Sustained stimulation of exocytosis triggers continuous membrane retrieval in rat pituitary somatotrophs

Resistance exercise stress: theoretical mechanisms for growth hormone processing and release from the anterior pituitary somatotroph

Heavy resistance exercise (HRE) is the most effective method for inducing muscular hypertrophy and stimulating anabolic hormones, including growth hormone, into the blood. In this review, we explore possible mechanisms within the GH secretory pathway of the pituitary somatotroph, which are likely to modulate the flow of hormone synthesis and packaging as it is processed prior to exocytosis. Special emphasis is placed on the secretory granule and its possible role as a signaling hub. We also review data that summarize how HRE affects the quality and quantity of the secreted hormone. Finally, these pathway mechanisms are considered in the context of heterogeneity of the somatotroph population in the anterior pituitary.

Alcohol metabolite acetic acid activates BK channels in a pH-dependent manner and decreases calcium oscillations and exocytosis of secretory granules in rat pituitary GH3 cells

Acetaldehyde and acetic acid/acetate, the active metabolites of alcohol (ethanol, EtOH), generate actions of their own ranging from behavioral, physiological, to pathological/cancerogenic effects. EtOH and acetaldehyde have been studied to some depth, whereas the effects of acetic acid have been less well explored. In this study, we investigated the effect of acetic acid on big conductance calcium-activated potassium (BK) channels present in GH3 rat pituitary tumor cells in more detail. In whole cell voltage clamp recordings, extracellular application of acetic acid increased total outward currents in a dose-dependent manner. This effect was prevented after the application of the specific BK channel blocker paxilline. Acetic acid action was pH-dependent-in whole cell current and single BK channel recordings, open probability (Po) was significantly increased by extracellular pH reduction and decreased by neutral or base pH. Acetic acid hyperpolarized the membrane potential, whereas acidic physiological solution had a depolarizing effect. Moreover, acetic acid reduced calcium (Ca2+) oscillations and exocytosis of growth hormone contained secretory granules from GH3 cells. These effects were partially prevented by BK inhibitors-tetraethylammonium or paxillin. In conclusion, our experiments indicate that acetic acid activates BK channels in GH3 cells which eventually contribute to acetic acid-induced membrane hyperpolarization, cessation of Ca2+ oscillations, and decrease of growth hormone release.

Number of Secretory Vesicles in Growth Hormone Cells of the Pituitary Remains Unchanged After Secretion

Immunogold-labeled transmission electron microscopy (TEM) was used to determine the total number of secretory vesicles in resting and in growth hormone (GH)-stimulated porcine pituitary cells. We identified three categories of vesicles: filled, empty, and partly empty. Resting GH cells contained more than twice as many filled vesicles than did the stimulated ones. Stimulated cells, however, contained nearly twice as many empty vesicles and 2.5 times more partly empty vesicles than did resting cells. Secretory vesicles in GH cells further revealed the localization of GH only in electron-dense vesicles in both resting and stimulated cells. The total number of secretory vesicles did not change after secretion. These results are consistent with a mechanism that, after stimulation of secretion, vesicles transiently dock and fuse at the fusion pore to release vesicular contents.

Homocysteine augments BK channel activity and decreases exocytosis of secretory granules in rat GH3 cells

In this study, we investigated the effects of L-homocysteine (Hcy) on maxi calcium-activated potassium (BK) channels and on exocytosis of secretory granules in GH3 rat pituitary-derived cells. A major finding of our study indicates that short-term application of Hcy increased the open probability of oxidized BK channels in inside-out recordings. Whole-cell recordings show that extracellular Hcy also augmented BK currents during long-term application. Furthermore, Hcy decreased the exocytosis of secretory granules. This decrease was partially prevented by the BK channel inhibitor paxilline and fully prevented by N-acetylcysteine, a reactive oxygen species scavenger. Taken together, our data show that elevation of cellular Hcy level induces oxidative stress, increases BK channel activity, and decreases exocytosis of secretory granules. These findings may provide insight into some of the developmental impairments and neurotoxicity associated with Hyperhomocysteinemia (HHcy), a disease arising due to abnormally elevated levels of Hcy in the plasma.

