Role of proton gradients and vacuola H(+)-ATPases in the refilling of intracellular calcium stores in exocrine cells

NAADP-sensitive Ca2+ stores in permeabilized rat hepatocytes

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a nucleotide that is potent to release calcium from intracellular stores in different cell types. NAADP was shown to target specific type of intracellular store namely endolysosomal system or acidic store. Despite intense studies, its effect on endoplasmatic reticulum (ER) still remains to be elucidated. The main aim of our work was to investigate NAADP-sensitive store in permeabilized rat hepatocytes monitoring the level of Ca2+ inside intracellular organelles using chlorotetracycline (CTC). We have shown that NAADP triggered changes of stored Ca2+ in rat hepatocytes are dependent on concentration of EGTA-Ca2+-buffer in cell incubation medium, i.e. the higher is the EGTA concentration in incubation medium the smaller or absent is the effect of NAADP. Besides, the effect of NAADP was more pronounced upon cells pretreatment with the inhibitory concentration of ryanodine (100 μM). This might suggest that the effect of NAADP is dependent on ER luminal calcium. We have also found that NAADP-evoked Ca2+ release in permeabilized hepatocytes is sensitive to nigericin, bafilomycin A and thapsigargin. Additionally, NAADP triggered changes in stored Ca2+ were completely abolished by NED-19 as antagonist of NAADP.

Propagation of Intracellular Ca2+ Signals in Aged Exocrine Cells

There is little information on the effects of aging in the propagation of calcium signals and its underlying mechanisms. We studied the effects of aging on propagation of Ca(2+) signals in pancreatic acinar cells. Fura-2 loaded cells isolated from young (3-4 months old) and aged (24 months old) mouse responded to acetylcholine (ACh) and cholecystokinin (CCK) with a polarized Ca(2+) response initiated at the secretory pole before spreading to the basal one. Aging slowed down the propagation of the response to ACh but enhanced the velocity of the CCK response. This pattern can be explained by the age-induced depolarization of mitochondria, because it can be reproduced in young cells by mitochondrial inhibitors. Aging also increased the role of acidic stores in the CCK signal, as judged by the folimycin-induced suppression of the polarization in aged but not in young cells. The involvement of ryanodine receptors in the ACh response was also enhanced, as indicated by the loss of polarization after the treatment with 8Br-cyclic ADP ribose. Therefore, we conclude that aging modifies differentially the propagation of ACh and CCK-evoked Ca(2+) signals through mitochondrial depolarization and changes in the role of the acidic Ca(2+) stores and ryanodine receptors in the initiation of the signals.

The cardiac (pro)renin receptor is primarily expressed in myocyte transverse tubules and is increased in experimental diabetic cardiomyopathy

BACKGROUND

The pro(renin) receptor is a 350 amino acid transmembrane protein, that on ligand binding, increases the catalytic efficiency of angiotensinogen cleavage by both prorenin and renin, augmenting angiotensin I formation at the cell surface. While implicated in a broad range of diseases, studies to date have focused on the kidney, particularly in the diabetic context. We sought to examine the site-specific expression of the pro(renin) receptor within the heart.

METHODS

Using confocal microscopy, site-specific markers and transmission electron microscopy we assessed the location of the pro(renin) receptor in the heart at both cellular/sub-cellular levels. We assessed pro(renin) receptor expression in the setting of disease and blockade of the renin-angiotensin system, using the TGR[m(Ren2)-27] model of diabetic cardiomyopathy and the direct renin inhibitor, aliskiren.

RESULTS

The pro(renin) receptor was found predominantly at the Z-disc and dyad of cardiac myocytes coinciding closely with the distributions of the vacuolar H⁺-ATPase and ryanodine receptor, known to be located within T-tubules and the sarcoplasmic reticulum's terminal cisternae, respectively. Pro(renin) receptor mRNA/protein abundance were increased ∼3-fold in the hearts of diabetic rats in association with diastolic dysfunction, myocyte hypertrophy and interstitial fibrosis (all P < 0.01). Direct renin inhibition reduced cardiac pro(renin) receptor expression in association with improved cardiac structure/function (all P < 0.05).

