Risk Factors and Genetics of Atrial Fibrillation.

Atrial fibrillation (AF) is by far the most common sustained tachyarrhythmia, affecting 1% to 2% of the general population. AF prevalence and the total annual cost for treatment are alarming, emphasizing the need for an urgent attention to the problem. Thus, having up-to-date information on AF risk factors and appreciating how they promote maintenance of AF maintenance are essential. This article presents a simplified examination of AF risk factors, including emerging genetic risks.

Dynamic reciprocity of sodium and potassium channel expression in a macromolecular complex controls cardiac excitability and arrhythmia.

The cardiac electrical impulse depends on an orchestrated interplay of transmembrane ionic currents in myocardial cells. Two critical ionic current mechanisms are the inwardly rectifying potassium current (I(K1)), which is important for maintenance of the cell resting membrane potential, and the sodium current (I(Na)), which provides a rapid depolarizing current during the upstroke of the action potential. By controlling the resting membrane potential, I(K1) modifies sodium channel availability and therefore, cell excitability, action potential duration, and velocity of impulse propagation. Additionally, I(K1)-I(Na) interactions are key determinants of electrical rotor frequency responsible for abnormal, often lethal, cardiac reentrant activity. Here, we have used a multidisciplinary approach based on molecular and biochemical techniques, acute gene transfer or silencing, and electrophysiology to show that I(K1)-I(Na) interactions involve a reciprocal modulation of expression of their respective channel proteins (Kir2.1 and Na(V)1.5) within a macromolecular complex. Thus, an increase in functional expression of one channel reciprocally modulates the other to enhance cardiac excitability. The modulation is model-independent; it is demonstrable in myocytes isolated from mouse and rat hearts and with transgenic and adenoviral-mediated overexpression/silencing. We also show that the post synaptic density, discs large, and zonula occludens-1 (PDZ) domain protein SAP97 is a component of this macromolecular complex. We show that the interplay between Na(v)1.5 and Kir2.1 has electrophysiological consequences on the myocardium and that SAP97 may affect the integrity of this complex or the nature of Na(v)1.5-Kir2.1 interactions. The reciprocal modulation between Na(v)1.5 and Kir2.1 and the respective ionic currents should be important in the ability of the heart to undergo self-sustaining cardiac rhythm disturbances.

Regulation of cardiac inward rectifier potassium current (I(K1)) by synapse-associated protein-97.

Synapse-associated protein-97 (SAP97) is a membrane-associated guanylate kinase scaffolding protein expressed in cardiomyocytes. SAP97 has been shown to associate and modulate voltage-gated potassium (Kv) channel function. In contrast to Kv channels, little information is available on interactions involving SAP97 and inward rectifier potassium (Kir2.x) channels that underlie the classical inward rectifier current, I(K1). To investigate the functional effects of silencing SAP97 on I(K1) in adult rat ventricular myocytes, SAP97 was silenced using an adenoviral short hairpin RNA vector. Western blot analysis showed that SAP97 was silenced by approximately 85% on day 3 post-infection. Immunostaining showed that Kir2.1 and Kir2.2 co-localize with SAP97. Co-immunoprecipitation (co-IP) results demonstrated that Kir2.x channels associate with SAP97. Voltage clamp experiments showed that silencing SAP97 reduced I(K1) whole cell density by approximately 55%. I(K1) density at -100 mV was -1.45 +/- 0.15 pA/picofarads (n = 6) in SAP97-silenced cells as compared with -3.03 +/- 0.37 pA/picofarads (n = 5) in control cells. Unitary conductance properties of I(K1) were unaffected by SAP97 silencing. The major mechanism for the reduction of I(K1) density appears to be a decrease in Kir2.x channel abundance. Furthermore, SAP97 silencing impaired I(K1) regulation by beta(1)-adrenergic receptor (beta1-AR) stimulation. In control, isoproterenol reduced I(K1) amplitude by approximately 75%, an effect that was blunted following SAP97 silencing. Our co-IP data show that beta1-AR associates with SAP97 and Kir2.1 and also that Kir2.1 co-IPs with protein kinase A and beta1-AR. SAP97 immunolocalizes with protein kinase A and beta1-AR in the cardiac myocytes. Our results suggest that in cardiac myocytes SAP97 regulates surface expression of channels underlying I(K1), as well as assembles a signaling complex involved in beta1-AR regulation of I(K1).

