Tài liệu Báo cáo khoa học: Synchronization of Ca2+ oscillations: a coupled oscillator-based mechanism in smooth muscle ppt

MINIREVIEW
Synchronization of Ca
2+
oscillations: a coupled
oscillator-based mechanism in smooth muscle
Mohammad S. Imtiaz
1
, Pierre-Yves von der Weid
1
and Dirk F. van Helden
2
1 Department of Physiology and Pharmacology, University of Calgary, Alberta, Canada
2 School of Biomedical Sciences, University of Newcastle, Callaghan, NSW, Australia
Long-range signaling
Biological organs display coordinated activities that
can extend over large distances. The spatial extent of
signaling required for such long-distance coordination
is many orders of magnitude greater than the size of
the participating cells; for example, coordinated con-
tractions of the intestine can occur over 250 cm
lengths [1], whereas smooth muscle cells are small
(typical size range 50–200 lm [2]). The problem is
further exacerbated when one considers that millions
of cells, each with its own intrinsic rhythm, partici-
pate in this ‘mob action’, and yet a meaningful global
outcome emerges. It is fascinating that in systems
such as the gut, even isolated muscle tissue
preparations continue to show coordinated rhythmic
contractions in the absence of any external neural
control [3]; thus, in such systems, the synchronizing
mechanism is embedded within the rhythmically oscil-

lating cells themselves. In this article, we review a
long-range signaling mechanism in smooth muscle
that explains global outcomes of local interactions [4–
10]. The main feature of this signaling mechanism is
coupled oscillator-based synchronization of Ca
2+
oscillations across cells, which drives membrane
potential changes and causes coordinated contrac-
tions. The key elements of this mechanism are a
Ca
2+
release–refill cycle of endoplasmic reticulum ⁄
Keywords
Ca
2+
oscillations; Ca
2+
stores; coupled
oscillators; lymphatics; slow waves;
synchronization
Correspondence
M. S. Imtiaz, Department of Physiology &
Pharmacology, Faculty of Medicine,
University of Calgary, Health Sciences
Centre, 3330 Hospital Drive NW, Calgary,
Alberta T2N 4N1, Canada
Fax: +1 403 210 8195
Tel: +1 403 210 9838
E-mail:
(Received 31 March 2009, revised

11 September 2009, accepted 14
October 2009)
doi:10.1111/j.1742-4658.2009.07437.x
Entrained oscillations in Ca
2+
underlie many biological pacemaking phe-
nomena. In this article, we review a long-range signaling mechanism in
smooth muscle that results in global outcomes of local interactions. Our
results are derived from studies of the following: (a) slow-wave depolariza-
tions that underlie rhythmic contractions of gastric smooth muscle; and (b)
membrane depolarizations that drive rhythmic contractions of lymphatic
smooth muscle. The main feature of this signaling mechanism is a coupled
oscillator-based synchronization of Ca
2+
oscillations across cells that
drives membrane potential changes and causes coordinated contractions.
The key elements of this mechanism are as follows: (a) the Ca
2+
release–
refill cycle of endoplasmic reticulum Ca
2+
stores; (b) Ca
2+
-dependent
modulation of membrane currents; (c) voltage-dependent modulation of
Ca
2+
store release; and (d) cell–cell coupling through gap junctions or
other mechanisms. In this mechanism, Ca
2+

stores alter the frequency of
adjacent stores through voltage-dependent modulation of store release.
This electrochemical coupling is many orders of magnitude stronger than
the coupling through diffusion of Ca
2+
or inositol 1,4,5-trisphosphate, and
thus provides an effective means of long-range signaling.
Abbreviations
[Ca
2+
]
c
, cytosolic Ca
2+
concentration; 18-b-GA, 18-b-glycyrrhetinic acid; ICC, interstitial cell of Cajal; Ins(1,4,5)P
3
, inositol 1,4,5-trisphosphate.
278 FEBS Journal 277 (2010) 278–285 ª 2009 The Authors Journal compilation ª 2009 FEBS
sarcoplasmic reticulum Ca
2+
stores, Ca
2+
-dependent
modulation of membrane currents, voltage-dependent
modulation of store release, and cell–cell coupling
through gap junctions or other mechanisms.
Ca
2+
store-based pacemaking
Gastric smooth muscle slow waves

