Mechanisms of cell death after pancreatic duct obstruction in the opossum and the rat

BACKGROUND & AIMS

Mechanisms of cell death in human and experimental pancreatitis remain poorly understood. The aim of this study was to determine the mechanisms of cell death, apoptosis vs. necrosis, in models of pancreatitis induced by pancreatic duct ligation in the opossum and rat.

METHODS

Morphological changes were shown by light and electron microscopy, chromatin condensation and DNA breaks were assessed using Hoechst 33258 staining and DNA nickend labeling, and DNA fragmentation was characterized b gel electrophoresis.

RESULTS

Cells with morphology of both necrosis and apoptosis were found in pancreata from duct-ligated animals; however, the apoptosis-necrosis ratio was different for the opossum and the rat. Both apoptotic and necrotic cells stained for breaks in DNA, suggesting that both apoptosis and necrosis are associated with DNA fragmentation and that DNA end labeling cannot specifically detect apoptotic cells. No significant oligonucleosomal DNA fragmentation was found in pancreas from duct-ligated animals, although the endonuclease responsible for oligonucleosomal DNA fragmentation was present in pancreatic nuclei.

CONCLUSIONS

Duct ligation resulted predominantly in apoptosis in the rat and necrosis in the opossum. The differences in types of cell death in the two models may result from differences in inflammatory cell infiltration.

Isolation of subcellular agonist-sensitive calcium stores from the pancreatic acinar cell

The purpose of the present study was to develop a technique to identify, isolate and partially purify these membrane bound compartments for further characterizations of their Ca2+ transport and storage mechanisms. We 45Ca(2+)-loaded the agonist-sensitive Ca2+ stores in rat pancreatic acini. The loading was accomplished by first depleting the stores with carbachol stimulation followed by the addition of 45Ca2+ and atropine to the extracellular media. After homogenization of the 45Ca(2+)-loaded acini, subcellular fractions were resolved on sucrose and Nycodenz gradients. 45Ca2+ fluxes were minimized during these procedures by inclusion in the media of LaCl3. Five subcellular fractions were identified that specifically accumulated 45Ca2+ after carbachol stimulation. Electron microscopic observations of the fractions demonstrated that three of the fractions consisted of rough membrane vesicles; that one consisted of a mixture of rough and smooth membrane vesicles; and that one consisted of smooth membrane vesicles. All fractions were enriched in glucose-6-phosphatase. All 5 fractions demonstrated ATP dependent 45Ca2+ uptake. By Western blot analysis, all fractions contained calnexin, p58, sarcoplasmic reticulum type Ca(2+)-ATPase, and IP3 receptor. These results demonstrated that the 45Ca(2+)-loading technique can be used to isolate and characterize distinct compartments of the agonist-sensitive Ca2+ store in the pancreatic acinar cell.

Parathyroid hormone-related protein expression in human gastric adenocarcinomas not associated with hypercalcemia

OBJECTIVE

Our evaluation of a patient with a parathyroid hormone-related protein (PTHrP)-producing gastric adenocarcinoma and hypercalcemia prompted us to study the expression of PTHrP in 13 additional patients with gastric cancer and in 10 control cases. Our objective was to investigate by immunohistology the expression of PTHrP in gastric cancer.

METHODS

Immunohistology studies were conducted with two murine monoclonal antibodies to synthetic peptides of human PTHrP, 9H7, and 8B12. The 9H7 antibody was raised to the carboxy-terminal amino acid fragment (109-141) of PTHrP, and the 8B12 antibody was raised to the amino-terminal amino acid fragment (1-34) of PTHrP.

RESULTS

Paraffin-embedded tumor specimens from 13 of 14 cases of gastric adenocarcinoma stained positively with the antibody to the carboxy terminus of the PTHrP peptide, but none stained positively with the antibody directed against the amino terminus. None of 10 control cases stained positively with either antibody. The staining was predominately evident in the cytoplasm of the tumor cells. Except for the sentinel case, none of the other patients with gastric adenocarcinoma had hypercalcemia. Thus, gastric adenocarcinoma seems to rarely result in systemic hypercalcemia.

