Unité d'Endocrinologie et Métabolisme, University of Louvain Faculty of Medicine, UCL 55.30, B-1200 Brussels, Belgium
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ABSTRACT |
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Thapsigargin (TG), a blocker of
Ca2+ uptake by the endoplasmic reticulum (ER), was used to
evaluate the contribution of the organelle to the oscillations of
cytosolic Ca2+ concentration
([Ca2+]c) induced by repetitive
Ca2+ influx in mouse pancreatic -cells. Because TG
depolarized the plasma membrane in the presence of glucose alone,
extracellular K+ was alternated between 10 and 30 mM in the
presence of diazoxide to impose membrane potential (MP) oscillations.
In control islets, pulses of K+, mimicking regular MP
oscillations elicited by 10 mM glucose, induced
[Ca2+]c oscillations whose nadir remained
higher than basal [Ca2+]c. Increasing the
depolarization phase of the pulses while keeping their frequency
constant (to mimic the effects of a further rise of the glucose
concentration on MP) caused an upward shift of the nadir of
[Ca2+]c oscillations that was reproduced by
raising extracellular Ca2+ (to increase Ca2+
influx) without changing the pulse protocol. In TG-pretreated islets,
the imposed [Ca2+]c oscillations were of much
larger amplitude than in control islets and occurred on basal levels.
During intermittent trains of depolarizations, control islets displayed
mixed [Ca2+]c oscillations characterized by a
summation of fast oscillations on top of slow ones, whereas no
progressive summation of the fast oscillations was observed in
TG-pretreated islets. In conclusion, the buffering capacity of the ER
in pancreatic
-cells limits the amplitude of
[Ca2+]c oscillations and may explain how the
nadir between oscillations remains above baseline during regular
oscillations or gradually increases during mixed
[Ca2+]c oscillations, two types of response
observed during glucose stimulation.
cytosolic Ca2+ concentration oscillations; endoplasmic
reticulum; pancreatic -cell; thapsigargin
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INTRODUCTION |
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GLUCOSE STIMULATES INSULIN
SECRETION by producing, triggering, and amplifying signals in
pancreatic -cells (22). The triggering signal, an
increase in cytosolic free Ca2+ concentration
([Ca2+]c), is the outcome of the following
sequence of events: the acceleration of metabolism augments the ATP/ADP
ratio, which closes ATP-sensitive K+ channels
(K+-ATP channels) in the plasma membrane, leading to
membrane depolarization, opening of voltage-dependent calcium channels,
and Ca2+ influx (5, 22, 39).
Although single-cell studies have been extremely informative on the
regulation of ionic currents and Ca2+ handling in -cells
(4, 10, 20), intact islets, in particular mouse islets,
are a more physiological preparation, permitting comparison of the
changes in membrane potential and
[Ca2+]c (13, 38, 43). The
validity of the model is attested by the similarity of the changes in
electrical activity and [Ca2+]c in vitro and
in vivo (15). Stimulation by a sudden increase in the
glucose concentration typically induces an initially small lowering of
[Ca2+]c, followed by a biphasic rise
consisting of a pronounced first peak, lasting 1-3 min, and a
second phase consisting of a sustained elevation with
superimposed [Ca2+]c oscillations (3,
13, 45). These oscillations are regular and rapid (frequency of
~2.5/min), regular and slow (frequency of ~0.2/min), or mixed, with
rapid oscillations superimposed on slow ones (23). They
are associated with periodic depolarizations of the
-cell membrane
(10, 13, 43) and abolished by omission of extracellular
Ca2+ or blockade of voltage-dependent Ca2+
channels (10, 13).
We recently reported that the endoplasmic reticulum (ER) takes up
Ca2+ during the upstroke of the
[Ca2+]c rise induced by an imposed
depolarization and releases it slowly upon repolarization
(12). Because this slow release prolongs the period of
[Ca2+]c elevation, we hypothesized that it
might prevent [Ca2+]c from returning to basal
levels when oscillations occur at a high frequency and thus underlie
the sustained elevation of [Ca2+]c on top of
which the oscillations appear. The aim of the present study was thus to
evaluate the possible contribution of the ER to the -cell
[Ca2+]c response to glucose. Thapsigargin
(TG) was used to inhibit the sarco-endoplasmic reticulum
Ca2+-ATPase (SERCA) pump (41) and prevent the
uptake of Ca2+ by the ER. However, Ca2+
emptying of the ER by TG causes sustained membrane depolarization and
stable elevation of [Ca2+]c in
glucose-stimulated
-cells (6, 12, 33, 45). The contribution of the ER to [Ca2+]c
oscillations cannot be studied under these conditions. Therefore, [Ca2+]c oscillations similar to those
produced by glucose were imposed by repetitive depolarizations of the
plasma membrane with elevated concentrations of K+ in the
presence of diazoxide, an opener of K+-ATP channels. The
results demonstrate that the ER markedly dampens the amplitude of
[Ca2+]c oscillations and contributes to
maintain a steady-state elevation of [Ca2+]c
when [Ca2+]c oscillations occur at a
frequency (2-3/min) similar to that of the fast oscillations
produced by glucose.
