The effects of an opiate agonist and antagonist on the human upper gastrointestinal tract

Endogenous opiates may play a role in regulating intestinal function and we have investigated this possibility in the human upper gastrointestinal tract using loperamide as a locally-acting opiate agonist and naloxone as antagonist. Loperamide (4 mg) had no effect on the fasting pattern of motor activity in the antrum, duodenum and proximal jejunum. However, naloxone (40 micrograms kg-1 h-1, i.v.) abolished interdigestive motor activity in five of ten subjects and reduced antroduodenal motility index per cycle in all subjects (antrum: 1230 +/- 120 to 130 +/- 140 mm2; duodenum: 1120 +/- 150 to 225 +/- 160 mm2; P less than 0.005). These effects were not significantly affected by loperamide. Plasma motilin exhibited cyclic variation with significantly higher concentrations during antroduodenal phase 3 than during phases 1 and 2 of the interdigestive cycle. Plasma motilin was not altered by naloxone or loperamide, and neither drug had any effect on basal gastric acid secretion, duodenal output of amylase and bile salt or duodenogastric reflux. The results suggest that endogenous opiates regulate fasting motor activity in the human gastrointestinal tract but do not influence basal secretory function of the stomach, pancreas or biliary tree.