ryanodine and Atrial-Fibrillation

Atrial fibrillation (AF) is the most common heart rhythm disorder. Transient postoperative AF can be elicited by high sympathetic nervous system activity. Catecholamines and serotonin cause arrhythmias in atrial trabeculae from patients with sinus rhythm (SR), but whether these arrhythmias occur in patients with chronic AF is unknown. We compared the incidence of arrhythmic contractions caused by norepinephrine, epinephrine, serotonin, and forskolin in atrial trabeculae from patients with SR and patients with AF. In the patients with AF, arrhythmias were markedly reduced for the agonists and abolished for forskolin, whereas maximum inotropic responses were markedly blunted only for serotonin. Serotonin and forskolin produced spontaneous diastolic Ca(2+) releases in atrial myocytes from the patients with SR that were abolished or reduced in myocytes from the patients with AF. For matching L-type Ca(2+)-current (ICa,L) responses, serotonin required and produced ∼ 100-fold less cAMP/PKA at the Ca(2+) channel domain compared with the catecholamines and forskolin. Norepinephrine-evoked ICa,L responses were decreased by inhibition of Ca(2+)/calmodulin-dependent kinase II (CaMKII) in myocytes from patients with SR, but not in those from patients with AF. Agonist-evoked phosphorylation by CaMKII at phospholamban (Thr-17), but not of ryanodine2 (Ser-2814), was reduced in trabeculae from patients with AF. The decreased CaMKII activity may contribute to the blunting of agonist-evoked arrhythmias in the atrial myocardium of patients with AF.

Topics: Atrial Fibrillation; Calcium; Calcium Channels, L-Type; Calcium-Binding Proteins; Calcium-Calmodulin-Dependent Protein Kinase Type 2; Cardiotonic Agents; Catecholamines; Chronic Disease; Colforsin; Cyclic AMP; Female; Heart Atria; Humans; Male; Myocardial Contraction; Phosphorylation; Ryanodine; Serotonin; Serotonin Receptor Agonists

The pulmonary vein, which returns oxygenated blood to the left atrium, is ensheathed by a population of unique, myocyte-like cells called pulmonary vein sleeve cells (PVCs). These cells autonomously generate action potentials that propagate into the left atrial chamber and cause arrhythmias resulting in atrial fibrillation; the most common, often sustained, form of cardiac arrhythmia. In mice, PVCs extend along the pulmonary vein into the lungs, and are accessible in a lung slice preparation. We exploited this model to study how aberrant Ca(2+) signaling alters the ability of PVC networks to follow electrical pacing. Cellular responses were investigated using real-time 2-photon imaging of lung slices loaded with a Ca(2+)-sensitive fluorescent indicator (Ca(2+) measurements) and phase contrast microscopy (contraction measurements). PVCs displayed global Ca(2+) signals and coordinated contraction in response to electrical field stimulation (EFS). The effects of EFS relied on both Ca(2+) influx and Ca(2+) release, and could be inhibited by nifedipine, ryanodine or caffeine. Moreover, PVCs had a high propensity to show spontaneous Ca(2+) signals that arose via stochastic activation of ryanodine receptors (RyRs). The ability of electrical pacing to entrain Ca(2+) signals and contractile responses was dramatically influenced by inherent spontaneous Ca(2+) activity. In PVCs with relatively low spontaneous Ca(2+) activity (<1 Hz), entrainment with electrical pacing was good. However, in PVCs with higher frequencies of spontaneous Ca(2+) activity (>1.5 Hz), electrical pacing was less effective; PVCs became unpaced, only partially-paced or displayed alternans. Because spontaneous Ca(2+) activity varied between cells, neighboring PVCs often had different responses to electrical pacing. Our data indicate that the ability of PVCs to respond to electrical stimulation depends on their intrinsic Ca(2+) cycling properties. Heterogeneous spontaneous Ca(2+) activity arising from stochastic RyR opening can disengage them from sinus rhythm and lead to autonomous, pro-arrhythmic activity.

