Ventricular filling slows epicardial conduction and increases action potential duration in an optical mapping study of the isolated rabbit heart

Cardiac Conduction Velocity, Remodeling and Arrhythmogenesis

Cardiac electrophysiological disorders, in particular arrhythmias, are a key cause of morbidity and mortality throughout the world. There are two basic requirements for arrhythmogenesis: an underlying substrate and a trigger. Altered conduction velocity (CV) provides a key substrate for arrhythmogenesis, with slowed CV increasing the probability of re-entrant arrhythmias by reducing the length scale over which re-entry can occur. In this review, we examine methods to measure cardiac CV in vivo and ex vivo, discuss underlying determinants of CV, and address how pathological variations alter CV, potentially increasing arrhythmogenic risk. Finally, we will highlight future directions both for methodologies to measure CV and for possible treatments to restore normal CV.

Computationally efficient model of myocardial electromechanics for multiscale simulations

A model of myocardial electromechanics is suggested. It combines modified and simplified versions of previously published models of cardiac electrophysiology, excitation-contraction coupling, and mechanics. The mechano-calcium and mechano-electrical feedbacks, including the strain-dependence of the propagation velocity of the action potential, are also accounted for. The model reproduces changes in the twitch amplitude and Ca2+-transients upon changes in muscle strain including the slow response. The model also reproduces the Bowditch effect and changes in the twitch amplitude and duration upon changes in the interstimulus interval, including accelerated relaxation at high stimulation frequency. Special efforts were taken to reduce the stiffness of the differential equations of the model. As a result, the equations can be integrated numerically with a relatively high time step making the model suitable for multiscale simulation of the human heart and allowing one to study the impact of myocardial mechanics on arrhythmias.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

Changes in speckle-tracking-derived mechanical dispersion index are associated with 30-day readmissions in acute heart failure

Background

The objective of the present study was to evaluate the relationship between speckle-tracking-derived parameters left ventricular (LV) mechanical dispersion index (MDI), defined as the standard deviation of the time-to-peak longitudinal strain of all segments analyzed of the LV, and global longitudinal strain (GLS) and 30-day post-discharge outcomes (death and readmission to the hospital) in patients with acute heart failure (AHF).

Methods

We performed a prospective observational study of selected emergency department patients with a primary diagnosis of AHF. Point-of-care echocardiograms were performed at baseline (prior to, or concurrent with the initiation of treatment) and 23 h post-enrollment. Offline speckle-tracking analysis was utilized to calculate GLS and MDI. The primary outcome was 30-day readmissions.

Results

A total of 31 patients were included, 13 of whom were readmitted within 30 days. Patients who were not readmitted to the hospital experienced an average relative improvement in MDI of 24% from baseline to 23 h (84 ms to 64 ms), while patients who were readmitted experienced an average relative worsening in MDI of 6% (66 ms to 70 ms) from baseline to 23 h.

Conclusions

MDI has promise as a treatment response variable in admitted patients with AHF; however, further study is needed.

The Ability to Reproducibly Record Cardiac Action Potentials From Multiple Anatomic Locations: Endocardially and Epicardially, In Situ and In Vitro

OBJECTIVE

For cardiac arrhythmia mapping and ablation procedures, the ability to record focal cardiac action potentials could aid in precisely identifying lesions, scarred tissue, and/or arrhythmic foci. Our study objective was to validate the electrophysiologic properties of a routinely employed large mammalian in vitro working heart model.

METHODS

Monophasic action potentials (MAPs) were recorded from 18 swine hearts during viable hemodynamic function both in situ (postmedian sternotomy) and in vitro (using Visible Heart methodologies). We placed specially designed mapping catheters in epicardial and endocardial locations. High-quality MAP signals were recorded for up to 2 h, and MATLAB was utilized to evaluate relative duration and temporal/regional changes in waveform morphology.

RESULTS

MAPs were reproducibly recorded from both epicardial and endocardial locations in situ and in vitro. No significant differences were noted in right atrial endocardial, right ventricular endocardial, right ventricular epicardial, or left atrial epicardial waveforms, when baseline recordings were compared to all other in situ and in vitro time points. Furthermore, MAP duration between right ventricular endocardial and epicardial waveforms was not significantly different, in situ or in vitro.

CONCLUSION

The use of in vitro models like the Visible Heart is considered invaluable for the study of cardiac arrhythmias, the development of novel therapies, and/or preclinical testing of future cardiac mapping catheters and systems.

SIGNIFICANCE

Preclinical studies assessing in situ and/or in vitro recorded cardiac monophasic action potentials could be critical for the future development and validation of cardiac devices.

Rabbit models of cardiac mechano-electric and mechano-mechanical coupling

Cardiac auto-regulation involves integrated regulatory loops linking electrics and mechanics in the heart. Whereas mechanical activity is usually seen as 'the endpoint' of cardiac auto-regulation, it is important to appreciate that the heart would not function without feed-back from the mechanical environment to cardiac electrical (mechano-electric coupling, MEC) and mechanical (mechano-mechanical coupling, MMC) activity. MEC and MMC contribute to beat-by-beat adaption of cardiac output to physiological demand, and they are involved in various pathological settings, potentially aggravating cardiac dysfunction. Experimental and computational studies using rabbit as a model species have been integral to the development of our current understanding of MEC and MMC. In this paper we review this work, focusing on physiological and pathological implications for cardiac function.