Cardiac strong inward rectifier potassium channels.

Cardiac I(K1) and I(KACh) are the major potassium currents displaying classical strong inward rectification, a unique property that is critical for their roles in cardiac excitability. In the last 15 years, research on I(K1) and I(KACh) has been propelled by the cloning of the underlying inwardly rectifying potassium (Kir) channels, the discovery of the molecular mechanism of strong rectification and the linking of a number of disorders of cardiac excitability to defects in genes encoding Kir channels. Disease-causing mutations in Kir genes have been shown experimentally to affect one or more of the following channel properties: structure, assembly, trafficking, and regulation, with the ultimate effect of a gain- or a loss-of-function of the channel. It is now established that I(K1) and I(KACh) channels are heterotetramers of Kir2 and Kir3 subunits, respectively. Each homomeric Kir channel has distinct biophysical and regulatory properties, and individual Kir subunits often display different patterns of regional, cellular, and membrane distribution. These differences are thought to underlie important variations in the physiological properties of I(K1) and I(KACh). It has become increasingly clear that the contribution of I(K1) and I(KACh) channels to cardiac electrical activity goes beyond their long recognized role in the stabilization of resting membrane potential and shaping the late phase of action potential repolarization in individual myocytes but extends to being critical elements determining the overall electrical stability of the heart.

A single-cell model of phase-driven control of ventricular fibrillation frequency.

The mechanisms controlling the rotation frequency of functional reentry in ventricular fibrillation (VF) are poorly understood. It has been previously shown that Ba2+ at concentrations up to 50 mumol/L slows the rotation frequency in the intact guinea pig (GP) heart, suggesting a role of the inward rectifier current (I(K1)) in the mechanism governing the VF response to Ba2+. Given that other biological (e.g., sinoatrial node) and artificial systems display phase-locking behavior, we hypothesized that the mechanism for controlling the rotation frequency of a rotor by I(K1) blockade is phase-driven, i.e., the phase shift between transmembrane current and voltage remains constant at varying levels of I(K1) blockade. We measured whole-cell admittance in isolated GP myocytes and in transfected human embryonic kidney (HEK) cells stably expressing Kir 2.1 and 2.3 channels. The admittance phase, i.e., the phase difference between current and voltage, was plotted versus the frequency in control conditions and at 10 or 50 micromol/L Ba2+ (in GP heart cells) or 1 mM Ba2+ (in HEK cells). The horizontal distance between plots was called the "frequency shift in a single cell" and analyzed. The frequency shift in a single cell was -14.14 +/- 5.71 Hz (n = 14) at 10 microM Ba2+ and -18.51 +/- 4.00 Hz (n = 10) at 50 microM Ba2+, p < 0.05. The values perfectly matched the Ba2+-induced reduction of VF frequency observed previously in GP heart. A similar relationship was found in the computer simulations. The phase of Ba2+-sensitive admittance in GP cells was -2.65 +/- 0.32 rad at 10 Hz and -2.79 +/- 0.26 rad at 30 Hz. In HEK cells, the phase of Ba2+-sensitive admittance was 3.09 +/- 0.03 rad at 10 Hz and 3.00 +/- 0.17 rad at 30 Hz. We have developed a biological single-cell model of rotation-frequency control. The results show that although rotation frequency changes as a result of I(K1) blockade, the phase difference between transmembrane current and transmembrane voltage remains constant, enabling us to quantitatively predict the change of VF frequency resulting from I(K1) blockade, based on single-cell measurement.

SAP97 regulates Kir2.3 channels by multiple mechanisms.

