Characterization of [3H]atipamezole as a radioligand for alpha 2-adrenoceptors.
Atipamezole (MPV-1248, 4-(2-ethyl-2,3-dihydro-1H-inden-2-yl)-1H-imidazole), a potent alpha 2-adrenoceptor antagonist, was tritiated to high specific activity. We then compared [3H]atipamezole and [3H]rauwolscine as radioligands for alpha 2-adrenoceptors in rat cerebral cortex, neonatal rat lung, and human platelets. (-)-Noradrenaline and phentolamine were used to define specific alpha 2-adrenergic binding. Unlabelled atipamezole was used in a similar manner to define saturable, high-affinity non-adrenergic binding. [3H]Atipamezole binding to human platelets (Kd 1.3 nM) and rat brain membranes (Kd 0.5 nM) equilibrated rapidly and was displaced in the expected manner by alpha 2-adrenergic ligands. In contrast, [3H]atipamezole binding in neonatal rat lung membranes was only effectively inhibited by unlabelled atipamezole, and by high concentrations of idazoxan. The total density of binding sites for [3H]atipamezole was clearly in excess of the density of alpha 2-adrenoceptors in this tissue, as defined by [3H]rauwolscine binding. We conclude that [3H]atipamezole binds with high affinity to alpha 2-adrenoceptors in human platelets and rat cerebral cortex, and that the compound can be used to investigate alpha 2-adrenoceptor properties and drug actions in these tissues. In neonatal rat lung, [3H]atipamezole identified an additional population of binding sites, distinct from both classical alpha 2-adrenoceptors and idazoxan-defined imidazoline receptors. The pharmacological identity of these binding sites remains to be elucidated. This non-adrenergic component in the binding characteristics of [3H]atipamezole complicates its use as a general alpha 2-adrenoceptor radioligand.
Department of Pharmacology, University of Turku, Finland.
B
B
Sjöholm
R
R
Voutilainen
K
K
Luomala
J M
JM
Savola
M
M
Scheinin
D000317:Adrenergic alpha-Antagonists / Q000378:metabolism*; D000818:Animals; D000831:Animals, Newborn / Q000378:metabolism; D001792:Blood Platelets / Q000378:metabolism / Q000648:ultrastructure; D002540:Cerebral Cortex / Q000378:metabolism / Q000648:ultrastructure; D005260:Female; D006801:Humans; D007093:Imidazoles / Q000378:metabolism*; D007700:Kinetics; D008168:Lung / Q000378:metabolism / Q000648:ultrastructure; D008297:Male; D008566:Membranes / Q000378:metabolism; D011869:Radioligand Assay; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D014316:Tritium; D015016:Yohimbine / Q000378:metabolism
D000317:Adrenergic alpha-Antagonists; D007093:Imidazoles; D011942:Receptors, Adrenergic, alpha; C050701:atipamezole; D014316:Tritium; D015016:Yohimbine