Can Too Much Acid Sour Your Pancreas?

Abstract

See “Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats” by Bhoomagoud M, Jung T, Atladottir J, et al, on page 1083. See “Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats” by Bhoomagoud M, Jung T, Atladottir J, et al, on page 1083. Acute pancreatitis afflicts approximately 50,000 Americans annually, is associated with significant morbidity and mortality, and has limited treatment options. Treatment is supportive, and in patients with potentially severe acute pancreatitis includes aggressive fluid resuscitation, hemodynamic monitoring, enteral feeding, and tempered use of endoscopic retrograde cholangiopancreatography.1Pandol S.J. Saluja A.K. Imrie C.W. et al.Acute pancreatitis: bench to the bedside.Gastroenterology. 2007; 132: 1127-1151Abstract Full Text Full Text PDF PubMed Scopus (470) Google Scholar, 2DiMagno M.J. Wamsteker E.J. Debenedet A.T. Advances in managing acute pancreatitis.F1000 Medicine Reports. 2009Google Scholar To develop specific prevention and treatment options for acute pancreatitis, there has been considerable interest in developing and characterizing animal models. Premature activation of pancreatic proteases, especially trypsin, is an important initiating component of the disease. Current interest on how and where the activation of trypsin occurs focuses on missorting of digestive and lysosomal enzymes and altered autophagy. Evidence is robust that acidic organelles from lysosomes/endosomes participate in intracellular trypsin activation and the initiation of cellular models of pancreatitis because dissipating local acidic compartments in acinar cells prevents premature trypsin activation. Owing to the potential importance of intracellular acidic compartments and clinical data “linking” acidosis to pancreatitis, Bhoomagoud et al,3Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar in this issue of Gastroenterology, evaluated how both an acidic environment in vitro and systemic acidosis in vivo affect secretagogue induced zymogen activation and acinar cell injury. Reducing the pH of an acinar cell suspension had no effect on its own, but potentiated secretagogue-induced trypsin and chymotrypsin activation and cell damage. Moreover, in vivo acid infusion had similar effects. The authors conclude that this effect was because of changes in intracellular pH. However, to evaluate their experiments and pose further questions, we first review acinar cell pH regulation and then examine how the acidic milieu investigated by Bhoomagoud et al3Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar might affect trypsin activation with resultant cell damage, thereby contributing to acute pancreatitis in human diseases associated with acidosis. Homeostatic mechanisms are designed to maintain the constancy of the “milieu intérieur.” Cells must maintain their cytoplasmic pH in response to changes in plasma pH, a continued acid load induced by cell metabolism, passive influx of H+, and for epithelia, the presence of transepithelial transport (for a comprehensive review of cellular pH homeostasis, the reader is referred to Boron4Boron W.F. Regulation of intracellular pH.Adv Physiol Educ. 2004; 28: 160-179Crossref PubMed Scopus (251) Google Scholar). Within the cell, the relatively constant cytoplasmic pH allows the maintenance of organelles with differing pH's such as the lysosome where a vacuolar H+-ATPase pumps H+ into the vacuolar lumen. Mean intracellular pH (pHi) for most cells, including acinar cells, is around 7.0. Rodent acinar pHi values vary from 6.77 to 7.28, as measured by the distribution of weak base,5Preissler M. Williams J.A. Pancreatic acinar cell function: measurement of intracellular ions and pH and their relation to secretion.J Physiol. 1981; 321: 437-448PubMed Google Scholar NMR,6Dufresne M. Bastie M.J. Vaysse N. et al.The amiloride sensitive Na+/H+ antiport in guinea pig pancreatic acini Characterization and stimulation by caerulein.FEBS Lett. 1985; 187: 126-130Crossref PubMed Scopus (15) Google Scholar and fluorescent probes.7Hellmessen W. Christian A.L. Fasold H. et al.Coupled Na+-H+ exchange in isolated acinar cells from rat exocrine pancreas.Am J Physiol. 1985; 249: G125-G136PubMed Google Scholar, 8Muallem S. Pandol S.J. Beeker T.G. Modulation of agonist-activated calcium influx by extracellular pH in rat pancreatic acini.Am J Physiol. 1989; 257: G917-G924PubMed Google Scholar, 9Carter K.J. Rutledge P.L. Steer M.L. et al.Secretagogue-induced changes in intracellular pH and amylase release in mouse pancreatic acini.Am J Physiol. 1987; 253: G690-G696PubMed Google Scholar, 10Bastie M.J. Williams J.A. Gastrointestinal peptides activate Na(+)-H+ exchanger in AR42J cells by increasing its affinity for intracellular H+.Am J Physiol. 