Potent Antisecretory Agents Research Articles

Helicobacter pyloriInfection and Upper Gastrointestinal Disorders

It has been thirty years since the two Australians Robin Warren and Barry Marshall discovered Helicobacter pylori (H. pylori) in 1983 [1]. In order to fulfil the Koch's postulates, Marshall and Morris drank a solution which was a suspension of H. pylori. This produced gastritis from which the bacteria could be reisolated [2]. It will be interesting to see how the approach to treatment has progressed in these three decades after the discovery of the organism-based both on consensus guidelines and other research in this field. The changes in the consensus statements have been highlighted. The isolation of H. pylori from the gastric mucosa and the report of the organism's urease activity generated excitement especially when it was postulated by Marshall that these microorganisms could be the cause of gastritis and could be a dominant etiological factor in the pathogenesis of peptic ulcer disease (PUD). With the isolation of H. pylori, floodgates opened to a new era of discovery and understanding of gastroduodenal pathology. These results were a paradigm shift from the earlier belief that PUD disease was related to stress, lifestyle, and acid secretion based on the dictum of Schwarz “no acid no ulcer.” The early nineteen eighties when Warren and Marshall reported their findings coincided with omeprazole belonging to the group of proton pump inhibitors (PPIs) being introduced. This PPI was documented as a potent antisecretory agent which yielded very good results for ulcer healing and achieving a potential cure for patients with PUD when compared to the earlier drugs belonging to the group of H2 receptor antagonists. Hence there was lot of scepticism in the gastroenterology community world over to accept that PUD was the result of infection. However, it was found that patients with PUD continued to have remission of the disease even after cessation of antisecretory therapy.

Attenuation of stress-induced gastric lesions by lansoprazole, PD-136450 and ranitidine in rats

Combining restraint with cold temperature (4°C) consistently induces gastric ulceration in rats after 3.5 h. The cold restraint-stress (CRS) method provides a suitable model for acute ulcer investigations. This study compares the antiulcer activities of lansoprazole (a proton pump inhibitor), PD-136450 (CCK(2)/gastrin receptor antagonist) and ranitidine (histamine H(2) receptor antagonist) on CRS-induced gastric ulcers in rats. The results have shown that lansoprazole, which is a potent anti-secretory agent, provides complete protection in this model of ulcer formation. The use of indomethacin pretreatment to inhibit the prostaglandin (PG) synthesis and N(G)-nitro L-arginine methyl ester (L-NAME) pretreatment to inhibit nitric oxide synthase did not alter the lansoprazole-induced inhibition of ulcer index obtained in the untreated Wistar rats indicating that these two systems were not involved in the activation of lansoprazole. PD-136450, an effective anti-secretory agent against gastrin- but not dimaprit-induced stimulation, evoked a dose-dependent inhibition of CRS-induced gastric ulcers. The results show that both PG and nitric oxide pathways can influence the inhibitory effect of PD-136450 against CRS-induced gastric ulcer. The antiulcer activities of both lansoprazole and PD-136450 were compared to that of ranitidine. The results showed that ranitidine was more potent than lansoprazole and PD-136450 in inhibiting CRS-induced gastric ulcers and its effect was shown to be influenced by PG as well as nitric oxide synthase. The results of this study have demonstrated that although lansoprazole, PD-136450 and ranitidine were protective against CRS-induced gastric ulcers, the antiulcer activities of PD-136450 and ranitidine involved both PG and nitric oxide pathways, while lansoprazole acted independently of these two systems during CRS.

False-Positive Secretin Stimulation Test for Gastrinoma Associated With the Use of Proton Pump Inhibitor Therapy

We report the case of a 22-year-old woman who was referred for evaluation of possible Zollinger-Ellison syndrome because of hypergastrinemia and a positive secretin stimulation test. She was being treated with proton pump inhibitors (PPIs) for severe gastroesophageal reflux disease during her initial evaluation. At cessation of PPI therapy, her fasting serum gastrin levels normalized, as did her response to secretin injection. Previous reports describing false-positive secretin tests have been limited to cases of hypergastrinemia in the setting of chronic atrophic gastritis, presumably a result of achlorhydria. This case represents a clearly documented instance of PPI-related hypergastrinemia with a false-positive secretin test, with subsequent normalization of serum gastrin and a negative secretin test after withdrawal of PPI therapy. The current case emphasizes the need to assess the acid secretory status of individuals with hypergastrinemia and to discontinue the use of potent antisecretory agents, principally PPIs, to avoid the erroneous diagnosis of gastrinoma and before embarking on expensive and potentially invasive evaluations for the purpose of tumor localization.