Insights into the Nanobiology of Growth Hormone Secretion

The fact that partially empty vesicles are generated following cell secretion suggested that secretory vesicles do not collapse at the cell plasma membrane but, rather, transiently dock and fuse at the plasma membrane to expel a portion of their contents before retracting or undergoing endocytosis into the cell. Such a process has also been referred to in the literature as a "kiss-and-run" mechanism. This mechanism of cell secretion was conclusively demonstrated following the discovery of permanent cup-shaped lipoprotein structures at the cell plasma membrane, called "porosomes", where secretory vesicles transiently dock and fuse to expel intravesicular contents from the cell. Porosomes are present in all secretory cells, from the digestive enzyme-secreting pancreatic acinar cells, to the hormone-releasing growth hormone cells, mast cells, chromaffin cells, hair cells of the inner ear, to neurons secreting neurotransmitters. Hence, it can be asserted that porosomes are the universal secretory machinery in the plasma membrane of secretory cells. Therefore, the discovery of the porosome has resulted in a paradigm shift in our understanding of cell secretion. Rapid transport of secretory vesicles containing hormones to the plasma membrane is powered by high-energy molecules such as ATP, GTP or NADH. Immunogold labeled transmission electron microscopy (TEM) was used to determine the total number of secretory vesicles in resting and in GH-stimulated porcine pituitary cells. We identified three categories of vesicles: filled, empty, and partly empty. Resting GH cells contained more than twice as many filled vesicles than did the stimulated ones. However, stimulated cells contained nearly twice as many empty vesicles and 2.5 times more partly empty vesicles than did resting cells. Secretory vesicles in GH cells further revealed the localization of GH only in electron dense vesicles in both resting and stimulated cells. No change in the total number of secretory vesicles following secretion was observed. These results are consistent with a mechanism that, after stimulation of secretion, vesicles transiently dock and fuse at the porosome to establish a fusion pore, through which intravesicular contents are released.

Voltage-gated Ca2+ influx and mitochondrial Ca2+ initiate secretion from Aplysia neuroendocrine cells

Neuroendocrine secretion often requires prolonged voltage-gated Ca(2+) entry; however, the ability of Ca(2+) from intracellular stores, such as endoplasmic reticulum or mitochondria, to elicit secretion is less clear. We examined this using the bag cell neurons, which trigger ovulation in Aplysia by releasing egg-laying hormone (ELH) peptide. Secretion from cultured bag cell neurons was observed as an increase in plasma membrane capacitance following Ca(2+) influx evoked by a 5-Hz, 1-min train of depolarizing steps under voltage-clamp. The response was similar for step durations of ≥ 50 ms, but fell off sharply with shorter stimuli. The capacitance change was attenuated by replacing external Ca(2+) with Ba(2+), blocking Ca(2+) channels, buffering intracellular Ca(2+) with EGTA, disrupting synaptic protein recycling, or genetic knock-down of ELH. Regarding intracellular stores, liberating mitochondrial Ca(2+) with the protonophore, carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazone (FCCP), brought about an EGTA-sensitive elevation of capacitance. Conversely, no change was observed to Ca(2+) released from the endoplasmic reticulum or acidic stores. Prior exposure to FCCP lessened the train-induced capacitance increase, suggesting overlap in the pool of releasable vesicles. Employing GTP-γ-S to interfere with endocytosis delayed recovery (presumed membrane retrieval) of the capacitance change following FCCP, but not the train. Finally, secretion was correlated with reproductive behavior, in that neurons isolated from animals engaged in egg-laying presented a greater train-induced capacitance elevation vs quiescent animals. The bag cell neuron capacitance increase is consistent with peptide secretion requiring high Ca(2+), either from influx or stores, and may reflect the all-or-none nature of reproduction.

Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system

We review the evidence of exocytosis from extrasynaptic sites in the soma, dendrites, and axonal varicosities of central and peripheral neurons of vertebrates and invertebrates, with emphasis on somatic exocytosis, and how it contributes to signaling in the nervous system. The finding of secretory vesicles in extrasynaptic sites of neurons, the presence of signaling molecules (namely transmitters or peptides) in the extracellular space outside synaptic clefts, and the mismatch between exocytosis sites and the location of receptors for these molecules in neurons and glial cells, have long suggested that in addition to synaptic communication, transmitters are released, and act extrasynaptically. The catalog of these molecules includes low molecular weight transmitters such as monoamines, acetylcholine, glutamate, gama-aminobutiric acid (GABA), adenosine-5-triphosphate (ATP), and a list of peptides including substance P, brain-derived neurotrophic factor (BDNF), and oxytocin. By comparing the mechanisms of extrasynaptic exocytosis of different signaling molecules by various neuron types we show that it is a widespread mechanism for communication in the nervous system that uses certain common mechanisms, which are different from those of synaptic exocytosis but similar to those of exocytosis from excitable endocrine cells. Somatic exocytosis has been measured directly in different neuron types. It starts after high-frequency electrical activity or long experimental depolarizations and may continue for several minutes after the end of stimulation. Activation of L-type calcium channels, calcium release from intracellular stores and vesicle transport towards the plasma membrane couple excitation and exocytosis from small clear or large dense core vesicles in release sites lacking postsynaptic counterparts. The presence of synaptic and extrasynaptic exocytosis endows individual neurons with a wide variety of time- and space-dependent communication possibilities. Extrasynaptic exocytosis may be the major source of signaling molecules producing volume transmission and by doing so may be part of a long duration signaling mode in the nervous system.

