Characterization of opioid receptors in intestinal muscle cells by selective radioligands and receptor protection

Abstract

Opioid receptors were characterized on muscle cells isolated separately from the circular and longitudinal muscle layers of rabbit intestine. Selective radioligands for kappa- ([3H]U69,593), delta- ([3H][D-Pen2,5]enkephalin, DPDPE), and mu- ([3H][D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin, DAGO) opioid receptors were used in conjunction with a technique of receptor protection designed to enrich cells with a specific receptor type. Binding was observed only in cells from the circular muscle layer. Binding was rapid (peak within 2 min), temperature dependent, and concentration dependent. Dissociation constants (Kd) for high-affinity binding sites derived from saturation curves (1.1 +/- 0.3 nM for U69,593, 0.39 +/- 0.04 nM for DPDPE, and 1.9 +/- 0.3 nM for DAGO) were similar to Kd values derived from competition curves. In competition studies, the order of potency with which opioid ligands inhibited binding depended on the radioligand used: U69,593 (Kd 1.5 +/- 0.2 nM) inhibited preferentially the binding of [3H]U69,593, DPDPE (Kd 0.72 +/- 0.16 nM) the binding of [3H]DPDPE and DAGO (Kd 1.2 +/- 0.3 nM) the binding of [3H]DAGO. In each instance the other two ligands were 400-12,000 times less potent. In cells enriched with one receptor type, binding and contraction were observed only with the corresponding selective ligand. The potency of the ligand was slightly enhanced, whereas the potencies of the other two ligands were further reduced (greater than 10,000-fold). We conclude that distinct kappa-, delta-, and mu-opioid receptors are present on muscle cells of the circular but not longitudinal muscle layer of the intestine.