention in the research literature over the past decade. Significant progress has been made in identifying these changes using non-invasive imaging, voltage mapping, and electrical recordings. Advances in computed tomography and cardiac magnetic resonance imaging can now provide insight regarding the presence and extent of cardiac fibrosis. Additionally, multiple technologies able to identify electrical targets during AF have emerged. However, an organized strategy to employ these resources in the targeted treatment of AF remains elusive. In this work, we will discuss the basis for mechanistic importance of atrial fibrosis and scar as potential sites promoting AF and emerging technologies to identify and target these structural and functional substrates in the electrophysiology laboratory. We also propose an approach to the use of such technologies to serve as a basis for ongoing work in the field.

Neural Remodeling of the Left Atrium in Rats by Rosuvastatin Following Acute Myocardial Infarction

Objective

This study aims to investigate the effect of rosuvastatin on sympathetic neural remodeling of the left atrium (LA) in rats after myocardial infarction (MI).

Methods

Rats were randomly divided into a three groups: sham group, statin group, and MI group. The mRNA expression levels of the growth-associated protein-43 (GAP43) and nerve growth factor (NGF) were measured by RT-PCR. Immunohistochemistry was used to detect the distribution and density of GAP43- and NGF-positive nerves. The expression levels of these proteins were quantified by Western blot.

Results

Compared with the sham group, the average optical density (AOD) values of GAP43 and nerve growth factor (NGF)-positive substances in the LA in the statin and MI groups were significantly higher (P<0.01), but the AOD values in the statin group were lower than of those in the MI group (P<0.01). Furthermore, the AOD values of GAP43 and NGF positive nerves in the left stellate ganglion in the statin and MI groups were significantly higher (P<0.01), but the AOD values in the statin group were lower, when compared with the MI group (P<0.01).

Conclusion

Rosuvastatin could effectively improve the sympathetic neural remodeling of LA in MI rats.

Dynamic clamping human and rabbit atrial calcium current: narrowing ICaL window abolishes early afterdepolarizations

Key points

Early‐afterdepolarizations (EADs) are abnormal action potential oscillations and a known cause of cardiac arrhythmias. Ventricular EADs involve reactivation of a Ca²⁺ current (I_CaL) in its ‘window region’ voltage range. However, electrical mechanisms of atrial EADs, a potential cause of atrial fibrillation, are poorly understood.

Atrial cells were obtained from consenting patients undergoing heart surgery, as well as from rabbits. I_CaL was blocked with nifedipine and then a hybrid patch clamp/mathematical‐modelling technique, ‘dynamic clamping’, was used to record action potentials at the same time as injecting an artificial, modifiable, I_CaL (I_CaL,D‐C).

Progressively widening the I_CaL,D‐C window region produced EADs of various types, dependent on window width. EAD production was strongest upon moving the activation (vs. inactivation) side of the window.

EADs were then induced by a different method: increasing I_CaL,D‐C amplitude and/or K⁺ channel‐blockade (4‐aminopyridine). Narrowing of the I_CaL,D‐C window by ∼10 mV abolished these EADs.

Atrial I_CaL window narrowing is worthy of further testing as a potential anti‐atrial fibrillation drug mechanism.

Abstract

Atrial early‐afterdepolarizations (EADs) may contribute to atrial fibrillation (AF), perhaps involving reactivation of L‐type Ca²⁺ current (I_CaL) in its window region voltage range. The present study aimed (i) to validate the dynamic clamp technique for modifying the I_CaL contribution to atrial action potential (AP) waveform; (ii) to investigate the effects of widening the window I_CaL on EAD‐propensity; and (iii) to test whether EADs from increased I_CaL and AP duration are supressed by narrowing the window I_CaL. I_CaL and APs were recorded from rabbit and human atrial myocytes by whole‐cell‐patch clamp. During AP recording, I_CaL was inhibited (3 µm nifedipine) and replaced by a dynamic clamp model current, I_CaL,D‐C (tuned to native I_CaL characteristics), computed in real‐time (every 50 µs) based on myocyte membrane potential. I_CaL,D‐C‐injection restored the nifedipine‐suppressed AP plateau. Widening the window I_CaL,D‐C, symmetrically by stepwise simultaneous equal shifts of half‐voltages (V_0.5) of I_CaL,D‐C activation (negatively) and inactivation (positively), generated EADs (single, multiple or preceding repolarization failure) in a window width‐dependent manner, as well as AP alternans. A stronger EAD‐generating effect resulted from independently shifting activation V_0.5 (asymmetrical widening) than inactivation V_0.5; for example, a 15 mV activation shift produced EADs in nine of 17 (53%) human atrial myocytes vs. 0 of 18 from inactivation shift (P < 0.05). In 11 rabbit atrial myocytes in which EADs were generated either by increasing the conductance of normal window width I_CaL,D‐C or subsequent 4‐aminopyridine (2 mm), window I_CaL,D‐C narrowing (10 mV) abolished EADs of all types (P < 0.05). The present study validated the dynamic clamp for I_CaL, which is novel in atrial cardiomyocytes, and showed that EADs of various types are generated by widening (particularly asymmetrically) the window I_CaL, as well as abolished by narrowing it. Window I_CaL narrowing is a potential therapeutic mechanism worth pursuing in the search for improved anti‐AF drugs.

