Enkephalin inhibits relaxation of the lower oesophageal sphincter

Five healthy volunteers were studied for the effect on oesophageal motility of a single subcutaneous injection of a synthetic analogue of enkephalin as compared with an injection of an equivalent volume (0.5 ml) of saline. The injections were given at random on separate days, and each was followed after 40 minutes by 2 mg naloxone given intravenously. Pressures were measured by manometry after dry and wet (5 ml) swallows at one-minute intervals, and traces were coded and analysed "blind". Twenty-five minutes after the injection of enkephalin the percentage relaxation of the lower oesophageal sphincter pressure was significantly less (p less than 0.005) than at the same time after saline. Within two minutes after intravenous naloxone this effect had disappeared completely. Enkephalin had no noticeable effect on pressure of the sphincter or on amplitude and duration of oesophageal peristalsis. The mechanism of action of enkephalin in selectively inhibiting relaxation of the lower oesophageal sphincter remains to be determined. That naloxone rapidly reversed the inhibition may be relevant in achalasia and warrants further study.