Dynamics of full fusion during vesicular exocytotic events: release of adrenaline by chromaffin cells

Modulating the lipid profile of blastocyst cell membrane with DPPC multilamellar vesicles

The aim of this study was to evaluate the effect of multilamellar vesicles (MLVs) of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in co-culture with in vitro-produced bovine embryos (IVPEs). The stability of five concentrations of MLVs (1.0, 1.25, 1.5, 1.75, and 2.0 mM) produced using ultrapure water or embryonic culture medium with 24 or 48 h of incubation at 38.5 °C with 5% CO₂ was assessed. In addition, the toxicity of MLVs and their modulation of the lipid profile of the plasma membrane of IVPEs were evaluated after 48 h of co-culture. Both media allowed the production of MLVs. Incubation (24 and 48 h) did not impair the MLV structure but affected the average diameter. The rate of blastocyst production was not reduced, demonstrating the nontoxicity of the MLVs even at 2.0 mmol/L. The lipid profile of the embryos was different depending on the MLV concentration. In comparison with control embryos, embryos cultured with MLVs at 2.0 mmol/L had a higher relative abundance of six lipid ions (m/z 720.6, 754.9, 759.0, 779.1, 781.2, and 797.3). This study sheds light on a new culture system in which the MLV concentration could change the lipid profile of the embryonic cell membrane in a dose-dependent manner.

Multiphase Methods in Organic Electrosynthesis

With water providing a highly favored solution environment for industrial processes (and in biological processes), it is interesting to develop water-based electrolysis processes for the synthesis and conversion of organic and biomass-based molecules. Molecules with low solubility in aqueous media can be dispersed/solubilized (i) by physical dispersion tools (e.g., milling, power ultrasound, or high-shear ultraturrax processing), (ii) in some cases by pressurization/supersaturation (e.g., for gases), (iii) by adding cosolvents or "carriers" such as chremophor EL, or (iv) by adding surfactants to generate micelles, microemulsions, and/or stabilized biphasic conditions. This Account examines and compares methodologies to bring the dispersed or multiphase system into contact with an electrode. Both the microscopic process based on individual particle impact and the overall electro-organic transformation are of interest. Distinct mechanistic cases for multiphase redox processes are considered. Most traditional electro-organic transformations are performed in homogeneous solution with reagents, products, electrolyte, and possibly mediators or redox catalysts all in the same (usually organic) solution phase. This may lead to challenges in the product separation step and in the reuse of solvents and electrolytes. When aqueous electrolyte media are used, reagents and products (or even the electrolyte) may be present as microdroplets or nanoparticles. Redox transformations then occur during interfacial "collisions" under multiphase conditions or within a reaction layer when a redox mediator is present. Benefits of this approach can be (i) the use of a highly conducting aqueous electrolyte, (ii) simple separation of products and reuse of the electrolyte, (iii) phase-transfer conditions in redox catalysis, (iv) new reaction pathways, and (v) improved sustainability. In some cases, a surface phase or phase boundary processes can lead to interesting changes in reaction pathways. Controlling the reaction zone within the multiphase redox system poses a challenge, and methods based on microchannel flow reactors have been developed to provide a higher degree of control. However, detrimental effects in microchannel systems are also observed, in particular for limited current densities (which can be very low in microchannel multiphase flow) or in the development of technical solutions for scale-up of multiphase redox transformations. This Account describes physical approaches (and reactor designs) to bring multiphase redox systems into effective contact with the electrode surface as well as cases of important electro-organic multiphase transformations. Mechanistic cases considered are "impacts" by microdroplets or particles at the electrode, effects of dissolved intermediates or redox mediators, and effects of dissolved redox catalysts. These mechanistic cases are discussed for important multiphase transformations for gaseous, liquid, and solid dispersed phases. Processes based on mesoporous membranes and hydrogen-permeable palladium membranes are discussed.

