Dynamic changes of cardiac conduction during rapid pacing

Slow conduction and unidirectional conduction block (UCB) are key mechanisms of reentry. Following abrupt changes in heart rate, dynamic changes of conduction velocity (CV) and structurally determined UCB may critically influence arrhythmogenesis. Using patterned cultures of neonatal rat ventricular myocytes grown on microelectrode arrays, we investigated the dynamics of CV in linear strands and the behavior of UCB in tissue expansions following an abrupt decrease in pacing cycle length (CL). Ionic mechanisms underlying rate-dependent conduction changes were investigated using the Pandit-Clark-Giles-Demir model. In linear strands, CV gradually decreased upon a reduction of CL from 500 ms to 230–300 ms. In contrast, at very short CLs (110–220 ms), CV first decreased before increasing again. The simulations suggested that the initial conduction slowing resulted from gradually increasing action potential duration (APD), decreasing diastolic intervals, and increasing postrepolarization refractoriness, which impaired Na+ current ( INa) recovery. Only at very short CLs did APD subsequently shorten again due to increasing Na+/K+ pump current secondary to intracellular Na+ accumulation, which caused recovery of CV. Across tissue expansions, the degree of UCB gradually increased at CLs of 250–390 ms, whereas at CLs of 180–240 ms, it first increased and subsequently decreased. In the simulations, reduction of inward currents caused by increasing intracellular Na+ and Ca2+ concentrations contributed to UCB progression, which was reversed by increasing Na+/K+ pump activity. In conclusion, CV and UCB follow intricate dynamics upon an abrupt decrease in CL that are determined by the interplay among INa recovery, postrepolarization refractoriness, APD changes, ion accumulation, and Na+/K+ pump function.

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Palmitoyl-L-carnitine acts like ouabain on voltage, current, and contraction in guinea pig ventricular cells

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1995.268.3.h1027 ◽

1995 ◽

Vol 268 (3) ◽

pp. H1027-H1036 ◽

Cited By ~ 1

Author(s):

J. B. Shen ◽

A. J. Pappano

Keyword(s):

Guinea Pig ◽

Ventricular Myocytes ◽

Present Report ◽

Ionic Currents ◽

Pump Current ◽

Membrane Potentials ◽

Cell Shortening ◽

Ventricular Cells ◽

Ramp Voltage ◽

Inward Currents

We previously showed that palmitoyl-L-carnitine (L-PC) inhibits the Na/K pump current (INa/K). In the present report, we test the hypothesis that L-PC, like ouabain, should increase myocyte shortening. Membrane potentials or ionic currents were recorded simultaneously with cell shortening in single guinea pig ventricular myocytes at room temperature (22 degrees C). Like ouabain, L-PC (1 microM) reversibly depolarized the resting membrane, decreased action potential duration, and increased the amplitude of myocyte contractions. Neither L-PC nor ouabain had a significant effect on Ca current (ICa). When L-PC increased cell shortening during ramp voltage clamp, membrane current shifted inward at voltages negative to -20 mV and shifted outward at more positive voltages. Similar to toxic concentrations of ouabain, L-PC induced transient inward currents and aftercontractions. At concentrations that inhibit INa/K, L-PC acted like ouabain to produce characteristic effects on membrane potentials, currents, and cell contractions that were unrelated to significant changes in ICa. L-PC reduces surface negative charge of erythrocytes and myocytes (C. Gruver and A. J. Pappano, J. Mol. Cell. Cardiol. 25: 1275–1284, 1993), and we speculate that L-PC inhibits INa/K by this mechanism.

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Ionic Mechanisms of Impulse Propagation Failure in the FHF2-Deficient Heart

Circulation Research ◽

10.1161/circresaha.120.317349 ◽

2020 ◽

Vol 127 (12) ◽

pp. 1536-1548 ◽

Cited By ~ 1

Author(s):

David S. Park ◽

Akshay Shekhar ◽

John Santucci ◽

Gabriel Redel-Traub ◽

Sergio Solinas ◽

...

