Changes In Action Potential Morphology Research Articles

GLP-1 increases heart rate by a direct action on the sinus node.

Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) are increasingly used to treat type 2 diabetes and obesity. Albeit cardiovascular outcomes generally improve, treatment with GLP-1 RAs is associated with increased heart rate, the mechanism of which is unclear. We employed a large animal model, the female landrace pig, and used multiple in-vivo and ex-vivo approaches including pharmacological challenges, electrophysiology and high-resolution mass spectrometry to explore how GLP-1 elicits an increase in heart rate. In anaesthetized pigs, neither cervical vagotomy, adrenergic blockers (alpha, beta or combined alpha-beta blockade), ganglionic blockade (hexamethonium) nor inhibition of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (ivabradine) abolished the marked chronotropic effect of GLP-1. GLP-1 administration to isolated perfused pig hearts also increased heart rate, which was abolished by GLP-1 receptor blockade. Electrophysiological characterization of GLP-1 effects in vivo and in isolated perfused hearts localized electrical modulation to the atria and conduction system. In isolated sinus nodes, GLP-1 administration shortened action potential cycle length of pacemaker cells and shifted the site of earliest activation. The effect was independent of HCN blockade. Collectively, these data support a direct effect of GLP-1 on GLP-1 receptors within the heart. Consistently, single nucleus RNA sequencing (snRNAseq) showed GLP-1 receptor expression in porcine pacemaker cells. Quantitative phosphoproteomics analyses of sinus node samples revealed that GLP-1 administration leads to phosphorylation changes of calcium cycling proteins of the sarcoplasmic reticulum, known to regulate heart rate. GLP-1 has direct chronotropic effects on the heart mediated by GLP-1 receptors in pacemaker cells of the sinus node, inducing changes in action potential morphology and the leading pacemaker site through a calcium signaling response characterized by PKA-dependent phosphorylation of Ca2+ cycling proteins involved in pace making. Targeting the pacemaker calcium clock may be a strategy to lower heart rate in GLP-1 RA recipients.

In-silico analysis of aging mechanisms of action potential remodeling in human atrial cardiomyocites

Electrophysiology of cardiomyocytes changes with aging. Agerelated ionic remodeling in cardiomyocytes may increase the incidence and prevalence of atrial fibrillation (AF) in the elderly and affect the efficiency of antiarrhythmic drugs. There is the deep lack of experimental data on an action potential and transmembrane currents recorded in the healthy human cardiomyocytes of different age. Experimental data in mammals is also incomplete and often contradicting depending on the experimental conditions. In this in-silico study, we used a population of ionic models of human atrial cardiomyocytes to transfer data on the age- related ionic remodeling in atrial cardiomyocytes from canines and mice to predict possible consequences for human cardiomyocyte activity. Based on experimental data, we analyzes two hypotheses on the aging effect on the ionic currents using two age-related sets of varied model parameters and evaluated corresponding changes in action potential morphology with aging. Using the two populations of aging models, we analyzed the agedependent sensitivity of atrial cardiomyocytes to Dofetilide which is one of the antiarrhythmic drugs widely used in patients with atrial fibrillation.

Temporal irregularity quantification and mapping of optical action potentials using wave morphology similarity

BackgroundCardiac optical mapping enables direct and high spatio-temporal resolution recording of action potential (AP) morphology. Temporal alterations in AP morphology are both predictive and consequent of arrhythmia. Here we sought to test if methods that quantify regularity of recorded waveforms could be applied to detect and quantify periods of temporal instability in optical mapping datasets in a semi-automated, user-unbiased manner. Methods and resultsWe developed, tested and applied algorithms to quantify optical wave similarity (OWS) to study morphological temporal similarity of optically recorded APs. Unlike other measures (e.g. alternans ratio, beat-to-beat variability, arrhythmia scoring), the quantification of OWS is achieved without a restrictive definition of specific signal points/features and is instead derived by analysing the complete morphology from the entire AP waveform.Using model datasets, we validated the ability of OWS to measure changes in AP morphology, and tested OWS mapping in guinea pig hearts and mouse atria. OWS successfully detected and measured alterations in temporal regularity in response to several proarrhythmic stimuli, including alterations in pacing frequency, premature contractions, alternans and ventricular fibrillation. ConclusionOWS mapping provides an effective measure of temporal regularity that can be applied to optical datasets to detect and quantify temporal alterations in action potential morphology. This methodology provides a new metric for arrhythmia inducibility and scoring in optical mapping datasets.

