Thermodynamic parameters DeltaG degrees , DeltaH degrees and DeltaS degrees of the binding equilibrium of 12 ligands (six agonists and six antagonists) to the A(2B) adenosine receptor subtype have been determined by affinity measurements carried out on HEK 293 cells stably transfected with human A(2B) adenosine receptors at six different temperatures (4, 10, 15, 20, 25, 30 degrees C) and van't Hoff plot analysis have been performed. Affinity constants were obtained from saturation experiments of [(3)H]MRE 2029-F20 or by its displacement in inhibition assays for the other compounds. van't Hoff plots were essentially linear in the temperature range investigated, showing that the DeltaC(p) degrees of the binding equilibrium is nearly zero. Thermodynamic parameters are in the range 7< or =DeltaH degrees < or =23 kJ mol(-1)and 123< or =DeltaS degrees < or =219 JK(-1)mol(-1) for agonists and -40 < or =DeltaH degrees < or =-20 kJ mol(-1) and 10< or =DeltaS degrees < or =91 JK(-1)mol(-1) for antagonists indicating that agonistic binding is always totally entropy-driven while antagonistic binding is enthalpy and entropy-driven. In the -TDeltaS degrees versus DeltaH degrees plot the thermodynamic data are clearly arranged in separate clusters for agonists and antagonists, which, therefore, turn out to be thermodynamically discriminated.