Kinetic analysis of the trafficking of muscarinic acetylcholine receptors between the plasma membrane and intracellular compartments.

We have studied the kinetics of muscarinic receptor trafficking between the plasma membrane and intracellular compartments in NG108-15 cells. A model of trafficking is proposed that includes four transport steps, delivery to the plasma membrane from the Golgi complex (linear) and endosomes (exponential) internalization from the plasma membrane into endosomes (exponential), and a degradative route from endosomes into lysosomes (linear). Based upon this model, a general equation has been derived that describes changes in receptor number at the plasma membrane and in endosomes in response to agonist stimulation. By following the movement of receptors into and out of the plasma membrane under various experimental conditions, it has been possible to determine the values of the four rate constants in the general equation and also the size of the endosomal receptor pool created by agonist stimulation. The consequences of changes in these constants for receptor trafficking are demonstrated. The model accounts for the effect of varying the duration of agonist stimulation on the size of the endosomal receptor pool and also permits an estimation of the receptor trafficking that underlies the well-established phenomenon of agonist-induced receptor internalization.