From the * Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development; and Computational Biology and Engineering Laboratory, Center for Information Technology, National Institutes of Health, Bethesda, Maryland 20892
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ABSTRACT |
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Differences in the calcium sensitivity of individual secretory vesicles can explain a defining feature of calcium-regulated exocytosis, a graded response to calcium. The role of the time dependence of calcium delivery in defining the observed differences in the calcium sensitivity of sea urchin egg secretory vesicles in vitro was examined. The calcium sensitivity of individual secretory vesicles (i.e., the distribution of calcium thresholds) is invariant over a range of calcium delivery rates from faster than micromolar per millisecond to slower than micromolar per second. Any specific calcium concentration above threshold triggers subpopulations of vesicles to fuse, and the size of these subpopulations is independent of the time course required to reach that calcium concentration. All evidence supports the hypothesis that the magnitude of the free calcium is the single controlling variable that determines the fraction of vesicles that fuse, and that this fraction is established before the application of calcium. Submaximal responses to calcium cannot be attributed to alterations in the calcium sensitivity of individual secretory vesicles arising from the temporal properties of the calcium delivery. Models that attempt to explain the cessation of fusion using changes in the distribution of calcium thresholds arising from the rate of calcium delivery and/or adaptation are not applicable to this system, and thus cannot be general.
Key words: cytoplasmic vesicles; fertilization; membrane fusion; sea urchins; secretion ![]() |
INTRODUCTION |
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Calcium has long been recognized as an essential intracellular messenger involved in many pivotal biochemical pathways. The biochemically relevant information
attributed to this ionic species is believed to be encoded in the concentration and spatio-temporal properties of the calcium signal. The relationship that exists between calcium and the regulated response has been
attributed to the complexity of the calcium signal triggering the physiological response (Knight and Baker,
1987). In this paper, we focus on one of the temporal
properties of the calcium signal, the rate of calcium delivery. The role of the time dependence of calcium delivery in defining the observed differences in the calcium sensitivity of sea urchin egg secretory vesicles was
examined. No evidence was found to support the hypothesis that the calcium sensitivity of individual secretory vesicles is dependent upon the rate of calcium delivery.
Since a graded response to calcium represents a
defining feature of calcium-regulated exocytosis, several classes of limiting mechanisms have been proposed. These include heterogeneity in the calcium sensitivity of individual vesicles, calcium-dependent inactivation, and rate-dependent inactivation through a
calcium-dependent intermediate (Knight and Baker,
1982). Recently, adaptation has been proposed as a
property of calcium-regulated exocytosis in presynaptic
terminals in which the sensitivity of synaptic vesicles to
calcium was reported to decrease in response to either
multiple exposures or different rates of calcium (Hsu
et al., 1996
). Our result that the calcium sensitivity of
secretory vesicles is invariant with the rate of calcium
delivery supports the hypothesis that the calcium sensitivity of individual vesicles within a population of exocytotic vesicles is heterogeneous before initiating the fusion process (Blank et al., 1998
) and that the magnitude of the calcium stimulus is the single controlling
variable that determines which subpopulations of vesicles enter the fusion process. These results are incompatible with several classes of models that have been
used to describe calcium-regulated exocytosis.
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MATERIALS AND METHODS |
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Materials and methods related to the use of the two specimens of
sea urchins, Stronglyocentrotus purpuratus and Lytechinus pictus are
described in the companion paper (Blank et al., 1998). The rate
of calcium delivery was varied using either perfusion or UV photolysis of DM-nitrophen (Kaplan and Ellis-Davies, 1988
). The
time-dependent increase in calcium at the sample is referred to
as a calcium "ramp" even if the change in calcium activity with
time was not linear. Since the calcium concentration can never
change instantaneously, there will always exist a period of time
over which the calcium concentration varies. Turbulent exchange of solutions within the flow chamber, described in the companion paper (Blank et al., 1998
) and referred to as a calcium "step," corresponds to a calcium ramp occurring in <300 ms.
