The effect of arachidonic acid and its metabolites on acid production in isolated human parietal cells

The Physiology of the Gastric Parietal Cell

Parietal cells are responsible for gastric acid secretion, which aids in the digestion of food, absorption of minerals, and control of harmful bacteria. However, a fine balance of activators and inhibitors of parietal cell-mediated acid secretion is required to ensure proper digestion of food, while preventing damage to the gastric and duodenal mucosa. As a result, parietal cell secretion is highly regulated through numerous mechanisms including the vagus nerve, gastrin, histamine, ghrelin, somatostatin, glucagon-like peptide 1, and other agonists and antagonists. The tight regulation of parietal cells ensures the proper secretion of HCl. The H+-K+-ATPase enzyme expressed in parietal cells regulates the exchange of cytoplasmic H+ for extracellular K+. The H+ secreted into the gastric lumen by the H+-K+-ATPase combines with luminal Cl- to form gastric acid, HCl. Inhibition of the H+-K+-ATPase is the most efficacious method of preventing harmful gastric acid secretion. Proton pump inhibitors and potassium competitive acid blockers are widely used therapeutically to inhibit acid secretion. Stimulated delivery of the H+-K+-ATPase to the parietal cell apical surface requires the fusion of intracellular tubulovesicles with the overlying secretory canaliculus, a process that represents the most prominent example of apical membrane recycling. In addition to their unique ability to secrete gastric acid, parietal cells also play an important role in gastric mucosal homeostasis through the secretion of multiple growth factor molecules. The gastric parietal cell therefore plays multiple roles in gastric secretion and protection as well as coordination of physiological repair.

Gastrointestinal effects of aspirin

Aspirin is being used as an effective analgesic and anti-inflammatory agent at doses >325 mg daily. At low doses (75-325 mg daily), aspirin is the key antiplatelet drug in the pharmacological prevention of cardiovascular diseases. Topical and systemic effects of aspirin in the gastrointestinal mucosa are associated with mucosal damage in the upper and lower gastrointestinal tract. The risk of upper gastrointestinal bleeding with aspirin is increased with old age, male sex, ulcer history and concomitant medication with NSAIDs, cyclooxygenase 2 selective inhibitors, corticosteroids or other antithrombotic agents. In some patients, the cardiovascular benefits of low-dose aspirin might be overcome by the risk of gastrointestinal complications, but withdrawal of aspirin therapy can precipitate a cardiovascular event. These patients will need concomitant therapy with antisecretory agents, especially PPIs, to reduce the gastrointestinal risk. Eradication of Helicobacter pylori infection might be an additional option in patients with a history of ulcer. Furthermore, there is growing evidence that long-term use of aspirin decreases the risk of colorectal cancer, even at low doses. As aspirin is one of the most prescribed drugs worldwide and its clinical impact is huge, physicians need to consider the benefits and harms for each individual patient in order to maximize the benefits of aspirin.

Gastroduodenal toxicity of low-dose acetylsalicylic acid: a comparison with non-steroidal anti-inflammatory drugs

BACKGROUND

Low-dose acetylsalicylic acid (ASA; aspirin; 75-325 mg/day) is effective for the prevention of cardiovascular events, and its use in this indication is rapidly increasing. However, the use of ASA and, indeed, other non-steroidal anti-inflammatory drugs (NSAIDs) is limited by the incidence of adverse gastroduodenal events. OBJECTIVES AND SCOPE: To review the clinical evidence for, and the pharmacodynamic basis of, ASA-induced gastroduodenal toxicity in comparison with NSAIDs, and address the question of whether low-dose ASA is 'safe' from a gastroduodenal perspective. This was a narrative, descriptive review, rather than a formal systematic review.

FINDINGS

Adverse gastroduodenal effects, which are well known to occur with NSAIDs, are also prevalent in patients receiving low-dose ASA for cardiovascular protection even at doses as low as 75 mg/day. The risk of gastroduodenal toxicity is particularly high among 'at-risk' low-dose ASA patients (aged >70 years, previous ulcer or upper gastrointestinal bleeding and users of antiplatelets or NSAIDs). There are important differences in the mechanism of ASA-induced gastroduodenal toxicity, relative to NSAIDs. These differences include the effects on the cyclooxygenase (COX)-1 isoenzyme, local effects on the gastroduodenal mucosa specific to ASA and a reduction in platelet aggregation.

CONCLUSION

Data suggest that ASA causes significant gastroduodenal damage even at the low doses used for cardiovascular protection. These effects (both systemic and possibly local) may be pharmacodynamically distinct from the gastroduodenal toxicity seen with NSAIDs. Studies are required to establish strategies for improving the tolerability of low-dose ASA, allowing patients to continue to benefit from the cardiovascular protection associated with such therapy.

