GASTROENTEROLOGY. Official Publication of the American Gastroenterological Association. COPTBIGHT 1969 THE W,LLIAMS & W,LDN8 Co.

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1 GASTROENTEROLOGY Official Publication of the American Gastroenterological Association COPTBIGHT 1969 THE W,LLIAMS & W,LDN8 Co. VOLUME 56 April 1969 NUMBER 4 EFFECT OF THE VAGUS NERVE AND SALICYLATE ADMINISTRATION ON THE PERMEABILITY CHARACTERISTICS OF THE RAT GASTRIC MUCOSAL BARRIER BERGEIN F. OVERHOLT, M.D., DAVID A. BRODIE, PH.D., AND BARBARA J. CHASE Section of Gastroenterology, Department of Intemnl Medicine, New York Hospital Cornell Medical Center, New York, New York, and Merck Institute for Therapeutic Research, West Point, Pennsylvania The influence of the vagus nerve and of salicylates on the permeability characteristics and on damage of the rat gastric mucosal barrier was studied. Results indicate the following. (1) The vagus nerve has no direct effect on the maintainence of the permeability characteristics of the gastric mucosal barrier. (2) Back diffusion of acid probably occurs normally in significant quantities. (3) Vagal or other forms of secretory stimulation can mask acid back diffusion by increasing acid output enough to exceed the amount diffusing into the mucosa. (4) Aspirin damages the mucosal barrier of the vagotomized rat stomach as in the vagotomized canine stomach. (5) Aspirin probably damages the vagally intact, secreting stomach without necessarily producing evidence of acid back diffusion. (6) Large doses of parenteral aspirin damage the mucosal barrier. Studies on unstimulated, vagally denervated pouches of the oxyntic gland area of canine stomachs have shown that the gastric mucosa contains a barrier to the diffusion of large quantities of hydrogen Received September 16,1968. Accepted November 15,1968. Address requests for reprints to: Dr. Bergein F. Overholt, New York Hospital-Cornell Medical Center, 525 East 68th Street, New York, New York This study was supported in part by a grant from the John A. Hartford Foundation. The authors gratefully acknowledge the assistance of Dr. Ralph Hirschman, Merck and Company, Rahway, New Jersey, in supplying the gastrin tetrapeptide tmd of Dr. Lawrence J. Fischer, Merck and Company, West Point, Pennsylvania, for performing the scintillation counting. ions from the lumen into the mucosa. Introduction of salicylates, bile salts, or other agents in acid solution into the denervated pouch disrupts the barrier and allows significantly increased diffusion of acid into the mucosa with an accompanying outpouring of fluid sodium and potassium ions. I - 3 Several recent observations have suggested that the vagus nerve might play a role in determining the permeability characteristics of the mucosal barrier. Acid instilled into the stomach of the vagotomized rat produced more mucosal ulcerations than in the sham-operated control, suggesting that vagal activity might influence the mucosal barrier.4 Wlodek's observation that antral stimulation, 651

2 652 OVERHOLT ET AL. Vol. 56, No. 4 whether it be of vagal or other origin, appeared to "tighten" the barrier against back diffusion of acid, also suggested a possible role of the vagus, 5 but other studies demonstrated no effect of the vagus nerve on the canine mucosal barrier. 