Effects of (D-Ala2,D-Leu5)enkephalinamide on isolated longitudinal muscle guinea pig and rat intestines.

The effects of (D-Ala2,D-Leu5)enkephalinamide, a synthetic analogue of (Leu5) enkephalin, on the contractile activity of isolated segments from guinea-pig and rat colon and rectum were investigated and compared to the effects of natural (Leu5)enkephalin. It was found that the contractile effects of (D-Ala2,D-Leu5)enkephalinamide vary between species and according to the route of administration (application of single doses of cumulative application). (D-Ala2,D-Leu5)enkephalinamide was more potent than (Leu5)enkephalin. It is suggested that in the separate regions of the guinea-pig and rat intestines (D-Ala2,D-Leu5)enkephalinamide activates the same opiate receptors as (Leu5)enkephalin. The higher potency of (D-Ala2,D-Leu5)enkephalinamide could be due to its retarded enzyme degradation because of the replacement of Gly2 by D-Ala and of Leu5 by D-Leu. A selective activation of delta type opiate receptors by (D-Ala2, D-Leu5)enkephalinamide is also assumed.