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

Role of inflammation in the initiation and maintenance of atrial fibrillation and the protective effect of atorvastatin in a goat model of aseptic pericarditis

The present study was designed to determine the association between atrial fibrillation (AF) and inflammation in a goat sterile pericarditis model and to assess the effect of atorvastatin, a cholesterol-reducing drug, on AF. A total of 15 adult male goats were randomly divided into control, untreated pericarditis and atorvastatin-treated pericarditis groups. Pericarditis was induced via thoracotomy and atorvastatin was administered orally (60 mg/day) to the goats in the latter group for the duration of the study, commencing 1 week prior to surgery. The levels of high-sensitivity C-reactive protein (hs-CRP), interleukin(IL)-6 and tumor necrosis factor-α (TNF-α) were significantly elevated following surgery in the untreated pericarditis and atorvastatin groups compared with the control group (P<0.05). However, lower levels of hs-CRP, IL-6 and TNF-α were observed in the atorvastatin group compared with the untreated pericarditis group (P<0.05). Additionally, the animals in the atorvastatin-treated pericarditis group had a longer effective refractory period (ERP) and a higher rate adaptation of the ERP compared with those in the untreated pericarditis group (P<0.05). There was a significant negative correlation between the levels of ERP and hs-CRP in the untreated pericarditis group. The inducibility of AF in the left atrium and the duration of AF in the untreated pericarditis and atorvastatin-treated groups increased significantly following surgery (P<0.05). The pericarditis group, however, had a longer duration of AF compared with the atorvastatin group (P<0.05). Thus, inflammation may promote AF by shortening atrial ERP and by reducing the rate adaptation of ERP. These results suggested that atorvastatin can attenuate AF by inhibiting inflammation and may assist in preventing the occurrence and recurrence of AF following cardiac surgery.

Dosage of angiotensin-II receptor blockers in heart failure patients following changes in Danish drug reimbursement policies

PURPOSE

National reimbursement policies in Denmark were changed in November 2010 favouring a shift in angiotensin-II receptor blocker (ARB) treatment to generic losartan for heart failure (HF) patients. We examined how changes in reimbursement policies affected the fraction of HF patients up-titrated to optimal or suboptimal ARB dosage.

METHODS

A historical cohort study was performed including HF patients with at least one prescription of ARB in the months of May-Jul 2010 (baseline). Patients were considered up-titrated at doses 100, 16 or 160 mg for losartan, valsartan and candesartan, respectively. Individual-level linkage of nationwide registries of hospitalization and drug dispensing in Denmark was used to describe patterns of ARB prescriptions and estimate dosage before and after November 2010. Logistic regression models were used to assess the probability for being up-titrated in the period.

RESULTS

Of 6036 individuals included (mean age 73.5 [standard deviation 11.2] years; 51% males), 3346 (55.4%) used losartan, 541 (9.0%) valsartan and 2149 (35.6%) candesartan at inclusion, respectively. 2887 (47.8%) were up-titrated at baseline (May-Jul 2010), followed by 2878 (48.2%) in the three months before the policy change (Aug-Oct 2010), and 2492 (43.7%) in the first months after the policy change (Feb-Apr 2011). Odds ratios for being up-titrated according to time period were 1.02 [0.95-1.09] in Aug-Oct 2010 (before policy change) and 0.84 [0.78-0.90] in Feb-Apr 2011 (after policy change), compared with May-Jul 2010 (reference).

CONCLUSION

Probability of being up-titrated in ARB treatment was reduced 20% following changes in drug reimbursement policies.

The clinical profile and pathophysiology of atrial fibrillation: relationships among clinical features, epidemiology, and mechanisms

Atrial fibrillation (AF) is the most common arrhythmia (estimated lifetime risk, 22%-26%). The aim of this article is to review the clinical epidemiological features of AF and to relate them to underlying mechanisms. Long-established risk factors for AF include aging, male sex, hypertension, valve disease, left ventricular dysfunction, obesity, and alcohol consumption. Emerging risk factors include prehypertension, increased pulse pressure, obstructive sleep apnea, high-level physical training, diastolic dysfunction, predisposing gene variants, hypertrophic cardiomyopathy, and congenital heart disease. Potential risk factors are coronary artery disease, kidney disease, systemic inflammation, pericardial fat, and tobacco use. AF has substantial population health consequences, including impaired quality of life, increased hospitalization rates, stroke occurrence, and increased medical costs. The pathophysiology of AF centers around 4 general types of disturbances that promote ectopic firing and reentrant mechanisms, and include the following: (1) ion channel dysfunction, (2) Ca(2+)-handling abnormalities, (3) structural remodeling, and (4) autonomic neural dysregulation. Aging, hypertension, valve disease, heart failure, myocardial infarction, obesity, smoking, diabetes mellitus, thyroid dysfunction, and endurance exercise training all cause structural remodeling. Heart failure and prior atrial infarction also cause Ca(2+)-handling abnormalities that lead to focal ectopic firing via delayed afterdepolarizations/triggered activity. Neural dysregulation is central to atrial arrhythmogenesis associated with endurance exercise training and occlusive coronary artery disease. Monogenic causes of AF typically promote the arrhythmia via ion channel dysfunction, but the mechanisms of the more common polygenic risk factors are still poorly understood and under intense investigation. Better recognition of the clinical epidemiology of AF, as well as an improved appreciation of the underlying mechanisms, is needed to develop improved methods for AF prevention and management.

Atrial tachyarrhythmias induced by the combined effects of β1/2-adrenergic autoantibodies and thyroid hormone in the rabbit

Activating autoantibodies (AAb) to β-adrenergic receptors (βAR) are associated with atrial fibrillation in patients with Graves' disease. In the present study, we examined the interaction of thyroid hormone with β1/2AR-AAb in inducing atrial tachyarrhythmias in the rabbit. Immunization of rabbits with a β1AR or β2AR second extracellular loop peptide produced high titers of β1AR-AAb or β2AR-AAb. Thyroid hormone in combination with β1AR-AAb or β2AR-AAb induced a significant number of sustained sinus tachycardia and atrial tachycardia, respectively. Both combinations resulted in significantly increased inductions of sustained arrhythmias compared to AAb alone. Thyroid hormone alone induced sustained sinus and junctional tachycardia. Sera from immunized rabbits specifically bound to and activated β1AR or β2AR in transfected cells in vitro. This study demonstrates thyroid hormone qualitatively accentuates the specific arrhythmogenic action of these AAb and quantitatively enhances their rate. Our data support a dual role of AAb and thyroid hormone in Graves'-associated tachyarrhythmias.