CONCLUSION

Together, these findings are consistent with the notion that the pro(renin) receptor is a component of the vacuolar H⁺-ATPase, and that like the latter, is increased in the setting of cardiac stress and lowered by the administration of an ostensibly cardioprotective agent.

STIM1 and STIM2 are located in the acidic Ca2+ stores and associates with Orai1 upon depletion of the acidic stores in human platelets

Mammalian cells accumulate Ca2+ into agonist-sensitive acidic organelles, vesicles that possess a vacuolar proton-ATPase. Acidic Ca2+ stores include secretory granules and lysosome-related organelles. Current evidence clearly indicates that acidic Ca2+ stores participate in cell signaling and function, including the activation of store-operated Ca2+ entry in human platelets upon depletion of the acidic stores, although the mechanism underlying the activation of store-operated Ca2+ entry controlled by the acidic stores remains unclear. STIM1 has been presented as the endoplasmic reticulum Ca2+ sensor, but its role sensing intraluminal Ca2+ concentration in the acidic stores has not been investigated. Here we report that STIM1 and STIM2 are expressed in the lysosome-related organelles and dense granules in human platelets isolated by immunomagnetic sorting. Depletion of the acidic Ca2+ stores using the specific vacuolar proton-ATPase inhibitor, bafilomycin A1, enhanced the association between STIM1 and STIM2 as well as between these proteins and the plasma membrane channel Orai1. Depletion of the acidic Ca2+ stores also induces time-dependent co-immunoprecipitation of STIM1 with the TRPC proteins hTRPC1 and hTRPC6, as well as between Orai1 and both TRPC proteins. In addition, bafilomycin A1 enhanced the association between STIM2 and SERCA3. These findings demonstrate the location of STIM1 and STIM2 in the acidic Ca2+ stores and their association with Ca2+ channels and ATPases upon acidic stores discharge.

Pancreatic calcium signaling: role in health and disease

In order to control cell functions, extracellular agents, such as hormones or neurotransmitters among others, generate a diversity of calcium (Ca(2+)) signals in target cells. Here, we review the components involved in Ca(2+) handling and effectors, both members of the known calcium signaling pathways. In the pancreas, Ca(2+) signal appears as local increases, global elevations or Ca(2+) oscillations. Ca(2+) plays a key role in the pancreatic cells, regulating secretion in exocrine cells, a widely used model for studying the coupling between Ca(2+) signaling and secretion, and the release of insulin, glucagon and somatostatin in the exocrine pancreas. Interestingly, Ca(2+) deregulations have been related to pancreatitis and aging of the pancreas, and treatment with melatonin has shown beneficial effects suggesting that melatonin could be an adequate therapeutic approach.

Polarized calcium signaling in exocrine gland cells

Cytosolic Ca2+ signals are crucial for the control of fluid and enzyme secretion from exocrine glands. The highly polarized exocrine acinar cells have evolved sophisticated and complex Ca2+ signaling mechanisms that exercise precise control of the secretory events occurring across the apical plasma membrane bordering the gland lumen. Ca2+ stores in the endoplasmic reticulum, the secretory granules, the lysosomes, and the endosomes all play important roles in the generation of the local apical Ca2+ spikes that switch on Cl(-) channels in the apical plasma membrane as well as exocytotic export of enzymes. The mitochondria are crucial not only for ATP generation but also for the physiologically important subcellular compartmentalization of the cytosolic Ca2+ signals.