Calcium ramps generated using a gradient maker (385; Bio-Rad Laboratories, Hercules, CA) and a peristaltic pump (7445-10 with 7013-20 Head; Cole Parmer Inst. Co., Vernon Hills, IL) operating at a flow rate of 3.5 ml/min and total volumes of 40, 80, or
120 ml correspond to linear gradients of total calcium lasting
~11, 23, and 34 min, respectively. Light scattering from a dilute
suspension of 0.1 µm polystyrene latex spheres was used to verify
the linearity of the ramp with respect to the mixing of the two
compartments. The calcium activity along the ramp ranged from
nominally zero to >10 mM and was roughly linear in pCa as determined by discrete sampling at different time points and subsequent measurement using a calcium electrode as described previously (Blank et al., 1998). If both the initial solution composition
and the rate of removal from the gradient mixer are kept constant, then the temporal response of the sample to ramps created
using different total volumes can be compared by renormalizing
time to the beginning and ending of the ramp. This normalization procedure provides a relationship between the calcium concentration at the sample and the fusion response. Fusion change
in response to a calcium ramp is designated the "dynamic response." Ramps are useful for comparing manipulations that are
believed to alter the calcium dependence of exocytosis and conditions where the calcium dependence of exocytosis may be different. However, depending upon the rate of delivery, it may be
inappropriate to use calcium ramps to determine the steady state
calcium dependence of vesicle fusion or the kinetics of fusion.
For a given ramp protocol, variability between experiments was observed. These differences were due to errors in precisely defining the zero time point, reproducing the exact volume flow rate, and perhaps to variability in the distribution of thresholds between preparations. The pH of samples taken at different time points dropped by a maximum of only 0.15 U for the standard solutions containing 50 mM PIPES, and was constant for solutions containing 100 mM PIPES. Increasing pH buffering capacity did not affect the fusion response to the calcium ramp. Since treatment with pH 6.0 solutions (but not pH 6.4) results in vesicles that do not fuse (data not shown), care must be taken to avoid conditions that lead to acidification of the preparation as pH-dependent effects could be mistaken for calcium-dependent effects.
Calcium ramps generated using photolysis of DM-nitrophen,
as described in Shafi et al. (1994), correspond to an ~60 µM
change over ~1 s, a rate of delivery intermediate between turbulent exchange and slow perfusion. Photolysis of DM-nitrophen
was required to evaluate the relationship between changes in calcium concentration and the first fusion event after an initial fusion response, because transient flexing of the chamber during
turbulent exchange alters the optical path. The change in optical
path during turbulent exchange makes it difficult to evaluate
when the first fusion event occurs because the high volume flow
(~8 ml/s) lasts, typically, for 0.5 s. This limitation does not exist
with photolysis.
Free calcium concentrations are estimated using the fluorescent indicator, Rhod-2 (Molecular Probes, Inc., Eugene, OR)
and calibrated as described previously (Shafi et al., 1994). Error in the estimation of the free calcium concentration arises primarily from error in Kd, contributions from fluorescence beyond the
plane of focus, and misregistration between the planes of UV and
fluorescence excitation and emission. Propagation of the error in
Kd (4.0 ± 0.8 µM; Shafi et al., 1994) leads to an error of ~0.2 pCa U. The other two optical properties are expected to bias the determinations towards an underestimation of the calcium concentration. For calcium concentrations in the neighborhood of the
midpoint of the calcium activation curve, the uncertainty in Kd
introduces an uncertainty in the estimation of the free calcium
concentration by a factor of ~2. In the photolysis double challenge experiments, light scattering and fluorescence were monitored simultaneously during a transient increase in calcium (first
challenge) and increases to high concentrations (>500 µM, second challenge). The first detectable fusion event during the second challenge was correlated with the calcium concentration. The criteria for the first detectable fusion event was the first light
scattering (% fusion) value outside the envelope defined by ±5%
fusion. This range in fusion, ±5% fusion, corresponded to the
steady state peak to peak noise in the light scattering signal. This
criteria represents a conservative estimate for detection and
places an upper limit on the calcium concentration required for
subsequent fusion in the photolysis double-challenge protocol. The results from a total of 30 perfusion ramps and 48 photolysis ramps are reported.
