Articles in PresS. Am J Physiol Heart Circ Physiol (January 10, 2014). doi:10.1152/ajpheart.00760.2013
1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Title: Mechanisms of Cardiac Conduction: A History of Revisions Authors: Rengasayee Veeraraghavan Ph.D,1 Robert Gourdie Ph.D,1,2 Steven Poelzing Ph.D1,2 Affiliations: 1 Virginia Tech Carilion Research Institute, and Center for Heart and Regenerative Medicine, Virginia Polytechnic University, Roanoke, VA 2 School of Biomedical Engineering and Sciences, Virginia Polytechnic University, Blacksburg, VA
Short Title: Cardiac Conduction
21 22 23 24
Address correspondence to: Steven Poelzing, Ph.D. Virginia Tech Carilion Research Institute 2 Riverside Circle Roanoke, VA 24016 TEL: (540) 526-2108 FAX: (540) 985-3373 e-mail:
[email protected]
Copyright © 2014 by the American Physiological Society.
2 Mechanisms of Cardiac Conduction: A History of Revisions
25 26 27
Abstract
28
Cardiac conduction is the process by which electrical excitation spreads
29
through the heart, triggering individual myocytes to contract in synchrony.
30
Defects in conduction disrupt synchronous activation and are associated with life-
31
threatening arrhythmias in many pathologies. Therefore, it is scarcely surprising
32
that this phenomenon continues to be the subject of active scientific inquiry. Here
33
we provide a brief review of how the conceptual understanding of conduction has
34
evolved over the last century and highlight recent, potentially paradigm-shifting
35
developments.
36 37
Background
38
Cardiac conduction is the process by which electrical activation is
39
communicated between myocytes, triggering their synchronous contraction.
40
Impulses originating in the sinoatrial node spread to the atria and via the
41
specialized His-Purkinje conduction system to the ventricles. The sequence of
42
activation thus achieved is key in translating the lengthwise contraction of
43
individual myocytes into the complex three-dimensional pumping motion of the
44
heart.
45
It is well established that aberrant ventricular conduction is associated with
46
a high risk of sudden cardiac death, presumably due to ventricular
47
arrhythmias.(67)
The
prevention
of
arrhythmias
caused
by
conduction
3 48
abnormalities remains a topic of intense research in part because there are many
49
factors that are thought to govern cardiac conduction. This review will focus on
50
canonical determinants of conduction -- cellular excitability, gap junctions and
51
tissue architecture in the context of the historical development of the theoretical
52
understanding of conduction in the ventricular myocardium. Additionally, long
53
theorized and new experimental evidence will be discussed concerning
54
alternative modes of action potential propagation from myocyte-to-myocyte.
55 56
The study of conduction
57
Dr. Theodor Wilhelm Engelmann is credited with determining in 1875 that
58
electrical activity in strips of frog atrial muscle spread through muscular
59
tissue.(27) It took 39 years before the first direct measurements of cardiac
60
conduction velocity were reported, when Dr. Thomas Lewis and his colleagues
61
first quantified the velocity of cardiac conduction from canine myocardium.(61)
62
Conduction velocity is still the primary metric for quantifying the spread of
63
electrical activity in cardiac muscle in part because of its conceptual simplicity
64
and the ease of measuring the time it takes for the electrical wavefront to travel a
65
known distance. More importantly, velocity as a metric of conduction has yielded
66
insights into the mechanisms of electrical activity spread, as conduction velocity
67
is not uniform in all directions from the point of stimulation. This issue of
68
direction-specific conduction spread has provided the foundation of scientific
69
inquiry and debate. But before researchers could even begin to understand the
70
biophysical mechanisms governing cardiac conduction velocity, they first had to
4 71
determine what caused the action potential, and to do that, the determinants of
72
tissue excitability had to be understood.
73 74
Tissue excitability
75
The definition of the term excitability has evolved with our understanding
76
of the mechanisms underlying this phenomenon. For the purpose of this review,
77
it is convenient to think of excitability in terms of the probability that a propagated
78
action potential will be triggered in response to some quantity of charge entering
79
a cell over a period of time. In neurons, Drs. Wallace Fenn and Doris Cobb(28)
80
suggested in 1936 that a propagated action potential was caused by sodium ions
81
entering an excitable cell and depolarizing the membrane. Subsequently, and
82
perhaps more famously, Drs. Alan Lloyd Hodgkin, Andrew Huxley and Bernard
83
Katz published a series of manuscripts on the topic of neuronal excitation.(39-46)
84
Finally, Drs. Alan Lloyd Hodgkin and Paul Horowicz demonstrated that
85
excitability in cardiac myocytes is based on similar mechanisms of sodium
86
entry.(38) These early studies, demonstrated that sodium conductance through
87
the cell membrane is very low when the cell is electrically quiescent, but
88
increases dramatically during the depolarization phase of the action potential.
89
These and other findings prompted Hodgkin and Huxley to suggest that there
90
may be specialized “pores” or channels through which Na+ permeates the cell
91
membrane.(44) In 1964, Dr. Toshio Narahashi and colleagues reported on the
92
sodium current blocking properties of tetrodotoxin,(72) and the use of
93
pharmacological blockers to elucidate properties of the sodium current began in
5 94
earnest. This line of investigation culminated in the landmark 1976 paper by Drs.
95
Erwin Neher and Bert Sackmann’s, in which currents were recorded from single
96
ion channels,(73)
97
voltage-gated sodium current (Nav1.5)(30) responsible for depolarizing the
98
preponderance of cardiac myocytes. Overall, it is now well-accepted that sodium
99
channel availability is an important determinant of cellular excitability.(95)
and finally, the identification of the cardiac isoform of the
100 101
Potassium Channels and Excitability
102
Potassium channels provide outward current that acts to set the resting
103
membrane potential and repolarize the membrane when it is depolarized. Thus,
104
these channels shape the action potential after the upstroke and their relevance
105
to conduction has been thought to be limited to an influence on resting
106
membrane potential. Under pathological conditions such as following ischemia,
107
changes in resting membrane potential are determined by potassium currents,
108
particularly the inward-rectifier current IK1 and the ATP-sensitive potassium
109
current IKATP. Since the resting membrane potential is a key determinant of
110
sodium channel availability, potassium currents have a significant influence on
111
conduction under such conditions. However, in recent years it has been
112
demonstrated that modulating potassium currents can also affect conduction
113
velocity and its dependence on the sodium current, independent of changes in
114
resting membrane potential: Partial inhibition of IK1 can speed up conduction
115
under normal physiological conditions but not when sodium channel availability is
116
compromised.(119) On the other hand, pharmacological activation of IKATP and
6 117
the slow component of the delayed-rectifier potassium current IKs both slow
118
conduction under normal physiological conditions; however, only the latter slows
119
conduction when sodium channel availability is reduced.(118) Overall, these
120
findings suggest that modulating voltage-dependent K+ currents affects
121
conduction independent of Na+ channel availability, whereas modulating K+
122
currents that do not display voltage-dependent kinetics only affect conduction
123
when Na+ channel availability is not reduced.
124
Further, ongoing studies suggest a co-dependence between the
125
membrane expression levels of inward rectifier potassium channels (Kir2.1) and
126
sodium channels (Nav1.5).(68) An additional line of evidence suggesting an
127
interrelationship between the two channel types comes from the recently
128
identified long QT syndrome type 9, where mutations in the scaffolding protein
129
caveolin-3 was associated with alterations in the biophysical properties of both
130
Kir2.1 and Nav1.5 channels. It is therefore likely that potassium channels will
131
continue to be a significant area of research focus in coming years.
