Phosphospecific site tyrosine phosphorylation of p125FAK and proline-rich kinase 2 is differentially regulated by cholecystokinin receptor type A activation in pancreatic acini

The focal adhesion kinases, p125FAK and proline-rich kinase 2 (PYK2), are involved in numerous processes as adhesion, cytoskeletal changes, and growth. These kinases have 45% homology and share three tyrosine phosphorylation (TyrP) sites. Little information exists on the ability of stimulants to cause TyrP of each kinase site and the cellular mechanism involved. We explored the ability of the neurotransmitter/hormone, CCK, to stimulate TyrP at each site. In rat pancreatic acini, CCK stimulated TyrP at each site in both kinases. TyrP was rapid except for pY397FAK. The magnitude of TyrP differed with the different FAK and PYK2 sites. The CCK dose-response curve for TyrP for sites in each kinase was similar. CCK-JMV, an agonist of the high affinity receptor state and antagonist of the low affinity receptor state, was less efficacious than CCK at each FAK/PYK2 site and inhibited CCK maximal stimulation. Thapsigargin decreased CCK-stimulated TyrP of pY402PYK2 and pY925FAK but not the other sites. GF109203X reduced TyrP of only the PYK2 sites, pY402 and pY580. GF109203X with thapsigargin decreased TyrP of pY402PYK2 and the three FAK sites more than either inhibitor alone. Basal TyrP of pY397FAK was greater than other sites. These results demonstrate that CCK stimulates tyrosine phosphorylation of each of the three homologous phosphorylation sites in FAK and PYK2. However, CCK-stimulated TyrP at these sites differs in kinetics, magnitude, and participation of the high/low affinity receptor states and by protein kinase C and [Ca2+]i. These results show that phosphorylation of these different sites is differentially regulated and involves different intracellular mechanisms in the same cell.

Molecular localization of the inhibitory arachidonic acid binding site to the pore of hIK1

We previously demonstrated that the endogenously expressed human intermediate conductance, Ca(2+)-activated K(+) channel (hIK1) was inhibited by arachidonic acid (AA) (Devor, D. C., and Frizzell, R. A. (1998) Am. J. Physiol. 274, C138-C148). Here we demonstrate, using the excised, inside-out patch-clamp technique, that hIK1, heterologously expressed in HEK293 cells, is inhibited 82 +/- 2% (n = 16) with 3 microm AA, being half-maximally inhibited (IC(50)) at 1.4 +/- 0.7 microm. In contrast, AA does not inhibit the Ca(2+)-dependent, small conductance K(+) channel, rSK2, another member of the KCNN gene family. Therefore, we utilized chimeric hIK1/rSK2 channels to define the AA binding domain on hIK1 to the S5-Pore-S6 region of the channel. Subsequent site-directed mutagenesis revealed that mutation of Thr(250) to Ser (T250S) resulted in a channel with limited sensitivity to block by AA (8 +/- 2%, n = 8), demonstrating that Thr(250) is a key molecular determinant for the inhibition of hIK1 by AA. Likewise, when Val(275) in S6 was mutated to Ala (V275A) AA inhibited only 43 +/- 11% (n = 9) of current flow. The double mutation T250S/V275A eliminated the AA sensitivity of hIK1. Introducing the complimentary single amino acid substitutions into rSK2 (S359T and A384V) conferred partial AA sensitivity to rSK2, 21 +/- 3% and 31 +/- 3%, respectively. Further, introducing the double mutation S359T/A384V into rSK2 resulted in a 63 +/- 8% (n = 9) inhibition by AA, thereby demonstrating the ability to introduce this inhibitory AA binding site into another member of the KCNN gene family. These results demonstrate that AA interacts with the pore-lining amino acids, Thr(250) and Val(275) in hIK1, conferring inhibition of hIK1 by AA and that AA and clotrimazole share similar, if not identical, molecular sites of interaction.

Intracellular Ca(2+) regulates the cellular iron uptake in K562 cells

Fluorescence quenching was used to study the kinetics of the transferrin receptor (TfR)-mediated iron uptake in the calcein-loaded K562 cells. It was found that elevation of intracellular free Ca(2+) ([Ca(2+)](i)) by thapsigargin (TG) speeds up the initial rate of iron uptake and increases the overall capacity of the cells in taking up iron. Depletion of intracellular Ca(2+) or complete chelation of extracellular Ca(2+) results in complete inhibition of the iron uptake in cells. To gain insight into molecular mechanism, IANBD-labeled transferrin (Tf) and microscopic fluorescence imaging were used to observe the endocytosis and recycling of the Tf-TfR complex in single live cells. The study showed that the preincubation of cells with TG or phorbol myristate acetate (PMA), the direct activator of protein kinase C (PKC), accelerated the endocytosis and recycling of the complex in a dose-dependent manner. W-7, the calmodulin antagonist, and GF109203X, a selected cell-permeant inhibitor of PKC, can reverse the acceleration. Analysis of actin polymerization in controlled, [Ca(2+)](i)-elevated and W-7-treated cells revealed that the actin polymerization is enhanced as [Ca(2+)](i) is raised, but reduced by W-7. The results suggest that the regulation of actin polymerization by intracellular Ca(2+) may play a central role in Ca(2+)-dependent iron uptake.

Early changes in pancreatic acinar cell calcium signaling after pancreatic duct obstruction

Intracellular Ca(2+)-changes not only participate in important signaling pathways but have also been implicated in a number of disease states including acute pancreatitis. To investigate the underlying mechanisms in an experimental model mimicking human gallstone-induced pancreatitis, we ligated the pancreatic duct of Sprague-Dawley rats and NMRI mice for up to 6 h and studied intrapancreatic changes including the dynamics of [Ca(2+)](i) in isolated acini. In contrast to bile duct ligation, pancreatic duct obstruction induced intra-pancreatic trypsinogen activation, leukocytosis, hyperamylasemia, and pancreatic edema and increased lung myeloperoxidase activity. Although resting [Ca(2+)](i) in isolated acini rose by 45% to 205 +/- 7 nmol, the acetylcholine- and cholecystokinin (CCK)-stimulated calcium peaks as well as the amylase secretion declined, but neither the [Ca(2+)](i)-signaling pattern nor the amylase output in response to the Ca(2+)-ATPase inhibitor thapsigargin nor the secretin-stimulated amylase release were impaired by pancreatic duct ligation. On the single cell level pancreatic duct ligation reduced the percentage of cells in which submaximal secretagogue stimulation was followed by a physiological response (i.e. Ca(2+) oscillations) and increased the percentage of cells with a pathological response (i.e. peak plateau or absent Ca(2+) signal). Moreover, it reduced the frequency and amplitude of Ca(2+) oscillation as well as the capacitative Ca(2+) influx in response to secretagogue stimulation. Serum pancreatic enzyme elevation as well as trypsinogen activation was significantly reduced by pretreatment of animals with the calcium chelator BAPTA-AM. These experiments suggest that pancreatic duct obstruction rapidly changes the physiological response of the exocrine pancreas to a Ca(2+)-signaling pattern that has been associated with premature digestive enzyme activation and the onset of pancreatitis, both of which can be prevented by administration of an intracellular calcium chelator.

Statins stimulate arachidonic acid release and prostaglandin I2 production in rat liver cells

Statins inhibit 3-hydroxy-3-methylglutaryl (HMG-CoA) reductase, the rate limiting step in cholesterol synthesis. They are, therefore, used clinically to lower cholesterol and prevent atherosclerosis. Statins have beneficial effects on multiple organ systems. Some of these effects are found in the absence of significant changes in cholesterol levels. Polyunsaturated fatty acids also inhibit HMG-CoA reductase and have many of the same beneficial effects of statins. Four statins (mevastatin, lovastatin, simvastatin and atorvastatin) have been tested in rat liver cells for their effect on arachidonic acid (AA) release and prostaglandin I2 production induced in the presence of lactacystin and 12-O-tetradecanoylphorbol-13-acetate. Each statin stimulated release of AA and induced prostaglandin I2 production. Mevalonate, the product of HMG-CoA reductase, did not reduce the stimulation observed in the presence of simvastatin indicating that HMG-CoA reductase activity is not involved. In view of the multiple biologic properties of AA, the AA released as a result of the action of the statins may play a role in some of the pharmacological effects attributed to these drugs.

