PEPSIN STIMULATED BY TOPICAL HYDROCHLORIC AND ACETIC ACIDS

Transcription

1 GASTROENTEROLOGY Copyright 1972 by The Williams & Wilkins Co. Vol. 62, No.1 Printed in U.S.A. PEPSN STMULATED BY TOPCAL HYDROCHLORC AND ACETC ACDS LEONARD R. JOHNSON, PH.D. Department of Physiology and Biophysics, University of Oklahoma Medical Center, Oklahoma City, Oklahoma Normally the gastric mucosa is relatively impermeable to hydrogen ions. When, however, the barrier to diffusion is broken, acid enters the mucosa, causing damage. Back diffusion of H+ and damage in dogs produces conditions similar to those existing in patients with gastric ulcer, and it has been suggested that the primary defect in gastric ulcer disease may be an abnormally permeable gastric mucosa. Pepsin secretion during the presence of noninjurious and damaging amounts of hydrochloric and acetic acid was studied in 4 dogs with vagally denervated pouches of the oxyntic gland area. rrigation with 200 mn Hel caused a 40% rise in pepsin output, 250 mn Hel increased pepsin output 75%, and 300 mn Hel caused a 300% increase over control levels obtained during irrigation with 100 mn Hel. Pepsin outputs were directly proportional to the flux of H + out of the pouch. rrigation with 100 mn acetic acid caused a 3-fold increase in pepsin output as well as increases in Na+ and K+ movement into the pouch fluid and H + movement out. ntravenous atropine (100 J.Lg per kg-hr) prevented the acetic acid induced increase in pepsin secretion without significantly altering ion fluxes. t is concluded that: (1) back diffusion of H+ stimulates pepsin output, (2) a local cholinergic reflex is probably involved in this stimulation, and (3) the increased pepsin output is due to actual secretion, The gastric mucosal barrier and the alterations in the physiology of the barrier which occur during damage provide a meaningful model of the events which may lead to the formation of a gastric ulcer. The gastric mucosa is relatively impermeable to hydrogen ions. f, however, the Received July 8, Accepted August 5, These results were presented in part to the American Gastroenterological Association, Miami, Florida, May Address requests for reprints to: Leonard R. Johnson, Ph.D., Department of Physiology and Biophysics, 800 N.E. 13th Street, Oklahoma City, Oklahoma This study was supported by a grant from the G, A. Manahan Trust Fund and by National nstitutes of Health Grant AM The expert technical assistance of Louise H. Wyss is gratefully acknowledged. 33 barrier is weakened, back diffusion of acid occurs and damage results. Davenport l - 3 has shown that damaging the canine Heidenhain pouch (vagally denervated) with acetic or salicylic acid results in excessive movement of hydrogen ion out of the pouch and increased flux of sodium and potassium into the lumen. n addition, fluid accumulates in the pouch along with plasma protein.' f damage is severe, bleeding occurs. During damage, histamine is released into the mucosa 5 and the activity of the enzyme which forms histamine increases. 6 The higher levels of intramucosal histamine probably add to the damage by causing edema, fluid production, and increased mucosal permeability and acid secretion. t has recently been shown that pepsin output increases when pouches are dam-

