Subcellular distribution of Ca2+ release channels underlying Ca2+ waves and oscillations in exocrine pancreas

Cholecystokinin-stimulated enzyme secretion from dispersed rabbit pancreatic acinar cells: phosphorylation-dependent changes in potency and efficacy

In order to establish a regulatory role for phosphoproteins in receptor-stimulated enzyme secretion, dispersed rabbit pancreatic acinar cells were stimulated with the COOH-terminal octapeptide of cholecystokinin (CCK8) in the absence and presence of staurosporine and/or 12-O-tetradecanoylphorbol 13-acetate (TPA) or forskolin. The dose/response curve for the stimulatory effect of CCK8 on amylase secretion was biphasic, with a mean half-maximal concentration (EC50) of 21 pM. Staurosporine (1 microM) did not affect secretion elicited by CCK8 concentrations below 0.1 nM, but reduced the response to CCK8 concentrations above 0.1 nM. As a result, the mean EC50 for CCK8 decreased to 8 pM and its efficacy to 70%. The phorbol ester TPA (0.1 microM) attenuated secretion evoked by CCK8 concentrations below 0.1 nM and potentiated the response to CCK8 concentrations above 0.1 nM. As a result, the mean EC50 for CCK8 increased to 0.14 nM and its efficacy to 300%. Staurosporine abolished both the inhibitory and the potentiating effect of TPA, thereby turning the inhibitory effect into a strong potentiating effect. As a result, the mean EC50 for CCK8 decreased to 3 pM, whereas its efficacy increased to 190%. Forskolin (30 microM) potentiated the response to both the lower and the higher CCK8 concentrations. As a result, the mean EC50 for CCK8 increased to 28 pM and its efficacy to 300%. Staurosporine enhanced the potentiating effect of forskolin at CCK8 concentrations below 0.1 nM, but abolished potentiation at CCK8 concentrations above 0.1 nM. As a result, the mean EC50 for CCK8 decreased to 1.4 pM, whereas its efficacy increased to 260%. The data presented demonstrate that the apparent sensitivity of dispersed pancreatic acinar cells to stimulation of the process of enzyme secretion by CCK8 decreases when kinases are activated and increases when kinases are inactivated. Moreover, they show that the efficacy of CCK8 increases by the action of kinases, both sensitive and insensitive to staurosporine.

Subcellular calcium oscillators and calcium influx support agonist-induced calcium waves in cultured astrocytes

We have analysed Ca2+ waves induced by norepinephrine in rat cortical astrocytes in primary culture using fluorescent indicators fura-2 or fluo-3. The temporal pattern of the average [Ca2+]i responses were heterogeneous from cell to cell and most cells showed an oscillatory response at concentrations of agonist around EC50 (200 nM). Upon receptor activation, [Ca2+]i signals originated from a single cellular locus and propagated throughout the cell as a wave. Wave propagation was supported by specialized regenerative calcium release loci along the length of the cell. The periods of oscillations, amplitudes, and the rates of [Ca2+]i rise of these subcellular oscillators differ from each other. These intrinsic kinetic properties of the regenerative loci support local waves when stimulation is continued over long periods of time. The presence of local waves at specific, invariant cellular sites and their inherent kinetic properties provide for the unique and reproducible pattern of response seen in a given cell. We hypothesize that these loci are local specializations in the endoplasmic reticulum where the magnitude of the regenerative Ca2+ release is higher than other regions of the cell. Removal of extracellular Ca2+ or blockade of Ca2+ channels by inorganic cations (Cd2+ and Ni2+) during stimulation of adrenergic receptors alter the sustained plateau component of the [Ca2+]i response. In the absence of Ca2+ release, due to store depletion with thapsigargin, agonist occupation alone does not induce Ca2+ influx in astrocytes. This finding suggests that, under these conditions, receptor-operated Ca2+ entry is not operative. Furthermore, our experiments provide evidence for local Ca2+ oscillations in cells which can support both wave propagation as well as spatially discrete Ca2+ signalling.

Differential expression of type 2 and type 3 inositol 1,4,5-trisphosphate receptor mRNAs in various mouse tissues: in situ hybridization study

The inositol 1,4,5-trisphosphate receptor (IP3R) is an intracellular Ca2+ release channel responsible for mobilizing stored Ca2+. Three different receptor types have been molecularly cloned, and their genes have been classified into a family. The gene for the type 1 receptor (IP3R1) is predominantly expressed in cerebellar Purkinje neurons, but its gene product is localized widely in a variety of tissues; however, there is little information on what types of cells express the other two receptor types, type 2 and type 3 (IP3R2 and IP3R3, respectively). We studied the expression of the IP3R gene family in various mouse tissues by in situ hybridization histochemistry. Compared with IP3R1, the levels of expression of IP3R2 and IP3R3 mRNAs were low in all of the tissues tested. IP3R2 mRNA was localized in the intralobular duct cells of the submandibular gland, the urinary tubule cells of the kidney, the epithelial cells of epididymal ducts and the follicular granulosa cells of the ovary, while the IP3R3 mRNA was distributed in gastric cells, salivary and pancreatic acinar cells and the epithelium of the small intestine. All of these cells which express either IP3R2 or IP3R3 mRNA are known to have a secretory function in which IP3/Ca2+ signalling has been shown to be involved, and thus either IP3R2 or IP3R3 may be a prerequisite to secretion in these cells.