Evidence for secretion-like coupling involving pp60src in the activation and maintenance of store-mediated Ca2+ entry in mouse pancreatic acinar cells

Store-mediated Ca2+ entry (SMCE) is one of the main pathways for Ca2+ influx in non-excitable cells. Recent studies favour a secretion-like coupling mechanism to explain SMCE, where Ca2+ entry is mediated by an interaction of the endoplasmic reticulum (ER) with the plasma membrane (PM) and is modulated by the actin cytoskeleton. To explore this possibility further we have now investigated the role of the actin cytoskeleton in the activation and maintenance of SMCE in pancreatic acinar cells, a more specialized secretory cell type which might be an ideal cellular model to investigate further the properties of the secretion-like coupling model. In these cells, the cytoskeletal disrupters cytochalasin D and latrunculin A inhibited both the activation and maintenance of SMCE. In addition, stabilization of a cortical actin barrier by jasplakinolide prevented the activation, but not the maintenance, of SMCE, suggesting that, as for secretion, the actin cytoskeleton plays a double role in SMCE as a negative modulator of the interaction between the ER and PM, but is also required for this mechanism, since the cytoskeleton disrupters impaired Ca2+ entry. Finally, depletion of the intracellular Ca2+ stores induces cytoskeletal association and activation of pp60(src), which is independent on Ca2+ entry. pp60(src) activation requires the integrity of the actin cytoskeleton and participates in the initial phase of the activation of SMCE in pancreatic acinar cells.

Dimethylsphingosine increases cytosolic calcium and intracellular pH in human T lymphocytes

N,N-Dimethyl-D-erythro-sphingosine (DMS) is the N-methyl derivative of sphingosine; both are activators of sphingosine-dependent protein kinases. The aim of this work was to study the effect of DMS on cytosolic calcium and intracellular pH (pHi) in human T lymphocytes. The variations of calcium and pH were determined by fluorescence digital imaging using Fura-2-AM and BCECF-AM, respectively. DMS increased both pHi and Ca(2+)-cytoslic in human T lymphocytes. These effects were dose-dependent. This drug induced a fast increase in pHi and a release of calcium from different intracellular calcium pools than thapsigargin. DMS also induced a Ca(2+)-influx different from the store-operated calcium channels, since drug effect was not modified by 30 microM SKF 96365. The influx of calcium induced by DMS was completely blocked by preincubation in the presence of nickel, or lanthanum, while the increase in pHi was no affected. However, the presence of cadmium reduced but does not block Ca(2+)-influx. The inhibition of G-protein by 100 ng/mL pertussis toxin, and the inhibition of tyrosine kinases by genistein significantly reduced the cytosolic calcium increase induced by DMS by an inhibition of both, release of calcium from intracellular pools and influx from extracellular medium. The inhibition of pools emptiness by these drugs was related with the inhibition that they induce in the DMS cytosolic alcalinization. In summary, DMS increases pHi and as consequence releases calcium from intracellular pools, and it increases calcium-influx through a channel different from store-operated channel (SOC). Both cytosolic calcium and pHi increase are modulated by G-proteins and tyrosine kinases.

Cholecystokinin-stimulated tyrosine phosphorylation of PKC-delta in pancreatic acinar cells is regulated bidirectionally by PKC activation

PKC-delta is important in cell growth, apoptosis, and secretion. Recent studies show its stability is regulated by tyrosine phosphorylation (TYR-P), which can be stimulated by a number of agents. Many of these stimuli also activate phospholipase C (PLC) cascades and little is known about the relationship between these cascades and PKC-delta TYR-P. Cholecystokinin (CCK) stimulates PKCs but it is unknown if it causes PKC-delta TYR-P and if so, the relationship between these cascades is unknown. In rat pancreatic acini, CCK-8 stimulated rapid PKC-delta TYR-P by activation of the low affinity CCK(A) receptor state. TPA had a similar effect. BAPTA did not decrease CCK-stimulated PKC-delta TYR-P but instead, increased it. A23187 did not stimulate PKC-delta TYR-P. Wortmannin and LY 294002 did not alter CCK-stimulated PKC-delta TYR-P. GF 109203X, at low concentrations, increased PKC-delta TYR-P stimulated by CCK or TPA and at higher concentrations, inhibited it. The cPKC inhibitors, Gö 6976 and safingol, caused a similar increase in TPA- and CCK-stimulated PKC-delta TYR-P. These results demonstrate that CCK(A) receptor activation causes PKC-delta TYR-P through activation of only one of its two receptor affinity states. This PKC-delta TYR-P is not directly influenced by changes in [Ca(2+)](i); however, the resultant activation of PKC-alpha has an inhibitory effect. Therefore, CCK activates both stimulatory and inhibitory PKC cascades regulating PKC-delta TYR-P and, hence, likely plays an important role in regulating PKC-delta degradation and cellular abundance.

Mitochondrial reactive oxygen species regulate spatial profile of proinflammatory responses in lung venular capillaries

Cytokine-induced lung expression of the endothelial cell (EC) leukocyte receptor P-selectin initiates leukocyte rolling. To understand the early EC signaling that induces the expression, we conducted real-time digital imaging studies in lung venular capillaries. To compare receptor- vs nonreceptor-mediated effects, we infused capillaries with respectively, TNF-alpha and arachidonate. At concentrations adjusted to give equipotent increases in the cytosolic Ca(2+), both agents increased reactive oxygen species (ROS) production and EC P-selectin expression. Blocking the cytosolic Ca(2+) increases abolished ROS production; blocking ROS production abrogated P-selectin expression. TNF-alpha, but not arachidonate, released Ca(2+) from endoplasmic stores and increased mitochondrial Ca(2+). Furthermore, Ca(2+) depletion abrogated TNF-alpha responses partially, but arachidonate responses completely. These differences in Ca(2+) mobilization by TNF-alpha and arachidonate were reflected in spatial patterning in the capillary in that the TNF-alpha effects were localized at branch points, while the arachidonate effects were nonlocalized and extensive. Furthermore, mitochondrial blockers inhibited the TNF-alpha- but not the arachidonate-induced responses. These findings indicate that the different modes of Ca(2+) mobilization determined the spatial patterning of the proinflammatory response in lung capillaries. Responses to TNF-alpha revealed that EC mitochondria regulate the proinflammatory process by generating ROS that activate P-selectin expression.

Nuclear receptor agonists stimulate release of arachidonic acid from rat liver cells

Release of arachidonic acid (AA) from rat liver cells is stimulated after a 6 h incubation with compounds that are members of the nuclear receptor superfamily, including vitamin D(3), clofibrate, 22(R) OH cholesterol, farnesol, progesterone, testosterone, 17beta-estradiol, hydrocortisone, 3,3'5 triiodothyronine, juvenile hormone III, WY14643, L -thyroxine, the tyrosine analog of thiazolidinediones, GW7845, tamoxifen, hydroxytamoxifen, 17alpha-estradiol and D -thyroxine. Squaline, lanosterol, cholesterol and the 17beta-estradiol antagonist, ICI-182,780, do not stimulate. ICI-182,780 inhibits the release stimulated by 17beta-estradiol, vitamin D(3), 22(R) OH cholesterol, celecoxib or indomethacin. Actinomycin D abolishes the release stimulated by 15-deoxy-delta(12,14) PGJ(2), but is less effective at inhibiting the release stimulated by all of the agonists listed above as well as the release stimulated by 9-cis retinoic acid, all trans -retinoic acid, the thiazidinedione, ciglitazone and the non-steroidal anti-inflammatory drugs, indomethacin and celecoxib. Based on the effects of the 17beta-estradiol antagonist, ICI-182,780, the release of AA appears to be a membrane effect and may not be mediated by the classical estrogen receptors. From the results obtained with actinomycin D, some stimulations may require transcription.

NAADP mobilizes Ca(2+) from reserve granules, lysosome-related organelles, in sea urchin eggs

Nicotinic acid adenine dinucleotide phosphate (NAADP) mobilizes Ca(2+) in many cells and species. Unlike other Ca(2+)-mobilizing messengers, NAADP mobilizes Ca(2+) from an unknown store that is not the endoplasmic reticulum, the store traditionally associated with messenger-mediated Ca(2+) signaling. Here, we demonstrate the presence of a Ca(2+) store in sea urchin eggs mobilized by NAADP that is dependent on a proton gradient maintained by an ATP-dependent vacuolar-type proton pump. Moreover, we provide pharmacological and biochemical evidence that this Ca(2+) store is the reserve granule, the functional equivalent of a lysosome in the sea urchin egg. These findings represent an unsuspected mechanism for messenger-mediated Ca(2+) release from lysosome-related organelles.

