Assessment of the potential agonistic and antagonistic properties of ketamine at opiate receptors in the guinea-pig ileum.

The anesthetic agent ketamine was studied for its ability to interact with opiate receptors in the longitudinal smooth muscle-myenteric plexus preparation of the guinea-pig ileum. The drug was found to possess agonistic, but not antagonistic, activity on opiate receptors. Naloxone antagonized the effect of ketamine although to a lesser degree than the antagonism exhibited toward morphine. In addition to the opiate action, ketamine also produced a depression of the contractile responses of the ileal smooth muscle to acetylcholine and histamine. The concentrations of ketamine that produced this non-specific depression were generally higher than those needed to demonstrate the opiate effect. However, some overlap in the concentration ranges for the two actions were observed. Although the nature of the opiate action of ketamine suggested that it interacts with opiate receptors or sub-types of these differently than does morphine, the non-specific action of the drug on the ileal smooth muscle precluded a definitive analysis of differences in the opiate receptor preferences of the drug.