Gastroenterology
Volume 137, Issue 3 , Pages 779-782, September 2009

Can Too Much Acid Sour Your Pancreas?

  • John A. Williams

      Affiliations

    • Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, Michigan
    • Corresponding Author InformationReprint requests Address requests for reprints to: John A. Williams, MD, PhD, Department of Molecular & Integrative Physiology, University of Michigan, 7708 MS II, Ann Arbor, Michigan 48109-5622
    • ,
  • Matthew J. DiMagno

      Affiliations

    • Department of Internal Medicine, Division of Gastroenterology and Hepatology, University of Michigan, Ann Arbor, Michigan
    • Corresponding Author InformationReprint requests Address requests for reprints to: Matthew J. DiMagno, MD, Department of Internal Medicine, Division of Gastroenterology and Hepatology, 6510 MSRB 1, Ann Arbor, Michigan, 48109

published online 30 July 2009.

Article Outline

 

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.1, 2

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,3 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 al3 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 Boron4). 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,5 NMR,6 and fluorescent probes.7, 8, 9, 10, 11 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.

  • View full-size image.
  • Figure 1. 

    Simplified model of acinar cell pH regulation and trypsinogen activation. The basolateral membrane adjacent to capillaries is on the left and the luminal or secretory membrane is on the right. The basolateral Na+-K+ ATPase is responsible for maintaining a low intracellular Na+, high K+ and a negative intracellular potential. Thus both Na+ and H+ are diffusing into the cell down their electrochemical gradient and the energy stored in the Na+ gradient is used to pump H+ out by the Na+-H+ exchanger or HCO3− in by the Na+-HCO3− cotransporter. Cl- is close to electrochemical equilibrium and at rest the Cl−-HCO3− exchanger serves to move HCO3− out of the cell. Note when cells are incubated in HEPES Ringer medium not all of these mechanisms are activated. Lysosomes maintain an acid pH by virtue of a vacuolar(v)-type H+-ATPase. The low pH environment is necessary for activation of trypsinogen by cathepsins.

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.12 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 al3 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.13 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.8, 14 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,3 other acute in vitro studies reported less amylase secretion (rather than no change) when pHe decreased below 7.0,5, 8, 14 perhaps owing to a decrease in Ca2+ signaling.

Changes in cytoplasmic pH could affect trypsin activation. Although Bhoomagould et al3 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 al3 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,3 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 al3: 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 children15 and is a known side effect of the branched-chain organic compound valproate, an antiepileptic medication.16 Kahler et al15 speculated that branched-chain organic compounds may predispose to acute pancreatitis by impairing mitochondrial function. Although Bhoomagoud et al3 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.17, 18

Back to Article Outline

Lactic acidosis develops from cell hypoxia and disorders of cell metabolism 

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.19 In severe acute pancreatitis, volume depletion is a leading contributor to necrotizing pancreatitis and requires early, aggressive fluid resuscitation.1, 2, 20 Niederau et al21 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.21 A systematic review provides additional insight that treating lactic acidosis with bicarbonate has no clear impact on clinical outcomes.22 Hence, lactic acidosis in acute pancreatitis is usually a secondary effect of hypoperfusion and hypoxic injury, and should be treated with fluid resuscitation.

Back to Article Outline

NRTI treatment for HIV is known to cause lactic acidosis and pancreatitis 

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.23 NRTIs may predispose to acute pancreatitis by inducing moderate hypertriglyceridemia and/or by inhibiting mitochondrial oxidative phosphorylation in a variety of cell types,24 including the exocrine pancreas,25 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.

Back to Article Outline

DKA has an unclear relationship with acute pancreatitis 

A prospective study clarified this relationship by investigating 100 consecutive episodes of DKA.26 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.26 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.

