Simultaneous capacitance and amperometric measurements of exocytosis: a comparison

Combined electrochemistry and mass spectrometry imaging to interrogate the mechanism of action of modafinil, a cognition-enhancing drug, at the cellular and sub-cellular level

Modafinil is a mild psychostimulant-like drug enhancing wakefulness, improving attention and developing performance in various cognitive tasks, but its mechanism of action is not completely understood. This is the first combination of amperometry, electrochemical cytometry and mass spectrometry to interrogate the mechanism of action of a drug, here modafinil, at cellular and sub-cellular level. We employed single-cell amperometry (SCA) and intracellular vesicle impact electrochemical cytometry (IVIEC) to investigate the alterations in exocytotic release and vesicular catecholamine storage following modafinil treatment. The SCA results reveal that modafinil slows down the exocytosis process so that, the number of catecholamines released per exocytotic event is enhanced in the modafinil-treated cells. Also, IVIEC results offer an upregulation effect of modafinil on the vesicular catecholamine storage. Mass spectrometry imaging by time-of-flight secondary ion mass spectrometry (ToF-SIMS) illustrates that treatment with modafinil reduces the cylindrical-shaped phosphatidylcholine at the cellular membrane, while the high curvature lipids with conical structures such as phosphatidylethanolamine and phosphatidylinositol are elevated after modafinil treatment. Combining the results obtained by SCA, IVIEC and ToF-SIMS suggests that modafinil-treated cells release a larger portion of their vesicular content at least in part by changing the lipid composition of the cell membrane, suggesting regulation of cognition.

Electrochemistry of Single-Vesicle Events

Neuronal transmission relies on electrical signals and the transfer of chemical signals from one neuron to another. Chemical messages are transmitted from presynaptic neurons to neighboring neurons through the triggered fusion of neurotransmitter-filled vesicles with the cell plasma membrane. This process, known as exocytosis, involves the rapid release of neurotransmitter solutions that are detected with high affinity by the postsynaptic neuron. The type and number of neurotransmitters released and the frequency of vesicular events govern brain functions such as cognition, decision making, learning, and memory. Therefore, to understand neurotransmitters and neuronal function, analytical tools capable of quantitative and chemically selective detection of neurotransmitters with high spatiotemporal resolution are needed. Electrochemistry offers powerful techniques that are sufficiently rapid to allow for the detection of exocytosis activity and provides quantitative measurements of vesicle neurotransmitter content and neurotransmitter release from individual vesicle events. In this review, we provide an overview of the most commonly used electrochemical methods for monitoring single-vesicle events, including recent developments and what is needed for future research.

Expanding neurochemical investigations with multi-modal recording: simultaneous fast-scan cyclic voltammetry, iontophoresis, and patch clamp measurements

Multi-modal recording describes the simultaneous collection of information across distinct domains. Compared to isolated measurements, such studies can more easily determine relationships between varieties of phenomena. This is useful for neurochemical investigations which examine cellular activity in response to changes in the local chemical environment. In this study, we demonstrate a method to perform simultaneous patch clamp measurements with fast-scan cyclic voltammetry (FSCV) using optically isolated instrumentation. A model circuit simulating concurrent measurements was used to predict the electrical interference between instruments. No significant impact was anticipated between methods, and predictions were largely confirmed experimentally. One exception was due to capacitive coupling of the FSCV potential waveform into the patch clamp amplifier. However, capacitive transients measured in whole-cell current clamp recordings were well below the level of biological signals, which allowed the activity of cells to be easily determined. Next, the activity of medium spiny neurons (MSNs) was examined in the presence of an FSCV electrode to determine how the exogenous potential impacted nearby cells. The activities of both resting and active MSNs were unaffected by the FSCV waveform. Additionally, application of an iontophoretic current, used to locally deliver drugs and other neurochemicals, did not affect neighboring cells. Finally, MSN activity was monitored during iontophoretic delivery of glutamate, an excitatory neurotransmitter. Membrane depolarization and cell firing were observed concurrently with chemical changes around the cell resulting from delivery. In all, we show how combined electrophysiological and electrochemical measurements can relate information between domains and increase the power of neurochemical investigations.

Imaging FITC-dextran as a Reporter for Regulated Exocytosis

Regulated exocytosis is a process by which cargo, which is stored in secretory granules (SGs), is released in response to a secretory trigger. Regulated exocytosis is fundamental for intercellular communication and is a key mechanism for the secretion of neurotransmitters, hormones, inflammatory mediators, and other compounds, by a variety of cells. At least three distinct mechanisms are known for regulated exocytosis: full exocytosis, where a single SG fully fuses with the plasma membrane, kiss-and-run exocytosis, where a single SG transiently fuses with the plasma membrane, and compound exocytosis, where several SGs fuse with each other, prior to or after SG fusion with the plasma membrane. The type of regulated exocytosis undertaken by a cell is often dictated by the type of secretory trigger. However, in many cells, a single secretory trigger can activate multiple modes of regulated exocytosis simultaneously. Despite their abundance and importance across cell types and species, the mechanisms that determine the different modes of secretion are largely unresolved. One of the main challenges in investigating the different modes of regulated exocytosis, is the difficulty in distinguishing between them as well as exploring them separately. Here we describe the use of fluorescein isothiocyanate (FITC)-dextran as an exocytosis reporter, and live cell imaging, to differentiate between the different pathways of regulated exocytosis, focusing on compound exocytosis, based on the robustness and duration of the exocytic events.

On the Action of General Anesthetics on Cellular Function: Barbiturate Alters the Exocytosis of Catecholamines in a Model Cell System

General anesthetics are essential in many areas, however, the cellular mechanisms of anesthetic-induced amnesia and unconsciousness are incompletely understood. Exocytosis is the main mechanism of signal transduction and neuronal communication through the release of chemical transmitters from vesicles to the extracellular environment. Here, we use disk electrodes placed on top of PC12 cells to show that treatment with barbiturate induces fewer molecules released during exocytosis and changes the event dynamics perhaps by inducing a less stable fusion pore that is prone to close faster during partial exocytosis. Larger events are essentially abolished. However, use of intracellular vesicle impact electrochemical cytometry using a nano-tip electrode inserted into a cell shows that the distribution of vesicle transmitter content does not change after barbiturate treatment. This indicates that barbiturate selectively alters the pore size of larger events or perhaps differentially between types of vesicles. Alteration of exocytosis in this manner could be linked to the effects of general anesthetics on memory loss.