Ca2+ extrusion in aged smooth muscle cells

We investigated the effects of aging in Ca(2+) extrusion mechanisms in smooth muscle bladder cells from 4 and 20-24-month-old guinea pigs using fluorescence microscopy and fura-2. Cells were challenged with a pulse of KCl immediately before perfusion with a Ca(2+) free solution containing no inhibitors (control, untreated cells) or inhibitors of plasma membrane Ca(2+) pump (PMCA, 1mM La(3+)), Na(+)/Ca(2+) exchanger (NCX, 1 microM SEA0400) or the sarcoendoplasmic Ca(2+) pump (SERCA, 1 microM thapsigargin). Treatment of young adult cells with the inhibitors allowed estimating a relative contribution of 55% for NCX, 27% for PMCA and 31% for SERCA. Combination of two inhibitors at the same time showed the presence of interaction between extrusion mechanisms. In aged cells the [Ca(2+)](i) extrusion was impaired due to decrease of PMCA activity, as revealed by the loss of effect of La(3+), and to inhibitory interactions between NCX and SERCA activities, indicated by acceleration of decay in response to their respective inhibitors. In conclusion, in smooth muscle cells aging decreases the overall Ca(2+) extrusion activity and modifies the interactions between the activities of the main Ca(2+) removing mechanisms.

Interaction of STIM1 with endogenously expressed human canonical TRP1 upon depletion of intracellular Ca2+ stores

STIM1 (stromal interaction molecule 1) has recently been proposed to communicate the intracellular Ca(2+) stores with the plasma membrane to mediate store-operated Ca(2+) entry. Here we describe for the first time that Ca(2+) store depletion stimulates rapid STIM1 surface expression and association with endogenously expressed human canonical TRP1 (hTRPC1) independently of rises in cytosolic free Ca(2+) concentration. These events require the support of the actin cytoskeleton in human platelets, as reported for the coupling between type II inositol 1,4,5-trisphosphate receptor in the Ca(2+) stores and hTRPC1 in the plasma membrane, which has been suggested to underlie the activation of store-operated Ca(2+) entry in these cells. Electrotransjection of cells with anti-STIM1 antibody, directed toward the N-terminal sequence that includes the Ca(2+)-binding region, prevented the migration of STIM1 toward the plasma membrane, the interaction between STIM1 and hTRPC1, the coupling between hTRPC1 and type II inositol 1,4,5-trisphosphate receptor, and reduced store-operated Ca(2+) entry. These findings provide evidence for a role of STIM1 in the activation of store-operated Ca(2+) entry probably acting as a Ca(2+) sensor.

NAADP, cADPR and IP3 all release Ca2+ from the endoplasmic reticulum and an acidic store in the secretory granule area

Inositol trisphosphate and cyclic ADP-ribose release Ca2+ from the endoplasmic reticulum via inositol trisphosphate and ryanodine receptors, respectively. By contrast, nicotinic acid adenine dinucleotide phosphate may activate a novel Ca2+ channel in an acid compartment. We show, in two-photon permeabilized pancreatic acinar cells, that the three messengers tested could each release Ca2+ from the endoplasmic reticulum and also from an acid store in the granular region. The nicotinic acid adenine dinucleotide phosphate action on both types of store, like that of cyclic ADP-ribose but unlike inositol trisphosphate, depended on operational ryanodine receptors, since it was blocked by ryanodine or ruthenium red. The acid Ca2+ store in the granular region did not have Golgi or lysosomal characteristics and might therefore be associated with the secretory granules. The endoplasmic reticulum is predominantly basal, but thin extensions penetrate into the granular area and cytosolic Ca2+ signals probably initiate at sites where endoplasmic reticulum elements and granules come close together.

