Inhibitory effect of enkephalins on contractions of the guinea-pig ileum elicited by PGE1

Methionine-enkephalin and leucine-enkephalin (m-enk and 1-enk) as well as D-Ala2-methionine-enkephalin amide have been shown to antagonize contractions of the isolated guinea-pig intestine elicited by PGE1, the last mentioned being the most potent of the three. The inhibitory effect of these pentapeptides is abolished by naloxone.