Effect of Plasma Membrane Semipermeability in Making the Membrane Electric Double Layer Capacitances Significant

Electric double layers (or EDLs) formed at the membrane-electrolyte interface (MEI) and membrane-cytosol interface (MCI) of a charged lipid bilayer plasma membrane develop finitely large capacitances. However, these EDL capacitances are often much larger than the intrinsic capacitance of the membrane, and all of these capacitances are in series. Consequently, the effect of these EDL capacitances in dictating the overall membrane-EDL effective capacitance C_eff becomes negligible. In this paper, we challenge this conventional notion pertaining to the membrane-EDL capacitances. We demonstrate that, on the basis of the system parameters, the EDL capacitance for both the permeable and semipermeable membranes can be small enough to influence C_eff. For the semipermeable membranes, however, this lowering of the EDL capacitance can be much larger, ensuring a reduction of C_eff by more than 20-25%. Furthermore, for the semipermeable membranes, the reduction in C_eff is witnessed over a much larger range of system parameters. We attribute such an occurrence to the highly nonintuitive electrostatic potential distribution associated with the recently discovered phenomena of charge-inversion-like electrostatics and the attainment of a positive zeta potential at the MCI for charged semipermeable membranes. We anticipate that our findings will impact the quantification and the identification of a large number of biophysical phenomena that are probed by measuring the plasma membrane capacitance.

TRPM7 kinase activity regulates murine mast cell degranulation

KEY POINTS

The Mg(2+) and Ca(2+) conducting transient receptor potential melastatin 7 (TRPM7) channel-enzyme (chanzyme) has been implicated in immune cell function. Mice heterozygous for a TRPM7 kinase deletion are hyperallergic, while mice with a single point mutation at amino acid 1648, silencing kinase activity, are not. As mast cell mediators trigger allergic reactions, we here determine the function of TRPM7 in mast cell degranulation and histamine release. Our data establish that TRPM7 kinase activity regulates mast cell degranulation and release of histamine independently of TRPM7 channel function. Our findings suggest a regulatory role of TRPM7 kinase activity on intracellular Ca(2+) and extracellular Mg(2+) sensitivity of mast cell degranulation.

ABSTRACT

Transient receptor potential melastatin 7 (TRPM7) is a divalent ion channel with a C-terminally located α-kinase. Mice heterozygous for a TRPM7 kinase deletion (TRPM7(+/∆K) ) are hypomagnesaemic and hyperallergic. In contrast, mice carrying a single point mutation at amino acid 1648, which silences TRPM7 kinase activity (TRPM7(KR) ), are not hyperallergic and are resistant to systemic magnesium (Mg(2+) ) deprivation. Since allergic reactions are triggered by mast cell-mediated histamine release, we investigated the function of TRPM7 on mast cell degranulation and histamine release using wild-type (TRPM7(+/+) ), TRPM7(+/∆K) and TRPM7(KR) mice. We found that degranulation and histamine release proceeded independently of TRPM7 channel function. Furthermore, extracellular Mg(2+) assured unperturbed IgE-DNP-dependent exocytosis, independently of TRPM7. However, impairment of TRPM7 kinase function suppressed IgE-DNP-dependent exocytosis, slowed the cellular degranulation rate, and diminished the sensitivity to intracellular calcium (Ca(2+) ) in G protein-induced exocytosis. In addition, G protein-coupled receptor (GPCR) stimulation revealed strong suppression of histamine release, whereas removal of extracellular Mg(2+) caused the phenotype to revert. We conclude that the TRPM7 kinase activity regulates murine mast cell degranulation by changing its sensitivity to intracellular Ca(2+) and affecting granular mobility and/or histamine contents.

Mechanisms of granule membrane recapture following exocytosis in intact mast cells

In secretory cells, several exocytosis-coupled forms of endocytosis have been proposed including clathrin-mediated endocytosis, kiss-and-run endocytosis, cavicapture, and bulk endocytosis. These forms of endocytosis can be induced under different conditions, but their detailed molecular mechanisms and functions are largely unknown. We studied exocytosis and endocytosis in mast cells with both perforated-patch and whole-cell configurations of the patch clamp technique using cell capacitance measurements in combination with amperometric serotonin detection. We found that intact mast cells exhibit an early endocytosis that follows exocytosis induced by compound 48/80. Direct observation of individual exocytic and endocytic events showed a higher percentage of capacitance flickers (27.3%) and off-steps (11.4%) in intact mast cells than in dialyzed cells (5.4% and 2.9%, respectively). Moreover, we observed a type of endocytosis of large pieces of membrane that were likely formed by cumulative fusion of several secretory granules with the cell membrane. We also identified "large-capacitance flickers" that occur after large endocytosis events. Pore conductance analysis indicated that these transient events may represent "compound cavicapture," most likely due to the flickering of a dilated fusion pore. Using fluorescence imaging of individual exocytic and endocytic events we observed that granules can fuse to granules already fused with the plasma membrane, and then the membranes and dense cores of fused granules are internalized. Altogether, our results suggest that stimulated exocytosis in intact mast cells is followed by several forms of compensatory endocytosis, including kiss-and-run endocytosis and a mechanism for efficient retrieval of the compound membrane of several secretory granules through a single membrane fission event.

Single cell amperometry reveals glycocalyx hinders the release of neurotransmitters during exocytosis

The diffusional hindrance of the glycocalyx along the cell surface on exocytotic peaks, observed with single cell amperometry, was investigated. Partial digestion of the glycocalyx with neuraminidase led to the observation of faster peaks, as shown by varied peak parameters. This result indicates that diffusion of small molecules in the partially digested glycocalyx is 2.2 faster than in the intact glycocalyx. Similarly, neutralization of the negative charges present in the cell microenvironment led to faster peak kinetics. The analysis of the vesicular efflux indicates that the diffusion coefficient of dopamine at the cell surface is at most 45% of the diffusion coefficient in free solution. This study shows that the glycocalyx plays an important role in the diffusion kinetics of processes along the cell surface, including exocytotic events.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

High resolution electrophysiological techniques for the study of calcium-activated exocytosis

BACKGROUND

Neurotransmitters, neuropeptides and hormones are released from secretory vesicles of nerve terminals and neuroendocrine cells by calcium-activated exocytosis. A key step in this process is the formation of a fusion pore between the vesicle membrane and the plasma membrane. Exocytotic fusion leads to an increase in plasma membrane area that can be measured as a proportional increase in plasma membrane capacitance.

