Histamine inhibits prostaglandin E2-stimulated rabbit duodenal bicarbonate secretion via H2 receptors and enteric nerves

GASTROENTEROLOGY1995;108:1676-1682

Histamine Inhibits Prostaglandin E2-Stimulated Rabbit Duodenal Bicarbonate Secretion Via H2 Receptors and Enteric Nerves DANIEL L. HOGAN, BIGUANG YAO, KIM E. BARRETT, and JON I. ISENBERG Division of Gastroenterology, Department of Medicine, Universityof California, San Diego, School of Medicine, San Diego, California

Background~Aims: The gastroduodenal epithelium is

protected from acid peptic damage by an adherent mucus-bicarbonate layer. Bicarbonate is secreted by the surface epithelial cells into this mucus layer. Patients with duodenal ulcer disease have impaired proximal duodenal bicarbonate secretion. Mast cells, present in large numbers in the duodenal mucosa, release a number of inflammatory mediators, including histamine. Release of such mast cell mediators has been implicated in ulcer disease. In this study, the ability of histamine to regulate bicarbonate secretion was examined. Methods: Bicarbonate secretion by rabbit proximal duodenal mucosa was examined in vitro, and the effects of histamine, its agonists, and its antagonists were studied. Results: Histamine essentially eliminated prostaglandin E2-stimulated duodenal mucosal bicarbonate secretion, an effect reversed both by the neurotoxin, tetrodotoxin, and the histamine H2-receptor antagonist, cimetidine, as well as reproduced by the Hwreceptor agonist, dimaprit. Conclusions: In addition to the stimulatory action of histamine on gastric acid secretion, histamine expresses an additional antidefensive action by inhibiting prostaglandin E2-stimulated duodenal epithelial bicarbonate secretion. This effect of histamine is likely mediated via H2 receptors located on enteric nerves.

n adherent layer of mucus covers the gastroduodenal epithelium. Bicarbonate secreted into its base provides a p H gradient that neutralizes hydrogen ions secreted by the stomach, thereby protecting the surface epithelial ceils from acid-peptic damage.1 Duodenal epithelial bicarbonate secretion has been shown in all species tested ranging from amphibia to humans. 2-13 Bicarbonate transport involves active transport ( ~ 6 0 % ) , passive paracellular diffusion ( ~ 3 0 % ) , and secretion of endogenous intracellular HCO3- (<10%); active transport requires an oxygen-dependent metabolic process and a basolateral membrane Na+,K+-adenosine triphosphatase. 1-5'16'17 Two potent physiological stimuli of bicarbonate secretion are luminal acidification, as occurs with the entry of gastric acid into the duodenum, and prosta-

glandins (PGs) of the E class. 8'9'12'I3 In fact, PGs are believed to be the primary physiological mediators of the acid-stimulated bicarbonate secretory response. 1'12'13 Thus, PG analogues are prescribed to prevent mucosal injury. 18 Interestingly, patients with duodenal ulcer disease have markedly diminished duodenal mucosal bicarbonate secretion, indicating that this transport process may play a protective role in normal physiology. ~9-2. Mast ceils, which underlie the gastroduodenal epithelium in large numbers, 22 have been implicated in the pathogenesis of experimental ulcer disease. 23-27 H u m a n duodenum contains about 20,000 mast cells/mm 3 compared with 7000 mast cells/mm 3 for human skin. 22'28 Compounds that inhibit the release of mast cell mediators reduce the severity of both stress and nonsteroidal antiinflammatory d r u g - i n d u c e d ulcers. 24 28 Further, mast cell-deficient mice develop significantly less mucosal damage in response to ethanol than their mast cell replete, normal littermates. 29'3° Histamine, a major mast cell mediator, induces duodenal villous damage in fasting rats, an effect that is inhibited by histamine Hg-receptor antagonists or administration of bicarbonate. 23 Duodenal ulcer disease is found both in systemic mastocytosis 31 and in chronic myelogenous leukemia when there is an increase in histamine-containing circulating basophils. 32,33 Although the ability of histamine to induce duodenal mucosal damage might be secondary to its ability to stimulate gastric acid secretion, we postulated that histamine possesses additional effects at the level of the duodenum. Histamine is known to alter ion transport in the ileum and colon, acting via histamine H1 receptors on enterocytes and H2 and H 3 receptors on submucosal neurons.34 36 In this study, we examined the hypothesis that Abbreviations used in this paper: Bt2 cAMP, dibutyryl 5'-cyclic adenosine monophosphate; ED~o, dose causing 50% of maximal response; EFS, electrical field stimulation; Isc, short-circuit current; TTX, tetrodotoxin. © 1995 by the American Gastroenterological Association 0016-5085/95/53.00