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MATERIALS AND METHODS |
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Preparation of islets and islet cell clusters.
The research project was approved by, and the experiments were
conducted in accordance with, the guidelines of the Commission d'Ethique d'Expérimentation Animale of the University of
Louvain School of Medicine. Islets were aseptically isolated after
collagenase digestion of the pancreas of fed female NMRI mice
(25-30 g) killed by decapitation. When needed, they were dispersed
into clusters of islet cells as previously described (25).
Islets or clusters were cultured for 1-3 days in RPMI 1640 culture
medium (GIBCO, Paisley, UK) containing 10% heat-inactivated fetal calf
serum, 100 IU/ml penicillin, 100 µg/ml streptomycin, and 10 mM
glucose. Except for the experiments of Figs. 1A,
6A, and some control tests performed with overnight cultured
islets, most experiments were carried out with islets or cells cultured
for 2-3 days. This culture period was necessary for the tissue to
firmly attach to glass coverslips and sustain frequent solution changes
at a high flow rate used subsequently during
[Ca2+]c measurements. Suction with glass
micropipettes proved unsatisfactory to hold the islets motionless.
However, the few successful experiments performed with this technique
showed that the results were not different between islets cultured
overnight or for 2-3 days.
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Solutions and drugs. The medium used for the isolation of islets and for all experiments was a bicarbonate-buffered solution containing (in mM) 120 NaCl, 4.8 KCl, 2.5 CaCl2, 1.2 MgCl2, and 24 NaHCO3. It was gassed with O2-CO2 (94:6) to maintain pH 7.4 at 37°C. Except for electrophysiological experiments, it was supplemented with 1 mg/ml BSA (fraction V, Boehringer Mannheim, Mannheim, Germany). When the concentration of KCl was increased, that of NaCl was decreased accordingly to keep the osmolarity of the medium unchanged. TG was obtained from Sigma (St. Louis, MO) and diazoxide from Schering-Plough Avondale (Rathdrum, Ireland).
[Ca2+]c measurements.
Cultured islets or clusters of islet cells were loaded with,
respectively, 2 µM fura PE3-AM (Teflabs, Austin, TX) for 90-120 min or 1 µM fura 2-AM (Molecular Probes, Eugene, OR) for 40 min at
37°C in a bicarbonate-buffered solution containing 10 mM glucose. When needed, 1 µM TG was added to the loading solution. The coverslip was then used as the bottom of a 200-µl temperature-controlled perifusion chamber (Intracell, Royston, Herts, UK) mounted on the stage
of an inverted microscope. The flow rate was 2 ml/min, and the
temperature within the chamber was 37°C. The different solutions were
introduced into the chamber through separate quartz capillaries placed
just in front of the studied islet or cluster and were rapidly changed
by computer-controlled Iso-Latch valves (Parker Hannifin, Fairfield,
NY). Control experiments with two solutions containing different
concentrations of K+ showed that as little as 0.2 s
was enough to start changing [Ca2+]c in
-cells with this system. [Ca2+]c was
measured at 10 Hz by dual-wavelength (340 and 380 nm) excitation spectrofluorimetry, using a photometric-based system (Photon
Technologies International, Princeton, NJ) to capture the emitted
fluorescence at 510 nm. [Ca2+]c was
calculated by comparing the ratio of the 510-nm signals successively
acquired at 340 and 380 nm with a calibration curve based on the
equation of Grynkiewicz et al. (18) and was established by
filling the chamber with an intracellular type of solution containing
10 µM fura PE3 or fura 2 free acid, and ~10 mM or <1 nM free
Ca2+. The dissociation constants, or
Kd, for the fura PE3-Ca2+ and fura
2-Ca2+ complexes of, respectively, 290 (44)
and 224 nM (18) were used. Previous experiments have shown
that the Ca2+ probe is not significantly compartmentalized
in islet cells and that the changes in fluorescence report changes in
[Ca2+] within the cytosol, not the ER (12).
