From the Department of Pathology, Anatomy, and Cell
Biology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107 and the ¶ Department of Pharmacology and Physiology, University of
Medicine and Dentistry of New Jersey, New Jersey Medical School,
Newark, New Jersey 07103
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ABSTRACT |
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The concerted action of inositol
1,4,5-trisphosphate (IP3) and Ca2+ on the
IP3 receptor Ca2+ release channel
(IP3R) is a fundamental step in the generation of cytosolic
Ca2+ oscillations and waves, which underlie
Ca2+ signaling in many cells. Mitochondria appear in close
association with regions of endoplasmic reticulum (ER) enriched in
IP3R and are particularly responsive to
IP3-induced increases of cytosolic Ca2+
([Ca2+]c). To determine whether feedback
regulation of the IP3R by released Ca2+ is
modulated by mitochondrial Ca2+ uptake, the interactions
between ER and mitochondrial Ca2+ pools were examined by
fluorescence imaging of compartmentalized Ca2+ indicators
in permeabilized hepatocytes. IP3 decreased luminal ER
Ca2+ ([Ca2+]ER), and this was
paralleled by an increase in mitochondrial matrix Ca2+
([Ca2+]m) and activation of
Ca2+-sensitive mitochondrial metabolism. Remarkably, the
decrease in [Ca2+]ER evoked by submaximal
IP3 was enhanced when mitochondrial Ca2+ uptake
was blocked with ruthenium red or uncoupler. Moreover, subcellular
regions that were relatively deficient in mitochondria demonstrated
greater sensitivity to IP3 than regions of the cell with a
high density of mitochondria. These data demonstrate that Ca2+ uptake by the mitochondria suppresses the local
positive feedback effects of Ca2+ on the IP3R,
giving rise to subcellular heterogeneity in IP3 sensitivity
and IP3R excitability. Thus, mitochondria can play an
important role in setting the threshold for activation and establishing
the subcellular pattern of IP3-dependent
[Ca2+]c signaling.
The mobilization of intracellular Ca2+ stores in
response to receptor-stimulated formation of inositol
1,4,5-trisphosphate
(IP3)1 is
dependent on IP3 receptor Ca2+ channels
(IP3R) in the endoplasmic reticulum (ER) (1-4). Both activation and deactivation of the IP3R is regulated by
cytosolic [Ca2+] ([Ca2+]c) (5-9),
and this feedback control of IP3R function by released
Ca2+ gives rise to the complex spatio-temporal organization
of IP3-induced Ca2+ release. Because the
regulation of [Ca2+]c involves a number of other
Ca2+ transport mechanisms (reviewed in Ref. 10),
Ca2+ feedback on IP3R may be modulated by other
organelles that transport Ca2+.
Mitochondria are well known to participate in intracellular
Ca2+ homeostasis, although mitochondrial Ca2+
uptake is relatively insensitive to submicromolar increases of [Ca2+]c (reviewed in Refs. 10 and 11). Rizzuto,
Pozzan, and co-workers (12-14) have demonstrated that
IP3R-mediated [Ca2+]c signals are
associated with large increases of mitochondrial matrix
[Ca2+] ([Ca2+]m). Furthermore, we
have found that IP3R-mediated [Ca2+]c
oscillations are transmitted into the mitochondria and appear in the
form of [Ca2+]m oscillations (15). The high
efficiency of Ca2+ signal transmission between the ER and
mitochondria is likely to be established by a privileged or local
transfer of Ca2+ from ER release sites to the mitochondrial
Ca2+ uptake pathway (12-15). Close associations of ER and
mitochondrial membranes (14, 16) and clustering of IP3R in
ER membranes facing mitochondria (17-19) are consistent with such
local Ca2+ signaling. Although it is also becoming apparent
that mitochondria modulate cytosolic Ca2+ signaling
(20-25), it is not clear whether mitochondria can exert a local
control over the feedback effects of IP3-induced
Ca2+ release on the IP3R itself.
In the present study we demonstrate that Ca2+ uptake by the
mitochondria suppresses the positive feedback effects of
Ca2+ on the IP3R in permeabilized hepatocytes.
