Regional differences in transient outward current density and inhomogeneities of repolarization in rabbit right atrium

A Novel Computational Model of the Rabbit Atrial Cardiomyocyte With Spatial Calcium Dynamics

Models of cardiac electrophysiology are widely used to supplement experimental results and to provide insight into mechanisms of cardiac function and pathology. The rabbit has been a particularly important animal model for studying mechanisms of atrial pathophysiology and atrial fibrillation, which has motivated the development of models for the rabbit atrial cardiomyocyte electrophysiology. Previously developed models include detailed representations of membrane currents and intracellular ionic concentrations, but these so-called “common-pool” models lack a spatially distributed description of the calcium handling system, which reflects the detailed ultrastructure likely found in cells in vivo. Because of the less well-developed T-tubular system in atrial compared to ventricular cardiomyocytes, spatial gradients in intracellular calcium concentrations may play a more significant role in atrial cardiomyocyte pathophysiology, rendering common-pool models less suitable for investigating underlying electrophysiological mechanisms. In this study, we developed a novel computational model of the rabbit atrial cardiomyocyte incorporating detailed compartmentalization of intracellular calcium dynamics, in addition to a description of membrane currents and intracellular processes. The spatial representation of calcium was based on dividing the intracellular space into eighteen different compartments in the transversal direction, each with separate systems for internal calcium storage and release, and tracking ionic fluxes between compartments in addition to the dynamics driven by membrane currents and calcium release. The model was parameterized employing a population-of-models approach using experimental data from different sources. The parameterization of this novel model resulted in a reduced population of models with inherent variability in calcium dynamics and electrophysiological properties, all of which fall within the range of observed experimental values. As such, the population of models may represent natural variability in cardiomyocyte electrophysiology or inherent uncertainty in the underlying experimental data. The ionic model population was also able to reproduce the U-shaped waveform observed in line-scans of triggered calcium waves in atrial cardiomyocytes, characteristic of the absence of T-tubules, resulting in a centripetal calcium wave due to subcellular calcium diffusion. This novel spatial model of the rabbit atrial cardiomyocyte can be used to integrate experimental findings, offering the potential to enhance our understanding of the pathophysiological role of calcium-handling abnormalities under diseased conditions, such as atrial fibrillation.

Sinus node-like pacemaker mechanisms regulate ectopic pacemaker activity in the adult rat atrioventricular ring

In adult mammalian hearts, atrioventricular rings (AVRs) surround the atrial orifices of atrioventricular valves and are hotbed of ectopic activity in patients with focal atrial tachycardia. Experimental data offering mechanistic insights into initiation and maintenance of ectopic foci is lacking. We aimed to characterise AVRs in structurally normal rat hearts, identify arrhythmia predisposition and investigate mechanisms underlying arrhythmogenicity. Extracellular potential mapping and intracellular action potential recording techniques were used for electrophysiology, qPCR for gene and, Western blot and immunohistochemistry for protein expression. Conditions favouring ectopic foci were assessed by simulations. In right atrial preparations, sinus node (SN) was dominant and AVRs displayed 1:1 impulse conduction. Detaching SN unmasked ectopic pacemaking in AVRs and pacemaker action potentials were SN-like. Blocking pacemaker current I_f, and disrupting intracellular Ca²⁺ release, prolonged spontaneous cycle length in AVRs, indicating a role for SN-like pacemaker mechanisms. AVRs labelled positive for HCN4, and SERCA2a was comparable to SN. Pacemaking was potentiated by isoproterenol and abolished with carbachol and AVRs had abundant sympathetic nerve endings. β₂-adrenergic and M₂-muscarinic receptor mRNA and β₂-receptor protein were comparable to SN. In computer simulations of a sick SN, ectopic foci in AVR were unmasked, causing transient suppression of SN pacemaking.

The Development of Compartmentation of cAMP Signaling in Cardiomyocytes: The Role of T-Tubules and Caveolae Microdomains

3′-5′-cyclic adenosine monophosphate (cAMP) is a signaling messenger produced in response to the stimulation of cellular receptors, and has a myriad of functional applications depending on the cell type. In the heart, cAMP is responsible for regulating the contraction rate and force; however, cAMP is also involved in multiple other functions. Compartmentation of cAMP production may explain the specificity of signaling following a stimulus. In particular, transverse tubules (T-tubules) and caveolae have been found to be critical structural components for the spatial confinement of cAMP in cardiomyocytes, as exemplified by beta-adrenergic receptor (β-ARs) signaling. Pathological alterations in cardiomyocyte microdomain architecture led to a disruption in compartmentation of the cAMP signal. In this review, we discuss the difference between atrial and ventricular cardiomyocytes in respect to microdomain organization, and the pathological changes of atrial and ventricular cAMP signaling in response to myocyte dedifferentiation. In addition, we review the role of localized phosphodiesterase (PDE) activity in constraining the cAMP signal. Finally, we discuss microdomain biogenesis and maturation of cAMP signaling with the help of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Understanding these mechanisms may help to overcome the detrimental effects of pathological structural remodeling.

Direct Evidence for Microdomain-Specific Localization and Remodeling of Functional L-Type Calcium Channels in Rat and Human Atrial Myocytes

Supplemental Digital Content is available in the text.

Distinct subpopulations of L-type calcium channels (LTCCs) with different functional properties exist in cardiomyocytes. Disruption of cellular structure may affect LTCC in a microdomain-specific manner and contribute to the pathophysiology of cardiac diseases, especially in cells lacking organized transverse tubules (T-tubules) such as atrial myocytes (AMs).

