Exposure to Secondhand Smoke and Arrhythmogenic Cardiac Alternans in a Mouse Model

Chronic nicotine exposure is associated with electrophysiological and sympathetic remodeling in the intact rabbit heart

Nicotine is the primary addictive component of tobacco products. Through its actions on the heart and autonomic nervous system, nicotine exposure is associated with electrophysiological changes and increased arrhythmia susceptibility. To assess the underlying mechanisms, we treated rabbits with transdermal nicotine (NIC, 21 mg/day) or control (CT) patches for 28 days before performing dual optical mapping of transmembrane potential (RH237) and intracellular Ca2+ (Rhod-2 AM) in isolated hearts with intact sympathetic innervation. Sympathetic nerve stimulation (SNS) was performed at the first to third thoracic vertebrae, and β-adrenergic responsiveness was additionally evaluated following norepinephrine (NE) perfusion. Baseline ex vivo heart rate (HR) and SNS stimulation threshold were higher in NIC versus CT (P = 0.004 and P = 0.003, respectively). Action potential duration alternans emerged at longer pacing cycle lengths (PCL) in NIC versus CT at baseline (P = 0.002) and during SNS (P = 0.0003), with similar results obtained for Ca2+ transient alternans. SNS shortened the PCL at which alternans emerged in CT but not in NIC hearts. NIC-exposed hearts tended to have slower and reduced HR responses to NE perfusion, but ventricular responses to NE were comparable between groups. Although fibrosis was unaltered, NIC hearts had lower sympathetic nerve density (P = 0.03) but no difference in NE content versus CT. These results suggest both sympathetic hypoinnervation of the myocardium and regional differences in β-adrenergic responsiveness with NIC. This autonomic remodeling may contribute to the increased risk of arrhythmias associated with nicotine exposure, which may be further exacerbated with long-term use.NEW & NOTEWORTHY Here, we show that chronic nicotine exposure was associated with increased heart rate, increased susceptibility to alternans, and reduced sympathetic electrophysiological responses in the intact rabbit heart. We suggest that this was due to sympathetic hypoinnervation of the myocardium and diminished β-adrenergic responsiveness of the sinoatrial node following nicotine treatment. Though these differences did not result in increased arrhythmia propensity in our study, we hypothesize that prolonged nicotine exposure may exacerbate this proarrhythmic remodeling.

The heart-bone connection: relationships between myocardial infarction and osteoporotic fracture

Myocardial infarction (MI) and osteoporotic fracture (Fx) are two of the leading causes of mortality and morbidity worldwide. Although these traumatic injuries are treated as if they are independent, there is epidemiological evidence linking the incidence of Fx and MI, thus raising the question of whether each of these events can actively influence the risk of the other. Atherosclerotic cardiovascular disease and osteoporosis, the chronic conditions leading to MI and Fx, are known to have shared pathoetiology. Furthermore, sustained systemic inflammation after traumas such as MI and Fx has been shown to exacerbate both underlying chronic conditions. However, the effects of MI and Fx outside their own system have not been well studied. The sympathetic nervous system (SNS) and the complement system initiate a systemic response after MI that could lead to subsequent changes in bone remodeling through osteoclasts. Similarly, SNS and complement system activation following fracture could lead to heart tissue damage and exacerbate atherosclerosis. To determine whether damaging bone-heart cross talk may be important comorbidity following Fx or MI, this review details the current understanding of bone loss after MI, cardiovascular damage after Fx, and possible shared underlying mechanisms of these processes.

Chronic nicotine exposure is associated with electrophysiological and sympathetic remodeling in the intact rabbit heart

Nicotine is the primary addictive component in tobacco products. Through its actions on the heart and autonomic nervous system, nicotine exposure is associated with electrophysiological changes and increased arrhythmia susceptibility. However, the underlying mechanisms are unclear. To address this, we treated rabbits with transdermal nicotine (NIC, 21 mg/day) or control (CT) patches for 28 days prior to performing dual optical mapping of transmembrane potential (RH237) and intracellular Ca 2+ (Rhod-2 AM) in isolated hearts with intact sympathetic innervation. Sympathetic nerve stimulation (SNS) was performed at the 1 st - 3 rd thoracic vertebrae, and β-adrenergic responsiveness was additionally evaluated as changes in heart rate (HR) following norepinephrine (NE) perfusion. Baseline ex vivo HR and SNS stimulation threshold were increased in NIC vs. CT ( P = 0.004 and P = 0.003 respectively). Action potential duration alternans emerged at longer pacing cycle lengths (PCL) in NIC vs. CT at baseline ( P = 0.002) and during SNS ( P = 0.0003), with similar results obtained for Ca 2+ transient alternans. SNS reduced the PCL at which alternans emerged in CT but not NIC hearts. NIC exposed hearts also tended to have slower and reduced HR responses to NE perfusion. While fibrosis was unaltered, NIC hearts had lower sympathetic nerve density ( P = 0.03) but no difference in NE content vs. CT. These results suggest both sympathetic hypo-innervation of the myocardium and diminished β-adrenergic responsiveness with NIC. This autonomic remodeling may underlie the increased risk of arrhythmias associated with nicotine exposure, which may be further exacerbated with continued long-term usage.

