Purkinje Fibres Structure and Function PDF

Preview

Summary

Dr. Agyei Boateng Stephen...

Description

CELL TO BEDSIDE

Cardiac Purkinje cells Penelope A. Boyden, PhD,* Masanori Hirose, MD, PhD, † Wen Dun, MD, PhD* From the *Department of Pharmacology, Center for Molecular Therapeutics, Columbia University, New York, New York, and †Department of Cardiology, Tohoku University School of Medicine, Sendai, Japan. Purkinje cells are specialized for rapid propagation in the heart. Furthermore, Purkinje fibers as the source as well as the perpetuator of arrhythmias is a familiar finding. This is not surprising considering their location in the heart and their unique cell ultrastructure, cell electrophysiology, and mode of excitation– contraction coupling. This review touches on each of these points as we outline what is known today about Purkinje fibers/cells.

ABBREVIATIONS AP ⫽ action potential; APD90 ⫽ action potential duration at 90% repolarization; AV ⫽ atrioventricular; DAD ⫽ delayed afterdepolarization; EAD ⫽ early afterdepolarization; IZPC ⫽ Purkinje cell from 48-hour infarcted heart; RyR ⫽ ryanodine receptor; SR ⫽ sarcoplasmic reticulum

KEYWORDS Arrhythmia; Calcium; Ion channel; Pacemaker activity; Purkinje cell

(Heart Rhythm 2010;7:127–135) © 2010 Heart Rhythm Society. Published by Elsevier Inc. All rights reserved.

Introduction

Many ventricular arrhythmias are initiated in the Purkinje fiber conduction system (Figure 2).4 –7 Both reentrant’ triggered and enhanced automatic rhythms can arise from the Purkinje fiber network in the presence of acquired disease or gene-based ion channelopathies. For instance, in the presence of valvular disease, dilated cardiomyopathy, and severe left ventricular dysfunction, a macroreentrant circuit through bundle branches with slowed conduction has been described.8 Bundle branch reentrant tachyarrhythmias (cycle length 300 ms) also can occur in patients with heart disease. Here the reentrant circuit is due to impulse propagation up one bundle and down the other, giving rise to a QRS configuration seen during sinus rhythm. For some intrafascicular verapamil-sensitive tachycardias, reentry in Purkinje fiber bundles is thought to be the mechanism. On the other hand, polymorphic ventricular tachycardias in patients with heart disease can be due to several mechanisms, but triggered beats preceding some of those reentrant tachycardias can arise from the Purkinje network. In the case of the initiating beats of catecholaminergic polymorphic ventricular tachycardia in patients, Purkinje cells with abnormal Ca2⫹ cycling most likely underlie the varying QRS morphologies, as shown in a mouse model of catecholaminergic polymorphic ventricular tachycardia.9 The same may be true for initiating arrhythmic beats in patients with long QT syndrome. In one study of post myocardial infarction patients with rapid polymorphic ventricular tachycardias, ablation of Purkinje regions eliminated premature depolarizations and the 10 arrhythmias. Even earlier mapping studies had implicated a role of Purkinje fibers in the initiation and maintenance of 11 ventricular fibrillation in patients with long QT syndrome, 12 11 ischemia, and Brugada syndrome. Mapping experiments

Johannes (Jan) Purkinje was a Czech phenomenologist who in the 19th century carefully described the now famous 1 subendocardial Purkinje fibers of the heart. At that time, he was “inclined to regard this new tissue as cartilage.” Sixty 2 years later, Tawara, in describing the connections of the atrioventricular (AV) node, showed that the connection between the AV bundle and Purkinje fibers was integral to electrical impulse propagation in the heart. The Purkinje fiber network is large, with free-running fibers (false tendons) as well as a subendocardial network. In all species, this highly specialized cell network provides a vital role in conduction from the AV downward to the endocardial ventricular muscle. In situ bipolar recordings of the His Bundle were first performed in 1959.3 At that time, it was shown that the sharp rapid deflection of the His electrogram followed atrial activity by 20 to 30 ms and in normal sinus rhythm preceded ventricular activity (Figure 1A). Conduction through the His bundle (conduction velocity 1–3 m/s) and bundle branches occurs during the isoelectric interval between the end of the P wave and the beginning of the QRS complex. Conduction through His Purkinje cells is little affected by vagal stimulation, epinephrine, or removal of stellate ganglia.

Supported by Grant HL67449 from the National Heart, Lung, and Blood Institute. Address reprint requests and correspondence: Dr. Penelope A. Boyden, Department of Pharmacology, Columbia College of Physicians and Surgeons, 630 West 168th Street, New York, New York 10032. E-mail address: [email protected]. (Received May 18, 2009; accepted September 9, 2009.)