SCOPE OF REVIEW

High resolution capacitance measurements in single cells, nerve terminals and small membrane patches have become possible with the development of the patch clamp technique. This review discusses the methods of whole cell patch clamp capacitance measurements and their use in conjunction with voltage clamp pulse stimulation and with stimulation by photorelease of caged calcium. It also discusses patch capacitance measurements for the study of single exocytotic events and fusion pore properties in neuroendocrine cells and nerve terminals.

MAJOR CONCLUSIONS

Capacitance measurements provide high resolution information on the extent and time course of fusion for the characterization of vesicle pools and the kinetics of exocytosis. They allow the characterization of the mode of fusion including distinction of single vesicle full fusion, transient kiss-and-run fusion or multivesicular compound exocytosis. Furthermore, measurement of fusion pore conductances and their dynamic behavior has enabled the characterization of fusion pore properties in a way that resembles the characterization of ion channel function through single channel recordings.

GENERAL SIGNIFICANCE

The combination of patch clamp capacitance measurements with pharmacological and molecular manipulations of exocytosis is emerging as a powerful approach to investigate the molecular mechanisms of calcium-activated exocytotic fusion pore formation. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Calcium dependencies of regulated exocytosis in different endocrine cells

Exocytotic machinery in neuronal and endocrine tissues is sensitive to changes in intracellular Ca(2+) concentration. Endocrine cell models, that are most frequently used to study the mechanisms of regulated exocytosis, are pancreatic beta cells, adrenal chromaffin cells and pituitary cells. To reliably study the Ca(2+) sensitivity in endocrine cells, accurate and fast determination of Ca(2+) dependence in each tested cell is required. With slow photo-release it is possible to induce ramp-like increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) that leads to a robust exocytotic activity. Slow increases in the [Ca(2+)](i) revealed exocytotic phases with different Ca(2+) sensitivities that have been largely masked in step-like flash photo-release experiments. Strikingly, in the cells of the three described model endocrine tissues (beta, chromaffin and melanotroph cells), distinct Ca(2+) sensitivity 'classes' of secretory vesicles have been observed: a highly Ca(2+)-sensitive, a medium Ca(2+)-sensitive and a low Ca(2+)-sensitive kinetic phase of secretory vesicle exocytosis. We discuss that a physiological modulation of a cellular activity, e.g. by activating cAMP/PKA transduction pathway, can switch the secretory vesicles between Ca(2+) sensitivity classes. This significantly alters late steps in the secretory release of hormones even without utilization of an additional Ca(2+) sensor protein.

cAMP increases the sensitivity of exocytosis to Ca²+ primarily through protein kinase A in mouse pancreatic beta cells

Cyclic AMP regulates the late step of Ca²+-dependent exocytosis in many secretory cells through two major mechanisms: a protein kinase A-dependent and a cAMP-GEF/Epac-dependent pathway. We designed a protocol to characterize the role of these two cAMP-dependent pathways on the Ca²+ sensitivity and kinetics of regulated exocytosis in mouse pancreatic beta cells, using a whole-cell patch-clamp based capacitance measurements. A train of depolarizing pulses or slow photo-release of caged Ca²+ were stimuli for the exocytotic activity. In controls, due to exocytosis after slow photo-release, the C(m) change had typically two phases. We observed that the Ca²+-dependency of the rate of the first C(m) change follows saturation kinetics with high cooperativity and half-maximal rate at 2.9±0.2 μM. The intracellular depletion of cAMP did not change amp1, while rate1 and amp2 were strongly reduced. This manipulation pushed the Ca²+-dependency of the exocytotic burst to significantly lower [Ca²+](i). To address the question of which of the cAMP-dependent mechanisms regulates the observed shifts in Ca²+ dependency we included regulators of PKA and Epac2 activity in the pipette solution. PKA activation with 100 μM 6-Phe-cAMP or inhibition with 500 μM Rp-cAMPs in beta cells significantly shifted the EC(50) in the opposite directions. Specific activation of Epac2 did not change Ca²+ sensitivity. Our findings suggest that cAMP modulates Ca²+-dependent exocytosis in mouse beta cells mainly through a PKA-dependent mechanism by sensitizing the insulin releasing machinery to [Ca²+](i); Epac2 may contribute to enhance the rates of secretory vesicle fusion.

Unperturbed islet α-cell function examined in mouse pancreas tissue slices

Critical investigation into α-cell biology in health and diabetes has been sparse and at times inconsistent because of the technical difficulties with employing conventional strategies of isolated islets and dispersed single cells. An acute pancreas slice preparation was developed to overcome the enzymatic and mechanical perturbations inherent in conventional islet cell isolation procedures. This preparation preserves intra-islet cellular communication and islet architecture in their in situ native state. α-Cells within tissue slices were directly assessed by patch pipette and electrophysiologically characterized. The identity of the patched cells was confirmed by biocytin dye labelling and immunocytochemistry. α-Cells in mouse pancreas slices exhibited well-described features of I(Na) (excitable at physiological membrane potential), I(KATP), small cell size, low resting membrane conductance, and inducible low and high voltage-activated I(Ca), the latter correlating with exocytosis determined by capacitance measurements. In contrast to previous reports, our large unbiased sampling of α-cells revealed a wide range distribution of all of these parameters, including the amount of K(ATP) conductance, Na+ and Ca2+ current amplitudes, and capacitance changes induced by a train of depolarization pulses. The proposed pancreas slice preparation in combination with standard patch-clamping technique allowed large sampling and rapid assessment of α-cells, which revealed a wide distribution in α-cell ion channel properties. This specific feature explains the apparent inconsistency of previous reports on these α-cell ion channel properties. Our innovative approach will enable future studies into elucidating islet α-cell dysregulation occurring during diabetes.