1990; 258: G958-G966PubMed Google Scholar, 11Muallem S. Loessberg P.A. Intracellular pH-regulatory mechanisms in pancreatic acinar cells II. Regulation of H+ and HCO3- transporters by Ca2(+)-mobilizing agonists.J Biol Chem. 1990; 265: 12813-12819Abstract Full Text PDF PubMed Google Scholar Acinar cells, similar to other cells, contain proteins and small molecules that provide intrinsic buffer capacity to cells. The CO2-HCO3− system also conveys buffering amounting to slightly over half of cellular buffering. Hence, when cells are studied in a TRIS or HEPES buffer without CO2 or HCO3−, not all buffering is evaluated. In addition to the intrinsic buffer capacity, cells defend their pH by specific transport mechanisms. The main acid extruding mechanism is Na+–H+ exchange that is activated by a falling pHi and Ca2+ mobilizing secretagogues. This and other key cellular mechanisms (Figure 1) are responsible for maintaining a relatively constant acinar cell pHi, even when extracellular pH (pHe) varies from 7.2 to 7.5. Within cells, the relatively constant cytoplasmic pH allows maintaining organelles of different pHs. The lysosome pumps H+ into the vacuolar lumen with the vacuolar H+-ATPase and the pH falls until active influx is balanced by passive efflux through the membrane. Lysosomes/endosomes have a pH around 5.0. In chromaffin or islet β cells secretory granules are acidic, which allows them to take up and sequester amines. In exocrine cells, trans-Golgi cisternae and condensing vacuoles (immature granules) are acidic, but mature granules are not.12Orci L. Ravazzola M. Anderson R.G. The condensing vacuole of exocrine cells is more acidic than the mature secretory vesicle.Nature. 1987; 326: 77-79Crossref PubMed Scopus (97) Google Scholar These organelle acidic compartments can be dissipated with H+-ATPase inhibitors such as bafilomycin or H+ ionophores such as monensin. Where does the change in pHe studied by Bhoomagould et al3Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar affect trypsin activation and cell damage? The location could be extracellular, cytoplasmic, or in the lysosome. Changes in pHe can alter receptor ligand interaction or signal transduction. Cholecystokinin (CCK) receptor binding to pancreatic membranes has a pH optima of 5.5 and lowering pHe should increase CCK binding.13Steigerwalt R.W. Williams J.A. Characterization of cholecystokinin receptors on rat pancreatic membranes.Endocrinology. 1981; 109: 1746-1753Crossref PubMed Scopus (74) Google Scholar CCK is a Ca2+ mobilizing secretagogue, but in intact acinar cells decreasing pHe reduced Ca2+ influx, the height of the sustained Ca2+ plateau and the amount of intracellular Ca2+ stores.8Muallem S. Pandol S.J. Beeker T.G. Modulation of agonist-activated calcium influx by extracellular pH in rat pancreatic acini.Am J Physiol. 1989; 257: G917-G924PubMed Google Scholar, 14Tsunoda Y. Stuenkel E.L. Williams J.A. Characterization of sustained [Ca2+]i increase in pancreatic acinar cells and its relation to amylase secretion.Am J Physiol. 1990; 259: G792-G801PubMed Google Scholar Thus, it seems unlikely that the effect of lowering pHe on trypsin activation and cell damage is mediated extracellularly. However, in contrast with the work of Bhoomagoud et al,3Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar other acute in vitro studies reported less amylase secretion (rather than no change) when pHe decreased below 7.0,5Preissler M. Williams J.A. Pancreatic acinar cell function: measurement of intracellular ions and pH and their relation to secretion.J Physiol. 1981; 321: 437-448PubMed Google Scholar, 8Muallem S. Pandol S.J. Beeker T.G. Modulation of agonist-activated calcium influx by extracellular pH in rat pancreatic acini.Am J Physiol. 1989; 257: G917-G924PubMed Google Scholar, 14Tsunoda Y. Stuenkel E.L. Williams J.A. Characterization of sustained [Ca2+]i increase in pancreatic acinar cells and its relation to amylase secretion.Am J Physiol. 1990; 259: G792-G801PubMed Google Scholar perhaps owing to a decrease in Ca2+ signaling. Changes in cytoplasmic pH could affect trypsin activation. Although Bhoomagould et al3Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar did not measure cytosolic pH it is reasonable to assume it was acidic. However, it would be useful to measure pHi and to investigate the effects of HEPES and HCO3− buffers. They concluded that the effect of lowering pHe was cytoplasmic because adding Na-propionate, a weak acid, acidified the cytoplasm measured with BCECF, even though the pHe was constant. However, the effect of 50 mmol/L propionate on pHi was much greater (a pHi of 6) than expected for reducing pHe from 7.4 to 7.0 which might decrease pHi by only 0.1–0.2 units. Propionate (25 mmol/L), which should decrease pHi to a lesser extent, had only a small effect on trypsin activation. Thus, further studies comparing pHe, pHi, and trypsin