Characterization of the opiate receptor in the guinea-pig ileal mucosa

Several opioid peptides and narcotic drugs reduced transepithelial potential difference (PD) and short circuit current (Isc) in guinea-pig ileal mucosa measured in vitro in Ussing chambers. [D-Ala²,D-Leu⁵]enkephalin was the most potent peptide tested. Enkephalin analogues with altered C-terminal amino acids were less potent, as were ß-endorphin and dermorphin. Etorphine produced potent effects whereas morphine and SKF 10,047 were inactive. Ethylketazocine produced a biphasic dose-response curve. When added by themselves diprenorphine and naloxone produced small increases in Isc. This effect was not seen when Cl^- and HCO₃^- in the Ringer were replaced by SO₄^2-. Diprenorphine and naloxone were able to shift the dose response curves for all agonists to the right, with the exception of that for ethylketazocine. Diprenorphine was a more potent antagonist than naloxone. SKF 10,047 also acted as a pure antagonist. Morphine and ethylketazocine had no antagonist effects. It is concluded that the opiate receptor in the guinea-pig ileal mucosa is similar to a δ-opiate receptor as defined by ligand binding studies, but that some differences also exist.