Mechanism of down-regulation of alpha-2 adrenergic receptor subtypes.

Long-term exposure to agonist down-regulates receptor expression for many G protein-coupled receptors. This decrease in receptor density could occur through either an increase in receptor degradation or a decrease 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 receptor endogenous to the HT29 cell line. The rate constants for receptor appearance and disappearance were calculated from the recovery of receptor expression after irreversible inactivation of the existing receptor population with an alkylating agent. In the presence of the agonist norepinephrine, the receptor subtypes in all three cell lines down-regulated to about 50% with a half-time of 2.5 hr. When recovering in the presence of norepinephrine after irreversible inactivation, the rate of receptor degradation increased approximately 2-fold for all three cell lines with little change in the rate of synthesis. During this recovery, the transfected alpha-2A receptor exhibited a half-life of 3.0 hr, which agrees with the 2.7-hr half-time of down-regulation in the presence of norepinephrine. In contrast, the transfected alpha-2B receptor and the endogenous alpha-2A receptor had a half-life of 1.2 hr and 8.9 hr, respectively. For only the endogenous alpha-2A receptor, pertussis toxin increased the half-time of down-regulation to 9.8 hr, similar to the 8.9-hr receptor half-life in the presence of norepinephrine during recovery after irreversible inactivation. Our results indicate that the mechanism of down-regulation of the alpha-2A and -2B adrenergic receptor subtypes is an increase in the rate of receptor degradation.