signal transduction, calcium and acute pancreatitis

Invited Review

Pancreatology 2003;3:497–505DOI: 10.1159/000075581

Signal Transduction, Calcium and AcutePancreatitis

Robert Suttona David Criddlea,b Michael G.T. Raratya Alexei Tepikinb

John P. Neoptolemosa Ole H. Petersenb

Departments of aSurgery and bPhysiology, University of Liverpool, Liverpool, UK

Published online: December 11, 2003

Prof. Robert Sutton, DPhil, FRCSDepartment of Surgery, University of Liverpool5th Floor UCD Block, Royal Liverpool University HospitalDaulby Street, Liverpool L69 3GA (UK)Tel. +44 151 7064170, Fax +44 151 7065826, E-Mail [email protected]

ABCFax + 41 61 306 12 34E-Mail [email protected]

© 2003 S. Karger AG, Basel1424–3903/03/0036–0497$19.50/0

Accessible online at:www.karger.com/pan

Key WordsCell signalling W Calcium W Acute pancreatitis W

Pathogenesis

AbstractEvidence consistently suggests that the earliest changesof acute pancreatitis are intracellular, the hallmark ofwhich is premature intracellular activation of digestivezymogens, accompanied by disruption of normal signaltransduction and secretion. Principal components ofphysiological signal transduction include secretagogue-induced activation of G-protein-linked receptors, fol-lowed by generation of inositol 1,4,5-trisphosphate, nico-tinic acid adenine dinucleotide phosphate and cyclicADP-ribose. In response, calcium is released from endo-plasmic reticulum terminals within the apical, granularpole of the cell, where calcium signals are usually con-tained by perigranular mitochondria, in turn respondingby increased metabolism. When all three intracellularmessengers are administered together, even at thresh-old concentrations, dramatic potentiation results in sus-tained, global, cytosolic calcium elevation. Prolonged,global elevation of cytosolic calcium is also induced byhyperstimulation, bile salts, alcohol and fatty acid ethylesters, and depends on continued calcium entry into thecell. Such abnormal calcium signals induce intracellularactivation of digestive enzymes, and of nuclear factor ÎB,

as well as the morphological changes of acute pancreati-tis. Depletion of endoplasmic reticulum calcium and mi-tochondrial membrane potential may contribute to fur-ther cell injury. This review outlines current understand-ing of signal transduction in the pancreas, and its appli-cation to the pathophysiology of acute pancreatitis.

Copyright © 2003 S. Karger AG, Basel

Premature Enzyme Activation Is the Hallmarkof Acute Pancreatitis

The incidence of acute pancreatitis is of the order of 40per 100,000 per year and is increasing [1]. Severe compli-cations that require major intervention and intensive caresupport develop in 20–30% of patients, and carry a mor-tality of 20–50% [1]. Despite recognition of precipitatingfactors, understanding of the pathogenesis of acute pan-creatitis has been limited, and randomised trials havelargely failed to show consistent benefits in favour of anytreatment [1].

Acute pancreatitis is an autolytic process, with acti-vated pancreatic enzymes identifiable within [2–4] andsubsequently around acinar cells, precipitated by ductalhypertension, bile salts, ethanol, ischaemia, hypercal-caemia, hyperlipidaemia, viral infections, drugs or toxins[1, 5]. Evidence consistently suggests that the initialchanges are intracellular: apical enzyme activation, cyto-

498 Pancreatology 2003;3:497–505 Sutton/Criddle/Raraty/Tepikin/Neoptolemos/Petersen

skeletal disruption, loss of secretory polarity, vacuolisa-tion, co-localisation of zymogen granules and lysosomes,and expression of pro-inflammatory cytokines all occur[6–10]. Co-localisation of zymogen granules and lyso-somes may be an initiating event [4]; alternatively, suchco-localisation may be protective [8]. It is also uncertainwhich of the many acinar cell pro-enzymes can initiateacute pancreatitis; substantial evidence favours trypsino-gen as the critical mediator [11–14]; pro-cathepsin B andgranular pro-elastase also have important early effects[15, 16].

