Understanding gastric acid secretion in dogs

Recent research has looked into the effectiveness of gastric acid suppression of famotidine, two omeprazole formulations (reformulated paste and enteric-coated tablets) and a placebo.1 The researchers used a continuous telemetric pH monitoring system to analyze intragastric pH.

The telemetric system, in comparison to previously used invasive procedures or indirect intragastric pH measurements, is catheterless and expected to have minimal effects on gastric physiology. Capsules were attached directly to the fundic gastric mucosa via gastroscopy.

Results indicated that both oral omeprazole preparations were superior to famotidine for suppressing gastric acid secretion in dogs. Oral famotidine, even though administered at a relatively high dose, resulted in surprisingly poor gastric acid suppression. In essence, the omeprazole preparations consistently kept the gastric acid levels lower for a longer period.

Gastric acid suppression overview

Gastric acid secretion is regulated by a variety of neurochemical and neurohormonal stimuli. Acid secretion is stimulated by luminal peptides, digested protein, acetylcholine, gastrin and histamine. Acid secretion is inhibited by somatostatin. Ultimately, all these stimuli affect the hydrogen-potassium ATPase acid pump located in the parietal cells in the gastric mucosa. Hydrogen ions are transported into the gastric lumen in exchange for potassium.

Three main types of gastric acid suppressants are used in veterinary medicine today—antacids, histamine H2-receptor antagonists and proton pump inhibitors (PPI). Antacids (calcium carbonate, aluminum hydroxide, magnesium carbonate) are generally not sufficiently effective and have a short duration of action. H2-receptor antagonists (famotidine, ranitidine, cimetidine) act as competitive inhibitors at the histamine receptor on the parietal cells. They do not block other stimulants for acid secretion. PPIs (omeprazole, lansoprazole, esomeprazole) inhibit the final step of acid production—the hydrogen-potassium ATPase pump—and, thus, prevent histamine, acetylcholine and gastric-induced acid secretion.2 This explains why this study found that omeprazole appeared to be more effective at gastric acid suppression in comparison to famotidine.

Other less commonly used drugs for reducing gastric acid secretion include proglumide (cholecystokinin antagonist), pirenzepine (an M2-receptor antagonist), misoprostol (a prostaglandin-2 analogue) and octreotide (a somatostatin analogue). Other means of decreasing gastric acid secretion not yet available include gastrin-receptor antagonists, gastric-releasing peptide receptor antagonists and potassium-competitive acid blockers at the level of the hydrogen-potassium ATPase pump.

Conclusion

Gastric acid suppressants are used to manage and treat a variety of conditions (Table 1). This study indicates a superior effect of omeprazole on gastric acid secretion. It would be logical to consider using omeprazole (and other PPIs) instead of famotidine (or other H2-receptor antagonists) in certain diseases for optimal increase in gastric pH (acid reduction). Unfortunately, injectable omeprazole is not available in the United States. Injectable famotidine can be used initially in cases in which vomiting or inability to swallow is an issue. If indicated, treatment can be followed with oral omeprazole.

Dr. Lyman is a graduate of The Ohio State University College of Veterinary Medicine. He completed a formal internship at the Animal Medical Center in New York City. Lyman is a co-author of chapters in the 2000 editions of Kirk's Current Veterinary Therapy XIII and Quick Reference to Veterinary Medicine.

Dr. Runde is a graduate of the University of Pennsylvania School of Veterinary Medicine. He completed an internship at Hollywood Animal Hospital. He is an associate veterinarian at the Animal Emergency and Referral Center, Ft. Pierce, Fla.

REFERENCES

1. Tolbert K, Bissett S, King A, et al. Efficacy of oral famotidine and 2 omeprazole formulations for the control of intragastric pH in dogs. J Vet Intern Med 2011;25(1):47-54.

2. Simpson KW. Diseases of the stomach. In: Ettinger SJ, Feldman EC, eds. Textbook of veterinary internal medicine: diseases of the dog and cat. 7th ed. St. Louis, Mo: Saunders Elsevier, 2010;1504-1526.