Comparative antagonism by naltrexone and naloxone of mu, kappa, and delta agonists

In the guinea pig ileum preparation, naltrexone was 3.5 to 5 times more potent than naloxone in antagonizing morphine but the antagonists were equipotent in antagonizing ethylketazocine. In the mouse vas deferens preparation, naltrexone and naloxone were equipotent in antagonizing both morphine and [D-Ala2-D-Leu5]enkephalin. The data provide evidence that the naltrexone-reversible mu population of receptors in the guinea pig ileum are different from those in the mouse vas deferens. These findings also point out the uniqueness of the mu receptor system in the guinea pig ileum which is reflected by the potency differences between naltrexone and naloxone.