A two-step model for acetylcholine control of exocytosis via nicotinic receptors

The view that Ca²⁺ entry through voltage-dependent Ca²⁺ channels (VDCC) and through nicotinic receptors for acetylcholine (nAChRs) causes equal catecholamine release responses in chromaffin cells, was reinvestigated here using new protocols. We have made two-step experiments consisting in an ACh prepulse followed by a depolarizing pulse (DP). In voltage-clamped bovine chromaffin cells an ACh prepulse caused a slow-rate release but augmented 4.5-fold the much faster exocytotic response triggered by a subsequent depolarizing pulse (measured with capacitance and amperometry). If the ACh prepulse was given with mecamylamine or in low external Ca²⁺, the secretion increase disappeared. This suggests a two-step model for the effects of ACh: (1) meager Ca²⁺ entry through nAChRs mostly serves to keep loaded with vesicles the secretory machine; and (2) in this manner, the cell is prepared to respond with an explosive secretion of catecholamine upon depolarization and fast high Ca²⁺ entry through VDCC.     

A Sequential Vesicle Pool Model with a Single Release Sensor and a Ca2+-Dependent Priming Catalyst Effectively Explains Ca2+-Dependent Properties of Neurosecretion

Neurotransmitter release depends on the fusion of secretory vesicles with the plasma membrane and the release of their contents. The final fusion step displays higher-order Ca²⁺ dependence, but also upstream steps depend on Ca²⁺. After deletion of the Ca²⁺ sensor for fast release – synaptotagmin-1 – slower Ca²⁺-dependent release components persist. These findings have provoked working models involving parallel releasable vesicle pools (Parallel Pool Models, PPM) driven by alternative Ca²⁺ sensors for release, but no slow release sensor acting on a parallel vesicle pool has been identified. We here propose a Sequential Pool Model (SPM), assuming a novel Ca²⁺-dependent action: a Ca²⁺-dependent catalyst that accelerates both forward and reverse priming reactions. While both models account for fast fusion from the Readily-Releasable Pool (RRP) under control of synaptotagmin-1, the origins of slow release differ. In the SPM the slow release component is attributed to the Ca²⁺-dependent refilling of the RRP from a Non-Releasable upstream Pool (NRP), whereas the PPM attributes slow release to a separate slowly-releasable vesicle pool. Using numerical integration we compared model predictions to data from mouse chromaffin cells. Like the PPM, the SPM explains biphasic release, Ca²⁺-dependence and pool sizes in mouse chromaffin cells. In addition, the SPM accounts for the rapid recovery of the fast component after strong stimulation, where the PPM fails. The SPM also predicts the simultaneous changes in release rate and amplitude seen when mutating the SNARE-complex. Finally, it can account for the loss of fast- and the persistence of slow release in the synaptotagmin-1 knockout by assuming that the RRP is depleted, leading to slow and Ca²⁺-dependent fusion from the NRP. We conclude that the elusive ‘alternative Ca²⁺ sensor’ for slow release might be the upstream priming catalyst, and that a sequential model effectively explains Ca²⁺-dependent properties of secretion without assuming parallel pools or sensors.     

Two distinct secretory vesicle�Cpriming steps in adrenal chromaffin cells

Priming of large dense-core vesicles (LDCVs) is a Ca²⁺-dependent step by which LDCVs enter a release-ready pool, involving the formation of the soluble N-ethyl-maleimide sensitive fusion protein attachment protein (SNAP) receptor complex consisting of syntaxin, SNAP-25, and synaptobrevin. Using mice lacking both isoforms of the calcium-dependent activator protein for secretion (CAPS), we show that LDCV priming in adrenal chromaffin cells entails two distinct steps. CAPS is required for priming of the readily releasable LDCV pool and sustained secretion in the continued presence of high Ca²⁺ concentrations. Either CAPS1 or CAPS2 can rescue secretion in cells lacking both CAPS isoforms. Furthermore, the deficit in the readily releasable LDCV pool resulting from CAPS deletion is reversed by a constitutively open form of syntaxin but not by Munc13-1, a priming protein that facilitates the conversion of syntaxin to the open conformation. Our data indicate that CAPS functions downstream of Munc13s but also interacts functionally with Munc13s in the LDCV-priming process.     

