Inositol 1,4,5-trisphosphate-dependent oscillations of luminal [Ca2+] in permeabilized HSY cells

Local anesthetics inhibit muscarinic acetylcholine receptor-mediated calcium responses and the recruitment of β-arrestin in HSY human parotid cells

OBJECTIVES

Local anesthetics act on G protein-coupled receptors (GPCRs); thus, their potential as allosteric modulators of GPCRs has attracted attention. Intracellular signaling via GPCRs involves both G-protein- and β-arrestin-mediated pathways. To determine the effects of local anesthetics on muscarinic acetylcholine receptors (mAChR), a family of GPCRs, we analyzed the effects of local anesthetics on mAChR-mediated Ca2+ responses and formation of receptor-β-arrestin complexes in the HSY human parotid cell line.

METHODS

Ca2+ responses were monitored by fura-2 spectrofluorimetry. Ligand-induced interactions between mAChR and β-arrestin were examined using a β-arrestin GPCR assay kit.

RESULTS

Lidocaine reduced mAChR-mediated Ca2+ responses but did not change the intracellular Ca2+ concentration in non-stimulated cells. The membrane-impermeant lidocaine analog QX314 and procaine inhibited mAChR-mediated Ca2+ responses, with EC50 values of 48.0 and 20.4 μM, respectively, for 50 μM carbachol-stimulated Ca2+ responses. In the absence of extracellular Ca2+, the pretreatment of cells with QX314 reduced carbachol-induced Ca2+ release, indicating that QX314 reduced Ca2+ release from intracellular stores. Lidocaine and QX314 did not affect store-operated Ca2+ entry as they did not alter the thapsigargin-induced Ca2+ response. QX314 and procaine reduced the carbachol-mediated recruitment of β-arrestin, and administration of procaine suppressed pilocarpine-induced salivary secretion in mice.

CONCLUSION

Local anesthetics, including QX314, act on mAChR to reduce carbachol-induced Ca2+ release from intracellular stores and the recruitment of β-arrestin. These findings support the notion that local anesthetics and their derivatives are starting points for the development of functional allosteric modulators of mAChR.

[Advances in methods for analyzing IP3 signaling and understanding of coupled Ca2+ and IP3 oscillations]

Inositol 1,4,5-trisphosphate (IP₃) is an important intracellular messenger produced by phospholipase C via the activation of G-protein-coupled receptor- or receptor-tyrosine-kinase-mediated pathways, and is involved in numerous responses to hormones, neurotransmitters, and growth factors through the releases of Ca²⁺ from intracellular stores via IP₃ receptors. IP₃-mediated Ca²⁺ signals often exhibit complex spatial and temporal organizations, such as Ca²⁺ oscillations. Recently, new methods have become available to measure IP₃ concentration ([IP₃]) using AlphaScreen technology, fluorescence polarization, and competitive ligand binding assay (CFLA). These methods are useful for the high throughput screening in drug discovery. Calcium ions generate versatile intracellular signals such as Ca²⁺ oscillations and waves. Fluorescent sensors molecules to monitor changes in [IP₃] in single living cells are crucial to study the mechanism for the spatially and temporally regulated Ca²⁺ signals. In particular, FRET-based IP₃ sensors are useful for the quantitative monitoring intracellular [IP₃], and allowed to uncovered the oscillatory IP₃ dynamics in association with Ca²⁺ oscillations. A mathematical model of coupled Ca²⁺ and IP₃ oscillations predicts that Ca²⁺ oscillations are the result of modulation of the IP₃ receptor by intracellular Ca²⁺, and that the period is modulated by the accompanying IP₃ oscillations. These model predictions have also been confirmed experimentally. At present, however, usefulness of FRET-based IP₃ sensors are limited by their relatively small change in fluorescence. Development of novel IP₃ sensors with improve dynamic range would be important for understanding the regulatory mechanism of Ca²⁺ signaling and for in vivo IP₃ imaging.

A mathematical model of calcium dynamics in HSY cells

Saliva is an essential part of activities such as speaking, masticating and swallowing. Enzymes in salivary fluid protect teeth and gums from infectious diseases, and also initiate the digestion process. Intracellular calcium (Ca2+) plays a critical role in saliva secretion and regulation. Experimental measurements of Ca2+ and inositol trisphosphate (IP3) concentrations in HSY cells, a human salivary duct cell line, show that when the cells are stimulated with adenosine triphosphate (ATP) or carbachol (CCh), they exhibit coupled oscillations with Ca2+ spike peaks preceding IP3 spike peaks. Based on these data, we construct a mathematical model of coupled Ca2+ and IP3 oscillations in HSY cells and perform model simulations of three different experimental settings to forecast Ca2+ responses. The model predicts that when Ca2+ influx from the extracellular space is removed, oscillations gradually slow down until they stop. The model simulation of applying a pulse of IP3 predicts that photolysis of caged IP3 causes a transient increase in the frequency of the Ca2+ oscillations. Lastly, when Ca2+-dependent activation of PLC is inhibited, we see an increase in the oscillation frequency and a decrease in the amplitude. These model predictions are confirmed by experimental data. We conclude that, although concentrations of Ca2+ and IP3 oscillate, Ca2+ oscillations in HSY cells are the result of modulation of the IP3 receptor by intracellular Ca2+, and that the period is modulated by the accompanying IP3 oscillations.

