Although desensitization of the beta AR-coupled adenylate cyclase system has been stressed in this chapter, it appears to be a useful model. Carbachol induces sequestration of muscarinic receptors in several cell lines. More recently, agonist-mediated phosphorylation of muscarinic receptors in chick heart has been observed. Based on the predicted amino acid sequence from the recently cloned cDNA, the muscarinic receptor resembles beta AR and rhodopsin with seven membrane-spanning regions and several potential phosphorylation sites in the carboxy-terminal region. Thus, receptor phosphorylation and sequestration may be a general mechanism of desensitization.