Adenosine receptor agonists: Binding and adenylate cyclase stimulation in rat liver plasma membranes

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Abstract
N6-Cyclohexyl[3H]adenosine([3H]CHA),[3H]adenosine, and 5′N-ethylcarboxamide[3H]adenosine ([3H]NECA), potent agonists in adenosine-responsive cellular systems, have been used to identify adenosine binding sites in rat liver plasma membranes. Endogenous ligands were removed by prior dialysis of the membranes. Specific binding of the ligands tested was characterized by rapid forward and reverse kinetics and heterogeneity as indicated by curvilinear Scatchard plots. The KD in the high affinity range was 80 nM for [3H]adenosine, 84 nM for [3H]NECA, and 168 nM for [3H]CHA; the respective binding capacities of 1.19, 1.03, and 1.05 pmol/mg protein were of virtually the same magnitude, suggesting labeling of identical sites. However, all ligands also displayed binding to large numbers of low affinity sites. This high level of apparently non-receptor binding markedly influenced the adenosine structure-activity profile of [3H]CHA displacement, which differs with pharmacological findings. — NECA and CHA stimulated hepatic adenylate cyclase with an apparent ED50 of 60 and 580 nM, respectively; adenosine was stimulatory at a concentration range from 0.1–2.0 μM, but inhibitory at higher concentrations. Hence, estimation of the true ED50 was not possible. Because the KD of high affinity binding and the ED50 of the biological effect of NECA and CHA are in the same range, it may be reasonable to assume that the high affinity sites represent adenosine receptors, recently classified as Ra-site receptors.