Abstract
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.
Footnotes
- Received December 2, 1999.
- Accepted May 24, 2000.
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Send reprint requests to: Dr. R. B. Clark, Department of Integrative Biology and Pharmacology, The University of Texas Medical School, 6431 Fannin, Houston, TX 77030. E-mail:dclark{at}farmr1.med.uth.tmc.edu
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This work was supported by National Institutes of Health Grants GM 31208 (to R.B.C.) and RR07710 (to R.B.).
- The American Society for Pharmacology and Experimental Therapeutics
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