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RESULTS |
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The average dynamic response of S. purpuratus and L.
pictus to an 11-min ramp in the total calcium is shown
in Fig. 1 A. The dynamic response of L. pictus, compared with S. purpuratus, is shifted in time, indicating
that higher calcium concentrations are required for
the same response (Fig. 1 B). The time to 50% response is significantly different between the two species
(4.75 ± 0.12 and 5.40 ± 0.19 min for S. purpuratus, n = 6, and L. pictus, n = 2, mean ± SEM or range, respectively). This phenotypic variation with species is consistent with the shift in the calcium activity curves measured using step changes in calcium (Blank et al.,
1998). For the same rate of calcium delivery, S. purpuratus is more sensitive to calcium than L. pictus.
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The dynamic responses observed using these ramps
were shifted along the pCa axis compared with the stable, submaximal responses obtained with step changes
in the calcium concentration (Blank et al., 1998). This
shift is expected if overlap between the characteristic times associated with the exocytotic process and the calcium ramp is significant because the dynamic response
is strongly influenced by the convolution of the calcium
delivery with the underlying fusion kinetics. Vesicle fusion, detected at a later time, may have entered the fusion process at an earlier time in response to a lower
calcium concentration. Whether this shift is secondary to the relationship between calcium and the extent of
fusion was tested. If the calcium-delivery history has no
effect on the steady state extent of fusion, then interrupting the ramp and holding the calcium concentration constant should result in extents of fusion identical to those produced by step changes to the same calcium concentration.
The dynamic response to an 11-min ramp with and
without interruption is shown in Fig. 2. Interrupted
ramps resulted in 70 ± 9% (mean ± SEM, n = 3) fusion at a calcium concentration of ~60 µM. Interrupting the ramp and maintaining the preparation at this
calcium concentration produced submaximal extents
of fusion that are indistinguishable from those produced by step changes to the same calcium concentration. This is indicated by the overlap of the interrupted
ramp data with the calcium activation curve described
by the log-normal cumulative distribution function
(this paper, see Fig. 8 A, ; Blank et al., 1998
, Fig. 7 A).
For these experiments, the more highly buffered solution (6 mM EGTA, 6 mM HEDTA, 3 mM nitrilotriacetic
acid) was used. To what extent, if any, is the behavior of
the unfused vesicles altered by the kinetics of calcium
delivery?
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After the establishment of a stable, submaximal extent of fusion, the calcium ramp was continued and the exocytotic response of the remaining vesicles was compared with the response obtained from a continuous ramp. When the time axis is rescaled to the concentration of calcium present at the sample, no systematic shifts in the response are observed for interrupted and continuous ramps. The pCa at 85% fusion in the dynamic responses of the interrupted and continuous ramp are identical (pCa85% = 3.88 ± 0.05). As a further test, the shape of the dynamic response was compared by rescaling the time axis after continuation of the interrupted calcium ramp. The rescaled plot of the final 30% fusion for interrupted and continuous ramps is shown in Fig. 3 A. The slope of the cross-correlation between the fusion response for interrupted and continuous ramps using normalized time as the parametric variable is 1.01 ± 0.00 (Fig. 3 B). There are no significant differences between the dynamic response of the remaining unfused vesicles in either a continuous or interrupted calcium ramp.
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If the cessation of fusion occurs because a specific population of vesicles is removed through fusion, then changing the rate at which a defined concentration of calcium is delivered may alter the fusion kinetics but not the underlying relationship between calcium and the extent of fusion. This hypothesis was tested by altering the buffering properties of the solution, total concentration of calcium in the second chamber of the gradient maker, or increasing the ramp duration from 11 to 34 min. Examples of the dynamic response to both 11- and 23-min ramps are shown in Fig. 4 A. When the time axis is rescaled to the concentration of calcium present at the sample, no systematic shifts in the response are observed and 100% fusion is always obtained (Fig. 4 B). The difference in pCa at 50% fusion in the dynamic response to the two ramps is not significant (pCa50% = 3.49 ± 0.05 and 3.57 ± 0.05 for 11- and 23-min ramps, respectively, P < 0.05). As a further test, the shape of the dynamic response was compared by rescaling the time axis about 50% fusion after renormalizing the time axis by the gradient volumes. This converts the time axis to an axis proportional to the extent of the ramp where 0 and 1 correspond to the concentrations at the beginning and end of the ramp. If the dynamic response is invariant to the delivery rate, then the slope of the cross-correlation between the two different fusion responses will be one. The slope of the cross-correlation between the fusion response at the two different calcium-delivery rates using normalized time as the parametric variable is 0.97 ± 0.00; the fusion response is invariant to a factor of two in the rate of calcium delivery.