Despite a series of safeguards, the central role of tryp-sinogen as a one-way master switch for digestive enzymeactivation carries a liability: this molecule, and thus thepancreas, is vulnerable to premature activation. Muta-tions of the PRSS1 cationic trypsinogen gene (includingR122H and N29I) render prematurely activated intracel-lular trypsinogen resistant to normal, protective inactiva-tion, causing hereditary pancreatitis [11, 12]. Hereditarypancreatitis is a rare autosomal dominant disorder result-ing in recurrent acute pancreatitis, leading to chronic pan-creatitis and a lifetime risk of pancreatic cancer of 40–50%. In addition, mutations of the pancreatic secretorytrypsin inhibitor contribute to progression of the disease[13]. The importance of trypsin is underlined by ourrecent demonstration that on admission to hospital, uri-nary trypsinogen activation peptide levels are highly pre-dictive of both the presence and severity of acute pancre-atitis [14].

Abnormal Elevation in Cytosolic CalciumInitiates Enzyme Activation

Stimulation of acinar cells with a physiological concen-tration of cholecystokinin (up to 10 pM) elicits repetitivecytosolic calcium spikes, primarily due to release of cal-cium from intracellular stores [17, 18]. Such calcium spik-ing evokes normal secretion, but not intracellular trypsinactivation [19]. On the other hand, cholecystokinin hyper-stimulation (10 nM) evokes a sustained elevation of thecytosolic calcium level that induces trypsinogen activa-tion [19]; in vivo, cholecystokinin hyperstimulation hasbeen found to induce acute pancreatitis in all species thathave been examined [2–10, 19].

Abnormally high levels of cytosolic calcium are instru-mental in damage to other cell types, including from abu-sive stimulation, ischaemia-reperfusion, toxins, viruses,in immune cell killing and apoptosis [5, 17, 20, 21]. Pro-longed elevations of cytosolic calcium can activate de-

gradative calcium-dependent enzymes, leading to necro-sis or apoptosis, depending on the degree of damage [21].We have proposed [5] and tested [19] the hypothesis thatacute pancreatitis is caused by pathological increases inacinar cytosolic calcium. Our results [19] agree with thoseof others [22, 23] in support of this, as discussed below(‘Abnormal Elevation of Cytosolic Calcium TriggersAcute Pancreatitis’). However, before considering the evi-dence further, it is appropriate to review normal pancreat-ic acinar cell signalling, discussed in the next section [andmore extensively in 17, 18 and 24].

Signal Transduction Has Been Defined inNormal Acinar Cells

The normal calcium-signalling process within the pan-creatic acinar cell has been worked out in some detail [17,18, 24–39]. Acetylcholine binding to muscarinic receptorsactivates phospholipase C, generating inositol 1,4,5-tris-phosphate. This messenger binds to specific receptors(calcium channels) in the endoplasmic reticulum mem-brane, causing release of calcium stored inside the endo-plasmic reticulum. Since acetylcholine-induced calciumspikes are blocked by ryanodine as well as inositol 1,4,5-receptor antagonists, the spiking process is probably dueto calcium-mediated actions on both sets of receptors (seefig. 1) [25].

In contrast to the actions of acetylcholine, physiologi-cal concentrations of cholecystokinin (up to 10 pM) gener-ate very little or no inositol trisphosphate [25, 26]. Chole-cystokinin-induced calcium spiking is totally dependenton functional nicotinic acid adenine dinucleotide phos-phate receptors, and normally requires operational cyclicADP-ribose receptors [25]. Nevertheless, cholecystoki-nin-induced calcium spiking is also dependent on func-tional inositol 1,4,5-trisphosphate receptors [25]. In addi-tion, apical calcium spikes induced by cholecystokinin aredramatically potentiated (globalised) by inositol 1,4,5-trisphosphate, but not by cyclic ADP-ribose nor nicotinicacid adenine dinucleotide phosphate, whereas calciumresponses to acetylcholine are globalised by intracellularcyclic ADP-ribose and nicotinic acid adenine dinucleo-tide phosphate, but not inositol 1,4,5-trisphosphate [27].When all three intracellular messengers are administeredtogether at threshold concentrations, dramatic potentia-tion results in sustained, global, cytosolic calcium eleva-tion [27]. Thus, two different intracellular signal path-ways converge on a common oscillator unit (see fig. 1)[25].


