Protean Agonism at α2A-Adrenoceptors

  1. Christian C. Jansson1,3,
  2. Jyrki P. Kukkonen1,2,4,
  3. Johnny Näsman4,
  4. Ge Huifang1,3,
  5. Siegfried Wurster3,5,
  6. Raimo Virtanen5,
  7. Juha-Matti Savola5,
  8. Vic Cockcroft5 and
  9. Karl E. O. Åkerman4
  1. 1Department of Biochemistry and Pharmacy (C.C.J., J.P.K., G.H.),2Turku Centre for Biotechnology (J.P.K.), Åbo Akademi University, Turku, Finland, 3Department of Pharmacology and Clinical Pharmacology, University of Turku, Turku, Finland (C.C.J., G.H., S.W.), 4Department of Physiology and Medical Biophysics, Uppsala University, Uppsala, Sweden (J.P.K., K.E.O.A, J.N.), and 5Orion-Corporation, Orion-Pharma, Turku, Finland (S.W., R.V., J.-M.S., V.C.)

    Abstract

    The coupling of the endogenously expressed α2A-adrenoceptors in human erythroleukemia cells (HEL 92.1.7) to Ca2+ mobilization and inhibition of forskolin-stimulated cAMP production was investigated. The two enantiomers of medetomidine [(±)-[4-(1-[2,3-dimethylphenyl]ethyl)-1H-imidazole]HCl] produced opposite responses. Dexmedetomidine behaved as an agonist in both assays (i.e., it caused Ca2+ mobilization and depressed forskolin-stimulated cAMP production). Levomedetomidine, which is a weak agonist in some test systems, reduced intracellular Ca2+ levels and further increased forskolin-stimulated cAMP production and therefore can be classified as an inverse agonist. A neutral ligand, MPV-2088, antagonized responses to both ligands. Several other, chemically diverse α2-adrenergic ligands also were tested. Ligands that could promote increases in Ca2+ levels and inhibition of cAMP production could be classified as full or partial agonists. Their effects could be blocked by the α2-adrenoceptor antagonist rauwolscine and by pertussis toxin treatment. Some typical antagonists such as rauwolscine, idazoxan, and atipamezole had inverse agonist activity like levomedetomidine. The results suggest that the α2A-adrenoceptors in HEL 92.1.7 cells exist in a precoupled state with pertussis toxin-sensitive G proteins, resulting in a constitutive mobilization of intracellular Ca2+ and inhibition of cAMP production in the absence of agonist. This constitutive activity can be antagonized by inverse agonists such as levomedetomidine and rauwolscine. Levomedetomidine can be termed a “protean agonist” because it is capable of activating uncoupled α2-adrenoceptors in other systems and inhibiting the constitutive activity of precoupled α2-adrenoceptors in HEL 92.1.7 cells. With this class of compounds, the inherent receptor “tone” could be adjusted, which should provide a new therapeutic principle in receptor dysfunction.

    Footnotes

    • Send reprint requests to: Karl E. O. Åkerman, Uppsala University, Department of Physiology and Medical Biophysics, BMC, P.O. Box 572, S-75123 Uppsala, Sweden. E-mail: karl.akerman{at}fysiologi.uu.se

    • This work was supported by the Borg Foundation, Magnus Ehrnrooth Foundation, Technology Development Center of Finland, Sigrid Jusélius Foundation, Swedish Medical Research Council, and Cancer Research Fund of Sweden.

    • C.C.J. and J.P.K. contributed equally to this work.

    • Abbreviations:
      [Ca2+]i
      intracellular free Ca2+ concentration
      IBMX
      3-isobuthyl-1-methyl-xanthine
      TES
      2-([2-hydroxy-1,1-bis(hydroxymethyl)ethyl]amino)ethanesulfonic acid
      TBM
      TES-buffered medium
      clonidine
      2-(2,6-dichloroaniline)-2-imidazoline HCl
      guanabenz
      1-(2,6-dichlorobenzylideneamino)guanidine)
      guanfacine
      N-(aminoiminomethyl)-2,6-dichlorobenzenacetamide
      idazoxan
      (±)-2-(1,4-benzodioxan-2-yl)-2-imidazoline HCl
      oxymetazoline
      (3-[4,5-dihydro-1H-imidazol-2-yl]-methyl)-6-(1,1-dimethylethyl)-2,4-dimethylphenol HCl
      propranolol
      (±)-1-(isopropylamino)-3-(1-naphthyloxy)-2-propanol HCl
      naphazoline
      4,5-dihydro-2-(1-naphthalenylmethyl)-1H-imidazole
      xylazine
      N-(2,6-dimethylphenyl)-5,6-dihydro-4H-1,3-thiazin-2-amine HCl
      rauwolscine
      17α-hydroxy-20α-yohimban-16β-carboxylic acid methyl ester HCl
      tizanidine
      5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3-benzothiadiazole
      atipamezole
      4(5)-(2-ethyl-indan-2-yl)imidazole HCl
      detomidine
      4(5)-(2,3-dimethylbenzyl)imidazole]HCl
      medetomidine
      (±)-[4-(1-[2,3-dimethylphenyl]ethyl)-1H-imidazole]HCl
      dexmedetomidine
      (+)-(S)-4-(1-[2,3-dimethylphenyl]ethyl)-1H-imidazole]HCl
      levomedetomidine (−)-(R)-4-(1-[2
      3-dimethylphenyl]ethyl)-1H-imidazole HCl
      EGTA
      ethylene glycol bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid
      HEPES
      4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
      • Received September 10, 1997.
      • Accepted February 4, 1998.
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    1. Molecular Pharmacology May 1, 1998 vol. 53 no. 5 963-968
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