Vesicle Motion during Sustained Exocytosis in Chromaffin Cells: Numerical Model Based on Amperometric Measurements

Chromaffin cells release catecholamines by exocytosis, a process that includes vesicle docking, priming and fusion. Although all these steps have been intensively studied, some aspects of their mechanisms, particularly those regarding vesicle transport to the active sites situated at the membrane, are still unclear. In this work, we show that it is possible to extract information on vesicle motion in Chromaffin cells from the combination of Langevin simulations and amperometric measurements. We developed a numerical model based on Langevin simulations of vesicle motion towards the cell membrane and on the statistical analysis of vesicle arrival times. We also performed amperometric experiments in bovine-adrenal Chromaffin cells under Ba²⁺ stimulation to capture neurotransmitter releases during sustained exocytosis. In the sustained phase, each amperometric peak can be related to a single release from a new vesicle arriving at the active site. The amperometric signal can then be mapped into a spike-series of release events. We normalized the spike-series resulting from the current peaks using a time-rescaling transformation, thus making signals coming from different cells comparable. We discuss why the obtained spike-series may contain information about the motion of all vesicles leading to release of catecholamines. We show that the release statistics in our experiments considerably deviate from Poisson processes. Moreover, the interspike-time probability is reasonably well described by two-parameter gamma distributions. In order to interpret this result we computed the vesicles’ arrival statistics from our Langevin simulations. As expected, assuming purely diffusive vesicle motion we obtain Poisson statistics. However, if we assume that all vesicles are guided toward the membrane by an attractive harmonic potential, simulations also lead to gamma distributions of the interspike-time probability, in remarkably good agreement with experiment. We also show that including the fusion-time statistics in our model does not produce any significant changes on the results. These findings indicate that the motion of the whole ensemble of vesicles towards the membrane is directed and reflected in the amperometric signals. Our results confirm the conclusions of previous imaging studies performed on single vesicles that vesicles’ motion underneath plasma membranes is not purely random, but biased towards the membrane.

Cdc42 controls the dilation of the exocytotic fusion pore by regulating membrane tension

On exocytosis, membrane fusion starts with the formation of a narrow fusion pore that must expand to allow the release of secretory compounds. The GTPase Cdc42 promotes fusion pore dilation in neuroendocrine cells by controlling membrane tension.

Membrane fusion underlies multiple processes, including exocytosis of hormones and neurotransmitters. Membrane fusion starts with the formation of a narrow fusion pore. Radial expansion of this pore completes the process and allows fast release of secretory compounds, but this step remains poorly understood. Here we show that inhibiting the expression of the small GTPase Cdc42 or preventing its activation with a dominant negative Cdc42 construct in human neuroendocrine cells impaired the release process by compromising fusion pore enlargement. Consequently the mode of vesicle exocytosis was shifted from full-collapse fusion to kiss-and-run. Remarkably, Cdc42-knockdown cells showed reduced membrane tension, and the artificial increase of membrane tension restored fusion pore enlargement. Moreover, inhibiting the motor protein myosin II by blebbistatin decreased membrane tension, as well as fusion pore dilation. We conclude that membrane tension is the driving force for fusion pore dilation and that Cdc42 is a key regulator of this force.

Two modes of exocytosis in an artificial cell

The details of exocytosis, the vital cell process of neuronal communication, are still under debate with two generally accepted scenarios. The first mode of release involves secretory vesicles distending into the cell membrane to release the complete vesicle contents. The second involves partial release of the vesicle content through an intermittent fusion pore, or an opened or partially distended fusion pore. Here we show that both full and partial release can be mimicked with a single large-scale cell model for exocytosis composed of material from blebbing cell plasma membrane. The apparent switching mechanism for determining the mode of release is demonstrated to be related to membrane tension that can be differentially induced during artificial exocytosis. These results suggest that the partial distension mode might correspond to an extended kiss-and-run mechanism of release from secretory cells, which has been proposed as a major pathway of exocytosis in neurons and neuroendocrine cells.

A planar microelectrode array for simultaneous detection of electrically evoked dopamine release from distinct locations of a single isolated neuron

Neurotransmission is a key process of communication between neurons. Although much is known about this process and the influence it has on the function of the body, little is understood about the dynamics of signalling from structural regions of a single neuron. In this study we have fabricated and characterised a microelectrode array (MEA) which was utilised for simultaneous multi-site recordings of dopamine release from an isolated single neuron. The MEA consisted of gold electrodes that were created in plane with the insulation layer using a chemical mechanical planarization process. The detection limit for dopamine measurements was 11 ± 3 nM and all the gold electrodes performed in a consistent fashion during amperometric recordings of 100 nM dopamine. Fouling of the gold electrode was investigated, where no significant change in the current was observed over 4 hours when monitoring 100 nM dopamine. The MEA was accessed using freshly isolated dopaminergic somas from the pond snail, Lymnaea stagnalis, where electrically evoked dopamine release was clearly observed. Measurements were conducted at four structural locations of a single isolated neuron, where electrically evoked dopamine release was observed from the cell body, axonal regions and the terminal. Over time, the release of dopamine varied over the structural regions of the neuron. Such information can provide an insight into the signalling mechanism of neurons and how they potentially form synaptic connections.

