A comparison of transient outward currents in canine cardiac Purkinje cells and ventricular myocytes

Prediction of experimental cardiac magnetostimulation thresholds using pig-specific body models

PURPOSE

Modern high-amplitude gradient systems can be limited by the International Electrotechnical Commission 60601-2-33 cardiac stimulation (CS) limit, which was set in a conservative manner based on electrode experiments and E-field simulations in uniform ellipsoidal body models. Here, we show that coupled electromagnetic-electrophysiological modeling in detailed body and heart models can predict CS thresholds, suggesting that such modeling might lead to more detailed threshold estimates in humans. Specifically, we compare measured and predicted CS thresholds in eight pigs.

METHODS

We created individualized porcine body models using MRI (Dixon for the whole body, CINE for the heart) that replicate the anatomy and posture of the animals used in our previous experimental CS study. We model the electric fields induced along cardiac Purkinje and ventricular muscle fibers and predict the electrophysiological response of these fibers, yielding CS threshold predictions in absolute units for each animal. Additionally, we assess the total modeling uncertainty through a variability analysis of the 25 main model parameters.

RESULTS

Predicted and experimental CS thresholds agree within 19% on average (normalized RMS error), which is smaller than the 27% modeling uncertainty. No significant difference was found between the modeling predictions and experiments (p < 0.05, paired t-test).

CONCLUSION

Predicted thresholds matched the experimental data within the modeling uncertainty, supporting the model validity. We believe that our modeling approach can be applied to study CS thresholds in humans for various gradient coils, body shapes/postures, and waveforms, which is difficult to do experimentally.

Polymorphic Ventricular Tachycardia Storm After Coronary Artery Bypass Graft Surgery: A Form of 'Angry Purkinje Syndrome'

BACKGROUND

Polymorphic ventricular tachycardia (PMVT) is a highly lethal arrhythmia which is commonly caused by acute myocardial ischaemia. PMVT mediated by short-coupled ventricular ectopy patients with ischaemic heart disease but in the absence of acute ischaemia may relate to transient peri-infarct Purkinje fibre irritability and has been termed 'Angry Purkinje Syndrome'.

METHODS

We present a case series of three patients with PMVT storm 3-5 days following coronary artery bypass graft surgery (CABG). In all three cases, recurrent episodes of PMVT were initiated by monomorphic ventricular ectopy with a short coupling interval. Acute coronary ischaemia was excluded in all three patients with a coronary angiogram and graft study. Two out of three of the patients commenced oral quinidine sulphate with subsequent rapid suppression of arrhythmia. Implantable cardiac defibrillators were implanted in all three patients and revealed no recurrence of PMVT following hospital discharge.

CONCLUSION

The Angry Purkinje Syndrome is a rare but important cause of ventricular tachycardia storm after CABG surgery and is mediated by short-coupled ventricular ectopy in the absence of acute myocardial ischaemia. This arrhythmia may be highly responsive to quinidine.

Species dependent differences in the inhibition of various potassium currents and in their effects on repolarization in cardiac ventricular muscle

Even though rodents are accessible model animals, their electrophysiological properties are deeply different from that of human, making the translation of rat studies to human rather difficult. We compared the mechanisms of ventricular repolarization in various animal models to those of human by measuring cardiac ventricular action potentials from ventricular papillary muscle preparations using conventional microelectrodes, and applying selective inhibitors of various potassium transmembrane ion currents. Inhibition of the IK1 current (10 µM barium chloride) significantly prolonged rat ventricular repolarization, but only slightly prolonged it in dog, and did not affect it in human. On the contrary, IKr inhibition (50 nM dofetilide) significantly prolonged repolarization in human, rabbit, and dog, but not in rat. Inhibition of the IKur current (1 µM XEN-D0101) only prolonged rat ventricular repolarization, and had no effect in human or dog. Inhibition of the IKs (500 nM HMR-1556) and Ito currents (100 µM chromanol-293B) elicited similar effects in all investigated species. We conclude that dog ventricular preparations have the strongest, and rat ventricular preparations have the weakest translational value in cardiac electrophysiological experiments.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Quinidine-responsive out-of-hospital polymorphic ventricular tachycardia in patients with coronary heart disease

Aims

We recently reported that patients with coronary artery disease (CAD) who develop polymorphic ventricular tachycardia (VT) during the healing phase of an acute coronary event, generally fail to respond to revascularization or standard antiarrhythmic therapy but respond immediately to quinidine therapy. Here, we describe that CAD patients presenting with out-of-hospital polymorphic VT without a recent coronary event or an obvious precipitating factor, also respond uniquely to quinidine therapy.

Methods and results

Retrospective study of patients with unheralded, mainly out-of-hospital, polymorphic VT related to CAD but without evidence of acute myocardial ischaemia. We identified 20 patients who developed polymorphic VT without precipitating factors. The polymorphic VT events were triggered by extrasystoles with short (376 ± 49 ms) coupling interval. Arrhythmic storms occurred in 70% patients. These arrhythmic storms were generally refractory to conventional antiarrhythmic therapy but invariably responded to quinidine therapy. Revascularization was antiarrhythmic in 3 patients despite the absent clinical or ECG signs of ischaemia. During long-term follow-up (range 2 months to 11 years), 3 (15%) of patients not receiving quinidine developed recurrent polymorphic VT. There were no recurrent arrhythmias during long-term quinidine therapy.

