Author + information
- Received November 30, 2012
- Revision received January 16, 2013
- Accepted January 20, 2013
- Published online July 2, 2013.
- Eduard Guasch, MD∗,†,
- Begoña Benito, MD∗,†,‡,
- Xiaoyan Qi, PhD∗,
- Carlo Cifelli, PhD§,
- Patrice Naud, PhD∗,
- Yanfen Shi, PhD∗,
- Alexandra Mighiu, BSc§,
- Jean-Claude Tardif, MD∗,
- Artavazd Tadevosyan, MSc∗,
- Yu Chen, MSc∗,
- Marc-Antoine Gillis, MSc∗,
- Yu-Ki Iwasaki, MD∗,
- Dobromir Dobrev, MD⋮,
- Lluis Mont, MD§,¶,
- Scott Heximer, PhD§ and
- Stanley Nattel, MD∗∗ ()
- ∗Research Center and Department of Medicine, Montreal Heart Institute and University of Montreal, Montreal, Quebec, Canada
- †Department of Medicine, University of Barcelona, Barcelona, Catalonia, Spain
- ‡Cardiology Department, Hospital del Mar, Parc de Salut Mar, Barcelona, Catalonia, Spain
- §Department of Physiology, Heart and Stroke/Richard Lewar Centre of Excellence in Cardiovascular Research and University of Toronto, Toronto, Ontario, Canada
- ⋮Institute of Pharmacology, University of Duisburg-Essen, Essen, Germany
- ¶Unitat de Fibrillació Auricular, Hospital Clínic, University of Barcelona, Institut d'Investigacions Biomédiques August Pi i Sunyer (IDIBAPS), Barcelona, Catalonia, Spain
- ↵∗Reprint requests and correspondence:
Dr. Stanley Nattel, Montreal Heart Institute, University of Montreal Research Center, 5000 Belanger Street E., Montreal, Quebec H1T 1C8, Canada.
Objectives The goal of this study was to assess mechanisms underlying atrial fibrillation (AF) promotion by exercise training in an animal model.
Background High-level exercise training promotes AF, but the underlying mechanisms are unclear.
Methods AF susceptibility was assessed by programmed stimulation in rats after 8 (Ex8) and 16 (Ex16) weeks of daily 1-h treadmill training, along with 4 and 8 weeks after exercise cessation and time-matched sedentary (Sed) controls. Structural remodeling was evaluated by using serial echocardiography and histopathology, autonomic nervous system with pharmacological tools, acetylcholine-regulated potassium current (IKACh) with patch clamp recording, messenger ribonucleic acid expression with quantitative polymerase chain reaction, and regulators of G protein–signaling (RGS) 4 function in knockout mice.
Results AF inducibility increased after 16 weeks of training (e.g., AF >30 s in 64% of Ex16 rats vs 15% of Sed rats; p < 0.01) and rapidly returned to baseline levels with detraining. Atropine restored sinus rhythm in 5 of 5 Ex rats with AF sustained >15 min. Atrial dilation and fibrosis developed after 16 weeks of training and failed to fully recover with exercise cessation. Parasympathetic tone was increased in Ex16 rats and normalized within 4 weeks of detraining. Baroreflex heart rate responses to phenylephrine-induced blood pressure elevation and IKACh sensitivity to carbachol were enhanced in Ex16 rats, implicating both central and end-organ mechanisms in vagal enhancement. Ex rats showed unchanged cardiac adrenergic and cholinergic receptor and IKACh-subunit gene expression, but significant messenger ribonucleic acid downregulation of IKACh-inhibiting RGS proteins was present at 16 weeks. RGS4 knockout mice showed significantly enhanced sensitivity to AF induction in the presence of carbachol.
Conclusions Chronic endurance exercise increased AF susceptibility in rats, with autonomic changes, atrial dilation, and fibrosis identified as potential mechanistic contributors. Vagal promotion is particularly important and occurs via augmented baroreflex responsiveness and increased cardiomyocyte sensitivity to cholinergic stimulation, possibly due to RGS protein downregulation.
The cardiovascular benefits of regular physical exercise are well recognized (1). However, there is emerging evidence that the cardiac remodeling induced by sustained high-level exercise training may also carry some risk. Recent reports emphasize the development of deleterious right ventricular remodeling, which may present as arrhythmogenic cardiomyopathy-like manifestations, as a result of high-intensity training (2). In a rat model of chronic high-intensity endurance training, we found a substrate for ventricular arrhythmia vulnerability associated with right ventricular fibrosis, dilation, and dysfunction (3).