The immediately releasable vesicle pool: highly coupled secretion in chromaffin and other neuroendocrine cells

In neuroendocrine cells, such as adrenal chromaffin cells, the exocytosis of hormone-filled vesicles is triggered by a localized Ca(2+) increase that develops after the activation of voltage-dependent Ca(2+) channels. To reach the fusion competent state, vesicles have to go through a series of maturation steps that involve the detachment from cytoskeletal proteins, docking and priming. However, the fusion readiness of vesicles will also depend on their proximity to the calcium source. The immediately releasable pool is a small group of ready-to-fuse vesicles, whose fusion is tightly coupled to Ca(2+) entry through channels. Recent work indicates that such coupling is not produced by a random distribution between vesicles and channels, but would be the result of a specific interaction of immediately releasable vesicles with particular Ca(2+) channel subtypes. The immediately releasable pool is able to sustain, with high efficiency, the secretion triggered by the small and localized Ca(2+) gradients produced by brief depolarizations at low frequencies, like action potentials at basal conditions in adrenal chromaffin cells.     

Maintenance of quantal size and immediately releasable granules in rat chromaffin cells by glucocorticoid

Glucocorticoid is reported to regulate catecholamine synthesis and storage. However, it is not clear whether the actual amount of catecholamine released from individual granules (quantal size, Q) in mature chromaffin cells is affected by glucocorticoid. Using carbon fiber amperometry, we found that dexamethasone did not affect mean cellular Q or the proportional release from different populations of granules in rat chromaffin cells cultured for 1 day in a serum-free defined medium. After two extra days of culture in the defined medium, there was a rundown in mean cellular Q, and it was associated with a shift in the proportional release from the different granule populations. This phenomenon could not be rescued by serum supplementation but could be prevented by dexamethasone via an action that was independent of changes in voltage-gated Ca²⁺ channel (VGCC) density. Using simultaneous measurements of membrane capacitance and cytosolic Ca²⁺ concentration, we found that for cells cultured in defined medium dexamethasone enhanced the exocytotic response triggered by a brief depolarization (50 ms) without affecting the VGCC density or the fast exocytotic response triggered via flash photolysis of caged Ca²⁺. Thus glucocorticoid may regulate the number of immediately releasable granules that are in close proximity to a subset of VGCC. Because chromaffin cells in vivo are exposed to high concentrations of glucocorticoid, our findings suggest that the paracrine actions of glucocorticoid maintain the mean catecholamine content in chromaffin cell granules as well as the colocalization of releasable granules with VGCCs. catecholamines; paracrine action; exocytosis; calcium channels     

Pathways that control cortical F-actin dynamics during secretion

Chromaffin cells possess a mesh of filamentous actin underneath the plasma membrane which acts as a barrier to the chromaffin vesicles access to exocytotic sites. Disassembly of cortical F-actin in response to stimulation allows the movement of vesicles from the reserve pool to the release-ready vesicle pool and, therefore, to exocytotic sites. The dynamics of cortical F-actin is controlled by two mechanisms: a) stimulation-induced Ca²⁺ entry and scinderin activation and b) protein kinase C (PKC) activation and MARCKS phosphorylation as demonstrated here by experiments with recombinant proteins, antisense olygodeoxynucleotides and vector mediated transient expressions. Under physiological conditions (i.e., cholinergic receptor stimulation followed by Ca²⁺ entry), mechanism (a) is the most important for the control of cortical F-actin network whereas when Ca²⁺ is released from intracellular stores (i.e., histamine stimulation) cortical F-actin is regulated mainly by mechanism b.     

Ouabain enhances exocytosis through the regulation of calcium handling by the endoplasmic reticulum of chromaffin cells