Action potential shortening rescues atrial calcium alternans.

Cardiac alternans refers to a beat-to-beat alternation in contraction, action potential (AP) morphology and Ca2+ transient (CaT) amplitude, and represents a risk factor for cardiac arrhythmia, including atrial fibrillation. We developed strategies to pharmacologically manipulate the AP waveform with the goal to reduce or eliminate the occurrence of CaT and contraction alternans in atrial tissue. With combined patch-clamp and intracellular Ca2+ measurements we investigated the effect of specific ion channel inhibitors and activators on alternans. In single rabbit atrial myocytes, suppression of Ca2+ -activated Cl- channels eliminated AP duration alternans, but prolonged the AP and failed to eliminate CaT alternans. In contrast, activation of K+ currents (IKs and IKr ) shortened the AP and eliminated both AP duration and CaT alternans. As demonstrated also at the whole heart level, activation of K+ conductances represents a promising strategy to suppress alternans, and thus reducing a risk factor for atrial fibrillation. At the cellular level alternans is observed as beat-to-beat alternations in contraction, action potential (AP) morphology and magnitude of the Ca2+ transient (CaT). Alternans is a well-established risk factor for cardiac arrhythmia, including atrial fibrillation. This study investigates whether pharmacological manipulation of AP morphology is a viable strategy to reduce the risk of arrhythmogenic CaT alternans. Pacing-induced AP and CaT alternans were studied in rabbit atrial myocytes using combined Ca2+ imaging and electrophysiological measurements. Increased AP duration (APD) and beat-to-beat alternations in AP morphology lowered the pacing frequency threshold and increased the degree of CaT alternans. Inhibition of Ca2+ -activated Cl- channels reduced beat-to-beat AP alternations, but prolonged APD and failed to suppress CaT alternans. In contrast, AP shortening induced by activators of two K+ channels (ML277 for Kv7.1 and NS1643 for Kv11.1) abolished both APD and CaT alternans in field-stimulated and current-clamped myocytes. K+ channel activators had no effect on the degree of Ca2+ alternans in AP voltage-clamped cells, confirming that suppression of Ca2+ alternans was caused by the changes in AP morphology. Finally, activation of Kv11.1 channel significantly attenuated or even abolished atrial T-wave alternans in isolated Langendorff perfused hearts. In summary, AP shortening suppressed or completely eliminated both CaT and APD alternans in single atrial myocytes and atrial T-wave alternans at the whole heart level. Therefore, we suggest that AP shortening is a potential intervention to avert development of alternans with important ramifications for arrhythmia prevention and therapy.

Complex restitution behavior and reentry in a cardiac tissue model for neonatal mice.

Spatiotemporal dynamics in cardiac tissue emerging from the coupling of individual cardiomyocytes underlie the heart's normal rhythm as well as undesired and possibly life‐threatening arrhythmias. While single cells and their transmembrane currents have been studied extensively, systematically investigating spatiotemporal dynamics is complicated by the nontrivial relationship between single‐cell and emergent tissue properties. Mathematical models have been employed to bridge this gap and contribute to a deepened understanding of the onset, development, and termination of arrhythmias. However, no such tissue‐level model currently exists for neonatal mice. Here, we build on a recent single‐cell model of neonatal mouse cardiomyocytes by Wang and Sobie (Am. J. Physiol. Heart Circ. Physiol. 294:H2565) to predict properties that are commonly used to gauge arrhythmogenicity of cardiac substrates. We modify the model to yield well‐defined behavior for common experimental protocols and construct a spatially extended version to study emergent tissue dynamics. We find a complex action potential duration (APD) restitution behavior characterized by a nonmonotonic dependence on pacing frequency. Electrotonic coupling in tissue leads not only to changes in action potential morphology but can also induce spatially concordant and discordant alternans not observed in the single‐cell model. In two‐dimensional tissue, our results show that the model supports stable functional reentry, whose frequency is in good agreement with that observed in adult mice. Our results can be used to further constrain and validate the mathematical model of neonatal mouse cardiomyocytes with future experiments.