Both hypothyroidism and hyperthyroidism increase atrial fibrillation inducibility in rats

BACKGROUND

Evidence indicates that cardiac hypothyroidism may contribute to heart failure progression. It is also known that heart failure is associated with an increased risk of atrial fibrillation (AF). Although it is established that hyperthyroidism increases AF incidence, the effect of hypothyroidism on AF is unclear. This study investigated the effects of different thyroid hormone levels, ranging from hypothyroidism to hyperthyroidism on AF inducibility in thyroidectomized rats.

METHODS AND RESULTS

Thyroidectomized rats with serum-confirmed hypothyroidism 1 month after surgery were randomized into hypothyroid (N=9), euthyroid (N=9), and hyperthyroid (N=9) groups. Rats received placebo, 3.3-mg l-thyroxine (T4), or 20-mg T4 pellets (60-day release form) for 2 months, respectively. At the end of treatment, hypothyroid, euthyroid, and hyperthyroid status was confirmed. Hypothyroid animals showed cardiac atrophy and reduced cardiac systolic and diastolic functions, whereas hyperthyroid rats exhibited cardiac hypertrophy and increased cardiac function. Hypothyroidism and hyperthyroidism produced opposite electrophysiological changes in heart rates and atrial effective refractory period, but both significantly increased AF susceptibility. AF incidence was 78% in hypothyroid, 67% in hyperthyroid, and the duration of induced AF was also longer, compared with 11% in the euthyroid group (all P<0.05). Hypothyroidism increased atrial interstitial fibrosis, but connexin 43 was not affected.

CONCLUSIONS

Both hypothyroidism and hyperthyroidism lead to increased AF vulnerability in a rat thyroidectomy model. Our results stress that normal thyroid hormone levels are required to maintain normal cardiac electrophysiology and to prevent cardiac arrhythmias and AF.

The developmental basis of adult arrhythmia: atrial fibrillation as a paradigm

Normal cardiac rhythm is one of the most fundamental physiologic phenomena, emerging early in the establishment of the vertebrate body plan. The developmental pathways underlying the patterning and maintenance of stable cardiac electrophysiology must be extremely robust, but are only now beginning to be unraveled. The step-wise emergence of automaticity, AV delay and sequential conduction are each tightly regulated and perturbations of these patterning events is now known to play an integral role in pediatric and adult cardiac arrhythmias. Electrophysiologic patterning within individual cardiac chambers is subject to exquisite control and is influenced by early physiology superimposed on the underlying gene networks that regulate cardiogenesis. As additional cell populations migrate to the developing heart these too bring further complexity to the organ, as it adapts to the dynamic requirements of a growing organism. A comprehensive understanding of the developmental basis of normal rhythm will inform not only the mechanisms of inherited arrhythmias, but also the differential regional propensities of the adult heart to acquired arrhythmias. In this review we use atrial fibrillation as a generalizable example where the various factors are perhaps best understood.

Contrast Induced Thyrotoxicosis in a Patient with New onset Atrial Fibrillation: A Case Report and Review

The development of thyrotoxicosis following the administration of iodinated contrast is a rare occurrence. The effect, referred to as the Jod-Basedow effect, is often observed in patients with underlying thyroid disease who develop thyrotoxicosis subsequent to the exposure of exogenous iodide. An example of an iatrogenic cause for this event may be seen when a large iodide load is given intravenously for studies or procedures. Thyrotoxicosis can also lead to cardiac arrhythmias including atrial fibrillation. This is a case presentation of a 74 year old female who developed thyrotoxicosis as well as new onset atrial fibrillation approximately one week after receiving iodinated contrast dye for a diagnostic CT of the abdomen. We further review the prior published literature in regard to atrial fibrillation and thyrotoxicosis.

New-onset atrial fibrillation is a predictor of subsequent hyperthyroidism: a nationwide cohort study

AIMS

To examine the long-term risk of hyperthyroidism in patients admitted to hospital with new-onset AF. Hyperthyroidism is a well-known risk factor for atrial fibrillation (AF), but it is unknown whether new-onset AF predicts later-occurring hyperthyroidism.

METHODS AND RESULTS

All patients admitted with new-onset AF in Denmark from 1997-2009, and their present and subsequent use of anti-thyroid medication was identified by individual-level linkage of nationwide registries. Patients with previous thyroid diagnosis or thyroid medication use were excluded. Development of hyperthyroidism was assessed as initiation of methimazole or propylthiouracil up to a 13-year period. Risk of hyperthyroidism was analysed by Poisson regression models adjusted for important confounders such as amiodarone treatment. Non-AF individuals from the general population served as reference. A total of 145,623 patients with new-onset AF were included (mean age 66.4 years [SD ±13.2] and 55.3% males) of whom 3% (4,620 events; 62.2% women) developed hyperthyroidism in the post-hospitalization period compared to 1% (48,609 events; 82% women) in the general population (n = 3,866,889). In both women and men we found a significantly increased risk of hyperthyroidism associated with new-onset AF compared to individuals in the general population. The highest risk was found in middle-aged men and was consistently increased throughout the 13-year period of observation. The results were confirmed in a substudy analysis of 527,352 patients who had thyroid screening done.

CONCLUSION

New-onset AF seems to be a predictor of hyperthyroidism. Increased focus on subsequent risk of hyperthyroidism in patients with new-onset AF is warranted.

Association of inflammation with atrial fibrillation in hyperthyroidism

Objectives

The aim of this study was to evaluate the relationship between inflammation and development of atrial fibrillation (AF) in patients with hyperthyroidism.

Methods

A total of 65 patients with newly diagnosed hyperthyroidism, 35 of whom were in sinus rhythm and 30 of whom in AF. Thirty five age- and gender-matched patients in a control group were included in the study. Factors associated with the development of AF were evaluated by multivariate regression analysis.

Results

Factors associated with AF in multivariate analysis included high sensitivity C reactive protein (HsCRP) [odds ratio (OR): 11.19; 95% confidence interval (95% CI): 1.80-69.53; P = 0.003], free T4 (OR: 8.76; 95% CI: 2.09–36.7; P = 0.003), and left atrial diameter (OR: 1.25; 95% CI: 1.06–1.47; P = 0.008).

Conclusions

The results of the present study suggest that HsCRP, an indicator of inflammation, free T4 and left atrial diameter are associated with the development AF in patients with hyperthyroidism.

