Inhibitory effect of methionine- and leucine-enkephalin on contractions of the guinea-pig ileum elicited by PGE1

Methionine-enkephalin and leucine-enkephalin (m-enk and l-enk) have been shown to antagonize contractions of the isolated guinea-pig intestine elicited by PGE1. The inhibitory effect of these 2 pentapeptides is abolished by naloxone.