Calcium oscillations in electrically non-excitable cells

Inhibitory action of dilazep on histamine-stimulated cytosolic Ca2+ increase in cultured human endothelial cells

Using a fluorescent Ca(2+)-sensitive dye, fura-2, and photometric fluorescence microscopy, we measured changes in cytosolic Ca2+ concentration ([Ca2+]i) in cultured human endothelial cells and studied the effect of dilazep on [Ca2+]i elevation induced by histamine. Histamine (1 microM) caused a rapid transient peak in the average [Ca2+]i of a group of cells (approximately 10(2) cells), followed by a decrease to a sustained elevation. Dilazep as well as diltiazem (1.0 to 100 microM) concentration-dependently inhibited the latter sustained elevation, which was eliminated by removal of extracellular Ca2+, while the initial transient response was not changed by dilazep at concentrations up to 100 microM. The IC50 values of dilazep and diltiazem were 16 and 58 microM, respectively. The patterns of the [Ca2+]i elevation responses to histamine were variable among individual cells. Some single cells showed a transient peak and a sustained elevation as observed in a group of cells. Some single cells caused a repetitive spikelike elevation of [Ca2+]i. Dilazep lowered the sustained elevation to the resting level and in some single cells, changed the sustained elevation to the spikelike elevation. The frequency of the spikelike [Ca2+]i elevation was also decreased by dilazep. Decrease in extracellular [Ca2+] showed the same pattern of inhibitory actions as dilazep did. These results indicate that dilazep inhibits the extracellular Ca2+ influx in endothelial cells.

Phase-dependent contributions from Ca2+ entry and Ca2+ release to caffeine-induced [Ca2+]i oscillations in bullfrog sympathetic neurons

Sympathetic neurons display robust [Ca2+]i oscillations in response to caffeine and mild depolarization. Oscillations occur at constant membrane potential, ruling out voltage-dependent changes in plasma membrane conductance. They are terminated by ryanodine, implicating Ca(2+)-induced Ca2+ release. Ca2+ entry is necessary for sustained oscillatory activity, but its importance varies within the oscillatory cycle: the slow interspike rise in [Ca2+]i requires Ca2+ entry, but the rapid upstroke does not, indicating that it reflects internal Ca2+ release. Sudden alterations in [Ca2+]o, [K+]o, or [caffeine]o produce immediate changes in d[Ca2+]i/dt and provide information about the relative rates of surface membrane Ca2+ transport as well as uptake and release by internal stores. Based on our results, [Ca2+]i oscillations can be explained in terms of coordinated changes in Ca2+ fluxes across surface and store membranes.

Cerebellar neurons and glia respond differentially to endothelins and sarafotoxin S6b

Endothelins (ETs) and sarafotoxin-S6b belong to a family of extremely potent vasoconstrictors which may also have a role as neuropeptides or neuromodulators in the central nervous system (CNS). We show, using single cell dynamic video imaging of intracellular free calcium ions ([Ca2+]i), that binding of ET to its receptors modulates [Ca2+]i of neurons as well as glial cells in primary cultures of rat cerebellum. At least two receptor subtypes, differing in both their ligand specificity and distribution, appear to be involved in the action of ETs and sarafotoxin S6b on these cells. One of these receptors may be a previously undescribed neuronal form of ET receptor. This is the first demonstration of a direct effect of ETs on neurons as well as glia in the CNS. These data support a possible role for ET as a neurotransmitter or neuromodulator in the CNS.

Do submaximal InsP3 concentrations only induce the partial discharge of permeabilized hepatocyte calcium pools because of the concomitant reduction of intraluminal Ca2+ concentration?

In several types of cells whose cytoplasmic Ca2+ is regulated by inositol phosphate derivatives, low concentrations of InsP3 added to permeabilized cell suspensions induce the rapid discharge of part of the InsPs-sensitive Ca2+ pool instead of slow monophasic release of Ca2+ from the entire pool. As a tentative explanation for this puzzling observation, sometimes called 'quantal release', it was suggested that the reduced intraluminal Ca2+ concentration remaining in the Ca2+ pool after a certain amount of Ca2+ had been released might allosterically reduce the channels' affinity for InsP3 and the corresponding InsP3-dependent Ca2+ efflux, and thus result in partial pool discharge (Irvine, R.F. (1990) FEBS Lett. 263, 5-9). We have tested this hypothesis by manipulating the Ca2+ pool contents with ionophore, and found that the rate of InsP3-dependent Ca2+ efflux after ionophore-induced partial discharge of the Ca2+ pools was much faster than what was predicted on the basis of this hypothesis. Heterogeneity of the Ca2+ pools appears to be a more likely reason for the 'quantal release' behavior.