132 133
Early Revisions and Models of Conduction
134
Cable Theory
135
In 1952 Dr. Silvio Weidmann demonstrated that electrical communication
136
between cardiac myocytes could be described using a theory first developed in
137
the 19th century by William Thomson (aka Lord Kelvin) to account for the
138
transmission of electrical signals through a transatlantic telegraph cable.(113,
139
114) Importantly, the application of what is known as cable or the continuous
7 140
core conductor theory supposes that myocardial tissue is a syncytium coupled
141
through purely resistive pathways. While the theory was initially applied to
142
understand action potential propagation through nerves, which are readily
143
conceived as a core of conductive material surrounded by a non-conductive
144
sheath, Weidman extended these treatments to understand electrical impulse
145
propagation
146
structure.(124)
through
cardiac
Purkinje
fibers,
which
are
cable-like
in
147
Briefly, for a fiber of radius a with intracellular resistance per unit length ri
148
and extracellular resistance per unit length re bounded by a membrane with
149
resistance per unit length rm and capacitance per unit length cm, the
150
transmembrane potential vm is can be described by:
151 rm ∂ 2 v m ∂v − c m rm m − v m = 0 2 ri + re ∂x ∂t
152 153 154
By extension, it was demonstrated that conduction velocity is related to ri and re
155
by:
156 157
θ=
k ri + re
158 159
where k is a constant representing membrane properties.(101) In the heart, ri and
160
re are determined by various anatomical structures that compose the electric
161
current path inside, outside and between myocytes.
8 162 163
Gap junctions
164
Up until 1954, many theorized that cytoplasmic continuity between
165
myocytes, but then Drs. Fritiof S. Sjostrand and Ebba Andersson showed by use
166
of electron microscopy that myocytes are fully bounded by a membrane.(96)
167
Therefore, the physical nature of electrical connectivity between cells remained
168
speculative. In the 1960’s, Dr. Lloyd Barr demonstrated the existence of low
169
resistance pathways between cardiac myocytes(5, 21), which he termed the
170
nexus. These structures have come to be known more widely as gap junctions,
171
the name having been coined by Drs. Milton W. Brightman and Thomas S.
172
Reese in 1969.(10)
173
Since their discovery, gap junctions have been intensely studied, with over
174
14,000 publications in the literature as of 2013. Each gap junction channel
175
consists of two hexameric connexon hemichannels, each in turn composed of 6
176
molecules from the connexin protein family.(97)
177
connexins 40, 43 and 45 are expressed(8, 9, 53) (122) which have different
178
conductances, permeabilities and cardiac tissue-specific expression patterns(15,
179
35). Further, different connexin isoforms can combine to form heterotypic and
180
heteromultimeric gap junctions which demonstrate composition-dependent
181
properties.(25, 69) However, the situation in mammalian ventricular myocardium
182
is somewhat simplified by the fact that it predominantly expresses connexin 43
183
(Cx43). Moreover, in the ventricle of humans, and many other mammals, Cx43 is
In the heart, three isoforms
9 184
almost exclusively localized to the intercalated disks(34). Thus myocytes are
185
coupled by gap junction channels in primarily end-to-end fashion.(49)
186
Since the majority of early conduction measurements in myocardial tissue
187
were performed macroscopically on Purkinje fibers, which resemble a cable, it is
188
unsurprising that cable theory continued to fit the available data well. To
189
reconcile the discovery of gap junctions with what appeared to be continuous
190
conduction, many deemed gap junctional conductance to be sufficiently high as
191
to render the cytoplasms of coupled myocytes electrically contiguous.(123, 125)
192
Gap junctional conductance was therefore, often incorporated into the
193
intracellular resistance term ri.
194 195
Anisotropic conduction
196
Although thought to be a true syncitium through the first half of the 20th
197
century, ultrastructural studies in the 1950s revealed the cellular nature of
198
cardiac muscle.(78, 81, 96) Atrial and ventricular myocardium came to be seen
199
as brick-wall-like structures composed of myocytes 100-150µm long and 10-
200
20µm wide.(19) Therefore, cardiac tissue is anisotropic, with lengthwise
201
orientation of cardiac myocytes and predominantly end-to-end gap junctional
202
coupling(49). This tissue architecture suggests that cardiac conduction should be
203
different parallel to the long axis relative to the short axis of myocytes. Drs. J.
204
Walter Woodbury and Wayne E. Crill showed in the late 1950’s that myocardium
205
exhibited a direction-dependent spatial decay of current injected into a point of
206
myocardium, with the longest decay, or space-constant, occurring parallel to the
10 207
long axis of moycytes and the shortest decay occurring perpendicular to
208
myocytes.(16) Continuous cable theory was quickly updated to 2 and 3-
209
dimensional models of anisotropic tissue to incorporate the vectors of directional
210
resistivity both inside (ri) and outside (re) cells.(50, 74, 75) From here, these
211
models as a group came to be referred to as ‘bidomain models’ and predicted
212
that cardiac conduction velocity in 2- and 3- dimensional tissue should be
213
anisotropic. By assuming that cardiac myocardium is a continuous but
214
anisotropic medium, Dr. L. Clerc demonstrated that conduction velocity parallel
215
and transverse to fibers is predicted by an inverse square relationship to total
216
axial resistance, similar to what would be anticipated from cable theory.(12)
217
Microelectrode recordings of conduction velocity from multiple sites of myocardial
218
tissue agreed well with these mathematical treatments, and therefore, anisotropic
219
conduction was linked to fiber orientation.(22, 93) In this context, it is important to
220
note that the continuous anisotropic resistivity envisaged by bidomain models is
221
a theoretical approximation. In tissue, end-to-end contacts between myocytes
222
can mediate both longitudinal and transverse coupling patterns.
223 224
Discontinuous Conduction
225
As technology improved to allow for electrical measurements at greater
226
spatial and temporal resolution, studies in the 1970’s revealed yet more
227
complications with the understanding of cardiac conduction. Were the
228
myocardium to behave as a syncytium, the time constant of slow depolarization
229
preceding sodium channel activation (τfoot) should depend on only axial and
11 230
membrane resistance and therefore, be independent of conduction velocity (θ).
231
The model also predicted that the maximal rate of rise of the transmembrane
232
potential (dV/dtmax) should depend only on sodium channel availability; therefore,
233
faster conduction should be associated with a larger dV/dtmax.(100) However,
234
ground-breaking microscopic measurements made by Dr. Madison Spach and
235
colleagues in both atrial and ventricular myocardium revealed a very different
236
picture: Longitudinal conduction, which is faster, was associated with a longer
237
τfoot and smaller dV/dtmax, relative to slower transverse conduction. (105, 106)
238
These experiments began to call into question the use of models based on
239
continuous cable theory to describe anisotropic conduction.
240
In order to explain these new discrepancies between measurements made
241
in tissue and those mathematically modeled, the concept of discontinuous
242
conduction was proposed since gap junctions may represent high resistance
243
pathways between myocytes, rather than the low resistance structures previously
244
assumed. In fact, direct measurements of gap junctional resistance revealed that
245
resistance at the intercalated disks is approximately equal to the axial resistance
246
of a myocyte.(87) Importantly, discontinuous conduction suggested that
247
conduction transverse to the myocyte axis might be slower and also more
248
discontinuous relative to conduction parallel to myocytes, and further
249
experimental evidence supported this assessment.(100, 106)
250
The development of discontinuous conduction theory needed to account
251
for a more complex type of axial resistance that included the following important
252
parameters. First, the mechanisms underlying the resistance of the intercalated
12 253
disks needed to be explored since the number and function of gap junctions can
254
dramatically affect gap junctional conductance. Second, a conduction wavefront
255
will encounter more discontinuities caused by gap junctions when it travels
256
transverse to myocytes relative to their longitudinal axis, as myocytes are shorter
257
than they are long. Since myocytes organize in a roughly brick-like structure in a
258
sheet or bundle, and the orientation of these sheets and bundles maintain a 3-
259
dimensional geometry that is optimized for the efficient propulsion of blood,
260
tissue geometry could not be treated as a simple 2 or 3-dimensional sheet. (101)
261
Case-in-point, myocyte orientation changes from the epicardium to the
262
endocardium,(60, 80, 110) and this complex rotational anisotropy has been
263
shown
264
conduction.(111)
to
produce
important
differences
when
measuring
cardiac
265 266
Contemporary Understanding of Anisotropic Conduction
267
Myocyte geometry
268
Cardiac myocytes do not maintain the same size and geometry over an
269
organism’s lifetime.(63, 103) Changes in cell size and composition can alter
270
cytoplasmic and extracellular resistances in direction-dependent manners
271
leading to altered conduction anisotropy(92, 106) (the ratio of longitudinal to
272
transverse conduction velocities). Once again, changes in myocyte geometry will
273
also alter the number of gap junctions encountered by the electrical wavefront
274
over a given distance.(58, 87, 89) Additionally, cellular hypertrophy over
275
postnatal life is accompanied by changes in GJ localization from uniform
13 276
distribution around the cell to predominantly intercalated disk localization(3, 36) -
277
further confounding the problem of understanding growth-related changes in
278
conduction.