Conclusions

Patients with CAD may develop polymorphic VT in the absence of obvious acute ischaemia or apparent precipitating factors, presenting as out-of-hospital polymorphic VT with high risk of arrhythmic storms that respond uniquely to quinidine therapy.

Cardiac voltage-gated ion channels in safety pharmacology: Review of the landscape leading to the CiPA initiative

Voltage gated ion channels are central in defining the fundamental properties of the ventricular cardiac action potential (AP), and are also involved in the development of drug-induced arrhythmias. Many drugs can inhibit cardiac ion currents, including the Na⁺ current (I_Na), L-type Ca²⁺ current (Ica-L), and K⁺ currents (I_to, I_K1, I_Ks, and I_Kr), and thereby affect AP properties in a manner that can trigger or sustain cardiac arrhythmias. Since publication of ICH E14 and S7B over a decade ago, there has been a focus on drug effects on QT prolongation clinically, and on the rapidly activating delayed rectifier current (I_Kr), nonclinically, for evaluation of proarrhythmic risk. This focus on QT interval prolongation and a single ionic current likely impacted negatively some drugs that lack proarrhythmic liability in humans. To rectify this issue, the Comprehensive in vitro proarrhythmia assay (CiPA) initiative has been proposed to integrate drug effects on multiple cardiac ionic currents with in silico modelling of human ventricular action potentials, and in vitro data obtained from human stem cell-derived ventricular cardiomyocytes to estimate proarrhythmic risk of new drugs with improved accuracy. In this review, we present the physiological functions and the molecular basis of major cardiac ion channels that contribute to the ventricle AP, and discuss the CiPA paradigm in drug development.

Cardiac Purkinje fibers and arrhythmias; The GK Moe Award Lecture 2015

Purkinje fibers/cells continue to be a focus of arrhythmologists. Here we review several new ideas that have emerged in the literature and fold them into important new points. These points include the following: some proteins in Purkinje cells are specific to Purkinjes; pacemaker function in Purkinje may be similar to that of the sinus node cell; sink-source concerns about tracts/sheets of Purkinje fibers; role of Ito in arrhythmias; and genetic lesions in Purkinjes and their high impact on cardiac rhythm. Although new ideas about the remodeled Purkinje cell are not the focus of this review, one can easily imagine how Purkinjes and their function may be altered in diseased hearts.

The importance of Purkinje activation in long duration ventricular fibrillation

BACKGROUND

The mechanisms that maintain long duration ventricular fibrillation (LDVF) are unclear. The difference in distribution of the Purkinje system in dogs and pigs was explored to determine if Purkinje activation propagates to stimulate working myocardium (WM) during LDVF and WM pacing.

METHODS AND RESULTS

In-vivo extracellular recordings were made from 1044 intramural plunge and epicardial plaque electrodes in 6 pig and 6 dog hearts. Sinus activation propagated sequentially from the endocardium to the epicardium in dogs but not pigs. During epicardial pacing, activation propagated along the endocardium and traversed the LV wall almost parallel to the epicardium in dogs, but in pigs propagated away from the pacing site approximately perpendicular to the epicardium. After 1 minute of VF, activation rate near the endocardium was significantly faster than near the epicardium in dogs (P<0.01) but not pigs (P>0.05). From 2 to 10 minutes of LDVF, recordings exhibiting Purkinje activations were near the endocardium in dogs (P<0.01) but were scattered transmurally in pigs, and the WM activation rate in recordings in which Purkinje activations were present was significantly faster than the WM activation rate in recordings in which Purkinje activations were absent (P<0.01). In 10 isolated perfused dog hearts, the LV endocardium was exposed and 2 microelectrodes were inserted into Purkinje and adjacent myocardial cells. After 5 minutes of LDVF, mean Purkinje activation rate was significantly faster than mean WM activation rate (P<0.01).

CONCLUSION

These extracellular and intracellular findings about activation support the hypothesis that Purkinje activation propagates to stimulate WM during sinus rhythm, pacing, and LDVF.

DIFFERENCES IN IONIC CURRENTS BETWEEN CANINE MYOCARDIAL AND PURKINJE CELLS

An electrophysiological analysis of canine single ventricular myocardial (VM) and Purkinje (P) cells was carried out by means of whole cell voltage clamp method. The following results in VM versus P cells were obtained. I_Na3 was present, had a threshold negative to the fast activating-inactivating I_Na1, its slow inactivation was cut off by I_Na1, and contributed to Na⁺ influx at I_Na1 threshold. I_Na1 was smaller and had a less negative threshold. There was no comparable slowly inactivating I_Na2, accounting for the shorter action potential. Slope conductance at resting potential was about double and decreased to a minimum value at the larger and less negative I_K1 peak. The negative slope region of I-V relation was smaller during fast ramps and larger during slow ramps than in P cells, occurred in the voltage range of I_K1 block by Mg²⁺, was not affected by a lower V_h and TTX and was eliminated by Ba²⁺, in contrast to P cells. I_Ca was larger, peaked at positive potentials and was eliminated by Ni²⁺. I_to was much smaller, began at more positive values, was abolished by less negative V_h and by 4-aminopyridine, included a sustained current that 4-aminopyridine decreased but did not eliminate. Steeper ramps increased I_K1 peak as well as the fall in outward current during repolarization, consistent with a time-dependent block and unblock of I_K1 by polyamines. During repolarization, the positive slope region was consistently present and was similar in amplitude to I_K1 peak, whereas it was small or altogether missing in P cells. The total outward current at positive potentials comprised a larger I_K1 component whereas it included a larger I_to and sustained current in P cells. These and other results provide a better understanding of the mechanisms underlying the action potential of VM and P cells under normal and some abnormal (arrhythmias) conditions.

