(3H)dipyridamole and (3H)nitrobenzylthioinosine binding sites at the human parietal cortex and erythrocyte adenosine transporter: a comparison.

We compared the binding sites of the adenosine transport inhibitors (3H)dipyridamole (DPR) and (3H)nitrobenzylthioinosine (NBI) in human parietal cortex and erythrocytes. In comparison with guinea pig (3H)DPR marked only slightly more binding sites than (3H)NBI with a Bmax of 1080 +/- 29 and 780 +/- 7 fmol/mg protein respectively in parietal cortex and 24288 +/- 2725 and 20875 +/- 1905 fmol/mg protein respectively in erythrocytes. NBI displaced (3H)DPR binding completely from its binding sites at about KD/2 concentrations in parietal cortex as well as erythrocytes with inhibition constants comparable to its dissociation constants. Lineweaver-Burke analysis in erythrocytes indicated a competitive inhibition of (3H)DPR binding by NBI. Pharmacological characterization of (3H)DPR binding sites in human erythrocytes is consistent with their localization on adenosine transporters. These findings provide evidence that as opposed to guinea pig (3H)DPR and (3H)NBI largely label binding sites to the same adenosine transporter in human erythrocytes and parietal cortex.