A Ca-dependent early step in the release of catecholamines from adrenal chromaffin cells

Spontaneous and gonadotropin-releasing hormone-stimulated cytosolic calcium rises in individual goldfish gonadotrophs

The cytosolic free calcium concentration, [Ca2+]i, was monitored in single isolated goldfish gonadotrophs with the fluorescent probe Indo-1. It was found that goldfish gonadotrophs exhibit both spontaneous and secretagogue-induced [Ca2+]i rises. Spontaneous [Ca2+]i transients showed striking kinetic features and a sensitivity to external Ca2+ suggesting that they were the consequence of transient Ca2+ entries. Two kinetically distinct patterns of [Ca2+]i rises were generated in response to the two native forms of gonadotropin-releasing hormone (GnRH), salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II). In a part of the gonadotrophs, GnRHs triggered a plateau [Ca2+]i rise whereas in other responsive cells they induced a series of [Ca2+]i bursts, each consisting of grouped [Ca2+]i transients. Both plateau and burst [Ca2+]i response patterns were due to Ca2+ entry through plasma membrane Ca2+ channels, inasmuch as they were suppressed with external Ca2+ removal. No contribution of Ca2+ release from thapsigargin-sensitive stores was observed in either response pattern. While in mammalian gonadotrophs GnRH rises [Ca2+] by mostly acting on internal Ca2+ sequestering stores, our results show that GnRH-stimulated goldfish gonadotrophs rapidly increase Ca2+ entry to enhance their [Ca2+]i levels.

Membrane fusion protein synexin (annexin VII) as a Ca2+/GTP sensor in exocytotic secretion

Exocytotic membrane fusion and secretion are promoted by the concerted action of GTP and Ca2+, although the precise site(s) of action in the process are not presently known. However, the calcium-dependent membrane fusion reaction driven by synexin (annexin VII) is an in vitro model for this process, which we have now found to be further activated by GTP. The mechanism of fusion activation depends on the unique ability of synexin to bind and hydrolyze GTP in a calcium-dependent manner, both in vitro and in vivo in streptolysin O-permeabilized chromaffin cells. The required [Ca2+] for GTP binding by synexin is in the range of 50-200 microM, which is known to occur at exocytotic sites in chromaffin cells, neurons, and other cell types. Previous immunolocalization studies place synexin at exocytotic sites in chromaffin cells, and we conclude that synexin is an atypical G protein that may be responsible for both detecting and mediating the Ca2+/GTP signal for exocytotic membrane fusion.

Patch-clamp capacitance analysis of the effects of alpha-SNAP on exocytosis in adrenal chromaffin cells

We have examined the effect of alpha-SNAP on exocytosis in adrenal chromaffin cells by direct assay of exocytosis using patch-clamp capacitance analysis. Cells were recorded using the whole cell patch-clamp configuration and the cells dialysed with control pipette solution or with a pipette solution containing alpha-SNAP or the deletion mutant alpha-SNAP(41–295). The deletion mutant was found to be unable to bind to syntaxin allowing a test of the requirement for syntaxin-binding for any effect of alpha-SNAP on exocytosis. Following cell dialysis for 10 minutes, cells were depolarised five times at 2 minute intervals. At each depolarisation step cells dialysed with alpha-SNAP showed a significant increase in both the initial rate and extent of exocytosis which was seen as a rise in membrane capacitance. This increase in exocytosis was not observed with alpha-SNAP(41–295) which instead produced some inhibition of the extent but had no effect on the initial rate of exocytosis. These results show directly that alpha-SNAP has a specific and marked stimulatory effect on exocytosis in chromaffin cells.

Neurotransmitter-induced inhibition of exocytosis in insulin-secreting beta cells by activation of calcineurin

Neurotransmitters and hormones such as somatostatin, galanin, and adrenalin reduce insulin secretion. Their inhibitory action involves direct interference with the exocytotic machinery. We have examined the molecular processes underlying this effect using high resolution measurements of cell capacitance. Suppression of exocytosis was maximal at concentrations that did not cause complete inhibition of glucose-stimulated electrical activity. This action was dependent on activation of G proteins but was not associated with inhibition of the voltage-dependent Ca2+ currents or adenylate cyclase activity. The molecular processes initiated by the agonists culminate in the activation of the Ca(2+)-dependent protein phosphatase calcineurin, and suppression of the activity of this enzyme abolishes their action on exocytosis. We propose that mechanisms similar to those we report here may contribute to adrenergic and peptidergic inhibition of secretion in other neuroendocrine cells and in nerve terminals.

Simultaneous capacitance and amperometric measurements of exocytosis: a comparison

We measured the exocytotic response induced by flash photolysis of caged compounds in isolated mast cells and chromaffin cells. Vesicle fusion was measured by monitoring the cell membrane capacitance. The release of vesicular contents was followed by amperometry. In response to a GTP gamma S stimulus we found that the time integral of the amperometric current could be superimposed on the capacitance trace. This shows that the integrated amperometric signal provides an alternative method of measuring the extent and kinetics of the secretory response. Very different results were obtained when photolysis of caged Ca2+ (DM-nitrophen) was used to stimulate secretion. In mast cells, there was an immediate, graded increase in membrane capacitance that was followed by step increases (indicative of granule fusion). During the initial phase of the capacitance increases, no release of oxidizable secretory products was detected. In chromaffin cells we also observed a considerable delay between increases in capacitance, triggered by uncaging Ca2+, and the release of oxidizable secretory products. Here we demonstrate that there can be large increases in the membrane capacitance of a secretory cell, triggered by flash photolysis of DM-nitrophen, which indicate events that are not due to the fusion of granules containing oxidizable substances. These results show that increases in capacitance that are not resolved as steps cannot be readily interpreted as secretory events unless they are confirmed independently.

Rapid exocytosis and endocytosis in nerve terminals of the rat posterior pituitary

1. Ca(2+)-induced exocytosis and endocytosis were studied by measuring the membrane capacitance of voltage-clamped peptidergic nerve terminals in slices prepared from the rat posterior pituitary. 2. Depolarizing pulses produced rapid increases in capacitance. These increases varied in parallel with Ca2+ current as voltage was varied. Elimination of Ca2+ current blocked depolarization-induced capacitance changes. 3. Depolarization-induced capacitance changes increased with pulse duration. Capacitance changes also increased with integrated Ca2+ influx, but saturated at high levels of Ca2+ entry. This saturation allowed us to estimate a pool size of 190 vesicles, assuming each vesicle has a capacitance of 1 fF. Vesicles from this pool fused with a time constant of 0.43 s. The capacitance change increased with the first power of integrated Ca2+ influx. 4. Experiments with briefer pulses revealed a rapid component of exocytosis comprising a pool of forty vesicles that fuse with a time constant of 14 ms. This rapid process may reflect a final Ca(2+)-regulated triggering step, which is distinct from the slower kinetic step revealed by longer duration pulses. The slower step may reflect a priming of vesicles prior to exocytosis. 5. Depolarization-induced capacitance increases in most cases were followed by a rapid decay in capacitance, reflecting membrane reuptake tightly coupled to exocytosis. A variable amount of rapid endocytosis followed depolarization-induced capacitance increases. The time constant for rapid endocytosis to baseline was 0.44 s. Excess endocytosis was occasionally observed, with capacitance decaying below the pre-stimulus baseline with a time constant of 2.1 s. 6. Rapid endocytosis was slower after pulses that produced greater increases in intracellular Ca2+, consistent with the hypothesis that intracellular Ca2+ inhibits rapid endocytosis. 7. Exocytosis follows depolarization with no detectable delay, indicating that Ca2+ triggers neuropeptide secretion from nerve terminals with kinetics comparable to that observed in other rapidly secreting systems.