Nanobiology and physiology of growth hormone secretion

Growth hormone (GH) secretion is controlled by hypothalamic releasing hormones from the median eminence together with hormones and neuropeptides produced by peripheral organs. Secretion of GH involves movement of secretory vesicles along microtubules, transient ‘docking’ with the porosome in the cell membrane and subsequent release of GH. Release of GH is stimulated by GH releasing hormone (GHRH) and inhibited by somatostatin (SRIF). Ghrelin may be functioning to stimulate GH release from somatotropes acting via the GH secretagogue (GHS) receptor (GHSR). However, recent physiological studies militate against this. In addition, ghrelin does influence GH release acting within the hypothalamus. Release of GH from the somatotropes involves the GH-containing secretory granules moving close to the cell surface followed by transitory fusion of the secretory granules with the porosomes located in multiple secretory pits in the cell membrane. Other peptides/proteins can influence GH secretion, particularly in species of non-mammalian vertebrates.

Colocalization of FM1-43, Bassoon, and GnRH-1: GnRH-1 release from cell bodies and their neuroprocesses

Pulsatile release of GnRH-1 is critical for reproductive function. However, the cellular mechanism of GnRH-1 neurosecretion is still elusive. In this study, we examined the neurosecretory process of GnRH-1 neurons using time-lapse image acquisition followed by immunocytochemistry with confocal microscopy. To monitor exocytotic processes, cultured GnRH-1 neurons derived from monkey embryos were labeled with the lipophilic dye, FM1-43, or its fixable form FM1-43Fx, in the presence or absence of depolarization signals, and changes in vesicles labeled with FM1-43 were analyzed. The results show FM1-43 was taken up into the cell and labeled puncta in the soma and neuroprocesses in the absence of depolarization signals, indicating that GnRH-1 neurons were spontaneously active. Depolarization of GnRH-1 neurons with high K+ or veratridine challenge increased the intensity and size of puncta in both soma and neuroprocesses, and the veratridine-induced changes in puncta were blocked by tetrodotoxin, indicating that changes in the puncta intensity and size reflect neurosecretory activity. Subsequent double immunocytochemistry for GnRH-1 and the synaptic vesicle marker, vesicle-associated membrane protein, demonstrated that the FM1-43Fx-labeled puncta were synaptic vesicles with the GnRH-1 peptide. Additional double immunocytochemistry for GnRH-1 and the marker of the neurosecretory active zone, Bassoon, indicated that the FM1-43Fx-labeled puncta were located at the sites of neurosecretory active zones in GnRH-1 neurons. These results suggest that GnRH-1 neurons have the capacity to release the peptide from the soma and dendrites. Collectively, we hypothesize that soma-dendritic release of the peptide may be a mechanism of synchronized activity among GnRH-1 neurons.

Regulated exocytosis and vesicle trafficking in astrocytes

Astrocytes are increasingly viewed as crucial cells supporting and integrating brain functions. It is thought that the release of gliotransmitters into the extracellular space by regulated exocytosis supports a significant part of communication between astrocytes and neurons. Prior to exocytosis, the membrane-bound vesicles are transported through the astrocyte cytoplasm. Our recent studies have revealed new insights into vesicle trafficking in the cytoplasm of astrocytes and are reviewed in this article. The prefusion mobility of fluorescently labeled peptidergic vesicles was studied in cultured rat and mouse astrocytes. Vesicle delivery to the plasma membrane involved an interaction with the cytoskeleton, in particular with microtubules and actin filaments. Interestingly, vesicle mobility in mouse astrocytes deficient in intermediate filaments show impaired directionality of peptidergic vesicle mobility. To explore whether stimuli that increase the concentration of free calcium ions in the cytoplasm triggered vesicular ATP release from astrocytes, human embryonic kidney-293T cells transfected with a P2X(3) receptor were used as sniffers to detect ATP release. Glutamate stimulation of astrocytes was followed by an increase in the incidence of small, transient, inward currents in sniffer cells, reminiscent of postsynaptic quantal events observed at synapses. Some of the membrane-bound vesicles are retrieved from the plasma membrane to be recycled back into the cytosol. Trafficking velocity of postfusion (recycling) atrial natriuretic peptide vesicles was one order of magnitude slower in comparison to the mobility of prefusion vesicles. However, transport of all vesicle types studied required an intact cytoskeleton.

