Abstract 17032: Substrate Stiffness Alters the Kinetics of Sodium Channel Nav1.5 and Depolarization of Cardiomyocytes

Background: Myocardial scarring after infarction (MI) can create areas of slowed conduction, which can lead to re-excitation of the heart, or re-entry. Reentrant arrhythmia is one of the electrophysiological mechanisms responsible for ventricular tachycardia after acute myocardial infarction (AMI). Since the sodium channel (Na v 1.5) is a major contributor to cardiac electrical conduction, the objective of this study was to evaluate the effect of cell substrate stiffness on the kinetics of the Na v 1.5. Methods: MI was created by permanent ligation of the left anterior descending artery in rats. After 7 days, hearts were decellularized and grossed into 1 mm rings for measuring stiffness by using an atomic force microscope. The hearts had an elastic modulus of 263.33±76.8 kPa at scar, and 27.26±3.97 kPa at a remote area (n=3). The surface of p olydimethylsiloxane (PDMS) gels was tuned to match the stiffness of decellularized infarcted rat hearts (17.04±2.02 kPa, 110.56±8.70 kPa and 328.4±45.72 kPa, respectively, n=3). Human embryonic kidney 293 (HEK-293) cells were induced to stably express human Na v 1.5. HEK-293 and human pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were separately cultured on the PDMS substrate that mimicked stiffness of infarcted rat hearts for 24 h. Sodium channel currents (I Na ) and action potentials (APs) were recorded by patch clamp techniques. Results: As substrate stiffness increased, HEK-293 voltage dependent activation of Na v 1.5 (I Na ) shifted significantly towards more positive voltage (Vhalf: -28.42±5.12 mV, -33.47±6.98 mV, -19.84±5.24 mV, respectively, p<0.01 in one-way ANOVA, n=12), and the transition from closed-state into inactivation was faster (tau: 48.47±4.60ms, 40.67±8.07ms, 81.20±9.54ms, p<0.05, n=12). However, the current density, the steady-state inactivation curve and the recovery time were comparable between different PDMS. In iPSC-CMs, the slope of AP upstroke was decreased when stiffness was increased (30.00±1.38V/s, 23.91±0.65V/s, 20.93±0.34V/s, p<0.05, n=15). Conclusion: Increased substrate stiffness, similar to myocardial scar, alters the kinetics of Na v 1.5 and affects the depolarization of cardiomyocytes, which likely contributes to slow conduction after MI.

Intracardiac echocardiography to guide catheter ablation of an accessory pathway in Ebstein’s anomaly. A case report

We present the case of a 17-year-old girl with Ebstein anomaly and repeated episodes of reentrant tachycardia due to a right posterior accessory pathway. Catheter ablation was performed using intracardiac echocardiography. A ViewFlex Xtra probe was inserted and showed an anormal tricuspid valve with elongated anterior leaflet and low insertion of the septal leaflet towards the apex. The anatomical annulus was identified by the course of the right coronary artery. RF application on the posterior annulus stopped the reentrant arrhythmia. After ablation, programmed stimulation showed absence of both antegrade and retrograde conduction through the accessory pathway.

Integrated rate-dependent and dual pathway AV nodal functions: principles and assessment framework

The atrioventricular (AV) node conducts slowly and has a long refractory period. These features sustain the filtering of atrial impulses and hence are often modulated to optimize ventricular rate during supraventricular tachyarrhythmias. The AV node is also the site of a clinically common reentrant arrhythmia. Its function is assessed for a variety of purposes from its responses to a premature protocol (S1S2, test beats introduced at different cycle lengths) repeatedly performed at different basic rates and/or to an incremental pacing protocol (increasingly faster rates). Puzzlingly, resulting data and interpretation differ with protocols as well as with chosen recovery and refractory indexes, and are further complicated by the presence of built-in fast and slow pathways. This problem applies to endocavitary investigations of arrhythmias as well as to many experimental functional studies. This review supports an integrated framework of rate-dependent and dual pathway AV nodal function that can account for these puzzling characteristics. The framework was established from AV nodal responses to S1S2S3 protocols that, compared with standard S1S2 protocols, allow for an orderly quantitative dissociation of the different factors involved in changes in AV nodal conduction and refractory indexes under rate-dependent and dual pathway function. Although largely based on data from experimental studies, the proposed framework may well apply to the human AV node. In conclusion, the rate-dependent and dual pathway properties of the AV node can be integrated within a common functional framework the contribution of which to individual responses can be quantitatively determined with properly designed protocols and analytic tools.

Functional reentry and circus movement arrhythmias in the small intestine of normal and diabetic rats

In a few recent studies, the presence of arrhythmias based on reentry and circus movement of the slow wave have been shown to occur in normal and diseased stomachs. To date, however, reentry has not been demonstrated before in any other part of the gastrointestinal system. No animals had to be killed for this study. Use was made of materials obtained during the course of another study in which 11 rats were treated with streptozotocin and housed with age-matched controls. After 3 and 7 mo, segments of duodenum, jejunum, and ileum were isolated and positioned in a tissue bath. Slow wave propagation was recorded with 121 extracellular electrodes. After the experiment, the propagation of the slow waves was reconstructed. In 10 of a total of 66 intestinal segments (15%), a circus movement of the slow wave was detected. These reentries were seen in control ( n = 2) as well as in 3-mo ( n = 2) and 7-mo ( n = 6) diabetic rats. Local conduction velocities and beat-to-beat intervals during the reentries were measured (0.42 ± 0.15 and 3.03 ± 0.67 cm/s, respectively) leading to a wavelength of 1.3 ± 0.5 cm and a circuit diameter of 4.1 ± 1.5 mm. This is the first demonstration of a reentrant arrhythmia in the small intestine of control and diabetic rats. Calculations of the size of the circuits indicate that they are small enough to fit inside the intestinal wall. Extrapolation based on measured velocities and rates indicate that reentrant arrhythmias are also possible in the distal small intestine of larger animals including humans.

The Effect of Low Extracellular Potassium on Cardiac Arrhythmia Vulnerability: A Computer Simulation Study

Unidirectional conduction block of cardiac excitation wave is one of the necessary conditions leading to initiation of reentrant arrhythmia. Temporal vulnerable window (VWtime) is an important measure for arrhythmia vulnerability. In this study, we examine theoretically how low extracellular K concentration ([K]o) control the VWtime by a single premature extrasystole. A numerical modified LR91 one dimensional heterogeneous ventricular model is used to quantitatively investigate the relation of low [K]o with conduction velocity (CV), VWtime, and electrical dynamic factor CV restitution. Morever, we characterize the distribution of Na channel conductance (gNa) and gating factors of premature beat applied at different time of the VWtime with different [K]o. The results show that lowering [K]o enlarge the VWtime magnitude gradually. The CV of S1 beat slows down with the [K]o in a reverse-[K]o dependent manner. Low [K]o promote the recovery of maximum Na conductance (GNa) from inactivation. However, decreasing [K]o can increase heterogeneity of gNa along the cable, and modulate CV restitution curve, which induce larger transmural dispersion of refratoriness for the premature beat and enlarge the VWtime.