Poisson-distributed active fusion complexes underlie the control of the rate and extent of exocytosis by calcium

We have investigated the consequences of having multiple fusion complexes on exocytotic granules, and have identified a new principle for interpreting the calcium dependence of calcium-triggered exocytosis. Strikingly different physiological responses to calcium are expected when active fusion complexes are distributed between granules in a deterministic or probabilistic manner. We have modeled these differences, and compared them with the calcium dependence of sea urchin egg cortical granule exocytosis. From the calcium dependence of cortical granule exocytosis, and from the exposure time and concentration dependence of N-ethylmaleimide inhibition, we determined that cortical granules do have spare active fusion complexes that are randomly distributed as a Poisson process among the population of granules. At high calcium concentrations, docking sites have on average nine active fusion complexes.

Two components of exocytosis and endocytosis in phaeochromocytoma cells studied using caged Ca2+ compounds

1. Changes in membrane capacitance evoked by the rapid photolysis of a caged Ca2+ compound, DM-nitrophen or nitrophenyl-EGTA, were investigated in undifferentiated PC12 cells. They were interpreted as representing exocytosis and endocytosis. 2. The Ca2+ jumps evoked two components of exocytosis. Slow exocytosis was selectively evoked with small increases in intracellular Ca2+ concentration between 5 and 10 microM, while fast exocytosis preceded the slow one at [Ca2+]i greater than 10 microM. 3. The release rates of the two components of exocytosis depended steeply on [Ca2+]i. A half-maximal release rate was achieved at 8 and 24 microM for the slow and fast exocytoses, respectively. 4. Prior Ca2+ rises did not augment the fast exocytosis. 5. The fast exocytosis was often followed by a rapid decrease in membrane capacitance, representing endocytosis, after a delay of 0.5-2 s. The speed and delay in the fast endocytosis were Ca2+ dependent. Amounts of the fast endocytosis tended to balance with those of the fast exocytosis evoked by the same Ca2+ jumps. 6. The slow exocytosis was followed by a sluggish endocytosis that was associated with large capacitance steps indicative of secretory processes involving large dense-core vesicles. The onset of the slow endocytosis exhibited a complex Ca2+ dependence. The amounts of the slow endocytosis appeared to parallel those of the slow exocytosis. Prior induction of the slow exocytosis gave rise to selective excess retrieval of membrane during the slow endocytosis. 7. These data indicate the existence of two distinct populations of secretory vesicles in PC12 cells. They seem to couple selectively with specific endocytotic mechanisms. Our data suggest that the two vesicles belong to two distinct secretory pathways.

Ca(2+)-dependent exocytosis in the somata of dorsal root ganglion neurons

Using capacitance measurements and the single-cell immunoblot assay to study secretion in dorsal root ganglion neurons, we found that the somata underwent robust exocytosis upon depolarization and released substance P, in response to KCl stimulation. The parallel changes between capacitance responses and intracellular Ca2+ concentration ([Ca2+]i) at different membrane potentials and the inhibition of exocytosis by Ca2+ chelators suggest that soma release is Ca(2+)-dependent. We also assessed the level of Ca2+ required for exocytosis by raising the average [Ca2+]i with the Ca2+ ionophore, ionomycin. Capacitance changes were triggered by cytosolic Ca2+ > 0.6 microM; the [Ca2+]i at the release sites during depolarizations was estimated to be 3-10 microM. These Ca2+ levels are similar to those obtained from neuroendocrine cells, but are at least 10 times lower than those required for transmitter release from nerve terminals.

Novel isoforms of synexin in Xenopus laevis: multiple tandem PGQM repeats distinguish mRNAs in specific adult tissues and embryonic stages

Synexin (annexin VII) is a calcium-dependent, phospholipid-binding and membrane fusion protein in the annexin gene family, which forms calcium channels and may play a role in exocytotic secretion. We report here the cloning and characterization of five novel isoforms of cDNAs encoding Xenopus synexin from brain, oocyte and stage 24 cDNA libraries. The most prevalent Xenopus synexin has 1976 bp of cDNA sequence, which contains a 1539 bp open reading frame of 512 amino acids encoding a 54 kDa protein. This Xenopus protein is 6 kDa larger than the previously reported human and mouse synexins with which it shares approx. 73% identity in the C-terminal region and approx. 44% identity in the N-terminal region. Further studies with PCR revealed the molecular basis of the substantial divergence in the Xenopus synexin's N-terminal domain. The domain equivalent to the mammalian tissue-specific cassette exon occurs at a different position and is variable in size and sequence. The most interesting observation relates to the occurrence of different forms of synexin due to the varying numbers of tandem PGQM repeats that are expressed differently in different adult tissues and embryonic stages. For these reasons we have labelled this set of unique isoforms annexin VIIb, referring to mammalian forms, which lack the PGQM tandem repeats, as annexin VIIa. In spite of these differences from annexin VIIa, the form of recombinant annexin VIIb with three PGQM repeats was found to be catalytically active. We interpret these results to indicate that the actual calcium and phospholipid binding sites are conserved in Xenopus, and that the variations observed between members of the synexin gene family in the regulatory domain clearly point towards the tissue- and stage-specific roles of individual members, possibly involving the exocytotic process.

Protein kinase C enhances exocytosis from chromaffin cells by increasing the size of the readily releasable pool of secretory granules

We have used membrane capacitance measurements to assay Ca2+-triggered exocytosis in single bovine adrenal chromatin cells. Brief application of phorbol ester (PMA) enhances depolarization-evoked exocytosis severalfold while actually decreasing the Ca2+ current. Ca2+ metabolism is unchanged. Three different protocols were used to show that PMA increases the size of the readily releasable pool of secretory granules. PMA treatment leads to a large increase in amplitude, but little change in the time course of the exocytic burst that results from rapid elevation of [Ca2+]i upon photolysis of DMI-Nitrophen. Thus, PKC appears to affect a late step in secretion but not the Ca2+ sensitivity of the final step.

