The effects of aspirin on gastric mucosal integrity, surface hydrophobicity, and prostaglandin metabolism in cyclooxygenase knockout mice.

Insight into the role of the different cyclooxygenase isoforms in prostaglandin biosynthesis, surface hydrophobicity, and gastric mucosal barrier integrity can be gained by comparing the effects of luminal damaging agents in wild-type and cyclooxygenase knockout mice. Fasted wild-type, cyclooxygenase-1, and cyclooxygenase-2 knockout mice were intragastrically administered saline, 0.6N HCl, or aspirin (aspirin 20 mmol/L) in combination with 0.6N HCl and killed 1 hour later, at which time the gastric lesion score was assessed and biopsy samples were taken for surface, biochemical, and morphological analyses. The gastric mucosa of cyclooxygenase-1 knockout mice was more severely injured by both HCl alone and aspirin/HCl than that of wild-type and cyclooxygenase-2 knockout mice. HCl alone and aspirin/HCl also induced a more profound decrease in surface hydrophobicity in cyclooxygenase-1 knockout mice than in wild-type mice, whereas this surface property was unaffected in cyclooxygenase-2 knockout mice. The gastric injury induced by aspirin/HCl in cyclooxygenase-1 knockout mice could be prevented if the animals were treated with phosphatidylcholine-associated aspirin. Aspirin/HCl, in comparison to saline or HCl alone, induced a 4-6-fold increase in gastric mucosal prostaglandin E(2) concentration in the cyclooxygenase-1 knockout mice, whereas it decreased prostaglandin E(2) levels in wild-type and cyclooxygenase-2 knockout mice. This paradoxical aspirin-induced increase in gastric prostaglandin E(2) in cyclooxygenase-1 knockout mice seemed to correspond to an increase in cyclooxygenase-2 messenger RNA and protein expression. The gastric lesion score seemed to be significantly associated with alterations in surface hydrophobicity but not with mucosal prostaglandin E(2) concentration. Our evidence on cyclooxygenase knockout mice suggests that aspirin predominantly causes gastric injury by a non-prostaglandin mechanism, perhaps by attenuating surface hydrophobicity, a possibility supported by the low gastric toxicity of phosphatidylcholine/aspirin. However, prostaglandins generated by cyclooxygenase-1 may play an important permissive role in maintaining gastric mucosal barrier integrity. Aspirin seems to paradoxically increase the gastric mucosal prostaglandin E(2) concentration in cyclooxygenase-1 knockout mice, possibly by the induction of cyclooxygenase-2.