Electrophysiology.
The membrane potential of a single -cell within an islet was
continuously recorded with a high-resistance (~200-M
)
intracellular microelectrode (32).
-Cells were
identified by the typical electrical activity that they display in the
presence of 10 mM glucose.
Presentation of the results. The experiments are illustrated by traces (means ± SE) or by recordings that are representative of results obtained with the indicated number of islets from at least three different preparations. The statistical significance between means was assessed by paired or unpaired Student's t-test as appropriate. Differences were considered significant at P < 0.05.
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RESULTS |
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Design of the experimental conditions to test the effect of TG.
Because TG depolarizes -cells and usually abolishes the oscillations
of membrane potential and [Ca2+]c induced by
glucose (6, 12, 33, 45), the drug cannot be used to
evaluate directly the contribution of the ER to glucose-induced [Ca2+]c oscillations. Therefore, all
experiments were performed in the presence of 250 µM diazoxide,
which, by opening K+-ATP channels (42), clamps
the plasma membrane at the resting potential and prevents the
depolarizing effect of TG (12, 33). Because diazoxide also
prevents the depolarizing effect of glucose, islet cells were
cyclically depolarized and repolarized to potentials close to those of
oscillations of the membrane potential induced by glucose in the
absence of diazoxide. The K+ concentrations that best
mimicked these potential changes were determined by recording the
-cell membrane potential with intracellular microelectrodes. After
initial recordings in the presence of 10 mM glucose alone, diazoxide
was added, and the K+ concentration of the medium was
changed stepwise (4.8, 8, 10, 12, 30, and 45 mM). Each step caused a
depolarization of the
-cell membrane (Fig.
1A). At concentrations of
K+
10 mM, the membrane potential varied linearly with
the logarithm of the K+ concentration. The slope of the
best linear fit for this relationship was 59 mV for a 10-fold change of
the K+ concentration, which is close to the theoretical
value of 61.5 mV (at 37°C) given by the Nernst equation (Fig.
1B). This value is higher than that of 47 mV measured in
experiments performed in 0-2.8 mM glucose (32),
probably because diazoxide opens K+-ATP channels more
efficiently than the simple omission of glucose. Below 10 mM
K+, the slope decreased, indicating that the membrane
potential of
-cells is not exclusively determined by the
K+ permeability.
Role of the ER on
[Ca2+]c in conditions
mimicking a change of the glucose concentration from 3 to 10 mM.
Figure 2A shows
[Ca2+]c changes occurring in a mouse islet
stimulated by a rise in the glucose concentration from 3 to 10 and then
15 mM. A small initial drop was followed by a large increase, and
eventually by rapid oscillations occurring above a sustained elevation.
These [Ca2+]c changes are known to result
from concomitant changes in membrane potential (13, 38,
45).
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Role of the ER on
[Ca2+]c in conditions
mimicking a change of the glucose concentration from 10 to 15 mM.
Membrane potential recordings of -cells within an islet have shown
that raising the glucose concentration of the medium from 10 to 15 mM
under control conditions increases the duration of the depolarization
phase without affecting the frequency of membrane potential
oscillations (21). Figure 2A illustrates
[Ca2+]c changes in an islet in response to a
similar rise of the glucose concentration. The peak of
[Ca2+]c oscillations was increased, and the
nadir between the oscillations was slightly shifted upward. To mimic
these changes in islets treated with diazoxide, the frequency of
K+ pulses was not changed, but the duration of the
depolarizing phase (K+30) was increased while that of the
repolarizing phase (K+10) was decreased, both to 12 s,
in accordance with the measurements of the membrane potential of islets
perifused with 15 mM glucose (21). The glucose
concentration was increased from 10 to 15 mM simultaneously with the
change in pulse protocol. Neither in control nor in TG-pretreated
islets did this protocol affect the amplitude of
[Ca2+]c oscillations (Table 1, lines
1 and 2). However, the nadir between the oscillations
became more elevated, the difference being larger (P < 0.05) in the absence of TG (33 ± 7 nM , n = 4) than in
its presence (12 ± 4 nM, n = 5). Similar
qualitative results were obtained in clusters of islet cells. Thus the
rise of the nadir brought about by the change of the protocol was
larger in control than in TG-pretreated cells (88 ± 14 nM,
n = 4 vs. 14 ± 7 nM, respectively, n = 4, P < 0.01; Table 1, lines 5 and 6).
This highlights again the role of the ER in the sustained elevation of
[Ca2+]c above which the oscillations of
[Ca2+]c occur in control
-cells.