1547-5271/$ -see front matter © 2010 Heart Rhythm Society. Published by Elsevier Inc. All rights reserved.

doi:10.1016/j.hrthm.2009.09.017

128

Heart Rhythm, Vol 7, No 1, January 2010 emphasized these early findings, that is, Purkinje cells are quite different from ventricular cells at both the histologic and ultrastructural levels. Histologically, Purkinje cells stain lightly, presumably due to the reduced, but still significant, myofibrillar content and enhanced glycogen. Electron microscopic studies show that Purkinje cells lack t-tubules and the all important core dyad.16 Recent immunostaining studies have confirmed distinguishable Purkinje cell-to-cell junctions and the existence of the connexin40 (Cx40) protein as an important Purkinje connexin isoform.17 All of these features help to dictate the Purkinje fiber’s specialized function in the heart.

Cell electrophysiology Action potentials

Figure 1 A: Bipolar recordings from a specialized conducting system at various locations (1– 6) in the in situ canine heart. Upper electrode is fixed on the His-Purkinje bundle. Note H deflection is His activation. Lower electrode records at sites from right bundle branch (1) to left anterior false tendon (6). P indicates activation of Purkinje fibers. Time lines ⫽ 40 ms.3 B: Transmembrane action potential of kid Purkinje fiber during spontaneous activity. Numbers indicate value of membrane resistance during electrical activity.73

using an animal model of nonischemic cardiomyopathy also have implicated a role for the Purkinje fiber network in ventricular tachycardias,13 similar to that shown to occur in 14 patients. Thus, Purkinje fibers as the source as well as the perpetuator of arrhythmias is a familiar finding. This is not surprising considering their location in the heart and their unique cell ultrastructure, cell electrophysiology, and mode of excitation– contraction coupling. This review touches on each of these points as we outline what is known today about Purkinje fibers/cells. Purkinje’s definition of specific Purkinje cells in the heart led to a detailed analysis and a comparison with the working ventricular cell by Sommer and Johnson15 in 1968. Subsequent qualitative and quantitative studies have only

Purkinje cell action potentials (APs) are longer than their ventricular counterparts.18 In rabbit and kid (Figure 1B), Purkinje cells exhibit a prominent phase 1 of repolarization, a more negative plateau, and a significantly longer action potential duration at 90% repolarization (APD90 ) than do ventricular cells.19 In canine Purkinje fibers, APD50 and APD90 are prolonged compared with their ventricular counterparts,20,21 yet there is no difference in resting potential between the two cell types. However, the total amplitude of the AP and the maximal rate of rise of the AP upstroke (Vmax) are larger in Purkinje fibers. Women are susceptible to the development of torsades de pointes, a life-threatening polymorphic ventricular tachycardia that may originate in Purkinje fibers. Gender differences in APs have been observed in canine Purkinje fibers, 22 where APD40, APD50 , APD 70, and APD90 of Purkinje fibers from female hearts have been found to be significantly longer compared with their male counterparts. However, no differences in resting potential, AP amplitude, and Vmax between male and female Purkinje cells have been reported. Although human Purkinje cell APs are assumed to be longer than ventricular APs, this has not yet been shown by any systematic study. Inward rectifier K⫹ channels—IK1 (Kir2.1, Kir2.2, Kir2.3), IKATP (Kir6.1, Kir6.2), and IKACh (Kir3.1, Kir3.4)— conduct ⫹ K ions more in the inward direction than the outward direction and play an important role in setting the resting potential close to the equilibrium potential for K⫹ in cardiac cells. Although it has long been known that Purkinje fibers display two levels of resting potential,23 this situation is significantly different in myocytes. Purkinje cells have reasonably strong IK1, 24 but some characteristics (e.g., nature of the negative slope) differ from those of ventricular cells,25 impacting not only the resting potential but also the slope of phase 3. A complete review of ion channel functions (INa, ICaL, I K , etc.) of both Purkinje and ventricular cells is given in Dun and Boyden.26

Pacemaker currents The automaticity that occurs in the isolated normal multicellular Purkinje fiber strand is an example of normal au-

Boyden et al

Cardiac Purkinje Cells

129

Figure 2 Recordings from distal poles of a mapping catheter in a patient with ventricular tachycardia. Evidence for participation of Purkinje fibers in the ventricular tachycardia circuit is shown by the presence of the Purkinje potential during ventricular tachycardia (Map) as well as during sinus rhythm at the same site (A, B). Pace mapping at the Purkinje site matched ventricular tachycardia morphology (C, D).7