We examined the impact of coexpressing the inwardly rectifying potassium channel, Kir2.3, with the scaffolding protein, synapse-associated protein (SAP) 97, and determined that coexpression of these proteins caused an approximately twofold increase in current density. A combination of techniques was used to determine if the SAP97-induced increase in Kir2.3 whole cell currents resulted from changes in the number of channels in the cell membrane, unitary channel conductance, or channel open probability. In the absence of SAP97, Kir2.3 was found predominantly in a cytoplasmic, vesicular compartment with relatively little Kir2.3 localized to the plasma membrane. The introduction of SAP97 caused a redistribution of Kir2.3, leading to prominent colocalization of Kir2.3 and SAP97 and a modest increase in cell surface Kir2.3. The median Kir2.3 single channel conductance in the absence of SAP97 was approximately 13 pS, whereas coexpression of SAP97 led to a wide distribution of channel events with three distinct peaks centered at 16, 29, and 42 pS. These changes occurred without altering channel open probability, current rectification properties, or pH sensitivity. Thus association of Kir2.3 with SAP97 in HEK293 cells increased channel cell surface expression and unitary channel conductance. However, changes in single channel conductance play the major role in determining whole cell currents in this model system. We further suggest that the SAP97 effect results from SAP97 binding to the Kir2.3 COOH-terminal domain and altering channel conformation.

Fatty Infiltration of the Myocardium and Arrhythmogenesis: Potential Cellular and Molecular Mechanisms.

Anatomical evidence in several species shows highly heterogeneous fat distribution in the atrial and ventricular myocardium. Atrial appendages have fat deposits, and more so on the posterior left atrium. Although such fat distributions are considered normal, fatty infiltration is regarded arrhythmogenic, and various cardiac pathophysiological conditions show excess myocardial fat deposits, especially in the epicardium. Hypotheses have been presented for the physiological and pathophysiological roles of epicardial fat, however this issue is poorly understood. Therefore, this mini-review will focus on epicardial fat distribution and the (patho)-physiological implications of this distribution. Potential molecular mechanisms that may drive structural and electrical myocardial remodeling attendant to fatty infiltration of the heart are also reviewed.

Kir2.3 isoform confers pH sensitivity to heteromeric Kir2.1/Kir2.3 channels in HEK293 cells.

BACKGROUND: Data on pH regulation of the cardiac potassium current I(K1) suggest species-dependent differences in the molecular composition of the underlying Kir2 channel proteins. OBJECTIVE: The purpose of this study was to test the hypothesis that the presence of the Kir2.3 isoform in heterotetrameric channels modifies channel sensitivity to pH. METHODS: Voltage clamp was performed on HEK293 cells stably expressing guinea pig Kir2.1 and/or Kir2.3 isoforms and on sheep cardiac ventricular myocytes at varying extracellular pH (pH(o)) and in the presence of CO(2) to determine the sensitivity of macroscopic currents to pH. Single-channel activity was recorded from the HEK293 stables to determine the mechanisms of the changes in whole cell current. RESULTS: Biophysical characteristics of whole-cell and single-channel currents in Kir2.1/Kir2.3 double stables displayed properties attributable to isoform heteromerization. Whole-cell Kir2.1/Kir2.3 currents rectified in a manner reminiscent of Kir2.1 but were significantly inhibited by extracellular acidification in the physiologic range (pK(a) approximately 7.4). Whole-cell currents were more sensitive to a combined extracellular and intracellular acidification produced by CO(2). At pH(o) = 6.0, unitary conductances of heteromeric channels were reduced. Ovine cardiac ventricular cell I(K1) was pH(o) and CO(2) sensitive, consistent with the expression of Kir2.1 and Kir2.3 in this species. CONCLUSION: Kir2.1 and Kir2.3 isoforms form heteromeric channels in HEK293. The presence of Kir2.3 subunit(s) in heteromeric channels confers pH sensitivity to the channels. The single and double stable cells presented in this study are useful models for studying physiologic regulation of heteromeric Kir2 channels in mammalian cells.

Unique Kir2.x properties determine regional and species differences in the cardiac inward rectifier K+ current.