2 34 JOHNSON Vol. 62, No.1 aged by high concentrations of hydrochloric acid in the presence of a subthreshold (for acid secretion) infusion of pentagastrin. 7 This study demonstrates that this increased pepsin output takes place in the absence of gastrin, is related to the back diffusion of hydrogen ion, and is secretory in nature. Methods Four dogs were surgically prepared with a gastric fistula drained by a Thomas cannula and a vagally denervated (Heidenhain) pouch with a Gregory9 cannula. These particular experiments were begun about 2 years after surgery. The animals were fasted for at least 18 hr but were allowed water before an experiment. At the start of an experiment the dogs were placed on tables where they stood quietly supported, in part, by slings. The gastric fistula was opened and the stomach was rinsed with saline. The fistula was left open for the duration of the experiment to prevent gastric juice from entering the duodenum. The metal cannula of the Heidenhain pouch was connected by rubber tubing to a reservoir with 50 ml of a control solution containing 100 mn HCl and 50 mn NaC. The solution was allowed to enter and leave the pouch influenced only by gravity and pouch contractions. The level of fluid in the reservoir was about 20 cm above the pouch. At the end of each 15-min period the reservoir and the pouch were drained and the solution was replaced with 50 ml of fresh acid. Volumes of recovered fluid ranged from 48 to 51 ml, and, after the first two 15-min periods, which were not included in the experiment, were usually 50 ml. Collections of the control solution continued for four 15-min periods. The pouches were then irrigated with 200 mn HCl, 250 mn HCl, 300 mn HCl, or 100 mn acetic acid plus 50 mn NaC for four periods. n each case the 100 mn HCl control solution was made isosmotic with the test solution by the addition of NaC. A final hour of collections with the control solution ended the experiment. At the beginning of an experiment in the acetic acid series an intravenous infusion of saline was started in a leg vein and maintained for the duration of the experiment at a rate of 60 ml per hr by a Harvard peristaltic pump. On some days atropine was added to the infusion to give a dose of 100 ~g per kg-hr. Other experiments consisted entirely of irrigating the pouch with the control solution. After each 15-min collection the volume of the sample was measured and the acid concentration was determined by titration to ph 7.0 with 0.2 N NaOH on an Autoburette and ph meter (Radiometer, Copenhagen). The Na+ and K + concentrations of each sample were determined by flame photometry (L model 143, nstrumentation Laboratory, nc., Lexington, Mass.). A volume of each sample was immediately diluted so that it contained 50 mn HC. The pepsin concentration of this sample was determined using a modification 10 of the Anson 11 hemoglobin method and expressed as milligrams of pepsin per milliliter by reading the trichloroacetic acid supernatant fluid at 280 m~ and comparing it with solutions incubated with different pepsin standards (Hog pepsin, three times crystallized, Pentex Biochemicals, Kankakee, 111.). Pepsin outputs were calculated, taking into consideration the dilution of the assay, and were expressed as milligrams of pepsin per 15 min. The data from the experiments with hydrochloric acid were expressed as the percentage of the mean of the two control periods during which the pouches were irrigated with 100 mn HCl and 50 mn NaC. This was done to eliminate some of the variability between individual animals which had consistent but different basal levels of pepsin secretion. The mean pepsin value of the periods taken as 100% was 2.8 mg per 15 min. Total amounts of Na+, K +, and H + in the recovered fluid were calculated for each period during the acetic acid experiments. The net flux of each ion was determined by subtracting the amount put in the pouch from the amount recovered and expressed as microequivalents per 15 min. Negative numbers indicate a net loss of an ion from the pouch, i.e., mucosal to serosal movement. Experiments were done on alternate days and no more than three times a week. Within each group of experiments the order was randomly assigned. Results of the HCl study represent the means of three observations in each of four dogs. Following the completion of this series 1 animal developed a leaky pouch and was dropped from the study. Therefore, the acetic acid experiments were run on 3 animals and the data are expressed as the means and standard errors of the means of nine observations. Results rrigation of the pouches with 200 and 250 mn Hel significantly increased pepsin output over that seen in response to 100 mn Hel (P < 0.05, periods 4 and 5, fig. 1). rrigation with 300 mn Hel produced a

