Zusammenfassung
Stimulation of beta 2-adrenergic receptors in intact cells causes,
first, rapid functional uncoupling from Gs, which is triggered by
receptor phosphorylation, and, second, somewhat slower sequestration
of the receptors to an internal compartment. The present study addresses
a possible role of sequestration in the resensitization of desensitized
beta 2-adrenergic receptors in human A431 cells. Exposure of these
cells to isoproterenol caused rapid phosphorylation, desensitization
(as assessed in adenylyl cyclase assays), and sequestration of the
receptors. Subsequent removal of the agonist led to recycling of
the receptors to the cell surface, dephosphorylation, and restoration
of receptor function. These effects occurred without any change in
the total receptor number. The rate constant of agonist-induced sequestration
was 0.03/min; the rate constant of receptor recycling was 0.06/min
and was not markedly altered by the presence of agonist. Blockade
of sequestration with concanavalin A or 0.6 M sucrose prevented receptor
dephosphorylation as well as receptor resensitization. Inhibition
of protein phosphatases with calyculin A caused a similar blockade
of beta 2-adrenergic receptor resensitization; the effects of maximally
effective concentrations of concanavalin A and calyculin A were not
additive. Monensin impaired recycling of desensitized beta 2-adrenergic
receptors to the cell surface and also prevented receptor resensitization.
We conclude that sequestration of beta 2-adrenergic receptors, followed
by dephosphorylation and recycling to the cell surface, may serve
to restore the function of desensitized receptors.
Nutzer