| Abstract: | We studied, by ligand binding methods, the two adenosine receptors, A1 and A2, in rat and pig cerebral microvessels and pig choroid plexus. Ligand binding to cerebral microvessels was compared with that to membranes of the cerebral cortex. 3H]Cyclohexyladenosine and 3H]L-phenylisopropyladenosine were the ligands used for A1-receptors, and 3H]5'-N-ethylcarboxamide adenosine (3H]NECA) was used to assess A2-receptors. We report that cerebral microvessels and choroid plexus exhibit specific 3H]NECA binding, but have no appreciable A1-receptor ligand binding sites. Specific binding of 3H]NECA to cerebral microvessels, choroid plexus, and cerebral cortex was saturable and suggested the existence of two classes of A2-receptor sites: high-affinity (Kd approximately 250 nM) and low-affinity (Kd approximately 1-2 microM) sites. The Kd and Bmax of NECA binding to cerebral microvessels and cerebral cortex were similar within each species. Our results, indicating the existence of A2-receptors in cerebral microvessels, are consistent with results of increased adenylate cyclase activity by adenosine and some of its analogues in these micro-vessels. |
