Repetitive endocytosis and recycling of the β₂-adrenergic receptor during agonist-induced steady state redistribution

The human β₂-adrenergic receptor (β₂AR) rapidly internalizes after binding agonist, resulting in a dramatic redistribution of receptors from the plasma membrane and into endocytic vesicles. We sought to determine whether intracellular receptors constitute a static pool or represent a fraction of dynamically internalizing and recycling receptors. Using cells expressing a β₂AR with an epitope tag at its amino-terminal ectodomain, changes in surface receptor levels were measured by flow cytometry and radioligand binding assays. The addition of a saturating level of a strong agonist (isoproterenol) caused the endocytosis of receptors with first- order kinetics (k(e) for naive cells, 0.222 min^-1). After 10 min, the level of surface receptors remained stable at ~20% that of untreated cells, even though endocytosis continued with similar kinetics (k(e) for pretreated cells, 0.258 min^-1), suggesting that internalized receptors were cycling in steady state with surface receptors. This prediction was confirmed directly by showing that internalized β₂ARs recycled to the cell surface in the continued presence of agonist. The calculated transit times (1/k) in the presence of isoproterenol were 3.9 min for endocytosis and 11.2 min for recycling. The endocytic rate constant and the steady state redistribution to the internal pool were much lower after treatment with the partial agonist albuterol, suggesting a correlation between the efficiency of endocytosis and that of receptor coupling to the downstream signal transduction pathway. These findings indicate that in the presence of agonist, β₂ARs are in a dynamic steady state between the plasma membrane and endosomes that is regulated principally by agonist efficacy.