Moreover, our data demonstrate that the mitochondrial modulation of
IP3-induced Ca2+ release is limited to those
elements of the ER Ca2+ stores in proximity with the
mitochondria, giving rise to subcellular heterogeneity in
IP3 sensitivity and IP3R excitability. These properties allow the mitochondria to play a key role in orchestrating the subcellular pattern of [Ca2+]c signaling.
Hepatocytes plated on polylysine-coated coverslips were
maintained in primary culture for 18-24 h (15, 26). Cytosolic [Ca2+] waves in fura2-loaded intact hepatocytes were
measured essentially as described previously (15, 27). The cells were
stimulated with vasopressin (2-20 nM) prior to and after
addition of mitochondrial inhibitors or solvent in sequential runs, and
the rate of wave propagation was determined in each condition (15, 27).
For permeabilized cell experiments, cells were loaded with fluorescent dyes (obtained from Molecular Probes or Teflabs) by incubation for
30-60 min at 37 °C in medium composed of 121 mM NaCl, 5 mM NaHCO3, 10 mM Na-HEPES, 4.7 mM KCl, 1.2 mM KH2PO4,
1.2 mM Mg2SO4, 2 mM
CaCl2, 10 mM glucose, and 2% bovine serum
albumin, pH 7.4, essentially as described previously (9, 15, 26). Dye
concentrations were: 150 nM MitoTracker Green, 2 µM rhod2/AM, 5 µM fura2FF/AM, and 5 µM fluo3FF/AM. We have shown previously that
compartmentalization of rhod2 occurs in the mitochondria (15) and
fura2FF is trapped in the ER (26) of hepatocytes using this loading
protocol. Dye-loaded cells were washed with Ca2+-free
buffer and then permeabilized by incubation for 6 min with 15 µg/ml
digitonin in intracellular medium (ICM) composed of 120 mM
KCl, 10 mM NaCl, 1 mM
KH2PO4, 20 mM Tris-HEPES at pH 7.2 with 2 mM MgATP and 1 µg/ml each of antipain,
leupeptin, and pepstatin. ICM was passed through a Chelex column prior
to addition of ATP and protease inhibitors to lower the ambient
[Ca2+]. Labeling of cells with CaGreenC18 (2.5 µM) was carried out during permeabilization. After
permeabilization, the cells were washed into fresh buffer without
digitonin and incubated in the imaging chamber, at 35 °C. Digital
image time series were obtained using a Bio-Rad MRC 600 confocal
microscope equipped for dual emission or a Photometrics cooled CCD
camera system using a filter wheel and multiwavelength
beamsplitter/emission filter combination that allows simultaneous
measurement of fura2FF and rhod2 fluorescence. Calibration of fura2FF
signals in permeabilized hepatocytes gave values of 300-1000
µM for [Ca2+]ER
(Kd = 35 µM, A. Minta, TEFLABS) (26).
The fluorescence of rhod2 (Frhod2), CaGreenC18
(FCaGr), and NAD(P)H (FNAD(P)H) are expressed
as arbitrary units. The fluorescence of rhod2 and CaGreenC18 was not
calibrated in terms of absolute [Ca2+], because these are
not ratiometric dyes, and photobleaching resulted in a gradual decrease
of fluorescence during confocal imaging measurements.
Experiments were carried out with at least three different cell
preparations. Traces represent single cell responses unless indicated
otherwise. Data are presented as the means ± S.E. Significance of
differences from the relevant controls was calculated by Student's t test.
In previous studies we have demonstrated that global application
of IP3 to permeabilized hepatocytes results in oscillatory release and reuptake of [Ca2+]ER and that
this reproduces the basic mechanism of [Ca2+]c
oscillations in intact cells treated with hormones (26). We have used a
similar approach to examine the interactions between mitochondrial and
ER Ca2+ stores. [Ca2+]m was monitored
with compartmentalized rhod2 (15, 28). Double labeling with the vital
mitochondrial dye MitoTracker Green (29) demonstrated that rhod2
fluorescence was completely coincident with the mitochondria in
permeabilized hepatocytes (Fig.
1A). Moreover, the
[Ca2+]m decrease elicited by uncoupler was
manifest in a reduction of rhod2 fluorescence for all of the
intracellular structures that were double labeled with MitoTracker
(compare overlay panels iii and iv of Fig.
