Effects of morphine, hypoxaemia and hypercapnia on the rat stomach

The effects of morphine, hypoxaemia or hypercapnia on gastric acid secretion, gastric mucus synthesis and the gastric mucosa were studied in conscious rats with pyloric occlusion. Hypoxaemia and hypercapnia were induced by morphine 32 mg/kg given i.p., or each condition was produced separately by adjusting the composition of respired air in the chamber where the animals were kept during the experimental period. Hypoxia significantly enhanced gastric mucus synthesis whereas hypercapnia significantly reduced gastric acid secretion. These effects were significantly alleviated by atropine pretreatment. Morphine-treated rats exhibited decreased gastric acid secretion, increased gastric mucus synthesis and a higher mean ulcer index but only the reduced gastric acid output was significantly prevented by atropine. It is suggested that the effect of morphine on gastric acid secretion may result from its respiratory depressant action and consequent acute stress production. However, the mechanisms by which morphine can increase mucus synthesis and produce ulceration remain obscure.