Ca(2+)-independent regulation of neurosecretion by intracellular Na+

Sodium channel NaV1.3 is important for enterochromaffin cell excitability and serotonin release

In the gastrointestinal (GI) epithelium, enterochromaffin (EC) cells are enteroendocrine cells responsible for producing >90% of the body's serotonin (5-hydroxytryptamine, 5-HT). However, the molecular mechanisms of EC cell function are poorly understood. Here, we found that EC cells in mouse primary cultures fired spontaneous bursts of action potentials. We examined the repertoire of voltage-gated sodium channels (Na_V) in fluorescence-sorted mouse EC cells and found that Scn3a was highly expressed. Scn3a-encoded Na_V1.3 was specifically and densely expressed at the basal side of both human and mouse EC cells. Using electrophysiology, we found that EC cells expressed robust Na_V1.3 currents, as determined by their biophysical and pharmacologic properties. Na_V1.3 was not only critical for generating action potentials in EC cells, but it was also important for regulating 5-HT release by these cells. Therefore, EC cells use Scn3a-encoded voltage-gated sodium channel Na_V1.3 for electrical excitability and 5-HT release. Na_V1.3-dependent electrical excitability and its contribution to 5-HT release is a novel mechanism of EC cell function.

Mechanism of alpha-latrotoxin action at nerve endings of neurohypophysis

The neurotoxin alpha-latrotoxin elicits spontaneous exocytosis of neurotransmitter from neurons and peptide hormones from endocrine cells. While the mechanism of action is not fully understood, both Ca(2+)-dependent and Ca(2+)-independent pathways participate in the facilitation of release, with the relative contribution of the pathways differing among neuronal and endocrine cell types. Here, we investigate the actions of alpha-latrotoxin on neuroendocrine nerve endings that emanate from central nervous system neurons and, therefore, are unique in that they possess properties of central nerve endings and endocrine cells. Using intracellular [Ca(2+)] measurements both calcium-independent receptors for latrotoxin (CIRL or latrophilin) and neurexin 1 alpha receptors were found to be functionally present. Interaction of alpha-latrotoxin with these receptors stimulated secretion of vasopressin and oxytocin neuropeptide. The secretory response was entirely dependent upon toxin-mediated extracellular Ca(2+) influx, although alpha-latrotoxin also consistently triggered mobilization of Ca(2+) from an intracellular store. The mobilization of intracellular Ca(2+) relied on alpha-latrotoxin-mediated Na(+) influx and was blocked by the protonophore FCCP, thereby implicating mitochondria as the Ca(2+) store being mobilized. Using the whole cell recording configuration of the patch clamp, we report that alpha-latrotoxin interaction with the CIRL receptor on these nerve endings resulted in ionic pore formation, generating unitary inward current steps of 20 pA and a channel conductance of approximately 220 pS in Ca(2+)-free saline. Thus, alpha-latrotoxin stimulates Ca(2+)-dependent exocytosis in neurohypophysial nerve endings through receptor interaction and insertion of Ca(2+) permeable membrane pores. While alpha-latrotoxin mobilizes intracellular Ca(2+) stores the elevation in [Ca(2+)] reached is insufficient to trigger measurable exocytosis.

Effects of depolarization evoked Na+ influx on intracellular Na+ concentration at neurosecretory nerve endings