Emerging data also underline the importance of intense exercise training in atrial fibrillation (AF). Marathon runners (4), elite cyclists (5), and cross-country skiers (6) are at particularly high risk. However, the AF risk is not confined to elite athletes: a dose–response relationship has been seen in men aged younger than 50 years and in joggers (7).
The mechanisms of AF promotion by regular high-intensity exercise are unclear. Proposed contributors include atrial dilation, inflammatory changes, vagal enhancement, and structural alterations (8). Improved understanding of the mechanisms of exercise-related AF may help in the development of novel therapeutic approaches (9) and evidence-based management guidelines (10). We designed the current studies to determine whether endurance training increases AF vulnerability in a rat model, and if so, to assess potential underlying mechanisms.
This section contains brief summaries of principal methods. For complete details, please see the Online Appendix.
Wistar rats were randomly assigned to matched sedentary (Sed) or intensive exercise-training (Ex) groups as previously described (3). Ex rats underwent 1-h/day treadmill training 5 days per week. The protocol included an initial 1-week progressive training program, increasing to steady-state 60-min running at 28 m/min. Animals that did not readily master the running program according to a specific running score (Online Appendix) were excluded. Thereafter, animals were trained at this level for 8 (Ex8) or 16 (Ex16) weeks. This training protocol was shown not to increase distress levels as measured with a previously validated distress score (Online Fig. 1). An additional group of Ex16 rats underwent discontinuation of exercise for 4 (DEx4) or 8 (DEx8) weeks. Time-matched parallel Sed rats were controls for each Ex and DEx group. At study end, rats underwent an in vivo electrophysiological study (EPS), followed by cardiac excision and formalin preservation (for histology), snap-freezing in liquid nitrogen (molecular biology), or retrograde perfusion (cardiomyocyte isolation).
Transthoracic echocardiographic studies were performed with a phased-array probe 10S (4.5 to 11.5 MHz) in a Vivid 7 Dimension system under 2% isoflurane anesthesia. Repeated measurements were made in 2 study series: a training evolution set (baseline, 8-week, and 16-week Ex) and a detraining set (16-week Ex, DEx4, and DEx8), along with corresponding Sed controls. Detailed measurements and methods are described in the Online Appendix. Recordings and analyses were blinded to group assignment.
In vivo EPS was performed under 2% isoflurane anesthesia. The distal dipole of a 1.9-F octapolar catheter in the right atrium was used for programmed stimulation.
Spontaneous (drug-free) arrhythmia inducibility was tested with double and triple extrastimuli during sinus rhythm and 9-beat trains at a cycle length (CL) of 150 ms and also with burst pacing at 40- and 60-ms CLs for 7.5, 15, and 30 s. AF was defined as >1 s of irregular atrial electrograms (≥800 beats/min) with irregular ventricular response. AF lasting 1 to 30 s was considered nonsustained, ≥30 s was sustained AF, and >15 min was long-lasting AF. If no sustained AF was induced, inducibility was retested 10 minutes after cumulative carbachol doses of 25 and 50 μg/kg intraperitoneally. The right atrial effective refractory period (RAERP) was measured in Ex16 rats and corresponding Sed rats with 9-beat basic pacing-stimulus (S1) trains (CL = 150 ms) followed by a premature extrastimulus (S2) with coupling interval (S1S2) reduced in 1-ms decrements, with RAERP defined as the longest S1S2 failing to produce a propagated response.
All surgical/instrumentation procedures were performed under 2% isoflurane anesthesia. Experiments were performed at least 48 hours postsurgery.
In 15 rats (8 Ex rats, 7 parallel Sed rats), repeated 24-h electrocardiography (ECG) recordings were obtained with implanted telemetry devices at baseline, 8-week Ex, 16-week Ex, DEx4, and DEx8. RR intervals were analyzed with commercial software. Recordings were reviewed manually to detect spontaneous arrhythmias.