Pancreatology 2003;3:497–505 499

Fig. 1. Physiological calcium release mecha-nisms in normal pancreatic acinar cells[from 25]. CCK = Cholecystokinin; ACh =acetylcholine; NAADP = nicotinic acid ade-nine dinucleotide phosphate; cADPR =cyclic adenosine diphosphate ribose;Ins(1,4,5)P3 = inositol 1,4,5-trisphosphate;Ca2+ = calcium; Ca-Cam = calcium-calmo-dulin complex; NAADPR = NAADP recep-tor; RyR = ryanodine receptor; InsP3R =InsP3 receptor.

Signal transduction in the pancreatic acinar cell ishighly polarised, which is suited to its principal functionof digestive enzyme secretion. As described, physiologicalcytosolic calcium signals occur principally in the apical,granular area of the cell, regulating exocytotic secretion[17, 18]. The functionally important calcium releasechannels are clustered in thin endoplasmic reticulum ele-ments extending into the apical pole [28]; followingrelease, most apical, cytosolic calcium signals are prevent-ed from spreading to the basolateral part of the cell by amitochondrial buffer barrier that surrounds the granularregion [29]. Such apical signals do not result in significantdepletion of the endoplasmic reticulum calcium store andcan go on for some time, in the complete absence of exter-nal calcium [30]. However, to restore normal apical cyto-solic levels, calcium is extruded by calcium-ATPase, athigh concentration within the apical plasma membrane[31]; calcium is also extruded by granular exocytosis [32].These small calcium fluxes prompt re-entry of small quan-tities of calcium through store-operated channels in thebasal membrane, independently of ATP, but driven byendoplasmic reticulum uptake via calcium-ATPase [33],and through endocytosis [34].

In contrast, maximal or supramaximal stimulationcauses substantial calcium depletion of the endoplasmicreticulum [30]; under these circumstances, the cytosoliccalcium signals spread from the apical region throughout

the cell [35]. Calcium-induced calcium release is themajor force behind such global transients, the mapping ofwhich (with calcium uncaging experiments) has shownthe extreme sensitivity of the apical region, in contrast tothe basolateral region [36]. With global calcium release,sustained elevations of cytosolic calcium become depen-dent on external calcium [19]. The entry of such externalcalcium is again through store-operated channels in thebasal membrane, with calcium-ATPase-mediated uptakeinto the endoplasmic reticulum, within which calciummoves rapidly across the cell [28, 30]. Whilst basal cyto-solic levels are re-established by calcium extrusion withinseconds of the cessation of stimulation, endoplasmicreticulum calcium-ATPase refilling takes several minutes[37]. Also, the endoplasmic reticulum calcium reuptakerate declines until equilibrium is reached with the passiveendoplasmic reticulum leak rate, at a calcium concentra-tion (100–300 ÌM) far higher than within the cytosol(!1 ÌM) [37].

Perigranular mitochondria take up calcium during api-cal cytosolic calcium spiking [38], not only restrictingcytosolic calcium release to the apical pole [29], but alsotriggering increased mitochondrial metabolism during ex-ocytosis [39]. There are also peripheral mitochondriapositioned close to the basolateral plasma membrane thattake up calcium during store-operated calcium entry, andperinuclear mitochondria that prevent or delay calcium


entry into the nucleoplasm; calcium uptake by all groupsof mitochondria are linked to specific subcellular meta-bolic demands [38, 39].