NEW & NOTEWORTHY

Here we show that chronic nicotine exposure was associated with increased heart rate, lower threshold for alternans and reduced sympathetic electrophysiological responses in the intact rabbit heart. We suggest that this was due to the sympathetic hypo-innervation of the myocardium and diminished β- adrenergic responsiveness observed following nicotine treatment. Though these differences did not result in increased arrhythmia propensity in our study, we hypothesize that prolonged nicotine exposure may exacerbate this pro-arrhythmic remodeling.

E-cigarettes and arrhythmogenesis: a comprehensive review of pre-clinical studies and their clinical implications

Electronic cigarette use has grown exponentially in recent years, and while their popularity has increased, the long-term effects on the heart are yet to be fully studied and understood. Originally designed as devices to assist with those trying to quit traditional combustible cigarette use, their popularity has attracted use by teens and adolescents who traditionally have not smoked combustible cigarettes. Acute effects on the heart have been shown to be similar to traditional combustible cigarettes, including increased heart rate and blood pressure. The main components of electronic cigarettes that contribute to these arrhythmic effects are found in the e-liquid that is aerosolized and inhaled, comprised of nicotine, flavourings, and a combination of vegetable glycerin (VG) and propylene glycol (PG). Nicotine can potentially induce both ventricular and atrial arrhythmogenesis, with both the atrial and ventricular effects resulting from the interactions of nicotine and the catecholamines they release via potassium channels. Atrial arrhythmogenesis, more specifically atrial fibrillation, can also occur due to structural alterations, which happens because of nicotine downregulating microRNAs 133 and 590, both post-transcriptional growth factor repressors. Liquid flavourings and the combination of PG and VG can possibly lead to arrhythmic events by exposing users to acrolein, an aldehyde that stimulates TRPA1 that in turn causes a change towards sympathetic activation and autonomic imbalance. The design of these electronic delivery devices is constantly changing; therefore, it has proven extremely difficult to study the long-term effects on the heart caused by electronic cigarettes but will be important to understand given their rising popularity. The arrhythmic effects of electronic cigarettes appear similar to traditional cigarettes as well; however, a comprehensive review has not been compiled and is the focus of this article.

Effects of chronic secondhand smoke exposure on cardiovascular regulation and the role of soluble epoxide hydrolase in mice

Background: Secondhand smoke (SHS) is a significant risk factor for cardiovascular morbidity and mortality with an estimated 80% of SHS-related deaths attributed to cardiovascular causes. Public health measures and smoking bans have been successful both in reducing SHS exposure and improving cardiovascular outcomes in non-smokers. Soluble epoxide hydrolase (sEH) inhibitors have been shown to attenuate tobacco exposure-induced lung inflammatory responses, making them a promising target for mitigating SHS exposure-induced cardiovascular outcomes. Objectives: The objectives of this study were to determine 1) effects of environmentally relevant SHS exposure on cardiac autonomic function and blood pressure (BP) regulation and 2) whether prophylactic administration of an sEH inhibitor (TPPU) can reduce the adverse cardiovascular effects of SHS exposure. Methods: Male C57BL/6J mice (11 weeks old) implanted with BP/electrocardiogram (ECG) telemetry devices were exposed to filtered air or 3 mg/m3 of SHS (6 hr/d, 5 d/wk) for 12 weeks, followed by 4 weeks of recovery in filtered air. Some mice received TPPU in drinking water (15 mg/L) throughout SHS exposure. BP, heart rate (HR), HR variability (HRV), baroreflex sensitivity (BRS), and BP variability were determined monthly. Results: SHS exposure significantly decreased 1) short-term HRV by ∼20% (p < 0.05) within 4 weeks; 2) overall HRV with maximum effect at 12 weeks (-15%, p < 0.05); 3) pulse pressure (-8%, p < 0.05) as early as week 4; and 4) BRS with maximum effect at 12 weeks (-11%, p < 0.05). Four weeks of recovery following 12 weeks of SHS ameliorated all SHS-induced cardiovascular detriments. Importantly, mice exposed to TPPU in drinking water during SHS-related exposure were protected from SHS cardiovascular consequences. Discussion: The data suggest that 1) environmental relevant SHS exposure significantly alters cardiac autonomic function and BP regulation; 2) cardiovascular consequences from SHS can be reversed by discontinuing SHS exposure; and 3) inhibiting sEH can prevent SHS-induced cardiovascular consequences.