Chromatographic analysis of age-related changes in mucosal serotonin transmission in the murine distal ileum

BACKGROUND

In the upper bowel, alterations in motility and absorption of key nutrients have been observed as part of the normal ageing process. Serotonin (5-HT) is a key signalling molecule in the gastrointestinal tract and is known to influence motility, however little is known of how the ageing process alters 5-HT signalling processes in the bowel.

RESULTS

An isocratic chromatographic method was able to detect all 5-HT precursors and metabolites. Using extracellular and intracellular sampling approaches, we were able to monitor all key parameters associated with the transmission process. There was no alteration in the levels of tryptophan and 5-HTP between 3 and 18 month old animals. There was a significant increase in the ratio of 5-HT:5-HTP and an increase in intracellular 5-HT between 3 and 18 month old animals suggesting an increase in 5-HT synthesis. There was also a significant increase in extracellular 5-HT with age, suggesting increased 5-HT release. There was an age-related decrease in the ratio of intracellular 5-HIAA:extracellular 5-HT, whilst the amount of 5-HIAA did not change with age. In the presence of an increase in extracellular 5-HT, the lack of an age-related change in 5-HIAA is suggestive of a decrease in re-uptake via the serotonin transporter (SERT).

CONCLUSIONS

We have used intracellular and extracellular sampling to provide more insight into alterations in the neurotransmission process of 5-HT during normal ageing. We observed elevated 5-HT synthesis and release and a possible decrease in the activity of SERT. Taken together these changes lead to increased 5-HT availability and may alter motility function and could lead to the changes in adsorption observed in the elderly.

Influence of cholesterol on cellular signaling and fusion pore kinetics

Cholesterol is an important lipid component of cellular membranes. Recent studies have shown that changes in cellular cholesterol level can affect cellular functions. Here, we summarize our recent findings on the impact of cholesterol on the glucose-stimulated Ca(2+) signaling in rat pancreatic β cells and the fusion pore kinetics of large dense core granules in rat chromaffin cells. In mouse pancreatic β cells, pharmacological elevation of cellular cholesterol (but not cholesterol extraction) reduced the current density of the delayed rectifier K(+) channels, the ATP-dependent K(+) channels, and voltage-gated Ca(2+) channels. Importantly, cholesterol enrichment impaired glucose-stimulated Ca(2+) signaling in mouse pancreatic β cells via a suppression of voltage-gated Ca(2+) channels and a decrease in mitochondrial ATP production, which in turn led to a reduction in the glucose-evoked depolarization. In rat chromaffin cells, we found that the persistence of the semi-stable fusion pore was increased by cholesterol enrichment, and acute cholesterol extraction from the cytosolic side of the cell destabilized the semi-stable fusion pore. Overall, our findings highlight the importance of cholesterol in the regulation of cellular signaling and exocytosis.

Hydrodynamic flow in a synaptic cleft during exocytosis

It is shown that exocytosis in a chemical synapse may be accompanied by "microjet" formation due to the overpressure that exists in the vesicles. This mechanism may take place either at complete fusion of a vesicle with the presynaptic membrane or in the so-called kiss-and-run mode of neurotransmitter release. A simple hydrodynamic model of the viscous incompressible flow arising in the synaptic cleft is suggested. The occurrence of hydrodynamic flow (microjet) leads to more efficient transport of neurotransmitter than in the case of classical diffusive transport.

Electroanalytical approaches to study signaling mechanisms in the gastrointestinal tract

Electroanalytical techniques over the past few years have been applied to study real-time release of various signaling molecules in the GI tract. These approaches have become highly attractive as they provide dynamic spatial information on the amount of signaling molecules released. Although these approaches are relatively new to the field, the studies to date have provided useful insights into the alterations in signaling mechanisms during maturation, obesity and in a model of colitis. New methods and techniques have also allowed the possibility to obtain information on the signaling process and future developments will provide a wide diverse array of information that will be of benefit to all researchers in the field of gastroenterology. This review focuses on the types of techniques utilized, the information they can provide, their potential advantages and disadvantages in monitoring signaling processes in the gastrointestinal tract, the existing scientific studies that have utilized electroanalytical methods to date and the future potential impact of such approaches.