CONCLUSIONS

Our studies demonstrated that abnormal PTHrP production can occur in malignant cells without producing hypercalcemia. PTHrP may play a role in the pathogenesis of gastric adenocarcinoma that is independent of its hypercalcemic effects. PTHrP measurements may be clinically valuable in patients with cancer who are not hypercalcemic.

Cross-talk between calcium and cAMP-dependent intracellular signaling pathways. Implications for synergistic secretion in T84 colonic epithelial cells and rat pancreatic acinar cells

Treatment of various cells with combinations of agents that increase either cAMP or cytosolic calcium can lead to synergistic responses. This study examined interactions, or cross-talk, between these two intracellular messengers and its implication for signaling in two secretory cell types, T84 human colonic epithelial cells and rat pancreatic acinar cells. T84 cell chloride secretion was measured in Ussing chambers. Acinar cell activation was monitored as amylase secretion. Cytosolic calcium was assessed via fura-2 microfluorimetry. A cell-permeant analogue of cAMP synergistically enhanced secretory responses to calcium-mobilizing hormones in both cell types, but paradoxically reduced overall calcium mobilization. The reduction in calcium mobilization could be attributed to an inhibition of calcium influx in T84 cells, although a different mechanism likely operates in acinar cells. The effects of the cAMP analogue were reproduced by other agents that increase cAMP. Furthermore, econazole, an inhibitor of calcium influx, potentiated secretory responses to calcium-dependent stimulation in T84 cells without itself inducing secretion. We conclude that there is cross-talk between calcium and cAMP-dependent signaling pathways at the level of second messenger generation in two secretory cell types. This cross-talk appears to regulate the extent of secretory responses.

Dual regulation of cGMP formation by calcium in pancreatic acinar cells

Regulation of guanosine 3',5'-cyclic monophosphate (cGMP) formation by calcium and calcium-binding proteins was studied at the levels of nitric oxide synthase (NOS) and guanylyl cyclase (GC) in dispersed pancreatic acini isolated from guinea pig. In intact cells, in the cytosol, and on diethylaminoethyl fractions from cytosolic proteins, GC activity was negatively regulated by Ca2+. An increase in Ca2+ concentration ([Ca2+]) from 25 to 950 nM suppressed cGMP formation by 85%. On the other hand, NOS was stimulated by agents increasing cytosolic [Ca2+] and inhibited by intracellular Ca2+ chelators. Thus Ca2+ regulates cGMP production in opposite directions by activating NOS and inhibiting GC. Calmodulin antagonists W-7, trifluoperazine, and R-24571 inhibited NOS, suggesting that the enzyme is regulated by calmodulin as in other cell types. Calmodulin antagonists appeared to inhibit GC. In particular, 200 microM W-7 completely abolished the cGMP rise evoked by the nitric oxide donor, nitroprusside. The effect was not reversed by addition of excess calmodulin. The findings suggest that the negative regulation of GC by Ca2+ is due to factors other than calmodulin but affected by calmodulin antagonists.

Effect of chronic ethanol feeding on digestive enzyme synthesis and mRNA content in rat pancreas

Chronic ethanol ingestion is a primary factor in the development of pancreatitis in humans. Alterations in both enzyme secretion and protein synthesis have been implicated as causative factors. We determined the effect of chronic ethanol feeding on the content and synthesis rates of digestive enzymes in dispersed acini from rats that were pair-fed isocaloric diets with or without ethanol for 3-6 months. Total protein content and synthesis were unchanged. The relative synthetic rates of individual digestive enzymes were analyzed using scanning laser densitometry of 1-D sodium dodecylsulfate (SDS) gels. The content of all measurable digestive enzymes except amylase increased in acini from ethanol-fed rats. Relative synthetic rates were examined in pancreatic acini labeled in vitro with [35S]methionine. Liquid scintillation counting of radiolabeled digestive enzymes extracted from gel slices revealed that amylase synthesis in ethanol-fed rats decreased 2.8-fold compared with controls whereas the synthetic rates of proelastase 1 and 2, chymotrypsinogen, and trypsinogen increased by 1.5-, 1.4-, 1.8-, and 1.6-fold, respectively. Total cellular RNA was extracted from control and ethanol-fed rats and subjected to Northern and dot blot analysis. Amylase mRNA decreased in ethanol-fed rats whereas chymotrypsinogen and trypsinogen mRNA content increased, indicating that the effect of ethanol on expression of these genes was regulated at a step prior to translation. Elastase mRNA content was not altered, suggesting that the increased synthesis of proelastase may be regulated posttranscriptionally.