June 1995

histamine has regulatory effects on duodenal mucosal bicarbonate secretion.

M a t e r i a l s and M e t h o d s Study Design Studies of duodenal mucosal bicarbonate secretion were performed in segments of proximal duodenum from overnightfasted male New Zealand White rabbits (2.5-3.0 kg) using techniques that have been described previously. 9a° All studies were approved by the University of California San Diego Committee on Investigations Involving Animal Subjects. In brief, tissue segments were excised, stripped of external serosal and muscle layers by sharp dissection, and mounted vertically in Ussing chambers (aperture of 1.23 cm2). Tissues were bathed at 37°C with an unbuffered, HCO3--free , Ringer's-type solution on the luminal side and circulated with a 100% 02 gas lift system and with a buffered, HCO3--containing Ringer's solution circulated with 95% 02/5% CO2 on the nutrient side. The rate of luminal alkalinization (i.e., bicarbonate secretion) in response to the various experimental treatments was calculated from the volume of HC1 (25 mmol/L) infused to m a i n t a i n the luminal reservoir at pH 7.4 (pH Stat, RTS 822; Radiometer America, Westlake, OH). Experiments were performed under short-circuited conditions by m e a n s of a voltage clamp (model VCC 600; Physiological Instruments, San Diego, CA), which maintained the transepithelial potential difference (PD) at zero, except for brief periods ( < 2 seconds) at each time point (taken at 10-minute intervals) when the open-circuit PD was measured. All agonists and antagonists were added to the nutrient side of the tissue at times indicated in the Results section of this article. Known agonists of rabbit duodenal mucosal bicarbonate secretion, PGE2 (20 gmol/L), vasoactive intestinal polypeptide (VIP, 80 nmollL), dibutyryl 5'-cyclic adenosine monophosphate (Bt2 cAMP, 2 mmol/L), and carbachol (1.5 ~mol/ L) were used at the stated doses determined in previous studies to produce approximately 50% of their maximal effect (EDs0), a n d enteric nerves were stimulated using electrical field stimulation (EFS). 9'1° In brief, EFS was performed by passing an electrical current through the tissue parallel to the plane of the muscularis propria via a pair of aluminum foil electrodes placed on the submucosal surface of the tissue. The electrodes were connected to a stimulus isolation unit (model 850A; W-P Instruments, Hamden, CT). The stimulus waveform was a 500-microsecond, 100-V positive rectangular pulse separated from a 500-microsecond, 100-V negative pulse by a 10-microsecond interval. These stimuli were applied at a frequency of 10 Hz for 600 milliseconds each second with a stimulus interval generator (model 830; W-P Instruments). To determine the mechanism of the effect of histamine, the histamine antagonists diphenhydramine (H1, 1 - 1 0 0 btmol/L), cimetidine (H2, 1 - 1 0 0 btmol/L), and thioperamide (H3, 1 100 btmol/L) and the neurotoxin tetrodotoxin (TTX, 0.1 btmol/ L) were used. The ability of various histamine agonists, 2pyridylethylamine (H1, 1 mmol/L), dimaprit (82, 1 mmol/L),