Cholesterol effects on vesicle pools in chromaffin cells revealed by carbon-fiber microelectrode amperometry

Cell-cell communication is often achieved via granular exocytosis, as in neurons during synaptic transmission or neuroendocrine cells during blood hormone control. Owing to its critical role in membrane properties and SNARE function, cholesterol is expected to play an important role in the highly conserved process of exocytosis. In this work, membrane cholesterol concentration is systematically varied in primary culture mouse chromaffin cells, and the change in secretion behavior of distinct vesicle pools as well as pool recovery following stimulation is measured using carbon-fiber microelectrode amperometry. Amperometric traces obtained from activation of the younger readily releasable and slowly releasable pool (RRP/SRP) vesicles at depleted cholesterol levels showed fewer sustained fusion pore features (6.1 ± 1.1% of spikes compared with 11.2 ± 1.0% for control), revealing that cholesterol content influences fusion pore formation and stability during exocytosis. Moreover, subsequent stimulation of RRP/SRP vesicles showed that cellular cholesterol level influences both the quantal recovery and kinetics of the later release events. Finally, diverging effects of cholesterol on RRP and the older reserve pool vesicle release suggest two different mechanisms for the release of these two vesicular pools.

Critical role of membrane cholesterol in exocytosis revealed by single platelet study

Exocytosis is a fundamental cellular process, pivotal in a wide range of cell types, used to deliver chemical messengers from one cell to another cell or tissue. While a tremendous amount of knowledge has been gained in the past several decades about the exocytotic machinery, recently it has become clear that the role of membrane lipids is also crucial in this process. In particular, the critical role of the abundant and ubiquitous cholesterol molecules has not been well-defined. Early insight has been gleaned from single cell amperometric studies on several commonly used secretory cell models, including chromaffin cells and PC12 cells; however, these secretory cell models are not ideal because manipulations of membrane cholesterol content may influence downstream cholesterol-dependent processes, making data interpretation difficult. Herein, blood platelets are employed as a simpler secretory cell model based on their anuclear nature and unique chemical messenger exocytosis behavior. Carbon-fiber microelectrochemistry was employed to measure real-time exocytosis from single platelets with depleted or enriched cholesterol either in the naturally occurring form or as the synthetic analogue epicholesterol. The experimental results show that membrane cholesterol directly modulates the secretion efficiency of individual platelets, as well as the kinetics of secretion events. Moreover, substitution of platelet membrane cholesterol with epicholesterol yields exocytotic behavior indistinguishable from that of normal platelets, arguing against the possibility of cholesterol-specific interactions in regulating exocytosis. It is clear from this work that membrane cholesterol plays a critical biophysical, rather than biochemical, role in platelet exocytosis and likely in exocytosis in general.

Microelectrode investigation of neuroneal ageing from a single identified neurone

Microelectrode amperometry is uniquely suited for characterising the dynamics of neurotransmitter release, as it offers unparalleled spatial and temporal resolution. We have used carbon fibre microelectrodes to study release of the monoamine neurotransmitter serotonin (5-HT) and the gaseous transmitter nitric oxide (NO) in intact central nervous system of the water snail, Lymnaea stagnalis. Analysis of spontaneous vesicular release of 5-HT and depolarisation-induced release of NO reveals significant differences with ageing that may be associated with changes in protein structure and function.

Understanding changes in uptake and release of serotonin from gastrointestinal tissue using a novel electroanalytical approach

