Comparison of methionine-enkephalin and morphine in the stimulation of gastric acid secretion in the dog

In dogs with Heidenhain pouches and gastric fistulas, we studied acid secretion in response to systemic or portal infusion of methionine-enkephalin (met-enkephalin), enkephalin-analog, or morphine. All these opiate compounds caused a dose-dependent increase in acid secretion under basal conditions and resulted in a significant rise in pentagastrin- or histamine-induced acid secretion. The stimulation by opiates of gastric secretion was accompanied by an increase in the mucosal blood flow but without any significant change in serum gastrin concentration. Gastric acid stimulation by met-enkephalin and morphine was strongly inhibited not only by naloxone, an opiate antagonist, but also by blockers of H2-receptors (metiamide) or cholinergic receptors (atropine), suggesting a cooperative interaction between opiates and other stimuli of parietal cells. The gastric stimulation by met-enkephalin and its analog, but not by morphine, was markedly reduced by portal administration of these compounds, indicating a marked inactivation of opiate peptides by hepatic transit. This study shows that enkephalin and morphine stimulate gastric acid secretion by a gastrin-independent mechanism sensitive to atropine and H2-blocker and probably involving opiate receptors.