Topics: Action Potentials; Animals; Atrial Fibrillation; Caffeine; Calcium Signaling; Electric Stimulation; Fluorescence; Lung; Mice; Microscopy, Phase-Contrast; Myocytes, Smooth Muscle; Nifedipine; Pulmonary Veins; Ryanodine; Ryanodine Receptor Calcium Release Channel; Statistics, Nonparametric

Calcium transient triggered firing (CTTF) is induced by large intracellular calcium (Ca(i)) transient and short action potential duration (APD). We hypothesized that CTTF underlies the mechanisms of early afterdepolarization (EAD) and spontaneous recurrent atrial fibrillation (AF) in transgenic (Tx) mice with overexpression of transforming growth factor β1 (TGF-β1).. MHC-TGFcys(33)ser Tx mice develop atrial fibrosis because of elevated levels of TGF-β1. We studied membrane potential and Ca(i)transients of isolated superfused atria from Tx and wild-type (Wt) littermates. Short APD and persistently elevated Ca(i) transients promoted spontaneous repetitive EADs, triggered activity and spontaneous AF after cessation of burst pacing in Tx but not Wt atria (39% vs. 0%, P=0.008). We were able to map optically 4 episodes of spontaneous AF re-initiation. All first and second beats of spontaneous AF originated from the right atrium (4/4, 100%), which is more severely fibrotic than the left atrium. Ryanodine and thapsigargin inhibited spontaneous re-initiation of AF in all 7 Tx atria tested. Western blotting showed no significant changes of calsequestrin or sarco/endoplasmic reticulum Ca(2+)-ATPase 2a.. Spontaneous AF may occur in the Tx atrium because of CTTF, characterized by APD shortening, prolonged Ca(i) transient, EAD and triggered activity. Inhibition of Ca(2+) release from the sarcoplasmic reticulum suppressed spontaneous AF. Our results indicate that CTTF is an important arrhythmogenic mechanism in TGF-β1 Tx atria.

Topics: Action Potentials; Animals; Atrial Fibrillation; Atrial Function; Blotting, Western; Calcium Signaling; Cardiac Pacing, Artificial; Disease Models, Animal; Electrophysiologic Techniques, Cardiac; Enzyme Inhibitors; Fibrosis; Heart Atria; Heart Conduction System; Mice; Mice, Transgenic; Ryanodine; Sarcoplasmic Reticulum Calcium-Transporting ATPases; Thapsigargin; Time Factors; Transforming Growth Factor beta1; Up-Regulation

Aging and pulmonary veins (PVs) play a critical role in the pathophysiology of atrial fibrillation. Abnormal Ca(2+) regulation and ryanodine receptors are known to contribute to PV arrhythmogenesis.. The purpose of this study was to investigate whether aging alters PV electrophysiology, Ca(2+) regulation proteins, and responses to rapamycin, FK-506, ryanodine, and ouabain.. Conventional microelectrodes were used to record action potential and contractility in isolated PV tissue samples in 15 young (age 3 months) and 16 aged (age 3 years) rabbits before and after drug administration. Expression of sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a), ryanodine receptor, and Na(+)/Ca(2+) exchanger was evaluated by western blot.. Aged PVs had larger amplitude of delayed afterdepolarizations, greater depolarized resting membrane potential, longer action potential duration, and higher incidence of action potential alternans and contractile alternans with increased expression of Na(+)/Ca(2+) exchanger and ryanodine receptor and decreased expression of SERCA2a. Rapamycin (1,10,100 nM), FK-506 (0.01, 0.1, 1 microM), ryanodine (0.1, 1 microM), and ouabain (0.1, 1 microM) concentration-dependently increased PV spontaneous rates and the incidence of delayed afterdepolarizations in young and aged PVs. Compared with results in young PVs, rapamycin and FK-506 in aged PVs increased PV spontaneous rates to a greater extent and exhibited a larger delayed afterdepolarization amplitude. In PVs without spontaneous activity, rapamycin and FK-506 induced spontaneous activity only in aged PVs, but ryanodine and ouabain induced spontaneous activity in both young and aged PVs.. Aging increases PV arrhythmogenesis via abnormal Ca(2+) regulation. These findings support the concept that ryanodine receptor dysfunction may result in high PV arrhythmogenesis and aging-related arrhythmogenic vulnerability.

