Ascorbate/Fe(3+)-induced peroxidation and inhibition of the binding of A1 adenosine receptor ligands in rat brain membranes.
The effect of peroxidation induced by the ascorbate/Fe3+ system on the binding properties of the A1 adenosine receptor, was studied in rat brain membranes, using the agonist, [3H]R-N6-phenylisopropyladenosine ([3H]R-PIA), and the antagonist, [3H]1,3-dipropyl-8-cyclopentylxanthine ([3H]DPCPX). For the agonist, as well as for the antagonist, the number of binding sites (Bmax) was significantly (P < 0.05) reduced after pretreatment of the membranes with ascorbate/Fe3+. The affinity of the agonist for the binding sites was not statistically modified (P > 0.05) after ascorbate/Fe3+ pretreatment, whereas the Kd value of the antagonist was increased (P < 0.05) by a factor of 2. Ascorbate/Fe3+ pretreatment affected agonist binding in the presence of GTP in a similar way as that observed in the absence of GTP, suggesting that peroxidation also affects agonist binding to A1 adenosine receptors uncoupled to G-proteins. The results suggest that when brain membranes suffer free radical oxidative damage, the adenosine modulation of neuronal activity through A1 receptors could be less efficient.