Whole-heart multiparametric optical imaging reveals sex-dependent heterogeneity in cAMP signaling and repolarization kinetics

Cyclic adenosine 3',5'-monophosphate (cAMP) is a key second messenger in cardiomyocytes responsible for transducing autonomic signals into downstream electrophysiological responses. Previous studies have shown intracellular heterogeneity and compartmentalization of cAMP signaling. However, whether cAMP signaling occurs heterogeneously throughout the intact heart and how this drives sex-dependent functional responses are unknown. Here, we developed and validated a novel cardiac-specific fluorescence resonance energy transfer-based cAMP reporter mouse and a combined voltage-cAMP whole-heart imaging system. We showed that in male hearts, cAMP was uniformly activated in response to pharmacological β-adrenergic stimulation. In contrast, female hearts showed that cAMP levels decayed faster in apical versus basal regions, which was associated with nonuniform action potential changes and notable changes in the direction of repolarization. Apical phosphodiesterase (PDE) activity was higher in female versus male hearts, and PDE inhibition prevented repolarization changes in female hearts. Thus, our imaging approach revealed sex-dependent regional breakdown of cAMP and associated electrophysiological differences.

Secondhand Smoke Decreased Excitability and Altered Action Potential Characteristics of Cardiac Vagal Neurons in Mice

Background: Secondhand smoke (SHS), a major indoor pollutant, is a significant risk factor for cardiovascular morbidity and mortality including arrhythmias and sudden cardiac death. Exposure to SHS can produce autonomic imbalance, as evidenced by reduced heart rate variability (HRV)—a clinical metric of cardiac vagal regulation. Currently, the mechanisms through which SHS changes the vagal preganglionic neuronal inputs to the heart to produce this remains unknown.

Objectives: To characterize the effect of SHS on both the excitability and action potential (AP) characteristics of anatomically identified cardiac vagal neurons (CVNs) in the nucleus ambiguus and examine whether SHS alters small conductance calcium-activated potassium (SK) channel activity of these CVNs.

Methods: Adult male mice were exposed to four weeks of filtered air or SHS (3 mg/m³) 6 h/day, 5 day/week. Using patch-clamp recordings on identified CVNs in brainstem slices, we determined neuronal excitability and AP characteristics with depolarizing step- and ramp-current injections.

Results: Four weeks of SHS exposure reduced spiking responses to depolarizing current injections and increased AP voltage threshold in CVNs. Perfusion with apamin (20 nM) magnified these SHS-induced effects, suggesting reduced SK channel activity may serve to minimize the SHS-induced decreases in CVNs excitability. Medium afterhyperpolarization (a measurement of SK channel activity) was smaller in the SHS group, further supporting a lower SK channel activity. AP amplitude, rise rate, fast afterhyperpolarization amplitude (a measurement of voltage-gated channel activity), and decay rate were higher in the SHS group at membrane voltages more positive to 0 mV, suggesting altered inactivation properties of voltage-dependent channels underlying APs.

Discussion: SHS exposure reduced neuronal excitability of CVNs with compensatory attenuation of SK channel activity and altered AP characteristics. Neuroplasticity of CVNs could blunt regulatory cardiac vagal signaling and contribute to the cardiovascular consequences associated with SHS exposure, including reduced HRV.

Secondhand Smoke Exposure Impairs Ion Channel Function and Contractility of Mesenteric Arteries

Cigarette smoke, including secondhand smoke (SHS), has significant detrimental vascular effects, but its effects on myogenic tone of small resistance arteries and the underlying mechanisms are understudied. Although it is apparent that SHS contributes to endothelial dysfunction, much less is known about how this toxicant alters arterial myocyte contraction, leading to alterations in myogenic tone. The study's goal is to determine the effects of SHS on mesenteric arterial myocyte contractility and excitability. C57BL/6J male mice were randomly assigned to either filtered air (FA) or SHS (6 h/d, 5 d/wk) exposed groups for a 4, 8, or 12-weeks period. Third and fourth-order mesenteric arteries and arterial myocytes were acutely isolated and evaluated with pressure myography and patch clamp electrophysiology, respectively. Myogenic tone was found to be elevated in mesenteric arteries from mice exposed to SHS for 12 wk but not for 4 or 8 wk. These results were correlated with an increase in L-type Ca2+ channel activity in mesenteric arterial myocytes after 12 wk of SHS exposure. Moreover, 12 wk SHS exposed arterial myocytes have reduced total potassium channel current density, which correlates with a depolarized membrane potential (Vm). These results suggest that SHS exposure induces alterations in key ionic conductances that modulate arterial myocyte contractility and myogenic tone. Thus, chronic exposure to an environmentally relevant concentration of SHS impairs mesenteric arterial myocyte electrophysiology and myogenic tone, which may contribute to increased blood pressure and risks of developing vascular complications due to passive exposure to cigarette smoke.