Ectopic and reentrant activation patterns in the posterior left atrium during stretch-related atrial fibrillation

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in humans and is predicted to dramatically increase its prevalence in the future. There is experimental evidence that increasing stretch increases the dominance of the pulmonary veins (PVs) during AF in isolated hearts and ectopic activity in the isolated PVs, but the ionic mechanisms underlying such effects are not clear and the ability of the PVs to favorably host functional reentry during stretch cannot be excluded. We used a combination of endocardial-epicardial optical mapping with phase and spectral analysis to study stretch-related AF (SRAF) in normal isolated sheep hearts. We have found rapid AF sources in the posterior left atrium (PLA) and PV region and their activation frequency and level of organization correlated with intra-atrial pressure. Analysis of the surfaces' optical mapping data in the phase domain reveals that activation of the PLA consisted of alternating patterns of breakthroughs, reentries and relatively simple waves swiping across the mapped field. The patterns on the endocardial and epicardial PLA surface at any given moment of time of the SRAF could be either identical or not identical, and the activity in the thickness of the PLA wall is hypothesized to conform to either ectopic discharge or scroll waves, but a definite evidence for the presence of such mechanisms is currently lacking. Thus the understanding of the manner by which the mechano-electric feedback effects in the PLA, including the PVs, become important in the initiation and maintenance of AF requires further detailed investigation.

Amyloid peptide regulates calcium homoeostasis and arrhythmogenesis in pulmonary vein cardiomyocytes

BACKGROUND

Amyloid peptides modulate cardiac calcium homoeostasis and play an important role in the pathophysiology of atrial fibrillation. Pulmonary veins (PVs) are critical in the genesis of atrial fibrillation and contain abundant amyloid peptides. Therefore, the purpose of this study is to investigate whether amyloid peptides may change the PV electrical activity through regulating calcium homoeostasis.

METHODS AND RESULTS

The channel and calcium-handling protein expressions, intracellular calcium and ionic currents were studied in isolated rabbit PV cardiomyocytes in the presence and absence (control) of beta-amyloid (Aβ(25-35) ) for 4-6 h, using Western blot analysis, indo-1 fluorimetric ratio and whole-cell patch clamp techniques. Aβ(25-35) decreased the expressions of Ca(V) 1.2, total or Ser16-phosphorylated phospholamban (p-PLB), p-PLB/PLB ratio, sodium/calcium exchanger, but did not change ryanodine receptor, sarcoplasmic reticulum (SR) ATPase and K(+) channel proteins (Kir2.1, Kir2.3, Kv1.4, Kv1.5 and Kv4.2). Aβ(25-35) -treated cardiomyocytes had smaller calcium transient, SR calcium store, L-type calcium current and sodium/calcium exchanger current than control cardiomyocytes. Moreover, Aβ(25-35) -treated cardiomyocytes (n = 20) had shorter 90% of the action potential duration (82 ± 3 vs. 93 ± 5 ms, P < 0·05) than control cardiomyocytes (n = 16).

CONCLUSION

Aβ(25-35) has direct electrophysiological effects on PV cardiomyocytes.

Cardiovascular involvement in patients with different causes of hyperthyroidism

Various clinical disorders can cause hyperthyroidism, the effects of which vary according to the patient's age, severity of clinical presentation and association with other comorbidities. Hyperthyroidism is associated with increased morbidity and mortality from cardiovascular disease, although whether the risk of specific cardiovascular complications is related to the etiology of hyperthyroidism is unknown. This article will focus on patients with Graves disease, toxic adenoma and toxic multinodular goiter, and will compare the cardiovascular risks associated with these diseases. Patients with toxic multinodular goiter have a higher cardiovascular risk than do patients with Graves disease, although cardiovascular complications in both groups are differentially influenced by the patient's age and the cause of hyperthyroidism. Atrial fibrillation, atrial enlargement and congestive heart failure are important cardiac complications of hyperthyroidism and are prevalent in patients aged > or = 60 years with toxic multinodular goiter, particularly in those with underlying cardiac disease. An increased risk of stroke is common in patients > 65 years of age with atrial fibrillation. Graves disease is linked with autoimmune complications, such as cardiac valve involvement, pulmonary arterial hypertension and specific cardiomyopathy. Consequently, the etiology of hyperthyroidism must be established to enable correct treatment of the disease and the cardiovascular complications.

Oxidative stress on pulmonary vein and left atrium arrhythmogenesis

BACKGROUND

Oxidative stress and pulmonary veins (PVs) play critical roles in the pathophysiology of atrial fibrillation. The purpose of the present study was to investigate whether oxidative stress and antioxidant agents can change the electrophysiological characteristics of the left atrium (LA) and PVs.

METHODS AND RESULTS

Conventional microelectrodes were used to record the action potentials (APs) in isolated rabbit PV and LA specimens before and after H(2)O(2) administration with or without ascorbic acid or N-mercaptopropionyl-glycine (N-MPG, a free radical .OH scavenger). H(2)O(2) (0.02 and 0.2 mmol/L) decreased the PV spontaneous rates from 2.0+/-0.1 Hz to 1.6+/-0.1 Hz, and 1.7+/-0.1 Hz (n=10, P<0.05), but H(2)O(2) (2 mmol/L) increased PV spontaneous rates from 2.0+/-0.1 Hz to 2.8+/-0.2 Hz. H(2)O(2) easily induced PV burst firing and early afterdepolarizations, but not in the LA. H(2)O(2) shortened the AP duration and increased the contractile force to a greater extent in the LA than in PVs. In addition, the H(2)O(2)-induced PV burst firing and increasing spontaneous rates were suppressed or attenuated by pretreatment with ascorbic acid (1 mmol/L) or N-MPG (10 mmol/L).

CONCLUSIONS

H(2)O(2) significantly changed the electrophysiological characteristics of PV and LA through activation of free radicals and may facilitate the occurrence of atrial fibrillation.

Nitroprusside modulates pulmonary vein arrhythmogenic activity

BACKGROUND

Pulmonary veins (PVs) are the most important sources of ectopic beats with the initiation of paroxysmal atrial fibrillation, or the foci of ectopic atrial tachycardia and focal atrial fibrillation. Elimination of nitric oxide (NO) enhances cardiac triggered activity, and NO can decrease PV arrhythmogenesis through mechano-electrical feedback. However, it is not clear whether NO may have direct electrophysiological effects on PV cardiomyocytes. This study is aimed to study the effects of nitroprusside (NO donor), on the ionic currents and arrhythmogenic activity of single cardiomyocytes from the PVs.

METHODS

Single PV cardiomyocytes were isolated from the canine PVs. The action potential and ionic currents were investigated in isolated single canine PV cardiomyocytes before and after sodium nitroprusside (80 muM,) using the whole-cell patch clamp technique.

RESULTS

Nitroprusside decreased PV cardiomyocytes spontaneous beating rates from 1.7 +/- 0.3 Hz to 0.5 +/- 0.4 Hz in 9 cells (P < 0.05); suppressed delayed after depolarization in 4 (80%) of 5 PV cardiomyocytes. Nitroprusside inhibited L-type calcium currents, transient outward currents and transient inward current, but increased delayed rectified potassium currents.

CONCLUSION

Nitroprusside regulates the electrical activity of PV cardiomyocytes, which suggests that NO may play a role in PV arrhythmogenesis.

