[3H]Dipyridamole Binding to Guinea Pig Brain Membranes: Possible Heterogeneity of Central Adenosine Uptake Sites

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Abstract
The binding of [3H]dipyridamole ([3H]DPR) to guinea pig brain membranes is described and compared to that of [3H]nitrobenzylthioinosine ([3H]NBI). The binding of [3H]DPR is saturable, reversible, and specific with phar-macologic evidence indicating that this ligand is binding to the adenosine uptake site. Compared to [3H]NBI the binding of [3H]DPR is of higher capacity (Bmax= 208 ±16 fmol/ mg protein for [3H]NBI and 530 ± 40 fmol/mg protein for [3H]DPR) and lower affinity (KD= 0.35 ± 0.02 nM for [3H]NBI and 7.6 ± 0.7 nM for [3H]DPR). The adenosine uptake inhibitors are the most potent inhibitors of binding (Ki of 10−8-10−7M) whereas adenosine receptor ligands such as cyclohexyladenosine, 2-chloroadenosine, and various methylxanthines are several orders of magnitude less potent (Ki 10−5-10−2). The inhibition of [3H]DPR binding by NBI is biphasic, with only 40% of binding being susceptible to inhibition by NBI concentrations < 10−5M. The tissue distribution of [3H]DPR binding parallels that of [3H]NBI although in most cases significantly more sites are observed with [3H]DPR. Calcium channel blocking agents such as nifedipine, nimodipine, and verapamil are also inhibitors of [3H]DPR binding with potencies in the micromolar range. The data are consistent with [3H]DPR being a useful additional ligand for the adenosine uptake site and provide evidence that multiple uptake binding sites exist of which only about 40% are NBI-sensitive.