Post-partum variation in the expression of paternal care is unrelated to urinary steroid metabolites in marmoset fathers

The organization and activation of maternal care are known to be highly regulated by hormones and there is growing evidence that expression of paternal care is also related to endocrine substrates. We examined the relationship between paternal behavior and steroid hormones in marmoset fathers (Callithrix geoffroyi) and evaluated whether hormone-paternal behavior relationships were altered by previous offspring-care experience in males. Based on previous findings, we predicted that testosterone, estradiol, and cortisol would decrease following the birth of offspring and would be lowest during the period of maximal infant carrying. Furthermore, we predicted that post-partum changes in carrying effort and hormone levels would be influenced by the level of offspring-care experience. Carrying effort and other paternal care behaviors underwent temporal changes over the post-partum period, but these patterns were not related to variation in hormone concentrations over the same period. There was a limited effect of offspring-care experience on hormone concentrations, but experience was found to play a role in the expression of paternal care, with experienced fathers engaging in significantly more infant allogrooming than inexperienced fathers. Furthermore, inexperienced fathers increased the frequency of food sharing in response to infant begging across the post-partum period, while experienced fathers displayed consistently low levels. We posit that a combination of experiential factors and an increased role for alloparents in offspring-care led to these changes. However, it appears that hormonal changes may not influence paternal responsiveness in white-faced marmoset fathers and that hormone-paternal behavior relationships are not critically dependent on a male's previous offspring-care experience.

Unique cardiac Purkinje fiber transient outward current β-subunit composition: a potential molecular link to idiopathic ventricular fibrillation

RATIONALE

A chromosomal haplotype producing cardiac overexpression of dipeptidyl peptidase-like protein-6 (DPP6) causes familial idiopathic ventricular fibrillation. The molecular basis of transient outward current (I(to)) in Purkinje fibers (PFs) is poorly understood. We hypothesized that DPP6 contributes to PF I(to) and that its overexpression might specifically alter PF I(to) properties and repolarization.

OBJECTIVE

To assess the potential role of DPP6 in PF I(to).

METHODS AND RESULTS

Clinical data in 5 idiopathic ventricular fibrillation patients suggested arrhythmia origin in the PF-conducting system. PF and ventricular muscle I(to) had similar density, but PF I(to) differed from ventricular muscle in having tetraethylammonium sensitivity and slower recovery. DPP6 overexpression significantly increased, whereas DPP6 knockdown reduced, I(to) density and tetraethylammonium sensitivity in canine PF but not in ventricular muscle cells. The K(+)-channel interacting β-subunit K(+)-channel interacting protein type-2, essential for normal expression of I(to) in ventricular muscle, was weakly expressed in human PFs, whereas DPP6 and frequenin (neuronal calcium sensor-1) were enriched. Heterologous expression of Kv4.3 in Chinese hamster ovary cells produced small I(to); I(to) amplitude was greatly enhanced by coexpression with K(+)-channel interacting protein type-2 or DPP6. Coexpression of DPP6 with Kv4.3 and K(+)-channel interacting protein type-2 failed to alter I(to) compared with Kv4.3/K(+)-channel interacting protein type-2 alone, but DPP6 expression with Kv4.3 and neuronal calcium sensor-1 (to mimic PF I(to) composition) greatly enhanced I(to) compared with Kv4.3/neuronal calcium sensor-1 and recapitulated characteristic PF kinetic/pharmacological properties. A mathematical model of cardiac PF action potentials showed that I(to) enhancement can greatly accelerate PF repolarization.

CONCLUSIONS

These results point to a previously unknown central role of DPP6 in PF I(to), with DPP6 gain of function selectively enhancing PF current, and suggest that a DPP6-mediated PF early-repolarization syndrome might be a novel molecular paradigm for some forms of idiopathic ventricular fibrillation.

Role of the transient outward potassium current in the genesis of early afterdepolarizations in cardiac cells

AIMS

The transient outward potassium current (I(to)) plays important roles in action potential (AP) morphology and dynamics; however, its role in the genesis of early afterdepolarizations (EADs) is not well understood. We aimed to study the effects and mechanisms of I(to) on EAD genesis in cardiac cells using combined experimental and computational approaches.

METHODS AND RESULTS

We first carried out patch-clamp experiments in isolated rabbit ventricular myocytes exposed to H(2)O(2) (0.2 or 1 mM), in which EADs were induced at a slow pacing rate. EADs were eliminated by either increasing the pacing rate or blocking I(to) with 2 mM 4-aminopyridine. In addition to enhancing the L-type calcium current (I(Ca,L)) and the late sodium current, H(2)O(2) also increased the conductance, slowed inactivation, and accelerated recovery from the inactivation of I(to). Computer simulations showed that I(to) promoted EADs under the condition of reduced repolarization reserve, consistent with the experimental observations. However, EADs were only promoted in the intermediate ranges of the I(to) conductance and the inactivation time constant. The underlying mechanism is that I(to) lowers the AP plateau voltage into the range at which the time-dependent potassium current (namely I(Ks)) activation is further slowed and I(Ca,L) is available for reactivation, leading to voltage oscillations to manifest EADs. Further experimental studies in cardiac cells of other species validated the theoretical predictions.