Increased cell membrane capacitance is the dominant mechanism of stretch-dependent conduction slowing in the rabbit heart: a computational study

Volume loading of the cardiac ventricles is known to slow electrical conduction in the rabbit heart, but the mechanisms remain unclear. Previous experimental and modeling studies have investigated some of these mechanisms, including stretch-activated membrane currents, reduced gap junctional conductance, and altered cell membrane capacitance. In order to quantify the relative contributions of these mechanisms, we combined a monomain model of rabbit ventricular electrophysiology with a hyperelastic model of passive ventricular mechanics. First, a simplified geometric model with prescribed homogeneous deformation was used to fit model parameters and characterize individual MEF mechanisms, and showed good qualitative agreement with experimentally measured strain-CV relations. A 3D model of the rabbit left and right ventricles was then compared with experimental measurements from optical electrical mapping studies in the isolated rabbit heart. The model was inflated to an end-diastolic pressure of 30 mmHg, resulting in epicardial strains comparable to those measured in the anterior left ventricular free wall. While the effects of stretch activated channels did alter epicardial conduction velocity, an increase in cellular capacitance was required to explain previously reported experimental results. The new results suggest that for large strains, various mechanisms can combine and produce a biphasic relationship between strain and conduction velocity. However, at the moderate strains generated by high end-diastolic pressure, a stretch-induced increase in myocyte membrane capacitance is the dominant driver of conduction slowing during ventricular volume loading.

Biomechanics of cardiac electromechanical coupling and mechanoelectric feedback

Cardiac mechanical contraction is triggered by electrical activation via an intracellular calcium-dependent process known as excitation-contraction coupling. Dysregulation of cardiac myocyte intracellular calcium handling is a common feature of heart failure. At the organ scale, electrical dyssynchrony leads to mechanical alterations and exacerbates pump dysfunction in heart failure. A reverse coupling between cardiac mechanics and electrophysiology is also well established. It is commonly referred as cardiac mechanoelectric feedback and thought to be an important contributor to the increased risk of arrhythmia during pathological conditions that alter regional cardiac wall mechanics, including heart failure. At the cellular scale, most investigations of myocyte mechanoelectric feedback have focused on the roles of stretch-activated ion channels, though mechanisms that are independent of ionic currents have also been described. Here we review excitation-contraction coupling and mechanoelectric feedback at the cellular and organ scales, and we identify the need for new multicellular tissue-scale model systems and experiments that can help us to obtain a better understanding of how interactions between electrophysiological and mechanical processes at the cell scale affect ventricular electromechanical interactions at the organ scale in the normal and diseased heart.

Caveolae in ventricular myocytes are required for stretch-dependent conduction slowing

Mechanical stretch of cardiac muscle modulates action potential propagation velocity, causing potentially arrhythmogenic conduction slowing. The mechanisms by which stretch alters cardiac conduction remain unknown, but previous studies suggest that stretch can affect the conformation of caveolae in myocytes and other cell types. We tested the hypothesis that slowing of action potential conduction due to cardiac myocyte stretch is dependent on caveolae. Cardiac action potential propagation velocities, measured by optical mapping in isolated mouse hearts and in micropatterned mouse cardiomyocyte cultures, decreased reversibly with volume loading or stretch, respectively (by 19±5% and 26±4%). Stretch-dependent conduction slowing was not altered by stretch-activated channel blockade with gadolinium or by GsMTx-4 peptide, but was inhibited when caveolae were disrupted via genetic deletion of caveolin-3 (Cav3 KO) or membrane cholesterol depletion by methyl-β-cyclodextrin. In wild-type mouse hearts, stretch coincided with recruitment of caveolae to the sarcolemma, as observed by electron microscopy. In myocytes from wild-type but not Cav3 KO mice, stretch significantly increased cell membrane capacitance (by 98±64%), electrical time constant (by 285±149%), and lipid recruitment to the bilayer (by 84±39%). Recruitment of caveolae to the sarcolemma during physiologic cardiomyocyte stretch slows ventricular action potential propagation by increasing cell membrane capacitance.

Living cardiac tissue slices: an organotypic pseudo two-dimensional model for cardiac biophysics research

Living cardiac tissue slices, a pseudo two-dimensional (2D) preparation, have received less attention than isolated single cells, cell cultures, or Langendorff-perfused hearts in cardiac biophysics research. This is, in part, due to difficulties associated with sectioning cardiac tissue to obtain live slices. With moderate complexity, native cell-types, and well-preserved cell-cell electrical and mechanical interconnections, cardiac tissue slices have several advantages for studying cardiac electrophysiology. The trans-membrane potential (Vm) has, thus far, mainly been explored using multi-electrode arrays. Here, we combine tissue slices with optical mapping to monitor Vm and intracellular Ca(2+) concentration ([Ca(2+)]i). This combination opens up the possibility of studying the effects of experimental interventions upon action potential (AP) and calcium transient (CaT) dynamics in 2D, and with relatively high spatio-temporal resolution. As an intervention, we conducted proof-of-principle application of stretch. Mechanical stimulation of cardiac preparations is well-established for membrane patches, single cells and whole heart preparations. For cardiac tissue slices, it is possible to apply stretch perpendicular or parallel to the dominant orientation of cells, while keeping the preparation in a constant focal plane for fluorescent imaging of in-slice functional dynamics. Slice-to-slice comparison furthermore allows one to assess transmural differences in ventricular tissue responses to mechanical challenges. We developed and tested application of axial stretch to cardiac tissue slices, using a manually-controlled stretching device, and recorded Vm and [Ca(2+)]i by optical mapping before, during, and after application of stretch. Living cardiac tissue slices, exposed to axial stretch, show an initial shortening in both AP and CaT duration upon stretch application, followed in most cases by a gradual prolongation of AP and CaT duration during stretch maintained for up to 50 min. After release of sustained stretch, AP duration (APD) and CaT duration reverted to shorter values. Living cardiac tissue slices are a promising experimental model for the study of cardiac mechano-electric interactions. The methodology described here can be refined to achieve more accurate control over stretch amplitude and timing (e.g. using a computer-controlled motorised stage, or by synchronising electrical and mechanical events) and through monitoring of regional tissue deformation (e.g. by adding motion tracking).