The inwardly rectifying potassium (Kir) 2.x channels mediate the cardiac inward rectifier potassium current (I(K1)). In addition to differences in current density, atrial and ventricular I(K1) have differences in outward current profiles and in extracellular potassium ([K+]o) dependence. The whole-cell patch-clamp technique was used to study these properties in heterologously expressed Kir2.x channels and atrial and ventricular I(K1) in guinea pig and sheep hearts. Kir2.x channels showed distinct rectification profiles: Kir2.1 and Kir2.2 rectified completely at potentials more depolarized than -30 mV (I approximately 0 pA). In contrast, rectification was incomplete for Kir2.3 channels. In guinea pig atria, which expressed mainly Kir2.1, I(K1) rectified completely. In sheep atria, which predominantly expressed Kir2.3 channels, I(K1) did not rectify completely. Single-channel analysis of sheep Kir2.3 channels showed a mean unitary conductance of 13.1+/-0.1 pS in 15 cells, which corresponded with I(K1) in sheep atria (9.9+/-0.1 pS in 32 cells). Outward Kir2.1 currents were increased in 10 mmol/L [K+]o, whereas Kir2.3 currents did not increase. Correspondingly, guinea pig (but not sheep) atrial I(K1) showed an increase in outward currents in 10 mmol/L [K+]o. Although the ventricles of both species expressed Kir2.1 and Kir2.3, outward I(K1) currents rectified completely and increased in high [K+]o-displaying Kir2.1-like properties. Likewise, outward current properties of heterologously expressed Kir2.1-Kir2.3 complexes in normal and 10 mmol/L [K+]o were similar to Kir2.1 but not Kir2.3. Thus, unique properties of individual Kir2.x isoforms, as well as heteromeric Kir2.x complexes, determine regional and species differences of I(K1) in the heart.

Cholinergic atrial fibrillation: I(K,ACh) gradients determine unequal left/right atrial frequencies and rotor dynamics.

OBJECTIVE: We tested the hypothesis that left atrial (LA) myocytes are more sensitive to acetylcholine (ACh) than right atrial (RA) myocytes, which results in a greater dose-dependent increase in LA than RA rotor frequency, increased LA-to-RA frequency gradient and increased incidence of wavelet formation during atrial fibrillation (AF). METHODS AND RESULTS: AF was induced in seven Langendorff-perfused sheep hearts in the presence of ACh (0.1-4.0 microM) and studied using optical mapping and bipolar recordings. Dominant frequencies (DFs) were determined in optical and electrical signals and phase movies were used to identify rotors and quantify their dynamics. DFs in both atria increased monotonically with ACh concentration until saturation, but the LA frequency predominated at all concentrations. Rotors were also seen more often in the LA, and although their life span decreased, their frequency and number of rotations increased. Patch-clamp studies demonstrated that ACh-activated potassium current (I(K,ACh)) density was greater in LA than RA sheep myocytes. Additionally, ribonuclease protection assay demonstrated that Kir3.4 and Kir3.1 mRNAs were more abundant in LA than in RA. CONCLUSIONS: A greater abundance of Kir3.x channels and higher I(K,ACh) density in LA than RA myocytes result in greater ACh-induced speeding-up of rotors in the LA than in the RA, which explains the ACh dose-dependent changes in overall AF frequency and wavelet formation.

Free Fatty Acid Effects on the Atrial Myocardium: Membrane Ionic Currents Are Remodeled by the Disruption of T-Tubular Architecture.

BACKGROUND: Epicardial adiposity and plasma levels of free fatty acids (FFAs) are elevated in atrial fibrillation, heart failure and obesity, with potentially detrimental effects on myocardial function. As major components of epicardial fat, FFAs may be abnormally regulated, with a potential to detrimentally modulate electro-mechanical function. The cellular mechanisms underlying such effects of FFAs are unknown. OBJECTIVE: To determine the mechanisms underlying electrophysiological effects of palmitic (PA), stearic (SA) and oleic (OA) FFAs on sheep atrial myocytes. METHODS: We used electrophysiological techniques, numerical simulations, biochemistry and optical imaging to examine the effects of acutely (≤ 15 min), short-term (4-6 hour) or 24-hour application of individual FFAs (10 µM) on isolated ovine left atrial myocytes (LAMs). RESULTS: Acute and short-term incubation in FFAs resulted in no differences in passive or active properties of isolated left atrial myocytes (LAMs). 24-hour application had differential effects depending on the FFA. PA did not affect cellular passive properties but shortened (p<0.05) action potential duration at 30% repolarization (APD30). APD50 and APD80 were unchanged. SA had no effect on resting membrane potential but reduced membrane capacitance by 15% (p<0.05), and abbreviated APD at all values measured (p≤0.001). OA did not significantly affect passive or active properties of LAMs. Measurement of the major voltage-gated ion channels in SA treated LAMs showed a ~60% reduction (p<0.01) of the L-type calcium current (ICa-L) and ~30% reduction (p<0.05) in the transient outward potassium current (ITO). A human atrial cell model recapitulated SA effects on APD. Optical imaging showed that SA incubated for 24 hours altered t-tubular structure in isolated cells (p<0.0001). CONCLUSIONS: SA disrupts t-tubular architecture and remodels properties of membrane ionic currents in sheep atrial myocytes, with potential implications in arrhythmogenesis.