Neuronal activity triggers calcium waves in hippocampal astrocyte networks

The recent discovery that the neurotransmitter glutamate can trigger actively propagating Ca2+ waves in the cytoplasm of cultured astrocytes suggests the possibility that synaptically released glutamate may trigger similar Ca2+ waves in brain astrocytes in situ. To explore this possibility, we used confocal microscopy and the Ca2+ indicator fluo-3 to study organotypically cultured slices of rat hippocampus, where astrocytic and neuronal networks are intermingled in their normal tissue relationships. We find that astrocytic Ca2+ waves are present under these circumstances and that these waves can be triggered by the firing of glutamatergic neuronal afferents with latencies as short as 2 s. The Ca2+ waves closely resemble those previously observed in cultured astrocytes: they propagate both within and between astrocytes at velocities of 7-27 microns/s at 21 degrees C. The ability of tissue astrocyte networks to respond to neuronal network activity suggests that astrocytes may have a much more dynamic and active role in brain function than has been generally recognized.

Calcium entry-dependent oscillations of cytoplasmic calcium concentration in cultured endothelial cell monolayers

Bovine endothelial cell monolayers grown to confluence and stimulated with bradykinin responded with periodic fluctuations in intracellular Ca2+ concentration ([Ca2+]i) when exposed to K(+)-free Hepes-buffered saline. The fluctuations in [Ca2+]i measured with fura-2 were synchronized among the population of cells observed and were sensitive to extracellular Ca2+ concentration ([Ca2+]o). Thapsigargin, which inhibits the endoplasmic reticular Ca2(+)-ATPase, did not inhibit the [Ca2+]i oscillations. Removal of extracellular Ca2+ or inhibition of Ca2+ entry by using La3+ or 1-(beta- [3-(4-methoxyphenyl)proproxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SKF 96365) abolished the [Ca2+]i oscillations in endothelial cell monolayers. The fluctuations in [Ca2+]i were therefore dependent on Ca2+ influx rather than Ca2+ mobilization from intracellular stores. Simultaneous measurements of membrane potential (Em) using the potential-sensitive bisoxonol dye bis(1,3-dibutylbarbituric acid)trimethine oxonol [Di-BAC4(3)] and [Ca2+]i using fura-2 showed that Em oscillated at the same frequency as the fluctuations in [Ca2+]i. The peak depolarization signal coincided with the maximum rate of increase in the [Ca2+]i signal. Oscillations in the Em signal were inhibited by removal of Ca2+ or by addition of 1 mM Ni2+ to the external solution. Taken together, these observations suggest that the change in Em is the consequence of oscillatory changes in a membrane conductance that also allows Ca2+ to enter the cell. Oscillations in the DiBAC4(3) signal may reflect a rhythmic entry of Ca2+ through nonselective cation channels.

Modified kinetics of platelet-derived growth factor-induced Ca2+ increases in NIH-3T3 cells overexpressing phospholipase C gamma 1

The effects of platelet-derived growth factor (PDGF) on cytosolic free Ca2+ concentration ([Ca2+]i) and inositol phosphates were studied in NIH-3T3 fibroblasts transfected with cDNA for phospholipase C gamma 1 (PLC gamma 1) to yield a 7-fold overexpression of this enzyme, compared with cells containing normal levels of PLC gamma 1. In a study published recently [Margolis, Zilberstein, Franks, Felder, Kremer, Ullrich, Rhee, Skorecki & Schlessinger (1990) Science 248, 607-610] it was reported that this overexpression of PLC gamma 1 caused a specific potentiation of the inositol phosphate response to PDGF, but this was not associated with an enhancement of the [Ca2+]i response. In the present study, measurements of the time course and isomeric profile of PDGF-induced inositol phosphate formation demonstrated that the initial rate of Ins(1,4,5)P3 formation was also enhanced in the PLC gamma 1-overexpressing cells, yielding a 10-fold greater increase at 1 min compared with the parental NIH-3T3 cells. By contrast, bradykinin-induced phosphoinositide metabolism was unchanged in PLC gamma 1-transfected cells. Measurements of [Ca2+]i in cell populations and single cells showed a significant latent period following PDGF addition prior to the [Ca2+]i increases in both cell lines, which decreased in a dose-dependent manner with increasing PDGF concentration. The duration of the latent period was decreased and the maximal rate of [Ca2+]i rise was increased in the PLC gamma 1-overexpressing cells at all doses of PDGF examined. In single-cell measurements these cells also responded to PDGF with a greater peak amplitude of [Ca2+]i. Both intracellular Ca2+ mobilization and Ca2+ influx across the plasma membrane were enhanced in the PLC gamma 1-overexpressing cells. There was no difference between the two cell lines in either the latency or the magnitude of the [Ca2+]i increases induced by bradykinin. These data provide further evidence that PLC gamma 1 is responsible for the PDGF-induced stimulation of Ins(1,4,5)P3 formation. Moreover, in contrast to earlier conclusions, the modified kinetics of the [Ca2+]i changes in PLC gamma 1-overexpressing cells suggest that Ins(1,4,5)P3 does play a predominant second messenger role in the PDGF-induced [Ca2+]i increases. The data also indicate that the latent period may be a function of the time required to reach a threshold level of Ins(1,4,5)P3, rather than an intrinsic property of the PDGF receptor.