1A), showing that rhod2 selectively monitors [Ca2+]m in this preparation. To determine whether
IP3-induced Ca2+ release led to an increase in
[Ca2+]m, we used compartmentalized rhod2 to
monitor [Ca2+]m while simultaneously measuring
Ca2+ release from the ER. To follow IP3-induced
Ca2+ release, [Ca2+]ER was
measured with low affinity Ca2+ indicators (fura2FF and
fluo3FF) as described previously (26). As an alternative approach,
Ca2+ at the cytosolic face of intracellular membranes
([Ca2+]memb) was measured with the lipophilic
indicator CaGreenC18 (30). CaGreenC18 labeled membranes throughout the
cell, apart from the nuclear matrix, whereas MitoTracker Red
fluorescence was predominantly perinuclear, consistent with the
subcellular location of mitochondria in these cells (Fig.
1B). This differential distribution is also shown in the
overlaid CaGreenC18 and rhod2 dual label images of Fig. 1C.
A similar global distribution of the ER Ca2+ stores was
observed with fluo3FF or fura2FF, and the perinuclear organization of
the mitochondria was also demonstrated based on pyridine nucleotide
fluorescence and the alkaline pH of the mitochondrial matrix (see Fig.
3).
INTRODUCTION
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES
EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES
RESULTS AND DISCUSSION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES
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Fig. 1.
IP3-induced mitochondrial
Ca2+ uptake in permeabilized hepatocytes.
A, localization of mitochondria and measurement of
[Ca2+]m using dual emission confocal imaging of
permeabilized hepatocytes loaded with MitoTracker Green and rhod2 (show
in green and red, respectively). These images are
overlaid in the right two panels to show the coincidence of
the labeled organelles (Overlay) and the decrease in the
rhod2 signal following 5 min of treatment with 5 µg/ml uncoupler 1799 and 5 µg/ml oligomycin (+Uncoupler). B, dual
emission confocal images of CaGreenC18 and MitoTracker red (left
two panels) and overlay of these images in permeabilized
hepatocytes. C, overlay image prior to IP3
addition taken from a dual emission confocal image series using
CaGreenC18 (green) and rhod2 (red) to obtain
simultaneous measurements of [Ca2+]memb and
[Ca2+]m. D, time course of
[Ca2+]memb and [Ca2+]m
response to a supramaximal dose of IP3 (10 µM) recorded from upper cell in panel C.
E, simultaneous measurements of IP3-induced
changes in [Ca2+]ER and
[Ca2+]m using compartmentalized fura2FF and rhod2
in permeabilized hepatocytes. Additions were: 7 µM
IP3, 200 µg/ml heparin, 5 µg/ml 1799 plus 5 µg/ml
oligomycin (Uncoupler), and 10 µM ionomycin
(Iono). F, effect of IP3-induced
Ca2+ mobilization on the redox state of mitochondrial
NAD(P)H. The trace shows the increases in NAD(P)H
fluorescence (360 nm excitation) elicited by sequential additions of
100 nM and 7.5 µM IP3. Increases
of NAD(P)H fluorescence evoked by sequential additions of 100 nM and 7.5 µM IP3 were 2.9 ± 0.2% (p < 0.001) and 5.2 ± 0.4%
(p < 0.025) in 42 cells. The data are representative
of experiments with three or four separate cell preparations.
Intracellular stores were loaded with Ca2+ by incubating the permeabilized cells in the presence of ATP without Ca2+ buffers, essentially as described previously (26). Addition of maximal IP3 to cells loaded with CaGreenC18 and rhod2 resulted in rapid Ca2+ release that was detected as an increase in [Ca2+]memb and a simultaneous increase in [Ca2+]m (Fig. 1D). Nevertheless, the CaGreenC18 and rhod2 signals responded differently to uncoupler, which selectively reduced [Ca2+]m (not shown). The IP3-induced decrease in [Ca2+]ER could be monitored directly with luminal fura2FF (26), and this was also accompanied by a rapid increase in [Ca2+]m measured simultaneously with rhod2 (Fig. 1E). At the maximal levels of IP3 used in Fig. 1E, the ER remained depleted of Ca2+, but [Ca2+]m declined after the peak, as reported previously for [Ca2+]m in intact cells stimulated with a maximal dose of hormone (15). Addition of heparin to block the IP3 receptor allowed recovery of [Ca2+]ER with essentially no effect on [Ca2+]m, whereas addition of uncoupler to collapse the mitochondrial membrane potential caused [Ca2+]m to decrease without affecting [Ca2+]ER (Fig. 1E). The residual ER Ca2+ could be released with ionophore. Several intramitochondrial dehydrogenases are activated by elevated [Ca2+]m (31), and this activation can be monitored fluorometrically through changes in pyridine nucleotide redox state in intact hepatocytes (15, 32). Fig. 1F shows that the IP3-induced Ca2+ release led to an increase in NAD(P)H fluorescence in permeabilized hepatocytes, reflecting the Ca2+-dependent dehydrogenase activation. Taken together, the data of Fig. 1 demonstrate that mitochondrial Ca2+ uptake and the consequent regulation of intramitochondrial metabolism is coupled to IP3-induced Ca2+ release from the ER in permeabilized hepatocytes. Because the Ca2+ released by IP3 plays a key role in both positive and negative feedback regulation of the IP3 receptor Ca2+ channel (5-9), we used this system to investigate whether mitochondrial Ca2+ uptake modulates IP3-induced Ca2+ release.
