Characterization of specific δ-opioid binding sites in the distal small intestine of swine

Delta-opioid receptors modulate neurogenic smooth muscle contractions and mucosal ion transport in the porcine distal small intestine. We compared specific δ-opioid binding sites in neuronal membranes isolated from the inner submucosal plexus and myenteric plexus of this gut region. In both preparations, the δ-opioid receptor antagonist naltrindole displaced [³H]diprenorphine from two binding sites. [³H]Naltrindole bound specifically to a single high-affinity site that was displaced by δ-opioid receptor ligands with the rank order of affinity: naltriben >7-benzylidenenaltrexone>deltorphin II>[D-Pen², D-Pen ⁵] enkephalin. Relative decreases in δ-opioid receptor agonist affinity in the presence of Na⁺ and other ions differed in submucosal and myenteric membranes. The κ-opioid receptor antagonist norbinaltorphimine displaced [³H]naltrindole binding in myenteric membranes (K_i=7.2 nM). Delta-opioid receptors in submucosal and myenteric plexuses of porcine ileum appear to differ in some respects.