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Role of the ER on
[Ca2+]c during mixed
[Ca2+]c oscillations.
During continuous stimulation with 10 mM glucose, the
[Ca2+]c response of many islets displays a
mixed pattern characterized by slow oscillations with superimposed fast
ones of variable frequencies (Fig.
6A). Trains of five pulses of
K+30/K+10, at different frequencies and
durations, were applied to the islets following the protocol shown
above Fig. 6B. In control islets, this protocol of
depolarization produced mixed [Ca2+]c
oscillations similar to those occurring in spontaneously oscillating islets (compare mean traces of Fig. 6, A and B).
Importantly, peak [Ca2+]c increased with the
repetition of the rapid pulses, and the recovery of a basal
[Ca2+]c at the end of the last pulse was
slow. The pattern of the mixed oscillations was very different in
TG-pretreated islets (Fig. 6C). The largest
[Ca2+]c peak coincided with the first pulse
of each train, and [Ca2+]c quickly returned
to basal levels after the last pulse. Moreover, the fast oscillations
occurred on nearly basal levels when the frequency of the oscillations
was 2.5/min (first half of Fig. 6C). This shows that the
sustained [Ca2+]c phase that slowly develops
during mixed [Ca2+]c oscillations in control
islets also involves the ER.
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DISCUSSION |
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The present study demonstrates that the ER limits the amplitude of
[Ca2+]c oscillations and keeps
[Ca2+]c elevated between oscillations when
these are induced by repetitive Ca2+ influx through
voltage-dependent Ca2+ channels in pancreatic -cells.
A summation of [Ca2+]c transients has been
reported in patch-clamped single -cells submitted to short
depolarization steps applied at a high frequency (
30/min) to mimic
rapid action potentials (2, 37). Here, the concentration
of extracellular K+ was repetitively changed at a frequency
of 2.5/min to mimic the oscillations of the membrane potential
occurring in
-cells within islets during stimulation with glucose.
This resulted in [Ca2+]c oscillations whose
nadir remained well above basal [Ca2+]c. In
other words, the imposed [Ca2+]c oscillations
appeared on top of a sustained elevation of
[Ca2+]c, as do glucose-induced
[Ca2+]c oscillations. Suppression of this
elevation by TG identifies the ER as a major contributor to its development.
We have previously suggested that periodic influx of Ca2+
into -cells induces parallel oscillations of the Ca2+
concentration in both cytosol and ER (12). Oscillations of the Ca2+ concentration in the ER are attributed to
Ca2+ uptake during the upstroke of
[Ca2+]c oscillations, followed by
Ca2+ release when Ca2+ influx stops. The
sustained [Ca2+]c elevation above which
[Ca2+]c oscillations occur would result from
differences in the kinetics of uptake and release of Ca2+
by the ER, the uptake being much faster than the release
(12). The fast kinetics of the uptake by the ER can be
inferred from the observation that inactivation of the SERCA by TG
nearly doubled the amplitude of the
[Ca2+]c rise produced by short
depolarizations (3 s). The mechanism of the slow release process has
not been identified, but Ca2+- or inositol
phosphate-induced Ca2+ release, two fast processes, are not
involved (12). We suggest that it reflects slow leakage of
Ca2+ from the ER. Although such a mechanism may seem
surprising, it is fully compatible with numerous observations showing
that blockade of the SERCA pump by TG in a Ca2+-free medium
induces a transient rise in [Ca2+]c by
emptying of the ER (28, 33-35). Because of its slow
kinetics, this passive Ca2+ release from the ER prolongs
[Ca2+]c oscillations when influx of
Ca2+ has stopped and prevents
[Ca2+]c from returning to basal levels during
fast [Ca2+]c oscillations like those induced
by glucose.
An important contribution of this study was to show that the
unexplained mixed [Ca2+]c oscillations
observed in islets (23) and clusters of -cells (25, 36) stimulated by glucose can be reproduced by trains of depolarizations interrupted by longer pauses. It is therefore tempting to ascribe them to the mixed oscillations of the membrane potential that occur in some
-cells during glucose stimulation (8, 24, 36), and to the activity of the ER that permits summation of these [Ca2+]c signals. This does
not imply that summation of fast oscillations is the only mechanism to
generate slow oscillations. Indeed, slow oscillations can occur in the
absence of fast oscillations, probably as a result of long bursts of
action potentials (13). The mechanisms that we propose to
explain the mixed pattern of [Ca2+]c
oscillations are thus different from those suggested by Liu et al.