CONCLUSION

In cardiac cells, I(to), with a proper conductance and inactivation speed, potentiates EADs by setting the AP plateau into the voltage range where I(Ca,L) reactivation is facilitated and I(Ks) activation is slowed.

The safety profile of dalfampridine extended release in multiple sclerosis clinical trials

BACKGROUND

Dalfampridine (fampridine outside the United States) is a broad-spectrum potassium channel blocker. Dalfampridine extended-release tablets have been approved by the US Food and Drug Administration to improve walking in patients with multiple sclerosis (MS).

OBJECTIVE

The objective of this article is to review the safety profile of dalfampridine extended-release tablets with respect to its expected use in patients with MS.

METHODS

We reviewed published data relevant to patient safety profiles based on searches of articles in PubMed published up to December 31, 2010, using the search terms fampridine OR dalfampridine OR 4-aminopyridine AND (multiple sclerosis) in combination with toxicity, safety, clinical trial, pharmacokinetics, and seizures. These searches were supplemented with data derived from the approved package insert and relevant sections of the New Drug Application (22-250) as submitted to the US Food and Drug Administration.

RESULTS

The literature searches returned 58 unique citations, of which 26 were considered relevant for characterizing the safety profile of dalfampridine; excluded citations were as follows: reviews (19), evaluation of 3,4-diaminopyridine (4), intravenous dosing (2), inadequate information on patient doses (2), preclinical models (2), and "other" (3). Dalfampridine is nearly completely (approximately 96%) eliminated unchanged in urine, with limited transformation to 2 inactive metabolites and low risk for interaction with drugs metabolized by hepatic P450 cytochromes. However, in patients with renal impairment (creatinine clearance [CrCl], ≤80 mL/min), mean peak plasma concentrations were 68%-101% higher and apparent clearance was 43%-73% lower relative to those without impairment, precluding dalfampridine use in patients with moderate (CrCl, 30-50 mL/min) or severe renal impairment (CrCl, <30 mL/min). Dalfampridine has a narrow therapeutic range. At the therapeutic dose of 10 mg twice daily, adverse events were generally mild to moderate and, consistent with the mechanism of action of dalfampridine, were primarily related to stimulatory effects on the nervous system. A thorough QT study suggested a low risk of induction of QT prolongation and associated cardiac arrhythmias in healthy individuals at therapeutic (10 mg, twice daily) or supratherapeutic (30 mg, twice daily) doses. Although the incidence of seizures was dose related, data from the clinical trials of dalfampridine extended-release tablets suggest that the risk of seizure at the therapeutic dose, in patients with no history of seizure, is not likely to be higher than background rates in MS.

CONCLUSION

In patients with MS, dalfampridine has a narrow therapeutic range but an acceptable safety profile when used at the therapeutic dose of 10 mg twice daily.

Analysis of the contribution of I(to) to repolarization in canine ventricular myocardium

BACKGROUND AND PURPOSE

The contribution of the transient outward potassium current (I(to)) to ventricular repolarization is controversial as it depends on the experimental conditions, the region of myocardium and the species studied. The aim of the present study was therefore to characterize I(to) and estimate its contribution to repolarization reserve in canine ventricular myocardium.

EXPERIMENTAL APPROACH

Ion currents were recorded using conventional whole-cell voltage clamp and action potential voltage clamp techniques in canine isolated ventricular cells. Action potentials were recorded from canine ventricular preparations using microelectrodes. The contribution of I(to) to repolarization was studied using 100 µM chromanol 293B in the presence of 0.5 µM HMR 1556, which fully blocks I(Ks).

KEY RESULTS

The high concentration of chromanol 293B used effectively suppressed I(to) without affecting other repolarizing K(+) currents (I(K1), I(Kr), I(p)). Action potential clamp experiments revealed a slowly inactivating and a 'late' chromanol-sensitive current component occurring during the action potential plateau. Action potentials were significantly lengthened by chromanol 293B in the presence of HMR 1556. This lengthening effect induced by I(to) inhibition was found to be reverse rate-dependent. It was significantly augmented after additional attenuation of repolarization reserve by 0.1 µM dofetilide and this caused the occurrence of early afterdepolarizations. The results were confirmed by computer simulation.

CONCLUSIONS AND IMPLICATIONS

The results indicate that I(to) is involved in regulating repolarization in canine ventricular myocardium and that it contributes significantly to the repolarization reserve. Therefore, blockade of I(to) may enhance pro-arrhythmic risk.

A model of canine purkinje cell electrophysiology and Ca(2+) cycling: rate dependence, triggered activity, and comparison to ventricular myocytes

