A comparison of the receptor constants of morphine and ethylketocyclazocine for analgesia and inhibition of gastrointestinal transit in the rat

Frank Porreca, Robert B. Raffa, Alan Cowan, Ronald J. Tallarida

Research output: Contribution to journalArticle

7 Scopus citations


The efficacies and dissociation constants of proposed mu and kappa receptor agonists (morphine and ethylketocyclazocine, respectively) were compared using the method of partial irreversible blockade (with buprenorphine) and Stephenson's theory of drug action. While there was good agreement between the dissociation constant (KA) of morphine in analgesia (3.3 × 10-5 M) and in inhibition of gastrointestinal transit (1.1 × 10-5 M), the KA of ethylketocyclazocine differed by an order of magnitude in these endpoints (3.2 × 10-6 M and 6.7 × 10-5 M, respectively). The efficacies of morphine were found to be similar for the two effects studied (4.23 and 5.26), while those for ethylketocyclazocine differed markedly (2.06 and 10.39). The fraction of receptors remaining unblocked after buprenorphine was consistent for the test but not for the agonist, indicating a different distribution of receptors for the two endpoints. Our results strongly suggest that morphine induces analgesia, and slows transit in the small intestine, through the same type of receptor. The same conclusion cannot be drawn for ethylketocyclazocine.

Original languageEnglish (US)
Pages (from-to)1955-1961
Number of pages7
JournalLife Sciences
Issue number18
StatePublished - Nov 1 1982
Externally publishedYes


ASJC Scopus subject areas

  • Biochemistry, Genetics and Molecular Biology(all)
  • Pharmacology, Toxicology and Pharmaceutics(all)

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