Autonomic tone assessment
Parasympathetic and sympathetic tone was evaluated based on heart rate changes due to sequential pharmacological blockade. Experiments were performed on separate days in awake, nonrestrained rats with chronically implanted ECG telemetry and intravenous access systems. In preliminary time-course studies (Online Fig. 2A), atropine and propranolol effects were maximal within 1 min and maintained for >30 min. Definitive studies were designed based on the preliminary data. Parasympathetic tone index was defined according to the difference between heart rate after propranolol alone versus propranolol + atropine (Online Fig. 2B, top). Sympathetic tone index was the heart rate difference between atropine alone and atropine + propranolol (Online Fig. 2B, bottom).
Baroreflex sensitivity was assessed in conscious, unrestrained rats. Maximum changes in blood pressure and heart rate in response to progressively increasing doses of phenylephrine and nitroprusside were determined at each dose (Online Fig. 3).
Fibrosis was quantified blinded to group assignment, with Picrosirius red staining as described previously (3). Atrial photomicrographs were obtained with an Olympus BX60 (Olympus Microscopes, Richmond Hill, Ontario, Canada) microscope and QImaging QCam (Olympus Canada), and quantified with color recognition software.
Acetylcholine-dependent potassium current measurement
Cardiomyocytes were isolated by enzymatic digestion. Acetylcholine-dependent potassium current (IKACh) was obtained by digital subtraction of baseline current from current in the presence of carbachol. Concentration-response curves were obtained with the equation: Y = Min + (Δ/(1 + 10LogEC50-X), where Y = effect at concentration X, Min = minimum-response constant, Δ = maximum drug effect, and EC50 = 50% maximal effect concentration.
Messenger ribonucleic acid quantification
Ribonucleic acid was purified, quantified, and DNAse treated. Polymerase chain reaction reactions were performed on TaqMan low-density arrays. Glyceraldehyde-3-phosphate dehydrogenase served as the reference gene.
Regulator of G protein–signaling 4 knock-out mice
The regulator of G protein–signaling (RGS) 4 knock-out (RGS4KO) mouse (Rgs4tm1Dge1/J) strain was backcrossed onto a C57Bl/6 background. In vivo EPS was conducted via right jugular vein access with a 2-F octopolar catheter. AF induction was attempted with burst pacing at 10- to 40-ms pacing intervals and triple extrastimulus techniques at baseline (drug-free) and after 10 μg/kg intraperitoneally and 30 μg/kg of carbachol.
Quantitative variables are shown as mean ± SEM. For details of statistical comparisons for each analysis, see the Online Appendix. Most comparisons were performed by using linear mixed-effects modeling, with main factor and all interaction effects included. When significant interactions were found, pairwise comparisons were executed with a least significant difference test (this test does not offer actual correction for multiplicity).
Nonnormally distributed variables were compared by using the Mann-Whitney U test. Categorical variables were compared by using the Fisher exact test. Analyses were processed with SPSS version 19.0 (IBM SPSS Statistics, IBM Corporation, Armonk, New York) and GraphPad version 5.0 (GraphPad Software, Inc., La Jolla, California). A p value <0.05 was considered statistically significant.
Endurance-training promotes AF vulnerability
Exercise training did not alter PR, QRS, or QT intervals (Online Table 1). Electrophysiological variables were unchanged at 8-week exercise, but 16-week exercise significantly increased sinus rhythm CL and Wenckebach CL, consistent with enhanced vagal tone (Online Table 1). RAERP averaged 34.4 ± 0.7 ms in Ex16 rats compared with 37.0 ± 0.9 ms in corresponding Sed controls (p < 0.05).
Figure 1A shows representative recordings during AF induction. In the majority of Sed rats, no AF could be induced (upper left). In all Ex16 rats, AF was inducible, in some cases self-terminating (nonsustained AF, lower left), in others sustained for considerable periods (right). Overall spontaneous (carbachol-free) inducibility rates are shown in Figure 1B. No clear AF-promoting effects were evident with 8-week exercise training, but 16-week exercise training strongly increased AF inducibility (e.g., sustained AF in 9 of 14 [64%] Ex rats vs. 3 of 20 [15%] Sed rats; p < 0.01). Sustained AF was induced by extrastimulation (Fig. 1C) in 5 of 14 Ex rats versus none of the Sed rats (p < 0.01). The mean duration of sustained AF in Ex16 rats was 8.8 ± 2.5 min.