Purkinje cells (Pcell) are characterized by different electrophysiological properties and Ca(2+) cycling processes than ventricular myocytes (Vcell) and are frequently involved in ventricular arrhythmias. Yet, the mechanistic basis for their arrhythmic vulnerability is not completely understood. The objectives were to: (1) characterize Pcell electrophysiology, Ca(2+) cycling, and their rate dependence; (2) investigate mechanisms underlying Pcell arrhythmogenicity; and compare Pcell and Vcell electrophysiology, Ca(2+) cycling, and arrhythmic properties. We developed a new mathematical model of Pcell. The Ca(2+) subsystem includes spatial organization and receptors distribution unique to Pcell. Results were: (1) in Pcell and Vcell, Na(+) accumulation via its augmentation of repolarizing I(NaK) dominates action potential duration adaptation and, in Pcell, I(NaL) contributes additional action potential duration shortening at short cycle length; (2) steep Pcell restitution is attributable to slow recovery of I(NaL); (3) biphasic Ca(2+) transients of Pcell reflect the delay between Ca(2+) release from junctional sarcoplasmic reticulum and corbular sarcoplasmic reticulum; (4) Pcell Ca(2+) alternans, unlike Vcell, can develop without inducing action potential alternans; (5) Pcell action potential alternans develops at a shorter cycle length than Vcell, with increased subcellular heterogeneity of Ca(2+) cycling attributable to refractoriness of Ca(2+) release from corbular sarcoplasmic reticulum and junctional sarcoplasmic reticulum; (6) greater Pcell vulnerability to delayed afterdepolarizations is attributable to higher sarcoplasmic reticulum Ca(2+) content and ionic currents that reduce excitation threshold and promote triggered activity; and (7) early after depolarizations generation in Pcell is mostly attributable to reactivation of I(NaL2), whereas I(CaL) plays this role in Vcell. Steeper rate dependence of action potential and Ca(2+) transients, central peripheral heterogeneity of Ca(2+) cycling, and distinct ion channel profile underlie greater arrhythmic vulnerability of Pcell compared to Vcell.

Effect of 4-Aminopyridine on Action Potential Parameters in Isolated Dog Purkinje Fibers

Introduction

4-Aminopyridine (fampridine), a potassium channel blocker, has demonstrated efficacy in improving lower extremity strength and walking speed in patients with multiple sclerosis. Since in vitro electrophysiologic studies are recommended for evaluating a drug's potential to prolong the QT interval and induce such cardiac arrhythmias as Torsades de Pointes, we examined the electrophysiologic effects of 4-aminopyridine (0.5, 5.0, 50, and 500 µM) on isolated canine Purkinje fibers.

Methods

Microelectrodes monitored the resting membrane potential, overshoot, amplitude of action potential (AP), and maximal rate of depolarization of the AP upstroke in Purkinje fibers stimulated at 0.5 and 1.0 Hz.

Results

None of the above variables were altered in the presence of 4-aminopyridine. The AP duration at 30%, 50%, and 90% repolarization was also monitored, with only the 500-µM concentration at the 1.0-Hz frequency significantly increasing these values with respect to baseline (P < 0.05). However, the small sample size (N = 4) was small. The proportional increases, and their 95% confidence intervals, were 90.8% (−36.4%, 218.0%), 25.8% (11.9%, 39.7%), and 22.0% (14.9%, 29.1%) for APD 30%, 50%, and 90% repolarization, respectively. Reverse rate dependence was not observed, suggesting inhibition of ion channels other than those contributing to QT interval prolongation.

Assessment of the cardiac safety of fampridine-SR sustained-release tablets in a thorough QT/QTc evaluation at therapeutic and supratherapeutic doses in healthy individuals

OBJECTIVE

To characterize the effects of a sustained-release formulation of fampridine (fampridine-SR) on QT interval in healthy subjects.

METHODS

In a double-blind, double-dummy trial, healthy subjects were randomized to 5 days treatment with fampridine-SR at therapeutic (10 mg twice daily) or supratherapeutic (30 mg twice daily) doses, placebo or moxifloxacin (400 mg on treatment day 5). Digital 12-lead electrocardiograms were recorded before treatment and on day 5; blood samples determined fampridine concentrations. Central tendency analysis determined whether the upper limit of the CI for the QT (individual-corrected QT; QTcI) interval change exceeded 10 ms. Outlier analysis determined new-onset QT (corrected QT; QTc) intervals; maximum change in QTc from baseline of 30 - 60 ms and maximum change from baseline >or= 60 ms. The relationship between pharmacokinetic parameters and QTcI values is explored.

RESULTS

Moxifloxacin was associated with a QTcI interval increase > 5 ms at 7 time points; no increase was observed with either dose of fampridine-SR; there were no fampridine outliers. Pharmacokinetic evaluation failed to find dose-dependent cardiac effects. Fampridine was well tolerated, with a higher frequency of adverse events at the supratherapeutic dose.

CONCLUSION

This study showed that fampridine-SR at therapeutic and supratherapeutic doses was not associated with QT prolongation in healthy subjects.

Purkinje-mediated effects in the response of quiescent ventricles to defibrillation shocks

In normal cardiac function, orderly activation of the heart is facilitated by the Purkinje system (PS), a specialized network of fast-conducting fibers that lines the ventricles. Its role during ventricular defibrillation remains unelucidated. Physical characteristics of the PS make it a poor candidate for direct electrical observation using contemporary experimental techniques. This study uses a computer modeling approach to assess contributions by the PS to the response to electrical stimulation. Normal sinus rhythm was simulated and epicardial breakthrough sites were distributed in a manner consistent with experimental results. Defibrillation shocks of several strengths and orientations were applied to quiescent ventricles, with and without PS, and electrical activation was analyzed. All shocks induced local polarizations in PS branches parallel to the field, which led to the rapid spread of excitation through the network. This produced early activations at myocardial sites where tissue was unexcited by the shock and coupled to the PS. Shocks along the apico-basal axis of the heart resulted in a significant abbreviation of activation time when the PS was present; these shocks are of particular interest because the fields generated by internal cardioverter defibrillators tend to have a strong component in the same direction. The extent of PS-induced changes, both temporal and spatial, was constrained by the amount of shock-activated myocardium. Increasing field strength decreased the transmission delay between PS and ventricular tissue at Purkinje-myocardial junctions (PMJs), but this did not have a major effect on the organ-level response. Weaker shocks directly affect a smaller volume of myocardial tissue but easily excite the PS, which makes the PS contribution to far field excitation more substantial than for stronger shocks.