Three-dimensional computer model of the right atrium including the sinoatrial and atrioventricular nodes predicts classical nodal behaviours

The aim of the study was to develop a three-dimensional (3D) anatomically-detailed model of the rabbit right atrium containing the sinoatrial and atrioventricular nodes to study the electrophysiology of the nodes. A model was generated based on 3D images of a rabbit heart (atria and part of ventricles), obtained using high-resolution magnetic resonance imaging. Segmentation was carried out semi-manually. A 3D right atrium array model (∼3.16 million elements), including eighteen objects, was constructed. For description of cellular electrophysiology, the Rogers-modified FitzHugh-Nagumo model was further modified to allow control of the major characteristics of the action potential with relatively low computational resource requirements. Model parameters were chosen to simulate the action potentials in the sinoatrial node, atrial muscle, inferior nodal extension and penetrating bundle. The block zone was simulated as passive tissue. The sinoatrial node, crista terminalis, main branch and roof bundle were considered as anisotropic. We have simulated normal and abnormal electrophysiology of the two nodes. In accordance with experimental findings: (i) during sinus rhythm, conduction occurs down the interatrial septum and into the atrioventricular node via the fast pathway (conduction down the crista terminalis and into the atrioventricular node via the slow pathway is slower); (ii) during atrial fibrillation, the sinoatrial node is protected from overdrive by its long refractory period; and (iii) during atrial fibrillation, the atrioventricular node reduces the frequency of action potentials reaching the ventricles. The model is able to simulate ventricular echo beats. In summary, a 3D anatomical model of the right atrium containing the cardiac conduction system is able to simulate a wide range of classical nodal behaviours.

Roles of hyperpolarization-activated current If in sinoatrial node pacemaking: insights from bifurcation analysis of mathematical models

To elucidate the roles of hyperpolarization-activated current (I(f)) in sinoatrial node (SAN) pacemaking, we theoretically investigated 1) the effects of I(f) on stability and bifurcation during hyperpolarization of SAN cells; 2) combined effects of I(f) and the sustained inward current (I(st)) or Na(+) channel current (I(Na)) on robustness of pacemaking against hyperpolarization; and 3) whether blocking I(f) abolishes pacemaker activity under certain conditions. Bifurcation analyses were performed for mathematical models of rabbit SAN cells; equilibrium points (EPs), periodic orbits, and their stability were determined as functions of parameters. Unstable steady-state potential region determined with applications of constant bias currents shrunk as I(f) density increased. In the central SAN cell, the critical acetylcholine concentration at which bifurcations, to yield a stable EP and quiescence, occur was increased by smaller I(f), but decreased by larger I(f). In contrast, the critical acetylcholine concentration and conductance of gap junctions between SAN and atrial cells at bifurcations progressively increased with enhancing I(f) in the peripheral SAN cell. These effects of I(f) were significantly attenuated by eliminating I(st) or I(Na), or by accelerating their inactivation. Under hyperpolarized conditions, blocking I(f) abolished SAN pacemaking via bifurcations. These results suggest that 1) I(f) itself cannot destabilize EPs; 2) I(f) improves SAN cell robustness against parasympathetic stimulation via preventing bifurcations in the presence of I(st) or I(Na); 3) I(f) dramatically enhances peripheral cell robustness against electrotonic loads of the atrium in combination with I(Na); and 4) pacemaker activity of hyperpolarized SAN cells could be abolished by blocking I(f).

Electrophysiological substrate for a dominant reentrant source during atrial fibrillation

Experimentally observed differences in the action potential (AP) properties between the left (LA) and right (RA) atria are believed to be important in maintaining reentrant sources during atrial fibrillation. We incorporate AP models for single LA and RA cells, as well as major intra- and interatrial conduction pathways, into a 2D atrial tissue model and study the role of tissue heterogeneity in global interactions between reentrant spiral waves in both atria. Our simulations show that shorter AP refractoriness in the LA translates into a shorter period of spiral rotation, and as a result, reentry in the LA dominates the overall excitation patterns in the atria.

Mechanisms of transition from normal to reentrant electrical activity in a model of rabbit atrial tissue: interaction of tissue heterogeneity and anisotropy

Experimental evidence suggests that regional differences in action potential (AP) morphology can provide a substrate for initiation and maintenance of reentrant arrhythmias in the right atrium (RA), but the relationships between the complex electrophysiological and anatomical organization of the RA and the genesis of reentry are unclear. In this study, a biophysically detailed three-dimensional computer model of the right atrial tissue was constructed to study the role of tissue heterogeneity and anisotropy in arrhythmogenesis. The model of Lindblad et al. for a rabbit atrial cell was modified to incorporate experimental data on regional differences in several ionic currents (primarily, I(Na), I(CaL), I(K1), I(to), and I(sus)) between the crista terminalis and pectinate muscle cells. The modified model was validated by its ability to reproduce the AP properties measured experimentally. The anatomical model of the rabbit RA (including tissue geometry and fiber orientation) was based on a recent histological reconstruction. Simulations with the resultant electrophysiologically and anatomically detailed three-dimensional model show that complex organization of the RA tissue causes breakdown of regular AP conduction patterns at high pacing rates (>11.75 Hz): as the AP in the crista terminalis cells is longer, and electrotonic coupling transverse to fibers of the crista terminalis is weak, high-frequency pacing at the border between the crista terminalis and pectinate muscles results in a unidirectional conduction block toward the crista terminalis and generation of reentry. Contributions of the tissue heterogeneity and anisotropy to reentry initiation mechanisms are quantified by measuring action potential duration (APD) gradients at the border between the crista terminalis and pectinate muscles: the APD gradients are high in areas where both heterogeneity and anisotropy are high, such that intrinsic APD differences are not diminished by electrotonic interactions. Thus, our detailed computer model reconstructs complex electrical activity in the RA, and provides new insights into the mechanisms of transition from focal atrial tachycardia into reentry.

Characterization and comparison of Na+, K+ and Ca2+ currents between myocytes from human atrial right appendage and atrial septum

Atrial pacing to reduce paroxysmal atrial fibrillation recurrences is performed in right atrial appendage (RAA) traditionally. However, recent studies indicate that atrial septal (AS) pacing produces better outcomes than the RAA pacing. The underlying mechanisms for this difference remained unclear. One possible explanation for the superiority of AS pacing over RAA pacing is that the two different regions have distinct electrophysiological properties. The study was to explore whether there indeed exist regional differences of electrical activities between RAA and AS, using whole-cell patch clamp techniques. The results showed that RAA cells had longer action potential duration, more negative resting potential and greater amplitude of action potential, whereas AS cells had more rapid depolarizing velocity. The sodium current was significantly smaller in RAA cells, whereas the calcium current was markedly smaller in AS cells. The transient outward K(+) current was similar in both regions. The ultrarapid delayed rectifier K(+) current was greater in RAA than that in AS cells. The inward rectifier K(+) current was similar at potentials more negative to -60 mV in both regions. The results indicate that RAA and AS of patients with rheumatic heart disease possess distinct electrophysiological properties. These differences provided a rational explanation for the different efficacies in treating atrial fibrillation by atrial pacing in RAA and AS regions.

