Temperature-dependent binding of hydrophilic beta-adrenergic receptor ligands to intact human lymphocytes

Important differences in binding characteristics between agonists and antagonists of the beta-adrenergic receptor have been described. However, these observations have been complicated since most available antagonists are much more lipophilic than agonists. In order to separate out those binding characteristics of agonist vs. antagonist from those characteristics of lipophilic vs. hydrophilic ligands, we have studied competition of the hydrophilic ligands isoproterenol (agonist) and CGP-12177 (antagonist) with [125I]iodopindolol binding in intact human lymphocytes. Analyzing competition curves from assays performed at 13 degrees C, 25 degrees C and 37 degrees C we demonstrated that at lower temperatures there was a decrease in IC50 for isoproterenol but not for CGP-12177. Using cells preincubated with isoproterenol then extensively washed, competition curves with both isoproterenol and CGP-12177 were biphasic, and characterized by the appearance of a population of receptors with a low affinity for both hydrophilic ligands. Furthermore, at lower temperatures the biphasic nature of these curves was accentuated and was characterized by a 6-fold and 40-fold increase in the apparent KD of a population of low affinity sites for isoproterenol and CGP-12177, respectively.