Ca2+ waves in PC12 neurites: a bidirectional, receptor-oriented form of Ca2+ signaling

Spatial and temporal aspects of Ca2+ signaling were investigated in PC12 cells differentiated with nerve growth factor, the well known nerve cell model. Activation of receptors coupled to polyphosphoinositide hydrolysis gave rise in a high proportion of the cells to Ca2+ waves propagating non decrementally and at constant speed (2-4 microns/s at 18 degrees C and approximately 10-fold faster at 37 degrees C) along the neurites. These waves relied entirely on the release of Ca2+ from intracellular stores since they could be generated even when the cells were incubated in Ca(2+)-free medium. In contrast, when the cells were depolarized with high K+ in Ca(2+)-containing medium, increases of cytosolic Ca2+ occurred in the neurites but failed to evolve into waves. Depending on the receptor agonist employed (bradykinin and carbachol versus ATP) the orientation of the waves could be opposite, from the neurite tip to the cell body or vice versa, suggesting different and specific distribution of the responsible surface receptors. Cytosolic Ca2+ imaging results, together with studies of inositol 1,4,5-trisphosphate generation in intact cells and inositol 1,4,5-trisphosphate-induced Ca2+ release from microsomes, revealed the sustaining process of the waves to be discharge of Ca2+ from the inositol 1,4,5-trisphosphate- (and not the ryanodine-) sensitive stores distributed along the neurites. The activation of the cognate receptor appears to result from the coordinate action of the second messenger and Ca2+. Because of their properties and orientation, the waves could participate in the control of not only conventional cell activities, but also excitability and differential processing of inputs, and thus of electrochemical computation in nerve cells.

Characterization of single potassium channels in mouse pancreatic acinar cells

1. Single K(+)-selective channels with a conductance of about 48 pS (pipette, 145 mM KCl; bath, 140 mM NaCl + 4.7 mM KCl) were recorded in the patch-clamp whole-cell configuration in isolated mouse pancreatic acinar cells. 2. Neither application of the secretagogues acetylcholine (second messenger, inositol 1,4,5-trisphosphate) or secretin (second messenger, cAMP), nor addition of the catalytic subunit of protein kinase A to the pipette solution changed the activity of the 48 pS K+ channel. 3. Intracellular acidification with sodium propionate (20 mM) diminished activity of the 48 pS channel, whereas channel open probability was increased by cytosolic alkalization with 20 mM NH4Cl. 4. BaCl2 (5 mM), TEA (10 mM) or apamin (1 microM) added to the bath solution had no obvious effect on the kinetics of the 48 pS channel. Similarly, glibenclamide and diazoxide failed to influence the channel activity. 5. When extracellular NaCl was replaced by KCl, whole-cell recordings revealed an inwardly rectifying K+ current carried by a 17 pS K+ channel. 6. The inwardly rectifying K+ current was not pH dependent and could largely be blocked by Ba2+ but not by TEA. 7. Since the 48 pS K+ channel is neither Ca2+ nor cAMP regulated, we suggest that this channel could play a role in the maintenance of the negative cell resting potential.

Region-specific activity of the plasma membrane Ca2+ pump and delayed activation of Ca2+ entry characterize the polarized, agonist-evoked Ca2+ signals in exocrine cells

The initial release of Ca2+ from the intracellular Ca2+ stores is followed by a second phase during which the agonist-dependent Ca2+ response becomes sensitive to the extracellular Ca2+, indicating the involvement of the plasma membrane (PM) Ca2+ transport systems. The time course of activation of these transport systems, which consist of both Ca2+ extrusion and Ca2+ entry pathways, is not well established. To investigate the participation of these processes during the agonist-evoked Ca2+ response, isolated pancreatic acinar cells were exposed to maximal concentrations of an inositol 1,4,5-trisphosphate-mobilizing agonist (acetylcholine, 10 microM) in different experimental conditions. Following the increase of [Ca2+]i, there was an almost immediate activation of the PM Ca2+ extrusion system, and maximal activity was reached within less than 2s. The rate of Ca2+ extrusion was dependent on the level of [Ca2+]i, with a steep activation at values just above the resting [Ca2+]i and reached a plateau value at 700 nM Ca2+. In contrast, the PM Ca2+ entry pathway was activated with a much slower time course. There was also a delay of 3-4 s between the maximal effective depletion of the intracellular Ca2+ stores and the activation of this entry pathway. By use of digital imaging data, the PM Ca2+ transport systems were also analyzed independently in two regions of the cells, the lumenal and the basal poles. With respect to the activation of the Ca2+ entry pathways, no significant difference existed between these two regions. In contrast, the PM Ca2+ pump displayed a different pattern of activity in these regions. In the basal pole, the pump activity was more sensitive to changes of [Ca2+]i and had a higher maximal activity. Also, in the lumenal pole, the pump became saturated at values of [Ca2+]i around 700 nM, whereas at the basal pole [Ca2+]i had a biphasic effect on the pump activity, and higher [Ca2+]i inhibited the pump. It is argued that these differences in sensitivity to the levels of [Ca2+]i and the different relationship between [Ca2+]i and the rate of extrusion at the two functional poles of the pancreatic acinar cells indicate that the plasma membrane Ca2+ ATPase might play an important role in the polarization of the Ca2+ response.

Inositol trisphosphate receptor mediated spatiotemporal calcium signalling

Spatiotemporal Ca2+ signalling in the cytoplasm is currently understood as an excitation phenomenon by analogy with electrical excitation in the plasma membrane. In many cell types, Ca2+ waves and Ca2+ oscillations are mediated by inositol 1,4,5-trisphosphate (IP3) receptor/Ca2+ channels in the endoplasmic reticulum membrane, with positive feedback between cytosolic Ca2+ and IP3-induced Ca2+ release creating a regenerative process. Remarkable advances have been made in the past year in the analysis of subcellular Ca2+ microdomains using confocal microscopy and of Ca2+ influx pathways that are functionally coupled to IP3-induced Ca2+ release. Ca2+ signals can be conveyed into the nucleus and mitochondria. Ca2+ entry from outside the cell allows repetitive Ca2+ release by providing Ca2+ to refill the endoplasmic reticulum stores, thus giving rise to frequency-encoded Ca2+ signals.

Inositol 1,4,5-trisphosphate receptors, secretory granules and secretion in endocrine and neuroendocrine cells

Recent studies have revealed the presence of inositol 1,4,5-trisphosphate receptors in the secretory granules of endocrine and neuroendocrine cells. This distribution suggests that inositol 1,4,5-trisphosphate-regulated release of granule stores of Ca2+ might facilitate the secretory process. In addition, inositol 1,4,5-trisphosphate receptors might participate directly in the biogenesis of secretory granules. The presence of inositol 1,4,5-trisphosphate receptors in synaptic nerve terminals raises the possibility that they might also be involved in the control of neurotransmitter release.

