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Aspirin causes peptic ulcers predominately by reducing gastric mucosal cyclooxygenase (COX) activity and prostaglandin synthesis. Because aspirin circulates for only a few hours, we hypothesized that aspirin's inhibitory effect on gastric COX activity must be prolonged. We performed a placebo-controlled experiment in healthy humans to determine the duration of inhibition of aspirin on gastric mucosal COX activity (PGE2 and PGF2α synthesis rates). Recovery of gastric COX activity after stopping aspirin was slow and linear. Seventy-two hours after 325-mg aspirin, gastric COX activity was still reduced by 57% ( P < 0.001). Duration of inhibition of gastric COX activity was estimated to be 7–8 days after 325-mg aspirin and 5 days after 81-mg aspirin. Recovery of gastric prostaglandin synthesis after 325-mg but not after 81-mg aspirin occurred at slower rates in subjects with Helicobacter pylori-associated gastritis than in those with normal histology. In conclusion, aspirin inhibits gastric COX activity for much longer than predicted from its pharmacokinetic profile, explaining why aspirin at widely spaced intervals is ulcerogenic.

Gastric acid secretion, gastrin release, gastric emptying, and gastroesophageal acid reflux were measured in asymptomatic individuals before and after elimination of Helicobacter pylorigastritis. After basal gastric acid secretion and serum gastrin concentrations were measured, meal-stimulated gastric acid secretion and gastrin release were assessed during in vivo intragastric titration to pH 3. Experiments were repeated 4 wk after treatment with lansoprazole, amoxicillin, and clarithromycin. Esophageal pH was also monitored for 24 h before and after therapy. Basal gastric acidity increased ∼20 mmol/l in subjects whose infection was eradicated ( P < 0.05) but not in those with persistent infection. Basal and meal-stimulated gastric acid secretion did not change after H. pylorieradication, despite a 41% reduction in meal-stimulated gastrin release ( P < 0.05). Gastroesophageal acid reflux increased two- to threefold after successful treatment ( P < 0.05) but did not change in subjects with persistent infection. Thus elimination of H. pylori gastritis increases gastric acidity, probably by reducing nonparietal alkaline secretion, and this may facilitate gastroesophageal acid reflux.

We examined the relationship between gastric HCO3- and Na+ secretion under fasting and sham-fed conditions in nine healthy men and also evaluated the effect of the carbonic anhydrase inhibitor acetazolamide on gastric secretion of HCO3- and Na+. Secretion of H+, K+, and Cl- were also measured. Gastric HCO3- secretion rates under fasting and sham-fed conditions closely paralleled Na+ secretion rates. A maximally tolerated intravenous dose (10 mg/kg) of acetazolamide significantly inhibited H+, Cl- and K+ secretion but did not significantly affect Na+ or HCO3- secretion. Thus the gastric mucosa secretes HCO3- and Na+ in parallel in humans both under fasting and sham-fed conditions. Relative to parietal secretion of HCl, nonparietal secretion of HCO3- and Na+ is resistant to carbonic anhydrase inhibition.

The partial pressure of CO2 (PCO2) in gastric juice often exceeds the PCO2 of blood. CO2 in gastric juice may originate from blood and enter luminal fluid by diffusion, or CO2 may be produced in the lumen of the stomach from the reaction of HCO3- and H+. Because CO2 production from HCO3- is dependent on acid (low pH), we suppressed acid secretion with intravenous cimetidine to estimate to what extent appearance of CO2 in luminal fluid is due to production from HCO3-. When denervated fundic pouches of dogs were distended with saline, the PCO2 of the solution increased to the PCO2 of blood in approximately 20 min, with the initial rate of appearance of CO2 in the pouch solution only minimally affected by cimetidine. Thereafter, PCO2 of luminal fluid continued to increase to 50-60 mmHg in the absence of cimetidine, whereas PCO2 of luminal fluid remained approximately equal to that of blood when cimetidine was infused (P less than 0.001, cimetidine vs. control). The mean pH in the pouch solution remained between 6.3 and 6.9 during cimetidine infusion but decreased to 4.75 without cimetidine (P less than 0.001). In additional experiments, an acidic solution with high PCO2 (242 +/- 3 mmHg) was infused into the fundic pouches. PCO2 in luminal fluid decreased rather slowly toward plasma PCO2, requiring 240 min for luminal fluid PCO2 to decrease to 56 +/- 2 mmHg. Thus the permeability of the gastric mucosa to luminal CO2 was relatively low.(ABSTRACT TRUNCATED AT 250 WORDS)

We evaluated the effect of two beta-adrenergic agonists, isoproterenol (nonselective agonist) and terbutaline (selective beta 2-agonist), on gastric acid secretion stimulated by intravenous pentagastrin, bethanechol, or histamine in dogs with gastric fistulas. Intravenous infusion of isoproterenol or terbutaline inhibited pentagastrin-stimulated acid secretion to a significantly greater extent than they inhibited bethanechol- or histamine-stimulated acid secretion. For example, isoproterenol (12 micrograms X kg-1 X h-1) reduced mean pentagastrin-, bethanechol-, and histamine-stimulated acid output by 86, 63, and 14%, respectively. Percent inhibition of acid secretion with terbutaline (30 micrograms X kg-1 X h-1) averaged 60, 17, and 24% for pentagastrin, bethanechol, and histamine, respectively. Terbutaline also inhibited pentagastrin-stimulated acid secretion from vagally denervated fundic pouches in a dose-related manner. Plasma somatostatin-like immunoreactivity was significantly higher during infusion of terbutaline plus pentagastrin than during infusion of pentagastrin alone. However, an intravenous infusion of 0.3 microgram X kg-1 X h-1 somatostatin-14 had no effect on pentagastrin-stimulated acid secretion from the gastric fistula, even though this infusion increased plasma somatostatin-like immunoreactivity to the same extent as terbutaline plus pentagastrin infusion. Thus the amount of somatostatin released during terbutaline infusion was not sufficient to explain the inhibition of pentagastrin-stimulated acid secretion observed.