Electrophysiological measurements of voltage-dependent Na+ influx using patch-clamp methodology were combined with optical monitoring of the free intracellular Na+ concentration in isolated rat neurohypophysial nerve endings to determine the relationship between Na+ influx generated by repetitive stimulation and change in [Na+]i. Application of step depolarizations under voltage-clamp-evoked tetrodotoxin-sensitive inward currents that were dependent upon extracellular Na+ and that exhibited rapid activation and inactivation properties. These characteristics substantiated the evoked current as a voltage-dependent Na+ current. Application of stimulus trains consisting of step depolarizations that mimick in frequency and duration those of action potentials were found to result in increases in [Na+]i. The induced change in [Na+]i was found to be related to the frequency and period of stimulation. Changes in [Na+]i were greatest at frequencies of 40 Hz and gave maximal changes with 30 s of continuous stimulation of approximately 2.4 mM. Sodium influx expressed as a molar quantity resulted in a nearly directly proportional increase in [Na+]i during the initial period of stimulation at low Na+ loads. When expressed as a charge density (pC/microm2) Na+ influx was found to increase with smaller diameter nerve endings as did the rate of change in [Na+]i in response to applied repetitive step depolarizations. Repetitive step depolarizations which simulate impulse activity that invade neuroendocrine nerve endings in vivo in response to physiological demand for hormone secretion resulted in an increased [Na+]i. It is postulated that this increased [Na+]i may provide a modulatory influence on the secretory response indirectly via alteration of intracellular calcium regulation or, perhaps, via a direct action on the secretory mechanism.

Sodium-dependent increase in quantal secretion induced by brevetoxin-3 in Ca2+-free medium is associated with depletion of synaptic vesicles and swelling of motor nerve terminals in situ

Brevetoxin-3 at nanomolar concentrations markedly enhanced spontaneous quantal transmitter release from neuromuscular junctions equilibrated in a Ca2+-free EGTA medium. After about 3 h, the sustained increase in miniature endplate potential frequency led to an exhaustion of transmitter release. This increase still occurred after loading the nerve terminals with the Ca2+ chelator bis-(aminophenoxy)ethanetetra-acetate or after pretreatment with various pharmacological agents known to prevent Ca2+ release from intracellular pools, but was completely prevented by the Na+ channel blocker tetrodotoxin. Brevetoxin-3 also increased miniature endplate potential frequency from junctions treated with botulinum type-A toxin, but to a smaller extent than at normal junctions. At normal junctions, brevetoxin-3 exposure for 2 h increased the three-dimensional projected area of living motor nerve terminals in situ by about 74% while at botulinum type-A poisoned junctions a similar toxin exposure caused only a 29% increase. Tetrodotoxin prevented such effects, indicating that they are related to both Na+ entry into the terminals and increased quantal transmitter release. Ultrastructural examination of nerve terminals from junctions exposed for 3 h to brevetoxin-3 revealed profound depletions of clear and large dense core synaptic vesicles and an increase in coated vesicles and axolemma infoldings. These results indicate that brevetoxin-3 impairs the recycling of clear synaptic vesicles and are consistent with our immunofluorescent observations showing that synaptophysin epitopes can be revealed without nerve terminal permeabilization. In contrast, no such changes were detected in nerve terminals poisoned with botulinum type-A toxin which, after 3 h exposure to brevetoxin-3, retained their synaptic vesicles and had a normal appearance. We conclude that tetrodotoxin-sensitive Na+ entry into motor nerve terminals induced by brevetoxin-3 triggers external Ca2+-independent asynchronous quantal transmitter release, blocks synaptic vesicle recycling and induces swelling of the terminals. We suggest that an excess of cytoplasmic Na+ per se can activate the asynchronous neurotransmitter release process.

A role of intracellular Na+ in the regulation of synaptic transmission and turnover of the vesicular pool in cultured hippocampal cells

Propagation of action potentials in axons and dendrites increases intracellular Na+ ([Na+]i) and Ca2+ concentrations ([Ca2+]i). While the importance of [Ca2+]i in synaptic transmission is well established, a possible functional role of [Na+]i is unclear. In cultured hippocampal cells, [Na+]i was increased by veratridine. We have then measured spontaneous excitatory postsynaptic currents (sEPSCs) and, by means of fluorescent dyes, changes in [Na+]i, in [Ca2+]i, and in the turnover of the vesicular pool of individual boutons. An elevation of [Na+]i and a concomitant rise in [Ca2+]i, led to a large increase in sEPSC frequency and in the turnover of the presynaptic vesicular pool. Extracellular Ca2+ was essential for these effects of elevated [Na+]i on synaptic transmission. They probably occur via Na+/Ca2+ exchange.