The augmentation of neurotransmitter and hormone release produced by ouabain inhibition of plasmalemmal Na⁺/K⁺-ATPase (NKA) is well established. However, the mechanism underlying this action is still controversial. Here we have shown that in bovine adrenal chromaffin cells ouabain diminished the mobility of chromaffin vesicles, an indication of greater number of docked vesicles at subplasmalemmal exocytotic sites. On the other hand, ouabain augmented the number of vesicles undergoing exocytosis in response to a K⁺ pulse, rather than the quantal size of single vesicles. Furthermore, ouabain produced a tiny and slow Ca²⁺ release from the endoplasmic reticulum (ER) and gradually augmented the transient elevations of the cytosolic Ca²⁺ concentrations ([Ca²⁺]_c) triggered by K⁺ pulses. These effects were paralleled by gradual increments of the transient catecholamine release responses triggered by sequential K⁺ pulses applied to chromaffin cell populations treated with ouabain. Both, the increases of K⁺-elicited [Ca²⁺]_c and secretion in ouabain-treated cells were blocked by thapsigargin (THAPSI), 2-aminoethoxydiphenyl borate (2-APB) and caffeine. These results are compatible with the view that ouabain may enhance the ER Ca²⁺ load and facilitate the Ca²⁺-induced-Ca²⁺ release (CICR) component of the [Ca²⁺]_c signal generated during K⁺ depolarisation. This could explain the potentiating effects of ouabain on exocytosis.     

Early requirement for alpha-SNAP and NSF in the secretory cascade in chromaffin cells.

NSF and alpha-SNAP have been shown to be required for SNARE complex disassembly and exocytosis. However, the exact requirement for NSF and alpha-SNAP in vesicular traffic through the secretory pathway remains controversial. We performed a study on the kinetics of exocytosis from bovine chromaffin cells using high time resolution capacitance measurement and electrochemical amperometry, combined with flash photolysis of caged Ca2+ as a fast stimulus. alpha-SNAP, a C-terminal mutant of alpha-SNAP, and NEM were assayed for their effects on secretion kinetics. Two kinetically distinct components of catecholamine release can be observed upon fast step-like elevation of [Ca2+]i. One is the exocytotic burst, thought to represent the readily releasable pool of vesicles. Following the exocytotic burst, secretion proceeds slowly at maintained high [Ca2+]i, which may represent vesicle maturation/recruitment, i.e. some priming steps after docking. alpha-SNAP increased the amplitude of both the exocytotic burst and the slow component but did not change their kinetics, which we examined with millisecond time resolution. In addition, NEM only partially inhibited the slow component without altering the exocytotic burst, fusion kinetics and the rate of endocytosis. These results suggest a role for alpha-SNAP/NSF in priming granules for release at an early step, but not modifying the fusion of readily releasable granules.     

Rab3 Proteins Involved in Vesicle Biogenesis and Priming in Embryonic Mouse Chromaffin Cells

The four Rab3 paralogs A–D are involved in exocytosis, but their mechanisms of action are hard to study due to functional redundancy. Here, we used a quadruple Rab3 knockout (KO) (rab3a, rab3b, rab3c, rab3d null, here denoted as ABCD^?/?) mouse line to investigate Rab3 function in embryonic mouse adrenal chromaffin cells by electron microscopy and electrophysiological measurements. We show that in cells from ABCD^?/? animals large dense‐core vesicles (LDCVs) are less abundant, while the number of morphologically docked granules is normal. By capacitance measurements, we show that deletion of Rab3s reduces the size of the releasable vesicle pools but does not alter their fusion kinetics, consistent with an altered function in vesicle priming. The sustained release component has a sigmoid shape in ABCD^?/? cells when normalized to the releasable pool size, indicating that vesicle priming follows at a higher rate after an initial delay. Rescue experiments showed that short‐term (4–6 h) overexpression of Rab3A or Rab3C suffices to rescue vesicle priming and secretion, but it does not restore the number of secretory vesicles. We conclude that Rab3 proteins play two distinct stimulating roles for LDCV fusion in embryonic chromaffin cells, by facilitating vesicle biogenesis and stabilizing the primed vesicle state.     

Inhibition of Ca<Superscript>2+</Superscript> Channels and Adrenal Catecholamine Release by G Protein Coupled Receptors

Catecholamines and other transmitters released from adrenal chromaffin cells play central roles in the “fight-or-flight” response and exert profound effects on cardiovascular, endocrine, immune, and nervous system function. As such, precise regulation of chromaffin cell exocytosis is key to maintaining normal physiological function and appropriate responsiveness to acute stress. Chromaffin cells express a number of different G protein coupled receptors (GPCRs) that sense the local environment and orchestrate this precise control of transmitter release. The primary trigger for catecholamine release is Ca²⁺ entry through voltage-gated Ca²⁺ channels, so it makes sense that these channels are subject to complex regulation by GPCRs. In particular G protein βγ heterodimers (Gβγ) bind to and inhibit Ca²⁺ channels. Here I review the mechanisms by which GPCRs inhibit Ca²⁺ channels in chromaffin cells and how this might be altered by cellular context. This is related to the potent autocrine inhibition of Ca²⁺ entry and transmitter release seen in chromaffin cells. Recent data that implicate an additional inhibitory target of Gβγ on the exocytotic machinery and how this might fine tune neuroendocrine secretion are also discussed.     