The ryanodine receptor/calcium channel genes are widely and differentially expressed in murine brain and peripheral tissues

Ryanodine receptors (RyRs) are intracellular calcium release channels that participate in controlling cytosolic calcium levels. At variance with the probably ubiquitous inositol 1,4,5-trisphosphate-operated calcium channels (1,4,5-trisphosphate receptors), RyRs have been mainly regarded as the calcium release channels controlling skeletal and cardiac muscle contraction. Increasing evidence has recently suggested that RyRs may be more widely expressed, but this has never been extensively examined. Therefore, we cloned three cDNAs corresponding to murine RyR homologues to carry a comprehensive analysis of their expression in murine tissues. Here, we report that the three genes are expressed in almost all tissues analyzed, where tissue-specific patterns of expression were observed. In the uterus and vas deferens, expression of RyR3 was localized to the smooth muscle component of these organs. In the testis, expression of RyR1 and RyR3 was detected in germ cells. RyR mRNAs were also detected in in vitro-cultured cell lines. RyR1, RyR2, and RyR3 mRNA were detected in the cerebrum and in the cerebellum. In situ analysis revealed a cell type-specific pattern of expression in the different regions of the central nervous system. The differential expression of the three ryanodine receptor genes in the central nervous system was also confirmed using specific antibodies against the respective proteins. This widespread pattern of expression suggests that RyRs may participate in the regulation of intracellular calcium homeostasis in a range of cells wider than previously recognized.

Function and characteristics of repetitive calcium waves associated with meiosis

BACKGROUND

Internal calcium waves and oscillations are now recognized as universal features of cellular activation, but their exact role remains uncertain. In mammalian and ascidian eggs, a large, sperm-triggered calcium activation wave crosses the egg at fertilization, followed by a series of periodic increases in intracellular calcium concentration ([Ca2+]i). We have previously shown that, in eggs of the ascidian Phallusia mammillata, these periodic, post-activation [Ca2+]i increases are in the form of waves, the origin of which relocalizes to a pacemaker region, and that they stop seconds before the completion of meiosis.

RESULTS

We show here that the origin of the first one to four post-activation calcium waves in P. mammillata eggs transfers progressively from the site of sperm entry, usually in the animal hemisphere, towards an endoplasmic reticulum (ER)-rich contraction pole in the vegetal hemisphere, a process that takes about five minutes. Once the origin of these repetitive post-activation calcium waves has reached the contraction pole, all subsequent calcium waves originate from the domain of ER concentrated there, which acts as a pacemaker. The first few post-activation calcium waves are faster than the activation wave and, like the activation wave, they propagate homogeneously throughout the cytoplasm. Approximately five to ten minutes after fertilization, the post-activation calcium waves begin to propagate preferentially in the egg cortex. By manipulating intracellular calcium levels with caged inositol 1,4,5 trisphosphate (InsP3) and a competitive inhibitor of InsP3-induced calcium release, we show that the activation wave induced by the sperm is sufficient to induce extrusion of the first polar body, but that additional [Ca2+]i increases are necessary for completion of the second meiotic division. However, periodic calcium waves per se do not seem to be strictly necessary for the completion of meiosis, as a persistent and homogeneous increase in calcium, induced by the calcium ionophore ionomycin, is sufficient to cause second polar body formation and allow completion of meiosis on time.

CONCLUSION

These results clearly show that, in the ascidian egg, post-activation calcium waves are required to complete meiosis. They also show that following a period of progressive relocalization of the wave origin, which lasts approximately five minutes, an ER-rich domain at the contraction pole finally becomes a pacemaker from which the calcium waves originate. Once their origin becomes stably localized, the calcium waves begin to propagate preferentially around the cortex of the egg rather than throughout the egg cytoplasm.

Cell membrane stretch in osteoclasts triggers a self-reinforcing Ca2+ entry pathway

Many cell types respond to mechanical membrane perturbation with intracellular Ca2+ responses. Stretch-activated (SA) ion channels may be involved in such responses. We studied the occurrence as well as the underlying mechanisms of cell membrane stretch-evoked responses in fetal chicken osteoclasts using separate and simultaneous patch-clamp and Ca2+ imaging measurements. In the present paper, evidence is presented showing that such responses involve a self-reinforcing mechanism including SA channel activity, Ca(2+)-activated K+ (KCa) channel activity, membrane potential changes and local and general intracellular Ca2+ ([Ca2+]i) increases. The model we propose is that during membrane stretch, both SA channels and KCa channels open at membrane potential values near the resting membrane potential. SA channel characterization showed that these SA channels are permeable to Ca2+. During membrane stretch, Ca2+ influx through SA channels and hyperpolarization due to KCa channel activity serve as positive feedback, leading ultimately to a Ca2+ wave and cell membrane hyperpolarization. This self-reinforcing mechanism is turned off upon SA channel closure after cessation of membrane stretch. We suggest that this Ca2+ entry mechanism plays a role in regulation of osteoclast activity.

ATP-dependent accumulation and inositol trisphosphate- or cyclic ADP-ribose-mediated release of Ca2+ from the nuclear envelope

Uptake and release of Ca2+ from isolated liver nuclei were studied with fluorescent probes. We show with the help of digital imaging and confocal microscopy that the Ca(2+)-sensitive fluorescent probe Fura 2 is concentrated in or around the nuclear envelope and that the distribution of Fura 2 fluorescence is similar to that of an endoplasmic reticulum marker. The previously demonstrated ATP-dependent uptake of Ca2+ into isolated nuclei and release of the accumulated Ca2+ by inositol 1,4,5-trisphosphate (IP3) are therefore due to transport of Ca2+ into and out of the nuclear envelope and not the nucleoplasm. Dextrans labeled with fluorescent Ca2+ indicators (calcium-Green 1 and Fura 2) are distributed uniformly in the nucleoplasm and can be used to show that changes in the external Ca2+ concentration produce rapid changes in the nucleoplasmic Ca2+ concentration. Nevertheless, IP3 and cyclic ADP-ribose evoke transient intranuclear Ca2+ elevations. The release from the Ca2+ stores in or around the nuclear envelope appears to be directed into the nucleoplasm from where it can diffuse out through the permeable nuclear pore complexes.

