Molecular and cellular aspects of re-entrant arrhythmias

In recent years it has become evident that myocardial tissue undergoes remodeling in diseased states such as myocardial infarction and hypertrophy which affects membrane channels, cell-to-cell coupling as well as the connective tissue matrix. Although the detailed mechanisms of ventricular arrhythmias in ventricular hypertrophy are not known, studies carried, out by computer simulations or high resolution mapping of electrical activity have suggested a complex interaction between changing ionic currents at the level of the cell membranes, altered cell-to-cell coupling and altered macroscopic structure. The present report summarises these recent developments and their potential relevance for arrhythmogenesi

Virtual cardiac monolayers for electrical wave propagation

The complex structure of cardiac tissue is considered to be one of the main determinants of an arrhythmogenic substrate. This study is aimed at developing the first mathematical model to describe the formation of cardiac tissue, using a joint in silico-in vitro approach. First, we performed experiments under various conditions to carefully characterise the morphology of cardiac tissue in a culture of neonatal rat ventricular cells. We considered two cell types, namely, cardiomyocytes and fibroblasts. Next, we proposed a mathematical model, based on the Glazier-Graner-Hogeweg model, which is widely used in tissue growth studies. The resultant tissue morphology was coupled to the detailed electrophysiological Korhonen-Majumder model for neonatal rat ventricular cardiomyocytes, in order to study wave propagation. The simulated waves had the same anisotropy ratio and wavefront complexity as those in the experiment. Thus, we conclude that our approach allows us to reproduce the morphological and physiological properties of cardiac tissue

Remodeling of cardiac passive electrical properties and susceptibility to ventricular and atrial arrhythmias

Coordinated electrical activation of the heart is essential for the maintenance of a regular cardiac rhythm and effective contractions. Action potentials spread from one cell to the next via gap junction channels. Because of the elongated shape of cardiomyocytes, longitudinal resistivity is lower than transverse resistivity causing electrical anisotropy. Moreover, non-uniformity is created by clustering of gap junction channels at cell poles and by non-excitable structures such as collagenous strands, vessels or fibroblasts. Structural changes in cardiac disease often affect passive electrical properties by increasing non-uniformity and altering anisotropy. This disturbs normal electrical impulse propagation and is, consequently, a substrate for arrhythmia. However, to investigate how these structural changes lead to arrhythmias remains a challenge. One important mechanism, which may both cause and prevent arrhythmia, is the mismatch between current sources and sinks. Propagation of the electrical impulse requires a sufficient source of depolarizing current. In the case of a mismatch, the activated tissue (source) is not able to deliver enough depolarizing current to trigger an action potential in the non-activated tissue (sink). This eventually leads to conduction block. It has been suggested that in this situation a balanced geometrical distribution of gap junctions and reduced gap junction conductance may allow successful propagation. In contrast, source-sink mismatch can prevent spontaneous arrhythmogenic activity in a small number of cells from spreading over the ventricle, especially if gap junction conductance is enhanced. Beside gap junctions, cell geometry and non-cellular structures strongly modulate arrhythmogenic mechanisms. The present review elucidates these and other implications of passive electrical properties for cardiac rhythm and arrhythmogenesis

Anisotropic Cardiac Conduction.

Anisotropy is the property of directional dependence. In cardiac tissue, conduction velocity is anisotropic and its orientation is determined by myocyte direction. Cell shape and size, excitability, myocardial fibrosis, gap junction distribution and function are all considered to contribute to anisotropic conduction. In disease states, anisotropic conduction may be enhanced, and is implicated, in the genesis of pathological arrhythmias. The principal mechanism responsible for enhanced anisotropy in disease remains uncertain. Possible contributors include changes in cellular excitability, changes in gap junction distribution or function and cellular uncoupling through interstitial fibrosis. It has recently been demonstrated that myocyte orientation may be identified using diffusion tensor magnetic resonance imaging in explanted hearts, and multisite pacing protocols have been proposed to estimate myocyte orientation and anisotropic conduction in vivo. These tools have the potential to contribute to the understanding of the role of myocyte disarray and anisotropic conduction in arrhythmic states

