MECHANISM OF DOWN-REGULATION OF ALPHA-2-ADRENERGIC RECEPTOR SUBTYPES

Long-term exposure to agonist down-regulates receptor expression for m any G protein-coupled receptors. This decrease in receptor density cou ld occur through either an increase in receptor degradation or a decre ase in receptor synthesis, We studied the mechanism of down-regulation of the alpha-2A and alpha-2B adrenergic receptor subtypes transfected into the Chinese hamster ovary cell line as well as the alpha-2A rece ptor endogenous to the HT29 cell line. The rate constants for receptor appearance and disappearance were calculated from the recovery of rec eptor expression after irreversible inactivation of the existing recep tor population with an alkylating agent. In the presence of the agonis t norepinephrine, the receptor subtypes in all three cell lines down-r egulated to about 50% with a half-time of 2.5 hr. When recovering in t he presence of norepinephrine after irreversible inactivation, the rat e of receptor degradation increased approximately 2-fold for all three cell lines with little change in the rate of synthesis. During this r ecovery, the transfected alpha-2A receptor exhibited a half-life of 3. 0 hr, which agrees with the 2.7-hr half-time of downregulation in the presence of norepinephrine. In contrast, the transfected alpha-2B rece ptor and the endogenous alpha-2A receptor had a half-life of 1.2 hr an d 8.9 hr, respectively. For only the endogenous alpha-2A receptor, per tussis toxin increased the half-time of down-regulation to 9.8 hr, sim ilar to the 8.9-hr receptor half-life in the presence of norepinephrin e during recovery after irreversible inactivation. Our results indicat e that the mechanism of down-regulation of the alpha-2A and -2B adrene rgic receptor subtypes is an increase in the rate of receptor degradat ion.