Action potential experiments complete hERG assay and QT-interval measurements in cardiac preclinical studies

INTRODUCTION

The ICHS7B guideline focused on hERG and QT assays, although other factors have also been linked with the induction of severe arrhythmias. Thus, the aim of the present study was to demonstrate that two in vitro action potential recordings constitute convincing models of predictive drug-induced Torsades de pointes (TdP) and re-entry arrhythmias.

METHODS

The effects of D,L-sotalol, flecainide and quinidine were investigated on potassium (hERG) and sodium (Na(V)1.5) currents transfected in HEK-293 cells to determine the repercussion of the blockade of these currents on rabbit Purkinje fibre (PF) and atrial action potentials. Atrial conduction velocity was also investigated as a model of re-entry arrhythmias.

RESULTS

hERG channels were blocked by D,L-sotalol, quinidine and flecainide (IC(50): 69, 0.33 and 0.74 micromol/L, respectively). D,L-sotalol (30 micromol/L) induced reverse-use dependent increases in action potential duration (APD(90): +31.7% and +81.2% at 1 and 0.2 Hz) and triangulation (APD(90-40): +34.7% and +73.6% at 1 and 0.2 Hz) in PF but not in atria. Quinidine (10 micromol/L) also increased APD(90) (+14.5% and +68.5% at 1 and 0.2 Hz) and APD(90-40) (+73.3% and +152.1% at 1 and 0.2 Hz) in PF. Flecainide (10 micromol/L) shortened APD(90) in PF (-26.0% and - 22.2% at 1 and 0.2 Hz). Quinidine and flecainide blocked Na(V)1.5 channels by 32.3% and 73.1%, respectively, and produced decreases in dV/dt(max) which were more marked in atria (-20.4% and -31.9%) compared to PF (-12.8% and 22.4%) at 1 Hz. Finally, quinidine and flecainide decreased atrial conduction speed by 14.6% and 30.8%, respectively.

CONCLUSION

Results obtained with flecainide demonstrate that use of the hERG channel alone should not be considered as a useful single assay. Rabbit Purkinje fiber action potentials can be considered as a comparable model for detection of reverse-use dependent APD prolongation and triangulation whereas the rabbit atria can be considered as a useful model for detection of sodium channel blockade associated with decreases in dV/dt(max) and conduction velocity.

Modeling transmural heterogeneity of K(ATP) current in rabbit ventricular myocytes

To investigate the mechanisms regulating excitation-metabolic coupling in rabbit epicardial, midmyocardial, and endocardial ventricular myocytes we extended the LabHEART model (Puglisi JL and Bers DM. Am J Physiol Cell Physiol 281: C2049-C2060, 2001). We incorporated equations for Ca(2+) and Mg(2+) buffering by ATP and ADP, equations for nucleotide regulation of ATP-sensitive K(+) channel and L-type Ca(2+) channel, Na(+)-K(+)-ATPase, and sarcolemmal and sarcoplasmic Ca(2+)-ATPases, and equations describing the basic pathways (creatine and adenylate kinase reactions) known to communicate the flux changes generated by intracellular ATPases. Under normal conditions and during 20 min of ischemia, the three regions were characterized by different I(Na), I(to), I(Kr), I(Ks), and I(Kp) channel properties. The results indicate that the ATP-sensitive K(+) channel is activated by the smallest reduction in ATP in epicardial cells and largest in endocardial cells when cytosolic ADP, AMP, PCr, Cr, P(i), total Mg(2+), Na(+), K(+), Ca(2+), and pH diastolic levels are normal. The model predicts that only K(ATP) ionophore (Kir6.2 subunit) and not the regulatory subunit (SUR2A) might differ from endocardium to epicardium. The analysis suggests that during ischemia, the inhomogeneous accumulation of the metabolites in the tissue sublayers may alter in a very irregular manner the K(ATP) channel opening through metabolic interactions with the endogenous PI cascade (PIP(2), PIP) that in turn may cause differential action potential shortening among the ventricular myocyte subtypes. The model predictions are in qualitative agreement with experimental data measured under normal and ischemic conditions in rabbit ventricular myocytes.

Action potential morphology heterogeneity in the atrium and its effect on atrial reentry: a two-dimensional and quasi-three-dimensional study

Atrial fibrillation (AF) is believed to be perpetuated by recirculating spiral waves. Atrial structures are often characterized with action potentials of varying morphologies; however, the role of the structure-dependent atrial electrophysiological heterogeneity in spiral wave behaviour is not well understood. The purpose of this study is to determine the effect of action potential morphology heterogeneity associated with the major atrial structures in spiral wave maintenance. The present study also focuses on how this effect is further modulated by the presence of the inherent periodicity in atrial structure. The goals of the study are achieved through the simulation of electrical behaviour in a two-dimensional atrial tissue model that incorporates the representation of action potentials in various structurally distinct regions in the right atrium. Periodic boundary conditions are then imposed to form a cylinder (quasi three-dimensional), thus allowing exploration of the additional effect of structure periodicity on spiral wave behaviour. Transmembrane potential maps and phase singularity traces are analysed to determine effects on spiral wave behaviour. Results demonstrate that the prolonged refractoriness of the crista terminalis (CT) affects the pattern of spiral wave reentry, while the variation in action potential morphology of the other structures does not. The CT anchors the spiral waves, preventing them from drifting away. Spiral wave dynamics is altered when the ends of the sheet are spliced together to form a cylinder. The main effect of the continuous surface is the generation of secondary spiral waves which influences the primary rotors. The interaction of the primary and secondary spiral waves decreased as cylinder diameter increased.

Monophasic action potential duration at the crista terminalis in patients with sinus node disease

BACKGROUND

The repolarization properties of the crista terminalis (CT) cells have not been elucidated in patients with sinus node disease (SND). In the present study a new technique of recording the monophasic action potential (MAP) at the CT was used to examine the repolarization of the right atrium (RA) in SND patients.