(29). From studies using ob/ob mouse islets,
these authors also emphasize the role of depolarization-induced
Ca2+ influx, but they attribute the mixed pattern of slow
and superimposed fast oscillations to separate cell populations, each
showing one type of response. They also invoke spiking Ca2+
release from the ER in some cells to coordinate the fast
[Ca2+]c oscillations. We do not believe that
this proposal is applicable to islets from normal mice for three
reasons. First, [Ca2+]c transients resulting
from Ca2+ mobilization in response to glucose and
cAMP-producing agents are much more common in
-cells from these
leptin-deficient ob/ob mice than in normal mice
(1). Second, experiments using two intracellular
microelectrodes in the same islet have shown that the electrical
activity is similar and synchronized in all
-cells from normal mice
(11, 31). Third, mixed [Ca2+]c
oscillations have been observed in clusters of 2-20 cells
(25, 36) and even in single
-cells (26)
from normal mice.
A rise in the glucose concentration lengthens the depolarized, active phase and shortens the repolarized, silent phase of membrane potential oscillations without affecting their frequency (21). It also causes an upward shift of the nadir between [Ca2+]c oscillations (i.e., prevents [Ca2+]c from intermittently returning to basal levels) (14, 38). A similar shift occurs when islets are subjected to a depolarizing protocol that mimics the changes in membrane potential normally produced by raising glucose from 10 to 15 mM. This elevation of the nadir cannot be explained by too short a repolarization for extrusion of all Ca2+ from the cytosol after Ca2+ influx has stopped, or by insufficient repolarization for abolition of Ca2+ influx. Thus, in TG-pretreated islets, short periods of repolarizations (9 s) are usually sufficient for restoration of low [Ca2+]c during the nadir, although the kinetics of the changes in membrane potential induced by high and low K+ solutions are not different from those in control islets (12).
When the external Ca2+ concentration was raised stepwise
without changing the pulse protocol, the nadir between
[Ca2+]c oscillations was progressively
shifted to higher values in control but not in TG-pretreated islets.
Our interpretation is that the ER takes up more Ca2+ when
Ca2+ influx is larger at higher extracellular
Ca2+ and therefore releases more Ca2+ during
the repolarization intervals. This is consistent with the observations
that high K+ raises the Ca2+ concentration in
the ER of insulin-secreting rat insulinoma cells (30) and ob/ob -cells (40), and
that the filling state of the ER is directly proportional to the
steady-state [Ca2+]c level (7, 9, 12,
34). We suggest that the same mechanism explains the upward
shift of the nadir observed when the fraction of depolarization time
increases, for instance when the glucose concentration of the medium is
raised from 10 to 15 mM. Under these conditions, Ca2+
pumping into the ER probably results from a direct activation of SERCA
by Ca2+. An increase in cell metabolism is unlikely to be
involved, because the change in glucose concentration (10 to 15 mM) did
not affect the nadir of [Ca2+]c oscillations
when the pulse protocol was kept constant. This is not surprising,
because glucose-induced filling of the ER is nearly maximal at 10 mM
(40).
The strong buffering capacity of the ER is illustrated by the much smaller amplitude of [Ca2+]c oscillations in control than in TG-pretreated islets. Because voltage-dependent Ca2+ currents are not affected by TG (12), this difference cannot be ascribed to greater Ca2+ influx. Whereas the buffering properties of the ER are evident during intermittent Ca2+ influx and [Ca2+]c oscillations, they are not during sustained Ca2+ influx, probably because the Ca2+ concentration within the ER is then in equilibrium with a steady-state [Ca2+]c ensured by extrusion mechanisms in the plasma membrane.
In conclusion, our study demonstrates that the ER has a major role in the control of [Ca2+]c oscillations induced by repetitive Ca2+ influx through voltage-dependent Ca2+ channels. The ER limits the amplitude of [Ca2+]c oscillations by a fast uptake and a slow release of the ion. This slow release keeps the nadir of regular [Ca2+]c oscillations elevated (higher than basal [Ca2+]c) or gradually sums the Ca2+ signals to result in mixed [Ca2+]c oscillations. This buffering effect of the ER with cycles of Ca2+ uptake and release could serve several functions. Prolongation of [Ca2+]c elevation between oscillations might recruit insulin secretory granules into a pool from which they can be released once [Ca2+]c is high enough to promote exocytosis, e.g., during the next [Ca2+]c oscillation (17). Pumping of Ca2+ into the ER could prevent [Ca2+]c from reaching too high levels that might be cytotoxic (27) and refill the organelle in which Ca2+ affects processes such as protein synthesis or maturation. Thus the proteolytic processing of proinsulin within the ER involves specific Ca2+-dependent maturation steps, and depletion of intracellular Ca2+ pools impairs proinsulin conversion and intracellular transport (19). The Ca2+ content of the ER might also modulate cell survival, because depletion of ER Ca2+ stores triggers apoptosis (46). Finally, because the Ca2+ content of the ER (45) influences the membrane potential, the periodic emptying and refilling of the ER during [Ca2+]c oscillations may participate in feedback mechanisms controlling membrane potential oscillations (12).