6 The present study was designed to assess the vagal influence on the maintainence of the permeability characteristics of the gastric mucosal barrier. Materials and Methods Male Holtzman rats weighing between 180 g and 220 g were fasted for 24 hr and anesthetized with sodium pentobarbital, 50 mg per kg intraperitoneally. The esophagus7 and trachea were cannulated with PE 205 (Intramedic) tubing and either vagotomy or sham vagotomy was performed in the neck. The stomach was then exposed through a midline incision and the pylorous ligated. A small incision was made in the rumen and the gastric contents were expressed gently. A 1-inch length of Vivosil tubing (0.011 cm inside diameter; Becton Dickinson and Company no ) was inserted into the rumen through the incision and secured with a ligature. The stomach of each animal was rinsed with 2-ml volumes of the test solution until the return was clear; the last rinse was saved for analysis as the initial sample. The Vivosil tubing was quickly removed, the fluid remaining in the stomach was gently expressed, and the tubing reinserted and secured in place. A 3-ml volume of the test solution containing, as a nonabsorbable marker, 14C-polyethylene glycol (New England Nuclear Corporation, Boston, Mass.; 2 /-IC per liter, specific activity 0.43 mc per g) was pipetted into the stomach. One milliliter was immediately removed by pipette after mixing and saved for 14C counting to determine the initial instilled volume. The tube was clamped and the wound closed. One hour later the gastric contents were recovered. The rats were divided into five experimental situations (table 1) with 8 rats in each group (A to P). 1. Effect of vagotomy on acid secretion. A 154 mm NaCI solution was infused into the stomach in groups A, B, and C to determine the effect of sham vagotomy, vagotomy, and sham vagotomy plus atropine (8 mg per kg subcutaneous, 30 min prior to surgery), respectively, on acid secretion. 2. Effect of vagotomy on acid back diffusion. An acid solution (100 mm HCI, 15 mm NaCI, 78 mm mannitol, 14C-polyethylene glycol 2 /-IC per liter) was instilled into the stomachs of groups D, E, and F to determine the effect of sham vagotomy, vagotomy, and sham vagotomy plus atropine (8 mg per kg subcutaneous, 30 min prior to surgery), respectively, on net ion fluxes across the gastric mucosa. 3. Effect of bethanechol, gastrin tetrapeptide, and histamine stimulation on acid back diffusion in the vagotomized stomach. Immediately following instillation of the above acid solution into the vagotomized stomachs of groups G to J, the rats were injected with bethanechol, 0.4 mg per kg (group G); gastrin tetrapeptide, 8 mg per kg (group H); gastrin tetrapeptide, 16 mg per kg (group n; or histamine base 8 mg per kg (group J). 4. Effect of intragastric acetylsalicylic acid on acid back diffusion in the sham vagotomized and vagotomized stomach. Acetylsalicylic acid (ASA) was added to the acid test solution (20 mm ASA, 100 mm HCI, 15 mm NaCI, 78 mm mannitol, 14C-polyethylene glycol 2 /-IC per liter) to determine the effect of aspirin on net ion movement in the sham vagotomized (group K) and vagotomized (group L) preparations. The ASA concentration in the test solution was increased to 60 mm in group M (sham vagotomized). 5. Effect of parenteral acetylsalicylic acid on acid back diffusion. The same 100 mm HCI acid solution was instilled into groups N to P to determine the effect of intraperitoneal saline on net ion movement (1 ml of 154 mm NaCI 2 hr prior to surgery; group N), ASA, 128 mg per kg (2 hr prior to surgery; group 0), and ASA, 256 mg per kg (2 hr prior to surgery; group P). The ASA was suspended in a 1 % solution of methylcellulose and administered intraperitoneally in equal volume. The total amount of each ion initially instilled and later recovered from the stomach after 1 hr was calculated from the measured initial and recovered test solutions correctej for initial and final residual volumes. From these values net ion movement and volume changes were determined using the equations given below: Net ion movement = net ion recovered - net ion instilled (1) Net volume change = corrected volume recovered - corrected volume instilled (2) Positive values indicated a net gain of ion or