Slow waves are rhythmic electrical depolarizations that
control the mechanical activity of many smooth mus-
cles [1,11–13] (Fig. 1). Slow waves cause entry of Ca
2+
through opening of L-type Ca
2+
channels and contrac-
tions of the smooth muscle. Cyclical release of Ca
2+
from inositol 1,4,5-trisphosphate [Ins(1,4,5)P
3
]-sensitive
endoplasmic Ca
2+
stores underlies the generation of
slow waves [12–15]. The store-generated change in
cytosolic Ca
2+
concentration ([Ca
2+
]
c
) causes opening
of excitatory channels, which allows inward current
flow and generates rhythmic pacemaker depolarization
[4,16–18]. However, the difficulty with oscillatory
Ca
2+
release providing a pacemaker mechanism is that
it requires synchronization of large numbers of stores

across many cells [4,19]. Gastric smooth muscle cells
and associated interstitial cells of Cajal (ICCs) form a
syncytium interconnected by gap junctions. Such syn-
cytia have low input impedance, and hence require a
massive amount of current to cause pacemaker depo-
larization. On the basis of experimental and theoretical
considerations, we now consider how Ca
2+
oscillations
can be synchronized across multiple cells in a syn-
cytium.
Synchronization of Ca
2+
oscillations
One reported means by which stores achieve local syn-
chrony is by Ca
2+
waves, a significant form of signal-
ing in living organisms [20–22]. Ca
2+
waves are
considered to be generated by the release of Ca
2+
from a dominant store, triggering Ca
2+
-induced Ca
2+
release from adjacent stores, and the continuation of
this process along the array of stores. However, Ca
2+

waves propagate relatively slowly, typically at
< 0.1 mmÆ s
)1
. Thus, Ca
2+
waves cannot explain the
synchrony of Ca
2+
oscillations underlying slow waves,
which appear to be conducted at velocities of many
millimeters per second.
Coupled oscillators
Another means by which stores can synchronize their
Ca
2+
release cycle is by coupled oscillator-based interac-
tions. The theory of coupled oscillators emerged from a
fortuitous observation of pendulum clocks by the Dutch
physicist Christiaan Huygens [23]. He noted that clock
pendulums could synchronize their oscillations even
though they were separated by distances of meters. This
synchronization of clock pendulums occurred through
coupling between the pendulums by transmission of
minute vibrations through the wall. An example of cou-
pled oscillators is a group of pendulums that are con-
nected to each other by springs. When all pendulums
are randomly set to swing, over time, interactions
through the springs result in the appearance of a global
synchrony pattern involving all the pendulums.
Fig. 1. Central interruption of intercellular

connectivity decouples slow waves.
Pacemaker potentials ⁄ slow waves simulta-
neously recorded at two sites along a
guinea pig gastric smooth muscle tissue
strip before (1), during (2) and after (3)
central application of 60 l
M 18-b-GA.
Decoupling commenced  1.5 min after
application of the blocker and was not
phase-locked, as more slow waves occurred
at site 2 than at site 1. For example, upon
commencement of decoupling, four slow
waves occurred at site 1 and five at site 2,
with delays between the slow waves
(site 2 ) site 1) of 0.8, 3.2, 7.9 and 9.5 s for
the first five sequential slow waves.
Nifedipine (1 l
M) was present throughout.
V
m
= )59 mV. Adapted from [8].
M. S. Imtiaz et al. Synchronization by voltage-modulated store release
FEBS Journal 277 (2010) 278–285 ª 2009 The Authors Journal compilation ª 2009 FEBS 279
An experiment that illustrates the underlying cou-
pled oscillator nature of slow waves involved a single
bundle strip of circular smooth muscle dissected from
the guinea pig gastric pylorus (Fig. 1). Initially, slow
waves occurred synchronously in the strip, as mea-
sured with two intracellular microelectrodes. When the
gap junction blocker 18-b-glycyrrhetinic acid (18-b-

