Desensitization and redistribution of beta-adrenergic receptors on human mononuclear leukocytes

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Abstract
We have used intact human mononuclear leukocytes (MNL) to examine desensitization of beta-adrenergic receptors in normal mammalian cells. MNL were prepared and radioligand binding experiments were performed at 4 degrees C. At this temperature the ligand [125I]iodocyanopindolol ([125I]ICYP) identified the same number of receptors as at 37 degrees C, and the agonist isoproterenol competed for this binding with high affinity (dissociation constant, Ki = 20 nM). At 37 degrees C, results were similar when the binding incubation was terminated after 1 min, but the apparent affinity of the receptors for isoproterenol was several 100-fold lower when the incubation was allowed to reach steady state. In desensitized MNL (prepared by incubating whole blood with 10 microM isoproterenol at 37 degrees C for 10 min, and then isolating and washing the MNL at 4 degrees C), isoproterenol-stimulated cAMP accumulation was reduced 63 +/- 4%. After desensitization, the total number of beta-receptors was unchanged, but isoproterenol and the hydrophilic antagonist CGP-12177 were able to compete with [125I]ICYP for binding to only 18 +/- 6% of these sites. Direct binding with [3H]CGP-12177 yielded similar results. These results demonstrate that isoproterenol promotes a rapid desensitization of beta-adrenergic receptors on MNL and a concomitant redistribution of receptors into a cellular compartment to which some ligands (including catecholamines) have restricted access. The findings demonstrate that redistribution of beta-receptors may be a mechanism mediating desensitization to catecholamines in normal mammalian cells.