Vernakalant does not alter early repolarization or contractility in normal and electrically remodelled atria.

Besides the inhibition of the sodium inward current, vernakalant also inhibits the ultra rapid rectifier (IKur) and transient outward current (Ito). Inhibition of these currents increases contractility in canine atrial myocytes and goat atria. We investigated the effect of vernakalant on early repolarization and contractility in normal and electrically remodelled atria. Goats were implanted a pressure catheter, piezoelectric crystals, and electrodes to obtain atrial contractility and effective refractory period (ERP). The active component in pressure distance loops was used to compute the atrial work index (AWI). Experiments were performed in normal and electrically remodelled atria at clinically relevant plasma levels of vernakalant. As a positive control, the Ito/IKur blocker AVE0118 was investigated. Monophasic action potentials were recorded in anaesthetized goats and in explanted hearts to determine changes in action potential morphology. Vernakalant did not affect atrial work loops during sinus rhythm. Likewise vernakalant did not increase atrial fractional shortening or AWI during pacing with fixed heart rate and AV-delay. In contrast, AVE0118 did increase AWI, with a positive force frequency relation. Both in normal and remodelled atria, vernakalant strongly increased ERP but did not prolong early repolarization. In goat atria, vernakalant does not have an atrial positive inotropic effect and does not affect early repolarization. At high rates vernakalant may even have a negative inotropic effect.

Effect of Drugs on Repolarization of IPSCD-Cardiomyocytes

The FDA has proposed a Comprehensive in Vitro Pro-Arrhythmia Assay (CiPA Initiative) to address the clinical potential for cardiotoxicity during pre-clinical drug development in part, this involves the use of human-induced pluripotent adult stem cell-derived cardiac myocytes (hiPSC-CM) as a model system to measure the effects of drugs on the cardiac action potential (AP). hiPSC-CMs, however, have little or no IK1, resulting in unstable APs and anomalous behavior in the presence of some drugs. We stabilized APs from hiPSC-CMs through the electronic addition of IK1 via dynamic clamp. hiPSC-CMs (iCell, Inc., WI) were cultured according to manufacturer's instructions, and studied using the Amphotericin B perforated patch technique. Cardiac APs (1.5-2.0 nA stimulus pulse for 1.5 ms, 0.5 Hz) were measured in the absence and presence of increasing drug concentrations. As expected, quinidine, dofetilide, cisapride and sotalol (known IKR blockers) caused dose-dependent increases in APD. In 10 μM quinidine APD90 increased 144±12%; 1420 nM dofetilide increased APD90 128±3%; 103 nM cisapride increased, APD90 170±6%; 324 μM sotalol increased APD90 139±14%). Further, qualitative differences in potentiation were observed in that cisapride and sotalol, but not dofetilide, caused a pronounced flattening in Phase 2 of the AP. Quinidine (10 μM) also decreased dV/dtmax by 68±18%, which is consistent with Na+ channel block. Nifedipine (300nM), a Ca2+ channel blocker, significantly decreased APD90 by 35±1%. The ability to measure stable cardiac APs by the electronic addition of IK1 allows for more accurate comprehensive evaluation of drug effects on the ion channel components comprising the cardiac AP, particularly in the case of sodium channel block, and may aid in the identification of pro-arrhythmic changes in AP morphology.

Defects in brainstem neurons associated with breathing and motor function in the Mecp2R168X/Y mouse model of Rett syndrome.