Mathematical models of the electrical action potential of Purkinje fibre cells

Early development of ionic models for cardiac myocytes, from the pioneering modification of the Hodgkin-Huxley giant squid axon model by Noble to the iconic DiFrancesco-Noble model integrating voltage-gated ionic currents, ion pumps and exchangers, Ca(2+) sequestration and Ca(2+)-induced Ca(2+) release, provided a general description for a mammalian Purkinje fibre (PF) and the framework for modern cardiac models. In the past two decades, development has focused on tissue-specific models with an emphasis on the sino-atrial (SA) node, atria and ventricles, while the PFs have largely been neglected. However, achieving the ultimate goal of creating a virtual human heart will require detailed models of all distinctive regions of the cardiac conduction system, including the PFs, which play an important role in conducting cardiac excitation and ensuring the synchronized timing and sequencing of ventricular contraction. In this paper, we present details of our newly developed model for the human PF cell including validation against experimental data. Ionic mechanisms underlying the heterogeneity between the PF and ventricular action potentials in humans and other species are analysed. The newly developed PF cell model adds a new member to the family of human cardiac cell models developed previously for the SA node, atrial and ventricular cells, which can be incorporated into an anatomical model of the human heart with details of its electrophysiological heterogeneity and anatomical complexity.

The Purkinje cell; 2008 style

Cardiac Purkinje fibers, due to their unique anatomical location, cell structure and electrophysiologic characteristics, play an important role in cardiac conduction and arrhythmogenesis. Purkinje cell action potentials are longer than their ventricular counterpart, and display two levels of resting potential. Purkinje cells provide for rapid propagation of the cardiac impulse to ventricular cells and have pacemaker and triggered activity, which differs from ventricular cells. Additionally, a unique intracellular Ca2+ release coordination has been revealed recently for the normal Purkinje cell. However, since the isolation of single Purkinje cells is difficult, particularly in small animals, research using Purkinje cells has been restricted. This review concentrates on comparison of Purkinje and ventricular cells in the morphology of the action potential, ionic channel function and molecular determinants by summarizing our present day knowledge of Purkinje cells.

Transient outward potassium current, 'Ito', phenotypes in the mammalian left ventricle: underlying molecular, cellular and biophysical mechanisms

At least two functionally distinct transient outward K(+) current (I(to)) phenotypes can exist across the free wall of the left ventricle (LV). Based upon their voltage-dependent kinetics of recovery from inactivation, these two phenotypes are designated 'I(to,fast)' (recovery time constants on the order of tens of milliseconds) and 'I(to,slow)' (recovery time constants on the order of thousands of milliseconds). Depending upon species, either I(to,fast), I(to,slow) or both current phenotypes may be expressed in the LV free wall. The expression gradients of these two I(to) phenotypes across the LV free wall are typically heterogeneous and, depending upon species, may consist of functional phenotypic gradients of both I(to,fast) and I(to,slow) and/or density gradients of either phenotype. We review the present evidence (molecular, biophysical, electrophysiological and pharmacological) for Kv4.2/4.3 alpha subunits underlying LV I(to,fast) and Kv1.4 alpha subunits underlying LV I(to,slow) and speculate upon the potential roles of each of these currents in determining frequency-dependent action potential characteristics of LV subepicardial versus subendocardial myocytes in different species. We also review the possible functional implications of (i) ancillary subunits that regulate Kv1.4 and Kv4.2/4.3 (Kvbeta subunits, DPPs), (ii) KChIP2 isoforms, (iii) spider toxin-mediated block of Kv4.2/4.3 (Heteropoda toxins, phrixotoxins), and (iv) potential mechanisms of modulation of I(to,fast) and I(to,slow) by cellular redox state, [Ca(2)(+)](i) and kinase-mediated phosphorylation. I(to) phenotypic activation and state-dependent gating models and molecular structure-function relationships are also discussed.

Phenotypic differences in transient outward K+ current of human and canine ventricular myocytes: insights into molecular composition of ventricular Ito

The Ca(2+)-independent transient outward K(+) current (I(to)) plays an important electrophysiological role in normal and diseased hearts. However, its contribution to ventricular repolarization remains controversial because of differences in its phenotypic expression and function across species. The dog, a frequently used model of human cardiac disease, exhibits altered functional expression of I(to). To better understand the relevance of electrical remodeling in dogs to humans, we studied the phenotypic differences in ventricular I(to) of both species with electrophysiological, pharmacological, and protein-chemical techniques. Several notable distinctions were elucidated, including slower current decay, more rapid recovery from inactivation, and a depolarizing shift of steady-state inactivation in human vs. canine I(to). Whereas recovery from inactivation of human I(to) followed a monoexponential time course, canine I(to) recovered with biexponential kinetics. Pharmacological sensitivity to flecainide was markedly greater in human than canine I(to), and exposure to oxidative stress did not alter the inactivation kinetics of I(to) in either species. Western blot analysis revealed immunoreactive bands specific for Kv4.3, Kv1.4, and Kv channel-interacting protein (KChIP)2 in dog and human, but with notable differences in band sizes across species. We report for the first time major variations in phenotypic properties of human and canine ventricular I(to) despite the presence of the same subunit proteins in both species. These data suggest that differences in electrophysiological and pharmacological properties of I(to) between humans and dogs are not caused by differential expression of the K channel subunit genes thought to encode I(to), but rather may arise from differences in molecular structure and/or posttranslational modification of these subunits.

