Pro-Arrhythmic Effects of Discontinuous Conduction at the Purkinje Fiber-Ventricle Junction Arising From Heart Failure-Induced Ionic Remodeling – Insights From Computational Modelling

Heart failure is associated with electrical remodeling of the electrical properties and kinetics of the ion channels and transporters that are responsible for cardiac action potentials. However, it is still unclear whether heart failure-induced ionic remodeling can affect the conduction of excitation waves at the Purkinje fiber-ventricle junction contributing to pro-arrhythmic effects of heart failure, as the complexity of the heart impedes a detailed experimental analysis. The aim of this study was to employ computational models to investigate the pro-arrhythmic effects of heart failure-induced ionic remodeling on the cardiac action potentials and excitation wave conduction at the Purkinje fiber-ventricle junction. Single cell models of canine Purkinje fiber and ventricular myocytes were developed for control and heart failure. These single cell models were then incorporated into one-dimensional strand and three-dimensional wedge models to investigate the effects of heart failure-induced remodeling on propagation of action potentials in Purkinje fiber and ventricular tissue and at the Purkinje fiber-ventricle junction. This revealed that heart failure-induced ionic remodeling of Purkinje fiber and ventricular tissue reduced conduction safety and increased tissue vulnerability to the genesis of the unidirectional conduction block. This was marked at the Purkinje fiber-ventricle junction, forming a potential substrate for the genesis of conduction failure that led to re-entry. This study provides new insights into proarrhythmic consequences of heart failure-induced ionic remodeling.


S1.1.3 Intracellular Ca 2+ transient: [Ca 2+ ]i 13
The formulation for the [ INaL was modified by increasing its maximum conductance and inactivation time 27 constant by 30% and 34% respectively based on the experimental data of Maltsev et 28 al. (2007). As shown in Supplementary Figure S3, the simulated INaL showed an 29 increased current density and a slowed decay time in the HF condition, which was 30 consistent with experimental findings (see Supplementary Table S2). 31

S1.2.3 Transient outward K + current: Ito1 32
Experimental studies have shown that in canine ventricles, HF did not alter the 33 kinetics of Ito1, but reduced its current density as well as Kv 4.3 expressions (see 34 Supplementary Table S3). In the simulation of HF, the maximal conductance of Ito1 35 was reduced by 43% in the Endo and Epi cells and 45% in the M cell (Li et al., 2002). 36

S1.2.4
Inward rectifier K + current: IK1 37 In the simulation of HF, IK1 maximal conductance were reduced by 41.1%, 40.7% and 38 40.9% in the Endo, M and Epi cells respectively based on the experimental data as 39 shown in Supplementary Table S4. Simulations results were shown in Supplementary 40 Figure S1. 41

S1.2.5 Fast and slow delayed rectifier K + current: IKr and IKs 42
In the simulation of HF, IKs was modified and IKr remained the same as the CTL 43 condition (see Supplementary Table S5) Some studies showed the evidence (see Supplementary Table S6) of ICaL remodelled  49 by HF in ventricular cells. In the simulation of HF, modifications to ICaL was mainly 50 based on the data from human on changed ICaL kinetics (Chen et al., 2002) and the 51 data from canine on altered ICaL density (O' Rourke et al., 1999). The steady state 52 activation curve of ICaL was shifted 7.64 mV to the left (Supplementary Figure S4). 53

S1.2.7 Intracellular Ca 2+ transient: [Ca 2+ ]i 54
There is abundant evidence from various studies (see Supplementary Table S7) 55 showing that in the HF condition, the systolic [Ca 2+ ]i level was reduced while the 56 decay process of [Ca 2+ ]i from the peak to the resting concentration levels was slowed 57 down in ventricular cells. There is also evidence of a reduced activity in the SR 58 uptake (Gupta et al., 1997) and down regulation of Ca 2+ uptake current (Whitmer et 59 al., 1988). Piacentino et al. (2003) showed a ～40% reduction in the SR content in 60 human ventricular cells and Pogwizd et al. (2001) observed a similar change in rabbit. 61 There is no experimental data available for the NCX density for canine ventricular 62 cells in the HF condition. But several studies have reported an up regulation of its 63 protein and mRNA levels in human ventricular cells. 64 In the simulation of HF, the experimental observations mentioned above were taken 65 into account for simulating HF-induced remodelling on the intracellular Ca 2+ 66 regulation. Specifically, the Ca 2+ uptake to the NSR was reduced as well as the Ca 2+ 67 leak in the SR and the Ca 2+ release in the junctional sarcoplasmic reticulum (JSR). 68 These were achieved by reducing the maximal sarcoplasmic reticulum Ca 2+ -ATPase 69 (SERCA) uptake (~87.5%), SR leak (~35%) and JSR release (~40%) coefficients. 70 The modifications resulted in a ~40% reduction in both NSR and JSR Ca 2+ contents. 71 In addition, the NCX density was increased by 20% in the HF condition. 72 where Vm is the membrane potential, EK the K + reversal potential, g K1 the maximal 99 conductance, c the half maximal voltage, α the fraction of the voltage-independent 100 conductance, β the fraction of the voltage-dependent conductance and b the steepness 101 of the GK1-voltage relationship.  Figure S5A&B). Simulation data also suggested that there were slight 138 differences in steady states of the activation and inactivation curves between the CTL 139 and HF conditions, but the amplitude of the slow inactivation time constant was 140 increased (Supplementary Figure S5D