Calcium Signalling Is Progressively Disruptedin Acute Pancreatitis

We have shown progressive disruption of acinar cellcalcium signalling to be an integral feature of early caeru-lein-induced acute pancreatitis in mice [9]. Followinginduction of acute pancreatitis in vivo, acinar cells andcell clusters were harvested by collagenase digestion invitro, and calcium signalling examined by digital imagingmicrofluorimetry with Fura-2. In isolated acinar cells fol-lowing only a single dose of caerulein in vivo, typical high-frequency oscillations of 2–3 per min followed expo-sure to quasi-physiological doses of acetylcholine (25–100 nmol/l) and slower transients every 2–3 min followedcholecystokinin (10–20 pmol/l). With cells isolated afteran increasing number of doses of caerulein in vivo,responses became more frequently abnormal, and in-cluded transient single increases without further re-sponses, as well as slow initiation of responses, minimal oreven absent responses. Thus, normal oscillatory calciumsignals evoked by low agonist concentrations were selec-tively abolished in acute pancreatitis, with the apical poleof the acinar cell as the site of primary abnormality [9].This was especially clear in experiments that showed aprogressive loss of the normal pattern of global calciumrelease following supramaximal stimulation. In healthycells, such stimulation initiates cytosolic calcium releasewithin the apical pole, that spreads to the basolateralregions within 200–300 ms [35]. With increasing doses ofcaerulein in vivo, this sequence was progressively defi-cient, such that after 7 injections, more than 50% of cellshad lost this pattern, displaying simultaneous apical andbasal calcium release. Furthermore, the basolateral re-gions showed relatively sparing of calcium-signalling ab-normalities, since both calcium reuptake and extrusionwere found to be intact. Thus, all groups of experimentaland control cells demonstrated typical patterns of cyto-solic calcium release following inhibition of the endoplas-mic reticulum calcium reuptake pump with thapsigargin.Recently, similar overall changes in calcium signallinghave been found in isolated acini following pancreaticduct ligation-induced acute pancreatitis in the rat [40].

The demonstration of a primary signal transductionabnormality at the apical pole provides an explanation asto why digestive enzyme secretion is diminished in acute

pancreatitis [6, 9]. These data also point to a pathogeneticprocess that may be initiated within the apical pole, sinceearly in the course of the disease other areas of the cellremain intact.

Abnormal Elevation of Cytosolic CalciumTriggers Acute Pancreatitis

We have examined the role of abnormal calcium sig-nals in the initiation of acute pancreatitis, through investi-gation of acinar cell changes induced in vitro by hyper-stimulation of isolated acinar cells and cell clusters iso-lated from normal mouse acini [19]. Trypsinogen activa-tion and the occurrence of vacuoles were seen in the apicalpole of cells hyperstimulated with cholecystokinin orexposed to thapsigargin for 60 min [19], treatments thatproduce prolonged elevations in cytosolic calcium. Theresulting changes could be prevented by preincubationwith a membrane-permeable specific calcium chelator(BAPTA-AM, see fig. 2) or by simply removing extracel-lular calcium [19]. Since chelation of cytosolic calciumwith BAPTA does not inhibit agonist-evoked loss of cal-cium from the endoplasmic reticulum [37], it wouldappear to be the sustained rise in cytosolic calcium thatcauses the intracellular changes, as reported by others [22,23]. Nor were changes in mitochondrial membrane poten-tial implicated, as the inner mitochondrial membranemaintained its potential throughout the 60 min of hyper-stimulation [19]. After the peak of calcium release, a smallelevation of calcium persists with continuing hyperstimu-lation, that as indicated depends on the continued entry ofcalcium from the external medium, and which results intrypsinogen activation [19, 22, 23]. Higher concentrationsof external calcium result in greater amounts of trypsino-gen activation [41]; indeed hypercalcaemia without hy-perstimulation can cause acute pancreatitis [42]. Further-more, abnormal cytosolic calcium release has now beenfound to induce trypsinogen activation, vacuolisation andacinar cell damage by a number of investigators [19, 22,23, 41]. Together, these data suggest a key role for abnor-mal, sustained elevations of cytosolic calcium in thepathogenesis of acute pancreatitis, that contrast with thenormal oscillatory changes, and again implicate the gran-ule-rich apical pole as the site of primary injury [19, 20].