3 April 1969 RA T GASTRIC MUCOSAL BARRIER 653 TABLE 1. Summary of procedures, drugs, and intragastric instillations on groups of rats Group Procedurea Drug Irrigation fluid Experiment 1 A SV 154 mm NaCI B V 154 mm NaCI C SV Atropine, 8 mg/kg, sc b 154 mm NaCI Experiment 2 D SV 100 mm HCle E V 100 mm HCI F SV Atropine, 8 mg/kg, sc 100 mm HCI Experiment 3 G SV Bethanechol, 0.4 mg/kg, sc 100 mm HCI H SV Gastrin tetrapeptide, 8 mg/kg, sc 100 mm HCI I SV Gastrin tetrapeptide, 16 mg/kg, sc 100 mm HCI J , SV Histamine, 8 mg/kg, sc 100 mm HOI Experiment 4d K SV 100 mm HCI:20 mm ASA- L V 100 mm HCI:20 mm ASA M SV 100 mm HCI:60 mm ASA Experiment 5 N SV 154 mm NaCI, 1 ml, ipl 100 mm HCI SV ASA, 128 mg/kg, ip 100 mm HCI P SV ASA, 256 mg/kg, ip 100 mm HCI a SV, sham vagotomy; V, vagotomy. b sc, subcutaneous. e 100 mm HCI = 100 mm HCI, 15 mm NaCI, 78 mm mannitol, and HC-polyethylene glycol, 2j.1c perliter. d Controls, groups D and E. e ASA, acetylsalicylic acid. I ip, intraperitoneal. 3.0 VOLUME CONCE NTRATION meqfl OUTPUT peq (mi.) -120 H H L L + 60 L SV V A sv V A SV V A FIG. 1. Effect of sham vagotomy (SV), vagotomy ( V), and sham vagotomy plus atropine (A) on volume and acid secretion at 1 hr following saline instillation (154 mm NaC!) in the stomach. Results are expressed as mean changes from instilled values. volume; negative values indicate a loss of volume or ion from the lumen. Hydrogen ion concentration was determined electrometrically by titration with 0.01 N NaOH to ph 7.0. Sodium, potassium, and chloride concentrations were measured on the Technicon Autoanalyzer. Salicylate concentration in groups K, L, and M was determined by the method of Brodie et al. 8 In group P the salicylate concentration in plasma and gastric contents was determined by the more sensitive method of Chirigos and Udenfriend. 9 The molar concentration of salicylate was subtracted from the hydrogen ion titration values in appropriate groups (K, L, M). I C counting was performed in the Packard Tri-Carb Liquid Scintillation Spectrometer with internal standard corrections. Statistical analysis was done by computer using the covariance analyses. Results 1. Effect of vagotomy on acid secretion. The effect of medical and surgical vagotomy on acid secretion by the rat is shown in figure 1. In the sham vagotomized control, a net hourly secretion of 238 ~Eq H+ and 1.70 ml was found. In contrast, the vagotomized rat had an acid secretion of only 11.6 ~Eq H+ (P < 0.001) and showed

4 654 OVERHOLT ET AL. Vol. 56, No.4 a net loss of 0.03 ml (P < 0.001) of the gastric volume. The effect of atropinization was similar to surgical vagotomy with a significantly depressed volume and acid output (P < 0.001). 2. Effect of vagotomy on acid back diffusion (fig. 2). Acid instillation in the sham vagotomized rats (group D) was followed by a net hourly increase of 164 ~Eq H+, 104 ~Eq Na+, and 1.70 ml, whereas in the vagotomized rats (group E) there was a significant back diffusion of H+ (~64 ~Eq; P < 0.001) with an increase of Na+ (63 ~Eq) and volume (0.13 ml) (table 2). Similar changes were found in the atropinized rats (group F). 