Fast steps in exocytosis and endocytosis studied by capacitance measurements in endocrine cells

The past year has witnessed progress in identifying late steps in exocytosis that are so short-lived as to be difficult to study biochemically. Recent studies have also revealed a novel and surprisingly fast mechanism of endocytosis that may be triggered by a rise in the intracellular concentration of Ca2+ and that retrieves exocytosed membrane in seconds.

Calcium- and barium-dependent exocytosis from the rat insulinoma cell line RINm5F assayed using membrane capacitance measurements and serotonin release

Electrophysiological measurements of cell capacitance (Cm) and biochemical assays of [3H] serotonin ([3H]5-hydroxytryptamine or [3H]5-HT) release were combined to study the control of secretion in rat insulinoma RINm5F cells. Depolarizing pulses produced Cm changes (DeltaCm), indicative of exocytosis, with the same voltage and Ca2+ dependency as the inward Ca2+ currents (ICa). Ba2+ was able to substitute for Ca2+ in stimulating exocytosis, but not endocytosis. However, both the relative potency and kinetics of Ca2+-versus Ba2+-triggered exocytosis differed significantly. 5-HT synthesis and uptake were demonstrated in RINm5F cells. This allowed the use of [3H]5-HT to study hormone release from cell populations. [3H]5-HT was released in a depolarization-, Ca2+- and time-dependent manner. Ba2+ also substituted for Ca2+ in depolarization-induced [3H]5-HT release. Thapsigargin, used to deplete Ca2+ stores, had no effects on Ca2+-triggered Cm increases, but Ca2+-triggered [3H]5-HT release was abolished. Ba2+-triggered [3H]5-HT release, however, was only slightly affected by Ca2+ store depletion. Ba2+ was found to act directly as a secretagogue of [3H]5-HT in intact cells, but not in Cm measurements of voltage-clamped cells, suggesting that cell depolarization is a prerequisite for this action.

Selective activation of exocytosis by low concentrations of ACh in rat pancreatic acinar cells

1. We have monitored changes in membrane capacitance (delta C) and conductance (delta G) induced by muscarinic acetylcholine stimulation in single rat pancreatic acinar cells. 2. Acetylcholine (ACh, 500 nM) induced simultaneous increases of delta C and delta G. In contrast, a low concentration (50 nM) of ACh exclusively induced delta C increases without delta G. These responses were abolished by the internal perfusion of heparin. This indicates that inositol 1,4,5-trisphosphate-mediated internal Ca2+ mobilization either simultaneously activates exocytosis and ion channels or exclusively initiates exocytosis. In comparison, a low concentration of A23187 selectively activated ion channels but a high concentration activated exocytosis and ion channels simultaneously. 3. These selective response patterns of delta C and delta G depend on the choice of agonist and the internal EGTA concentration. From this, we postulated two explanations for the selective action of muscarinic ACh stimulation on exocytosis. First, an area of high [Ca2+]i, spatially close to secretory granules, activates exocytosis. Second, an as yet unknown signalling factor sensitizes the Ca2+ affinity of the exocytotic apparatus.

Millisecond studies of calcium-dependent exocytosis in pituitary melanotrophs: comparison of the photolabile calcium chelators nitrophenyl-EGTA and DM-nitrophen

DM-nitrophen (DMN) is a photolabile calcium chelator that has been used extensively to study calcium-triggered exocytosis. Nitrophenyl-EGTA (NPE) is a recently synthesized photolabile calcium chelator that, unlike DMN, selectively binds calcium over magnesium. Here, we compare NPE and DMN for their effectiveness in raising cytosolic calcium ([Ca]i) to trigger exocytosis. The whole cell patch clamp technique was used to monitor membrane capacitance (Cm) and to load both calcium indicator dye and photolabile chelators into rat pituitary melanotrophs prior to flash photolysis. In cells dialysed with DMN, a transient increase in [Ca]i was observed immediately after continuity between the patch pipette and the cell cytosol was achieved. This 'loading transient' reflects the release of calcium from DMN during the binding of intracellular magnesium. No such transient was seen with NPE, consistent with the negligible binding of magnesium to this chelator. Following flash photolysis of DMN or NPE, [Ca]i increased, triggering both a rapid exocytic burst and slower sustained phases of exocytosis. When flashes of the same intensity were compared, the photolysis of NPE resulted in smaller increases in [Ca]i and slower exocytic bursts than that of DMN. These findings are in accordance with the properties of the two compounds [Ellis-Davies G.C.R., Kaplan J.H. Nitrophenyl-EGTA, a photolabile chelator that selectively binds Ca2+ with high affinity and releases it rapidly upon photolysis. Proc Natl Acad Sci USA 1994; 91: 187-191] and the calcium dependency of the exocytic burst [Thomas P., Wong J.G., Lee A.K., Almers W. A low affinity Ca2+ receptor controls the final steps in peptide secretion from pituitary melanotrophs. Neuron 1993; 11: 93-104]. Although NPE is somewhat less effective than DMN in raising [Ca]i, this chelator promises to be a useful and interesting tool for the time-resolved study of calcium-dependent exocytosis in the presence of physiological concentrations of magnesium.

Correlation between secretagogue-induced Ca2+ influx, intracellular Ca2+ levels and secretion of catecholamines in cultured adrenal chromaffin cells

Catecholamine secretion induced by various secretagogues in cultured bovine chromaffin cells has been correlated with Ca2+ influx and intracellular Ca2+ concentrations. Nicotine and high K+ caused prompt secretion of catecholamines from cells. Coincidently, both secretagogues evoked 45[Ca2+] influx with a parallel increase in free intracellular Ca2+ concentration, as determined by Quin 2 fluorescence. However, the rate of return of Ca2+ level to baseline after nicotine stimulation was more rapid than after K+ stimulation. In comparison, stimulation with veratridine produced a slow and prolonged Ca2+ influx accompanied by lower levels of intracellular Ca2+ than those observed after nicotine or K+ stimulation. Yet, during 15 min of stimulation, veratridine induced a substantial catecholamine release, which was larger than that obtained after nicotine or K+ stimulations. The Ca2+ ionophore A23187 (1 microM) induced a pronounced increase in intracellular Ca2+ levels, but did not evoke any significant catecholamine release. Finally, addition of the Ca2+ channel blocker verapamil following stimulation, at a time when intracellular Ca2+ concentration was at its peak level, did not affect the rate of decline in intracellular free Ca2+ concentration but promptly blocked Ca2+ uptake and catecholamine secretion. These findings suggest that the rate of Ca2+ influx, rather than the absolute level of intracellular Ca2+ concentration, determines the rate and extent of catecholamine release.

