High expression of nitrobenzylthioinosine-insensitive dipyridamole binding sites in postmortem human ependymal tissue

The presence of both nitrobenzylthioinosine-sensitive and nitrobenzylthioinosine-insensitive dipyridamole binding sites in postmortem human ependymal tissue is reported. Displacement studies using 15 nM [3H]dipyridamole revealed 50-60% of the sites were sensitive to nitrobenzylthioinosine. Non-linear analysis of binding isotherms to estimate the density of nitrobenzylthioinosine-sensitive and -insensitive sites revealed a maximum number of nitrobenzylthioinosine-sensitive binding sites (Bmax) of 395 +/- 19 fmol/mg protein and a nitrobenzylthioinosine-insensitive Bmax of 3910 +/- 700 fmol/mg protein (corresponding Kd values of 0.1 nM and 114 nM respectively). Thus there are approximately 10 times as many nitrobenzylthioinosine-insensitive sites as nitrobenzylthioinosine-sensitive [3H]dipyridamole binding sites in human ependymal membranes.