Nitric oxide inhibits capacitative Ca2+ entry by suppression of mitochondrial Ca2+ handling

1. Nitric oxide (NO) is a key modulator of cellular Ca(2+) signalling and a determinant of mitochondrial function. Here, we demonstrate that NO governs capacitative Ca(2+) entry (CCE) into HEK293 cells by impairment of mitochondrial Ca(2+) handling. 2. Authentic NO as well as the NO donors 1-[2-(carboxylato)pyrrolidin-1-yl]diazem-1-ium-1,2-diolate (ProliNO) and 2-(N,N-diethylamino)-diazenolate-2-oxide (DEANO) suppressed CCE activated by thapsigargin (TG)-induced store depletion. Threshold concentrations for inhibition of CCE by ProliNO and DEANO were 0.3 and 1 micro M, respectively. 3. NO-induced inhibition of CCE was not mimicked by peroxynitrite (100 micro M), the peroxynitrite donor 3-morpholino-sydnonimine (SIN-1, 100 micro M) or 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP, 1 mM). In addition, the guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazole[4,3-a] quinoxalin-1-one (ODQ, 30 micro M) failed to antagonize the inhibitory action of NO on CCE. 4. DEANO (1-10 micro M) suppressed mitochondrial respiration as evident from inhibition of cellular oxygen consumption. Experiments using fluorescent dyes to monitor mitochondrial membrane potential and mitochondrial Ca(2+) levels, respectively, indicated that DEANO (10 micro M) depolarized mitochondria and suppressed mitochondrial Ca(2+) sequestration. The inhibitory effect of DEANO on Ca(2+) uptake into mitochondria was confirmed by recording mitochondrial Ca(2+) during agonist stimulation in HEK293 cells expressing ratiometric-pericam in mitochondria. 5. DEANO (10 micro M) failed to inhibit Ba(2+) entry into TG-stimulated cells when extracellular Ca(2+) was buffered below 1 micro M, while clear inhibition of Ba(2+) entry into store depleted cells was observed when extracellular Ca(2+) levels were above 10 micro M. Moreover, buffering of intracellular Ca(2+) by use of N,N'-[1,2-ethanediylbis(oxy-2,1-phenylene)] bis [N-[25-[(acetyloxy) methoxy]-2-oxoethyl]]-, bis[(acetyloxy)methyl] ester (BAPTA/AM) eliminated inhibition of CCE by NO, indicating that the observed inhibitory effects are based on an intracellular, Ca(2+) dependent-regulatory process. 6. Our data demonstrate that NO effectively inhibits CCE cells by cGMP-independent suppression of mitochondrial function. We suggest disruption of local Ca(2+) handling by mitochondria as a key mechanism of NO induced suppression of CCE.

Old players in a new role: mitochondria-associated membranes, VDAC, and ryanodine receptors as contributors to calcium signal propagation from endoplasmic reticulum to the mitochondria

In many cell types, IP(3) and ryanodine receptor (IP(3)R/RyR)-mediated Ca(2+) mobilization from the sarcoendoplasmic reticulum (ER/SR) results in an elevation of mitochondrial matrix [Ca(2+)]. Although delivery of the released Ca(2+) to the mitochondria has been established as a fundamental signaling process, the molecular mechanism underlying mitochondrial Ca(2+) uptake remains a challenge for future studies. The Ca(2+) uptake can be divided into the following three steps: (1) Ca(2+) movement from the IP(3)R/RyR to the outer mitochondrial membrane (OMM); (2) Ca(2+) transport through the OMM; and (3) Ca(2+) transport through the inner mitochondrial membrane (IMM). Evidence has been presented that Ca(2+) delivery to the OMM is facilitated by a local coupling between closely apposed regions of the ER/SR and mitochondria. Recent studies of the dynamic changes in mitochondrial morphology and visualization of the subcellular pattern of the calcium signal provide important clues to the organization of the ER/SR-mitochondrial interface. Interestingly, key steps of phospholipid synthesis and transfer to the mitochondria have also been confined to subdomains of the ER tightly associated with the mitochondria, referred as mitochondria-associated membranes (MAMs). Through the OMM, the voltage-dependent anion channels (VDAC, porin) have been thought to permit free passage of ions and other small molecules. However, recent studies suggest that the VDAC may represent a regulated step in Ca(2+) transport from IP(3)R/RyR to the IMM. A novel proposal regarding the IMM Ca(2+) uptake site is a mitochondrial RyR that would mediate rapid Ca(2+) uptake by mitochondria in excitable cells. An overview of the progress in these directions is described in the present paper.

Contribution of different phospholipases and arachidonic acid metabolites in the response of gallbladder smooth muscle to cholecystokinin

Guinea pig gallbladder muscle strips were used to investigate the contribution of different sources of diacylglicerol (DAG) in the cholecystokinin (CCK)-induced contraction. The involvement of arachidonic acid (AA) in this response was also investigated. Three distinct pathways for DAG production were investigated with specific phospholipase (PL) inhibitors. U-73122 (10 microM) was used for inhibition of phosphoinositide-specific-PLC (PI-PLC), D-609 (100 microM) for phosphatidylcholine specific-PLC (PC-PLC), and propranolol (100 microM) for phospholipase D (PLD). Separate or combined inhibition of each of these enzymes showed that the CCK-induced output of DAG involves the parallel activation of each of these phospholipases. Thus, after inhibition of a PL subtype, the remaining subtypes were able to functionally compensate in mediating CCK-induced contraction. Inhibition of AA production via DAG-lipase or phospholipase A(2) (PLA(2)) was accomplished using RHC-80267 (40 microM), mepacrine (100 microM) and 4-BPB (100 microM). These inhibitors diminished contractile response, indicating that AA is an important modulator of CCK-induced contraction. Indomethacin (10 microM) and nordihydroguaiaretic acid (NDGA, 100 microM), which inhibit subsequent steps in AA metabolism through the cyclooxygenase and 5-lipooxygenase pathways, also inhibited contractions. Taken together, these results show that CCK redundantly activates PC-PLC, PI-PLC and PLD, to produce DAG, which in turn stimulates PKC and provides a substrate for the generation of AA. sPLA(2) is also a source of AA, whose metabolites are, in part, responsible for determining the magnitude of the CCK-evoked contraction.

On the conservation of fast calcium wave speeds

Calcium waves were first seen about 25 years ago as the giant, 10 micro m/s wave or tsunami which crosses the cytoplasm of an activating medaka fish egg [J Cell Biol 76 (1978) 448]. By 1991, reports of such waves with approximately 10 micro m/s velocities through diverse, activating eggs and with approximately 30 micro m/s velocities through diverse, fully active systems had been compiled to form a class of what are now called fast calcium waves [Proc Natl Acad Sci USA 88 (1991) 9883; Bioessays 21 (1999) 657]. This compilation is now updated to include organisms from algae and sponges up to blowflies, squid and men and organizational levels from mammalian brains and hearts as well as chick embryos down to muscle, nerve, epithelial, blood and cancer cells and even cell-free extracts. Plots of these data confirm the narrow, 2-3-fold ranges of fast wave speeds through activating eggs and 3-4-fold ones through fully active systems at a given temperature. This also indicate Q(10)'s of 2.7-fold per 10 degrees C for both activating eggs and for fully activated cells.Speeds through some ultraflat preparations which are a few-fold above the conserved range are attributed to stretch propagated calcium entry (SPCE) rather than calcium-induced calcium release (CICR).

The pre-synaptic blocker toosendanin does not inhibit secretion in exocrine cells

AIM

Toosendanin is a pre-synaptic blocker at the neuromuscular junction and its inhibitory effect is divided into an initial facilitative/stimulatory phase followed by a prolonged inhibitory phase. The present study investigated whether the subsequent inhibitory phase was due to exhaustion of the secretory machinery as a result of extensive stimulation during the initial facilitative phase. More specifically, this paper examined whether toosendanin could directly inhibit the secretory machinery in exocrine cells.

METHODS

Rat pancreatic acinar cells were isolated by collagenase digestion. Secretion was assessed by measuring the amount of amylase released into the extracellular medium as a percentage of the total present in the cells before stimulation. Cholecystokinin (CCK)-induced increases in intracellular calcium in single cells were measured with fura-2 microfluorometry.

RESULTS

Effects of toosendanin on CCK-induced amylase secretion and calcium oscillations were investigated. Toosendanin of 87-870 microM had no effect on 10 pM-100 nM CCK-stimulated amylase secretion, nor did 8.7-870 microM toosendanin inhibit 5 pM CCK-induced calcium oscillations. In contrast, 10 nM CCK(1) receptor antagonist FK 480 completely blocked 5 pM CCK-induced calcium oscillations.

CONCLUSION

The pre-synaptic "blocker" toosendanin is a selective activator of the voltage-dependent calcium channels, but does not interfere with the secretory machinery itself.

The recombinant human TRPV6 channel functions as Ca2+ sensor in human embryonic kidney and rat basophilic leukemia cells

The activation mechanism of the recently cloned human transient receptor potential vanilloid type 6 (TRPV6) channel, originally termed Ca(2+) transporter-like protein and Ca(2+) transporter type 1, was investigated in whole-cell patch-clamp experiments using transiently transfected human embryonic kidney and rat basophilic leukemia cells. The TRPV6-mediated currents are highly Ca(2+)-selective, show a strong inward rectification, and reverse at positive potentials, which is similar to store-operated Ca(2+) entry in electrically nonexcitable cells. The gating of TRPV6 channels is strongly dependent on the cytosolic free Ca(2+) concentration; lowering the intracellular free Ca(2+) concentration results in Ca(2+) influx, and current amplitude correlates with the intracellular EGTA or BAPTA concentration. This is also the case for TRPV6-mediated currents in the absence of extracellular divalent cations; compared with endogenous currents in nontransfected rat basophilic leukemia cells, these TRPV6-mediated monovalent currents reveal differences in reversal potential, inward rectification, and slope at very negative potentials. Release of stored Ca(2+) by inositol 1,4,5-trisphosphate and/or the sarco/endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin appears not to be involved in TRPV6 channel gating in both cell lines but, in rat basophilic leukemia cells, readily activates the endogenous Ca(2+) release-activated Ca(2+) current. In conclusion, TRPV6, expressed in human embryonic kidney cells and in rat basophilic leukemia cells, functions as a Ca(2+)-sensing Ca(2+) channel independently of procedures known to deplete Ca(2+) stores.