Cellular mechanisms and physiological implications of quantal Ca2+ release in pancreatic acinar cells

Previous studies in pancreatic acini demonstrated that submaximal concentrations of agonist release only a portion of Ca2+ from the intracellular pool during the first few seconds of stimulation. Despite continued stimulation, no further release of pool Ca2+ takes place. This process has been termed quantal release. Previous hypotheses have proposed that quantal release results from varying sensitivities of the intracellular pool to inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release. The purpose of the present experiments was to further characterize the cellular mechanism of quantal release and to determine the role of quantal release in pancreatic enzyme secretion. The results indicate that the phenomenon of quantal calcium release is not due to varying sensitivities of the intracellular Ca2+ stores to IP3. Quantal release results from both an intrinsic property of the IP3 receptor, which causes it to transport Ca2+ transiently, and from the transient nature of the increase in IP3 generated by submaximal agonist concentration. With this mechanism, sequential additions of physiological concentrations of agonist cause transient increases in intracellular Ca2+ concentration leading to bursts of enzyme secretion.

Sphingosine regulates Ca(2+)-ATPase and reloading of intracellular Ca2+ stores in the pancreatic acinar cell

The purpose of present study was to examine the effects of sphingosine on cellular Ca2+ transports using dispersed rat pancreatic acini. The results demonstrated that sphingosine had a specific effect to inhibit Ca2+ uptake into the cell's agonist-sensitive pool as well as inhibiting microsomal Ca(2+)-ATPase. The ability of sphingosine to inhibit Ca2+ uptake resulted in both augmentation of Ca2+ release from the pool by inositol 1,4,5-trisphosphate (IP3) and conversion of the Ca2+ release by inositol 1,4,5-trisphosphate from a transient response to a sustained response. Furthermore, by preventing Ca2+ pool refilling sphingosine mimicked the effect of the agonist, carbachol, to maintain an increased [Ca2+]i during sustained stimulation. These results suggest that regulation of Ca(2+)-ATPase by sphingosine or a sphingosine-like agent mediates some of the effects of agonist on cell Ca2+ transports.

Cellular mechanisms mediating agonist-stimulated calcium influx in the pancreatic acinar cell

Figure 1 summarizes our current concept of a signaling mechanism to explain agonist-induced Ca2+ entry in the pancreatic acinar cell. We propose that cGMP can modulate Ca2+ entry under conditions of internal Ca2+ store depletion and that the NO signaling system may be involved in coupling Ca2+ depletion to cGMP formation. The finding that Ca2+ entry after Ca2+ store depletion can occur with no elevation in [Ca2+]i37 raises the possibility that alternative signaling pathways may converge to stimulate cGMP formation or that additional messengers may activate plasmalemmal Ca2+ entry mechanisms in parallel.

Cholecystokinin JMV-180 and caerulein effects on the pancreatic acinar cell cytoskeleton

We previously demonstrated that the supramaximally effective concentrations of caerulein caused marked changes in the apical cytoskeleton of the rat pancreatic acinar cell. These changes included ablation of microvilli, the terminal actin web, and intermediate filament bands. The present study was designed to elucidate part of the intracellular signalling mechanism mediating these changes. For these studies we used a cholecystokinin (CCK) analogue, CCK-JMV-180, that has been previously demonstrated not to inhibit enzyme secretion and to prevent the inhibition caused by caerulein. We investigated the effects of CCK-JMV-180 alone and in combination with supramaximal concentrations of caerulein on the morphology of the apical structures, on 1,2-diacylglycerol production (a measure of phospholipase C activity), and on amylase secretion in rat pancreatic acini. Supramaximally effective concentrations of caerulein caused inhibition of enzyme secretion. CCK-JMV-180 had no effect on the ultrastructure of the apical region of the acinar cell and it prevented the ablation of apical cytoskeleton induced by a supramaximal concentration of caerulein (10 nM). CCK-JMV-180 inhibited the increase in 1,2-diacylglycerol formation and the inhibition of amylase release caused by 10 nM caerulein. Mimicking the effect of 1,2-diacylglycerol on activation of protein kinase C with phorbol 12-myristate 13-acetate and reproducing changes in [Ca2+]i caused by 10 nM caerulein with 100 nM bombesin did not alter the apical cytoskeleton. These results suggest that the cytoskeletal changes observed with inhibitory concentrations of caerulein are caused by the phospholipase C effects of caerulein on membrane phospholipids.