Ca(2+)-induced Ca2+ release in insulin-secreting cells

The sulphydryl reagent thimerosal (50 microM) released Ca2+ from a non-mitochondrial intracellular Ca2+ pool in a dose-dependent manner in permeabilized insulin-secreting RINm5F cells. This release was reversed after addition of the reducing agent dithiothreitol. Ca2+ was released from an Ins(1,4,5)P3-insensitive pool, since release was observed even after depletion of the Ins(1,4,5)P3-sensitive pool by a supramaximal dose of Ins(2,4,5)P3 or thapsigargin. The Ins(1,4,5)P3-sensitive pool remained essentially unaltered by thimerosal. Thimerosal-induced Ca2+ release was potentiated by caffeine. These findings suggest the existence of Ca(2+)-induced Ca2+ release also in insulin-secreting cells.

Activation of endothelin receptors by sarafotoxin regulates Ca2+ homeostasis in cerebellar astrocytes

We carried out experiments designed to investigate the effects of sarafotoxin-6B (SFTx) on [Ca2+]i in cerebellar astrocytes using the Ca2+ indicator fura-2. Both endothelin-1 and sarafotoxin-6B increased [Ca2+]i in individual cerebellar astrocytes in cell culture. The shape of the response was variable but usually consisted of an initial peak of [Ca2+]i followed by an extended plateau increase in [Ca2+]i. In Ca(2+)-free medium only the initial peak was observed. If Ca2+ was subsequently readmitted to the external medium a plateau was now formed. When external Ca2+ was removed during a plateau, [Ca2+]i rapidly declined; replacing the external Ca2+ reversed this decline. The plateau was also reversibly reduced by addition of Ni2+ (5 mM) to the external medium. Addition of 50 mM K+ produced a small increase in [Ca2+]i in most cells. This response was blocked by nimodipine. However, nimodipine only slightly blocked the plateau increase in [Ca2+]i that was formed following activation of endothelin receptors. Furthermore, perfusion of cells with 50 mM K+ during the plateau portion of a response to SFTx reduced [Ca2+]i. In some cells addition of a phorbol ester produced a sustained increase in [Ca2+]i that was blocked by nimodipine. In conclusion, activation of endothelin receptors by SFTx in cerebellar astrocytes produces both Ca2+ mobilization and Ca2+ influx. The pathway for Ca2+ influx is predominantly a non-voltage-dependent one, although some entry through a dihydropyridine-sensitive pathway also appears to occur. Furthermore, activation of protein kinase C in cerebellar astrocytes activates voltage-sensitive Ca2+ channels.

Oscillations of cytosolic sodium during calcium oscillations in exocrine acinar cells

In acinar cells from rat salivary glands, cholinergic agonists cause oscillations in cytoplasmic free calcium concentration, which then drive oscillations of cell volume that reflect oscillating cell solute content and fluid secretion. By quantitative fluorescence ratio microscopy of an intracellular indicator dye for sodium, it has now been shown that large amplitude oscillations of sodium concentration were associated with the calcium and cell volume oscillations. Both calcium and sodium oscillations were dependent on the continued presence of calcium in the extracellular medium and were abolished by the specific sodium-potassium adenosine triphosphatase inhibitor ouabain. Thus, calcium oscillations in salivary acinar cells, by modulating the activities of ion transport pathways in the plasma membrane, can cause significant oscillations of monovalent ions that may in turn feed back to regulate calcium oscillations and fluid secretion.