METHODS AND RESULTS

Symptomatic SND (n=13) patients and age-, sex-matched control patients (n=13) were tested. The MAP duration (MAPD) at a basic cycle length of 600 ms was recorded at the CT in the superior vena cava - RA junction and at the middle - anterior RA with the effective refractory period (ERP) at the high RA. In 6 controls and 4 SND patients, the effect of adenosine triphosphate on the MAPD was examined. The MAPD at the CT exceeded that at the middle - anterior RA in both groups. The MAPD at the CT in the SND group was significantly prolonged compared with the control group (CT: 358+/-39 ms vs 289+/-43 ms). Between the SND and control groups, the MAPD at the middle - anterior RA (278+/-36 ms vs 265+/-39 ms) and ERP (294+/-42 ms vs 266+/-41 ms) did not differ. Both the corrected-sinus node recovery time and sinoatrial conduction time were better correlated with the MAPD at the CT than the MAPD at the middle - anterior RA and ERP. Adenosine triphosphate shortened the MAPD, which was augmented at the CT in the SND patients.

CONCLUSION

A novel method of estimating the MAP at the CT revealed the characteristics of atrial repolarization in SND patients.

Bistability and correlation with arrhythmogenesis in a model of the right atrium

Rapid pacing is an important tool for understanding cardiac arrhythmias. A recent experiment involving rapid pacing of sheep atria indicated that the initiation of atrial arrhythmias may be related to the 1:1/2:1 bistability. To elucidate the mechanism of this relation, this study applied the pacing protocol from the sheep study to an idealized model of the right atrium. The model included all major anatomical features, the sino-atrial node, and the regional differences in the action potential duration (APD). A pacing protocol was applied, in which the basic cycle length (BCL) was decreased in steps of 10 ms until the response switched to 2:1, then BCL was increased. The 1:1-to-2:1 transitions occurred at shorter BCLs than the 2:1-to-1:1 transitions yielding a global bistability window of 60ms. As in the sheep study, idiopathic waves were observed at BCLs within or near the bistability window. The model was used to quantify the types, prevalence, and persistence of idiopatic waves, study their initiation and termination, and relate them to the model components. The results demonstrate that idiopatic waveforms move with the shift of the bistability window and that they disappear when bistability is eliminated. Thus, this modeling study supports causal relationship between the 1:1/2:1 bistability and the initiation of arrhythmias.

Recurrence of Atrial Fibrillation After Internal Cardioversion of Persistent Atrial Fibrillation Prognostic Importance of Electrophysiologic Parameters

BACKGROUND

The purpose of this study was to determine whether the extent of atrial electrical remodeling affects the recurrence of atrial fibrillation (AF) after cardioversion of persistent AF (PAF).

METHODS AND RESULTS

Internal atrial cardioversion was performed in 47 patients with PAF. The right atrial monophasic action potential duration (RA-MAPD) at pacing cycle lengths (PCLs) of 800-300 ms and P wave signal-averaged electrocardiogram were recorded after cardioversion. Bepridil (150-200 mg/day) and carvedilol (10 mg/day) were administered to all patients after cardioversion. Of the 47 patients, 20 had recurrent AF within 3 months. No relation was observed between age, left atrial dimension, left ventricular ejection fraction, and AF recurrence. The AF duration was significantly longer (p<0.05) and RA-MAPD at PCLs of 800 to 300 ms were significantly shorter (p<0.05) in patients with AF recurrence than in those without recurrence. The mean slope of the RA-MAPD for PCLs between 600 and 300 ms did not differ between the patients with and without AF recurrence. The filtered P-wave duration (FPD) was significantly longer in the patients with AF recurrence than in those without (p<0.05). Multivariate analysis also showed that the RA-MAPD at a PCL of 300 ms and FPD were predictors of AF recurrence (RAMAPD: p=0.038; FPD: p=0.052).

CONCLUSION

These results suggest that electrical remodeling related to the repolarization and depolarization may be the main contributors to early AF recurrence after cardioversion under the administration of bepridil and carvedilol.

Heterogeneity of action potential durations in isolated mouse left and right atria recorded using voltage-sensitive dye mapping

An imaging system for di-4-ANEPPS (4-[beta-[2-(di-n-butylamino)-6-naphthylvinyl]pyridinium]) voltage-sensitive dye recordings has been adapted for recording from an in vitro mouse heart preparation that consists of both atria in isolation. This approach has been used to study inter- and intra-atrial activation and conduction and to monitor action potential durations (APDs) in the left and right atrium. The findings from this study confirm some of our previous findings in isolated mouse atrial myocytes and demonstrate that many electrophysiological properties of mouse atria closely resemble those of larger mammals. Specifically, we made the following observations: 1) Activation in mouse atria originates in the sinoatrial node and spreads into the right atrium and, after a delay, into the left atrium. 2) APD in the left atrium is shorter than in the right atrium. 3) Sites in the posterior walls have longer APDs than sites in the atrial appendages. 4) Superfusion of this preparation with 4-aminopyridine and tetraethylammonium resulted in increases in APD, consistent with their inhibitory effects on the K+ currents known to be expressed in mouse atria. 5) The muscarinic agonist carbachol shortened APD in all areas of the preparation, except the left atrial appendage, in which carbachol had no statistically significant effect on APD. These results validate a new approach for monitoring activation, conduction, and repolarization in mouse atria and demonstrate that the physiological and pharmacological properties of mouse atria are sufficiently similar to those of larger animals to warrant further studies using this preparation.