Cyclic GMP modulates depletion-activated Ca2+ entry in pancreatic acinar cells

In the pancreatic acinar cell, hormonal stimulation causes a rise in the intracellular free Ca2+ concentration by activating the inositol 1,4,5-trisphosphate-mediated release of Ca2+ from intracellular stores (Berridge, M. J., and Irvine, R. F. (1989) Nature 341, 197-205). The released Ca2+ is, for the most part, extruded from the cell, necessitating a mechanism for Ca2+ entry and reloading of intracellular Ca2+ stores (Putney, J. W., Jr. (1990) Cell Calcium 11, 611-624; Rink, T. J. (1990) FEBS Lett. 268, 381-385). However, neither the mechanism of depletion-activated Ca2+ entry nor the signal that activates it is known. We report here that a sustained inward current of depletion-activated Ca2+ entry can be measured in pancreatic acinar cells using patch-clamp recording methods. Furthermore, the current can be blocked by an inhibitor of guanylyl cyclase, can be reactivated by 8-bromo-cGMP after inhibition, and can be activated in the absence of Ca2+ depletion by perfusing the cell with cGMP, but not cAMP. Intracellular perfusion with 1,3,4,5-inositol tetrakisphosphate did not activate an inward current, whereas perfusion with 2,4,5-inositol trisphosphate did activate an inward current. We conclude that cGMP may be an intracellular messenger that regulates depletion-activated Ca2+ entry.

Cholecystokinin-induced changes in polysome structure regulate protein synthesis in pancreas

Translational regulation of digestive enzyme synthesis during short-term stimulation by cholecystokinin-octapeptide (CCK-OP), was examined in minced rabbit pancreas by measuring protein synthesis and monitoring alterations in the size of polysomes attached to the rough endoplasmic reticulum (RER). The effect of CCK-OP on protein synthesis was determined by measuring [3H]leucine incorporation into trichloroacetic-acid-precipitable proteins. Concentrations of CCK-OP that caused maximal enzyme secretion (10 and 30 nM) decreased protein synthesis by approx. 50% compared to control. Protein synthesis returned to the control level 60 min after terminating the action of CCK-OP. Autoradiography of [35S]methionine-labeled proteins separated by one-dimensional SDS-polyacrylamide gel electrophoresis demonstrated that CCK-OP reversibly inhibited the synthesis of all of the major groups of digestive enzymes. Northern blot analysis revealed that CCK-OP did not alter the cellular content of amylase and elastase mRNA. Incubation with CCK-OP caused a decrease in the size distribution of RER-bound polysomes. Polysome profiles returned to the control pattern 60 min following termination of the stimulus. These results suggest that the inhibitory effects of CCK-OP on the synthesis of digestive enzymes is regulated at translation by decreasing the number of RER-bound ribosomes that are actively translating digestive enzyme mRNA.

Quantal calcium release and calcium entry in the pancreatic acinar cell

In the past decade, there have been remarkable advances in our understanding of the calcium messenger system that mediates the effects of various agonists. The purpose of the present article is to describe two areas of current interest in the calcium signaling field--quantal calcium release and calcium entry into the cell--using the pancreatic acinar cell as a model. Proposed mechanisms describing these phenomena and the role they play in the kinetics of calcium movements in the cell are discussed.