Toward an understanding of the molecular mechanisms of ventricular fibrillation.

A major goal of basic research in cardiac electrophysiology is to understand the mechanisms responsible for ventricular fibrillation (VF). Here we review recent experimental and numerical results, from the ion channel to the organ level, which might lead to a better understanding of the cellular and molecular mechanisms of VF. The discussion centers on data derived from a model of stable VF in the Langendorff-perfused guinea pig heart that demonstrate distinct patterns of organization in the left (LV) and right (RV) ventricles. Analysis of optical mapping data reveals that VF excitation frequencies are distributed throughout the ventricles in clearly demarcated domains. The highest frequency domains are usually found on the anterior wall of the LV, demonstrating that a high frequency reentrant source (a rotor) that remains stationary in the LV is the mechanism that sustains VF in this model. Computer simulations predict that the inward rectifying potassium current (IK1) is an essential determinant of rotor stability and rotation frequency, and patch-clamp results strongly suggest that the outward component of the background current (presumably IK1) of cells in the LV is significantly larger in the LV than in the RV. These data have opened a new and potentially exciting avenue of research on the possible role played by inward rectifier channels in the mechanism of VF and may lead us toward an understanding of its molecular basis and hopefully lead to new preventative approaches.

Risk factors and genetics of atrial fibrillation.

Atrial fibrillation (AF) is by far the most common sustained tachyarrhythmia, affecting 1% to 2% of the general population. AF prevalence and the total annual cost for treatment are alarming, emphasizing the need for an urgent attention to the problem. Thus, having up-to-date information on AF risk factors and appreciating how they promote maintenance of AF maintenance are essential. This article presents a simplified examination of AF risk factors, including emerging genetic risks.

The ionic bases of the action potential in isolated mouse cardiac Purkinje cell.

BACKGROUND: Collecting electrophysiological and molecular data from the murine conduction system presents technical challenges. Thus, only little advantage has been taken of numerous genetically engineered murine models to study excitation through the cardiac conduction system of the mouse. OBJECTIVE: To develop an approach for isolating murine cardiac Purkinje cells (PCs), to characterize major ionic currents and to use the data to simulate action potentials (APs) recorded from PCs. METHODS: Light microscopy was used to isolate and identify PCs from apical and septal cells. Current and voltage clamp techniques were used to record APs and whole cell currents. We then simulated a PC AP on the basis of our experimental data. RESULTS: APs recorded from PCs were significantly longer than those recorded from ventricular cells. The prominent plateau phase of the PC AP was very negative (≈-40 mV). Spontaneous activity was observed only in PCs. The inward rectifier current demonstrated no significant differences compared to ventricular myocytes (VMs). However, sodium current density was larger, and the voltage-gated potassium current density was significantly less in PCs compared with myocytes. T-type Ca(2+) currents (I(Ca,T)) were present in PCs but not VMs. Computer simulations suggest that I(Ca,T) and cytosolic calcium diffusion significantly modulate AP profile recorded in PCs, as compared to VMs. CONCLUSIONS: Our study provides the first comprehensive ionic profile of murine PCs. The data show unique features of PC ionic mechanisms that govern its excitation process. Experimental data and numerical modeling results suggest that a smaller voltage-gated potassium current and the presence of I(Ca,T) are important determinants of the longer and relatively negative plateau phase of the APs.

Inhibition of platelet-derived growth factor-AB signaling prevents electromechanical remodeling of adult atrial myocytes that contact myofibroblasts.