Activity-induced internalization and rapid degradation of sodium channels in cultured fetal neurons

A regulatory mechanism for neuronal excitability consists in controlling sodium channel density at the plasma membrane. In cultured fetal neurons, activation of sodium channels by neurotoxins, e.g., veratridine and alpha-scorpion toxin (alpha-ScTx) that enhance the channel open state probability induced a rapid down-regulation of surface channels. Evidence that the initial step of activity-induced sodium channel down-regulation is mediated by internalization was provided by using 125I-alpha-ScTx as both a channel probe and activator. After its binding to surface channels, the distribution of 125I-alpha-ScTx into five subcellular compartments was quantitatively analyzed by EM autoradiography. 125I-alpha-ScTx was found to accumulate in tubulovesicular endosomes and disappear from the cell surface in a time-dependent manner. This specific distribution was prevented by addition of tetrodotoxin (TTX), a channel blocker. By using a photoreactive derivative to covalently label sodium channels at the surface of cultured neurons, we further demonstrated that they are degraded after veratridine-induced internalization. A time-dependent decrease in the amount of labeled sodium channel alpha subunit was observed after veratridine treatment. After 120 min of incubation, half of the alpha subunits were cleaved. This degradation was prevented totally by TTX addition and was accompanied by the appearance of an increasing amount of a 90-kD major proteolytic fragment that was already detected after 45-60 min of veratridine treatment. Exposure of the photoaffinity-labeled cells to amphotericin B, a sodium ionophore, gave similar results. In this case, degradation was prevented when Na+ ions were substituted by choline ions and not blocked by TTX. After veratridine- or amphotericin B-induced internalization of sodium channels, breakdown of the labeled alpha subunit was inhibited by leupeptin, while internalization was almost unaffected. Thus, cultured fetal neurons are capable of adjusting sodium channel density by an activity-dependent endocytotic process that is triggered by Na+ influx.

Sodium-evoked, calcium-independent vasopressin release from rat isolated neurohypophysial nerve endings

1. The effects of Na+ on vasopressin release and on redistribution of Ca2+, Na+ and H+ in isolated rat neurohypophysial nerve endings have been studied. 2. Substituting Na+ for a non-permanent cation produced a pronounced and sustained release of vasopressin. This increase occurred in the absence of external Ca2+ and in nerve endings loaded with the Ca2+ chelator dimethyl-BAPTA (1,2-bis-(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid). 3. The effect of Na+ was independent of a rise in intracellular Ca2+ as judged by the measurement of [Ca2+]i using the indicator fura-2 and 45Ca2+ efflux studies. Although Na+ could release Ca2+ from internal reservoirs the small elevation in [Ca2+]i induced by Na+ could not explain the large and sustained increase in vasopressin secretion. 4. The channel blockers TTX (tetrodotoxin), D888 (desmethyoxyverapamil), N144 (5-nitro-2-(phenylpropylamino)-benzoic acid) or SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) could not prevent the Na(+)-dependent increase in vasopressin release. Similarly this increase was not affected by metabolic inhibitors (Ruthenium Red and KCN) nor by CCCP (carbonyl cyanide m-chlorophenylhydrazone), an uncoupler of oxidative phosphorylation. 5. Selectivity among monovalent cations to promote secretion was found with the largest effect on the secretory response being produced by Na+. Similarly Cl- was found to be the most potent anion studied for inducing, in the presence of Na+, an increase in neurohormone release. 6. Measuring [Na+]i by means of the Na+ indicator SBFI showed that the extent of the secretory response was correlated with the intraterminal Na+ concentration. 7. The Na(+)-induced, Ca(2+)-independent release of vasopressin occurred by exocytosis as judged (i) by the linear relationship between the amount of vasopressin secreted and that of the co-localized neurophysin and (ii) by the demonstration that the extracellular marker horseradish peroxidase was only found in endocytotic vacuoles and not in the cytoplasm of the stimulated nerve endings. 8. The Na(+)-dependent secretory response found on addition of extracellular Na+ was not the result of the change in internal pH as measured with the indicator BCECF and as mimicked by addition of propionic acid. 9. Addition of Na+ to digitonin- or streptolysin-O-permeabilized nerve endings in the presence or absence of Ca2+ also gave rise to an increase in vasopressin secretion. 10. It is concluded that an increase in internal Na+ per se can promote, in the absence of a rise in intracellular Ca2+, an increase in neuropeptide secretion.

