Sequestration and recycling of beta 2-adrenergic receptors permit receptor resensitization

Stimulation of beta 2-adrenergic receptors in intact cells causes, first, rapid functional uncoupling from Gs, which is triggered by receptor phosphorylation, and, second, somewhat slower sequestration of the receptors to an internal compartment. The present study addresses a possible role of sequestration in the resensitization of desensitized beta 2-adrenergic receptors in human A431 cells. Exposure of these cells to isoproterenol caused rapid phosphorylation, desensitization (as assessed in adenylyl cyclase assays), and sequestration of the receptors. Subsequent removal of the agonist led to recycling of the receptors to the cell surface, dephosphorylation, and restoration of receptor function. These effects occurred without any change in the total receptor number. The rate constant of agonist-induced sequestration was 0.03/min; the rate constant of receptor recycling was 0.06/min and was not markedly altered by the presence of agonist. Blockade of sequestration with concanavalin A or 0.6 M sucrose prevented receptor dephosphorylation as well as receptor resensitization. Inhibition of protein phosphatases with calyculin A caused a similar blockade of beta 2-adrenergic receptor resensitization; the effects of maximally effective concentrations of concanavalin A and calyculin A were not additive. Monensin impaired recycling of desensitized beta 2-adrenergic receptors to the cell surface and also prevented receptor resensitization. We conclude that sequestration of beta 2-adrenergic receptors, followed by dephosphorylation and recycling to the cell surface, may serve to restore the function of desensitized receptors.