In-Depth Review Acid-Base Disturbances in Gastrointestinal Disease F. John Gennari and Wolfgang J. Weise Department of Medicine, University of Vermont College of Medicine, Burlington, Vermont Disruption of normal gastrointestinal function as a result of infection, hereditary or acquired diseases, or complications of surgical procedures uncovers its important role in acid-base homeostasis. Metabolic acidosis or alkalosis may occur, depend- ing on the nature and volume of the unregulated losses that occur. Investigation into the specific pathophysiology of gastrointestinal disorders has provided important new insights into the normal physiology of ion transport along the gut and has also provided new avenues for treatment. This review provides a brief overview of normal ion transport along the gut and then discusses the pathophysiology and treatment of the metabolic acid-base disorders that occur when normal gut function is disrupted. Clin J Am Soc Nephrol 3: 1861–1868, 2008. doi: 10.2215/CJN.02450508 he gastrointestinal tract is a slumbering giant with re- the stomach and the exocrine pancreas, secretion of fluid and ϩ gard to acid-base homeostasis. Large amounts of H electrolytes occurs primarily in a subset of cells in the epithelial Ϫ and HCO traverse the specialized epithelia of the crypts with unique ion transport properties. Throughout the T 3 various components of the gut every day, but under normal gut, fluid and electrolyte transport (both absorption and secre- ϩ ϩ conditions, only a small amount of alkali (approximately 30 to tion) is driven primarily by Na /K -ATPase transport activity 40 mmol) is lost in the stool (1,2). In contrast to the kidney, acid across the basolateral membrane of epithelial cells. Several key and alkali transport in the gut is adjusted for efficient absorp- apical membrane electrolyte transporters participate, including Ϫ Ϫ ϩ ϩ tion of dietary constituents rather than for acid-base homeosta- Cl /HCO3 and Na /H ion exchangers and the cystic fibro- Ϫ sis. The small amount of alkali lost as a byproduct of these sis transmembrane conductance regulator (CFTR) Cl channel, transport events is easily regenerated by renal net acid excre- all of which are present in many segments of the gut, and ϩ ϩ tion, which is regulated by the kidney to maintain body alkali H /K -ATPases, which are confined to the stomach and colon stores. Disruption of normal gut function, however, uncovers (Figure 1). Disruption of function or abnormal stimulation of its power to overwhelm acid-base homeostasis. Acid-base dis- these ion transporters underlies a variety of gastrointestinal orders can vary from severe acidosis to severe alkalosis, de- disorders that are associated with acid-base and electrolyte pending on the site along the gastrointestinal tract affected and disorders. A full description of the ion transport processes that the nature of the losses that ensue. These disruptions in acid- participate in normal gut function is beyond the scope of this base equilibrium are associated with disorders of potassium review, and reader is referred to other sources for more detail (1). balance, often leading to either hypo- or hyperkalemia. Major sodium and chloride losses also occur, sometimes causing life- Mouth and Throat threatening volume depletion and almost always contributing Secretion of salivary fluid occurs via the parotid and salivary to the acid-base abnormalities. glands, ultimately producing a hypotonic alkaline solution when stimulated by activation of muscarinic receptors. Acinar Normal Physiology of Gut Fluid and cells secrete fluid that is similar in composition to serum, and ϩ Ϫ Electrolyte Transport then the secreted fluid is modified by Na and Cl absorption Ϫ During the course of each day, secretion as well as absorption and HCO3 secretion. The apical membrane transport proteins of fluid and electrolytes occurs along the gastrointestinal tract. that alter the composition under conditions of stimulation re- Ϫ Normally 7 to8Loffluid is secreted each day, far exceeding main to be defined, but HCO3 secretion may occur via an Ϫ dietary consumption, and almost all of these secretions, as well anion channel, possibly the CFTR Cl channel (3). When stim- as any ingested fluid, are absorbed by the end of the colon (1). ulated, up to 1 L/d fluid is produced (Table 1). This alkaline The gastrointestinal tract is divided into sequential segments, secretion likely serves to protect the mucosa of the mouth, Ϫ each with a distinct group of ion transporters and channels that throat, and esophagus and has little impact on serum [HCO3 ]. interact with one another to determine the electrolyte content It is more than counterbalanced by gastric acid secretion in the and volume of the fluid in the gut lumen. With the exception of stomach. Published online ahead of print. Publication date available at www.cjasn.org. Stomach