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Counterpoint: M cells do not have a functional role in the ventricular myocardium of the intact heart

2011; Elsevier BV; Volume: 8; Issue: 6 Linguagem: Inglês

10.1016/j.hrthm.2010.10.048

1556-3871

Michiel J. Janse, Ruben Coronel, Tobias Opthof,

Cardiac Arrhythmias and Treatments

Transmural heterogeneities in repolarization between the subendocardial, subepicardial myocardium, and a so-called layer of M cells are thought to be implied in the genesis of the T-wave and in arrhythmogenesis. We address these issues herein. The work of Antzelevitch et al1Yan G.X. Antzelevitch C. Cellular basis for the normal T wave and the electrocardiographic manifestations of the long QT syndrome.Circulation. 1998; 98: 1928-1936Crossref PubMed Scopus (807) Google Scholar on the arterially perfused canine wedge preparation has given rise to the concept that there is a considerable transmural gradient in repolarization that is responsible for the T-wave in the electrocardiogram (ECG). In the wedge preparation, the peak of the T-wave coincides with the end of epicardial repolarization, and the end of the T-wave with the end of repolarization of the midmural M cells. The Tpeak-end interval was proposed as an index of transmural repolarization. The fact that in the pseudo ECG of the wedge preparation there is concordance between the QRS complex and the T-wave, whereas in the intact dog there is discordance in most leads, suggests that a wedge preparation is not a suitable model for studying repolarization patterns in the intact heart.2Janse M.J. Sosunov E.A. Coronel R. et al.Repolarization gradients in the canine left ventricle before and after induction of short-term cardiac memory.Circulation. 2005; 112: 1711-1718Crossref PubMed Scopus (89) Google Scholar There are numerous studies on intact hearts in which intramural measurements have shown the absence of relevant transmural gradients in repolarization despite the presence of gradients in action potential duration (see Opthof et al3Opthof T. Coronel R. Janse M.J. Is there a significant transmural gradient in repolarization time in the intact heart?.Circ Arrhythmia Electrophysiol. 2009; 2: 89-96Crossref PubMed Scopus (97) Google Scholar for references). Figure 1 shows strength-interval curves for cathodal stimuli determined at successive intramural electrodes in a study that was performed some 50 years ago.4Van Dam R.T.H. Durrer D. Experimental study on the intramural distribution of the excitability cycle and on the form of the epicardial T wave in the dog heart in situ.Am Heart J. 1961; 61: 537-542Abstract Full Text PDF PubMed Scopus (42) Google Scholar In the middle layers, recovery of excitability occurs earlier than at epicardial and endocardial layers, and the sum of activation time (AT) and refractory period, which is equivalent to repolarization time (RT), is longest in the epicardium, contrary to what modern textbooks describe. Later studies showed that in the intact dog heart, the Tpeak-end interval of 42 ms far exceeded the transmural dispersion of RT of 2.7 ± 4.2 ms. Differences in RTs between apex and base and between anterior and posterior regions of the left ventricle accounted for the dispersion of RT in the whole heart.5Opthof T. Coronel R. Wilms-Schopman F.J.L. et al.Dispersion of repolarization in canine left ventricle and the electrocardiographic T wave: T(p-e) interval does not reflect transmural dispersion.Heart Rhythm. 2007; 4: 341-348Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar Thus, although the Tpeak-end interval may constitute a useful parameter in arrhythmogenesis, which is not at debate, it simply has a different meaning in a wedge preparation and in an intact heart. Figures 2A and 2B show ATs versus activation-recovery intervals (ARIs) and versus RTs, respectively, at many intramural and epicardial sites in the left ventricle of an open-chest dog.3Opthof T. Coronel R. Janse M.J. Is there a significant transmural gradient in repolarization time in the intact heart?.Circ Arrhythmia Electrophysiol. 