Exocytosis in islet beta-cells

The development of technologies that allow for live optical imaging of exocytosis from beta-cells has greatly improved our understanding of insulin secretion. Two-photon imaging, in particular, has enabled researchers to visualize the exocytosis of large dense-core vesicles (LDCVs) containing insulin from beta-cells in intact islets of Langerhans. These studies have revealed that high glucose levels induce two phases of insulin secretion and that this release is dependent upon cytosolic Ca(2+) and cAMP. This technology has also made it possible to examine the spatial profile of insulin exocytosis in these tissues and compare that profile with those of other secretory glands. Such studies have led to the discovery of the massive exocytosis of synaptic-like microvesicles (SLMVs) in beta-cells. These imaging studies have also helped clarify facets of insulin exocytosis that cannot be properly addressed using the currently available electrophysiological techniques. This chapter provides a concise introduction to the field of optical imaging for those researchers who wish to characterize exocytosis from beta-cells in the islets of Langerhans.

Measuring secretion in chromaffin cells using electrophysiological and electrochemical methods

Our present understanding of exocytosis of catecholamines has benefited tremendously from the arrival of single-cell electrochemical methods (amperometry and voltammetry), electrophysiological techniques (whole-cell and patch capacitance) and from the combination of both techniques (patch amperometry). In this brief review, we will outline the strengths and limitations of amperometric and electrophysiological methods and highlight the major contribution obtained with the use of these techniques in chromaffin cells.

Relationship between amperometric pre-spike feet and secretion granule composition in chromaffin cells: an overview

Amperometry is a simple and powerful technique to study exocytosis at the single cell level. By positioning and polarizing (at an appropriate potential at which the molecules released by the cell can be oxidized) a carbon fiber microelectrode at the top of the cell, each exocytotic event is detected as an amperometric spike. More particularly, a portion of these spikes has previously been shown to present a foot, i.e. a small pedestal of current that precedes the spike itself. Among the important number of works dealing with the monitoring of exocytosis by amperometry under different conditions, only a few studies focus on amperometric spikes with a foot. In this work, by coupling our previous and recent experiments on chromaffin cells (that release catecholamines after stimulation) with literature data, we bring more light on what an amperometric foot and particularly its features, represents.

Two-photon excitation imaging of exocytosis and endocytosis and determination of their spatial organization

Two-photon excitation imaging is the least invasive optical approach to study living tissues. We have established two-photon extracellular polar-tracer (TEP) imaging with which it is possible to visualize and quantify all exocytic events in the plane of focus within secretory tissues. This technology also enables estimate of the precise diameters of vesicles independently of the spatial resolution of the optical microscope, and determination of the fusion pore dynamics at nanometer resolution using TEP-imaging based quantification (TEPIQ). TEP imaging has been applied to representative secretory glands, e.g., exocrine pancreas, endocrine pancreas, adrenal medulla and a pheochromocytoma cell line (PC12), and has revealed unexpected diversity in the spatial organization of exocytosis and endocytosis crucial for the physiology and pathology of secretory tissues and neurons. TEP imaging and TEPIQ analysis are powerful tools for elucidating the molecular and cellular mechanisms of exocytosis and certain related diseases, such as diabetes mellitus, and the development of new therapeutic agents and diagnostic tools.

Different effects on fast exocytosis induced by synaptotagmin 1 and 2 isoforms and abundance but not by phosphorylation

Synaptotagmins comprise a large protein family, of which synaptotagmin 1 (Syt1) is a Ca2+ sensor for fast exocytosis, and its close relative, synaptotagmin 2 (Syt2), is assumed to serve similar functions. Chromaffin cells express Syt1 but not Syt2. We compared secretion from chromaffin cells from Syt1 null mice overexpressing either Syt isoform. High time-resolution capacitance measurement showed that Syt1 null cells lack the exocytotic phase corresponding to the readily-releasable pool (RRP) of vesicles. Comparison with the amperometric signal confirmed that the missing phase of exocytosis consists of catecholamine-containing vesicles. Overexpression of Syt1 rescued the RRP and increased its size above wild-type values, whereas the size of the slowly releasable pool decreased, indicating that the availability of Syt1 regulates the relative size of the two releasable pools. The RRP was also rescued by Syt2 overexpression, but the kinetics of fusion was slightly slower than in cells expressing Syt1. Biochemical experiments showed that Syt2 has a slightly lower Ca2+ affinity for phospholipid binding than Syt1 because of a difference in the C2A domain. These data constitute evidence for the function of Syt1 and Syt2 as alternative, but not identical, calcium-sensors for RRP fusion. By overexpression of Syt1 mutated in the shared PKC/calcium/calmodulin-dependent kinase phosphorylation site, we show that phorbol esters act independently and upstream of Syt1 to regulate the size of the releasable pools. We conclude that exocytosis from mouse chromaffin cells can be modified by the differential expression of Syt isoforms and by Syt abundance but not by phosphorylation of Syt1.

Screening populations of individual cells for secretory heterogeneity

Many common metabolic and neurological disorders are related to defective regulation of exocytosis at the level of single cells. In exocytosis, vesicles containing the secretory product of a given cell type fuse with the plasma membrane allowing release of the vesicular contents into the extracellular environment where the physiological action can be exerted. The typical secretory vesicle contains between 0.15 and 10 attomoles of material that is released on a millisecond timescale. Hence, detection of this process presents several chemical and analytical challenges. In this work, we utilize the native ATP, stored at high concentrations within the secretory vesicles of most neuroendocrine cells and co-released during exocytosis and during cell lysis, as a universal tracer of cellular secretion events. Organisms studied include pancreatic islets, mast cells, and Escherischia coli. Cellular processes investigated include exocytotic release, stimulated cell lysis, and programmed cell lysis.

Function of the t-SNARE SNAP-23 and secretory carrier membrane proteins (SCAMPs) in exocytosis in mast cells

Mast cells acutely respond to allergens and other stimuli by releasing accumulated internal stores of inflammatory mediators and other secretory products by "compound" exocytosis involving massive granule-to-plasma membrane and granule-to-granule fusion. Our recent findings implicate the SNAP receptor (SNARE) protein SNAP-23 and secretory carrier membrane protein 2 (SCAMP2) as regulators of this process. We summarize evidence indicating that stimulus-induced relocation of SNAP-23 from foci in the plasma membrane to putative sites of membrane fusion both at the cell surface and intracellularly between granules is an essential link in coupling stimulation to exocytosis. We also review recent findings showing that the candidate exocytotic SNAREs SNAP-23 and syntaxin 4 colocalize with SCAMPs, especially SCAMP2, and that SCAMP2 is likely to be a partner in the compound exocytotic process.