EAAT2 density at the astrocyte plasma membrane and Ca(2 + )-regulated exocytosis

We studied whether regulated exocytosis affects the glutamate transporter density in cultured astrocytes, in which the expression of a fluorescently labeled excitatory amino acid transporter 2 (EAAT2-EGFP) predominantly labeled the plasma membrane. The addition of ionomycin that elevates cytosolic Ca(2+) strongly increased the fluorescence of FM 4-64 membrane area dye, confirming the presence of regulated exocytosis in transfected astrocytes. However, concomitant with Ca(2+)-dependent FM 4-64 fluorescence increase, ionomycin induced a significant steady-state decrease in EAAT2-EGFP fluorescence. This is likely due to a secondary inner filter effect since,(i) in the absence of FM 4-64, ionomycin stimulation was ineffective in changing the EAAT2-EGFP fluorescence, and (ii) fluorescence changes in FM 4-64 and EAAT2-EGFP were inversely correlated. To test whether subcellular EAAT2-EGFP structures are translocated from the cytoplasm to the plasma membrane during ionomycin stimulation, EAAT2-EGFP fluorescence was monitored locally at the plasma membrane and a few microns away in the adjacent cytoplasm. Measurements revealed sites with an increase in EAAT2-EGFP plasma membrane fluorescence correlated with a fluorescence decrease beneath the plasma membrane, and sites with plasma membrane fluorescence decrease correlated with fluorescence increase within the adjacent cytoplasm. The sites of rapid translocation/retrieval of EAAT2-EGFP structures to/from the plasma membrane appeared to be distributed in a punctuate pattern around the cell perimeter. The density of EAAT2-EGFP was regulated in a Ca(2+)-dependent manner, since in the absence of extracellular Ca(2+) local translocation/retrieval events were absent, revealing rapid surface density regulation of EAAT2 in astrocytes by regulated exo/endocytosis.

Evidence for AMPK-dependent regulation of exocytosis of lipoproteins in a model liver cell line

5'-AMP-activated kinase (AMPK) plays a key role in the regulation of cellular lipid metabolism. The contribution of vesicular exocytosis to this regulation is not known. Accordingly, we studied the effects of AMPK on exocytosis and intracellular lipid content in a model liver cell line. Activation of AMPK by metformin or 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) increased the rates of constitutive exocytosis by about 2-fold. Stimulation of exocytosis by AMPK occurred within minutes, and persisted after overnight exposure to metformin or AICAR. Activation of AMPK also increased the amount of triacylglycerol (TG) and apolipoprotein B (apoB) secreted from lipid-loaded cells. These effects were accompanied by a decrease in the intracellular lipid content indicating that exocytosis of lipoproteins was involved in these lipid-lowering effects. While AMPK increased the rates of fatty acid oxidation (FAO), the lipid-lowering effects were quantitatively significant even after inhibition of FAO with R-etomoxir. These results suggest that hepatic AMPK stimulates constitutive exocytosis of lipoproteins, which may function in parallel with FAO to regulate intracellular lipid content.

Dual control of cardiac Na+ Ca2+ exchange by PIP(2): electrophysiological analysis of direct and indirect mechanisms

Cardiac Na(+)-Ca(2+) exchange (NCX1) inactivates in excised membrane patches when cytoplasmic Ca(2+) is removed or cytoplasmic Na(+) is increased. Exogenous phosphatidylinositol-4,5-bis-phosphate (PIP(2)) can ablate both inactivation mechanisms, while it has no effect on inward exchange current in the absence of cytoplasmic Na(+). To probe PIP(2) effects in intact cells, we manipulated PIP(2) metabolism by several means. First, we used cell lines with M1 (muscarinic) receptors that couple to phospholipase C's (PLCs). As expected, outward NCX1 current (i.e. Ca(2+) influx) can be strongly inhibited when M1 agonists induce PIP(2) depletion. However, inward currents (i.e. Ca(2+) extrusion) without cytoplasmic Na(+) can be increased markedly in parallel with an increase of cell capacitance (i.e. membrane area). Similar effects are incurred by cytoplasmic perfusion of GTPgammaS or the actin cytoskeleton disruptor latrunculin, even in the presence of non-hydrolysable ATP (AMP-PNP). Thus, G-protein signalling may increase NCX1 currents by destabilizing membrane cytoskeleton-PIP(2) interactions. Second, to increase PIP(2) we directly perfused PIP(2) into cells. Outward NCX1 currents increase as expected. But over minutes currents decline substantially, and cell capacitance usually decreases in parallel. Third, using BHK cells with stable NCX1 expression, we increased PIP(2) by transient expression of a phosphatidylinositol-4-phosphate-5-kinase (hPIP5KIbeta) and a PI4-kinase (PI4KIIalpha). NCX1 current densities were decreased by > 80 and 40%, respectively. Fourth, we generated transgenic mice with 10-fold cardiac-specific overexpression of PI4KIIalpha. This wortmannin-insensitive PI4KIIalpha was chosen because basal cardiac phosphoinositides are nearly insensitive to wortmannin, and surface membrane PI4-kinase activity, defined functionally in excised patches, is not blocked by wortmannin. Both phosphatidylinositol-4-phosphate (PIP) and PIP(2) were increased significantly, while NCX1 current densities were decreased by 78% with no loss of NCX1 expression. Most mice developed cardiac hypertrophy, and immunohistochemical analysis suggests that NCX1 is redistributed away from the outer sarcolemma. Cholera toxin uptake was increased 3-fold, suggesting that clathrin-independent endocytosis is enhanced. We conclude that direct effects of PIP(2) to activate NCX1 can be strongly modulated by opposing mechanisms in intact cells that probably involve membrane cytoskeleton remodelling and membrane trafficking.