Effects of mechano-electric feedback on scroll wave stability in human ventricular fibrillation

Recruitment of stretch-activated channels, one of the mechanisms of mechano-electric feedback, has been shown to influence the stability of scroll waves, the waves that underlie reentrant arrhythmias. However, a comprehensive study to examine the effects of recruitment of stretch-activated channels with different reversal potentials and conductances on scroll wave stability has not been undertaken; the mechanisms by which stretch-activated channel opening alters scroll wave stability are also not well understood. The goals of this study were to test the hypothesis that recruitment of stretch-activated channels affects scroll wave stability differently depending on stretch-activated channel reversal potential and channel conductance, and to uncover the relevant mechanisms underlying the observed behaviors. We developed a strongly-coupled model of human ventricular electromechanics that incorporated human ventricular geometry and fiber and sheet orientation reconstructed from MR and diffusion tensor MR images. Since a wide variety of reversal potentials and channel conductances have been reported for stretch-activated channels, two reversal potentials, −60 mV and −10 mV, and a range of channel conductances (0 to 0.07 mS/µF) were implemented. Opening of stretch-activated channels with a reversal potential of −60 mV diminished scroll wave breakup for all values of conductances by flattening heterogeneously the action potential duration restitution curve. Opening of stretch-activated channels with a reversal potential of −10 mV inhibited partially scroll wave breakup at low conductance values (from 0.02 to 0.04 mS/µF) by flattening heterogeneously the conduction velocity restitution relation. For large conductance values (>0.05 mS/µF), recruitment of stretch-activated channels with a reversal potential of −10 mV did not reduce the likelihood of scroll wave breakup because Na channel inactivation in regions of large stretch led to conduction block, which counteracted the increased scroll wave stability due to an overall flatter conduction velocity restitution.

Mechano-sensitivity of cardiac pacemaker function: pathophysiological relevance, experimental implications, and conceptual integration with other mechanisms of rhythmicity

Cardiac pacemaker cells exhibit spontaneous, rhythmic electrical excitation, termed automaticity. This automatic initiation of action potentials requires spontaneous diastolic depolarisation, whose rate determines normal rhythm generation in the heart. Pacemaker mechanisms have been split recently into: (i) cyclic changes in trans-sarcolemmal ion flows (termed the ‘membrane-clock’), and (ii) rhythmic intracellular calcium cycling (the ‘calcium-clock’). These two ‘clocks’ undoubtedly interact, as trans-sarcolemmal currents involved in pacemaking include calcium-carrying mechanisms, while intracellular calcium cycling requires trans-sarcolemmal ion flux as the mechanism by which it affects membrane potential. The split into separate ‘clocks’ is, therefore, somewhat arbitrary. Nonetheless, the ‘clock’ metaphor has been conceptually stimulating, in particular since there is evidence to support the view that either ‘clock’ could be sufficient in principle to set the rate of pacemaker activation.

Of course, the same has also been shown for sub-sets of ‘membrane-clock’ ion currents, illustrating the redundancy of mechanisms involved in maintaining such basic functionality as the heartbeat, a theme that is common for vital physiological systems.

Following the conceptual path of identifying individual groups of sub-mechanisms, it is important to remember that the heart is able to adapt pacemaker rate to changes in haemodynamic load, even after isolation or transplantation, and on a beat-by-beat basis. Neither the ‘membrane-’ nor the ‘calcium-clock’ do, as such, inherently account for this rapid adaptation to circulatory demand (cellular Ca²⁺ balance changes over multiple beats, while variation of sarcolemmal ion channel presence takes even longer). This suggests that a third set of mechanisms must be involved in setting the pace. These mechanisms are characterised by their sensitivity to the cyclically changing mechanical environment, and – in analogy to the above terminology – this might be considered a ‘mechanics-clock’.

In this review, we discuss possible roles of mechano-sensitive mechanisms for the entrainment of membrane current dynamics and calcium-handling. This can occur directly via stretch-activation of mechano-sensitive ion channels in the sarcolemma and/or in intracellular membrane compartments, as well as by modulation of ‘standard’ components of the ‘membrane-’ or ‘calcium-clock’. Together, these mechanisms allow rapid adaptation to changes in haemodynamic load, on a beat-by-beat basis.

Additional relevance arises from the fact that mechano-sensitivity of pacemaking may help to explain pacemaker dysfunction in mechanically over- or under-loaded tissue. As the combined contributions of the various underlying oscillatory mechanisms are integrated at the pacemaker cell level into a single output – a train of pacemaker action potentials – we will not adhere to a metaphor that implies separate time-keeping units (‘clocks’), and rather focus on cardiac pacemaking as the result of interactions of a set of coupled oscillators, whose individual contributions vary depending on the pathophysiological context. We conclude by considering the utility and limitations of viewing the pacemaker as a coupled system of voltage-, calcium-, and mechanics-modulated oscillators that, by integrating a multitude of inputs, offers the high level of functional redundancy that is vitally important for cardiac automaticity.