2009; 2: 89-96Crossref PubMed Scopus (97) Google Scholar Subendocardial, midmural, subepicardial, and epicardial sites appear in different colors in planes parallel to the epicardial surface at distances of 13, 9, 5, 1, and 0 mm, respectively. The regression line has a slope of −0.6 (Figure 2A) and the regression line of AT versus RT has (therefore) a slope of +0.4 (Figure 2B). The longer action potential duration in early activated sites and the shorter action potential duration in late activated sites decreases dispersion in repolarization. In this particular heart, the dispersion in AT was 35 ms, in ARI 44 ms, and in RT 32 ms. Within each plane (colour) parallel to the cardiac surface substantial dispersion exists, primarily in ARIs (Figure 2A), but also in RTs (Figure 2B). In another report, we have shown that transmural dispersion in RTs is minimal.5Opthof T. Coronel R. Wilms-Schopman F.J.L. et al.Dispersion of repolarization in canine left ventricle and the electrocardiographic T wave: T(p-e) interval does not reflect transmural dispersion.Heart Rhythm. 2007; 4: 341-348Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar There is no midmural zenith in either ARI or in RT in intact hearts, including humans6Taggart P. Sutton P.M.I. Opthof T. et al.Transmural repolarisation in the left ventricle in humans during normoxia and ischaemia.Cardiovasc Res. 2001; 50: 454-462Crossref PubMed Scopus (124) Google Scholar, 7Conrath C.E. Wilders R. Coronel R. et al.Intercellular coupling through gap junctions masks M cells in the human heart.Cardiovasc Res. 2004; 62: 407-414Crossref PubMed Scopus (93) Google Scholar (see Opthof et al3Opthof T. Coronel R. Janse M.J. Is there a significant transmural gradient in repolarization time in the intact heart?.Circ Arrhythmia Electrophysiol. 2009; 2: 89-96Crossref PubMed Scopus (97) Google Scholar for other references). Finally, Figure 2C compares the activation pattern of the left ventricle with the repolarization pattern. We have marked the earliest AT and earliest RT in red and ensuing activity in orange, yellow, and green. Although the repolarization pattern is not an exact copy of the activation pattern, the resemblance between AT and RT maps is striking. This underscores that AT and RT patterns roughly follow the same path, and not opposite paths as in canine wedge preparations. Both in dogs (Figure 2B) and in humans,7Conrath C.E. Wilders R. Coronel R. et al.Intercellular coupling through gap junctions masks M cells in the human heart.Cardiovasc Res. 2004; 62: 407-414Crossref PubMed Scopus (93) Google Scholar the AT-RT relationship is positive along the transmural axis. However, along the apicobasal (epicardial) axis, the AT-RT relationship is positive in the dog (Figure 2B), but negative in humans.8Cowan J.C. Hilton C.J. Griffiths C.J. et al.Sequence of epicardial repolarisation and configuration of the T wave.Br Heart J. 1988; 60: 424-433Crossref PubMed Scopus (122) Google Scholar The fact that in most standard leads of the ECG the T-wave is discordant in the dog, but concordant in humans, indicates that the RT pattern along the apicobasal (epicardial) axis and not the RT pattern along the transmural axis determines the polarity of the T-wave. There are several studies on wedge preparations that failed to show a midmural maximum in action potential duration, one on a canine wedge from the interventricular septum,9Morita S.T. Zipes D.P. Morita H. Wu J. Analysis of action potentials in the canine ventricular septum: no phenotypic expression of M cells.Cardiovasc Res. 2007; 74: 96-103Crossref PubMed Scopus (16) Google Scholar the other from the rabbit left ventricle.10Myles R.C. Bernus O. Burton F.L. Cobbe S.M. Smith G.L. The effect of activation sequence on transmural patterns of repolarization and action potential duration in rabbit ventricular myocardium.Am J Physiol Heart Circ Physiol. 2010; 299: H1812-H1822Crossref PubMed Scopus (41) Google Scholar In a study on human wedge preparations from human left ventricles, isolated islands of cells with long action potentials were found in the deep subendocardium, but no continuous midmural layer of prolonged action potentials.11Glukhov A.V. Fedorov V.V. Lou Q. et al.Transmural dispersion of repolarization in failing and nonfailing human ventricle.Circ Res. 2010; 106: 981-991Crossref PubMed Scopus (255) Google Scholar Although the investigators describe a transmural gradient in action potential duration, it would have been better to calculate RTs, i.e., the sum of AT and action potential duration. Thus, judging from their Figure 5,11Glukhov A.V. Fedorov V.V. Lou Q. et al.Transmural dispersion of repolarization in failing and nonfailing human ventricle.Circ Res. 2010; 106: 981-991Crossref PubMed Scopus (255) Google Scholar at a pacing cycle length of 1,000 ms, AT at the epicardium is about 50 ms, action potential duration is about 370 ms, yielding an RT of 420 ms. At the endocardium, AT is zero, action potential duration about 450 ms. The gradient in repolarization is thus 30 ms, with the epicardium