279 280
It is noteworthy though that there remain open questions regarding the
281
relationship between cell size and conduction velocity. Some in silico studies
282
have suggested a positive correlation between cell size and conduction
283
velocity,(32, 103) while experiments in hypertrophied myocardium have
284
suggested a negative correlation between myocyte diameter and conduction
285
velocity.(65) A combined experimental and simulation study by Dr. Rob F.
286
Wiegerinck and colleagues found increased longitudinal, but not transverse
287
conduction velocity in failing rabbit hearts; however, the degree of increase was
288
insufficient to compensate for the increased path length resulting from
289
hypertrophy resulting in an increased total activation time.(126) To complicate
290
matters further, a recent study by Dr. Thomas Seidel and colleagues suggests
291
that myocyte shape may be as important as myocyte size, if not more so, with
292
respect to modulating conduction velocity in a predictable manner.(94)
293
It continues to be a challenge to understand the role that heterogeneous
294
cell geometry has on conduction in diseases such as cardiac hypertrophic
295
cardiomyopathy,(121) because these conditions often also affect the expression
296
and localization of membrane proteins, especially connexins. Additionally,
297
myocyte geometry can also change in the acute time scale in response to
298
ischemia(115) or osmolarity for example.(18)
14 299 300
Non-Myocytes
301
In addition to cardiac myocytes, the heart also consists of fibroblasts and
302
other non-myocyte cells. Indeed, these cells outnumber the myocytes although
303
the latter account for the majority of the heart's volume.(128) It has been
304
proposed that fibroblasts could influence cardiac conduction by either acting as
305
passive obstacles, or by coupling to myocytes and/or each other and acting as
306
either a current sink, or even participating actively in conduction. Additionally,
307
under pathophysiological conditions, fibroblasts can transform into myofibroblasts
308
which participate in inflammatory signaling and have also been proposed to
309
modulate conduction.(6) However, the extent to which fibroblasts and
310
myofibroblasts
311
myocardium remains a topic of ongoing inquiry.(51, 91, 127) For more detailed
312
discussion of the role of non-myocytes in conduction, the reader is referred to
313
reviews by Drs. Peter Kohl (55, 56) and Heather Duffy(6, 23).
electrotonically
couple
with
myocytes
in
the
ventricular
314 315
In the normal heart, fibroblasts are thought to provide structural support
316
directly, as well as by laying down the extracellular matrix.(6) However, under
317
pathophysiological conditions, excessive deposition of extracellular collagen can
318
occur, electrically isolating myocytes from each other, forming barriers to
319
conduction. Indeed, in an elegant study in aging human atrial fiber bundles, Dr.
320
Madison Spach demonstrated how collagenous septa deposited between
321
myocytes
create
a
tortuous
electrical
path
and
facilitate
reentrant
15 322
arrhythmias.(104) Likewise, in the ventricles, fibrosis can create resistive barriers
323
to conduction and act as an arrhythmogenic substrate. While a certain degree of
324
fibrosis occurs in aging hearts, it can be greatly exacerbated in a variety of
325
pathological conditions.(11, 33, 48)
326 327
Gap junctions
328
Given the role gap junctions play in electrically coupling myocytes, and
329
because they are remodeled so frequently in cardiac disease, they have received
330
the preponderance of experimental attention. While slowed conduction is well
331
correlated with an increased risk of arrhythmias, there is disagreement in the
332
literature concerning the relationship between the degree of gap junctional
333
uncoupling and conduction velocity changes. It is well established that
334
pharmacologically uncoupling gap junctions slows cardiac conduction.(57, 87,
335
88) With this observation comes an important experimental caveat: Most
336
manuscripts only report data from doses of gap junction uncouplers that
337
measurably slow conduction.(120) This binary data representation therefore
338
excludes the linear correlation between the degree of gap junction uncoupling
339
and conduction slowing – as well as non-specific (i.e., non-gap junction-related)
340
effects of the drugs. Importantly, pharmacologic gap junction inhibition studies
341
have provided sufficient evidence that gap junctions at the ends of myocytes
342
constitute the primary source of delay during microscopic conduction.(58, 89)
343
Thus, modulating gap junctional coupling preferentially impacts transverse
344
conduction velocity leading to altered anisotropy. (90, 92, 106)
16 345
Contrast pharmacologic gap junction manipulation with genetically
346
manipulating gap junction functional expression. There are a variety of studies on
347
cardiac conduction utilizing the same transgenic mouse lineage expressing 50%
348
of the wild-type levels of connexin43, the primary ventricular gap junction protein.
349
Some groups have reported significantly slowed conduction in mice with a 50%
350
reduction in Cx43 expression relative to wild-type mice,(26, 37) while others
351
could not measure a difference between them. (7, 17, 71, 109, 112, 116, 117)
352
These very basic studies are critically important to understanding cardiac
353
arrhythmias because gap junction remodeling is a hallmark of cardiac disease.
354
To make matters worse, in disease, the relationship between gap junction
355
remodeling and conduction is even less clear. For example, a reduction of total
356
Cx43 expression in a canine pacing induced heart failure model was associated
357
with aberrant ventricular conduction and increased arrhythmogenesis.(79) In a
358
similar model, gap junction remodeling (relocation) and conduction slowing
359
preceded loss of Cx43 expression.(2)
360
The phrase, “gap junctional remodeling” is a catchall term to describe
361
connexin redistribution at the cellular level, post-translational modification, and
362
total protein expression changes. While it has been demonstrated that cellular
363
redistribution of connexins to the lateral membrane is associated with altered
364
conduction, in hypertrophy for example, lateralization occurs concomitantly with
365
changes in myocyte geometry.(102) Whether or not lateralized Cx43 in the
366
pathological myocardium forms functional gap junctions remains an important
367
and complex question.(24) The connexin life-cycle is also dynamically regulated
17 368
by
a
variety
of
biochemical
pathways
including
phosphorylation
and
369
dephosphorylation of specific amino acid residues of the channel.(4, 82, 98, 99)
370
Therefore, gap junctions do not exist in isolation and are part of larger
371
macromolecular complexes and biological pathways.
372 373
Dynamic Determinants of Conduction:
374
In addition to the structural substrate, conduction is also modulated by
375
dynamic functional changes. Primarily, these dynamics result from the interplay
376
between the strength of the excitatory impulse - the source - and the electrical
377
load represented by the tissue it must excite - the sink. In the adult canine
378
ventricular myocardium, each myocyte is coupled to an average of 11±3 other
379
myocytes.(92) As an activation wavefront spreads through the myocardium, the
380
amount of source available per unit mass of tissue is determined by its
381
excitability whereas the balance between source and sink is determined by the
382
curvature of the wavefront and its interaction with the architecture of the
383
myocardium - intercellular coupling, fiber orientation, rotational anisotropy,
384
branching tissue geometry etc.(58, 88, 92, 106) Since local excitability in tissue is
385
dynamically modulated by changes in the shape and duration of action
386
potentials, mismatch between source and sink can arise locally and dynamically,
387
creating
388
Pathophysiological gap junction remodeling and fibrosis can exacerbate source-
389
sink mismatch and thereby the propensity for arrhythmias. For a more detailed
a
functional
substrate
for
arrhythmogenic
conduction
defects.