GA; 40 mm) was applied centrally in a narrow stream
approximately 0.5 mm wide to this strip, slow waves
recorded at the two electrodes continued to occur but
were no longer synchronized. When 18-b-GA was
removed, slow waves in the two regions resynchro-
nized.
What is the mechanism of coupling
between Ca
2+
stores?
Oscillating Ca
2+
stores can interact by altering the
phase of adjacent oscillators through Ca
2+
-induced-
Ca
2+
release. Here, coupling by exchange of Ca
2+
[and ⁄ or Ins(1,4,5)P
3
for Ins(1,4,5)P
3
receptor-operated
stores] through gap junctions could serve as the spring
joining the pendulums in the above analogy. However,
coupling through release of Ca
2+
results in very weak

coupling, as the effective diffusion of Ca
2+
is limited
to very short distances ( 5 lm) [24]. The same applies
to coupling through diffusion of second messengers
such as Ins(1,4,5)P
3
, even though the effective diffu-
sion of Ins(1,4,5)P
3
is approximately three times higher
than that of Ca
2+
[24]. However, a candidate mecha-
nism that could serve as a coupling spring involves
electrical membrane potential changes caused by Ca
2+
store-activated inward current flow [5,8,18,25]. Electri-
cal coupling can be 100–1000 times stronger than
chemical coupling, as the electrical length constant of
smooth muscle (i.e. the distance needed for a steady-
state voltage resulting from current injection to
decrease to  37% of its original size) is typically in
the range 2–3 mm [26].
Finding experimental evidence that electrical cou-
pling is the key ‘spring’ interlinking the Ca
2+
stores
has involved repeating the decoupling experiment of
Fig. 1, but inhibiting the oscillators (i.e. the Ca

2+
stores) while leaving the connectivity between cells
intact [8]. An example of such an experiment is pre-
sented in which caffeine was used to block store Ca
2+
release and resulting slow-wave potentials (Fig. 2A).
Application of the caffeine-containing physiological sal-
ine solution to the central region of a single bundle
strip of guinea pig gastric circular smooth muscle
caused decoupling when the store inhibitor was applied
in a very wide stream about 5 mm in width, but not
when the stream was narrower (e.g. 3 mm; Fig. 2B).
These distances are commensurate with coupling being
20 mV
10 mV
2 min
F
F
0
=1
Ca
3.0 mm 5.0 mm
20 s
B
A
Caffeine
Caffeine
Control
Control
el1

el2
Fig. 2. Central interruption of stored Ca
2+
release decouples slow waves. (A) Caffeine
(0.5 m
M), applied to an Oregon Green-
loaded guinea pig gastric smooth muscle
tissue strip, blocked slow waves (upper
trace) and underlying Ca
2+
release-associ-
ated increases in [Ca
2+
]
c
(lower trace). F
0
,
baseline fluorescence; F, fluorescence;
nF ⁄ F
0
, relative change in fluorescence
normalized to baseline. (B) Slow waves
recorded at two sites 6 mm apart along a
strip before, during and after central applica-
tion of 1 m
M caffeine applied at widths of 3
and 5 mm. The 3 mm stream markedly
increased jitter between the delays. By con-
trast, the 5 mm stream decoupled the slow

waves. Decoupling commenced  1 min
after application of the blocker and was not
phase-locked, with slow waves at the two
recording sites now occurring at significantly
different frequencies (P < 0.05; frequencies
3.7 ± 0.1 per min and 4.4 ± 0.1 per min at
electrodes 1 and 2, respectively; n = 10).
Nifedipine (1 l
M) was present throughout in
(A) and (B). V
m
: (A) )56 mV; (B) ) 67 mV.
Adapted from [8].
Synchronization by voltage-modulated store release M. S. Imtiaz et al.
280 FEBS Journal 277 (2010) 278–285 ª 2009 The Authors Journal compilation ª 2009 FEBS
mediated by intercellular current flow in these strips,
which exhibited a length constant of about 3 mm. This
and related experiments [8] fit the hypothesis that oscil-
lations in stored Ca
2+
couple intercellularly across the
syncytial smooth muscle by electrical coupling to gener-
ate highly synchronous slow waves.
Modeling studies
As considered above, electrical conduction is many
orders of magnitude stronger than chemical coupling,
and this provides the ‘spring’ that underlies entrainment
of Ca
2+
stores to pace tissue syncytia. However, cou-