Rett Syndrome (RTT) is an X-linked neurodevelopmental disorder caused mostly by disruption of the MECP2 gene. Among several RTT-like mouse models, one of them is a strain of mice that carries an R168X point mutation in Mecp2 and resembles one of the most common RTT-causing mutations in humans. Although several behavioral defects have previously been found in the Mecp2R168X/Y mice, alterations in nerve cells remain unknown. Here we compare several behavioral and cellular outcomes between this Mecp2R168X/Y model and a widely used Mecp2Bird/Y mouse model. With lower body weight and shorter lifespan than their wild-type littermates, the Mecp2R168X/Y mice showed impairments of breathing and motor function. Thus we studied brainstem CO2-chemosensitive neurons and propriosensory cells that are associated with these two functions, respectively. Neurons in the locus coeruleus (LC) of both mutant strains showed defects in their intrinsic membrane properties, including changes in action potential morphology and excessive firing activity. Neurons in the mesencephalic trigeminal nucleus (Me5) of both strains displayed a higher firing response to depolarization than their wild-type littermates, likely attributable to a lower firing threshold. Because the increased excitability in LC and Me5 neurons tends to impact the excitation-inhibition balances in brainstem neuronal networks as well as their associated functions, it is likely that the defects in the intrinsic membrane properties of these brainstem neurons contribute to the breathing abnormalities and motor dysfunction. Furthermore, our results showing comparable phenotypical outcomes of Mecp2R168X/Y mice with Mecp2Bird/Y mice suggest that both strains are valid animal models for RTT research.

Efficacy of selective NCX inhibition by ORM-10103 during simulated ischemia/reperfusion

In this study we evaluated the effects of selective Na+/Ca2+ exchanger (NCX) inhibition by ORM-10103 on the [Ca2+]i transient (CaT), action potential (AP), and cell viability in isolated canine ventricular cardiomyocytes exposed to a simulated ischemia/reperfusion protocol performed either alone (modeling moderate low-flow ischemia) or with simultaneous strophantidine challenge (modeling more severe low-flow ischemia). CaTs were monitored using a Ca2+-sensitive fluorescent dye, APs were recorded by intracellular microelectrodes, and anaerobic shifts in cellular metabolism were verified via monitoring native NADH fluorescence. Simulated ischemia increased the NADH fluorescence, reduced the amplitudes of the AP and CaT and induced membrane depolarization. APs moderately shortened, CaTs prolonged. Diastolic [Ca2+]i ([Ca2+]iD) level increased significantly during ischemia and further elevated following strophantidine application. Reperfusion normalized the NADH level, the amplitude of the AP and duration of the [Ca2+]i transient, but only partially restored action potential triangulation and the amplitude of the CaT. [Ca2+]iD decreased in untreated, but further increased in strophantidine-treated cells. 10µM ORM-10103 significantly reduced the ischemic [Ca2+]i raise in both untreated and strophantidine-treated cells. During reperfusion ORM-10103 decreased [Ca2+]i and eliminated its diastolic elevation in untreated and strophantidine-treated cardiomyocytes. Following the application of ORM-10103 the detrimental effect of ischemia/reperfusion on cell viability and the reperfusion-induced increase in AP and CaT variabilities were substantially reduced, but ischemia-induced shifts in AP morphology were barely influenced. In conclusion, selective NCX inhibition by ORM-10103 is highly effective against ischemia/reperfusion induced pathologic alterations in [Ca2+]i homeostasis, however, it fails to normalize untoward arrhythmogenic changes in AP morphology.

Contribution of Ca-Regulated Ion Currents to the Action Potential Morphology During Cardiac Alternans