HISTAMINE AND DUODENAL BICARBONATE SECRETION

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N-ot-methylhistamine (H3, 0.1 mmol/L) to reproduce the effects of histamine was also tested. and

Materials Bt2 cAMP, histamine, diphenhydramine, cimetidine, 2-pyridylethylamine, TTX (Sigma Chemical Co., St. Louis, MO); dimaprit (ICN, Irvine, CA), thioperamide (Research Biochemicals, Natich, MA), and N-o~-methylhistamine (Calbiochem, San Diego, CA) were purchased from the sources indicated. PGE2 and VIP were the generous gifts of the late Dr. Andre Robert (Upjohn Co., Kalamazoo, MI) and Dr. Jean Rivier (Salk Institute, La Jolla, CA), respectively. All other chemicals used were of at least analytical grade and were obtained commercially.

Statistical Analyses Experimental treatments were compared in paired tissue specimens, a n d at least four tissues were studied in each series of experiments. Results are expressed as mean _+ SEM. Statistically significant (P < 0.05 or less) differences between treatments were assessed by means of the two-tailed Student's t test.

Results Effect of Histamine on Basal and PGE2Stimulated Bicarbonate Secretion Histamine (1 mmol/L) failed to alter basal bicarbonate transport (addition of histamine, 1.29 + 0.02 b t m o l ' h -1" cm-2; removal of histamine, 1.30 + 0.02 btmol" h -1" cm2). However, histamine significantly increased short-circuit current (Isc), which is a measure of n e t ion flux (net increase, 7.3 + 1.2 g A / c m 2) and P D (net increase, 0.5 + 0.1 mV), indicating stimulation of a transport pathway other than bicarbonate secretion; it is likely that of electrogenic chloride secretion as previously shown in the ileum and colon. 34-39 PGE2 was a potent agonist of bicarbonate secretion, significantly (P < 0.001) stimulating both alkaline secretion and Isc (Figure 1). However, when histamine was added before PGE2, the bicarbonate secretory response was inhibited almost completely ( > 8 5 % ; P < 0.001; Figure 1). Removal of histamine reversed the inhibitory effect, and reapplication of PGE2 again induced a significant bicarbonate secretory response. Addition of PGE2 also resulted in a p r o m p t and significant increase in Isc that corresponded to the periods of bicarbonate secretion. Therefore, alt h o u g h both histamine and PGE2 independently stimulate duodenal mucosal ion transport, histamine also has a marked inhibitory effect on PGEe-stimulated bicarbonate secretion.

Role of Enteric Nervous System The duodenal mucosa contains a dense network of nerve fibers that extend into the crypts and villi. 1°

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HOGAN ET AL.

GASTROENTEROLOGY Vo1.108, No. 6

Some of the effects of histamine on ion transport in other regions of the gastrointestinal tract may be mediated via such enteric nerves. 34 36 Thus, to determine if the inhibitory effect of histamine on PGE2-induced bicarbonate transport was due to a direct effect on epithelial cells or via submucosal neurons, the effect of the neurotoxin T T X was examined. Inhibition of neurotransmission was accomplished by adding 0.1 btmol/L T T X to the nutrient side of the tissue 10 minutes before histamine. T T X alone had no effect on either the bicarbonate secretory or Isc response to PGE2 (Figure 2). However, in the presence of T T X , histamine no longer inhibited PGE2stimulated bicarbonate secretion (Figure 2). These find-

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Figure 1. Histamine inhibits PGE2-stimulated bicarbonate secretion. The effect of PGE2 (20 pmol/L) on (A) duodenal bicarbonate secretion and (B) Isc in the absence (0) or presence (0) of histamine (Hist; 1 mmol/L) is shown. When histamine was removed by washing the tissue, reapplication of PGE2 stimulated secretion (P < 0.05). Isc also increased in response to histamine and to PGE2; however, the response with histamine + PGE2was significantly less compared with PGE2 alone, indicating a decreased rate of secretion due to histamine. Results here and in subsequent figures are means _+ SEM; n = 4. *P < 0.01 vs. histamine + PGE2.