18 390
discussion of source-sink mismatch, the reader is referred to the in depth review
391
of electrotonic conduction by Drs. Andre Kleber and Yoram Rudy.(54)
392 393
Ion Channels at the Intercalated Disk: Functional Implications
394
The major impetus for reassessing our understanding of conduction arises
395
from structural insights into the subcellular localization of ion channels. In 1996,
396
Dr. Sidney A. Cohen published the first immunofluorescence images of rat TTX-
397
resistant sodium channels (rH1) demonstrating their strong localization at the
398
intercalated disk of cardiac myocytes.(13) While the importance of ion channels
399
at the intercalated disk has been long postulated as a mechanism of non-gap
400
junction mediated coupling, this work was mostly the domain of mathematical
401
models.(14, 59, 66, 70, 107, 108, 129) These models envision intercellular
402
coupling as occurring thusly: A depolarized myocyte withdraws sodium ions from
403
the restricted junctional cleft via its intercalated disk-localized Nav1.5 channels
404
(figure 1A). The resulting depletion of positive charge from the junctional cleft
405
would render the local extracellular electrical potential more negative.
406
Consequently, the transmembrane potential across the apposed membrane of
407
the neighboring myocyte becomes more positive, causing the activation of
408
Nav1.5 channels (figure 1B). Thus electrical activation is communicated from one
409
cell to another without the direct transfer of ions between them (figure 1C).
410
Dr. Nicholas Sperelakis is perhaps best known for championing these
411
mechanisms which he summarized in a 2002 review.(108) Later that year, Drs.
412
Jan P. Kucera, Stephan Rohr, and Yoram Rudy confirmed the presence of both
19 413
sodium channels and connexins at the intercalated disc.(59) More importantly,
414
they revised mathematical models of cardiac conduction in a one-dimensional
415
strand demonstrating that the high density of sodium channels at the intercalated
416
disk could impact cardiac conduction in previously un-appreciated ways.
417
Specifically, they concluded that gap junctions are still likely the principal
418
mechanism of electrical transmission between cells, but sodium channels at the
419
intercalated disk could modulate the conduction velocity - gap junction
420
relationship particularly if the space between the myocytes were very small and
421
densely packed with sodium channels.
422
Since then much evidence has been uncovered to support the existence
423
at the intercalated disk of a macromolecular complex containing the gap junction
424
protein Cx43, as well as cardiac sodium channels (Nav1.5). Indeed, Cx43 and
425
Nav1.5 were found to co-immunoprecipitate from mouse heart lysates(64) and
426
more recently to colocalize at the intercalated disk(76). Work from the group of
427
Dr. Mario Delmar has demonstrated that mechanical adhesion proteins first
428
localize to sites of cell-cell contact followed by recruitment of Cx43 gap junctions
429
and ankyrin-G, a sub-membrane adapter protein involved in localizing cardiac
430
sodium channels (Nav1.5) in the membrane.(31) Recent results from the
431
laboratory of Dr. Mario Delmar demonstrate a loss of Nav1.5 from the membrane
432
in conditional Cx43 knockout mice(52) and even suggest that Cx43 is important
433
for the recruitment of Nav1.5 channels into the membrane at the intercalated
434
disk.(1, 20)
20 435
In addition to sodium channels, evidence has also emerged placing
436
various potassium channel isoforms at the intercalated disk - specifically the
437
inward rectifier potassium channel (Kir2.1)(68), the ATP-sensitive potassium
438
channel (Kir6.2)(47), the delayed rectifier potassium channel (KvLQT1 encoded
439
by KCNQ1)(83) and the 'rapid' delayed rectifier potassium channel (Kv11.1
440
encoded by hERG)(130). As previously discussed, potassium channels can have
441
significant effects on conduction. Additionally, being localized at the intercalated
442
disk, they could also play a role in intercellular coupling via a potassium-
443
mediated ephaptic mechanism. Briefly, potassium efflux from a depolarized
444
myocyte could lead to a transient accumulation of potassium in the narrow
445
junctional cleft, causing the membrane of the neighboring myocyte to depolarize
446
via an inward potassium current.(108)
447
Although the presence of ion channels at the intercalated disk is
448
suggestive, the ephaptic coupling hypothesis has yet to be experimentally tested
449
in the heart. A key missing element has been the identification of a well-defined
450
structure that could serve as a functional unit of ephaptic coupling, an ephapse.
451
The localization of ion channels at the intercalated disk could have important
452
implications in this regard given the necessity of close apposition between
453
membranes of adjacent cells for ephaptic coupling.(62, 70) Taking the
454
experimental evidence together with predictions made by the models,
455
intermembrane spacing could be a key variable in identifying specific
456
microdomains within the intercalated disk that could function as an ephapse.
457
21 458
Interstitial Volume
459
As with any electrical circuit, one must not only consider electrical
460
conduction “forward” through the circuit, but also how the circuit is completed by
461
a current return path. In tissue, the return path can be the interstitial space
462
between the myocytes. At this point, it is useful to re-visit cable theory for a
463
moment, as there is no other mathematical theory that has been applied so
464
rigorously to cardiac conduction. Since myocyte geometry is anisotropic, the
465
interstitial space outside the cells is also anisotropic and can affect axial
466
resistance and thereby conduction in an anisotropic manner.(77, 101) Cable
467
theory-based models predict a direct proportionality between the volume of the
468
extracellular space and conduction velocity,32,81 and this has been experimentally
469
supported in the cable-like papillary muscle.(29)
470
However, one of our recent manuscripts provides evidence that
471
modulating the extracellular volume in a heart changes epicardial conduction in a
472
manner inconsistent with bidomain models of cardiac conduction that lack
473
ephaptic coupling.(120) Additionally, we demonstrated that small degrees of gap
474
junction uncoupling that did not alter conduction normally, significantly slowed
475
conduction when the interstitial space was increased. This study suggested that
476
the effects of small degrees of gap junctional uncoupling on cardiac conduction
477
can be unmasked by increasing the interstitial volume. What remains unknown
478
though is how increasing interstitial volume produces changes in cardiac
479
conduction inconsistent with bi-domain models.
22 480
As mentioned earlier, non-gap junction mediated coupling or "ephaptic"
481
coupling had been proposed by mathematical models, and the distance between
482
cells at the intercalated disk might be an important factor for mediating this type
483
of coupling. A candidate structure that could serve as a functional ephaptic unit
484
as definitive as a synapse or gap junction emerges from recent work by an
485
author of this review together with his colleague Dr. J Matthew Rhett. In these
486
studies a new feature of cardiac ultrastructure was described: The perinexus - a
487
juxta-gap
488
hemichannels wherein interaction between connexin43 and the cardiac sodium
489
channel (Nav1.5) occurs. Given the close (0-20 nm) apposition of membranes
490
from adjacent myocytes in the vicinity of the gap junction, the perinexus emerges
491
as a strong candidate structure for the cardiac ephapse.(84-86)
junction
membrane
microdomain
rich
in
undocked
connexin
492
As it stands, there is evidence from both experiments and mathematical
493
models to suggest that ephaptic coupling could be important to cardiac
494
conduction. While previously viewed as a possible alternative to electrotonic
495
coupling, ephaptic coupling has since come to be viewed as operating in tandem
496
with gap junctions, helping sustain conduction when gap junctional coupling is
497
compromised. To fully appreciate the role of ephaptic coupling, a multi-pronged
498
strategy will be necessary: a) Further whole-heart experiments will need to be
499
performed to generate additional examples of complex conduction; b) The
500
biochemical and functional ultrastructure of the ephaptic machinery will need to
501
be dissected by high resolution microscopic, molecular and physiological
23 502
methods and c) Multi-dimensional mathematical models will need to be revised to
503
incorporate ephaptic coupling and tested against the new experimental data.
504 505
Conduction: A new multi-factorial understanding
506
Like all science, the understanding of cardiac conduction has undergone
507
revisions and refinements over the last 130 years, and it appears that discoveries
508
will only continue to accelerate. The picture that is emerging though is that
509
cardiac conduction is not a simple phenomenon mechanistically determined by a
510
few independent biophysical parameters. Rather, cellular excitability, gap
511
junctional conductance, cell size, gap junction localization, sub-cellular
512
architecture, and ion channel localization are important and inter-related
513
determinants of the conduction phenomenon. While these factors were initially
514
studied vis-à-vis conduction in isolation using a reductionist approach, we are
515
beginning to appreciate that changes in one determinant can potentiate the
516
effects of altering another. This type of higher-level understanding of conduction
517
is critical for determining why certain therapies developed to treat conduction
518
defects are sometimes ineffective or even produce deleterious effects. While it
519
could be argued the inter-related nature and complexity of biological processes
520
means that we will never completely prevent conduction-related arrhythmias, we
521
would suggest that understanding the myriad factors underpinning conduction
522
could eventually provide individualized therapeutic targets that might provide for
523
improved treatment of patients suffering from disease of the heart.