Keyword(s):

Connexin 43 ◽

Sodium Current ◽

Conduction Block ◽

Tissue Level ◽

Axial Current ◽

Epileptic Encephalopathy ◽

Cardiac Conduction ◽

Impulse Propagation ◽

Ionic Mechanisms ◽

Gap Junctional

Rationale: FHFs (fibroblast growth factor homologous factors) are key regulators of sodium channel (Na V ) inactivation. Mutations in these critical proteins have been implicated in human diseases including Brugada syndrome, idiopathic ventricular arrhythmias, and epileptic encephalopathy. The underlying ionic mechanisms by which reduced Na v availability in Fhf2 knockout ( Fhf2 KO ) mice predisposes to abnormal excitability at the tissue level are not well defined. Objective: Using animal models and theoretical multicellular linear strands, we examined how FHF2 orchestrates the interdependency of sodium, calcium, and gap junctional conductances to safeguard cardiac conduction. Methods and Results: Fhf2 KO mice were challenged by reducing calcium conductance (gCa V ) using verapamil or by reducing gap junctional conductance (Gj) using carbenoxolone or by backcrossing into a cardiomyocyte-specific Cx43 (connexin 43) heterozygous background. All conditions produced conduction block in Fhf2 KO mice, with Fhf2 wild-type ( Fhf2 WT ) mice showing normal impulse propagation. To explore the ionic mechanisms of block in Fhf2 KO hearts, multicellular linear strand models incorporating FHF2-deficient Na v inactivation properties were constructed and faithfully recapitulated conduction abnormalities seen in mutant hearts. The mechanisms of conduction block in mutant strands with reduced gCa V or diminished Gj are very different. Enhanced Na v inactivation due to FHF2 deficiency shifts dependence onto calcium current (I Ca ) to sustain electrotonic driving force, axial current flow, and action potential (AP) generation from cell-to-cell. In the setting of diminished Gj, slower charging time from upstream cells conspires with accelerated Na v inactivation in mutant strands to prevent sufficient downstream cell charging for AP propagation. Conclusions: FHF2-dependent effects on Na v inactivation ensure adequate sodium current (I Na ) reserve to safeguard against numerous threats to reliable cardiac impulse propagation.

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Effect of intracellular and extracellular acidosis on sodium current in ventricular myocytes

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1995.268.4.h1749 ◽

1995 ◽

Vol 268 (4) ◽

pp. H1749-H1756 ◽

Cited By ~ 1

Author(s):

C. L. Watson ◽

M. R. Gold

Keyword(s):

Steady State ◽

Time Course ◽

Ventricular Myocytes ◽

Sodium Current ◽

Essential Element ◽

Neonatal Rat ◽

Cardiac Conduction ◽

Inactivation Kinetics ◽

Extracellular Acidosis ◽

Steady State Inactivation

Conduction slowing is an essential element in the generation of ischemic ventricular arrhythmias and is determined in part by the inward Na+ current (INa). Because intracellular acidosis is an early consequence of ischemia, we hypothesized that lowering intracellular pH (pHi) would reduce or kinetically modulate INa and thus affect cardiac conduction. To test this hypothesis, the whole cell patch-clamp method was used to measure INa in neonatal rat ventricular myocytes exposed to varying extracellular pH (pHo 6.4–7.4), while perfusing the cells with acidic solutions (pHi 6.2–7.2). With simultaneous acidification of pHo and pHi there was a progressive increase in time to peak current, a 31% decrease in peak INa (298 +/- 18 to 206 +/- 16 pA/pF), and a complex slowing of inactivation kinetics. At the most extreme levels of acidification, there was a 5-mV hyperpolarizing shift in steady-state inactivation and a 6-mV depolarizing shift in activation. Independent changes of pHo and pHi indicate that the reduction of peak INa is a function of pHo. However, steady-state inactivation is modulated by pHi. The time course of activation and inactivation appears to depend on both pHo and pHi. We conclude that both intracellular and extracellular acidosis are significant but distinct modulators of INa amplitude and kinetics in cardiac myocytes.

