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-

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+

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

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

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

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

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-

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-

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

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

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

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

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

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

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

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

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FEBS Journal 277 (2010) 278–285 ª 2009 The Authors Journal compilation ª 2009 FEBS 285