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ACKNOWLEDGEMENTS |
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This work was supported by Grant 3.4552.98 from the Fonds de la Recherche Scientifique Médicale (Brussels), Grant 1.5.212.00 from the Fonds National de la Recherche Scientifique (Brussels), Grant ARC 00/05-260 from the General Direction of Scientific Research of the French Community of Belgium, and by the Interuniversity Poles of Attraction Program (P4/21), Federal Office for Scientific, Technical and Cultural Affairs. P. Gilon is Maître de Recherches of the Fonds National de la Recherche Scientifique, Brussels.
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FOOTNOTES |
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Address for reprint requests and other correspondence: P. Gilon, Unité d'Endocrinologie et Métabolisme, Univ. of Louvain Faculty of Medicine, UCL 55.30, Ave. Hippocrate 55, B-1200 Brussels, Belgium (E-mail: gilon{at}endo.ucl.ac.be).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
First published January 2, 2002;10.1152/ajpendo.00347.2001
Received 1 August 2001; accepted in final form 18 December 2001.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Ahmed, M,
and
Grapengiesser E.
Pancreatic -cells from obese-hyperglycemic mice are characterized by excessive firing of cytoplasmic Ca2+ transients.
Endocrine
15:
73-78,
2001[ISI][Medline].
2.
Ämmälä, C,
Eliasson L,
Bokvist K,
Larsson O,
Ashcroft FM,
and
Rorsman P.
Exocytosis elicited by action potentials and voltage-clamp calcium currents in individual mouse pancreatic -cells.
J Physiol
472:
665-688,
1993[Abstract].
3.
Antunes, CM,
Salgado AP,
Rosario LM,
and
Santos RM.
Differential patterns of glucose-induced electrical activity and intracellular calcium responses in single mouse and rat pancreatic islets.
Diabetes
49:
2028-2038,
2000[Abstract].
4.
Ashcroft, FM,
Proks P,
Smith PA,
Ämmälä C,
Bokvist K,
and
Rorsman P.
Stimulus-secretion coupling in pancreatic -cells.
J Cell Biochem
55, Suppl:
54-65,
1994[ISI][Medline].
5.
Ashcroft, FM,
and
Rorsman P.
Electrophysiology of the pancreatic -cell.
Prog Biophys Mol Biol
54:
87-143,
1989[Medline].
6.
Bertram, R,
Smolen P,
Sherman A,
Mears D,
Atwater I,
Martin F,
and
Soria B.
A role for calcium release-activated current (CRAC) in cholinergic modulation of electrical activity in pancreatic -cells.
Biophys J
68:
2323-2332,
1995[Abstract].
7.
Camello, P,
Gardner J,
Petersen OH,
and
Tepikin AV.
Calcium dependence of calcium extrusion and calcium uptake in mouse pancreatic acinar cells.
J Physiol
490:
585-593,
1996[Abstract].
8.
Cook, DL.
Isolated islets of Langerhans have slow oscillations of electrical activity.
Metabolism
32:
681-685,
1983[ISI][Medline].
9.
Corkey, BE,
Deeney JT,
Glennon MC,
Matschinsky FM,
and
Prentki M.
Regulation of steady-state free Ca2+ levels by the ATP/ADP ratio and orthophosphate in permeabilized RINm5F insulinoma cells.
J Biol Chem
263:
4247-4253,
1988
10.
Dryselius, S,
Grapengiesser E,
Hellman B,
and
Gylfe E.
Voltage-dependent entry and generation of slow Ca2+ oscillations in glucose-stimulated pancreatic -cells.
Am J Physiol Endocrinol Metab
276:
E512-E518,
1999
11.
Eddlestone, GT,
Goncalves A,
Bangham JA,
and
Rojas E.
Electrical coupling between cells in islets of Langerhans from mouse.
J Membr Biol
77:
1-14,
1984[ISI][Medline].
12.
Gilon, P,
Arredouani A,
Gailly P,
Gromada J,
and
Henquin JC.