Receptor biology and intracellular regulatory mechanisms in pancreatic acinar cells

Continuing progress is being made in understanding the regulation of pancreatic acinar cell function by receptor-activated intracellular signaling mechanisms. Knowledge of how ligands interact at the molecular level with their receptors and activate heterotrimeric G proteins is increasing. In addition to inositol trisphosphate, intracellular messengers include cyclic ADP ribose, nicotinic acid adenine dinucleotide phosphate, arachidonic acid, and diacylglycerol. Ca signaling involves the interaction of inositol trisphosphate, cyclic ADP ribose, and nicotinic acid adenine dinucleotide phosphate with distinct subcellular Ca stores. Ca signals ultimately induce exocytosis of zymogen granules and identification of the proteins involved on the granule and plasma membrane, and understanding of their roles is continuing. Other receptor-activated signaling pathways primarily regulate nonsecretory events. Considerable progress has been made in understanding how the mammalian target of rapamycin pathway regulates protein synthesis through translation factors and ribosomal proteins. Other pathways in acinar cells include the mitogen-activated protein kinases, the tyrosine kinases, and the transforming growth factor-beta-Smad pathways.

Arachidonic acid and docosahexaenoic acid suppress thrombin-evoked Ca2+ response in rat astrocytes by endogenous arachidonic acid liberation

Arachidonic (AA) and docosahexaenoic acid (DHA) are the major polyunsaturated fatty acids (PUFAs) in the brain. However, their influence on intracellular Ca2+ signalling is still widely unknown. In astrocytes, the amplitude of thrombin- induced Ca2+ response was time-dependently diminished by AA and DHA, or by the AA tetraynoic analogue ETYA, but not by eicosapentaenoic acid (EPA). Thrombin-elicited Ca2+ response was reduced (20-30%) by 1-min exposure to AA or DHA. Additionally, 1-min application of AA or DHA together with thrombin in Ca2+-free medium blocked Ca2+ influx, which followed after readdition of extracellular Ca2+. EPA and ETYA, however, were ineffective. Long-term treatment of astrocytes with AA and DHA, but not EPA reduced the amplitude of the thrombin-induced Ca2+ response by up to 80%. AA and DHA caused a comparable decrease in intracellular Ca2+ store content. Only DHA and AA, but not EPA or ETYA, caused liberation of endogenous AA by cytosolic phospholipase A2 (cPLA2). Therefore, we reasoned that the suppression of Ca2+ response to thrombin by AA and DHA could be due to release of endogenous AA. Possible participation of AA metabolites, however, was excluded by the finding that specific inhibitors of the different oxidative metabolic pathways of AA were not able to abrogate the inhibitory AA effect. In addition, thrombin evoked AA release via activation of cPLA2. From our data we propose a novel model of positive/negative-feed-back in which agonist-induced release of AA from membrane phospholipids promotes further AA release and then suppresses agonist-induced Ca2+ responses.

Differential activation of p42ERK2 and p125FAK by cholecystokinin and bombesin in the secretion and proliferation of the pancreatic amphicrine cell line AR42J

BACKGROUND

AR42J rat pancreatic acinar carcinoma cells have retained the potential to secrete digestive enzymes in addition to their ability to proliferate upon stimulation with regulatory peptides. We investigated the involvement of p42ERK2 and p125FAK (extracellular signal-regulated protein kinase and focal adhesion protein kinase, respectively) by cholecystokinin and bombesin stimulation with regard to secretion and mitogenesis.

METHODS

The p42ERK2 activity was measured by kinase assay and the activation of p125FAK by antiphosphotyrosine Western blot analysis of p125FAK immunoprecipitates. The expression of both kinases was determined by Western blot analysis, the amylase secretion by colorimetry, and the DNA synthesis by [3H]thymidine incorporation.

RESULTS

p42ERK2 and p125FAK were activated by cholecystokinin and bombesin with maximum stimulation at concentrations above 10 nM. Bombesin was a weaker activator of p42ERK2 and p125FAK, causing only half of the kinase activity induced by stimulation with cholecystokinin. PD98059 was shown to inhibit p42ERK2, while tyrphostin 25 blocked p125FAK tyrosine phosphorylation. Preincubation of AR42J cells with PD98059 or tyrphostin 25 was without influence on cholecystokinin- or bombesin-stimulated secretion in normal or 72-hour dexamethasone-pretreated cells. In contrast, inhibition of both protein kinases leads to reduced cholecystokinin-stimulated [3H]thymidine incorporation rates.

CONCLUSIONS

Cholecystokinin induced proliferation of AR42J cells by strong activation of p42ERK2 and p125FAK. Bombesin failed to stimulate DNA synthesis, probably due to its reduced potency to stimulate these kinases. Both protein kinases are not implicated in the process of enzyme secretion.

Presynaptic mitochondrial calcium sequestration influences transmission at mammalian central synapses

Beyond their role in generating ATP, mitochondria have a high capacity to sequester calcium. The interdependence of these functions and limited access to presynaptic compartments makes it difficult to assess the role of sequestration in synaptic transmission. We addressed this important question using the calyx of Held as a model glutamatergic synapse by combining patch-clamp with a novel mitochondrial imaging method. Presynaptic calcium current, mitochondrial calcium concentration ([Ca(2+)](mito), measured using rhod-2 or rhod-FF), cytoplasmic calcium concentration ([Ca(2+)](cyto), measured using fura-FF), and the postsynaptic current were monitored during synaptic transmission. Presynaptic [Ca(2+)](cyto) rose to 8.5 +/- 1.1 microM and decayed rapidly with a time constant of 45 +/- 3 msec; presynaptic [Ca(2+)](mito) also rose rapidly to >5 microM but decayed slowly with a half-time of 1.5 +/- 0.4 sec. Mitochondrial depolarization with rotenone and carbonyl cyanide p-trifluoromethoxyphenylhydrazone abolished mitochondrial calcium rises and slowed the removal of [Ca(2+)](cyto) by 239 +/- 22%. Using simultaneous presynaptic and postsynaptic patch clamp, combined with presynaptic mitochondrial and cytoplasmic imaging, we investigated the influence of mitochondrial calcium sequestration on transmitter release. Depletion of ATP to maintain mitochondrial membrane potential was blocked with oligomycin, and ATP was provided in the patch pipette. Mitochondrial depolarization raised [Ca(2+)](cyto) and reduced transmitter release after short EPSC trains (100 msec, 200 Hz); this effect was reversed by raising mobile calcium buffering with EGTA. Our results suggest a new role for presynaptic mitochondria in maintaining transmission by accelerating recovery from synaptic depression after periods of moderate activity. Without detectable thapsigargin-sensitive presynaptic calcium stores, we conclude that mitochondria are the major organelle regulating presynaptic calcium at central glutamatergic terminals.

Role of mitochondria in Ca(2+) homeostasis of mouse pancreatic acinar cells

The effects of mitochondrial Ca(2+) uptake on cytosolic Ca(2+) concentration ([Ca(2+)](c)) were investigated in mouse pancreatic acinar cells using cytosolic and/or mitochondrial Ca(2+) indicators. When calcium stores of the endoplasmic reticulum (ER) were emptied by prolonged incubation with thapsigargin (Tg) and acetylcholine (ACh), small amounts of calcium could be released into the cytosol (Delta[Ca(2+)](c)=46 +/- 6 nM, n=13) by applying mitochondrial inhibitors (combination of rotenone (R) and oligomycin (O)). However, applications of R/O, soon after the peak of Tg/Ach-induced Ca(2+) transient, produced a larger cytosolic calcium elevation (Delta[Ca(2+)](c)=84 +/- 6 nM, n=9), this corresponds to an increase in the total mitochondrial calcium concentration ([Ca(2+)](m)) by approximately 0.4 mM. In cells pre-treated with R/O or Ru360 (a specific blocker of mitochondrial Ca(2+) uniporter), the decay time-constant of the Tg/ACh-induced Ca(2+) response was prolonged by approximately 40 and 80%, respectively. Tests with the mitochondrial Ca(2+) indicator rhod-2 revealed large increases in [Ca(2+)](m) in response to Tg/ACh applications; this mitochondrial uptake was blocked by Ru360. In cells pre-treated with Ru360, 10nM ACh elicited large global increases in [Ca(2+)](c), compared to control cells in which ACh-induced Ca(2+) signals were localised in the apical region. We conclude that mitochondria are active elements of cellular Ca(2+) homeostasis in pancreatic acinar cells and directly modulate both local and global calcium signals induced by agonists.