Differences in uptake, storage and release properties between inositol trisphosphate-sensitive and -insensitive Ca2+ stores in permeabilized pancreatic acinar cells

Rabbit pancreatic acinar cells, permeabilized by saponin treatment, were used to study the kinetics of ATP-dependent Ca2+ uptake and release in inositol 1,4,5-trisphosphate (Ins-1,4,5-P3)-sensitive and -insensitive stores. Permeabilized acinar cells rapidly accumulated Ca2+ to steady-state. At steady state, approximately 60% of actively stored Ca2+ resided in the Ins-1,4,5-P3-sensitive store. Kinetic analysis of the Ca2+ uptake process revealed that the initial Ca2+ uptake rate was 1.7 times higher in the Ins-1,4,5-P3-insensitive store as compared to the Ins-1,4,5-P3-sensitive store. On the other hand, the Ca2+ uptake capacity was 1.6 times higher in the Ins-1,4,5-P3-sensitive store as compared to the Ins-1,4,5-P3-insensitive store. The Ca2+ uptake rate in the Ins-1,4,5-P3-sensitive store remained virtually constant for at least 4 min, whereas in the Ins-1,4,5-P3-insensitive Ca2+ store this rate progressively declined with time. These observations are compatible with: (i) an Ins-1,4,5-P3-sensitive store containing relatively few Ca2+ pumps but possessing a relatively high Ca2+ uptake capacity, which may reflect the presence of a substantial amount of Ca2+ binding protein; and (ii) an Ins-1,4,5-P3-insensitive Ca2+ store containing relatively many Ca2+ pumps but possessing a relatively low Ca2+ uptake capacity, which may reflect the presence of little if any Ca2+ binding protein. The data presented are consistent with the idea of a heterogeneous distribution of Ca2+ pumps, Ca2+ binding proteins and Ca2+ release channels between intracellular Ca2+ storage organelles.

Quantal puffs of intracellular Ca2+ evoked by inositol trisphosphate in Xenopus oocytes

1. Ca2+ liberation induced in Xenopus oocytes by a poorly metabolized derivative of inositol 1,4,5-trisphosphate (3-deoxy-3-fluoro-D-myo-inositol 1,4,5-trisphosphate; 3-F-InsP3) was visualized using a video-rate confocal microscope to image fluorescence signals reported by the indicator dye calcium green-1. 2. Low (10-30 nM) intracellular concentrations of 3-F-InsP3 evoked Ca2+ release as localized transient 'puffs'. Progressively higher concentrations (30-60 nM) gave rise to abortive Ca2+ waves triggered by puffs, and then (> 60 nM) to a sustained elevation of Ca2+ followed by the appearance of propagating Ca2+ waves. At concentrations up to that giving waves, the frequency of puffs increased as about the third power of [InsP3], whereas their amplitudes increased only slightly. 3. The rise of cytosolic Ca2+ during a puff began abruptly, and peaked within about 50 ms. The peak free Ca2+ level was about 180 nM, and the total amount of Ca2+ liberated was several attomoles (10(-18) mol), too much to be accounted for by opening of a single InsP3-gated channel. The subsequent decline of Ca2+ occurred over a few hundred milliseconds, determined largely by diffusion of Ca2+ away from the release site, rather than by resequestration. Lateral spread of Ca2+ was restricted to a few micrometres, consistent with an effective diffusion coefficient for Ca2+ ions of about 27 microns2 s-1. 4. The peak amplitudes of puffs recorded at a given site were distributed in a roughly Gaussian manner, and a small proportion of sites consistently gave puffs much larger than the main population. Intervals between successive puffs at a single site were exponentially distributed, except for a progressive fall-off in puffs seen at intervals shorter than about 10 s. Thus, triggering of puffs appeared to be stochastically determined after recovery from a refractory period. 5. There was little correlation between the occurrence of puffs at sites more than a few micrometres apart, indicating that puff sites can function autonomously, but closely (ca 2 microns) adjacent sites showed highly correlated behaviour. 6. Puffs arose from sites-present at a density of about 1 per 30 microns2 in the animal hemisphere, located within a narrow band about 5-7 microns below the plasma membrane. 7. We conclude that Ca2+ puffs represent a 'quantal' unit of InsP3-evoked Ca2+ liberation, which may arise because local regenerative feedback by cytosolic Ca2+ ions causes the concerted opening of several closely clustered InsP3 receptor channels.

Co-localization of L-type Ca2+ channels and insulin-containing secretory granules and its significance for the initiation of exocytosis in mouse pancreatic B-cells

We have monitored L-type Ca2+ channel activity, local cytoplasmic Ca2+ transients, the distribution of insulin-containing secretory granules and exocytosis in individual mouse pancreatic B-cells. Subsequent to the opening of the Ca2+ channels, exocytosis is initiated with a latency < 100 ms. The entry of Ca2+ that precedes exocytosis is unevenly distributed over the cell and is concentrated to the region with the highest density of secretory granules. In this region, the cytoplasmic Ca2+ concentration is 5- to 10-fold higher than in the remainder of the cell reaching concentrations of several micromolar. Single-channel recordings confirm that the L-type Ca2+ channels are clustered in the part of the cell containing the secretory granules. This arrangement, which is obviously reminiscent of the 'active zones' in nerve terminals, can be envisaged as being favourable to the B-cell as it ensures that the Ca2+ transient is maximal and restricted to the part of the cell where it is required to rapidly initiate exocytosis whilst at the same time minimizing the expenditure of metabolic energy to subsequently restore the resting Ca2+ concentration.