The Effect of Gap Junctional Coupling on the Spatiotemporal Patterns of Ca2+ Signals and the Harmonization of Ca2+-Related Cellular Responses

Calcium ions (Ca²⁺) are important mediators of a great variety of cellular activities e.g. in response to an agonist activation of a receptor. The magnitude of a cellular response is often encoded by frequency modulation of Ca²⁺ oscillations and correlated with the stimulation intensity. The stimulation intensity highly depends on the sensitivity of a cell to a certain agonist. In some cases, it is essential that neighboring cells produce a similar and synchronized response to an agonist despite their different sensitivity. In order to decipher the presumed function of Ca²⁺ waves spreading among connecting cells, a mathematical model was developed. This model allows to numerically modifying the connectivity probability between neighboring cells, the permeability of gap junctions and the individual sensitivity of cells to an agonist. Here, we show numerically that strong gap junctional coupling between neighbors ensures an equilibrated response to agonist stimulation via formation of Ca²⁺ phase waves, i.e. a less sensitive neighbor will produce the same or similar Ca²⁺ signal as its highly sensitive neighbor. The most sensitive cells within an ensemble are the wave initiator cells. The Ca²⁺ wave in the cytoplasm is driven by a sensitization wave front in the endoplasmic reticulum. The wave velocity is proportional to the cellular sensitivity and to the strength of the coupling. The waves can form different patterns including circular rings and spirals. The observed pattern depends on the strength of noise, gap junctional permeability and the connectivity probability between neighboring cells. Our simulations reveal that one highly sensitive region gradually takes the lead within the entire noisy system by generating directed circular phase waves originating from this region.

The calcium ion (Ca²⁺), a universal signaling molecule, is widely recognized to play a fundamental role in the regulation of various biological processes. Agonist–evoked Ca²⁺ signals often manifest as rhythmic changes in the cytosolic free Ca²⁺ concentration (c_cyt) called Ca²⁺ oscillations. Stimuli intensity was found to be proportional to the oscillation frequency and the evoked down-steam cellular response. Stochastic receptor expression in individual cells in a cell population inevitably leads to individually different oscillation frequencies and individually different Ca²⁺-related cellular responses. However, in many organs, the neighboring cells have to overcome their individually different sensitivity and produce a synchronized response. Gap junctions are integral membrane structures that enable the direct cytoplasmic exchange of Ca²⁺ ions and InsP₃ molecules between neighboring cells. By simulations, we were able to demonstrate how the strength of intercellular gap junctional coupling in relation to stimulus intensity can modify the spatiotemporal patterns of Ca²⁺ signals and harmonize the Ca²⁺-related cellular responses via synchronization of oscillation frequency. We demonstrate that the most sensitive cells are the wave initiator cells and that a highly sensitive region plays an important role in the determination of the Ca²⁺ phase wave direction. This sensitive region will then also progressively determine the global behavior of the entire system.

Routes of Ca2+ Shuttling during Ca2+ Oscillations: FOCUS ON THE ROLE OF MITOCHONDRIAL Ca2+ HANDLING AND CYTOSOLIC Ca2+ BUFFERS

In some cell types, Ca²⁺ oscillations are strictly dependent on Ca²⁺ influx across the plasma membrane, whereas in others, oscillations also persist in the absence of Ca²⁺ influx. We observed that, in primary mesothelial cells, the plasmalemmal Ca²⁺ influx played a pivotal role. However, when the Ca²⁺ transport across the plasma membrane by the “lanthanum insulation method” was blocked prior to the induction of the serum-induced Ca²⁺ oscillations, mitochondrial Ca²⁺ transport was found to be able to substitute for the plasmalemmal Ca²⁺ exchange function, thus rendering the oscillations independent of extracellular Ca²⁺. However, in a physiological situation, the Ca²⁺-buffering capacity of mitochondria was found not to be essential for Ca²⁺ oscillations. Moreover, brief spontaneous Ca²⁺ changes were observed in the mitochondrial Ca²⁺ concentration without apparent changes in the cytosolic Ca²⁺ concentration, indicating the presence of a mitochondrial autonomous Ca²⁺ signaling mechanism. In the presence of calretinin, a Ca²⁺-buffering protein, the amplitude of cytosolic spikes during oscillations was decreased, and the amount of Ca²⁺ ions taken up by mitochondria was reduced. Thus, the increased calretinin expression observed in mesothelioma cells and in certain colon cancer might be correlated to the increased resistance of these tumor cells to proapoptotic/pronecrotic signals. We identified and characterized (experimentally and by modeling) three Ca²⁺ shuttling pathways in primary mesothelial cells during Ca²⁺ oscillations: Ca²⁺ shuttled between (i) the endoplasmic reticulum (ER) and mitochondria, (ii) the ER and the extracellular space, and (iii) the ER and cytoplasmic Ca²⁺ buffers.

