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Southern Illinois University School of Medicine, Department of
Pharmacology, Springfield, Illinois 62794
Previous studies have indicated that desensitization of the
A1 adenosine receptor (A1AR), unlike other
adenosine receptor subtypes and G protein-coupled receptors, required
prolonged exposure to agonists. We more closely studied this
observation by focusing on changes in the A1AR signal
transduction pathway after short term agonist exposure (0.5-4 hr) in
the hamster vas deferens smooth muscle cell line (DDT1MF-2
cells). Incubation of these cells with 1 µM
(R)-phenylisopropyladenosine
[(R)-PIA] produced a time-dependent loss in
binding of the agonist radioligand
[125I]N6-2-(4-amino-3-iodophenyl)ethyladenosine
but not of the antagonist radioligand
[3H]8-cyclopentyl-1,3-dipropylxanthine. This was
accompanied by a reduction in the high affinity (G protein-coupled)
state of this receptor from 63 ± 8% to 37 ± 12% after
treatment for 4 hr. Moreover, cells treated with
(R)-PIA demonstrated reduced agonist-stimulated GTPase activity and diminished inhibition of adenylyl cyclase activity
but no change in expression of
i and
subunits. The decreases in agonist binding in the desensitized cells were reversible after treatment of DDT1MF-2 cell membranes with alkaline
phosphatase or protein phosphatases 1 and 2A, suggesting a role of
phosphorylation in the uncoupling and desensitization of the
A1AR. Incubation of cells with
(R)-PIA led to rapid translocation of G
protein-coupled receptor kinase (GRK) from the cytosol to the plasma
membrane within 1 hr of exposure. In addition, purified preparations of the A1AR that were phosphorylated with purified recombinant
GRK-2 demonstrated enhanced affinity for arrestin over
Gi/Go. These results indicate rapid and
functional desensitization of the A1AR by brief exposure to
agonist. The mechanism underlying this event seems to involve
phosphorylation of the A1AR, presumably by the GRK or GRKs.
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