Flow cytometric measurements of intracellular Ca2+ mobilization in isolated rat pancreatic acinar cells after hormone stimulation and action of lectins

Isolated rat pancreatic acinar cells were loaded with the Ca(2+)-sensitive fluorescence dye Fluo 3 in vitro and the intracellular Ca2+ changes were analysed by flow cytometry. Morphology, viability, and loading with the dye were studied by light microscopy. Stimulation with cholecystokinin/pancreozymin (CCK) and its agonist caerulein as well as with carbamylcholine (Jestryl) led to an increase of intracellular calcium ions and a fluorescence peak. The slope and height of the Ca2+ signals were found to be influenced by preincubation of cells with some plant lectins (WGA, UEA, PHA, Con A, LCA, PNA). These effects are discussed with respect to the interaction of lectins with the carbohydrate chains of cell membrane receptors.

A mathematical model of agonist-induced propagation of calcium waves in astrocytes

In astrocytes, calcium signals evoked by neurotransmitters appear as waves within single cells, which spread to other cells in the network. Recent analysis has shown that waves are initiated at a single invariant site in the cell and propagated within the cell in a nonlinear and saltatory manner by regenerative amplification at specific predestined cellular sites. In order to gain insight into local cellular waves and wave collisions we have developed a mathematical model of cellular wave amplification loci. This model is in good agreement with experimental data which includes: ambient calcium gradients in resting cells, wave origination and local amplification and generation of local waves. As observed in experiments, the model also predicts that different locations in the cell can have different frequencies of oscillation. The amplification loci are thought to be specialized areas of the endoplasmic reticulum membrane containing a higher density or higher sensitivity of IP3 receptors. Our analysis suggests that the cellular loci act as weakly coupled oscillators each with its intrinsic latency and frequency of oscillation. Thus the appearance of the propagated calcium wave may be a reflection of these differences rather than an actual diffusional wave propagation.

Inositol trisphosphate and cyclic ADP ribose as long range messengers generating local subcellular calcium signals

The process of messenger-mediated release of Ca2+ from intracellular stores, which is of great importance in virtually all cell types including neurons, can best be studied in cells lacking voltage-gated Ca2+ channels in the plasma membrane. In pancreatic acinar cells agonist-evoked repetitive cytosolic Ca2+ spikes are due to release of Ca2+ via inositoltrisphosphate (IP3) and ryanodine receptors and reuptake into the stores via thapsigargin-sensitive Ca2+ pumps. At low acetylcholine (ACh) or cholecystokinin concentrations the cytosolic Ca2+ spikes are mostly confined to the secretory granule area of the polarized pancreatic acinar cells. Similar results can be obtained by intracellular infusion of IP3 (or one of its non-metabolizable analogues) or cyclic ADP ribose. This suggests that high affinity IP3 and ryanodine receptors are concentrated in the secretory granule area. We have generated an 'artificial synapse' on isolated acinar cells by having a cell-attached patch pipette filled with ACh on the basal membrane. Initially, ACh is prevented from making contact with the receptors by the negative potential applied to the pipette. When the pipette polarity is switched to positive ACh can bind to its receptors. Using digital Ca2+ imaging it could be seen that the first cytosolic rise often occurred in the secretory granule area, a considerable distance away from the site of the agonist-receptor interaction. This shows the long-range action of the messenger(s) IP3 and or cyclic ADP ribose generated by the ACh-receptor interaction. The local Ca2+ spikes in the secretory granule area are sufficient for exocytotic secretory responses as seen in capacitance measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatiotemporal dynamics of intracellular [Ca2+]i oscillations during the growth and meiotic maturation of mouse oocytes

Calcium oscillations occur during meiotic maturation of mouse oocytes. They also trigger activation at fertilization. We have monitored [Ca2+]i in oocytes at different stages of growth and maturation to examine how the calcium release mechanisms alter during oogenesis. Spontaneous calcium oscillations occur every 2–3 minutes in the majority of fully grown (but immature) mouse oocytes released from antral follicles and resuming meiosis. The oscillations last for 2–4 hours after release from the follicle and take the form of global synchronous [Ca2+]i increases throughout the cell. Rapid image acquisition or cooling the bath temperature from 28 degrees C to 16 degrees C did not reveal any wave-like spatial heterogeneity in the [Ca2+]i signal. Calcium appears to reach highest levels in the germinal vesicle but this apparent difference of [Ca2+] in nucleus and cytoplasm is an artifact of dye loading. Smaller, growing immature oocytes are less competent: about 40% are able to resume meiosis and a similar proportion of these oocytes show spontaneous calcium oscillations. [Ca2+]i transients are not seen in oocytes that do not resume meiosis spontaneously in vitro. Nonetheless, these oocytes are capable of [Ca2+]i oscillations since they show them in response to the addition of carbachol or thimerosal. To examine how the properties of calcium release change during meiotic maturation, a calcium-releasing factor from sperm was microinjected into fully grown immature and mature oocytes. The sperm-factor-induced oscillations were about two-fold larger and longer in mature oocytes compared to immature oocytes. Calcium waves travelling at 40–60 microns/second were generated in mature oocytes, but not in immature oocytes. In some mature oocytes, successive calcium waves had different sites of origin. The modifications in the size and spatial organization of calcium transients during oocyte maturation may be a necessary prerequisite for normal fertilization.

Ca(2+)-mobilizing hormones induce sequentially ordered Ca2+ signals in multicellular systems of rat hepatocytes

The development of hormone-mediated Ca2+ signals was analysed in polarized doublets, triplets and quadruplets of rat hepatocytes by video imaging of fura2 fluorescence. These multicellular models showed dilated bile canaliculi, and gap junctions were observed by using an anti-connexin-32 antibody. They also showed highly organized Ca2+ signals in response to vasopressin or noradrenaline. Surprisingly, the primary rises in intracellular Ca2+ concentration ([Ca2+]i) did not start randomly from any cell of the multiplet. It originated invariably in the same hepatocyte (first-responding cell), and then was propagated in a sequential manner to the nearest connected cells (cell 2, then 3, in triplets; cell 2, 3, then 4 in quadruplets). The sequential activation of the cells appeared to be an intrinsic property of multiplets of rat hepatocytes. (1) In the continued presence of hormones, the same sequential order was observed up to six times, i.e. at each train of oscillations occurring between the cells. (2) The order of [Ca2+]i responses was modified neither by the repeated addition of hormones nor by the hormonal dose. (3) The mechanical disruption of an intermediate cell slowed down the speed of the propagation, suggesting a role of gap junctions in the rapidity of the sequential activation of cells. (4) The same multiplet could have a different first-responding cell for vasopressin or noradrenaline, suggesting a role of the hormonal receptors in the sequentiality of cell responses. It is postulated that a functional heterogeneity of hormonal receptors, and the presence of functional gap junctions, are involved in the existence of sequentially ordered hormone-mediated [Ca2+]i rises in the multiplets of rat hepatocytes.