Aging Disrupts Normal Time-of-Day Variation in Cardiac Electrophysiology

BACKGROUND

Cardiac gene expression and arrhythmia occurrence have time-of-day variation; however, daily changes in cardiac electrophysiology, arrhythmia susceptibility, and Ca²⁺ handling have not been characterized. Furthermore, how these patterns change with age is unknown.

METHODS

Hearts were isolated during the light (zeitgeber time [ZT] 4 and ZT9) and dark cycle (ZT14 and ZT21) from adult (12-18 weeks) male mice. Hearts from aged (18-20 months) male mice were isolated at ZT4 and ZT14. All hearts were Langendorff-perfused for optical mapping with voltage- and Ca²⁺-sensitive dyes (n=4-7/group). Cardiac gene and protein expression were assessed with real-time polymerase chain reaction (n=4-6/group) and Western blot (n=3-4/group).

RESULTS

Adult hearts had the shortest action potential duration (APD) and Ca²⁺ transient duration (CaTD) at ZT14 (APD₈₀: ZT4: 45.4±4.1 ms; ZT9: 45.1±8.6 ms; ZT14: 34.7±4.2 ms; ZT21: 49.2±7.6 ms, P<0.05 versus ZT4 and ZT21; and CaTD₈₀: ZT4: 70.1±3.3 ms; ZT9: 72.7±2.7 ms; ZT14: 64.3±3.3 ms; ZT21: 74.4±1.2 ms, P<0.05 versus other time points). The pacing frequency at which CaT alternans emerged was faster, and average CaT alternans magnitude was significantly reduced at ZT14 compared with the other time points. There was a trend for decreased spontaneous premature ventricular complexes and pacing-induced ventricular arrhythmias at ZT14, and the hearts at ZT14 had diminished responses to isoproterenol compared with ZT4 (ZT4: 49.5.0±5.6% versus ZT14: 22.7±9.5% decrease in APD, P<0.01). In contrast, aged hearts exhibited no difference between ZT14 and ZT4 in nearly every parameter assessed (except APD₈₀: ZT4: 39.7±1.9 ms versus ZT14: 33.8±3.1 ms, P<0.01). Gene expression of KCNA5 (potassium voltage-gated channel subfamily A member 5; encoding Kv1.5) was increased, whereas gene expression of ADRB1 (encoding β1-adrenergic receptors) was decreased at ZT14 versus ZT4 in adult hearts. No time-of-day changes in expression or phosphorylation of Ca²⁺ handling proteins (SERCA2 [sarco/endoplasmic reticulum Ca²⁺-ATPase], RyR2 [ryanodine receptor 2], and PLB [phospholamban]) was found in ex vivo perfused adult isolated hearts.

CONCLUSIONS

Isolated adult hearts have strong time-of-day variation in cardiac electrophysiology, Ca²⁺ handling, and adrenergic responsiveness, which is disrupted with age.

Is second hand smoking associated with atrial fibrillation risk among women in Israel? A case-control study

BACKGROUND

The first complete ban on indoor smoking in all public spaces (including bars and restaurants) occurred in 1994 in Israel, because of clear scientific evidence that SHS (Second Hand Smoking) is dangerous to non-smokers. Despite the smoking-ban law warning about the dangers of smoking, SHS remains in houses. The role of cigarette smoking on cardiac arrhythmia is less clearly defined and secondhand smoke (SHS) impact on the risk of atrial fibrillation (AF) remains unknown. The aim of this study was to evaluate the relationship between SHS and AF risk in Israeli women.

METHODS

This was a population-based case-control study consisting of never-smoking women aged 30-80 from Israel: 102 cases (diagnosed) of AF and 109 population- based controls. All participants were interviewed using a socio-demographic questionnaire that also related to past and current exposure to SHS.