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Late Na+ Current Is [Ca2+]i-Dependent in Canine Ventricular Myocytes

Pharmaceuticals ◽

10.3390/ph14111142 ◽

2021 ◽

Vol 14 (11) ◽

pp. 1142

Author(s):

Dénes Kiss ◽

Balázs Horváth ◽

Tamás Hézső ◽

Csaba Dienes ◽

Zsigmond Kovács ◽

...

Keyword(s):

Action Potential ◽

Ventricular Myocytes ◽

Sodium Current ◽

Na Channel ◽

Dependent Manner ◽

Calcium Calmodulin Dependent ◽

Rate Dependent ◽

Ventricular Cells ◽

Inward Currents ◽

Dependent Protein

Enhancement of the late sodium current (INaL) increases arrhythmia propensity in the heart, whereas suppression of the current is antiarrhythmic. In the present study, we investigated INaL in canine ventricular cardiomyocytes under action potential voltage-clamp conditions using the selective Na+ channel inhibitors GS967 and tetrodotoxin. Both 1 µM GS967 and 10 µM tetrodotoxin dissected largely similar inward currents. The amplitude and integral of the GS967-sensitive current was significantly smaller after the reduction of intracellular Ca2+ concentration ([Ca2+]i) either by superfusion of the cells with 1 µM nisoldipine or by intracellular application of 10 mM BAPTA. Inhibiting calcium/calmodulin-dependent protein kinase II (CaMKII) by KN-93 or the autocamtide-2-related inhibitor peptide similarly reduced the amplitude and integral of INaL. Action potential duration was shortened in a reverse rate-dependent manner and the plateau potential was depressed by GS967. This GS967-induced depression of plateau was reduced by pretreatment of the cells with BAPTA-AM. We conclude that (1) INaL depends on the magnitude of [Ca2+]i in canine ventricular cells, (2) this [Ca2+]i-dependence of INaL is mediated by the Ca2+-dependent activation of CaMKII, and (3) INaL is augmented by the baseline CaMKII activity.

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Spatially discordant alternans in cardiomyocyte monolayers

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.01233.2007 ◽

2008 ◽

Vol 294 (3) ◽

pp. H1417-H1425 ◽

Cited By ~ 31

Author(s):

Carlos de Diego ◽

Rakesh K. Pai ◽

Amish S. Dave ◽

Adam Lynch ◽

Mya Thu ◽

...

Keyword(s):

Cardiac Tissue ◽

Ventricular Myocytes ◽

Conduction Block ◽

Three Dimensional ◽

Bay K 8644 ◽

Neonatal Rat ◽

Two Dimensional ◽

Nodal Lines ◽

Dynamic Factors ◽

Rapid Pacing

Repolarization alternans is a harbinger of sudden cardiac death, particularly when it becomes spatially discordant. Alternans, a beat-to-beat alternation in the action potential duration (APD) and intracellular Ca (Cai), can arise from either tissue heterogeneities or dynamic factors. Distinguishing between these mechanisms in normal cardiac tissue is difficult because of inherent complex three-dimensional tissue heterogeneities. To evaluate repolarization alternans in a simpler two-dimensional cardiac substrate, we optically recorded voltage and/or Cai in monolayers of cultured neonatal rat ventricular myocytes during rapid pacing, before and after exposure to BAY K 8644 to enhance dynamic factors promoting alternans. Under control conditions ( n = 37), rapid pacing caused detectable APD alternans in 81% of monolayers, and Cai transient alternans in all monolayers, becoming spatially discordant in 62%. After BAY K 8644 ( n = 28), conduction velocity restitution became more prominent, and APD and Cai alternans developed and became spatially discordant in all monolayers, with an increased number of nodal lines separating out-of-phase alternating regions. Nodal lines moved closer to the pacing site with faster pacing rates and changed orientation when the pacing site was moved, as predicted for the dynamically generated, but not heterogeneity-based, alternans. Spatial APD gradients during spatially discordant alternans were sufficiently steep to induce conduction block and reentry. These findings indicate that spatially discordant alternans severe enough to initiate reentry can be readily induced by pacing in two-dimensional cardiac tissue and behaves according to predictions for a predominantly dynamically generated mechanism.