Back to Article Outline

References 

  1. Pandol SJ, Saluja AK, Imrie CW, et al. Acute pancreatitis: bench to the bedside. Gastroenterology. 2007;132:1127–1151
  2. DiMagno MJ, Wamsteker EJ, Debenedet AT. Advances in managing acute pancreatitis. F1000 Medicine Reports. 2009;
  3. Bhoomagoud M, Jung T, Atladottir J, et al. Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats. Gastroenterology. 2009;137:1083–1092
  4. Boron WF. Regulation of intracellular pH. Adv Physiol Educ. 2004;28:160–179
  5. Preissler M, Williams JA. Pancreatic acinar cell function: measurement of intracellular ions and pH and their relation to secretion. J Physiol. 1981;321:437–448
  6. Dufresne M, Bastie MJ, Vaysse N, et al. The amiloride sensitive Na+/H+ antiport in guinea pig pancreatic acini (Characterization and stimulation by caerulein). FEBS Lett. 1985;187:126–130
  7. Hellmessen W, Christian AL, Fasold H, et al. Coupled Na+-H+ exchange in isolated acinar cells from rat exocrine pancreas. Am J Physiol. 1985;249:G125–G136
  8. Muallem S, Pandol SJ, Beeker TG. Modulation of agonist-activated calcium influx by extracellular pH in rat pancreatic acini. Am J Physiol. 1989;257:G917–G924
  9. Carter KJ, Rutledge PL, Steer ML, et al. Secretagogue-induced changes in intracellular pH and amylase release in mouse pancreatic acini. Am J Physiol. 1987;253:G690–G696
  10. Bastie MJ, Williams JA. Gastrointestinal peptides activate Na(+)-H+ exchanger in AR42J cells by increasing its affinity for intracellular H+. Am J Physiol. 1990;258:G958–G966
  11. Muallem S, Loessberg PA. Intracellular pH-regulatory mechanisms in pancreatic acinar cells (II. Regulation of H+ and HCO3- transporters by Ca2(+)-mobilizing agonists). J Biol Chem. 1990;265:12813–12819
  12. Orci L, Ravazzola M, Anderson RG. The condensing vacuole of exocrine cells is more acidic than the mature secretory vesicle. Nature. 1987;326:77–79
  13. Steigerwalt RW, Williams JA. Characterization of cholecystokinin receptors on rat pancreatic membranes. Endocrinology. 1981;109:1746–1753
  14. Tsunoda Y, Stuenkel EL, Williams JA. Characterization of sustained [Ca2+]i increase in pancreatic acinar cells and its relation to amylase secretion. Am J Physiol. 1990;259:G792–G801
  15. Kahler SG, Sherwood WG, Woolf D, et al. Pancreatitis in patients with organic acidemias. J Pediatr. 1994;124:239–243
  16. Badalov N, Baradarian R, Iswara K, et al. Drug-induced acute pancreatitis: an evidence-based review. Clin Gastroenterol Hepatol. 2007;5:648–661
  17. Bultron G, Seashore MR, Pashankar DS, et al. Recurrent acute pancreatitis associated with propionic acidemia. J Pediatr Gastroenterol Nutr. 2008;47:370–371
  18. Burlina AB, Dionisi-Vici C, Piovan S, et al. Acute pancreatitis in propionic acidaemia. J Inherit Metab Dis. 1995;18:169–172
  19. Schuster HP. Prognostic value of blood lactate in critically ill patients. Resuscitation. 1984;11:141–146
  20. Gardner TB, Vege SS, Pearson RK, et al. Fluid resuscitation in acute pancreatitis. Clin Gastroenterol Hepatol. 2008;6:1070–1076
  21. Niederau C, Crass RA, Silver G, et al. Therapeutic regimens in acute experimental hemorrhagic pancreatitis (Effects of hydration, oxygenation, peritoneal lavage, and a potent protease inhibitor). Gastroenterology. 1988;95:1648–1657
  22. Forsythe SM, Schmidt GA. Sodium bicarbonate for the treatment of lactic acidosis. Chest. 2000;117:260–267
  23. Moreno A, Quereda C, Moreno L, et al. High rate of didanosine-related mitochondrial toxicity in HIV/HCV-coinfected patients receiving ribavirin. Antivir Ther. 2004;9:133–138
  24. Brinkman K, ter Hofstede HJ, Burger DM, et al. Adverse effects of reverse transcriptase inhibitors: mitochondrial toxicity as common pathway. AIDS. 1998;12:1735–1744
  25. Yeo TK, 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;26:911–921
  26. Nair S, Yadav D, Pitchumoni CS. Association of diabetic ketoacidosis and acute pancreatitis: observations in 100 consecutive episodes of DKA. Am J Gastroenterol. 2000;95:2795–2800

 Conflicts of interest The authors disclose no conflicts.

PII: S0016-5085(09)01196-2

doi:10.1053/j.gastro.2009.07.036

Refers to article:

  • Editorial Accompanies ArticlePodcastAdditional Online Content Available Reducing Extracellular pH Sensitizes the Acinar Cell to Secretagogue-Induced Pancreatitis Responses in Rats , 18 May 2009

    Madhavi Bhoomagoud, Thomas Jung, Jorunn Atladottir, Thomas R. Kolodecik, Christine Shugrue, Anamika Chaudhuri, Edwin C. Thrower, Fred S. Gorelick
    Gastroenterology September 2009 (Vol. 137, Issue 3, Pages 1083-1092)

Gastroenterology
Volume 137, Issue 3 , Pages 779-782, September 2009

Article Tools

* Image Usage & Resolution