The effects of leucine-enkephalin on the spontaneous motility of the circular muscle of the cat colon.

Since the effects of endogenously occurring opiates in the regulation of colonic motility are unknown, we set out to study the actions of leucine-enkephalin on the spontaneous contractile and electrical activity of the isolated circular muscle of the cat colon. The muscle strips exhibited regular rhythmic contractions and myoelectrical slow waves 3.67 +/- 0.16 (SEM) times/min. Mean contraction force was 1.20 +/- 0.09 mN. Leucine-enkephalin caused a dose-dependent increase in contractile activity by a factor of 8.24 +/- 2.68; ED50 was 2.23 X 10(-9) M, ED100 4.33 X 10(-8) M. Methionine-enkephalin was equally effective, but morphine was much less effective. Naloxone acted as a specific antagonist. Concentrations of 10(-8) and 10(-7) M shifted the dose-response curve to the right, at the ED50 of leucine-enkephalin by 0.66 and 1.63 log units, respectively. Naloxone exhibited the characteristics of competitive antagonism; pA2 was -8.69, the slope was 1.06. Other antagonists did not affect the enkephalin actions on the muscle. The results of this study suggest that leucine-enkephalin acts on specific receptors of the circular muscle of the cat colon. It is conceivable, therefore, that leucine-enkephalin exerts its effects under physiological conditions to increase segmenting motor activity.