Glutamate stimulation increases hormone release in rat melanotrophs

In melanotrophs, neuroendocrine cells from the intermediate lobe of the rat pituitary gland, glutamate causes a rise in intracellular [Ca2+] suggesting the presence of ionotropic NMDA and non-NMDA AMPA/K receptors. However, the Ca(2+)-dependent release of the major peptide hormone, alpha-melanocyte stimulating hormone (alpha-MSH), in response to glutamate stimulation has not been studied yet in this cell model. Significant spontaneous secretion of the peptide, which results in hormone deposits on the perimeter of the cells, has been confirmed by using confocal microscopy. Co-staining with a membrane area marker FM 1-43, which co-localized with the immunocytochemically marked hormone deposits, showed that fusion-competent sites on the plasma membrane coincided with secretion-competent sites. Stimulation of the cells with glutamate and high K+ saline induced a significant increase in the plasma membrane area covered with alpha-MSH deposits compared to control cells incubated with glutamate and CNQX, a glutamate channel blocker. The optical approach to monitor the secretory activity of a single neuroendocrine cell revealed that glutamate stimulates the release of alpha-MSH at distinct exocytotic membrane domains only.

Extracellular ATP stimulates exocytosis via localized Ca(2+) release from acidic stores in rat pancreatic beta cells

Three different methods, membrane capacitance (C(m)) measurement, amperometry and FM dye labeling were used to investigate the role of extracellular ATP in insulin secretion from rat pancreatic beta cells. We found that extracellular application of ATP mobilized intracellular Ca(2+) stores and synchronously triggered vigorous exocytosis. No influence of ATP on the readily releasable pool of vesicles was observed, which argues against a direct modulation of the secretory machinery at a level downstream of Ca(2+) elevation. The stimulatory effects of ATP were greatly reduced by intracellular perfusion of BAPTA but not EGTA, suggesting a close spatial association of fusion sites with intracellular Ca(2+) releasing sites. ATP-induced Ca(2+) transients and exocytosis were not blocked by thapsigargin (TG), by a ryanodine receptor antagonist or by dissipation of pH in acidic stores by monensin alone, but they were greatly attenuated by IP(3) receptor inhibition as well as ionomycin plus monensin, suggesting involvement of IP(3)-sensitive acidic Ca(2+) stores. Taken together, our data suggest that extracellular ATP triggers exocytosis by mobilizing spatially limited acidic Ca(2+) stores through IP(3) receptors. This mechanism may explain how insulin secretion from the pancreas is coordinated through diffusible ATP that is co-released with insulin.

Small-cell lung cancer (human): potentiation of endocytic membrane activity by voltage-gated Na(+) channel expression in vitro

The possible functional role of voltage-gated Na(+) channel (VGSC) expression in controlling endocytic membrane activity in human small-cell lung cancer (SCLC) cell lines (H69, H209, H510) was studied using uptake of horseradish peroxidase (HRP). The normal human airway epithelial (16HBE14o) cell line was used in a comparative approach. Uptake of HRP was vesicular, strongly temperature-sensitive and suppressed by cytoskeletal poisons (cytochalasin D and colchicine), consistent with endocytosis. Compared with the normal cells, HRP uptake into SCLC cells was kinetically more efficient, resulting in more than four-fold higher uptake under optimized conditions. Importantly, HRP uptake into SCLC cells was inhibited significantly by the specific VGSC blocker tetrodotoxin, as well as lidocaine and phenytoin. These effects were dose-dependent. None of these drugs had any effect on the uptake into the 16HBE14o cells. Uptake of HRP into SCLC cells was reduced by approximately 66% in Na(+)-free medium and was partially ( approximately 30%) dependent on extracellular Ca(2+). The possibility that the endocytic activity in the H510 SCLC cells involved an endogenous cholinergic system was investigated by testing the effects of carbachol (a cholinergic receptor agonist) and eserine (an inhibitor of acetylcholinesterase). Both drugs inhibited HRP uptake, thereby suggesting that basal cholinergic activity occurred. It is concluded that VGSC upregulation could enhance metastatic cell behavior in SCLC by enhancing endocytic membrane activity.

