title: Opioid agonist binding affinity is increased by magnesium in the presence of guanosine diphosphate but decreased by magnesium in the presence of guanyl-5'-yl imidodiphosphate.
abstract: Opioid receptors exist in multiple affinity states for agonists. Cations and guanine nucleotides affect mu-agonist affinity by modulating the interaction between receptor molecule and G-protein. In this study, we systematically examined the effects of cations and guanine nucleotides on the binding of mu-agonist [D-Ala2, N-Me-Phe4, Met-(O)5-ol]enkephalin in rat spinal cord and brain membranes. A homogeneous class of low-affinity receptors was obtained by conducting binding assays in the presence of Na+ (100 mM) + guanosine diphosphate (100 microM). The addition of Mg++ (5 mM) shifted a portion of the low-affinity receptors to the high-affinity state. In membranes preincubated with Na+ + guanosine diphosphate and with the binding assay performed in the presence of Mg++, a homogeneous class of high-affinity receptors was induced by the agonist. Further addition of Na+ + guanyl-5'-yl imidodiphosphate (30 microM) to the above Mg++-containing medium converted all receptors to a low-affinity state, whereas Na+ + guanyl-5'-yl imidodiphosphate produced a mixture of high- and low-affinity states. Our results suggest that, depending on which guanine nucleotide is bound with G-proteins, mg++induces opposite effects on the states of mu-receptor affinity: high affinity with GDP and low affinity with guanyl-5'-yl imidodiphosphate. Sodium ion promotes the low-affinity receptor conformation and increases total low-affinity binding sites. The affinity of the opiod antagonist naloxone is not sensitive to regulation by Mg++ and guanine nucleotides, except that the total receptor binding is increased by Na+.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Pharmacology, Duke University Medical Center, Durham, North Carolina.
authors.forename: C S
authors.initials: CS
authors.lastname: Wong
authors.forename: Y F
authors.initials: YF
authors.lastname: Su
authors.forename: W D
authors.initials: WD
authors.lastname: Watkins
authors.forename: K J
authors.initials: KJ
authors.lastname: Chang
mesh_terms: D000818:Animals; D001921:Brain / Q000378:metabolism; D005405:D-Ala(2),MePhe(4),Met(0)-ol-enkephalin / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000494:pharmacology*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology*; D008274:Magnesium / Q000494:pharmacology*; D009270:Naloxone / Q000378:metabolism; D051381:Rats; D017450:Receptors, Opioid, mu / Q000378:metabolism*; D012964:Sodium / Q000494:pharmacology; D013116:Spinal Cord / Q000378:metabolism
chemical_list: D017450:Receptors, Opioid, mu; D006153:Guanosine Diphosphate; D006165:Guanylyl Imidodiphosphate; D009270:Naloxone; D005405:D-Ala(2),MePhe(4),Met(0)-ol-enkephalin; D012964:Sodium; D008274:Magnesium