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CARP Plays a Key Role in Cellular Growth Under Hypertrophic Stimuli in Neonatal Rat Ventricular Myocytes

Vanderbilt Undergraduate Research Journal ◽

10.15695/vurj.v9i0.3790 ◽

2013 ◽

Vol 9 ◽

Author(s):

Tara A Shrout

Keyword(s):

Gene Expression ◽

Ventricular Myocytes ◽

Cellular Growth ◽

Neonatal Rat ◽

Transcriptional Level ◽

Dual Nature ◽

Hypertrophic Growth ◽

Rat Ventricular Myocytes ◽

Neonatal Rat Ventricular Myocytes ◽

Over Time

Cardiac hypertrophy is a growth process that occurs in response to stress stimuli or injury, and leads to the induction of several pathways to alter gene expression. Under hypertrophic stimuli, sarcomeric structure is disrupted, both as a consequence of gene expression and local changes in sarcomeric proteins. Cardiac-restricted ankyrin repeat protein (CARP) is one such protein that function both in cardiac sarcomeres and at the transcriptional level. We postulate that due to this dual nature, CARP plays a key role in maintaining the cardiac sarcomere. GATA4 is another protein detected in cardiomyocytes as important in hypertrophy, as it is activated by hypertrophic stimuli, and directly binds to DNA to alter gene expression. Results of GATA4 activation over time were inconclusive; however, the role of CARP in mediating hypertrophic growth in cardiomyocytes was clearly demonstrated. In this study, Neonatal Rat Ventricular Myocytes were used as a model to detect changes over time in CARP and GATA4 under hypertrophic stimulation by phenylephrine and high serum media. Results were detected by analysis of immunoblotting. The specific role that CARP plays in mediating cellular growth under hypertrophic stimuli was studied through immunofluorescence, which demonstrated that cardiomyocyte growth with hypertrophic stimulation was significantly blunted when NRVMs were co-treated with CARP siRNA. These data suggest that CARP plays an important role in the hypertrophic response in cardiomyocytes.

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Cardiac mitofusin-1 is declined in non-responding patients with idiopathic dilated cardiomyopathy

European Heart Journal ◽

10.1093/ehjci/ehaa946.3601 ◽

2020 ◽

Vol 41 (Supplement_2) ◽

Author(s):

Y Hsiao ◽

I Shimizu ◽

T Wakasugi ◽

S Jiao ◽

T Watanabe ◽

...

Keyword(s):

