Skip Navigation

Cardiovascular Research 2000 45(4):952-960; doi:10.1016/S0008-6363(99)00418-6
© 2000 by European Society of Cardiology
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Stankovicova, T.
Right arrow Articles by Sipido, K. R
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Stankovicova, T.
Right arrow Articles by Sipido, K. R
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

Copyright © 2000, European Society of Cardiology

M cells and transmural heterogeneity of action potential configuration in myocytes from the left ventricular wall of the pig heart

Tania Stankovicova1, Monika Szilard, I De Scheerder and Karin R Sipido*

Laboratory of Experimental Cardiology, University of Leuven, Campus Gasthuisberg O/N 7th floor, Herestraat 49, B-3000 Leuven, Belgium

* Corresponding author. Tel.: +32-16-347-153; fax: +32-16-345-844 karin.sipido{at}med.kuleuven.ac.be

Received 7 June 1999; accepted 1 December 1999


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objective: Heterogeneity of action potential configuration in the left ventricle (LV), and the contribution of M cells to it, has been observed in the human heart and is important for arrhythmogenesis. Whether the pig heart has similar properties remains a controversial but important issue as the pig heart is currently under study for use in xenotransplantation. Methods: Single myocytes were enzymatically isolated from the epicardium (EPI, ncells=29), midmyocardium (MID, ncells=38), and endocardium (ENDO, ncells=13) of the free LV wall (npigs=26, 14–22 weeks old, 55–80 kg), and studied at different stimulation rates during whole-cell recording (normal Tyrode's solution, K+-aspartate-based pipette solution, 50 µM K5fluo-3 as [Ca2+]i indicator, 37°C). Standard six-lead ECGs were recorded from anesthetized pigs. Results: The action potential duration (APD) was not significantly different at 0.25 Hz vs. 2 Hz for the majority of cells in all three layers. However, a subpopulation of cells behaved like M cells and had a very steep frequency response (APD90 at 0.25 Hz 538±30 ms, vs. 337±9 ms at 2 Hz, P<0.05, n=22). These cells were found predominantly in the MID layer (34% of cells), but also (24%) in EPI. M cells had a more pronounced spike-and-dome configuration, with a significantly larger phase 1 magnitude and plateau voltage. The frequency response of these parameters was different from the other cell types. [Ca2+]i transients tended to be larger in M cells. For the in vivo ECG of anesthetized pigs, the QT time was close to the APD90 of M cells, and J waves were seen in 7/12 recordings. Conclusions: In young adult pigs, M cells can be identified by a steep frequency response of the APD and by a spike-and-dome configuration. These cells are mostly, but not exclusively, found in the midmyocardium, and could contribute to the ECG characteristics. Their properties may however be different from those of other species, including humans.

KEYWORDS Membrane potential; Myocytes; ECG


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
The existence of heterogeneity of the electrical properties of the myocytes in the different layers of the left ventricle is now well established, and has been described for several animal species, as well as in the human heart [1–5]. In addition, differences between left and right ventricle, as well as variability in the apex to base regions, have been described [6–10]. Regional variation in remodeling during hypertrophy may contribute to increased and potentially arrhythmogenic heterogeneity [8,11]. Within the different layers of the free ventricular wall, i.e. epicardial, midmyocardial and endocardial, the midmyocardial layer has been described to contain cells which have action potentials with a particular configuration and frequency response, the so-called M cells [2,3,12]. These cells have a prominent spike and dome morphology, as also found in the epicardium, but the maximal rate of rise of the action potential upstroke is considerably greater than in either endocardium or epicardium. The hallmark of the M cells is the pronounced prolongation of action potential duration (APD) with deceleration of the stimulation rate, leading to a very steep rate dependence of the APD, compared to epicardium and endocardium. This feature of M cells resembles the APD rate relationships of Purkinje fibers, but M cells have no phase 4 depolarization [1]. The presence of M cells in the ventricular wall will contribute to the T wave morphology, the U wave and the J wave of the surface ECG [3,13–15]. In the normal heart in situ electronic coupling is expected to reduce heterogeneity, but M cells can still exert an important influence [16,17]. It is thought that M cells have a prominent role in arrhythmogenesis as differential response of these cells to e.g. drugs or ischemia may increase or decrease heterogeneity of conduction and refractoriness, thereby increasing/decreasing the dispersion and vulnerability of the ventricle to arrhythmias [1,18]).