Topics: Age Factors; Animals; Atrial Fibrillation; Blotting, Western; Calcium; Dose-Response Relationship, Drug; Electrocardiography; Heart Atria; Microelectrodes; Myocardial Contraction; Ouabain; Pulmonary Veins; Rabbits; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum; Sarcoplasmic Reticulum Calcium-Transporting ATPases; Signal Processing, Computer-Assisted; Sirolimus; Sodium-Calcium Exchanger; Tacrolimus; Tissue Culture Techniques

Rapid firing within pulmonary vein sleeves frequently initiates atrial fibrillation. The role of the autonomic nervous system in facilitating spontaneous firing is unknown.. The purpose of this study was to determine if autonomic nerve stimulation within canine atrium and pulmonary vein sleeves initiates arrhythmia formation.. Extracellular bipolar and intracellular microelectrode recordings were obtained from isolated superfused canine pulmonary veins (N = 28) and right atrium (N = 5) during local autonomic nerve stimulation.. Autonomic nerve stimulation decreased pulmonary vein sleeve action potential duration (APD90 = 160 +/- 17 to 92 +/- 24 ms; P < .01) and initiated rapid (782 +/- 158 bpm) firing from early afterdepolarizations in 22 of 28 pulmonary vein preparations. The initial spontaneous beat had a coupling interval of 97 +/- 26 ms. Failure to induce arrhythmia was associated with a failure to shorten APD90 (151 +/- 18 to 142 +/- 8 ms; P = .39). Muscarinic receptor blockade (atropine: 3.2 x 10(-8) M) prevented APD90 shortening in 8 of 8 preparations and suppressed firing in 6 of 8 preparations, whereas beta1-adrenergic receptor blockade (atenolol: 3.2 x 10(-8) M) suppressed firing in 8 of 8 preparations. Suppression of the Ca transient with ryanodine (10(-5) M) completely suppressed firing in 6 of 6 preparations. Inhibition of forward Na/Ca exchange by a transient increase in [Ca+2]o completely suppressed firing in 4 of 6 preparations. The same stimulus trains produce atropine-suppressed APD90 shortening in superfused right atrial free wall but fail to produce triggered arrhythmia.. The data demonstrate triggered firing within canine pulmonary veins with combined parasympathetic and sympathetic nerve stimulation. Both an enhanced Ca transient and increased Na/Ca exchange may be required for arrhythmia formation.

Topics: Animals; Atenolol; Atrial Fibrillation; Atropine; Autonomic Nervous System; Dogs; Electric Stimulation; Electrophysiologic Techniques, Cardiac; Heart Atria; Male; Pulmonary Veins; Ryanodine; Tetrodotoxin

Topics: Animals; Atenolol; Atrial Fibrillation; Atropine; Autonomic Nervous System; Dogs; Electric Stimulation; Electrophysiologic Techniques, Cardiac; Heart Atria; Male; Pulmonary Veins; Ryanodine; Tetrodotoxin

In subsets of patients paroxysmal firing of ectopic foci in pulmonary veins or coronary sinus is an important cause of atrial fibrillation. This appears to represent a rare event overriding a dominant sinus mechanism to alter the rhythmic firing of the atrium. Hence, we tested the hypothesis that a rare stimulation pattern might alter the myocardial substrate, making it more susceptible to the initiation of arrhythmias.. In isolated right and left rabbit atria, a "rare" burst pacing protocol (BPP) was applied as follows: over 3 h, preparations were driven for 4.5 min from sinus node (SN) or Bachmann's bundle (BB) regions at cycle length (CL)=400 ms followed by 30 s of stimulation from coronary sinus (CS) or pulmonary vein (PV) at CL=200 ms. Microelectrodes were used to record action potentials at the end of 4.5 min of pacing at CL=400 ms. We then intervened with 5-min bigeminal pacing to probe atrial vulnerability to arrhythmias: S1 was delivered from SN or BB and S2 from CS or PV, respectively. S1-S2 interval was the shortest eliciting a propagated response.. BPP shortened repolarization in CS and PV regions but not in SN or BB, resulting in increased dispersion of repolarization in right and decreased in left atria. Propranolol, atropine and losartan failed to alter the decrease in repolarization induced by BPP whereas apamin, nifedipine and ryanodine prevented BPP effects. Before BPP, bigeminy did not induce arrhythmias in either atrium, but after BPP, bigeminy significantly increased the incidence of arrhythmias in the right atrium.. BPP from foci outside the regions of dominant activation alters dispersion of atrial repolarization. Modulation of apamin-sensitive channels may contribute to the shortening of repolarization in CS and PV regions. Alterations of atrial repolarization gradient create an arrhythmogenic substrate and may be an early step in atrial electrophysiologic remodeling.

