Agonist and antagonist actions of buprenorphine on three types of opioid receptor in isolated preparations.

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Both agonist and antagonist actions of buprenorphine on isolated preparations were studied. The K<SUB>e</SUB> (equilibrium dissociation constant) values of both naloxone and Mr 2266 [(-)-2-(3-furylmethyl)-5, 9-diethyl-2'-hydroxy-6, 7-henzomorphan] against buprenorphine and the ratio of IC50 (concentration of the drug to produce 50% inhibition of the twitch) value of buprenorphine after to before exposure of mouse vas deferens to β-FNA (β-fumaramate methyl ester derivatives of naltrexone), an irreversible mu antagonist, suggest that buprenorphine acts as both a mu and kappa agonist on mouse vas deferens. The agonist effect of buprenorphine at relatively high doses on guinea-pig ileum and mouse vas deferens and the negative agonist effect on both rat and rabbit vas deferens indicate that buprenorphine acts as a partial agonist on isolated preparations. The K<SUB>e</SUB> values of buprenorphine show that buprenorphine has about equal antagonist effectiveness against a mu and kappa agonist with approximately five-fold lower effectiveness against a delta agonist. The possible mechanisms for the several characteristic actions of buprenorphine on guinea-pig ileum such as the slow onset of action, the increased magnitude of inhibition after washing the tissue, the negative elimination of the inhibition by either washing the tissue or the naloxone administration, and the negative elimination of the antagonist action by washing the tissue were discussed.