Exocytotic dynamics and calcium cooperativity effects in the calyx of Held synapse: a modelling study

A stochastic computational approach to the study of secretory processes at the calyx of Held synapse is presented in this paper. The calyx of Held is a giant synapse located in the brainstem which is widely used for experimental recording of neurotransmitter release. We focus on the study of the exocytotic dynamics for a pool of readily releasable vesicles using a Monte Carlo simulation scheme that includes models for the P-type calcium channels, the kinetic reactions of endogenous and exogenous (mobile) buffers, the kinetic reactions for the secretory vesicles, as well as the microscopic diffusion of mobile buffers and calcium ions. The simulations are performed in a 3-D orthogonal grid which approximates a cylindrical domain representing an active zone of the presynaptic terminal of the calyx. For this domain, we quantify the release rates related to calcium currents in response to depolarizing voltage pulses. The influence on simulated pulse/action potential depolarization protocols of the kinetic scheme for the calcium sensor of vesicles and the geometry of calcium channels for the kinetic cooperativity for release, is analyzed at a microdomain level. Among other aspects, our results suggest that the spatial organization of Ca ^2 + channels could have measurable effects in the kinetic cooperativity which could reflect developing changes in the calyx of Held synapse.     

Docking of chromaffin granules—A necessary step in exocytosis?

Putative docking of secretory vesicles comprising recognition of and attachment to future fusion sites in the plasma membrane has been investigated in chromaffin cells of the bovine adrenal medulla and in rat phaeochromocytoma (PC 12) cells. Upon permeabilization with digitonin, secretion can be stimulated in both cell types by indreasing the free Ca²⁺-concentration to M levels. Secretory activity can be elicited up to 1 hr after starting permeabilization and despite the loss of soluble cytoplasmic components indicating a stable attachment of granules to the plasma membrane awaiting the trigger for fusion. Docked granules can be observed in the electron microscope in permeabilized PC 12 cells which contain a large proportion of their granules aligned underneath the plasma membrane. The population of putatively docked granules in chromaffin cells cannot be as readily discerned due to the dispersal of granules throughout the cytoplasm. Further experiments comparing PC 12 and chromaffin cells suggest that active docking but not transport of granules can still be performed by permeabilized cells in the presence of Ca²⁺: a short (2 min) pulse of Ca²⁺ in PC 12 cells leads to the secretion of almost all releasable hormone over a 15 min observation period whereas, in chromaffin cells, with only a small proportion of granules docked, withdrawal of Ca²⁺ leads to an immediate halt in secretion. Transport of chromaffin granules from the Golgi to the plasma membrane docking sites seems to depend on a mechanism sensitive to permeabilization. This is shown by the difference in the amount of hormone released from the two permeabilized cell types, reflecting the contrast in the proportion of granules docked to the plasma membrane in PC 12 or chromaffin cells. Neither docking nor the docked state are influenced by cytochalasine B or colchicine. The permeabilized cell system is a valuable technique for thein vitro study of interaction between secretory vesicles and their target membrane.     

v-SNAREs control exocytosis of vesicles from priming to fusion

SNARE proteins (soluble NSF-attachment protein receptors) are thought to be central components of the exocytotic mechanism in neurosecretory cells, but their precise function remained unclear. Here, we show that each of the vesicle-associated SNARE proteins (v-SNARE) of a chromaffin granule, synaptobrevin II or cellubrevin, is sufficient to support Ca(2+)-dependent exocytosis and to establish a pool of primed, readily releasable vesicles. In the absence of both proteins, secretion is abolished, without affecting biogenesis or docking of granules indicating that v-SNAREs are absolutely required for granule exocytosis. We find that synaptobrevin II and cellubrevin differentially control the pool of readily releasable vesicles and show that the v-SNARE's amino terminus regulates the vesicle's primed state. We demonstrate that dynamics of fusion pore dilation are regulated by v-SNAREs, indicating their action throughout exocytosis from priming to fusion of vesicles.     