Variety of Ca(2+)-permeable channels in human carcinoma A431 cells

Patch-clamp methods were used to search for and characterize channels that mediate calcium influx through the plasma membrane of human carcinoma A431 cells. Here we present four Ca(2+)-permeable channel types referred to as SG, G, 1 and BI. With 105 mM Ca2+ as the charge carrier, at 30-33 degrees C their mean unitary conductances (in pS) are: 1.3 (SG), 2.4 (G), 3.7 (I) and 12.8 (BI). SG and G channels are activated by nonhydrolyzable analogues of guanosine 5-triphosphate (GTP) applied to the inside of the membrane, suggesting an involvement of G-proteins in the control of their activity. I and BI channels are activated by inositol 1,4,5-trisphosphate (InsP3). G, I, BI and possibly SG channels are activated from the extracellular side of the membrane by epidermal growth factor (EGF) and histamine. It is assumed that all identified Ca2+ channels take part in the generation of the agonist-induced intracellular Ca2+ signal. The variety of Ca-channel types seems to be necessary to tune cell responses according to the respective type and level of an external signal, on the one hand, and to the functional state of the cell, on the other.

Spiking of intracellular calcium ion concentration in single cultured pig aortic endothelial cells stimulated with ATP or bradykinin

Single pig aortic endothelial cells in culture loaded with the Ca(2+)-sensitive fluorescent dye Indo-1 were stimulated with ATP (0.1-100 microM) or bradykinin (0.1-5.0 nM). Spiking or oscillations of [Ca2+]i were seen in approx. 50% of cells stimulated with either agonist. Non-spiking or transient responses in which [Ca2+]i returned to pre-stimulation levels rapidly 9120-250 s), or sustained responses in which [Ca2+]i remained elevated for many minutes, were seen in a further 20% of cells in each case, stimulated with either agonist. There was a marked variation between individual cells in the latency, magnitude, frequency and overall pattern of oscillations induced by ATP and bradykinin, although the patterns of response to bradykinin were less variable. In cells where repetitive spikes were seen, a relation between concentration of ATP and the latency of the response and the frequency of spiking was evident. Effects of removal of extracellular Ca2+, elevation of extracellular K+ concentration (35 or 70 mM) or exposure to phorbol 12,13-dibutyrate or 1,2-dioctanoyl-sn-glycerol were tested on the spiking Ca2+ responses. Each of these procedures reversibly slowed or prevented Ca2+ spiking evoked by ATP or bradykinin. In contrast, the inactive phorbol ester 4 alpha-phorbol didecanoate had no effect on Ca2+ spiking evoked by these hormones. Our results thus indicate that the responses of single cells to ATP or bradykinin exhibit marked heterogeneity, and suggest that secretory events driven by extracellular Ca2+ may be regulated by repetitive spikes or oscillations of Ca2+.

Dose-dependency in spatial dynamics of [Ca2+]c in pancreatic acinar cells

Spatial dynamics of cytosolic concentration of Ca2+, [Ca2+]c, in stimulus-secretion coupling of rat pancreatic acinar cell was monitored by a digital image analysing technique using Fura-2. When freshly isolated acini were stimulated with lower concentrations of CCK-8 (5-30 pM), [Ca2+]c increase began at the region beneath the basolateral membrane and the [Ca2+]c increase depended on the presence of extracellular Ca2+ ([Ca2+]o). CCK-8 at higher concentrations (100 pM and 1 nM), however, caused [Ca2+]c increase even in the absence of [Ca2+]o. Low concentrations of G-protein activator, NaF (10 mM or lower), caused [Ca2+]o-dependent increase in [Ca2+]c, whereas higher concentrations of NaF (15 mM or higher) increased [Ca2+]c in the absence of [Ca2+]o. These results are compatible with the view that G-protein activated by a physiological concentration of secretagogue accelerates Ca2+ entry. This process is in contrast to the process of Ca2+ release from intracellular stores, which can be predominant when pharmacological or toxic concentration of the secretagogue was applied.