Electron Microscopy ◽

Heart Failure ◽

Ventricular Myocytes ◽

Severe Heart Failure ◽

Left Ventricular ◽

Neonatal Rat ◽

Heart Failure Patients ◽

Underlying Mechanisms ◽

Neonatal Rat Ventricular Myocytes ◽

Mitofusin 1

Abstract Background/Introduction Mitochondria are dynamic regulators of cellular metabolism and homeostasis. The dysfunction of mitochondria has long been considered a major contributor to aging and age-related diseases. The prognosis of severe heart failure is still unacceptably poor and it is urgent to establish new therapies for this critical condition. Some patients with heart failure do not respond to established multidisciplinary treatment and they are classified as “non-responders”. The outcome is especially poor for non-responders, and underlying mechanisms are largely unknown. Purpose Studies indicate mitochondrial dysfunction has causal roles for metabolic remodeling in the failing heart, but underlying mechanisms remain to be explored. This study tried to elucidate the role of Mitofusin-1 in a failing heart. Methods We examined twenty-two heart failure patients who underwent endomyocardial biopsy of intraventricular septum. Patients were classified as non-responders when their left-ventricular (LV) ejection fraction did not show more than 10% improvement at remote phase after biopsy. Fourteen patients were classified as responders, and eight as non-responders. Electron microscopy, quantitative PCR, and immunofluorescence studies were performed to explore the biological processes or molecules involved in failure to respond. In addition to studies with cardiac tissue specific knockout mice, we also conducted functional in-vitro studies with neonatal rat ventricular myocytes. Results Twenty-two patients with IDCM who underwent endomyocardial biopsy were enrolled in this study, including 14 responders and 8 non-responders. Transmission electron microscopy (EM) showed a significant reduction in mitochondrial size in cardiomyocytes of non-responders compared to responders. Quantitative PCR revealed that transcript of mitochondrial fusion protein, Mitofusin-1, was significantly reduced in non-responders. Studies with neonatal rat ventricular myocytes (NRVMs) indicated that the beta-1 adrenergic receptor-mediated signaling pathway negatively regulates Mitofusin-1 expression. Suppression of Mitofusin-1 resulted in a significant reduction in mitochondrial respiration of NRVMs. We generated left ventricular pressure overload model with thoracic aortic constriction (TAC) in cardiac specific Mitofusin-1 knockout model (c-Mfn1 KO). Systolic function was reduced in c-Mfn1 KO mice, and EM study showed an increase in dysfunctional mitochondria in the KO group subjected to TAC. Conclusions Mitofusin-1 becomes a biomarker for non-responders with heart failure. In addition, our results suggest that therapies targeting mitochondrial dynamics and homeostasis would become next generation therapy for severe heart failure patients. Funding Acknowledgement Type of funding source: None

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The Ku Protein Complex Interacts with YY1, Is Up-Regulated in Human Heart Failure, and Represses α Myosin Heavy-Chain Gene Expression

Molecular and Cellular Biology ◽

10.1128/mcb.24.19.8705-8715.2004 ◽

2004 ◽

Vol 24 (19) ◽

pp. 8705-8715 ◽

Cited By ~ 31

Author(s):

Carmen C. Sucharov ◽

Steve M. Helmke ◽

Stephen J. Langer ◽

M. Benjamin Perryman ◽

Michael Bristow ◽

...

Keyword(s):

Heart Failure ◽

Myosin Heavy Chain ◽

Heavy Chain ◽

Human Heart ◽

Ventricular Myocytes ◽

Major Effect ◽

Neonatal Rat ◽

Chain Gene ◽

Human Heart Failure ◽

Peptide Mass

ABSTRACT Human heart failure is accompanied by repression of genes such as α myosin heavy chain (αMyHC) and SERCA2A and the induction of fetal genes such as βMyHC and atrial natriuretic factor. It seems likely that changes in MyHC isoforms contribute to the poor contractility seen in heart failure, because small changes in isoform composition can have a major effect on the contractility of cardiac myocytes and the heart. Our laboratory has recently shown that YY1 protein levels are increased in human heart failure and that YY1 represses the activity of the human αMyHC promoter. We have now identified a region of the αMyHC promoter that binds a factor whose expression is increased sixfold in failing human hearts. Through peptide mass spectrometry, we identified this binding activity to be a heterodimer of Ku70 and Ku80. Expression of Ku represses the human αMyHC promoter in neonatal rat ventricular myocytes. Moreover, overexpression of Ku70/80 decreases αMyHC mRNA expression and increases skeletal α-actin. Interestingly, YY1 interacts with Ku70 and Ku80 in HeLa cells. Together, YY1, Ku70, and Ku80 repress the αMyHC promoter to an extent that is greater than that with YY1 or Ku70/80 alone. Our results suggest that Ku is an important factor in the repression of the human αMyHC promoter during heart failure.

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