Ca2+ accumulation into acidic organelles mediated by Ca2+- and vacuolar H+-ATPases in human platelets

Most physiological agonists increase cytosolic free [Ca2+]c (cytosolic free Ca2+ concentration) to regulate a variety of cellular processes. How different stimuli evoke distinct spatiotemporal Ca2+ responses remains unclear, and the presence of separate intracellular Ca2+ stores might be of great functional relevance. Ca2+ accumulation into intracellular compartments mainly depends on the activity of Ca2+- and H+-ATPases. Platelets present two separate Ca2+ stores differentiated by the distinct sensitivity to thapsigargin and TBHQ [2,5-di-(t-butyl)-1,4-hydroquinone]. Although one store has long been identified as the dense tubular system, the nature of the TBHQ-sensitive store remains uncertain. Treatment of platelets with GPN (glycylphenylalanine-2-naphthylamide) impaired Ca2+ release by TBHQ and reduced that evoked by thrombin. In contrast, GPN did not modify Ca2+ mobilization stimulated by ADP or AVP ([arginine]vasopressin). Treatment with nigericin, a proton carrier, and bafilomycin A1, an inhibitor of the vacuolar H+-ATPase, to dissipate the proton gradient into acidic organelles induces a transient increase in [Ca2+]c that was abolished by previous treatment with the SERCA (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase) 3 inhibitor TBHQ. Depleted acidic stores after nigericin or bafilomycin A1 were refilled by SERCA 3. Thrombin, but not ADP or AVP, reduces the rise in [Ca2+]c evoked by nigericin and bafilomycin A1. Our results indicate that the TBHQ-sensitive store in human platelets is an acidic organelle whose Ca2+ accumulation is regulated by both Ca2+- and vacuolar H+-ATPases.

Physiological characterization of 45Ca2+ and 65Zn2+ transport by lobster hepatopancreatic endoplasmic reticulum

The crustacean hepatopancreas is an epithelial-lined, multifunctional organ that, among other activities, regulates the flow of calcium into and out of the animal's body throughout the life cycle. Transepithelial calcium flow across this epithelial cell layer occurs by the combination of calcium channels and cation exchangers at the apical pole of the cell and by an ATP-dependent, calcium ATPase in conjunction with a calcium channel and an Na+/Ca2+ antiporter in the basolateral cell region. The roles of intracellular organelles such as mitochondria, lysosomes, and endoplasmic reticulum (ER) in transepithelial calcium transport or in transient calcium sequestration are unclear, but may be involved in transferring cytosolic calcium from one cell pole to the other. The ER membrane has a complement of ATP-dependent calcium ATPases (SERCA) and calcium channels that regulate the uptake and possible transfer of calcium through this organelle during periods of intense calcium fluxes across the epithelium as a whole. This investigation characterized the mechanisms of calcium transport by lobster hepatopancreatic ER vesicles and the effects of drugs and heavy metals on them. Kinetic constants for 45Ca2+ influx under control conditions were K(n) (m)=10.38+/-1.01 microM, J(max)=14.75+/-1.27 pmol/mg protein x sec, and n=2.53+/-0.46. The Hill coefficient for 45Ca2+ influx under control conditions, approximating 2, suggests that approximately two calcium ions were transported for each transport cycle in the absence of ATP or the inhibitors. Addition of 1 mM ATP to the incubation medium significantly (P<0.01) elevated the rate of 45Ca2+ influx at all calcium activities used and retained the sigmoidal nature of the transport relationship. The kinetic constants for 45Ca2+ influx in the presence of 1 mM ATP were K(n) (m)=12.76+/-0.91 microM, J(max)=25.46+/-1.45 pmol/mg protein x sec, and n=1.95+/-0.15. Kinetic analyses of ER 65Zn2+ influx resulted in a sigmoidal relationship between transport rate and zinc activity under control conditions (K(n) (m)=38.63+/-0.52 microM, J(max)=19.35+/-0.17 pmol/mg protein x sec, n=1.81+/-0.03). The Addition of 1 mM ATP enhanced 65Zn2+ influx at each zinc activity, but maintained the overall sigmoidal nature of the kinetic relationship. The kinetic constants for zinc influx in the presence of 1 mM ATP were K(n) (m)=34.59+/-2.31 microM, J(max)=26.09+/-1.17 pmol/mg protein x sec, and n=1.96+/-0.17. Both sigmoidal and ATP-dependent calcium and zinc influxes by ER vesicles were reduced in the presence of thapsigargin and vanadate. This investigation found that lobster hepatopancreatic ER exhibited a thapsigargin- and vanadate-inhibited, SERCA-like, calcium ATPase. This transporter displayed cooperative calcium transport kinetics (Hill coefficient, n approximately 2.0) and was inhibited by the heavy metals zinc and copper, suggesting that the metals may reduce the binding and transport of calcium when they are present in the cytosol.