Properties of intracellular Ca2+ waves generated by a model based on Ca(2+)-induced Ca2+ release

Cytosolic Ca2+ waves occur in a number of cell types either spontaneously or after stimulation by hormones, neurotransmitters, or treatments promoting Ca2+ influx into the cells. These waves can be broadly classified into two types. Waves of type 1, observed in cardiac myocytes or Xenopus oocytes, correspond to the propagation of sharp bands of Ca2+ throughout the cell at a rate that is high enough to permit the simultaneous propagation of several fronts in a given cells. Waves of type 2, observed in hepatocytes, endothelial cells, or various kinds of eggs, correspond to the progressive elevation of cytosolic Ca2+ throughout the cell, followed by its quasi-homogeneous return down to basal levels. Here we analyze the propagation of these different types of intracellular Ca2+ waves in a model based on Ca(2+)-induced Ca2+ release (CICR). The model accounts for transient or sustained waves of type 1 or 2, depending on the size of the cell and on the values of the kinetic parameters that measure Ca2+ exchange between the cytosol, the extracellular medium, and intracellular stores. Two versions of the model based on CICR are considered. The first version involves two distinct Ca2+ pools sensitive to inositol 1,4,5-trisphosphate (IP3) and Ca2+, respectively, whereas the second version involves a single pool sensitive both to Ca2+ and IP3 behaving as co-agonists for Ca2+ release. Intracellular Ca2+ waves occur in the two versions of the model based on CICR, but fail to propagate in the one-pool model at subthreshold levels of IP3. For waves of type 1, we investigate the effect of the spatial distribution of Ca(2+)-sensitive Ca2+ stores within the cytosol, and show that the wave fails to propagate when the distance between the stores exceeds a critical value on the order of a few microns. We also determine how the period and velocity of the waves are affected by changes in parameters measuring stimulation, Ca2+ influx into the cell, or Ca2+ pumping into the stores. For waves of type 2, the numerical analysis indicates that the best qualitative agreement with experimental observations is obtained for phase waves. Finally, conditions are obtained for the occurrence of "echo" waves that are sometimes observed in the experiments.

Two inositol 1,4,5-trisphosphate binding sites in rat basophilic leukemia cells: relationship between receptor occupancy and calcium release

Quantal calcium release is a novel paradigm for second messenger signal transduction which provides spatial and temporal control of calcium release from intracellular stores by inositol 1,4,5-trisphosphate (InsP3). We have proposed a mechanism to account for this phenomenon [Kindman, L. A., & Meyer, T. (1993) Biochemistry 32, 1270-1277], which hypothesized the existence of five channels, each with a different affinity for InsP3. As a direct test of this hypothesis, InsP3 binding to microsomes from RBL cells was examined under conditions similar to those used for calcium release. Scatchard analyses performed under a variety of conditions indicates the presence of high affinity (KD = 0.9 +/- 0.3 nM) and low affinity (KD = 47 +/- 5 nM) InsP3 binding sites. The low affinity sites are more prevalent, constituting 82 +/- 5% of the total. Both sites are identified in the presence and absence of MgATP. Moreover, both sites are selective for InsP3 over InsP4, through high concentrations of InsP4 displace InsP3 from each site (with inhibition constants of 16 and 267 nM InsP4, respectively). The relative abundance of the two InsP3 binding sites is Ca2+ dependent. An increase in Ca2+ from 0.1 to 0.5 microM results in the apparent conversion of a portion of the low affinity sites into high affinity sites into high affinity sites. Ca2+ (0.5 microM) also increased the KD of the low affinity InsP3 binding site. Given the presence of both high and low affinity InsP3 binding sites, two simple mathematical models describing both the kinetics of calcium release and quantal calcium release from RBL cells were developed.(ABSTRACT TRUNCATED AT 250 WORDS)

Identical regional mechanisms of intracellular free Ca2+ concentration increase during polarized agonist-evoked Ca2+ response in pancreatic acinar cells

The initial increase of intracellular free Ca2+ concentration ([Ca2+]i) following agonist stimulation is spatially restricted to one pole of the cell, from where a wave of [Ca2+]i spreads across the cytosol. In the present study we have investigated the dynamic properties of the agonist-activated Ca(2+)-release mechanisms in different regions of the acinar cell and show that, during maximal agonist stimulation, the rate of [Ca2+]i increase at the secretory pole is identical with that recorded at the basal pole. Furthermore, the relationship between [Ca2+]i and the apparent rate of [Ca2+]i increase is similar in both regions of the cell. The data show that whereas the sensitivity to the Ca(2+)-releasing agent is different in different regions of the cell, the process of [Ca2+]i increase, once triggered, will proceed in an identical fashion, irrespective of the area of the cell.