Cardiac alternans, described as periodic beat-to-beat alternations in contraction, action potential (AP) morphology or cytosolic Ca transient (CaT) amplitude, is recognized as high risk indicator for cardiac arrhythmias, stroke and sudden cardiac death.We investigated mechanisms of cardiac alternans in single rabbit atrial myocytes. CaTs were monitored simultaneously with membrane currents or APs recorded with the patch clamp technique, and revealed a strong correlation between CaT amplitude and AP duration (APD) during alternans. Investigation of [Ca]i-Vm interactions indicates that disturbances in Ca signaling are the primary events mediating activity of Ca-regulated ion channels and leading to the changes in AP morphology. Voltage-clamp in form of pre-recorded APs (AP clamp) was used to assess the contribution of Ca-regulated membrane currents by measuring the difference between currents recorded during large and small alternans CaTs. Ca-dependent currents exhibited a large outward component (2.9 ± 1.1 pA/pF in amplitude) during AP phases 1 and 2 that was followed by an inward current (0.5 ± 0.1 pA/pF in amplitude; n=7) during AP repolarization. We also investigated contributions of Na/Ca exchange (NCX), L-type Ca, Ca-activated Cl and small conductance Ca-activated K currents to the AP morphology during Ca alternans. ∼90 % of the initial outward current was blocked by substitution of Cl ions or application of Cl channel blocker (DIDS) indicating that this current is mainly carried by Ca-activated Cl channels. In summary, during Ca alternans the prominent prolongation of AP at APD10 to APD30 repolarization levels during the small CaT is due to reduced Ca-activated Cl current while the overall effect of other Ca-sensitive currents (NCX, L-type Ca, small conductance K) cause slight shortening of AP in the APD50 to APD90 range.

Toward an Integrative Computational Model of the Guinea Pig Cardiac Myocyte

The local control theory of excitation-contraction (EC) coupling asserts that regulation of calcium (Ca2+) release occurs at the nanodomain level, where openings of single L-type Ca2+ channels (LCCs) trigger openings of small clusters of ryanodine receptors (RyRs) co-localized within the dyad. A consequence of local control is that the whole-cell Ca2+ transient is a smooth continuous function of influx of Ca2+ through LCCs. While this so-called graded release property has been known for some time, its functional importance to the integrated behavior of the cardiac ventricular myocyte has not been fully appreciated. We previously formulated a biophysically based model, in which LCCs and RyRs interact via a coarse-grained representation of the dyadic space. The model captures key features of local control using a low-dimensional system of ordinary differential equations. Voltage-dependent gain and graded Ca2+ release are emergent properties of this model by virtue of the fact that model formulation is closely based on the sub-cellular basis of local control. In this current work, we have incorporated this graded release model into a prior model of guinea pig ventricular myocyte electrophysiology, metabolism, and isometric force production. The resulting integrative model predicts the experimentally observed causal relationship between action potential (AP) shape and timing of Ca2+ and force transients, a relationship that is not explained by models lacking the graded release property. Model results suggest that even relatively subtle changes in AP morphology that may result, for example, from remodeling of membrane transporter expression in disease or spatial variation in cell properties, may have major impact on the temporal waveform of Ca2+ transients, thus influencing tissue level electromechanical function.

Trpc3-Mediated Electrical Remodeling of Cardiac Myocytes

Recent evidence suggests a key role of transient receptor potential (TRP) channels in cardiac pathophysiology with TRPC3 as one potential key player in cardiac remodeling. TRPC3 is typically up-regulated by hypertrophic stimuli and may be involved in distorted Ca2+ signaling that drives pathological remodeling. As TRPC proteins generate non-selective cation conductances, we hypothesized that these channels may not only govern Ca2+-mediated gene expression but exert in addition a severe impact on basic electrical properties and excitability of the myocardium. Utilizing the patch clamp technique we characterized membrane currents and electrical properties of cardiomyocytes in response to enhanced TRPC3 expression in the murine HL-1 model. Stimulation of TRPC3-overexpressing HL-1 cells with endothelin-1 [100 nM] (ET-1) as a Gqi-PLC-activator gave rise to a conductance with features distinctly different from the properties described for TRPC3 conductance in expression systems. In HL-1 cells, the TRPC3 over-expression-induced conductance in physiological solutions reversed at about −50 mV, displayed profound outward rectification and was suppressed by the TRPC3-inhibitor Pyr3 [10 µM] (ethyl-1-(4-(2,3,3-trichloroacrylamide)phenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylate). As a result of this conductance, action potential duration was effectively shortened by ET-1 in HL-1 myocytes over-expressing the TRPC3, while this effect was minute in wild-type myocytes. Moreover, TRPC3 over-expression enabled significant depolarizing effects of ET-1 along with action potential shortening and reduction of refractory period. Our results suggests that increased expression of TRPC3 in cardiomyocytes may significantly contribute to electrical remodeling in hypertrophic hearts, generating changes in action potential morphology that are likely to promote arrhythmias.