Heterogeneous changes in [Ca2+]i induced by glucose, tolbutamide and K+ in single rat pancreatic B cells

The effects of glucose, tolbutamide and K+ on cytosolic free Ca2+ ([Ca2+]i) in single rat pancreatic B cells were examined using Fura-2 and dual wavelength microfluorimetry. At basal glucose concentration (2.8 mM), about half of the cells were found to display spontaneous Ca2+ oscillations. Glucose (greater than or equal to 11.1 mM), tolbutamide (greater than or equal to 50 microM) and K+ (50 mM) induced rises in [Ca2+]i that could be inhibited by the Ca2+ channel blocker D600. The pattern of response and the sensitivity to the secretagogues were characterized by a marked heterogeneity. The majority of the cells responded to glucose and tolbutamide by a progressive increase in [Ca2+]i onto which sinusoidal oscillations were superimposed. The periodicity of these oscillations was about 2.5/min. Occasionally, some cells displayed slow and major waves in Ca2+ levels (about 0.2/min). None of the cells responded to glucose by displaying an initial decrease in [Ca2+]i. Likewise, the sugar failed to decrease [Ca2+]i in the absence of extracellular Ca2+. The present study shows that, despite a large heterogeneity of the response, the majority of the pancreatic B cells respond to different secretagogues by displaying fast [Ca2+]i oscillations that are reminiscent of the bursts of electrical activity recorded in B cells.

[Ca2+]i oscillations from internal stores sustain exocytic secretion from the chromaffin cells of the rat

A large (65%) fraction of in vitro cultured rat chromaffin cells exhibit spontaneous [Ca2+]i oscillations, and the rest can be recruited to oscillate by appropriate stimulations. Based on fura-2 single cell [Ca2+]i measurements, evidence is provided that these oscillations originate, via the activation of Ca(2+)-induced Ca(2+)-release, from intracellular Ca2+ stores in rapid equilibrium with extracellular Ca2+. By combining [Ca2+]i measurements with a specific plaque secretion assay we demonstrate that oscillating cells exhibit a spontaneous exocytic secretory activity whereas the cells with stable [Ca2+]i do not. [Ca2+]i oscillations appear therefore to account for the high unstimulated catecholamine release, an activity typical of the chromaffin cells of the rat.

Cyclic AMP-evoked oscillations of intracellular [Ca2+] in guinea-pig hepatocytes

The effects of the beta-adrenoceptor agonist isoprenaline and cyclic AMP (cAMP) on cytosolic free Ca2+ ([Ca2+]i) were studied in the single guinea-pig hepatocyte. In common with InsP3-dependent agonists such as noradrenaline or angiotensin II, isoprenaline (0.5-10 microM) and cAMP (50-100 mM, perfused into the cell via the patch-pipette), were able to generate fast and slow fluctuations of [Ca2+]i. Responses to isoprenaline and cAMP also were observed in the absence of external Ca2+. Isoprenaline-evoked [Ca2+]i rises were not blocked by the intracellular perfusion of heparin, suggesting that these fluctuations are independent of the binding of InsP3 to its receptor.

Cytosolic Ca2+ oscillations in REF52 fibroblasts: Ca(2+)-stimulated IP3 production or voltage-dependent Ca2+ channels as key positive feedback elements

Oscillations in cytosolic free calcium concentrations ([Ca2+]i) can be elicited in REF52 fibroblasts by three different modes of stimulation. We have previously demonstrated that [Ca2+]i oscillations result when these cells are simultaneously depolarized and stimulated with a hormone linked to phosphoinositide breakdown. Further evidence is now presented that such oscillations are linked to fluctuations in the concentration of IP3 and the Ca2+ content of an IP3-sensitive Ca2+ store. [Ca2+]i oscillations can also be generated in REF52 cells either by direct stimulation of G-proteins with GTP gamma S or AlF4- or by destabilizing the membrane potential and opening voltage-dependent calcium channels. This report compares the different types of oscillations and their mechanisms.

Calcium oscillations in endothelial cells

Several different types of endothelial cells are now known to respond to agonist stimulation with oscillations of cytosolic free [Ca2+] ([Ca2+]i). The oscillations can be repetitive [Ca2+]i spikes or sinusoidal-like oscillations according to the type of endothelial cell. Several properties of these oscillations are described including the effect of removal of extracellular Ca2+ and of changes in membrane potential, and the spatial heterogeneity of the oscillations. Results obtained with human umbilical vein endothelial cells are assessed in relation to a model for [Ca2+]i oscillations that involves Ca(2+)-induced Ca2+ release. In some preparations the oscillations are synchronized in neighbouring cells, whereas in other preparations they are not. The degree of synchrony may have functional implications and this is discussed with respect to control of blood flow and transmural permeability. A third functional implication of oscillations, their possible effect on desensitization, is also discussed.