Transmembrane action potential heterogeneity in the canine isolated arterially perfused right atrium: effect of IKr and IKur/Ito block

The role of electrical heterogeneity in development of cardiac arrhythmias is well recognized. The extent to which transmembrane action potential (TAP) heterogeneity contributes to the normal electrophysiology of well-oxygenated atria is not well defined. The principal objective of the present study was to define regional and transmural differences in characteristics of the TAP in isolated superfused and arterially perfused canine right atrial (RA) preparations under baseline, rapidly activating delayed rectifier K(+) current (I(Kr)) block, and combined block of ultrarapid delayed rectifier and transient outward K(+) current (I(Kur)/I(to) block). Superfused preparations that survived generally displayed a triangle-shaped TAP. Exceptions included cells from the crista terminalis, where TAPs with a normal plateau could be recorded. In contrast, most TAPs recorded from throughout the perfused RA displayed a spike-and-dome and/or plateau morphology. The perfused RA displayed a heterogeneous distribution of repolarization, V(max), and spike-and-dome morphology along the epicardial and endocardial surfaces as well as transmurally, in the region of the upper crista terminalis. I(Kr) block with E-4031 prolonged repolarization homogeneously in the perfused RA, whereas I(Kur)/I(to) block using low concentrations of 4-aminopyridine abbreviated action potential duration at 90% repolarization heterogeneously, leading to a reduction in dispersion of repolarization. Our data indicate that the electrical heterogeneities, previously described for the canine ventricle, also exist within the atria and that I(Kr) block does not accentuate and I(Kur)/I(to) block reduces RA dispersion of repolarization. Our study also points to major differences in the transmembrane activity recorded using superfused vs. arterially perfused atrial preparations.

The muscular network of the sheep right atrium and frequency-dependent breakdown of wave propagation

The complex branching structure of the right atrium (RA) muscular network may provide the substrate for complex patterns of propagation during atrial fibrillation (AF). As AF results in some cases from stable sources in the left atrium (LA) with fibrillatory conduction toward the RA, we hypothesize that periodic input to the RA at an exceedingly high frequency results in disorganized wave propagation associated with the complex structure of the RA. Optical mapping was performed in isolated coronary-perfused sheep RA. Rhythmic pacing of Bachmann's bundle allowed well-controlled and realistic conditions for LA-driven RA. Pacing at increasingly higher frequencies led to increasing delays in activation distal to major branching sites of the Crista terminalis and pectinate bundles, culminating in spatially distributed intermittent blockade at and above approximately 6.5 Hz. At this breakdown frequency, the dominant frequencies of the RA response activity became spatially nonuniform. Such frequency-dependent changes were independent of action potential duration. Rather, the spatial boundaries between proximal and distal frequencies correlated well with branch sites of the pectinate musculature. Thus, there exists a breakdown frequency in the sheep RA below which activity is periodic throughout the atrium and above which it is fibrillation-like, consistent with the ideas that during AF, high-frequency activation initiated in the LA undergoes fibrillatory conduction toward the RA, and that sink-to-source mismatch effect at branch points of the Crista terminalis and pectinate muscles is important in determining the complexity of the arrhythmia.

Comparison of time- and voltage-dependent K+ currents in myocytes from left and right atria of adult mice

Consistent differences in K+ currents in left and right atria of adult mouse hearts have been identified by the application of current- and voltage-clamp protocols to isolated single myocytes. Left atrial myocytes had a significantly (P < 0.05) larger peak outward K+ current density than myocytes from the right atrium. Detailed analysis revealed that this difference was due to the rapidly activating sustained K+ current, which is inhibited by 100 muM 4-aminopyridine (4-AP); this current was almost three times larger in the left atrium than in the right atrium. Accordingly, 100 muM 4-AP caused a significantly (P < 0.05) larger increase in action potential duration in left than in right atrial myocytes. Inward rectifier K+ current density was also significantly (P < 0.05) larger in left atrial myocytes. There was no difference in the voltage-dependent L-type Ca2+ current between left and right atria. As expected from this voltage-clamp data, the duration of action potentials recorded from single myocytes was significantly (P < 0.05) shorter in myocytes from left atria, and left atrial tissue was found to have a significantly (P < 0.05) shorter effective refractory period than right atrial tissue. These results reveal similarities between mice and other mammalian species where the left atrium repolarizes more quickly than the right, and provide new insight into cellular electrophysiological mechanisms responsible for this difference. These findings, and previous results, suggest that the atria of adult mice may be a suitable model for detailed studies of atrial electrophysiology and pharmacology under control conditions and in the context of induced atrial rhythm disturbances.

Alternans of atrial action potentials during atrial flutter as a precursor to atrial fibrillation

BACKGROUND

The mechanisms underlying the transition of typical atrial flutter (Afl) to fibrillation (AF) remain unclear. We set out to test the hypothesis that Afl disorganizes to AF via alternans of atrial action potentials.

METHODS AND RESULTS

In 38 patients with Afl, monophasic action potentials (MAPs) were recorded at the isthmus and either high or low right atrium (HRA, LRA) during overdrive pacing to 160 ms or to the initiation of AF, whichever came first. MAP duration measured at 90% repolarization was longer at the isthmus in all patients, and failed to shorten with rate, compared with the HRA (n=38) or LRA (n=5). In 20 patients who developed AF, progressive pacing first caused alternans of isthmus MAP duration and amplitude at mean cycle length of 219+/-45 ms, followed by AF at a mean onset cycle length of 184+/-38 ms. Subsets of this group showed spontaneous action potential duration alternans at the isthmus (11 of 20 patients) and 2:1 isthmus conduction block immediately preceding AF (4 of 20 patients). In the 18 patients who did not develop AF, MAP alternans was less common (9 of 18 patients; P<0.0003), and occurred only at faster pacing (cycle length=169+/-25 ms; P<0.05).

CONCLUSIONS

In patients with typical Afl, action potential duration rate maladaptation at the isthmus may lead to action potential duration alternans and conduction block preceding the transition to AF. These isthmus characteristics may enable the spontaneous initiation of AF through wavefront fractionation and may explain the benefits of isthmus ablation in preventing AF recurrence.