repolarizing earlier than the endocardium. At a pacing cycle length of 500 ms, AT at the epicardium is about 60 ms, action potential duration 280 ms, and RT 340 ms, which is 20 ms later than the RT at the endocardium of 320 ms.11Glukhov A.V. Fedorov V.V. Lou Q. et al.Transmural dispersion of repolarization in failing and nonfailing human ventricle.Circ Res. 2010; 106: 981-991Crossref PubMed Scopus (255) Google Scholar In the rabbit wedge, both endocardial and epicardial stimulation were used. The investigators write: "In both activation sequences, transmural repolarization followed activation, and action potential duration shortened along the activation path such that significant transmural gradients of repolarization did not occur."10Myles R.C. Bernus O. Burton F.L. Cobbe S.M. Smith G.L. The effect of activation sequence on transmural patterns of repolarization and action potential duration in rabbit ventricular myocardium.Am J Physiol Heart Circ Physiol. 2010; 299: H1812-H1822Crossref PubMed Scopus (41) Google Scholar This is very similar to our results in intact hearts shown in Figure 2. Optical mapping in a canine wedge preparation revealed a transmural dispersion in action potential duration.12Strom M. Wan X. Poelzing S. Ficker E. Rosenbaum D.S. Gap junction heterogeneity as mechanism for electrophysiologically distinct properties across the ventricular wall.Am J Physiol Heart Circ Physiol. 2010; 298: H787-H794Crossref PubMed Scopus (22) Google Scholar However, at a conduction velocity of about 20 cm/sec one can assume that during endocardial pacing and a wall thickness of 1 cm, the epicardium will be activated after 50 ms. Their Figure 112Strom M. Wan X. Poelzing S. Ficker E. Rosenbaum D.S. Gap junction heterogeneity as mechanism for electrophysiologically distinct properties across the ventricular wall.Am J Physiol Heart Circ Physiol. 2010; 298: H787-H794Crossref PubMed Scopus (22) Google Scholar shows that action potential duration at the epicardium is 240 ms, which gives an RT of 290 ms. At the endocardium, action potential duration is 290 ms, AT is zero, resulting in the same RT of 290 ms. Interestingly, after administration of the cellular uncoupler carbenoxoline, a subepicardial layer of long action potentials was found. This may indicate that cellular uncoupling may unmask M cells, whereas in well-coupled tissue, electrotonic current flow between cells with intrinsically long action potentials and intrinsically short action potentials will attenuate the differences in action potential duration.7Conrath C.E. Wilders R. Coronel R. et al.Intercellular coupling through gap junctions masks M cells in the human heart.Cardiovasc Res. 2004; 62: 407-414Crossref PubMed Scopus (93) Google Scholar It is possible that at the cut end of a wedge preparation partial cellular uncoupling occurs, which may account for the differences in findings in some, but not all, wedge preparations and in intact hearts. In the canine wedge preparation, drugs such as d-sotalol13Shimizu W. Antzelevitch C. Sodium channel block with mexiletine is effective in reducing dispersion of repolarization and preventing torsade des pointes in LQT2 and LQT3 models of the long QT syndrome.Circulation. 1997; 96: 2038-2047Crossref PubMed Scopus (459) Google Scholar create a transmural gradient in repolarization by excessive prolongation of the action potentials of the M cells. In these preparations both spontaneous and induced torsade de pointes occurred, which were thought to be the result of both triggered activity based on early afterdepolarizations and transmural reentry. In the intact porcine heart, perfused according to the Langendorff method with a 1:1 mixture of blood and Tyrode solution, infusion of a bolus of d-sotalol in the left anterior descending coronary artery (LAD) caused a uniform prolongation of RT of all transmural layers of about 100 ms.14Coronel R. Wilms-Schopman F.J.G. Opthof T. Janse M.J. Dispersion of repolarization and arrhythmogenesis.Heart Rhythm. 2009; 6: 537-543Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar Despite the huge repolarization gradient between the LAD area and the rest of the heart, no spontaneous arrhythmias occurred.3Opthof T. Coronel R. Janse M.J. Is there a significant transmural gradient in repolarization time in the intact heart?.Circ Arrhythmia Electrophysiol. 2009; 2: 89-96Crossref PubMed Scopus (97) Google Scholar, 14Coronel R. Wilms-Schopman F.J.G. Opthof T. Janse M.J. Dispersion of repolarization and arrhythmogenesis.Heart Rhythm. 