RESULTS

SHS was associated with AF risk with adjusted odds ratio (OR) of 3.81 (95% confidence interval, CI 2.02-7.18). Higher exposure to SHS was associated with higher risk of AF compared to never-exposed women. Those exposed to SHS during one, two, or three life-periods (childhood, adolescence or adulthood) had an OR of 1.71 (95% CI 0.76-3.86), 2.87 (95% CI 1.25-6.56), and 9.14 (95% CI 4.09-20.44), respectively. Moreover, exposure to one pack/day increased the risk of AF by 2.89 times compared to 'never exposed' (95% CI 2.05-4.09).

CONCLUSIONS

SHS exposure in women who never smoked is associated with increased risk for AF.

Hyperglycemia Acutely Increases Cytosolic Reactive Oxygen Species via O-linked GlcNAcylation and CaMKII Activation in Mouse Ventricular Myocytes

RATIONALE

Diabetes mellitus is a complex, multisystem disease, affecting large populations worldwide. Chronic CaMKII (Ca²⁺/calmodulin-dependent kinase II) activation may occur in diabetes mellitus and be arrhythmogenic. Diabetic hyperglycemia was shown to activate CaMKII by (1) O-linked attachment of N-acetylglucosamine (O-GlcNAc) at S280 leading to arrhythmia and (2) a reactive oxygen species (ROS)-mediated oxidation of CaMKII that can increase postinfarction mortality.

OBJECTIVE

To test whether high extracellular glucose (Hi-Glu) promotes ventricular myocyte ROS generation and the role played by CaMKII.

METHODS AND RESULTS

We tested how extracellular Hi-Glu influences ROS production in adult ventricular myocytes, using DCF (2',7'-dichlorodihydrofluorescein diacetate) and genetically targeted Grx-roGFP2 redox sensors. Hi-Glu (30 mmol/L) significantly increased the rate of ROS generation-an effect prevented in myocytes pretreated with CaMKII inhibitor KN-93 or from either global or cardiac-specific CaMKIIδ KO (knockout) mice. CaMKII KO or inhibition also prevented Hi-Glu-induced sarcoplasmic reticulum Ca²⁺ release events (Ca²⁺ sparks). Thus, CaMKII activation is required for Hi-Glu-induced ROS generation and sarcoplasmic reticulum Ca²⁺ leak in cardiomyocytes. To test the involvement of O-GlcNAc-CaMKII pathway, we inhibited GlcNAcylation removal by Thiamet G (ThmG), which mimicked the Hi-Glu-induced ROS production. Conversely, inhibition of GlcNAcylation (OSMI-1 [(αR)-α-[[(1,2-dihydro-2-oxo-6-quinolinyl)sulfonyl]amino]-N-(2-furanylmethyl)-2-methoxy-N-(2-thienylmethyl)-benzeneacetamide]) prevented ROS induction in response to either Hi-Glu or ThmG. Moreover, in a CRSPR-based knock-in mouse in which the functional GlcNAcylation site on CaMKIIδ was ablated (S280A), neither Hi-Glu nor ThmG induced myocyte ROS generation. So CaMKIIδ-S280 is required for the Hi-Glu-induced (and GlcNAc dependent) ROS production. To identify the ROS source(s), we used different inhibitors of NOX (NADPH oxidase) 2 (Gp91ds-tat peptide), NOX4 (GKT137831), mitochondrial ROS (MitoTempo), and NOS (NO synthase) pathway inhibitors (L-NAME, L-NIO, and L-NPA). Only NOX2 inhibition or KO prevented Hi-Glu/ThmG-induced ROS generation.

CONCLUSIONS

Diabetic hyperglycemia induces acute cardiac myocyte ROS production by NOX2 that requires O-GlcNAcylation of CaMKIIδ at S280. This novel ROS induction may exacerbate pathological consequences of diabetic hyperglycemia.

Optocardiography and Electrophysiology Studies of Ex Vivo Langendorff-perfused Hearts

Small animal models are most commonly used in cardiovascular research due to the availability of genetically modified species and lower cost compared to larger animals. Yet, larger mammals are better suited for translational research questions related to normal cardiac physiology, pathophysiology, and preclinical testing of therapeutic agents. To overcome the technical barriers associated with employing a larger animal model in cardiac research, we describe an approach to measure physiological parameters in an isolated, Langendorff-perfused piglet heart. This approach combines two powerful experimental tools to evaluate the state of the heart: electrophysiology (EP) study and simultaneous optical mapping of transmembrane voltage and intracellular calcium using parameter sensitive dyes (RH237, Rhod2-AM). The described methodologies are well suited for translational studies investigating the cardiac conduction system, alterations in action potential morphology, calcium handling, excitation-contraction coupling and the incidence of cardiac alternans or arrhythmias.