Monitoring Activity-Dependent Peptide Release from the CNS Using Single-Bead Solid-Phase Extraction and MALDI TOF MS Detection

To investigate dynamic peptidergic cell-cell communication, single micrometer-sized solid-phase extraction (SPE) beads were used to collect peptides from specific locations of well-characterized neurosecretory structures and even individual neuronal processes for off-line MALDI MS analyses. Peptide binding parameters of single SPE beads, including limits of collection, detection, and saturation capacity, were tested with 14C-labeled cytochrome c as well as with mixtures of multiple neuropeptides (bradykinin, Aplysia acidic peptide 1-20, and insulin). MALDI MS detection of secreted peptides was demonstrated in two well-characterized neurosecretory structures, the rat pituitary gland and single cultured Aplysia bag cell neurons. With cultured cells, precise placement of SPE beads allowed peptide collection from distinct neurites with spatial localization on the order of 200 microm, and SPE beads could be replaced within time frames that allowed analyte collection before and after cell stimulation paradigms. Comparison between pre- and poststimulation peptide profiles in both model systems allowed a directed strategy to determine which compounds were released with neuronal activity. Single SPE bead MALDI MS offers a novel approach to investigate peptide signaling that allows the detection and discovery of unknown intercellular signals secreted from a large variety of biological tissues.

Inhibition of cellular responses to insulin in a rat liver cell line. A role for PKC in insulin resistance

The initial response of liver cells to insulin is mediated through exocytosis of Cl- channel-containing vesicles and a subsequent opening of plasma membrane Cl- channels. Intracellular accumulation of fatty acids leads to profound defects in metabolism, and is closely associated with insulin resistance. It is not known whether the activity of Cl- channels is altered in insulin resistance and by which mechanisms. We studied the effects of fatty acid accumulation on Cl- channel opening in a model liver cell line. Overnight treatment with amiodarone increased the fat content by approximately 2-fold, and the rates of gluconeogenesis by approximately 5-fold. The ability of insulin to suppress gluconeogenesis was markedly reduced indicating that amiodarone treatment induces insulin resistance. Western blot analysis showed that these cells express the same number of insulin receptors as control cells. However, insulin failed to activate exocytosis and Cl- channel opening. These inhibitory effects were mimicked in control cells by exposures to arachidonic acid (15 microm). Further studies demonstrated that fatty acids stimulate the PKC activity, and inhibition of PKC partially restored exocytosis and Cl- channel opening in insulin-resistant cells. Accordingly, activation of PKC with PMA in control cells potently inhibited the insulin responses. These results suggest that stimulation of PKC activity in insulin resistance contributes to the inhibition of cellular responses to insulin in liver cells.

Retention of peptide hormones during partial secretion in pituitary somatotrophs and corticotrophs

The secretion of peptide hormones during exocytosis of an individual vesicle can result in either complete discharge of vesicle content or can occur in a partial manner in which some hormone is retained during transient fusion. In anterior pituitary lactotrophs, the retained hormone prolactin was internalized and recycled into a pool of vesicles that underwent preferential use during subsequent exocytic stimulations [Bauer et al., (2004) J Cell Sci. 117:2193-2202]. The aim of the present study was to determine whether retention and preferential recycling of retained hormones occurred in other anterior pituitary cells. Stimulation of somatotrophs with high K+ resulted in 50 discrete puncta per cell that were positive for growth hormone immunoreactivity. Identical stimulation of corticotrophs resulted in 150 puncta per cell that were anti-adrenocorticotrophic hormone (ACTH) positive. However, unlike what was observed for lactotrophs, the number of structures containing retained growth hormone and ACTH decreased to less than 10% of the initial value in 80 min in somatotrophs and in less than 10 min in corticotrophs. Our results indicate that functional recycling of retained hormones is not shared by all anterior pituitary cell types.

Retention and stimulus-dependent recycling of dense core vesicle content in neuroendocrine cells

We have used fluorescence imaging of individual exocytic events in combination with immunogold electron microscopy and FM1-43 photoconversion to study the stimulus-dependent recycling of dense core vesicle content in isolated rat pituitary lactotrophs. Secretory stimulation with high external [K+] resulted in 100 exocytic sites per cell that were labeled by extracellular antibodies against the peptide hormone prolactin. Morphological analysis demonstrated that the prolactin was retained and internalized in intact dense cores. Vesicles containing non-secreted, internalized prolactin did not colocalize with DiI-LDL that had been chased into lysosomes but did transiently colocalize with internalized transferrin. The recycling vesicles also trafficked through a syntaxin 6-positive compartment but not the TGN38-positive trans-Golgi. Recycling vesicles, which returned to the cell surface in a slow basal manner, could also be stimulated to undergo exocytosis with a high release probability during subsequent exocytic stimulation with external K+. These studies suggest a functional role for recycling vesicles that retain prolactin.

