Regulation of Mu and Delta Opioid Action in Normal and Morphine-Tolerant Cells and Cell Membrane Preparations
UNIFORMED SERVICES UNIV OF THE HEALTH SCIENCES BETHESDA MD
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Na and guanyl nucleotides have profound effects on opioid action. Analysis of these effects is confounded by the presence of heterogeneous populations of opioid sites in most tissues investigated. To avoid this complication, the regulatory functions of Na and guanyl nucleotides were studied in cells carrying only delta NG108-15 or mu 7315c receptors respectively. Binding studies were performed at 37 C in Krebs-Hepes buffer at pH 7.4. Na and guanyl nucleotides inhibited agonist binding in each cell type, whereas antagonist binding was not affected. Although Na appeared to regulate both mu and delta receptors through intracellular sites, its effects were not identical at each receptor type. Na regulation of opioid inhibition of adenylyl cyclase activity also differed between cell types. High and low affinity components of binding were characterized by competition of unlabeled mu or delta agonists against antagonist binding in the absence or presence of GTP or GTPgammaS in both cell types. In contrast, only single agonist affinity states were observed in the presence of GDPgammaS or in membranes from pertussis toxin treated cells. Data suggest regulation of agonist binding by guanyl nucleotides may be mediated through interconversions among multiple affinity states of the delta and mu opioid receptors in both cell types. These states may represent several forms of the agonist-receptor-guanyl nucleotide protein complex in the transmembrane signalling cycle.
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