Differential binding properties of adenosine receptor agonists and antagonists in brain.
The binding properties of N6-cyclohexyl [3H]adenosine ( [3H]CHA) and 1,3-diethyl-8-[3H]phenylxanthine ( [3H]DPX) in rat forebrain membrane are compared. The kinetic parameters of binding for each ligand are quite distinct, with [3H]CHA displaying two populations of binding sites (KD = 0.4 +/- 0.05 nM and 4.2 +/- 0.3 nM; Bmax = 159 +/- 17 and 326 +/- 21 fmol/mg protein), whereas [3H]DPX yielded monophasic Scatchard plots (KD = 13.9 +/- 1.1 nM; Bmax = 634 +/- 27 fmol/mg protein). The metals copper, zinc, and cadmium are potent inhibitors of [3H]CHA binding, with respective IC50 concentrations of 36 microM, 250 microM, and 70 microM. Copper is a much less potent inhibitor of [3H]DPX binding (IC50 = 350 microM). The inhibitory effect of copper on both [3H]CHA and [3H]DPX binding is apparently irreversible, as membranes pretreated with copper cannot be washed free of its inhibitory effect. The inhibitory effect of both copper and zinc on [3H]CHA binding was reversed by the guanine nucleotide Gpp(NH)p. [3H]DPX binding is only partially inhibited by zinc and cadmium (60% of specific binding remains unaffected), suggesting that this adenosine receptor ligand binds to two separate sites. Guanine nucleotides had no effect on the inhibition of [3H]DPX binding by either copper or zinc. Differential thermal and proteolytic denaturation profiles are also observed for [3H]CHA and [3H]DPX binding, with the former ligand binding site being more labile in both cases. Stereospecificity is observed in the inhibition of both [3H]CHA and [3H]DPX binding, with L-N-phenylisopropyladenosine (PIA) being 50-fold more potent than D-PIA in both cases. Evidence is therefore provided that adenosine receptor agonists and antagonists have markedly different binding properties to brain adenosine receptors.
D000241:Adenosine / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D002216:Captopril / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D007700:Kinetics; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000302:isolation & purification / Q000378:metabolism*; D011983:Receptors, Purinergic; D013329:Structure-Activity Relationship; D014970:Xanthines / Q000378:metabolism*
D011956:Receptors, Cell Surface; D011983:Receptors, Purinergic; D014970:Xanthines; C027512:1,3-diethyl-8-phenylxanthine; C027513:N(6)-cyclohexyladenosine; D002216:Captopril; D000241:Adenosine