TY - JOUR T1 - Kinetic Analysis of Agonist-Induced Down-Regulation of the β<sub>2</sub>-Adrenergic Receptor in BEAS-2B Cells Reveals High- and Low-Affinity Components JF - Molecular Pharmacology JO - Mol Pharmacol SP - 421 LP - 430 DO - 10.1124/mol.58.2.421 VL - 58 IS - 2 AU - Bruce R. Williams AU - Roger Barber AU - Richard B. Clark Y1 - 2000/08/01 UR - http://molpharm.aspetjournals.org/content/58/2/421.abstract N2 - We examined the interrelationships of internalization and down-regulation of the β2-adrenergic receptor in response to treatment of the BEAS-2B human epithelial cell line with both a series of agonists at high occupancy and with various concentrations of fenoterol that gave occupancies from 0.93 to 0.001. We found that the extent of internalization measured after a 30-min treatment increased as a function of coupling efficiency, with ephedrine, dobutamine, albuterol, fenoterol, and epinephrine giving 0, 7, 17, 48, and 55% internalization, respectively. With the exception of dobutamine, the rates of down-regulation (k deg) also showed a dependence on agonist coupling efficiency, giving (in terms of fraction of receptors lost/h) 0.082 with ephedrine, 0.250 with dobutamine, 0.148 with albuterol, 0.194 with fenoterol, and 0.212 with epinephrine. Comparison of down-regulation to internalization showed that weak agonists caused down-regulation in the absence of significant internalization. The extent of internalization caused by fenoterol over a 1000-fold range of occupancy was proportional to agonist occupancy. However, although no internalization was observed with the low concentrations (0.2 and 2 nM fenoterol), these concentrations did cause significant down-regulation. Thus, as with partial agonists, it was clear that down-regulation occurred in the absence of measurable internalization. The kinetics of agonist-induced down-regulation are consistent with a scheme in which down-regulation proceeds by two pathways; a high-affinity, low-capacity component (EC50 = 0.5 nM) clearly dissociated from internalization and a low-affinity, high-capacity component (EC50 = 160 nM) closely correlated with internalization. ER -