BACKGROUND: Persistent atrial fibrillation (PAF) results in electromechanical and structural remodeling by mechanisms that are poorly understood. Myofibroblast proliferation and fibrosis are major sources of structural remodeling in PAF. Myofibroblasts also interact with atrial myocytes via direct physical contact and release of signaling molecules, which may contribute to remodeling. OBJECTIVE: To determine whether myofibroblasts contribute to atrial myocyte electromechanical remodeling via direct physical contact and platelet-derived growth factor (PDGF) signaling. METHODS: Myofibroblasts and myocytes from adult sheep atria were co-cultured for 24 hours. Alternatively adult sheep atrial myocytes were exposed to 1 ng/mL recombitant PDGF AB peptide for 24 hours. RESULTS: Myocytes making contact with myofibroblasts demonstrated significant reduction (P ≤ .05) in peak L-type calcium current density, shortening of action potential duration (APD), and reduction in calcium transients. These effects were blocked by pretreatment with a PDGF-AB neutralizing anti-body. Heterocellular contact also severely disturbed the localization of the L-type calcium channel. Myocytes exposed to recombinant PDGF-AB peptide for 24 hours demonstrated reduced APD50, APD80 and Peak L-type calcium current. Pretreatment with a PDGF-AB neutralizing antibody prevented these effects. Finally, while control atrial myocytes did not respond in a 1:1 manner to pacing frequencies of 3 Hz or higher, atrial myocytes from hearts that were tachypaced for 2 months and normal myocytes treated with PDGF-AB for 24 hours could be paced up to 10 Hz. CONCLUSIONS: In addition to leading to fibrosis, atrial myofibroblasts contribute to electromechanical remodeling of myocytes via direct physical contact and release of PDGF-AB, which may be a factor in PAF-induced remodeling.

Subcellular heterogeneity of sodium current properties in adult cardiac ventricular myocytes.

BACKGROUND: Sodium channel α-subunits in ventricular myocytes (VMs) segregate either to the intercalated disc or to lateral membranes, where they associate with region-specific molecules. OBJECTIVE: To determine the functional properties of sodium channels as a function of their location in the cell. METHODS: Local sodium currents were recorded from adult rodent VMs and Purkinje cells by using the cell-attached macropatch configuration. Electrodes were placed either in the cell midsection (M) or at the cell end (area originally occupied by the intercalated disc [ID]). Channels were identified as tetrodotoxin (TTX)-sensitive (TTX-S) or TTX-resistant (TTX-R) by application of 100 nM of TTX. RESULTS: Average peak current amplitude was larger in ID than in M and largest at the site of contact between attached cells. TTX-S channels were found only in the M region of VMs and not in Purkinje myocytes. TTX-R channels were found in both M and ID regions, but their biophysical properties differed depending on recording location. Sodium current in rat VMs was upregulated by tumor necrosis factor-alpha. The magnitude of current increase was largest in the M region, but this difference was abolished by application of 100 nM of TTX. CONCLUSIONS: Our data suggest that (a) a large fraction of TTX-R (likely Na(v)1.5) channels in the M region of VMs are inactivated at normal resting potential, leaving most of the burden of excitation to TTX-R channels in the ID region; (b) cell-cell adhesion increases functional channel density at the ID; and (c) TTX-S (likely non-Na(v)1.5) channels make a minimal contribution to sodium current under control conditions, but they represent a functional reserve that can be upregulated by exogenous factors.

Action potential duration restitution portraits of mammalian ventricular myocytes: role of calcium current.

Construction of the action potential duration (APD) restitution portrait allows visualization of multiple aspects of the dynamics of periodically paced myocytes at various basic cycle lengths (BCLs). For the first time, we obtained the restitution portrait of isolated rabbit and guinea pig cardiac ventricular myocytes and analyzed the time constant, tau, of APD accommodation and the slopes of different types of restitution curves, Sdyn and S12, measured at varying BCLs. Our results indicate that both tau and the individual slopes are species and pacing dependent. In contrast, the mutual relationship between slopes Sdyn and S12 does not depend on pacing history, being a generic feature of the species. In addition, the maximum slope S12, measured in the restitution portrait at the lowest BCL, predicts the onset of alternans. Further, we investigated the role of the L-type calcium current, ICa-L, in the restitution portrait. We found that ICa-L dramatically affects APD accommodation, as well as the individual slopes Sdyn and S12 measured in the restitution portrait. However, peak calcium current plays a role only at small values of BCL. In conclusion, the results demonstrate that the restitution portrait is a powerful technique to investigate restitution properties of periodically paced cardiac myocytes and the onset of alternans, in particular. Moreover, the data also show that ICa-L plays a crucial role in multiple aspects of cardiac dynamics measured through the restitution portrait.

