Adenosine receptors in brain membranes: binding of N6-cyclohexyl3Hadenosine and 1,3-diethyl-8-3Hphenylxanthine.

N6-Cyclohexyl3Hadenosine (3HCHA) and 1,3-diethyl-8-3Hphenylxanthine (3HDPX) to bind to adenosine receptors in brain membranes. The agonist 3HCHA has high affinity in both bovine and guinea pig brain (Kd, 0.7 nM and 6 nM, respectively). 3HCHA binding kinetics are slow (dissociation t1/2;60 min); binding is much higher at 25 degrees C than at 0 degrees C and is inhibited by guanine nucleotides. Potencies of nucleosides and xanthines in competing for 3HCHA sites indicate that specific binding is entirely to A1 adenosine receptors. In bovine brain, the antagonist 3HDPX exhibits high-affinity binding (Kd, 5 nM) to the same A1 receptors that bind 3HCHA. Binding kinetics are rapid (dissociation t1/2, 1 min), and binding is moderately higher at 0 degrees C than at 25 degrees C. In guinea pig brain, 3HDPX binding has only moderate affintiy (Kd 50 nM), and about 60% of specific binding is to sites that resemble A2 adenosine receptors.