Mechanisms determining the time course of secretion in neuroendocrine cells

Transmitter release from chromaffin cells differs from that in synapses in that it persists for a longer time after Ca2+ entry has stopped. This prolonged secretion is not due to a delay between vesicle fusion and transmitter release, nor to slow detection of released substance: step increases in capacitance due to single vesicle fusion precede the release detected by amperometry by only a few milliseconds. The persistence of secretion after a depolarization is reduced by addition of mobile calcium buffer. This suggests that most of the delay is due to diffusion of Ca2+ between channels and release sites, implying that Ca2+ channels and secretory vesicles are not colocalized in chromaffin cells, in contrast to presynaptic active zones.

Temporally resolved, independent stages of individual exocytotic secretion events

The stages of the complex events involved in exocytotic secretion after vesicle-cell membrane fusion have been examined at the level of individual vesicles. Catecholamine flux from single bovine adrenal medullary cells was measured with carbon-fiber microelectrodes firmly touching the cell surface. The data reveal that secretion during exocytotic events has three distinct stages: a small increase in catecholamine flux, a rapid, but not instantaneous, rise to a maximum, followed by an exponential decrease in the flux. These stages are interpreted in the following ways. The initial stage corresponds to catecholamine secretion through a fusion pore. The rate of pore expansion appears to control the rise time of the flux to its maximum value. The final exponential stage is consistent with chemical dissociation of the intravesicular matrix or gel.

Dynamics of the cationic, bioelectrical and secretory responses to formycin A in pancreatic islet cells

The dynamics of the cationic, bioelectrical and secretory responses to formycin A were monitored in pancreatic islet cells in order to assess whether this adenosine analogue, which is known to be converted to formycin A 5'-triphosphate in isolated islets, triggers the same sequence of ionic events as that otherwise involved in the process of nutrient-stimulated insulin release and currently attributed to an increase in adenosine 5'-triphosphate (ATP) generation rate. Unexpectedly, formycin A first increased 86Rb outflow, decreased 45Ca outflow and inhibited insulin release from prelabelled islets perifused at physiological or higher concentrations of D-glucose. This early inhibitory effect of formycin A upon insulin release coincided, in perforated patch whole-cell recordings, with an initial transient increase of ATP-sensitive K+ channel activity. A positive secretory response to formycin A, still not associated with any decrease in K+ conductance, was only observed either immediately after formycin A administration to islets already exposed to glibenclamide or during prolonged exposure to the adenosine analogue. This coincided with an increase of cytosolic Ca2+ concentration in intact B-cells and a greater increase of membrane capacitance in response to depolarization in B-cells examined in the perforated patch whole-cell configuration. The latter stimulation of exocytotic activity could not be attributed, however, to any increase in peak or integrated Ca2+ current. Thus, the mode of action of formycin A, or its 5'-triphosphate ester, in islet cells obviously differs from that currently ascribed to endogenous ATP in the process of nutrient-stimulated insulin release.

Swelling-induced catecholamine secretion recorded from single chromaffin cells

We have studied osmotically induced catecholamine secretion from bovine adrenal chromaffin cells by combining patch-clamp measurements, electrochemical detection of secretion, and Fura-2 measurements of intracellular free calcium concentration ([Ca2+]i). We find that osmotically induced catecholamine release is exocytotic and calcium dependent. Furthermore, we demonstrate that cell swelling is coupled to such secretion via a volume-activated current, carrying predominantly chloride, which causes a plateau depolarization of the cell membrane potential and thus promotes voltage-activated calcium influx. Therefore, cell volume changes may modulate the secretory activity.

Effects of tyramine and calcium on the kinetics of secretion in intact and electroporated chromaffin cells superfused at high speed

Fast superfusion of electroporated bovine adrenal chromaffin cells with a K+ glutamate-based solution containing 50 nM free Ca2+ and 2 mM adenosine 5'-triphosphate, dipotassium salt (K2ATP), produced a steady-state low catecholamine secretion, measured on-line with an electrochemical detector (about 20 nA). Rapid switching to electroporation solutions containing increasing Ca2+ concentrations ([Ca2+]) produced a rapid increase in the rate and peak secretion, followed by a decline. At intermediate [Ca2+] (3-100 microM), a fast peak and a slow secretory plateau were distinguished. The fast secretory peak identifies a readily releasable catecholamine pool consisting of about 200-400 vesicles per cell. Pretreatment of cells with tyramine (10 microM for 4 min before electroporation) supressed the initial fast secretory peak, leaving intact the slower phase of secretion. With [Ca2+] in the range of 0.1-3 microM, the activation rate of secretion increased from 2.3 to 35.3 nA.s-1, reached a plateau between 3-30 microM and rose again from 100 to 1000 microM [Ca2+] to a maximum of 91.9 nA.s-1. In contrast, total secretion first increased (0.1-1 microM Ca2+), then plateaud (1-100 microM Ca2+) and subsequently decreased (100-1000 microM Ca2+). At 30 and 1000 microM extracellular [Ca2+] or [Ca2+]o, the activation rates of secretion from intact cells depolarised with 70 mM K+ were close to those obtained in electroporated cells. However, secretion peaks were much lower in intact (93 nA at 30 microM Ca2+) than in electroporated cells (385 nA). On the other hand, inactivation of secretion was much faster in intact than in electroporated cells; as a consequence, total secretion in a 5-min period was considerably smaller in intact (10.6 microA.s at 1000 microM Ca2+) than in electroporated cells (42.4 microA.s at 1 microM Ca2+). Separation of the time-courses of changes in intracellular [Ca2+] or [Ca2+]i and secretion in intact chromaffin cells depolarised with 70 mM K+ was demonstrated at different [Ca2+]o. The increase in the rate of catecholamine release was substantially higher than the increase of the average [Ca2+]i. In contrast, the decline of secretion was faster than the decline of the peak [Ca2+]i. The results are compatible with the idea that the peak and the amount of catecholamine released from depolarised intact cells is determined essentially by plasmalemmal factors, rather than by vesicle supply from reserve pools. These plasmalemmal factors limit the supply of Ca2+ by the rates of opening and closing of voltage-dependent Ca2+ channels of the L- and Q-subtypes, which control the local [Ca2+]i near to exocytotic sites.

Docked granules, the exocytic burst, and the need for ATP hydrolysis in endocrine cells

Ca(2+)-triggered exocytosis was studied in single rat melanotrophs and bovine chromaffin cells by capacitance measurements. Sustained exocytosis required MgATP, but even in the absence of MgATP, Ca2+ could trigger exocytosis of 2700 granules in a typical melanotroph and of 840 granules in a chromaffin cell. Granules undergoing ATP-independent exocytosis were similar in number to those appearing docked to the plasmalemma in quickly frozen unfixed sections (3300 in a melanotroph and 830 in a chromaffin cell). Most exocytosis required tens of seconds, but a small pool of granules was released in tens of milliseconds. Evidently, only a small subset of docked granules is rapidly releasable. We suggest that, temporally, the last ATP-dependent step in exocytosis is closely associated with docking and that docked granules reach fusion competence only after subsequent steps.