Spatial and temporal organization of calcium signalling in hepatocytes

Treatment of hepatocytes with agonists which act via the second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), results in increases of cytosolic free Ca2+ [( Ca2+]i) which are manifest as a series of discrete [Ca2+]i transients or oscillations. With increasing agonist dose [Ca2+]i oscillation frequency increases and the initial latent period decreases, but the amplitude of the [Ca2+]i oscillations remains constant. Studies of these [Ca2+]i oscillations at the subcellular level have indicated that the [Ca2+]i changes do not occur synchronously throughout the cell, but initiate at a specific subcellular domain, adjacent to a region of the plasma membrane, and then propagate through the cell as a [Ca2+]i wave. For a given ceil, the locus of [Ca2+]i wave initiation is constant for every oscillation in a series and is also identical when the cell is sequentially stimulated with different agonists or when the phospholipase C-linked G protein is activated directly using AIF4-. The kinetics of the [Ca2+]i waves indicate that a Ca(2+)-activated mechanism is involved in propagating the oscillatory [Ca2+]i increases throughout the cell, and the data appear to be most consistent with a process of Ca(2+)-induced Ca2+ release. It is proposed that the ability to propagate [Ca2+]i oscillations into regions of the cell distal to the region in which the signal transduction apparatus is localized could serve an important function in allowing all parts of the cell to respond to the stimulus.

Signal-induced Ca2+ oscillations: properties of a model based on Ca(2+)-induced Ca2+ release

We consider a simple, minimal model for signal-induced Ca2+ oscillations based on Ca(2+)-induced Ca2+ release. The model takes into account the existence of two pools of intracellular Ca2+, namely, one sensitive to inositol 1,4,5 trisphosphate (InsP3) whose synthesis is elicited by the stimulus, and one insensitive to InsP3. The discharge of the latter pool into the cytosol is activated by cytosolic Ca2+. Oscillations in cytosolic Ca2+ arise in this model either spontaneously or in an appropriate range of external stimulation; these oscillations do not require the concomitant, periodic variation of InsP3. The following properties of the model are reviewed and compared with experimental observations: (a) Control of the frequency of Ca2+ oscillations by the external stimulus or extracellular Ca2+; (b) correlation of latency with period of Ca2+ oscillations obtained at different levels of stimulation; (c) effect of a transient increase in InsP3; (d) phase shift and transient suppression of Ca2+ oscillations by Ca2+ pulses, and (e) propagation of Ca2+ waves. It is shown that on all these counts the model provides a simple, unified explanation for a number of experimental observations in a variety of cell types. The model based on Ca(2+)-induced Ca2+ release can be extended to incorporate variations in the level of InsP3 as well as desensitization of the InsP3 receptor; besides accounting for the phenomena described by the minimal model, the extended model might also account for the occurrence of complex Ca2+ oscillations.

Ca2+ waves in astrocytes

The glial cell is the most numerous cell type in the central nervous system and is believed to play an important role in guiding brain development and in supporting adult brain function. One type of glial cell, the astrocyte also may be an integral computational element in the brain since it undergoes neurotransmitter-triggered signalling. Here we review the role of the astrocyte in the central nervous system, emphasizing receptor-mediated Ca2+ physiology. One focus is the recent discovery that the neurotransmitter glutamate induces a variety of intracellular Ca2+ changes in astrocytes. Simple Ca2+ spikes or intracellular Ca2+ oscillations often appear spatially uniform. However, in many instances, the Ca2+ rise has a significant spatial dimension, beginning in one part of the cell it spreads through the rest of the cell in the form of a wave. With high enough agonist concentration an astrocyte syncitium supports intercellular waves which propagate from cell to cell over relatively long distances. We present results of experiments using more specific pharmacological glutamate receptor agonists. In addition to describing the intercellular Ca2+ wave we present evidence for another form of intercellular signalling. Some possible functions of a long-range glial signalling system are also discussed.

Cytoplasmic calcium oscillations: a two pool model

Cytosolic calcium oscillations induced by a wide range of agonists, particularly those which stimulate phosphoinositide metabolism, are the result of a periodic release of stored calcium. The formation of inositol 1,4,5 trisphosphate (Ins(1,4,5)P3) seems to play an important role because it can initiate this periodic behaviour when injected or perfused into a variety of cells. A two pool model has been developed to explain how Ins(1,4, 5)P3 sets up these calcium oscillations. It is proposed that Ins(1,4,5)P3 acts through its specific receptor to create a constant influx of primer calcium (Ca2+p) made up of calcium released from the Ins(1,4,5)P3-sensitive pool (ISCS) together with an influx of external calcium. This Ca2+p fails to significantly elevate cytosolic calcium because it is rapidly sequestered by the Ins(1,4,5)P3-insensitive (IICS) stores of calcium distributed throughout the cytosol. Once the latter have filled, they are triggered to release their stored calcium through a process of calcium-induced calcium release to give a typical calcium spike (Ca2+s). In many cells, each Ca2+s begins at a discrete initiation site from which it then spreads through the cell as a wave. The two pool model can account for such waves if it is assumed that calcium released from one IICS diffused across to excite its neighbours thereby setting up a self-propagating wave based on calcium-induced calcium release.