The zebrafish as a novel animal model to study the molecular mechanisms of mechano-electrical feedback in the heart

Altered mechanical loading of the heart leads to hypertrophy, decompensated heart failure and fatal arrhythmias. However, the molecular mechanisms that link mechanical and electrical dysfunction remain poorly understood. Growing evidence suggest that ventricular electrical remodeling (VER) is a process that can be induced by altered mechanical stress, creating persistent electrophysiological changes that predispose the heart to life-threatening arrhythmias. While VER is clearly a physiological property of the human heart, as evidenced by "T wave memory", it is also thought to occur in a variety of pathological states associated with altered ventricular activation such as bundle branch block, myocardial infarction, and cardiac pacing. Animal models that are currently being used for investigating stretch-induced VER have significant limitations. The zebrafish has recently emerged as an attractive animal model for studying cardiovascular disease and could overcome some of these limitations. Owing to its extensively sequenced genome, high conservation of gene function, and the comprehensive genetic resources that are available in this model, the zebrafish may provide new insights into the molecular mechanisms that drive detrimental electrical remodeling in response to stretch. Here, we have established a zebrafish model to study mechano-electrical feedback in the heart, which combines efficient genetic manipulation with high-precision stretch and high-resolution electrophysiology. In this model, only 90 min of ventricular stretch caused VER and recapitulated key features of VER found previously in the mammalian heart. Our data suggest that the zebrafish model is a powerful platform for investigating the molecular mechanisms underlying mechano-electrical feedback and VER in the heart.

Ranolazine decreases mechanosensitivity of the voltage-gated sodium ion channel Na(v)1.5: a novel mechanism of drug action

BACKGROUND

Na(V)1.5 is a mechanosensitive voltage-gated sodium-selective ion channel responsible for the depolarizing current and maintenance of the action potential plateau in the heart. Ranolazine is a Na(V)1.5 antagonist with antianginal and antiarrhythmic properties.

METHODS AND RESULTS

Mechanosensitivity of Na(V)1.5 was tested in voltage-clamped whole cells and cell-attached patches by bath flow and patch pressure, respectively. In whole cells, bath flow increased peak inward current in both murine ventricular cardiac myocytes (24±8%) and human embryonic kidney 293 cells heterologously expressing Na(V)1.5 (18±3%). The flow-induced increases in peak current were blocked by ranolazine. In cell-attached patches from cardiac myocytes and Na(V)1.5-expressing human embryonic kidney 293 cells, negative pressure increased Na(V) peak currents by 27±18% and 18±4% and hyperpolarized voltage dependence of activation by -11 mV and -10 mV, respectively. In human embryonic kidney 293 cells, negative pressure also increased the window current (250%) and increased late open channel events (250%). Ranolazine decreased pressure-induced shift in the voltage dependence (IC(50) 54 μmol/L) and eliminated the pressure-induced increases in window current and late current event numbers. Block of Na(V)1.5 mechanosensitivity by ranolazine was not due to the known binding site on DIVS6 (F1760). The effect of ranolazine on mechanosensitivity of Na(V)1.5 was approximated by lidocaine. However, ionized ranolazine and charged lidocaine analog (QX-314) failed to block mechanosensitivity.

CONCLUSIONS

Ranolazine effectively inhibits mechanosensitivity of Na(V)1.5. The block of Na(V)1.5 mechanosensitivity by ranolazine does not utilize the established binding site and may require bilayer partitioning. Ranolazine block of Na(V)1.5 mechanosensitivity may be relevant in disorders of mechanoelectric dysfunction.

Extracting surface activation time from the optically recorded action potential in three-dimensional myocardium

Optical mapping has become an indispensible tool for studying cardiac electrical activity. However, due to the three-dimensional nature of the optical signal, the optical upstroke is significantly longer than the electrical upstroke. This raises the issue of how to accurately determine the activation time on the epicardial surface. The purpose of this study was to establish a link between the optical upstroke and exact surface activation time using computer simulations, with subsequent validation by a combination of microelectrode recordings and optical mapping experiments. To simulate wave propagation and associated optical signals, we used a hybrid electro-optical model. We found that the time of the surface electrical activation (t(E)) within the accuracy of our simulations coincided with the maximal slope of the optical upstroke (t(F)*) for a broad range of optical attenuation lengths. This was not the case when the activation time was determined at 50% amplitude (t(F50)) of the optical upstroke. The validation experiments were conducted in isolated Langendorff-perfused rat hearts and coronary-perfused pig left ventricles stained with either di-4-ANEPPS or the near-infrared dye di-4-ANBDQBS. We found that t(F)* was a more accurate measure of t(E) than was t(F50) in all experimental settings tested (P = 0.0002). Using t(F)* instead of t(F50) produced the most significant improvement in measurements of the conduction anisotropy and the transmural conduction time in pig ventricles.

Electromechanical feedback with reduced cellular connectivity alters electrical activity in an infarct injured left ventricle: a finite element model study

Myocardial infarction (MI) significantly alters the structure and function of the heart. As abnormal strain may drive heart failure and the generation of arrhythmias, we used computational methods to simulate a left ventricle with an MI over the course of a heartbeat to investigate strains and their potential implications to electrophysiology. We created a fully coupled finite element model of myocardial electromechanics consisting of a cellular physiological model, a bidomain electrical diffusion solver, and a nonlinear mechanics solver. A geometric mesh built from magnetic resonance imaging (MRI) measurements of an ovine left ventricle suffering from a surgically induced anteroapical infarct was used in the model, cycled through the cardiac loop of inflation, isovolumic contraction, ejection, and isovolumic relaxation. Stretch-activated currents were added as a mechanism of mechanoelectric feedback. Elevated fiber and cross fiber strains were observed in the area immediately adjacent to the aneurysm throughout the cardiac cycle, with a more dramatic increase in cross fiber strain than fiber strain. Stretch-activated channels decreased action potential (AP) dispersion in the remote myocardium while increasing it in the border zone. Decreases in electrical connectivity dramatically increased the changes in AP dispersion. The role of cross fiber strain in MI-injured hearts should be investigated more closely, since results indicate that these are more highly elevated than fiber strain in the border of the infarct. Decreases in connectivity may play an important role in the development of altered electrophysiology in the high-stretch regions of the heart.