Experiments with fluorescent dyes coupled to enzyme-specific amino acid moieties (without cleavage of whichfluorescence does not occur) and visualised by confocalmicroscopy showed that within 300 s of the onset ofhyperstimulation, fluorescence occurred within multiple,



discrete, rounded compartments, each up to 1 Ìm diame-ter and situated throughout the granular region of the cell[19, 43]. These enlarged modestly during hyperstimula-tion and became less distinct as fluorescence appeared inthe surrounding apical cytosol. By 60 min, typical vac-uoles were demonstrable by electron microscopy. Whenthe abnormal calcium signals were attenuated with thecalcium chelator BAPTA, no trypsinogen or cathepsin Bactivation occurred, and no morphological abnormalitiesdeveloped, again confirming their dependence on abnor-mal calcium signalling. The spatio-temporal characteris-tics of intracellular enzyme activation seen in these exper-iments suggested that activation first occurs within thezymogen granules themselves, and that activated granulesbecome vacuoles, which then leak activated enzymes intothe surrounding cytosol [43].

Abnormal, prolonged cytosolic calcium may provokeacinar cell enzyme activation through excessive depletionof calcium from and potassium entry into zymogen gran-ules [19]. Zymogen granules are calcium stores [32], with-in which calcium maintains granular integrity, and anacidic pH maintains the condensed, inactive state of theenclosed digestive enzymes [44]. Zymogen and othersecretory granules release calcium through inositol 1,4,5-trisphosphate and cyclic ADP-ribose triggered channels[32, 45, 46], contributing to the apical cytosolic calciumrelease that leads to exocytosis [19]. Zymogen granule cal-cium release is followed by potassium entry [45], account-ing for a modest pH increase within maturing granulesand preparing enzymes for secretion. If granular calciumdepletion is excessive before exocytosis is possible, potas-sium entry may induce not only unfolding but also prema-ture activation of digestive enzymes [47].

Recent experiments have examined the potential roleof factors that precipitate acute pancreatitis in producingabnormal signal transduction. Thus, we [48] and others[49] have found various naturally occurring bile salts toinduce abnormal, prolonged elevations in cytosolic cal-cium that are similar to those induced by hyperstimula-tion, and are likely to be associated with acute pancreati-tis. We found bile salts to induce global calcium oscilla-tions and extended calcium transients that spread fromthe apical pole, unaffected by atropine, but abolished bycaffeine or by prior depletion of calcium stores [48]. Suchcalcium release may thus occur through activation of ino-sitol 1,4,5-trisphosphate receptors, although blockade ofthe endoplasmic reticulum calcium reuptake pump hasalso been suggested [49]. Even though the debate stillrages over the clinical importance of retrograde passage ofbile into the pancreatic duct, put forward by Opie [50]

Fig. 2. Effect of cholecystokinin on cytosolic calcium (thin line) andtrypsinogen activation (thick line) in the pancreatic acinar cell[adapted from 19]. A Physiological stimulation producing normalcalcium oscillations and no trypsinogen activation (arbitrary units).B Hyperstimulation producing abnormal, prolonged elevation of cal-cium and increasing trypsinogen activation, reduced by a membrane-permeant trypsin inhibitor, benzamidine (rapid washout of rhodam-ine 110). C After loading with the calcium chelator BAPTA, hyper-stimulation producing reduced elevation of calcium and no trypsino-gen activation.

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100 years ago to account for fatal acute pancreatitis in 2patients who were found at post mortem to have gall-stones impacted at the sphincter of Oddi, bile salts perfu-sion of the pancreatic duct is an established experimentalmodel of acute pancreatitis [6]. Thus, signal transductionexperiments with bile salts provide further evidence insupport of an important role for abnormal, prolonged ele-vations of cytosolic calcium in the pathogenesis of experi-mental acute pancreatitis.