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Calcium ramps generated using different UV excitation intensities resulted in calcium ramps intermediate between turbulent exchange and slow perfusion. Examples of the average dynamic response to different photolysis-derived ramps (n = 6) is shown in Fig. 5 A. The UV excitation intensity was varied using a glass plate. No systematic shifts in the response are observed when the time axis is rescaled to the concentration of calcium present at the sample using time as the parametric variable; the two traces overlap (Fig. 5 B). The dynamic response was not altered by changes in the rate of calcium delivery intermediate between turbulent exchange and slow perfusion.
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If vesicles respond only to the magnitude of the calcium stimulus, then the calcium concentration associated with the first detectable fusion event after a stable, submaximal response must be greater then the concentration that produced the initial fusion response. This hypothesis was tested using photolysis double challenge experiments. Calcium transients resulting from the photolysis of DM-nitrophen lead to stable, submaximal extents of fusion (Fig. 6 A). After a return to base-line calcium concentrations, a ramp in the calcium concentration was used to initiate fusion of the remaining vesicles. The concentration of calcium associated with the first detectable fusion event after this second challenge was correlated with the calcium concentration present at the peak of the first challenge (Fig. 6 B). In 35 of 36 cases, the first detectable fusion event always occurred at a higher calcium concentration independent of the calcium rate of delivery in the second challenge (Fig. 7). That the data is shifted to the left of the identity line is expected because in these experiments the noise envelope for % fusion is approximately ±5% (Fig. 6 B). The one anomalous event may be due to the stable, submaximal response not being reached before initiating the second calcium challenge. The calcium concentration associated with the first detectable fusion event after a stable, submaximal fusion response is greater than the concentration that produced the initial fusion response. However, does the change in calcium required to detect the first fusion event, but monitored using the fluorescent indicator Rhod-2, agree with the steady state calcium activation curve derived from rapid perfusion (turbulent exchange) of solutions with defined calcium concentrations?
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Results from photolysis, interrupted perfusion, and
turbulent exchange are summarized in Fig. 8 A. The
solid line represents the log-normal cumulative distribution function previously shown to parameterize the
calcium activation curve (Blank et al., 1998). A shift of
~0.2 pCa U in this distribution function describes the
average data derived from photolysis in which the relationship between peak fusion and calcium was monitored using the fluorescent indicator Rhod-2 (Fig. 8 A,
dotted line). However, this shift is not considered significant because the error in Kd alone is sufficient to account
for a difference of 0.2 pCa U in a single determination
and <0.1 pCa U for averaged data. If the calcium concentrations were systematically underestimated, then
comparing the change in calcium required to detect
the first fusion event as a function of the initial level of
fusion with the same quantity derived from the cumulative distribution function provides an alternative method
for comparing the response to calcium step changes
and photolytic release. The translational invariance of the calcium activation curve along the pCa axis justifies
this approach.
The change in calcium required for a subsequent
10% change in fusion, starting from any initial level of
fusion, was derived from the slope of the log-normal cumulative distribution. The infinitesimal change in calcium
with fusion, 1/(d% fusion/d pCa), for a log-normal cumulative distribution function is d pCa/d% fusion = 0.01 · (2)0.5 · W · exp({[pCa + log(10-6 M)]/(2W)}2).