Simultaneous optical mapping of transmembrane potential and wall motion in isolated, perfused whole hearts

Optical mapping of cardiac propagation has traditionally been hampered by motion artifact, chiefly due to changes in photodetector-to-tissue registration as the heart moves. We have developed an optical mapping technique to simultaneously record electrical waves and mechanical contraction in isolated hearts. This allows removal of motion artifact from transmembrane potential (V(m)) recordings without the use of electromechanical uncoupling agents and allows the interplay of electrical and mechanical events to be studied at the whole organ level. Hearts are stained with the voltage-sensitive dye di-4-ANEPPS and ring-shaped markers are attached to the epicardium. Fluorescence, elicited on alternate frames by 450 and 505 nm light-emitting diodes, is recorded at 700 frames∕ per second by a camera fitted with a 605 ± 25 nm emission filter. Marker positions are tracked in software. A signal, consisting of the temporally interlaced 450 and 505 nm fluorescence, is collected from the pixels enclosed by each moving ring. After deinterlacing, the 505 nm signal consists of V(m) with motion artifact, while the 450 nm signal is minimally voltage-sensitive and contains primarily artifacts. The ratio of the two signals estimates V(m). Deformation of the tissue enclosed by each set of 3 rings is quantified using homogeneous finite strain.

Mechanoelectrical coupling enhances initiation and affects perpetuation of atrial fibrillation during acute atrial dilation

BACKGROUND

Acute atrial dilation increases the susceptibility to atrial fibrillation (AF). However, the mechanisms by which atrial stretch may contribute to the initiation and perpetuation of AF remain to be determined.

OBJECTIVE

The purpose of this study was to use a novel multiscale model of atrial electromechanics and mechanoelectrical feedback to test the hypothesis that acute stretch increases vulnerability to AF by heterogeneous activation of stretch-activated channels.

METHODS

Human atria were represented by a triangular mesh obtained from magnetic resonance imaging data. Atrial trabecular bundle structure was incorporated by varying thicknesses of the atrial wall. Atrial membrane behavior was modeled by the Courtemanche-Ramirez-Nattel model with the addition of a nonselective stretch-activated cation current (I(sac)). Mechanical behavior was modeled by a series elastic, a contractile, and a parallel elastic element in which contractile force was related to intracellular concentration of free calcium and sarcomere length.

RESULTS

Acute atrial dilation was simulated by increasing stretch throughout the atrial wall. Stimulation near the pulmonary vein ostia at an interval of 600 ms induced AF at an overall stretch ratio of 1.10. Initiation and perpetuation of AF in our model were related to increased dispersion of effective refractory period, conduction slowing, and local conduction block, all related to heterogeneous activation of I(sac). Upon local contraction, mechanoelectrical coupling affects perpetuation of AF by temporarily changing local excitability.

CONCLUSION

During acute atrial dilation, heterogeneous activation of I(sac) enhances initiation and can affect perpetuation of AF.

Structural heterogeneity in the ventricular wall plays a significant role in the initiation of stretch-induced arrhythmias in perfused rabbit right ventricular tissues and whole heart preparations

RATIONALE

Mechanical stress is known to alter the electrophysiological properties of the myocardium and may trigger fatal arrhythmias when an abnormal load is applied to the heart.

OBJECTIVE

We tested the hypothesis that the structural heterogeneity of the ventricular wall modulates globally applied stretches to create heterogeneous strain distributions that lead to the initiation of arrhythmias.

METHODS AND RESULTS

We applied global stretches to arterially perfused rabbit right ventricular tissue preparations. The distribution of strain (determined by marker tracking) and the transmembrane potential (measured by optical mapping) were simultaneously recorded while accounting for motion artifacts. The 3D structure of the preparations was also examined using a laser displacement meter. To examine whether such observations can be translated to the physiological condition, we performed similar measurements in whole heart preparations while applying volume pulses to the right ventricle. At the tissue level, larger stretches (> or = 20%) caused synchronous excitation of the entire preparation, whereas medium stretches (10% and 15%) induced focal excitation. We found a significant correlation between the local strain and the local thickness, and the probability for focal excitation was highest for medium stretches. In the whole heart preparations, we observed that such focal excitations developed into reentrant arrhythmias.

CONCLUSIONS

Global stretches of intermediate strength, rather than intense stretches, created heterogeneous strain (excitation) distributions in the ventricular wall, which can trigger fatal arrhythmias.