Histological substrate of human atrial fibrillation

Histologic and ultrastructural examination of atrial tissue regarding the main entities responsible of human atrial fibrillation, is reported. The pathologic changes deriving from various disorders, like degenerative, inflammatory, ischemic diseases as well as from cardiac aging and hormonal imbalance are analysed. Structural changes associated with lone atrial fibrillation and investigated by atrial biopsy are also described, as being able to provide useful information on the disease's etiology, prognosis and treatment.

P-wave dispersion in endogenous and exogenous subclinical hyperthyroidism

BACKGROUND

The aim of this study was to measure maximum P wave duration (Pmax) and P wave dispersion (PWD), which can be indicators for the risk of paroxysmal atrial fibrillation when increased, and to reveal their relationship with thyroid hormone levels in patients with endogenous and exogenous subclinical hyperthyroidism.

METHODS

Seventy-one patients with sublinical thyrotoxicosis (34 endogenous, 37 exogenous) and 69 healthy individuals were enrolled in the study. Pmax and minimum P wave duration (Pmin) on electrocardiogram recordings were measured and PWD was calculated as Pmax-Pmin.

RESULTS

Pmax (p<0.001) and PWD (p<0.001) values were significantly higher in patients with endogenous subclinical hyperthyroidism compared with the control group. Pmax (p<0.001) and PWD (p<0.001) values were significantly higher in patients with exogenous subclinical thyrotoxicosis compared with the control group. Pmax (p=0.710) and PWD (p=0.127) were not significantly different in patients with endogenous subclinical hyperthyroidism compared with exogenous subclinical hyperthyroid patients. Pmax and PWD negatively associated with TSH in endogenous and exogenous subclinical hyperthyroidism.

CONCLUSION

In the present study, we observed that Pmax and PWD were longer in patients with endogenous and exogenous subclinical hyperthyroidism. Lack of a difference in Pmax and PWD between patients with endogenous and exogenous subclinical hyperthyroidism seems to support the idea that hormone levels rather than the etiology of thyrotoxicosis affect the heart.

Activating autoantibodies to the beta-1 adrenergic and m2 muscarinic receptors facilitate atrial fibrillation in patients with Graves' hyperthyroidism

OBJECTIVES

We studied activating autoantibodies to beta-1 adrenergic receptors (AAbeta1AR) and activating autoantibodies to M2 muscarinic receptors (AAM2R) in the genesis of atrial fibrillation (AF) in Graves' hyperthyroidism.

BACKGROUND

Atrial fibrillation frequently complicates hyperthyroidism. Both AAbeta1AR and AAM2R have been described in some patients with dilated cardiomyopathy and AF. We hypothesized that their copresence would facilitate AF in autoimmune Graves' hyperthyroidism.

METHODS

Immunoglobulin G purified from 38 patients with Graves' hyperthyroidism with AF (n=17) or sinus rhythm (n=21) and 10 healthy control subjects was tested for its effects on isolated canine Purkinje fiber contractility with and without atropine and nadolol. Immunoglobulin G electrophysiologic effects were studied using intracellular recordings from isolated canine pulmonary veins. Potential cross-reactivity of AAbeta1AR and AAM2R with stimulating thyrotropin receptor (TSHR) antibodies was evaluated before and after adsorption to Chinese hamster ovary cells expressing human TSHRs using flow cytometry and enzyme-linked immunosorbent assays.

RESULTS

The frequency of AAbeta1AR and/or AAM2R differed significantly between patients with AF and sinus rhythm (AAbeta1AR=94% vs. 38%, p<0.001; AAM2R=88% vs. 19%, p<0.001; and AAbeta1AR+AAM2R=82% vs. 10%, p<0.001). The copresence of AAbeta1AR and AAM2R was the strongest predictor of AF (odds ratio: 33.61, 95% confidence interval: 1.17 to 964.11, p=0.04). Immunoglobulin G from autoantibody-positive patients induced hyperpolarization, decreased action potential duration, enhanced early afterdepolarization formation, and facilitated triggered firing in pulmonary veins by local autonomic nerve stimulation. Immunoadsorption studies showed that AAbeta1AR and AAM2R were immunologically distinct from TSHR antibodies.

CONCLUSIONS

When present in patients with Graves' hyperthyroidism, AAbeta1AR and AAM2R facilitate development of AF.

Thyroid hormones and cardiac arrhythmias

Thyroid hormone plays an important role in cardiac electrophysiology and Ca2+ handling through both genomic and nongenomic mechanisms of action, while both actions can interfere. Chronic changes in the amount of circulating thyroid hormone due to thyroid dysfunction or systemic disease result in structural, electrophysiological and Ca2+ handling remodeling, while acute changes may affect basal activity of cardiac cells membrane systems. Consequently, long-term or rapid modulation of sarcolemmal ion channels, Ca2+ cycling proteins and intercellular communicating channels by thyroid hormone may affect heart function as well as susceptibility of the heart to arrhythmias. This aspect including pro- and anti-arrhythmic potential of thyroid hormone is highlighted in this review.

The role of pulmonary veins in atrial fibrillation: a complex yet simple story

Atrial fibrillation (AF) is the most common cardiac arrhythmia, with increased incidence among the elderly population. The concept that ectopic activity in pulmonary veins (PVs) could be responsible for triggering AF has been put forward, and the inter-relationship between PVs and left atrium has been the subject of many anatomical and physiological investigations. Variable configuration of action potentials among various PV cardiomyocytes has been reported. PV myocytes were shown to have a higher resting membrane potential and a lower action potential amplitude and duration than the left atrium. Much evidence has accumulated to indicate that spontaneous depolarization and/or re-entry from PVs could be the mode by which AF is initiated and/or sustained. Attempts have been made to link AF in certain pathophysiological states, notably, congestive heart failure, valvular disease and hyperthyroidism to PVs. There has been evidence to suggest that an increase in PV diameter may be the trigger for initiating AF. However, there is limited clinical knowledge available on the nature of the antiarrhythmic drugs that act upon PVs to alleviate AF. Most drugs currently employed are the standard agents generally utilized for the treatment of AF. Radiofrequency ablation (RFA) of the PVs and its isolation from the left atrium has become a major curative measure of AF. It is also possible that pharmacotherapy may be more effective or provide extra benefit to patients after a RFA procedure. The trend of the clinical evidence seems to suggest that a hybrid treatment may be beneficial in some population of patients.

Spontaneous onset of atrial fibrillation

Most commonly, atrial fibrillation is triggered by rapid bursts of electrical impulses originating in the myocardial sleeves of pulmonary veins (PVs). However, the nature of such bursts remains poorly understood. Here, we propose a mechanism of bursting consistent with the extensive empirical information about the electrophysiology of the PVs. The mechanism is essentially non-local and involves the spontaneous initiation of non-sustained spiral waves in the distal end of the muscle sleeves of the PVs. It reproduces the experimentally observed dynamics of the bursts, including their frequency, their intermittent character, and the unusual shape of the electrical signals in the pulmonary veins that are reminiscent of so-called early afterdepolarizations (EADs).