Silence of Synaptotagmin VII inhibits release of dense core vesicles in PC12 cells

Synaptotagmin VII (Syt VII), which has a higher Ca²⁺ affinity and slower disassembly kinetics with lipid than Syt I and Syt IX, was regarded as being uninvolved in synaptic vesicle (SV) exocytosis but instead possibly as a calcium sensor for the slower kinetic phase of dense core vesicles (DCVs) release. By using high temporal resolution capacitance and amperometry measurements, it was demonstrated that the knockdown of endogenous Syt VII attenuated the fusion of DCV with the plasma membrane, reduced the amplitude of the exocytotic burst of the Ca²⁺-triggered DCV release without affecting the slope of the sustained component, and blocked the fusion pore expansion. This suggests that Syt VII is the Ca²⁺ sensor of DCV fusion machinery and is an essential factor for the establishment and maintenance of the pool size of releasable DCVs in PC12 cells.     

Neuronal Calcium Sensor-1 Regulation of Calcium Channels,Secretion, and Neuronal Outgrowth

Calcium (Ca²⁺) is an important intracellular messenger underlying cell physiology. Ca²⁺ channels are the main entry route for Ca²⁺ into excitable cells, and regulate processes such as neurotransmitter release and neuronal outgrowth. Neuronal Calcium Sensor-1 (NCS-1) is a member of the Calmodulin superfamily of EF-hand Ca²⁺ sensing proteins residing in the subfamily of NCS proteins. NCS-1 was originally discovered in Drosophila as an overexpression mutant (Frequenin), having an increased frequency of Ca²⁺-evoked neurotransmission. NCS-1 is N-terminally myristoylated, can bind intracellular membranes, and has a Ca²⁺ affinity of 0.3 μM. Over 10 years ago it was discovered that NCS-1 overexpression enhances Ca²⁺-evoked secretion in bovine adrenal chromaffin cells. The mechanism was unclear, but there was no apparent direct effect on the exocytotic machinery. It was revealed, again in chromaffin cells, that NCS-1 regulates voltage-gated Ca²⁺ channels (Cavs) in G-Protein Coupled Receptor (GPCR) signaling pathways. This work in chromaffin cells highlighted NCS-1 as an important modulator of neurotransmission. NCS-1 has since been shown to regulate and/or directly interact with many proteins including Cavs (P/Q, N, and L), TRPC1/5 channels, GPCRs, IP3R, and PI4 kinase type IIIβ. NCS-1 also affects neuronal outgrowth having roles in learning and memory affecting both short- and long-term synaptic plasticity. It is not known if NCS-1 affects neurotransmission and synaptic plasticity via its effect on PIP2 levels, and/or via a direct interaction with Ca²⁺ channels or their signaling complexes. This review gives a historical account of NCS-1 function, examining contributions from chromaffin cells, PC12 cells and other models, to describe how NCS-1’s regulation of Ca²⁺ channels allows it to exert its physiological effects.     

Paradoxical facilitation of exocytosis by inhibition of L-type calcium channels of bovine chromaffin cells

Ca²⁺ entry through the L-subtype (α_1D, Ca_v1,3) of voltage-dependent calcium channels (VDCCs) seems to selectively regulate the endocytotic response after the application of a single depolarizing pulse to voltage-clamped bovine chromaffin cells. Here we have found that L channel blockade with nifedipine transformed the exocytotic responses elicited by a double-pulse protocol, from depression to facilitation. This apparent paradoxical effect was mimicked by pharmacological interventions that directly block endocytosis namely, dynasore, calmidazolium, GTP-γS and GDP-βS. This reinforces our view that Ca²⁺ entry through PQ channels (α_1A; Ca_v2.1) regulates fast exocytosis while Ca²⁺ entry through L channels preferentially controls rapid endocytosis.     