The calcium signal in human neutrophils and its relation to exocytosis investigated by patch-clamp capacitance and Fura-2 measurements

Intracellular calcium ([Ca2+]i) and exocytosis of human neutrophils were investigated with patch-clamp capacitance and Fura-2 fluorescence measurements. Intracellular application of GTP gamma S induces a calcium transient and exocytosis. The onset of degranulation occurs at the time where the maximal [Ca2+]i is reached. Despite the close correlation in time, buffering [Ca2+]i at the resting level or at approximately 2 microM leaves the extent and the time course of degranulation unchanged. The decay of the calcium transient is due to diffusional equilibration between the cytosol and the pipette volume. GTP gamma S activates no cellular mechanisms for Ca2+ reuptake or extrusion. The endogenous calcium buffer capacity can be estimated to be as low as that of approximately 90 microM Fura-2. Stimulation with fMLP also induces degranulation and a calcium transient. The decay of fMLP-induced calcium transients is much faster than that of GTP gamma S-induced transients and is independent of diffusion indicating that fMLP also induces rapid reuptake or extrusion of Ca2+. Degranulation but not the calcium transient requires the presence of intracellular GTP. Different signalling pathways appear to be involved in GTP gamma S- and fMLP-stimulated calcium signals. The intracellular calcium release is not an essential signal to initiate exocytosis in neutrophils.

A calcium influx is neither strictly associated with nor necessary for exocytotic membrane fusion in Paramecium cells

Exocytosis of trichocysts in Paramecium cells was generally believed to depend on extracellular Ca, since it is accompanied by a Ca influx and not seen in the absence of Ca. However, by short term removal of Ca we showed recently that only extrusion of secretory contents, but not membrane fusion after stimulation with aminoethyldextran (AED), depends on extracellular Ca. We have now extended these studies to longer times and shown that membrane fusion is stimulated by AED even after 1 min at low Ca (< or = 30 nM). At prolonged times membrane fusion was induced by sole removal of Ca. In the presence of AED, trichocyst contents were slowly extruded followed by resealing of the fused membranes, indicating independency of endocytotic membrane fusion from extracellular Ca (though we observed aberrant resealing). Later on, Ca removal is followed by cell death. By using videomicroscopy, we further provide the first evidence that exocytosis is not necessarily accompanied by an influx of Ca in the presence of the usual high concentrations (1 mM), since local exocytosis at the rear end of the cells is not followed by ciliary reversal which is triggered by Ca influx. We conclude that a Ca influx is neither regularly associated with, nor necessary for, induction of exocytotic membrane fusion in Paramecium cells. As a source for a possible alternative intracellular liberation of calcium during exocytosis, we analyzed the subplasmalemmal alveolar sac system by electron spectroscopic imaging and found indications for Ca redistributions shortly after stimulation.

Lack of involvement of [Ca2+]i in the external Ca(2+)-independent release of acetylcholine evoked by veratridine, ouabain and alpha-latrotoxin: possible role of [Na+]i

Synaptosomes were challenged by veratridine, ouabain and alpha-latrotoxin, and the release of 14C-acetylcholine (ACh) was measured in the absence of external Ca2+. We wished to test whether Ca2+ mobilized from internal stores triggered the ACh release that was independent of external Ca2+. We found that none of the agents altered the [Ca2+]i in a Ca(2+)-free medium. Buffering the intracellular Ca2+ concentration with BAPTA did not prevent the increase in release of 14C-ACh by veratridine or ouabain in the absence of Ca2+, however, it greatly reduced the release evoked in a Ca(2+)-containing medium. In parallel samples the release of ACh and the change in the internal Na+ concentration ([Na+]i) were measured. It was found that veratridine, ouabain and alpha-latrotoxin all enhanced [Na+]i in a concentration-dependent manner and a good quantitative relationship existed between the increase in [Na+]i and the release of ACh.

