Air Pollution and Cardiac Arrhythmias: From Epidemiological and Clinical Evidences to Cellular Electrophysiological Mechanisms

Cardiovascular disease is the leading cause of death worldwide and kills over 17 million people per year. In the recent decade, growing epidemiological evidence links air pollution and cardiac arrhythmias, suggesting a detrimental influence of air pollution on cardiac electrophysiological functionality. However, the proarrhythmic mechanisms underlying the air pollution-induced cardiac arrhythmias are not fully understood. The purpose of this work is to provide recent advances in air pollution-induced arrhythmias with a comprehensive review of the literature on the common air pollutants and arrhythmias. Six common air pollutants of widespread concern are discussed, namely particulate matter, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen dioxide, and ozone. The epidemiological and clinical reports in recent years are reviewed by pollutant type, and the recently identified mechanisms including both the general pathways and the direct influences of air pollutants on the cellular electrophysiology are summarized. Particularly, this review focuses on the impaired ion channel functionality underlying the air pollution-induced arrhythmias. Alterations of ionic currents directly by the air pollutants, as well as the alterations mediated by intracellular signaling or other more general pathways are reviewed in this work. Finally, areas for future research are suggested to address several remaining scientific questions.

Late Na+ Current Is [Ca2+]i-Dependent in Canine Ventricular Myocytes

Enhancement of the late sodium current (I_NaL) increases arrhythmia propensity in the heart, whereas suppression of the current is antiarrhythmic. In the present study, we investigated I_NaL in canine ventricular cardiomyocytes under action potential voltage-clamp conditions using the selective Na⁺ channel inhibitors GS967 and tetrodotoxin. Both 1 µM GS967 and 10 µM tetrodotoxin dissected largely similar inward currents. The amplitude and integral of the GS967-sensitive current was significantly smaller after the reduction of intracellular Ca²⁺ concentration ([Ca²⁺]_i) either by superfusion of the cells with 1 µM nisoldipine or by intracellular application of 10 mM BAPTA. Inhibiting calcium/calmodulin-dependent protein kinase II (CaMKII) by KN-93 or the autocamtide-2-related inhibitor peptide similarly reduced the amplitude and integral of I_NaL. Action potential duration was shortened in a reverse rate-dependent manner and the plateau potential was depressed by GS967. This GS967-induced depression of plateau was reduced by pretreatment of the cells with BAPTA-AM. We conclude that (1) I_NaL depends on the magnitude of [Ca²⁺]_i in canine ventricular cells, (2) this [Ca²⁺]_i-dependence of I_NaL is mediated by the Ca²⁺-dependent activation of CaMKII, and (3) I_NaL is augmented by the baseline CaMKII activity.

Inhibition of Ca2+-dependent protein kinase C rescues high calcium-induced pro-arrhythmogenic cardiac alternans in rabbit hearts

Cardiac alternans closely linked to calcium dysregulation is a crucial risk factor for fatal arrhythmia causing especially sudden death. Calcium overload is well-known to activate Ca²⁺-dependent protein kinase C (PKC); however, the effects of PKC on arrhythmogenic cardiac alternans have not yet been investigated. This study aimed to determine the contributions of PKC activities in cardiac alternans associated with calcium cycling disturbances. In the present study, action potential duration alternans (APD-ALT) induced by high free intracellular calcium ([Ca²⁺]_i) exerted not only in a calcium concentration-dependent manner but also in a frequency-dependent manner. High [Ca²⁺]_i-induced APD-ALT was suppressed by not only BAPTA-AM but also nifedipine. On the other hand, PKC inhibitors BIM and Gö 6976 eliminated high [Ca²⁺]_i-induced APD-ALT, and PKC activator PMA was found to induce APD-ALT at normal [Ca²⁺]_i condition. Furthermore, BIM effectively prevented calcium transient alternans (CaT-ALT) and even CaT disorders caused by calcium overload. Moreover, BIM not only eliminated electrocardiographic T-wave alternans (TWA) caused by calcium dysregulation, but also lowered the incidence of ventricular arrhythmias in isolated hearts. What's more, BIM prevented the expression of PKC α upregulated by calcium overload in high calcium-perfused hearts. We firstly found that pharmacologically inhibiting Ca²⁺-dependent PKC over-activation suppressed high [Ca²⁺]_i-induced cardiac alternans. This recognition indicates that inhibition of PKC activities may become a therapeutic target for the prevention of pro-arrhythmogenic cardiac alternans associated with calcium dysregulation.