pled oscillator interactions also require chemical cou-
pling, in that store-generated changes in [Ca
2+
]
c
are
required to activate inward membrane current, with the
resulting membrane depolarization activating or
advancing the phase of other Ca
2+
stores. The electrical
and chemical transduction pathways are as depicted in
Fig. 3. The key mechanisms are as follows: (a) cyclical
release of Ca
2+
from stores can occur spontaneously
and is modulated by two signals – Ca
2+
and
Ins(1,4,5)P
3
; (b) release of Ca
2+
from stores activates
an inward current and depolarizes the membrane [18] –
thus, store oscillations are transformed into membrane
potential oscillations; (c) membrane potential can mod-
ulate store excitability ⁄ oscillations by modulating Ca
2+
and ⁄ or Ins(1,4,5)P

3
concentrations in the cytosol – this
provides a pathway for transforming electrical signals
into chemical signals to which the stores respond; (d)
cells are connected by gap junctions and form a syncy-
tium, so stores can now interact across cells through
electrical signals; and (e) the effective distance that
Ca
2+
and Ins(1,4,5)P
3
can diffuse is very short, in the
low micrometer range, whereas electrical coupling is in
the order of millimeters – thus, whereas stores are
weakly coupled through chemical diffusion, they are
strongly interconnected by electrical coupling.
We now illustrate the coupling mechanism outlined
above with a two-cell model example (Fig. 4). This sys-
tem is based on gastric smooth muscle, where depolar-
ization of the membrane is modeled to cause an
increase in Ins(1,4,5)P
3
concentration in the cytosol
[25]. Cytosolic Ca
2+
concentrations of two uncoupled
model cells are shown in Fig. 4A. Cell 1 (solid line) is
more sensitive to Ins(1,4,5)P
3
, and is therefore oscillat-

ing, whereas cell 2 (dashed line) is quiescent, because it
is less sensitive to Ins(1,4,5)P
3
. Electrical coupling is
then instituted between the two cells, and because of
voltage coupling-based interactions, cell 2 begins to
oscillate (Fig. 4B). This occurs because the oscillatory
Ca
2+
release from cell 1 (Fig. 4C) activates an inward
current, which, owing to electrical coupling, now depo-
larizes both cells (Fig. 4D). Depolarization in cell 2
causes an increase in cytosolic Ins(1,4,5)P
3
concen-
tration through voltage-dependent activation of
Ins(1,4,5)P
3
(Fig. 3), with the increased cytosolic
Ins(1,4,5)P
3
concentration causing generation of oscil-
lations in cell 2. Importantly, although the frequency
of the oscillations in cell 2 might be different to that of
cell 1, coupled oscillator interactions advance or retard
the cycle of each cell so that they remain entrained.
Chemical versus electrochemical
coupling
A similar sequence of events occurs when the above
example of two oscillators is extended to a system

Cytosol-Ca
2+
Ca
2+
St or e
+/ –
+/ –
Ins(1,4,5)P
3
(V) or Ca
2+
(V)
Local
oscillato
r
Ca
2+
V
AT Pase
Cytosol-Ca
2+
Ca
2+
St or e
+/ –
+/ –
Local
oscillato
r
Ca

