Enkephalin inhibits presynaptically the contractility of urinary tract smooth muscle.

The influence of methionine-enkephalin and leucine-enkephalin on human detrusor and on pig detrusor, trigone, bladder neck and urethral smooth muscle was investigated in vitro. The enkephalins inhibited the smooth muscle contractility presynaptically. Methionine-enkephalin was about 40% more potent than leucine-enkephalin. Phentolamine-resistant contractions were less inhibited by the enkephalins than atropine-resistant contractions and contractions that were not blocked. The inhibitory nerve responses of the trigone, bladder neck and urethra were unaffected.