Characterization of K Opioid Receptors in Neurosecretosomes from Bovine Posterior Pituitary |
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Authors: | Guido Pesce Michael A Lang James T Russell David Rodbard Harold Gainer |
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Institution: | Laboratory of Theoretical and Physical Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland, U.S.A.;Laboratory of Neurochemistry and Neuroimmunology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland, U.S.A. |
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Abstract: | The binding properties of opioid receptors on isolated nerve terminals (neurosecretosomes) from bovine posterior pituitaries were characterized. Both 3H]etorphine and 3H]ethylketocyclazocine (3H]EKC) showed high-affinity binding with complex binding isotherms, consistent with the presence of multiple classes of binding sites. D-Ala2,D-Leu5]enkephalin showed no specific binding and failed to displace 3H]etorphine at high concentrations, indicating the absence of mu, delta, or benzomorphan (kappa 2) sites. Mathematical modelling of the data suggested the presence of three classes of binding sites. The first was of high affinity with Kd values of 0.9 and 2.0 nM for etorphine and EKC, respectively. The second class of sites appeared to bind etorphine with a KD of 150 nM, and EKC with extremely low affinity (unmeasurable binding). The third class of sites was characterized by KD values of 7 and 2 microM for etorphine and EKC, respectively. These results indicate that the nerve terminals of bovine posterior pituitary contain opioid binding sites of the kappa type. Furthermore, these binding sites appear heterogeneous, consisting of at least two and possibly more subtypes or states. |
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Keywords: | Neurosecretosomes Opioid receptors (K) Posterior pituitary Dynorphin Mathematical modelling Etorphine Ethylketocyclazo-cine |
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