The [Ca2+]ER decrease elicited by submaximal
and maximal IP3 was measured under the conditions described
above, where the mitochondria were able to take up part of the released
Ca2+, and compared with conditions where mitochondrial
Ca2+, uptake was blocked with ruthenium red or uncoupler
(Fig. 2). These inhibitors affect neither
the steady state [Ca2+]c nor the amount of
Ca2+ released by IP3 in liver microsomes,
suggesting that they have no direct effect on Ca2+ release
from ER in hepatocytes (33, 34). Despite the fact that the
mitochondrial blockers removed a sink for the released Ca2+, the extent of ER Ca2+ release at
submaximal IP3 was actually increased in the presence of
ruthenium red or uncoupler (Fig. 2A). Under the experimental conditions used in Fig. 2A, the Ca2+ release
response to 100 nM IP3 was increased from
8.6 ± 1.8% under control conditions to 12.1 ± 1.6% in the
presence of ruthenium red (p < 0.01, n = 4). This did not reflect a change in the size of the releasable ER
Ca2+ store, because there was no significant difference in
the extent of Ca2+ release in response to maximal
IP3 (Fig. 2A; 25.3 ± 3.5 and 25.1 ± 2.8% in the absence and presence of ruthenium red, respectively; n = 4).
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We hypothesized that the paradoxical increased efficacy of submaximal IP3 to release Ca2+ when the mitochondria are no longer available to act as a sink for this released Ca2+ reflects the feedback effects of Ca2+ on the IP3R. To examine this possibility, we repeated the experiments of Fig. 2A in the presence of BAPTA to clamp [Ca2+]c at the prestimulation level and prevent local feedback regulation by [Ca2+]c. The [Ca2+]ER decrease in response to submaximal IP3 was smaller in the presence of BAPTA, presumably because the positive feedback effects of [Ca2+]c were prevented (Fig. 2B). Alternatively, this may be explained by a decrease in IP3 sensitivity due to the pharmacological effect of BAPTA (35). More importantly, the potentiation by mitochondrial inhibitors at submaximal IP3 was completely eliminated when the Ca-BAPTA buffer was included. Thus, mitochondrial Ca2+ uptake in the immediate vicinity of the IP3-activated Ca2+ release sites can suppress the positive feedback effects of released Ca2+ that would otherwise facilitate activation of neighboring IP3Rs.
The data of Fig. 2 were averaged over a number of cells in the imaging
field. However, because mitochondria show a perinuclear distribution in
individual hepatocytes, it might be expected that the modulation of
IP3-induced Ca2+ release would occur
heterogeneously at the subcellular level. The confocal image of Fig.
3A (panel i) shows
that the entire reticular network is labeled with compartmentalized
fluo3FF in permeabilized hepatocytes. The decrease of
[Ca2+]ER in response to maximal
IP3 occurred homogeneously throughout each cell, apart from
the nuclear matrix, as shown by the difference image of Fig.