The relationship between (d pCa/d% fusion) and %
fusion was derived from a parametric plot of (d pCa/d%
fusion) and % fusion using pCa as the parametric variable. When multiplied by 10%, this relationship describes the change in calcium associated with a 10%
change in fusion, or pCa per 10% fusion. The solid
line in Fig. 8 B represents this function. A 10% change
in fusion was chosen because the noise in the light scattering signal of the photolysis experiments corresponded
to ~10% peak to peak and represents the minimum
change detectable in these experiments. When the initial extent of fusion is low or high, a large change in calcium is required for a subsequent 10% change in fusion,
while in the transition region, small changes in calcium
are required. The change in calcium required to detect
the first fusion event in the photolysis experiments (Fig. 8,
) is in agreement with the relationship derived from the calcium activation curve for rapid perfusion of solutions with defined calcium concentrations
(Fig. 8 B, solid line). In the linear region of the calcium
activation curve, a change in calcium of 0.22 ± 0.03 pCa U (mean ± SEM, n = 14) is sufficient for additional subpopulations to fuse. The agreement between
photolysis, step perfusion, and ramp perfusion data,
despite the very different time courses associated with
the calcium delivery, is consistent with the hypothesis
that the concentration of free calcium is the single controlling variable that determines the fraction of the vesicles that fuse.
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DISCUSSION |
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By combining slow perfusion of calcium in a chamber optimized for microscopic examination with rapid production of calcium by photolysis of chelator, we have tested the often-debated suggestion that stimulus-secretion coupling depends on the rate of calcium delivery. The calcium sensitivity of individual secretory vesicles was invariant to calcium delivery rates from faster than micromolar per millisecond to slower than micromolar per second. All experimental evidence supports the hypothesis that vesicles respond to the magnitude of the calcium stimulus independent of the time course over which it is delivered. No evidence was found to support the hypothesis that a decrease in the calcium sensitivity of individual secretory vesicles arising from the temporal properties of the calcium delivery is a limiting mechanism in establishing a graded response to calcium. These results have profound implications for models that attempt to explain the mechanism of calcium-regulated exocytosis.
Elimination of Models for Calcium-triggered Exocytosis
The properties of sea urchin egg secretory vesicle exocytosis in vitro in response to both multiple solution exchange protocols and different rates of calcium delivery
are incompatable with several classes of kinetic reactions
that have been incorporated into models of calcium-regulated exocytosis. Kinetic reactions that are not applicable are summarized here (1) A B (two-state reaction);
(2) A
B
C (consecutive reaction); (3) A
B
C with A, B
Iirreversible or A, B
Ireversible (consecutive
reaction with inhibitory branches); (4) Ai
Ai* (i = 1, 2, ...) and Bj
Bj* (j = 1, 2, ...) (parallel reactions with
fusion
Ai*N Bj*M); and (5) adapting quantity.
All homogeneous population models (reactions 1, 2, and 3) with or without reversible or irreversible inhibitory states fail to describe the properties of this calcium-triggered exocytotic system. The first two models are
simple linear reaction schemes in which reactant A is
converted to product in one or two steps. If fusion is
proportional to the concentration of product, then there can be no submaximal levels of fusion because all
reactants are converted to product. Including an inhibitory branch in the reaction (reaction 3) does not alter
the final outcome of a linear reaction scheme, conversion to product, unless the branch is irreversible. However, an irreversible branch means that the pool of reactants that are irreversibly inhibited can never take part
in the reaction. This property is not observed: there was
no irreversible loss in the ability of the remaining vesicles to fuse (this paper and Blank et al., 1998). Parallel
reaction models (reaction 4) in which fusion is proportional to the product of individual transitions allows for
submaximal conversion if one class of transitions is reversible. However, this model has the property of reset
through the reversible transition. Removing calcium
would reset the reversible transition such that a second
challenge of the same concentration would result in fusion of the remaining vesicles to the same fractional extent as before. This property is not observed: the first
detectable fusion event after the development of a stable, submaximal response occurs at a higher calcium
concentration than the concentration that produced
the initial response. All attempts to reveal a limiting reaction other than a decrease in the number of vesicles
in different subpopulations were unsuccessful.
