Abstract

Full and partial agonists of the A1 adenosine receptor were characterized with respect to their influence on G protein activation and their thermodynamic parameters of receptor binding in rat brain membranes. G protein activation was determined through measurement of [35S]guanosine-5'-(gamma-thio)triphosphate ([35S]GTP[S]) binding, and receptor binding was studied under identical conditions through the displacement of [3H]-1,3-dipropyl-8-cyclopentylxanthine ([3H]DPCPX) in equilibrium binding studies. The intrinsic activity in stimulating [35S]GTP[S] binding did not correlate with the affinity of the ligands. 5'-Deoxy-5'-methylthioadenosine, 2-phenylaminoadenosine, and 2-chloro-2'-deoxyadenosine were identified as partial A1 agonists in the G protein activation assay. Depending on the temperature, these ligands showed agonistic and antagonistic properties to varying extents. EC50 values for G protein stimulation and KH and KL values of the partial agonists decreased when the incubations were performed at lower temperatures, indicating a mainly enthalpy-driven process of interaction with the receptor. Thermodynamic parameters of receptor binding of the partial agonists resembled the characteristics of the antagonist DPCPX more closely than those of the agonist 2-chloro-N6-cyclopentyladenosine. In addition, partial agonists detected fewer A1 adenosine receptors in the high affinity state than did full agonists. The lower efficacy in stimulation of the binding of [35S]GTP[S] is probably the consequence of an impaired ability of the partial agonists to release GDP from the G protein, as was shown by an impaired release of prebound [35S]GDP[S] from the membranes.