Characterization of [(3)H]idazoxan binding sites on human platelets.
Human platelets possess at least two non-adrenoceptor binding sites pharmacologically distinct from the alpha(2)-adrenoceptors. The effects of various imidazol(in)es on platelet aggregation have suggested that these compounds may interact with these non-adrenoceptor binding sites on platelets. [(3)H]Idazoxan is an antagonist of the alpha(2)-adrenoceptors frequently used to characterize imidazoline I(2) receptors. We evaluated the binding of [(3)H]idazoxan to human platelet membranes. In saturation experiments [(3)H]idazoxan (1.25-32 nM) recognized a single, saturable binding site with high affinity. However, competition assays revealed the presence of alpha(2A)-adrenoceptors and a non-adrenoceptor minor population (25-39%) recognized with high affinity by the imidazoline drug with low affinity for alpha(2)-adrenoceptors 2-BFI. After the addition of (-)adrenaline (5 microM) to mask alpha(2)-adrenoceptors, competition curves against [(3)H]idazoxan binding were biphasic. The imidazoline I(1) receptor-selective drugs, efaroxan and rilmenidine, recognized the minor component with high affinity, whereas the imidazoline I(2) receptor-selective drugs, guanabenz and 2-BFI, bound with high affinity to the major component. Further masking experiments in the presence of efaroxan (2 microM) or guanabenz (1 microM) confirmed that [(3)H]idazoxan labels two non-adrenoceptor binding sites pharmacologically compatible with imidazoline I(1) and I(2) receptors as well as alpha(2A)-adrenoceptors in human platelets.
Ruiz
Javier
J
Department of Pharmacology, University of the Basque Country, E-48940 Leioa, Bizkaia, Spain.
Barinagarrementeria
Guillermo
G
Martín-Gómez
J Ignacio
JI
Callado
Luis F
LF
Meana
J Javier
JJ
D000317:Adrenergic alpha-Antagonists / Q000378:metabolism*; D001665:Binding Sites; D001667:Binding, Competitive; D001792:Blood Platelets / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D019329:Idazoxan / Q000378:metabolism*; D054755:Imidazoline Receptors; D011869:Radioligand Assay; D018341:Receptors, Adrenergic, alpha-2 / Q000032:analysis / Q000378:metabolism; D011955:Receptors, Drug / Q000032:analysis / Q000378:metabolism; D014316:Tritium
C547768:ADRA2A protein, human; D000317:Adrenergic alpha-Antagonists; D054755:Imidazoline Receptors; D018341:Receptors, Adrenergic, alpha-2; D011955:Receptors, Drug; D014316:Tritium; D019329:Idazoxan