Action potential remodeling in the human right atrium with chronic lone atrial fibrillation

It has been shown in animal experiments that recurrent induction of atrial fibrillation (AF) or long-lasting atrial pacing causes a shortening of the atrial effective refractory period (ERP) and action potential duration (APD) and a loss of their physiological adaptation to rate. Much remains to be clarified as to the electrical remodeling in human patients with chronic AF. We recorded monophasic action potentials (MAPs) from the right atrium at pacing cycle lengths (CLs) of 300, 333, 400, 500, 600, and 750 ms after external cardioversion in 13 patients with chronic lone AF. Their configuration was compared with those obtained from 13 control patients. APDs at 50% and 90% repolarization (APD50, APD90) at the shortest CL (300 ms) in control and AF patients were 131 +/- 14, 211 +/- 19 ms and 136 +/- 12, 210 +/- 22 ms, respectively (mean +/- SD). APDs in control patients increased linearly with increases of CL, reaching maximal values of 174 +/- 30 ms (APD50) and 277 +/- 38 ms (APD90) at a CL of 750 ms. In AF patients, the steady-state CL-APD relation was shifted downward and flattened at CLs > 500 ms; APD50 and APD90 at a CL of 750 ms were 158 +/- 19 ms, 232 +/- 28 ms, respectively. APD90s at CLs of 600 and 750 ms were significantly shorter in AF than in control patients. No statistically significant difference was obtained in APD50 between the two groups at any CL tested. MAP configuration in AF patients was characterized by an acceleration of the late repolarization. The difference between APD90 and APD50 (APD90-50) in control patients was increased with increases of CL, reaching a plateau at a CL of 600 ms. This CL dependent slowing of the late repolarization of MAPs was abolished in AF patients. The atrial ERP, measured at CLs of 400 and 600 ms, showed changes parallel to those of APD90. ERP at a CL of 600 ms in AF patients (224 +/- 13 ms) was significantly shorter than that in control patients (247 +/- 25 ms). We conclude that chronic lone AF leads to electrical remodeling in the human atrium, which causes a loss of rate response of the late repolarization of action potential, leading to a shortening of APD and ERP at slower heart rates.

Altered transient outward current in human atrial myocytes of patients with reduced left ventricular function

INTRODUCTION

Electrophysiologic remodeling is involved in the self-perpetuation of atrial fibrillation. To define whether differences in atrial electrophysiology already are present in patients with increased susceptibility for atrial fibrillation, we compared patients in sinus rhythm with and without heart failure.

METHODS AND RESULTS

Atrial specimens were obtained from patients with reduced left ventricular ejection fraction (LVEF; n = 10) and normal LVEF (n = 16) who were undergoing aortocoronary bypass surgery and from donor hearts (n = 4). Enzymatically isolated atrial myocytes were investigated by whole cell, patch clamp techniques. Total outward current was significantly larger in myocytes of hearts with low LVEF than normal LVEF (19.4 +/- 1.3 vs 15.1 +/- 1.2 pA/pF at pulses to +60 mV, respectively). Analysis of inactivation time courses of different outward current components revealed that the observed current difference is due to the transient calcium-independent outward current I(to1) which is twice as large in the low LVEF group than in the normal LVEF group (9.4 +/- 0.9 vs 4.7 +/- 0.4 pA/pF at pulses to +60 mV, respectively). I(to1) recovery from inactivation was significantly more rapid in myocytes of hearts with low LVEF, and action potential plateau in these cells was significantly shorter. The results of I(to1) and action potential measurements in atrial myocytes of donor hearts were very similar to the results of patients with preserved heart function.

CONCLUSION

I(to1) in human atrial myocytes of patients with reduced LVEF has an increased density and altered kinetics in sinus rhythm. These differences in outward current may explain the reduced plateau phase of action potentials.

Properties of the transient outward current in rabbit sino-atrial node cells

The electrophysiological properties of the transient outward current were investigated in voltage-clamped single cells from the rabbit sino-atrial node. To make a regional comparison, some experiments were repeated in atrial myocytes. The current-voltage relationship showed a characteristic outward rectification with an activation threshold of -30 mV. External 4-aminopyridine (0.01-5 mM) inhibited this current in a dose-dependent manner (IC50 = 0.28 mM, Hill coefficient = 1.38). The steady-state inactivation exhibited a half-maximum voltage of -35 mV and a slope factor of -.4 mV. The current density of the transient outward current was 6.3 +/- 0.5 pA/pF in sino-atrial node cells and 12.3 +/- 1.2 pA/pF in atrial cells. The inactivation time constant was faster in sino-atrial node cells (time constants 4.2 +/- 0.5 and 26.0 +/- 0.6 ms, respectively, for the fast and slow components) than in atrial cells (9.7 +/- 1.2 and 44.8 +/- 3.2 ms, respectively). Recovery from inactivation was much faster in sino-atrial node cells (time constants 44.7 +/- 9.0 ms) than in atrial cells (time constants 1.39 +/- 0.32 and 6.70 +/- 0.1 s, respectively, for the fast and slow components). These results suggest that the kinetic properties, as well as the current density, of the transient outward current differs between sino-atrial node and atrial cells. Taking the current density of Ito at +10 mV as 2.5 +/- 0.3 pA/pF gives a total Ito of approximately 100 pA at the peak of the action potential in rabbit sino-atrial node cells. The action potential duration was increased by 24.8 +/- 1.3% by 0.5 mM 4-AP. Thus, Ito may contribute significantly to the repolarization phase in mammalian sino-atrial node cells.

Post-partum prolongation of the atrial repolarization in rabbit

Female sexual steroids are known to modify the expression of various K+ channels and thus they can alter cardiac repolarization. In the present work, using conventional microelectrode techniques, action potential characteristics were studied in atrial myocardium isolated from virgin, late pregnant, early (1-3 days) post-partum and late (2-3 weeks) post-partum rabbits. No changes in action potential configuration were observed during pregnancy. However, the duration, overshoot and amplitude of action potentials were significantly increased in the early (1-3 days) post-partum period. Resting potential and maximum rate of depolarization remained unchanged. The observed changes were transient, normal action potential characteristics were obtained at weeks 2-3 post-partum. 4-aminopyridine (1 mmol L(-1)). caused a marked lengthening of action potential duration in all preparations obtained from non-pregnant and pregnant rabbits, whereas this 4-aminopyridine-induced prolongation was moderate in those preparations excised from the hearts of early post-partum animals. Action potential configuration was not affected by pinacidil (10 micromol L(-1)) or glibenclamide (5 micromol L(-1)) in non-pregnant or pregnant animals. In preparations obtained from early post-partum rabbits, pinacidil significantly shortened action potential duration, which was reverted by glibenclamide. The lengthening of action potential duration together with the decreased sensitivity to 4-aminopyridine observed in early post-partum animals may probably be caused by reduction of the transient outward K+ current at this stage. The results also suggest that electrophysiological alterations in the early post-partum period may probably be more pronounced than those associated with pregnancy itself.

