Binding of the adenosine A2 receptor ligand [3H]CGS 21680 to human and rat brain: evidence for multiple affinity sites.
A new radiolabeled adenosine receptor agonist, 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadeno sin e (CGS 21680), apparently specific for high-affinity binding sites of the A2 subtype in rat brain, was used to identify and pharmacologically characterize adenosine receptors in human brain. The binding of [3H]CGS 21680, as determined by standard radioligand binding technique in the presence of exogenously added adenosine deaminase, reached equilibrium after 40 min at 25 degrees C. In saturation studies, a single class of high-affinity binding sites with values for KD of 22 +/- 0.5 nM and Bmax of 444 +/- 63 fmol/mg of protein were observed. Similar binding characteristics were observed regardless of whether rapid filtration or centrifugation was used to separate bound versus free ligand. Of the 14 brain regions examined, [3H]CGS 21680 binding was highest in putamen, followed by globus pallidus and caudate nucleus. The level of [3H]CGS 21680 binding in these areas of basal ganglia was identical to 5'-N-[3H]ethylcarboxamidoadenosine ([3H]NECA) binding in the presence of 50 nM N6-cyclopentyladenosine (CPA). The rank order of agonist potencies as determined by a series of competition experiments was NECA greater than or equal to CGS 21680 greater than 2-chloroadenosine greater than N6-(R)-phenylisopropyladenosine greater than N6-cyclohexyladenosine greater than N6-(S)-phenylisopropyladenosine. This potency order was the same for the binding of [3H]CGS 21680 to rat, and of [3H]NECA in the presence of 50 nM CPA to rat and human, brain membranes.(ABSTRACT TRUNCATED AT 250 WORDS)
Wan
W
W
Department of Pharmacology and Therapeutics, University of Manitoba Faculty of Medicine, Winnipeg, Canada.
Sutherland
G R
GR
Geiger
J D
JD
D000241:Adenosine / Q000031:analogs & derivatives* / Q000378:metabolism; D019830:Adenosine-5'-(N-ethylcarboxamide); D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D006801:Humans; D008024:Ligands; D008297:Male; D008875:Middle Aged; D010627:Phenethylamines / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011983:Receptors, Purinergic / Q000378:metabolism*; D014018:Tissue Distribution; D014316:Tritium
D008024:Ligands; D010627:Phenethylamines; D011983:Receptors, Purinergic; D014316:Tritium; C061282:2-(4-(2-carboxyethyl)phenethylamino)-5'-N-ethylcarboxamidoadenosine; D019830:Adenosine-5'-(N-ethylcarboxamide); C027513:N(6)-cyclohexyladenosine; D000241:Adenosine