Turnover of adrenergic receptors under normal and desensitized conditions

Alpha 1 and beta adrenergic receptor metabolism was investigated by studying receptor reappearance after an irreversible blockade. Phenoxybenzamine was used to irreversibly block alpha 1 adrenergic receptors both in vitro in the BC3H1 cell line and in vivo in rat submaxillary glands. In these two systems, the alpha 1 adrenergic receptor reappearance followed a monoexponential kinetic allowing to determine the half-life of the receptor (23h in vitro, 33h in vivo) as well as the rate of receptor synthesis and degradation. the receptor reappearance was due to receptor synthesis since it was blocked by cycloheximide. The irreversible blockade of beta adrenergic receptors was done with an alkylating beta adrenergic antagonist that we recently developed: Br-pindolol (1). This ligand has high efficiency and blocked at 10(-7)M 80-90% of the beta adrenergic receptors present in C6 glioma cells in culture. After this irreversible blockade, receptors reappeared only during cell division. At confluency, when cells did not significantly divide, receptor synthesis could hardly be detectable. Therefore, at confluency, the metabolic stability of the beta adrenergic receptor is considerable, compared to that of the alpha 1 adrenergic receptor. This stability was confirmed by the observation that after an almost complete "down-regulation" of the beta adrenergic receptor, receptor repopulation of the C6 glioma cells was total and occurred in the presence of cycloheximide.