Adaptation (reaction 5) has been proposed as a
property of calcium-triggered exocytosis in presynaptic
terminals (Hsu et al., 1996). Models based on an adapting quantity that is proportional to a linear combination of receptor states predict that the removal of calcium should lead to recovery of sensitivity, and the activity of the adapting quantity should be dependent
upon the rate of change of calcium (Segel et al., 1986
;
Knox et al., 1986
). The previous paper (Blank et al.,
1998
) demonstrated that the fusion process cannot be
reset: two identical pulses separated in time failed to
elicit the same response. Here, we demonstrate that the
same extent of fusion is achieved for different rates of
delivery of the same calcium concentration. Together,
these results rule out these kinetic models based on an
adapting quantity that is proportional to a linear combination of receptor states.
One can speculate that the existence of multiple, calcium-dependent states in the exocytotic process before
fusion may be branch points for calcium-dependent inactivation. For example, calcium could have a dual effect on
a "priming" state where the main kinetic branch proceeds
to fusion and a second branch leads to a nonfusing state
that can proceed to fusion only by a second calcium-
dependent path with a higher threshold for calcium. An extension of this process could lead to multiple, nonfusing states where each state has an alternate kinetic pathway requiring higher concentrations of calcium for subsequent fusion. Calcium could create a distribution of nonfusing, "priming" states, each state having a pathway to
fusion with differing calcium requirements. The question
of heterogeneity would be shifted from an initial population of vesicles with different calcium thresholds to a calcium-induced population of priming states. Each priming
state would have alternate fusion pathways with different
calcium requirements. However, the data are difficult to
reconcile with this proposal. If these nonfusing states are
in equilibrium with "upstream" states, then the removal of
calcium should reset the system. Removal of calcium does
not reset the system. One would have to postulate, post
hoc, that only the forward rate of reaction creating the nonfusing state was calcium dependent and that the backward reactions were dependent on factors no longer available; in essence, locking the system into new, calcium-dependent, metastable states with the potential to proceed to fusion with different, but higher, concentrations
of calcium. Without detailed knowledge of the calcium triggering event, it is difficult to evaluate these complicated kinetic alternatives. However, there exists a simpler
description of the kinetics of exocytosis that is consistent
with the observed behavior. We have proposed that the
number and distribution of activated fusion complexes
underlie the control of the rate and extent of triggered
exocytosis (Vogel et al., 1996). This model has been extended and describes the kinetics of fusion in response to
all experimental calcium challenges (single, double, and
ramp) used to date (Blank, P.S., S.S. Vogel, J. Malley, and
J. Zimmerberg, manuscript in preparation).
In summary, we have determined that the calcium sensitivity of individual secretory vesicles is invariant with the
rate of calcium delivery. The graded responses observed
in calcium-regulated exocytosis are explained by earlier
differences in the calcium sensitivity of individual secretory vesicles (Blank et al., 1998), and these differences in
calcium sensitivity are not modulated by the rate of calcium delivery over a range faster than millimolar per second to slower than micromolar per second. Our working
hypothesis is that differences in calcium sensitivity, together with a stochastic variability in the number of fusion complexes active on individual vesicles, explains the
kinetics and extent of calcium-regulated exocytosis. Exocytosis occurs when a vesicle has at least one fusion complex active; the rate of exocytosis is a function of the
number of active fusion complexes per vesicle. Exocytosis ceases after all the vesicles with active fusion complexes
have fused: the release of this subpopulation of vesicles
explains the graded response to calcium.
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FOOTNOTES |
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Address correspondence to Paul S. Blank, LCMB/NICHD/NIH, Bldg. 10, Rm. 10D14, 10 Center Drive MSC 1855, Bethesda, MD 20892-1855. Fax: 301-480-0857; E-mail: psblank{at}helix.nih.gov
Original version received 12 January 1998 and accepted version received 15 September 1998.
Portions of this work have been previously presented in abstract form (Blank, P.S., M.S. Cho, S.S. Vogel, D. Kaplan, D. Bhuva, A. Kang, J. Malley, and J. Zimmerberg. 1997. J. Gen. Physiol. 110:17a).We thank Nadeem I. Shafi for preliminary work on exocytosis using photolysis of DM-nitrophen.
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REFERENCES |
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Submaximal responses in calcium-triggered exocytosis are explained by differences in the calcium sensitivity of individual secretory vesicles.
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