Adrenal medullary opiate receptors. Pharmacological characterization in bovine adrenal medulla and a human pheochromocytoma.

We have characterized the opiate binding sites on the membranes of bovine adrenal medulla and human pheochromocytoma, using 3H-labeled D-Ala2-D-Leu5-enkephalin ( [3H]DADLE), [3H]etorphine, and [3H]ethylketocyclazocine ( [3H]EKC). Binding was stereoselective in both membrane preparations. Association and dissociation kinetics showed that steady state was achieved after 20-25 min of incubation at 37 degrees. Saturation experiments were performed in the absence or in the presence of morphiceptin (1 microM), which masks the mu sites, D-Ser2-Leu-enkephalin-Thr6 (100 nM), which masks delta sites, or DADLE (5 microM), which was found to mask the delta, mu, and benzomorphan receptor. Taking into consideration the affinities of the three radioligands used (DADLE identifying the delta and mu sites when used in the nanomolar range; etorphine identifying the delta, mu, and benzomorphan sites; EKC identifying the delta, mu, kappa, and benzomorphan receptors) we have characterized pharmacologically the opiate sites present on bovine and human membranes. Human pheochromocytoma membranes contained (a) mu binding sites (15 fmoles/mg of protein, KD [3H]etorphine 1.0 nM, [3H]EKC 5.4 nM, [3H]DADLE 5.6 nM); (b) kappa sites (41 fmoles/mg of protein, KD [3H]EKC 1.0 nM); (c) benzomorphan sites (115 fmoles/mg of protein, KD [3H]etorphine and [3H]EKC 1.0 nM). On bovine membranes we have detected (a) delta binding sites (10 fmoles/mg of protein, KD [3H]DADLE 0.7 nM); (b) mu sites (24 fmoles/mg of protein, KD [3H]DADLE 2.9 nM, [3H]etorphine 0.2 nM, [3H]EKC 3.4 nM); (c) kappa sites (12 fmoles/mg of protein, KD [3H]EKC 0.4 nM); (d) benzomorphan sites (80 fmoles/mg of protein, KD [3H]etorphine 0.2 nM, [3H]EKC 1.3 nM); (e) a residual high-affinity (20 fmoles/mg of protein, KD 0.2 nM) site identified by [3H]etorphine in the presence of 5 microM DADLE. The relative proportions of benzomorphan sites were equal in both tissues (65% of the high-affinity sites) whereas kappa receptors were more abundant on human membranes (25%) than on bovine membranes (9% of the high-affinity sites).