Early‐afterdepolarizations (EADs) are abnormal action potential oscillations and a known cause of cardiac arrhythmias. Ventricular EADs involve reactivation of a Ca²⁺ current (I_CaL) in its ‘window region’ voltage range. However, electrical mechanisms of atrial EADs, a potential cause of atrial fibrillation, are poorly understood.

Atrial cells were obtained from consenting patients undergoing heart surgery, as well as from rabbits. I_CaL was blocked with nifedipine and then a hybrid patch clamp/mathematical‐modelling technique, ‘dynamic clamping’, was used to record action potentials at the same time as injecting an artificial, modifiable, I_CaL (I_CaL,D‐C).

Progressively widening the I_CaL,D‐C window region produced EADs of various types, dependent on window width. EAD production was strongest upon moving the activation (vs. inactivation) side of the window.

EADs were then induced by a different method: increasing I_CaL,D‐C amplitude and/or K⁺ channel‐blockade (4‐aminopyridine). Narrowing of the I_CaL,D‐C window by ∼10 mV abolished these EADs.

Atrial I_CaL window narrowing is worthy of further testing as a potential anti‐atrial fibrillation drug mechanism.

Increased susceptibility of spontaneously hypertensive rats to ventricular tachyarrhythmias in early hypertension

Hypertension is a risk factor for sudden cardiac death caused by ventricular tachycardia and fibrillation (VT/VF). We hypothesized that, in early hypertension, the susceptibility to stress-induced VT/VF increases. We compared the susceptibility of 5- to 6-month-old male spontaneously hypertensive rats (SHR) and age/sex-matched normotensive rats (NR) to VT/VF during challenge with oxidative stress (H2 O2 ; 0.15 mmol l(-1) ). We found that only SHR hearts exhibited left ventricular fibrosis and hypertrophy. H2 O2 promoted VT in all 30 SHR but none of the NR hearts. In 33% of SHR cases, focal VT degenerated to VF within 3 s. Simultaneous voltage-calcium optical mapping of Langendorff-perfused SHR hearts revealed that H2 O2 -induced VT/VF arose spontaneously from focal activations at the base and mid left ventricular epicardium. Microelectrode recording of SHR hearts showed that VT was initiated by early afterdepolarization (EAD)-mediated triggered activity. However, despite the increased susceptibility of SHR hearts to VT/VF, patch clamped isolated SHR ventricular myocytes developed EADs and triggered activity to the same extent as NR ventricular myocytes, except with larger EAD amplitude. During the early stages of hypertension, when challenged with oxidative stress, SHR hearts showed an increased ventricular arrhythmogenicity that stems primarily from tissue remodelling (hypertrophy, fibrosis) rather than cellular electrophysiological changes. Our findings highlight the need for early hypertension treatment to minimize myocardial fibrosis, ventricular hypertrophy, and arrhythmias.

The effect of the left stellate ganglion on sympathetic neural remodeling of the left atrium in rats following myocardial infarction

BACKGROUND

The neural remodeling of the atrium plays an important role in the initiation of atrial fibrillation after myocardial infarction (MI); however, the effects of the left stellate ganglion (LSG) on the neural remodeling of the atrium remain incompletely understood. Thus, this study investigated the mechanism by which the LSG mediates sympathetic neural remodeling of the left atrium (LA) in rats after MI.

METHODS

Sixty rats were randomly divided into a Sham group and an MI group. The expression levels of growth-associated protein-43 (GAP43) and nerve growth factor (NGF) messenger ribonucleic acid (mRNA) were measured by reverse transcription polymerase chain reaction. Immunohistochemistry was used to detect the distribution and density of GAP43- and NGF-positive nerves. The expression levels of the proteins were quantified by Western blotting.

RESULTS

Compared with the Sham group, GAP43 mRNA expression in the LSG was increased in the MI group (P < 0.01), but not significantly increased in the LA. Immunohistochemical analysis demonstrated that in both the LSG and the LA, the mean densities of GAP43- and NGF-positive nerves in the MI group were increased (P < 0.01). In both the LSG and the LA, the protein levels of GAP43 and NGF in the MI group were increased relative to the Sham group (P < 0.01).

CONCLUSIONS

The increased levels of NGF and GAP43 proteins can induce sympathetic nerve hyperinnervation in the LSG and the LA after MI. The increased GAP43 proteins in the LA, which may have been transported from the LSG, accelerated LA sympathetic neural remodeling in rats.