Alpha4 integrins and the immune response

The alpha4 integrins (alpha4beta1 and alpha4beta7) play multiple roles in the immune system. Alpha4 integrins impact hematopoiesis, leukocyte trafficking in immune surveillance and inflammation, and leukocyte activation and survival. To perform these functions, alpha4 integrins act as both adhesive and signaling receptors. Paxillin, a signaling adapter molecule, binds directly to the alpha4 subunit cytoplasmic domain, and its binding is regulated by serine phosphorylation of the alpha4 subunit. This regulated interaction of paxillin with the alpha4 subunit is likely to regulate the diverse functions of alpha4 integrins in the immune system. Furthermore, this protein-protein interaction may provide novel targets for the modulation of the immune response.

Increase in cytosolic Ca2+ levels through the activation of non-selective cation channels induced by oxidative stress causes mitochondrial depolarization leading to apoptosis-like death in Leishmania donovani promastigotes

Reactive oxygen species are important regulators of protozoal infection. Promastigotes of Leishmania donovani, the causative agent of Kala-azar, undergo an apoptosis-like death upon exposure to H2O2. The present study shows that upon activation of death response by H2O2, a dose- and time-dependent loss of mitochondrial membrane potential occurs. This loss is accompanied by a depletion of cellular glutathione, but cardiolipin content or thiol oxidation status remains unchanged. ATP levels are reduced within the first 60 min of exposure as a result of mitochondrial membrane potential loss. A tight link exists between changes in cytosolic Ca2+ homeostasis and collapse of the mitochondrial membrane potential, but the dissipation of the potential is independent of elevation of cytosolic Na+ and mitochondrial Ca2+. Partial inhibition of cytosolic Ca2+ increase achieved by chelating extracellular or intracellular Ca2+ by the use of appropriate agents resulted in significant rescue of the fall of the mitochondrial membrane potential and apoptosis-like death. It is further demonstrated that the increase in cytosolic Ca2+ is an additive result of release of Ca2+ from intracellular stores as well as by influx of extracellular Ca2+ through flufenamic acid-sensitive non-selective cation channels; contribution of the latter was larger. Mitochondrial changes do not involve opening of the mitochondrial transition pore as cyclosporin A is unable to prevent mitochondrial membrane potential loss. An antioxidant like N-acetylcysteine is able to inhibit the fall of the mitochondrial membrane potential and prevent apoptosis-like death. Together, these findings show the importance of non-selective cation channels in regulating the response of L. donovani promastigotes to oxidative stress that triggers downstream signaling cascades leading to apoptosis-like death.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Effects of reactive oxygen species on actin filament polymerisation and amylase secretion in mouse pancreatic acinar cells

The present study investigates the effect of reactive oxygen species (ROS) on actin filament reorganisation and its relevance to exocytosis in pancreatic acinar cells. Treatment of pancreatic acini with cholecystokinin (CCK-8) induced spatial and temporal changes in actin filament reorganisation with an initial depolymerisation of the apical actin barrier followed by an increase in the actin filament content in the subapical area leading to amylase release. Hydrogen peroxide (H(2)O(2)) increased actin filament content and potentiated the polymerizing effects of CCK-8 in these cells but abolished the disruption of the apical actin layer and amylase release induced by CCK-8. Similar to CCK-8, ROS generated by the oxidation of hypoxanthine (HX) with xanthine oxidase (XOD) induced an initial decrease in actin filaments located under the apical membrane followed by a smaller increase in the content of actin filaments in the subapical area. XOD-generated ROS are able to increase amylase release in pancreatic acini although combination with CCK-8 leads to abnormal exocytosis. We provide evidence that indicates that CCK-8- and ROS-induced actin reorganisation is entirely dependent on Ca(2+) mobilisation and independent of PKC activation. The regulation of the actin cytoskeleton by ROS might be involved in radical-induced cell injury in pancreatic acinar cells.

Cell side-specific sensitivities of intracellular Ca2+ stores for inositol 1,4,5-trisphosphate, cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate in permeabilized pancreatic acinar cells from mouse

In pancreatic acinar cells hormonal stimulation leads to a cytosolic Ca(2+) wave that starts in the apical cell pole and subsequently propagates toward the basal cell side. We used permeabilized pancreatic acinar cells from mouse and the mag-fura-2 technique, which allows direct monitoring of changes in [Ca(2+)] of intracellular stores. We show here that Ca(2+) can be released from stores in all cellular regions by inositol 1,4,5-trisphosphate. Stores at the apical cell pole showed a higher affinity to inositol 1,4,5-trisphosphate (EC(50) = 89 nm) than those at the basolateral side (EC(50) = 256 nm). In contrast, cADP-ribose, a modifier of Ca(2+)-induced Ca(2+) release, and nicotinic acid adenine dinucleotide phosphate (NAADP) were able to release Ca(2+) exclusively from intracellular stores located at the basolateral cell side. Our data agree with observations that upon stimulation Ca(2+) is released initially at the apical cell side and that this is caused by high affinity inositol 1,4,5-trisphosphate receptors. Moreover, our findings allow the conclusion that in Ca(2+) wave propagation from the apical to the basolateral cell side observed in pancreatic acinar cells Ca(2+)-induced Ca(2+) release, modulated by cADP-ribose and/or NAADP, might be involved.

Modelling of simple and complex calcium oscillations. From single-cell responses to intercellular signalling

This review provides a comparative overview of recent developments in the modelling of cellular calcium oscillations. A large variety of mathematical models have been developed for this wide-spread phenomenon in intra- and intercellular signalling. From these, a general model is extracted that involves six types of concentration variables: inositol 1,4,5-trisphosphate (IP3), cytoplasmic, endoplasmic reticulum and mitochondrial calcium, the occupied binding sites of calcium buffers, and the fraction of active IP3 receptor calcium release channels. Using this framework, the models of calcium oscillations can be classified into 'minimal' models containing two variables and 'extended' models of three and more variables. Three types of minimal models are identified that are all based on calcium-induced calcium release (CICR), but differ with respect to the mechanisms limiting CICR. Extended models include IP3--calcium cross-coupling, calcium sequestration by mitochondria, the detailed gating kinetics of the IP3 receptor, and the dynamics of G-protein activation. In addition to generating regular oscillations, such models can describe bursting and chaotic calcium dynamics. The earlier hypothesis that information in calcium oscillations is encoded mainly by their frequency is nowadays modified in that some effect is attributed to amplitude encoding or temporal encoding. This point is discussed with reference to the analysis of the local and global bifurcations by which calcium oscillations can arise. Moreover, the question of how calcium binding proteins can sense and transform oscillatory signals is addressed. Recently, potential mechanisms leading to the coordination of oscillations in coupled cells have been investigated by mathematical modelling. For this, the general modelling framework is extended to include cytoplasmic and gap-junctional diffusion of IP3 and calcium, and specific models are compared. Various suggestions concerning the physiological significance of oscillatory behaviour in intra- and intercellular signalling are discussed. The article is concluded with a discussion of obstacles and prospects.

Distinct effects of different calcium-mobilizing agents on cell death in NG108-15 neuroblastoma X glioma cells

The effects of different calcium-mobilizing agents on cell death were characterized in NG108-15 neuroblastoma x glioma hybrid cells. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) increased the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and caused cell death. Thapsigargin (TG) not only increased the [Ca(2+)](i) and caused cell death but also induced neurite outgrowth via activation of phospholipase A(2) and cytochrome P450 epoxygenase. In contrast, bradykinin increased the [Ca(2+)](i), but had no effect on cell morphology or cell death. Cell death occurred by two different mechanisms, one of which was caspase-3-dependent and the other caspase-3-independent. Caspase-3 activation was Ca(2+)-dependent, whereas neurite outgrowth was Ca(2+)-independent. TG- or FCCP-induced caspase-3 activation occurred at the same time, but the cell death induced by TG was delayed. TG treatment did not enhance the generation of nitric oxide or cAMP or secretion of glial-derived neurotrophic factor or neurotrophin-3, but activated sphingosine kinase. Furthermore, inhibition of sphingosine kinase accelerated TG-induced cell death, and exogenous sphingosine 1-phosphate (S1P) protected cells from FCCP-induced cell death by about 60%. These results indicate that, in these cells, depletion of intracellular nonmitochondrial or mitochondrial Ca(2+) stores causes cell death, that TG activates phospholipase A(2) and sphingosine kinase, and that arachidonic acid induces neurite outgrowth, whereas S1P delays cell death.