Calcium-stimulated parathyroid hormone-like protein secretion: potentiation through a protein kinase-C pathway

The effect of altering intracellular calcium levels on PTH-like protein (PLP) secretion was assessed in the NCI-H727 cell line. Ionomycin stimulated PLP secretion in a dose- and time-dependent manner, with a maximum rate of secretion occurring within minutes. Phorbol esters (phorbol 12-myristate 13-acetate and phorbol 12,13-didecanoate) stimulated PLP secretion at 10(-7)-10(-5) M, whereas forskolin treatment had no effect. When cells were treated initially with phorbol esters, ionomycin-stimulated PLP secretion was potentiated. The kinetics of this effect were rapid (t1/2, less than 1 min), and pretreatment with cycloheximide or actinomycin-D before phorbol ester and ionomycin treatment did not alter PLP secretion. Pretreatment for 24 h with phorbol 12-myristate 13-acetate to desensitize the cells or use of inactive phorbol ester isomer 4 alpha-phorbol 12,13-didecanoate did not potentiate the ionomycin-stimulated PLP secretion. Treatment with exogenously added phospholipase-C caused a dose-dependent increase in PLP secretion. These data indicate that PLP secretion can be stimulated through a phospholipase-C-mediated mechanism.

Intracellular mechanisms involved in short-term regulation of net protein synthesis in pancreatic acini

The mechanisms regulating the net synthesis of digestive enzymes during short-term stimulation by agonists were examined in pancreatic acini isolated from the rat. Dispersed pancreatic acini were stimulated for up to 60 min with various concentrations of cholecystokinin octapeptide (CCK-OP), carbachol, A23187, 4 beta-phorbol 12-myristate 13-acetate (PMA). The effects of these agonists on net protein synthesis was determined by measuring the incorporation of [3H]leucine or [35S]methionine into protein. Carbachol, PMA, A23187 and concentrations of CCK-OP of 100 pM and greater caused inhibition of protein synthesis. Fluorography of [35S]methionine labeled acinar cell proteins separated by one-dimensional SDS-polyacrylamide gel electrophoresis demonstrated that the agonists inhibited the synthesis of the digestive enzymes. Northern blot analysis using cDNA probes revealed that CCK-OP, carbachol and PMA did not alter the cellular content of amylase, lipase and elastase mRNA. The protein kinase C inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) and staurosporine failed to reverse the inhibitory effects of CCK-OP, carbachol and PMA on protein synthesis. CCK-OP and PMA activated phospholipase A (PLA) which liberated lysophosphatidylcholine (LPC) and free fatty acids from membrane phosphatidylcholine. Exogenously added PLA2 (Naja naja venom) inhibited protein synthesis and increased LPC to a similar extent as CCK and PMA. The results suggest that the inhibitory effects of CCK and carbachol on net protein synthesis are due to their effects on intracellular calcium and PLA-mediated breakdown of phosphatidylcholine rather than protein kinase C activation.

Dual pathways for agonist-stimulated arachidonic acid release in pancreatic acini: roles in secretion

The present experiments were performed to determine pathways responsible for arachidonic acid release stimulated by cholecystokinin (CCK) and phorbol ester, 4 beta-phorbol 12-myristate 13-acetate (PMA), and the roles of pathways in the secretory response in dispersed acini from guinea pig pancreas. Both CCK-octapeptide (CCK-OP) and PMA increased intracellular arachidonic acid. To determine the source of released arachidonic acid, we measured the effects of PMA and CCK-OP on cellular 1,2-diacylglycerol and lysophosphatidylcholine (LPC) and of diglyceride lipase inhibitor RHC 80267 on [3H]arachidonic acid release. Both PMA and CCK-OP increased 1,2-diacylglycerol and LPC. RHC 80267 had no effect on LPC but inhibited the increase in [3H]arachidonic acid release with a concentration of CCK-OP that was maximal for enzyme secretion. The increase in [3H]arachidonic acid release with PMA or a supramaximal concentration of CCK-OP was not inhibited by RHC 80267. In parallel fashion, RHC 80267 inhibited amylase release caused by maximally effective concentrations of CCK-OP but not that caused by PMA or by supramaximally effective concentrations of CCK-OP. Arachidonic acid stimulated amylase release. Exogenous addition of phospholipase A2 caused increases in [3H]arachidonic acid release, LPC formation, and amylase release. The results indicate that there are at least two pathways responsible for the increase in free cellular arachidonic acid stimulated by pancreatic agonists. One is sequential action of phospholipase C and diglyceride lipase on phosphatidylinositol. The other is a phospholipase A action on phosphatidylcholine. The results also suggest a stimulatory role for both pathways in the secretory response.

