Comparison of A4 and A2a binding sites in striatum and COS cells transfected with adenosine A2a receptors

A putative A4 adenosine receptor is characterized by a distinct structure activity profile of compounds in competition for [3H]2-phenylaminoadenosine ([3H]CV 1808) binding sites on rat brain membranes assayed at 4 degrees C. We now confirm that A4 binding sites can be demonstrated on ice-cold membranes of rat striatum and demonstrate a similar binding site on COS cells transfected with rat A2a adenosine receptors (COS/A2a). The characteristic A4 potency order is: CV 1808 > [1R-(1 alpha, 2 alpha, 3 beta, 5 beta)]-3-(2,6-diamino-N2-(3-carbethoxyphenyl)-9 H-purin-9-yl)-5'-(N-ethylcarbamoyl)-1,2-cyclopentanediol (CGS 22988) >> 5'-N-ethylcarboxamidoadenosine (NECA) > or = 2-[4-(2- carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680); 9-chloro-2-(2-furyl)[1,2,4]-triaolo[1,5-c]-quinazolin-5-a min e (CGS 15943) only partially inhibits binding at 1 microM. If [3H]CGS 21680 is used for ice-cold assays, or if either [3H]CV 1808 or [3H]CGS 21680 are used for assays at 21 degrees C, the potency order of competing compounds changes markedly and becomes characteristic of A2a adenosine receptor binding sites; CGS 15943 > or = CGS 21680 congruent to NECA > CGS 22988 > or = CV 1808. Binding of [3H]CGS 21680, but not [3H]CV 1808, is significantly enhanced by the pore-forming antibiotic, alamethicin. Guanosine 5'-O-(3-thiotriphosphate) decreases the binding of both radioligands to striatal membranes at 21 degrees C significantly more than to membranes on ice.(ABSTRACT TRUNCATED AT 250 WORDS)