Five rats in the Ex group showed long-lasting induced AF (>15 min). To assess the role of vagal tone in long-lasting AF, these rats were given intraperitoneal atropine (2 mg/kg) (Online Fig. 4), which terminated AF in all of them. In 4 of the 5 rats, AF could not be reinduced after atropine administration. In rats without inducible AF at baseline, inducibility was reassessed after cholinergic stimulation with carbachol. Carbachol enhanced AF inducibility and persistence in both Sed and Ex rats, with sustained AF rates postcarbachol of 45% (9 of 20) in Sed rats and 100% (14 of 14) in Ex rats (p < 0.001 vs. Sed rats) (Online Fig. 5). No spontaneous AF or atrial tachycardia episodes occurred on 24-hour telemetry, and the frequency of premature atrial contractions did not differ between Sed rats and Ex rats (6 ± 2 vs. 4 ± 2 premature atrial contractions/100,000 beats respectively; p = 0.61).
Autonomic tone changes
Mean hourly heart rates in conscious, unrestrained rats are shown in Figure 2. Ex rats displayed slowing of heart rate within 8 weeks (Fig. 2A), which was maintained at 16 weeks (Fig. 2B). Parasympathetic tone clearly increased at 16-week Ex (Fig. 2C), but sympathetic tone was not significantly altered by exercise. Intrinsic heart rate (in presence of both atropine and propranolol) was not significantly affected by exercise (318 ± 8 beats/min for Sed rats vs. 338 ± 8 beats/min for Ex rats at 16 weeks), excluding primary sinus-node alterations.
Figure 3A shows the evolution of atrial size with exercise. Left atrial (LA) dimension in atrial-diastole increased significantly at 16 weeks. Because exercise-related differences might be obscured by body size discrepancies, we examined dimensions normalized to body weight (Fig. 3B). Atrial dimension/body weight ratios decreased with aging-related growth in both groups, but by 16 weeks, Ex rat LA diameters increased versus those in Sed rats by 34% and right atrial (RA) diameters by 19%. Full echocardiographic data are provided in Online Table 2. Within 8 weeks, exercise training caused left ventricular hypertrophy and dilation, LA dilation, and impaired left ventricular diastolic function. LA dilation was accompanied by atrial cardiomyocyte hypertrophy, cell capacitance averaging 129 ± 9 pF in Ex16 rats versus 74 ± 5 pF (p < 0.001) in Sed rats.
Representative Picrosirius red–stained photomicrographs are shown in Figure 3C. Colorimetric image analysis indicated a 60% statistically significant increase of fibrous tissue content in both atria with exercise training (Fig. 3D).
Our data show that intense exercise training causes several changes known to promote AF (9): enhanced vagal tone, atrial dilation, and atrial fibrosis. To obtain additional insights, we studied the time course of changes after cessation of 16-week exercise training. All electrophysiological differences between exercise-trained and Sed rats, including AF indices, resolved within 4 weeks of detraining (Online Table 1, Fig. 4A). Similarly, diurnal heart rate differences and parasympathetic tone returned to Sed values within 4 weeks of detraining (Fig. 2E). Structural remodeling responded quite differently. Fibrous tissue content remained increased after 8 weeks of detraining (Fig. 4B). LA enlargement similarly failed to recover with detraining (Fig. 4C, Online Table 3). RA enlargement did recover at DEx8, but significant RA enlargement remained after 4 weeks of detraining. Overall, these results demonstrate relatively rapid reversibility of AF susceptibility, vagal enhancement, and associated heart rate changes, in contrast to delayed and/or incomplete recovery of fibrotic and other atrial structural alterations. In conjunction with the strongly suppressive effect of atropine on AF in exercise-trained rats, these results point to vagal enhancement as an important component of AF promotion with exercise. We therefore directed our attention to the mechanisms by which exercise training enhances vagal tone.