Effects of caerulein on the apical cytoskeleton of the pancreatic acinar cell

In this study experiments were performed to correlate the rate of digestive enzyme secretion to morphologic observations of the apical cytoskeleton using dispersed rat pancreatic acini with various concentrations of caerulein. Caerulein at concentrations of 10 pM to 0.1 nM stimulated increasing rates of secretion of amylase, a digestive enzyme. Greater concentrations of caerulein caused progressively less amylase secretion. Transmission electron microscopy demonstrated several characteristics of the apical cytoskeleton in untreated acini that were altered with the "inhibitory" concentrations of caerulein. In control acini and acini stimulated with concentrations of caerulein up to 0.1 nM, the micrographs reveal an apical actin network extending into microvilli, an intermediate filament band, and electron-dense structures contained in both the actin filament network and the intermediate filament band. With concentrations of caerulein greater than 0.1 nM, these structures were progressively ablated. The findings with respect to the actin filament network were confirmed with light microscopic observations of dispersed acini stained with rhodamine-phalloidin. These results indicate that caerulein has marked morphologic effects on the pancreatic acinar cell cytoskeleton and that the cytoskeletal changes may modulate the secretory response.

Cyclic GMP mediates the agonist-stimulated increase in plasma membrane calcium entry in the pancreatic acinar cell

The present studies were performed to determine the role of cyclic GMP in regulating agonist mediated calcium entry in the pancreatic acinar cell. In guinea pig-dispersed pancreatic acini the findings demonstrated that carbachol stimulated a transient 20-40-fold rise in cellular cyclic GMP followed by a sustained 3-4-fold rise in cellular cyclic GMP. The guanylate cyclase inhibitor, 6-anilino-5,8-quinolinedione (LY83583), caused a dose-dependent inhibition of carbachol-stimulated increases in cellular cyclic GMP both during the initial transient large increase in cyclic GMP and the sustained increase in cyclic GMP. LY83583 also inhibited cellular Ca2+ influx during carbachol stimulation and reloading of the agonist-sensitive pool of Ca2+ at the termination of carbachol stimulation with atropine. The effect of the inhibition on reloading of the agonist-sensitive pool was secondary to its effects on the plasma membrane C2+ entry. The addition of dibutyryl cyclic GMP to LY83583-treated acini restored Ca2+ influx across the plasma membrane. Nitroprusside increased both cellular cyclic GMP and the rate of Ca2+ influx. During periods when plasma membrane Ca2+ entry was activated, cellular cyclic GMP levels were increased. These results suggest that agonist-induced increases in cellular cyclic GMP are necessary and sufficient to mediate the effects of the agonist on the plasma membrane Ca2+ entry mechanism.

Cyclic GMP regulates free cytosolic calcium in the pancreatic acinar cell

The present studies were performed in order to measure the effects of cyclic GMP (cGMP) on the regulation of free cytosolic calcium [( Ca2+]i) in the pancreatic acinar cell. In guinea pig dispersed pancreatic acini the findings demonstrated that the Ca2+ ionophore, Br A23187, caused a sustained increase in [Ca2+]i in the presence of 3 mM CaCl2 in the media and a transient 20 fold rise in cellular cGMP followed by a sustained 3-4 fold rise in cellular cGMP. Increasing cellular cGMP with nitroprusside, hydroxylamine or dibutyryl cGMP had no effect on resting [Ca2+]i. However, these agents attenuated the increase in [Ca2+]i resulting from Br A23187-induced Ca2+ influx. Nitroprusside also attenuated the carbachol-induced sustained rise in [Ca2+]i that resulted from Ca2+ influx. The nitroprusside effect on carbachol-stimulated acini occurred without decreasing Ca2+ influx across the plasma membrane or alteration in the mobilization of Ca2+ from the intracellular agonist-sensitive pool. Inhibition of the increase in cellular cGMP caused by Br A23187 by the guanylate cyclase inhibitor, 6-anilino-5,8-quinolinedione (LY83583), resulted in augmentation of the increase in [Ca2+]i. This augmentation was reversed with dibutyryl cGMP. These results indicated that cGMP regulated [Ca2+]i in the pancreatic acinar cell. The mechanism involves the removal of Ca2+ from the cytoplasm.