Arrhythmogenic and antiarrhythmic actions of late sustained sodium current in the adult human heart

Late sodium current (late INa) inhibition has been proposed to suppress the incidence of arrhythmias generated by pathological states or induced by drugs. However, the role of late INa in the human heart is still poorly understood. We therefore investigated the role of this conductance in arrhythmias using adult primary cardiomyocytes and tissues from donor hearts. Potentiation of late INa with ATX-II (anemonia sulcata toxin II) and E-4031 (selective blocker of the hERG channel) slowed the kinetics of action potential repolarization, impaired Ca2+ homeostasis, increased contractility, and increased the manifestation of arrhythmia markers. These effects could be reversed by late INa inhibitors, ranolazine and GS-967. We also report that atrial tissues from donor hearts affected by atrial fibrillation exhibit arrhythmia markers in the absence of drug treatment and inhibition of late INa with GS-967 leads to a significant reduction in arrhythmic behaviour. These findings reveal a critical role for the late INa in cardiac arrhythmias and suggest that inhibition of this conductance could provide an effective therapeutic strategy. Finally, this study highlights the utility of human ex-vivo heart models for advancing cardiac translational sciences.

Isoliensinine Eliminates Afterdepolarizations Through Inhibiting Late Sodium Current and L-Type Calcium Current

Isoliensinine (IL) extracted from lotus seed has a good therapeutic effect on cardiovascular diseases. However, its effect on ion channels of ventricular myocytes is still unclear. We used whole-cell patch-clamp techniques to detect the effects of IL on transmembrane ion currents and action potential (AP) in isolated rabbit left ventricular myocytes. IL inhibited the transient sodium current (I_NaT), late sodium current (I_NaL) enlarged by sea anemone toxin (ATX II) and L-type calcium current (I_CaL) in a concentration-dependent manner without affecting inward rectifier potassium current (I_K1) and delayed rectifier potassium current (I_K). These inhibitory effects are mainly manifested as reduced the AP amplitude (APA) and maximum depolarization velocity (V_max) and shortened the action potential duration (APD), but had no significant effect on the resting membrane potential (RMP). Moreover, IL significantly eliminated ATX II-induced early afterdepolarizations (EADs) and high extracellular calcium-induced delayed afterdepolarizations (DADs). These results revealed that IL effectively eliminated EADs and DADs through inhibiting I_NaL and I_CaL in ventricular myocytes, which indicates it has potential antiarrhythmic action.

Antiarrhythmic effect of crotonoside by regulating sodium and calcium channels in rabbit ventricular myocytes

AIMS

Detect the antiarrhythmic effect of crotonoside (Cro).

MAIN METHODS

We used whole-cell patch-clamp techniques to detect the effects of Cro on action potentials (APs) and transmembrane ion currents in isolated rabbit left ventricular myocytes. We also verified the effect of Cro on ventricular arrhythmias caused by aconitine in vivo.

KEY FINDINGS

Cro reduced the maximum depolarization velocity (V_max) of APs and shortened the action potential duration (APD) in a concentration-dependent manner, but it had no significant effect on the resting membrane potential (RMP) or action potential amplitude (APA). It also inhibited the peak sodium current (I_Na) and L-type calcium current (I_CaL) in a concentration-dependent manner with half-maximal inhibitory concentrations (IC₅₀) of 192 μmol/L and 159 μmol/L, respectively. However, Cro had no significant effects on the inward rectifier potassium current (I_K1) or rapidly activating delayed rectifier potassium current (I_Kr). Sea anemone toxin II (ATX II) increased the late sodium current (I_NaL), but Cro abolished this effect. Moreover, Cro significantly abolished ATX II-induced early afterdepolarizations (EADs) and high extracellular Ca²⁺ concentration (3.6 mmol/L)-induced delayed afterdepolarizations (DADs). We also verified that Cro effectively delayed the onset time and reduced the incidence of ventricular arrhythmias caused by aconitine in vivo.