Further work has demonstrated global pancreatic aci-nar cell cytosolic calcium release in response to ethanoland free fatty acid ethyl esters [51], important pancreaticmetabolites of ethanol, which also induce acute pancreati-tis [52]. In addition, oxidative stress, associated withknown precipitants of acute pancreatitis, causes sustainedacinar cell free ionised cytosolic calcium release [53].Water immersion stress, which induces heat-shock pro-teins and prevents hyperstimulation-induced acute pan-creatitis, attenuates the calcium release normally conse-quent upon caerulein hyperstimulation [54]. It is also ofnote that nuclear factor ÎB activation, which leads to aci-nar cell cytokine expression independently of trypsinogenactivation, has been found to depend on abnormal acinarcell cytosolic calcium release [54–56].

Cytokine Expression Is Dependent on CalciumSignalling

Cells produce cytokines in response to injury, to enrolvital reparative responses [57]. Acute pancreatitis inducesacinar cell expression of cytokines (e.g., tumour necrosisfactor ·) [10, 57], including chemokines (e.g., monocytechemotactic protein (1) that signal to leukocytes [55, 58].We and others have found isolated acinar cell chemokineexpression to occur by a nuclear factor ÎB-dependentmechanism within 30 min of cholecystokinin hyperstimu-lation [55, 58, 59], through prolonged elevation of cyto-solic calcium [55, 59]. However, the mechanisms induc-ing acinar cell cytokine expression are not fully under-stood, as the isolation process per se can induce cytokineexpression, again via nuclear factor ÎB [60]. Nuclear fac-tor ÎB appears to be essential to the pathogenesis of acutepancreatitis [61], although of itself does not cause trypsin-ogen activation [62]. It is thus of considerable interest thatactivation of nuclear factor ÎB requires not only proteinkinase C activation, but also an elevation of cytosolic cal-cium [54–56].

Subcellular Organelle Stress Contributes toPathogenesis

We have shown that the initial phase (within the first30 min) of acinar cell enzyme activation is criticallydependent on the prolonged elevation of cytosolic cal-cium, and not depletion of calcium within the endoplas-mic reticulum [19]. However, calcium depletion fromwithin the endoplasmic reticulum is a recognised sourceof cell stress, the end result of which may include misfold-ing of proteins and impaired export through the Golgiapparatus [63]. Excessive accumulation of misfolded pro-teins in the endoplasmic reticulum may in turn exacer-bate calcium release, causing the production of reactiveoxygen species and activation of nuclear factor ÎB [63,64]. The action of store-operated calcium, which may notonly activate but also control enzyme secretion by meansof a switch between phospholipase-Cß or phosphoinosi-tide 3-kinase signalling, may be central in this [65]. Thus,wortmannin, which inhibits phosphoinositide 3-kinases,also protects against experimentally-induced acute pan-creatitis [66]. Further evidence of the importance of cal-cium depletion of the endoplasmic reticulum comes fromexperiments with thapsigargin [19]. This substance,which blocks calcium-ATPase and depletes endoplasmicreticulum calcium, produced more dramatic acinar cellvacuolisation than cholecystokinin hyperstimulation,which does deplete endoplasmic reticulum calcium to thesame extent [19]. Furthermore, the calcium chelatorBAPTA was less effective in preventing vacuolisationproduced by thapsigargin than by cholecystokinin hyper-stimulation [19].

Lysosomes have previously been implicated in thepathogenesis of acute pancreatitis, although our recentevidence [43] brings an initiating role [4] into question(see above: ‘Abnormal Elevation of Cytosolic CalciumTriggers Acute Pancreatitis’). Lysosomes are a potentialsource of calcium release, but whether their calcium con-tributes to pathogenesis remains undetermined.