Mechanisms of enhanced vagal effects
The determinants of vagal effects can be broadly divided into 2 components: the control of vagal nerve activity by autonomic reflex arcs such as the baroreflex and the end-organ response. Baroreflex sensitivity data from one 16-week Sed rat and 1 Ex16 rat are plotted in Figure 5A. Although the tachycardic responses to nitroprusside-induced pressure reductions follow similar slopes, the heart rate slowing caused by phenylephrine was greater for any given blood pressure increase in the Ex rat, producing a steeper CL–blood pressure slope. Overall, the bradycardic response to blood pressure elevation with phenylephrine, reflecting vagal enhancement, was unaffected at 8 weeks of exercise (Fig. 5B) but was approximately doubled (p < 0.01) at 16 weeks. Consistent with indices of vagal tone (sinus CL, Wenckebach CL during EPS, and diurnal heart rates on telemetry), the bradycardic response normalized rapidly with detraining. The cardiac end-organ response (IKACh amplitude produced by muscarinic cholinergic stimulation) to 10-nM, 100-nM, and 1-μM carbachol in 16-week Sed rat and Ex rat atrial cardiomyocytes is illustrated in Figure 6A. Mean current density voltage relations indicate enhanced responses to carbachol at the 2 lower concentrations, with no intergroup differences at the highest concentration; these results were confirmed with 10 μM of carbachol (Online Fig. 6A). Current kinetics were unaltered (Online Fig. 6B). Figure 6B shows IKACh concentration-response curves, which suggest enhanced sensitivity to cholinergic stimulation at 16 weeks of exercise (EC50 199 vs. 722 nmol/l, Ex vs Sed).
To explore potential underlying molecular mechanisms, we assessed the gene expression of relevant subunits. The enhanced IKACh response cannot be attributed to increased expression of IKACh channel subunits, because Kir3.4 was unchanged and Kir3.1 slightly down-regulated by exercise (Fig. 6C). We therefore examined components of the G protein–coupled receptor-signaling systems that mediate/modulate IKACh. There were no significant changes in the expression of alpha-adrenergic, beta-adrenergic, or muscarinic cholinergic receptors (Online Fig. 7A). Rats with long-lasting AF had significantly greater Kir3.4-subunit and M3-receptor mRNA expression than rats with shorter-lasting AF, along with a trend to slower heart rates (Online Fig. 8), suggesting that cholinergic molecular determinants might contribute to AF maintenance.
The principal G proteins mediating cholinergic signal transduction (Gαi1-3) were unchanged with exercise (Online Fig. 7). Gαo was down-regulated but does not control vagal responses (11). Gαq was up-regulated, possibly contributing to exercise-induced structural remodeling (12). RGS proteins modulate cardiac autonomic responses (13), particularly RGS4, which negatively controls cholinergic signaling (14). We observed significant changes in many members of the RGS family, primarily down-regulation of RGS3, 4, 10, 12, 14, and 16, in Ex rats versus Sed rats (Fig. 7A). To assess the potential role of RGS down-regulation in increased AF susceptibility, we evaluated arrhythmia inducibility in RGS4 KO-mice versus wild-type littermate controls. Both wild-type and RGS4KO-mice had normal ECG and intracardiac ECG traces at baseline (Fig. 7B). AF could not be induced under any of the conditions tested in wild-type mice but was readily induced in the presence of carbachol in RGS4KO mice (Fig. 7C). Overall, RGS4 deficiency strongly enhanced carbachol-related AF susceptibility (Fig. 7D). These observations are consistent with the notion that exercise training–induced RGS down-regulation contributes to cholinergic enhancement and associated AF susceptibility.
We developed, for the first time, an animal model of the AF-promoting substrate due to endurance exercise training, an important clinical paradigm (4–8), and explored the underlying mechanisms. We found several potential pathophysiological determinants, including vagal enhancement, atrial dilation, and atrial fibrosis. The time-course data and response to atropine suggest that vagal enhancement is particularly important. We then analyzed mechanisms underlying vagal changes, finding both baroreflex enhancement and increased sensitivity of IKACh to cholinergic stimulation, and identified a potential molecular basis, down-regulation of RGS proteins, that negatively controls the G protein signaling mediating cholinergic activation of IKACh.
Endurance training and AF-related remodeling
Our exercise rats displayed a cardiac phenotype similar to trained athletes, including eccentric ventricular hypertrophy and atrial dilation (15). Enhanced AF vulnerability was not evident at 8 weeks of exercise training, requiring 16 weeks to develop (Fig. 1). Similarly, athletes only develop AF after prolonged high-intensity training (4–6), with shorter training periods not appreciably increasing AF risk (16). Ongoing exercise practice is an important factor (17), consistent with our data showing relatively rapid resolution of AF promotion with detraining.