Thimerosal causes calcium oscillations and sensitizes calcium-induced calcium release in unfertilized hamster eggs

Calcium-induced-calcium-release (CICR) was assayed in unfertilized golden hamster eggs by injecting Ca2+ and monitoring Ca2(+)-dependent hyperpolarizing responses (HRs) and Ca2(+)-sensitive fluo-3 fluorescence. Incubating eggs in the sulfhydryl reagent thimerosal caused [Ca2+]i oscillations as monitored by Ca2(+)-dependent HRs and decreased approximately 10-fold the Ca2+ injection current required to generate an HR and cause a large intracellular Ca2+ increase. Thimerosal also enhanced the sensitivity of eggs to Ca2+ injection in a calcium-free medium. The effects of thimerosal on CICR were prevented by dithiothreitol and were not mimicked by injecting inositol 1,4,5-trisphosphate. The data suggest that thimerosal may be an alternative agent for studying CICR in caffeine-insensitive cells.

Generation of calcium oscillations in fibroblasts by positive feedback between calcium and IP3

A wide variety of nonexcitable cells generate repetitive transient increases in cytosolic calcium ion concentration ([Ca2+]i) when stimulated with agonists that engage the phosphoinositide signalling pathway. Current theories regarding the mechanisms of oscillation disagree on whether Ca2+ inhibits or stimulates its own release from internal stores and whether inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DG) also undergo oscillations linked to the Ca2+ spikes. In this study, Ca2+ was found to stimulate its own release in REF52 fibroblasts primed by mitogens plus depolarization. However, unlike Ca2+ release in muscle and nerve cells, this amplification was insensitive to caffeine or ryanodine and required hormone receptor occupancy and functional IP3 receptors. Oscillations in [Ca2+]i were accompanied by oscillations in IP3 concentration but did not require functional protein kinase C. Therefore, the dominant feedback mechanism in this cell type appears to be Ca2+ stimulation of phospholipase C once this enzyme has been activated by hormone receptors.

Regulation of intracellular calcium in cerebellar granule neurons: effects of depolarization and of glutamatergic and cholinergic stimulation

The regulation of the cytosolic free Ca2+ concentration ([Ca2+]i) was investigated by microfluorimetry in single cerebellar granule neurons exposed to various treatments (high K+, glutamate, or acetylcholine) and drugs. The responses to the treatments developed asynchronously during cell culture, with high K+ and glutamate reaching their maxima at 6 and 7 days in vitro and acetylcholine at 9 days in vitro. The biphasic [Ca2+]i transients induced by high K+ (an initial peak, followed by a plateau 30-40% of the peak, both sustained by dihydropyridine-sensitive voltage-gated Ca2+ channels) were dissipated by washing with fresh medium or, more rapidly, by addition of excess EGTA (t1/2 = 11 +/- 2 and 3 +/- 0.6 s, respectively). Compared to those induced by high K+, the [Ca2+]i transients induced by glutamate administered in Mg2(+)-free medium were much more variable. An initial peak, sustained by voltage-gated Ca2+ channels, was visible in only approximately 50% of the cells and disappeared when multiple glutamate pulses were administered. In the rest of the population, the transients were monophasic, with persistent plateaus sustained only in part (30-40%) by voltage-gated Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization of intracellular calcium signals generated by inositol lipid-dependent hormones

Recent studies at the single cell level have demonstrated hitherto unsuspected complexities in the organization of intracellular Ca2+ homeostasis in both the temporal and spatial domains. Activation of receptors coupled to the phosphoinositide signalling system has been shown to generate [Ca2+]i oscillations in many cell types. These oscillations display diverse patterns, with variations in oscillation amplitude, latency and frequency which are often tissue and/or agonist dose specific. Furthermore, increases in [Ca2+]i can either occur uniformly or originate from a specific region and propagate throughout the cell in the form of a Ca2+ wave. The significance and underlying mechanisms responsible for these phenomena are discussed.