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Figure 2. The enteric nervous system is involved in the inhibitory effect of histamine on stimulated bicarbonate secretion. The effect of PGE2 (20 pmol/L) on (A) duodenal bicarbonate secretion and (B) Isc in the presence of the neurotoxin TTX (0.1 ~Lmol/L) with (A) or without (A) histamine (1 mmol/L) pretreatment is shown. Application of TTX completely reversed the inhibitory effect of histamine on PGE2stimulated bicarbonate secretion (see Figure 1), indicating mediation by subepithelial enteric nerves.

June 1995

HISTAMINE AND DUODENAL BICARBONATE SECRETION

lated bicarbonate secretion (Figure 3), the histamine Hereceptor antagonist, cimetidine (100 ~tmol/L), almost completely eliminated this inhibitory effect. Furthermore, evaluating these antagonists at varying concentrations confirmed that only cimetidine reversed histamine's inhibitory effect on PGEe-activated bicarbonate secretion in a dose-dependent fashion (Figure 4), whereas diphenhydramine and thioperamide were without effect at any dose tested. None of the antagonists had an effect on Isc (Figure 3). To provide further evidence that the Hehistamine receptor is involved in the inhibitory effect of histamine, specific agonists were used. A comparable concentration of the histamine He-receptor agonist, dimaprit, duplicated the inhibitory effect of histamine on

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PGEe-stimulated bicarbonate secretion, whereas histamine H1 and H 3 agonists (2-pyridylethylamine and NO~-methylhistamine, respectively) were without effect (Figure 3).

Effect of Histamine on Bicarbonate Secretion Stimulated by Other Agents In contrast to tile findings with PGEe, histamine failed to inhibit the stimulatory effects (on bicarbonate secretion, Isc, and PD) of a number of other agonists, including VIP, Bt2 cAMP, carbachol, and EFS. In general, each are potent agonists of duodenal bicarbonate secretion and of ion transport in the small intestine. 9,t° In this study, each agonist was examined at its approximate EDs0 for bicarbonate secretion. Net bicarbonate secretion (~tmol" h 1. cm-e) induced by each agonist in the absence or presence of histamine was 0.37 + 0.03 vs. 0.37 -+ 0.08 (VIP; 80 nmol/L), 0.46 + 0.07 vs. 0.40 + 0.05 (Bt2 cAMP; 2 mmol/L), 0.28 + 0.02 vs. 0.29 -+ 0.03 (carbachol; 1.5 btmol/L), and 0.46 + 0.04 vs. 0.43 + 0.05 (EFS; 1010 microseconds, 10 Hz, 100 V), respectively. All data are means ___ SEM (n = 4). Thus, the inhibitory action of histamine is highly specific for PGEe-stimulated bicarbonate secretion.

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Our studies have shown that histamine, the primary amine mediator of intestinal mucosal mast cells, is an important regulator of a duodenal defensive factor, namely, duodenal mucosal bicarbonate secretion. Histamine selectively eliminated PGEe-stimulated bicarbonate

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secretion is mediated via H2 receptors. The effect of PGE2 (20 ~Jmol/ L) on (A) bicarbonate secretion and (B) Isc in the presence of histamine and either an Hz-, H2-, or H3-histamine receptor antagonist is shown. Of three histamine receptor antagonists tested, only the H2receptor antagonist cimetidine (0.1 mmol/L; • ) reversed the inhibitory effect of histamine. The histamine antagonists diphenhydramine (HI; 0.1 mmol/L; D) and thioperamide (H3; 1 pmol/L; V) had no effect. *P < 0.01 vs. H1 and H3 antagonists.

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1680 HOGANET AL.