Little is known about the mechanisms of AF promotion by high-intensity endurance training in humans, despite its clinical importance (4–8). We noted 3 potential contributors in our model: atrial dilation, fibrosis, and enhanced vagal tone. Atrial dilation, a typical feature of athlete's heart (15), increases atrial substrate size, a significant determinant of reentrant AF susceptibility (18). Atrial dilation per se is not enough to sustain AF in young competitive athletes (16). Similarly, atrial dilation alone was insufficient to promote AF vulnerability in our model: AF vulnerability resolved rapidly with detraining despite slow or absent resolution of atrial dilation (Fig. 4). The limited reversibility of LA dilation in our model is consistent with clinical observations of limited reversibility and persistent LA dilation with detraining (19–21).
Myocardial fibrosis is a recognized pathophysiological factor; both physical interruption of cardiac muscle bundle continuity (22) and fibroblast cardiomyocyte interactions (23) may contribute. The 30% to 60% increase in fibrous tissue content we observed is well below the 5- to 15-fold increase in AF associated with heart failure in dog models (22,24) and in a mouse model of transforming growth factor beta overactivity (25). This limited fibrous tissue accumulation may explain the lack of AF promotion, despite persistent fibrosis, in the DEx4 and DEx8 groups (Fig. 4). Thus, as for atrial dilation, while tissue fibrosis likely contributes to exercise-related AF promotion, it is by itself insufficient.
AF in athletes who are otherwise healthy has generally been considered “lone AF.” Because the presence of atrial fibrosis and dilation in trained rats indicates atrial structural remodeling, AF cannot be considered “lone” in our model. Atrial dilation is also reported clinically in athletes (15,19,20); thus, the “lone” classification of AF in highly trained athletes should perhaps be reconsidered. It would be interesting to evaluate the possibility of atrial fibrosis in endurance athletes by using noninvasive methods such as delayed-enhancement magnetic resonance imaging.
Enhanced parasympathetic tone was clearly important in AF promotion in our model. Atropine terminated long-lasting AF and suppressed AF inducibility in Ex rats, and the time course of vagal indices matched that of AF inducibility during exercise training and detraining. Vagal activation is well recognized to promote AF by inducing spatially heterogeneous atrial refractoriness decreases (18). Consistent with this mechanism, RAERP was shortened in our trained rats. Vagal enhancement is a well-recognized consequence of endurance training, and most AF episodes in athletes develop in situations of predominant vagal tone (26). Conversely, vagal enhancement alone is insufficient to fully explain exercise-related AF because autonomic tone changes within weeks of clinical exercise training onset (27), whereas AF susceptibility takes longer to develop (4–6,16). Thus, enhanced vagal tone is likely a necessary but insufficient condition for exercise-induced AF promotion, with other factors such as fibrosis and atrial dilation also contributing.
Molecular mechanisms underlying exercise-induced atrial remodeling
Although vagal enhancement is a characteristic feature of highly trained athletes, the underlying basis is largely unknown. Exercise training enhances baroreflex sensitivity in patients with heart failure (28), consistent with our observations. Our study is the first to examine IKACh changes in an exercise model, and our findings suggest enhanced cardiac responsiveness. RGS protein down-regulation likely played a significant role in increasing cholinergic sensitivity, as most endogenous RGS proteins are negative regulators of cholinergic effects (13,14,29). RGS4 reduces cholinergic effects through βγ5 G-protein subunit modulation (14). RGS10 similarly accelerates IKACh deactivation (30). Our novel observation that RGS4 deletion enhances cholinergically mediated AF vulnerability is a proof of principle for the potential arrhythmogenic role of exercise-induced RGS dysregulation. Of note, RGS9 was the only RGS-protein that was up-regulated with exercise in our study. In contrast to other RGS proteins, RGS9 positively regulates muscarinic cholinergic signaling (31). The increase in Gαq-expression that we noted may have contributed to atrial fibrosis (12).
Extrapolation from animal models to humans should always be done with caution. In a previous work with this model, we estimated that a 16-week exercise program in the rat corresponds roughly to approximately 10 years of exercise training in humans, at about 85% to 90% maximum oxygen uptake (3). In a young individual, running at 7 miles/h or swimming at 75 yards/min would provide comparable exercise intensity (32).
The nature and reversibility of changes in atrial size, ventricular dimensions, mass, and function in the current model correlate well with observations in humans (15,16). In a previous study of ventricular function and ventricular tachycardia susceptibility in the same animal model (3), we noted activation of atrial collagen genes with exercise training, which reversed with detraining. The persistent fibrosis with detraining noted in our study suggests that although collagen synthesis may return to baseline with detraining, the already formed collagen is not broken down.
