Characterization of delta-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-op ioid 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 constan t (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 rec eptors suggests the possibility that enkephalins directly control ion trans port in enterocytes. In order to verify this hypothesis, investigations wer e designed to determine whether these receptors are functional and whether enkephalins can inhibit the stimulation of adenosine 3',5' cyclic monophosp hate (cAMP) synthesis induced by cholera toxin. The increase in cAMP synthe sis 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 in hibitor thiorphan potentiated this effect on IRD 98 cells, which contain en kephalinase. The action of DSLET was increased by 40% in the presence of th is inhibitor. This effect was reversed by naltrindole, a potent delta-antag onist. Enkephalins can regulate intestinal secretion by acting directly on enterocytes; they thus have an antidiarrheal role, especially in the presen ce of an enkephalinase inhibitor.