The Role of Chemoprevention in Barrett Esophagus and Esophageal Adenocarcinoma

Esophageal adenocarcinoma (EAC) is a highly lethal cancer and is the most rapidly increasing cancer in the Western countries. In the United States, the incidence of EAC has increased 6-fold and despite recent advances, the 5-year survival is appalling. The high proportion of patients presenting with advanced disease, the poor response to therapy, and low survival rates warrants identification of potential agents for chemoprevention. Barrett esophagus (BE) is a well-established premalignant condition for the development of EAC. A better understanding of the pathogenesis and development of strategies to halt progression along the metaplasia-dysplasia-carcinoma sequence are desirable. The utility of rigorous acid inhibition with proton pump inhibitors (PPIs) as chemopreventive agents has been contested. Acid exposure increases cell proliferation and decreases apoptosis in BE and EAC. Therefore, preventing exposure of the esophageal mucosa to gastric acid by potent antisecretory agents may decrease cancer risk in BE patients. Studies have shown that PPI therapy does not lead to reliable regression of BE. Although, recent studies suggest decreased risk of dysplasia with PPIs, the effect of PPIs on progression of BE is unclear. There is epidemiologic and experimental evidence to suggest that chemoprevention using nonsteroidal anti-inflammatory drugs, aspirin, and selective cyclo-oxygenase-2 (COX-2) inhibitors may reduce the risk of cancer in BE patients. Their chemopreventive action is mediated by inhibition of COX-2 and resultant decrease in eicosanoid-prostaglandin E2 production. Increased COX-2 expression has been demonstrated in BE including a progressive increase along the metaplasia-dysplasia-carcinoma sequence contributing to increased cell proliferation, decreased apoptosis, and increased angiogenesis. Increased cardiovascular toxicity associated with prolonged use of selective COX-2 inhibitors tempers enthusiasm for any further investigation of this agent class in this context. Results of ongoing studies investigating effect of aspirin with PPI therapy on neoplastic progression in BE are awaited. Other agents that have shown promising initial results include troglitazone, a peroxisome proliferators-activated receptor gamma ligand that reduces ornithine decarboxylase activity; telomerase inhibitors that induce apoptosis; and all transretinoic acid that induces apoptosis as well. Large randomized controlled prospective clinical studies are needed to validate the effect of chemopreventive strategies in high-risk populations; results of which could profoundly change the natural history and management of this disease.

Eradication Therapy with Rabeprazole versus Omeprazole in the Treatment of Active Duodenal Ulcer

Background: Rabeprazole has been demonstrated to be a potent antisecretory agent and has been shown to be clinically effective in the treatment of acid-related diseases. Aims: It was to determine the efficacy of rabeprazole at 20 and 40 mg in addition to amoxicillin and clarithromycin in the treatment of active Helicobacter pylori-positive duodenal ulcers compared with omeprazole 40 mg. Patients and Methods: One hundred and twenty-seven patients were randomised into three treatment groups: 40 patients were treated with rabeprazole 40 mg daily, 42 patients with rabeprazole 20 mg daily and 45 patients with omeprazole 40 mg daily for 10 days. All patients received amoxicillin 1 g twice a day and clarithromycin 500 mg twice a day for 5 days. All patients were re-assessed at least 4 weeks after the end of the treatment. Results: According to the intention-to-treat (ITT) protocol, ulcer healing was observed in 90% of patients in the rabeprazole 40 group, in 85.7% in the rabeprazole 20 group and in 93.3% in the omeprazole 40 group. We observed H. pylori eradication in 90% ITT in the rabeprazole 40 group, in 80.9% ITT in the rabeprazole 20 group and in 88.8% ITT in the omeprazole 40 group. Statistical analysis did not show significant differences among the three groups. Conclusions: A 10-day rabeprazole 20 mg regimen represents an efficacious and safe regimen for H. pylori eradication and ulcer healing.

