REVIEW
Duodenal Carbonic Anhydrase: Mucosal Protection, Luminal
Chemosensing, and Gastric Acid Disposal
Jonathan D. Kaunitz1-4 and Yasutada Akiba2-4

Greater Los Angeles Veterans Affairs Healthcare System; 2CURE: Digestive Diseases
Research Center; 3Department of Medicine, School of Medicine, University of California
Los Angeles, and 4Brentwood Biomedical Research Institute, Los Angeles, CA, U.S.A.
1(Received for publication on April 19, 2006)(Revised for publication on June 27, 2006)(Accepted for publication on July 20, 2006)

Abstract. The duodenum serves as a buffer zone between the stomach and jejunum. Overa length of only 25 cm, large volumes of strong acid secreted by the stomach must be converted to the neutral-alkaline chyme of the hindgut lumen, generating large volumes ofCO2, which the duodenum then absorbs. The duodenal mucosa consists of epithelial cells
connected by low-resistance tight junctions, forming a leaky epithelial barrier. Despite
this high permeability, the epithelial cells, under intense stress from luminal mineralacid and highly elevated Pco2, maintain normal functioning. Furthermore, the duodenum
plays an active role in foregut acid-base homeostasis, absorbing large amounts of H＋ andCO 2 that are recycled by the gastric parietal cells. Prompted by the high expression ofcytosolic and membrane carbonic anhydrase (CAs) in duodenal epithelial cells, and the
intriguing observation that CA activity appears to augment cellular acid stress, we formulated a novel hypothesis regarding the role of CA in duodenal acid absorption, epithelial protection, and chemosensing. In this review, we will describe how luminal CO2 /H＋traverses the duodenal epithelial cell brush border membrane, acidifies the cytoplasm, andis sensed in the subepithelium. (Keio J Med 55 (3): 96－106, September 2006)
Key words: intracellular pH, carbonic anhydrase, duodenum, vanilloid receptor, carbon dioxide

Introduction

ing this acid onslaught, the duodenum also functions toabsorb virtually all of the acid secreted by the stomach,while sensing the acidity of the luminal content. Thisdichotomous role is unique for the duodenum, in that theother acid-exposed organs of the GI tract, although able
to sense luminal acid, do not absorb significant amountsof acid or acid equivalents.10 -13
Over its short length, the duodenum serves several essential functions. Unlike its more distal counterparts,
the duodenum does not absorb significant quantities of
fluid and electrolytes, although it is an important site formultivalent ion absorption.1 4-1 7 The bulk of the ~400mmol/24 hr of gastric H＋ secretion is neutralized in the
duodenum by secreted HCO 3- , generating equimolar
amounts of CO2. The duodenum also plays an important

Located between the pylorus and ligament of Treitz,
the duodenum routinely experiences 5-log changes of[H＋] and PCO2 > 500 Torr,1 the most extreme environment in terms of rapid changes and absolute concentrations of H＋ and PCO2 in the body.2-5 This milieu places
the mucosa at risk for irreversible cellular acidification
with subsequent necrosis or apoptosis.6 -9 Furthermore,
the duodenal mucosa is leaky (transepithelial resistance= ~50Ω·cm,2 com·pared with gastric transepithelial resistance ~1500Ω·cm2), the epithelial functions normally,
due to the development of a potent defensive system
geared towards preserving intracellular pH (pHi) in theface of extreme environmental acid stress. While resist-

Reprint Requests to: Jonathan D. Kaunitz, M.D., Bldg. 114, Room 217, West Los Angeles VA Medical Center,11301 Wilshire Blvd., Los Angeles, CA 90073
USA, E-mail: jake@ucla.edu
96Keio J Med 2006; 55 (3): 96－106