524
24 525
WORK CITED
526
1.
527
E, and Delmar M. A novel noncanonical role of cx43 in the heart: ensuring the arrival of
528
nav1.5 to the intercalated disk. Heart rhythm : the official journal of the Heart Rhythm
529
Society 10: 1742, 2013.
530
2.
531
D, Tunin RS, Kass DA, and Tomaselli GF. Dynamic changes in conduction velocity
532
and gap junction properties during development of pacing-induced heart failure.
533
American journal of physiology Heart and circulatory physiology 293: H1223-1230, 2007.
534
3.
535
Magee AI, and Gourdie RG. Dissociated spatial patterning of gap junctions and cell
536
adhesion junctions during postnatal differentiation of ventricular myocardium. Circulation
537
research 80: 88-94, 1997.
538
4.
539
Rathlou NH, Andersen S, Jensen ON, Hennan JK, and Kjolbye AL. Identification of
540
ischemia-regulated phosphorylation sites in connexin43: A possible target for the
541
antiarrhythmic peptide analogue rotigaptide (ZP123). Journal of molecular and cellular
542
cardiology 40: 790-798, 2006.
543
5.
544
Structure of the Nexus in Cardiac Muscle. J Gen Physiol 48: 797-823, 1965.
545
6.
546
about? Journal of cardiovascular pharmacology 57: 376-379, 2011.
547
7.
548
Weingart R, Saffitz JE, and Kleber AG. Electrical propagation in synthetic ventricular
549
myocyte strands from germline connexin43 knockout mice. Circulation research 95: 170-
550
178, 2004.
Agullo-Pascual E, Lin X, Pfenniger A, Lubkemeier I, Willecke K, Rothenberg
Akar FG, Nass RD, Hahn S, Cingolani E, Shah M, Hesketh GG, DiSilvestre
Angst BD, Khan LU, Severs NJ, Whitely K, Rothery S, Thompson RP,
Axelsen LN, Stahlhut M, Mohammed S, Larsen BD, Nielsen MS, Holstein-
Barr L, Dewey MM, and Berger W. Propagation of Action Potentials and the
Baum J and Duffy HS. Fibroblasts and myofibroblasts: what are we talking
Beauchamp P, Choby C, Desplantez T, de Peyer K, Green K, Yamada KA,
25 551
8.
552
gap junction proteins. The Journal of biological chemistry 265: 14439-14443, 1990.
553
9.
554
homologous to a gap junction protein from liver. The Journal of cell biology 105: 2621-
555
2629, 1987.
556
10.
557
membranes in the vertebrate brain. The Journal of cell biology 40: 648-677, 1969.
558
11.
559
cardiology 38: 103-123, 2013.
560
12.
561
mammalian heart. J Physiol 255: 335-346, 1976.
562
13.
563
heart atria and ventricle. Presence in terminal intercalated disks. Circulation 94: 3083-
564
3086, 1996.
565
14.
566
junctional electric potential. Journal of mathematical biology 57: 265-284, 2008.
567
15.
568
preferentially associated with the ventricular conduction system in mouse and rat heart.
569
Circulation research 82: 232-243, 1998.
570
16.
571
transmembrane potentials of cultured chick embryo heart cells. Am J Physiol 197: 733-
572
735, 1959.
573
17.
574
Modulation of cardiac gap junction expression and arrhythmic susceptibility. Circulation
575
research 95: 1035-1041, 2004.
Beyer EC. Molecular cloning and developmental expression of two chick embryo
Beyer EC, Paul DL, and Goodenough DA. Connexin43: a protein from rat heart
Brightman MW and Reese TS. Junctions between intimately apposed cell
Calkins H. Arrhythmogenic right ventricular dysplasia. Current problems in
Clerc L. Directional differences of impulse spread in trabecular muscle from
Cohen SA. Immunocytochemical localization of rH1 sodium channel in adult rat
Copene ED and Keener JP. Ephaptic coupling of cardiac cells through the
Coppen SR, Dupont E, Rothery S, and Severs NJ. Connexin45 expression is
Crill WE, Rumery RE, and Woodbury JW. Effects of membrane current on
Danik SB, Liu F, Zhang J, Suk HJ, Morley GE, Fishman GI, and Gutstein DE.
26 576
18.
577
failing heart. Opposite effects of angiotensin II and angiotensin (1-7) on cell volume
578
regulation. Mol Cell Biochem 330: 211-217, 2009.
579
19.
De Mello WC. Electrical phenomena in the heart: Academic Press, 1972.
580
20.
Delmar M. Connexin43 regulates sodium current; ankyrin-G modulates gap
581
junctions: the intercalated disc exchanger. Cardiovascular research 93: 220-222, 2012.
582
21.
583
the Nexus. Science 137: 670-672, 1962.
584
22.
585
tissues of various mammals. Q J Exp Physiol Cogn Med Sci 44: 91-109, 1959.
586
23.
587
Journal of cardiovascular pharmacology 57: 373-375, 2011.
588
24.
589
the official journal of the Heart Rhythm Society 9: 1331-1334, 2012.
590
25.
591
docking of Cx43 and Cx45 connexons blocks fast voltage gating of Cx43. Biophysical
592
journal 81: 1406-1418, 2001.
593
26.
594
Rosenbaum DS. High resolution optical mapping reveals conduction slowing in
595
connexin43 deficient mice. Cardiovascular research 51: 681-690, 2001.
596
27.
597
11: 465-480, 1875.
598
28.
599
American journal of physiology 115: 345-356, 1936.
De Mello WC. Cell swelling, impulse conduction, and cardiac arrhythmias in the
Dewey MM and Barr L. Intercellular Connection between Smooth Muscle Cells:
Draper MH and Mya-Tu M. A comparison of the conduction velocity in cardiac
Duffy HS. Fibroblasts, myofibroblasts, and fibrosis: fact, fiction, and the future.
Duffy HS. The molecular mechanisms of gap junction remodeling. Heart rhythm :
Elenes S, Martinez AD, Delmar M, Beyer EC, and Moreno AP. Heterotypic
Eloff BC, Lerner DL, Yamada KA, Schuessler RB, Saffitz JE, and
Engelmann TW. Ueber die Leitung der Erregung im Herzmuskel. Pflüger, Arch
Fenn W and Cobb D. Electrolyte changes in muscle during activity. The
27 600
29.
601
and microvascular space as determinants of the extracellular electrical field and velocity
602
of propagation in ventricular myocardium. Circulation 92: 587-594, 1995.
603
30.
604
currents. Cardiovascular research 55: 1-8, 2002.
605
31.
606
Russell MW. Ordered assembly of the adhesive and electrochemical connections within
607
newly formed intercalated disks in primary cultures of adult rat cardiomyocytes. J
608
Biomed Biotechnol 2010: 624719, 2010.
609
32.
610
reduced conduction reserve in the diabetic rat heart: response to uncoupling and
611
reduced excitability. Ann Biomed Eng 38: 1415-1425, 2010.
612
33.
613
fibrosis. 2. Contributory pathways leading to myocardial fibrosis: moving beyond
614
collagen expression. American journal of physiology Cell physiology 304: C393-402,
615
2013.
616
34.
617
mammalian myocardium revealed by an anti-peptide antibody and laser scanning
618
confocal microscopy. Journal of cell science 99 ( Pt 1): 41-55, 1991.
619
35.
620
Evidence for a distinct gap-junctional phenotype in ventricular conduction tissues of the
621
developing and mature avian heart. Circulation research 72: 278-289, 1993.
622
36.
623
patterns of gap junction connexins in the developing and mature rat heart. Anatomy and
624
embryology 185: 363-378, 1992.