Properties of potassium currents in Purkinje cells of failing human hearts

Cardiac Purkinje fibers play an important role in cardiac arrhythmias, but no information is available about ionic currents in human cardiac Purkinje cells (PCs). PCs and midmyocardial ventricular myocytes (VMs) were isolated from explanted human hearts. K(+) currents were evaluated at 37 degrees C with whole cell patch clamp. PCs had clear inward rectifier K(+) current (I(K1)), with a density not significantly different from VMs between -110 and -20 mV. A Cs(+)-sensitive, time-dependent hyperpolarization-activated current was measurable negative to -60 mV. Transient outward current (I(to)) density was smaller, but end pulse sustained current (I(sus)) was larger, in PCs vs. VMs. I(to) recovery was substantially slower in PCs, leading to strong frequency dependence. Unlike VM I(to), which was unaffected by 10 mM tetraethylammonium, Purkinje I(to) was strongly inhibited by tetraethylammonium, and Purkinje I(to) was 10-fold more sensitive to 4-aminopyridine than VM. PC I(sus) was also reduced strongly by 10 mM tetraethylammonium. In conclusion, human PCs demonstrate a prominent I(K1), a time-dependent hyperpolarization-activated current, and an I(to) with pharmacological sensitivity and recovery kinetics different from those in the atrium or ventricle and compatible with a different molecular basis.

Comparison of ion-channel subunit expression in canine cardiac Purkinje fibers and ventricular muscle

Although Purkinje fibers (PFs) play an important role in cardiac electrophysiology, almost nothing is known about the expression of ion-channel subunits in PFs. We applied competitive reverse transcription-polymerase chain reaction, Western blotting, and immunocytochemistry to compare the expression of ion-channel subunit mRNA and protein in canine PFs versus ventricular muscle (VM). For transient outward current-related subunits, Kv4.2 was not detected, and Kv1.4 expression was extremely low. Kv4.3 expression was of the same order for VM and PFs. The tetraethylammonium chloride-sensitive subunit Kv3.4 was expressed much more strongly in PFs than in VM, and Kv channel-interacting protein transcript expression was 25-fold stronger in VM than in PFs. For delayed rectifiers, ERG and KvLQT1 expression was lower in PFs at both mRNA and protein levels. Although minK transcripts were more numerous in PFs, minK protein was significantly more strongly expressed in VM. L-type Ca2+ current alpha-subunit (Ca(V)1.2) and Na+-Ca2+ exchanger proteins were more strongly expressed in VM than in PFs. For T-type Ca2+ current, Ca(V)3.1, Ca(V)3.2, and Ca(V)3.3 transcripts were all more strongly expressed in PFs. For the nonselective cation current, hyperpolarization-activated cation channel 1 (HCN1) expression was subquantifiable, HCN2 transcript expression was comparable in PFs and VM, and HCN4 mRNA expression was strong in PFs but below the detection threshold in VM. HCN2 and HCN4 protein expression was much stronger in PFs than in VM. We conclude that ion-channel subunit expression in PFs differs from that in VM in ways that are consistent with, and shed light on the molecular basis of, well-recognized fundamental PF ionic properties.

Endogenous Kv channels in human embryonic kidney (HEK-293) cells

The human embryonic kidney cells (HEK-293) have been widely used as one mammalian expression system in the study of voltage-gated K+ (Kv) channels. Understanding the endogenous Kv channels in these cells is the prerequisite for the characterization of the heterogeneously expressed Kv channels in these cells. In the present study we screened the transcriptional expression of different Kv genes in HEK-293 cells using reverse transcribed DNApolymerase chain reaction (RT-PCR) method. Among 16 Kv genes examined in native HEK-293 cells 10 Kv genes were reproducibly amplified, including those Kv a subunits encoding for the delayed rectifier (IK) [Kv1.1, Kv1.2, Kv1.3, Kv1.6, and Kv3.1], and for the transient outward Kv channels (IA) [Kv1.4, Kv3.3, Kv3.4, and Kv4.1] as well as a Kvbeta2 subunit. The whole-cell outward rectifier IK currents in the native HEK-293 cells were recorded (203 +/- 13 pA at +30 mV, n = 82) with the patch-clamp technique. In about 42% of the examined cells, IA coexisted with IK currents. IK currents were inhibited by tetraethylammonium chloride (TEA) at 1 and 10 mM by 39.5 and 48.4%, respectively. A 39.6% inhibition of IK currents was also observed in the presence of4-aminopyridine (4-AP, 5 mM). Interestingly, both TEAand 4-AP also inhibited IA currents. 4-acelamido-4'-isothiocyanalostilbene-2, 2'-disulfonic acid (1 mM), a Cl- channel blocker, had no effect on the endogenous outward currents. We concluded that multiple endogenous Kv genes were expressed in native HEK-293 cells, which possessed significant endogenous IK and IA currents with unique pharmacological properties.

