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

CS Wong, YF Su, WD Watkins and KJ Chang

Department of Pharmacology, Duke University Medical Center, Durham, North Carolina.

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)

Volume 268, Issue 2, pp. 653-661, 02/01/1994
Copyright © 1994 by American Society for Pharmacology and Experimental Therapeutics

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