Changes in the mechanism of Ca2(+) mobilization during the differentiation of BC3H1 muscle cells

Ca2+ sequestration and release in BC3H1 muscle cells is strongly dependent on the stage of differentiation. In proliferating cells, more than 90% of the sequestered Ca2+ was Ins(1,4,5)P3-sensitive and 25% was caffeine-sensitive. In differentiated cells, the Ca2+ accumulation was 5-fold higher and was InsP3-insensitive, but about 60% of the sequestered Ca2+ was caffeine-sensitive. These changes were reversible upon addition of growth stimuli. Similarly, by measuring the intracellular Ca2+ concentration in single intact BC3H1 cells, it was found that the number of histamine-responsive cells decreased and the number of caffeine-responsive cells increased during muscle cell differentiation. These data indicate that the development of the muscle phenotype in BC3H1 myoblasts induces a major rearrangement of the mechanisms for Ca2+ mobilization.

Cytosolic calcium ion regulation in cultured endothelial cells

Mechanisms involved in Ca2+ homeostasis and stimulus-secretion coupling in cultured endothelial cells in response to humoral and physical stimuli include passive leak, activation of ion channels, and chemical second messengers. Calcium entry is controlled by receptor activation, passive leak, and mechanical stretch. The rate at which Ca2+ enters the cell through these pathways is dependent on the transmembrane potential which governs the electrochemical gradient for Ca2+ and which is set by participation of various K channels.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.

Synchronized repetitive spikes in cytoplasmic calcium in confluent monolayers of human umbilical vein endothelial cells

Synchronized repetitive spikes in cytoplasmic free calcium concentration, [Ca2+]i, are evoked by histamine in confluent monolayers of human endothelial cells. The repetitive spikes, which are apparently dependent upon the establishment of cell coupling, are also induced by caffeine, indicating that they may be due to an oscillatory release of Ca2+ from the endoplasmic reticulum, and may not involve oscillations in inositol phosphates. It is suggested that synchronized repetitive spikes in [Ca2+]i might lead to oscillatory release of endothelial-derived substances such as prostacyclin, nitric oxide and endothelin, which have potent effects on the vascular system.

Homologous desensitization of ATP-mediated elevations in cytoplasmic calcium and prostacyclin release in human endothelial cells does not involve protein kinase C

Single human umbilical-vein endothelial cells in culture loaded with the Ca2(+)-sensitive dye fura-2 exhibited characteristic increases in cytosolic Ca2+ concentrations [( Ca2+]i) in response to extracellular ATP. The rapid decline of [Ca2+]i to prestimulated levels in the continued presence of ATP, with in most cells no sustained or oscillatory increase in [Ca2+]i, indicated desensitization. This was agonist-specific, and contrasted with the [Ca2+]i response to histamine, though each agonist mobilized Ca2+ from the same internal store. In populations of cells, when desensitization was variably induced by a second challenge with ATP after different times, desensitization of the initial peak [Ca2+]i was directly related to desensitization of prostacyclin release. This was not affected by treatment with the protein kinase C inhibitor staurosporine, under conditions where a similar degree of desensitization of peak [Ca2+]i induced by phorbol 12-myristate 13-acetate was blocked. Sequential addition of ATP to cell populations cumulatively desensitized the peak elevation of [Ca2+]i, but did not block the second, sustained, phase of the response. We conclude that desensitization of prostacyclin synthesis by ATP is likely to be due to uncoupling of the P2Y purinoceptor from phosphoinositidase C, but does not involve protein kinase C activation.

2,5-Di-(tert-butyl)-1,4-benzohydroquinone mobilizes inositol 1,4,5-trisphosphate-sensitive and -insensitive Ca2+ stores

In permeabilized rat hepatocytes a maximal concentration (25 microM) of 2,5-di-(tert-butyl)-1,4-benzohydroquineone (tBuBHQ) mobilized 70% of sequestere Ca2+ and a half-maximal effect was produced by 1.7 microM tBuBHQ. Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) stimulated release of about 40% of the intracellular Ca2+ stores. Combined applications of a range of tBuBHQ concentrations with a maximal concentration of Ins(1,4,5)P3 demonstrated that tBuBHQ has slight selectivity for the Ca2+ transport process of the Ins(1,4,5)P3-sensitive stores. We conclude that the Ins(1,4,5)P3-sensitive stores are a subset of those sensitive to tBuBHQ and that the latter is therefore unlikely to prove useful as a tool to discriminate Ins(1,4,5)P3-sensitive and -insensitive Ca2+ stores though it may provide opportunities to design more selective agents.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.