Comparison of the kinetics and extent of muscarinic M1-M5 receptor internalization, recycling and downregulation in Chinese hamster ovary cells

We characterized agonist-induced internalization, recycling and downregulation of each muscarinic receptor subtype (M-1-M-5) stably expressed in Chinese hamster ovary (CHO) cells. The radioligands [H-3]QNB and [H-3] NMS were used to measure the total and plasma membrane populations of muscarinic receptors, respectively. Following carbachol treatment (1 mM), the rank orders for the rate of carbachol-induced internalization of the muscarinic subtypes were M-2>M-4 = M-5>M-3 = M-1, respectively. Unlike the M-2 receptor, M-1, M-3, M-4 and M-5 receptors recycled back to the plasma membrane after 1 h carbachol treatment. The receptor downregulation elicited to 24 h carbachol treatment was similar for M-2, M-3, M-4 and M-5 receptors, whereas that for the M-1 receptor was greater. Our results indicate that there are subtype-specific differences in the rate and extent of agonist-induced muscarinic receptor internalization, recycling and downregulation in CHO cells. (C) 2010 Elsevier B.V. All rights reserved.