Serotonin (5-HT) is well known to be a key neurotransmitter within the gastrointestinal (GI) tract, where it is responsible for influencing motility. Obtaining dynamic information about the neurotransmission process (specifically the release and reuptake of 5-HT) requires the development of new approaches to measure the extracellular 5-HT concentration profile. In this work constant-potential amperometry has been utilised at +650 mV vs. Ag|AgCl to measure in vitro the overflow of 5-HT. Steady-state levels of 5-HT have been observed, due to continuous mechanical stimulation of the tissue from the experimental protocol. Measurements are conducted at varying tissue-electrode distances in the range of 5 to 1100 microm. The difference in the current from the bulk media and that from each tissue-electrode distance is obtained, and the natural log of this current is plotted versus the tissue-electrode distance. The linear fit to the log of the current is derived, and its intercept, I(0), with the vertical axis and its slope are calculated. The reciprocal of the slope, indicated as slope(-1), is used as a marker of reuptake. The ratio between intercept, I(0), and the reciprocal of the slope, I(0)/slope(-1), is a measure of the flux at the tissue surface and it can be used as a marker for the 5-HT release rate. Current measurements for ileum and colon tissue indicated a significantly higher reuptake rate in the colon, showed by a lower slope(-1). In addition, the ratio, I(0)/slope(-1), indicated that the colon has a higher 5-HT flux compared to the ileum. Following the application of the serotonin selective reuptake inhibitor (SSRI), fluoxetine, both tissues showed a higher value of slope(-1), as the reuptake process is blocked preventing clearance of 5-HT. No differences were observed in the ratio, I(0)/slope(-1), in the ileum, but a decrease was observed in the colon. These results indicate that ileum and colon are characterised by different reuptake and release processes. The new approach we propose provides pivotal information on the variations in the signalling mechanism, where steady state levels are observed and can be a vital tool to study differences between normal and diseased tissue and also the efficacy of pharmacological agents.

Bioanalytical tools for single-cell study of exocytosis

Regulated exocytosis is a fundamental biological process used to deliver chemical messengers for cell-cell communication via membrane fusion and content secretion. A plethora of cell types employ this chemical-based communication to achieve crucial functions in many biological systems. Neurons in the brain and platelets in the circulatory system are representative examples utilizing exocytosis for neurotransmission and blood clotting. Single-cell studies of regulated exocytosis in the past several decades have greatly expanded our knowledge of this critical process, from vesicle/granule transport and docking at the early stages of exocytosis to membrane fusion and to eventual chemical messenger secretion. Herein, four main approaches that have been widely used to study single-cell exocytosis will be highlighted, including total internal reflection fluorescence microscopy, capillary electrophoresis, single-cell mass spectrometry, and microelectrochemistry. These techniques are arranged in the order following the route of a vesicle/granule destined for secretion. Within each section, the basic principles and experimental strategies are reviewed and representative examples are given revealing critical spatial, temporal, and chemical information of a secretory vesicle/granule at different stages of its lifetime. Lastly, an analytical chemist's perspective on potential future developments in this exciting field is discussed.

Invariance of exocytotic events detected by amperometry as a function of the carbon fiber microelectrode diameter

Etched carbon fiber microelectrodes of different radii have been used for amperometric measurements of single exocytotic events occurring at adrenal chromaffin cells. Frequency, kinetic, and quantitative information on exocytosis provided by amperometric spikes were analyzed as a function of the surface area of the microelectrodes. Interestingly, the percentage of spikes with foot (as well as their own characteristics), a category revealing the existence of sufficient long-lasting fusion pores, was found to be constant whatever the microelectrode diameter was, whereas the probability of overlapping spikes decreased with the electrode size. This confirmed that the prespike foot could not feature accidental superimposition of separated events occurring at different places. Moreover, the features of amperometric spikes investigated here (charge, intensity and kinetics) were found constant for all microelectrode diameters. This demonstrated that the electrochemical measurement does not introduce significant bias onto the kinetics and thermodynamics of release during individual exocytotic events. All in all, this work evidences that information on exocytosis amperometrically recorded with the usual 7 microm diameter carbon fiber electrodes is biologically relevant, although the frequent overlap between spikes requires a censorship of the data during the analytical treatment.

Quantitative investigations of amperometric spike feet suggest different controlling factors of the fusion pore in exocytosis at chromaffin cells

Around 30% of exocytosis events recorded by amperometry at carbon fiber microelectrodes exhibit a pre-spike feature (PSF) termed a "foot". This wave is associated with the release of the neurotransmitters via a transitory fusion pore, whilst the large, main exocytotic spike is due to complete release. The amperometric data reported herein were obtained using bovine chromaffin cells stimulated with either potassium or barium ions, two commonly-employed elicitors of exocytosis. Identical trends are observed with both activators: (i) they induce the same ratio (close to 30%) of events with a foot in the population of amperometric spikes, and (ii) spikes with a foot can be divided into two primary categories, depending on the temporal variation of the current wave (viz. as a ramp, or a ramp followed by a plateau). Correlations between the characteristics of the whole current spike, and of its observed foot, have been sought; such analyses demonstrate that the maximum current of both foot and spike signals are highly correlated, but, in contrast, the integrated charges of both are poorly correlated. Moreover, the temporal duration of the PSF is fully uncorrelated with any parameter pertaining to the main current spike. On the basis of these reproducible observations, it is hypothesized that the characteristics (dimensions and topology, at least) of each secretory vesicle determine the probability of formation of the fusion pore and its maximum size, whilst molecular factors of the cell membrane control its duration, and, consequently, the amount delivered prior to the massive exocytosis of catecholamines observed as a spike in amperometry.