3 January 1972 ACD AND PEPSN SECRETON ~ 30 C o u '0 z in Cl. UJ Cl ! 2 Hel V,. " r-'" '1300mN Hel ' 250 mn Hel ~--l mN Hel min PERODS 100mN Hel FG. 1. Pepsin secretion as percentage of control from vagally denervated canine pouches. During periods 3 to 6 the pouches were irrigated with the concentrations of HC indicated in the margin. During the remaining time, the irrigation solution contained 100 mn HC plus NaC to make the solution isotonic with the acid during periods 3 to 6. Means and standard errors of the means are shown for three observations in each of 4 dogs. dramatic 3- to 4-fold increase in pepsin secretion. t can be seen in figure 1 that pepsin secretion was approximately the same during the control periods even though the osmolarity of the solutions differed. These relatively high concentrations of Hel were used to increase the diffusion gradient for H +, thereby causing a flux of H+ through the normal mucosa approximately equivalent to what would be expected if the barrier had been broken by an extraneous agent such as alcohol, acetic acid, or aspmn. These agents were avoided at the outset of the experiments in order to be able to attribute any stimulation in pepsin secretion to H+ itself without having to worry about unphysiological anions or substances like alcohol which could stimulate on their own. Pepsin secretion was directly proportional to the concentration of acid placed in the pouches for 100, 200, and 250 mn Hel (fig. 2). At 300 mn Hel this relationship no longer existed for pepsin output in~ creased out of proportion to the increase in acidity. When, however, the data were replotted, pepsin secretion was found to be directly related to the net flux of hydrogen ion out of the pouch (solid line, fig. 2; r = ). Thus, in addition to a large increase in pepsin secretion in going from 250 to 300 mn Hel, one also finds a concurrent increase in H+ flux. rrigation of the pouches with 100 mn acetic acid (fig. 3) caused a 3-fold increase C 1.2 _-11- ~ 250 E... ~o~ tl Z :::: E ~ >>is CT UJ ~(,e qo z E ~ ("o,~ Q t- ::> :r...j 0 ~ 0.6 <J)...J :: 150 L u ::> t;j Cl. z J 0 i PEPSN OUTPUT as % Control FG. 2. Peak pepsin outputs from figure 1 plotted against the concentration of acid in the irrigating solutions and against the net flux of H + out of the pouch during the corresponding period. 10 J 1/ /1, i HAc E s.o j ::;,.. z 0; 6.0 /1 " '" "- '._r ~ ~ 15 min PERODS FG. 3. Pepsin outputs in response to control period and irrigation with 100 mn acetic acid solutions. n one set of experiments atropine was infused intravenously. Means and standard errors of the means of three observations in each of 3 animals are shown. i

4 36 JOHNSON Vol. 62, No.1 in pepsin output over control periods before and during acetic acid irrigation (i.e., periods 4 and 6, P < 0.01). Following acetic acid irrigation, 100 mn Hel caused a further rise in pepsin secretion (P < 0.01, periods 9, 10, 11, 12) indicating that the barrier to ion diffusion had been disrupted by the acetic acid. Figure 3 also demonstrates that 100 Jlg per kg-hr atropine infused intravenously for the duration of the experiment completely blocked the increase in pepsin secretion in response to acetic acid. The ability of acetic acid to damage the mucosal barrier was relatively unaffected by atropine. This is shown by the similarity in the volume changes and fluxes of Na+, K+, and H+ during intravenous saline TABLE 1. Net changes across Heidenhain pouches of 3 dogs during intravenous saline infusiona Solution Period Volume Na K H' Control: 100 mn HC and 50 mn NaC mn acetic acid and 50 mn NaC mn HC and 50 mn NaC a Net volume and ion fluxes per 15-min period during the acetic acid experiments shown in figure 3. Saline was infused intravenously throughout. Numbers are means of three observations in each of 3 animals. Negative numbers indicate mucosal to serosal flux. ml ~Eq TABLE 2. Net changes across Heidenhain pouches of 3 dogs during intravenous infusion of atropine (100 p,g per kg-hr)a Solution Period Volume Na K H' ml ~Eq Control: 100 mn HC and 50 mn NaC m N acetic acid and 50 mn NaC mn HC and 50 mn NaC a Same as table 1 except that data were taken from the experiments in figure 3 in which atropine was infused throughout.