Stimulation of catecholamine secretion from adrenal chromaffin cells by 14-3-3 proteins is due to reorganisation of the cortical actin network

Catecholamine release from digitonin-permeabilized adrenal chromaffin cells is increased by exogenous 14-3-3 proteins. In order to determine how 14-3-3 proteins stimulate exocytosis their effect on the cortical actin network was examined. Increased amounts of beta and gamma isoforms of 14-3-3 proteins were associated with the Triton-insoluble cytoskeleton of chromaffin cells following incubation with exogenous 14-3-3 proteins. The stimulation of catecholamine release by 14-3-3 proteins was abolished by prior incubation with the actin filament stabilising drug phalloidin. Rhodamine phalloidin staining showed that the cortical actin network was disassembled and actin reorganised into intracellular foci following treatment with 14-3-3 proteins. These data suggest that 14-3-3 proteins enhance catecholamine release in permeabilized chromaffin cells by reorganisation of the cortical actin barrier to allow increased availability of secretory vesicles for exocytosis.

The kinetics of quantal transmitter release from retinal amacrine cells

Exocytosis of transmitter at most synapses is a very fast process triggered by the entry of Ca2+ during an action potential. A reasonable expectation is that the fast step of exocytosis is followed by slow steps readying another vesicle for exocytosis but the identity and kinetics of these steps are presently unclear. By voltage clamping both pre- and postsynaptic neurons in an isolated pair of retinal amacrine cells, we have measured evoked synaptic currents and responses to single vesicles of transmitter (minis). From these currents, we have computed the rate of exocytosis during a sustained presynaptic depolarization. We show here that for these cells, release is consistent with a scheme of "fire and reload." Large Ca2+ influx causes the rapid release of a small number of vesicles, typically approximately 10 per presynaptic neuron, likely corresponding to those vesicles already docked. After this spike of exocytosis whose peak is 150 quanta per release site per s, continued Ca2+ influx sustains release at only 22 quanta per release site per s, probably rate-limited by the docking of fresh vesicles.

Kinetics of receptor-mediated endocytosis of albumin in cells derived from the proximal tubule of the kidney (opossum kidney cells): influence of Ca2+ and cAMP

In this study we investigated the effects of Ca2+ and cyclic adenosine monophosphate (cAMP) on the kinetic of receptor-mediated (RME) and fluid-phase (FPE) endocytosis in opossum kidney (OK) cells, derived from the proximal tubule of the kidney. We used fluorescein isothiocyanate (FITC)-labelled albumin and FITC-labelled dextran as endocytotic substrates for RME and FPE, respectively. Removal of extracellular Ca2+ led to a dramatic decrease of the apparent affinity of RME, but did not influence the maximum endocytotic uptake rate (Jmax). Reduction of extracellular Ca2+ to 1 mumol/1 had no effect. Apparent affinity of specific binding of albumin to the plasma membrane was increased to 200% of control in the absence of extracellular Ca2+, whereas maximum binding capacity was slightly decreased. FPE was not affected by removal of extracellular Ca2+. Additional removal of cytoplasmic Ca2+, using ionomycin, had no further effect on RME and did not affect FPE. Increases of cytoplasmic (using ionomycin at extracellular Ca2+ concentrations of 1 mumol/l or 1.2 mmol/l) or extracellular Ca2+ did not alter the kinetics of RME or FPE. Dibutyryl-cAMP reduced Jmax but left the apparent affinity of RME unchanged. FPE and albumin binding to the plasma membrane were not changed in the presence of cAMP. Removal of extracellular Ca2+ and addition of cAMP led to an alkalinization of endocytotic vesicles. Yet the alkalinization induced by removal of Ca2+ was significantly greater as compared to the alkalinization in the presence of cAMP. Endosomal alkalinization with bafilomycin A1 had no further effect in the absence of Ca2+, but reduced RME in the presence of cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Fast exocytosis and endocytosis triggered by depolarisation in single adrenal chromaffin cells before rapid Ca2+ current run-down

The kinetics of exocytosis and membrane retrieval (endocytosis) were examined in bovine chromaffin cells using membrane capacitance measurement during whole-cell recording. At early times after breakthrough to the whole-cell recording mode, depolarisation for 1 s resulted in a fast (600 vesicles per s) exocytotic response and efficient membrane retrieval with a time constant of 25 s. The ability to activate fast exocytosis and retrieval was lost during intracellular dialysis, with a time constant of 40 s. At later times, a slow exocytotic response could be elicited with no membrane retrieval following single depolarisations. The wash-out of the responses appeared to be due to a rapid loss of a portion of the Ca2+ current. Trains of depolarisation at late times after breakthrough could elicit a fast (time constant 4 s) retrieval. These data show that in addition to a previously studied slow Ca(2+)-independent retrieval mechanism, chromaffin cells also possess an efficient and rapid retrieval pathway coupled to exocytosis that can be activated following depolarisation. The fast endocytosis appears to have a higher threshold for activation than exocytosis, probably due to a higher Ca2+ requirement. Rapid membrane retrieval appears to occur via a clathrin-independent pathway in chromaffin cells.

A novel Ca(2+)-dependent step in exocytosis subsequent to vesicle fusion

Exocytosis begins with formation of a small fusion pore which then expands allowing rapid release of granular contents. We studied the influence of cytoplasmic free Ca2+ ([Ca2+]i) on the conductance of the initial pore and on the dynamics of subsequent expansion in horse eosinophils using the patch clamp technique. The mean initial conductance is approximately 200 pS independent of [Ca2+]i. This value is close to that previously found in beige mouse mast cells. The pore subsequently expands by 18 nS/s at [Ca2+]i < 10 nM, by 40 nS/s at [Ca2+]i = 1.5 microM and by 90 nS/s at [Ca2+]i = 10 microM. These results show that the structure of the initial fusion pore is independent of cytoplasmic Ca2+. However, the pore expansion is a Ca(2+)-dependent process modulating secretion at a step later than vesicle-plasma membrane fusion.

Ca2+ and secretory-vesicle dynamics

Exocytosis in neurones and neuroendocrine cells is triggered by an increase in the cytosolic concentration of Ca2+, and is followed by endocytotic membrane retrieval. Electrophysiological studies have characterized the nature of the Ca2+ signal that is required for exocytosis, and have defined the Ca(2+)-dependent steps in exocytotic and endocytotic vesicle cycling. In parallel, biochemical approaches have led to the discovery of a range of proteins that appears to function in synaptic- and secretory-vesicle dynamics. The nature of the Ca(2+)-binding proteins, and how they interact with the identified components of the exocytotic and endocytotic machinery, remain key unresolved issues. However, it is apparent that exocytosis involves multiple Ca(2+)-binding proteins with different affinities, and that the Ca2+ sensor involved in the final membrane-fusion step has different affinities for Ca2+ in synapses and neuroendocrine cells.