Formyl peptides and ATP stimulate Ca2+ and Na+ inward currents through non-selective cation channels via G-proteins in dibutyryl cyclic AMP-differentiated HL-60 cells. Involvement of Ca2+ and Na+ in the activation of beta-glucuronidase release and superoxide production

In human neutrophils, the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) induces increases in the intracellular free Ca2+ concentration ([Ca2+]i) with subsequent activation of beta-glucuronidase release and superoxide (O2-) production. Results from several laboratories suggest that the increase in [Ca2+]i is due to activation of non-selective cation (NSC) channels. We studied the biophysical characteristics, pharmacological modulation and functional role of NSC channels in dibutyryl cyclic AMP (Bt2cAMP)-differentiated HL-60 cells. fMLP increased [Ca2+]i by release of Ca2+ from intracellular stores and influx of Ca2+ from the extracellular space. fMLP also induced Mn2+ influx. Ca2+ and Mn2+ influxes were inhibited by 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SK&F 96365). Under whole-cell voltage-clamp conditions, fMLP and ATP (a purinoceptor agonist) activated inward currents characterized by a linear current-voltage relationship and a reversal potential near 0 mV. NSC channels were substantially more permeable to Na+ than to Ca2+. SK&F 96365 inhibited fMLP- and ATP-stimulated currents with a half-maximal effect at about 3 microM. Pertussis toxin prevented stimulation by fMLP of NSC currents and reduced ATP-stimulated currents by about 80%. Intracellular application of the stable GDP analogue, guanosine 5'-O-[2-thio]diphosphate, completely blocked stimulation by agonists of NSC currents. In excised inside-out patches, single channel openings with an amplitude of 0.24 pA were observed in the presence of fMLP and the GTP analogue, guanosine 5'-O-[3-thio]triphosphate. The bath solution contained neither Ca2+ nor ATP. The current/voltage relationship was linear with a conductance of 4-5 pS and reversed at about 0 mV. fMLP-induced beta-glucuronidase release and O2- production were substantially reduced by replacement of extracellular CaCl2 or NaCl by ethylenebis(oxyethylenenitrilo)tetra-acetic acid and choline chloride respectively. In the absence of Ca2+ and Na+, fMLP was ineffective. SK&F 96365 inhibited fMLP-induced beta-glucuronidase release and O2- production in the presence of both Ca2+ and Na+, and in the presence of Ca2+ or Na+ alone. NaCl (25-50 mM) enhanced the basal and absolute extent of fMLP-stimulated GTP hydrolysis of heterotrimeric regulatory G-proteins in HL-60 membranes. The order of effectiveness of salts in enhancing GTP hydrolysis was LiCl > KCl > NaCl > choline chloride.(ABSTRACT TRUNCATED AT 400 WORDS)

Endothelin regulation of neuropeptide release from nerve endings of the posterior pituitary

We have investigated the role of endothelin (ET) in the stimulus-secretion coupling mechanism in the posterior pituitary. We report that isolated nerve endings contain immunoreactive endothelin, the level of which is regulated by homeostatic mechanisms involved in control of water balance. ET-1 and ET-3 potentiate vasopressin release induced by depolarization through interaction with specific receptors of the ETA subtype and this response is antagonized by sarafotoxin S6b. The second messenger for this effect, however, remains unknown since the potentiation of depolarization-induced vasopressin release occurs in the absence of an increase in cellular calcium.

Exocytosis and its control at the synapse

Several new approaches have given fresh insight into the mechanism and control of exocytosis. Electrophysiological and morphological studies show that many or all of the intramembrane particles at presynaptic active zones are voltage-gated Ca2+ channels. The sensitivity and time resolution of voltammetry allow the time course with which a single vesicle releases transmitter to be studied. Membrane proteins of the cell surface and synaptic vesicles have been shown to interact, and may join to form the fusion-pore complex.