5 January 1972 ACD AND PEPSN SECRETON 37 (table 1) and atropine (table 2) infusion. n other words, the pepsin output was the only entity altered by atropine. Discussion A previous study demonstrated that high concentrations of Hel could increase pepsin output in the presence of a background intravenous infusion of gastrin. 7 This work shows that pepsin output increases in response to the back diffusion of H + in the absence of gastrin stimulation. f the primary defect in some cases of gastric ulcer is, as Davenport 12 suggests, an abnormally permeable gastric mucosa, the concurrent increase in pepsin output could play a significant causative role in the production of gastric ulcers. Large increases in pepsin output did not occur in response to Hel until sufficient acid was used to break the mucosal barrier to back diffusion. As is readily apparent in figure 2 the flux of H+ did not increase out of proportion to the concentration put into the pouch until 300 mn Hel was used. Therefore, the threshold concentration of Hel which damages the barrier is somewhere between 250 and 300 mn. This is the same approximation arrived at by Davenport. 12 The increased pepsin output in response to nondamaging concentrations of Hel is extremely interesting from the standpoint of the physiological control of pepsin secretion. These data indicate that the normal gastric mucosa contains a mechanism sensitive to H + which stimulates pepsin secretion, and we are currently involved in experiments to determine whether pepsin secretion is stimulated by the presence of physiological amounts of acid within the pouches. The large increase in pepsin output in response to 100 mn acetic acid (fig. 3) indicates that the effect is not peculiar to damage by high (300 mn) concentrations of hydrogen ion. As Davenport 3. 4 has previously demonstrated, this concentration of acetic acid breaks the barrier to ion diffusion, resulting in the loss of H + from the pouch and a net gain of Na+ and K+ similar to what was observed in the current experiments (table 1). Although the barrier was broken by a much lower concentration of acetic acid than Hel, the relationship between pepsin output and H+ back diffusion still holds. During acetic acid irrigation maximal pepsin secretion was roughly 300% greater than the control. This corresponded to a back diffusion of 916 J.LEq of H+. n the flux curve drawn for Hel and pepsin output (fig. 2) a 300% increase in pepsin corresponds to the back diffusion of 900 J.LEq of H +. The observation that atropine blocks pepsin output during damage by acetic acid without affecting ion fluxes indicates that the increased output of pepsin is due to actual secretion. Since damage still occurs with atropine, the absence of the pepsin effect dissociates the source of pepsin from damage per se, that is, sloughed or broken cells. This observation also indicates the involvement of a local intramural cholinergic reflex in the stimulation of pepsin secretion. From these experiments we conclude that (1) the back diffusion of H+ increases the output of pepsin into the canine Heidenhain pouch, (2) the amount of pepsin secreted is directly related to the amount of H + lost from the pouch, (3) the increased pepsin output represents actual secretion, and (4) a cholinergic reflex is involved in the stimulation of this secretion. n addition, it is tempting to speculate that normal amounts of Hel in the stomach may actually enhance the physiological secretion of pepsinogen as well as activate it. REFERENCES 1. Davenport HW: Gastric mucosal injury by fatty and acetylsalicylic acids. Gastroenterology 46: , Davenport HW: Damage to the gastric mucosa: effects of salicylates and stimulation. Gastroenterology 49: , Davenport HW: Potassium fluxes across the resting and stimulated gastric mucosa: injury by salicylic and acetic acids. Gastroenterology 49: , Davenport HW: Fluid produced by the gastric mucosa during damage by acetic and salicylic acids. Gastroenterology 50: , 1966

6 38 JOHNSON Vol. 62, No Johnson LR, Overholt BF: Release of histamine into gastric venous blood following injury by acetic or salicylic acid. Gastroenterology 52: , Johnson LR: The source of histamine released during damage to the gastric mucosa by acetic acid. Gastroenterology 54:8-15, Johnson LR: Pepsin output from the damaged canine Heidenhain pouch. Am J Dig Dis 16: , Thomas JE: An improved cannula for gastric and intestinal fistulas. Proc Soc Exp Bio Med 46: , Gregory RA: Gastric secretory responses after portal venous ligation. J Physiol (Lond) 144: , Northrup JH, Kunitz, M, Herriot RM: Crystalline Enzymes. Second edition. New York, Columbia University Press, 1948, p Anson ML: The estimation of pepsin, trypsin, papain and cathe pepsin with hemoglobin. J Gen Physiol 22:79-89, Davenport HW: Back diffusion of acid through the gastric mucosa and its physiological consequences, Progress in Gastroenterology, vol 2. Edited by GBJ Glass. New York, Grune and Stratton, 1970, p 42-56