Heterogeneity of 4-aminopyridine-sensitive current in rabbit sinoatrial node cells

The electrophysiological properties of sinoatrial (SA) node pacemaker cells vary in different regions of the node. In this study, we have investigated variation of the 4-aminopyridine (4-AP)-sensitive current as a function of the size (as measured by the cell capacitance) of SA node cells to elucidate the ionic mechanisms. The 10 mM 4-AP-sensitive current recorded from rabbit SA node cells was composed of transient and sustained components (Itrans and Isus, respectively). The activation and inactivation properties [activation: membrane potential at which conductance is half-maximally activated (Vh) = 19.3 mV, slope factor (k) = 15.0 mV; inactivation: Vh = -31.5 mV, k = 7.2 mV] as well as the density of Itrans (9.0 pA/pF on average at +50 mV) were independent of cell capacitance. In contrast, the density of Isus (0.97 pA/pF on average at +50 mV) was greater in larger cells, giving rise to a significant correlation with cell capacitance. The greater density of Isus in larger cells (presumably from the periphery) can explain the shorter action potential in the periphery of the SA node compared with that in the center. Thus variation of the 4-AP-sensitive current may be involved in regional differences in repolarization within the SA node.

Downward gradient in action potential duration along conduction path in and around the sinoatrial node

Regional differences in electrical activity in rabbit sinoatrial node have been investigated by recording action potentials throughout the intact node or from small balls of tissue from different regions. In the intact node, action potential duration was greatest at or close to the leading pacemaker and declined markedly in all directions from it, e.g., by 74 +/- 4% (mean +/- SE, n = 4) to the crista terminalis. Similar data were obtained from the small balls. The gradient is down the conduction pathway and will help prevent reentry. In the intact node, a zone of inexcitable tissue with small depolarizations of <25 mV or stable resting potentials was discovered in the inferior part of the node, and this will again help prevent reentry. The intrinsic pacemaker activity of the small balls was slower in tissue from more inferior (as well as more central) parts of the node [e.g., cycle length increased from 339 +/- 13 ms (n = 6) to 483 +/- 13 ms (n = 6) in transitional tissue from more superior and inferior sites], and this may help explain pacemaker shift.

Regional differences in effects of 4-aminopyridine within the sinoatrial node

4-Aminopyridine (4-AP)-sensitive transient outward current (Ito) has been observed in the sinoatrial node, but its role is unknown. The effect of block of Ito by 5 mM 4-AP on small ball-like tissue preparations (diameter approximately 0.3-0.4 mm) from different regions of the rabbit sinoatrial node has been investigated. 4-AP elevated the plateau, prolonged the action potential, and decreased the maximum diastolic potential. Effects were greater in tissue from the periphery of the node than from the center. In peripheral tissue, 4-AP abolished the action potential notch, if present. 4-AP slowed pacemaker activity of peripheral tissue but accelerated that of central tissue. Differences in the response to 4-AP were also observed between tissue from more superior and inferior regions of the node. In the intact sinoatrial node, 4-AP resulted in a shift of the leading pacemaker site consistent with the regional differences in the response to 4-AP. It is concluded that 4-AP-sensitive outward current plays a major role in action potential repolarization and pacemaker activity in the sinoatrial node and that its role varies regionally.

Effects of pirmenol on action potentials and membrane currents in single atrial myocytes

Electrophysiological effects of pirmenol hydrochloride (pirmenol) were investigated in single atrial myocytes obtained from rabbit and guinea-pig hearts by using a whole-cell clamp technique. Under current clamp conditions, pirmenol (2-30 microM) prolonged action potential duration in a concentration-dependent manner without affecting resting membrane potential in rabbit atrial myocytes. However, in the presence of 4-aminopyridine (4 mM), pirmenol (10 microM) failed to prolong the action potential duration further. Pirmenol also suppressed acetylcholine-induced hyperpolarization and action potential duration shortening, resulting in a significant prolongation of the action potential duration in the presence of acetylcholine. Under voltage clamp conditions, pirmenol (1-1000 microM) inhibited transient outward current (I(to)) in a concentration-dependent manner. The concentration for half-maximal inhibition (IC50) of pirmenol on I(to) was about 18 microM. Pirmenol did not show the use and frequency dependent inhibition of I(to). The voltage dependence of the steady-state inactivation of I(to) and the recovery from inactivation were not significantly affected by pirmenol. Pirmenol accelerated the inactivation of I(to) and blocked I(to) as an exponential function of time, consistent with a time-dependent open channel blockade. Pirmenol (30 microM) did not affect the inwardly rectifying K+ current significantly, but it decreased the voltage-dependent L-type Ca2+ current by about 20%. In guinea-pig atrial myocytes, both acetylcholine and adenosine induced a specific K+ current activated by GTP-binding proteins. Pirmenol suppressed both the acetylcholine- and adenosine-induced K+ current effectively. The IC50 of pirmenol for acetylcholine- and adenosine-induced current was about 1 and 8 microM, respectively. The present results suggest that pirmenol prolongs the action potential duration by primarily inhibiting the transient outward current in atrial myocytes. In addition, since pirmenol inhibits acetylcholine- and adenosine-induced K+ current, pirmenol may effectively prolong the action potential duration in atrial myocytes under various physiological conditions as in the whole heart or ischemia.

Electrophysiologic effects of SD-3212, a new class I antiarrhythmic drug, on canine atrial flutter and atrial action-potential characteristics