tomaticity and is thought to underlie junctional rhythms. A Purkinje fiber does not show automaticity when it is being overdriven by excitatory waves from the rapidly firing sinus node. This phenomenon, called overdrive suppression, results from enhanced electrogenic Na/K pump activity.27 Enhanced pump-induced current results from the increased influx of sodium that occurs at fast rates. Interestingly, although overdrive suppression is the common outcome in Purkinje fibers, postdrive rate acceleration also has been reported. These models of automaticity are enhanced using ouabain or other forms of digitalis, suggesting a role for intracellular Ca2⫹ in initiation of nondriven electrical activity in Purkinje cells (see below). During pacemaker activity, a Purkinje fiber’s transmembrane AP shows a slow diastolic depolarization during phase 4 (Figure 1B) that precedes a nondriven AP. Several types of ionic currents existing in the Purkinje myocyte could play a role in generating phase 4 depolarization and thus underlie normal automaticity. These currents include If, which some consider to be the main current.28,29 In wholecell patch-clamp studies, steady-state If activation occurs in Purkinje cells at physiologic potentials (– 80 to –130 mV), whereas in ventricular cells If activation occurs at more 30 negative potentials (–120 to –170 mV), consistent with the activation curve of If for ventricular cells being shifted approximately 30 mV in the negative direction compared with Purkinje cells. In other Purkinje fiber studies, a critical role of If for normal automaticity is less clear. Lidoflazine, a drug that decreases If in sheep Purkinje fibers,31 had no effect on normal canine Purkinje fiber automaticity.32 At first, po33 tassium currents were thought to deactivate (IK2 ), giv-

ing rise to a slow phase 4 diastolic depolarization due to the presence of a persistent inward Na⫹ current in Purkinje cells. However, when outward currents are blocked, the If current is revealed and controls automaticity.28 Recently, another K⫹ current, IKdd, has been defined in Purkinje fibers. This current deactivates at more positive potentials than Purkinje If , suggesting a role in pacemaking.34 Although the molecular nature of If has been ascribed to both HCN4 and HCN2 proteins in Purkinje fibers,35 the molecular nature of the other currents remains unresolved. Additional questions arise from single myocytes dispersed from Purkinje fiber bundles. The normally polarized (– 85 mV) individual canine Purkinje cell was found to be quiescent, lacking normal automaticity in the absence as well as in the presence of catecholamines (Figure 3)36 despite the fact that If had been identified in the single Purkinje cell.37 Our laboratory has shown that focally arising, cell-wide intracellular Ca2⫹ waves occur in normal canine Purkinje cells in the absence of electrical stimulation.38 These Ca2⫹ waves originate at cell borders, can propagate the full extent of an aggregate, and initiate membrane depolarization and/or nondriven electrical activity of the well-polarized Purkinje aggregate. Importantly, ryanodine and thapsigargin suppress the Ca2⫹ wave and the accompanying membrane depolarization. This is an example of reverse excitation– contraction coupling, where intracellular Ca2⫹ causes a change in ionic conductances to change membrane voltage.39 Thus, spontaneous Ca 2⫹ release and ensuing Ca2⫹ waves modulate normal Purkinje cell pacemaker function (see below).

130

Heart Rhythm, Vol 7, No 1, January 2010

Figure 3 Lack of spontaneous impulse generation in single canine Purkinje cells. A: Fine-tip microelectrode recordings of a single Purkinje cell show small depolarizations during diastole in a well-polarized cell (–90 mV). B: Paced Purkinje cell with a normal diastolic potential but no spontaneous activity. C: Absence of pacemaker activity in canine Purkinje cell in the presence of 2.7 mM Ko (–90 mV).36

The Purkinje cell: Forward and reverse mode excitation– contraction coupling 2⫹

Because intracellular Ca has been shown to have a pacemaker effect in Purkinje cells, it is important to understand forward mode excitation– contraction coupling in these cells. In free-running, multicellular Purkinje fiber strands, 2⫹ electrical excitation induces a biphasic Ca transient (la40 beled as L1 [Light 1] and L 2 by Wier and Isenberg ). Although it was assumed that some of the features of excitation– contraction coupling in ventricular myocytes were similar to those of Purkinje cells from normal hearts, few details of this process were known. Excitation– contraction 2⫹ coupling and Ca transients have been studied in various 41,42 multicellular Purkinje fiber preparations, but few stud2⫹ ies have defined the spatial/temporal changes of Ca in Purkinje cells. A preliminary report suggested that AP2⫹ evoked Ca transients in rabbit Purkinje cells were initiated at the periphery and then by either simple diffusion or 2⫹ subsequent triggered release Ca traveled to the center of 43 the cell. Purkinje myocytes lack t-tubules and thus have two types of sarcoplasmic reticulum (SR): junctional and 15 corbular (or nonjunctional). The mechanism of propaga2⫹ tion is not electrical, but chemical. Ca influx binds to the 2⫹ regulatory domain of the Ca release ryanodine receptor (RyR) channel. This leads to opening of the RyR channel 2⫹ and release of a greater amount of Ca from the SR. This 2⫹ 2⫹ Ca diffuses to its neighboring RyRs to cause further Ca 2⫹ release from SR stores. Hence, released Ca continues to propagate along a cell. This is called Ca2⫹-induced Ca 2⫹ release. In canine Purkinje cells upon electrical stimulation, there 2⫹ is a uniform Ca rise just under the membrane of the 44 Purkinje cell aggregate (Figure 4A (a)). Presumably this is 2⫹ 2⫹ 2⫹ due to Ca influx via the L-type Ca channel. This Ca 2⫹ then propagates to the central core region via Ca -induced 2⫹ Ca release (Figure 4A (b, c)). Implicit in this process is

