Naunyn-Schmiedeberg's Archives of Pharmacology

, Volume 308, Issue 3, pp 191–198 | Cite as

Demonstration of α-adrenoceptors in the rabbit heart by [3H]-dihydroergocryptine binding

  • H. J. Schümann
  • O. -E. Brodde
Article
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Summary

For direct identification of α-adrenoceptors in a membrane fraction of the rabbit heart the potent α-adrenoceptor antagonist [3H]-dihydroergocryptine ([3H]-DHE) was used.
  1. 1.

    The binding of [3H]-DHE was saturable with 80 fmol of [3H]-DHE bound/mg protein and of high affinity with an equilibrium dissociation constant (KD) of 11.5 nM. Binding of [3H]-DHE (6 nM) was rapid (t1/2=2 min) and readily reversible. From the ratio of the rate constants for forward (K1=1.97×107 M−1 min−1) and reverse (K2=0.206 min−1) reactions a KD-value of 10 nM was calculated, which is in good agreement with that obtained by equilibrium studies.

     
  2. 2.

    Adrenergic agonists compete for [3H]-DHE binding in an order of potency: (−)adrenaline > (−)phenylephrine≫ (−)isoprenaline and adrenergic antagonists in the order: phentolamine > yohimbine≫ (−)propranolol. Binding is stereospecific as indicated by the greater potency of (−)adrenaline than (±)adrenaline in displacing [3H]-DHE from the binding sites.

     
  3. 3.

    For comparison the binding of the potent β-adrenoceptor antagonist (−)[3H]-dihydroalprenolol ((−)[3H]-DHA) was measured in the same membrane fraction. The number and affinity of β-adrenoceptors amounted to 115 fmol of (−)[3H]-DHA bound/mg protein at saturation and KD=7.9 nM. Adrenergic agonists compete for (−)[3H]-DHA binding in an order of potency: (−)isoprenaline > (−)adrenaline > (−)phenylephrine; and adrenergic antagonists in the order: (−)propranolol≫ phentolamine.

     
  4. 4.

    It is concluded that in a membrane fraction of the rabbit heart there exist binding sites for [3H]-DHE which have characteristics indistinguishable from α-adrenoceptors. Thus the present results are in agreement with previously reported data on the existence of cardiac α-adrenoceptors in the rabbit heart (Schümann et al., 1974; Endoh et al., 1976b).

     

Key words

Cardiac α- and β-adrenoceptors Radioligand binding Rabbit heart [3H]-Dihydroergocryptine (−)[3H]-Dihydroalprenolol 

Abbreviations

[3H]-DHE

[3H]-Dihydroergocryptine

(−)[3H]-DHA

(−)[3H]-dihydroalprenolol

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Copyright information

© Springer-Verlag 1979

Authors and Affiliations

  • H. J. Schümann
    • 1
  • O. -E. Brodde
    • 1
  1. 1.Pharmakologisches InstitutUniversität EssenEssenFederal Republic of Germany

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