SD-3212 (levo-semotiadil fumarate) is a newly developed compound that exhibits potent antiarrhythmic activity because of its inhibitory action on sodium and calcium channels. In animal models, SD-3212 suppressed ventricular tachyarrhythmias, but the effects of this drug on atrial tachyarrhythmias have not been reported. We investigated the electrophysiologic effects of SD-3212 on canine atrial flutter induced after placement of the intercaval obstacle and on atrial action-potential characteristics. In all seven dogs, SD-3212 (1.9 +/- 0.3 mg/kg) terminated atrial flutter after significant increase in atrial flutter cycle length from 126 +/- 5 to 166 +/- 14 ms (increase, 31 +/- 8%; p < 0.005). SD-3212 increased right atrial effective refractory period (RAERP) significantly from 126 +/- 7 to 149 +/- 11 ms at a basic cycle length of 300 ms. The increases in RAERP after SD-3212 at basic cycle lengths of 300, 200, and 150 ms did not differ (increase, 18 +/- 4%, 17 +/- 3%, and 19 +/- 3%, respectively). Interatrial conduction time (IACT) was prolonged after SD-3212 from 63 +/- 4 to 81 +/- 6 ms (increase, 31 +/- 6%) at a basic cycle length of 150 ms. Prolongation of IACT was frequency dependent. The plasma concentration of SD-3212 after the termination of atrial flutter was 187 +/- 56 ng/ml in four dogs tested. In vitro study by using standard microelectrode techniques showed SD-3212 at concentrations of 1-3 microM significantly prolonged action-potential duration at 90% repolarization. Vmax was decreased by SD-3212 in a concentration-dependent manner (0.3-3 microM), and the inhibitory effect on Vmax was greatest at the highest stimulation frequency of 3.3 Hz. These results indicate that a new antiarrhythmic drug, SD-3212, is effective in interrupting canine atrial flutter, possibly by suppressing atrial conduction, and might be effective for the treatment of clinical atrial tachyarrhythmias.

Heterogeneity in K+ channel transcript expression detected in isolated ferret cardiac myocytes

The molecular basis of the potassium ion (K+) channels that generate repolarization in heart tissue remains uncertain, in part because of the molecular diversity of the voltage-gated K+ channel family. In our investigation, we used fluorescent labeled oligonucleotide probes to perform in situ hybridization studies on enzymatically isolated myocytes to determine the identity, regional distribution, and cellular distribution of voltage-gated K+ channel, alpha-subunit mRNA expressed in ferret heart. The regions studied were from the sinoatrial node (SA), right and left atrium, right and left ventricle, and interatrial and interventricular septa. Kv1.5 and Kv1.4 were the most widely distributed K+ channel transcripts in the ferret heart (present in approximately 70%-86% and approximately 46%-95% of tested myocytes, respectively), followed by Kv1.2, Kv2.1, and Kv4.2. In addition, many myocytes contain transcripts for Kv1.3, Kv2.2, Kv4.1, Kv5.1, and members of the Kv3 family. Kv1.1, Kv1.6, and Kv6.1 were rarely expressed in working myocytes, but were more commonly expressed in SA nodal cells. Two other transcripts whose genes have been implicated in the long QT syndrome, erg and KvLQT1, were common in all regions (approximately 41%-58% and 52%-72%, respectively). These results show that both the diversity and heterogeneity of K+ channel mRNA in heart tissue is greater than previously suspected.

The upper turnover site in the reentry circuit of common atrial flutter

The upper turnover site of the reentry circuit of common atrial flutter was examined with the uses of atrial activation mapping and extrastimulus techniques during atrial flutter. The findings suggest that it is anterior to the orifice of the superior vena cava, i.e., between the superior vena cava and tricuspid annulus.

Triple atrioventricular nodal pathways can be masked

In summary, CL modulates the dispersion in refractoriness of triple nodal pathways by causing greater changes in refractoriness in the slower pathways and thus determines the feasibility of manifestation.

The shape of human atrial action potential accounts for different frequency-related changes in vitro

We aimed at investigating frequency-related changes of human atrial action potential (AP) in vitro to see whether baseline AP shape might account for different responses to increasing stimulation rates. Human right atrial trabeculae (n = 48) obtained from adult (n = 38, mean age 59 +/- 8, range 45-72 years) consecutive patients (approximately equal to 30% of those operated upon by a single surgeon; 1.26 preparations per patient, range 1-2) were superfused in an organ bath with oxygenated (O2 content 16 ml/l) and modified (NaHCO3 25.7 mmol/l) Tyrode's solution at 31 degrees C. Baseline electrophysiology (pacing: 1 ms duration, 2-4 mA current intensity) at cycle length (CL) of 1000 ms was recorded in 90% (43 out of 48) of the preparations. The frequency-related protocol (CL from 1600 to 300 ms) was, however, undertaken in 23 (48%) preparations because 20 (42%) became pacing unresponsive immediately after baseline recordings. No statistical differences were seen when baseline electrophysiological parameters (mean +/- SD) were grouped according to late pacing responsiveness (n = 43 vs. n = 23): respectively, resting membrane potential (RMP) was -74 +/- 6 vs. -75 +/- 4 mV, maximal upstroke velocity (Vmax) 172 +/- 60 vs. 173 +/- 39 V/s, AP amplitude (APA) 89 +/- 11 vs. 91 +/- 8 mV and AP durations were at 30% (APD30%) 10 +/- 13 vs. 13 +/- 18 ms, 50% (APD50%) 45 +/- 79 vs. 62 +/- 91 ms and 90% (APD90%) 383 +/- 103 vs. 407 +/- 108 ms. To classify baseline AP shape, two criteria were adopted: criterion 1 ("objective"), based on APA (cut-off 90 mV) and APD90% (cut-off 500 ms) computed values and criterion 2 ("visual") derived from the literature. These criteria enabled us to differentiate three AP shape types: type 1 (spike and dome), type 3 (no dome) and type 4 (extremely prolonged). At baseline, the two criteria diagnosed different proportions of AP shape types. There were, however, no intra-type statistical differences among electrophysiological parameters. By criterion 1, analysis of variance (ANOVA) showed significant inter-type differences of RMP,Vmax, APA, APD50 and 90% and by criterion 2 of APA, APD30, 50 and 90%, respectively. To facilitate comparisons with previous published data, criterion 2 was selected to analyse frequency-related changes of AP shape types. At low stimulation rate, ANOVA for repeated measures (with Greenhouse-Geisser epsilon correction) showed inter-type differences for APD30, 50 and 90% (P = 0.00005). RMP, Vmax, APA and APD90% were overall frequency-related (P = 0.00005). Inter-type frequency-related differences were however seen only for APD90%. Human atrial AP durations (30, 50 and 90%) enable differentiation among AP shape types (1, 3 and 4). By a standardized use-dependent protocol overall RMP, Vmax, APA and APD90% are frequency-related. AP shape accounts for frequency-related changes of APD90% only. A type 4 AP shape with much prolonged AP duration had a flat frequency dependence. At high stimulation rates, adult type 1 and 3 AP shapes are indistinguishable. Use-dependent and pharmacological investigations in human atrial myocytes need to take AP shape into account.