Topics: Action Potentials; Adrenergic beta-Antagonists; Animals; Apamin; Atrial Fibrillation; Atropine; Calcium Channel Blockers; Calcium Channels, L-Type; Cardiac Pacing, Artificial; Coronary Vessels; Dogs; Electrocardiography; Female; Heart Atria; Heart Conduction System; Losartan; Nifedipine; Potassium Channel Blockers; Propranolol; Pulmonary Veins; Rabbits; Receptor, Angiotensin, Type 1; Receptors, Muscarinic; Ryanodine

To investigate the expression and function changes of inositol 1,4,5-triphosphate receptor (IP3R) and ryanodine receptor (RyR) in the atrial myocytes during atrial fibrillation.. Ten adult mongrel dogs were randomly divided into 2 groups: 5 dogs underwent continuous rapid atrial pacing (500 beats/min) for twenty-four weeks to create persistent atrial fibrillation, and the other 5 size-matched dogs without pacemaker implantation were used as controls. Twenty-four weeks after the dogs' hearts were taken out and the canine atrial myocytes were isolated by enzymatic dissociation: fluorescent indicator Fluo-3/AM was added into the buffer to load the myocytes and then the Ca(2+) concentration was determined by confocal microscopy. BODIPY TR-X ryanodine was added into the buffer to stain the myocytes. Caffeine and ATP were added separately to stimulate the release of Ca(2+) from RyR.. (1) The expression of RyR in the sarcoplasmic reticulum of the atrial myocytes of the control group was (2.70 +/- 0.23), significantly higher than that of the atrial fibrillation group (0.25 +/- 0.14, P < 0.05). RyR was expressed mostly around the nucleus and only expressed in a small amount in the nucleus in the atrial fibrillation group. However, it was not expressed in the nucleus of the control group. The expression of IP3R in the atrial fibrillation group was significantly higher than that of the control group (P < 0.05). (2) After caffeine stimulation, the concentration in the atrial myocytes of the control group was (1.74 +/- 0.16), significantly higher than that of the fibrillation group (1.26 +/- 0.06, P < 0.05). (3) After ATP stimulation the Ca(2+) concentration in the atrial myocytes of the control group was (1.23 +/- 0.23), not significantly increased in comparison with that before ATP stimulation; however, the Ca(2+) concentration in the atrial myocytes of the fibrillation group after ATP stimulation was (2.29 +/- 0.65), significantly increased in comparison with that before ATP stimulation (P < 0.05).. (1) The expression of RyR is down-regulated, the function of RyR is decreased, and it is expressed in the nucleus during atrial fibrillation which shows that RyR is possibly translocated into the nucleus. (2) The expression of IP3R is up-regulated and the function of IP3R is increased during atrial fibrillation, which may be one of the major mechanisms of intracellular Ca(2+)-overload during atrial fibrillation.

Topics: Animals; Atrial Fibrillation; Atrial Function; Calcium Channels; Dogs; Female; Inositol 1,4,5-Trisphosphate; Inositol 1,4,5-Trisphosphate Receptors; Male; Myocytes, Cardiac; Receptors, Cytoplasmic and Nuclear; Ryanodine; Ryanodine Receptor Calcium Release Channel

Recent clinical electrophysiology studies and successful results of radiofrequency catheter ablation therapy suggest that high-frequency focal activity in the pulmonary veins (PVs) plays important roles in the initiation and perpetuation of atrial fibrillation, but the mechanisms underlying the focal arrhythmogenic activity are not understood.. Extracellular potential mapping of rabbit right atrial preparations showed that ryanodine (2 micromol/L) caused a shift of the leading pacemaker from the sinoatrial node to an ectopic focus near the right PV-atrium junction. The transmembrane potential recorded from the isolated myocardial sleeve of the right PV showed typical atrial-type action potentials with a stable resting potential under control conditions. Treatment with ryanodine (0.5 to 2 micromol/L) resulted in a depolarization of the resting potential and a development of pacemaker depolarization. These changes were enhanced transiently after an increase in the pacing rate: a self-terminating burst of spontaneous action potentials (duration, 33.6+/-5.0 s; n=32) was induced by a train of rapid stimuli (3.3 Hz) applied after a brief rest period. The pacing-induced activity was attenuated by either depletion of the sarcoplasmic reticulum of Ca2+ or blockade of the sarcolemmal Na+-Ca2+ exchanger or Cl- channels and potentiated by beta-adrenergic stimulation.. PV myocardial sleeves have the potential to generate spontaneous activity, and such arrhythmogenic activity is uncovered by modulation of intracellular Ca2+ dynamics.