97

Duodenal CAa ) CA expression in duodenal epithelium

Fig. 1 Expression of membrane-bound carbonic anhydrases(CAs) in duodenal villous epithelium. Duodenal epithelial cells
express CA IX on the basolateral membrane (B) and XIV on theapical membrane (D) of villous epithelial cells, whereas CA IV isexpressed in the capillary endothelium and interstitial cells in thevilli (A) and XII in the cytoplasm of the epithelial cells (C). Bar =50 µm. (adapted from 32)

role in the regulation of traditional epithelial defense
mechanisms, such as HCO3- and mucus secretion, andin the augmentation of mucosal blood flow.1 8 , 1 9 Unlike
the stomach or esophagus, the duodenum must absorb
the gastric acid load while maintaining epithelial integrity. Furthermore, regulation of protective mechanisms isreliant on an ability of the mucosa to sense luminal acidity.2 0 The need for acid absorption necessitates transmucosal permeability or ‘leakiness’ in the context ofconstant exposure to high H＋ and PCO2 a situation generally associated with poor resistance to injury. In thisreview, we will describe hypotheses regarding the role ofcarbonic anhydrase (CA) in facilitating these dual, andseemingly contradictory functions.
We now will show data to suggest that the ability ofthe duodenal mucosa to transport large amounts of acidacross the mucosa, of the subepithelium to sense luminal
acid, and of the epithelial cells to resist acid injury, is dueto the function of membrane-bound and cellular CAs. In
this fashion, CAs help the duodenum fulfill its uniquedual purpose of efficiently absorbing luminal acid while
preserving epithelial cell pHi.

CAs are heavily expressed in duodenal epithelial cells.
Histochemistry reveals high CA activity localized tothe brush border and basolateral membranes, and in thecytoplasm. 2 1 CA II is believed to be the predominantcytoplasmic isoform, whereas CA IX and others are expressed in the epithelial cell plasma membranes.22-25
Traditionally, CAs have been classified into soluble
and membrane-bound enzymes. Since activities were
initially measured in red cells, until recently only four
isoforms had been identified. Soluble CA II is the mostaccepted cytoplasmic CA in intestine, playing a major
function in the hydration of cellular CO2 to HCO3- andH＋, serving as a major source of cellular HCO3-, whichis then transported into the lumen as HCO3- secretion, an
accepted mucosal defense mechanism.26-28
Histochemical studies, however, have additionallyidentified CA activity closely linked with the brush border membrane of the intestinal epithelial cells. 2 1 The
function of this membrane-bound (ecto) CA activity inintestine is not readily apparent. In the past years, many
additional CA isoforms have been identified. Most importantly, several isoforms have been localized to themembranes of HCO3- transporting epithelia, such as renal proximal tubule29 , 30 and in intestine.22-24 , 31
Although multiple CA isoforms have been discovered recently, there has been no systematic survey ofintestinal CA expression. Using real-time RT-PCR andconfocal microscopy of histologic sections with immunofluorescence staining for known CA isoforms, we
conducted a survey of duodenal CA expression, focusing on the expression of membrane CA. 3 2 As seen in
Fig. 1, known membrane CA isoforms IV, IX, XII, and
XIV are expressed in the duodenum, with immunostaining indicating that CA XIV is the predominant apicalisoform, whereas CA IX is the predominant basolateralisoform, similar to the expression pattern found in proximal tubule.2 9 , 3 0 We hypothesize that the apical CA XIV
facilitates CO 2 uptake across the apical membrane ofduodenal epithelial cell, whereas the basolateral CA IX
converts CO2 diffusing across the basolateral membrane
into interstitial H＋, or converts H＋ transported across thebasolateral membrane into interstitial CO2.
b) Role of CAs in epithelial protection
We here describe duodenal defenses in a novel manner, implicating the function of CAs in duodenal acidbase physiology. Interest in CAs in this regard originatedwith the realization that cytosolic and membrane boundCAs are highly expressed in the duodenal epithelium.21 , 3 1
Cytosolic CA, by virtue of its conversion of CO2 to H＋,

Kaunitz JD, et al: Duodenal Carbonic Anhydrases

98

Fig. 2 The Jacobs-Stewart Cycle. Originally described in red
cells, the Jacobs-Stewart cycle is a mechanism by which the net
transfer of H＋ between medium and cytosol can occur via CO2
uptake in the presence of extracellular and intracellular carbonic
anhydrases combined with a plasma membrane anion exchanger.
In the duodenal epithelium, the Jacobs-Stewart cycle is a likely
means by which luminal CO2 is absorbed as H＋ equivalents into
the epithelial cells, from which it exits via the basolateral NHE1
into the interstitium and portal vein.