3. Effect of bethanechol, gastrin tetrapeptide, and histamine stimulation on acid back diffusion in the vagotomized stomach (fig. 3). With bethanechol (group G), 0.4 mg per kg, volume, acid concentration, and acid output were significantly increased over the unstimulated, vagotomized control group (P < 0.001). No change in sodium concentration or output was noted. With gastrin tetrapeptide (group H), 8 mg per kg, volume and acid concentration were increased over the control (P < 0.001), but still a net disappearance of acid was found (P < 0.001) even in view of the increased volume of secretion. Sodium output was also increased, but not significantly (P > 0.05) compared to the control. When 16 mg per kg of the tetrapeptide was used, the volume and acid concentration and output were all significantly increased (P < 0.001), thus masking the back diffusion that most certainly was occurring. Sodium output was higher than the control but not significantly increased (P > 0.05). Histamine (group J), 8 mg per kg, produced the greatest increases in volume, acid concentration, and output (P < 0.001) but the least change in sodium fluxes (43 ~Eq Na+). 4. Effect of intragastric acetylsalicylic acid on acid back diffusion in the sham vagotomized and vagotomized stomach. When 20 mm ASA (table 2) was added to the acid test solution in the sham vagotomized rat (group K), the sodium output was significantly greater (P < 0.001) than in control animals (group D) but volume and acid secretion did not change. In contrast, the 20 mm ASA solution produced greater evidence of barrier disruption in the vagotomized (group L) rats with greater back diffusion of H+ (P < 0.01); however, a nonsignificant increase in sodium efflux was found. When the ASA concentration was increased to 60 mm in the sham vagotomized rat (group M), a slight increase in volume of 1.84 ml wa. found; the increment of 175 ~Eq Na+ and only 91 ~Eq H+ suggested back diffusion CONCENTRATION meq/l VOLUME ml ~ ~ ~ ~ OUTPUT }JEQ ~~~_C_I_ t SV V A FIG. 2. Effect of sham vagotomy (SV), vagotomy Oil, ana sham vagotomy plus atropine '(A) on volume and ion changes at 1 hr following intra gastric instillation of an acid solution (see text), Results are means of changes from the instilled solution,

5 April 1969 RA T GASTRIC MUCOSAL BARRIER t:. 1.5 vo~ymf '-----' t:. H O ~--~~~,..r~~~.. ~~r-.. ~~--.. ~~~~--~~ [ ~ ~~---~~~~~~~~~~~~~~ : 1~~f~.u~L-~ ~ mm~.u~~l-~~~~~ ~.u~~l-~~~~ O ~"~~--.. ~~L--- ~~~"~~L-~~~~~"~~ - 50 SHAM SALINE GTP 8.0 GTP 16.0 B 0. 4 H 8.0 DOSE VAG V. V. V. V. V. mg/kg S.C. ; t:. CONCENTRATION meq/l ~ ; t:. OUTPUT ~ JJEQ FIG. 3. Changes in volume (milliliters) and electrolytes (concentrations: milliequivalents per liter; output: microequivalents) following intragastric instillation of a 100 mm HCI solution into the sham vagotomized (Sham Vag), vagotomized (V) or stimulated-vagotomized stomach. (GT? 8.0, gastrin tetrapeptide, 8.0 mg per kg; GT? 16.0, gastrin tetrapeptide, 16.0 mg per kg; B 0.4, bethanechol: HCI 0.4 mg per kg; H 8.0, histamine base 8.0 mg per kg.) Results are means of changes from the instilled solution. TABLE 2. Effect of 100 mal HCI and 100 mal HCI-acetylsalicylic acid solutions on net volume (milliliters) and ion movements (microequivalents) across the gastric mucosa of sham vagotomized (groups D, K, M) and vagotomized rats (groups E, L)a Volume H + Na+ K + Sham Vagotomy Sham I V Sham I Vagotomy Sham I V vagotomy vagotomy agotomy vagotomy vago,tomy agotomy I ml /leg /leg /leq 100 mm HClb 1.70 (D) 0.13 (E) ± ±0.42 ±0.14 ±39 ±16 ±19 ±2.1 ± mm HCI:20 mm 2.13 (K) 0.06 (L) ± ASAc ±0.69 ±0.20 ±114 ±20 ±30 ±5.1 ± mm HCI:60 mm 1.84 (M) ASAd ±0.22 ±60.1 ±36 I ±3.4 a Results are expressed as means with one standard deviation. b 100 mm HCI = 100 mm HCI, 15 mm NaCI, 78 mm mannitol, and 14C-polyethylene glycol, 2 p.c per liter. c 100 mm HCI:20 mm ASA = above acid solution plus 20 millimoles acetylsalicylic acid. d 100 mm HCI:60 mm ASA = above acid solution plus 60 millimoles acetylsalicylic acid.

