Comparative effects of somatostatin and enkephalins on the guinea pig ileum and the rat vas deferens.

The action of somatostatin (SRIF (somatotrophin release inhibiting factor)) was compared with that of Met-enkephalin (Tyr-Gly-Gly-Phe-Met) in the electrically stimulated guinea pig ileum myenteric plexus longitudinal muscle and with that of an enkephalin analogue (FK 33-824 (Tyr-D-Ala-Gly-MePhe-Met-(O)-ol)) in the rat vas deferens. In both tissues SRIF produced a twitch inhibition which was not antagonized by naloxone and which showed a long-lasting tachyphylaxis. The enkephalins tested produced a naloxone-antagonizable inhibition of twitch in both tissues but no tachyphylaxis. Therefore we conclude that SRIF is not acting at opiate receptors in these tissues.