1. Endogenous opioid peptides are found in the enteric nervous system in the gastrointestinal tract. The opioid peptide enkephalin, which has anti-secretory actions in the small intestine, is also contained in nerves in the gall-bladder wall.
2. In experimental cholecystitis, there is active fluid secretion by the epithelial cells into the gall-bladder lumen, when the intraluminal hydrostatic pressure is low. This fluid secretion to the lumen was abolished by intravenous administration of enkephalin, an effect that was blocked by naloxone pretreatment. The flux of fluid into the lumen was also abolished when the intraluminal hydrostatic pressure was raised to the level initially observed in the inflamed and obstructed gall bladder. Fluid absorption in the normal gall bladder was unaffected by enkephalin.
3. In experimental cholecystitis, naloxone, used as a non-specific antagonist of opiate action, did not affect the gall-bladder mucosal fluid transport observed at a low intraluminal hydrostatic pressure, but it induced fluid secretion when this pressure was high.
4. It is suggested that a raised intraluminal hydrostatic pressure in experimental cholecystitis, which distends the gall bladder, releases endogenous opioids that inhibit active fluid secretion by the gall-bladder epithelial cells. This response may represent a defence mechanism that could be present also in the gastrointestinal tract. In the obstructed and inflamed gall bladder it may prevent progressive distension, ischaemia and perforation of the wall.