Multiscale modelling of saliva secretion

We review a multiscale model of saliva secretion, describing in brief how the model is constructed and what we have so far learned from it. The model begins at the level of inositol trisphosphate receptors (IPR), and proceeds through the cellular level (with a model of acinar cell calcium dynamics) to the multicellular level (with a model of the acinus), finally to a model of a saliva production unit that includes an acinus and associated duct. The model at the level of the entire salivary gland is not yet completed. Particular results from the model so far include (i) the importance of modal behaviour of IPR, (ii) the relative unimportance of Ca(2+) oscillation frequency as a controller of saliva secretion, (iii) the need for the periodic Ca(2+) waves to be as fast as possible in order to maximise water transport, (iv) the presence of functional K(+) channels in the apical membrane increases saliva secretion, (v) the relative unimportance of acinar spatial structure for isotonic water transport, (vi) the prediction that duct cells are highly depolarised, (vii) the prediction that the secondary saliva takes at least 1mm (from the acinus) to reach ionic equilibrium. We end with a brief discussion of future directions for the model, both in construction and in the study of scientific questions.

Expression of a truncated form of the endoplasmic reticulum chaperone protein, σ1 receptor, promotes mitochondrial energy depletion and apoptosis

The σ1 receptor (σ(1)R) regulates endoplasmic reticulum (ER)/mitochondrial interorganellar Ca(2+) mobilization through the inositol 1,4,5-trisphosphate receptor (IP(3)R). Here, we observed that expression of a novel splice variant of σ(1)R, termed short form σ(1)R (σ(1)SR), has a detrimental effect on mitochondrial energy production and cell survival. σ(1)SR mRNA lacks 47 ribonucleotides encoding exon 2, resulting in a frameshift and formation of a truncated receptor. σ(1)SR localizes primarily in the ER at perinuclear regions and forms a complex with σ(1)R but not with IP(3)R in the mitochondrion-associated ER membrane. Overexpression of both σ(1)R and the truncated isoform promotes mitochondrial elongation with increased ER mitochondrial contact surface. σ(1)R overexpression increases the efficiency of mitochondrial Ca(2+) uptake in response to IP(3)R-driven stimuli, whereas σ(1)SR overexpression reduces it. Most importantly, σ(1)R promotes ATP production via increased mitochondrial Ca(2+) uptake, promoting cell survival in the presence of ER stress. By contrast, σ(1)SR suppresses ATP production following ER stress, enhancing cell death. Taken together, the newly identified σ(1)SR isoform interferes with σ(1)R function relevant to mitochondrial energy production under ER stress conditions, promoting cellular apoptosis.

Use of Fluorescence Resonance Energy Transfer-based Biosensors for the Quantitative Analysis of Inositol 1,4,5-Trisphosphate Dynamics in Calcium Oscillations

Inositol 1,4,5-trisphosphate (IP(3)) is an intracellular messenger that elicits a wide range of spatial and temporal Ca(2+) signals, and this signaling versatility is exploited to regulate diverse cellular responses. In this study, we have developed a series of IP(3) biosensors that exhibit strong pH stability and varying affinities for IP(3), as well as a method for the quantitative measurement of cytosolic concentrations of IP(3) ([IP(3)](i)) in single living cells. We applied this method to elucidate IP(3) dynamics during agonist-induced Ca(2+) oscillations, and we demonstrated cell type-dependent differences in IP(3) dynamics, a nonfluctuating rise in [IP(3)](i) and repetitive IP(3) spikes during Ca(2+) oscillations in COS-7 cells and HSY-EA1 cells, respectively. The size of the IP(3) spikes in HSY-EA1 cells varied from 10 to 100 nm, and the [IP(3)](i) spike peak was preceded by a Ca(2+) spike peak. These results suggest that repetitive IP(3) spikes in HSY-EA1 cells are passive reflections of Ca(2+) oscillations, and are unlikely to be essential for driving Ca(2+) oscillations. In addition, the interspike periods of Ca(2+) oscillations that occurred during the slow rise in [IP(3)](i) were not shortened by the rise in [IP(3)](i), indicating that IP(3)-dependent and -independent mechanisms may regulate the frequency of Ca(2+) oscillations. The novel method described herein as well as the quantitative information obtained by using this method should provide a valuable and sound basis for future studies on the spatial and temporal regulations of IP(3) and Ca(2+).

Localized accumulation of angiotensin II and production of angiotensin-(1-7) in rat luteal cells and effects on steroidogenesis

These studies aim to investigate subcellular distribution of angiotensin II (ANG II) in rat luteal cells, identify other bioactive angiotensin peptides, and investigate a role for angiotensin peptides in luteal steroidogenesis. Confocal microscopy showed ANG II distributed within the cytoplasm and nuclei of luteal cells. HPLC analysis showed peaks that eluted with the same retention times as ANG-(1-7), ANG II, and ANG III. Their relative concentrations were ANG II >or= ANG-(1-7) > ANG III, and accumulation was modulated by quinapril, an inhibitor of angiotensin-converting enzyme (ACE), Z-proprolinal (ZPP), an inhibitor of prolyl endopeptidase (PEP), and parachloromercurylsulfonic acid (PCMS), an inhibitor of sulfhydryl protease. Phenylmethylsulfonyl fluoride (PMSF), a serine protease inhibitor, did not affect peptide accumulation. Quinapril, ZPP, PCMS, and PMSF, as well as losartan and PD-123319, the angiotensin receptor type 1 (AT1) and type 2 (AT2) receptor antagonists, were used in progesterone production studies. ZPP significantly reduced luteinizing hormone (LH)-dependent progesterone production (P < 0.05). Quinapril plus ZPP had a greater inhibitory effect on LH-stimulated progesterone than either inhibitor alone, but this was not reversed by exogenous ANG II or ANG-(1-7). Both PCMS and PMSF acutely blocked LH-stimulated progesterone, and PCMS blocked LH-sensitive cAMP accumulation. Losartan inhibited progesterone production in permeabilized but not intact luteal cells and was reversed by ANG II. PD-123319 had no significant effect on luteal progesterone production in either intact or permeabilized cells. These data suggest that steroidogenesis may be modulated by angiotensin peptides that act in part through intracellular AT1 receptors.

