μ, but not κ, opioid agonists induce contractions of the canine small intestine ex vivo

The proposed κ opioid receptor agonists ethylketocyclazocine (EK), nalorphine, bremazocine and U-50,488H were evaluated for their ability to produce contractions of isolated, vascularly perfused canine small intestinal segments. Responses to these agonists were compared to those of morphine and phenazocine, a μ benzomorphan. Morphine (0.04-25 μg) and phenazocine (0.01-3.0 μg) both produced naloxone-reversible contractions, suggesting that the responses were mediated largely by μ opioid receptors. In contrast, the proposed κ agonists were ineffective in producing intestinal stimulation, with only EK (1-100 μg) showing minimal but significant activity at very high doses. We suggest that the effects of EK may be mediated through μ opioid receptors and that κ receptors appear not to be involved in the contractile response of the dog small intestine to opioids.