Mitochondrial production of oxidants is necessary for physiological calcium oscillations

Mitochondrial involvement in Ca2+ signaling is thought to be due to the effect of mitochondrial Ca2+ removal from and Ca2+ release to cytosolic domains close to ryanodine and IP3 Ca2+ channels. However, mitochondria are a source of low levels of endogenous reactive oxygen species, and Ca2+ release channels are known to be redox-sensitive. In the present work, we studied the role of mitochondrial production of oxygen species in Ca2+ oscillations during physiological stimulation. Mitochondria-targeted antioxidants and mitochondrial inhibitors quickly inhibited calcium oscillations in pancreatic acinar cells stimulated by postprandial levels of the gut hormone cholecystokinin. Confocal microscopy using different redox-sensitive dyes showed that cholecystokinin-induced oscillations are associated with mitochondrial production of reactive oxygen species. This production is inhibited by application of mitochondria-targeted antioxidants and mitochondrial inhibitors. In addition, we found no correlation between inhibition of oscillations and mitochondrial depolarization. We conclude that low level production of reactive oxygen species by mitochondria is a necessary element in the development of Ca2+ oscillations during physiological stimulation. This study unveils a new and unexplored aspect of the participation of mitochondria in calcium signals.

Vacuolar ATPase regulates zymogen activation in pancreatic acini

Supramaximal concentrations of cholecystokinin or its analogue caerulein have been shown to stimulate the proteolytic activation of zymogens within the pancreatic acinar cell and initiate acute pancreatitis. Previous studies suggest that a low pH compartment might be required for activation. To test this hypothesis, the effects of agents that modulate intracellular pH on caerulein-induced trypsin and chymotrypsin activation were studied. Pretreatment of pancreatic acini with the proto-ionophore monensin (10 microM) and the weak base chloroquine (40 microM) inhibited activation. Pre-incubation with the vacuolar ATPase (V-ATPase) inhibitors bafilomycin A(1) and concanamycin A also decreased activation in a concentration-dependent manner with 50% inhibition at approximately 50 and 25 nM, respectively. Caerulein stimulation caused a time- and concentration-dependent translocation of soluble V-ATPase V(1) subunits to a membrane fraction, a marker of V-ATPase activation. Carbachol also stimulated translocation at supramaximal concentrations. Elevation of cytosolic Ca(2+) by thapsigargin was sufficient to induce translocation. Thus, stimulation of V-ATPase activity appears to be required for agonist-induced zymogen activation in the pancreatic acinar cell.

The V-type H(+)-ATPase in Malpighian tubules of Aedes aegypti: localization and activity

The V-type H(+)-ATPase is thought to provide the driving force for transepithelial electrolyte and fluid secretion in Malpighian tubules. To confirm the presence of this proton pump in Malpighian tubules of the yellow fever mosquito Aedes aegypti, we used several antibodies raised against the V-type H(+)-ATPase of Manduca sexta. Western blot analysis confirmed the presence of the V-type H(+)-ATPase in Malpighian tubules of Aedes aegypti. In situ immunostaining identified the V-type H(+)-ATPase at the apical membrane of the mitochondrion-rich brush border of principal cells. The V-type H(+)-ATPase was not found in stellate cells. Measurements of ATPase activity revealed that bafilomycin-sensitive and NO(3)(-)-sensitive ATPase activity accounted for 50-60% of total ATPase activity in crude extracts of Malpighian tubules. No significant ouabain- or vanadate-sensitive Na(+)/K(+)-ATPase activity was detected. These results support the conclusion reached previously in electrophysiological studies that the mechanisms for transepithelial electrolyte secretion in the Aedes Malpighian tubules rely on the V-type H(+)-ATPase as the principal energizer of epithelial transport. Measures of transepithelial Na(+) and K(+) secretion and estimates of the H(+) flux mediated by the V-type H(+)-ATPase suggest a 1:1 stoichiometry for Na(+)/H(+) and K(+)/H(+) exchange transport across the apical membrane.