Differing effects of ethanol on in vitro stimulated pancreatic enzyme secretion in ethanol-fed and control rats

The single most important risk factor for chronic and acute pancreatitis is the abuse of ethanol, which, theoretically, could affect the pancreas by interacting either with some of its cell receptors or with any of its intracellular signal transduction mechanisms. Therefore, we determined in isolated pancreatic acini from 3 month ethanol-fed rats and controls the dose-response effects of secretagogues on enzyme secretion, both in the presence and absence of ethanol in the incubation medium. In ethanol-fed rats, pancreatic amylase activity was decreased by 40%, compared to controls (with identical carbohydrates intakes), whereas lipase and trypsinogen activities were unaffected. With no ethanol in the incubation medium, basal enzyme releases and enzyme dose-response curves to CCK-8, VIP, secretin, bombesin, and bethanechol were essentially unchanged in ethanol-fed rats compared to controls. In contrast, with 0.1 M ethanol present in the medium, enzyme responses to VIP, secretin, and CCK-8 were inhibited and that to CCK-8 also shifted to the right in ethanol-fed rats compared to controls. Hence, if rechallenged to ethanol, acini from ethanol-fed rats show inhibited secretions, in response to two secretagogues acting through the release of cyclic AMP, and an inhibited and right-shifted secretory response to CCK.

Modulation of agonist-activated calcium influx by extracellular pH in rat pancreatic acini

The biochemical and Ca2+ transport pathways involved in generating the hormone-evoked Ca2+ signal are reported to be influenced by pH. The present study was designed to determine the effect of extracellular pH (pHo) and intracellular pH (pHi) on hormone-stimulated Ca2+ transport. We used rat pancreatic acini and measured free cytosolic Ca2+ concentration ([Ca2+]i) with fura-2, pHi with 2,7-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and Ca2+ fluxes with 45Ca2+. In the presence of external Ca2+, increasing pHo increased steady-state [Ca2+]i during sustained agonist stimulation; in the absence of external Ca2+, this increase in [Ca2+]i did not occur. The addition of an antagonist or blocking plasma membrane Ca2+ influx with La3+ in stimulated cells suspended at pHo 8.2 resulted in a reduction in [Ca2+]i. Increasing pHo increased the rate and extent of 45Ca2+ uptake into stimulated cells and the rate and extent of Ca2+ reloading of intracellular stores. The increased Ca2+ content of the intracellular stores with increased pHo indicated that at physiological pHo and pHi the agonist-mobilizable internal stores are not saturated with Ca2+. Changes in pHo affected pHi. However, changes in pHi at constant pHo had no effect on hormone-evoked [Ca2+]i increase, reduction in [Ca2+]i after hormone stimulation, or reloading of intracellular stores. We conclude that the hormone-activated plasma membrane Ca2+ entry pathway responsible for Ca2+ reloading is directly modulated by external H+.

Cl- secretion induced by bile salts. A study of the mechanism of action based on a cultured colonic epithelial cell line

When applied to the basolateral (serosal) side of the T84 colonic epithelial monolayer, taurodeoxycholate caused net Cl- secretion in a dose-dependent manner with a threshold effect observed at 0.2 mM. In contrast, when applied to the apical (luminal) surface, concentrations of taurodeoxycholate below 1 mM had little or no effect. Only when the concentration of taurodeoxycholate present on the apical side was greater than or equal to 1 mM did apical addition results in an electrolyte transport effect. This apical effect on electrolyte transport was associated with an abrupt increase in the permeability of the monolayer. Cyclic AMP and cyclic GMP in the T84 monolayers were not increased by the bile salt, but in the presence of extracellular Ca2+, free cytosolic Ca2+ increased with a graded dose effect and time course that corresponded approximately to the changes in short circuit current (Isc). The results suggest that luminal bile salts at a relatively high concentration (greater than or equal to 1 mM) increase tight junction permeability. Once tight junction permeability increases, luminal bile salts could reach the basolateral membrane of the epithelial cells where they act to increase free cytosolic Ca2+ from extracellular sources. The resulting increases in free cytosolic Ca2+, rather than in cyclic nucleotides, appear to be involved in transcellular Cl- secretion.