Monitoring of vesicular exocytosis from single cells using micrometer and nanometer-sized electrochemical sensors

Communication between cells by release of specific chemical messengers via exocytosis plays crucial roles in biological process. Electrochemical detection based on ultramicroelectrodes (UMEs) has become one of the most powerful techniques in real-time monitoring of an extremely small number of released molecules during very short time scales, owing to its intrinsic advantages such as fast response, excellent sensitivity, and high spatiotemporal resolution. Great successes have been achieved in the use of UME methods to obtain quantitative and kinetic information about released chemical messengers and to reveal the molecular mechanism in vesicular exocytosis. In this paper, we review recent developments in monitoring exocytosis by use of UMEs-electrochemical-based techniques including electrochemical detection using micrometer and nanometer-sized sensors, scanning electrochemical microscopy (SECM), and UMEs implemented in lab-on-a-chip (LOC) microsystems. These advances are of great significance in obtaining a better understanding of vesicular exocytosis and chemical communications between cells, and will facilitate developments in many fields, including analytical chemistry, biological science, and medicine. Furthermore, future developments in electrochemical probing of exocytosis are also proposed.

The nature and efficiency of neurotransmitter exocytosis also depend on physicochemical parameters

Exocytosis is an important biological process used by cells to deliver messengers or effectors to target cells with high spatial, quantitative, and kinetic precision. This process occurs by interaction and fusion of vesicles containing the (bio)chemical information with the cell membrane to release their contents into the surrounding medium. Because of its importance for life, this mechanism underlies many biological controlling factors, including different families of proteins and enzymes. Tremendous efforts have been made over the last decade toward their determination. However, in parallel, many studies have also shown that the physical and chemical characteristics of the exocytosis actors (vesicle, membrane, and extracellular medium) could directly affect the quantitative or kinetic features of secretion. The major pieces evidence for this influence, which have been reported in the literature, are reviewed herein. It demonstrates undoubtedly that pure biological aspects cannot be segregated from the physicochemical context of living mechanisms.

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.

Vesicular exocytosis under hypotonic conditions shows two distinct populations of dense core vesicles in bovine chromaffin cells

Several previous reports have discussed the effects of external osmolarity on vesicular exocytotic processes. However, few of these studies considered hypotonic conditions on chromaffin cells. Herein, the exocytosis of catecholamines by chromaffin cells was investigated in a medium of low osmolarity (200 mOsm) by amperometry at carbon fiber microelectrodes. It is observed that the frequency of the exocytotic events is significantly higher under hypotonic conditions than under physiological conditions (315 mOsm). This further confirms that the swelling of the polyelectrolytic matrix (which follows ionic exchanges) contained in dense core vesicles is the energetic driving force of the exocytotic phenomenon, being favored by a lower osmolarity. The mean amount of catecholamines released during secretory events also increases importantly under the hypotonic condition. This may be rationalized by the coexistence of two distinct populations of dense core vesicles with a relative content ratio of 4.7. The larger content population is favored under hypotonic conditions but plays only a side role under isotonic conditions.

Regulation of exocytosis in chromaffin cells by trans-insertion of lysophosphatidylcholine and arachidonic acid into the outer leaflet of the cell membrane

Vesicular exocytosis is an important complex process in the communication between cells in organisms. It controls the release of chemical and biochemical messengers stored in an emitting cell. In this report, exocytosis is studied amperometrically (at carbon fiber ultramicroelectrodes) at adrenal chromaffin cells, which release catecholamines after appropriate stimulation, while testing the effects due to trans-insertion of two exogenous compounds (lysophosphatidylcholine (LPC) and arachidonic acid (AA)) on the kinetics of exocytotic events. Amperometric analyses showed that, under the present conditions (short incubation times and micromolar LPC or AA solutions), LPC favors catecholamine release (rate, event frequency, charge released) while AA disfavors the exocytotic processes. The observed kinetic features are rationalized quantitatively by considering a stalk model, for the fusion pore formation, and the physical constraints applied to the cell membrane by the presence of small fractions of LPC and AA diluted in its external leaflet (trans-insertion). We also observed that the detected amount of neurotransmitters in the presence of LPC was larger than under control conditions, while the opposite trend is observed with AA.