Nicotinic acetylcholine receptors do not mediate excitatory transmission in young rat carotid body

Carotid body chemoreceptors transduce a decrease in arterial oxygen tension into increased action potential (AP) activity on the sinus nerve, which increases the drive to breathe. The mechanism by which AP activity increases is unresolved, but acetylcholine (ACh), acting through nicotinic receptors, is postulated to be a major contributor to nerve excitation based partly on the demonstration that pharmacological antagonism of nicotinic receptors reduces the afferent nerve response in some studies. However, most previous studies relied on indirect measures of chemoreceptor activity or utilized a recording configuration that is sensitive to AP morphology in addition to AP frequency. In the present study, single-unit AP activity was recorded from the soma of rat chemoreceptor neurons in vitro. The nicotinic blocker mecamylamine (50 microM) ablated the excitatory actions of exogenous ACh and increased, rather than decreased, AP activity during moderate hypoxia. At higher dosage (500 microM) AP height was reduced, conduction velocity slowed, and conduction failure occurred, especially during hypoxia, producing the appearance of a decreased response to hypoxia. Recovery from mecamylamine block was slow (>10 min). In contrast to mecamylamine, suramin, a P2X receptor blocker, reversibly inhibited the response to hypoxia, suggesting relatively free diffusion of drugs to the glomus cell/nerve synaptic site. These results strongly suggest that ACh acting through nicotinic receptors does not mediate excitatory transmission in rat carotid body and that previous results demonstrating such a role may have been partially influenced by changes in AP morphology or conduction failure.

Effects of articaine on action potential characteristics and the underlying ion currents in canine ventricular myocytes

In spite of its widespread clinical application, there is little information on the cellular cardiac effects of articaine. In the present study, the concentration-dependent effects of articaine on action potential morphology and the underlying ion currents were studied in isolated canine ventricular cardiomyocytes. Action potentials were recorded from the enzymatically dispersed myocytes using sharp microelectrodes (16 cells from 3 dogs). Conventional patch clamp and action potential voltage clamp arrangements were used to study the effects of articaine on transmembrane ion currents (37 cells from 14 dogs). Articaine-induced concentration-dependent changes in action potential configuration including shortening of the action potentials, reduction of their amplitude and maximum velocity of depolarization (V(max)), suppression of early repolarization and depression of plateau. The EC50 value obtained for the V(max) block was 162 (sd 30) microM. Both the reduction of V(max) and action potential shortening were frequency dependent: the former was more prominent at shorter, while the latter at longer pacing cycle lengths. A rate dependent V(max) block, having rapid offset kinetics [tau = 91 (20) ms], was observed in addition to tonic block. Under voltage clamp conditions, a variety of ion currents were blocked by articaine: I(Ca) [EC50 = 471 (75) microM], I(to) [EC50 = 365 (62) microM], I(K1) [EC50 = 372 (46) microM], I(Kr) [EC50 = 278 (79) microM], and I(Ks) [EC50 = 326 (65) microM]. Hill coefficients were close to unity indicating a single binding site for articaine, except for I(K1). Articaine can modify cardiac action potentials and ion currents at concentrations higher than the therapeutic range which can be achieved only by accidental venous injection. Since its suppressive effects on the inward and outward currents are relatively well balanced, the articaine-induced changes in action potential morphology may be moderate even in the case of overdose.