Biosynthesis, glycosylation, and enzymatic processing in vivo of human tripeptidyl-peptidase I

Human tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal serine protease that removes tripeptides from the free N termini of small polypeptides and also shows a minor endoprotease activity. Due to various naturally occurring mutations, an inherited deficiency of TPP I activity causes a fatal lysosomal storage disorder, classic late infantile neuronal ceroid lipofuscinosis (CLN2). In the present study, we analyzed biosynthesis, glycosylation, transport, and proteolytic processing of this enzyme in stably transfected Chinese hamster ovary cells as well as maturation of the endocytosed proenzyme in CLN2 lymphoblasts, fibroblasts, and N2a cells. Human TPP I was initially identified as a single precursor polypeptide of approximately 68 kDa, which, within a few hours, was converted to the mature enzyme of approximately 48 kDa. Compounds affecting the pH of intracellular acidic compartments, those interfering with the intracellular vesicular transport as well as inhibition of the fusion between late endosomes and lysosomes by temperature block or 3-methyladenine, hampered the conversion of TPP I proenzyme into the mature form, suggesting that this process takes place in lysosomal compartments. Digestion of immunoprecipitated TPP I proenzyme with both N-glycosidase F and endoglycosidase H as well as treatment of the cells with tunicamycin reduced the molecular mass of TPP I proenzyme by approximately 10 kDa, which indicates that all five potential N-glycosylation sites in TPP I are utilized. Mature TPP I was found to be partially resistant to endo H treatment; thus, some of its N-linked oligosaccharides are of the complex/hybrid type. Analysis of the effect of various classes of protease inhibitors and mutation of the active site Ser(475) on human TPP I maturation in cultured cells demonstrated that although TPP I zymogen is capable of autoactivation in vitro, a serine protease that is sensitive to AEBSF participates in processing of the proenzyme to the mature, active form in vivo.

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.

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.

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.

The endoplasmic reticulum: one continuous or several separate Ca(2+) stores?

The Ca2+ store and sink in the endoplasmic reticulum (ER) is important for Ca2+ signal integration and for conveyance of information in spatial and temporal domains. Textbooks regard the ER as one continuous network, but biochemical and biophysical studies revealed apparently discrete ER Ca2+ stores. Recent direct studies of ER lumenal Ca2+ movements show that this organelle system is one continuous Ca2+ store, which can function as a Ca2+ tunnel. The concept of a fully connected ER network is entirely compatible with evidence indicating that the distribution of Ca2+ -release channels in the ER membrane is discontinuous with clustering in certain localities.

The effects of aging on gene expression in the hypothalamus and cortex of mice

A better understanding of the molecular effects of aging in the brain may help to reveal important aspects of organismal aging, as well as processes that lead to age-related brain dysfunction. In this study, we have examined differences in gene expression in the hypothalamus and cortex of young and aged mice by using high-density oligonucleotide arrays. A number of key genes involved in neuronal structure and signaling are differentially expressed in both the aged hypothalamus and cortex, including synaptotagmin I, cAMP-dependent protein kinase C beta, apolipoprotein E, protein phosphatase 2A, and prostaglandin D. Misregulation of these proteins may contribute to age-related memory deficits and neurodegenerative diseases. In addition, many proteases that play essential roles in regulating neuropeptide metabolism, amyloid precursor protein processing, and neuronal apoptosis are up-regulated in the aged brain and likely contribute significantly to brain aging. Finally, a subset of these genes whose expression is affected by aging are oppositely affected by exposure of mice to an enriched environment, suggesting that these genes may play important roles in learning and memory.