activation are warranted. Because the cytoplasm provides the external milieu for the organelles, it is possible that lowering cytoplasmic pH will lower lysosomal/endosomal vacuolar pH and thereby increase activity of lysosomal cathepsin enzymes that activate trypsin. The findings that concanamycin A and bafilomycin blocked augmentation of trypsin activation by low pHe suggests a lysosomal site for trypsin activation might be important. It would be useful to confirm that concanamycin did not affect cytoplasmic pH. Overall, the studies of Bhoomagould et al3Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar indicate that further investigation focused on pancreatic acinar cytoplasmic and organelle pH regulation are worthwhile. However, in vivo studies are necessary to determine the clinical relevance of pH regulation to pancreatitis. Bhoomagould et al,3Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar in their preliminary studies, found that acidosis enhanced cerulein-induced pancreatitis. To interpret the clinical significance of these findings, we review the incidence and possible pathogenesis of acute pancreatitis associated with metabolic acidoses, specifically the clinical conditions referred to by Bhoomagoud et al3Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar: organic acidemias, lactic acidosis induced by hypovolemia and reverse transcriptase inhibitor (NRTI) therapy for HIV, and diabetic ketoacidosis (DKA). Branched-chain organic acidemias, including the classic form propionic acidemia, are a group of inborn errors of metabolisms characterized by impaired catabolism of amino acids, generation of toxic intermediates and development of ketoacidosis, coma, death, or neurologic sequelae. Acute pancreatitis occurs in up to 6.5% of affected children15Kahler S.G. Sherwood W.G. Woolf D. et al.Pancreatitis in patients with organic acidemias.J Pediatr. 1994; 124: 239-243Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar and is a known side effect of the branched-chain organic compound valproate, an antiepileptic medication.16Badalov N. Baradarian R. Iswara K. et al.Drug-induced acute pancreatitis: an evidence-based review.Clin Gastroenterol Hepatol. 2007; 5: 648-661Abstract Full Text Full Text PDF PubMed Scopus (355) Google Scholar Kahler et al15Kahler S.G. Sherwood W.G. Woolf D. et al.Pancreatitis in patients with organic acidemias.J Pediatr. 1994; 124: 239-243Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar speculated that branched-chain organic compounds may predispose to acute pancreatitis by impairing mitochondrial function. Although Bhoomagoud et al3Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar show that sodium propionate sensitizes rats to pancreatic zymogen activation and injury, the relationship between propionic acidemia and clinical acute pancreatitis remains unclear because we identified only 5 individual cases in the literature, and only 2 cases had imaging or histologic evidence of pancreatitis.17Bultron G. Seashore M.R. Pashankar D.S. et al.Recurrent acute pancreatitis associated with propionic acidemia.J Pediatr Gastroenterol Nutr. 2008; 47: 370-371Crossref PubMed Scopus (30) Google Scholar, 18Burlina A.B. Dionisi-Vici C. Piovan S. et al.Acute pancreatitis in propionic acidaemia.J Inherit Metab Dis. 1995; 18: 169-172Crossref PubMed Scopus (23) Google Scholar Lactic acidosis resulting from hypoperfusion and hypoxic injury of organs is a useful prognostic indicator and a tool to monitor the adequacy of fluid resuscitation in critically ill patients.19Schuster H.P. Prognostic value of blood lactate in critically ill patients.Resuscitation. 1984; 11: 141-146Abstract Full Text PDF PubMed Scopus (25) Google Scholar In severe acute pancreatitis, volume depletion is a leading contributor to necrotizing pancreatitis and requires early, aggressive fluid resuscitation.1Pandol S.J. Saluja A.K. Imrie C.W. et al.Acute pancreatitis: bench to the bedside.Gastroenterology. 2007; 132: 1127-1151Abstract Full Text Full Text PDF PubMed Scopus (470) Google Scholar, 2DiMagno M.J. Wamsteker E.J. Debenedet A.T. Advances in managing acute pancreatitis.F1000 Medicine Reports. 2009Google Scholar, 20Gardner T.B. Vege S.S. Pearson R.K. et al.Fluid resuscitation in acute pancreatitis.Clin Gastroenterol Hepatol. 2008; 6: 1070-1076Abstract Full Text Full Text PDF PubMed Scopus (125) Google Scholar Niederau et al21Niederau C. Crass R.A. Silver G. et al.Therapeutic regimens in acute experimental hemorrhagic pancreatitis Effects of hydration, oxygenation, peritoneal lavage, and a potent protease inhibitor.Gastroenterology. 