Exocytosis of catecholamine (CA)-containing and CA-free granules in chromaffin cells

Recent evidence suggests that endocytosis in neuroendocrine cells and neurons can be tightly coupled to exocytosis, allowing rapid retrieval from the plasma membrane of fused vesicles for future use. This can be a much faster mechanism for membrane recycling than classical clathrin-mediated endocytosis. During a fast exo-endocytotic cycle, the vesicle membrane does not fully collapse into the plasma membrane; nevertheless, it releases the vesicular contents through the fusion pore. Once the vesicle is depleted of transmitter, its membrane is recovered without renouncing its identity. In this report, we show that chromaffin cells contain catecholamine-free granules that retain their ability to fuse with the plasma membrane. These catecholamine-free granules represent 7% of the total population of fused vesicles, but they contributed to 47% of the fusion events when the cells were treated with reserpine for several hours. We propose that rat chromaffin granules that transiently fuse with the plasma membrane preserve their exocytotic machinery, allowing another round of exocytosis.

Temperature and PMA affect different phases of exocytosis in bovine chromaffin cells

Amperometry was used to study secretory kinetics of single bovine chromaffin cells stimulated by transient depolarizations at different temperatures. The initial rate of release was moderately enhanced when the temperature was raised from 18 to 22 and 37 degrees C. Secretion increased drastically at a later period, 5-10 s after the initiation of stimulus. Interestingly, incubation of the cells with phorbol 12-myristate 13-acetate (PMA) clearly enhanced fast secretory components. In addition, the rate of secretion of the slower component recruited by prolonged depolarizations (t > 30 s) was unaffected at the range of temperatures normally used in secretory experiments (22-37 degrees C). A 'counting events' analysis of secretion, which avoids the influence of event charge changes, showed specific increases in a population of vesicles fusing between 7 and 12 s over the same range of temperatures, and a marked increase in vesicles fusing during the initial phase (1-5 s), of PMA-treated cell secretion. An analysis of temperature influence on transient components released by high sucrose, the secretion elicited by cell permeabilization with digitonin, and studies of the individual characteristics of amperometric events, allow us to conclude that an increase in the size of a secondary-released vesicle population is the main factor contributing to temperature-dependent enhancement of secretion, in clear contrast to the enhancement of fast releasable pools caused by phorbol esters.

Amperometric analysis of exocytosis at chromaffin cells from genetically distinct mice

Amperometry is a very powerful technique for investigating the role(s) specific proteins play in exocytosis at the single-cell level. In this study, amperometry has been used to investigate possible changes in exocytosis at chromaffin cells isolated from coloboma and tottering mutant mice. Coloboma mice possess a deletion mutation that encompasses the gene for the presynaptic protein SNAP-25 and tottering mice carry a mutation of the alpha(1A) subunit gene, which encodes the pore-forming region of P/Q-type calcium channels. Although amperometric data measured from tottering and coloboma cells are not significantly different from that measured at wild-type control cells, significant differences are found when groups of wild-type chromaffin cells are analyzed at room temperature and at 37 degrees C. Due to the large variability inherent to amperometric data, it is possible that changes in release resulting from some genetic differences cannot be detected. To fully exploit the technical advantages of using mouse chromaffin cells, experimental guidelines are described which should maximize changes in release resulting from genetic differences and increase the likelihood of detecting a change in amperometric data.

Stimulation of Ca(2+)-independent exocytosis in rat pituitary gonadotrophs by G-protein

We employed the whole-cell recording technique in conjunction with fluorometry to measure cytosolic Ca(2+) concentration ([Ca(2+)](i)) and exocytosis (capacitance measurement) in single, identified rat gonadotrophs. Direct activation of G-protein (via intracellular dialysis of non-hydrolysable analogues of GTP, but not of GDP) triggered a slow rise in capacitance even in the presence of a fast intracellular Ca(2+) chelator. The broad-spectrum kinase inhibitors H7 and staurosporine did not prevent this Ca(2+)-independent exocytosis, ruling out the involvement of the cAMP and PKC pathways. AlF(4)(-), a potent stimulator of heterotrimeric G-proteins, failed to stimulate any exocytosis when the intracellular Ca(2+) store was depleted, implicating the involvement of AlF(4)(-)-insensitive G-protein(s). Maximal stimulation of Ca(2+)-independent exocytosis by GTP analogues did not reduce the number of readily releasable granules that were available subsequently for Ca(2+)-dependent release. The last finding raises the possibility that the G-protein-stimulated Ca(2+)-independent exocytosis may regulate a pool of granules that is distinct from the Ca(2+)-dependent pool.

Measurement of exocytosis by amperometry in adrenal chromaffin cells: effects of clostridial neurotoxins and activation of protein kinase C on fusion pore kinetics

We have used carbon-fibre amperometry to examine the kinetics of individual secretory granule fusion/release events in bovine adrenal chromaffin cells. Transfection with plasmids encoding the light chains of botulinum neurotoxins (BoNTs) was used to investigate the effects of cleavage of syntaxin or SNAP-25 on exocytosis. Expression of BoNT/C1 or BoNT/E inhibited the extent of exocytosis that was evoked by application of digitonin/Ca(2+) to permeabilise and stimulate single chromaffin cells. Following neurotoxin expression, the residual release events were no different from those of control cells in their magnitude and kinetics from analysis of the amperometric spikes. In contrast, activation of protein kinase C (PKC) resulted in a modification of the kinetics of single granule release events. Following phorbol ester treatment, the amperometric spikes showed a significant decrease in their total charge due to a decrease in their mean half-width with increases in the rate of the initial rise and also the fall to baseline of the spikes. These changes were prevented by pre-treatment with the PKC inhibitor bisindolylmaleimide. These results suggest that PKC regulates the rate of fusion pore expansion and also subsequent pore closure or granule retrieval. A PKC-mediated regulation of kiss-and-run fusion may, therefore, control the extent of catecholamine release from single secretory granules. The experimental approach used here may provide further information on the protein constituents and regulation of the fusion pore machinery.

