Characterization of a low affinity binding site for N6-substituted adenosine derivatives in rat testis membranes.
The binding characteristics of radiolabeled N6-(cyclohexyl)adenosine ([3H]CHA), N6-(R-phenylisopropyl)adenosine ([3H]R-PIA), 5'-N-ethyl-carboxamidoadenosine ([3H]NECA), and 2-[4-(2-carboxyethyl)phenyl]ethyl-amino-5'-N-ethylcarboxamidoadenosine ([3H]CGS 21680), to rat testis membranes were investigated. Specific binding of [3H]CGS 21680, a selective agonist for the A2a adenosine receptor, was very modest whilst the nonselective agonist [3H]NECA bound to rat testis membranes showing high binding capacity. At least two types of binding sites for [3H]NECA could be identified in rat testis membranes: high affinity sites and high capacity sites. Selective agonists for the A1 adenosine receptor, [3H]CHA and [3H]R-PIA bound with high affinity to a single class of binding sites. This high affinity binding site showed the typical pharmacological specificity of the A1 adenosine receptor with a potency order for agonists of CHA > or = R-PIA > NECA > N6-(S-phenylisopropyl)adenosine (S-PIA). In order to detect the presence of the A3 adenosine receptor in these membranes we selectively blocked the A1 receptor with a large molar excess of a xanthine antagonist, either 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) or xanthine amine congener (XAC). In the presence of an antagonist a low affinity binding site for [3H]CHA and [3H]R-PIA was detected. This low affinity binding site showed a different pharmacological specificity than the high affinity binding site. In fact the potency order for agonists was CHA > or = NECA = R-PIA > S-PIA. This finding suggests that the low affinity binding site represents the A3 adenosine receptor.
Mazzoni
M R
MR
Istituto Policattedra di Discipline Biologiche University of Pisa, Italy.
Buffoni
R S
RS
Giusti
L
L
Lucacchini
A
A
D000241:Adenosine / Q000031:analogs & derivatives / Q000378:metabolism; D019830:Adenosine-5'-(N-ethylcarboxamide); D000818:Animals; D001665:Binding Sites; D008297:Male; D010627:Phenethylamines / Q000378:metabolism; D010660:Phenylisopropyladenosine / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D018047:Receptors, Purinergic P1 / Q000032:analysis* / Q000378:metabolism; D013737:Testis / Q000737:chemistry*; D014970:Xanthines / Q000378:metabolism
D010627:Phenethylamines; D018047:Receptors, Purinergic P1; D014970:Xanthines; C061282:2-(4-(2-carboxyethyl)phenethylamino)-5'-N-ethylcarboxamidoadenosine; D010660:Phenylisopropyladenosine; D019830:Adenosine-5'-(N-ethylcarboxamide); C027513:N(6)-cyclohexyladenosine; C051360:1,3-dipropyl-8-cyclopentylxanthine; D000241:Adenosine