Mechanisms of conduction slowing during myocardial stretch by ventricular volume loading in the rabbit

Acute ventricular loading by volume inflation reversibly slows epicardial electrical conduction, but the underlying mechanism remains unclear. This study investigated the potential contributions of stretch-activated currents, alterations in resting membrane potential, or changes in intercellular resistance and membrane capacitance. Conduction velocity was assessed using optical mapping of isolated rabbit hearts at end-diastolic pressures of 0 and 30 mmHg. The addition of 50 microM Gd3+ (a stretch-activated channel blocker) to the perfusate had no effect on slowing. The effect of volume loading on conduction velocity was independent of changes in resting membrane potential created by altering the perfusate potassium concentration between 1.5 and 8 mM. Bidomain model analysis of optically recorded membrane potential responses to a unipolar stimulus suggested that the cross-fiber space constant and membrane capacitance both increased with loading (21%, P = 0.006, and 56%, P = 0.004, respectively), and these changes, when implemented in a resistively coupled one-dimensional network model, were consistent with the observed slowing (14%, P = 0.005). In conclusion, conduction slowing during ventricular volume loading is not attributable to stretch-activated currents or altered resting membrane potential, but a reduction of intercellular resistance with a concurrent increase of effective membrane capacitance results in a net slowing of conduction.

Experimental and computational studies of strain-conduction velocity relationships in cardiac tissue

Velocity of electrical conduction in cardiac tissue is a function of mechanical strain. Although strain-modulated velocity is a well established finding in experimental cardiology, its underlying mechanisms are not well understood. In this work, we summarized potential factors contributing to strain-velocity relationships and reviewed related experimental and computational studies. We presented results from our experimental studies on rabbit papillary muscle, which supported a biphasic relationship of strain and velocity under uni-axial straining conditions. In the low strain range, the strain-velocity relationship was positive. Conduction velocity peaked with 0.59 m/s at 100% strain corresponding to maximal force development. In the high strain range, the relationship was negative. Conduction was reversibly blocked at 118+/-1.8% strain. Reversible block occurred also in the presence of streptomycin. Furthermore, our studies revealed a moderate hysteresis of conduction velocity, which was reduced by streptomycin. We reconstructed several features of the strain-velocity relationship in a computational study with a myocyte strand. The modeling included strain-modulation of intracellular conductivity and stretch-activated cation non-selective ion channels. The computational study supported our hypotheses, that the positive strain-velocity relationship at low strain is caused by strain-modulation of intracellular conductivity and the negative relationship at high strain results from activity of stretch-activated channels. Conduction block was not reconstructed in our computational studies. We concluded this work by sketching a hypothesis for strain-modulation of conduction and conduction block in papillary muscle. We suggest that this hypothesis can also explain uni-axially measured strain-conduction velocity relationships in other types of cardiac tissue, but apparently necessitates adjustments to reconstruct pressure or volume related changes of velocity in atria and ventricles.

The role of mechanoelectric feedback in vulnerability to electric shock

Experimental and clinical studies have shown that ventricular dilatation is associated with increased arrhythmogenesis and elevated defibrillation threshold; however, the underlying mechanisms remain poorly understood. The goal of the present study was to test the hypothesis that (1) stretch-activated channel (SAC) recruitment and (2) geometrical deformations in organ shape and fiber architecture lead to increased arrhythmogenesis by electric shocks following acute ventricular dilatation. To elucidate the contribution of these two factors, the study employed, for the first time, a combined electro-mechanical simulation approach. Acute dilatation was simulated in a model of rabbit ventricular mechanics by raising the LV end-diastolic pressure from 0.6 (control) to 4.2 kPa (dilated). The output of the mechanics model was used in the electrophysiological model. Vulnerability to shocks was examined in the control, the dilated ventricles, and in the dilated ventricles that also incorporated currents through SAC as a function of local strain, by constructing vulnerability grids. Results showed that dilatation-induced deformation alone decreased upper limit of vulnerability (ULV) slightly and did not result in increased vulnerability. With SAC recruitment in the dilated ventricles, the number of shock-induced arrhythmia episodes increased by 37% (from 41 to 56) and the lower limit of vulnerability (LLV) decreased from 9 to 7 V/cm, while ULV did not change. The heterogeneous activation of SAC caused by the heterogeneous fiber strain in the ventricular walls was the main reason for increased vulnerability to electric shocks since it caused dispersion of electrophysiological properties in the tissue, resulting in postshock unidirectional block and establishment of reentry.

Cell cultures as models of cardiac mechanoelectric feedback

Although stretch-activated currents have been extensively studied in isolated cells and intact heart in the context of mechanoelectric feedback (MEF) in the heart, quantitative data regarding other mechanical parameters such as pressure, shear, bending, etc, are still lacking at the multicellular level. Cultured cardiac cell monolayers have been used increasingly in the past decade as an in vitro model for the studies of fundamental mechanisms that underlie normal and pathological electrophysiology at the tissue level. Optical mapping makes possible multisite recording and analysis of action potentials and wavefront propagation, suitable for monitoring the electrophysiological activity of the cardiac cell monolayer under a wide variety of controlled mechanical conditions. In this paper, we review methodologies that have been developed or could be used to mechanically perturb cell monolayers, and present some new results on the acute effects of pressure, shear stress and anisotropic strain on cultured neonatal rat ventricular myocyte (NRVM) monolayers.

The role of stretch-activated channels in atrial fibrillation and the impact of intracellular acidosis

The incidence of atrial fibrillation correlates with increasing atrial size. The electrical consequences of atrial stretch contribute to both the initiation and maintenance of atrial fibrillation. It is suggested that altered calcium handling and stretch-activated channel activity could explain the experimental findings of stretch-induced depolarisation, shortened refractoriness, slowed conduction and increased heterogeneity of refractoriness and conduction. Stretch-activated channel blocking agents protect against these pro-arrhythmic effects. Gadolinium, GsMTx-4 toxin and streptomycin prevent the stretch-related vulnerability to atrial fibrillation without altering the drop in refractory period associated with stretch. Changes the activity of two-pore K+ channels, which are sensitive to stretch and pH but not gadolinium, could underlie the drop in refractoriness. Intracellular acidosis induced with propionate amplified the change in refractoriness with stretch in the isolated rabbit heart model in keeping with the clinical observation of increased propensity to atrial fibrillation with acidosis. We propose that activation of non-specific cation stretch-activated channels provides the triggers for acute atrial fibrillation with high atrial pressure while activation of atrial two-pore K+ channels shortens atrial refractory period and increases heterogeneity of refractoriness, providing the substrate for atrial fibrillation to be sustained. Stretch-activated channel blockade represents an exciting target for future antiarrhythmic drugs.