Regulation of quantal size by presynaptic mechanisms

Quantal size is often modeled as invariant, although it is now well established that the number of transmitter molecules released per synaptic vesicle during exocytosis can be modulated in central and peripheral synapses. In this review, we suggest why presynaptically altered quantal size would be important at social synapses that provide extrasynaptic neurotransmitter. Current techniques used to measure quantal size are reviewed with particular attention to amperometry, the first approach to provide direct measurement of the number of molecules and kinetics of presynaptic quantal release, and to CNS dopamine neuronal terminals. The known interventions that alter quantal size at the presynaptic locus are reviewed and categorized as (1) alteration of transvesicular free energy gradients, (2) modulation of vesicle transmitter transporter activity, (3) modulation of fusion pore kinetics, (4) altered transmitter degranulation, and (5) changes in synaptic vesicle volume. Modulation of the number of molecules released per quantum underlies mechanisms of drug action of L-DOPA and the amphetamines, and seems likely to be involved in both normal synaptic modification and disease states. Statistical analysis for examining quantal size and data presentation is discussed. We include detailed information on performing nonparametric resampling statistical analysis, the Kolmogorov-Smirnov test for two populations, and random walk simulations using spreadsheet programs.

Quantal analysis of 5-hydroxytryptamine release from mouse pancreatic beta-cells

1. A combination of patch-clamp, amperometric and fluorimetric methods were used to investigate the Ca2+ dependence and kinetics of secretion from pancreatic beta-cells elicited by voltage-gated Ca2+ entry. 2. Whether measured by the change in cell capacitance or by amperometric detection of 5-hydroxytryptamine (5-HT) release, the voltage dependence of the amount of secretion mirrored that of both the peak Ca2+ current and Ca2+ entry. 3. The magnitude of secretion elicited by a single pulse could be entirely accounted for by a readily releasable pool of approximately 200 vesicles. Neither depression nor potentiation of release was observed with 0.1 Hz pulse trains. 4. Transient amperometric currents were detected, which occurred independently of each other and were attributed to the fusion of single vesicles. 5. The time course of the macroscopic amperometric current could be accurately reconstructed by convolution of the all-events latency distribution and the unitary amperometric current. 6. In response to membrane depolarisation, secretion was initiated with a variable latency: approximately 95 % of the first secretory events occurred at least 50 ms after the start of the voltage pulse (and Ca2+ influx). Secretion fell rapidly on membrane repolarisation, even though the average intracellular calcium concentration ([Ca2+]i) was still elevated. 7. The [Ca2+] in the locality of the release site was estimated from the all-events latency distribution. [Ca2+] rose during a voltage pulse and secretion was elicited at > 0.4 microM and peaked at approximately 2-10 microM.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Electrophysiology of synaptic vesicle cycling

Patch-clamp capacitance measurements can monitor in real time the kinetics of exocytosis and endocytosis in living cells. We review the application of this technique to the giant presynaptic terminals of goldfish bipolar cells. These terminals secrete glutamate via the fusion of small, clear-core vesicles at specialized, active zones of release called synaptic ribbons. We compare the functional characteristics of transmitter release at ribbon-type and conventional synapses, both of which have a unique capacity for fast and focal vesicle fusion. Subsequent rapid retrieval and recycling of fused synaptic vesicle membrane allow presynaptic terminals to function independently of the cell soma and, thus, as autonomous computational units. Together with the mobilization of reserve vesicle pools, local cycling of synaptic vesicles may delay the onset of vesicle pool depletion and sustain neuronal output during high stimulation frequencies.

Multiple kinetic components and the Ca2+ requirements of exocytosis

The use of caged-Ca2+ compounds to stimulate Ca(2+)-dependent exocytosis has substantially increased our understanding of this complex process. By this approach, the existence of multiple kinetic components of exocytosis has been established. These components may correspond to a series of sequential steps that lead to a single fusion-ready state (sequential mechanism) or, alternatively, to heterogeneity in secretory vesicles or in fusion-ready states (parallel mechanism). It is suggested that both of these mechanisms can underlie exocytosis of a single type of vesicle (mixed sequential-parallel mechanism). Studies with caged-Ca2+ compounds have also indicated that the Ca2+ requirement for exocytosis is substantially greater than that suggested by conventional methodologies. This discrepancy is mainly attributable to the underestimation, by imaging studies with high-affinity Ca2+ indicators (due to dye saturation), of the local increases in cytosolic Ca2+ concentration that trigger the exocytosis of individual vesicles. The effects of local saturation of such indicators are explored by means of a simple theory.

Comparative biology of Ca2+-dependent exocytosis: implications of kinetic diversity for secretory function

The application of caged-Ca2+ compounds to the study of Ca2+-dependent exocytosis has begun to reveal kinetic intermediates in this important process. The time course of exocytosis varies greatly among different cell and vesicle types, even in response to Ca2+ 'jumps' of identical amplitude. The kinetics of the binding of Ca2+ to the putative Ca2+ sensor for exocytosis also vary. Theoretical analysis reveals that the kinetic diversity of exocytotic and Ca2+-binding reactions has distinct roles in determining the probability of exocytosis occurring. It is proposed that both of these reactions are optimized for the secretory function of specific cell types and that the exocytotic reaction includes vesicle translocation in addition to the fusion of vesicles with the plasma membrane.

Post-priming actions of ATP on Ca2+-dependent exocytosis in pancreatic beta cells

The role of cytosolic ATP in exocytosis was investigated by using amperometric measurement of insulin exocytosis in pancreatic beta cells, which were stimulated with photolysis of caged Ca2+ compounds. Insulin exocytosis occurred with two rates. We found that ATP hastened and augmented the exocytosis via selective enhancement of the exocytosis with the faster rate. A nonhydrolysable analog of ATP, adenosine 5'-O-(3-thiotriphosphate), which blocks ATPase, was even more effective than ATP, indicating that the phosphorylation event occurred downstream of ATP-dependent vesicle transportation and priming. The action of ATP was eliminated by a competitive antagonist of cAMP, and by an inhibitor of adenylate cyclase. These data characterize an ATP sensing mechanism for the Ca2+-dependent exocytosis involving adenylate-cyclase, cAMP-dependent protein kinase, and, possibly, the fusion machinery itself. Thus, the fast exocytotic machinery requires both phosphorylation and Ca2+ for the final triggering and likely constitutes a distal ATP sensor for insulin exocytosis that acts in concert with ATP-sensitive K+ channels.