Calcium-dependent enzyme activation and vacuole formation in the apical granular region of pancreatic acinar cells

The pancreatic acinar cell produces powerful digestive enzymes packaged in zymogen granules in the apical pole. Ca(2+) signals elicited by acetylcholine or cholecystokinin (CCK) initiate enzyme secretion by exocytosis through the apical membrane. Intracellular enzyme activation is normally kept to a minimum, but in the often-fatal human disease acute pancreatitis, autodigestion occurs. How the enzymes become inappropriately activated is unknown. We monitored the cytosolic Ca(2+) concentration ([Ca(2+)](i)), intracellular trypsin activation, and its localization in isolated living cells with specific fluorescent probes and studied intracellular vacuole formation by electron microscopy as well as quantitative image analysis (light microscopy). A physiological CCK level (10 pM) eliciting regular Ca(2+) spiking did not evoke intracellular trypsin activation or vacuole formation. However, stimulation with 10 nM CCK, evoking a sustained rise in [Ca(2+)](i), induced pronounced trypsin activation and extensive vacuole formation, both localized in the apical pole. Both processes were abolished by preventing abnormal [Ca(2+)](i) elevation, either by preincubation with the specific Ca(2+) chelator 1, 2-bis(O-aminophenoxy)ethane-N,N-N',N'-tetraacetic acid (BAPTA) or by removal of external Ca(2+). CCK hyperstimulation evokes intracellular trypsin activation and vacuole formation in the apical granular pole. Both of these processes are mediated by an abnormal sustained rise in [Ca(2+)](i).

Effects of antioxidants on calcium signal induced by cholecystokinin in mouse pancreatic acinar cells

Digital imaging fluorescence microscopy was used to study the effect of two antioxidants, N-acetyl-cysteine (NAC) and glutathione, on the cytosolic free calcium concentration ([Ca2+]i) induced by cholecystokinin-octapeptide (CCK-8) of mouse pancreatic acinar cells. When acinar cells were preincubated with either NAC or glutathione, subsequent stimulation with CCK-8 in the presence of each antioxidant had no significant effect on the typical pattern of [Ca2+]i transient evoked by the gastrointestinal hormone. However, application of NAC to acinar cells pretreated for 60 min with the same antioxidant, strongly blocked the oscillatory pattern initiated by CCK-8, inhibiting both amplitude and frequency of calcium oscillations. By contrast, glutathione had no effect on the oscillatory pattern evoked by CCK-8. The present results allow us to speculate that during [Ca2+]i oscillation there is a production of oxidants that facilitate oscillations by enhancing release of calcium from internal stores.

Differential involvement of vacuolar H(+)-ATPase in the refilling of thapsigargin- and agonist-mobilized Ca(2+) stores

Our objective was to evaluate the role of vacuolar H(+)-ATPase and proton gradients in the refilling of Ca(2+) stores in fura-2-loaded pancreatic acinar cells. Once depleted with a high level of ACh, the Ca(2+) stores were replenished with a Ca(2+)-containing solution. The degree of refilling was estimated with a second release in response to either ACh (ACh-releasable store) or thapsigargin (thapsigargin-releasable store), a specific inhibitor of the endoplasmic reticulum Ca(2+) pumps. Both the protonophore nigericin and folimycin, a specific inhibitor of the vacuolar H(+)-ATPase, reduced reuptake into the ACh-mobilized stores but not into the thapsigargin-releasable pools. These treatments effectively dissipated the subcellular pH gradients (revealed by confocal observation of the distribution of a marker for acidic compartments), and did not impair the [Ca(2+)](i) response to ACh in control cells. Our results indicate that thapsigargin and ACh release heterogeneous Ca(2+) stores which are differently operated by vacuolar proton ATPase.