Store depletion-induced calcium influx in rat cerebellar astrocytes

1. In rat cerebellar astrocytes, intracellular Ca(2+) store depletion by receptor agonists or sarco(endo)plasmic reticulum Ca(2+) ATPase inhibitors induced a transient increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) in the absence of extracellular Ca(2+) and a sustained increase in its presence. 2. After 10 min treatment with thapsigargin, the [Ca(2+)](i) was unaffected by removal of thapsigargin, but fell rapidly to the basal level when extracellular Ca(2+) was removed, suggesting the involvement of capacitative Ca(2+) entry (CCE); this effect was not seen until cells had been exposed to thapsigargin for at least 2 min. 3. Using the whole cell voltage clamp technique, a 60-100 pA inward current was activated by store depletion, the reversal potential ranging from -5 to 0 mV. 4. When extracellular Na(+) was isotonically replaced by Tris, the thapsigargin-induced [Ca(2+)](i) increase was enhanced, while the inward current was reduced, indicating that store-operated Ca(2+) channels were permeable to Na(+); however, they were not permeable to Sr(2+) or Ba(2+). 5. Thapsigargin-induced CCE remained the same in the presence of nifedipine, La(3+) or Cd(2+), while it was inhibited in the presence of SK&F96365. 6. In cerebellar astrocytes, inhibition of protein serine/threonine phosphorylation promoted CCE. 7. In conclusion, in rat cerebellar astrocytes, store depletion activated a CCE via channels which were permeable to Ca(2+) and Na(+) and regulated by phosphorylation.

The TRP family of cation channels: probing and advancing the concepts on receptor-activated calcium entry

Stimulation of membrane receptors linked to a phospholipase C and the subsequent production of the second messengers diacylglycerol and inositol-1,4,5-trisphosphate (InsP(3)) is a signaling pathway of fundamental importance in eukaryotic cells. Signaling downstream of these initial steps involves mobilization of Ca(2+) from intracellular stores and Ca(2+) influx through the plasma membrane. For this influx, several contrasting mechanisms may be responsible but particular relevance is attributed to the induction of Ca(2+) influx as consequence of depletion of intracellular calcium stores. This phenomenon (frequently named store-operated calcium entry, SOCE), in turn, may be brought about by various signals, including soluble cytosolic factors, interaction of proteins of the endoplasmic reticulum with ion channels in the plasma membrane, and a secretion-like coupling involving translocation of channels to the plasma membrane. Experimental approaches to analyze these mechanisms have been considerably advanced by the discovery of mammalian homologs of the Drosophila cation channel transient receptor potential (TRP). Some members of the TRP family can be expressed to Ca(2+)-permeable channels that enable SOCE; other members form channels activated independently of stores. TRP proteins may be an essential part of endogenous Ca(2+) entry channels but so far expression of most TRP cDNAs has not resulted in restitution of channels found in any mammalian cells, suggesting the requirement for further unknown subunits. A major exception is CaT1, a TRP channel demonstrated to provide Ca(2+)-selective, store-operated currents identical to those characterized in several cell types. Ongoing and future research on TRP channels will be crucial to understand the molecular basis of receptor-mediated Ca(2+) entry, with respect to the structure of the entry channels as well as to the mechanisms of its activation and regulation.

Dissociation of the store-operated calcium current I(CRAC) and the Mg-nucleotide-regulated metal ion current MagNuM

Rat basophilic leukaemia cells (RBL-2H3-M1) were used to study the characteristics of the store-operated Ca(2+) release-activated Ca(2+) current (I(CRAC)) and the magnesium-nucleotide-regulated metal cation current (MagNuM) (which is conducted by the LTRPC7 channel). Pipette solutions containing 10 mM BAPTA and no added ATP induced both currents in the same cell, but the time to half-maximal activation for MagNuM was about two to three times slower than that of I(CRAC). Differential suppression of I(CRAC) was achieved by buffering free [Ca(2+)](i) to 90 nM and selective inhibition of MagNuM was accomplished by intracellular solutions containing 6 mM Mg.ATP, 1.2 mM free [Mg(2+)](i) or 100 microM GTP-gamma-S, allowing investigations on these currents in relative isolation. Removal of extracellular Ca(2+) and Mg(2+) caused both currents to be carried significantly by monovalent ions. In the absence or presence of free [Mg(2+)](i), I(CRAC) carried by monovalent ions inactivated more rapidly and more completely than MagNuM carried by monovalent ions. Since several studies have used divalent-free solutions on either side of the membrane to study selectivity and single-channel behaviour of I(CRAC), these experimental conditions would have favoured the contribution of MagNuM to monovalent conductance and call for caution in interpreting results where both I(CRAC) and MagNuM are activated.

Bcl-2-dependent modulation of Ca(2+) homeostasis and store-operated channels in prostate cancer cells

Antiapoptotic oncoprotein Bcl-2 has extramitochondrial actions due to its localization on the endoplasmic reticulum (ER); however, the specific mechanisms of such actions remain unclear. Here we show that Bcl-2 overexpression in LNCaP prostate cancer epithelial cells results in downregulation of store-operated Ca(2+) current by decreasing the number of functional channels and inhibiting ER Ca(2+) uptake through a reduction in the expression of calreticulin and SERCA2b, two key proteins controlling ER Ca(2+) content. Furthermore, we demonstrate that Ca(2+) store depletion by itself is not sufficient to induce apoptosis in Bcl-2 overexpressing cells, and that sustained Ca(2+) entry via activated store-operated channels (SOCs) is required as well. Our data therefore suggest the pivotal role of SOCs in apoptosis and cancer progression.

Correlation of NADH and Ca2+ signals in mouse pancreatic acinar cells

Relationships between calcium signals and NADH responses were investigated in pancreatic acinar cells stimulated with calcium-releasing secretagogues. Cytosolic calcium signals were studied using Fura Red or calcium-sensitive Cl(-) current. Mitochondrial calcium was measured using Rhod-2. The highest levels of NADH autofluorescence were found around the secretory granule region. Stimulation of cells with physiological doses of cholecystokinin (CCK) triggered slow oscillations of NADH autofluorescence. NADH oscillations were clearly resolved in the mitochondrial clusters around secretory granules. Very fast apical calcium signals induced by acetylcholine (ACh) produced no detectable changes in NADH; slightly more extended apical (or preferentially apical) calcium transients triggered clear NADH responses. Triple combined recordings of cytosolic calcium, mitochondrial calcium and NADH revealed the sequence of development of individual signals: an increase in cytosolic calcium was accompanied by a slower mitochondrial calcium response followed by a delayed increase in NADH fluorescence. Recovery of cytosolic calcium was faster than recovery of mitochondrial calcium. NADH recovery occurred at elevated mitochondrial calcium levels. During the transient cytosolic calcium oscillations induced by intermediate doses of ACh, there was an initial increase in NADH fluorescence following the first calcium transient; each of the subsequent calcium responses produced biphasic NADH changes comprising an initial small decline followed by restoration to an elevated calcium level. During the higher-frequency sinusoidal calcium oscillations induced by higher doses of ACh, NADH responses fused into a smooth rise followed by a slow decline. Supramaximal doses of ACh and CCK produced single large NADH transients.

Two modes of secretion in pancreatic acinar cells: involvement of phosphatidylinositol 3-kinase and regulation by capacitative Ca(2+) entry

In pancreatic acinar cells, muscarinic agonists stimulate both the release of Ca(2+) from intracellular stores and the influx of extracellular Ca(2+). The part played by Ca(2+) released from intracellular stores in the regulation of secretion is well established; however, the role of Ca(2+) influx in exocytosis is unclear. Recently, we observed that supramaximal concentrations of acetylcholine (>or=10 microM) elicited an additional component of exocytosis despite reducing Ca(2+) influx. In the present study, we found that supramaximal exocytosis was substantially inhibited (approximately 70%) by wortmannin (100 nM), an inhibitor of phosphatidylinositol 3-kinase. In contrast, exocytosis evoked by a lower concentration of acetylcholine (1 microM) was potentiated (approximately 45%) by wortmannin. Exocytosis stimulated by 1 microM acetylcholine in the absence of extracellular Ca(2+) was, like supramaximal exocytosis, inhibited by wortmannin. The switch to a wortmannin-inhibitable form of exocytosis depended upon a reduction in Ca(2+) entry through store-operated Ca(2+) channels, as the switch in exocytotic mode could also be brought about by the selective blockade of these channels by Gd(3+) (2 microM), but not by inhibition of noncapacitative Ca(2+) entry by SB203580 (10 microM). We conclude that supramaximal doses of acetylcholine lead to a switch in the mode of zymogen granule exocytosis by inhibiting store-dependent Ca(2+) influx.

Acute pancreatitis decreases pancreas phospholipid levels and increases susceptibility to lipid peroxidation in rat pancreas

The objective of this study was to analyze whether acute pancreatitis leads to changes in the lipid composition and susceptibility to lipid peroxidation of pancreatic membranes. Total lipids, cholesterol, phospholipids, FA, and lipid peroxidation were determined in the pancreatic tissue of rats treated with cerulein and of control rats. In pancreatitic rats, significant decreases in membrane total phospholipid contents (P < 0.05) and in choline and ethanolamine glycerophospholipid levels (P < 0.05 and P < 0.01, respectively), with concomitant significantly higher values of their lysoderivative forms, were found. The cholesterol/phospholipid molar ratio increased by 26%. The unsaturation index of the FA profile decreased significantly (P < 0.01) as a consequence of a decrease in the arachidonic acid content. Incubation of membranes with xanthine oxidase/hypoxanthine-Fe2+/ADP resulted in an increase in the production of TBARS in pancreatitic rats compared to controls. In summary, acute pancreatitis causes changes in the lipid composition of rat pancreatic crude membranes and a greater susceptibility of these membranes to lipid peroxidation.

