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.