Fluorescent biosensor for quantitative real-time measurements of inositol 1,4,5-trisphosphate in single living cells

The second messenger inositol 1,4,5-trisphosphate (IP(3)) plays a central role in the generation of a variety of spatiotemporally complex intracellular Ca(2+) signals involved in the regulation of many essential physiological processes. Here we describe the development of "LIBRA", a novel ratiometric fluorescent IP(3) biosensor that allows for the quantitative monitoring of intracellular IP(3) concentrations in single living cells in real time. LIBRA consists of the IP(3)-binding domain of the rat type 3 IP(3) receptor fused between the fluorescence resonance energy transfer pair cyan fluorescent protein and yellow fluorescent protein and preceded by a membrane-targeting signal. We show that the LIBRA fluorescent signal is highly selective for IP(3) and unaffected by concentrations of Ca(2+) and ATP in the physiological range. In addition, LIBRA can be calibrated in situ. We demonstrate the utility of LIBRA by monitoring the temporal relationship between the responses intracellular IP(3) and Ca(2+) concentrations in SH-SY5Y cells following acetylcholine stimulation.

Possible mechanisms regulating ATP- and thimerosal-induced Ca(2+) oscillations in the HSY salivary duct cell line

The ATP-induced oscillatory changes in cytosolic Ca(2+) concentration ([Ca(2+)](i)) were analysed in HSY cells, a salivary ductal cell line from human parotid, using a fluorescence ratio imaging system. At concentrations higher than 1 microM, ATP caused sinusoidal [Ca(2+)](i) oscillations due to the periodic release and reuptake of Ca(2+) by intracellular Ca(2+) stores. The phorbol ester 4beta-phorbol 12,13-dibutyrate (PDBu) changed the [Ca(2+)](i) oscillations to a single spike. The inhibitory effect of PDBu on the [Ca(2+)](i) signals was reversed by protein kinase C (PKC) inhibitors such as staurosporine and chelerythrine chloride. However, preincubation of the cells with the PKC inhibitors did not affect the pattern of the ATP-induced [Ca(2+)](i) oscillations. The desensitization of the [Ca(2+)](i) response observed during prolonged stimulation with ATP was also not prevented by the PKC inhibitors. Incubation of HSY cells with the sulphydryl reagent thimerosal, which enhances the sensitivity of inositol 1,4,5-trisphosphate (IP(3)) receptors, caused repetitive Ca(2+) release from intracellular Ca(2+) stores resulting in baseline spikes of [Ca(2+)](i). The thimerosal-induced [Ca(2+)](i) oscillations did not change in the presence of PDBu and the phospholipase C inhibitor U73122. Thus, we could not provide evidence that negative feedback by PKC plays a central role in the regulation of ATP-induced [Ca(2+)](i) oscillations. These results suggest that the [Ca(2+)](i) oscillations, at least the baseline spikes, in HSY cells can be generated without stimulating the formation of IP(3).

Multiple isoforms of the ryanodine receptor are expressed in rat pancreatic acinar cells

Regulation of cytosolic Ca(2+) is important for a variety of cell functions. The ryanodine receptor (RyR) is a Ca(2+) channel that conducts Ca(2+) from internal pools to the cytoplasm. To demonstrate the presence of the RyR in the pancreatic acinar cell, we performed reverse transcriptase (RT)-PCR, Western blot, immunocytochemistry and microscopic Ca(2+)-release measurements on these cells. RT-PCR showed the presence of mRNA for RyR isoforms 1, 2 and 3 in both rat pancreas and dispersed pancreatic acini. Furthermore, mRNA expression for RyR isoforms 1 and 2 was demonstrated by RT-PCR in individual pancreatic acinar cells selected under the microscope. Western-blot analysis of acinar cell immunoprecipitates, using antibodies against RyR1 and RyR2, showed a high-molecular-mass (>250 kDa) protein band that was much less intense when immunoprecipitated in the presence of RyR peptide. Functionally, permeablized acinar cells stimulated with the RyR activator, palmitoyl-CoA, released Ca(2+) from both basolateral and apical regions. These data show that pancreatic acinar cells express multiple isoforms of the RyR and that there are functional receptors throughout the cell.

Molecular and binding characteristics of IP3 receptors in bovine spermatozoa

We have shown the presence of inositol 1,4,5-triphosphate (IP3) receptors in bovine spermatozoa. These receptors are mainly localized and functionally associated with the acrosome region. Molecular characterization of these bovine IP3 receptors has shown that the functional size of the IP3 binding domain is a protein of 66+/-2 kDa, in agreement with the size of both bovine adrenal cortex and bovine adrenal medullar chromaffin cells IP3 receptors. In contrast, bovine cerebellum IP3 receptor displays molecular weight of 220+/-5 kDa, a value in agreement with data in the literature. Bovine IP3 receptors have a one-affinity state characterized by a low affinity (Kd 750 nM) and a relatively high density (7.5 pmol/mg protein). They are functional and release internal calcium upon the binding of the second messenger. Moreover, the finding that the specific A1 adenosine receptor agonist R-PIA elicits almost the same effect as IP3 might be of some help in understanding the physiological role of these inhibitory adenosine receptors in mammalian spermatozoa.