Pharmacological and therapeutic properties of lafutidine (stogar and protecadin), a novel histamine H2 receptor antagonist with gastroprotective activity

Potent antisecretory agents, such as histamine H2-receptor antagonists and proton pump inhibitors, have achieved great improvement in peptic ulcer therapy. It has, however, been reported that incidence of ulcer relapse is high after discontinuation of these drugs. Insufficient efficacy against NSAID-induced ulcers is also critical. Lafutidine is a novel histamine H2 antagonist with gastroprotective activity. Lafutidine exhibited potent and long-lasting H2 antagonism and prolonged antisecretion. In addition, lafutidine showed a gastroprotective effect against noxious agents-induced gastric mucosal damage through capsaicin-sensitive afferent nerves. Lafutidine showed antiulcer activities against acute ulcer models, prevented gastric ulcer relapse of acetic ulcer, and accelerated the healing of indomethacin-induced antral ulcers in rats. These results suggest the advantage of the combined antisecretory and gastroprotective activities. In clinical studies, lafutidine showed prolonged antisecretion, healing effect against gastric and duodenal ulcers and gastritis, and its potency was equal or superior to that of conventional H2 antagonists. Additionally, lafutidine induced a high transition rate to the E0 stage determined by endoscopical ultrasonography, suggesting the high quality of ulcer healing. Furthermore, effectiveness of lafutidine against NSAIDs-induced ulcer was high. From these results, lafutidine is equal or superior to conventional H2 antagonists in antiulcer potency, and it may be useful for the prevention of ulcer relapse and or treatment of NSAIDs-induced gastroduodenal damage.

Zollinger-Ellison Syndrome: A Case Report

Zollinger-Ellison syndrome (ZES) involves hypergastinemia produced by a gastrin-secreting tumor. Not only can it cause an ulcer but may also behave as a malignant lesion, metastasizing to the liver or other organs. The development of potent antisecretory agents for controlling acid secretion as well as techniques for localizing these islet cell tumors, has led to greatly improved survival rates. We describe a case of Zollinger-Ellison syndrome, emphasising the radiologic findings, and including a review of the literature.

Identification and characterisation of heterogeneous somatostatin binding sites in rat distal colonic mucosa.

We have previously shown that the somatostatin (SRIF) sst2 receptor-selective peptide, BIM-23027, is a potent antisecretory agent in rat isolated distal colonic mucosa (RDCM) and in radioligand binding studies in RDCM membranes, it only maximally inhibited approximately 40% of [125I]-Tyr11-SRIF-14 binding (McKeen ES, Feniuk W, Humphrey PPA (1995) Naunyn-Schmiedeberg's Arch Pharmacol 352:402-411). The aim of this study was to characterise the BIM-23027-sensitive and -insensitive SRIF binding sites in more detail and to compare their properties with those of the recombinant sst2 receptor stably expressed in mouse fibroblast (Ltk-) cells. SRIF-14, SRIF-28, CGP-23996 and D Trp8-SRIF-14 abolished [125I]-Tyr11-SRIF-14 binding (pIC50 values, 8.7-9.7) but the competition curves had Hill slopes which were less than unity. Octreotide and L-362,855 inhibited binding over a wide concentration range (0.1 nM-1 microM) and inhibition of binding was incomplete at the highest concentration studied. BIM-23056 (pIC50 < 6.5) was a weak inhibitor of [125]-Tyr11-SRIF-14 binding. GTP gamma S decreased [125I]-Tyr11-SRIF-14 binding by 40%. Further binding experiments with [125I]-Tyr11-SRIF-14 were carried out in RDCM in the continuous presence of BIM-23027 (1 microM). Under these conditions, seglitide had no effect on [125I]-Tyr11-SRIF-14 binding at concentrations up to 10 microM, whilst SRIF-14 and SRIF-28 abolished specific [125I]-Tyr11-SRIF-14 binding in a manner which was consistent with the ligand binding to two sites. SRIF-14 and SRIF-28 displayed high affinity (pIC50 values of 7.8 and 7.3) for the remaining sites. Octreotide, L-362,855 and BIM-23056 were weak inhibitors of [125I]-Tyr11-SRIF-14 binding (pIC50 < 6.5). [125I]-BIM-23027 labelled a single population of SRIF binding sites in RDCM membranes and mouse fibroblast (Ltk-) cells stably expressing the human recombinant sst2 receptor. There was a significant correlation between the affinity estimates of a range of SRIF analogues at inhibiting [125I]-BIM-23027 binding in RDCM membranes and binding to the recombinant sst2 receptor in Ltk- cells, suggesting that the sites labelled by [125I]-BIM-23027 in RDCM are similar to the sst2 receptor. GTP gamma S (100 microM) decreased [125I]-BIM-23027 binding in RDCM by 60%. The results from these studies demonstrate that [125I]-Tyr11-SRIF-14 labels a heterogeneous population of high affinity SRIF binding sites in RDCM membranes. The majority of these sites are insensitive to GTP gamma S and display negligible affinity for the cyclic hexapeptides, BIM-23027 and seglitide. The remaining high affinity binding sites can be selectively labelled with [125I]-BIM-23027, are sensitive to GTP gamma S and show similar characteristics to the recombinant sst2 receptor which appears to mediate the antisecretory effects of SRIF in the mucosa (McKeen ES, Feniuk W, Humphrey PPA (1995) Naunyn-Schmiedeberg's Arch Pharmacol 352:402-411).