3A (panel ii). By contrast, subsequent staining of the mitochondria with the pH-sensitive dye fluorescein diacetate revealed the more centralized mitochondrial distribution (Fig. 3A, panel iii). Thus, although the
IP3-sensitive Ca2+ store appears to be
distributed throughout the hepatocyte, the modulation of
IP3 sensitivity by the mitochondria may occur predominately in the central domain of each cell. Evidence in support of this is
shown in Fig. 3B, where the spatial pattern of
[Ca2+]ER decrease evoked by submaximal and
maximal IP3 is compared with the distribution of the
mitochondria. Compartmentalized fura2FF was used to monitor
[Ca2+]ER (Fig. 3B, panel
i), and the mitochondria were localized functionally by their
redox response to the mitochondrial substrate -hydroxybutyrate (yellow overlay in Fig. 3B, panel ii).
The functional mitochondria showed the same perinuclear distribution
observed with other techniques in Figs. 1 and 3A. Addition
of 100 nM IP3 elicited a partial decrease of
[Ca2+]ER (purple overlays) in
cells 1 and 2, and this response was larger in the peripheral regions
than in the central domains where the mitochondria were located
(compare panels ii and iii of Fig. 3B). By contrast, subsequent addition of maximal
IP3 elicited a larger decrease in
[Ca2+]ER in the mitochondria-rich domains of
these cells (Fig. 3B, panel iv), which primarily
reflects the prior depletion of peripheral [Ca2+]ER by the submaximal IP3
dose. Time courses of [Ca2+]ER change in
cells 1 and 2 are shown below the images of Fig. 3B
(panels i-iv) for regions with high
mitochondrial density (traces 1A and 1B) and for
regions that were relatively deficient in mitochondria (traces
1B and 2B).
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Similar differences in IP3 sensitivity between regions with high and low mitochondrial density were observed in every cell in the imaging field, but because the responses were asynchronous they do not all show in the images. In addition, some cells gave [Ca2+]ER oscillations and waves at submaximal IP3 (26). For cell 3 of Fig. 3B, addition of 100 nM IP3 did not cause an immediate Ca2+ release. Instead, the [Ca2+]ER decrease elicited in cell 2 propagated into cell 3 as a slow wave of Ca2+ release (not shown). Significantly, the greatest magnitude and rate of [Ca2+]ER decrease occurred in the distal part of cell 3, which was largely devoid of mitochondria. [Ca2+]ER recovered in this oscillating cell and then after about 90 s in the continuing presence of 100 nM IP3 there was a second wave of [Ca2+]ER decrease that was intrinsic to cell 3. This intrinsic [Ca2+]ER wave propagated from the mitochondrial-deficient region of the cell (Fig. 3B, panels v-vii). The suppression of IP3 sensitivity in subcellular regions that were rich in mitochondria relative to other subcellular regions was observed in all experiments of the type shown in Fig. 3B.
To further evaluate the role mitochondrial Ca2+ uptake in shaping the subcellular pattern of Ca2+ release, the effects of mitochondrial inhibitors on the spatial distribution of [Ca2+]ER decrease induced by IP3 was examined in the permeabilized cells. Fig. 3C shows that the peripherial distribution of [Ca2+]ER decrease observed during the first stimulation with submaximal IP3 (Fig. 3C, panel i) was replaced by an essentially uniform response when the same cell was restimulated with the same dose of IP3 in the presence of mitochondrial uncoupler. Consistent with the idea that mitochondrial Ca2+ uptake suppresses IP3-mediated Ca2+ mobilization in intact cells, the rate of propagation of global [Ca2+]c waves evoked by the IP3-linked agonist vasopressin in intact hepatocytes was increased by 92 ± 24% (n = 7 cells, p < 0.005) when the cells were restimulated in the presence of uncoupler (1799+ oligomycin, 5 µg/ml each). By contrast, oligomycin alone had no significant effect on vasopressin-induced [Ca2+]c waves (27 ± 18% of control, n = 5).
Taken together the findings described above demonstrate that the mitochondrial modulation of IP3-induced Ca2+ release is limited to those elements of the ER Ca2+ stores in proximity with the mitochondria. As a result, the distribution of mitochondria establishes spatial heterogeneity in IP3 sensitivity, such that regions lacking mitochondria are most likely to respond first and/or with a greater amplitude of [Ca2+]ER release. Thus, the major finding of the present study is that mitochondrial Ca2+ uptake exerts strong control over local Ca2+ feedback regulation of IP3 receptors. This occurs because the mitochondria rapidly sequester a fraction of the released Ca2+, which presumably suppresses the positive feedback effects of this Ca2+ on neighboring IP3 receptors. Because this positive feedback is a key component of the mechanisms responsible for the initiation and propagation of [Ca2+]c waves, the mitochondria can play a key role in orchestrating the subcellular pattern of [Ca2+]c signaling.
