HETEROGENEITY OF [H-3] DIPYRIDAMOLE BINDING TO CNS MEMBRANES - CORRELATION WITH [3H]NITROBENZYLTHIOINOSINE BINDING AND [H-3] URIDINE INFLUX STUDIES

The relationship between the nucleoside transport system and the nitrobenzylthioinosine-sensitive and -resistant [H-3]dipyridamole binding sites was examined by comparing the characteristics of [H-3]dipyridamole binding with those of [H-3]nitrobenzylthioinosine binding and [H-3]uridine influx in rabbit and guinea pig cerebral cortical synaptosomes. Two distinct high-affinity synaptosomal membrane-associated [H-3]dipyridamole binding sites, with different sensitivities to inhibition by nitrobenzylthioinosine, were characterized in the presence of 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS, 0.01%) to prevent [H-3]dipyridamole binding to glass tubes and filters. The nitrobenzylthioinosine-resistant [H-3]dipyridamole binding sites represented a greater proportion of the total membrane sites in guinea pig than in rabbit (40 vs. 10% based on inhibition studies). In rabbit, nitrobenzylthioinosine-sensitive [H-3]dipyridamole binding (K(D) = 1.4 +/- 0.2 nM) and [H-3]nitrobenzylthioinosine binding (K(D) = 0.30 +/- 0.01 nM) appeared to involve the same membrane site associated with the nitrobenzylthioinosine-sensitive nucleoside transporter. By mass law analysis, [H-3]dipyridamole binding in guinea pig could be resolved into two components based on sensitivity to inhibition by 1-mu-M nitrobenzylthioinosine. The nitrobenzylthioinosine-resistant [H-3]dipyridamole binding sites were relatively insensitive to inhibition by all of the nucleoside transport substrates and inhibitors tested. with the exception of dipyridamole itself. In guinea pig synaptosomes, 100-mu-M dilazep blocked nitrobenzylthioinosine-resistant [H-3]uridine transport completely but inhibited the nitrobenzylthioinosine-resistant [H-3]dipyridamole binding component by only 20%. Furthermore, a greater percentage of the [H-3]dipyridamole binding was nitrobenzylthioinosine resistant in guinea pig compared with rabbit, yet both species had a similar percentage of nitrobenzylthioinosine-resistant [H-3]uridine transport. These data suggest that the nitrobenzylthioinosine-resistant [H-3]dipyridamole binding site(s) involves membrane components distinct from those associated with functional, nitrobenzylthioinosine-resistant. nucleoside transporters.