Adenosine receptors in brain membranes: binding of N6-cyclohexyl[3H]adenosine and 1,3-diethyl-8-[3H]phenylxanthine.

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Abstract
N6-Cyclohexyl[3H]adenosine ([3H]CHA) and 1,3-diethyl-8-[3H]phenylxanthine ([3H]DPX) bind to adenosine receptors in brain membranes. The agonist [3H]CHA has high affinity in bovine and guinea pig brain (Kd, 0.7 nM and 6 nM, respectively). [3H]CHA binding kinetics are slow (dissociation t1/2 [1/2 time]; 60 min); binding is much higher at 25.degree. C than at 0.degree. C and is inhibited by guanine nucleotides. Potencies of nucleosides and xanthines is competing for [3H]CHA sites imply specific binding is entirely to A1 adenosine receptors. In bovine brain the antagonist [3H]DPX exhibits high-affinity binding (Kd, 5 nM) to the same A1 receptors that bind [3H]CHA. Binding kinetics are rapid (dissociation t1/2, 1 min) and binding is moderately higher at 0.degree. C than at 25.degree. C. In guinea pig brain [3H]DPX binding has only moderate affinity (Kd, 50 nM) and about 60% of specific binding is to sites resembling A2 adenosine receptors.