Gap Junction Channels and Cardiac Impulse Propagation

The role of gap junction channels on cardiac impulse propagation is complex. This review focuses on the differential expression of connexins in the heart and the biophysical properties of gap junction channels under normal and disease conditions. Structural determinants of impulse propagation have been gained from biochemical and immunocytochemical studies performed on tissue extracts and intact cardiac tissue. These have defined the distinctive connexin coexpression patterns and relative levels in different cardiac tissues. Functional determinants of impulse propagation have emerged from electrophysiological experiments carried out on cell pairs. The static properties (channel number and conductance) limit the current flow between adjacent cardiomyocytes and thus set the basic conduction velocity. The dynamic properties (voltage-sensitive gating and kinetics of channels) are responsible for a modulation of the conduction velocity during propagated action potentials. The effect is moderate and depends on the type of Cx and channel. For homomeric-homotypic channels, the influence is small to medium; for homomeric-heterotypic channels, it is medium to strong. Since no data are currently available on heteromeric channels, their influence on impulse propagation is speculative. The modulation by gap junction channels is most prominent in tissues at the boundaries between cardiac tissues such as sinoatrial node-atrial muscle, atrioventricular node-His bundle, His bundle-bundle branch and Purkinje fibers-ventricular muscle. The data predict facilitation of orthodromic propagatio

Calcium Remodeling through Different Signaling Pathways in Heart Failure: Arrhythmogenesis Studies of Pyk2, Dystrophin, and β-adrenergic Receptor Signaling

Heart failure is a common clinical syndrome that ensues when the heart is no longer able to generate sufficient cardiac output to meet the demands of the body. It is one of the leading causes of death worldwide but with limited and non-ideal therapies at the moment. One reason behind this may be the complexity of significant alterations in multiple signaling pathways and concomitant structural and functional remodeling, especially Ca handling. Ca is critical in both the electrical and mechanical properties of cardiac myoctyes, and much is known about ionic currents and the normal excitation-contraction coupling process. In heart failure, distinct impaired signaling pathways induce significant alterations in how cardiac Ca handling is regulated. These alterations either directly cause certain arrhythmias or facilitate arrhythmias by association with electrical remodeling. The goal of this dissertation was to investigate the mechanisms of calcium remodeling through different signaling pathways in heart failure, and mechanisms on how the intricate and dynamic interactions between Ca handling and signaling pathways impairment facilitate arrhythmias in heart failure. To achieve this goal, a dual optical mapping system was designed to investigate electrical activity and Ca transient simultaneously. High spatio-temporal resolution mapping allows for quantifying conduction, repolarization and Ca cycling, especially on the interactions between action potential and Ca handling. In this dissertation, I investigated Ca remodeling in three different signaling pathways: stress activated signaling, cytoskeletal signaling and β adrenergic receptor signaling pathway. Proline-rich tyrosine kinase 2: Pyk2) is a non-receptor protein kinase regulated by intracellular Ca. It mediates a typical stress activated signaling pathways along with c-Src, P38 MAPK and regulates a broad range of key biological responses. By optically mapping the genetically engineered mouse model: Pyk2 knockout, I detected a protective role of Pyk2 with respect to ventricular tachyarrhythmia during parasympathetic stimulation by regulation of gene expression related to calcium handling. The mdx mouse model was introduced in the investigation of cytoskeletal signaling pathway. mdx mice is a common model for Duchenne muscular dystrophy, which is a clinical syndrome resulted from recessive of dystrophin and eventually develops into heart failure. The project suggested the association of mechanical stimulation and deficiency of dystrophin account for the cardiac mechanical defects and resulting Ca mishandling, but not either of the two above-mentioned entities alone. Ca mishandling leads to Ca cycling dispersion, which facilitates generation of arrhythmias. β Adrenergic receptor signaling pathway was investigated on explanted donor and failing human hearts. Distinct β adrenergic receptor subtypes were found to regulate remodeling differently. The association between remodeling of action potential and Ca transient provides crucial arrhythmic drivers and substrate in heart failure