The mechanisms of atrial fibrillation in hyperthyroidism

Atrial fibrillation (AF) is a complex condition with several possible contributing factors. The rapid and irregular heartbeat produced by AF increases the risk of blood clot formation inside the heart. These clots may eventually become dislodged, causing embolism, stroke and other disorders. AF occurs in up to 15% of patients with hyperthyroidism compared to 4% of people in the general population and is more common in men and in patients with triiodothyronine (T₃) toxicosis. The incidence of AF increases with advancing age. Also, subclinical hyperthyroidism is a risk factor associated with a 3-fold increase in development of AF. Thyrotoxicosis exerts marked influences on electrical impulse generation (chronotropic effect) and conduction (dromotropic effect). Several potential mechanisms could be invoked for the effect of thyroid hormones on AF risk, including elevation of left atrial pressure secondary to increased left ventricular mass and impaired ventricular relaxation, ischemia resulting from increased resting heart rate, and increased atrial eopic activity. Reentry has been postulated as one of the main mechanisms leading to AF. AF is more likely if effective refractory periods are short and conduction is slow. Hyperthyroidism is associated with shortening of action potential duration which may also contribute to AF.

Mechanical characterization of rabbit pulmonary vein sleeves in in vitro intact ring preparation

BACKGROUND

Pulmonary vein (PV) sleeves, composed of cardiomyocytes, play certain roles in arrhythmogenesis. In the literature, it has been frequently reported that PV sleeves possess intrinsic spontaneous pacemaking activity and triggered activity in normal dogs and rabbits. In contrast, other research groups presented totally opposite findings which showed absence of such pacemakers in dogs, rabbits and rats. The present study was designed to clarify this puzzle and contradiction.

METHODS

A novel methodology using in vitro experimentation was used to examine the electromechanical activity of whole segments of PV sleeves. The ring preparation was composed of a small piece of left atrial (LA) free wall, PV ostium and sleeve from rabbits. A circumferential contraction of the PV sleeve was measured when the preparation was electrically driven from the LA free wall. Mechanical force of the ring preparation was measured using a force transducer. The action potentials were recorded using conventional intracellular recording technique in strip preparation.

RESULTS

In 15 rabbits, no spontaneous pacemaking activity or triggered activity was found in the in vitro ring preparation of PV sleeve. The circumferential contraction of PV sleeves was external calcium-dependent. Frequency-force relation displayed a negative staircase at 0.1-0.5 Hz and a positive staircase at 1-5 Hz. Post-rest potentiation was prominent between 15 s and 120 s. Intracellular action potential recording did not display any automaticity or triggered activity in PV sleeves.

CONCLUSION

In an intact ring preparation of rabbit PV sleeves, intrinsic spontaneous pacemaking activity or triggered activity was not found.

Effects of alcohol on atrial fibrillation: myths and truths

Alcohol is the most consumed drug worldwide. Both acute and chronic alcohol use have been associated with cardiac arrhythmias, in particular atrial fibrillation, or so-called 'holiday heart syndrome'. Epidemiological, clinical and experimental studies have attempted to elucidate the mechanisms involved in this association. However, because most of these studies have shown conflicting results, the connection between ethanol and atrial arrhythmias remains controversial. Historical, epidemiological and pharmacological aspects of alcohol, as well as recent concepts on atrial fibrillation are reviewed. We then examine the literature and provide a critical point of view on the still elusive association between alcohol and atrial fibrillation.

Prevention of atrial fibrillation following cardiac surgery: basis for a novel therapeutic strategy based on non-hypoxic myocardial preconditioning

Atrial fibrillation is the most common complication of cardiac surgical procedures performed with cardiopulmonary bypass. It contributes to increased hospital length of stay and treatment costs. At present, preventive strategies offer only suboptimal benefits, despite improvements in anesthesia, surgical technique, and medical therapy. The pathogenesis of postoperative atrial fibrillation is considered to be multifactorial. However oxidative stress is a major contributory factor representing the unavoidable consequences of ischemia/reperfusion cycle occurring in this setting. Considerable evidence suggests the involvement of reactive oxygen species (ROS) in the pathogenic mechanism of this arrhythmia. Interestingly, the deleterious consequences of high ROS exposure, such as inflammation, cell death (apoptosis/necrosis) or fibrosis, may be abrogated by a myocardial preconditioning process caused by previous exposure to moderate ROS concentration known to trigger survival response mechanisms. The latter condition may be created by n-3 PUFA supplementation that could give rise to an adaptive response characterized by increased expression of myocardial antioxidant enzymes and/or anti-apoptotic pathways. In addition, a further reinforcement of myocardial antioxidant defenses could be obtained through vitamins C and E supplementation, an intervention also known to diminish enzymatic ROS production. Based on this paradigm, this review presents clinical and experimental evidence supporting the pathophysiological and molecular basis for a novel therapeutic approach aimed to diminish the incidence of postoperative atrial fibrillation through a non-hypoxic preconditioning plus a reinforcement of the antioxidant defense system in the myocardial tissue.

Swelling activated chloride currents in the electrical activity of pulmonary vein cardiomyocytes

BACKGROUND

Pulmonary veins (PVs) contain cardiomyocytes with a high arrhythmogenicity for inducing atrial fibrillation. The swelling-activated outwardly rectifying Cl(-) currents (I(Cl,swell)) are important in the electrical activity of cardiomyocytes. This study was to investigate whether I(Cl,swell) play a role in the PV electrophysiological characteristics.

MATERIALS AND METHODS

A whole-cell patch clamp was used to investigate the action potentials and I(Cl,swell) in isolated rabbit single PV and atrial cardiomyocytes during immersion in isotonic (290-300 mosm L(-1)) and hypotonic (220-230 mosm L(-1)) solutions. The cell length and cell width were measured using confocal microscopy.

RESULTS

Hypotonic solution induced larger I(Cl,swell) in the PV cardiomyocytes with pacemaker activity than those in the PV cardiomyocytes without pacemaker activity or atrial cardiomyocytes. Hypotonic solution shortened the action potential duration and increased the cell width to a greater extent in the PV cardiomyocytes than in the atrial cardiomyocytes. Moreover, hypotonic solution decreased the PV firing with a decrease in the transient inward currents and delayed after depolarizations.

CONCLUSIONS

These findings suggest that the I(Cl,swell) plays an important role in the electrical activity of the PV cardiomyocytes.