Uptake and release of Ca2+ by the endoplasmic reticulum contribute to the oscillations of the cytosolic Ca2+ concentration triggered by Ca2+ influx in the electrically excitable pancreatic -cell.
J Biol Chem
274:
20197-20205,
1999
13.
Gilon, P,
and
Henquin JC.
Influence of membrane potential changes on cytoplasmic Ca2+ concentration in an electrically excitable cell, the insulin-secreting pancreatic -cell.
J Biol Chem
267:
20713-20720,
1992
14.
Gilon, P,
and
Henquin JC.
Distinct effects of glucose on the synchronous oscillations of insulin release and cytoplasmic Ca2+ concentration measured simultaneously in single mouse islets.
Endocrinology
136:
5725-5730,
1995[Abstract].
15.
Gomis, A,
Sánchez-Andrés JV,
and
Valdeolmillos M.
Oscillatory patterns of electrical activity in mouse pancreatic islets of Langerhans recorded in vivo.
Pflügers Arch
432:
510-515,
1996[ISI][Medline].
16.
Gopel, SO,
Kanno T,
Barg S,
Eliasson L,
Galvanovskis J,
Renstrom E,
and
Rorsman P.
Activation of Ca2+-dependent K+ channels contributes to rhythmic firing of action potentials in mouse pancreatic -cells.
J Gen Physiol
114:
759-770,
1999
17.
Gromada, J,
Hoy M,
Renstrom E,
Bokvist K,
Eliasson L,
Gopel S,
and
Rorsman P.
CaM kinase II-dependent mobilization of secretory granules underlies acetylcholine-induced stimulation of exocytosis in mouse pancreatic -cells.
J Physiol
518:
745-759,
1999
18.
Grynkiewicz, G,
Poenie M,
and
Tsien RY.
A new generation of Ca2+ indicators with greatly improved fluorescence properties.
J Biol Chem
25:
3440-3450,
1985.
19.
Guest, PC,
Bailyes EM,
and
Hutton JC.
Endoplasmic reticulum Ca2+ is important for the proteolytic processing and intracellular transport of proinsulin in the pancreatic -cell.
Biochem J
323:
445-450,
1997[ISI][Medline].
20.
Hellman, B,
Gylfe E,
Grapengiesser E,
Lund PE,
and
Berts A.
Cytoplasmic Ca2+ oscillations in pancreatic -cells.
Biochim Biophys Acta
1113:
295-305,
1992[ISI][Medline].
21.
Henquin, JC.
ATP-sensitive K+ channels may control glucose-induced electrical activity in pancreatic -cells.
Biochem Biophys Res Commun
156:
769-775,
1988[ISI][Medline].
22.
Henquin, JC.
Triggering and amplifying pathways of regulation of insulin secretion by glucose.
Diabetes
49:
1751-1760,
2000[Abstract].
23.
Henquin, JC,
Jonas JC,
and
Gilon P.
Functional significance of Ca2+ oscillations in pancreatic -cells.
Diabetes Metab
24:
30-36,
1998[ISI][Medline].
24.
Henquin, JC,
Meissner HP,
and
Schmeer W.
Cyclic variations of glucose-induced electrical activity in pancreatic -cells.
Pflügers Arch
393:
322-327,
1982[ISI][Medline].
25.
Jonkers, FC,
and
Henquin JC.
Measurements of cytoplasmic Ca2+ in islet cell clusters show that glucose rapidly recruits -cells and gradually increases the individual cell response.
Diabetes
50:
540-550,
2001
26.
Jonkers, FC,
Jonas JC,
Gilon P,
and
Henquin JC.
Influence of cell number on the characteristics and synchrony of Ca2+ oscillations in clusters of mouse pancreatic islet cells.
J Physiol
520:
839-849,
1999
27.
Kass, GEN,
and
Orrenius S.
Calcium signaling and cytotoxicity.
Environ Health Perspect
107, Suppl1:
25-35,
1999[ISI][Medline].
28.
Liu, YJ,
and
Gylfe E.
Store-operated Ca2+ entry in insulin-releasing pancreatic -cells.
Cell Calcium
22:
277-286,
1997[ISI][Medline].
29.
Liu, YJ,
Tengholm A,
Grapengiesser E,
Hellman B,
and
Gylfe E.
Origin of slow and fast oscillations of Ca2+ in mouse pancreatic islets.
J Physiol
508:
471-481,
1998
30.
Maechler, P,
Kennedy ED,
Sebo E,
Valeva A,
Pozzan T,
and
Wollheim CB.
