Abstract:

Atrial fibrillation is the most common cardiac arrhythmia in humans. The precise cellular mechanisms underlying atrial fibrillation are still poorly understood. Recent studies have identified several genetic defects as predisposing factors for this pathology. One of the identified genetic defects is the mutation N588K, which affects the cardiac IKr channel. Genetic variants in this channel have been identified to modify ventricular repolarization. The aim of this work is to investigate the effect of this mutation on atrial repolarization and the predisposition to atrial fibrillation.Measured data obtained with whole cell voltage clamp technique of wild-type and mutated hERG channel were implemented in the Courtemanche et al. ionic model. For this purpose, channel kinetics and density of the model were adjusted using parameter fitting to the measured data. By this way, the effects of the mutation in the hERG channel could be analyzed in the whole cell and in tissue, as well. The channel mutation N588K showed a gain of function effect, causing a rapid repolarization and consequently, a shortening of the action potential duration. Computer simulations of a schematic anatomical model of the right atrium were then carried out to investigate the excitation propagation and the repolarization.The action potential duration of the mutant cell was reduced to 116 ms and the effective refractory period to 220 ms. Both factors are linked to a shortening of the wavelength, indicating that the mutation N588K predisposes the initiation and perpetuation of atrial fibrillation.

Abstract:

The inhibition of IKuris a promising approach in the pharmacotherapy of AF, since it ensures atrial APD prolongation without affecting ventricular electrophysiology. Clinical and experimental data published so far are unconvincing, though. A possible reason is that the kinetics of such channel blockers, which are thought to play a very important role for the success of the blockade, have not always been considered. In this work, this hypothesis was examined by testing potential I Kur blockers regarding their antiarrhythmic efficiency.Tsujimae and co workers [2] studied the effects of I Kur block kinetics on human APD prolongation. By introducing a voltage and time dependent description of the blockade, they were able to analyze several binding properties. The investigators concluded that different kinetics play, indeed, a very important role, being responsible for the antiarrhythmic success.To analyze this hypothesis, the mathematical description of I Kur blockade presented in [2] was implemented into the CRN model of atrial myocytes. The effects of several binding properties were then tested under pathological and physiological conditions by carrying out single cell and tissue simulations. For this purpose a model of cAF was necessary. Due to the discrepancies found in literature about remodeling, two cAF models were developed; one representing the modifications carried by electrical remodeling and the other including additional gap junctional remodeling. The different binding properties were included in the model by adapting onset and recovery time constants. These values were modified, according to existing compounds, which can be used for I Kur blockade, as presented by Lagrutta et al. [1]. Two agents were adopted from [1]; one pre- senting fast and the other one slow kinetics (for both, onset and recovery). For both of them, a concentration of 1μM was assumed. Moreover, a fictive compound with slow onset and fast recov- ery was used to show an example of inefficient APD prolongation. The compounds were compared to a blockade without transmembrane voltage dependence. By this, the effects of voltage and time dependence could be quantified. Finally, these four configurations were compared to the values proposed by Tsujimae et al.Single cell simulations showed APD prolongation for all configurations, except for the fictive one. In contrast, only three configurations presented a satisfactory increase of the WL in tissue: both with fast onset and the last configuration without kinetics. For that reason, only these three were considered for 2D simulations.2D simulations were carried out by using a simplified model, representing isotropic homogeneous atrial tissue. cAF could be initiated with a crossfield stimulation protocol. The excitation propagation was analyzed by recording APs, calculating pseudo-ECGs, and determining the DF. Further, an algorithm for the automatic detection of spiral waves was developed, which enabled the exami- nation of complex propagation patterns. This new method was especially useful for counting rotors before and after applying the blockade. By this, the direct effects of the I Kur inhibition could be quantified.After the initiation of fast onset I Kur blockade, fibrillatory activity was observed in the model of cAF including additional gap junctional remodeling. Furthermore, the DF was not significantly changed and the number of wavelets increased. Thus, AF was not suppressed. On the other hand, blockade with fast onset and slow recovery was able to terminate the rotating wave in the second cAF model. However, this cAF model did not support the presence of stable spiral waves. Therefore, whether the rotor was terminated because of the model instability or due to the effects of the blockade remains unclear. All other configurations did not affect significantly the dynamics of the original spiral waves, indicating that the blockade was ineffective for the suppression of rotors. All analysis were carried out for the control case, as well. Hereby, I Kur block modified the AP morphology severely, nevertheless it did not have an impact on the APD90. By driving the plateau phase towards more positive values, K+ channels were activated sooner, resulting in a faster re- polarization. The modified morphology caused unstable behavior, as observed in the restitution curves. In 2D simulations, this instability was reflected by a non physiological excitation propa- gation. Under I Kur blockade, neither the plateau phase, nor the repolarization, corresponded to those observed in healthy tissue.The results obtained in this study indicate that the characteristics presented by Tsujimae et al. do not present the expected efficiency regarding termination of AF. Most importantly, this block might even show proarrhythymic properties under control conditions.

Abstract:

In the course of this project, the effects of the hERG channel mutation N588K on atrial repolar- ization and the predisposition to atrial fibrillation have been analyzed. For this purpose, measured data obtained with whole cell voltage clamp technique of wild-type and mutated hERG channel were implemented in the Courtemanche et al. ionic model. Channel kinetics and density of the model were therefore adjusted using parameter fitting to the measured data. The effects of the mutation in the hERG channel could be analyzed in a single-cell and tissue environment. Hereby, the most relevant factors for cardiac arrhythmias, such as APD, rate dependence, CV, and ERP were determined. The excitation propagation, repolarization and pre- disposition for rotating waves were finally investigated using a schematic anatomical model of the right atrium.The results presented in chapter 5 underline that the proper implementation of measured data in electrophysiological cell models can be affected by the measurement protocol used in the voltage clamp-technique. Since the use of a different pulse duration and return pulse can alter the behavior of the ionic currents, in particular during the recovery from inactivation and deactivation processes, the clamp-protocol used is very important for an accurate analysis of the channel effects on the cell.Further, mutation N588K showed a gain of function effect of IKr caused by the shifted inactivation of the hERG channel towards more positive potentials. In single-cell, this resulted in a significant shortening of the APD to 116 ms. However, the effects of the mutation in tissue were not that strong as in a single-cell. This results from the characteristic behavior of the mutation that shows a dependence on the maximum upstroke reached by depolarization. As a consequence, the effective APD was 220 ms. The ERP was also reduced in the tissue simulations. Combined with the fact that the CV decreases by short BCL, this effect builds a substrate for the initiation and perpetuation of AF. The differences between physiological and mutated case were most visible in the two- dimensional model. Not only the repolarization was shorter, but the mutated model supported the initiation of rotating waves, in contrast to the physiological one. The generated results show that mutation N588K builds, indeed, a substrate that can support the predisposition of AF. For further investigations it is very important to implement measured data which includes the information of the channel processes during recovery from inactivation. This will lead to more accurate analysis about the effects of the mutation, and with it to a better understanding of the electrical behavior underlying cardiac arrhtymia.Finally, a complete three-dimensional anatomical model should be used to further analyze the effect of mutation N588K on the perpetuation of rotating waves. The model used in this work did not include important factors like curvature or sharp edges, which play a very important role for the conduction velocity. In a three-dimensional model the conduction velocity will decrease, resulting in a further shortening of the wavelength. By this way, the model could not only produce one rotating wave, but flutter or fibrillation.