enhances cellular acidification in the presence of a highPCO2 environment,3 3 , 3 4 which would seem to be the lastthing a cell would need in a strongly acid environment.
This prompted the formulation and testing of a novel
hypothesis regarding the overall role of CAs in duodenal
epithelial defense, acid disposal, and chemosensing.
CAs are the only known enzymes that facilitate conversion of H＋ to CO2 through the intermediary H2CO3.
CAs function in red cells to facilitate transmembrane H＋
movement by conversion of H＋ to CO2 outside the cell,and CO2 to H＋ inside the cell, functionally transferring
H＋ from blood to cytoplasm through a CO2 surrogate.3 5
By transporting H ＋ as CO 2, the cell is protected fromirreversible acidification due to environmental H＋, but
is able to transport large quantities of acid. CO2, being amembrane permeant gas, can readily cross most biological membranes, although recent studies are consistent
with the presence of membrane CO2 channels that facilitate CO2 permeability.3 6 - 4 0 Furthermore, since thecell always contains ~25 mM HCO3-, the lower limit ofcellular acidification, even with saturating PCO2 (e.g. >650 Torr), is ~pH 6.1. In this fashion, the cell can take uplarge amounts of acid while preserving pHi at an acceptable level. This mechanism was first discovered in the
red cell, where the existence of soluble and membranebound CAs and plasma membrane anion exchangers was

first hypothesized. In the presence of these three components, H＋ and CO2 are functionally equivalent. First described in 1942, the Jacobs-Stewart cycle is an acceptedmeans of quantitative transmembrane transport of H＋
by conversion to CO2, with transport of CO2 across thecell membrane.3 5 According to the mechanism, plasma
H＋ is neutralized by HCO3-, forming H2O and CO2. The
CO2 formed diffuses into the red cell, across the plasma
membrane where it is hydrated to H 2CO 3, which dissociated to H＋ and HCO3-. HCO3- is exchanged acrossthe plasma membrane for Cl- , where it can neutralizeanother H＋ (Fig. 2). The H＋ is carried by the red cell tothe lung, where the process is reversed. In this fashion,
H＋ enters the red cell without having to cross the plasma
membrane. Since the duodenal epithelium is equippedwith the same array of transporters, we hypothesize thattransapical H＋ transport occurs according to the samemechanism. Since H＋ does not actually cross the plasma
membrane, and since CO2 is buffered by cellular HCO3-,
the potentially deleterious effects of strong luminal acidon pHi are largely prevented.
Duodenal Acid Disposala) Overview
Duodenal acid disposal, though not commonly studied,plays an essential role in preserving the body’s acidbase status, while transmitting luminal chemical signals
to the subepithelium. The stomach secretes ~ 400 mmol
H ＋/day, almost all of which enters the duodenum. 4 1
The stomach cannot absorb this acid to any great extent,
based on studies of acid back-diffusion,1 2 , 4 2 , 4 3 and withthe observation that in the case of pyloric obstruction,
severe metabolic alkalosis develops, due to the lack of
substantial acid absorption through the gastric wall.4 4 , 4 5
Measured luminal P CO2 is maximal in the duodenum,with PCO2 rapidly normalizing to ~40 Torr in the distal
intestine, indicating that HCO3- neutralization by gastricH＋ mostly occurs in the duodenum1. These data imply
that nearly the entire gastric acid load is absorbed by theduodenum. Although the mechanism of duodenal H ＋
absorption is controversial, a predominance of opinion
has favored a combined mechanism of HCO3- neutralization and H＋ absorption in exchange for Na＋.4 6 -4 9 No
one, however, has previously addressed the mechanism
by which luminal H＋ or CO2 traverses the mucosa, especially since the duodenum on the one hand must quantitatively absorb acid, while its cells are protected fromirreversible acidification.
b) Mechanism
The mechanism by which H＋ is absorbed by cells falls
into at least four possibilities : direct diffusion across theKeio J Med 2006; 55 (3): 96－106