Contribution of L- and N-type calcium currents to exocytosis in rat adrenal medullary chromaffin cells

The excitation-secretion coupling process requires Ca2+ influx through voltage-dependent Ca2+ channels (VDCC) but the contribution of L-type or N-type VDCC during the secretion from adrenal chromaffin cell is still on debate. In this study we explored the contribution of each VDCC to exocytosis in single rat adrenal chromaffin cells. Chromaffin cells were voltage-clamped clamped in the whole-cell recording mode. Ca2+ inward current (ICa) was elicited by depolarization from -70 mV to +10 mV and the change in cell membrane capacitance (Cm) was monitored as an indicator of the resultant exocytosis. The increase in Cm had positive correlation with the amount of ICa and replacing the internal Ca2+ buffer to high EGTA (5 mM) decreased the sensitivity of Cm increase to Ca2+ influx. After blockage of ICa with 100 microM Cd2+, there was no increase in Cm following membrane potential depolarization while INa was intact. To clarify the contribution of each type of VDCC to induce exocytosis during membrane potential depolarization, L- and N-type ICa were blocked selectively by Ca2+ channel antagonists. After blockage of L-type ICa with nicardipine (1 microM), ICa was blocked to 35 +/- 6.2% (mean +/- standard error) of control and the resultant change in Cm was reduced to 38 +/- 4.6% of control. Bay K-8644 (1 microM) enhanced ICa and the similar proportion of Cm was increased by this L-type VDCC agonist. On the other hand omega-conotoxin GVIA (1 microM), an N-type VDCC antagonist, blocked ICa to 60 +/- 4.3% of control and reduced the change in Cm to 58 +/- 3.9% of control.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of protein synthesis inhibitors on synexin levels and secretory response in bovine adrenal medullary chromaffin cells

The effects of the protein synthesis inhibitors actinomycin D and cycloheximide on the cellular content of the calcium binding protein synexin, and on the secretory response of cultured bovine adrenal medullary chromaffin cells were determined. Both protein synthesis inhibitors produced a slow decrease in the cellular synexin content. The synexin level was reduced by 50% after 133 h of incubation in the presence of 2 micrograms/ml actinomycin D or 5 micrograms/ml cycloheximide. However, this was partly due to an artefactual stabilization of synexin, since metabolic labelling of synexin with [35S]methionine showed that the half-time of degradation was only 40 h. The secretory response of chromaffin cells was quickly diminished in the presence of protein synthesis inhibitors. Catecholamine secretion induced by membrane depolarization or barium stimulation of intact cells, or by calcium stimulation of digitonin-permeabilized cells was decreased by 77-82% after 24 h of incubation in the presence of 5 micrograms/ml cycloheximide. These results suggest that, in addition to synexin, at least one or more proteins with a shorter half-time of degradation than synexin are involved in the secretory response of adrenal chromaffin cells.

Correlation of real-time catecholamine release and cytosolic Ca2+ at single bovine chromaffin cells

Previous investigations of the role of Ca2+ in stimulus-secretion coupling have been undertaken in populations of adrenal chromaffin cells. In the present study, the simultaneous detection of intracellular Ca2+, with the fluorescent probe fura-2, and catecholamine release, using a carbon-fiber microelectrode, are examined at single chromaffin cells in culture. Results from classic depolarizing stimuli, high potassium (30-140 mM) and 1,1-dimethyl-4-phenylpiperazinium (3-50 microM), show a dependence of peak cytosolic Ca2+ concentration and catecholamine release on secretagogue concentration. Catecholamine release induced by transient high K+ stimulation increases logarithmically with K+ concentration. Continuous exposure to veratridine (50 microM) induces oscillations in intracellular Ca2+ and at higher concentrations (100 microM) concomitant fluctuation of cytosolic Ca2+ and catecholamine secretion. Mobilization of both caffeine- and inositol trisphosphate-sensitive intracellular Ca2+ stores is found to elicit secretion with or without extracellular Ca2+. Caffeine-sensitive intracellular Ca2+ stores can be depleted, refilled, and cause exocytosis in medium without Ca2+. Single cell measurement of exocytosis and the increase in cytosolic Ca2+ induced by bradykinin-activated intracellular stores reveal cell to cell variability in exocytotic responses which is masked in populations of cells. Taken together, these results show that exocytosis of catecholamines can be induced by an increase in cytosolic Ca2+ either as a result of transmembrane entry or by release of internal stores.

Discrete acetylcholine release from neuroblastoma or hybrid cells overexpressing choline acetyltransferase into the neuromuscular synaptic cleft

Neuroblastoma (clones NS-20Y, N1E-115, and Neuro2A) and neuroblastoma x glioma hybrid (NG108-15) cells were transfected with mouse choline acetyltransferase (ChAT) complementary DNA (cDNA) or vector DNA alone and stably transformed cell lines were established to examine their ability to secrete acetylcholine (ACh). Membrane potentials were recorded from either presynaptic neuroblastoma and hybrid cells or postsynaptic myotubes in co-culture. After transformation with ChAT, synapses were formed and miniature end-plate potentials (MEPPs) were recorded in myotubes co-cultured with Neuro2A and N1E-115 cells, while parental and mock-transfected control cells totally lacked this ability. The rate of synapse formation and/or MEPP frequency was higher in transformed NG108-15 hybrid and NS-20Y cells than that in the control cells. Action potentials of NS-20Y, Neuro2A or NG108-15 cells overexpressing ChAT were able to evoke end-plate potentials in myotubes, though the average quantum content of these cells was 0.04-0.14, which is as low as the control value. The results show that increased concentrations of ACh by ChAT cDNA transfection reveal a masked property in vesicular ACh release from Neuro2A and N1E-115 cells with no endogenous ChAT activity, or modify their secretory capacity upwardly from NG108-15 and NS-20Y cells with endogenous activity.

The biochemistry of neurotransmitter secretion

The progress that has resulted from the convergence of biochemistry with yeast genetics has accelerated the pace at which the molecular events of membrane transport are being elucidated. Future research will focus not only on testing the proposed sequence of protein-protein interactions but also on identifying how calcium regulation is imposed on this system. As our understanding of the basic mechanisms of neurosecretion increases, attention will undoubtedly shift to how the molecules of release are modified to produce changes in synaptic efficacy.

Chromaffin cell cortical actin network dynamics control the size of the release-ready vesicle pool and the initial rate of exocytosis

Morphological, biochemical, and membrane capacitance measurements were used to study the role of cortical filamentous actin (F-actin) in exocytosis. Fluorescence and electron microscopy of resting chromaffin cells revealed a cortical actin network that excluded secretory vesicles from the subplasmalemmal area. Phorbol ester (PMA) treatment disrupted cortical F-actin and increased both the number of vesicles within the 0-50 nm subplasmalemmal zone and the initial rate of stimulated catecholamine release. In PMA-pretreated cells, membrane capacitance studies showed an increased number of vesicles fusing with the plasmalemma during the first two depolarizations of a train. PMA did not affect voltage-dependent Ca2+ influx. The total number of vesicles fused with the plasma membrane correlated well with the number of vesicles occupying the 0-50 nm cortical zone. Therefore, cortical F-actin disassembly allows translocation of vesicles to the plasmalemma in preparation for exocytosis.