Somatostatin receptors mediating inhibition of basal and stimulated electrogenic ion transport in rat isolated distal colonic mucosa

The aim of this study was to examine the potencies of several recently identified selective somatostatin (SRIF)-receptor ligands as inhibitors of electrogenic ion transport in the rat distal colonic mucosa with the view to identifying the SRIF receptor type involved. Under basal conditions, cumulative administration of SRIF and SRIF28 decreased short circuit current (SCC), a measure of electrogenic ion transport, with EC50 values of 4 nM and 9 nM respectively. The peptidase inhibitors, phosphoramidon (1 microM) and amastatin (10 microM), has no effect on the potencies of either SRIF or SRIF28. The inhibitory action of SRIF on basal SCC was suppressed by piretanide and diphenylamine-2-carboxylate, compatible with the assumption that the Na+K+2Cl- co-transporter and Cl- channels, respectively, may be involved in this antisecretory action of SRIF. Tetrodotoxin (1 microM) had no effect on the antisecretory action of SRIF, suggesting that the process was not neuronally mediated. All of the SRIF analogues examined, with the exception of BIM-23056, maximally inhibited basal SCC to a similar extent as SRIF. Seglitide and octreotide were both more potent antisecretory agents than SRIF (respective EC50 values, 0.4 nM and 1.5 nM) suggesting that this effect was mediated by a receptor belonging to the SRIF1 receptor group. The most distinguishing feature of the rank order of agonist potencies was the high potency of the selective sst2 receptor ligand, BIM-23027 (EC50 value 0.32 nM), the weaker potency exhibited by the selective sst5 receptor ligand, L-362855 (EC50 value 21 nM), and the lack of agonist activity displayed by the selective sst3 receptor ligand, BIM-23056 (EC50 value > 1000 nM). This profile is comparable with that observed in binding studies on the recombinant sst2 receptor. Forskolin-stimulated secretion was suppressed by SRIF analogues with the rank order of agonist potencies BIM-23027 > SRIF > L-362855 >> BIM-23056 which resembled that exhibited under basal conditions. However, the absolute potencies of these agonists were lower (respective EC50 values 2 nM, 14 nM< 38 nM and > 1000 nM) whilst the magnitude of inhibition was about three fold greater. BIM-23027 and SRIF (both 30 nM) also inhibited carbachol-stimulated increases in basal SCC by 60-70%, while a similar concentration of L-362855 inhibited these responses by 11%. BIM-23056 (1 microM) had no effect on carbachol-simulated secretion. Radioligand binding studies on rat colonic mucosal membranes using [125I]-Tyr11-SRIF suggested heterogeneity of SRIF binding sites. Thus, SRIF and SRIF28 competed for binding (IC50 values, 0.32 and 0.63 nM, respectively) with Hill slopes less than unity; while seglitide and BIM-23027 both maximally displaced only 30-40% of specific binding with apparent high affinity (respective pIC50 values, 10.1 nM and 10.0). In conclusion, SRIF decreases basal as well as both cAMP and Ca(2+)-dependent Cl- secretion in rat colonic mucosa. The rank order of agonist potencies suggests that receptors resembling the recombinant sst2 receptor mediate inhibition of basal and forskolin-stimulated secretion. Radioligand binding studies suggest that BIM-23027 interacts with a sub-population of [125I]Tyr11-SRIF binding sites in rat colonic mucosal membranes which probably corresponds to the receptors mediating the antisecretory effects described here.

Alternatives to the acid-oriented approach to ulcer disease: does 'cytoprotection' exist in man? A new classification of antiulcer agents.

