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Vol. 297, Issue 3, 1201-1209, June 2001
Department of Medicinal Chemistry and Molecular Pharmacology,
Purdue University, West Lafayette, Indiana
Persistent activation of G
i/o-coupled receptors results
in an enhanced responsiveness of drug-stimulated adenylate cyclase activity through an unknown mechanism. This agonist-induced
heterologous sensitization of drug-stimulated cyclic AMP accumulation
has been proposed to be a mechanism by which cells adapt to prolonged
G
i/o activation. Heterologous sensitization was examined
in human embryonic kidney 293 cells stably expressing D2L
dopamine receptors in combination with recombinant isoforms of
adenylate cyclase. The ability of each isoform to be differentially
regulated by G protein subunits and other signaling intermediates
allowed us to identify potential mechanisms that are involved in
heterologous sensitization of adenylate cyclase. We now report that
both short- and long-term activation of D2L dopamine
receptors resulted in a marked degree of sensitization of ACI, ACII,
ACV, and ACIX, but not ACVIII. The effects of agonist treatment on ACI,
ACII, and ACVIII appeared to be dependent upon the ability of these
adenylate cyclase isoforms to synergistically respond to selective
activators in the presence of activated G
s.
Sensitization of ACV was characterized by enhanced cyclic AMP
accumulation following G
s or forskolin stimulation. Furthermore, agonist pretreatment enhanced the basal levels of cyclic
AMP accumulation in ACV/D2L cells, an effect that was not observed with the other adenylate cyclase isoforms. ACIX, which has no
known activators other than G
s, showed robust
agonist-induced sensitization of isoproterenol-stimulated cyclic AMP
accumulation. In summary, heterologous sensitization appeared to be
related to the ability of each adenylate cyclase isoform to be
modulated by G
s.
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