Receptor internalization delays m4 muscarinic acetylcholine receptor resensitization at the plasma membrane

We analyzed the role of receptor internalization and recycling in muscarinic acetylcholine receptor (mAChR) desensitization and resensitization. Incubation of Chinese hamster ovary cells stably expressing the m4 mAChR with 1 mM carbachol for 1 hr reduced cell surface receptor number by 50-60% with no change in total receptor number. Pretreatment of the cells with 450 mM sucrose, which did not affect the ability of m4 receptors to inhibit forskolin-stimulated cAMP accumulation, completely blocked receptor internalization. On the other hand, the carbachol treatment reduced the ability of m4 receptors to inhibit cAMP accumulation in both sucrose-treated and untreated cells, with a similar onset and to a similar extent. The EC50 value for carbachol was increased approximately 10-fold, and maximal inhibition determined at 100 microM carbachol was reduced approximately 50%. In contrast, thrombin-induced inhibition of cAMP accumulation was not affected. Recycled receptors in cells not treated with sucrose remained refractory to carbachol stimulation for > or = 2 hr after agonist removal, even though cell surface receptor number had recovered completely within 1 hr. In contrast, resensitization of receptor function was very rapid in cells treated with sucrose. Ten minutes on removal of agonist, mAChRs in the plasma membrane of sucrose-treated cells were fully resensitized. Also, an internalization-defective m4 mAChR mutant, T399A, that was found to desensitize similar to the wild-type receptor, resensitized more rapidly than the wild-type receptor. We conclude that desensitization and resensitization of m4 mAChRs in Chinese hamster ovary cells can occur at the plasma membrane and that receptor internalization strongly delays the process of resensitization of desensitized receptors.