Control of InsP3-induced Ca2+ oscillations in permeabilized blowfly salivary gland cells: contribution of mitochondria

Many agonists linked to the generation of inositol 1,4, 5-trisphosphate (InsP3) and release of Ca2+ from intracellular stores induce repetitive transients in cytosolic Ca2+ whose frequency increases over a certain range of agonist concentrations. In order to investigate the mechanisms underlying this frequency modulation, the fluorescent Ca2+ sensor mag-fura-2 was loaded into intracellular calcium stores and used to monitor InsP3-induced dynamics of the intraluminal calcium concentration ([Ca2+]L) in secretory cells of permeabilized blowfly Calliphora vicina salivary glands. In this preparation, increasing concentrations of InsP3 induced graded decreases in [Ca2+]L that were often superimposed with repetitive [Ca2+]L transients produced by sequential Ca2+ release and re-uptake. These [Ca2+]L oscillations developed at frequencies of 3-11 min-1 unrelated to the concentration of InsP3 present. In contrast, incremental concentrations of InsP3 applied in the presence of the oxidizable mitochondrial substrates citrate, succinate, or pyruvate-malate induced repetitive [Ca2+]L transients whose frequency increased with the concentration of InsP3. This InsP3 concentration-dependent modulation of oscillation frequency was abolished after dissipating the mitochondrial membrane potential (Delta psi m) by combined treatment with carbonyl cyanide p-trifluoromethoxyphenyl hydrazone + oligomycin or after application of Ruthenium Red, an inhibitor of mitochondrial Ca2+ uptake. Taken together, the data indicate that energized mitochondria exert negative control over the frequency of InsP3-induced Ca2+ oscillations. It is concluded that mitochondria play a crucial role in determining the duration of the interspike period and, therefore, for the encoding of amplitude-modulated, InsP3-liberating stimuli into the frequency of cytosolic Ca2+ oscillations.

Calcium signal transmission between ryanodine receptors and mitochondria

Control of energy metabolism by increases of mitochondrial matrix [Ca(2+)] ([Ca(2+)](m)) may represent a fundamental mechanism to meet the ATP demand imposed by heart contractions, but the machinery underlying propagation of [Ca(2+)] signals from ryanodine receptor Ca(2+) release channels (RyR) to the mitochondria remains elusive. Using permeabilized cardiac (H9c2) cells we investigated the cytosolic [Ca(2+)] ([Ca(2+)](c)) and [Ca(2+)](m) signals elicited by activation of RyR. Caffeine, Ca(2+), and ryanodine evoked [Ca(2+)](c) spikes that often appeared as frequency-modulated [Ca(2+)](c) oscillations in these permeabilized cells. Rapid increases in [Ca(2+)](m) and activation of the Ca(2+)-sensitive mitochondrial dehydrogenases were synchronized to the rising phase of the [Ca(2+)](c) spikes. The RyR-mediated elevations of global [Ca(2+)](c) were in the submicromolar range, but the rate of [Ca(2+)](m) increases was as large as it was in the presence of 30 microm global [Ca(2+)](c). Furthermore, RyR-dependent increases of [Ca(2+)](m) were relatively insensitive to buffering of [Ca(2+)](c) by EGTA. Therefore, RyR-driven rises of [Ca(2+)](m) appear to result from large and rapid increases of perimitochondrial [Ca(2+)]. The falling phase of [Ca(2+)](c) spikes was followed by a rapid decay of [Ca(2+)](m). CGP37157 slowed down relaxation of [Ca(2+)](m) spikes, whereas cyclosporin A had no effect, suggesting that activation of the mitochondrial Ca(2+) exchangers accounts for rapid reversal of the [Ca(2+)](m) response with little contribution from the permeability transition pore. Thus, rapid activation of Ca(2+) uptake sites and Ca(2+) exchangers evoked by RyR-mediated local [Ca(2+)](c) signals allow mitochondria to respond rapidly to single [Ca(2+)](c) spikes in cardiac cells.

Regulation of calcium in salivary gland secretion

Neurotransmitter-regulation of fluid secretion in the salivary glands is achieved by a coordinated sequence of intracellular signaling events, including the activation of membrane receptors, generation of the intracellular second messenger, inositol 1,4,5, trisphosphate, internal Ca2+ release, and Ca2+ influx. The resulting increase in cytosolic [Ca2+] ([Ca2+]i) regulates a number of ion transporters, e.g., Ca2+-activated K+ channel, Na+/K+/2Cl- co-transporter in the basolateral membrane, and the Ca2+-activated Cl- channel in the luminal membrane, which are intricately involved in fluid secretion. Thus, regulation of [Ca2+]i is central to the regulation of salivary acinar cell function and is achieved by the concerted activities of several ion channels and Ca2+-pumps localized in various cellular membranes. Ca2+ pumps, present in the endoplasmic reticulum and the plasma membrane, serve to remove Ca2+ from the cytosol. Ca2+ channels present in the endoplasmic reticulum and the plasma membrane facilitate rapid influx of Ca2+ into the cytosol from the internal Ca2+ stores and from the external medium, respectively. It is well-established that prolonged fluid secretion is regulated via a sustained elevation in [Ca2+]i that is primarily achieved by the influx of Ca2+ into the cell from the external medium. This Ca2+ influx occurs via a putative plasma-membrane-store-operated Ca2+ channel which has not yet been identified in any non-excitable cell type. Understanding the molecular nature of this Ca2+ influx mechanism is critical to our understanding of Ca2+ signaling in salivary gland cells. This review focuses on the various active and passive Ca2+ transport mechanisms in salivary gland cells--their localization, regulation, and role in neurotransmitter-regulation of fluid secretion. In addition to a historical perspective of Ca2+ signaling, recent findings and challenging problems facing this field are highlighted.

