Down-regulation of surface beta-adrenoceptors on intact human mononuclear leukocytes. Time-course and isoproterenol...

Incubation of human mononuclear leukocytes (MNL) in vitro with isoproterenol resulted in a rapid loss of surface beta-adrenoceptors, determined by radioligand binding at 4 degrees. Isoproterenol concentrations in the range of 10 nM to 100 microM resulted in significant down-regulation of beta-adrenoceptors. At a concentration of 1 microM isoproterenol, the time-dependent loss of surface beta-adrenoceptors closely paralleled the loss in isoproterenol-stimulated adenylate cyclase activity. If receptor number in intact cells was determined at 32 degrees, hardly any loss in receptor number was observed, due to reversal of down-regulation during the incubation period. When beta-adrenoceptor number in broken cell preparations was determined by [125I]cyanopindolol binding at 37 degrees no significant loss was observed, even after 2 hr of isoproterenol treatment, while [3H]CGP-12177 binding resulted in a similar reduction in binding sites as in intact cells. Reversal of loss in surface beta-adrenoceptors was rapid after 1 hr pretreatment with isoproterenol, but followed a biphasic time course after 4 hr pretreatment, with an initial rapid return of about 40% of the down-regulated receptors, followed by a slow, gradual reappearance of receptors. The results indicate that catecholamine exposure leads to a rapid sequestration of MNL surface beta-adrenoceptors away from the cell surface, to a compartment where they are inaccessible to the hydrophilic ligand [3H]CGP-12177 as well as to the lipophilic ligand [125I]cyanopindolol at 4 degrees. Up to 2 hr of isoproterenol treatment does not lead to any breakdown of sequestered beta-adrenoceptors, as they are still recognized by [125I]cyanopindolol binding in broken cell preparations.