Reflex peristalsis in the guinea pig isolated ileum is endogenously controlled by kappa opioid receptors

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Abstract
(1) Reflex peristalsis in the circular muscle of the guinea pig ileum was elicited in vitro by sustained luminal distension of the intestinal wall according to 2 cm H2O and evaluated in terms of the number of peristaltic waves within 15 min intervals. (2) The poorly μ-selective opioid antagonist naloxone at concentrations of 10−7 and 10−6 mol/l increased the frequency of peristaltic contractions within the first 15 min interval, and thereafter in a declining fashion, by 68 and 88%, respectively. The highly κ-selective opioid antagonist nor-binaltorphimine behaved similarly. It was, by one order of magnitude, more potent but a little less effective than naloxone, i.e., the maximum effect was 57% increase in peristaltic frequency at 10−8 mol/l. Concentrations of 10−7 and 10−6 mol/l had the same effect as 10−8 mol/l, and 10−9 mol/l were ineffective. The highly μ-selective antagonist CTOP-NH2 and the highly δ-selective antagonist ICI 174,864 were ineffective up to 10−6 mol/l. (3) It is concluded that predominantly ξ opioid receptors are used by endogenous opioids under the present conditions to inhibit reflex peristalsis.