Dual effects of botulinum neurotoxin A on the secretory stages of chromaffin cells

Truncation of the C-terminal domain of the synaptosomal associated protein of 25 kDa (SNAP-25) by botulinum neurotoxin A (BoNT A) has been shown to block neuroendocrine cell secretion. It is unclear, however, if toxin mechanism involved the affection of a single or more events of the exocytotic cascade. BoNT A induced changes in both the degree of inhibition and the kinetics of catecholamine secretion from populations of cultured bovine chromaffin cells. Ca2+-dependent secretion from digitonin-permeabilized cells showed partial inhibition associated with the alteration of a slow secretory phase at different toxin concentrations. In contrast, in intact cells stimulated by depolarization, cell treatment with low concentrations (1 nM) of the toxin affected the late phase of secretion, whereas 100 nM BoNT A-poisoned cells showed an alteration even of fast components. The high degree of inhibition associated with fast secretory component alteration was dependent on Ca2+ entry through the Ca2+ channels, as it was absent from cells permeated with the A23187 Ca2+ ionophore. Vesicle pools implicated in the effect of BoNT A on the secretory response from single cells were identified using amperometry. These studies supported the macroscopic view by showing that secretion from BoNT A-treated permeabilized cells presented specific inhibition of late vesicle fusions. Intact cells showed alterations in the late vesicle pool (t1/2 = 39 s) recruited during prolonged or repetitive KCI depolarizations using 1 nM BoNT A-treated cells as well as in an intermediate kinetic pool (t1/2 = 18 s) at higher toxin concentrations (100 nM). The faster resolved component (t1/2 = 8 s) or the membrane fusion event itself were not affected. Our results demonstrate that removal of the last nine C-terminal amino acids of SNAP-25 by BoNT A has a specific effect on two different and distal secretory stages in chromaffin cells.

Relocation of the t-SNARE SNAP-23 from lamellipodia-like cell surface projections regulates compound exocytosis in mast cells

For regulated secretion, mast cells and several other cell types utilize compound exocytosis, a combination of granule-plasma membrane and granule-granule fusions. The molecular machinery that controls this massive export process has not been identified. We report that SNAP-23, a t-SNARE related to SNAP-25, relocates in response to stimulation from plasma membrane lamellipodia-like projections to granule membranes in permeabilized mast cells. While relocation is a prerequisite for secretion, it can occur without membrane fusion and will expedite a subsequent secretory response. After relocation, SNAP-23 is required for exocytosis, implying a crucial role in promoting membrane fusion. Thus, relocation of this SNARE regulates compound exocytosis and links granule-plasma membrane and granule-granule fusions.

The annexin II-p11 complex is involved in regulated exocytosis in bovine pulmonary artery endothelial cells

Annexin II is a member of a multigene family of Ca2+-regulated, membrane-binding proteins implicated through biochemical and perforated cell experiments in Ca2+-triggered secretion. Within most cells annexin II resides in a tight heterotetrameric complex with a cellular protein ligand, p11, and complex formation is mediated via the N-terminal 14 residues of annexin II including the N-terminal acetyl group. To analyze at the single cell level whether the annexin II-p11 complex is involved in regulated secretion, we used membrane capacitance measurements to follow exocytotic fusion events in bovine aortic endothelial cells manipulated with respect to their annexin II-p11 complex formation. Upon guanosine 5'-O-(thiotriphosphate) (GTPgammaS) stimulation, the endothelial cells show a significant increase in membrane capacitance which is generally preceded by a transient rise in intracellular Ca2+ and thus indicative of the occurrence of Ca2+-regulated secretion. The GTPgammaS-induced capacitance increase is markedly reduced in cells loaded with a synthetic peptide, Ac1-14, which corresponds in sequence to the N-terminal 14 residues of annexin II in their correctly acetylated form and which is capable of disrupting preformed annexin II-p11 complexes. The effect of the peptide is highly specific as the nonacetylated variant, N1-14, which is incapable of disrupting annexin II-p11, does not interfere with the GTPgammaS-induced increase in membrane capacitance. These data show that intact annexin II-p11 complexes are indispensable for regulated exocytosis to occur in an efficient manner in endothelial cells.

Spatio-temporal resolution of exocytosis from individual cells

Biophysical events involved in late stages of exocytosis occur at highly localized areas of cells on millisecond and submillisecond time scales. Thus, methodologies with high spatio-temporal resolution are required to achieve measurements at individual secretory cells. Much has been learned about the mechanisms and kinetics of vesicular release through analysis with the carbon fiber microelectrode techniques amperometry and cyclic voltammetry. Coupling of these techniques with other methods such as patch-clamp continues to reveal details of the secretion process. It is now clear that extrusion of the vesicular contents is a more complex process than previously believed. Vesicle-cell fusion, revealed by cell capacitance measurements, is temporally dissociated from secretion measured amperometrically. The stability imparted by interaction and association of vesicle contents at rest results in a rate-limiting extrusion process after full fusion. Furthermore, the presence of partial fusion events and the occurrence of nonquantized release have been revealed with electrochemical tools.

Comparison of secretory responses as measured by membrane capacitance and by amperometry

We have compared capacitance and amperometric measurements in bovine chromaffin cells when secretion was elicited by flash photolysis of caged-calcium or step depolarizations. Total amperometric charge depended linearly on the amount of capacitance increase in both types of experiments. Furthermore, the properties of resolvable amperometric spikes after flashes were comparable to those observed after depolarizations, and their timing was compatible with the rate of capacitance increase. For a more detailed comparison, we used Monte Carlo simulations of multiple amperometric events occurring randomly over the surface of a sphere and summing together, to generate a reference amperometric signal for a given measured capacitance increase. Even after correction for endocytotic processes, the time courses of the integrated experimental records lagged behind the integrated Monte Carlo records by approximately 50 ms in flash and depolarization experiments. This delay was larger by approximately 40 ms than what can be expected from the "pre-foot delay" or the foot duration. Possible sources for the remaining delay could be diffusional barriers like the patch-pipette and the chamber bottom, which are not taken into account in the model. We also applied a novel type of fluctuation analysis to estimate the relative quantum size of an amperometric event. On average the estimates from experimental amperometric traces, in both flash and depolarization experiments, were 3-5 times smaller than estimates from simulated ones. This discrepancy can be due to contributions to the amperometric current from small vesicles, preferred release from cellular regions orientated toward the chamber bottom, or abundance of "foot-only" events. In conclusion, amperometric signals in flash and depolarization experiments displayed similar delayed average time courses and a lower estimate for the relative quantum size compared to the modeled amperometric signals. However, individual amperometric spikes were in agreement with expectations derived from capacitance signals.