Computer simulation of wild-type and mutant human cardiac Na+ current

Long QT syndrome (LQTS) and Brugada syndrome (BrS) are inherited diseases predisposing to ventricular arrhythmias and sudden death. Genetic studies linked LQTS and BrS to mutations in genes encoding for cardiac ion channels. Recently, two novel missense mutations at the same codon in the gene encoding the cardiac Na+ channel (SCN5A) have been identified: Y1795C (causing the LQTS phenotype) and Y1795H (causing the BrS phenotype). Functional studies in HEK293 cells showed that both mutations alter the inactivation of Na+ current and cause a sustained Na+ current upon depolarisation. In this paper, a nine state Markov model was used to simulate the Na+ current in wild-type Na+ cardiac channel and the current alterations observed in Y1795C and Y1795H mutant channels. The model includes three distinct closed states, a conducting open state and five inactivation states (one fast-, two intermediate- and two closed-inactivation). Transition rates between these states were identified on the basis of previously published voltage-clamp experiments. The model was able to reproduce the experimental Na+ current in mutant channels just by altering the assignment of model parameters with respect to wild-type case. Parameter assignment was validated by performing action potential clamp experiments and comparing experimental and simulated I(Na) current. The Markov model was subsequently introduced in the Luo-Rudy model of ventricular myocyte to investigate "in silico" the consequences on the ventricular cell action potential of the two mutations. Coherently with their phenotypes, the Y1795C mutation prolongs the action potential, while the Y1795H mutation causes only negligible changes in action potential morphology.

Action Potential Morphology Influences Intracellular Calcium Handling Stability and the Occurrence of Alternans

Instability in the intracellular Ca 2+ handling system leading to Ca 2+ alternans is hypothesized to be an underlying cause of electrical alternans. The highly coupled nature of membrane voltage and Ca 2+ regulation suggests that there should be reciprocal effects of membrane voltage on the stability of the Ca 2+ handling system. We investigated such effects using a mathematical model of the cardiac intracellular Ca 2+ handling system. We found that the morphology of the action potential has a significant effect on the stability of the Ca 2+ handling system at any given pacing rate, with small changes in action potential morphology resulting in a transition from stable nonalternating Ca 2+ transients to stable alternating Ca 2+ transients. This bifurcation occurs as the alternans eigenvalue of the system changes from absolute value <1 to absolute value >1. These results suggest that the stability of the intracellular Ca 2+ handling system and the occurrence of Ca 2+ alternans are not dictated solely by the Ca 2+ handling system itself, but are also modulated to a significant degree by membrane voltage (through its influence on sarcolemmal Ca 2+ currents) and, therefore, by all ionic currents that affect membrane voltage.

Differences in electrophysiological and contractile properties of mammalian cardiac tissues bathed in bicarbonate - and HEPES-buffered solutions.

The aim of this study was to compare the action potential configuration, contractility, intracellular Ca2+ and H+ concentrations in mammalian cardiac tissues bathed with Krebs and Tyrode solutions at 37 degrees C. In Langendorff-perfused guinea-pig hearts, loaded with the fluorescent Ca2+-indicator Fura-2, or H+-sensitive dye carboxy-SNARF, shifts from Krebs to Tyrode solution caused intra-cellular acidification, increased diastolic pressure and [Ca2+]i, decreased systolic pressure and [Ca2+]i, leading to a reduction in the amplitude of [Ca2+]i transients and pulse pressure. Contractility was also depressed in canine ventricular trabeculae when transferred from Krebs to Tyrode solution. Shifts from Krebs to Tyrode solution increased the duration of action potentials in multicellular cardiac preparations excised from canine and rabbit hearts but not in isolated cardiomyocytes. All these changes in action potential morphology, contractility, [Ca2+]i and [H+]i were readily reversible by addition of 26 mmol L(-1) bicarbonate to Tyrode solution. Effects of dofetilide and CsCl, both blockers of the delayed rectifier K current, on action potential duration were compared in Krebs and Tyrode solutions. Dofetilide lengthened rabbit ventricular action potentials in a significantly greater extent in Tyrode than in Krebs solution. Exposure of canine Purkinje fibres to CsCl evoked early after depolarizations within 40 min in all preparations incubated with Tyrode solution, but not in those bathed with Krebs solution. It is concluded that the marked differences in action potential morphology, [Ca2+]i, [H+]i and contractility observed between preparations bathed with Krebs and Tyrode solutions are more likely attributable to differences in the intracellular buffering capacities of the two media.