Time course of Ca2+ concentration triggering exocytosis in neuroendocrine cells

We have used the secretory response of chromaffin cells to estimate the submembrane intracellular Ca2+ concentration ([Ca2+]i) "seen" by secretory granules during short depolarizations. The rate of secretion during a depolarization was assessed by combining the electrochemical method of amperometry and electrical capacitance measurements. The rate was then related to [Ca2+]i based on a previous characterization of how Ca2+ affects the dynamics of vesicle priming and fusion in chromaffin cells [Heinemann, C., Chow, R. H., Neher, E. & Zucker, R. S. (1994) Biophys. J. 67, in press]. Calculated [Ca2+]i rose during the depolarization to a peak of < 10 microM, then decayed over tens of milliseconds. In synapses, vesicles are presumed to be located within nanometers of Ca2+ channels where [Ca2+]i is believed to rise in only microseconds to near steady-state levels of hundreds of micromolar. Channel closure should lead to a decrease in [Ca2+]i also in microseconds. Our findings of the slower time course and the lower peak [Ca2+]i suggest that in chromaffin cells, unlike synapses, Ca2+ channels and vesicles are not strictly colocalized. This idea is consistent with previously published data on dense-core vesicle secretion from diverse cell types.

Antidiuretic hormone and exocytosis: lessons from neurosecretion

Many cells, both single and epithelial, are programmed for exocytosis. In most cases, the contents of cytoplasmic vesicles are delivered rapidly and directly to the extracellular fluid. The process has been intensively studied in the chromaffin cell and the nerve terminal, where, as in other cells, exocytosis is under a complex type of cytoskeletal control. An array of vesicle-associated proteins mediates attachment of the vesicles to the cytoskeleton, their release, and their fusion with the plasma membrane. Two functional pools of vesicles, the releasable and reserve pool, carry out immediate and long-term secretory activity. Some of the mediators of neurotransmitter vesicle fusion, originally thought to be restricted to neurosecretory cells, have now been found in nonneuronal cells. The mammalian collecting duct and the amphibian bladder are also engaged in exocytosis. In both epithelia, antidiuretic hormone (ADH) induces the transfer of water channels from cytoplasmic vesicles to the apical cell membrane. The process is slower than in the nerve terminal and ends with channel placement rather than the extrusion of vesicular contents. Nevertheless, there are several respects in which cytoskeletal control, vesicle positioning in the cell, docking, and fusion may prove to resemble the events in neurosecretion. This review begins with a survey of cytoskeletal structure and function in the erythrocyte, the chromaffin cell, and the nerve terminal and then presents current studies of ADH-induced exocytosis, emphasizing common themes in cytoskeletal control.

Kinetics of the secretory response in bovine chromaffin cells following flash photolysis of caged Ca2+

The kinetics of the secretory response in bovine chromaffin cells following flash photolysis of caged Ca2+ were studied by capacitance (Cm) measurements with millisecond time resolution. After elevation of the internal Ca2+ concentration ([Ca2+]i), Cm rises rapidly with one or more exponentials. The time constant of the fastest component decreases for higher [Ca2+]i (range 3-600 microM) over three orders of magnitude before it saturates at approximately 1 ms. The corresponding maximal rates of secretion can be as fast as 100,000 fF/s or 40,000 vesicles/s. There is a Ca(2+)-dependent delay before Cm rises, which may reflect the kinetics of multiple Ca2+ ions binding to the secretory apparatus. The initial rise in Cm is described by models containing a sequence of two to four single Ca(2+)-binding steps followed by a rate-limiting exocytosis step. The predicted Ca2+ dissociation constant (Kd) of a single Ca(2+)-binding site is between 7 and 21 microM. At [Ca2+]i > 30 microM clear indications of a fast endocytotic process complicate the analysis of the secretory response.

Control of transmitter release from retinal amacrine cells by Ca2+ influx and efflux

Cultured retinal amacrine cells show quantal GABAergic synaptic transmission. Voltage clamping pre- and post-synaptic cells of an isolated pair has allowed us to examine the entry and removal of Ca2+ at synaptic terminals. Brief presynaptic Ca2+ currents elicit an initial postsynaptic current that probably reflects the roughly synchronous exocytosis of docked vesicles. Prolonged Ca2+ currents elicit an additional second phase of release whose time course can greatly exceed that of the presynaptic voltage step. The time course of this second phase reflects a sustained increase in cytosolic Ca2+ and is matched closely by the activity of the presynaptic Na-Ca exchanger, as revealed by an exchange current. Eliminating the activity of the exchanger by removal of external Na+ prolongs this second phase of transmission greatly. Because transmitter release at these synapses outlasts Ca+ channel opening, Na-Ca exchange plays a significant role in shaping transmission.

Releasable pools and the kinetics of exocytosis in adrenal chromaffin cells

Using high resolution capacitance (Cm) measurement techniques, we mapped the kinetics of exocytosis evoked by brief (5-100 ms) depolarizations that activate voltage-dependent Ca2+ channels in rat adrenal chromaffin cells. After correcting the Cm signal for contributions from Na+ channel-gating charge movements, the initial kinetics of exocytosis were consistent with depolarization-triggered release occurring initially from an immediately releasable pool of only approximately 17 secretory vesicles. In contrast, repetitive application of longer depolarizations evoked release from a distinct, larger readily releasable pool of approximately 170 vesicles. Our results suggest that the secretory response of a chromaffin cell to an action potential is limited by the size of the immediately releasable pool rather than by a fusion mechanism that is slower than that at synapses.

Rab3 proteins: key players in the control of exocytosis

Although some mechanistic aspects of exocytosis, such as fusion events, have been well documented by the technique of time-resolved membrane-capacitance measurement, it was only recently that new insights into the molecular mechanisms involved in the traffic of secretory vesicles were provided by the convergence of different lines of research. In this review Lledo et al. present some of the recent findings concerning small GTPases of the Rab3 subfamily which regulate hormone release, triggered by entry of Ca2+, in endocrine and neuroendocrine cells. In view of these new results, Rab proteins might be considered as candidates for inhibition or stimulation of specific steps involved in vesicle traffic.

The molecular machinery for fast and slow neurosecretion

Recent studies indicate that the molecular machinery for synaptic vesicle docking and fusion consists of a triad of botulinum/tetanus neurotoxin substrates (synaptobrevin, syntaxin, SNAP-25) that are homologues of proteins required for constitutive secretion. Proposed low-affinity Ca2+ sensors that regulate exocytosis remain to be identified, although recent studies on synaptotagmin suggest that it, along with other proteins, could play this role. Regulated peptide secretion from dense-core granules has been found to utilize a similar machinery for docking/fusion, and recent studies indicate that this pathway involves a pre-docking step that is regulated by a higher affinity Ca2+ sensor.