2+
V
Ins(1,4,5)
P
3
R
Ins(1,4,5)
P
3
R
AT Pase
Strong electrical couplin g
W eak
chemical coupling
Gap junction
Ins(1,4,5)P
3
(V)
or Ca
2+
(V)
Fig. 3. A schematic representation of the two-cell system. Each cell is a local oscillator composed of a cytosolic store Ca
2+
-excitable sys-
tem. The cytosolic Ca
2+
of each oscillator is transformed into membrane potential (V) oscillations by a Ca
2+
-activated inward current. The
membrane potentials of the cells are strongly linked. Each local oscillator is weakly linked to the membrane potential by a voltage-dependent

feedback loop such as voltage-dependent Ins(1,4,5)P
3
synthesis or voltage-dependent Ca
2+
influx. Ins(1,4,5)P
3
R, Ins(1,4,5)P
3
receptor;
ATPase, ATPase pump. Adapted from [37].
M. S. Imtiaz et al. Synchronization by voltage-modulated store release
FEBS Journal 277 (2010) 278–285 ª 2009 The Authors Journal compilation ª 2009 FEBS 281
composed of a large number of Ca
2+
store oscillators.
In this simulation, the intrinsic frequencies of oscilla-
tors are different from each other, and as the
[Ins(1,4,5)P
3
] is increased in the model tissue, a global
synchronous rhythm emerges following events that
grow from a noisy baseline (Fig. 5A).
The above simulation outcome is very similar to
what is observed in isolated gastric smooth muscle tis-
sue. When gastric smooth muscle is freshly dissected
and isolated, it usually remains quiescent, and mem-
brane potential recordings display a noisy baseline.
Confocal Ca
2+
imaging records obtained during this

time reveal asynchronous isolated Ca
2+
events [8] simi-
lar to those seen in the simulated voltage recordings of
Fig. 5B1. However, over time, these release events
begin to synchronize and summate to larger events
(Fig. 5B2), and finally a global synchronous rhythm
emerges (Fig. 5B3).
We tested the potency of electrochemical coupling
by running the same simulation but allowing no
voltage-dependent modulation of Ca
2+
store release.
This was achieved by blocking voltage-dependent syn-
thesis of Ins(1,4,5)P
3
. In this case, no global synchrony
emerged, and the baseline remained noisy even though
the cells were coupled both electrically and by diffu-
sion of Ca
2+
and Ins(1,4,5)P
3
(chemical coupling). In
fact, the outcome was very similar to what is seen
when no coupling exists between the cells (achieved by
deleting gap junctions in the simulation) [8,10]. This
example indicates that: (a) voltage-dependent modula-
tion of store release in electrically coupled cells is a
very efficient long-range coupling mechanism; and (b)

chemical coupling by itself is not sufficient to synchro-
nize Ca
2+
release events. In this regard, we note that a
modeling study by Koenigsberger et al. [6] showed that
diffusive coupling through Ca
2+
is sufficient to
40 42 44 46 48 50
0
1
2
3
[C a
2+
]
c
, Z (µM)
[C a
2+
]
c
, Z (µM)
[C a
2+
]
c
, Z (µM)
Ti me (min )
14

0 14 2 14 4 14 6 14 8 15 0
0
1
2
3
Ti me (min )
A B
14 1 14 2 14 3 14 4 145 146 147
0.25
0.3
0.35
Time
(
min
)

Time (min)
Time (min)
14
1 14 2 14 3 14 4 145 146 147
–70
–60
–50
–40
14 1 14 2 14 3
14
4
145 146 147
0.5
1

1.5
2
D
C
E
V (mV)
Cell 1
Cell 2
G
ap junction
Gap junction
Cell 1
Cell 2
Cell 1
Cell 2
[Ins(1,4,5)P
3
]
c
, (µM)
Fig. 4. Synchronization of a cell pair. A two-cell system shows how synchrony can be achieved through voltage-dependent modulation of
store release. (A, B) [Ca
2+
]
c
plot of cell 1 and cell 2 before (A) and after (B) coupling. (C, E) [Ca
2+
]
c
and [Ins(1,4,5)P