Mitochondrial Ca2+ uptake following IP3-induced Ca2+ release appears to be driven by the relatively large rapid changes in [Ca2+]c and the privileged access of the mitochondria to IP3R Ca2+ release sites in closely apposed regions of the ER (10-15). We have demonstrated that the [Ca2+]c oscillations elicited by hormones in intact hepatocytes are coupled to oscillations of [Ca2+]m (15). These frequency-modulated [Ca2+]m oscillations establish dynamic control of mitochondrial energy metabolism (15, 32). Although mitochondrial Ca2+ uptake clearly serves to transduce [Ca2+]c signals from the cytosol to regulate Ca2+-dependent processes in the mitochondrial matrix (15, 31), it is also becoming apparent that mitochondria modulate cytosolic Ca2+ signaling (20-25). The simplest way in which the mitochondrial Ca2+ transport pathways can modify [Ca2+]c signals is by acting as a slow buffer that accumulates Ca2+ during rapid [Ca2+]c increases and then returns the Ca2+ as [Ca2+]c declines. In this way the mitochondria can blunt and prolong a [Ca2+]c transient, as occurs during depolarization-induced Ca2+ influx in chromaffin cells (22). However, the present study demonstrates that mitochondria can also directly regulate the Ca2+ release function of the IP3R in the ER by modulating the feedback effects of cytosolic Ca2+. This process could account for the observation that mitochondrial energization in Xenopus oocytes enhances the organization of IP3-activated [Ca2+]c waves by decreasing frequency and increasing the amplitude of Ca2+ release (20). Specifically, mitochondrial suppression of the positive feedback effects of [Ca2+]c should reduce the excitability of the system. This stabilization of the basal state would lower Ca2+ wave frequency and ensure that a greater proportion of IP3Rs are in the resting state available to contribute to Ca2+ release when the activation threshold is finally achieved at the Ca2+ wave front. A different picture has emerged in oligodendrocytes, where mitochondria appear to be selectively localized at sites of Ca2+ wave amplification (23, 28). This could reflect a role for mitochondrial Ca2+-induced Ca2+ release, whereby the accumulation of [Ca2+]m elicits mitochondrial depolarization and consequent Ca2+ release (24). However, the mechanism described in the present work could also operate in this system, but instead of suppressing positive feedback effects of [Ca2+]c, the spatial and temporal properties of mitochondrial Ca2+ uptake in the oligodendrocyte may act predominantly to suppress the negative feedback effects of [Ca2+]c.
Overall, it appears that mitochondria can have a number of important
effects on cytosolic Ca2+ signaling. These effects are not
limited to simple Ca2+ buffering but include direct
modulation of the feedback effects of [Ca2+]c on
its own release. In addition to shaping the temporal and spatial
pattern of [Ca2+]c transients, the suppression of
IP3 sensitivity by mitochondria may also play a role in
stabilizing basal [Ca2+]c. This function of the
mitochondria in setting the threshold for [Ca2+]c
spikes, together with the effects on spatial organization and signal
amplification can all contribute to enhance the fidelity of
Ca2+ signaling.
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FOOTNOTES |
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* This work was supported by Grants DK38422 (to A. P. T.) and DK51526 (to G. H.) from the National Institutes of Health.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.
§ Recipient of a Burroughs Wellcome Fund Career Award in the Biomedical Sciences.
To whom correspondence should be addressed: Dept. of
Pharmacology and Physiology, New Jersey Medical School, UMDNJ, 185 South Orange Ave., University Heights, Newark, NJ 07103. Fax:
973-972-7950; E-mail: thomasap{at}umdnj.edu.
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ABBREVIATIONS |
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The abbreviations used are: IP3, inositol 1,4,5-trisphosphate; IP3R, IP3 receptor Ca2+ channel; ER, endoplasmic reticulum; [Ca2+]c, cytosolic Ca2+; [Ca2+]m, mitochondrial matrix Ca2+; [Ca2+]ER, luminal ER Ca2+; [Ca2+]memb, Ca2+ at the cytosolic face of intracellular membrane; ICM, intracellular medium; BAPTA, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid.
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