99

Fig. 3 Effect of extracellular carbonic anhydrase (CA) inhibition on luminal high CO 2 challenge in rat duodenum. A, B: The
extracellular CA inhibitor 6a or 14v dose-dependently inhibited CO 2-induced CO2 loss (A), while enhancing CO2-induced HCO3secretion (B). C, D: CO2-induced portal venous acidification (C) and Pco2 increase (D) were dose-dependently inhibited by 6a or 14v.
All data represent the values at 10 min after the CO2 challenge. Each datum is expressed as mean ± SEM (n=6). *p < 0.05 vs. salinegroup, †p < 0.05 vs. pH 6.4 CO2 group. (Adapted from 58).

apical membrane, diffusion through intercellular tightjunctions and then facilitated transport across the basolateral membrane, facilitated transport across the apicalmembrane, and luminal conversion to CO2 with diffusionof CO2 across the apical membrane. In practice, these
possibilities are difficult to differentiate, given the lackof effective means of differentiating bulk paracellularfrom transcellular transport.

single conductance in the pS range). Furthermore, theH ＋ diffusion rate decreases at low pH, and the apical membrane is less permeable than other membranes
making the possibility of direct diffusion of significant
H ＋ directly through the apical membrane unlikely. 5 0
Protons may cross the membrane by an existing plasma
membrane protein, the best candidate protein being
Na ＋/H ＋ exchangers (NHE). NHEs comprise a family of membrane proteins, the most common of whichis NHE1, which is one of the essential ‘housekeeping’i) Transport as H＋
proteins of almost all cells, whose function is to maintain
Luminal H＋ may simply diffuse across the epithelial pHi. NHE3 is present on the apical membrane of villous
cell plasma membrane. Of all ions, protons are the only intestinal epithelial cells,5 1 including duodenum,5 2 andspecies small and permeant enough to cross the lipid has been implicated in the mechanism of neutral NaCl
bilayer without facilitation by membrane proteins. This uptake. These proteins, however, can work in the reverse
possibility has been addressed directly by the measure- direction, i.e. load H＋ while extruding Na＋ into the lument of ionic diffusion through artificial lipid mem- men, depending on the ion gradients to which they arebranes. The H＋ permeability of lipid bilayers (0.5-7 nS・ exposed. When duodenal lumen pH is 2, for example,
cm - 2 ), is exceedingly low (typical ion channels have there is a 5 log order inward (lumen → cytoplasm) H＋

100

Kaunitz JD, et al: Duodenal Carbonic Anhydrases

Fig. 4 Model of transmucosal duodenal CO2/H＋ movement and sensing.
Luminal H ＋ from the stomach is neutralized with the secreted HCO3- in the duodenum, generating CO2. CO 2 diffuses into thecytoplasm through the apical membrane of the epithelial cells. CO2 is converted into H＋ and HCO3- by cytosolic CA. H＋ acidifies
cells and is extruded into the subepithelium via NHE1. H＋ stimulates acid sensors such as VR, followed by the release of CGRP andNO, producing blood flow increase. H＋ is also converted to CO2 by sub-epithelial CA, including the basolateral CA or vascular CA.
CO2 enters the PV blood and is carried by red blood cells, acidifying PV blood and increasing PV Pco2. Cytoplasmic HCO3- generated
from CO2 and loaded via NBC1, is secreted through an apical SLC26Ax anion exchanger or CFTR to the lumen. Luminal HCO3- isconverted to CO2 by apical extracellular CA. Note that net movement of CO2 is lumen → mucosa, whereas net HCO3- movement ismucosa → lumen. These simultaneous movements result in net H＋ absorption and net HCO3- secretion consistent with the functionof the Jacobs-Stewart cycle. Abbreviations: CA: carbonic anhydrase, NHE: Na ＋/H＋ exchanger, NBC: Na＋:HCO3- cotransporter,
SLC26A: solute carrier family 26Ax, CFTR: cystic fibrosis transmembrane conductance regulator, PV: portal vein, VR: vanilloid
receptor, CGRP: calcitonin gene-related peptide, NO: nitric oxide, BF: blood flow. (Adapted and modified from 58)

gradient placed across the apical cell membrane, which
far exceeds the ~10-fold inward sodium gradient. Under such conditions, the apical NHE-3 might reverse,
loading H＋ into the cell while extruding Na＋ into thelumen. This would lower pH i, as has been observed
previously,6 , 5 3-5 5 with the lower limit of the fall of pHilimited by [Na＋i]. This would keep [Na＋i] low, driving
HCO3- uptake via the basolateral sodium-bicarbonate
cotransporter (NBC), which would further limit the dropof pH i. There is thus a ‘safety’ mechanism that, evenin the presence of an apical proton transporter, would
limit cellular acidification by depleting intracellular Na＋
and activating HCO3- loading. Our data, however, does
not support the role of NHE3 in the mechanism of luminal H＋ entry into duodenal epithelial cells. We foundthat basal pH i, measured in vivo when perfusing theduodenum with a pH 7 solution, did not change when theselective NHE3 inhibitor S3226 was added to the perfusate.56 Further preliminary studies have revealed that mildacid (pH 4)-induced intracellular acidification of duodenal villus cells is enhanced by S3226, whereas S3226 has
no effect on intracellular acidification in the presence ofstrong luminal acid (pH 2.2), suggesting that NHE3 inthe ‘reverse mode’ is not involved in H＋ absorption in
the presence of luminal acid (unpublished observations).
ii) Transport as CO2
Acid equivalents may enter the cell as CO2 gas, whichis then hydrated by CA to HCO3- and H＋. Holm and coworkers tested this hypothesis by examining the effectof luminal perfusion of solutions of varying pH and CO2
content on alkaline secretion in rats,5 7 finding that solutions with calculated PCO2 > 120 Torr stimulated HCO3secretion to the same extent as did exposure to pH 2.0
solutions. Since PCO2 in the duodenum due to mixture ofconcentrated HCl with HCO3- can be > 400 Torr1, acidentry as CO 2 may play a role in duodenal defense bymaintaining pHi in the face of extreme acid stress. We
hypothesize that transepithelial duodenal H＋ transportKeio J Med 2006; 55 (3): 96－106

occurs by conversion of H＋ to CO2 with secreted HCO3in the lumen, diffusion of CO2 into the epithelial cells,
re-conversion of CO2 to H＋ in the cytoplasm, and transport of H＋ via basolateral NHE1 into the subepithelialinterstitium.2 0 , 3 2, 33, 58 Recently, we have studied H＋ andCO2 movement among lumen, mucosa, and portal veinin an in vivo perfused rat preparation. Measuring input(perfusate) and output (effluent) pH and [CO2], we wereable to measure uptake and secretion of H＋ and CO2 by
the mucosa, and also measure the transmucosal absorption of H＋ and CO2 into portal vein. Our data powerfully
implicates membrane-bound and cytosolic CAs in net
transmucosal acid movement.5 8 One particularly striking
finding was the reversal of net H＋ absorption to net CO2/
HCO3- secretion with selective inhibition of membrane
CAs (Fig. 3). Note that superfusion of a high PCO2 solution over the mucosa was associated with net loss of CO2
from lumen (3A), net HCO3- secretion to lumen (3B),
decreased portal venous pH (3C) and increased portal
venous PCO2 (3D), consistent with prior observations ofhigh PCO2 solutions augmenting HCO 3- secretion, although this happens later period3 3 , 5 7 and the assumptionthat CO2 diffuses down its concentration gradient. Unexpectedly, selective inhibition of membrane CA with experimental CA inhibitors, provided by Prof. C. Supuran(Firenze, Italy) reversed CO2 absorption to net secretion,
increased net HCO3- secretion, and inhibited portal venous acidification and PCO2 increase. These effects aremost consistent with membrane CAs facilitating CO2
uptake by the mucosa and conversion of HCO3- to CO2
in the lumen.
Once transported into the subepithelial interstitium,
H ＋ may interact directly with subepithelial acid sensors, or may be further converted to CO2 by basolateral
membrane CAs such as CA IX or vascular CA such as
CA IV. Possible mechanisms of CO2 /H＋ diffusion into
duodenal mucosa and portal venous blood with epithelialand sub-epithelial CAs are summarized in Fig. 4. Subepithelial acidity is presumably the chemical signal thatis transduced into efferent protective responses.
Luminal chemosensing
The mucosa can ‘taste’ the luminal contents with multiple sensors that identify specific components.5 9 One
well-established luminal component is acidity, which
is sensed by the upper gut. Responses to luminal acidinclude reflex, protective responses such as augmentation of mucosal blood flow (hyperemia), increased mucus gel thickness and mucus secretion, and augmented
HCO3- secretion.6 0 , 6 1 Clinically, excess luminal acid is
associated in some cases with functional symptoms, suchas heartburn and dyspepsia.6 2 If luminal acid provokes
these responses, a mechanism must exist that senses luminal acid and coordinates the protective responses. The

101

mechanism must be comprised of a means to transport
luminal acid or acid equivalents across the mucosa, acidsensors, and an efferent system to convert afferent acidinduced signals into responses such as increased bloodflow.
Our laboratory, having measured mucosal responses
to acid perfusion in a variety of systems, has been actively involved in identifying the nature of this sensing
mechanism.2 0 , 3 2, 6 3 , 6 4 Through these studies, we haveshown evidence that NHE1 and the vanilloid receptor(VR1) (now termed transient receptor potential vanilloid-1 or TRPV1) play important roles in acid chemosensing, with NHE1 transporting H ＋ out of the cells,
and TRPV-1 serving as the primary acid sensor.5 5 , 6 4
Data supporting this hypothesis include inhibitionof efferent responses by the NHE1 inhibitor dimethylamiloride (DMA), selective de-afferentation withcapsaicin, and with the capsaicin receptor antagonist
capsazepine (CPZ).5 5 , 6 4 Furthermore, TRPV1 is present in the submucosa and villous core of the duodenal
epithelium. 2 0 The net effect of the abolition of thesecompensatory mechanisms by capsaicin is the enhancement of the susceptibility of the duodenum to acid injury. 6 5 With cell acidification in response to elevatedluminal CO 2 and H＋, the next step is to transduce thesignal through the epithelium. With a fall in pH i, thebasolateral membrane NHE1 is activated, serving in its
well-accepted role of eliminating excess cellular acid.
The acid is thus transported to the subepithelial interstitium, where it then presumably interacts with acidsensors. The precise identity of these sensors is unknown, although there are accumulating data that these
are likely to be vanilloid receptors (TRPV1), which are
acid- and heat-sensing cation channels that are present in the gastroduodenal lamina propria mucosae andsubmucosa. 2 0 , 6 6-6 8 Activation of TRPV1 on afferent
nerves produces effector responses such as nitric oxide(NO) generation and the release of the vasoactive peptide calcitonin gene-related peptide (CGRP) or prostaglandins, eventuating in hyperemia, and HCO3- and mucus secretion. Furthermore, TRPV1 activation is likely
to produce clinical sensations such as heartburn and dyspepsia.6 2 , 6 9 , 7 0 Hyperemia in response to luminal perfusion of a high CO2 solution in rat duodenum, a response
inhibited by the inhibition of membrane-bound and cytosolic CAs, NHE1 and TRPV1.3 2 These data support our
hypothesis that luminal H＋ traverses the apical membrane
as CO2 facilitated by membrane-bound CA activity, CO2
acidifies the cells facilitated by cytosolic CA activity, H＋exits via NHE1, acidifying the interstitium, and finallysubepithelial interstitial H＋ activates TRPV1 on afferent
nerves followed by the release of vasodilatory CGRP andNO, producing hyperemia. Intrinsic and extrinsic afferent neurons express membrane-bound CA and TRPV1,3 2
further suggesting that a transport metabolon consisting

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Kaunitz JD, et al: Duodenal Carbonic Anhydrases

Fig. 5 Foregut acid-base balance. CO2 from the systemic circulation enters gastric parietal cells, where it is hydrated to carbonic acidby carbonic anhydrase. Carbonic acid is dissociated into a proton which is secreted by the apical membrane H, K ATPase, and HCO 3-,
which is secreted into the circulation via a basolateral anion exchanger, most likely AE2. Secreted acid enters the gastric lumen, whereperistalsis carries it to the duodenal lumen. HCO3- alkalinizes the circulation, creating the “alkaline tide”. This HCO3- is secreted by
the pancreatic duct cells, duodenal epithelial cells and possibly by the Brunner’s glands into the duodenal lumen, where it combineswith H＋, generating CO2. Luminal CO2 and H＋ are absorbed by the mucosa, enter the portal vein, and are taken up by the parietal
cells.

of CA and TRPV1 serves as a CO2 /H＋ sensor, similar to
the metabolon of CA and anion exchanger.7 1 , 7 2 A schematic illustrating our hypotheses regarding luminal acidchemosensing in duodenum is provided in Fig. 4.
The gastroduodenal ‘carbon cycle’Due to the massive amounts of acid and base that flow
past its mucosa every day, the duodenum is a major
acid-base regulatory organ. To understand upper GI acid-base balance, we will describe the basic mechanisms
underlying gastric acid secretion and duodenal function.
Parietal cells secrete concentrated HCl derived fromcirculating CO 2, which diffuses into the cells and isconverted to HCO 3- and H ＋ by cytosolic CA. H ＋ issecreted into the secretory canaliculus, which is contiguous with the gastric lumen. HCO3- exits the basolateral
membrane of the cell via an anion exchanger and enters
the circulation as the ‘alkaline tide’ . 7 3 The estimatedamount of H＋ and HCO3- secretion, as judged by gastric secretory rates, is approximately 400 mmol/24 hr.4 1
With antral peristalsis, the gastric acid content enters theduodenum, where it combines with HCO3- secreted by
the duodenal epithelium and by the liver and pancreas.
Estimates of total duodenal HCO3- secretion rate are notavailable, but, on the basis of measured human HCO3secretory rates of ~700 µmol/hr7 4 and a pancreatic secretory volume of ~ 100 ml/hr of 75 mM HCO3-,75 a total
pancreatico-duodenal HCO3- output of 200 mmol/24 hrs

or higher is easily justified. With the mixture of gastric
HCl and secreted HCO3- in the duodenal lumen, CO2
is generated in equimolar amounts. We have postulated
that this generated CO2 is transported from the lumen to
the subepithelial circulation, circulated through the pulmonary and systemic circulations, where it is then taken
up by parietal cells to complete the cycle.
We have addressed the mechanism of duodenal CO2
uptake using two approaches: measurement of portal
vein blood gas composition, and simultaneous and continuous measurement of perfusate pH and PCO2 during
luminal CO2 perfusion. Our data indicate that net CO 2
and H＋ loss from the lumen correlates with net H＋ andCO 2 gain in the portal vein, strongly consistent withtransmucosal CO2 and H＋ movement from the lumen to
the portal vein.58 This net transmucosal movement of H＋
and CO2 explains why the elevated PCO2 of the duodenal lumen does not occur in more distal segments, andsuggests the near-quantitative duodenal absorption of
acid1 , 76 (Fig. 5).
Clinical correlate- Helicobacter pylori, pyloric
obstruction, and CO2-induced visceral pain
Helicobacter pylori (Hp) infection is implicated in thepathogenesis of peptic ulcer disease in patients not taking nonsteroidal anti-inflammatory drugs (NSAIDs). 7 7
The pathogenesis of gastroduodenal injury due to Hp
remains incompletely understood. One possibility is that

Keio J Med 2006; 55 (3): 96－106

inflammation due to Hp infection releases neutrophil hypochlorous acid, which, combined with Hp-derived ammonia, yields highly toxic monochloramine.7 8-8 1 Since
Hp colonizes the antrum, and can colonize duodenal
gastric metaplasia, this mechanism may explain some
aspects of Hp-related gastroduodenal injury.7 7 , 8 2-8 4 Howthese Hp-related disturbances affect mucosal protective
and signaling mechanisms remains conjectural, although
recent data suggest that ammonium ion inhibits the
SLC26A9 anion exchanger, a paralog of the duodenal
anion exchangers implicated in HCO3- secretion.85 Furthermore, since active Hp infection reduces acid-induced
duodenal HCO3- secretion,8 6 and since HCO3- secretionis needed for transmucosal H＋ absorption5 8 and above,
Hp might affect duodenal protective and chemosensing
mechanisms.
Hp infection can be complicated by perforation, pyloric obstruction, or hemorrhage.8 7- 8 9 Obstruction of thepyloric valve or proximal duodenum, as a result of anacute pyloric channel ulcer, scarring from prior ulcer disease, or duodenal compression from a pancreatic mass,
diverts the acidic gastric content from neutralization by
the duodenum to elimination by vomiting. Despite theloss of HCl, alkaline tide HCO3- is still present in thecirculation. Since renal compensation for excess filtered
HCO3- is inefficient, a hypochloremic metabolic alkalosis develops, demonstrating the importance of duodenalacid and base transport in terms of overall acid - basehomeostasis,4 5 , 9 0 and also the importance of the gastroduodenal ‘carbon cycle’. The acid-base disturbance also
underscores the physiological concept that the gastricmucosa is unable to effectively absorb H＋ or CO2;9 1 , 9 2
Fig. 5.
CA and TRPV1 may also function as pain sensors,
producing burning or dyspeptic symptoms in response
to luminal acidity or CO 2 in acid-related symptomatic
gastrointestinal diseases such as functional dyspepsia
and irritable bowel syndrome. Although we will notinclude a detailed discussion about the role of TRPV1
function as a pain sensor in this review, due to the availability of excellent reviews,9 3 , 9 4 the colocalization of
membrane-bound CA and TRPV1 in duodenal afferent
neurons and nerves,3 2 suggests that CA and TRPV1 mayform a transport metabolon that senses CO2. This sensor may be analogous to chemosensors in the central
nervous system, in avian pulmonary chemosensors, andin the nasal epithelium.9 5-9 9 In epithelial cells, CA may
form a ‘transport metabolon’ with HCO 3- transporters to facilitate net CO2 transport through conversion to
HCO3- by CA and HCO3- transport by anion exchange
or cotransport.7 1 , 7 2 In this fashion, submucosal sensors
could detect changes of PCO2 through conversion of CO2
to H ＋ and HCO 3- , with subsequent activation of H ＋
sensors such as TRPV1 or acid-sensing cation channels(ASIC). Further evidence supporting the presence of a103

TRPV1-CA metabolon is that acetazolamide decreases
pain associated with abdominal CO2 insufflation prior tolaparoscopic surgery.100
Summary and conclusions
Study of duodenal acid movement and CA expression
and function has yielded fresh insights into how mucosal
surfaces protect themselves from a hostile environment
while absorbing large quantities of H＋ and sensing theluminal pH. The duodenum is also an important component of a larger system of acid-base balance involving
gastric acid secretion and carbonic anhydrases. In the
duodenal subepithelium, sensors detect interstitial acidification that is transduced from luminal H＋ via transepithelial H＋ and CO2 movement.
Through study of the duodenal mucosa, we hope tounderstand how environmentally exposed epithelia deal
with hostile conditions, with the hope that this may provide further understanding and insight into inflammatory
and neoplastic epithelial diseases.
Acknowledgements: We would like to thank Drs. Eli Engel and Paul
Guth, for their helpful advice, Drs. Tetsu Takeuchi and Misa Mizumori
for their great contribution to recent experimental data, andDrs. Masahiko Hirokawa, Kotaro Kaneko, Yasuhiro Nishizaki, andShin Tanaka for developing the experimental system used in thesestudies and for their invaluable intellectual contributions. Supported by
Department of Veterans Affairs Merit Review Funding and National
Institutes of Health, Digestive Diseases and Kidney Institute RO1 DK
54221 and P30 DK0413

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