Host-specific differences in the physiology of acid secretion related to prostaglandins may play a role in gastric inflammation and injury

Immune mediators are involved in strain-specific manifestations of Helicobacter pylori infection, and the type of immune response is associated with production of PGE(2), which in turn influences gastric acid secretion. Acid secretion plays a pivotal role, not only in the pattern of H. pylori-induced gastritis and its consequences, but also in nonsteroidal anti-inflammatory drug (NSAID)-induced gastropathies. Mice and their transgenic modifications are widely used in Helicobacter and eicosanoid research. Using [(14)C]aminopyrine accumulation and pylorus ligation, we aimed to study acid secretion in gastric gland preparations from the commonly used strains of BALB/c and C57BL/6 mice. We found that PGE(2) does not inhibit acid secretion in gastric glands from C57BL/6 mice, in contrast to the expected antisecretory effect of PGE(2) observed in BALB/c mice. In BALB/c mice the effect of histamine and carbachol was reduced by PGE(2), whereas in C57BL/6 mice dose-response curves to these secretagogues were not affected. EP(3) receptors are not involved in acid secretion in C57BL/6 mice, as confirmed by significantly lower expression of mRNA for the EP(3) receptor. These contrary findings are important to the interpretation of the antisecretory role of eicosanoids in BALB/c and C57BL/6 mouse strains and the involvement of prostanoids in the etiology of Helicobacter-induced inflammation and NSAID-induced gastropathies. We propose that the lack of antisecretory effect of PGE(2) observed in C57BL/6 mice could reflect the extent of Helicobacter-induced inflammation and status of acid secretion in response to anti-inflammatory drugs.

Non-steroidal anti-inflammatory drugs and prostaglandin effects on pepsinogen secretion by dispersed human peptic cells

The effects of aspirin and ibuprofen on pepsinogen secretion were studied in isolated human peptic cells prepared from endoscopically obtained biopsy specimens after collagenase digestion, mechanical disruption, and percoll gradient centrifugation. Pharmacological concentrations of aspirin and ibuprofen (10(-8)-10(-4) M), potentiated histamine (10(-6)-10(-4)M) and forskolin (10(-5)M) stimulated pepsinogen secretion without affecting basal secretion, acetylcholine (10(-6)M) stimulated pepsinogen secretion or cell vitality. Augmentation of secretagogue stimulated pepsinogen secretion was dependent on extracellular calcium because potentiation was abolished by calcium depletion of the medium. Cimetidine inhibited the potentiation effect on histamine but not on forskolin stimulated pepsinogen secretion, thus suggesting that this augmentation was independent of histamine H2 receptors. Of interest, potentiation was also independent of endogenous prostaglandin inhibition because exogenous addition of prostaglandin E2 and D2 increased both basal and acetylcholine stimulated pepsinogen secretion in a dose dependent way, but they did not modify histamine or histamine plus aspirin or ibuprofen stimulated pepsinogen secretion. In conclusion, aspirin and ibuprofen potentiate secretagogue stimulated pepsinogen secretion by dispersed human peptic cells and this might be an additional mechanism of non-steroidal anti-inflammatory drug (NSAID) induced gastric injury. This potentiation effect is regulated by calcium, independent of endogenous prostaglandin inhibition and seems to act on pepsinogen secretion at a post-receptor site.

Gastric prostaglandin E2 receptors are the common antisecretory target of mucosal prostanoids

The gastric mucosa produces all principal prostaglandin (PG) types, but receptor binding studies in this tissue have as yet been performed exclusively with [3H]PGE2. Therefore we compared the binding of different 3H-labelled prostanoids to fundic mucosal plasma membranes from the porcine stomach. Binding sites for [3H]PGE2, [3H]iloprost and [3H]PGF2 alpha had similar nanomolar dissociation constants with high affinities for unlabelled PGE2. Iloprost and PGF2 alpha were 10- and 100-fold less potent competitors with Hill slopes near unity in all cases. In further [3H]PGE2 competition studies the affinities of prostanoid ligands with selectivity for different PG receptor types correlated closely with their respective antisecretory potencies, as tested by [14C]aminopyrine uptake in isolated porcine parietal cells. We conclude that parietal cell PGE2 receptors are the common antisecretory target for all prostanoid types in the porcine stomach. There was no evidence for other mucosal PG receptors possibly involved in acid secretion.

Arachidonic acid and prostaglandin E2 activate small-conductance Cl- channels in the basolateral membrane of rabbit parietal cells

1. Cl- channels in the basolateral membrane of non-stimulated parietal cells in isolated rabbit gastric glands were studied by patch-clamp and noise analysis techniques. 2. Voltage-independent whole-cell currents were recorded from parietal cells equilibrated with Cl(-)-containing solutions. Upon reducing the Cl- concentration of the basolateral bathing solution, the current-voltage curve was rapidly shifted to the right. The reversal potential changed according to changes in the equilibrium potential for Cl-. 3. A Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) inhibited whole-cell Cl- currents. The half-maximal inhibitory concentration of NPPB was 300 microM, and the inhibition was reversible (at less than or equal to 200 microM). Another Cl- channel blocker, 4-acetamido-4'-isothiocyanatostilbene-2,2'-dilsulphonic acid or diphenylamine-2-carboxylate was much less effective. 4. The power spectra of whole-cell Cl- current fluctuations contained one Lorentzian component. The single Cl- channel conductance was estimated to be 0.35 pS by the variance noise analysis, which is in agreement with the value obtained by a method of power spectrum analysis (0.29 pS). 5. The addition of arachidonic acid (10 microM) to the basolateral medium markedly increased the whole-cell Cl- current. The combined application of a cyclo-oxygenase inhibitor, indomethacin (50 microM), and a lipoxygenase inhibitor, esculetin (100 microM), increased the Cl- current, whereas the administration of a phospholipase A2 inhibitor, mepacrine (100 microM), significantly decreased the whole-cell Cl- current. 6. A sizeable increase in the whole-cell Cl- current was also induced by a metabolite of arachidonic acid, prostaglandin E2 (10 microM), but not by leukotriene B4 (5 microM) or D4 (10 microM). 7. The present study has shown that small-conductance Cl- channels are present in the basolateral membrane of rabbit parietal cells, and that the channel was functionally regulated by arachidonic acid and prostaglandin E2.