Contribution of functional voltage-gated Na+ channel expression to cell behaviors involved in the metastatic cascade in rat prostate cancer: II. Secretory membrane activity

The secretory membrane activities of two rat prostate cancer cell lines of markedly different metastatic potential, and corresponding electrophysiological characteristics, were studied in a comparative approach. In particular, voltage-gated Na(+) channels (VGSCs) were expressed in the strongly metastatic MAT-LyLu but not in the closely related, but weakly metastatic, AT-2 cells. Uptake and release of the non-cytotoxic marker horseradish peroxidase (HRP) were used as indices of general endocytotic and exocytotic membrane activity, respectively. The amount of tracer present in a given experimental condition was quantified by light microscopic digital imaging. The uptake of HRP was an active process, abolished completely by incubating the cells at low temperature (5 degrees C) and suppressed by disrupting the cytoskeleton. Interestingly, the extent of HRP uptake into the strongly metastatic MAT-LyLu cells was almost twice that into the weakly metastatic AT-2 cells. Vesicular uptake of HRP occurred in a fast followed by a slow phase; these appeared to correspond to cytoplasmic and perinuclear pools, respectively. Importantly, the overall quantitative difference in the uptake disappeared in the presence of 1 microM tetrodotoxin which significantly reduced the uptake of HRP into the MAT-LyLu cells. There was no effect on the AT-2 cells, consistent with functional VGSC expression occurring selectively in the former. A similar effect was observed in Na(+)-free medium. The uptake was partially dependent upon extracellular Ca(2+) but was not affected by raising the extracellular K(+) concentration. We suggest that functional VGSC expression could potentiate prostate cancer cells' metastatic ability by enhancing their secretory membrane activity.

Calcium-dependent exocytosis of atrial natriuretic peptide from astrocytes

Astrocytes are non-neuronal cells in the CNS, which, like neurons, are capable of releasing neuroactive molecules. However, the mechanism of release is ill defined. In this study, we investigated the mechanism of release of atrial natriuretic peptide (ANP) from cultured cortical astrocytes by confocal microscopy. To study the discharge of this hormone, we transfected astrocytes with a construct to express pro-ANP fused with the emerald green fluorescent protein (ANP.emd). The transfection of cells with ANP.emd resulted in fluorescent puncta in the cytoplasm that represent secretory organelles. If ANP is released by exocytosis, in which the vesicle fuses with the plasma membrane, then the total intensity of the green fluorescing probe should decrease, whereas the vesicle membrane is incorporated into the plasma membrane. To monitor exocytosis, we labeled the membrane with the fluorescent styryldye FM 4-64, a reporter of cumulative exocytosis. The application of ionomycin to elevate cytoplasmic [Ca(2+)] increased the fluorescence intensity of FM 4-64, whereas that of ANP.emd decreased. These effects were not observed in the absence of extracellular Ca(2+), suggesting that ANP is released by regulated Ca(2+)-dependent exocytosis from astrocytes.

Somatic exocytosis of serotonin mediated by L-type calcium channels in cultured leech neurones

We studied somatic exocytosis of serotonin and its mediation by L-type calcium (Ca2+) channels in cultured Retzius neurones of the leech. Exocytosis was induced by trains of impulses at different frequencies or by depolarisation with 40 mM potassium (K+), and was quantified by use of the fluorescent dye FM 1-43. Stimulation increased the membrane fluorescence and produced a pattern of FM 1-43 fluorescent spots of 1.28 +/- 0.01 microm in diameter, provided that Ca2+ was present in the bathing fluid. Individual spots lost their stain during depolarisation with 40 mM K+. Electron micrographs showed clusters of dense core vesicles, some of which were in contact with the cell membrane. Presynaptic structures with clear vesicles were absent from the soma. The number of fluorescent spots per soma, but not their diameter or their fluorescence intensity, depended on the frequency of stimulation. Trains at 1 Hz produced 19.5 +/- 5 spots per soma, 77.9 +/- 13.9 spots per soma were produced at 10 Hz and 91.5 +/- 16.9 spots per soma at 20 Hz. Staining patterns were similar for neurones in culture and in situ. In the presence of the L-type Ca2+ channel blocker nimodipine (10 microM), a 20 Hz train produced only 22.9 +/- 6.4 spots per soma, representing a 75 % reduction compared to control cells (P < 0.05). Subsequent incubation with 10 mM caffeine to induce Ca2+ release from intracellular stores increased the number of spots to 73.22 +/- 12.5. Blockers of N-, P-, Q- or invertebrate Ca2+ channels did not affect somatic exocytosis. Our results suggest that somatic exocytosis by neurones shares common mechanisms with excitable endocrine cells.