This review summarizes the historical contradictions and inconsistencies that form the labile arguments advocating neutralization or inhibition of secretion of gastric acid for the prevention or treatment of gastroduodenal ulcers. Re-evaluation of old concepts is needed in the wake of recognition that even the most potent antisecretory agents do not change the natural history of ulcer disease; that is, the recurrence is high after termination of treatment. New biochemical, functional, and structural targets are listed for pharmacologic intervention in ulcer disease. As a supplement or alternative to the antisecretory agents, we should now consider prosecretory agents (for example, for bicarbonate and mucus secretion) and antioxidants (for example, free radical scavengers). Gastroduodenal motility, smooth muscle, the vascular endothelial cell, and the basement membrane seem to represent additional pharmacologic targets toward which new gastroprotective drugs can be directed even though the biochemical mechanism of action of these new agents may not be fully understood. New results suggest that these elements have a role in the pathogenesis of ulcer disease, and their modulations seem to exert a beneficial effect without inhibiting gastric secretion in rodents. In man, the acid antisecretory and cytoprotective doses seem to overlap, but arguments are presented to shift defining gastric 'cytoprotection' by the dose of drugs to the characterization of the phenomenon (for example, events such as the ethanol-induced hemorrhagic erosions which cannot be decreased by antisecretory agents). Furthermore, non-prostaglandin and non-H2-receptor antagonist drugs are available that exert acid-independent gastroprotection both in animals and humans. The future is thus bright for the development of new antiulcer agents.

Zollinger-Ellison syndrome. Current concepts in diagnosis and management.

The Zollinger-Ellison syndrome, although uncommon, is not rare, and most patients with the disorder present with clinical manifestations similar to those of patients with common peptic ulcer. Early studies emphasized death due to complications of massive gastric acid hypersecretion. However, with the availability of potent antisecretory agents to control acid secretion, death is now more frequently associated with the metastatic potential of slowly growing but malignant gastrinomas. Therefore, physicians should maintain a high degree of suspicion of the Zollinger-Ellison syndrome in assessing patients with either chronic peptic ulcer or unexplained secretory diarrhea. An evaluation aimed at early diagnosis of the Zollinger-Ellison syndrome should be instituted in such patients and should begin with a determination of the fasting serum gastrin level. At least 50 percent of patients with gastrinoma will have nondiagnostic serum gastrin concentrations and will therefore require provocative testing to establish the correct diagnosis. After the presence of the syndrome is established, patients should be treated with a potent antisecretory agent in doses sufficient to reduce basal acid output to less than 10 mmol in the hour preceding administration of the next dose. Although some patients may be maintained satisfactorily in this manner for extended periods, an approach aimed at tumor localization and extirpation is recommended in most patients. Preoperative evaluation should begin with CT scanning with intravenous contrast material. Selective angiography, and occasionally, portal venous sampling for gastrin, should be performed if the location and extent of tumor remain in question. If metastatic disease is demonstrated, or if MEN-I is present, surgery aimed at tumor resection, although it is occasionally effective, will probably be unsuccessful. Because of the considerable morbidity and mortality associated with pancreatoduodenectomy, it should not be performed for unresectable tumor in the head of the pancreas. In other patients with the Zollinger-Ellison syndrome, exploratory surgery should be performed; this should include a careful search for, and resection of, all pancreatic and extrapancreatic gastrinomas. With this approach, it is likely that at least 20 percent of all patients with the Zollinger-Ellison syndrome can be cured.

The refractory ulcer.

AbstractThe termrefractoryis applied to those ulcers that fail to heal despite 3 months of active treatment, recur within 1 year despite maintenance therapy with H2‐receptor antagonists, or, in the untreated state, persist without remission. Approximately 20–30% of ulcers fulfill 1 of these 3 criteria.Factors often associated with a refractory ulcer include large ulcer size, gastric outlet obstruction, hour‐glass deformity, penetration, Candida infection, and cimetidine resistance. Incomplete healing of the ulcer and/or initial treatment with H2‐receptor antagonists may predispose to a refractory long‐term course.Successful treatment of the refractory ulcer correlates more with duration of treatment than with the drug used. The more potent antisecretory agents, such as omeprazole, may be the drug of choice. The role of smoking and other environmental factors is not clear.

Sucralfate Overcomes Adverse Effect of Cigarette Smoking on Duodenal Ulcer Healing and Prolongs Subsequent Remission

