Characterization of δ-opioid receptors and effect of enkephalins on IRD 98 rat epithelial intestinal cell line

Using H-3-Tyr-D-Ala-Gly-Phe-D-Leu-OH (H-3-DADLE) as a radioligand, delta-opioid binding sites on the IRD 98 rat epithelial cell line were identified. These sites were found to be reversible, saturable, specific and displayed high affinity for DADLE. Scatchard analysis revealed a dissociation constant (K-d) Of 4.9+/-0.5 nmol/l, a maximum binding capacity (B-max) of 1.7 pmol/mg protein, and 5x10(5) binding sites per cell. The presence of opioid receptors suggests the possibility that enkephalins directly control ion transport in enterocytes. In order to verify this hypothesis, investigations were designed to determine whether these receptors are functional and whether enkephalins can inhibit the stimulation of adenosine 3',5' cyclic monophosphate (cAMP) synthesis induced by cholera toxin. The increase in cAMP synthesis induced by cholera toxin was inhibited in a dose-dependent manner by H-Tyr-D-Ser-Gly-Phe-Leu-Thr-OH (DSLET), a delta-agonist. The enkephalinase inhibitor thiorphan potentiated this effect on IRD 98 cells, which contain enkephalinase. The action of DSLET was increased by 40% in the presence of this inhibitor. This effect was reversed by naltrindole, a potent delta-antagonist. Enkephalins can regulate intestinal secretion by acting directly on enterocytes; they thus have an antidiarrheal role, especially in the presence of an enkephalinase inhibitor.