Fleischhauer J, Lehmann L, and Kleber AG. Electrical resistances of interstitial
Fozzard HA. Cardiac sodium and calcium channels: a history of excitatory
Geisler SB, Green KJ, Isom LL, Meshinchi S, Martens JR, Delmar M, and
Ghaly HA, Boyle PM, Vigmond EJ, Shimoni Y, and Nygren A. Simulations of
Goldsmith EC, Bradshaw AD, and Spinale FG. Cellular mechanisms of tissue
Gourdie RG, Green CR, and Severs NJ. Gap junction distribution in adult
Gourdie RG, Green CR, Severs NJ, Anderson RH, and Thompson RP.
Gourdie RG, Green CR, Severs NJ, and Thompson RP. Immunolabelling
28 625
37.
626
KA, and Saffitz JE. Slow ventricular conduction in mice heterozygous for a connexin43
627
null mutation. J Clin Invest 99: 1991-1998, 1997.
628
38.
629
The Journal of physiology 145: 405-432, 1959.
630
39.
631
giant axon of Loligo. The Journal of physiology 116: 473-496, 1952.
632
40.
633
through the membrane of the giant axon of Loligo. The Journal of physiology 116: 449-
634
472, 1952.
635
41.
636
conductance in the giant axon of Loligo. The Journal of physiology 116: 497-506, 1952.
637
42.
638
nervous activity. Cold Spring Harbor symposia on quantitative biology 17: 43-52, 1952.
639
43.
640
fibers. Proc R Soc Lond B Biol Sci 140: 177-183, 1952.
641
44.
642
and its application to conduction and excitation in nerve. The Journal of physiology 117:
643
500-544, 1952.
644
45.
645
in the membrane of the giant axon of Loligo. The Journal of physiology 116: 424-448,
646
1952.
647
46.
648
giant axon of the squid. The Journal of physiology 108: 37-77, 1949.
649
47.
650
Taskin E, Zhandre M, Reid DA, Rothenberg E, Delmar M, and Coetzee WA.
Guerrero PA, Schuessler RB, Davis LM, Beyer EC, Johnson CM, Yamada
Hodgkin AL and Horowicz P. Movements of Na and K in single muscle fibres.
Hodgkin AL and Huxley AF. The components of membrane conductance in the
Hodgkin AL and Huxley AF. Currents carried by sodium and potassium ions
Hodgkin AL and Huxley AF. The dual effect of membrane potential on sodium
Hodgkin AL and Huxley AF. Movement of sodium and potassium ions during
Hodgkin AL and Huxley AF. Propagation of electrical signals along giant nerve
Hodgkin AL and Huxley AF. A quantitative description of membrane current
Hodgkin AL, Huxley AF, and Katz B. Measurement of current-voltage relations
Hodgkin AL and Katz B. The effect of sodium ions on the electrical activity of
Hong M, Bao L, Kefaloyianni E, Agullo-Pascual E, Chkourko H, Foster M,
29 651
Heterogeneity of ATP-sensitive K+ channels in cardiac myocytes: enrichment at the
652
intercalated disk. The Journal of biological chemistry 287: 41258-41267, 2012.
653
48.
654
Coronel R. The Brugada ECG pattern: a marker of channelopathy, structural heart
655
disease, or neither? Toward a unifying mechanism of the Brugada syndrome. Circulation
656
Arrhythmia and electrophysiology 3: 283-290, 2010.
657
49.
658
structure of intercellular junctions in canine myocardium. Circulation research 64: 563-
659
574, 1989.
660
50.
661
Oxford: Clarendon Press, 1975.
662
51.
663
on resting potential, impulse propagation, and repolarization: insights from a
664
microstructure model. American journal of physiology Heart and circulatory physiology
665
294: H2040-2052, 2008.
666
52.
667
TJ, Mohler PJ, Vos MA, van Veen TA, de Bakker JM, Delmar M, and van Rijen HV.
668
Reduced heterogeneous expression of Cx43 results in decreased Nav1.5 expression
669
and reduced sodium current that accounts for arrhythmia vulnerability in conditional
670
Cx43 knockout mice. Heart rhythm : the official journal of the Heart Rhythm Society 9:
671
600-607, 2012.
672
53.
673
junction proteins. Circulation research 70: 438-444, 1992.
674
54.
675
associated arrhythmias. Physiological reviews 84: 431-488, 2004.
Hoogendijk MG, Opthof T, Postema PG, Wilde AA, de Bakker JM, and
Hoyt RH, Cohen ML, and Saffitz JE. Distribution and three-dimensional
Jack JJB, Noble D, and Tsien RW. Electric Current Flow in Excitable Cells.
Jacquemet V and Henriquez CS. Loading effect of fibroblast-myocyte coupling
Jansen JA, Noorman M, Musa H, Stein M, de Jong S, van der Nagel R, Hund
Kanter HL, Saffitz JE, and Beyer EC. Cardiac myocytes express multiple gap
Kleber AG and Rudy Y. Basic mechanisms of cardiac impulse propagation and
30 676
55.
677
implications for ventricular arrhythmogenesis. Heart rhythm : the official journal of the
678
Heart Rhythm Society 4: 233-235, 2007.
679
56.
680
rhythm : the official journal of the Heart Rhythm Society 9: 461-464, 2012.
681
57.
682
effects of carbenoxolone on human myocardial conduction: a tool to investigate the role
683
of gap junctional uncoupling in human arrhythmogenesis. J Am Coll Cardiol 48: 1242-
684
1249, 2006.
685
58.
686
from optical mapping at the cellular level. J Electrocardiol 34 Suppl: 57-64, 2001.
687
59.
688
intercalated disks modulates cardiac conduction. Circulation research 91: 1176-1182,
689
2002.
690
60.
691
structure of the heart: ventricular myocyte arrangement and connective tissue
692
architecture in the dog. American Journal of Physiology - Heart and Circulatory
693
Physiology 269: H571-H582, 1995.
694
61.
695
Part I. The Auricles. Philosophical Transactions of the Royal Society of London Series B,
696
Containing Papers of a Biological Character 205: 375-420, 1914.
697
62.
698
Biomed Eng 60: 576-582, 2013.
699
63.
700
Gourdie RG. The rate and anisotropy of impulse propagation in the postnatal terminal
Kohl P and Camelliti P. Cardiac myocyte-nonmyocyte electrotonic coupling:
Kohl P and Camelliti P. Fibroblast-myocyte connections in the heart. Heart
Kojodjojo P, Kanagaratnam P, Segal OR, Hussain W, and Peters NS. The
Kucera JP, Kleber AG, and Rohr S. Slow conduction in cardiac tissue: insights
Kucera JP, Rohr S, and Rudy Y. Localization of sodium channels in
LeGrice IJ, Smaill BH, Chai LZ, Edgar SG, Gavin JB, and Hunter PJ. Laminar
Lewis T, Meakins J, and White PD. The Excitatory Process in the Dog's Heart.
Lin J and Keener JP. Ephaptic coupling in cardiac myocytes. IEEE Trans
Litchenberg WH, Norman LW, Holwell AK, Martin KL, Hewett KW, and
31 701
crest are correlated with remodeling of Cx43 gap junction pattern. Cardiovascular
702
research 45: 379-387, 2000.
703
64.
704
and nonphosphorylated sodium channel beta1 subunits are differentially localized in
705
cardiac myocytes. The Journal of biological chemistry 279: 40748-40754, 2004.
706
65.
707
hypertrophied left ventricular myocardium. J Cardiovasc Electrophysiol 8: 887-894,
708
1997.
709
66.
710
in smooth muscle and myocardial tissues. Electrotonic interaction]. Biofizika 24: 135-
711
140, 1979.
712
67.
713
electrocardiology 40: S118-122, 2007.
714
68.
715
PB, Hou L, Hu B, Schumacher SM, Vaidyanathan R, Martens JR, and Jalife J.
716
Dynamic reciprocity of sodium and potassium channel expression in a macromolecular
717
complex controls cardiac excitability and arrhythmia. Proc Natl Acad Sci U S A 109:
718
E2134-2143, 2012.
719
69.
720
formed by cardiac connexins. Cardiovascular research 62: 276-286, 2004.
721
70.