XOD-catalyzed ROS generation mobilizes calcium from intracellular stores in mouse pancreatic acinar cells

In fura-2 loaded isolated mouse pancreatic acinar cells, xanthine oxidase (XOD)-catalyzed reactive oxygen species (ROS) generation caused an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](i)) by release of Ca(2+) from intracellular stores. The ROS-induced Ca(2+) signals showed large variability in shape and time-course and resembled in part Ca(2+) signals in response to physiological secretagogues. ROS-induced Ca(2+) mobilization started at the luminal cell pole and spread towards the basolateral side in a wave manner. ROS-evoked Ca(2+) responses were not inhibited by the phospholipase C (PLC) inhibitor U73122 (10 microM). Neither 2-aminoethoxy-diphenylborate (2-APB) (70 microM) nor ryanodine (50 microM) suppressed ROS-evoked Ca(2+) release. ROS still released Ca(2+) when the endoplasmic reticulum Ca(2+)-ATPase was blocked with thapsigargin (1 microM), or when rotenone (10 microM) was added to release Ca(2+) from mitochondria. Our results suggest that pancreatic acinar cells ROS do not unspecifically affect Ca(2+) homeostasis. ROS primarily affect Ca(2+) stores located in the luminal cell pole, which is also the trigger zone for agonist-induced Ca(2+) signals. Release of Ca(2+) induces Ca(2+) waves carried by Ca(2+)-induced Ca(2+) release and produces thereby global Ca(2+) signals. Under oxidative stress conditions, the increase in [Ca(2+)](i) could be one mechanism contributing to an overstimulation of the cell which could result in cell dysfunction and cell damage.

Role of mitochondria in Ca(2+) oscillations and shape of Ca(2+) signals in pancreatic acinar cells

We studied the role of mitochondria in Ca(2+) signals in fura-2 loaded exocrine pancreatic acinar cells. Mitochondrial depolarization in response to carbonylcyanide-p-tryfluoromethoxyphenyl hydrazone or rotenone (assessed by confocal microscopy using rhodamine-123) induced a partial but statistically significant reduction in the decay of Ca(2+) signals under different experimental conditions. Spreading of Ca(2+) waves evoked by the pancreatic secretagogue cholecystokinin cholecystokinin octapeptide was accelerated by mitochondrial inhibitors, whereas the cytosolic Ca(2+) concentration ([Ca(2+)](i)) oscillations in response to physiological levels of this hormone were suppressed by rotenone and carbonylcyanide-p-tryfluoromethoxyphenyl hydrazone. Oligomycin, an inhibitor of mitochondrial ATP synthase, did no affect either propagation of calcium waves nor [Ca(2+)](i) oscillations. Individual mitochondria of rhod-2 loaded acinar cells showed heterogeneous matrix Ca(2+) concentration increases in response to oscillatory and maximal levels of cholecystokinin octapeptide. On the other hand, using Ba(2+) for unequivocal study of capacitative calcium entry we found that mitochondrial inhibitors did not affect this process. Our results show that although the role of mitochondria as a Ca(2+) clearing system in exocrine cells is quantitatively secondary, they play an essential role in the spatial propagation of Ca(2+) waves and in the development of [Ca(2+)](i) oscillations.

Effect of xanthine oxidase-catalyzed reactive oxygen species generation on secretagogue-evoked calcium mobilization in mouse pancreatic acinar cells

In the present study we have employed fura-2 loaded isolated mouse pancreatic acinar cells to monitor the effect that xanthine oxidase (XOD)-catalyzed reactive oxygen species generation presents on Ca(2+) mobilization by the secretagogue cholecystokinin octapeptide (CCK-8). Our results show that perfusion of pancreatic acinar cells with CCK-8 at a physiological concentration (20 pM) induced low frequency oscillations in intracellular free calcium concentration ([Ca(2+)](i)) at a rate of 1 per minute; this oscillatory pattern was completely inhibited by the introduction in the perifusion medium of 20 mU/mL XOD to generate reactive oxygen species. In addition, perfusion of pancreatic acinar cells with 20 mU/mL XOD in the absence of extracellular calcium led to a transient increase in [Ca(2+)](i,) that blocked the initiation of the Ca(2+) signals in response to 20 pM CCK-8. Similarly, XOD was also able to block acetylcholine evoked Ca(2+) spikes. However, reactive oxygen species had no effect either on Ca(2+) extrusion or on re-uptake into intracellular stores, but CCK-8-evoked Ca(2+) entry was reduced by XOD. In conclusion, our results show that XOD-evoked reactive oxygen species generation leads to a reduction either of Ca(2+) mobilization, following stimulation of pancreatic acinar cells with the Ca(2+)-mobilizing agonists CCK-8 and acetylcholine, and Ca(2+) influx evoked by CCK-8 depletion of intracellular stores. The possible XOD inhibitory mechanism on Ca(2+) mobilization by agonists is discussed.

Participation of mitochondria in calcium signalling in the exocrine pancreas

This minireview is an attempt to put together some of the recent advances regarding the implications of mitochondria in Ca2+ homeostasis. Although the main role of this cytoplasmic organelle is ATP supply to the cell, during the past years strong evidence has been accumulated supporting an active role of these organelles in Ca2+ handling by the cell. The discovery of mitochondrial specific fluorescent dyes has permitted the study of these organelles within living cells. Due to its ubiquitous localisation within the cytosol, mitochondria would play an important role in the modulation of the subcellular patterns of Ca2+ signalling, and therefore would act as modulators of Ca2+-dependent cellular processes.

Ion transport and morphological changes of mitochondria in brown adipocytes of warm- and cold-acclimatized obese Zucker rats

Brown adipose tissue plays the dominant role in response to cold acclimatization through its capacity to produce heat. To demonstrate the cellular function for thermogenesis induced by cold acclimation in the brown adipose tissue of obese Zucker rats, we examined the changes for the area as well as the Na, K, Cl, and Ca concentrations in the mitochondria of brown adipocytes after the warm (25 degrees C, WG) and the cold acclimations (10 degrees C, CG). Moreover, the respiratory quotients (RQs) of these rats were measured. After the acclimations, the RQ in the CG was decreased and the oxygen consumption increased. A morphometric analysis of electron micrographs of brown adipocytes from the two groups of rats showed a marked increase in the area of the mitochondria in the CG. An electron probe X-ray microanalysis showed an increase in the Ca concentration and decreases in the Na and K concentrations in the matrix of the mitochondria of the cells in the CG. These results suggest that the reduction in the RQ of obese Zucker rats acclimated to cold is the consequence of the metabolism of a large quantity of lipid in the brown adipocytes. Our data also indicate that the observed change in the mitochondrial area and the increase for Ca in the mitochondria were associated with the cold-induced thermogenesis in brown adipocytes of obese Zucker rats.

Glutamate induces focal adhesion kinase tyrosine phosphorylation and actin rearrangement in heterologous mGluR1-expressing CHO cells via calcium/calmodulin signaling

Group 1 metabotropic glutamate receptors (mGluR1 and mGluR5) stimulate phospholipase C (PLC) and lead to mobilization of intracellular Ca(2+) and activation of protein kinase C (PKC). In this investigation, using heterologous receptor-expressing Chinese hamster ovary (CHO) cells, we showed that stimulation of mGluR1 or mGluR5 with glutamate rapidly increases tyrosine phosphorylation of focal adhesion kinase (FAK) (maximum at 1-3 min) in a dose-dependent manner (half-maximal responses at approximately 2 microM). In mGluR1-expressing cells, the glutamate-induced increase of FAK tyrosine phosphorylation was blocked by not only the PLC inhibitor, U73122, but also depletion of intracellular Ca(2+) and effectively abrogated by calmodulin (CaM) inhibitors, calmidazolium and fluphenazine. However, neither the PKC inhibitor, GF109203X, nor the CaM kinase II inhibitor, KN-62, inhibited glutamate-stimulated FAK tyrosine phosphorylation. Stimulation of mGluR1 caused a marked increase in actin stress fiber formation. Importantly, this actin rearrangement was prevented by the CaM inhibitor, but not by the PKC inhibitor and is thus in a good agreement with the signaling cascade of the mGluR1-FAK pathway. These results suggest that the Ca(2+)/CaM signaling and its downstream FAK tyrosine phosphorylation play an important role in cellular function of mGluR1.