Mitochondrial metabolism is necessary for the provi-sion of energy throughout the cell. We have previouslyshown that administration of menadione, an oxidant,activates the permeability transition pore, leading to lossof the ability of perinuclear mitochondria to delay cal-cium entry into the nucleoplasm [67]. Also, the combina-tion of induction of the permeability transition pore,which causes partial mitochondrial depolarisation, andcytosolic calcium release results in activation of caspases,following which the pancreatic acinar cell proceeds downan apoptotic pathway [67]. With more marked interfer-



Fig. 3. Contrasting models of (A) normal [adapted from 24] and (B)abnormal calcium signalling with attendant morphological changesin the pancreatic acinar cell. A Normal calcium signalling with(a) close contacts between plasma membrane and endoplasmic retic-ulum, allowing immediate calcium uptake into the endoplasmicreticulum via calcium-ATPase pumps; (b) accumulation in the cyto-plasm; (c) peripheral mitochondrial uptake priming metabolism,supplying energy for subsequent endoplasmic reticulum uptake viacalcium-ATPase pumps; (d) tunnelling of calcium through the endo-plasmic reticulum and nuclear envelope; (e) cytoplasmic diffusion;(f) calcium accumulation in the granules via the Golgi apparatus;(g) active calcium extrusion via plasma membrane calcium-ATPasepumps; (h) exocytic release of calcium via granules, following apicalcalcium spiking (which triggers stimulus-secretion coupling), normal-ly contained within the granular region by the perigranular mito-chondrial belt. B Abnormal calcium signalling with (a) global eleva-

tion of cytosolic calcium induced by hyperstimulation, bile salts, fat-ty acid ethyl esters, etc., sustained by continued entry of calcium intothe cell; (b) calcium release from zymogen granules, allowing potas-sium entry, zymogen unfolding, and enzyme activation, with the for-mation of vacuoles; (c) activated enzymes leaking into the surround-ing cytosol, contributing to cytoskeletal disruption, and subsequentplasma membrane damage; (d) basolateral secretion of zymogengranules into the pancreatic interstitium; (e) mitochondrial stressesmay lead to induction of the permeability transition pore, and fail tocontain the calcium signal to the apical region; (f) endoplasmic retic-ulum calcium depletion may contribute to cell death signals, and leadto lower Golgi and thus granular calcium; (g) sustained global cal-cium elevation induces cytokine signalling; (h) cell fate depends ondegree of enzyme activation, integrity of membranes, energy supplyand leukocyte response.

ence in mitochondrial metabolism, it would be likely thatcell death would be accelerated, and without adequateATP generation, by necrosis rather than apoptosis. Al-though activation of intracellular digestive enzymes doesnot depend on mitochondrial membrane depolarisation[19], the cell injury resulting from digestive enzyme acti-vation may be more severe when accompanied by mito-chondrial membrane depolarisation, as could occur invivo during acute pancreatitis.

Current Evidence Concerning SignalTransduction in the Pancreas

Experimental evidence incriminates pathophysiologi-cal calcium signalling as a trigger of acute pancreatitis [19,22, 23, 41] but the picture is by no means complete, and asyet there is no obvious target that can be manipulatedtherapeutically. Nevertheless, considerable progress hasbeen made in the study of both normal and more recentlyabnormal calcium signalling within the pancreatic acinarcell, our current view of which is summarised in figure 3.


Future research perspectives must address the generalrelevance of calcium signal transduction abnormalities,and determine whether the majority if not all precipitantsof acute pancreatitis initiate the disease by these means.In this regard, extension of these studies to other in vitroand in vivo models of acute pancreatitis will be useful [68,69]. The calcium-signalling abnormalities themselves re-quire further characterisation, to distinguish principalfeatures that are disease initiators, including amplitude,

timing and subcellular location. In turn, the effects ofthese abnormalities on several of the subcellular organelles may be revealing, including the endoplasmic reticu-lum, mitochondria and zymogen granules. As the treat-ment of acute pancreatitis is one of the most importantareas of pancreatology, it is to be hoped that continuedresearch of these phenomena will eventually lead to thedevelopment of more effective treatments.

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