6 656 OVERHOLT ET AL. Vol. 56, No.4 TABLE 3. Effect of intraperitoneal administration of varying doses of acetylsalicylic acid on net volume (milliliters) and ion movements (microequivalents) across the gastric mucosa of sham vagotomized rats during intragastric instillation of an acid solution (100 mm HCI, 15 mal NaCI, 78 mal mannitol, 14Cpolyethylene glycol, 2 /ole per liter)a Group Volume H+ Na+ K+ CI- I I - - I ml "Eq Saline control.. '",... N 1.78 ± ± ± ± ± 103 Acetylsalicylic acid, 128 mg per kg ± ± ± ± ± 64 Acetylsalicylic acid, 256 mg per kg... P 0.31 ± ± ± ± ± 43 a Results are expressed as means with one standard deviation. of acid and injury. No difference in the net absorption of acetylsalicylic acid was found in the sham vagotomized rat (group K; absorption of 35.1 J.tM) as compared to the vagotomized rat (group L; absorption of 37.1 J.tM). 5. Effect of parenteral acetylsalicylic acid on acid back diffusion (table 3). Acetylsalicylic acid, 128 mg per kg, produced a depression of volume secreted (P < 0.05) and a net gain of only 22 J.tEq H+ (P < 0.01) suggesting back diffusion of acid. Acetylsalicylic acid, 256 mg per kg, definitely reduced secretion (P < 0.001) and a significant back diffusion of acid (P < 0.001) but only a small increase of sodium was found. However, the concentration of intragastric Na+ was significantly increased (62 meq per liter compared to 30 meq per liter for the control; P < 0.001). Blood salicylate levels measured in group P at 1 hr averaged 450 J.tg per ml, but salicylate was not detected in the gastric contents. Mean recovery of the instilled 14C_poly_ ethylene glycol was 103% when the activity of the 6O-min aspirated sample was combined with the activity of the rinse (residual volume) sample, results similar to those of Wlodek and Leach. lo Discussion Work by several individuals has provided evidence for and against a vagal influence on the maintenance of the gastric mucosal barrier. Davenport studied the effect of histamine and bethanechol hydrochloride stimulation on the production of salicylate damage. He found evidence that salicylates damaged the stimulated Heidenhain pouch. II However, the salicylate-induced injury was initiated at the time the stimulant was injected; undoubtedly injury had occurred prior to the establishment of histamine or bethanecol secretion. One would be reluctant, therefore, to say that cholinergic stimulation did not prevent injury. Wlodek and Leach 5 suggested that the vagus nerve might influence the barrier. Based on sodium fluxes, they found that insulin stimulation of gastric secretion from the Pavlov pouch had no effect on "tightening" the barrier to back diffusion (inferred by increased Na+ efflux into the lumen) but that feeding or histamine stimulation did increase the competence of the barrier to prevent acid back diffusion (inferred by less Na+ efflux into the lumen). In a later study, they also found no significant difference in the permeability characteristics of the vagally innervated or denervated canine gastric pouch during acid instillation,6 suggesting that the vagus nerve has no role in determing the characteristics of the mucosal barrier. Brodie and Chase 4 found that vagotomy markedly decreased the incidence of aspirin-induced mucosal ulceration in the rat. However, when acid was instilled into the stomach, mucosal ulceration again occurred. As the pylorusligated rat secretes acid at a high rate (vagotomy markedly reduces the secretion), one might infer that the mucosal irritation seen in the sham vagotomized rat