Calmodulin kinase II inhibition prevents arrhythmic activity induced by alpha and beta adrenergic agonists in rabbit pulmonary veins

The autonomic nervous system and calcium regulation play important roles in the pathophysiology of atrial fibrillation. Calmodulin regulates the calcium homeostasis and may mediate the proarrhythmic effects of autonomic nervous agents. The purpose of this study was to compare the effects of beta- and alpha-adrenoceptor agonists on the pulmonary vein electrical activity and evaluate whether calmodulin kinase II inhibitors may change the effects of the adrenoceptor agonists on the pulmonary vein arrhythmogenesis. Conventional microelectrodes were used to record the action potentials in isolated rabbit pulmonary vein tissue specimens before and after the administration of isoproterenol, phenylephrine and KN-93 (a calmodulin kinase II inhibitor). In the tissue preparation, isoproterenol (0, 0.1, 3 microM) increased the beating rates (1.5+/-0.2, 1.6+/-0.2, 2.3+/-0.3 Hz, n=10, P<0.001) with the genesis of early afterdepolarizations (EADs, 0%, 40%, 50%, P<0.05) and increased the amplitude of the delayed afterdepolarizations (DADs, 0.6+/-0.3, 1.7+/-0.4, 3.9+/-1.0 mV, P<0.05). Phenylephrine (0, 1, 10 microM) also increased the beating rates (1.4+/-0.2, 1.6+/-0.2, 1.9+/-0.2 Hz, n=12, P<0.001), incidence of EADs (0%, 8%, 50%, P<0.05) and amplitude of the DADs (0.4+/-0.2, 1.2+/-0.4, 2.6+/-0.8 mV, P<0.05). KN-93 did not change the pulmonary vein beating rates or action potential duration. However, in the presence of KN-93 (1 microM), isoproterenol (3 microM) and phenylephrine (10 microM) did not induce any EADs or DADs in the pulmonary veins. In conclusion, calmodulin kinase II inhibition may prevent adrenergic induced pulmonary vein arrhythmogenesis.

Catheter ablation of atrial fibrillation in patients with hyperthyroidism

AIMS

To study the clinical efficacy of catheter ablation for treating patients with hyperthyroidism-related atrial fibrillation (AF).

MATERIALS AND METHODS

The study involved 16 patients (12 males; age, 59.8 +/- 11.3 years) with hyperthyroidism-related AF, who had all been euthyroid for more than 3 months but still suffered from highly symptomatic and antiarrhythmia drug (AAD)-refractory AF. Circumferential pulmonary vein ablation (CPVA) guided by a 3-D mapping system was carried out to encircle the ipsilateral pulmonary veins (PVs) with a procedural endpoint of continuity of the circular lesions and PV isolation. Success was defined as the absence of any atrial tachyarrhythmia (ATa) off AADs beyond the first 3 months after the procedure.

RESULTS

CPVA was safely carried out in each of the 16 patients without any complications. PV isolation was achieved in all the treated PVs. After a mean follow-up of 15.8 +/- 11.8 (range, 6-55) months, 9 patients (56%) were free of ATa without any AADs beyond the first 3 months. AF relapsed in the remaining 7 patients, among whom 4 responded to AAD therapy and 3 were totally unresponsive.

CONCLUSION

For patients suffering hyperthyroidism-related AF, CPVA guided by a 3-D mapping system could represent one of the therapeutic options.

Tumor necrosis factor-alpha alters calcium handling and increases arrhythmogenesis of pulmonary vein cardiomyocytes

Inflammation and abnormal calcium homeostasis play important roles in atrial fibrillation. Tumor necrosis factor-alpha (TNFalpha), a proinflammatory cytokine, can induce cardiac arrhythmias. Pulmonary veins (PVs) are critical in initiating paroxysmal atrial fibrillation. This study was designed to investigate whether TNFalpha may change the calcium handling and arrhythmogenic activity of PV cardiomyocytes. We used whole-cell patch clamp and indo-1 fluorimetric ratio technique to investigate the action potentials, ionic currents and intracellular calcium in isolated rabbit single PV cardiomyocytes with and without (control) incubation with TNFalpha (25 ng/ml) for 7-10 h. The expression of sarcoplasmic reticulum ATPase in the control and TNFalpha-treated PV cardiomyocytes was evaluated by confocal micrographs and Western blot. We found that the spontaneous beating rates were similar between the control (n=45) and TNFalpha-treated (n=28) PV cardiomyocytes. Compared with the control PV cardiomyocytes, the TNFalpha-treated PV cardiomyocytes had significantly a larger amplitude of the delayed afterdepolarizations (6.0+/-1.7 vs. 2.6+/-0.8 mV, P<0.05), smaller L-type calcium currents, larger transient inward currents, larger Na(+)-Ca(2+) exchanger currents, a smaller intracellular calcium transient, smaller sarcoplasmic reticulum calcium content, larger diastolic intracellular calcium, a longer decay portion of the calcium transient (Tau), and a decreased sarcoplasmic reticulum ATPase expression. In conclusion, TNFalpha can increase the PV arrhythmogenicity and induce an abnormal calcium homeostasis, thereby causing inflammation-related atrial fibrillation.

Mechanoelectrical feedback regulates the arrhythmogenic activity of pulmonary veins

BACKGROUND

Atrial fibrillation is commonly associated with dilated pulmonary veins. Stretch has been shown to have mechano-electrical effects.

OBJECTIVE

To investigate whether stretch can increase the arrhythmogenic activity of the pulmonary veins.

METHODS

The transmembrane action potentials were recorded from rabbit pulmonary veins before and after stretch (100 and 300 mg). Gadolinium and streptomycin (stretch-activated ion channel blockers) were each perfused into the pulmonary veins under a 300-mg stretch.

RESULTS

Stretch (0, 100 and 300 mg) force dependently increased the incidence of spontaneous activity (22%, 48% and 83%; p<0.05), mean (standard deviation (SD)) firing rates of spontaneous activity (1.7 (0.2), 2.1 (0.3) and 3 (0.2) Hz; p<0.05) and incidence of early post-depolarisations (9%, 26% and 61%; p<0.05) and delayed post-depolarisations (0%, 4% and 30%; p<0.05) in 23 pulmonary veins. In the seven preparations with spontaneous activity after the 300-mg stretch, gadolinium (1, 3 and 10 mumol/l) decreased the incidence of spontaneous activity by 43%, 29% and 14%, respectively (p<0.05), and decreased the firing rate from 2.9 (0.1) Hz to 0.8 (0.4), 0.3 (0.1) and 0.1 (0.1) Hz, respectively (p<0.05). Streptomycin (10 and 40 mumol/l) decreased the incidence of spontaneous activity by 71% and 29%, respectively (p<0.05), and decreased the firing rate from 2.9 (0.1) Hz to 1.6 (0.4) and 0.5 (0.3) Hz, respectively (p<0.05).