Cellular and molecular electrophysiology of atrial fibrillation initiation, maintenance, and progression

Atrial fibrillation (AF) is the most common clinically relevant arrhythmia and is associated with increased morbidity and mortality. The incidence of AF is expected to continue to rise with the aging of the population. AF is generally considered to be a progressive condition, occurring first in a paroxysmal form, then in persistent, and then long-standing persistent (chronic or permanent) forms. However, not all patients go through every phase, and the time spent in each can vary widely. Research over the past decades has identified a multitude of pathophysiological processes contributing to the initiation, maintenance, and progression of AF. However, many aspects of AF pathophysiology remain incompletely understood. In this review, we discuss the cellular and molecular electrophysiology of AF initiation, maintenance, and progression, predominantly based on recent data obtained in human tissue and animal models. The central role of Ca(2+)-handling abnormalities in both focal ectopic activity and AF substrate progression is discussed, along with the underlying molecular basis. We also deal with the ionic determinants that govern AF initiation and maintenance, as well as the structural remodeling that stabilizes AF-maintaining re-entrant mechanisms and finally makes the arrhythmia refractory to therapy. In addition, we highlight important gaps in our current understanding, particularly with respect to the translation of these concepts to the clinical setting. Ultimately, a comprehensive understanding of AF pathophysiology is expected to foster the development of improved pharmacological and nonpharmacological therapeutic approaches and to greatly improve clinical management.

Association between obstructive sleep apnea and premature supraventricular contractions

OBJECTIVE

The exact association between obstructive sleep apnea (OSA) and premature supraventricular contractions (PSVCs) has not been established.

METHODS

We prospectively performed polysomnography together with 24-hour Holter electrocardiography in 431 patients who were clinically suspected of having OSA and examined the association between OSA severity and PSVCs during wakefulness and sleep. The patients were classified into 4 groups according to the apnea-hypopnea index (AHI) quartiles (Q1=patients with AHI<13.8, Q2=those with 13.8≤AHI<28.8, Q3=those with 28.8≤AHI<48.1, Q4=those with AHI≥48.1).

RESULTS

The number of PSVCs/hour during sleep differed significantly among the 4 groups, but the number of PSVCs/hour during wakefulness did not. The prevalence of PSVC≥5/hour during sleep was significantly higher in Q4 (21.0%) than the other 3 groups (Q1, 9.0%; Q2, 8.0%; Q3, 6.0%; all p<0.05 for Q4), but the prevalence of PSVC≥5/hour during wakefulness did not differ among the 4 groups. A multivariate logistic regression analysis showed that the highest AHI quartile was significantly associated with PSVC≥5/hour during sleep (odds ratio 3.04, 95% confidence interval 1.44-6.42, p=0.004).

CONCLUSIONS

Severe OSA can cause PSVCs during sleep, but its effect appears not to be strong. Further studies are needed to clarify the clinical significance of this small but significant increase in PSVCs during sleep in severe OSA patients.

Chronic myocardial infarction promotes atrial action potential alternans, afterdepolarizations, and fibrillation

Aims

Atrial fibrillation (AF) is increased in patients with heart failure resulting from myocardial infarction (MI). We aimed to determine the effects of chronic ventricular MI in rabbits on the susceptibility to AF, and underlying atrial electrophysiological and Ca²⁺-handling mechanisms.

Methods and results

In Langendorff-perfused rabbit hearts, under β-adrenergic stimulation with isoproterenol (ISO; 1 µM), 8 weeks MI decreased AF threshold, indicating increased AF susceptibility. This was associated with increased atrial action potential duration (APD)-alternans at 90% repolarization, by 147%, and no significant change in the mean APD or atrial global conduction velocity (CV; n = 6–13 non-MI hearts, 5–12 MI). In atrial isolated myocytes, also under β-stimulation, L-type Ca²⁺ current (I_CaL) density and intracellular Ca²⁺-transient amplitude were decreased by MI, by 35 and 41%, respectively, and the frequency of spontaneous depolarizations (SDs) was substantially increased. MI increased atrial myocyte size and capacity, and markedly decreased transverse-tubule density. In non-MI hearts perfused with ISO, the I_CaL-blocker nifedipine, at a concentration (0.02 µM) causing an equivalent I_CaL reduction (35%) to that from the MI, did not affect AF susceptibility, and decreased APD.

Conclusion

Chronic MI in rabbits remodels atrial structure, electrophysiology, and intracellular Ca²⁺ handling. Increased susceptibility to AF by MI, under β-adrenergic stimulation, may result from associated production of atrial APD alternans and SDs, since steady-state APD and global CV were unchanged under these conditions, and may be unrelated to the associated reduction in whole-cell I_CaL. Future studies may clarify potential contributions of local conduction changes, and cellular and subcellular mechanisms of alternans, to the increased AF susceptibility.