7 April 1969 RA T GASTRIC MUCOSAL BARRIER (357 is related to the intact vagus nerve, possibly through its stimulatory effect on acid secretion. Our results, similar to those reported by Lindner, show that when acid is instilled into the medically or surgically vagotomized rat, significant back diffusion of H+ occurs. However, in the sham vagotomized preparation, no apparent back diffusion was noted. To determine whether the vagus nerve played a specific role in maintaining the permeability characteristics of the vagotomized barrier, three secretory stimulants were chosen to induce volume and acid output similar to. the control state. With histamine, gastrin tetrapeptide, or bethanechol the back diffusion could be reversed, but only when volume output and concentration were both raised. significantly over vagotomized levels. With the lower dose of the tetrapeptide, significant increases in volume were found but the acid concentration remained low. A net loss of -16 ",Eq H+ was found, almost "reversing" the apparent back diffusion. When the higher dose of tetrapeptide was used, the acid back diffusion was "reversed" with a net gain of 17 ",Eq H+. The sodium values were increased in the stimulated, vagotomized preparations indicating that probable acid back diffusion with an exchange of Na+ for H+ was occurring. These results suggest that acid back diffusion was occurring in the vagotomized preparation whether basal or stimulated, and that with sufficient secretion the back diffusion can be masked. They also provide evidence that the "protective" effect of the vagus is nonspecific and is secondary to the stimulation of a large output of acid which, by its bulk, masks the back diffusion that is occurring. If it is assumed that acid is secreted at 160 meq per liter,14 the secretion of 1.7 ml of gastric juice into the saline or acid-filled stomach should be accompanied by a net addition of 272 ",Eq H+. However, in the saline-filled stomach, only 238 ",Eq H+ was added and in the acid-filled stomach only 164 ",Eq H+. It would appear that back diffusion of 34 ",Eq H+ occurred in the saline preparation and of 108 ",Eq H+ occurred in the acid study. The differences. in the acid,.and saline preparations were m()st likely due to the different concentration gradients: the more acid in the lumen, the greater the back diffusion. In addition,. in the acid-filled, vagally intact stomach, a net increase of 104 ",Eq Na+ was found. Adding the 164 ",Eq H+ and the 104, ",.Eq Na+ yields 268 ",Eq, almost identical with what would be expected with.the secretion of 1.7 ml of gastric juice. In support of Teorell's Na+ : H + exchange theory of acid secretion,15 our results suggest that back diffusion of acid occurs to a significant degree in the secreting stomacn with ex~ change of Na+ for H+. The medically or surgically vagotomized rat stomach is comparable with the Heidenhain pouch in that minimal acid and volume secretion occurs in the unstimulated state (fig. 1). Acid instilled into the vagotomized rat stomach is associated with H+ back diffusion as in the canine. Heidenhain pouch.i,2 Likewise, as in the dog, the back diffusion in the vagotomized rat is significantly increased when salicylatesare added to the instillate. In contrast, when acid and salicylates are added to the sham vagotomized rat, no measurable back diffusion was found, although a slight increase in volume and significant increases in sodium and potassium outputs were detected. If the volume added to the lumen (group K) was acid secreted at 160 meq per liter, one would have expected a net gain of 341 ",Eq H+. An increase of only 177 ",Eq H+ was found, suggesting the back diffusion of 164 ",Eq H+. The H+ was most likely exchanged for Na+ as a net gain of 150 ",Eq Na+ was found. The increase in volume, sodium and potassium outputs, and probable acid back diffusion following acid-salicylate instillation in the vagally intact rat are similar to the changes found in human studies following salicylate instillation. 16 The results are also similar to those found in the stimulated, acid-salicylate filled Heidenhain pouch.ll This provides suggestive evidence of