Recently there has been some controversy on whether M cells were present in the pig heart as well [19–21]. This question goes beyond the general relevance of comparative physiology, as the pig has proven to be an extremely useful animal for the study of a ischemic heart disease [22,23], as well as hypertrophy and failure [24,25], and is the animal currently being evaluated for xenotransplantation [26–28]. The size of the pig heart, and its coronary anatomy are indeed quite close those of the human heart, notwithstanding some important differences [29]. The transmembrane action potential of the pig has been studied during ischemia in vivo [30] and in isolated hearts [31] or during drug treatment of papillary muscles [32]. There are however few studies on isolated cells; changes in action potential duration and in calcium current during transition to heart failure have been reported [33,34]. Study of the characteristics of the pig ventricular action potential in different layers has been restricted to studies on transmural slices [19]. We therefore have studied properties of the action potential of single cells isolated from the different layers of the left ventricle. We also looked for evidence of electrical heterogeneity in the ECG waveform.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
2.1 Animal characteristics and in vivo ECG recordings
A total of 26 domestic pigs of either sex were used; 22 animals were between 14 and 17 weeks old (body weight 55–70 kg), four animals were 22 weeks old (70–80 kg). In this strain, breeding age is around 24 weeks, and these animals can be considered to be young adults. Animal care and treatment conformed with the Guide for the Care and Use of Laboratory Animals (N.I.H. Publication No. 85-23, revised 1996). In 12 animals, a standard six-lead ECG (Elema AB and Mingograph 720, Siemens) was recorded under ketamine (10 mg/kg i.v.) anesthesia, following premedication with Stresnil (40 mg/kg i.m.), with the animal in the supine position, and the artificial ventilation arrested for the time of the recording. QT time was measured at a paper speed of 25 mm/s in lead II and III, from the onset of the QRS complex to the end of the T wave. Biphasic T waves were included, but U waves were not included in the measurement. For cell isolation, the anesthetized animal was sacrificed with an intravenous bolus of 10 ml of 1 M KCl, to arrest the heartbeat. Then the heart was quickly removed through midsternal thoracotomy and placed in cold Ca2+-free Tyrode's solution.

2.2 Cell isolation procedure
Ventricular myocytes were enzymatically isolated from a wedge of the left ventricular free wall, supplied by the left anterior descending coronary artery, using the same method we have described for isolation of human myocytes [35]. The coronary artery was cannulated, and small leaking branches at the edges of the preparation were ligated. The tissue was perfused at constant pressure (~80 mmHg); all solutions were gassed with 100% O2, and temperature was maintained at 37°C. First we perfused for 30 min with Ca2+-free Tyrode's solution (see below), next we added to this solution 1.4 mg/ml collagenase A (Boehringer Mannheim) and 0.1 mg/ml protease (type XIV, Sigma) for 30 min. The enzyme solution was washed out for 15 min with a low Ca2+ Tyrode's solution. The tissue was then sectioned perpendicular to the wall in slices of 2–4 mm thickness. From these slices the epicardial and endocardial parts (each 2–3 mm) were cut parallel to the surface and placed into separate dishes in low Ca2+ (0.18 mmol/l) Tyrode's solution; a central midmyocardial part of some 5 mm was also put in a dish. These preparations were minced, filtered through a nylon mesh, and the cells resuspended in low Ca2+ Tyrode's solution. After 20 min, the solution was slowly replaced with a normal Tyrode's solution containing 1.8 mmol/l CaCl2. Ca2+-tolerant cells were stored at room temperature and used within 8–10 h after isolation. The yield was 40–60%, only rod-shaped cells with clear cross striations, and without spontaneous contractions or significant granulation were used.

2.3 Solutions
The Ca2+-free Tyrode's solution used for cell isolation contained: (in mmol/l) NaCl 130, KCl 5.4, KH2PO4 1.2, MgSO4 1.2, Na–Hepes 6.0, glucose 10 (pH 7.20). The composition of normal Tyrode's solution used for recording of action potentials was: (in mmol/l) NaCl 137, KCl 5.4, MgCl2 0.5, CaCl2 1.8, Na–Hepes 11.8, glucose 10 (pH 7.40). [K+]o in our study was comparable to what has been used for pig myocardium before [19], but slightly higher than in previous studies in dogs [12]. The standard pipette solution contained: (in mmol/l) K+-aspartate 120, KCl 20, MgCl2 0.5, MgATP 5.0, K–Hepes 10, NaCl 10, K5fluo-3 0.05 (pH 7.20). With these solutions, a junction potential of +12 mV occurred, and was subtracted from the values of resting membrane potential, but other values were not corrected.

2.4 Membrane potential and [Ca2+]i measurements
All experiments were done at 36–37°C. Action potentials were measured in current clamp mode (Axopatch-1D, Axon Instruments) during whole-cell recording; patch pipettes typically had tip resistances of 2–3 MOhm. Cells were stimulated via the pipette with brief depolarizing current pulses, at different frequencies, each recorded repeatedly and in random order. Action potentials were analyzed after a steady state was reached, usually after 10 to 15 stimuli. Signals were sampled at 2 kHz on three channels, with an effective time resolution of 1.5 ms per signal, precluding a reliable measurement of the rate of rise of the action potential, Vmax, which was therefore not included in the present analysis. Cell capacitance was determined in voltage clamp mode, by integrating the capacitive transient during 5 mV hyperpolarizing steps from –70 mV. Values obtained with this method correlated well with values obtained by fitting of the decay phase.

The setup for [Ca2+]i measurements was as described before [36]. The fluorescence signals obtained with fluo-3 were corrected for the background recorded after seal formation, and normalized for baseline values at rest before stimulation. The pseudo-ratios thus obtained provide a reasonable estimate of [Ca2+]i, and have previously been used for group comparisons [37].

2.5 Data analysis
Group data are shown as mean±S.E.M., the number of cells used is given. One-way ANOVA was used to compare morphological parameters of different cell types, followed by Fischer's test if appropriate. ANOVA for repeated measurements was used to analyze frequency dependence of action potential parameters, followed by Bonferroni/Dunn's test to further identify differences between groups. P values <0.05 were considered significant.


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
3.1 Action potential configuration and frequency response of isolated cells
M cells have previously been characterized by their steep increase in action potential duration (APD) at low frequency. We therefore first analyzed the frequency response for cells in the different anatomical layers. Fig. 1 shows the APD at 90% repolarization (APD90) at stimulation frequencies of 0.25 and 2 Hz for all cells. In all three layers, the majority of cells had a rather shallow frequency dependence (see below). However, a subpopulation of cells had a pronounced increase in APD90 at the low frequency. This was most prominent in the midmyocardial layer, less so in the epicardial layers, and practically absent in the endocardial layer. To further analyze the properties of these cells we decided to group all cells with a APD90 of more than 420 ms at 0.25 Hz, as this group included exclusively cells with the steep frequency response. These cells we called M cells; cells that not fulfilled this criteria are further referred to according to their anatomical region of origin. Based on these criteria, the midmyocardial layer contained 34% M cells, the epicardial layer 24%, and the endocardial layer 15%.


Figure 1
View larger version (15K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 1 Action potential duration at 90% repolarization (APD90) at 0.25- and 2-Hz stimulation frequency of all cells, shown according to their anatomical layer of origin. The dashed line indicates the cut-off of 420 ms. This cut-off was chosen as all cells with an APD90 of more than 420 ms at 0.25 Hz have a steep frequency dependence; they were designated M cells. The prevalence of M cells in the layers is shown below.

 
Fig. 2 shows typical examples of the action potential configuration at different frequencies for each cell type. Following the action potential upstroke, all cell types had a clear phase 1 repolarization. This phase 1 repolarization could be followed by a small second depolarization, giving rise to the so-called spike-and-dome configuration. This pattern was almost completely restricted to the M cells, as it was observed in 18/22 M cells, versus 2/11 endo cells, 3/25 midmyocardial cells, and 5/22 epi cells. The action potential plateau declined slowly (phase 2) followed by a rapid phase 3 repolarization. We never observed phase 4 depolarization in any cell type.


Figure 2
View larger version (13K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 2 Configurations of action potentials recorded under steady state conditions at pacing frequencies of 0.25 (a), 0.5 (b), 1 (c), and 2 Hz (d) from different layers as indicated. The M cell shown was derived from the midmyocardial layer, all cells are from the same heart.

 
Mean resting membrane potential was comparable for all the cell types (endo cells –77±1 mV, midmyocardial cells –78±1 mV, epicardial cells –77±1 mV, and M cells –79±1 mV). Phase 0 amplitude was also not different between the different cell types. The phase 1 magnitude decreased with frequency in all cell types, except in the M cells where it increased (Fig. 3, upper panel). At 1 and 2 Hz, phase 1 magnitude was significantly larger in M cells than in MID or ENDO cells. To analyze the prevalence of a ‘dome’ in M cells, we measured the value of the membrane potential at 70 ms, as the peak of the dome typically occurred at 70–80 ms after the stimulus (Fig. 3, lower panel). In M cells, this value was significantly larger than in the other groups, and declined with increasing frequency, in contrast to the increase observed in the other cell types.


Figure 3
View larger version (10K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 3 (A) Phase 1 magnitude, i.e. the difference between peak and notch, measured at different frequencies, including after 1-min rest (data points close to 0). ANOVA for cell*frequency was, P<0.001, # indicates P<0.05 for M cells vs. all other cell types, * indicates P<0.05 for M cells vs. MID and ENDO after applying the Bonferroni/Dunn test; at 2 Hz, EPI is also different from MID and ENDO. Average values for 21 EPI cells, 25 MID cells, 11 ENDO cells, and 23 M cells. Frequency dependence was significantly different for M cells vs. other cell types. (B) Membrane potential of the dome or plateau, measured as the potential at 70 ms after the upstroke. Overall ANOVA P<0.0001, # indicates P<0.001 for M cells vs. all other cell types with Bonnferoni/Dunn test. Decrease with frequency for M cells, and increase for the other cell types was significant for all groups (P<0.01 for frequency up to 1 Hz, NS for 2 Hz).

 
The most striking characteristic of the M cells, the frequency dependence of the action potential duration is illustrated in Fig. 4. The APD90 and APD50 of M cells was significantly larger than of other cell types at all frequencies. Whereas M cells had a steep frequency dependence, the APD did not vary significantly with frequency in the other cells.


Figure 4
View larger version (11K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 4 Frequency-dependence of the average APD50 (top) and APD90 (bottom) for EPI (n=21), MID (n=25), ENDO (n=11), and M cells (n=23). The data points close to 0 represent values after 1-min rest. Data are expressed as mean±S.E.M. Overall ANOVA P<0.001,* indicates P<0.001 for M cells vs. other cell types with Bonferroni/Dunn test.

 
3.2 Characteristics of the body surface ECG of the anesthetized pig
We looked for evidence of heterogeneity in repolarization on the ventricular wall by analyzing QT(U) morphology of the body surface ECG. Since we were not able to reliably record ECG in awake animals, ECGs were recorded during anesthesia. In 12 animals, the mean heart rate was 112±35 beats per min (range 63–176). The mean QRS duration was 38±1 ms. The QT interval as a function of heart rate is shown in Fig. 5A (solid circles, shown with the linear regression line). For the longer QT intervals, biphasic T waves, or U waves could be observed (data points marked with a cross). An example of such a recording is shown in Fig. 5B. In this same recording a small J wave can be seen as well as in lead III. Such a J wave was seen in 7/12 recordings. The Q–J interval was between 30 and 54 ms, close to the range of the time to the notch (between 29 and 35 ms for M cells with spike-and-dome configuration).


Figure 5
View larger version (22K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 5 (A) The solid circles represent QT intervals as a function of heart rate measured from the ECG recorded in vivo under anesthesia for 12 pigs. The solid line is a linear regression through these data points (R=–0.78, P<0.005). The circles marked with a cross are from ECG recordings with biphasic T waves or U waves. For the data point marked by the arrow, the ECG is shown in panel (B). (B) Original ECG tracings illustrating long QT intervals, and biphasic T waves in lead II, III and aVF. A J wave is also present.

 
3.3 [Ca2+]i transients of M cells
Because of the pronounced differences in APD, we looked for differences in the behavior of the [Ca2+]i transient. The frequency behavior of the [Ca2+]i transient is illustrated for a M cell Fig. 6A, with average values for cells of all layers shown in Fig. 6B. After 1-min rest, M cells had small [Ca2+]i transients and the amplitude increased with increasing frequency of stimulation. This behavior was significantly different from that of the other cell types, where the amplitude of the [Ca2+]i transient varied very little with frequency. Comparing Fig. 6B to Fig. 3A, it can be seen that the frequency dependence of the [Ca2+]i transients closely resembles the frequency dependence of the phase 1 magnitude.


Figure 6
View larger version (9K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 6 (A) Typical example of the frequency response of [Ca2+]i transients, measured as the fluorescence ratio, in a M cell. (B) Average values for the peak F/F0 of the different cell types. Whereas the increase with frequency was significant for M cells (one-way ANOVA, P<0.001), in the other cell types, there was little change.

 
3.4 M cell morphology
The general appearance of M cells with regular transmitted light was not different from any other cell type. In particular, they did not have the smooth, elongated appearance of single Purkinje myocytes, and they had conspicuous edges at the intercalated disc areas. Average cell capacity was not different between the groups, although it tended to be less for M cells (P=NS), whereas cell length was slightly larger for M cells (P<0.05, Table 1).


View this table:
[in this window]
[in a new window]

  		Table 1 Capacitance and physical dimensions of cells from different layers of porcine left ventriclea

 

   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
In this study of single myocytes isolated from the different layers of the left ventricular wall of the pig heart, we found a subpopulation of cells with characteristics similar to the ones of M cells described previously in other adult mammalian species [1,2], including humans [3]. These M cells had a steep frequency response with long action potentials at low frequencies; most of them had a prominent spike-and-dome action potential configuration. The majority of these cells were found in the midmyocardial layer. Our findings support the idea put forward by Antzelevitch [20], that the absence of M cells in the pig heart reported earlier [19], may have been related to the young age of the pigs under study. We can at present not rule out that a difference in strain may also contribute to the contrasting findings. The distribution of the cells we find in the pig, is not completely similar to those of M cells previously described. A major difference is the fact that they are not confined to the midmyocardial layer, but are also present in the epicardial layer. One possible explanation is that our sampling of the epicardial layer should have been restricted to a more superficial layer. Another difference is the fact that a substantial portion of the midmyocardial layer consists of cells which lack the typical M cell features. This is in contrast to observations in transmural slices of the dog, where M cell behaviour was homogenous in the midmyocardial layer [9], although in isolated canine cells behaviour was more variable and transitional cells were observed [38].

The contribution of M cells to regional heterogeneity of repolarization in the intact heart has recently been the subject of some debate (for review see [39,50]). Indeed, in vivo with intact gap-junctional coupling, the differences in APD of the cell types are expected to be eliminated to a large extent through electrotonic interactions [40]. Our results cannot directly contribute to the resolution of this issue. The longer QT time at lower heart rates is compatible with an important contribution of cells with clear rate-dependent changes in APD, a feature we most clearly observed in M cells. Analysis of the ECG recordings of the anesthetized pigs suggested the presence of some heterogeneity of action potential configuration and repolarization by the presence of biphasic T waves and J waves [13–15], although this was observed only in some animals. We can at present not know whether this related to an effect of the anesthetic agents, but the observation indicates the presence and influence of myocardium with a more pronounced notch and longer action potential duration in these conditions. Our experiments on isolated cells offer most direct evidence for the existence of M cells, which could be a substrate for heterogeneity and pronounced rate-dependence of repolarization. To identify whether indeed these cells are responsible for the observed ECG characteristics, a combined measurement of electrograms across a larger multicellular preparation and transmembrane action potentials is needed (see e.g Ref. [41]).

Previous studies have pointed out that M cells share common characteristics with Purkinje cells [1]. Purkinje cells have a rather distinct appearance, with less striations than ventricular cells, and smooth edges, as described for dog [42], sheep [43], and rabbit heart [44]. In the present study, M cells had no distinct appearance or cell size, although on average they tended to be slightly longer. Their presence in the midmyocardial and epicardial layer, makes it further unlikely that they would represent intramyocardial Purkinje cells.

The ionic basis for the typical action potential configuration, and frequency response of M cells has been studied extensively in the dog. Several repolarizing currents are quantitatively different for M cells. They have a higher density of the transient outward K+ current, Ito1, compared to endocardial, but not to epicardial cells, and the (slow) delayed rectifier current, IKs, is smaller in M cells than in epicardial cells [1,38,45]. In the human heart as well, regional differences in Ito1 have been reported [46–48]. In the current study, the action potential of cells from all layers has a conspicuous phase 1 repolarization, reminiscent of action potentials in the dog, or human, where a transient outward current is known to be responsible for this phase. However, in these pig cells this phase seems to be smaller, as also noted by Rodriguez-Sinovas et al. [19]. In addition, the phase 1 repolarization is slower than e.g. in canine M cells, where we measured a time to the notch of 19±2 ms (ncells=7, K.R. Sipido and P.G.A. Volders, unpublished), versus 31±2 ms in pig M cells in this study, using identical recording conditions. The frequency dependent behavior of the notch in M cells is also different, as the phase 1 magnitude tends to increase rather than decrease with increasing frequency. These characteristics suggest that the underlying outward current may be different from the ‘classic’ outward K+ current in other species. In a recent study it was indeed reported that the current density of the transient outward K+ current in pig myocytes was very low (cells studied with intracellular Ca2+ buffering [49]). The absence of a transient outward K+ current may help to explain the differences in frequency dependent behaviour of early repolarization in the pig, compared to other species. The identity of other currents implicated in the regional heterogeneity of action potential configuration remains to be established. Since Ca2+ handling may also be different for M cells in the pig, Ca2+-dependent currents may play a role. The differences in APD and its frequency-dependent behaviour in M cells, are likely to be related to differences in delayed K+ currents, and/or in inward currents contributing to the plateau, such as the Ca2+ current, window Na+ current, or Na/Ca exchange current.

In conclusion, in single cells from the left ventricular wall of the young adult pig, we found a population of cells closely resembling M cells described in other species. These cells could contribute to the overall behaviour of the heart in vivo. However, pig myocardium also carries some distinctions, which may be important to take into account when extrapolating data from pig studies of electrical remodeling, or when evaluating the pig heart for xenotransplantation.

Time for primary review 31 days.


   Acknowledgements
 
The study was supported by the N.F.W.O., the Belgian Fund for Scientific research (K.R.S.). We gratefully acknowledge Prof. Dr. P. Svec in making the stay of Dr. T. Stankovicova in Leuven possible. The help of Dr. Wang Kai, and D. Decoux with the in vivo experiments, and of Dr. R. Willems for the statistics, was greatly appreciated.


   Notes
 
1 Present address: Faculty of Pharmacy, Comenius University, Bratislava, Slovak Republic. Back


   References
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 

  1. Antzelevitch C., Sicouri S., Litovsky S.H., et al. Heterogeneity within the ventricular wall. Electrophysiology and pharmacology of epicardial, endocardial, and M cells. Circ Res (1991) 69:1427–1449.[Free Full Text]
  2. Sicouri S., Quist M., Antzelevitch C. Evidence for the presence of M cells in the guinea pig ventricle. J Cardiovasc Electrophysiol (1996) 7:503–511.[ISI][Medline]
  3. Drouin E., Charpentier F., Gauthier C., Laurent K., Le Marec H. Electrophysiologic characteristics of cells spanning the left ventricular wall of human heart: evidence for presence of M cells. J Am Coll Cardiol (1995) 26:185–192.[Abstract]
  4. Fedida D., Giles W.R. Regional variations in action potentials and transient outward current in myocytes isolated from rabbit left ventricle. J Physiol (Lond) (1991) 442:191–209.[Abstract/Free Full Text]
  5. Bryant S.M., Wan X., Shipsey S.J., Hart G. Regional differences in the delayed rectifier current (IKr and IKs) contribute to the differences in action potential duration in basal left ventricular myocytes in guinea-pig. Cardiovasc Res (1998) 40:322–331.[Abstract/Free Full Text]
  6. Volders P.G.A., Sipido K.R., Carmeliet E., Spätjens R.L.H.M.G., Wellens H.J.J., Vos M.A. Repolarizing K+ currents are larger in right than left canine ventricular midmyocardium. Circulation (1999) 99:206–210.[Abstract/Free Full Text]
  7. Di Diego J.M., Sun Z.Q., Antzelevitch C. I(to) and action potential notch are smaller in left vs. right canine ventricular epicardium. Am J Physiol (1996) 271:H548–561.[Medline]
  8. Gomez A.M., Benitah J.P., Henzel D., Vinet A., Lorente P., Delgado C. Modulation of electrical heterogeneity by compensated hypertrophy in rat left ventricle. Am J Physiol (1997) 41:H1078–H1086.
  9. Sicouri S., Fish J., Antzelevitch C. Distribution of M cells in the canine ventricle. J Cardiovasc Electrophysiol (1994) 5:824–837.[ISI][Medline]
  10. Clark R.B., Bouchard R.A., Salinas Stefanon E., Sanchez Chapula J., Giles W.R. Heterogeneity of action potential waveforms and potassium currents in rat ventricle. Cardiovasc Res (1993) 27:1795–1799.[Abstract/Free Full Text]
  11. Shipsey S.J., Bryant S.M., Hart G. Effects of hypertrophy on regional action potential characteristics in the rat left ventricle: a cellular basis for T-wave inversion? Circulation (1997) 96:2061–2068.[Abstract/Free Full Text]
  12. Sicouri S., Antzelevitch C. A subpopulation of cells with unique electrophysiological properties in the deep subepicardium of the canine ventricle. The M cell. Circ Res (1991) 68:1729–1741.[Abstract/Free Full Text]
  13. Yan G.X., Antzelevitch C. Cellular basis for the normal T wave and the electrocardiographic manifestations of the long-QT syndrome. Circulation (1998) 98:1928–1936.[Abstract/Free Full Text]
  14. Lazzara R. The U wave and the M cell. J Am Coll Cardiol (1995) 26:193–194.[CrossRef][ISI][Medline]
  15. Yan G.X., Antzelevitch C. Cellular basis for the electrocardiographic J wave. Circulation (1996) 93:372–379.[Abstract/Free Full Text]
  16. Anyukhovsky E.P., Sosunov E.A., Rosen M.R. Regional differences in electrophysiological properties of epicardium, midmyocardium, and endocardium. In vitro and in vivo correlations. Circulation (1996) 94:1981–1988.[Abstract/Free Full Text]
  17. Anyukhovsky E.P., Sosunov E.A., Feinmark S.J., Rosen M.R. Effects of quinidine on repolarization in canine epicardium, midmyocardium, and endocardium. II. In vivo study. Circulation (1997) 96:4019–4026.[Abstract/Free Full Text]
  18. Drouin E., Lande G., Charpentier F. Amiodarone reduces transmural heterogeneity of repolarization in the human heart. J Am Coll Cardiol (1998) 32:1063–1067.[Abstract/Free Full Text]
  19. Rodriguez-Sinovas A., Cinca J., Tapias A., Armadans L., Tresanchez M., Soler-Soler J. Lack of evidence of M-cells in porcine left ventricular myocardium. Cardiovasc Res (1997) 33:307–313.[Abstract/Free Full Text]
  20. Antzelevitch C. Are M cells present in the ventricular myocardium of the pig? A question of maturity. Cardiovasc Res (1997) 36:127–128.[Free Full Text]
  21. Nabauer M. Electrical heterogeneity in the ventricular wall and the M cell. Cardiovasc Res (1998) 40:248–250.[Free Full Text]
  22. Verdouw P.D., Wolffenbuttel B.H.R., Van der Giessen W.J. Domestic pigs in the study of myocardial ischemia. Eur Heart J (1983) 4:61–67.
  23. Shen Y.T., Vatner S.F. Mechanism of impaired myocardial function during progressive coronary stenosis in conscious pigs. Hibernation versus stunning? Circ Res (1995) 76:479–488.[Abstract/Free Full Text]
  24. Spinale F.G., Fulbright B.M., Mukherjee R., et al. Relation between ventricular and myocyte function with tachycardia-induced cardiomyopathy. Circ Res (1992) 71:174–187.[Abstract/Free Full Text]
  25. Hinton R.B., Hebbar L., Cox M.H., et al. Left ventricular regional myocyte contractility in normal and heart failure states. J Mol Cell Cardiol (1997) 29:1939–1946.[CrossRef][ISI][Medline]
  26. Lambrigts D., Sachs D.H., Cooper D.K. Discordant organ xenotransplantation in primates: world experience and current status. Transplantation (1998) 66:547–561.[CrossRef][ISI][Medline]
  27. Minanov O.P., Itescu S., Michler R.E. Recent advances and the potential for clinical use of xenotransplantation. Curr Opin Cardiol (1996) 11:214–220.[CrossRef][ISI][Medline]
  28. Bailey L.L., Gundry S.R. Survival following orthotopic cardiac xenotransplantation between juvenile baboon recipients and concordant and discordant donor species: foundation for clinical trials. World J Surg (1997) 21:943–950.[CrossRef][ISI][Medline]
  29. Crick S.J., Sheppard M.N., Ho S.Y., Gebstein L., Anderson R.H. Anatomy of the pig heart: comparisons with normal human cardiac structure. J Anat (1998) 193:105–119.[CrossRef][ISI][Medline]
  30. Ejima J., Martin D., Engle C., Sherman Z., Kunimoto S., Gettes L.S. Experimental Cardiology Group, University of North Carolina at Chapel Hill. Ability of activation recovery intervals to assess action potential duration during acute no-flow ischemia in the in situ porcine heart. J Cardiovasc Electrophysiol (1998) 9:832–844.[ISI][Medline]
  31. Janse M.J. Electrophysiological effects of myocardial ischaemia. Relationship with early ventricular arrhythmias. Eur Heart J (1986) 7(Suppl_A):35–43.
  32. Berman N.D., Wang L.Y., Ahmed A. The cellular electropharmacology of mexiletine combined with sotalol in porcine papillary muscle and Purkinje fibre. II. Effects on action potential duration and refractoriness. Can J Cardiol (1990) 6:423–430.[ISI][Medline]
  33. Mukherjee R., Hewett K.W., Spinale F.G. Myocyte electrophysiological properties following the development of supraventricular tachycardia-induced cardiomyopathy. J Mol Cell Cardiol (1995) 27:1333–1348.[CrossRef][ISI][Medline]
  34. Mukherjee R., Hewett K.W., Walker J.D., Basler C.G., Spinale F.G. Changes in L-type calcium channel abundance and function during the transition to pacing-induced congestive heart failure. Cardiovasc Res (1998) 37:432–444.[Abstract/Free Full Text]
  35. Sipido K.R., Stankovicova T., Flameng W., Vanhaecke J., Verdonck F. Frequency dependence of Ca2+ release from the sarcoplasmic reticulum in human ventricular myocytes from end-stage heart failure. Cardiovasc Res (1998) 37:478–488.[Abstract/Free Full Text]
  36. Sipido K.R., Maes M.M., Van de Werf F. Low efficiency of Ca2+ entry through the Na/Ca exchanger as a trigger for Ca2+ release from the sarcoplasmic reticulum. Circ Res (1997) 81:1034–1044.[Abstract/Free Full Text]