Effects of wall stress on the dynamics of ventricular fibrillation: a simulation study using a dynamic mechanoelectric model of ventricular tissue

INTRODUCTION

To investigate the mechanisms underlying the increased prevalence of ventricular fibrillation (VF) in the mechanically compromised heart, we developed a fully coupled electromechanical model of the human ventricular myocardium.

METHODS AND RESULTS

The model formulated the biophysics of specific ionic currents, excitation-contraction coupling, anisotropic nonlinear deformation of the myocardium, and mechanoelectric feedback (MEF) through stretch-activated channels. Our model suggests that sustained stretches shorten the action potential duration (APD) and flatten the electrical restitution curve, whereas stretches applied at the wavefront prolong the APD. Using this model, we examined the effects of mechanical stresses on the dynamics of spiral reentry. The strain distribution during spiral reentry was complex, and a high strain-gradient region was located in the core of the spiral wave. The wavefront around the core was highly stretched, even at lower pressures, resulting in prolongation of the APD and extension of the refractory area in the wavetail. As the left ventricular pressure increased, the stretched area became wider and the refractory area was further extended. The extended refractory area in the wavetail facilitated the wave breakup and meandering of tips through interactions between the wavefront and wavetail.

CONCLUSIONS

This simulation study indicates that mechanical loading promotes meandering and wave breaks of spiral reentry through MEF. Mechanical loading under pathological conditions may contribute to the maintenance of VF through these mechanisms.

Mechanoelectric feedback leads to conduction slowing and block in acutely dilated atria: a modeling study of cardiac electromechanics

Atrial fibrillation, a common cardiac arrhythmia, is promoted by atrial dilatation. Acute atrial dilatation may play a role in atrial arrhythmogenesis through mechanoelectric feedback. In experimental studies, conduction slowing and block have been observed in acutely dilated atria. In the present study, the influence of the stretch-activated current ( Isac) on impulse propagation is investigated by means of computer simulations. Homogeneous and inhomogeneous atrial tissues are modeled by cardiac fibers composed of segments that are electrically and mechanically coupled. Active force is related to free Ca2+ concentration and sarcomere length. Simulations of homogeneous and inhomogeneous cardiac fibers have been performed to quantify the relation between conduction velocity and Isac under stretch. In our model, conduction slowing and block are related to the amount of stretch and are enhanced by contraction of early-activated segments. Conduction block can be unidirectional in an inhomogeneous fiber and is promoted by a shorter stimulation interval. Slowing of conduction is explained by inactivation of Na+ channels and a lower maximum upstroke velocity due to a depolarized resting membrane potential. Conduction block at shorter stimulation intervals is explained by a longer effective refractory period under stretch. Our observations are in agreement with experimental results and explain the large differences in intra-atrial conduction, as well as the increased inducibility of atrial fibrillation in acutely dilated atria.

Acute volume overload elevates T-wave alternans magnitude

The objective of this study was to determine whether acute volume loading elevates T-wave alternans (TWA) in dogs with structurally normal hearts. TWA predicts sudden cardiac arrest in patients with left ventricular dysfunction and congestive heart failure. However, volume load and ventricular stretch may themselves precipitate arrhythmias. It is unclear to what extent volume load causes TWA. In six male mongrel dogs [25.8 kg (SD 4.2)] under general anesthesia, we measured TWA during progressive atrial pacing to 160 beats/min. Pacing was performed at baseline, at the midpoint and peak of a saline infusion designed to induce acute CHF, and then during diuresis. Dog 1 was hypothermic throughout the protocol and excluded from analysis. For dogs 2– 6, 102 ml/kg (SD 30) were infused over 315 min (SD 50), causing pulmonary capillary wedge pressure to rise from 9.6 (SD 3.5) to 21.2 mmHg (SD 1.6) ( P < 0.01), and heart rate variability to fall ( P < 0.01). TWA magnitude (Valt) rose in all dogs with volume load ( P < 0.001). Compared with baseline, TWA at peak infusion had higher magnitude [Valt 3.4 (SD 1.95) vs. 0.5 μV (SD 0.35); P = 0.011] and occurred at lower heart rates [128 (SD 6) vs. 151 beats/min (SD 12); P = 0.008]. Net volume load was linearly related to Valt ( P < 0.01), with each 10 ml/kg net volume load increasing Valt by 0.23 μV. Acute volume overload elevates TWA in normal canine hearts. Although dramatic, however, this effect may contribute clinically to abnormal TWA only in patients with marked volume overload. Future studies should examine the interaction of fluid overload, myocardial disease, and arrhythmia susceptibility.