Design Considerations for Engineered Myocardium

The fabrication of biomimetic heart muscle suitable for pharmaceutical compound evaluation and disease modeling is hindered by limitations in our understanding of how to guide and assess the maturity of engineered myocardium in vitro. We hypothesized that tissue architecture serves as an important cue for directing the maturation of engineered heart tissues and that reliable assessment of maturity could be performed using a multi-parametric rubric utilizing cardiomyocytes of known developmental state as a basis for comparison. Physical micro-environmental cues are recognized to play a fundamental role in normal heart development, therefore we used micro-patterned extracellular matrix to direct isolated cardiac myocytes to self-assemble into anisotropic sheets reminiscent of the architecture observed in the laminar musculature of the heart. Comparison of global sarcomere alignment, gene expression, and contractile stress in engineered anisotropic myocardium to isotropic monolayers, as well as, adult ventricular tissue revealed that anisotropic engineered myocardium more closely matched the characteristics of adult ventricular tissue, than isotropic cultures of randomly organized cardiomyocytes. These findings support the notion that tissue architecture is an important cue for building mature engineered myocardium. Next, we sought to develop a quality assessment strategy that utilizes a core set of 64 experimental measurements representative of 4 major categories (i.e. gene expression, myofibril structure, electrical activity, and contractility) to provide a numeric score of how closely stem cell-derived cardiac myocytes match the physiological characteristics of mature, post-natal cardiomyocytes. The efficacy of this rubric was assessed by comparing anisotropic engineered tissues fabricated from commercially-available murine ES- (mES) and iPS- (miPS) derived myocytes against neonatal mouse ventricular myocytes. The quality index scores calculated for these cells revealed that the miPS-derived myocytes more closely resembled the neonate ventricular myocytes than the mES-derived myocytes. Taken together, the results of these studies provide valuable insight into the fabrication and validation of engineered myocardium that faithfully recapitulate the characteristics of mature ventricular myocardium found in vivo. These engineered tissue design and quality validation strategies may prove useful in developing heart muscle analogs from human stem cell-derived myocytes that more accurately predict patient response than currently used animal models.Engineering and Applied Science

Block of impulse propagation at an abrupt tissue expansion: evaluation of the critical strand diameter in 2- and 3-dimensional computer models

Objective: Unidirectional conduction block in the heart can occur at a site where the impulse is transmitted from a small to a large tissue volume. The aim of this study was to evaluate the occurrence of conduction block in a 2-dimensional and 3-dimensional computer model of cardiac tissue consisting of a narrow strand abruptly emerging into a large area. In this structure, the strand diameter critical for the occurrence of block, hc, was evaluated as a function of changes in the active and passive electrical properties of both the strand and the large medium. Methods: The effects of changes in the following parameters on hc were analysed: (1) maximum sodium conductance (gNamax), (2) longitudinal (Rx) and transverse (Ry) intracellular resistivities, and (3) inhomogeneities in gNamax and Rx and Ry between the strand and the large area. Three ionic models for cardiac excitation described by Beeler-Reuter, Ebihara-Johnson, and Luo-Rudy ionic current kinetics were compared. Results: In the 2-dimensional simulations, hc was 175 μm in Ebihara-Johnson and Beeler-Reuter models and 200 μm in the Luo-Rudy model. At the critical strand diameter, the site of conduction block was located beyond the transition, i.e. a small circular area was activated in the large medium, whereas with narrower strands conduction block occurred within the strands. The decrease of gNamax resulted in a large increase of hc. This increase was mainly due to the change of gNamax in the large area, while hc was almost independent of gNamax in the strand. Changing Rx had no effect on hc, whereas the increase of Ry decreased hc and reversed conduction block. Inhomogeneous changes of Rx and Ry in the strand versus the large medium had opposite effects on hc. When the resistivities of the strand alone were increased, hc also increased. In contrast, the increase of the resistivities in the large area reduced hc. In the 3-dimensional model, hc was 2.7 times larger than the corresponding 2-dimensional values at the various levels of gNamax and resistivity. Conclusions: (1) At physiological values for active and passive electrical properties, hc in the 2D simulations is close to 200 μm in all three ionic models. In the 3-dimensional simulations, hc is 2.7 larger than in the 2-dimensional models. (2) The excitable properties of the large area but not of the strand modify hc. The decrease of intercellular coupling in the large medium facilitates impulse conduction and reduces hc, while the same change in the strand increases hc. (3) Occurrence of conduction block at an abrupt geometrical transition can be explained by both the impedance mismatch at the transition site and the critical curvature beyond the transitio