SIGNIFICANCE

These results revealed that Cro effectively inhibits I_Na, I_NaL, and I_CaL in ventricular myocytes. Cro has antiarrhythmic potential and thus deserves further study.

Arrhythmogenic mechanisms of obstructive sleep apnea in heart failure patients

Heart failure (HF) affects 23 million people worldwide and results in 300000 annual deaths. It is associated with many comorbidities, such as obstructive sleep apnea (OSA), and risk factors for both conditions overlap. Eleven percent of HF patients have OSA and 7.7% of OSA patients have left ventricular ejection fraction <50% with arrhythmias being a significant comorbidity in HF and OSA patients. Forty percent of HF patients develop atrial fibrillation (AF) and 30%-50% of deaths from cardiac causes in HF patients are from sudden cardiac death. OSA is prevalent in 32%-49% of patients with AF and there is a dose-dependent relationship between OSA severity and resistance to anti-arrhythmic therapies. HF and OSA lead to various downstream arrhythmogenic mechanisms, including metabolic derangement, remodeling, inflammation, and autonomic imbalance. (1) Metabolic derangement and production of reactive oxidative species increase late Na+ currents, decrease outward K+ currents and downregulate connexin-43 and cell-cell coupling. (2) remodeling also features downregulated K+ currents in addition to decreased Na+/K+ ATPase currents, altered Ca2+ homeostasis, and increased density of If current. (3) Chronic inflammation leads to downregulation of both Nav1.5 channels and K+ channels, altered Ca2+ homeostasis and reduced cellular coupling from alterations of connexin expression. (4) Autonomic imbalance causes arrhythmias by evoking triggered activity through increased Ca2+ transients and reduction of excitation wavefront wavelength. Thus, consideration of these multiple pathophysiological pathways (1-4) will enable the development of novel therapeutic strategies that can be targeted against arrhythmias in the context of complex disease, such as the comorbidities of HF and OSA.

Increase in CO2 levels by upregulating late sodium current is proarrhythmic in the heart

BACKGROUND

Increased CO₂ levels in the general circulation and/or in the myocardium are common under pathologic conditions.

OBJECTIVE

The purpose of this study was to test the hypothesis that an increase in CO₂ levels, and not just the subsequent extra- or intracellular acidosis, would augment late sodium current (I_Na,L) and contribute to arrhythmogenesis in hearts with reduced repolarization reserve.

METHODS

Monophasic action potential durations at 90% completion of repolarization (MAPD₉₀) from isolated rabbit hearts, I_Na,L, and extra- (pH_o) and intracellular pH (pH_i) values from cardiomyocytes using the whole-cell patch-clamp techniques and 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, acetoxymethyl ester (BCECF-AM), respectively, were measured.

RESULTS

Increasing CO₂ levels from 5% to 10% and 20% and administration of 1 nM sea anemone toxin (ATX)-II increased I_Na,L and prolonged both epicardial and endocardial MAPD₉₀ (n = 7 and 10, respectively) without causing arrhythmic activities. Compared to 5% CO₂, 10% and 20% CO₂ decreased pH_o and pH_i in hearts treated with 1 nM ATX-II, caused greater prolongation of MAPD₉₀, and elicited ventricular tachycardias. Increasing CO₂ levels from 5% to 10% and 20% with pH_o maintained at 7.4 produced smaller changes in pH_i (P <.05) but similar increases in I_Na,L, prolongation of MAPD₉₀, and incidence of ventricular tachycardias (n = 8). Inhibition of I_Na,L reversed the increase in I_Na,L, suppressed MAPD₉₀ prolongations, and ventricular tachycardias induced by 20% CO₂. Increased phospho-calmodulin-dependent protein kinase II-δ (CaMKIIδ) and phospho-Na_V1.5 protein levels in hearts treated with 20% CO₂ was attenuated by eleclazine.