Calcium-dependent exocytosis of atrial natriuretic peptide from astrocytes

Astrocytes are non-neuronal cells in the CNS, which, like neurons, are capable of releasing neuroactive molecules. However, the mechanism of release is ill defined. In this study, we investigated the mechanism of release of atrial natriuretic peptide (ANP) from cultured cortical astrocytes by confocal microscopy. To study the discharge of this hormone, we transfected astrocytes with a construct to express pro-ANP fused with the emerald green fluorescent protein (ANP.emd). The transfection of cells with ANP.emd resulted in fluorescent puncta in the cytoplasm that represent secretory organelles. If ANP is released by exocytosis, in which the vesicle fuses with the plasma membrane, then the total intensity of the green fluorescing probe should decrease, whereas the vesicle membrane is incorporated into the plasma membrane. To monitor exocytosis, we labeled the membrane with the fluorescent styryldye FM 4-64, a reporter of cumulative exocytosis. The application of ionomycin to elevate cytoplasmic [Ca(2+)] increased the fluorescence intensity of FM 4-64, whereas that of ANP.emd decreased. These effects were not observed in the absence of extracellular Ca(2+), suggesting that ANP is released by regulated Ca(2+)-dependent exocytosis from astrocytes.

Optical monitoring of synaptic vesicle trafficking in ribbon synapses

Synaptic transmission constitutes the major basis of communication among nerve cells. Upon nerve terminal depolarisation, calcium influx triggers the exocytosis of synaptic vesicles at active zones. Vesicles are then retrieved by endocytosis, recycled and refilled with neurotransmitter. Fluorescent styryl dyes have proven very useful as tools for studying several aspects of the synaptic vesicle cycle. Here, we review recent imaging studies using styryl FM dyes and bipolar cells of goldfish retina, which have a giant synaptic terminal containing ribbon-type active zones. Optical techniques applied to this unique synaptic terminal have provided novel insights into the trafficking of synaptic vesicles during and following strong stimulation.

Exocytosis in bovine chromaffin cells: studies with patch-clamp capacitance and FM1-43 fluorescence

In response to physiological stimuli, neuroendocrine cells secrete neurotransmitters through a Ca(2+)-dependent fusion of secretory granules with the plasma membrane. We studied insertion of granules in bovine chromaffin cells using capacitance as a measure of plasma membrane area and fluorescence of a membrane marker FM1-43 as a measure of exocytosis. Intracellular dialysis with [Ca(2+)] (1.5-100 microM) evoked massive exocytosis that was sufficient to double plasma membrane area but did not swell cells. In principle, in the absence of endocytosis, the addition of granule membrane would be anticipated to produce similar increases in the capacitance and FM1-43 fluorescence responses. However, when endocytosis was minimal, the changes in capacitance were markedly larger than the corresponding changes in FM1-43 fluorescence. Moreover, the apparent differences between capacitance and FM1-43 fluorescence changes increased with larger exocytic responses, as more granules fused with the plasma membrane. In experiments in which exocytosis was suppressed, increasing membrane tension by osmotically induced cell swelling increased FM1-43 fluorescence, suggesting that FM1-43 fluorescence is sensitive to changes in the membrane tension. Thus, increasing membrane area through exocytosis does not swell chromaffin cells but may decrease membrane tension.

Heterotrimeric G-proteins activate Cl- channels through stimulation of a cyclooxygenase-dependent pathway in a model liver cell line

Circulating hormones produce rapid changes in the Cl(-) permeability of liver cells through activation of plasma membrane receptors coupled to heterotrimeric G-proteins. The resulting effects on intracellular pH, membrane potential, and Cl(-) content are important contributors to the overall metabolic response. Consequently, the purpose of these studies was to evaluate the mechanisms responsible for G-protein-mediated changes in membrane Cl(-) permeability using HTC hepatoma cells as a model. Using patch clamp techniques, intracellular dialysis with 0.3 mm guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) increased membrane conductance from 10 to 260 picosiemens/picofarads due to activation of Ca(2+)-dependent Cl(-) currents that were outwardly rectifying and exhibited slow activation at depolarizing potentials. These effects were mimicked by intracellular AlF(4)(-) (0.03 mm) and inhibited by pertussis toxin (PTX), consistent with current activation through Galpha(i). Studies using defined agonists and inhibitors indicate that Cl(-) channel activation by GTPgammaS occurs through an indomethacin-sensitive pathway involving sequential activation of phospholipase C, mobilization of Ca(2+) from inositol 1,4,5-trisphosphate-sensitive stores, and stimulation of phospholipase A(2) and cyclooxygenase (COX). Accordingly, the conductance responses to GTPgammaS or to intracellular Ca(2+) were inhibited by COX inhibitors. These results indicate that PTX-sensitive G-proteins regulate the Cl(-) permeability of HTC cells through Ca(2+)-dependent stimulation of COX activity. Thus, receptor-mediated activation of Galpha(i) may be essential for hormonal regulation of liver transport and metabolism through COX-dependent opening of a distinct population of plasma membrane Cl(-) channels.