Adenosine triphosphate and the late steps in calcium-dependent exocytosis at a ribbon synapse

The ATP dependence of the kinetics of Ca2+-dependent exocytosis after flash photolysis of caged Ca2+ was studied by capacitance measurements with submillisecond resolution in single synaptic terminals of retinal bipolar neurons. After control experiments verified that this combination of techniques is valid for the study of exocytosis in synaptic terminals, a comparison was made between the Ca2+ dependence of the rate of exocytosis in synaptic terminals internally dialyzed with MgATP, MgATP-gamma-S, or no added Mg2+ or nucleotide. The Ca2+ threshold for release, the maximum rate of release, and the overall relationship between the rate of synaptic vesicle fusion and [Ca2+]i were found to be independent of MgATP. A decrease in the average rate at near-threshold [Ca2+]i was observed in terminals with MgATP-gamma-S, but due to the small sample size is of unclear significance. The Ca2+ dependence of the delay between the elevation of [Ca2+]i and the beginning of the capacitance rise was also found to be independent of MgATP. In contrast, MgATP had a marked effect on the ability of terminals to respond to multiple stimuli. Terminals with MgATP typically exhibited a capacitance increase to a second stimulus that was >70% of the amplitude of the first response and to a third stimulus with a response amplitude that was >50% of the first, whereas terminals without MgATP responded to a second stimulus with a response <35% of the first and rarely responded to a third flash. These results suggest a major role for MgATP in preparing synaptic vesicles for fusion, but indicate that cytosolic MgATP may have little role in events downstream of calcium entry, provided that [Ca2+]i near release sites is elevated above approximately 30 microM.

Ultrastructural organization of bovine chromaffin cell cortex-analysis by cryofixation and morphometry of aspects pertinent to exocytosis

We have analyzed ultrathin sections from isolated bovine chromaffin cells grown on plastic support, after fast freezing, by quantitative electron microscopy. We determined the size and intracellular distribution of dense core vesicles (DVs or chromaffin granules) and of clear vesicles (CVs). The average diameter of DVs is 356 nm, and that of CVs varies between 35-195 nm (average 90 nm). DVs appear randomly packed inside cells. When the distance of the center of DVs to the cell membrane (CM) is analyzed, DV density is found to decrease as the CM is approached. According to Monte Carlo simulations performed on the basis of the measured size distribution of DVs, this decay can be assigned to a "wall effect." Any cortical barrier, regardless of its function, seems to not impose a restriction to a random cortical DV packing pattern. The number of DVs closely approaching the CM (docked DVs) is estimated to be between 364 and 629 (average 496), i.e., 0.45 to 0.78 DVs/micron2 CM. Deprivation of Ca2+, priming by increasing [Ca2+]i, or depolarization by high [K+]e for 10 s (the effect of which was controlled electrophysiologically and predicted to change the number of readily releasable granules [RRGs]) does not significantly change the number of peripheral DVs. The reason may be that (a) structural docking implies only in part functional docking (capability of immediate release), and (b) exocytosis is rapidly followed by endocytosis and replenishment of the pool of docked DVs. Whereas the potential contribution of DVs to CM area increase by immediate release can be estimated at 19-33%, that of CVs is expected to be in the range of 5.6-8.0%.

Effects of intravesicular H+ and extracellular H+ and Zn2+ on insulin secretion in pancreatic beta cells

The effects of extracellular Zn2+ and pH and intravesicular pH on insulin and 5-hydroxytryptamine (5-HT) secretion from pancreatic beta cells were investigated. Insulin and 5-HT secretion from single cells was detected by amperometry as a series of current spikes corresponding to detection of multimolecular packets secreted by exocytosis. Spike width was used as a measure of the kinetics of clearance from the cell and the area of spikes as a measure of amount released. Changes in extracellular pH from 6.9 to 7.9 caused insulin spikes to become narrower with no change in area, whereas the same treatments had no effect on 5-HT secretion. Treatment of cells with Bafilomycin A1 or N-ethylmaleimide, both of which are expected to increase intravesicular pH by inhibiting V-type H+-ATPase, had no effect on 5-HT secretion but caused insulin spikes to become more narrow. These results indicate that exposure to high pH, whether intravesicular or extracellular, accelerates release of insulin during exocytosis without affecting the amount of insulin released. Increasing extracellular Zn2+ concentration from 0 to 25 microM increased the width and decreased the area of insulin spikes without affecting 5-HT secretion. Zn2+ effects were likely exerted through a common-ion effect on Zn2+-insulin dissociation. It was concluded that intravesicular storage conditions and extracellular ions can affect free insulin concentration in the vicinity of beta cells during secretion.

Temporal and spatial monitoring of exocytosis with native fluorescence imaging microscopy

Exocytosis of rat peritoneal mast cells (RPMCs) was monitored temporally and spatially using native fluorescence imaging microscopy with 305-nm laser excitation. Real time chemical images of the relative amounts of serotonin and protein released from each cell are obtained. Individual cells released different amounts of material and the time delay of the release event after stimulation by polymyxin varied from cell to cell. Release consisted of a main burst of activity followed by slow sustained secretion over many seconds. The images show that different regions of a given cell behave asynchronously in releasing material into the surrounding medium. On rare occasions, highly localized fluorescence bursts can be seen in the vicinity of the cell. Presumably, these are due to delayed release of fluorescent mediators from single granules, following detachment of the latter from the cell. These quantitative fluorescence measurements allow one to follow the time-course of the physiologically important parameter---the amount of material that is secreted into the body fluid on stimulation.

Monitoring secretion in real time: capacitance, amperometry and fluorescence compared

Techniques for measuring exocytosis, endocytosis and vesicle cycling in living cells in real time have resulted in a rapid expansion in the knowledge of these processes in neurons and other secretory cells. Several experimental approaches, developed during the past decade, have played key roles in this expansion. In this review we focus on three techniques: electrophysiological methods for monitoring membrane capacitance, electrochemical methods for detecting released secretory contents and optical methods for imaging membranes of endosomes and recycled vesicles that are stained with fluorescent dyes. Each technique has contributed unique and complementary information about the vesicle cycle, advancing our knowledge of the kinetics of membrane fusion and retrieval, the identity of the secretory contents and the spatial patterns and directional pathways involved in secretory membrane recycling. Naturally, each technique has inherent limitations; some of these shortcomings have recently been resolved by using more than one method simultaneously.