GASTROENTEROLOGYVo1.108, No. 6

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The effect of PGE2 (20 pmol/L) on (A) bicarbonate secretion and (B) Isc in tissues treated with either an Hi-, H2-, or H3-histamine receptor agonist is shown. The inhibitory effect of histamine on PGE2stimulated bicarbonate secretion was reproduced by addition of the H2-receptor agonist dimaprit (Dim; 1 retool/L; 0). In contrast, 2pyridylethylamine (H1 agonist; 1 mmol/L; II), and N-c~-methylhistamine (H3 agonist; 0.1 mmol/L; T) had no effect. *P < 0.01 vs. H1 and H3 agonists.

secretion in rabbit duodenal mucosa, whereas the alkaline secretory responses to other agonists of this process were unaffected by histamine. It has been shown previously that PGs are key physiological regulators of duodenal bicarbonate secretion and that they are formed by the mucosa in response to luminal acidification; cyclooxygenase inhibitors decrease both basal and acid-stimulated epithelial bicarbonate secretion, t-7 In addition to being specific for PG-induced bicarbonate secretion, the inhibitory effect of histamine was reversed by both T T X and by histamine H 2 - r e c e p t o r antagonism and was reproduced by an H2 agonist. In total, these findings suggest that histamine, acting through H2 receptors, either directly or indirectly activates a subpopulation of nerves within the duodenal mucosa that con-

rain an inhibitory messenger. Presumably the pathway involves release of a neurotransmitter from the enteric nervous system. However, this transmitter does not seem to be cholinergic. It has been shown that carbachol is an agonist of bicarbonate secretion and that activation of all resident enteric nerves via EFS directly stimulates bicarbonate secretion, an effect that is inhibited by atropine. s° In this study, atropine did not alter the inhibitory effect of histamine on PGE2-induced bicarbonate secretion (unpublished observations, 1993). Furthermore, histamine had no effect on EFS- or carbachol-stimulated bicarbonate secretion. Rather, we propose that histamine, acting through H 2 receptors, activates a subpopulation of nerves within the mucosa that contain an inhibitory neuropeptide or other neurotransmitter. The simplest model that takes account of available data would place the H 2 receptors directly on a subset of enteric nerves. However, it is possible that H2 receptors are located on other cells in the lamina propria, which could release an intermediary messenger that in turn would activate a neural response. In either case, if the neurotransmitter is released, it would act on the epithelium to inhibit bicarbonate secretion. However, the identification of this neurotransmitter and the basis for its selectivity for PGE2-stimulated bicarbonate secretion will require further study. Possible candidates for the neurotransmitter would include somatostatin, peptide histidine isoleucine, neuropeptide Y, peptide YY, substance P, and others. In contrast to its failure to increase basal bicarbonate secretion and its inhibitory effect on PGE2-stimulated bicarbonate secretion, histamine had a stimulatory effect on Isc and PD in rabbit duodenal tissues. This presumably reflects stimulation of an alternate ion transport pathway, likely to be chloride transport as shown in other intestinal segments such as the ileum and colon of various species. 34-39 Histamine can induce chloride secretion via a direct interaction with H1 receptors located on epithelial cells, as evidenced by the ability of the amine to induce chloride secretion across the colonic epithelial cell line, T84.4° Histamine also has indirect actions on chloride secretion. In keeping with the effects of histamine on bicarbonate secretion reported in this study, histamine can activate recurrent cyclical increases in Isc (representing chloride secretion) in guinea pig colon via effects on H2 receptors (cimetidine-sensitive) on enteric nerves ( T T X sensitive). The H2-agonist dimaprit also evoked these recurrent cycles. 41 Similarly, histamine further contributes to the overall regulation of chloride secretion by binding to presynaptic, inhibitory H 3 receptors on submucous nerves. 36 Thus, there is ample precedent for physiological effects of histamine on the intestinal epithelium that are mediated via the enteric nervous system.

June 1995

Moreover, whereas histamine has direct calcium-mediated stimulatory effects on epithelial cells, it also interacts synergistically with other mediators using cAMP-dependent signaling pathways. Histamine has also been shown to evoke release of PGs, such as PGE2, PGI2, and PGD2, each of which have varying effects on epithelial ion transport. 34-41 At the high concentration of histamine used here, it is possible that the release of one or more of these prostanoids occurred, contributing to the increase in Isc. Likewise, a released prostanoid might contribute to the inhibition of bicarbonate secretion. However, histamine was without effect on basal bicarbonate secretion. Although these studies were performed in rabbit duodenal mucosa in vitro, it is possible that similar findings may be obtained in humans. This requires direct study. However, in the event that histamine inhibits human PGE2-stimulated duodenal mucosal bicarbonate secretion, this would suggest that duodenal mucosal mast cells may contribute to the pathogenesis of duodenal ulcer disease by releasing histamine and thereby inhibiting PGE2-stimulated epithelial bicarbonate secretion. Thus, in addition to histamine having an aggressive action due to its potent acid stimulatory effect, histamine may also have an antidefensive intraduodenal effect by suppressing duodenal bicarbonate secretion. Moreover, if similar events occur in humans, they may explain, at least in part, the therapeutic effect of histamine H 2receptor antagonists in duodenal ulcer disease as well as the observation that H2-receptor antagonists are beneficial in preventing nonsteroidal anti-inflammatory drug associated duodenal ulcer. 42'43 Likewise, previous studies from our laboratory examining human duodenal bicarbonate secretion have used H 2 antagonists to suppress gastric acid secretion. If the findings here can be extrapolated to humans, they may suggest that previous observations of acid-stimulated bicarbonate secretory responses (mediated by PGs) may have been underestimates. In summary, histamine, a prototype inflammatory mediator, eliminates PGE2-stimulated proximal duodenal mucosal bicarbonate secretion in rabbit duodenal mucosa. Because this response was blocked by the histamine H2-receptor antagonist cimetidine as well as the neurotoxin TTX, and reproduced by the histamine H2-receptor agonist dimaprit, the inhibitory effect of histamine is likely mediated by H 2 receptors on enteric neurons. Overall, the findings indicate that histamine is likely to play an important modulatory role that contributes to the overall level of duodenal mucosal defense.

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2. Flemstr6m G, Heylings JR, GarnerA. Gastric and duodenal HCO3transport in vitro: effects of hormones and local transmitters. Am J Physiol 1982;242:G100-G110. 3. Simson JN, Merhav A, Silen W. Alkaline secretion by amphibian duodenum. II. Short circuit current and Cl- fluxes. Am J Physiol 1981; 240:G472-G479. 4. Simson JN, Merhav A, Silen W. Alkaline secretion by amphibian duodenum. I. General characteristics. Am J Physiol 1981;240: G401-G408. 5. Simson JNL, Merhav A, Silen W. Alkaline secretion by amphibian duodenum. III. Effect of DBcAMP, theophylline, and prostaglandin. Am J Physiol 1981;241:G528-G536. 6. Flemstr6m G, Garner A, Nylander O, Hurst BC, Heylings JR. Surface epithelial HCOa- transport by mammalian duodenum in vivo. Am J Physiol 1982; 243:G348-G358. 7. Isenberg JI, Flemstr6m G, Johansson C. Mucosal bicarbonate secretion is significantly greater in the proximal versus distal duodenum in the in vivo rat. In: Allen A, Flemstr6m G, Garner A, Silen W, Turnberg LA, eds. Mechanisms of mucosal protection in the upper gastrointestinal tract. New York: Raven, 1 9 8 4 : 1 7 5 -

180. 8. Vattay P, Fell W, Klimesch S, Wenzl E, Starlinger M, Schiessel R. Acid stimulated alkaline secretion in the rabbit is passive and correlates with mucosal damage. Gut 1 9 8 8 ; 2 9 : 2 8 4 - 2 9 0 . 9. Yao BG, Hogan DL, Bukhave K, Koss MA, Isenberg JI. Bicarbonate transport by rabbit duodenum in vitro: effect of vasoactive intestinal polypeptide, prostaglandin E2 and cyclic adenosine monophosphate. Gastroenterology 1 9 9 3 ; 1 0 4 : 7 3 2 - 7 4 0 : 10. Hogan DL, Yao BG, Steinbach JH, Isenberg JI. The enteric nervous system modulates mammalian duodenal mucosal bicarbonate secretion. Gastroenterology 1 9 9 3 ; 1 0 5 : 4 1 0 - 4 1 7 . 11. Konturek SJ, Bilski J, Tasler J, Laskiewicz J. Gut hormones in stimulation of gastroduodenal alkaline secretion in conscious dogs. Am J Physiol 1985;24:G687-G691. 12. Konturek SJ, Bilski J, Tasler J, Konturek JW, Bielanski W, Kaminska A. Role of endogenous prostaglandins in duodenal alkaline response to luminal hydrochloric acid or arachidonic acid in conscious dogs. Digestion 1986; 3 4 : 2 6 8 - 2 7 4 . 13. Isenberg JI, Hogan DL, Koss MA, Selling JA. Human duodenal mucosal bicarbonate secretion. Evidence for basal secretion and stimulation by hydrochloric acid and a synthetic prostaglandin E~ analogue. Gastroenterology 1986; 9 1 : 3 7 0 - 3 7 8 . 14. Knutson L, Flemstr6m G. Duodenal mucosal bicarbonate secretion in man. Stimulation by acid and inhibition by the alpha-2adrenoceptor agonist, clonidine. Gut 1 9 8 9 ; 3 0 : 1 7 0 8 - 1 7 1 5 . 15. Ainsworth MA, Kjeldsen J, Schaffalitzky de Muckadell OB. Morphine inhibits secretion of bicarbonate from the human duodenal mucosa. Possible role of endogenous opioids in the regulation of human duodenal mucosal bicarbonate secretion. Scand J Gastroenterol 1 9 9 0 ; 2 5 : 1 0 6 6 - 1 0 7 5 . 16. Flemstr6m G. Gastric and duodenal mucosal bicarbonate secretion. In: Johnson LR, ed. Physiology of the gastrointestinal tract. 2nd ed. New York: Raven, 1 9 8 7 : 1 0 1 1 - 1 0 3 0 . 17. Flemstr6m G, Garner A. Gastroduodenal HC03 transport: characteristics and proposed role in acidity regulation and mucosal protection. Am J Physiol 1982;242:G183-G193. 18. Graham DY, White RH, Moreland LW, Schubert TT, Katz R, Jaszewski R, Tindall E, Triadafilopoulos G, Stromatt SC, Teoh LS, The Misoprostol Study Group. Duodenal and gastric ulcer prevention with misprostol in arthritis patients taking NSAIDs. Ann Intern Med 1993; 1 1 9 : 2 5 7 - 2 6 2 . 19. Isenberg JI, Selling JA, Hogan DL, Koss MA. Impaired proximal duodenal mucosal bicarbonate secretion in patients with duodenal ulcer. N Engl J Med 1 9 8 7 ; 3 1 6 : 3 7 4 - 3 7 9 . 20. Bukhave K, Rask-Madsen J, Hogan DL, Koss MA, Isenberg JI. Proximal duodenal prostaglandin E2 release and mucosal bicar-

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Received June 16, 1994. Accepted January 20, 1995. Address requests for reprints to: Kim E. Barrett, Ph.D., University of California, San Diego Medical Center, 8414, 200 West Arbor Drive, San Diego, California 92103-8414. Fax: (619) 543-6969. Supported by grants from the National Institutes of Health (AI24992 and DK33491) and a grant from Glaxo Pharmaceuticals. This work has been published in part in abstract form (Gastroenterology 1993; 104:A228). The authors thank the late Dr. Andr6 Robert and Dr. Jean Rivier for their gifts of reagents and Julie Lessem for her assistance with manuscript preparation.