CONCLUSION

Increased CO₂ levels enhance I_Na,L and are proarrhythmic factors in hearts with reduced repolarization reserve, possibly via mechanisms related to phosphorylation of CaMKIIδ and Na_V1.5.

Kinetic properties of persistent Na+ current orchestrate oscillatory bursting in respiratory neurons

The rhythmic pattern of breathing depends on the pre-Bötzinger complex (preBötC) in the brainstem, a vital circuit that contains a population of neurons with intrinsic oscillatory bursting behavior. Here, we investigate the specific kinetic properties that enable voltage-gated sodium channels to establish oscillatory bursting in preBötC inspiratory neurons, which exhibit an unusually large persistent Na⁺ current (I_NaP). We first characterize the kinetics of I_NaP in neonatal rat brainstem slices in vitro, using whole-cell patch-clamp and computational modeling, and then test the contribution of I_NaP to rhythmic bursting in live neurons, using the dynamic clamp technique. We provide evidence that subthreshold activation, persistence at suprathreshold potentials, slow inactivation, and slow recovery from inactivation are kinetic features of I_NaP that regulate all aspects of intrinsic rhythmic bursting in preBötC neurons. The slow and cumulative inactivation of I_NaP during the burst active phase controls burst duration and termination, while the slow recovery from inactivation controls the duration of the interburst interval. To demonstrate this mechanism, we develop a Markov state model of I_NaP that explains a comprehensive set of voltage clamp data. By adding or subtracting a computer-generated I_NaP from a live neuron via dynamic clamp, we are able to convert nonbursters into intrinsic bursters, and vice versa. As a control, we test a model with inactivation features removed. Adding noninactivating I_NaP into nonbursters results in a pattern of random transitions between sustained firing and quiescence. The relative amplitude of I_NaP is the key factor that separates intrinsic bursters from nonbursters and can change the fraction of intrinsic bursters in the preBötC. I_NaP could thus be an important target for regulating network rhythmogenic properties.

β-adrenergic regulation of late Na+ current during cardiac action potential is mediated by both PKA and CaMKII

Late Na⁺ current (I_NaL) significantly contributes to shaping cardiac action potentials (APs) and increased I_NaL is associated with cardiac arrhythmias. β-adrenergic receptor (βAR) stimulation and its downstream signaling via protein kinase A (PKA) and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) pathways are known to regulate I_NaL. However, it remains unclear how each of these pathways regulates I_NaL during the AP under physiological conditions. Here we performed AP-clamp experiments in rabbit ventricular myocytes to delineate the impact of each signaling pathway on I_NaL at different AP phases to understand the arrhythmogenic potential. During the physiological AP (2 Hz, 37 °C) we found that I_NaL had a basal level current independent of PKA, but partially dependent on CaMKII. βAR activation (10 nM isoproterenol, ISO) further enhanced I_NaL via both PKA and CaMKII pathways. However, PKA predominantly increased I_NaL early during the AP plateau, whereas CaMKII mainly increased I_NaL later in the plateau and during rapid repolarization. We also tested the role of key signaling pathways through exchange protein activated by cAMP (Epac), nitric oxide synthase (NOS) and reactive oxygen species (ROS). Direct Epac stimulation enhanced I_NaL similar to the βAR-induced CaMKII effect, while NOS inhibition prevented the βAR-induced CaMKII-dependent I_NaL enhancement. ROS generated by NADPH oxidase 2 (NOX2) also contributed to the ISO-induced I_NaL activation early in the AP. Taken together, our data reveal differential modulations of I_NaL by PKA and CaMKII signaling pathways at different AP phases. This nuanced and comprehensive view on the changes in I_NaL during AP deepens our understanding of the important role of I_NaL in reshaping the cardiac AP and arrhythmogenic potential under elevated sympathetic stimulation, which is relevant for designing therapeutic treatment of arrhythmias under pathological conditions.