Delay in granular fusion evoked by repetitive cytosolic Ca2+ spikes in mouse pancreatic acinar cells

Patch-clamp whole-cell recording in combination with a phase-sensitive detection method was applied to single, enzymatically isolated, mouse pancreatic acinar cells. Either muscarinic stimulation with a low concentration of ACh (50 nM) or cell infusion of inositol 1,4,5-trisphosphate (InsP3) induced repetitive spike-like increases of membrane capacitance (delta C), membrane conductance (delta G) and membrane current (delta I). Cellular perfusion of InsP3, 10 microM in patch-pipettes, induced baseline spikes in delta C and delta G, resembling those evoked by ACh. The result indicates that exocytotic granular fusion is primarily triggered by the InsP3-induced repetitive rise of [Ca2+]i. The ACh-induced delta C took off almost synchronously with delta G with an apparent delay of less than 40 ms in the initial spike response. This delay of delta C, however, becomes longer by a factor of 7-12 during repetitive Ca2+ spike cycles. Concomitantly a faster decrease in delta C spikes than delta G spikes was observed during the cycles. Two explanations are proposed. First, the Ca2+ sensitivity of granular fusion decreases during the repetitive Ca2+ spikes. This might be due to gradual washout of low molecular components responsible for exocytosis under the whole-cell recording condition. Second, the pool of immediately releasable or of primed zymogen granules is easily exhausted or desensitized during the Ca2+ spike cycles, and has to be supplied from newly primed or sensitized resources. The progressive delay in delta C during the spike cycle is interpreted as a delay in the process of supplying fusible granules.

The control of chloride conductance in rat parotid isolated acinar cells investigated by photorelease of caged compounds

The control of Cl- conductance in rat parotid isolated acinar cells was studied by combined use of whole-cell recording and flash photolysis techniques. Cells were voltage-clamped either at a membrane potential of -40 mV or stepped between -85 mV and 0 mV. Bath-applied carbachol and noradrenaline evoked Cl- current at -85 mV and K+ current at 0 mV. Similar current activations resulted from the photolytic release of either inositol trisphosphate (InsP3) or Ca2+ by a brief near-UV flash. The peak amplitudes of the Cl- conductance (at -85 mV), measured relative to the K+ conductance (at 0 mV), evoked by application of carbachol, noradrenaline or direct manipulation of cytosolic free calcium ([Ca2+]i), were very similar, being 0.56 +/- 0.09 (mean +/- SEM, n = 9), 0.52 +/- 0.01 (n = 7) and 0.46 +/- 0.06 (n = 7). In contrast, the relative amplitude of the Cl- conductance evoked by InsP3 was much larger: 1.49 +/- 0.24 (n = 9). Neither bath application of isoprenaline nor photolysis of "caged" cAMP induced any detectable membrane current. The most probable interpretation of these results is that the observed activation of Cl- conductance by agonists can be explained by the elevation of [Ca2+]i alone. In addition, the present results provide further support for the previously reported suggestion that the Cl- channels and the Ca(2+)-release sites are co-localised [10].

Stimulus-secretion coupling and Ca2+ dynamics in pancreatic acinar cells

1. Unique spatiotemporal dynamics in cytosolic Ca2+ concentration, [Ca2+]c, were characterized in various cell types. In pancreatic acinar cells, physiological concentrations of cholecystokinin octapeptide, CCK-8, (< 10 pM) induce repetitive [Ca2+]c spikes commonly termed Ca2+ oscillation, whereas relatively higher concentrations (30 pM-1 nM) evoke biphasic [Ca2+]c dynamics; a rapid transient peak followed by a sustained increase. Much higher concentrations (> 1 nM) induce a large transient followed by a steep decay. 2. These [Ca2+]c dynamics correspond to secretory responses. Repetitive [Ca2+]c change is attributable to the upstroke of the bell-shaped dose-response relationship and the biphasic change is responsible for the downstroke of the relation (so called high-dose inhibited secretion). The large transient [Ca2+]c increase is associated with morphological changes such as bleb formation. 3. Possible interrelation between dose of secretagogues, secretory responses, [Ca2+]c dynamics, IP3 production, receptor occupation and morphological change will be discussed from both pharmacological and physiological points of view.

Ca(2+)-dependent unidirectional vesicular release detected with a carbon-fibre electrode in rat pancreatic acinar cell triplets

An amperometric constant-voltage method for detection of serotonin oxidation currents was applied to pancreatic acinar cell triplets to determine the site of release of granular content following an increase in [Ca2+]i. The carbon fibre electrode, fabricated to be compatible with a conventional patch-clamp amplifier, was voltage-clamped at 600 mV exceeding the serotonin oxidation voltage, 300 mV. The electrode was placed on the different regions of cell surface of acinar cell triplets loaded with exogenous serotonin. Transient oxidation currents were detected only when the electrode was placed on the acinar lumen after stimulation with a Ca2+ ionophore, A23187, but never observed on the basal or lateral cell surface, or paracellular clefts. No such current responses were observed in the acinar cells without serotonin loading. The results indicate that the A23187-induced sustained increase in [Ca2+]i discharges serotonin specifically into the lumen, and provides direct evidence for the presence of Ca(2+)-dependent unidirectional release of granular contents in pancreatic acinar cells.

Induction of Ca2+ oscillations by selective, U73122-mediated, depletion of inositol-trisphosphate-sensitive Ca2+ stores in rabbit pancreatic acinar cells

The effect of the putative inhibitor of phospholipase C activity, U73122, on the Ca2+ sequestering and releasing properties of internal Ca2+ stores was studied in both permeabilized and intact rabbit pancreatic acinar cells. U73122 dose dependently inhibited ATP-dependent Ca2+ uptake in the inositol (1,4,5)-trisphosphate-[Ins(1,4,5)P3]-sensitive, but not the Ins(1,4,5)P3-insensitive, Ca2+ store in acinar cells permeabilized by saponin treatment. In a suspension of intact acinar cells, loaded with the fluorescent Ca2+ indicator, Fura-2, U73122 alone evoked a transient increase in average free cytosolic Ca2+ concentration ([Ca2+]i,av), which was largely independent of external Ca2+. Addition of U73122 to cell suspensions prestimulated with either cholecystokinin octapeptide or JMV-180 revealed an inverse relationship in size between the U73122- and the agonist-evoked [Ca2+]i,av transient. Moreover, thapsigargin-induced inhibition of intracellular Ca(2+)-ATPase activity resulted in a [Ca2+]i,av transient, the size of which was not different following maximal prestimulation with either U73122 or agonist. These observations suggest that U73122 selectively affects the Ins(1,4,5)P3- casu quo agonist-sensitive internal Ca2+ store, whereas thapsigargin affects both the Ins(1,4,5)P3-sensitive and -insensitive Ca2+ store. Digital-imaging microscopy of Fura-2-loaded acinar cells demonstrated that U73122, in contrast to thapsigargin, evoked sustained oscillatory changes in [Ca2+]i. The U73122-evoked oscillations were abolished in the absence of external Ca2+. The ability of U73122 to generate external Ca(2+)-dependent Ca2+ oscillations suggests that depletion of the agonist-sensitive store leads to an increase in Ca2+ permeability of the plasma membrane and that the Ins(1,4,5)P3-insensitive Ca2+ pool is necessary for the Ca2+ oscillations.