Differential distribution of cardiac ion channel expression as a basis for regional specialization in electrical function

The cardiac electrical system is designed to ensure the appropriate rate and timing of contraction in all regions of the heart, which are essential for effective cardiac function. Well-controlled cardiac electrical activity depends on specialized properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Cardiac electrical specialization was first recognized in the mid 1800s, but over the past 15 years, an enormous amount has been learned about how specialization is achieved by differential expression of cardiac ion channels. More recently, many aspects of the molecular basis have been revealed. Although the field is potentially vast, an appreciation of key elements is essential for any clinician or researcher wishing to understand modern cardiac electrophysiology. This article reviews the major regionally determined features of cardiac electrical function, discusses underlying ionic bases, and summarizes present knowledge of ion channel subunit distribution in relation to functional specialization.

Ionic remodeling of cardiac Purkinje cells by congestive heart failure

BACKGROUND

Cardiac Purkinje cells (PCs) are important for the generation of triggered arrhythmias, particularly in association with abnormal repolarization. The effects of congestive heart failure (CHF) on the ionic properties of PCs are unknown.

METHODS AND RESULTS

PCs were isolated from false tendons of control dogs and dogs with ventricular tachypacing-induced CHF. CHF PCs were hypertrophied (capacitance, mean+/-SEM, 149+/-4 pF, n=130; versus 128+/-3 pF, n=150, control; P<0.001). Transient outward current density was reduced in CHF PCs without change in voltage dependence or kinetics. CHF also reduced inward-rectifier current density, with no change in form of the current-voltage relationship. Densities of L- and T-type calcium, rapid and slow delayed rectifier, and Na(+)-Ca(2+) exchange currents were unaltered by CHF, but L-type calcium current inactivation was slowed at positive potentials. Purkinje fiber action potentials from CHF dogs showed decreased phase 1 amplitudes and elevated plateau voltages and demonstrated twice as much prolongation on exposure to the rapid delayed rectifier blocker E-4031 as control Purkinje fibers.

CONCLUSIONS

CHF causes remodeling of important K(+) and Ca(2+) currents in cardiac PCs, decreasing repolarization reserve and causing an exaggerated repolarization delay in response to a class III drug. These results have important potential implications regarding ventricular arrhythmogenesis, particularly related to triggered activity in PCs, in patients with CHF.

Differential contribution of sialic acid to the function of repolarizing K(+) currents in ventricular myocytes

We investigated the contribution of sialic acid residues to the K(+) currents involved in the repolarization of mouse ventricular myocytes. Ventricular K(+) currents had a rapidly inactivating component followed by slowly decaying and sustained components. This current was produced by the summation of three distinct currents: I(to), which contributed to the transient component; I(ss), which contributed to the sustained component; and I(K,slow), which contributed to both components. Incubation of ventricular myocytes with the sialidase neuraminidase reduced the amplitude of I(to) without altering I(K,slow) and I(ss). We found that the reduction in I(to) amplitude resulted from a depolarizing shift in the voltage of activation and a reduction in the conductance of I(to). Expression of Kv4.3 channels, a major contributor to I(to) in the ventricle, in a sialylation-deficient Chinese hamster ovary cell line (lec2) mimicked the effects of neuraminidase on the ventricular I(to). Furthermore, we showed that sialylated glycolipids have little effect on the voltage dependence of I(to). Finally, consistent with its actions on I(to), neuraminidase produced an increase in the duration of the action potential of ventricular myocytes and the frequency of early afterdepolarizations. We conclude that sialylation of the proteins forming Kv4 channels is important in determining the voltage dependence and conductance of I(to) and that incomplete glycosylation of these channels could lead to arrhythmias.

The molecular physiology of the cardiac transient outward potassium current (I(to)) in normal and diseased myocardium

G. Y. Oudit, Z. Kassiri, R. Sah, R. J. Ramirez, C. Zobel and P. H. Backx. The Molecular Physiology of the Cardiac Transient Outward Potassium Current (I(to)) in Normal and Diseased Myocardium. Journal of Molecular and Cellular Cardiology (2001) 33, 851-872. The Ca(2+)-independent transient outward potassium current (I(to)) plays an important role in early repolarization of the cardiac action potential. I(to)has been clearly demonstrated in myocytes from different cardiac regions and species. Two kinetic variants of cardiac I(to)have been identified: fast I(to), called I(to,f), and slow I(to), called I(to,s). Recent findings suggest that I(to,f)is formed by assembly of K(v4.2)and/or K(v4.3)alpha pore-forming voltage-gated subunits while I(to,s)is comprised of K(v1.4)and possibly K(v1.7)subunits. In addition, several regulatory subunits and pathways modulating the level and biophysical properties of cardiac I(to)have been identified. Experimental findings and data from computer modeling of cardiac action potentials have conclusively established an important physiological role of I(to)in rodents, with its role in large mammals being less well defined due to complex interplay between a multitude of cardiac ionic currents. A central and consistent electrophysiological change in cardiac disease is the reduction in I(to)density with a loss of heterogeneity of I(to)expression and associated action potential prolongation. Alterations of I(to)in rodent cardiac disease have been linked to repolarization abnormalities and alterations in intracellular Ca(2+)homeostasis, while in larger mammals the link with functional changes is far less certain. We review the current literature on the molecular basis for cardiac I(to)and the functional consequences of changes in I(to)that occur in cardiovascular disease.