[Inhibitory and excitatory effects of methionine-enkephalin in the isolated rabbit ileum]

Methionine-enkephalin first inhibited and then stimulated the spontaneous rhythmic activity of the rabbit isolated ileum. The inhibitory effect of methionine-enkephalin was antagonized by naloxone. On the contrary, atropine did not change significantly either the inhibitory or the stimulatory action of this peptide. Furthermore, methionine-enkephalin inhibited the peristaltic reflex of the rabbit isolated ileum as well. Naloxone completely antagonized the inhibition of peristalsis produced by methionine-enkephalin. On the other hand, acetylcholine reversed only the propulsive activity and the back pressure, but not the peristaltic movements of the longitudinal muscle previously abolished by methionine-enkephalin. It appears, therefore, that only the inhibitory effects of methionine-enkephalin are mediated through opioid receptors in the intestine.