1988; 95: 1648-1657Abstract Full Text PDF PubMed Scopus (66) Google Scholar investigated the importance of treating hypovolemia, hypoxia, and acidosis and reported that survival from a murine model of severe acute pancreatitis improved dramatically by fluid resuscitation but not by correcting hypoxia and acidosis.21Niederau C. Crass R.A. Silver G. et al.Therapeutic regimens in acute experimental hemorrhagic pancreatitis Effects of hydration, oxygenation, peritoneal lavage, and a potent protease inhibitor.Gastroenterology. 1988; 95: 1648-1657Abstract Full Text PDF PubMed Scopus (66) Google Scholar A systematic review provides additional insight that treating lactic acidosis with bicarbonate has no clear impact on clinical outcomes.22Forsythe S.M. Schmidt G.A. Sodium bicarbonate for the treatment of lactic acidosis.Chest. 2000; 117: 260-267Crossref PubMed Scopus (240) Google Scholar Hence, lactic acidosis in acute pancreatitis is usually a secondary effect of hypoperfusion and hypoxic injury, and should be treated with fluid resuscitation. Didanosine, combined with ribavarin (another NRTI that activates didanosine), has a high frequency of hyperlactatemia (23%), acute pancreatitis (28%), and hyperamylasemia (51%) in patients with HIV and hepatitis C.23Moreno A. Quereda C. Moreno L. et al.High rate of didanosine-related mitochondrial toxicity in HIV/HCV-coinfected patients receiving ribavirin.Antivir Ther. 2004; 9: 133-138PubMed Google Scholar NRTIs may predispose to acute pancreatitis by inducing moderate hypertriglyceridemia and/or by inhibiting mitochondrial oxidative phosphorylation in a variety of cell types,24Brinkman K. ter Hofstede H.J. Burger D.M. et al.Adverse effects of reverse transcriptase inhibitors: mitochondrial toxicity as common pathway.AIDS. 1998; 12: 1735-1744Crossref PubMed Scopus (757) Google Scholar including the exocrine pancreas,25Yeo T.K. Kintner J. Armand R. et al.Sublethal concentrations of gemcitabine (2′,2′-difluorodeoxycytidine) alter mitochondrial ultrastructure and function without reducing mitochondrial DNA content in BxPC-3 human pancreatic carcinoma cells.Hum Exp Toxicol. 2007; 26: 911-921Crossref PubMed Scopus (5) Google Scholar which could predispose to cellular necrosis. Hence, it seems more plausible that NRTI associated lactic acidosis is a sequela of a mitochondrial disorder that predisposes to pancreatitis rather than the possibility that lactic acidosis predisposes to or causes acute pancreatitis. A prospective study clarified this relationship by investigating 100 consecutive episodes of DKA.26Nair S. Yadav D. Pitchumoni C.S. Association of diabetic ketoacidosis and acute pancreatitis: observations in 100 consecutive episodes of DKA.Am J Gastroenterol. 2000; 95: 2795-2800Crossref PubMed Google Scholar The diagnosis of acute pancreatitis was based on conclusive computed tomographic imaging findings in patients with either abdominal pain and/or >3-fold elevation of serum lipase or amylase levels.26Nair S. Yadav D. Pitchumoni C.S. Association of diabetic ketoacidosis and acute pancreatitis: observations in 100 consecutive episodes of DKA.Am J Gastroenterol. 2000; 95: 2795-2800Crossref PubMed Google Scholar Eleven percent of patients had acute pancreatitis, but patients had a higher frequency of nonspecific hyperamylasemia (21%) and/or hyperlipasemia (29%). Hypertriglyceridemia was present in 30% and was the most common cause of acute pancreatitis. Other etiologies of acute pancreatitis were alcohol, drug induced, and idiopathic. Acute pancreatitis was associated with more severe metabolic acidosis and a higher mean blood glucose, findings most likely attributable to the composite effects of DKA and acute pancreatitis, because both conditions cause hyperglycemia, hypovolemia, and metabolic acidosis. Because 64% of cases had an identifiable cause of acute pancreatitis, it seems unlikely that acidosis or DKA triggered acute pancreatitis. Conversely, it is conceivable in some cases that acute pancreatitis triggered DKA or that an independent insult triggered DKA and acute pancreatitis simultaneously. Currently, immediate treatment of suspected severe acute pancreatitis focuses on aggressive fluid resuscitation to restore perfusion of organs and correct hypoxia and acidosis. There are insufficient clinical data to appraise and substantiate the postulated paradigm that metabolic acidemias predispose to or cause acute pancreatitis. To clarify the role of metabolic acidemia in acute pancreatitis, further research is necessary using in vitro and in vivo animal models and careful human studies. Reducing Extracellular pH Sensitizes the Acinar Cell to Secretagogue-Induced Pancreatitis Responses in RatsGastroenterologyVol. 137Issue 3PreviewProtease activation within the pancreatic acinar cell is a key early event in acute pancreatitis and may require low pH intracellular compartments. Clinical studies suggest that acidosis may affect the risk for developing pancreatitis. We hypothesized that exposure to an acid load might sensitize the acinar cell to secretagogue-induced pancreatitis. Full-Text PDF