Characteristics of intracellular calcium changes required for augmentation of phorbol ester-stimulated pancreatic enzyme secretion

Previous studies demonstrated that Ca2+ ionophores augment the pancreatic enzyme secretion caused by phorbol esters. The present study was performed to determine the nature of the cellular Ca2+ effects responsible for the augmentation. Relatively low concentrations (0.3-1.0 microM) of the nonfluorescent Ca2+ ionophore, 4-bromo-A23187 (Br-A23187), did not measurably increase free cytosolic Ca2+ ([Ca2+]i) and caused little or no enzyme release from guinea pig pancreatic acini. However, these concentrations of Br-A23187 augmented the amylase release caused by the phorbol ester, 4 beta-phorbol 12-myristate 13-acetate (PMA). This augmentation occurred in the absence of extracellular Ca2+ as long as the intracellular agonist-sensitive pool contained Ca2+. Greater concentrations of Br-A23187 (3-10 microM) alone caused transient increases in [Ca2+]i and transient increases in amylase release. Although not resulting in an increase in [Ca2+]i, the low concentrations of Br-A23187 caused release of Ca2+ from the intracellular agonist-sensitive pool. These results suggest that Ca2+ mediates enzyme release by two distinct mechanisms in the pancreatic acinar cell. First, an increase in [Ca2+]i alone mediates enzyme release. Second, Ca2+ release from the agonist-sensitive pool not resulting in a measurable increase in [Ca2+]i augments enzyme release stimulated by a phorbol ester. The second effect of Ca2+ may be due to a small localized change in cell Ca2+ or an induction of cytosolic Ca2+ oscillations.

Effect of VIP antagonist on VIP-, PGE2-, and acid-stimulated duodenal bicarbonate secretion

Vasoactive intestinal peptide (VIP), prostaglandin E2 (PGE2), and luminal acidification are each potent stimulants of duodenal mucosal bicarbonate secretion. The present experiments were performed to determine whether the recently described VIP antagonist, [4Cl-D-Phe6,Leu17]VIP, suppresses VIP-stimulated duodenal mucosal bicarbonate secretion and to determine whether VIP serves as a mediator of bicarbonate secretion stimulated by acid or PGE2. In anesthetized rats, the effects of intravenous VIP, intraluminal PGE2, and intraluminal HCl on duodenal mucosal bicarbonate secretion both in the presence and absence of [4Cl-D-Phe6,Leu17]VIP were measured. The VIP antagonist inhibited duodenal bicarbonate secretion stimulated by both intravenous VIP and luminal acidification but not luminal PGE2. These findings suggest that VIP could be one mediator of acid-induced duodenal bicarbonate secretion and that the mechanism of PGE2-stimulated bicarbonate secretion is independent of VIP.

Hormone-evoked calcium release from intracellular stores is a quantal process

Ca2+ mobilization by hormones, ionomycin, and inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) were studied to determine whether Ca2+ release is a continuous or a quantal process. Hormone-mediated Ca2+ release occurs only during the first 2-4 s of stimulation. Stimulation of acini with a maximal hormone concentration following stimulation with a submaximal concentration resulted in free cytosolic Ca2+ concentration ([Ca2+]i) increase and 45Ca efflux. The peak [Ca2+]i increase induced by a maximal concentration of agonist was nearly constant when cells were prestimulated with a submaximal dose for 1-15 min. Submaximal hormone concentrations release only a fraction of intracellular 45Ca2+, after which intracellular Ca2+ content remains constant. The partially released stores remain depleted until cell stimulation is terminated, at which time the stores reload with Ca2+. For comparison, increasing concentrations of ionomycin resulted in increasing rates of Ca2+ release. Each ionomycin concentration released all the Ca2+ from intracellular stores. We therefore conclude that hormone-evoked Ca2+ release is a quantal rather than a continuous process. In permeabilized cells, increasing concentrations of Ins-1,4,5-P3 resulted in an increased fraction of Ca2+ release. No submaximal Ins-1,4,5-P3 concentration was capable of releasing all the Ins-1,4,5-P3-mobilizable Ca2+. Therefore, it appears that the quantal properties of hormone-evoked Ca2+ release reflect the quantal properties of Ins-1,4,5-P3-mediated Ca2+ release from intracellular stores.