Dual stretch responses of mHCN2 pacemaker channels: accelerated activation, accelerated deactivation

Mechanoelectric feedback in heart and smooth muscle is thought to depend on diverse channels that afford myocytes a mechanosensitive cation conductance. Voltage-gated channels (e.g., Kv1) are stretch sensitive, but the only voltage-gated channels that are cation permeant, the pacemaker or HCN (hyperpolarization-activated cyclic nucleotide-gated) channels, have not been tested. To assess if HCN channels could contribute to a mechanosensitive cation conductance, we recorded I(HCN) in cell-attached oocyte patches before, during, and after stretch for a range of voltage protocols. I(mHCN2) has voltage-dependent and instantaneous components; only the former was stretch sensitive. Stretch reversibly accelerated hyperpolarization-induced I(mHCN2) activation (likewise for I(spHCN)) and depolarization-induced deactivation. HCN channels (like Kv1 channels) undergo mode-switch transitions that render their activation midpoints voltage history dependent. The result, as seen from sawtooth clamp, is a pronounced hysteresis. During sawtooth clamp, stretch increased current magnitudes and altered the hysteresis pattern consistent with stretch-accelerated activation and deactivation. I(mHCN2) responses to step protocols indicated that at least two transitions were mechanosensitive: an unspecified rate-limiting transition along the hyperpolarization-driven path, mode I(closed)-->mode II(open), and depolarization-induced deactivation (from mode I(open) and/or from mode II(open)). How might this affect cardiac rhythmicity? Since hysteresis patterns and "on" and "off" I(HCN) responses all changed with stretch, predictions are difficult. For an empirical overview, we therefore clamped patches to cyclic action potential waveforms. During the diastolic potential of sinoatrial node cell and Purkinje fiber waveforms, net stretch effects were frequency dependent. Stretch-inhibited (SI) I(mHCN2) dominated at low frequencies and stretch-augmented (SA) I(mHCN2) was progressively more important as frequency increased. HCN channels might therefore contribute to either SI or SA cation conductances that in turn contribute to stretch arrhythmias and other mechanoelectric feedback phenomena.

An ionic model of stretch-activated and stretch-modulated currents in rabbit ventricular myocytes

AIMS

To develop an ionic model of stretch-activated and stretch-modulated currents in rabbit ventricular myocytes consistent with experimental observations, that can be used to investigate the role of these currents in intact myocardium.

METHODS AND RESULTS

A non-specific cation-selective stretch-activated current I(ns), was incorporated into the Puglisi-Bers ionic model of epicardial, endocardial and midmyocardial ventricular myocytes. Using the model, we predict a reduction in action potential duration at 20% repolarization (APD(20)) and action potential amplitude, an elevated resting transmembrane potential and either an increase or decrease in APD(90), depending on the reversal potential of I(ns). A stretch-induced decrease in I(K1) (70%), plus a small I(ns) current (g(ns) = 10 pS), results in a reduction in APD(20) and increase in APD(90), and a reduced safety factor for conduction. Increasing I(K1) (150%) plus a large I(ns) current (g(ns) = 40 pS), also leads to a reduction in APD(20) and increase in APD(90), but with a greater safety factor. Endocardial and midmyocardial cells appear to be the most sensitive to stretch-induced changes in action potential. The addition of the K(+)-specific stretch-activated current (SAC) I(Ko) results in action potential shortening.

CONCLUSION

Transmural heterogeneity of I(Ko) may reduce repolarization gradients in intact myocardium caused by intrinsic ion channel densities, nonuniform strains and electrotonic effects.

Species- and preparation-dependence of stretch effects on sino-atrial node pacemaking

Acute dilation of the right atrium (e.g., via increased venous return) raises spontaneous beating rate (BR) of the heart in many species. Neural mechanisms contribute to this behavior in vivo, but a positive chronotropic response to stretch can also be observed in isolated right atrial tissue preparations and even at the level of single sino-atrial node (SAN) cells. The underlying mechanism has previously been reported to be compatible with stretch-activation of cation nonselective ion channels (SAC). This review reports species peculiarities in the chronotropic response of isolated SAN tissue strips to stretch: in contrast to guinea pig, murine SAN preparations respond to distension with a reduction in spontaneous BR. This differential response need not necessarily involve disparate (sub-)cellular mechanisms, as SAC activation would occur against the background of very different SAN electrophysiology in the two species. On the basis of single SAN cell action potential recordings, this review illustrates how this may give rise to potentially opposing effects on spontaneous BR. Interestingly, streptomycin (a useful SAC blocker in isolated cells) has no effect on stretch-induced chronotropy in situ, and this is interpreted as an indication of protection of SAC, in native tissue, from interaction with the drug.

Proarrhythmic consequences of a KCNQ1 AKAP-binding domain mutation: computational models of whole cells and heterogeneous tissue

The KCNQ1-G589D gene mutation, associated with a long-QT syndrome, has been shown to disrupt yotiao-mediated targeting of protein kinase A and protein phosphatase-1 to the I(Ks) channel. To investigate how this defect may lead to ventricular arrhythmia during sympathetic stimulation, we use integrative computational models of beta-adrenergic signaling, myocyte excitation-contraction coupling, and action potential propagation in a rabbit ventricular wedge. Paradoxically, we find that the KCNQ1-G589D mutation alone does not prolong the QT interval. But when coupled with beta-adrenergic stimulation in a whole-cell model, the KCNQ1-G589D mutation induced QT prolongation and transient afterdepolarizations, known cellular mechanisms for arrhythmogenesis. These cellular mechanisms amplified tissue heterogeneities in a three-dimensional rabbit ventricular wedge model, elevating transmural dispersion of repolarization and creating other T-wave abnormalities on simulated electrocardiograms. Increasing heart rate protected both single myocyte and the coupled myocardium models from arrhythmic consequences. These findings suggest that the KCNQ1-G589D mutation disrupts a critical link between beta-adrenergic signaling and myocyte electrophysiology, creating both triggers of cardiac arrhythmia and a myocardial substrate vulnerable to such electrical disturbances.