Control of guinea pig intestinal electrolyte secretion by a δ-opiate receptor

The effects of opioids on transepithelial potential difference and short-circuit current across guinea pig ileum stripped of one muscle layer were measured in vitro in Ussing chambers. Opioid peptides such as [DAla², DLeu⁵]enkephalin and [DAla², DMet⁵]enkephalin, which are primarily agonists at δ-opiate receptors, were able to reduce transepithelial potential difference and short-circuit current at concentrations as low as 1 nM. The narcotic drug etorphine was also very potent in reducing short-circuit current, but fentanyl and morphine, which are primarily agonists at μ-opiate receptors, were almost completely ineffective. Ketocyclazocine was relatively ineffective, and β-endorphin had intermediate potency. All opioid effects could be reversed by the opiate antagonist naloxone. Somatostatin also reduced short-circuit current, but its effect was not reduced by naloxone. Chloride flux measurements indicated that the effect of etorphine on short-circuit current is associated with an enhancement of active Cl^- absorption. The relative effects of opioids in this system suggest that their actions are being mediated by a specific δ-opiate receptor. In contrast, opioid effects on guinea pig intestinal smooth muscle seem to be primarily mediated by a μ-opiate receptor.