Remodeling in atrial fibrillation

Atrial fibrillation is associated with alterations in atrial electrophysiology that facilitate the initiation and persistence of the arrhythmia. This process was termed electrical remodeling in atrial fibrillation. The underlying cellular and molecular mechanisms have intensively been investigated over the past few years in patients with atrial fibrillation and in different experimental models. The results, that have substantially improved the understanding of the pathophysiology of atrial fibrillation, are reviewed. On the cellular level, atrial fibrillation leads to a strong shortening and an impaired rate adaptation of the action potential as well as to changes in action potential morphology. Atrial fibrillation is associated with an altered gene expression of the L-type calcium channel (ICa,L) and of potassium channels (Ito, IK1, IKACh). The molecular mechanisms of intraatrial conduction slowing are less well understood, changes in the expression or distribution of gap junction proteins or a decrease of the fast sodium inward channel (INa) have been reported in some studies. A trigger of initiation for electrical remodeling is an overload of the cytoplasm with Ca2+ and a consecutive decrease of the systolic calcium gradient, furthermore changes in calcium-handling proteins are detectable in atrial fibrillation. These changes in the cellular and molecular milieu importantly determine the clinical course and the efficacy of therapeutical interventions in atrial fibrillation. The clinical relevance and potential new therapeutic approaches are discussed in the last part.

Propagation in cardiac tissue adjacent to connective tissue: two-dimensional modeling studies.

The conditions for activation transmission across a region of extracellular space was demonstrated in two-dimensional preparations with results consistent with those previously seen in the one-dimensional fiber studies. In addition, one sees changes in action potential morphology which occur in the tissue nearest the connective-tissue border as well as changes in conduction velocity along the border. These results hinge on an adequate representation of the connective-tissue region achieved by careful implementation of the boundary conditions in the intracellular and interstitial spaces and the expansion of the connective-tissue discretization to a "double-tier network" description. Through a series of simulations, a clear dependence on fiber orientation is illustrated in the efficacy to transmit activation. The collision of a front with an embedded connective-tissue region was also examined. The results revealed that fibers aligned normal to a planar stimulus would more greatly disrupt the advancement of a planar front. Such pronounced disruptions have been shown to be proarrhythmic in the literature. The increasing evidence of the ability of connective tissue to transmit activation has implications in understanding spread of activation through infarcted tissues and through the healthy ventricular wall in the presence of connective-tissue sheets.

Electrophysiology of Rabbit Ventricular Myocytes Following Sustained Rapid Ventricular Pacing

The present study examined changes in electrophysiological properties of ventricular myocytes isolated from rabbit hearts after 2–3 weeks of rapid ventricular pacing. Left ventricular end-diastolic pressure at completion of the pacing period was nearly four-fold greater than in age-matched controls, although there was no significant change in heart weight/body weight ratio. Action potentials recorded in current-clamp mode at low stimulation frequencies were significantly longer in duration and phase 1 diminished in isolated myocytes from paced hearts compared with control. In voltage-clamp experiments,l-type Ca2+current (ICa) density was not different between groups of myocytes, but the maximum current (at+10 mV) elicited by 10μmisoproterenol was approximately 40% less in myocytes from paced hearts. In contrast, maximum ICaelicited by 10μmforskolin was similar in both groups. The 4-aminopyridine-sensitive, transient outward current (Ito) was 65% less (at+60 mV) in myocytes from paced hearts than from control. However, after approximately 24 h in culture, Itodensity in these myocytes returned toward control values. Despite marked reduction in Itodensity, the inward rectifier current (IK1) was not different between groups. These data demonstrate that Itois significantly and reversibly decreased in myocytes from rapidly paced hearts, which may partly account for marked changes in action potential morphology. Although basal ICawas not altered in this group of myocytes compared with control, its modulation byβ-agonists was markedly blunted, probably through a decrease in receptor density or coupling to adenylyl cyclase. These changes in myocyte K+and Ca2+channel behavior in paced hearts may relate to impaired contractility and arrhythmogenesis that is characteristic of the intact failing heart.