Digestion of many of the foods that we eat requires secre- Correspondence: Dr. F. John Gennari, 2319 Rehab, UHC Campus, Fletcher Allen Health Care, Burlington, VT 05401. Phone: 802-847-2534; Fax: 802-847-8736; E- tion of an acid solution into the lumen of the stomach. mail: [email protected] Secretion of this solution involves several linked transport Copyright © 2008 by the American Society of Nephrology ISSN: 1555-9041/306–1861 1862 Clinical Journal of the American Society of Nephrology Clin J Am Soc Nephrol 3: 1861–1868, 2008 Figure 1. Key apical membrane ion transporters and channels in various segments of the gastrointestinal tract. The driving force ϩ ϩ for most absorption and secretion across the apical membrane is the basolateral Na /K -ATPase shown. All other basolateral ion transporters and channels are deliberately omitted. CFTR, cystic fibrosis transmembrane conductance regulator; DRA, down- regulated in adenoma gene product; SCFA, short-chain fatty acids. Table 1. Volume and nature of the fluid leaving various segments of the gastrointestinal tract Gut Segment Range of Volume (L/d)a Nature of Fluid ϩ Salivary glands 0.20 to 1.00 Alkaline when stimulated, ͓K ͔ 20 to 30 mmol/L ϩ Stomach 0.50 to 2.00 Highly Acid when stimulated, ͓K ͔ approximately 10 mmol/L Biliary tree 1.00 Alkaline when stimulated ϩ Pancreas 1.00 to 2.00 Highly alkaline when stimulated, ͓K ͔ 5 to 10 mmol/L ϩ Jejunum/ileum 1.00 to 2.00 Alkaline, ͓K ͔ 5 to 10 mmol/L ϩ Ϫ Colon Ͻ0.15 Low pH, ͓Na ͔ and ͓Cl ͔Ͻ30 mmol/L, ϩ ͓K ͔ 55 to 75 mmol/L aFluid volume delivered beyond indicated portion of the gastrointestinal tract. ϩ proteins in the parietal cells that line the stomach (Figure 1) falls to as low as 1.0 (H ϭ 100 mmol/L) and volume ϩ ϩ (1). Of these, the gastric H /K -ATPase is essential for acid increases to as high as 7 ml/min. The surge in acid secretion Ϫ secretion; pharmacologic blockade of this transporter effec- transiently increases serum [HCO3 ] by 1 to 2 mmol/L, a tively blocks acidification of the gastric contents. Hydrogen change referred to as the alkaline tide, once used as a test of ϩ ion secretion by this transporter is facilitated by K recycling acid secretion. The increase is transient, because the process across the apical membrane via a selective cation channel. of acid secretion itself sets in motion countervailing alkali Ϫ Chloride is secreted by an as-yet-unidentified Cl channel, secretion by the exocrine pancreas into the duodenum. Gas- yielding hydrochloric acid under conditions of stimulation. tric secretion is usually 1 to 2 L/d (Table 1). Under basal conditions, the secreted fluid is primarily NaCl. The pH and volume of gastric secretions is regulated primar- Duodenum ily by gastrin, so maximal volume and acid secretion occurs Regardless of the pH and tonicity of the gut contents that only after a meal. At such times, the pH of the gastric fluid enter the duodenum, this segment of the gut restores isotonicity Clin J Am Soc Nephrol 3: 1861–1868, 2008 Acid-Base Disorders in GI Disease 1863 Ϫ ϩ Ϫ through water and solute absorption, and the pH rises to 7.0. and Cl from the gut lumen and secrete H and HCO3 into it. Ϫ Acid is neutralized by HCO3 addition in pancreatic and bili- The latter two ions combine, forming H2CO3, which dehydrates Ϫ Ϫ ary secretions as well as by direct secretion in Brunner’s glands to form CO2 in the intestinal lumen. The Cl /HCO3 ex- along the duodenum (1). The specific ion transporters involved changer in the small intestine is the “downregulated in ade- Ϫ in duodenal HCO3 secretion remain to be defined. noma” (DRA) gene product, also named CLD (for chloride Ϫ Ϫ diarrhea) (9). By the end of the ileum, Cl /HCO3 exchange Pancreas predominates, resulting in an alkaline solution (Table 1). Chlo- Entry of the acid gastric secretions into the duodenum signals ride secretion occurs in specialized cells in the intestinal crypts Ϫ Ϫ the pancreas to secrete its highly alkaline solution ([HCO3 ] via a series of apical Cl ion channels, one of which is the CFTR Ϫ approximately 70 to 120 mmol/L) into the gut. The anion channel, recycling Cl into the lumen (Figure 1). In contrast to transporter primarily responsible for this process is an apical the colon, the small intestinal secretory cells do not have an Ϫ Ϫ ϩ membrane Cl /HCO3 exchanger (Figure 1). The activity of apical K channel. Potassium movement across the membrane Ϫ this ion exchanger is regulated by the CFTR Cl channel, which is accounted for by passive diffusion and solvent drag, with Ϫ recycles Cl across the apical membrane (1,4). Under maximal absorption predominating (10). These absorptive and secretory Ϫ stimulation, some secreted HCO3 seems to enter the lumen processes leave the fluid that enters the colon slightly hypo- Ϫ Ϫ Ϫ directly via the CFTR channel as well as by Cl /HCO3 ex- tonic with a [HCO3 ] of approximately 30 mmol/L and with a Ϫ ϩ change (5).