Mitochondria suppress local feedback activation of inositol 1,4, 5-trisphosphate receptors by Ca2+

The concerted action of inositol 1,4,5-trisphosphate (IP3) and Ca2+ on the IP3 receptor Ca2+ release channel (IP3R) is a fundamental step in the generation of cytosolic Ca2+ oscillations and waves, which underlie Ca2+ signaling in many cells. Mitochondria appear in close association with regions of endoplasmic reticulum (ER) enriched in IP3R and are particularly responsive to IP3-induced increases of cytosolic Ca2+ ([Ca2+]c). To determine whether feedback regulation of the IP3R by released Ca2+ is modulated by mitochondrial Ca2+ uptake, the interactions between ER and mitochondrial Ca2+ pools were examined by fluorescence imaging of compartmentalized Ca2+ indicators in permeabilized hepatocytes. IP3 decreased luminal ER Ca2+ ([Ca2+]ER), and this was paralleled by an increase in mitochondrial matrix Ca2+ ([Ca2+]m) and activation of Ca2+-sensitive mitochondrial metabolism. Remarkably, the decrease in [Ca2+]ER evoked by submaximal IP3 was enhanced when mitochondrial Ca2+ uptake was blocked with ruthenium red or uncoupler. Moreover, subcellular regions that were relatively deficient in mitochondria demonstrated greater sensitivity to IP3 than regions of the cell with a high density of mitochondria. These data demonstrate that Ca2+ uptake by the mitochondria suppresses the local positive feedback effects of Ca2+ on the IP3R, giving rise to subcellular heterogeneity in IP3 sensitivity and IP3R excitability. Thus, mitochondria can play an important role in setting the threshold for activation and establishing the subcellular pattern of IP3-dependent [Ca2+]c signaling.

Transduction of basolateral-to-apical signals across epithelial cells: ligand-stimulated transcytosis of the polymeric immunoglobulin receptor requires two signals

Transcytosis of the polymeric immunoglobulin receptor (pIgR) is stimulated by binding of its ligand, dimeric IgA (dIgA). During this process, dIgA binding at the basolateral surface of the epithelial cell transmits a signal to the apical region of the cell, which in turn stimulates the transport of dIgA-pIgR complex from a postmicrotubule compartment to the apical surface. We have previously reported that the signal of stimulation was controlled by a protein-tyrosine kinase (PTK) activated upon dIgA binding. We now show that this signal of stimulation moves across the cell independently of pIgR movement or microtubules and acts through the tyrosine kinase activity by releasing Ca++ from inositol trisphosphate-sensitive intracellular stores. Surprisingly we have found that a second independent signal is required to achieve dIgA-stimulated transcytosis of pIgR. This second signal depends on dIgA binding to the pIgR solely at the basolateral surface and the ability of pIgR to dimerize. This enables pIgR molecules that have bound dIgA at the basolateral surface to respond to the signal of stimulation once they reach the postmicrotubule compartment. We propose that the use of two signals may be a general mechanism by which signaling receptors maintain specificity along their signaling and trafficking pathways.

Quasi-synaptic calcium signal transmission between endoplasmic reticulum and mitochondria

Transmission of cytosolic [Ca2+] ([Ca2+]c) oscillations into the mitochondrial matrix is thought to be supported by local calcium control between IP3 receptor Ca2+ channels (IP3R) and mitochondria, but study of the coupling mechanisms has been difficult. We established a permeabilized cell model in which the Ca2+ coupling between endoplasmic reticulum (ER) and mitochondria is retained, and mitochondrial [Ca2+] ([Ca2+]m) can be monitored by fluorescence imaging. We demonstrate that maximal activation of mitochondrial Ca2+ uptake is evoked by IP3-induced perimitochondrial [Ca2+] elevations, which appear to reach values >20-fold higher than the global increases of [Ca2+]c. Incremental doses of IP3 elicited [Ca2+]m elevations that followed the quantal pattern of Ca2+ mobilization, even at the level of individual mitochondria. In contrast, gradual increases of IP3 evoked relatively small [Ca2+]m responses despite eliciting similar [Ca2+]c increases. We conclude that each mitochondrial Ca2+ uptake site faces multiple IP3R, a concurrent activation of which is required for optimal activation of mitochondrial Ca2+ uptake. This architecture explains why calcium oscillations evoked by synchronized periodic activation of IP3R are particularly effective in establishing dynamic control over mitochondrial metabolism. Furthermore, our data reveal fundamental functional similarities between ER-mitochondrial Ca2+ coupling and synaptic transmission.