CONCLUSION

Stretch is an important factor in the electrical activity of the pulmonary vein. Stretch-induced arrhythmogenic activity of the pulmonary vein may contribute to the genesis of atrial fibrillation.

Thoracic Vein Arrhythmias

The thoracic veins are important foci for the genesis of ectopic atrial tachycardia and play a critical role in the pathophysiology of paroxysmal and permanent atrial fibrillation. The pulmonary veins have the highest arrhythmogenic activity and other venous structures (eg, superior vena cava, coronary sinus and ligament of Marshall) have also been shown arrhythmogenic potential. Thoracic veins contain cardiomyocytes with distinct electrical activities and complex anatomical structures. This review summaries the current understanding of the basic and clinical electrophysiology of thoracic vein arrhythmias.

Atrial fibrillation may be a vascular disease: the role of the pulmonary vein

Recent years have seen an enormous amount of experimental and clinical research into role of the pulmonary veins (PVs) in atrial fibrillation (AF). The PVs contain cardiomyocytes with easily inducible arrhythmogenic activity due to the enhanced automaticity, induction of triggered activity, and genesis of microreentrant circuits. The enhanced automaticity, induced triggered activity, either alone or in combination with the reentrant mechanisms, may play a role in the initiation of PVs AF. Detailed mapping studies suggest that reentry within the PVs is most likely responsible for their arrhythmogenicity. There is no doubt that the PVs represent the most important source of arrhythmogenic activity in patients with paroxysmal AF. In AF patients with risk factors for development of AF, the presence of the pathological situation is important in enhancing the PV arrhythmogenic activity. Coronary sinus or superior vena cava may also be a source of rapid repetitive electrical activity during AF. Thus, AF should be considered a kind of vascular disease. Moreover, in patients with paroxysmal AF originating from the PVs, a wide spectrum of atrial arrhythmias may coexist, including paroxysms of atrial premature, tachycardia, flutter and fibrillation. This kind of arrhythmias should be named as PV atrial arrhythmias. These new views will help understand the mechanism, diagnosis and treatment method for AF.

Electrophysiology of Pulmonary Veins

Pulmonary veins were found to be important foci for the genesis and maintenance of atrial fibrillation. Morphological studies have demonstrated the presence of complex anatomic structures and different types of cardiomyocytes in pulmonary veins. Numerous studies have suggested that the combination of reentrant and nonreentrant mechanisms (automaticity and triggered activity) are the underlying arrhythmogenic mechanisms of atrial fibrillation initiation from the pulmonary veins. Electropharmacological studies further indicated that pulmonary veins contained distinct arrhythmogenic activity. Several experimental models have been used to study the pulmonary vein electrical activity and demonstrate the precipitating factors for enhancing the pulmonary vein arrhythmogenic activity. The aim of this review article is to provide a critical overview of the current understanding of the basic and clinical electrophysiology of pulmonary veins and to underscore the importance of future research in this field.

Angiotensin II and angiotensin II receptor blocker modulate the arrhythmogenic activity of pulmonary veins

Angiotensin II receptor blockers (AIIRBs) have been shown to prevent atrial fibrillation. The pulmonary veins (PVs) are the most important focus for the generation of atrial fibrillation. The aim of this study was to evaluate whether angiotensin II or AIIRB may change the arrhythmogenic activity of the PVs. Conventional microelectrodes and whole-cell patch clamps were used to investigate the action potentials (APs) and ionic currents in isolated rabbit PV tissue and single cardiomyocytes before and after administering angiotensin II or losartan (AIIRB). In the tissue preparations, angiotensin II induced delayed after-depolarizations (1, 10, and 100 nM) and accelerated the automatic rhythm (10 and 100 nM). Angiotensin II (100 nM) prolonged the AP duration and increased the contractile force (10 and 100 nM). Losartan (1 and 10 microM) inhibited the automatic rhythm. Losartan (10 microM) prolonged the AP duration and reduced the contractile force (1 and 10 microM). Angiotensin II reduced the transient outward potassium current (I(to)) but increased the L-type calcium, delayed rectifier potassium (I(K)), transient inward (I(ti)), pacemaker, and Na(+)-Ca(2+) exchanger (NCX) currents in the PV cardiomyocytes. Losartan decreased the I(to), I(K), I(ti), and NCX currents. In conclusion, angiotensin II and AIIRB modulate the PV electrical activity, which may play a role in the pathophysiology of atrial fibrillation.

Inducibility of abnormal automaticity and triggered activity in myocardial sleeves of canine pulmonary veins

BACKGROUND

To study the cellular mechanisms governing cardiac atrial arrhythmias initiated by ectopic focus (or foci) from pulmonary veins (PVs).

METHODS

In the present in vitro study, we applied the conventional microelectrode technique to record intracellular action potentials in PV sleeves from dogs.

RESULTS

In 80 normal healthy dogs, all action potentials recorded in cardiomyocytes from PV sleeves were fast-response. The pharmacological responses to quinidine, nisoldipine, D-sotalol, 4-aminopyridine, isoproterenol, acetylcholine, and adenosine were characteristic of those in atrial cells. Diastolic depolarization and spontaneous activity could be induced by 1 mmol/L Ba2+ in all the 22 PV specimens being tested, but only in 3 of 11 of left atrial specimens (p<0.0001). In the presence of 1 mmol/L Ba2+, the diastolic slope was only slightly affected by Ni2+ (500 micromol/L), but was significantly suppressed by Cd2+ (200 micromol/L). Ryanodine (2 micromol/L) caused a transient increase, followed by a marked decrease of Ba2+-induced spontaneous activity. Isoproterenol shortened and acetylcholine prolonged the cycle length of the Ba2+-induced automatic activity. In the presence of isoproterenol, washout of acetylcholine induced a rebound phenomenon, which triggered a short period of spontaneous activity. The results suggest an important role of intracellular cytoplasmic Ca2+ loading. Under conditions that mimic ischemia/hypoxia, the resting membrane potential depolarized, upstroke of the action potential became depressed and the action potential duration shortened. In the presence of isoproterenol and elevated external K+, spontaneous activity was generated.

CONCLUSIONS

These findings indicate a lack of arrhythmogenic activity in normal healthy PV sleeves. Abnormal automaticity and triggered activity occurred exclusively under simulated pathologic conditions. Ba2+-induced automaticity was more easily induced in PV than in the left atrium. The same conditions might also favor the genesis of reentry in the in vivo condition.