Q- and L-type Ca2+ channels dominate the control of secretion in bovine chromaffin cells

Potassium-stimulated catecholamine release from superfused bovine adrenal chromaffin cells (70 mM K+ in the presence of 2 mM Ca2+ for 10 s, applied at 5-min intervals) was inhibited by the dihydropyridine furnidipine (3 microM) by 50%. omega-Conotoxin MVIIC (CTx-MVIIC, 3 microM) also reduced the secretory response by about half. Combined CTx-MVIIC plus furnidipine blocked 100% catecholamine release. 45Ca2+ uptake and cytosolic Ca2+ concentrations ([Ca2+]i) in K(+)-depolarized cells were partially blocked by furnidipine or CTx-MVIIC, and completely inhibited by both agents. The whole cell current through Ca2+ channels carried by Ba2+ (IBa) was partially blocked by CTx-MVIIC. Although omega-conotoxin GVIA (CTx-GVIA, 1 microM) and omega-agatoxin IVA (Aga-IVA, 0.2 microM) partially inhibited 45Ca2+ entry, IBa and the increase in [Ca2+]i, the combination of both toxins did not affect the K(+)-evoked secretory response. The results are compatible with the presence in bovine chromaffin cells of a Q-like Ca2+ channel which has a prominent role in controlling exocytosis. They also suggest that Q- and L-type Ca2+ channels, but not N- or P-types are localized near exocytotic active sites in the plasmalemma.

Localized L-type calcium channels control exocytosis in cat chromaffin cells

Depolarizing 1-s pulses to 0 mV from a holding potential of -70 mV, induced whole-cell currents through Ca2+ channels (ICa) in patch-clamped cat adrenal medulla chromaffin cells. The dihydropyridine (DHP) furnidipine (3 microM) reduced the peak current by 47% and the late current by 80%. omega-Conotoxin GVIA (CgTx, 1 microM) reduced the peak ICa by 42% and the late ICa by 55%. Pulses (10 s duration) with 70 mM K+/2.5 mM Ca2+ solution (70 K+/2.5 Ca2+), applied to single fura-2-loaded cat chromaffin cells increased the cytosolic Ca2+ concentration ([Ca2+]i) from 0.1 to 2.21 microM; this increase was reduced by 43.7% by furnidipine and by 42.5% by CgTx. In the perfused cat adrenal gland, secretion evoked by 10-s pulses of 70 K+/2.5 Ca2+ was reduced by 25% by CgTx and by 96% by furnidipine. Similar results were obtained when secretion from superfused isolated cat adrenal chromaffin cells was studied and when using a tenfold lower [Ca2+]o. The results are compatible with the existence of DHP-sensitive (L-type) as well as CgTx-sensitive (N-type) voltage-dependent Ca2+ channels in cat chromaffin cells. It seems, however, that though extracellular Ca2+ entry through both channel types leads to similar increments of averaged [Ca2+]i, the control of catecholamine release is dominated only by Ca2+ entering through L-type Ca2+ channels. This supports the idea of a preferential segregation of L-type Ca2+ channels to localized "hot spots" in the plasmalemma of chromaffin cells where exocytosis occurs.

Activation of exocytosis by GTP analogues in adrenal chromaffin cells revealed by patch-clamp capacitance measurement

The role of GTP-binding proteins in exocytosis in bovine adrenal chromaffin cells was examined using patch-clamp capacitance measurement. Internal dialysis with the non-hydrolysable GTP analogue guanosine 5'-[beta gamma-imido]triphosphate and xanthosine triphosphate (XTP) activated a capacitance increase. Exocytosis triggered by XTP was blocked by guanosine 5'-[beta-thio]diphosphate (GDP beta S) but Ca(2+)-induced exocytosis was unaffected. The capacitance increase due to XTP could not be explained by Ca2+ mobilisation since Ins(1,4,5)P3 and caffeine did not mimic the response. Chromaffin cells appear to possess a Ca(2+)-independent pathway for exocytosis that involves GTP-binding proteins. The magnitude of the response to XTP suggested that GTP analogues stimulate both exocytosis and recruitment of secretory granules.

Exocytosis in chromaffin cells: evidence for a MgATP-independent step that requires a pertussis toxin-sensitive GTP-binding protein

We have previously described that mastoparan, an amphiphilic tetradecapeptide that activates heterotrimeric G-proteins, inhibits Ca(2+)-induced MgATP-dependent secretion from streptolysin-O-permeabilized chromaffin cells [Vitale, Mukai, Rouot, Thiersé, Aunis and Bader (1993) J. Biol. Chem. 268, 14715-14723]. Our observations suggest the involvement of an inhibitory G(o)-protein, possibly located on the membrane of secretory granules, in the final stages of the exocytotic pathway in chromaffin cells. Here, we demonstrate that mastoparan is also able to stimulate the Ca(2+)-dependent secretion of catecholamines in the absence of MgATP in the medium. This MgATP-independent secretion is totally blocked by tetanus toxin, a potent inhibitor of exocytosis in all neurosecretory cells so far investigated, suggesting that the mastoparan target is a component of the exocytotic machinery. Mas17, a mastoparan analogue inactive on G-proteins, had no effect on catecholamine secretion whereas both Mas7, a highly active analogue of mastoparan, and AlF4-, which selectively activates trimeric G-proteins, triggered MgATP-independent secretion. Non-hydrolysable GTP analogues (GTP[S] and p[NH]ppG) mimicked the dual effects of mastoparan on secretion: they inhibited exocytosis in the presence of MgATP and stimulated MgATP-independent secretion. The different potencies displayed by these two analogues suggest the involvement of two distinct G-proteins. Accordingly, the mastoparan-induced MgATP-independent secretion is highly sensitive to pertussis toxin (PTX) whereas the inhibition by mastoparan of secretion in the presence of MgATP is resistant to PTX treatment. When permeabilized cells were incubated with mastoparan, the release of arachidonic acid increased in a PTX-sensitive manner. 7,7-Dimethyl-5,8-eicosadienoic acid, a potent inhibitor of intracellular phospholipase A2, inhibited both the arachidonate release and the MgATP-independent catecholamine secretion evoked by mastoparan. In contrast, neomycin, an inhibitor of phospholipase C, had no significant effect on either the release of arachidonic acid or the secretion of catecholamines provoked by mastoparan. We conclude that two distinct heterotrimeric G-proteins act in series in the exocytotic pathway in chromaffin cells: one controls an ATP-dependent priming step through an effector pathway that remains to be determined, and the second is involved in a late Ca(2+)-dependent step which does not require MgATP but possibly involves the generation of arachidonic acid.