P/Q Ca2+ channels are functionally coupled to exocytosis of the immediately releasable pool in mouse chromaffin cells

Chromaffin cell exocytosis is triggered by Ca(2+) entry through several voltage-dependent channel subtypes. Because it was postulated that immediately releasable vesicles are closely associated with Ca(2+) channels, we wondered what channel types are specifically coupled to the release of this pool. To study this question, cultured mouse chromaffin cell exocytosis was followed by patch-clamp membrane capacitance measurements. The immediately releasable pool was estimated using paired pulse stimulation, resulting in an upper limit of 31+/-3 fF for control conditions (I(Ca): 25+/-2 pA/pF). The N-type channel blocker omega-conotoxin-GVIA affected neither I(Ca) nor the immediately releasable pool exocytosis; although the L channel blocker nitrendipine decreased current by 50%, it did not reduce this pool significantly; and the R channel inhibitor SNX-482 significantly reduced the current but induced only a moderate decrease in the estimated IRP exocytosis. In contrast, the P/Q channel blocker omega-Agatoxin-IVA decreased I(Ca) by 37% but strongly reduced the immediately releasable pool (upper limit: 6+/-1 fF). We used alpha1A subunit knockout mice to corroborate that P/Q Ca(2+) channels were specifically linked to immediately releasable vesicles, and we found that also in this preparation the exocytosis of this pool was severely decreased (6+/-1 fF). On the other hand, application of a strong stimulus that caused the fusion of most of releasable vesicles (3 min, 50 mM K(+)) induced similar exocytosis for wild type and knockout cells. Finally, whereas application of train stimulation on chromaffin cells derived from wild type mice provoked typical early synchronous and delayed asynchronous exocytosis components, the knockout derived cells presented a strongly depressed early exocytosis but showed a prominent delayed asynchronous component. These results demonstrate that P/Q are the dominant calcium channels associated to the release of immediately releasable pool in mouse chromaffin cells.     

Ca2+ influx and clearance at hyperpolarized membrane potentials modulate spontaneous and stimulated exocytosis in neuroendocrine cells

Neuroendocrine adrenal chromaffin cells release neurohormones catecholamines in response to Ca²⁺ entry via voltage-gated Ca²⁺ channels (VGCCs). Adrenal chromaffin cells also express non-voltage-gated channels, which may conduct Ca²⁺ at negative membrane potentials, whose role in regulation of exocytosis is poorly understood. We explored how modulation of Ca²⁺ influx at negative membrane potentials affects basal cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and exocytosis in metabolically intact voltage-clamped bovine adrenal chromaffin cells. We found that in these cells, Ca²⁺ entry at negative membrane potentials is balanced by Ca²⁺ extrusion by the Na⁺/Ca²⁺ exchanger and that this balance can be altered by membrane hyperpolarization or stimulation with an inflammatory hormone bradykinin. Membrane hyperpolarization or application of bradykinin augmented Ca²⁺-carrying current at negative membrane potentials, elevated basal [Ca²⁺]_i, and facilitated synchronous exocytosis evoked by the small amounts of Ca²⁺ injected into the cell via VGCCs (up to 20 pC). Exocytotic responses evoked by the injections of the larger amounts of Ca²⁺ via VGCCs (> 20 pC) were suppressed by preceding hyperpolarization. In the absence of Ca²⁺ entry via VGCCs and Ca²⁺ extrusion via the Na⁺/Ca²⁺ exchanger, membrane hyperpolarization induced a significant elevation in [Ca²⁺]_i and asynchronous exocytosis. Our results indicate that physiological interferences, such as membrane hyperpolarization and/or activation of non-voltage-gated Ca²⁺ channels, modulate basal [Ca²⁺]_i and, consequently, segregation of exocytotic vesicles and their readiness to be released spontaneously and in response to Ca²⁺ entry via VGCCs. These mechanisms may play role in homeostatic plasticity of neuronal and endocrine cells.     

Mechanisms underlying phasic and sustained secretion in chromaffin cells from mouse adrenal slices.

Many neurosecretory preparations display two components of depolarization-induced exocytosis: a phasic component synchronized with Ca2+ channel opening, followed by a slower sustained component. We evaluated possible mechanisms underlying this biphasic behavior by stimulating mouse chromaffin cells in situ with both depolarizations and flash photolysis of caged Ca2+. From a direct comparison of the secretory responses to both stimuli, we conclude that phasic and sustained release components originate from a readily releasable pool (RRP) of equally fusion-competent vesicles, suggesting that differences in the vesicles' proximity to Ca2+ channels underlie the biphasic secretory behavior. An intermediate pool in dynamic equilibrium with the RRP ensures rapid recruitment of release-ready vesicles after RRP depletion. Our results are discussed in terms of a refined model for secretion in chromaffin cells.