Répercussions cardiaques des dysthyroïdies avérées

KEY POINTS

Thyroid hormones affect the vascular system, including the diastolic and systolic functioning of the heart. Resting heart rate increases early in hyperthyroidism (cardiac contractility expands due to improved ventricular loading and decreased systemic vascular resistance). Paradoxically, these hemodynamic alterations progressively reduce cardiac performance on effort (changes in diastolic, then systolic functioning) and finally at rest (modification in ventricular loading following tachycardia or atrial fibrillation), especially in cases of underlying heart disease (in the elderly). Hypothyroidism has an inverse hemodynamic effect and is less noisy, usually limited to relative bradycardia. The morbidity and mortality associated with hypothyroidism are apparently related to the atherogenic and prothrombotic vascular modifications that follow thyroid hormone deficiency, whereas heart failure and particularly atrial fibrillation and its thromboembolic complications are the primary consequences of hyperthyroidism. In both cases, return to normal thyroid levels corrects the cardiac abnormalities caused by the dysthyroidism. Dysthyroidism (hypo- or hyperthyroidism) occurs in 10 to 20% of the patients treated with amiodarone for arrhythmia. Because of its potential seriousness, some clinical or laboratory tests are necessary before initiating treatment, and specific clinical surveillance should be scheduled, including laboratory tests.

Thyroid hormone action in the heart

The heart is a major target organ for thyroid hormone action, and marked changes occur in cardiac function in patients with hypo- or hyperthyroidism. T(3)-induced changes in cardiac function can result from direct or indirect T(3) effects. Direct effects result from T(3) action in the heart itself and are mediated by nuclear or extranuclear mechanisms. Extranuclear T(3) effects, which occur independent of nuclear T(3) receptor binding and increases in protein synthesis, influence primarily the transport of amino acids, sugars, and calcium across the cell membrane. Nuclear T(3) effects are mediated by the binding of T(3) to specific nuclear receptor proteins, which results in increased transcription of T(3)-responsive cardiac genes. The T(3) receptor is a member of the ligand-activated transcription factor family and is encoded by cellular erythroblastosis A (c-erb A) genes. T(3) also leads to an increase in the speed of diastolic relaxation, which is caused by the more efficient pumping of the calcium ATPase of the sarcoplasmic reticulum. This T(3) effect results from T(3)-induced increases in the level of the mRNA coding for the sarcoplasmic reticulum calcium ATPase protein, leading to an increased number of calcium ATPase pump units in the sarcoplasmic reticulum.

Mechanisms of Atrial Fibrillation: Lessons From Animal Models

Studies in animal models have provided extremely important insights about atrial fibrillation (AF). The classic mechanisms that still form the framework for our understanding of AF (focal activity, single-circuit or "mother wave" reentry, and multiple circuit reentry) were established based on animal studies almost 100 years ago. The past 10 years have witnessed a tremendous acceleration of animal work in this area, including the development of a range of AF models in clinically relevant pathological substrates (eg, atrial tachycardia remodeling, congestive heart failure, pericarditis, ischemic heart disease, mitral valve disease, volume overload states, respiratory failure) and the establishment of an increasing number of genetically defined transgenic mouse models. This article reviews the contribution of animal models to our knowledge about AF mechanisms and to clinical management, dealing with such issues as the theory of reentry; the specific applications of various animal models and their contribution to our understanding of electrophysiologic, ionic, and molecular mechanisms; the role of the autonomic nervous system and regional factors; and the development of novel therapeutic approaches. The complementary nature of animal research and clinical investigation is emphasized and the clinical relevance of findings in experimental models is highlighted.

Effects of hyperthyroidism on delayed rectifier K+ currents in left and right murine atria

Hyperthyroidism has been associated with atrial fibrillation (AF); however, hyperthyroidism-induced ion channel changes that may predispose to AF have not been fully elucidated. To understand the electrophysiological changes that occur in left and right atria with hyperthyroidism, the patch-clamp technique was used to compare action potential duration (APD) and whole cell currents in myocytes from left and right atria from both control and hyperthyroid mice. Additionally, RNase protection assays and immunoblotting were performed to evaluate the mRNA and protein expression levels of K(+) channel alpha-subunits in left and right atria. The results showed that 1) in control mice, the APD was shorter and the ultra-rapid delayed rectifier K(+) conductance (I(Kur)) and the sustained delayed rectifier K(+) conductance (I(ss)) were larger in the left than in the right atrium; also, mRNA and protein expression levels of Kv1.5 and Kv2.1 were higher in the left atrium; 2) in hyperthyroid mice, the APD was shortened and I(Kur) and I(ss) were increased in both left and right atrial myocytes, and the protein expression levels of Kv1.5 and Kv2.1 were increased significantly in both atria; and 3) the influence of hyperthyroidism on APD and delayed rectifier K(+) currents was more prominent in right than in left atrium, which minimized the interatrial APD difference. In conclusion, hyperthyroidism resulted in more significant APD shortening and greater delayed rectifier K(+) current increases in the right vs. the left atrium, which can contribute to the propensity for atrial arrhythmia in hyperthyroid heart.

Atrial Tachycardia Remodeling of Pulmonary Vein Cardiomyocytes

Background— The pulmonary veins (PVs) are important in the pathophysiology of atrial fibrillation (AF), as is atrial tachycardia (AT) remodeling. The relative importance of AT remodeling in PVs versus other atrial sites is unknown. The present study assessed AT-induced cellular changes in PVs versus left atrium (LA) and their relationship to arrhythmogenesis.

Methods and Results— We studied ionic currents (single-cell patch clamp) and action potentials (APs; coronary-perfused multicellular preparations) in the PVs and LA free wall of dogs after 7-day AT pacing (400 bpm), as well as in nonpaced control dogs. In controls, rapid ( I Kr ) and slow ( I Ks ) delayed-rectifier currents were larger in PVs; transient-outward ( I to ), inward-rectifier ( I K1 ), and L-type Ca 2+ ( I Ca ) currents and AP duration were smaller. AT remodeling reduced I Ca and I to , left I Kr and I Ks unchanged, and increased I K1 in both LA and PV. AT reduced action potential duration in both LA and PV. LA–PV AP differences became smaller in AT than in control dogs. Premature extrastimuli induced atrial tachyarrhythmias at 4.5±2.8% (mean±SEM) sites in 6 control multicellular preparations compared with 64.2±7.3% sites in 9 AT-remodeled preparations ( P <0.001). Resection of all PVs failed to alter atrial tachyarrhythmia inducibility in AT-remodeled preparations (67.5±13.1%). PV resection did not significantly change tachyarrhythmia duration (mean 3.9 seconds per heart, range 0.7 to 15.7 seconds before resection; mean 7.0 seconds per heart, range 0.9 to 36.0 seconds after resection) or cycle length (120±6 ms before resection, 115±8 ms after resection).

Conclusions— AT produces qualitatively similar ionic remodeling in LA and PVs but reduces PV–LA AP differences. PVs are not essential for AT-induced atrial tachyarrhythmia promotion in this model, which may relate to the failure of PV isolation to prevent AF in some patient populations.