Activation of beta-adrenoceptors does not cause any change in cytosolic Ca2+ distribution in rat parotid acinar cells

The effects of the beta-adrenoceptor agonist isoproterenol on the distribution of cytosolic Ca2+ concentrations were studied with digital imaging microscopy in fura-2-loaded rat parotid acinar cells. At concentrations < 10 microM, isoproterenol did not cause any measurable change in cytosolic Ca2+ concentration ([Ca2+]i). Monitoring of [Ca2+]i in selected areas of the acinar cells failed to show that stimulation with isoproterenol causes a localized rise in [Ca2+]i at the apical region close to the lumen. As the maximum response of amylase exocytosis is observed at 0.1 or 1 microM isoproterenol [Tanimura, A., Matsumoto, Y., Tojyo, Y., 1990. Evidence that isoproterenol-induced Ca2+-mobilization in rat parotid acinar cells is not mediated by activation of beta-adrenoceptors. Biochim. Biophys. Acta, 1055, pp. 273-277], the data obtained here indicate that the isoproterenol-induced amylase exocytosis is not accompanied by Ca2+ mobilization. The high concentration (100 microM) of isoproterenol caused a small but significant increase in [Ca2+]i, particularly in the apical region. This response was completely attenuated by the alpha-adrenoceptor antagonist phentolamine, but not by the beta-adrenoceptor antagonist propranolol, indicating that the isoproterenol-induced increase in [Ca2+]i resulted from an activation of alpha-adrenoceptors. Further, the effect of cyclic AMP on Ca2+ release from intracellular Ca2+ stores was studied in saponin-permeabilized acinar cells using the lipophilic Ca2+ indicator Calcium Green C18. Cyclic AMP had no effect on the Ca2+ release, while the same acinar cells responded strongly to inositol 1,4,5-trisphosphate. This result does not support the hypothesis that cyclic AMP directly stimulates Ca2+ mobilization in rat parotid acinar cells.

Cellular mechanisms and role of endothelin-1-induced calcium oscillations in pulmonary arterial myocytes

The effect of endothelin (ET)-1 on both cytosolic Ca2+ concentration ([Ca2+]i) and membrane current in freshly isolated myocytes, as well as on the contraction of arterial rings, was investigated in rat main pulmonary artery (RMPA) and intrapulmonary arteries (RIPA). ET-1 (5-100 nM, 30 s) induced a first [Ca2+]i peak followed by 3-5 oscillations of decreasing amplitude. In RMPA, the ET-1-induced [Ca2+]i response was fully abolished by BQ-123 (0.1 microM). In RIPA, the response was inhibited by BQ-123 in only 21% of the cells, whereas it was abolished by BQ-788 (1 microM) in 70% of the cells. In both types of arteries, the response was not modified in the presence of 100 microM La3+ or in the absence of external Ca2+ but disappeared after pretreatment of the cells with thapsigargin (1 microM) or neomycin (0.1 microM). In RPMA myocytes clamped at -60 mV, ET-1 induced an oscillatory inward current, the reversal potential of which was close to the equilibrium potential for Cl-. This current was unaltered by the removal of external Ca2+ but was abolished by niflumic acid (50 microM). In arterial rings, the ET-1 (100 nM)-induced contraction was decreased by 35% in the presence of either niflumic acid (50 microM) or nifedipine (1 microM). These results demonstrate that ET-1 via the ETA receptor only in RMPA and both ETA and ETB receptors in RIPA induce [Ca2+]i oscillations due to iterative Ca2+ release from an inositol trisphosphate-sensitive Ca2+ store. Ca2+ release secondarily activates an oscillatory membrane Cl- current that can depolarize the cell membrane, leading to an influx of Ca2+, this latter contributing to the ET-1-induced vasoconstrictor effect.

Polarized Ca2+ release in saponin-permeabilized parotid acinar cells evoked by flash photolysis of 'caged' inositol 1,4,5-trisphosphate

In exocrine acinar cells, agonist stimulation results in a polarized Ca2+ signal, termed the 'Ca2+ wave', that propagates from the apical pole towards the basolateral region. We attempted to detect the inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ wave in saponin-permeabilized rat parotid acinar cells using a digital imaging system. The permeabilized acinar cells were labelled with the membrane-bound Ca2+ indicator Calcium Green C18 to detect changes in Ca2+ concentration adjacent to the membrane of intracellular organelles. Application of InsP3 was made by the photolysis of InsP3 P4(5)-1-(2-nitrophenyl)ethyl ester (caged InsP3) to expose simultaneously all regions of the permeabilized acinar cells to InsP3. The increase in fluorescence ratio following the photolysis of 0.5 microM caged InsP3 started at the apical region of the acinar cells within 0.1 s and spread towards the basolateral region, indicating that Ca2+ release from intracellular Ca2+ stores was initially evoked at the apical region. Pretreatment with thapsigargin, an inhibitor of endoplasmic reticulum Ca2+ pumps, failed to prevent the InsP3-induced Ca2+ wave, suggesting that the generation of the Ca2+ wave is not attributed to the polarized distribution of the Ca2+ pumps. The photolysis of a high concentration (10 microM) of caged InsP3 caused a homogeneous increase in the fluorescence ratio throughout the cells, indicating that all regions of intracellular Ca2+ stores similarly responded to the high concentration of InsP3. The present study is the first demonstration of the InsP3-induced Ca2+ wave in permeabilized exocrine acinar cells. The result provides fresh evidence that the apical region contains elements of intracellular Ca2+ stores particularly sensitive to InsP3 and that the Ca2+ wave results from a polarized distribution of InsP3-sensitive Ca2+ stores.