3
]
c
, respectively, for the
two cells after they are coupled. Note that the membrane potentials (D) for both cells are same, owing to large electrical coupling. Note that
changes in [Ins(1,4,5)P
3
]
c
for both cells follow changes in the membrane potential. Adapted from [10].
Synchronization by voltage-modulated store release M. S. Imtiaz et al.
282 FEBS Journal 277 (2010) 278–285 ª 2009 The Authors Journal compilation ª 2009 FEBS
synchronize Ca
2+
oscillations. However, their simula-
tion entailed only a small number of cells. Our findings
agree with those of Koenigsberger et al. for the case of
a small number of cells that have similar intrinsic oscil-
latory frequencies and that are not separated by large
distances, but their results do not apply to long-range
coupling involving large numbers of cells.
The electrochemical coupling of intracellular stores
is found, with variations, in other systems as well.
Below, we present some details that illustrate the same
principles of pacemaking and synchronization mecha-
nism in lymphatic smooth muscle.
Lymphatic pacemaking
A rhythmic constriction–relaxation cycle is displayed
by blood and lymphatic vessels, a phenomenon known
as vasomotion. Lymphatic vessels are divided into

chambers by interconnecting valves. Rhythmic constric-
tion and relaxation of these chambers propels lymph
fluid through the lymphatic vessels. The pacemaking
mechanism underlying contractions of lymphatic
smooth muscle has been found to be dependent on
Ins(1,4,5)P
3
-receptor operated Ca
2+
release from intra-
cellular Ca
2+
stores [19]. Spontaneous Ca
2+
releases
from Ins(1,4,5)P
3
receptor-operated Ca
2+
stores acti-
vate a transient inward current, causing a spontaneous
transient depolarization. However, the amount of Ca
2+
released from individual or small groups of stores
is small, and results in spontaneous transient
depolarizations that do not reach the threshold for
opening L-type Ca
2+
channels which underlie action
potential and constriction. This mechanism can only be

effective if there are cooperative interactions between
the release cycles of the Ca
2+
stores, as would be
effected by stores interacting as coupled oscillators [4].
Indeed, this is highly likely to be the situation underpin-
ning vasomotion in both blood and lymphatic vessels
[5,6,9]. The mechanism operates on the same principles
as outlined for gastrointestinal smooth muscle, but dif-
fers from it in that the ‘springs’ that couple the oscilla-
tors now rely on voltage coupling mediated by Ca
2+
entry through L-type Ca
2+
channels rather than volt-
age-dependent production of Ins(1,4,5)P
3
.
Ca
2+
oscillations in other cell types
Gastrointestinal store-based pacemaker activity is, in
fact, more complicated than considered so far, in that
the pacemaker cells driving the slow waves are the
ICCs [27–29]. These cells form networks in regions
such as the myenteric plexus (i.e. ICC-MY) and intra-
muscularly within the smooth muscle (i.e. ICC-IM),
interconnecting with each other and with adjacent
smooth muscle. As a consequence, the dominant Ca
2+

stores that underlie pacemaking reside in these cells
[8,14]. However, whether this is the case may depend
on the tissue. For example, the pacemaker activity that
generates vasomotion in blood and lymphatic vessels,
although Ca
2+
store-based, may be driven by Ca
2+
stores in the smooth muscle, as a role for ICC-like
cells has yet to be confirmed [5,9,19]. In contrast,
Ca
2+
store-based pacemaking in the rabbit urethra is
generated in ICC-like cells [13,30].
There is now evidence that sinoatrial cells that pace
the heart also show Ca
2+
store-based oscillation. This
0 20 40 60 80 100 120 140 160 180
–65
–60
–55
–50
–45
V (mV)
V (mV)
V (mV)
1
2
3