We showed increased AF inducibility and mechanistically characterized the arrhythmogenic substrate but did not observe spontaneous arrhythmic events. Patients with exercise-related AF must also have AF-inducing triggers, which were not evident in our model. We attempted to quantify RGS proteins by using western blotting with commercially available antibodies but were unsuccessful because of poor specificity. Therefore, our findings pointing to a role of RGS changes in parasympathetic enhancement are based solely on mRNA expression, which does not always correlate with protein expression.
We found that sustained, high-intensity endurance training induces an atrial arrhythmogenic substrate that includes atrial dilation, myocardial fibrosis, and autonomic imbalance. Vagal enhancement was an essential contributor to AF susceptibility, and was caused by increased baroreflex sensitivity and atrial myocyte acetylcholine-hyperresponsiveness related to RGS-protein downregulation.
The authors thank France Thériault for excellent secretarial help and Nathalie L'Heureux, Patrick Lawler, and Chantal St-Cyr for experimental assistance.
This research was funded by the Canadian Institutes of Health Research (MGP-6957 and MOP44365 for Dr. Nattel; MOP106670, for Dr. Heximer), Quebec Heart and Stroke Foundation, Fondation Leducq (European–North American Atrial Fibrillation Research Alliance), and the Deutsche Forschungsgemeinschaft (Do 769/1-3). Drs. Guasch and Benito, doctoral candidates at the University of Barcelona, were supported by Hospital Clínic and Rio Hortega awards (CM08/00201, CM06/00189) from the Spanish Health Ministry. Ms. Mighiu and Dr. Cifelli were supported by graduate studentships from the Canadian Institutes of Health Research. Mr. Tadevosyan is a recipient of a Le Fonds de la recherche en santé du Québec (FRSQ)-Le Réseau en santé cardiovasculaire (RSCV)/Heart and Stroke Foundation of Québec (HSFQ) FRSQ-RSCV/HSFQ doctoral scholarship. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose. The first 2 authors contributed equally to this work.
- Abbreviations and Acronyms
- cycle length
- discontinuation of exercise
- electrophysiological study
- exercise training
- acetylcholine-regulated potassium current
- left atrial
- messenger ribonucleic acid
- right atrial
- right atrial effective refractory period
- regulators of G-protein signaling
- G-protein–signaling 4 knock-out
- Wenckebach cycle length
- Received November 30, 2012.
- Revision received January 16, 2013.
- Accepted January 20, 2013.
- American College of Cardiology Foundation
- La G.A.,
- Robberecht C.,
- Kuiperi C.,
- et al.
- Benito B.,
- Gay-Jordi G.,
- Serrano-Mollar A.,
- et al.
- Molina L.,
- Mont L.,
- Marrugat J.,
- et al.
- Baldesberger S.,
- Bauersfeld U.,
- Candinas R.,
- et al.
- Mont L.,
- Elosua R.,
- Brugada J.
- Iwasaki Y.K.,
- Nishida K.,
- Kato T.,
- Nattel S.
- Cifelli C.,
- Rose R.A.,
- Zhang H.,
- et al.
- Fagard R.
- Pelliccia A.,
- Maron B.J.,
- Di Paolo F.M.,
- et al.
- Liu L.,
- Nattel S.
- Pelliccia A.,
- Maron B.J.,
- De L.R.,
- Di Paolo F.M.,
- Spataro A.,
- Culasso F.
- Burstein B.,
- Comtois P.,
- Michael G.,
- et al.
- Yue L.,
- Xie J.,
- Nattel S.
- Li D.,
- Fareh S.,
- Leung T.K.,
- Nattel S.
- Verheule S.,
- Sato T.,
- Everett T.,
- et al.
- Mont L.,
- Sambola A.,
- Brugada J.,
- et al.
- Pietila M.,
- Malminiemi K.,
- Vesalainen R.,
- et al.
- Fu Y.,
- Huang X.,
- Zhong H.,
- Mortensen R.M.,
- D'Alecy L.G.,
- Neubig R.R.
- Laroche G.,
- Giguere P.M.,
- Roth B.L.,
- Trejo J.,
- Siderovski D.P.