Topics: Action Potentials; Adrenergic beta-Agonists; Animals; Atrial Fibrillation; Atrial Function; Calcium; Cardiac Pacing, Artificial; Culture Techniques; Electrophysiology; Heart Atria; Isoproterenol; Pulmonary Veins; Rabbits; Ryanodine; Sarcoplasmic Reticulum; Strophanthidin

Atrial fibrillation (AF) at times recurs immediately after termination of the arrhythmia. The mechanism(s) responsible for the extrasystole that reinduces AF is largely unknown. We hypothesized that abbreviation of action potential duration (APD) would permit very rapid rates of excitation, known to induce intracellular calcium loading, which in turn could promote delayed and/or early afterdepolarizations (EADs).. Acetylcholine (ACh, 1 micromol/L) was used to abbreviate atrial APD and permit rapid-pacing induction of AF in isolated coronary-perfused canine right atria. Transmembrane action potentials, pseudo-ECG, and tension development were recorded. AF or rapid pacing was associated with an increase in tonic tension. Termination of AF or rapid pacing (cycle length, 150 to 80 ms) resulted in a dramatic rise of phasic tension, prolongation of repolarization of the initial beats at the regular rate (cycle length, 700 ms), and the development of late phase 3 EADs and extrasystoles. These extrasystoles initiated AF in 15 cases (involving 9 right atria) within the first 11 seconds after termination of AF or rapid pacing. This novel EAD mechanism is observed only in association with marked APD abbreviation. The calcium channel blocker nifedipine reduced, and the sarcoplasmic reticulum calcium release blocker ryanodine eliminated, the post-rapid pacing-induced increase in phasic tension, late phase 3 EADs, and extrasystoles that initiate AF.. These data suggest that calcium overload conditions present after termination of vagally mediated AF contribute to the development of late phase 3 EAD-induced triggered activity and that this mechanism may be responsible for the extrasystolic activity that reinitiates AF.

Topics: Acetylcholine; Action Potentials; Animals; Atrial Fibrillation; Atrial Function; Calcium Channel Blockers; Cardiac Pacing, Artificial; Culture Techniques; Dogs; Heart Atria; Kinetics; Models, Cardiovascular; Nifedipine; Recurrence; Ryanodine

To study the possible role of intracellular Ca2+ overload in initiation of cholinergic-dependent atrial fibrillation (AF), we tested the effects of ryanodine in canine models of AF. In anesthetized open-chest dogs (n=10) AF was induced by two methods: (I) perfusion (9 ml/min) with normal Tyrode solution containing acetylcholine (ACh) into the sinus node artery (SNA) and (II) stimulation of the right vagal nerve (VS, 5 sec train). AF was induced in all dogs: by perfusion with ACh (3.7-/+1.5 mcM) into the SNA in 97-/+3% of attempts and by VS in 78-/+6% of attempts. Intravenous infusion of ryanodine (5 mg/kg) did not prevent induction of AF during ACh perfusion (84-/+5%, NS) but completely prevented the induction of AF by VS (4-/+3%, p<0.001). Atrial activation mapping (112 unipolar electrodes) did not show any significant differences between the beginning of ACh-dependent AF in control and after ryanodine treatment. Ryanodine significantly reduced both systolic and diastolic arterial pressures but had no effect on heart rate, atrial effective refractory period (AERP) and conduction velocity for one hour after infusion. Ryanodine, itself, did not exert antivagal activity, so after ryanodine treatment in the presence of VS (8 Hz) the reduction of AERP and the deceleration of heart rate were similar to that in control. These data suggest that ryanodine can suppress the initiation of AF induced by VS but not AF induced by ACh perfusion. We can conclude that the initiation of AF during ACh perfusion unlikely relates to triggering activity induced by intracellular Ca2+ overload. In addition, we suggest that besides ACh some 'unclear' ryanodine sensitive factor(s) contribute to the initiation of AF induced by VS.

Topics: Animals; Atrial Fibrillation; Calcium Channels; Cholinergic Fibers; Disease Models, Animal; Dogs; Electrophysiology; Ryanodine; Ryanodine Receptor Calcium Release Channel; Vagus Nerve

Systemic infusion of ryanodine did not prevent induction of atrial fibrillation (AF) during acetylcholine (Ach) perfusion in frogs. The AF, however, appeared later as of the dogs/Ach perfusion start and lasted for a shorter time as compared with the control. The activation mapping of the right atrium showed no significant difference from the control. The findings suggest that the mechanism of AF induction is hardly related to triggering activity, at least in this particular model.

Topics: Acetylcholine; Animals; Atrial Fibrillation; Atrioventricular Node; Cholinergic Agents; Dogs; Electrophysiology; Ryanodine