2 min
10 s
3
20 mV
2
1
3
2
1
50 100 150 200 250 300
–65
–60
–55
Time (min)
Time (min)
Time (min)
0 20 40 60 80 100 120 140 160 180
–66
–64
–62
A
B
C
D
Fig. 5. Synchronization of a cell population. (A) The emergence of
synchronized global slow waves in a gap junction-coupled model
cell syncytium. (B) The emergence of slow waves in guinea pig
pyloric smooth muscle. Nifedipine (1 l
M) was present throughout.
The voltage scale bar applies to all records. Events marked with

labeled arrows are shown on an expanded time scale. The resting
membrane potential was )59 mV. Expanded regions 1, 2 and 3 are
similar to events similarly marked in the model syncytium mem-
brane potential in (A). (C) When voltage-dependent synthesis of
Ins(1,4,5)P
3
is blocked, no synchronous events arise in the model
syncytium, even though all of the other parameters are the same
as in (A). (D) Similarly, no synchronous events arise if gap junctions
are blocked in the model syncytium, even though all the parame-
ters are the same as in (A). Adapted from [37].
M. S. Imtiaz et al. Synchronization by voltage-modulated store release
FEBS Journal 277 (2010) 278–285 ª 2009 The Authors Journal compilation ª 2009 FEBS 283
operates together with the classic membrane oscillator
generated by voltage-dependent channels in the cell
membrane to drive the heart [31,32]. It differs from
the smooth muscle cell store oscillator in that it
utilizes ryanodine receptor-operated rather than
Ins(1,4,5)P
3
receptor-operated Ca
2+
stores. It remains
to be seen whether Ca
2+
stores have a role in the syn-
chronization of sinoatrial nodal cells. However, in the
heart muscle, increased Ca
2+
store excitability can

cause the emergence of unwanted pacemakers that
result in pathological waves of contractions known as
arrhythmias [33,34]. Indeed, this raises the question of
why stores in the atrial and ventricular muscle do not
normally synchronize, as they do in the pacemaker
node. This is, of course, a very important feature of
the heart, as otherwise the muscle systems themselves
would have autonomous pacemaker capability. The
reason for this needs to be explored, but there is a
very interesting analogous circumstance in the stom-
ach. Here, only the middle and lower sections of the
stomach exhibit slow waves and associated rhythmic
contractions; the upper region of the stomach (i.e. the
gastric fundus) is nonrhythmic. As has been noted,
slow waves are generated by stored Ca
2+
release [14],
a mechanism that requires long-range intercellular
synchronization of oscillatory stored Ca
2+
release [8].
The gastric fundus should exhibit slow waves, as it
has abundant pacemaker cells (i.e. ICCs) that exhibit
store Ca
2+
release coupled to membrane depolariza-
tion [35]. However, coupling does not happen! The
reason for this is that stores in this region lack a key
component of their coupling mechanism, namely the
feedback by which membrane depolarization causes

stored Ca
2+
release [35]. The consequence is that the
coupling link that allows long-range store coupling is
no longer functional, and hence store pacemaking
cannot occur in this smooth muscle.
Conclusion and future directions
In this article, we have reviewed long-range signaling
through Ca
2+
release from intracellular Ca
2+
stores,
which is a key determinant of whether stores can pro-
duce sufficient synchrony to act as a pacemaker mech-
anism. Voltage-dependent coupling between Ca
2+
stores is critical for such signaling, as it is several
orders of magnitude stronger than chemical coupling
through diffusion of Ca
2+
and ⁄ or Ins(1,4,5)P
3
. In our
model, electrochemical coupling was considered to
occur by intercellular current flow through presumed
gap junctions. However, such electrical coupling could
also occur wholly or in part by capacitive coupling, as
shown in the study of Yamashita [36] (see accompany-
ing review), and it will be interesting to determine the

relative role of this mechanism.
In summary, store-based pacemaking, whether oper-
ated by Ins(1,4,5)P
3
receptors or by ryanodine recep-
tors, has a role in a range of tissues where cells are
electrically connected. The key for a functional pace-
maker mechanism in such cell syncytia is that oscilla-
tory store Ca
2+
release generates inward currents and
resultant depolarization, that the cellular network
readily conducts currents, and that the conducted
depolarization in turn leads to activation of other
Ca
2+
stores. This latter step could be mediated by
depolarization-induced Ca
2+
entry and ⁄ or production
of Ins(1,4,5)P
3
[9,25].
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