Cellular and subcellular calcium signaling in gastrointestinal epithelium

Ca2+ is a critical second messenger in virtually all cell types, including the various epithelial cell types within the digestive system. When measured in cell populations, Ca2+ signals usually appear as a single transient or prolonged elevation. In individual epithelial cells, signaling patterns often vary from cell to cell and may contain more complex features such as Ca2+ oscillations. Subcellular Ca2+ signals show a further level of complexity, such as Ca2+ waves, and may relate to the polarized structure and function of epithelial cells. The approaches to detect cytosolic Ca2+ signals, the patterns and mechanisms of Ca2+ signaling, and the role of such signals in regulating the function of polarized epithelium within the gastrointestinal tract, pancreas, and liver are reviewed in this report.

Spatial and temporal signalling by calcium

Recent research has shown the importance of the spatial and temporal aspects of calcium signals, which depend upon regenerative properties of the inositol trisphosphate and ryanodine receptors that regulate the release of calcium from internal stores. Initiation sites have been found to spontaneously release calcium, recognized as 'hot spots' or 'sparks', and can trigger a wave that spreads through a process of calcium-induced calcium release.

Nonlinear propagation of agonist-induced cytoplasmic calcium waves in single astrocytes

In astrocytes in primary culture, activation of neurotransmitter receptors results in intracellular calcium signals that propagate as waves across the cell. Similar agonist-induced calcium waves have been observed in astrocytes in organotypic cultures in response to synaptic activation. By using primary cultured astrocytes grown on glass coverslips, in conjunction with fluorescence microscopy we have analyzed agonist-induced Ca2+ wave initiation and propagation in individual cells. Both norepinephrine and glutamate elicited Ca2+ signals which were initiated focally and discretely in one region of the cell, from where the signals spread as waves along the entire length of the cell. Analysis of the wave propagation and the waveform revealed that the propagation was nonlinear with one or more focal loci in the cytoplasm where the wave was regeneratively amplified. These individual loci appear as discrete focal areas 7-15 microns in diameter and having intrinsic oscillatory properties that differ from each other. The wave initiation locus and the different amplification loci remained invariant in space during the course of the experiment and supported an identical spatiotemporal pattern of signalling in any given cell in response to multiple agonist applications and when stimulated with different agonists which are coupled via InsP3. Cytoplasmic Ca2+ concentration at rest was consistently higher (17 +/- 4 nM, mean +/- S.E.M.) in the wave initiation locus compared with the rest of the cytoplasm. The nonlinear propagation results from significant changes in signal rise times, amplitudes, and wave velocity in cellular regions of active loci. Analysis of serial slices across the cell revealed that the rise times and amplitudes of local signals were as much as three- to fourfold higher in the loci of amplification. A phenomenon of hierarchy in local amplitudes of the signal in the amplification loci was observed with the wave initiation locus having the smallest and the most distal locus having the largest amplitude. By this mechanism locally very high concentrations of Ca2+ are achieved in strategic locations in the cell in response to receptor activation. While the average wave velocity calculated over the length of the cell was 10-15 microns/s, in the active loci rates as high as 40 microns/s were measured. Wave velocity was fivefold lower in regions of the cell separating active loci. The differences in the intrinsic oscillatory periods give rise to local Ca2+ waves that show the properties of collision and annihilation. It is hypothesized that the wave front provokes regenerative Ca2+ release from specialized areas in the cell where the endoplasmic reticulum is endowed with higher density of InsP3 receptor channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial dynamics of second messengers: IP3 and cAMP as long-range and associative messengers

Recent imaging experiments have revealed the distinct spatial dynamics of second-messenger actions. In general, actions of Ca2+ tend to be local, whereas those of other messengers such as inositol 1,4,5-trisphosphate (IP3) and cAMP are long range. In pancreatic acinar cells, IP3 generated at the base can diffuse across the cell and evoke a spatially confined Ca2+ signal in the apical pole, triggering enzyme and fluid secretion. Similar mechanisms might also operate in other cell types. We propose that the distinct dynamics of messengers might be relevant to neuronal function: IP3 and cAMP could convey signals over long distances along neurites, and serve as mediators for association and co-operation, for example, during learning.

Ca2+ oscillations in pancreatic acinar cells: spatiotemporal relationships and functional implications

The pancreatic acinar cells are of particular interest for the study of cytosolic Ca2+ signals, since they are morphologically polarized and generate agonist-specific Ca2+ oscillation patterns. Recent data obtained by combining digital video imaging of Fura-2 fluorescence with patch-clamp whole-cell current recording have provided new information on the spatiotemporal relationships of the cytosolic Ca2+ signals and the Ca(2+)-activated ionic currents. Low agonist concentrations evoke repetitive short-lasting local Ca2+ spikes in the secretory pole region that activate shortlasting current spikes. In the case of acetylcholine stimulation the spikes are confined to this region. When cholecystokinin is used the shortlasting local spikes precede longer Ca2+ transients that spread to the whole of the cell. Infusion of non-metabolizable inositol trisphosphate analogues can mimic these responses. The shortlasting local Ca2+ spikes are particularly sensitive to blockade by the inositol trisphosphate receptor antagonist heparin. These results show that the secretory pole region has a particularly high sensitivity to inositol trisphosphate probably due to clustering of high affinity receptors.