A unicenter, single-blind, randomized study was conducted on 283 patients with active duodenal ulcer to compare possible factors that may affect healing and relapse in patients treated with a potent antisecretory agent, cimetidine, or a site-protective and cytoprotective agent, sucralfate. The endoscopic healing rates at 4 wk were 76% and 79%, respectively, and cross-over treatment of the failures for a further 4 wk resulted in 68% healing with cimetidine and 69% healing with sucralfate, both differences being not statistically different. Unlike cimetidine, healing by sucralfate was unaffected by cigarette smoking, reluctance to give up smoking, habitual use of alcohol, high maximal acid output, and large ulcer diameter. In particular, the healing rate of smokers treated with sucralfate (82%) was significantly greater than that of smokers treated with cimetidine (63%). Duodenal bulb deformity significantly affected healing in both groups, and was the only offsetting factor identifiable for sucralfate out of 46 factors examined. Of the patients with healed ulcers, 238 participated in a 24-mo follow-up study consisting of interviews at 2-mo intervals and endoscopy at 4-mo intervals or whenever symptoms recurred. The cumulative relapse rate was significantly (p less than 0.007) greater in patients healed with cimetidine than with sucralfate, 50% relapse occurring at 6 and 12 mo, respectively. In both, the cumulative relapse rate was significantly greater in cigarette smokers than in nonsmokers, but smokers and nonsmokers treated with cimetidine relapsed (50% at 4 and 8 mo, respectively) faster than the corresponding smokers and nonsmokers treated with sucralfate (50% at 8 and 18 mo, respectively). Furthermore, in cimetidine- but not sucralfate-healed patients, early ulcer relapse (within 6 mo) was associated with short duration of illness, short remission period, long symptomatic spell, and reluctance to give up smoking. We conclude that smoking adversely affects duodenal ulcer healing by cimetidine and hastens subsequent relapse, and that sucralfate overcomes the adverse effect of smoking on healing as encountered with cimetidine, and results in a subsequent remission period double that of cimetidine.

Omeprazole: A Study of Its Inhibition of Gastric pH and Oral Pharmacokinetics After Morning or Evening Dosage

Pharmacodynamic and pharmacokinetic studies of omeprazole, a new gastric antisecretory agent, were undertaken in 8 healthy subjects. The drug was administered orally as an encapsulated enteric-coated granulate (40 mg daily at 9 am or 9 pm for 5 days), and its effect on the integrated 24-h gastric pH was determined, together with its apparent bioavailability. The pretreatment 24-h median pH was 1.9 (interquartile range 1.4-2.9). After 5 days of treatment, the median pH had risen to 5.0 (3.7-6.0) (p less than 0.01) with morning dosage and 4.5 (3.0-5.6) (p less than 0.01) with evening dosage. This corresponded to a greater than 99% reduction in 24-h median hydrogen ion activity, with morning dosage having a greater effect (from 9 am to 8 pm) (p less than 0.01) than evening dosage. The relative bioavailability of omeprazole increased twofold from day 1 to day 5 of treatment with morning dosage (p less than 0.02) and threefold with evening dosage (p less than 0.02), suggesting that increased absorption of this acid-labile drug occurs with increasing inhibition of acid secretion. We conclude that this formulation of omeprazole presently being used in clinical trials is a highly potent antisecretory agent in humans, although its optimal effect may not be observed for several days.

Effects of SCH 32651 on resting and stimulated acid secretion in guinea-pig isolated fundic mucosa.

Effects of SCH 32651, a novel antisecretory and cytoprotective agent, on resting and stimulated acid secretion by the guinea-pig isolated fundic mucosa were studied. SCH 32651 inhibited resting acid secretion in proportion to concentrations in serosal solution (0.1-10 microM), the IC50 being 4.4 microM. Cimetidine and atropine at concentrations up to 100 microM were inactive. Serosal application of SCH 32651 inhibited acid secretory responses to histamine (10 microM), methacholine (1 microM) or dibutyryl cyclic AMP (0.5 mM) plus theophylline (1 mM) in a concentration-dependent manner. The IC50S against histamine, methacholine and db cyclic AMP plus theophylline were 4.2 microM, 0.71 microM and 2.9 microM, respectively. In contrast, atropine and cimetidine each at 100 microM, a concentration that entirely abolished responses to methacholine and histamine, respectively, did not affect acid responses to db cyclic AMP plus theophylline. The inhibitory effects of SCH 32651 on resting and histamine-stimulated acid secretion were readily reversible upon washing. SCH 32651 0.1 mM in the mucosal solution also greatly suppressed the resting and stimulated acid secretion. In the presence of histamine treatment, SCH 32651 concomitantly caused a marked rise in K+ entry into the mucosal solution in parallel to a decline in the appearance of H+ in the same solution. The various events demonstrated by SCH 32651 in the present study are shared by omeprazole, a potent antisecretory agent working through inhibition of gastric H+/K+-ATPase. We conclude that SCH 32651 as a potent antisecretory agent seems to act directly on the parietal cell, near or at the site of H+/K+-ATPase which is a final step in the acid secretory process triggered by various stimuli.