8 658 OVERHOLT ET AL. Vol. 56, No. 4 aspirin-induced Injury to the innervated or secreting mucosa with the occurrence of "masked" acid back diffusion and indicates that in the secreting stomach, aspirin injury to the mucosal barrier can occur without demonstrable acid back diffusion. To determine whether a net back diffusion could be produced with aspirin, the concentration of the instilled acetylsalicylic acid was increased to 60 mm. Although a net addition of acid was found in the lumen, it was much less than in the control rat and suggests greater back diffusion. As with the lesser concentrations of aspirin, the marked increase in Na+ in the lumen is further evidence for back diffusion and injury to the barrier. The amount of salicylate absorbed was not influenced by vagotomy. The sham vagotomized rat absorbed a mean value of 35.1 Ilmoles of acetylsalicylic acid whereas the vagotomized preparation absorbed 37.1 Ilmoles. These results agree with those of Davenport et al. I7 who found no difference in the net absorption of weak acids in the stimulated and unstimulated Heidenhain pouch. Parenteral salicylates have been shown to produce bleeding from the gastrointestinal tract,18 but their effect on the mucosal barrier had not been previously determined. In the present study, acetylsalicylic acid at high doses in the vagally intact rat stomach produced changes of injury reflected in the altered permeability characteristics of the barrier. The salicylates were not detected in the gastric contents of the rats receiving intraperitoneal aspirin. It thus appears that salicylates can damage the gastric mucosal barrier through a mechanism not dependent upon direct surface contact with the mucosa. REFERENCES 1. Davenport, H. W Gastric mucosal injury by fatty and acetylsalicylic acids. Gastroenterology 46: Davenport, H. W., H. A. Warner, and C. F. Code Functional significance of gastric mucosal barrier to sodium. Gastroenterology 47: Davenport, H. W Destruction of the gastric mucosal barrier by detergents and urea. Gastroenterology 54: Brodie, D. A., and B. J. Chase Evaluation of gastric acid as a factor in drug-induced gastric hemorrhage in the rat. Gastroenterology 56: Wlodek, G. K., and R. K. Leach Effects of histamine, feeding, and insulin hypoglycemia on net ionic fluxes in gastric pouches. Arch. Surg. (Chicago) 93: Wlodek, G. K., and R. K. Leach The effect of histamine stimulation on the net ionic fluxes in Heidenhain and Pavlov fundic pouches. Canad. J. Surg. 10: Brodie, D. A., and P. G. Knapp The mechanism of the inhibition of gastric secretion produced by esophageal ligation in the pylorus-ligated rat. Gastroenterology 50: Brodie, B. B., S. Udenfriend, and A. F. Coburn The determination of salicylic acid in plasma. J. Pharmacol. Exp. Ther. 80: Chirigos, M. A., and S. Udenfriend A simple fluorometric procedure for determining salicylic acid in biologic tissues. J. Lab. Clin. Med. 54: Wlodek, G. K., and R. K. Leach A method for the continuous measurement of net ionic fluxes in gastric pouches. Canad. J. Surg. 9: Davenport, H. W Damage to the gastric mucosa: effects of salicylates and stimulation. Gastroenterology 49: Lindner, A. E Electrolyte changes in rat stomachs following instillation of acid solutions. Amer. J. Physiol. 207: Lindner, A. E Effect of histamine on electrolyte changes in solutions instilled in rat stomachs. Amer. J. Physiol. 210: Hollander, F Gastric secretion of electrolytes. Fed. Proc. 11: Teorell, T Electrolyte diffusion in relation to the acidity regulation of the gastric juice. Gastroenterology 9: Overholt, B. F., and H. M. Pollard Acetylsalicylic acid and ionic fluxes across the gastric mucosa of man. Gastroenterology 54: Davenport, H. W., W. S. Rehm, and B. F. Overholt Absorption through unstimulated and secreting canine oxyntic glandular mucosa. Proc. Soc. Exp. Bioi. Med. 126: Grossman, M. I., K. K. Matsumoto, and R. J. Lichter Fecal blood loss produced by oral and intravenous administration of various salicylates. Gastroenterology 40: