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 [(3)H]QNB and [(3)H]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 24h 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.