Secretagogues modulate the calcium concentration in the endoplasmic reticulum of insulin-secreting cells. Studies in aequorin-expressing intact and permeabilized Ins-1 cells.
J Biol Chem
274:
12583-12592,
1999
31.
Meissner, HP.
Electrophysiological evidence for coupling between cells of pancreatic islets.
Nature
262:
502-504,
1976[ISI][Medline].
32.
Meissner, HP,
Henquin JC,
and
Preissler M.
Potassium dependence of the membrane potential of pancreatic -cells.
FEBS Lett
94:
87-89,
1978[ISI][Medline].
33.
Miura, Y,
Henquin JC,
and
Gilon P.
Emptying of intracellular Ca2+ stores stimulates Ca2+ entry in mouse pancreatic -cells by both direct and indirect mechanisms.
J Physiol
503:
387-398,
1997[Abstract].
34.
Mogami, H,
Tepikin AV,
and
Petersen OH.
Termination of cytosolic Ca2+ signals: Ca2+ reuptake into intracellular stores is regulated by the free Ca2+ concentration in the store lumen.
EMBO J
17:
435-442,
1998
35.
Putney, JW, Jr,
Bird G,
and
St J.
The inositol phosphate-calcium signaling system in nonexcitable cells.
Endocr Rev
14:
610-631,
1993[ISI][Medline].
36.
Ravier, MA,
Eto K,
Jonkers FC,
Nenquin M,
Kadowaki T,
and
Henquin JC.
The oscillatory behavior of pancreatic islets from mice with mitochondrial glycerol-3-phosphate dehydrogenase knockout.
J Biol Chem
275:
1587-1593,
2000
37.
Rorsman, P,
Ämmälä C,
Berggren PO,
Bokvist K,
and
Larsson O.
Cytoplasmic calcium transients due to single action potentials and voltage-clamp depolarizations in mouse pancreatic -cells.
EMBO J
11:
2877-2884,
1992[Abstract].
38.
Santos, RM,
Rosario LM,
Nadal A,
Garcia-Sancho J,
Soria B,
and
Valdeolmillos M.
Widespread synchronous [Ca2+]i oscillations due to bursting electrical activity in single pancreatic islets.
Pflügers Arch
418:
417-422,
1991[ISI][Medline].
39.
Satin, LS.
Localized calcium influx in pancreatic -cells: its significance for Ca2+-dependent insulin secretion from the islets of Langerhans.
Endocrine
13:
251-262,
2000[ISI][Medline].
40.
Tengholm, A,
Hellman B,
and
Gylfe E.
Glucose regulation of free Ca2+ in the endoplasmic reticulum of mouse pancreatic -cells.
J Biol Chem
274:
36883-36890,
1999
41.
Thastrup, O,
Cullen PJ,
Drobak BK,
Hanley MR,
and
Dawson AP.
Thapsigargin, a tumor promoter, discharges intracellular Ca2+ stores by specific inhibition of the endoplasmic reticulum Ca2+-ATPase.
Proc Natl Acad Sci USA
87:
2466-2470,
1990[Abstract].
42.
Trube, G,
Rorsman P,
and
Ohno Shosaku T.
Opposite effects of tolbutamide and diazoxide on the ATP-dependent K+ channel in mouse pancreatic -cells.
Pflügers Arch
407:
493-499,
1986[ISI][Medline].
43.
Valdeolmillos, M,
Santos RM,
Contreras D,
Soria B,
and
Rosario LM.
Glucose-induced oscillations of intracellular Ca2+ concentration resembling bursting electrical activity in single mouse islets of Langerhans.
FEBS Lett
259:
19-23,
1989[ISI][Medline].
44.
Vorndran, C,
Minta A,
and
Poenie M.
New fluorescent calcium indicators designed for cytosolic retention or measuring calcium near membranes.
Biophys J
69:
2112-2124,
1995[Abstract].
45.
Worley, JF,
McIntyre MS,
Spencer B,
Mertz RJ,
Roe MW,
and
Dukes ID.
Endoplasmic reticulum calcium store regulates membrane potential in mouse islet -cells.
J Biol Chem
269:
14359-14362,
1994
46.
Zhou, YP,
Teng DL,
Dralyuk F,
Ostrega D,
Roe MW,
Philipson L,
and
Polonsky KS.
Apoptosis in insulin-secreting cellsevidence for the role of intracellular Ca2+ stores and arachidonic acid metabolism.
J Clin Invest
101:
1623-1632,
1998