Kinetic analysis of agonist-induced down-regulation of the beta(2)-adrenergic receptor in BEAS-2B cells reveals high- and low-affinity components.
We examined the interrelationships of internalization and down-regulation of the beta(2)-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 (EC(50) = 0.5 nM) clearly dissociated from internalization and a low-affinity, high-capacity component (EC(50) = 160 nM) closely correlated with internalization.
Williams
B R
BR
Department of Integrative Biology and Pharmacology, The University of Texas Medical School, Houston, Texas 77030, USA.
Barber
R
R
Clark
R B
RB
D000262:Adenylyl Cyclases / Q000378:metabolism; D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D002478:Cells, Cultured; D004280:Dobutamine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation / Q000187:drug effects; D004789:Enzyme Activation; D005280:Fenoterol / Q000494:pharmacology*; D006801:Humans; D007700:Kinetics; D018343:Receptors, Adrenergic, beta-2 / Q000187:drug effects / Q000378:metabolism*
D000318:Adrenergic beta-Agonists; D018343:Receptors, Adrenergic, beta-2; D005280:Fenoterol; D004280:Dobutamine; D000262:Adenylyl Cyclases