2009; 6: 537-543Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar Heterogeneity in repolarization facilitates the occurrence of unidirectional block and reentry, following a closely coupled premature beat emanating from the early repolarizing myocardium. Indeed, various indices for repolarization heterogeneity correlate with the occurrence of ventricular arrhythmias and sudden death.15Amlie J.P. Increased dispersion of repolarization: a major mechanism behind the genesis of malignant ventricular arrhythmias.in: Olsson S.B. Amlie J.P. Yuan S. Dispersion in ventricular repolarization: state of the art. Futura Publishing Company, Armonk, NY2000: 143-163Google Scholar It has been argued that an intramural region of delayed repolarization formed by M cells may provide a substrate for repolarization heterogeneity, unidirectional block, and reentry in the wedge preparation. A premature activation (S2) will encounter a line of conduction block if the arrival time at the area of delayed repolarization precedes local repolarization. The size of the region of delayed repolarization is important for the subsequent initiation of reentry. Figure 3 shows diagrammatic activation maps of the premature activation (S2). If the M layer completely separates the subendocardial tissue from the subepicardial tissue (Figure 3, top left) very slow conduction is required to overcome the difference in RT between the proximal area, where the tissue is stimulated, and the M cell layer. When M cells would be organized in islands, it is mandatory that the local activation (AT, closed star) is later than local RT (open star) to permit activation of the M cell island with prolonged RT (Figure 3, top middle). Whether or not reentry into the proximal area ensues depends on the restitution characteristics of the proximal area. Figure 3, top right panel, shows that if the area with delayed repolarization is too small, the premature wave front simply passes the M cell region, because this is still refractory. If we assume that the point of stimulation is in the center of the endocardium of a 2-dimensional plaque of tissue, the minimum circumference of the region of delayed repolarization to allow reentry when unidirectional block has occurred equals the length of the line of block (lb), which is given by the time between the arrival time of the activation wave (tA) at the proximal side of the line of block, local RT (tR) at the prospective site of reentry, and the averaged conduction velocity along the line of block (cv) as follows:lb=cv∗(tR−tA)∗2 A realistic value for the average conduction velocity in a wedge preparation is 30 cm/s.10Myles R.C. Bernus O. Burton F.L. Cobbe S.M. Smith G.L. The effect of activation sequence on transmural patterns of repolarization and action potential duration in rabbit ventricular myocardium.Am J Physiol Heart Circ Physiol. 2010; 299: H1812-H1822Crossref PubMed Scopus (41) Google Scholar The difference between tA and tR can be approximated by the difference in RT between the subendocardium and the M cell layer, and may be as large as 100 ms. It follows that the minimum circumference equals 6 cm. Even with this relatively low conduction velocity, this minimum size is too large to be contained in a small wedge preparation. Figure 3, bottom panel, also shows an epicardial S2-activation pattern recorded from an isolated perfused pig heart in which one coronary artery was perfused with sotalol and another branch with pinacidil, leading to a large repolarization gradient of 130 ms (not shown). Premature stimulation was performed in the area perfused with pinacidil. The bold line indicates the interface between the 2 perfusion domains where the activation wave did not enter the area with the prolonged repolarization. The length of this line of block in Figure 3 is approximately 6 cm. The contention that a preparation is more conducible to sustain reentry the larger the difference in RT, is therefore not true, in particular in small preparations. If reentry does occur in a small piece of tissue and in the presence of large heterogeneities in repolarization, this can only be achieved if conduction, at least in some parts, is severely impaired. The electrocardiographic T-wave is caused by global (i.e., involving the entire heart) rather than transmural heterogeneity in repolarization. In the left ventricle of intact heart, (1) there is no midmural layer of delayed repolarization, (2) the repolarization process roughly follows the activation sequence from endocardium to epicardium, and (3) large heterogeneities in repolarization do not lead to spontaneous arrhythmias.