722
model with 3D electrodiffusion. Proc Natl Acad Sci U S A 105: 6463-6468, 2008.
723
71.
724
Characterization of conduction in the ventricles of normal and heterozygous Cx43
725
knockout mice using optical mapping. J Cardiovasc Electrophysiol 10: 1361-1375, 1999.
Malhotra JD, Thyagarajan V, Chen C, and Isom LL. Tyrosine-phosphorylated
McIntyre H and Fry CH. Abnormal action potential conduction in isolated human
Medvinskii AB and Pertsov AM. [Fiber interaction during impulse propagation
Mehra R. Global public health problem of sudden cardiac death. Journal of
Milstein ML, Musa H, Balbuena DP, Anumonwo JM, Auerbach DS, Furspan
Moreno AP. Biophysical properties of homomeric and heteromultimeric channels
Mori Y, Fishman GI, and Peskin CS. Ephaptic conduction in a cardiac strand
Morley GE, Vaidya D, Samie FH, Lo C, Delmar M, and Jalife J.
32 726
72.
727
Conductance Increase in Lobster Giant Axons. J Gen Physiol 47: 965-974, 1964.
728
73.
729
denervated frog muscle fibres. Nature 260: 799-802, 1976.
730
74.
731
Biol 41: 183-192, 1979.
732
75.
733
Bull Math Biol 41: 163-181, 1979.
734
76.
735
Bittihn P, Luther S, Lehnart SE, Hatem SN, Coulombe A, and Abriel H. SAP97 and
736
dystrophin macromolecular complexes determine two pools of cardiac sodium channels
737
Nav1.5 in cardiomyocytes. Circulation research 108: 294-304, 2011.
738
77.
Plonsey R and Barr RC. Bioelectricity: a quantitative approach: Springer, 2007.
739
78.
Poche R and Lindner E. [Study of the intercalated discs of heart muscle tissue
740
in warm-blooded and cold-blooded animals]. Zeitschrift fur Zellforschung und
741
mikroskopische Anatomie 43: 104-120, 1955.
742
79.
743
arrhythmia substrate in heart failure. American journal of physiology Heart and
744
circulatory physiology 287: H1762-1770, 2004.
745
80.
746
nature of intercellular coupling across ventricular wall. American journal of physiology
747
Heart and circulatory physiology 289: H1428-1435, 2005.
748
81.
749
myocardium of dogs with particular reference to histologic study by electron microscopy.
750
The Journal of experimental medicine 101: 687-694, 1955.
Narahashi T, Moore JW, and Scott WR. Tetrodotoxin Blockage of Sodium
Neher E and Sakmann B. Single-channel currents recorded from membrane of
Peskoff A. Electric potential in cylindrical syncytia and muscle fibers. Bull Math
Peskoff A. Electric potential in three-dimensional electrically syncytial tissues.
Petitprez S, Zmoos AF, Ogrodnik J, Balse E, Raad N, El-Haou S, Albesa M,
Poelzing S and Rosenbaum DS. Altered connexin43 expression produces
Poelzing S, Roth BJ, and Rosenbaum DS. Optical measurements reveal
Price KC, Weiss JM, Hata D, and Smith JR. Experimental needle biopsy of the
33 751
82.
752
NH, Nielsen MS, and Braunstein TH. Phosphorylation of connexin43 on serine 306
753
regulates electrical coupling. Heart rhythm : the official journal of the Heart Rhythm
754
Society 6: 1632-1638, 2009.
755
83.
756
Jorgensen NK. Subcellular localization of the delayed rectifier K(+) channels KCNQ1
757
and ERG1 in the rat heart. American journal of physiology Heart and circulatory
758
physiology 286: H1300-1309, 2004.
759
84.
760
organization. Heart rhythm : the official journal of the Heart Rhythm Society 9: 619-623,
761
2012.
762
85.
763
Na(v)1.5 in the cardiomyocyte perinexus. J Membr Biol 245: 411-422, 2012.
764
86.
765
Sign-post on the path to a new model of cardiac conduction? Trends Cardiovasc Med,
766
2013.
767
87.
768
Cardiovascular research 62: 309-322, 2004.
769
88.
770
of a reduction of excitability versus a reduction of electrical coupling on microconduction.
771
Circulation research 83: 781-794, 1998.
772
89.
773
Chains of Single Ventricular Myocytes in Culture: Multiple Site Optical Recording of
774
Transmembrane Voltage (MSORTV) Suggests Propagation Delays at the Junctional
775
Sites Between Cells. The Biological Bulletin 183: 342-343, 1992.
Procida K, Jorgensen L, Schmitt N, Delmar M, Taffet SM, Holstein-Rathlou
Rasmussen HB, Moller M, Knaus HG, Jensen BS, Olesen SP, and
Rhett JM and Gourdie RG. The perinexus: a new feature of Cx43 gap junction
Rhett JM, Ongstad EL, Jourdan J, and Gourdie RG. Cx43 associates with
Rhett JM, Veeraraghavan R, Poelzing S, and Gourdie RG. The perinexus:
Rohr S. Role of gap junctions in the propagation of the cardiac action potential.
Rohr S, Kucera JP, and Kleber AG. Slow conduction in cardiac tissue, I: effects
Rohr S and Salzberg BM. Discontinuities in Action Potential Propagation Along
34 776
90.
777
voltage (MSORTV) in patterned growth heart cell cultures: assessing electrical behavior,
778
with microsecond resolution, on a cellular and subcellular scale. Biophys J 67: 1301-
779
1315, 1994.
780
91.
781
fibroblasts and their interaction with myocytes. Annals of biomedical engineering 36: 41-
782
56, 2008.
783
92.
784
determinants of anisotropic conduction velocity in canine atrial and ventricular
785
myocardium. Circulation research 74: 1065-1070, 1994.
786
93.
787
velocity in the cardiac ventricular syncytium studied by microelectrodes. Circulation
788
research 7: 262-267, 1959.
789
94.
790
and connexin lateralization in cardiac tissue. Biophys J 99: 2821-2830, 2010.
791
95.
792
Roles of the sodium and L-type calcium currents during reduced excitability and
793
decreased gap junction coupling. Circulation research 81: 727-741, 1997.
794
96.
795
of cardiac muscle tissue. Experientia 10: 369-370, 1954.
796
97.
797
Cardiovascular research 62: 228-232, 2004.
798
98.
799
biological effects. Biochem J 419: 261-272, 2009.
800
99.
801
gap junction life cycle. J Membr Biol 217: 35-41, 2007.
Rohr S and Salzberg BM. Multiple site optical recording of transmembrane
Sachse FB, Moreno AP, and Abildskov JA. Electrophysiological modeling of
Saffitz JE, Kanter HL, Green KG, Tolley TK, and Beyer EC. Tissue-specific
Sano T, Takayama N, and Shimamoto T. Directional difference of conduction
Seidel T, Salameh A, and Dhein S. A simulation study of cellular hypertrophy
Shaw RM and Rudy Y. Ionic mechanisms of propagation in cardiac tissue.
Sjostrand FS and Andersson E. Electron microscopy of the intercalated discs
Sohl G and Willecke K. Gap junctions and the connexin protein family.
Solan JL and Lampe PD. Connexin43 phosphorylation: structural changes and
Solan JL and Lampe PD. Key connexin 43 phosphorylation events regulate the
35 802
100.
803
cardiac conduction. Circulation research 92: 125-126, 2003.
804
101.
805
communication: role in structural and electrical development and remodeling of the
806
heart. Heart rhythm : the official journal of the Heart Rhythm Society 1: 500-515, 2004.
807
102.
808
conduction caused by remodeling cell size and the cellular distribution of gap junctions
809
and Na(+) channels. J Electrocardiol 34 Suppl: 69-76, 2001.
810
103.
811
of remodeling cardiac gap junctions and cell size: experimental and model studies of
812
normal cardiac growth. Circulation research 86: 302-311, 2000.
813
104.
814
conduction disturbances in aging human atrial bundles: experimental and model study.
815
Heart rhythm : the official journal of the Heart Rhythm Society 4: 175-185, 2007.
816
105.