Monitoring in real time with a microelectrode the release of reactive oxygen and nitrogen species by a single macrophage stimulated by its membrane mechanical depolarization

Macrophages are key cells of the immune system. During phagocytosis, the macrophage engulfs a foreign bacterium, virus, or particle into a vacuole, the phagosome, wherein oxidants are produced to neutralize and decompose the threatening element. These oxidants derive from in situ production of superoxide and nitric oxide by specific enzymes. However, the chemical nature and sequence of release of these compounds is far from being completely determined. The aim of the present work was to study the fundamental mechanism of oxidant release by macrophages at the level of a single cell, in real time and quantitatively. The tip of a microelectrode was positioned at a micrometric distance from a macrophage in a culture to measure oxidative-burst release by the cell when it was submitted to physical stimulation. The ensuing release of electroactive reactive oxygen and nitrogen species was detected by amperometry and the exact nature of the compounds was characterized through comparison with in vitro electrochemical oxidation of H2O2, ONOO-, NO*, and NO2(-) solutions. These results enabled the calculation of time variations of emission flux for each species and the reconstruction of the original flux of production of primary species, O2*- and NO*, by the macrophage.

Monitoring Dopamine Release from Single Living Vesicles with Nanoelectrodes

Carbon fiber nanoelectrodes (tip diameter = ca. 100 nm) have been first used to monitor real-time dopamine release from single living vesicles of single rat pheochromocytoma (PC12) cells. The experiments show that active and inactive release sites exist on the surface of cells, and the spatial distributions have been differentiated even in the same active release zone. It is first demonstrated that multiple vesicles can sequentially release dopamine at the same site of the cell surface, which possibly plays the main role in the dopamine release from PC12 cells.

Correlation between vesicle quantal size and fusion pore release in chromaffin cell exocytosis

A significant number of exocytosis events recorded with amperometry demonstrate a prespike feature termed a "foot" and this foot has been correlated with messengers released via a transitory fusion pore before full exocytosis. We have compared amperometric spikes with a foot with spikes without a foot at chromaffin cells and found that the probability of detecting a distinct foot event is correlated to the amount of catecholamine released. The mean charge of the spikes with a foot was found to be twice that of the spikes without a foot, and the frequency of spikes displaying a foot was zero for small spikes increasing to approximately 50% for large spikes. It is hypothesized that in chromaffin cells, where the dense core is believed to nearly fill the vesicle, the expanding core is a controlling factor in opening the fusion pore, that prefusion of two smaller vesicles leads to excess membrane, and that this slows pore expansion leading to an increased observation of events with a foot. Clearly, the physicochemical properties of vesicles are key factors in the control of the dynamics of release through the fusion pore and the high and variable frequency of this release makes it highly significant.

Insulin granule dynamics in pancreatic beta cells

Glucose-induced insulin secretion in response to a step increase in blood glucose concentrations follows a biphasic time course consisting of a rapid and transient first phase followed by a slowly developing and sustained second phase. Because Type 2 diabetes involves defects of insulin secretion, manifested as a loss of first phase and a reduction of second phase, it is important to understand the cellular mechanisms underlying biphasic insulin secretion. Insulin release involves the packaging of insulin in small (diameter approximately 0.3 micro m) secretory granules, the trafficking of these granules to the plasma membrane, the exocytotic fusion of the granules with the plasma membrane and eventually the retrieval of the secreted membranes by endocytosis. Until recently, studies on insulin secretion have been confined to the appearance of insulin in the extracellular space and the cellular events preceding exocytosis have been inaccessible to more detailed analysis. Evidence from a variety of secretory tissues, including pancreatic islet cells suggests, however, that the secretory granules can be functionally divided into distinct pools that are distinguished by their release competence and/or proximity to the plasma membrane. The introduction of fluorescent proteins that can be targeted to the secretory granules, in combination with the advent of new techniques that allow real-time imaging of granule trafficking in living cells (granule dynamics), has led to an explosion of our knowledge of the pre-exocytotic and post-exocytotic processes in the beta cell. Here we discuss these observations in relation to previous functional and ultra-structural data as well as the secretory defects of Type 2 diabetes.