Effects of morphine, hypoxaemia and hypercapnia on the rat stomach

Modulation by peripheral opioids of basal and distension-stimulated gastric acid secretion in the rat

1. The influence of opioids in modulating gastric acid secretory responses has been investigated in the continuously perfused stomach of the anaesthetized rat. 2. Intravenous administration of morphine (0.75-3 mg kg-1) or the peripherally acting enkephalin analogue, BW443C (0.75-3 mg kg-1), substantially augmented acid secretion in basal conditions. These effects were significantly inhibited by the opioid antagonists naloxone (1 mg kg-1) and the peripherally acting N-methylnalorphine (2 mg kg-1). When administered alone, neither opioid antagonist influenced basal acid output. 3. Acid secretory responses to different levels of gastric distension (5-20 cmH2O) were significantly and dose-dependently reduced in rats pretreated with morphine (3 mg kg-1) or BW443C (1.5 mg kg-1). Previous administration of either naloxone or N-methyl nalorphine reversed the inhibitory effects of opioids on gastric acid secretion stimulated by distension. Likewise, blockade of opioid receptors with naloxone or N-methylnalorphine significantly increased acid output induced by distension. 4. Levels of serum gastrin in control animals were not increased after intragastric distension (20 cmH2O). Pretreatment with BW443C (1.5 mg kg-1) did not modify the levels of gastrin present during basal or distension stimulated conditions. 5. Pretreatment with morphine or BW443C did not influence the acid responses to i.v. injection of pentagastrin (100 micrograms kg-1), histamine (5 mg kg-1) or carbachol (4 micrograms kg-1). Acid secretion induced by i.v. administration of 2-deoxy-D-glucose (150 mg kg-1) was reduced in rats pretreated with morphine but not with BW443C. Gastric secretory responses to insulin (0.3 i.u. kg-1) were not modified by i.v. morphine.6. These observations support a role for peripherally acting opioids in the regulation of gastric acid secretion during basal and distension-stimulated conditions.

Antisecretory and anti-ulcer effects of morphine in rats after gastric mucosal aggression

The effects of morphine on gastric secretion, barrier mucus and mucosal lesions were studied in pylorus-ligated rats treated with the ulcerogenic agents, indomethacin, aspirin or taurocholic acid. All three ulcerogenic agents induced significant mucosal lesions. Morphine decreased gastric acid secretion and suppressed the aspirin- and taurocholic acid-induced, but not the indomethacin-induced mucosal lesions. These results suggest that the ulcerogenic mechanisms of indomethacin and the other agents are not identical. Moreover the antiulcer effect of morphine appears to be mediated by an increased barrier mucus level: the amount of Alcian blue bound to the mucosa, an indirect estimate of the adherent mucus layer, was increased by morphine and correlated with its protective effect. All morphine effects were reversed by naloxone.

On the possible role of different endogenous opiate receptors in gastroduodenal ulceration of the rat

In Wistar rats of both sexes stress(restraint)- and drug (indomethacin)-induced gastric, as well, as cysteamine-induced duodenal ulceration was treated by morphine and nalorphine, given either alone or in combination. Neither morphine nor nalorphine had a significant anti-ulcerogenic effect. In contrast, the combined morphine + nalorphine treatment showed a highly significant anti-ulcerogenic action in the case of gastric ulceration but it was ineffective on duodenal ulcer formation. It seems that the chi and sigma endogenous opiate receptors play a significant role in the anti-ulcerogenic effect, while the mu receptors are involved to a considerably lesser degree. Moreover, the experimental results strengthen the view that gastric and duodenal ulceration are two different pathological entities and that ulcer formation is only a common consequence.