817
CE, Jr. The functional role of structural complexities in the propagation of depolarization
818
in the atrium of the dog. Cardiac conduction disturbances due to discontinuities of
819
effective axial resistivity. Circulation research 50: 175-191, 1982.
820
106.
821
Johnson EA. The discontinuous nature of propagation in normal canine cardiac muscle.
822
Evidence for recurrent discontinuities of intracellular resistance that affect the membrane
823
currents. Circulation research 48: 39-54, 1981.
824
107.
825
myocardial cells. Circulation research 91: 985-987, 2002.
826
108.
827
abutting excitable cells. IEEE Eng Med Biol Mag 21: 77-89, 2002.
Spach MS. Transition from a continuous to discontinuous understanding of
Spach MS, Heidlage JF, Barr RC, and Dolber PC. Cell size and
Spach MS, Heidlage JF, Dolber PC, and Barr RC. Changes in anisotropic
Spach MS, Heidlage JF, Dolber PC, and Barr RC. Electrophysiological effects
Spach MS, Heidlage JF, Dolber PC, and Barr RC. Mechanism of origin of
Spach MS, Miller WT, 3rd, Dolber PC, Kootsey JM, Sommer JR, and Mosher
Spach MS, Miller WT, 3rd, Geselowitz DB, Barr RC, Kootsey JM, and
Sperelakis N. An electric field mechanism for transmission of excitation between
Sperelakis N and McConnell K. Electric field interactions between closely
36 828
109.
829
Stuijvenberg L, van der Nagel R, Bezzina CR, Hauer RN, de Bakker JM, and van
830
Rijen HV. Combined reduction of intercellular coupling and membrane excitability
831
differentially affects transverse and longitudinal cardiac conduction. Cardiovascular
832
research 83: 52-60, 2009.
833
110.
834
Fiber orientation in the canine left ventricle during diastole and systole. Circulation
835
research 24: 339-347, 1969.
836
111.
837
and intramural excitation during ventricular pacing: effect of myocardial structure.
838
American journal of physiology Heart and circulatory physiology 294: H1753-1766, 2008.
839
112.
840
AG. Impulse propagation in synthetic strands of neonatal cardiac myocytes with
841
genetically reduced levels of connexin43. Circulation research 92: 1209-1216, 2003.
842
113.
843
Royal Society of London 8: 121-132, 1856.
844
114.
845
Society of London 7: 382-399, 1854.
846
115.
847
Durrer D. Tissue osmolality, cell swelling, and reperfusion in acute regional myocardial
848
ischemia in the isolated porcine heart. Circulation research 49: 364-381, 1981.
849
116.
850
Jalife J. Null mutation of connexin43 causes slow propagation of ventricular activation in
851
the late stages of mouse embryonic development. Circulation research 88: 1196-1202,
852
2001.
Stein M, van Veen TA, Remme CA, Boulaksil M, Noorman M, van
Streeter DD, Jr., Spotnitz HM, Patel DP, Ross J, Jr., and Sonnenblick EH.
Taccardi B, Punske BB, Macchi E, Macleod RS, and Ershler PR. Epicardial
Thomas SP, Kucera JP, Bircher-Lehmann L, Rudy Y, Saffitz JE, and Kleber
Thomson W. On Peristaltic Induction of Electric Currents. Proceedings of the
Thomson W. On the Theory of the Electric Telegraph. Proceedings of the Royal
Tranum-Jensen J, Janse MJ, Fiolet WT, Krieger WJ, D'Alnoncourt CN, and
Vaidya D, Tamaddon HS, Lo CW, Taffet SM, Delmar M, Morley GE, and
37 853
117.
854
Opthof T, and de Bakker JM. Slow conduction and enhanced anisotropy increase the
855
propensity for ventricular tachyarrhythmias in adult mice with induced deletion of
856
connexin43. Circulation 109: 1048-1055, 2004.
857
118.
858
Potassium channel activators differentially modulate the effect of sodium channel
859
blockade on cardiac conduction. Acta Physiol (Oxf) 207: 280-289, 2013.
860
119.
861
ventricular conduction sensitivity to flecainide challenge. Cardiovascular research 77:
862
749-756, 2008.
863
120.
864
the conduction velocity-gap junction relationship. American journal of physiology Heart
865
and circulatory physiology 302: H278-286, 2012.
866
121.
867
Delacretaz E. Connexin 43 expression in human hypertrophied heart due to pressure
868
and volume overload. Physiol Res 59: 35-42, 2010.
869
122.
870
differences in connexin expression in the human heart. Journal of molecular and cellular
871
cardiology 31: 991-1003, 1999.
872
123.
873
mammalian cardiac muscle. J Physiol 187: 323-342, 1966.
874
124.
875
1952.
876
125.
877
Physiol 210: 1041-1054, 1970.
van Rijen HV, Eckardt D, Degen J, Theis M, Ott T, Willecke K, Jongsma HJ,
Veeraraghavan R, Larsen AP, Torres NS, Grunnet M, and Poelzing S.
Veeraraghavan R and Poelzing S. Mechanisms underlying increased right
Veeraraghavan R, Salama ME, and Poelzing S. Interstitial volume modulates
Vetter C, Zweifel M, Zuppinger C, Carrel T, Martin D, Haefliger JA, and
Vozzi C, Dupont E, Coppen SR, Yeh HI, and Severs NJ. Chamber-related
Weidmann S. The diffusion of radiopotassium across intercalated disks of
Weidmann S. The electrical constants of Purkinje fibres. J Physiol 118: 348-360,
Weidmann S. Electrical constants of trabecular muscle from mammalian heart. J
38 878
126.
879
Opthof T, Wilders R, de Bakker JM, and Coronel R. Larger cell size in rabbits with
880
heart failure increases myocardial conduction velocity and QRS duration. Circulation
881
113: 806-813, 2006.
882
127.
883
fibroblast-myocyte coupling on cardiac conduction and vulnerability to reentry: A
884
computational study. Heart rhythm : the official journal of the Heart Rhythm Society 6:
885
1641-1649, 2009.
886
128.
887
cardiology 31: 211-219, 1973.
888
129.
889
intramural versus surface activation. Cardiovascular research 69: 98-106, 2006.
890
130.
891
Yang B, and Wang Z. Restoring depressed HERG K+ channel function as a mechanism
892
for insulin treatment of abnormal QT prolongation and associated arrhythmias in diabetic
893
rabbits. American journal of physiology Heart and circulatory physiology 291: H1446-
894
1455, 2006.
895
Wiegerinck RF, Verkerk AO, Belterman CN, van Veen TA, Baartscheer A,
Xie Y, Garfinkel A, Camelliti P, Kohl P, Weiss JN, and Qu Z. Effects of
Zak R. Cell proliferation during cardiac growth. The American journal of
Zemlin CW, Mironov S, and Pertsov AM. Near-threshold field stimulation:
Zhang Y, Xiao J, Wang H, Luo X, Wang J, Villeneuve LR, Zhang H, Bai Y,
39 896
Figure legend
897
Figure 1: Schematic cartoon illustrating the mechanism of ephaptic coupling. A) Sodium
898
channels (shown in red) on the depolarized (green) myocyte's membrane activate and
899
withdraw sodium ions (Na+) from the restricted extracellular cleft at the intercalated disk.
900
As a result, the transmembrane potential (Vm,1) of the first myocyte is elevated. B) The
901
concomitant depletion of positive charge from the extracellular cleft lowers the local
902
extracellular potential (Φe). This leads to an increase in the second, resting (red)
903
myocyte's transmembrane potential (Vm,2) - defined as the difference between its
904
intracellular potential and the extracellular potential (Φe). In turn, sodium channels
905
located at or near the intercalated disk of the second myocyte activate. C) Sodium
906
enters the second myocyte via these channels further depolarizing it and triggering an
907
action potential. Thus activation is communicated 'ephaptically' from cell-to-cell without
908
the direct transfer of ions between them.
909
A) + +
+
+
+
+ +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Vm,2
Vm,1 B)
+
+ +
+ +
+ +
+
+
+
+
Vm,2
Vm,1 C) + +
Vm,1
+ +
+ +
+ +
+
+
+
+
Vm,2