the idea that intracellular Ca2⫹ release units of the SR in the core of the Purkinje cell do not respond to membrane voltage and Ca2⫹ influx but rather to Ca2⫹ arrival as a wave. This differs considerably from the ventricular cell. From these findings of spatial and temporal measurements of epifluorescent Ca2⫹ in Purkinje cells came the determination of multiple subcellular components of Ca2⫹ release in Purkinje cells. If these subcellular components are spatially averaged, then the normal Purkinje cell has a complex global Ca2⫹ transient. The exact time course of this Ca2⫹ transient would depend on the experimental conditions (e.g., temperature, stimulus rate, [Ca2⫹]o). Thus, fundamental to the concept of excitation– contraction coupling in the forward direction is propagation of Ca2⫹ as a wave within a Purkinje cell (Figure 4B) as well as between cells.45

Spontaneous impulse generation: Normal Purkinje cells Although several investigators had reported diastolic oscillations in muscle force, potential, or aequorin luminescence with toxic levels of glycosides or high Cao in multicellular Purkinje fiber preparations,46 intracellular Ca2⫹ is commonly thought to play only a small, if any, role in the normal rhythmic activity or automaticity of the Purkinje fiber. However, if the interval between depolarizations is sufficient in normal Purkinje cell aggregates, spontaneous Ca2⫹ waves occur with a low probability. A portion of the Ca2⫹ of the wave is pumped out of the cell via the Na/Ca exchanger, and the resulting current depolarizes the membrane. This current, referred to as Iti in the literature,47 causes delayed afterdepolarizations (DADs). Iti has been the subject of many studies and in canine Purkinje cells appears to be dependent at least on current through the Na/Ca exchanger and Ca2⫹-dependent chloride currents.39 In rabbit Purkinje cells, Iti currents (transient inward) underlying DADs have been recorded both in the absence of spontaneous Ca2⫹ release and in the presence of variable spontaneous release.48 If conditions are correct, these Ca2⫹ waves can elicit APs.38 Wave conduction velocity decreases with thapsigargin and/or ryanodine, suggesting Ca2⫹-induced Ca2⫹ release as a mechanism of propagation of Purkinje cell aggregate Ca2⫹ waves.

Purkinje early afterdepolarizations and intracellular Ca2ⴙ 2⫹

The role of propagating Ca waves in membrane DADs is reasonably well accepted (see below). Early afterdepolarizations (EADs) in ventricular myocytes appear not to be 2⫹ due to a spontaneous regional increase in [Ca ] i or prop2⫹ agating Ca waves. Rather, during EADs, fluorescence transients show synchronous changes throughout the myo2⫹ 49 –52 cyte that lack distinct high peaks of [Ca ]i . These findings are consistent with the idea that a change in membrane potential primarily causes the observed increases in 2⫹ [Ca ] i during EADs. Evidence supporting a role for the 2⫹ L-type Ca window current in BayK-8644 –induced EADs

Boyden et al

Cardiac Purkinje Cells

131

Figure 4 Action potential– evoked Ca2⫹ transients (A) and spontaneous Ca2⫹ wave (B) in normal Purkinje cell aggregates. Top panels show fluorescence intensity of Ca2⫹ in two Purkinje cells during depolarization (A) and spontaneous Ca2⫹ wave (B). a– c correspond to three-dimensional surface plots of ratio images of a section of these aggregates. Ca2⫹ concentration is reflected by both the color and the height of the surface. The first response to a stimulus is an increase in Ca2⫹ (A, a), which is present mostly at the aggregate’s periphery (A, b). Peak Ca2⫹ change occurs later in the core of the aggregate (A, c). White line on surface plot (A, c) corresponds to 10 ␮ m. B: Spontaneous cell-wide Ca2⫹ wave moving along four regions of a normal Purkinje cell. A small Ca2⫹ transient appears at the edge of the aggregate (B, a) and induces the Ca2⫹ wave. Thick white line on surface plot (B, c) corresponds to 26 ␮m for all surface plots.44

in sheep Purkinje cells has been demonstrated by the appropriate voltage- and time-dependent properties of the whole-cell L-type Ca2⫹ current as well as of its single 53,54 2⫹ channel events. Reactivation of the Ca current and/or ⫹ the late Na current has been shown to underlie the inward currents contributing to EADs observed dur...