  37. Gomez A.M., Valdivia H.H., Cheng H., et al. Defective excitation–contraction coupling in experimental cardiac hypertrophy and heart failure. Science (1997) 276:800–806.[Abstract/Free Full Text]
  38. Liu D.W., Antzelevitch C. Characteristics of the delayed rectifier current (IKr and IKs) in canine ventricular epicardial, midmyocardial, and endocardial myocytes. A weaker IKs contributes to the longer action potential of the M cell. Circ Res (1995) 76:351–365.[Abstract/Free Full Text]
  39. Antzelevitch C., Shimizu W., Yan G.X., et al. The M cell: its contribution to the ECG and to normal and abnormal electrical function of the heart. J Cardiovasc Electrophysiol (1999) 10:1124–1152.[ISI][Medline]
  40. Viswanathan P.C., Shaw R.M., Rudy Y. Effects of IKr and IKs heterogeneity on action potential duration and its rate dependence: a simulation study. Circulation (1999) 99:2466–2474.[Abstract/Free Full Text]
  41. Shimizu W., Antzelevitch C. Cellular and ionic basis for T-wave alternans under long-QT conditions. Circulation (1999) 99:1499–1507.[Abstract/Free Full Text]
  42. Boyden P.A., Albala A., Dresdner K.P. Jr. Electrophysiology and ultrastructure of canine subendocardial Purkinje cells isolated from control and 24-h infarcted hearts. Circ Res (1989) 65:955–970.[Abstract/Free Full Text]
  43. Callewaert G., Carmeliet E., Vereecke J. Single cardiac Purkinje cells: general electrophysiology and voltage-clamp analysis of the pacemaker current. J Physiol (Lond) (1984) 349:643–661.[Abstract/Free Full Text]
  44. Sipido K.R., Callewaert G., Carmeliet E. [Ca2+]i transients and [Ca2+]i-dependent chloride current in single Purkinje cells from rabbit heart. J Physiol (Lond) (1993) 468:641–667.[Abstract/Free Full Text]
  45. Gintant G.A. Regional differences in IK density in canine left ventricle: role of IK,s in electrical heterogeneity. Am J Physiol (1995) 268:H604–613.[ISI][Medline]
  46. Wettwer E., Amos G.J., Posival H., Ravens U. Transient outward current in human ventricular myocytes of subepicardial and subendocardial origin. Circ Res (1994) 75:473–482.[Abstract/Free Full Text]
  47. Nabauer M., Beuckelmann D.J., Uberfuhr P., Steinbeck G. Regional differences in current density and rate-dependent properties of the transient outward current in subepicardial and subendocardial myocytes of human left ventricle. Circulation (1996) 93:168–177.[Abstract/Free Full Text]
  48. Konarzewska H., Peeters G.A., Sanguinetti M.C. Repolarizing K+ currents in nonfailing human hearts. Similarities between right septal subendocardial and left subepicardial ventricular myocytes. Circulation (1995) 92:1179–1187.[Abstract/Free Full Text]
  49. Nabauer M., Barth A., Kaab S. Repolarization in pig left ventricle: absence of a relevant Ito1 and control by Ikr and Iks. Circulation (1997) 96:I–500. Abstract.
  50. Anyukhovsky E.P., Sosunov E.A., Gainullin R.Z., Rosen M.R. The controversial M cell. J Cardiovasc Electrophysiol (1999) 10:244–260.[ISI][Medline]

Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?


This article has been cited by other articles:


Home page
 Circ. Res.Home page
S. Ozdemir, V. Bito, P. Holemans, L. Vinet, J.-J. Mercadier, A. Varro, and K. R. Sipido
Pharmacological Inhibition of Na/Ca Exchange Results in Increased Cellular Ca2+ Load Attributable to the Predominance of Forward Mode Block
Circ. Res., June 6, 2008; 102(11): 1398 - 1405.
[Abstract] [Full Text] [PDF]

Home page
 Cardiovasc ResHome page
M.N. M. N. Obreztchikova, E.A. E. A. Sosunov, A. Plotnikov, E.P. E. P. Anyukhovsky, R. Z. Gainullin, P. Danilo Jr., Z.-H. Yeom, R. B. Robinson, and M. R. Rosen
Developmental changes in IKr and IKs contribute to age-related expression of dofetilide effects on repolarization and proarrhythmia
Cardiovasc Res, August 1, 2003; 59(2): 339 - 350.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Stankovicova, T.
Right arrow Articles by Sipido, K. R
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Stankovicova, T.
Right arrow Articles by Sipido, K. R
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?