Does a decrease in subplasmalemmal Ca2+ explain how store-operated Ca2+ channels are opened?

The phenomenon of store-activated Ca2+ inflow (capacitative Ca2+ entry) in which the depletion of Ca2+ in the endoplasmic reticulum (ER) increases the probability of opening of store-operated Ca2+ channels (SOCs) located in the plasma membrane is ubiquitous in 'non-excitable' animal cells and is also found in some 'excitable' cells. At present, neither the structures of SOCs nor the mechanism(s) by which a decrease in Ca2+ in the lumen of the ER activates SOCs are well understood. This paper discusses the hypothesis that a decrease in the concentration of Ca2+ in restricted regions of the subplasmalemmal space (bounded by the plasma membrane and peripheral regions of the ER) is responsible for the activation of SOCs. The hypothesis rests on observations made by others that Ca2+ is a strong feed-back inhibitor of SOCs and of the endoplasmic reticulum (Ca(2+)+Mg2+)-ATPases (SERCAs), and on the concepts (developed previously by others) of a subplasmalemmal space and the directed flow of Ca2+ through SOCs into the lumen of the ER and from there to the deep cytoplasmic space. The way in which the hypothesis might explain the actions of agonists (acting via inositol 1,4,5-trisphosphate) and thapsigargin (an inhibitor of SERCAs) in activating SOCs under physiological conditions is described. The proposed involvement of thapsigargin-insensitive SERCAs, and possible limitations of the hypothesis are discussed.

Monitoring of Ca2+ release from intracellular stores in permeabilized rat parotid acinar cells using the fluorescent indicators Mag-fura-2 and calcium green C18

The operation of intracellular Ca2+ stores in saponin-permeabilized rat parotid acinar cells was studied by monitoring the Ca2+ concentration within organelles loaded with the low affinity Ca2+ indicator Mag-fura-2. Inositol 1, 4, 5-trisphosphate (InsP3) caused a decrease in the Mag-fura-2 ratio in a dose-dependent manner, and this effect was reversed by a removal of InsP3 or by an addition of the InsP3 receptor antagonist heparin. The changes in Mag-fura-2 ratio indicate the Ca2+ release from InsP3-sensitive Ca2+ stores and Ca2+ re-uptake into the stores in permeabilized acinar cells. The decrease in Mag-fura-2 ratio induced by InsP3 was observed at all regions of the acinar cells, suggesting that the InsP3-sensitive Ca2+ stores are located throughout the cells. The InsP3-induced Ca2+ release was also monitored using the membrane-bound Ca2+ indicator Calcium Green C18 which is sensitive to the changes in Ca2+ concentration immediately adjacent to the membrane of intracellular Ca2+ stores. InsP3 caused a large increase in the Calcium Green C18 fluorescence reflecting Ca2+ release from the stores. The Ca2+ pump inhibitor thapsigargin (ThG) itself had little or no effect on the Mag-fura-2 ratio or Calcium Green C18 fluorescence, but combined application of ThG with a low concentration of InsP3 evoked a significant decrease in the Mag-fura-2 ratio. This result supports the hypothesis that the ThG-induced Ca2+ release is due to InsP3-sensitive Ca2+ release which is mediated by the resting levels of InsP3. Further, none of cyclic ADP-ribose, caffeine or ryanodine changed the Mag-fura-2 ratio and Calcium Green C18 fluorescence, leading to the assumption that the ryanodine-sensitive Ca2+ stores are minor in rat parotid acinar cells.

Sensing and refilling calcium stores in an excitable cell

Inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ mobilization leads to depletion of the endoplasmic reticulum (ER) and an increase in Ca2+ entry. We show here for the gonadotroph, an excitable endocrine cell, that sensing of ER Ca2+ content can occur without the Ca2+ release-activated Ca2+ current (Icrac), but rather through the coupling of IP3-induced Ca2+ oscillations to plasma membrane voltage spikes that gate Ca2+ entry. Thus we demonstrate that capacitative Ca2+ entry is accomplished through Ca(2+)-controlled Ca2+ entry. We develop a comprehensive model, with parameter values constrained by available experimental data, to simulate the spatiotemporal behavior of agonist-induced Ca2+ signals in both the cytosol and ER lumen of gonadotrophs. The model combines two previously developed models, one for ER-mediated Ca2+ oscillations and another for plasma membrane potential-driven Ca2+ oscillations. Simulations show agreement with existing experimental records of store content, cytosolic Ca2+ concentration ([Ca2+]i), and electrical activity, and make a variety of new, experimentally testable predictions. In particular, computations with the model suggest that [Ca2+]i in the vicinity of the plasma membrane acts as a messenger for ER content via Ca(2+)-activated K+ channels and Ca2+ pumps in the plasma membrane. We conclude that, in excitable cells that do not express Icrac, [Ca2+]i profiles provide a sensitive mechanism for regulating net calcium flux through the plasma membrane during both store depletion and refilling.