Intracellular signaling mechanisms activated by cholecystokinin-regulating synthesis and secretion of digestive enzymes in pancreatic acinar cells

The intracellular signaling mechanisms by which cholecystokinin (CCK) and other secretagogues regulate pancreatic acinar function are more complex than originally realized. CCK couples through heterotrimeric G proteins of the Gq family to lead to an increase in intracellular free Ca2+, which shows spatial and temporal patterns of signaling. The actions of Ca2+ are mediated in part by activation of a number of Ca2+-activated protein kinases and the protein phosphatase calcineurin. By the process of exocytosis the intracellular messengers Ca2+, diacylglycerol, and cAMP activate the release of the zymogen granule content in a manner that is poorly understood. This fusion event most likely involves SNARE and Rab proteins present on zymogen granules and cellular membrane domains. More likely related to nonsecretory aspects of cell function, CCK also activates three MAPK cascades leading to activation of ERKs, JNKs, and p38 MAPK. Although the function of these pathways is not well understood, ERKs are probably related to cell growth, and through phosphorylation of hsp27, p38 can affect the actin cytoskeleton. The PI3K (phosphatidylinositol 3-kinase)-mTOR (mammalian target of rapamycin) pathway is important for regulation of acinar cell protein synthesis because it leads to both activation of p70S6K and regulation of the availability of eIF4E in response to CCK. CCK also activates a number of tyrosyl phosphorylation events including that of p125FAK and other proteins associated with focal adhesions.

Specific Ca2+ signaling evoked by cholecystokinin and acetylcholine: the roles of NAADP, cADPR, and IP3

In order to control cell functions, hormones and neurotransmitters generate an amazing diversity of Ca2+ signals such as local and global Ca2+ elevations and also Ca2+ oscillations. In pancreatic acinar cells, cholecystokinin (CCK) stimulates secretion of digestive enzyme and promotes cell growth, whereas acetylcholine (ACh) essentially triggers enzyme secretion. Pancreatic acinar cells are a classic model for the study of CCK- and ACh-evoked specific Ca2+ signals. In addition to inositol 1,4,5 trisphosphate (IP3), recent studies have shown that cyclic ADPribose (cADPr) and nicotinic acid adenine dinucleotide phosphate (NAADP) release Ca2+ in pancreatic acinar cells. Moreover, it has also been shown that both ACh and CCK trigger Ca2+ spikes by co-activation of IP3 and ryanodine receptors but by different means. ACh uses IP3 and Ca2+, whereas CCK uses cADPr and NAADP. In addition, CCK activates phospholipase A2 and D. The concept emerging from these studies is that agonist-specific Ca2+ signals in a single target cell are generated by combination of different intracellular messengers.

Mechanisms of capacitative calcium entry

Capacitative Ca2+ entry involves the regulation of plasma membrane Ca2+ channels by the filling state of intracellular Ca2+ stores in the endoplasmic reticulum (ER). Several theories have been advanced regarding the mechanism by which the stores communicate with the plasma membrane. One such mechanism, supported by recent findings, is conformational coupling: inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) receptors in the ER may sense the fall in Ca2+ levels through Ca2+-binding sites on their lumenal domains, and convey this conformational information directly by physically interacting with Ca2+ channels in the plasma membrane. In support of this idea, in some cell types, store-operated channels in excised membrane patches appear to depend on the presence of both Ins(1,4,5)P3 and Ins(1,4,5)P3 receptors for activity; in addition, inhibitors of Ins(1,4,5)P3 production that either block phospholipase C or inhibit phosphatidylinositol 4-kinase can block capacitative Ca2+ entry. However, the electrophysiological current underlying capacitative Ca2+ entry is not blocked by an Ins(1,4,5)P3 receptor antagonist, and the blocking effects of a phospholipase C inhibitor are not reversed by the intracellular application of Ins(1,4,5)P3. Furthermore, cells whose Ins(1,4,5)P3 receptor genes have been disrupted can nevertheless maintain their capability to activate capacitative Ca2+ entry channels in response to store depletion. A tentative conclusion is that multiple mechanisms for signaling capacitative Ca2+ entry may exist, and involve conformational coupling in some cell types and perhaps a diffusible signal in others.

Energized mitochondria increase the dynamic range over which inositol 1,4,5-trisphosphate activates store-operated calcium influx

In eukaryotic cells, activation of cell surface receptors that couple to the phosphoinositide pathway evokes a biphasic increase in intracellular free Ca2+ concentration: an initial transient phase reflecting Ca2+ release from intracellular stores, followed by a plateau phase due to Ca2+ influx. A major component of this Ca2+ influx is store-dependent and often can be measured directly as the Ca2+ release-activated Ca2+ current (I(CRAC)). Under physiological conditions of weak intracellular Ca2+ buffering, respiring mitochondria play a central role in store-operated Ca2+ influx. They determine whether macroscopic I(CRAC) activates or not, to what extent and for how long. Here we describe an additional role for energized mitochondria: they reduce the amount of inositol 1,4,5-trisphosphate (InsP3) that is required to activate I(CRAC). By increasing the sensitivity of store-operated influx to InsP3, respiring mitochondria will determine whether modest levels of stimulation are capable of evoking Ca2+ entry or not. Mitochondrial Ca2+ buffering therefore increases the dynamic range of concentrations over which the InsP3 is able to function as the physiological messenger that triggers the activation of store-operated Ca2+ influx.

Plasticity and adaptation of Ca2+ signaling and Ca2+-dependent exocytosis in SERCA2(+/-) mice

Darier's disease (DD) is a high penetrance, autosomal dominant mutation in the ATP2A2 gene, which encodes the SERCA2 Ca2+ pump. Here we have used a mouse model of DD, a SERCA2(+/-) mouse, to define the adaptation of Ca2+ signaling and Ca2+-dependent exocytosis to a deletion of one copy of the SERCA2 gene. The [Ca2+]i transient evoked by maximal agonist stimulation was shorter in cells from SERCA2(+/-) mice, due to an up-regulation of specific plasma membrane Ca2+ pump isoforms. The change in cellular Ca2+ handling caused approximately 50% reduction in [Ca2+]i oscillation frequency. Nonetheless, agonist-stimulated exocytosis was identical in cells from wild-type and SERCA2(+/-) mice. This was due to adaptation in the levels of the Ca2+ sensors for exocytosis synaptotagmins I and III in cells from SERCA2(+/-) mice. Accordingly, exocytosis was approximately 10-fold more sensitive to Ca2+ in cells from SERCA2(+/-) mice. These findings reveal a remarkable plasticity and adaptability of Ca2+ signaling and Ca2+-dependent cellular functions in vivo, and can explain the normal function of most physiological systems in DD patients.

Vanadate inhibits the calcium extrusion in rat pancreatic acinar cells

Our objective was to evaluate the role of vanadate on calcium extrusion in Fura-2-loaded rat pancreatic acinar cells by digital microscopic fluorimetry and spectrofluorimetry. In the absence of extracellular calcium, perfusion of pancreatic acinar cells with 1 nM CCK-8 and 1 mM vanadate did not significantly affect the typical transient calcium spike induced by CCK-8, but the plateau phase of calcium in response to CCK-8 remained elevated. In addition, vanadate was able to inhibit calcium efflux evoked by CCK-8 when we determined directly calcium transport across plasma membrane using Calcium Green-5N hexapotassium salt (cell impermeant form) in cell populations. The effect of vanadate on calcium extrusion was strongly blocked by the sulfhydryl-reducing agent dithiothreitol (DTT). The present results demonstrate that vanadate is able to irreversibly inhibit the calcium extrusion. This effect of vanadate can be blocked using DTT, indicating that its action is probably mediated by oxidation of sulfhydryl groups of Ca2+-ATPases.

Protective effect of long term high fiber diet consumption on rat exocrine pancreatic function after chronic ethanol intake

The effects of ethanol administration on exocrine pancreas have been widely studied, but little is known about the effect of dietary fiber in combination with chronic ethanol on exocrine pancreatic function. The aim of this work was to examine the chronic effects of a high fiber diet, ethanol ingestion, and a combination of both on the function of the rat exocrine pancreas. Four groups of rats were fed for six months the following diets: 1.- NW: standard laboratory diet; 2.- FW: high fiber diet (15% cellulose); 3.- NE: standard laboratory diet and 20% ethanol in the drinking water; and 4.- FE: high fiber diet and 20% ethanol. Cholecystokinin (CCK) and acetylcholine (Ach) effects on amylase release and intracellular calcium mobilization in pancreatic acini were studied. In rats fed a 20% ethanol (NE), both the basal amylase release and the basal [Ca(2+)](i) were significantly increased; nonetheless, CCK and Ach-induced amylase release were significantly reduced compared with control rats. Ach- but not CCK-stimulated [Ca(2+)](i) increase in NE rats was significantly decreased compared with NW. In rats fed a combination of ethanol and a high fiber diet (FE) all the parameters under study were not significantly affected compared to control rats (NW). In conclusion, high fiber consumption does not alter the function of the exocrine pancreas. However, it ameliorates the deleterious effect of chronic ethanol consumption on pancreatic amylase secretion and, at least partially, reverses the ethanol-induced alterations on [Ca(2+)](i) in the rat exocrine pancreas.