Ionic determinants of functional reentry in a 2-D model of human atrial cells during simulated chronic atrial fibrillation.

Recent studies suggest that atrial fibrillation (AF) is maintained by fibrillatory conduction emanating from a small number of high-frequency reentrant sources (rotors). Our goal was to study the ionic correlates of a rotor during simulated chronic AF conditions. We utilized a two-dimensional (2-D), homogeneous, isotropic sheet (5 x 5 cm(2)) of human atrial cells to create a chronic AF substrate, which was able to sustain a stable rotor (dominant frequency approximately 5.7 Hz, rosette-like tip meander approximately 2.6 cm). Doubling the magnitude of the inward rectifier K(+) current (I(K1)) increased rotor frequency ( approximately 8.4 Hz), and reduced tip meander (approximately 1.7 cm). This rotor stabilization was due to a shortening of the action potential duration and an enhanced cardiac excitability. The latter was caused by a hyperpolarization of the diastolic membrane potential, which increased the availability of the Na(+) current (I(Na)). The rotor was terminated by reducing the maximum conductance (by 90%) of the atrial-specific ultrarapid delayed rectifier K(+) current (I(Kur)), or the transient outward K(+) current (I(to)), but not the fast or slow delayed rectifier K(+) currents (I(Kr)/I(Ks)). Importantly, blockade of I(Kur)/I(to) prolonged the atrial action potential at the plateau, but not at the terminal phase of repolarization, which led to random tip meander and wavebreak, resulting in rotor termination. Altering the rectification profile of I(K1) also slowed down or abolished reentrant activity. In combination, these simulation results provide novel insights into the ionic bases of a sustained rotor in a 2-D chronic AF substrate.

Blockade of the inward rectifying potassium current terminates ventricular fibrillation in the guinea pig heart.

INTRODUCTION: Stable high-frequency rotors sustain ventricular fibrillation (VF) in the guinea pig heart. We surmised that rotor stabilization in the left ventricle (LV) and fibrillatory conduction toward the right ventricle (RV) result from chamber-specific differences in functional expression of inward rectifier (Kir2.x) channels and unequal IK1 rectification in LV and RV myocytes. Accordingly, selective blockade of IK1 during VF should terminate VF. METHODS AND RESULTS: Relative mRNA levels of Kir2.x channels were measured in LV and RV. In addition, LV (n = 21) and RV (n = 20) myocytes were superfused with BaCl2 (5-50 micromol/L) to study the effects on IK1. Potentiometric dye-fluorescence movies of VF were obtained in the presence of Ba2+ (0-50 micromol/L) in 23 Langendorff-perfused hearts. Dominant frequencies (DFs) were determined by spectral analysis, and singularity points were counted in phase maps to assess VF organization. mRNA levels for Kir2.1 and Kir2.3 were significantly larger in LV than RV. Concurrently, outward IK1 was significantly larger in LV than RV myocytes. Ba2+ decreased IK1 in a dose-dependent manner (LV change > RV change). In baseline control VF, the fastest DF domain (28-40 Hz) was located on the anterior LV wall and a sharp LV-to-RV frequency gradient of 21.2 +/- 4.3 Hz was present. Ba2+ significantly decreased both LV frequency and gradient, and it terminated VF in a dose-dependent manner. At 50 micromol/L, Ba2+ decreased the average number of wavebreaks (1.7 +/- 0.9 to 0.8 +/- 0.6 SP/sec x pixel, P < 0.05) and then terminated VF. CONCLUSION: The results strongly support the hypothesis that IK1 plays an important role in rotor stabilization and VF dynamics.